Stefaan Stevens modified ZSM-5 Transformation of bioethanol into hydrocarbons on

Stefaan Stevens

modified ZSM-5Transformation of bioethanol into hydrocarbons on

Academiejaar 2011-2012Faculteit Ingenieurswetenschappen en ArchitectuurVoorzitter: prof. dr. ir. Guy MarinVakgroep Chemische Proceskunde en Technische Chemie

Master in de ingenieurswetenschappen: chemische technologieMasterproef ingediend tot het behalen van de academische graad van

Begeleider: Kristof Van der BorghtPromotor: dr. Vladimir Galvita

FACULTEIT INGENIEURSWETENSCHAPPEN

EN ARCHITECTUUR

Laboratorium voor Chemische Technologie • Krijgslaan 281 S5, B-9000 Gent • www.lct.ugent.be

Secretariaat : T +32 (0)9 264 45 16 • F +32 (0)9 264 49 99 • [email protected]

Laboratorium voor Chemische Technologie

Verklaring in verband met de toegankelijkheid van de scriptie

Ondergetekende, Stefaan Stevens, afgestudeerd aan de UGent in het academiejaar

2011-2012 en auteur van de scriptie met als titel Transformation of bioethanol into

hydrocarbons on modified ZSM-5: .

verklaart hierbij:

1. dat hij/zij geopteerd heeft voor de hierna aangestipte mogelijkheid in verband

met de consultatie van zijn/haar scriptie:

de scriptie mag steeds ter beschikking gesteld worden van elke

aanvrager

de scriptie mag enkel ter beschikking gesteld worden met uitdrukkelijke,

schriftelijke goedkeuring van de auteur

de scriptie mag ter beschikking gesteld worden van een aanvrager na

een wachttijd van…………jaar

de scriptie mag nooit ter beschikking gesteld worden van een aanvrager

2. dat elke gebruiker te allen tijde gehouden is aan een correcte en volledige

bronverwijzing

Gent,

Vakgroep Chemische Proceskunde en Technische Chemie

Laboratorium voor Chemische Technologie

Directeur: Prof. Dr. Ir. Guy B. Marin

D

Dankwoord

First of all I would like to thank my promotor Vladimir Galvita to give me the opportunity to

work on this project and for his advice during this thesis project.

Ik wil in het bijzonder mijn begeleider Kristof bedanken. Zijn advies en steun zowel tijdens

de experimenten als bij het verwerken ervan was noodzakelijk om deze thesis tot een goede

einde te brengen, mijn welgemeende dank hiervoor !

Verder wil ik mijne medestudenten, in het bijzonder Ben, Jeroen en Matthias bedanken. Ik

denk dat het laatste jaar een serieuze uitdaging was, maar we kunnen er met een tevreden

gevoel op terugkijken. Veel succes gewenst in jullie verder carrière!

Tot slot, wil ik mijn vriendin Shanna nog bedanken, je stond steeds klaar na een intensieve

thesis-dag en had begrip voor beperkte hoeveelheid tijd, dat ik met je de laatste weken heb

doorgebracht.

Stefaan

Juli 2012

Table of contents

Chapter 1 Introduction 1

1.1 Biomass as feedstock 4

1.2 Bio ethanol production process 5

1.3 Bio ethanol intermediate or product 6

1.4 Bio ethanol as a feedstock 6

1.5 Scope of the thesis 8

1.6 References 9

Chapter 2 Literature study 11

2.1 Zeolites as suitable catalysts 12

2.2 Application of various catalysts for the ethanol to ethylene conversion 16

2.3 Application of various catalysts for the ethanol to hydrocarbon conversion 19

2.4 Ethanol to hydrocarbon conversion on HZSM5 19

2.4.1 Process conditions 21

2.4.2 Post synthesis modifications 25

2.5 Reaction mechanism 29

2.5.1 Introduction 29

2.5.2 Ethanol to ethylene 29

2.5.3 Ethanol to higher hydrocarbons 31

2.6 References 35

Chapter 3 Procedures 38

3.1 Preparation 38

3.1.1 Incipient wetness impregnation on HZSM-5 38

3.1.2 Ion Exchange 39

3.2 Characterization 39

3.2.1 Metal content determination 39

3.2.2 Hydrogen Temperature Programmed Reduction 39

3.2.3 Ammonia Temperature Programmed Desorption 40

3.2.4 BET surface area and total pore volume 40

3.3 Experimental setup 42

3.3.1 Reaction section 42

3.3.2 Analysis section 44

3.4 Validation of plug flow and intrinsic kinetics 44

3.5 Data treatment 44

3.5.1 GC Analysis 44

3.5.2 Calculation of conversion, yields and selectivity’s 47

3.6 Reference 48

Chapter 4 Experimental study on the ethanol conversion on HZSM-5 49

4.1 Introduction 50

4.2 Product distribution 51

4.3 Stability 51

4.4 Effect of process conditions 52

4.4.1 Space time 52

4.4.2 Temperature 53

4.4.3 Effect of water 54

4.5 Reaction mechanism elucidation 55

4.5.1 Experimental evidence 56

4.5.2 Products 59

4.6 Conclusion 63

4.7 References 64

Chapter 5 Tuning HZSM5 65

5.1 Tuning the acidity by changing the Si/Al ratio 66

5.1.1 Physical and acid properties 66

5.1.2 Performance 68

5.1.3 Linking properties and testing 70

5.2 Metal introduction 72

5.2.1 Physical and acid properties 72

5.2.2 Performance 78

5.2.3 Linking properties and testing 81

5.3 Conclusion 83

5.4 References 84

Chapter 6 Conclusions and future work 87

Appendix A 89

Appendix B: 91

Appendix C: 93

List of figures

Figure 1-1: relative oil depletion [1] .......................................................................................... 1

Figure 1-3: articles published regarding biomass as energy alternative fuel. ............................ 2

Figure 1-4:Main routes to biofuels.[4] ....................................................................................... 4

Figure 1-5: Most important bio ethanol conversion routes. ....................................................... 6

Figure 2-1: (a) primary TO4 structure (b) TO4 sharing a common oxygen atom.[7] ............... 12

Figure 2-2: Brönsted and Lewis acid sites. .............................................................................. 13

Figure 2-3: Kinetic diameter of some molecules and zeolites[10]. .......................................... 14

Figure 2-4: Primary shape selectivity mechanisms: a) reactant shape selectivity: cracking of

an n- and iso- ............................................................................................................................ 14

Figure 2-5: Influence of the acidity on the coke formation rate. [12] ...................................... 16

Figure 2-6:Process flow sheet of conversion of ethanol to ethylene. ....................................... 18

Figure 2-7: MFI structure viewed along [010] [21] ................................................................. 20

Figure 2-8: Effect of space time and water content on product distribution at a temperature of

350°C and 450°C. ( XE Weight fraction of ethene, based on the organic components [calculated as the (mass flow of ethene)/(mass

flow of organic components) ratio] Xi =Weight fraction of component i, based on the organic components [calculated as the (mass flow of

component i)/(mass flow of organic components) ratio] XWo =Water/ethanol ratio in mass, in the feed.catalyst HZSM5(Si/Al=24) .............22

Figure 2-9: (a) Conversion/yield(%) in function of Time on stream (TOS) (b) carbon content

in function of TOS. Reaction conditions: T=350°C, P=30bar, N2/EtOH=4, W/F=0.09 molEtOH/kgcat.s) catalyst:HZSM5(Si/Al=16).

.................................................................................................................................................. 23

Figure 2-10: GC-MS analysis of coke molecules extracted by CH2Cl2 after solubilization by

HF solution, of coked samples TOS = 1 h and 30h. (Reaction conditions: T=350°C, P=30bar, N2/EtOH=4,

W/F=0.09molETOH/kgcat.s, catalyst= HZSM-5 Si/Al=16). ...................................................................................... 24

Figure 2-11: Dehydration of ethanol. ....................................................................................... 30

2

Figure 2-12: Conventional mechanism and proposed mechanism. .......................................... 30

Figure 2-13: Simplified representation of hydrocarbon pool. .................................................. 32

Figure 2-14: Dual catalytic cycle. ............................................................................................ 33

Figure 2-15: Comparison between different hypothesizes. ..................................................... 34

Figure 3-1: Simplified schematic representation of reactor setup………………………... …42

Figure 4-1: Yield in function of temperature ........................................................................... 50

Figure 4-2: A typical product distribution reaction conditions: T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH,

EtOH/He=1/10,Ptot=1bar catalyst: HZSM5 (Si/Al=30). Detailed values are added in Appendix H. .............................................................. 51

Figure 4-3: Conversion/selectivity (mol%) i.f.o. TOS (hr): reaction conditions: T=350°C,Wcat/FEtOH= 16.6

kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, catalyst: HZSM5(Si/Al=30). Detailed values can be found in appendix H. ....................... 52

Figure 4-4: Conversion of ethanol and ethylene (mol%) in function of W/F [kgcat.s/molEtOH].

Reaction conditions: T=350°C, EtOH/He=1/10,Ptot=1bar, catalyst:HZSM5 (Si/Al=30). ...................................................... 53

Figure 4-5: conversion/selectivity (mol%) in function of temperature. Reaction conditions: Wcat/FEtOH=

16.6 kgcat.s/molEtOH , Ptot=1bar, TOS=3-5hr, catalyst: HZSM5 (Si/Al=30). ................................................................... 54

Figure 4-6:Ethylene conversion in function of feed composition. reaction conditions: T=350°C, ,Ptot=1bar,

TOS=3-5hr, catalyst: HZSM5 (Si/Al=30). feedstock:50wt% ethanol-50wt% water, EtOH/(He+H2O)=1/10 Wcat/FEtOH=16.6 kgcat.s/molEtOH55

Figure 4-7: Lumped reaction mechanism. ................................................................................ 55

Figure 4-8: Selectivity (mol%) i.f.o. ethylene conversion(mol%) reaction conditions: T=350°C,

EtOH/He=1/10,Ptot=1bar, TOS=3-8hr, variable W/F [8.3-55 kgcat.s/molEtOH]. catalyst: HZSM5 (Si/Al=30)................................................ 56

Figure 4-9: Selectivity (mol%) in function of conversion (mol%). reaction conditions: Solid data points

T=300-400°C, EtOH/He=1/10,Ptot=1bar, TOS=3-6hr, catalyst: HZSM5 (Si/Al=30) Wcat/FEtOH=16.6 kgcat.s/molEtOH. Not filled data points

T=350°C Wcat/FEtOH=8.8-55 kgcat.s/molEtOH . .................................................................................................................................................. 57

Figure 4-10: selectivity (mol%) i.f.o. ethylene conversion. reaction conditions: T=350°C, ,Ptot=1bar, TOS=3-

5hr, catalyst: HZSM5 (Si/Al=30). No filling data points: feed: ethanol, Wcat/FEtOH=8.8-55 kgcat.s/molEtOH EtOH/(He)=1/10. Dotted data

3

points: feed: ethanol, Wcat/FEtOH=16.6 kgcat.s/molEtOH EtOH/(He)=1/10 solid data points: feedstock:50wt% ethanol-50wt% water,

EtOH/(He+H2O)=1/10 Wcat/FEtOH=16.6 kgcat.s/molEtOH ................................................................................................................................. 58

Figure 4-11: Selectivity (mol%) in function of the ethylene conversion (mol%) reaction conditions:

T=350°C, EtOH/He=1/10,Ptot=1bar, TOS=3-8hr, variable W/F [8.3-55 kgcat.s/molEtOH] catalyst: HZSM5 (Si/Al=30) (a) selectivity of C3-C5

olefins, (b) selectivity of C1-C5 paraffin’s, (c) BTXE selectivity d) selectivity of C5+ in function of ethylene conversion. ........................... 59

Figure 4-12: Molar ratio of C2-C5 paraffins/C3-C5 olefins and C2-C5 paraffins/BTXE]. ( Reaction

conditions: T=350°C, EtOH/He=1/10,Ptot=1bar, variable W/F catalyst:HZSM5 Si/Al=30). .................................................. 61

Figure 5-1: NH3 TPD profiles of HZSM5 (Si/Al=30,Si/Al=50,Si/Al=80) (β=5 °C/min). ....... 67

Figure 5-2: ethylene conversion for HZSM5 (Si/Al=30), HZSM5 (Si/Al=50) and HZSM5

(Si/Al=80). reaction conditions: T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, TOS=4-6hr. ................... 69

Figure 5-3: Selectivity (mol%) i.f.o. ethylene conversion(mol%) Solid data points reaction conditions:

T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, TOS=4-6hr, catalysts: HZSM5 (Si/Al:30, 50 and 80). No filled data

points T=350°C Wcat/FEtOH=8.8-55 kgcat.s/molEtOH, EtOH/He=1/10, Ptot=1bar, TOS=4-6hr, catalyst: HZSM5 (Si/Al=30). .............................70

Figure 5-4: A Ethylene conversion in function of concentration strong acid sites [mol

NH3/kgcat] B Activity per active center(=AAS) in function of activity (ethylene

conversion(mol%)) Reaction conditions (cf.5.1.2). ................................................................ 71

Figure 5-5: a H2 TPR of Fe-ZSM5 b NH3 TPD of

Fe-ZSM5. 73

Figure 5-6: a H2 TPR of Ga-ZSM5 b NH3 TPD of

Ga-ZSM5. 75

Figure 5-7: H2 TPR of Ni-ZSM5 b NH3 TPD of

Ni-ZSM5. 76

Figure 5-8: Concentration of high temperature acid sites in function of metal content (%) .... 77

Figure 5-9: Yield (%) reaction conditions: : T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH,

EtOH/He=1/10,Ptot=1bar, TOS=4-6hr. ................................................................................... 78

Figure 5-10: Selectivity (mol%) in function of ethylene conversion(mol%) reaction conditions: M-

HZSM5: T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, TOS=4-6hr.solid data point=IMP arrayed data point=EX

reaction conditions HZSM5= T=350°C,Wcat/FEtOH=5.5- 55 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, TOS=4-6hr. .......................................80

4

Figure 5-12: ethylene conversion (mol%) versus total acid site concentration[mol NH3/kg].

reaction conditions: T=350°C, ,Ptot=1bar, TOS=3-5hr, Wcat/FEtOH=16.6 kgcat.s/molEtOH . ..................................................... 81

Figure 5-13: Activity per active center(=AAS) in function of activity (ethylene

conversion(mol%)) Reaction conditions (cf.5.1.2) ................................................................. 82

List of Tables

Table 1-1: Discussion of the most important bio ethanol conversion routes ............................. 6

Table 2-1: Application of zeolites and metallic catalysts in the chemical industry. [9] .... Error!

Bookmark not defined.

Table 2-2: Ethanol to ethylene catalytic conversion processes. ............................................... 17

Table 2-3: Effect of post synthesis modifications. Reaction conditions 4 T=400°C, W/F=0.16 mol EtOH/gcat

h-1 catalyst: modified (Incipient Wetness Impregnation) HZSM5, 10% metal loading except for Pt 2%. ...................... 25

Table 2-4: Rol of iron, nickel and gallium in hydrocarbon reactions and specific in the ethanol

to hydrocarbon conversion. [22]…………………………………………………………….. 26

Table 3-1: Experimental conditions of H2-TPR. ...................................................................... 40

Table 3-2: Experimental conditions of NH3-TPD. ................................................................... 40

Table 3-3: BET surface area and total pore volume. ................................................................ 41

Table 3-4: Interval of ethanol and helium flow. ....................................................................... 43

Table 3-5:Reference conditions................................................................................................ 43

Table 3-6: GC-settings……………………………………………………………………….. 44

Table 4-1: Reaction mechanism proposal. ............................................................................... 62

Table 5-1: BET surface area, total pore volume and concentration of strong acid sites for

HZSM5 catalyts. ....................................................................................................................... 66

Table 5-2: Aimed and actual (ICP) metal loading (%). ........................................................... 72

Table 5-3: BET surface area, total pore volume and concentration of strong acid sites for

HZSM (Si/Al=30) and M-ZSM5 catalysts. ............................................................................. 73

Table 5-4: Summary of best performance (conversion/selectivity)………………………… 79

Nomenclature

Nomenclature Acronym Description

AAGR Aggregate Annual Grow Rate

AAS Activity per Active Site

ALD Atomic Layer Deposition

Al-MAS- MR Aluminium Magic Angle Spinning Nuclear Magnetic Resonance

BET Brunauer-Emmett-Teller isotherm

BTXE Benzene Toluene Xylenes Ethylbenzene group

CVD Chemical Vapor Deposition

EFAL Extra Framework Aluminum species

ETG Ethanol to gasoline

EX Ion Exchange

FID Flame Ionization Detector

FT-IR Fourier Transform Infrared Spectroscopy

ICP Inductively Coupled plasma mass spectroscopy

IEA International Energy Agency

IMP Incipient Wetness Impregnation

H2 TPR Hydrogen Temperature Programmed Reduction

NNN Next nearest Neighbor theorem

NH3 TPD Ammonia Temperature Programmed Desorption

M/HZSM5 Metal Modified HZSM5

MS Mass Spectrometer

TCD Thermal Conductivity Detector

Roman symbols Description Units

Ai

CFi

Area of the peak of component i

Calibration factor of component i

F Molar flowrate Mol s-1

F Mass flowrate Kg s-1

n Number of carbon atoms -

N

MWi

Number of repetition experiments

Molecular Weight of component i

-

Kg/mol

Ptot Total pressure atm

PEtOH Ethanol partial pressure atm

S Estimated standard deviation -

T Temperature [°C]

TOS Time on stream h

HN Number of atoms of x in component i -

W Weight catalyst [kg]

XEtOH ethanol conversion mol mol-1

Y Yield

Greek symbols Description Units

Nomenclature

β Heating rate °C/min

Supscript Description Units

EtOH Ethanol

i Component index

Tot Total

Subscript

0 Indication for feed molecule

Transformation of bioethanol into hydrocarbons on modified ZSM-5

Stefaan Stevens

Begeleider: ir. Kristof Van der Borght Promotor: Dr. Vladimir Galvita

Abstract: In deze thesis is een systematische studie is uitgevoerd

naar de conversie van ethanol naar hogere koolwaterstoffen over

zowel een niet gemodificeerde als een gemodificeerde HZSM5

katalysator. Fysisch chemische karakterisatie methoden zijn

gecombineerd met de overeenkomstige katalystische prestaties Een

uitgebreide literatuurstudie is voorafgegaan aan deze

experimentele studie. Inzicht in het effect van verschillende

modificatie technieken en de huidige hypothesen omtrent het

reactie mechanisme zijn verkend. De prestaties van een niet

gemodificeerde HZSM5 katalysator zijn vervolgens bestudeerd op

een vast bed reactor. Modificatie is gebeurd door variatie van Si/Al

ratio met als doel het effect van zuurtegraad te bestuderen en

introductie van metalen door droge impregnatie en ion uitwisseling

met als doen additionele functionaliteiten te identificeren. Een

relatie is gevonden tussen het zuur karakter en de activiteit voor

alle verschillende toegepaste modificatie technieken.

Trefwoorden: HZSM5, ethanol, koolwaterstoffen, zuurgehalte,

metaal introductie

I. INTRODUCTIE

Belangrijke organische intermediaren (ethylene, propylene,

BTX,… ) en brandstof zijn voornamelijk afkomstig van

fossiele bronnen. Vanwege hun beperkte beschikbaarheid,

milieu onvriendelijk en politieke impact, zijn hernieuwbare

alternatieven noodzakelijk. Bio ethanol is hiervoor een goed

alternatief. Bio ethanol kan worden geproduceerd van

lignocellulosic biomassa zonder in competitie te zijn met de

voedingsindustrie [1]. Ethanol is al reeds geïncorporeerd als

brandstof in Brazilië en de Verenigde Staten. In dit werk,

wordt de verdere katalytische omzetting naar hogere

koolwaterstoffen onderzocht. Goede katalytische prestaties

zijn reeds vastgesteld voor HZSM5 [2]. Vele

wetenschappelijke artikels beweren een bevorderend effect

van de activiteit, selectiviteit en stabiliteit door toepassen van

modificatie technieken[3, 4]. Door het gebrek aan een

combinatie van gedetailleerde karakterisering en katalytische

testen, is het huidig inzicht beperkt.

II. PROCEDURES

HZSM5 katalysatoren zijn bereid door calcinatie van

commercieel beschikbare NH4-ZSM5 (Zeolyst) zeolieten.

Metalen zijn geïntroduceerd door impregnatie en ion

uitwisseling. NH3TPD en H2-TPR zijn uitgevoerd op

Micromertics Autochem 2910. Het specifieke oppervlak is

bepaald door stikstof adsorptie. De katalytische testen zijn

uitgevoerd op een vast bed reactor. Analyses van de product

samenstelling zijn gebeurd op een Chrompack GC met een

FID detector. De reactie condities zijn samengevat in tabel 1.

Tabel 1: Operationele instellingen.

T (°C) W/F[molEtOH.s/kgcat] Ptot PEtOH

200-400 5,5-55 1 0,1

III. EXPERIMENTELE STUDIE VAN ETHANOL NAAR

KOOLWATERSTOFFEN OVER HZSM5 KATALYSATOR

Vier temperatuurzones kunnen worden onderscheiden (cf.

figuur 1). Een temperatuur lager dan 220°C leidt tot partiële

dehydratatie van ethanol tot diethylether. Verdere toename

van de temperatuur (ǀǀ) zorgt voor verdere omzetting van

ethanol naar zowel ethyleen als diethylether. Verdere

temperatuur verhoging heeft tot gevolg dat ethanol volledig

wordt omgezet in uitsluitend ethyleen. Een temperatuur hoger

dan 300°C blijkt noodzakelijk te zijn voor de vorming van C2+

koolwaterstoffen.

Figuur 1:Opbrengst(%) in functie van reactie temperatuur. Wcat/FEtOH= 16.6 kgcat.s/molEtOH, HZSM5(Si/Al=30).

Reproduceerbare data in de afwezigheid van deactivatie is

bereikt. Omdat een hoge selectiviteit/opbrengst reeds kan

worden behaald zullen verdere testen enkel gebeuren op

hogere temperaturen.

In figuur 2 is de selectiviteit voor verschillende product

groepen weergegeven in functie van de ethyleen conversie.

Een toename van de ethyleen conversie leidt tot de reductie

van de selectiviteit ten opzichte van vluchtige alkenen

(Figuur 2A ) en de BTXE groep (Figuur 2C en D). Een

toename van de vluchtige alkanen (Figuur 2B) en een

maximum van de selectiviteit van de C6-C8 olefines (Figuur

2C) is eveneens merkbaar. Een mogelijke verklaring hiervoor

is de vorming van aromaten vertrekkende van cyclo alkenen

die aanleiding geven tot waterstof overdracht reacties tussen

alkenen en alkanen.

Figuur 2: Selectiviteit in functie van ethylene conversie (a) C3-C5

alkenen, (b) C1-C5 alkanen, (c) BTXE d) C5+. ( HZSM5 (Si/Al=30), T=350°C,

variable W/F [8.3-55 kgcat.s/molEtOH] )

IV. KATALYSATOR MODIFICATIE

A. Si/Al

Een toename van de Si/Al ratio veroorzaakt een afname van

de concentratie van sterke zure centra (cf. Figuur 4A) en een

afname van de activiteit op basis van de corresponderende

NH3 TPD profielen. Het is ook opgemerkt dat niet enkel de

concentratie maar ook de sterkte van de zure sites een

belangrijke rol speelt.

B. Metaal gemodificeerde HZSM5

Gallium, nikkel en ijzer zijn geïntroduceerd in verschillende

metaalhoeveelheden door ion uitwisseling en impregnatie.

Een toename van de metaal hoeveelheid zal op basis van de

corresponderende H2 TPR profielen, specifiek oppervlak en

totaal porie volume, resulteren in de vorming van

voornamelijk metaal oxiden en agglomeraten die de poriën

gaan blokkeren en leiden tot een reductie van de concentratie

van het aantal toegankelijke zure sites (cf. Figuur 4A).

Ondanks de reductie van de concentratie van toegankelijke

zure sites kan een toename in de activiteit worden opgemerkt

voor lage metaal beladingen (<1%). Op basis van figuren

3A,3B,3C en 3D) heeft de introductie van de verschillende

metalen geen significant effect op de selectiviteit.

Figuur 3: Selectiviteit (mol%) in function van de ethylene

conversie (mol%). (M-HZSM5: T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, arrayed data

point=EX reaction conditions HZSM5= T=350°C,Wcat/FEtOH=5.5-55 kgcat.s/molEtOH.)

Het bevorderend effect van metal introductie wordt duidelijk

op basis van figuur 4. De activiteit per actief site is uitgezet

voor alle verschillende geteste katalysatoren met uitzondering

van Ni-HZSM5. Dit vanwege onnauwkeurigheid in de

bepaling van de concentratie van zure centra. Als referentie is

HZSM5 met een variabele ruimtetijd ook weergegeven.

Figuur4: Activiteit per actief centrum(=AAC) in functie van de

ethylene conversie. (reaction conditions: T=350°C, variable W/F [8.3-55 kgcat.s/molEtOH].

voor HZSM5 (Si/Al=30,50,80),Fe/HZSM5 (verschillende beladingen),Ga/HZSM5 verschillende

beladingen.

Een hogere activiteit per actief site is voor alle metaal

gemodificeerde katalysatoren aanwezig. Dit illustreert het

promoverend effect van metaal introductie.

V. CONCLUSIE

Effect van temperatuur , ruimtetijd, en toevoeging van water

aan de voeding, zijn bestudeerd. Een voorstelling voor een

reactiemechanisme gebaseerd op deze experimentele data is

geformuleerd. Afname van de Si/Al verhouding en introductie

van een kleine metaalhoeveelheid (<1%) heeft telkens geleid

tot toename van de activiteit.

Een relatie tussen de concentratie zure centra concentratie en

de ethylene conversie is waargenomen. Door introductie van

metalen door zowel impregnatie als ion uitwisseling is een

sterke wijziging van de activiteit waar genomen

Ga/HZSM>Fe/HZSM5>HZSM5. De selectiviteit daarentegen

is niet significant gewijzigd.

REFERENCES

[1] 1. Centi, G., P. Lanzafame, and S. Perathoner, Analysis of the alternative routes in the catalytic transformation of lignocellulosic

materials. Catalysis Today, 2011. 167(1): p. 14-30.

[2] 2. Magnoux, P., et al., Ethanol transformation over HFAU, HBEA and HMFI zeolites presenting similar Bronsted acidity. Applied

Catalysis a-General, 2009. 367(1-2): p. 39-46.

[3] 3. Inaba, M., et al., Production of chemical compound from bio-ethanol by zeolite catalysts. Fourth International Symposium on

Environmentally Conscious Design and Inverse Manufacturing,

Proceedings, 2005: p. 866-872. [4] 4. Lu, J.Y., Y.C. Liu, and N. Li, Fe-modified HZSM-5 catalysts for

ethanol conversion into light olefins. Journal of Natural Gas

Chemistry, 2011. 20(4): p. 423-427.

Transformation of bioethanol into hydrocarbons on modified ZSM-5

Stefaan Stevens

Coach: ir. Kristof Van der Borght Supervisor(s): Dr. Vladimir Galvita

Abstract: In this work, a systematic study of the ethanol to

hydrocarbon conversion reaction over non modified and

modified HZSM5 catalysts is presented by linking the physical

chemical properties with catalytic testing results. First, an

extensive literature study is made focusing on the current

reaction mechanism hypothesizes and the tunable character of

HZSM5. Then is an experimental study performed over a

reference HZSM5 (Si/Al=30) catalyst. The effect of temperature

space time and co feeding water are determined. Indications

regarding the reaction mechanism are formulated based on the

obtained experimental data. Modifications to the reference

catalyst were done by varying the Si/Al ratio and introducing

metals through incipient wetness impregnation and ion

exchange. A relation has found for all these modification

methods between the activity and the acidity of the catalyst.

Keywords: HZSM5, ethanol, hydrocarbons, acidity, metal

modified

I. INTRODUCTION

Conventionally, chemical basic building blocks and transport

fuels originate primarily from fossil resources. However due

to their limited availability, environmental and political

impact the development of renewable alternatives becomes

urgent. One of these alternatives is bioethanol. Bioethanol can

be produced at low cost starting from lignocellulosic biomass

without having an impact on food production [1]. Ethanol is

already commonly used as a biofuel in countries like Brazil

and the USA. This work explores the further catalytic

conversion of ethanol to biogasoline and biochemicals.

According to literature [2] HZSM5 is a very suitable catalyst.

Many articles in literature claim promotion of the activity,

selectivity or stability of HZSM-5 through a variety of

modification techniques[3][4]. However due the absence of

the combination of a detailed catalyst characterization and

catalytic testing, only a limited knowledge on the origin of

these effects is present.

II. PROCEDURES

Through calcination, starting from NH4-ZSM5 (Zeolyst),

HZSM5 is prepared. Incipient wetness impregnation and ion

exchange are applied for metal introduction. The acidity is

determined by NH3TPD and the nature of the reducible

species by H2-TPR ( Micromertics Autochem 2910). Specific

surface area and total pore volume are determined by N2

adsorption (Micromeretics Gemini). Catalytic testing is

conducted on a fixed bed lab scale reactor. A Chrompack GC

equipped with a FID detector is used for product analysis. The

reaction conditions are added in

Table 1.

Table 1: Operating conditions.

T (°C) W/F[molEtOH.s/kgcat] Ptot PEtOH

200-400 5,5-55 1 0,1

III. EXPERIMENTAL STUDY OF ETHANOL TO HYDROCARBON

CONVERSION OVER HZSM5

Preliminary experiments led to the conclusion that four

temperature zones can be distinguished (cf. Figure 1).

At a temperature lower than 220°C (zone I) ethanol results

in intramolecular dehydration of ethanol to diethylether. In

this zone, there is no significant formation of ethylene and

higher hydrocarbons. In zone II (220-260°C), ethylene will be

formed which is accompanied by the decrease of diethylether.

Characteristic to this zone is the presence of both ethylene as

diethylether. Starting from 260°C (zone III) ethanol will be

completely converted to primarily ethylene and, in very small

quantities, further to higher hydrocarbons (primarily propene

and butenes). Under these conditions a maximum ethylene

yield of more than 93% can be achieved. Above 300°C (zone

IV), the ethylene yield will decrease, leading to a strong

increase of higher hydrocarbons.

Figure 1:Yield(%) in function of reaction temperature. (W/F= 16.6

kgcat.s/molEtOH.)

Reproducible experimental data can be obtained on the

reaction setup in the absence of significant deactivation. Since

ethylene can be very selectively produced, the further focus

will lie on the high temperature area (T>300°C) where a

higher added value is present for catalyst modifications.

In figure 2 are the product selectivities in function of the

ethylene conversion presented. It can be observed that an

increase of the ethylene conversion results in a decrease of the

selectivity towards gaseous olefins (Figure 2 A ), an increase

of both gaseous paraffins (Figure 2 B) and the BTXE group

(Figure 2 C and D). According to Figure 2 C is a maxima of

the selectivity towards C6-C8 olefins noticeable as well. A

possible corresponding reaction path is the formation of

aromatics from cyclic olefins releasing hydrides which can

serve in transformation of olefins into paraffins.

Figure 2: Selectivity in function of conversion (A) C3-C5 olefins, (B) C1-

C5 paraffins, (C) BTXE D) selectivity of C5+ ( HZSM5 (Si/Al=30), T=350°C,

variable W/F [8.3-55 kgcat.s/molEtOH] )

IV. ETHANOL CONVERSION OVER MODIFIED HZSM5

A. Effect Si/Al ratio

An increase of the Si/Al resulted in a decrease of

concentration of strong acid sites (cf. Figure 4B) and a

decrease of the activity. A correlation between the strong acid

site concentration, activity per active site (Figure 4C) and the

ethylene conversion is detected.

B. Effect metal introduction

Different loadings of gallium, iron and nickel loadings are

introduced through ion exchange and incipient wetness

impregnation. Increase in metal content led to a systematical

decrease of the specific surface area and pore volume. In

combination with the corresponding H2 TPR profiles , an

increase of the metal content results in the formation of

primarily metal oxides and agglomerates which can be

responsible for pore blockage and thus reducing the

concentration of accessible acid sites (cf. Figure 4A).

Despite the reduction of the concentration of accessible sites

an increase in activity (ethylene conversion) for low metal

contents (<1%) is noticed. No large change in product

selectivities (paraffins,olefins,C5+ hydrocarbons and BTXE) is

detected (cf. Figure 3). This is done through comparison with

HZSM5 (Si/Al=30) with variable space time.

Figure 3: Selectivity (mol%) in function of ethylene conversion (mol%). solid data point=incipient wetness impregnation, arrayed data point=ion exchange and not filled data

points =.HZSM5 M-HZSM5: W/F= 16.6 kgcat.s/molEtOH HZSM5Wcat/FEtOH=5.5-55 kgcat.s/molEtOH .

350°C

The promoting effect of the introduction of metals on the

activity is illustrated in Figure 4. The activity per active site

(AAS) is plotted for all the different catalysts except Ni-

HZSM5. (Due to inaccuracy in determination of the acid sites

concentration.)

Figure 4: Activity per active center(=AAS) in function of activity

(ethylene conversion(mol%). (catalysts: HZSM5 (Si/Al=30,50,80), Fe HZSM5 (different

loadings), Ga HZSM5 (different loadings). 350°C , W/F =16.6 kgcat.s/molEtOH)

It can be concluded that a higher activity per active site is

present for the metal modified HZSM catalysts (cf. Figure

4C). Indicating a promoting effect of these metal species on

the activity

V. CONCLUSIONS

Experimental data was found to be reproducible. No

deactivation have been observed. The effect of temperature,

space time and water have been investigated. A reaction

mechanism suggestion is made which is in line with the

experimental data. Modifications through variable Si/Al and

metal introduction affected only significantly the activity. The

activity per acid site was higher for metal modified HZSM5

catalyst in the following order: Ga-HZSM>Fe-

HZSM5>HZSM5 illustrating the promoting effect of metal

modification on the activity.

VI. REFERENCES

.

[1] 1. Centi, G., P. Lanzafame, and S. Perathoner, Analysis of the

alternative routes in the catalytic transformation of lignocellulosic materials. Catalysis Today, 2011. 167(1): p. 14-30.

[2] 2. Madeira, F.F., et al., Ethanol transformation over HFAU, HBEA and

HMFI zeolites presenting similar Bronsted acidity. Applied Catalysis a-General, 2009. 367(1-2): p. 39-46.

[3] 3. Lu, J.Y., Y.C. Liu, and N. Li, Fe-modified HZSM-5 catalysts for

ethanol conversion into light olefins. Journal of Natural Gas Chemistry, 2011. 20(4): p. 423-427.

[4] 4. Inaba, M., et al., Ethanol conversion to aromatic hydrocarbons over

several zeolite catalysts. Reaction Kinetics and Catalysis Letters, 2006. 88(1): p. 135-142.

Chapter 1

Introduction

A great fraction of the worldwide energy supply and important chemicals like ethylene,

propylene, benzene, etc. originate from fossil resources. Crude oil, is one of the most

abundant fossil resources and is primarily used as energy source for the transport sector (cf.

Figure 1-1). In this industry, which accounts for about one third of the world energy

consumption [1], about 80 million barrels were used in 2003. This increased to 89 barrels

anno 2011 and further increase is expected according to the International Energy Agency

(IEA) [2].

Figure 1-1: World oil consumption by sector 2008-2035 (million barrels per day) [3].

The limited availability, the environmental and political impact make the research and

development of economically feasible renewable energy sources urgent[4]. The majority of

the current renewable energy sources (wind, solar, etc. ) are suited to produce electrical

energy. The current energy storage capacity of the present batteries are about two orders of

magnitude lower than that of gasoline or diesel. This is one of the reasons why the IEA,

estimated that in 2030 only 1% in the car industry will be covered by electric cars [5].

2

The production of basic organic chemicals is strongly dependent of the fossil resources as

well. In Figure 1-2 is the relative important of several basic building blocks shown. Ethylene,

propylene and several aromatics contribute for about 70% of the total organic distribution.

Figure 1-2: European organic market of basic chemicals, 2005 (Adapted from[6])

Biomass is a renewable energy source which can lead to both energy applications and organic

building blocks in the chemical industry [5]. Biomass can be converted to ethanol which can

be directly used as a transportation fuel or further converted to important organic building

blocks. The application of biomass is in the next paragraph further discussed.

1.1 Biomass as a feedstock

Biomass used for the production as energy source of organic basic block can be divided into

two main “generations”. In the first generation is biomass in competition with the food

industry (sugarcane, corn, …) which is not the case for second generation biomass. Two types

of second biomass types can be distinguished: lignocellulosic biomass (corn stover, switch

grass,…) and microalgae.

Lignocellulosic materials are composed of lignin, hemicellulose en cellulose [7]. Cellulose is

a linear polysaccharide which forms the main structural component of the cell wall.

Hemicellulose is a polysaccharide as well but has a more branched structure. Lignin is a

3

polymer of phenyl propane units linked primarily by ether bonds. The structure of

lignocellulose biomass is depicted in Figure 1-3.

Figure 1-3: Structure of lignocellulosic biomass.[8]

Lignin is tightly bound to the carbohydrate polymers (hemicellulose and cellulose) acting as a

glue decreasing the activity of further conversion routes. This makes the conversion of

biomass to useful feedstock more difficult if compared with first generation biomass

processes. However, lignocellulosic biomass has the advantages to be not in competition

with the food industry and have a higher environmental performance [9].

The production of organic products through photosynthetic by microalgae, is suggested to

have a higher theoretical yield than processes starting from other types of biomass [10].

However, the major challenge is that the practical yield of outdoor algae cultures are only one

third to one tenth lower of the maximum theoretical yields [11]. This type of conversion route

is in a very early development stage and therefore in the short to medium term the conversion

of lignocellulosic biomass looks more promising.

Many routes exist for converting biomass to biochemical /fuels. An overview of the main

routes of biomass to biofuels are illustrated in Figure 1-4. Depending of the type of biomass

different conversion routes are possible.

The first generation biomass contains starch, sugars and lipids. Sugarcane can after a physical

pretreatment directly be transformed through fermentation to bio ethanol. Starch containing

4

bio ethanol requires an additional preliminary saccharification step. Lipids can through

catalytic transesterfication be converted into biodiesel.

Converting processes starting from second generation biomass are not commercially attractive

yet but offers, as mentioned before great advantages compared to the first generation biomass.

There are three main routes for the conversion of lignocellulosic materials into biofuels:

thermo chemical, bio chemical and chemo catalytic.

Thermo chemical processes include pyrolysis and gasification, which are processes at

elevated temperature (T>500°C) and are able to deal with a relative complex feedstock.

Chemo catalytic are typically at lower temperatures (but requires specific biomass derived

compounds (i.e. sugars). In the bio chemical way, a fermentation process, is the central

process such as for first generation starch and sugar however using lignocellulosic materials,

a more profound pretreatment is necessary.[5]

Figure 1-4:Main routes to biofuels[5].

A combination of these different biomass conversion processes will form the heart of the new

energy landscape (bio refinery)[5, 12].

5

The conversion route producing (bio) ethanol (water ethanol 12% mixture) from biomass has

the advantage of having a multipurpose character: ethanol can be used to obtain both biofuel

and suitable platform molecules, offering similar characteristics as crude oil.

1.2 Bio ethanol production process

The production of bio ethanol consists of the following main steps: pretreatment, conversion

of polysaccharides into fermentable sugars, fermentation and purification. The detailed

preferable procedure is strongly dependent of the type and composition of the biomass.

Sugar can directly be fermented without the preliminary steps unlike starch which has to

undergo a saccharification step. A fermentation process can be generally defined as: a

process of extracting energy from the oxidation of organic compounds using an endogeneous

electron acceptor. For obtaining ethanol from sugars, yeast will be used to break down the

sugars to ethanol and carbon dioxide.

Additional steps are required for the utilization of lignocellulosic biomass. Different

pretreatment methods are available: physicochemical (i.e. steam explosion), chemical (i.e.

acid hydrolysis, alkaline treatment, enzymatic hydrolysis,..),etcet. .The pretreatment method

strongly determines the increase in reactivity. An effective pretreatment method removes

lignin and/or reduces the cristallinity and polymerization degree and/or increases the available

surface area. [5, 13]

The difficulty lies in handling the complex feedstock. Therefore many efforts have been made

to produce microorganisms, which are able to produce ethanol from a variety of different

lignocellulosic materials [14].

After the decomposition of the polymer chains, a mixture of mainly ethanol and water is

obtained. Ethanol and water forms an azeotropic mixture. Commonly this separation process

exists of two parts. For the first part an ordinary distillation can be used. Further dehydration

can be done by: azeotropic distillation, adsorption techniques and membrane separation

techniques. The latter group is considered the most promising energy saving process for

separation of azeotropic mixtures [14, 15].

A lot of research in the biomass to ethanol conversion is therefore to (1) optimize the removal

of lignin using lignocellulose biomass and (2) to reduce the high separation costs. [5, 15].

6

1.3 Bio ethanol intermediate or product ?

Bio ethanol can be directly used or further converted to bio chemicals/gasoline. In Brazil ,

ethanol obtained from sugarcane is used as a mixture of conventional gasoline and ethanol

(E85) which is already incorporated as fuel. However, due to mainly the poor lubrication

properties, the higher corrosion character, the worse cold temperature behavior (i.e. starting

problems) and the lower energy content, adaptations to the conventional engine and the

infrastructure (i.e. filling stations) are necessary [16]. A high purification degree is required if

directly used as an engine fuel. Because the presence of water can cause phase separation in

the fuel tank which has negative consequences for the performance: the gasoline used in

ethanol/gasoline mixtures has a low octane number compared to regular gasoline (due to the

high octane number of ethanol)[17].

Bioethanol can also be further converted to chemicals. One of the advantages is that it is not

always necessary to remove all the water and therefore the energetic post treatment can be

partially skipped. In Figure 1-5 is an overview given of different product lumps obtainable

starting from

bioethanol.

Figure 1-5: Bio ethanol conversion routes.

7

It can be concluded that ethanol conversion processes are able to fill in both energy

applications as chemical building blocks.

In table 1 are the possible conversion routes of bio ethanol shown.

Table 1-:1: Discussion of the most important bio ethanol conversion routes.

Product Production process

Market/pro's Difficulties/ cons Current Bio ethanol feedstock

Hydrogen Reforming natural gas

Ethanol (steam) reforming [18]

Interesting energy carrier Low volumetric energy density used in car battery → storage

difficulties[19] production ammonia (Haber Bosch)

Ethylene feed (Wacker process)[20]

autothermal reforming[21] production methanol Relatively small market

Acetylene Dehydrogenation [22, 23] Production of acetic acid, butyl

alcohol,…

Ethylene

Thermal cracking

petroleum gas/naptha

Dehydration [24] [25]

Most important organic basic building block

Strong potential reduction GHG emission[26]

Very endothermic reaction [23]

Lower temperature and higher selectivity can be achieved[27]

higher hydrocarbons

Fossil resources

Consists of mainly two steps [28]:

1) Dehydratation to ethylene

2)Oligomerization/isomerizatio

n

Very large market : BTXE, gasoline, propene, butadiene,…[29, 30]

Development of active & selective catalyst with sufficient stability is

necessary

Potential strong reduction of fossil fuel dependence and strong

decrease GHG emissions

Compensating endothermic dehydration with exothermic

oligomerization steps

Hydrogen, acetylene, ethylene and higher hydrocarbons can be produced starting from (bio)

ethanol. Hydrogen is a potentially interesting energy carrier in the transport sector however

several practical issues are present. Acetylene can be produced from ethanol, however a

greater potential market exists for the conversion to ethylene or higher hydrocarbons. The

catalytic conversion of ethylene is already industrially applied [31] however a great energy

demand is necessary due to the high endothermic character of this reaction. A great fraction of

the current organic market can be achieved through further catalytic conversion to higher

hydrocarbons. An additional advantage is that the endothermic character is then (partially)

compensated through these reactions. Good catalytic performance is achieved through

8

application of zeolites, especially HZSM5. A more thoroughly discussion of the catalytic

conversion of ethanol to ethylene and higher hydrocarbons is done in the second chapter.

1.4 Scope of thesis

In this work, a systematic study of the ethanol to hydrocarbon reaction over HZSM5 and

modified HZSM5 catalysts is presented by combining of different characterization techniques

(the specific BET surface area, total pore volume, H2 TPR and NH3 TPD profiles) and the

corresponding catalytic performance over a fixed bed lab scale reactor.

First, a literature study is performed aiming to identify suitable catalysts, the potential

promoting effects of catalyst modification and to gain further insight in the current reaction

mechanism hypothesizes.

The ethanol to hydrocarbon reaction is first tested over reference HZSM5 catalyst. The effect

of temperature, space time and addition of water to the feed will be investigated. Based on

these experimental data, are indications regarding the reaction mechanism formulated.

Modifications to the reference catalyst will be done by varying the Si/Al ratio and

introducing metals through incipient wetness impregnation and ion exchange. The aim is then

to identify suitable catalytst and to link the physical chemical properties of the catalysts with

the corresponding catalytic performance.

1.5 References

1. IEA, World Energy Outlooks, 2009.

2. IEA, oil market report march 2012, 2012.

3. IEA, International energy outlook 2011, 2011.

4. Le Treut, H., R. Somerville, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T.

Peterson and M. Prather, Historical Overview of Climate Change Science, in Climate

Change 2007: The Physical Science Basis

2007, Cambridge University press: New York. p. 996.

9

5. Centi, G., P. Lanzafame, and S. Perathoner, Analysis of the alternative routes in the

catalytic transformation of lignocellulosic materials. Catalysis Today, 2011. 167(1):

p. 14-30.

6. European Union and national government statistics offices, A.o.P.P.i.E. Europe in

figures: Eurostat yearbook 2004-05. 2005.

7. Zinoviev, S., et al., Next-Generation Biofuels: Survey of Emerging Technologies and

Sustainability Issues. Chemsuschem, 2010. 3(10): p. 1106-1133.

8. Menon, V. and M. Rao, Trends in bioconversion of lignocellulose: Biofuels, platform

chemicals & biorefinery concept. Progress in Energy and Combustion Science, 2012.

38(4): p. 522-550.

9. International Energy Agency (Simms, R.T., M) from 1st to 2nd generation biofuel

technologies: an overview of current industry and RD&D activities. 2008.

10. Schenk, P.M., et al., Second Generation Biofuels: High-Efficiency Microalgae for

Biodiesel Production. Bioenergy Research, 2008. 1(1): p. 20-43.

11. Williams, P.J.L. and L.M.L. Laurens, Microalgae as biodiesel & biomass feedstocks:

Review & analysis of the biochemistry, energetics & economics. Energy &

Environmental Science, 2010. 3(5): p. 554-590.

12. Stocker, M., Biofuels and Biomass-To-Liquid Fuels in the Biorefinery: Catalytic

Conversion of Lignocellulosic Biomass using Porous Materials. Angewandte Chemie-

International Edition, 2008. 47(48): p. 9200-9211.

13. Kumar, P., et al., Methods for Pretreatment of Lignocellulosic Biomass for Efficient

Hydrolysis and Biofuel Production. Industrial & Engineering Chemistry Research,

2009. 48(8): p. 3713-3729.

14. Naik, S.N., et al., Production of first and second generation biofuels: A comprehensive

review. Renewable & Sustainable Energy Reviews, 2010. 14(2): p. 578-597.

15. Marris, E., Drink the best and drive the rest. Nature, 2006. 444(7120): p. 670-672.

16. Makarfi, Y.I., et al., Conversion of bioethanol over zeolites. Chemical Engineering

Journal, 2009. 154(1-3): p. 396-400.

17. Karaosmanoglu, F., A. Isigigur, and H.A. Aksoy, Effects of a new blending agent on

ethanol-gasoline fuels. Energy & Fuels, 1996. 10(3): p. 816-820.

18. Galvita, V.V., et al., Synthesis gas production by steam reforming of ethanol. Applied


19. Mori, D. and K. Hirose, Recent challenges of hydrogen storage technologies for fuel

cell vehicles. International Journal of Hydrogen Energy, 2009. 34(10): p. 4569-4574.

20. Chiusoli, G.P., formation of C-O bonds by oxidation, in Metal-Catalysis in Industrial

Organic Processes, 2008.

10

21. Leung, D.Y.C., M. Ni, and M.K.H. Leung, A review on reforming bio-ethanol for

hydrogen production. International Journal of Hydrogen Energy, 2007. 32(15): p.

3238-3247.

22. Guan, Y.J., et al., Controlling reaction pathways for alcohol dehydration and

dehydrogenation over FeSBA-15 catalysts. Catalysis Letters, 2007. 117(1-2): p. 18-24.

23. Hu, Y., et al., Selective dehydration of bio-ethanol to ethylene catalyzed by

lanthanum-phosphorous-modified HZSM-5: Influence of the fusel. Biotechnology

Journal, 2010. 5(11): p. 1186-1191.

24. Levanmao, R., T.M. Nguyen, and G.P. Mclaughlin, The Bioethanol-to-Ethylene (Bete)

Process. Applied Catalysis, 1989. 48(2): p. 265-277.

25. Levanmao, R., The Bioethanol-to-Ethylene (Bete) Process - Production of Ethylene

and Btx Aromatics from Biomass. Abstracts of Papers of the American Chemical

Society, 1990. 199: p. 119-IAEC.

26. Ramesh, K., et al., Structure and reactivity of phosphorous modified H-ZSM-5

catalysts for ethanol dehydration. Catalysis Communications, 2009. 10(5): p. 567-571.

27. Yang, X.X., et al., Comparison of four catalysts in the catalytic dehydration of ethanol

to ethylene. Microporous and Mesoporous Materials, 2008. 116(1-3): p. 210-215.

28. Magnoux, P., et al., Ethanol transformation over HFAU, HBEA and HMFI zeolites

presenting similar Bronsted acidity. Applied Catalysis a-General, 2009. 367(1-2): p.

39-46.

29. Coupard, V.M., S;, method of converting ethanol to base stock for diesel fuel, 2011.

30. Minoux;D., Process to Make Olefins from Ethanol, Total, Editor 2011.

31. Tsao, G.T., Biotechnology in China II: Chemicals, Energy and Environment. 2011.

264.

Chapter 2

Literature study

Although the ethanol to hydrocarbons process seems fairly simple, only very limited amount

of research papers are available in literature. Most papers focus on only a single modification

method to improve either the activity, selectivity or stability.

In the last decades are chemical plants are built for the conversion of (bio) ethanol to ethylene

[1]. For the conversion of ethanol to higher hydrocarbons, a growing interest is noticeable as

well. Multiple patents [2, 3] were published regarding the ethanol to olefin/gasoline

conversion illustrating the industrial potential of this conversion route. This chapter will

present a literature survey on the catalytic conversion of (bio)-ethanol to hydrocarbons,

focusing on the influence of the catalyst properties , the feedstock and the process conditions.

First will be explained why zeolites have interesting catalytic properties in many hydrocarbon

conversion processes. A comparison of the performance of several catalysts, in the ethanol to

ethylene and to higher hydrocarbons will then be discussed. Further focus is then given to the

performance of a particular zeolite, HZSM5 in the ethanol to hydrocarbon conversion. The

chapter will end by summarizing the current hypothesizes considering the reaction

mechanism.

2.1 Zeolites as suitable catalysts

Two important catalytst types in hydrocarbon conversion processes are acidic and metallic

catalysts. Acidic catalysts are mainly utilized for their isomerization, cracking and

oligomerization activity whereas metallic catalysts are used for primarily their

(de)hydrogenation activity.

Chapter 2: Literature Study

12

Multiple types of acidic catalysts are distinguishable: chloride catalysts (i.e. AlCl3), zirconia

catalysts (i.e. ZrO2), zeolites, molybdenum oxide based catalysts (MoO3), Al2O3, etcet. . In

cracking [4] and oligomerization [5] reactions it is suggested that a sufficient acid strength is

necessary or will enhance the activity/selectivity. The highest acid strength is present in

zeolites (except AlCl3 which is environmentally unfriendly) [6, 7].

Zeolites are crystalline aluminosilicates with the general formula M2/n.Al2O3.ySiO2.wH2O,

where n is the valence of the cation M and y may vary from 2 to infinity. Structurally, zeolites

are crystalline polymers based on a three-dimensional arrangement of tetrahedral SiO4 or

AlO4 interconnected by their oxygen atoms, hence, forming subunits and finally lattices by

repeating identical building blocks (unit cells). In a primary building block is shown, in

Figure 2-1 b two primary building blocks are connected through a common (bridging) oxygen

atom.

Figure 2-1: (a) Primary TO4 structure (b) TO4 sharing a common oxygen atom [8].

The combination of tetrahedronic structures can form cyclic structures, the secondary building

blocks. These secondary building blocks can be buildup by a different number of T (Si, Al,

Ga,..) atoms. The mean type of zeolite structure (MFI, TON,…) is determined by the way the

primary building blocks are connected with each other to form different ring sizes and

different three dimensional structures.

On a zeolite, most of the hydrocarbon reactions as well as many transformations of functional

compounds take place at the protonic (Brönsted) sites of the zeolites which are determined by

the number of acid sites (or density), the strength and their accessibility [9] . The number of

framework aluminum atoms determine the maximum number of protonic sites. Normally

aluminum can have three covalent bonds (3 valence electrons) however in the zeolite structure

it has 4 bonds therefore the interaction of aluminum with that fourth O weakens the OH bond

and an acid function is introduced. The maximum number of protonic sites is equal to the

number of framework aluminum atoms. Following the Lowenstein rule the nearest neighbor


13

(NN) of an Aluminum atom cannot be another Aluminum atom. This means that the

maximum number of protonic sites is obtained by a framework Si/Al of 1. But a ratio of one

is not stable, according to the Next Nearest Neighbors (NNN) concept the maximum strength

of a protonic Aluminum site is maximal when the next nearest neighbors atoms are not

aluminum atoms. Other parameters like bond angle (T-O-T), Brönsted-Lewis interactions,…

will influence the acid strength as well.[10]

Summarizing: A high Si/Al (Si/Al>100), means a low concentration of acid groups of high

strength. A low Si/Al (<30) is equal to high concentration of acid groups of moderate

strength. This leads to a tunable acidity which will has an effect on the acitivity and selectivity

of the zeolite.

Other acid type , are Lewis acid sites. Lewis acid sites have a vacant orbital. An unshared

electron pair can therefore form a covalent bond with this vacant orbital of the Lewis acid site.

Brönsted acid sites can donate protons (H+) [10].

Figure 2-2: Brönsted and Lewis acid sites.

The tunable acidity is not the only catalytic interesting feature of zeolites. The pore size of

several zeolites are in the same magnitude as the kinetic diameter of C2-C12 hydrocarbons

which is illustrated in Figure 2-3. This induces shape selectivity properties. It has to be noted

that not all zeolites have shape selective properties (i.e.Y zeolite).


14

Figure 2-3: Kinetic diameter of some molecules and zeolites [11].

Different shape selective mechanisms are possible. In Figure 2-4 is a distinction is made

between: a) reactant shape selectivity, b) product shape selectivity and c) transition state

shape selectivity.

Figure 2-4: Primary shape selectivity mechanisms: a) reactant shape selectivity: cracking of an n- and iso-

C6 mixture b) Product shape selectivity: disproportionation of toluene over HFMI-zeolite c) Transition

state shape selectivity; disproportionation of m-xylene over a HMOR-zeolite [12].

For shape selective zeolites selectivity/activity can be adapted by changing the characteristics

by changing the pore structure [10]. These properties had to led to several applications. In


15

Table 2-1: are several industrial applications of zeolites, bifunctional zeolites and metal

catalysts presented. Zeolites can be classified based on the ring size of the secondary

structure. Only (>8 membered rings zeolites) are applicable for catalytic applications. Catalyst

with smaller ring sizes are not interesting for catalytic reactions.

For completeness, not only 8,10 and 12 membered rings can exist but multiple pore zeolites

[13] exist and mesoporeus zeolites [14] (>12membered rings) as well exist having potentially

interesting properties for the conversion heavy hydrocarbon feeds.

Table 2-1: Application of zeolites and metallic catalysts in the chemical industry [10].

Zeolites Pore

size(nm) Catalysts Applications

8 Membered rings 0,3-0,45 A n/iso butene separation, ion exchange

10 Membered rings 0,45-0,6 MFI

xylene isomerization, isomerization,

dewaxing, methanol to olefins

Ga2O3 -HMFI aromatization

12 Membered rings 0,6-0,8 Y,Beta,FAU

cracking,isomerization, fluid catalytic

cracking

Pd/HFAU isomerization aromatics

Metallic catalysts >2nm Fe2O3, Cr2O3,Pt dehydrogenation, watergas shift

reaction, …

The selection of a catalyst is not only determined by activity & selectivity. The stability of the

catalyst (the maintaining of a high activity and selectivity with increasing time on stream) is

an important criteria as well. The main reason for the decreasing stability of a catalyst

employed in a hydrocarbon conversion is the formation of a carbonaceous residuel

(‘cokes’).This can lead to site coverage (poisoning of the active sites) or/and block the pores

blockage (inaccessibility of the active sites). Coke is a collective name for amorphous carbon

which consist of polyaromatic hydrocarbons , high molecular weight polycyclic

hydrocarbons, methylbenzenes, branched long chain alliphatics,… .

Cokes can be classified based on their solubility or insolubility in dichloromethane. The

zeolite is first treated with a hydrofluoric acid solution which leads to the destruction of the

zeolite matrix. This however does not alter the cokes. The soluble cokes are then extracted by

dicholomethane. Hydrocarbons up to 4 aromatic rings can be extracted this way. Generally


16

this accounts for only a small part of the total coke content (5 wt.%). The remaining cokes are

typically black particles and insoluble and are of very high molecular weight [15].

The chemical process of the coke formation is very complex. During the catalytic conversion

of hydrocarbons on zeolites the cokes consist mainly of stable polyaromatic compounds.

Besides the chemical aspect, the retention within the pores is essential. This retention can be

due to steric hindrance within the pores (entrapment), strong chemisorption on active sites

and their low volality (gas phase reaction) and solubility (liquid reaction). The way different

parameters (process parameters and catalyst properties) influence the coke formation in acid

catalyzed reactions is summarized in Figure 2-5 .

Figure 2-5: Influence of the acidity on the coke formation rate. [16]

Next to coke formation, irreversible changes to the zeolite framework can happen as well.

Dealumination is the process where alumina is irreversible removed (dealumination) from the

framework at high temperature (T > 500°C) and in the presence of water vapor. This will lead

to change in acidity and stability of the zeolite [17]. An example of this treatment is the

ultrastable Y zeolite (USY). It is reported that dealumination causes a decrease of the

cristallinity degree and creates mesopores [18].

2.2 Application of various catalysts for the ethanol to

ethylene conversion

Several catalysts are already tested for the catalytic dehydration of ethanol to ethylene. Table

2-2 reports the most important results that can be found in literature. A distinction can be

made between non zeolites (acidic & metallic) and zeolites catalyst in the dehydration of

http://www.sciencedirect.com/science/article/pii/S1381116908005098


17

ethanol. ( in the strict definitions (consists of of only silica and alumina) SAPO catalyst are

not zeolites, however due to very similar typical pore structure and strong acidity they are

here considered as zeolites.)

Table2-2: Ethanol to ethylene catalytic conversion processes.

Catalyst T(°C) Feedstock Yield (wt%) Year

non zeolites

Al2O3 450 ethanol 80 1976[19]

TiO2- Al2O3 410 ethanol 99.96 2007[20]

Fe2O3-MnO3-SiO2 450 ethanol 65 2005[21]

zeolites

SAPO11 320°C ethanol 90 1997[22] NiSAPO-34 370 ethanol 96.5 2008[23]

Mn-SAPO-34 340 ethanol 97.8 2010[24] H-ZSM5 300 ethanol 97.3 1979[25]

HZSM5 2% TFA 275 ethanol/water 95 1989[26]

First, Al2O3 have been tested for the ethanol to hydrocarbon reaction which leaded to and

ethylene yield of 80% at 450°C. Modifications through addition of primarily metal oxides

have led to an improvement of the yield at reduced temperature. Despite these improvement,

the reaction temperature required to obtain a significant ethylene yields remained higher than

400°C.

The best performance was obtained on SAPO11 and HZSM5 zeolites. The dehydration to

ethylene was on these zeolites possible at much lower temperature. Modifications of these

catalyst led to a higher yield and/or even lower reaction temperatures. Addition of

trifluoroacetic acid (TCA) to HZSM-5 led to high ethylene yield at a temperature of even

200°C.

Figure 2-6:Process flow sheet of conversion of ethanol to ethylene.

The ethanol to ethylene process can be divided in two main parts: the reaction and separation

section. In the former are multiple heaters necessary which have to compensate for the strong

endothermic character of this conversion illustrating the high energy requirement of the

process.


18

Industrially are primarily (modified) Al2O3 obtained [1]. The reasons that these catalyst are

used and not zeolites can be primarily attributed to the “proven technology” image of Al2O3 in

this process and the lower stability of zeolites towards coke formation. However, as shown in

Figure 2-6 the ethanol to ethylene conversion process is a very endothermic energetic process.

A lowering of the reaction temperature would drastically decrease the energy consumption.

This could be done through HZSM-5 zeolite catalyst which offers a low temperature

alternative with high selectivity and yield.

2.3 Application of various catalysts for the ethanol to

hydrocarbon conversion

In the ethanol to ethylene process a wide variety of both acidic and metallic catalytst can be

used to dehydrate ethanol to ethylene. For the conversion of ethanol to hydrocarbons only a

limited amount of catalysts can be used. This is mainly attributed to the requirement of strong

acid sites and shape selective properties . Consequently, only zeolites are qualified for this

reaction (cf. 2.1).

Madeira et al. made a comparison of HZSM5 (Si/Al=40), HFAU (Si/Al=20) and HBEA

(Si/Al=11) catalysts in ethanol to hydrocarbon conversion. This zeolites were chosen so that a

same number of acid sites were achieved. They found that, at a temperature of 350°C and a

pressure of 30 bar, a great fraction of liquid hydrocarbons were formed especially over a

HZSM5 catalyst. The other types of zeolites showed faster deactivation: they related this to

the larger pores size which facilitates coke formation. A similar study was done in the

methanol to olefins process (MTO) at a temperature of 370°C where HZSM5 was compared

to Mordenite, Y and Beta zeolites. HZSM5 appeared to be the most stable catalysts here as

well [27].

The best performance in the ethanol to hydrocarbon conversion is, according to open

literature, achieved through application of HZSM5. Further enhancement of the activity,

selectivity and stability is possible through altering the acidity and through post synthesis

treatments.


19

2.4 Ethanol to hydrocarbon conversion on HZSM5

The MFI zeolite structure, consists of 10 tetrahedral atoms in each ring leading to medium

pores ( 0.45-0.6 nm). HZSM5, is a MFI type zeolite, where H+ cations are present in the

zeolite pore structure acting as charge compensating cations. In Figure 2-7 is the MFI

structure shown. The characteristic pore structure of HZSM5 will have a strong contribution

to the high liquid hydrocarbon fraction and relatively high stability (compared to other

zeolites) in the ethanol to hydrocarbon conversion (cf. 2.3).

Figure 2-7: MFI structure [28].

The acidity of the zeolite (cf. 2.1) will have a large influence on the activity, selectivity and

stability of the catalyst as well. Initial studies indicate the dependence of Brönsted acid site

concentration and the oligomerization and aromatization activity in the ethanol to

hydrocarbon conversion reaction.

One way of altering the acidity of the catalyst is through changing the Si/Al ratio. For the

ethanol to hydrocarbon conversion on a HZSM5 catalyst different authors [29-31] reported

different trends of the influence of the Si/Al ratio on the product distribution and catalyst

stability. Makarfi et. al. , compared the performance of HZSM5 with variable Si/Al (30-90). A

minima in liquid hydrocarbon fraction was found for a Si/Al ratio of 50 at a temperature of

400°C [31]. However, the most authors [29, 30] conclude that a decrease of the Si/Al ratio

enhances the formation of the liquid hydrocarbon fraction (C5+) and lowers the C2-C4 olefins

lump. In catalytic cracking it is illustrated that there exists a maximum of the cracking


20

activity in function of Si/Al. .Indicating that an optimum between the number of active sites

and the individual strength.

The introduction of additives can lead to a drastic change in the selectivity and stability of the

catalyst as well. Introduction of additional functionalities is possible during (isomorphic

substitution) and after synthesis. Isomorphic substitution aims to integrate additional

compounds in the zeolite framework [32]. Isomorphic substitution of gallium in ZSM5

framework for example, led to an increased aromatization activity in the conversion of

propane[33].

Post synthesis methods, are more applied due to the reduced preparation time and higher

reproducibility [34]. These modification techniques will not lead primarily to an altering of

the zeolites framework but to formation of an additional phase (i.e. metal oxides) or to

compensating cations (partially) replacing H+ cations present in the pores. The applied post

synthesis method will have a major influence the state and location of these compounds. In

this thesis will ion exchange and incipient wetness be applied (cf.Chapter 3). Hence, detailed

characterization is essential. A more detailed discussion of the effects of post synthesis

modifications is done in 2.4.2.

Proper understanding of the effect of process conditions are necessary to make a critical

comparison about the different effects of modifications. Moreover, they can give insight in the

reaction mechanism.

2.4.1 Process conditions

2.4.1.1 Steady state behaviour

Temperature has an important effect on the conversion and selectivity of ethanol to

hydrocarbons over a HZSM5 catalyst. At temperatures lower than 200°C ethanol will be

dehydrated to diethylether [35]. Further increase of temperature will lead to a very selective

ethylene formation. For temperatures higher than 300°C a decrease of the ethylene

concentration is observed and C2+ hydrocarbons will be significantly be present. Further

increase in temperature lead to a further decrease of ethylene and to a larger fraction of liquid

hydrocarbons [36].

In Figure 2-8 are the organic yields plotted in function of space-time. This is plotted for a

temperature of 350°C and 450°C with a negligible water content (Figure 2-8 A and B) and at


21

a temperature of 450°C with higher water content of respectively 16% (Figure 2-8 C) and

50% (Figure 2-8 D).

Figure 2-8: Effect of space time and water content on product distribution at a temperature of 350°C and

450°C. ( XE Weight fraction of ethene, based on the organic components [calculated as the (mass flow of

ethene)/(mass flow of organic components) ratio] Xi =Weight fraction of component i, based on the organic

components [calculated as the (mass flow of component i)/(mass flow of organic components) ratio] XWo

=Water/ethanol ratio in mass, used catalyst= HZSM5 (Si/Al=24) [36].

Increase of space-time will lead, according to Figure 2-8 under all the different process

conditions to a higher C5+ fraction and paraffin fraction. A maximum is present for the lower

olefins in function of space-time. According to Figure 2-8 A en B, a temperature increase will

lead to a higher C5+ and paraffin fraction. Comparing Figure 2-8 B,C and D, at a temperature

of 450°C, a higher water content leads to a strong decrease in C5+ and paraffin formation

resulting in a higher ethylene and lower olefin mass fraction. However, according to

Talukdar et al.[30] , an increase of the water content of 20 - 80% led to an increase of the C6+

aromatics. Thus reflecting the non-agreement of the data found in open literature.

Two process conditions remain: total pressure and ethanol partial pressure. Increase of total

pressure or ethanol partial pressure will increase the liquid hydrocarbon fraction [37, 38].

Neglecting these effects, especially the partial ethanol pressure, could lead to incorrect

conclusions after altering the feedstock (i.e. co feeding water).

A

B

C

D


22

In the previous paragraphs are only the different effects summarized. A more thoroughly

analysis will be performed in this thesis based on less lump product selectivities leading to

increase of insight in the reaction mechanism and the inhibiting/promoting effect of process

conditions towards certain reaction steps.

2.4.1.2 Stability

Two types of deactivation can occur (cf. 2.1):

- Irreversible deactivation caused by dealumination

- Formation of carbonaceous residuals

Coke formation is a typical reversible side reaction for conversion reaction of hydrocarbons.

The presence of cokes will have a negative effect on the activity of the catalyst. In Figure 2-9

a is the conversion/yield plotted in function of time on stream for different product lumps. In

Figure 2-9 b is the corresponding coke formation in function of time on stream plotted. High

pressures were here applied, explaining the very high observed yield of C3+ hydrocarbons.

Figure 2-9: (a) Conversion/yield(%) in function of Time on stream (TOS) (b) carbon content in function of

TOS. Reaction conditions: T=350°C, P=30bar, N2/EtOH=4, W/F=0.09 molEtOH/kgcat.s)

catalyst:HZSM5(Si/Al=16).[39]

According to Figure 2-9 (a), initially, all the ethanol and ethylene converted to higher

hydrocarbons. However, the conversion of ethanol and ethylene to hydrocarbons decreases

with time on stream. In Figure 2-9 B, an abrupt increase of the total coke content is noticable

whereafter the coke formation rate decreases. Moreover, the coke formation led to, even

iniatilly, a strong reduction of the acid sites despite of the still high C3+ .hydrocarbons

Hence, the amount of coke formation gives no complete explanation for the product

distribution in function of time on stream.

A B


23

In Figure 2-10 are the corresponding coke composition after 1 and 30 hour(s) TOS depicted.

A strong evolution from relatively small aromatics to more bulky stable hydrocarbons is

observed. It is suggested that the nature of the cokes may play an important role in the

comprehension of the deactivation behaviour.

Figure 2-10: GC-MS analysis of coke molecules extracted by CH2Cl2 after solubilization by HF solution, of

coked samples TOS = 1 h and 30h. (Reaction conditions: T=350°C, P=30bar, N2/EtOH=4,

W/F=0.09molETOH/kgcat.s, catalyst= HZSM5 Si/Al=16) [39].

The addition of water may not only has an effect on the product distribution but on the

stability of the catalyst as well. According to literature [38, 40] water has an attenuating effect

on coke deposition. Two possible explanations are suggested. First water is in competition

with ethanol and hydrocarbons in the adsorption on the zeolite. Hence, the higher

hydrocarbon fraction in the product stream will be lower as well as the coke formation. The

other hypothesis is that the acid strength of the Brönsted sites lowers through hydration of

these sites.

At relative high temperature not only coke formation but dealumination (irreversible

deactivation) will occur as well (cf. 2.1). In the ethanol to hydrocarbon conversion

irreversible deactivation is only significantly present at temperatures above 500°C. However,

after addition of water to the feed (>50wt%) dealumination was significantly present at lower

temperatures (450°C) [36].


24

2.4.2 Post synthesis modifications

The effects of post modifications on the catalytic performance in hydrocarbon processes and

primarily the ethanol to hydrocarbon conversion process is here discussed. To illustrate the

potential effect, are, in Table 2-3, the ethanol conversion and carbon selectivity towards

different product lumps presented both before as after post synthesis modification with a

variety of additives.

Table2-3: Possible effect post synthesis modifications according to Inaba et al. (Reaction conditions:

T=400°C, W/F=0.16 mol EtOH/gcat h-1

catalyst: modified (Incipient Wetness Impregnation) HZSM5, 10%

metal loading except for Pt 2%) [41].

Ethanol conv. (%)

C-selectivity (%)

Not modified

Ethylene C2+ olefins Paraffins BTX

92,2 11 16 13 53

Modified

Mg 36,9 93 0 0 0

Fe 97 11 30 6 51

Ni 95,6 30 21 6 39

Pt 98,5 34 7 7 51

Ga 93,4 5 11 7 74

Cu 90,1 92 1 0 5

Cr 93,9 24 18 9 42

The addition of gallium, nickel , platina and iron lead to an increase of ethanol conversion

and/or C2+ olefin and BTX fraction. Explanation for this are however not given. In the

following paragraphs, various post synthesis treatments will be more thoroughly discussed.

2.4.2.1 Transition metals

The effects of introduction of transition metals on hydrocarbon processes and in the ethanol to

hydrocarbons conversion process is here discussed.

Transition metals have partially occupied d-orbitals which are suitable for participating in

chemical bond formation with neutral molecules. Through coordination of the substrate to the

metal, the activation energy can be lowered. Multiple substrates can interact with the same

metal which can lead a so-called coordinated complex. Once the ligands (molecules which

interact with the transition metal) are formed a large change in reactivity is possible.


25

The role of these metals is despite several industrial applications not completely understood

especially not for the ethanol to hydrocarbon reaction over a M/HZSM5, as in illustrated in

[37, 41].

First is the effect of gallium, nickel and iron in hydrocarbon conversion processes other then

the ethanol to hydrocarbons over M/HZSM5 discussed. Herafter is the same done for ethanol

to hydrocarbons over M/HZSM5.

Table 2-4: Effect iron, nickel and gallium in a variety of hydrocarbon reactions ( ethanol to hydrocarbons

over HZSM5 not included).

Iron Nickel Gallium

Process

A)benzene

oxidation[48] C)ethylene isomerization[42]

E)cracking n heptane[43]

B)catalytic cracking isobutane[44]

D)Fluidized catalytic cracking[45]

F)ethylene aromatization[46]

E) ethanol/ethene to propene[47]

G)heptane aromatization

A)oxidator

C)Oligomerization iniator

E)Paraffin activation

Suggested metal role

B)increase dehydrogenation activity

D)dehydrogenation F-G)Aromatization

E)methathesis

Iron containing catalysts are historically applied for a variety of hydrocarbons processes: the

Fischer Tropsch , Haber Bosch process, benzene oxidation to phenol [48], ethane

aromatization,… . Lu et al. [44] modified HZSM5 catalyst through incipient wetness

impregnation. The made catalyst was then applied in the catalytic cracking of isobutane which

led to increased dehydrogenation activity.

Nickel is traditionally applied for its dehydrogenation properties. Lallemand et al. prepared

mesoporeus catalyst (MCM-36,MCM-22) with nickel through ion exchange leading to nickel

present as primarily cations. This catalyst is then applied in ethylene oligomerization at

relatively high pressure feeding ethene. A high selectivity towards even oligomers was

noticed. The same authors suggest that nickel will enhance the oligomerization iniation

(dimerization). It has to be noted that these catalyst have no shape selective properties. Maia

et al. [50] introduced nickel both through ion exchange as through incipient wetness

impregnation on HZSM5. The catalyst is then applied in the cracking of n hexane at 500°C.


26

Increase in hexane conversion and a higher selectivy towards lower olefins, especially for the

ion exchanged catalyst was observed. Iwamoto et al. [47] described the ethanol to process on

nickel ion loading silica at 400°C. Here, Nickel was introduced before synthesis. The

introduction of nickel led to a strong increase of the propylene selectivity which was here

explained by the introduction of a metathesis function by nickel.

The introduction of gallium on HZSM5 has led in a variety of hydrocarbon processes to an

increase of the aromatic selectivity. Rane et al. [43] introduced gallium through different post

synthesis methods (chemical vapor deposition, incipient wetness impregnation and ion

exchange). These catalyst were then applied in the cracking of ne heptane. For all the different

catalyst is a increase in aromatization selectivity noticed. Choudhary et al. [46] introduced a

variety of gallium loadings (1.4% and higher loadings) through incipient wetness

impregnation. Increase in selectivity was noticed as well for the different catalysts. However

the effect was the most pronounced for a gallium loading of 1.4%.

In Table 2-5 are specific for metal modified HZSM5 catalyst in the ethanol to hydrocarbon

reactions the noticed effects an suggested explanation listed.

Table 2-5: Rol of iron, nickel and gallium in ethanol to hydrocarbons over HZSM5.

Iron Nickel Gallium

Effect Increase aromatization selectivity[51]

Increase C5+ hydrcarbon

selectivity

Increase C5+ hydrcarbon selectivity

Decrease aromatization selectivity[52]

Decrease aromatization selectivity[37]

Increase aromatization selectivity[41]

Increase selectivity lower olefins[52]

Increase aromatization selectivity[37]

Function metal

Decrease coke formation[52]

Increase propenes and butenes selectivivty [53]

Suppression hydrogen transfer

Decrease acid concentration[53]

Additional Brönsted sites[54]

Decrease strong acid sites concentration[52]


27

Lu et al. tested introducted different iron contents (1-16%) through incipient wetness

impregnation on HZSM5. Catalytic testing were performed at temperature of 450°C. The

ethanol conversion remained for all the different catalysts close to 100%. An increase of the

ethylene yield have been observed for all samples as well. The yield of the other product

groups was dependent of the metal loading. Iron loadings lower than 9% led to a decrease of

the paraffin and aromatic yield and an increase of the C2+ olefins. Further increase of the

metal loadings will led to a strong increase of the paraffin yield and a decrease of C3-4 olefins.

A minima for the paraffin fraction and a maxima for the lower olefin is achieved for a 9 wt%

metal loading. Inaba et al. compared the performance of Fe/HZSM5 in the ethanol to

hydrocarbon reaction for different loadings as well. A decrease of the paraffin and aromatic

yield and an increase of the lower olefins have been reported as well. Multiple authors[51, 52,

55] suggest that a lower coke formation is present for the iron loaded catalysts compared to

the non-loaded HZSM variant (at the same conditions). According to Lu et al. can the lower

aromatization activity and coke formation be ascribed to the reduction of strong acidic sites

through addition of iron. The lower paraffin yield is by same authors ascribed to the inhibiting

effect of iron to the hydrogen transfer reaction [52]. However no mechanistic explanation or

quantitative correlations are presented.

Aguayo et al. introduced nickel through impregnation with a metal loading between 0.5-

3.5%. At 400°C an increase in selectivity towards butenes and propenes were detected.

Analysis of the corresponding NH3 TPD profiles indicates that increase of metal content led

to a decrease of strong acid sites concentration suggesting necessary for the conversion to

liquid hydrocarbons and aromatics. A decrease of the irreversible deactivation after addition

of nickel have been noticed as well.

According to Inaba et al [41], after introduction of gallium through incipient wetness

impregnation a strong increase of paraffin and aromatic yield and a decrease of the yield

towards lower olefins is observed at 400°C.

2.4.2.2Alkaline and earth metals

The exchange of a zeolite with a less charge balancing cation such as Cs+ makes the zeolite

more basic. Indeed, according to Ammonia Temperature Desorption Profiles( NH3-TPD) of


28

alkaline treated ZSM5 catalysts, the strength of the strong acid sites was reduced [56]. This is

accompanied by the formation of mesopores

According to Sano et al. [57], the introduction of alkaline earth metals, led in the ethanol to

hydrocarbon process, at a temperature of 500°C, to a higher yield of light olefins and a higher

stability. The stability however can be questioned when working at such high temperatures in

the presence of water.

2.4.2.3Phosphor

Phosphorus modification is commonly used for the improvement of the hydrothermal stability

and influence the shape selectivity. It is suggested that it hinders structural changes of zeolite

results in a higher stability [58].

Treatment of HZSM5 with phosphor will neutralize the Brönsted sites primarily at the

entrance of the pores. The yield of the C2-C4 in the ethanol to hydrocarbon conversion

increased after modification with phosphor [59].


29

2.5 Reaction mechanism

The ethanol to hydrocarbon reaction over a HZSM5 catalyst consists of two main steps;

dehydration to ethylene and the further conversion to a higher hydrocarbon fraction. In the

first paragraph the proposed reaction scheme for the ethanol to hydrocarbon reaction will be

explained before discussing The different hypothesis for the further conversion.

2.5.1 Ethanol to ethylene

The dehydration of ethanol takes place by two competitive paths: the intramolecular

dehydration to ethylene and the intermolecular dehydration to diethylether and the

dehydration of diethylether to ethylene. The reaction scheme is proposed by Le Van Mao en

Nguyen (1990):

Figure 2-11: Dehydration of ethanol.

The direct route to ethylene becomes predominant starting from 250°C based on the

calculated (apparent) activation energies of the different dehydration routes. [60]

Historically, it is suggested that classic carbocation reaction mechanism is responsible for the

dehydration to ethylene. However based on in situ Fourier Transformed Infrared

Spectroscopy (FT-IR) experiments,[61] the dehydration to ethylene, will take place through

an alkoxy intermediates and not through carbocations (cf. Figure 2-12).

Figure 2-12: Conventional mechanism and proposed mechanism.[61]


30

2.5.2 Ethanol to higher hydrocarbons

Various hypothesis exist over the location and type of active centers responsible for these

reactions. According to Makarfi et al. , the active centers are acid centers located at the

external surface of the zeolite. This is based on experiments were, after introduction of

additives in the pores, no change in product distribution were noticed.

However, according to Madeira et. Al,[62] the active sites responsible are primarily present in

the zeolites pores. Here, experiments were performed passivating the external surface with

TEOS. Only small changes in product distribution were noticed.

Such conflicting results together with the stability behavior as discussed in 2.4.1.2, results in

several proposed mechanisms. First several mechanisms for the conversion of ethanol to C3+

hydrocarbons on a H-ZSM-5 zeolite will be presented where after these mechanism will be

discussed.

The difficulty lies in gathering information of a complex reaction mechanism inside the pores

on a very small timescale. The different theories are primarily based on indirect information

through IR spectroscopy and MNR and direct information of the product distribution.

The traditional oligomerization cracking mechanism is a direct mechanism where

carbocations are the reaction intermediates. The hydrocarbon pool mechanism is an indirect

mechanism where cyclic hydrocarbon carbocations act as co-catalysts. In the radical

mechanism, it is suggested that certain radical coke species have a catalytic effect on the

hydrocarbon conversion.

2.5.2.1Oligomerization isomerization cracking mechanism

The oligomerization-cracking mechanism applied to the ethene to hydrocarbon conversion

starts with the formation of ethene carbocations through protonation on Brönsted sites. It has

to be noted that the description of elementary steps over ZSM5 is based on primarily alkane

cracking at higher temperatures.

Formation of carbocations can be done through protonation by a Brönsted site. The first

carbocation formed is the ethyl carbocation. Through deprotonation ethylene can be formed.


31

It can further react as well through several elementary steps as well. The formation of α

olefins is possible through further oligomerization and deprotonation. Branched olefins can be

formed through (aklylshift, PCP branching,…). After the formation of a carbocations several

elemental steps can take place: hydride shift, aklylshift, PCP branching, B scission,….

leading to branched olefins. Cyclic and aromatic species can be created through cyclization

and aromatization steps. Paraffins can be formed through a bimolecular mechanism on a

Lewis acid site or through a mono molecular mechanism. [43, 62, 63]

2.5.2.2Hydrocarbon model

The hydrocarbon pool model states that organic reaction centers have a co catalytic effect

inside the zeolite pores. The exact nature of the hydrocarbon pool may vary, depending on

catalyst properties and reaction conditions.[64] These organic catalytic centers will switch

between neutral molecules and relatively stable carbenium ions. The principle of the

hydrocarbon pool is shown in Figure 2-13.

Figure 2-13: Simplified representation of hydrocarbon pool.

The ethanol to gasoline (ETG) has a similar product distribution compared to the methanol to

gasoline (MTG) and therefore several authors are suggesting that the reaction mechanism will

be similar as well. The MTG process has been extensively studied by Kolboe et. al.

Experiments[65, 66] were performed co feeding polymethylbenzenes, resulting in a higher

catalytic activity . Other experiments showed isotopic scrambling occurred after the 12C/13C

switch[67]. The product flow changed only gradually indicating entrapment of the organic

material.[68] HexaMB can be alkylated forming heptamethylbenzenium ion.[66] By NMR

spectroscopy of the retained material, the HEXAMB+ ion was located. Ab initio results


32

confirmed the high stability of cyclic organic molecules because of the zeolite framework

having an electronic confinement effect.[64, 69]

The presence of co catalytic organic centers allow the continuation of production of higher

hydrocarbons despite the lower acidity of the zeolite due to coke formation. Moreover it has

been suggested by Stocker et. al. in [70] that some “coke species” can be a part of these

trapped hydrocarbons. The hydrocarbon pool can co exist with the previously discussed

carbocation mechanism. A possible general mechanism is represented in

Figure 2-14: Hydrocarbon pool proposal for MTO process [71].

Two catalytic cycles are visible. In the first cycle, the formation of olefinic species are based

on methylation / cracking. Through hydrogen transfer reaction of higher alkanes with

naphtenic species, aromatics are being formed. By methylation reactions with the aromatic

species, ethylene is produced in the second cycle.

It has to be noted that the difference between methanol and ethanol dehydration, lies in the

direct dehydration of ethanol to ethylene. This reaction has no counterpart in methanol

dehydration (as shown in Figure 2-15). The hydrocarbon pool model mainly serves as a

mechanism to explain the formation of the first C-C bond. In ethanol conversion reactions this

is not necessary since here a very reactive molecule is present i.e. ethylene.

Figure 2-15:Iniation of (a) methanol to hydrocarbons (2) ethanol to hydrocarbons.


33

Propositions have been made for a hydrocarbon pool reaction mechanism applied to the

ethanol to hydrocarbon process as shown in Figure 2-16.

Figure 2-16: Dual catalytic cycle for ETO process[72]

2.5.2.3. Radical mechanism

Madeira et al. [73]suggested a radical mechanism as an alternative reaction mechanism for the

ethanol to hydrocarbon conversion. Through EPR (electron paramagnetic resonance)

techniques radical species were detected in the cokes. An altering of the cokes composition

was also noticable; from single highly alkylated rings to more bulky and more stable

hydrocarbons. The evolution of these species with time on stream seemed to be correlated

with the C3+ hydrocarbon conversion ( which possible offers an explanation for the high

catalytic performance despite great loses in acidity and microporosity).

In gas phase reactions over acid zeolites radicals acting als the reaction intermediates have

already been proposed [74].

2.5.2.4 Discussion

Based on previous paragraphs it can be concluded that none of the presented reaction

hypothesis are able to explain the experimental results. In Figure 2-17 a comparison is made.


34

Figure 2-17: Comparison between the different hypothesizes.

The oligomerization-isomerization-cracking mechanism is able to qualitative explain the

steady state behavior. However, no quantitative micro kinetic model have been established

yet in the ethanol to hydrocarbon reaction. For the previous described deactivation behavior

(strong decrease in acidity but still a relatively high C3+ formation), no explanation based on

this model, can be formulated.

The hydrocarbon pool however can explain the persisting C3+ formation despite the loss in

acidity through additional catalytic centers. The hydrocarbon pool is suggested to take place

in the micropores were certain cyclic hydrocarbons are retained. However, the great loss in

microporosity which makes it very difficult for the reactant/products to enter/leave the zeolite

structure makes this hypothesis questionable. [73]

Moreover, the evidence is based only on the methanol to olefin production where initially no

C-C bonds are present. This can lead to differences in the reaction mechanism despite the fact

that a similar product distribution have been noticed.

The radical chain reaction mechanism offers an explanation for persisting high selectivity

towards C3+ olefins despite drastic lowered acidity. However, besides the detection of small

electron spin resonance (ESR) signals no other experimental results were presented indicating

that radical intermediates play a key role in the reaction mechanism . The exact nature and


35

location (negligible microporosity still present as well as a passivated external surface area)

remains in this hypothesis unsolved as well. Moreover, it cannot be excluded that the radical

species are only responsible for the further coke growth [75].


36

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37

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25. Tsao, U., Production of ethylene from ethanol, L. company, Editor 1979: USA.

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Editor 1989: USA.

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30. Talukdar, A.K., K.G. Bhattacharyya, and S. Sivasanker, HZSM-5 catalysed conversion

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357-371.


38

31. Makarfi, Y.I., et al., Conversion of bioethanol over zeolites. Chemical Engineering

Journal, 2009. 154(1-3): p. 396-400.

32. Ribeiro, F.R., et al., Structure-Activity Relationship in Zeolites. Journal of Molecular

Catalysis a-Chemical, 1995. 96(3): p. 245-270.

33. Bhan, A. and W.N. Delgass, Propane aromatization over HZSM-5 and Ga/HZSM-5

catalysts. Catalysis Reviews-Science and Engineering, 2008. 50(1): p. 19-151.

34. Kirkby, S.J. and T.L. Schaeffer, Synthesis and characterization of large crystal

transition metal substituted zeolite ZSM-5. Abstracts of Papers of the American

Chemical Society, 2001. 222: p. U631-U631.

35. Deng, J.F., et al., Acidic Properties of Zsm-5 Zeolite and Conversion of Ethanol to

Diethyl-Ether. Applied Catalysis, 1988. 41(1-2): p. 13-22.

36. Aguayo, A.T., et al., Study of operating variables in the transformation of aqueous

ethanol into hydrocarbons on an HZSM-5 zeolite. Journal of Chemical Technology

and Biotechnology, 2002. 77(2): p. 211-216.

37. Schulz, J. and F. Bandermann, Conversion of Ethanol over Zeolite H-Zsm-5. Chemical

Engineering & Technology, 1994. 17(3): p. 179-186.

38. Costa, E., et al., Ethanol to Gasoline Process - Effect of Variables, Mechanism, and

Kinetics. Industrial & Engineering Chemistry Process Design and Development, 1985.

24(2): p. 239-244.

39. Madeira, F.F., et al., Radical Species Detection and Their Nature Evolution with

Catalyst Deactivation in the Ethanol-to-Hydrocarbon Reaction over HZSM-5 Zeolite.

Acs Catalysis, 2011. 1(4): p. 417-424.

40. Oudejans, J.C., P.F. Vandenoosterkamp, and H. Vanbekkum, Conversion of Ethanol

over Zeolite H-Zsm-5 in the Presence of Water. Applied Catalysis, 1982. 3(2): p. 109-

115.

41. Inaba, M., et al., Ethanol conversion to aromatic hydrocarbons over several zeolite

catalysts. Reaction Kinetics and Catalysis Letters, 2006. 88(1): p. 135-142.

42. Lallemand, M., et al., Ni-MCM-36 and Ni-MCM-22 catalysts for the ethylene

oligomerization. Zeolites and Related Materials: Trends, Targets and Challenges,

Proceedings of the 4th International Feza Conference, 2008. 174: p. 1139-1142.

43. Rane, N., et al., Cracking of n-heptane over Bronsted acid sites and Lewis acid Ga

sites in ZSM-5 zeolite. Microporous and Mesoporous Materials, 2008. 110(2-3): p.

279-291.

44. Lu, J.Y., et al., FeHZSM-5 molecular sieves - Highly active catalysts for catalytic

cracking of isobutane to produce ethylene and propylene. Catalysis Communications,

2006. 7(4): p. 199-203.


39

45. Tangstad, E., et al., Catalytic behaviour of nickel and iron metal contaminants of an

FCC catalyst after oxidative and reductive thermal treatments. Applied Catalysis a-

General, 2008. 346(1-2): p. 194-199.

46. Choudhary, V.R., et al., Aromatization of dilute ethylene over Ga-modified ZSM-5

type zeolite catalysts. Microporous and Mesoporous Materials, 2001. 47(2-3): p. 253-

267.

47. Iwamoto, M., One Step Formation of Propene from Ethene or Ethanol through

Metathesis on Nickel Ion-loaded Silica. Molecules, 2011. 16(9): p. 7844-7863.

48. Jia, J.F., K.S. Pillai, and W.M.H. Sachtler, One-step oxidation of benzene to phenol

with nitrous oxide over Fe/MFI catalysts. Journal of Catalysis, 2004. 221(1): p. 119-

126.

49. Lallemand, M., et al., NiMCM-36 and NiMCM-22 catalysts for the ethylene

oligomerization: Effect of zeolite texture and nickel cations/acid sites ratio. Applied


50. Maia, A.J., et al., Ni-ZSM-5 catalysts: Detailed characterization of metal sites for

proper catalyst design. Journal of Catalysis, 2010. 269(1): p. 103-109.

51. Inaba, M., K. Murata, and I. Takahara, Effect of Fe-loading and reaction temperature

on the production of olefins from ethanol by Fe/H-ZSM-5 zeolite catalysts. Reaction

Kinetics and Catalysis Letters, 2009. 97(1): p. 19-26.

52. Lu, J.Y., Y.C. Liu, and N. Li, Fe-modified HZSM-5 catalysts for ethanol conversion

into light olefins. Journal of Natural Gas Chemistry, 2011. 20(4): p. 423-427.

53. Gayubo, A.G., et al., Hydrothermal stability of HZSM-5 catalysts modified with Ni for

the transformation of bioethanol into hydrocarbons. Fuel, 2010. 89(11): p. 3365-3372.

54. Ausavasukhi, A. and T. Sooknoi, Additional Bronsted acid sites in [Ga]HZSM-5

formed by the presence of water. Applied Catalysis a-General, 2009. 361(1-2): p. 93-

98.

55. Machado, N.R.C.F., et al., Hydrocarbons from ethanol using [Fe,Al]ZSM-5 zeolites

obtained by direct synthesis. Applied Catalysis a-General, 2006. 311: p. 193-198.

56. Gayubo, A.G., et al., Kinetic Model for the Transformation of Bioethanol into Olefins

over a HZSM-5 Zeolite Treated with Alkali. Industrial & Engineering Chemistry

Research, 2010. 49(21): p. 10836-10844.

57. Sano, T., et al., Conversion of ethanol to propylene over HZSM-5 type zeolites

containing alkaline earth metals. Applied Catalysis a-General, 2010. 383(1-2): p. 89-

95.

58. Gayubo, A.G., et al., Selective production of olefins from bioethanol on HZSM-5

zeolite catalysts treated with NaOH. Applied Catalysis B-Environmental, 2010. 97(1-

2): p. 299-306.


40

59. Lu, J.L., J. Y. and Y.C. Liu, Effects of P content in a P/HZSM-5 catalyst on the

conversion of ethanol to hydrocarbons. Journal of Natural Gas Chemistry, 2011.

20(2): p. 162-166.

60. Phillips, C.B. and R. Datta, Production of ethylene from hydrous ethanol on H-ZSM-5

under mild conditions. Industrial & Engineering Chemistry Research, 1997. 36(11): p.

4466-4475.

61. Kondo, J.N., et al., An ethoxy intermediate in ethanol dehydration on bronsted acid

sites in zeolite. Journal of Physical Chemistry B, 2005. 109(21): p. 10969-10972.

62. Magnoux, P., et al., Mechanistic insights on the ethanol transformation into

hydrocarbons over HZSM-5 zeolite. Chemical Engineering Journal, 2010. 161(3): p.

403-408.

63. Kotrel, S., H. Knozinger, and B.C. Gates, The Haag-Dessau mechanism of protolytic

cracking of alkanes. Microporous and Mesoporous Materials, 2000. 35-6: p. 11-20.

64. Christensen, C.H., et al., The Hydrocarbon Pool in Ethanol-to-Gasoline over HZSM-5

Catalysts. Catalysis Letters, 2009. 127(1-2): p. 1-6.

65. Dahl, I.M. and S. Kolboe, On the reaction mechanism for hydrocarbon formation

from methanol over SAPO-34 .2. Isotopic labeling studies of the co-reaction of

propene and methanol. Journal of Catalysis, 1996. 161(1): p. 304-309.

66. Bjorgen, M., et al., The methanol-to-hydrocarbons reaction: insight into the reaction

mechanism from [C-12]benzene and [C-13]methanol coreactions over zeolite H-beta.

Journal of Catalysis, 2004. 221(1): p. 1-10.

67. Duffy, B.L., et al., Isotopic Labeling Studies of the Effects of Temperature, Water, and

Vanadia Loading on the Selective Catalytic Reduction of No with Nh3 over Vanadia-

Titania Catalysts. Journal of Physical Chemistry, 1994. 98(29): p. 7153-7161.

68. Seiler, M., U. Schenk, and M. Hunger, Conversion of methanol to hydrocarbons on

zeolite HZSM-5 investigated by in situ MAS NMR spectroscopy under flow conditions

and on-line gas chromatography. Catalysis Letters, 1999. 62(2-4): p. 139-145.

69. McCann, D.M., et al., A complete catalytic cycle for supramolecular methanol-to-

olefins conversion by linking theory with experiment. Angewandte Chemie-

International Edition, 2008. 47(28): p. 5179-5182.

70. Karlsson, A., M. Stocker, and R. Schmidt, Composites of micro- and mesoporous

materials: simultaneous syntheses of MFI/MCM-41 like phases by a mixed template

approach. Microporous and Mesoporous Materials, 1999. 27(2-3): p. 181-192.

71. Olsbye, U., et al., Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and

Pore Size Controls Product Selectivity. Angewandte Chemie-International Edition,

2012. 51(24): p. 5810-5831.

72. Bjorgen, M., et al., Conversion of methanol to hydrocarbons over zeolite H-ZSM-5:

On the origin of the olefinic species. Journal of Catalysis, 2007. 249(2): p. 195-207.


41

73. Madeira, F.F., et al., Mechanistic insights on the ethanol transformation into

hydrocarbons over HZSM-5 zeolite. Chemical Engineering Journal, 2010. 161(3): p.

403-408.

74. Arstad, B. and S. Kolboe, The reactivity of molecules trapped within the SAPO-34

cavities in the methanol-to-hydrocarbons reaction. Journal of the American Chemical

Society, 2001. 123(33): p. 8137-8138.

75. Marcus, J.F.H.a.D.M., Examples of Organic Reactions on Zeolites:

Methanol to Hydrocarbon Catalysis, in Handbook of zeolite science and technology, K.A.C.

Scott M. Auerbach, Prabir K. Dutta, Editor 2003. p. 1184.

Chapter 3

Procedures

This chapter comprises all the necessary procedures for the experimental work and data

progressing. This including catalyst preparation, characterization and testing.

3.1 Preparation

3.1 Unmodified HZSM-5

HZSM-5 catalysts are prepared starting from the NH4-ZSM5 precursor (Zeolyst). Conversion

to the H form is done by high temperature calcination. This treatment exists of heating the

catalyst (in air) with a heating rate of 2°C/min to 550°C which is maintained for 5 hours [1].

A small heating rate is taken to avoid collapse of the zeolite structure. During this procedure

gaseous NH3 is released resulting in the HZSM5 catalyst. The M-HZSM5 catalysts are

prepared starting from the calcinated H form.

3.1.1 Incipient wetness impregnation on HZSM-5

Incipient Wetness Impregnation (IMP) is a technique where primarily metal oxides and

clusters are deposited in the zeolite pores [1].

The precursors used for preparation of Fe-HZSM5, Ni-HZSM5 and Ga-HZSM5 catalysts are

respectively: Fe(NO3)3.9H2O, Ni(NO3)2.6H2O and Ga(NO3)3.xH2O.

First the precursors are brought into solution. To insure the metal is primarily present in the

zeolite pores and not at the external surface, the amount of solution added is limited to the

total pore volume of the zeolite. The maximum pore volume is determined through addition of

water to HZSM5 until a strong decrease of viscosity is noticed. The appropriate amount of

metal salt , to procedure 3g of M-ZSM aiming to a metal content of 1, 2, 5 and 10% is solved

Chapter 3: Procedures

44

in water. The dissolved metal is then mixed with the zeolite. Due to capillary forces, if the

metal solution is added to the zeolite, a deposit of these metals in the zeolite pores can be

achieved. After impregnation, the catalyst is dried (100°C) to remove the solvent and

calcinated (same procedure as for HZSM5) to remove the nitrates and to fixate the metal. The

success of impregnation was determined by inductively coupled plasma.

3.1.2 Ion Exchange

The framework of HZSM5 contains channels intersection where water and cations are

present. Ion exchange is a post synthesis technique aiming to (partially) exchange the

compensating cations present in the zeolite with metal cations.

A solution of 0.01M (precursors: : Fe(NO3)3.9H2O, Ni(NO3)2.6H2O and Ga(NO3)3.xH2O ) is

added to 2.05g HZSM5 zeolite. The mixture was brought into a small vessel where it was

continuously mixed at 60°C for 1day. Hereafter, the mixture is three times filtrated with

water, dried and calcinated.

3.2 Characterization

3.2.1 Metal content determination

The bulk chemical composition of the tested catalysts was determined by means of

inductively coupled plasma atomic emission spectrometry (ICP-AES) (IRIS Advantage

system, Thermo Jarrell Ash).

3.2.2 Hydrogen Temperature Programmed Reduction

Hydrogen Temperature Programmed Reduction (H2 TPR) is a characterization technique

aiming to identify the reducible species. For this the Autochem 2910 (Micromertics) is used.

The sample is placed at a fixed position in a tube(U shape). First, absorbed molecules (such as

water) are removed in an inert atmosphere through heating at 520°C. The flow is then

changed to a diluted hydrogen stream (5% H2/Ar). The temperature is under these conditions

lineary increased. Hydrogen will reduce (the reducible) metal species. A typical reduction

reaction is given by:


45

Fe2O3 + H2 → 2 Fe3O4 + H2O 3-1

The amount of hydrogen consumption is monitored by a thermal conductivity detector

(TCD). During the reduction, water is formed which has a very large effect on the TCD

signal. Hence a cold trap is utilized to remove the water from the gas stream. The reaction

conditions are summarized in Table 3-1.

Table 3-1: Experimental conditions of H2-TPR.

phase Feed T(°C) β (°C/min) Pre treatment He 25-520°C 20

H2-TPR H2 (4%)-He 25-900 10

3.2.3 Ammonia Temperature Programmed Desorption

Ammonia temperature Programmed Desorption (NH3-TPD) is a characterization technique

used for the identification of the quantity and strength of acidic sites. The same equipment, as

for the H2- TPR experiments, is used. After the pretreatment, the feed is changed to a diluted

ammonia feed (4%NH3/He). Ammonia will interact with the acidic sites through both

fysisorption as chemisorption. Hereafter, the feed is changed back to helium to flush to

remove the physisorped ammonia. The temperature is then set to increase lineary (in time).

Leading to desorption of ammonia which is detected by a TCD detector.

Table 3-2: Experimental conditions of NH3-TPD.

Phase Feed T(°C) β

(°C/min) Pretreatment He 25-520°C 20

0 (60min) NH3 addition NH3 (4%)-He 50 NH3 TPD He 50-600 300

3.2.4 BET surface area and total pore volume

The specific surface area is measured using nitrogen adsorption-desorption isotherms, under

assumptions suggested by Brunauer-Emmett-Teller (BET) [2] in a micromeritics Gemini

equipment. Prior to the analysis , the catalysts (0.25g) were heated to 120°C for one hour to

remove the water. The adsorption isotherm is obtained through physisorption of nitrogen

(77K) . For the calculation of the BET surface area only relative pressures (P/Po ) between


46

0.06 and 0.30 were used. Due to limitations of the setup, it was not possible to obtain the

desorption isotherm. Hence, no microporosity measurement was possible. However, the total

pore volume is calculated according to the method of Barrett, Joyner, and Halenda (BJH)

[3]. This procedure is based on the the Kelvin model of pore filling. It applies only to the

mesopore and small macropore size range.

3.3 Experimental setup

Experimental data is acquired on a lab scale plug flow reactor used for catalyst screening,

located at Laboratory at University of Ghent (LCT).

3.3.1 Reaction section

The reaction setup consisst of various sections: a feeding section, reaction section and a

analysis section. A simplified reaction scheme is presented in Figure 3-1.

Figure 3-1: Simplified schematic representation of reactor setup.

3.3.1.1Feed section

In the first section, pure ethanol (>99.8% absolute, Sigma Aldrich) is mixed and evaporated

with Helium. The pressure (1atm) ethanol partial pressure (0.1atm) is set constant for all the


47

experiments. The ethanol flow is regulated with a mass flow controller (Mini cori flow,

Bronkhorst) and the gasses are controlled with a thermal mass flow controller . A calibration

curve to determine the helium valve position is added in Appendix A (using a ADM Universal

Gas flowmeter, Agilent Technologies). The ethanol flow is checked by using a mass balance.

3.3.1.2Reaction section

The reaction section consists of a stainless steel tubular reactor (length=200mm, external

diameter=10mm, wall thickness=1mm. The reactor is heated by an electric heating mantle.

The temperature at the reactor wall and inside the reactor is measured. Temperature control is

done based on temperature inside, in the center of the catalytic bed, of the reactor by an PID

controller (Eurotherm). The process conditions are listed in Table 3-3.

Table 3-3:Reference conditions.

Temperature (°C) 170 - 400

Spacetime(molEtOH.s/kgcat ) 8.8-55

Ptot(atm) 1

PEtOH(atm) 0,1

3.3.2 Analysis section

By heating the effluent lines above 100°C, everything is kept in the gas phase. An online

sample is taken by means of a 4 way valve. Through appropiate position of a 4 way valve

present at the effluent line a sample can be taken and be send to an apolar (capillary, CP-

SilPONA CB) GC column (Chrompack). A Flame Ionization Detector (FID) is then used for

analysis. The detailed GC settings are added in Table 3-4. An example of a chromatogram and

a list of the retention times can be found in Appendix A.

Table 3-4: GC-settings.

Parameter Condition

Temperature: Inlet/Detector 240°C

Temperature profile -50°C →240°C β=6°C/min

Split: mode/ratio Split / 100:1

Column: length/inner diameter/film thickness 10.00 m / 0.21 mm / 0.25 µm

Carriergas: type/mode/flow He / constant flow /2.00 ml/min-1)


48

Detector: hydrogen flow rate/ make-up flow/ 26.7 Nml min

-1

make-up gas / 26.7 ml min-1

/ N2

Trigger External

3.4 Validation of plug flow and intrinsic kinetics

3.5 Data treatment

3.5.1 GC Analysis

At first, due to practical considerations, no internal standard could be added. During the

course of the thesis however it became possible. A description is therefore given of both the

normalization method and the internal standard method based on Toch et al. [4]

The mass composition of a component i calculation with the normalization method is given

by:

n

i

ii

iii

ACF

ACFx

1

3-2

with the calibration factor for component i and Ai the area of the peak for this component.

This factor links the area of a component with its concentration. After the incorporation of

the calibration factors, the obtained percentages are normalized to determine the real

composition of the outlet gas flow. The calibration factors are listed in Appendix A (obtained

by others [5]). Assuming a closed mass balance, the outgoing flows can then be calculated.

According to the internal standard method the outgoing mass flows of the different

components are calculated by:

3-3


49

Derivated quantities like mass fraction can then be calculated. Hence, the (component) mass

balances can be checked according by this method., the data obtained through applying the

normalization method is justified If the mass balance is closed.

For the calculation of the outlet flow rates, four balances are available: a total mass balance, a

carbon balance, hydrogen balance and oxygen mass balance. The column cannot detect water,

which is in large quantities formed through dehydration of ethanol. Under the reference

reaction conditions (T=350°C, W/F=16.6 molEtOH.s/kgcat, Pet=0.1atm) ethanol is very fast

dehydrated (>99.9%) to primarily ethylene and water (cf.chapter 2 and 4):

C2H5OH → C2H4 + H2O 3-4

Hence, the amount of water present in the outgoing flow can be calculated based on previous

reaction. component mass balances are calculated under these assumptions.

The equation for calculation the component mass balances is given by:

= 3-5

With:

= flow in of component i [kg/hr]

= Molecular mass of x (x=C,H,O) [kg/kmol]

HN= Number of atoms of x in component i

= Molecular mass of component i [kg/kmol]

Based on this formula the total mass balance and the hydrogen and carbon balance can be

calculated. The relative error was for all the balances less than 5%. Hence, the previous

results obtained through normalization method can be used.

For the analysis of the dehydration data (temperature range 170 – 250°C), water is calculated

from the mass balance and a mass balance check was done based on the carbon balance.


50

3.5.2 Calculation of conversion, yields and selectivities

The ethanol conversion is defined as follows:

1000

0

EtOH

EtOHEtOH

EtOHF

FFX

3-6

Due to the (almost) very fast and complete dehydration of ethanol to ethylene, can ethylene be

considered as reagent which leads to the following suitable definition for ethylene conversion:

= 1000

C

C

0

C

2

22

F

FF 3-7

In equation 3.2 is the “initial” ethylene molar flow calculated based on equation 3-1. The

yield of component i (i ethanol or ethylene) is given by :

1000

2C

ii

F

FX 3-8

Selectivities are defined as:

100

22

0

CC

ii

FF

FS 3-9

3.6 Error bars

The uncertainity of the data is determined through performance of several repetition

experiments (cf.appendix B). The population variance will be approximated by the estimated

standard deviation. The standard deviation is given by:

S= 3-10

In this formule represent N the number of repetitions experiments and is the average value.


51

The confidence interval (95%) can then be calculated (under the assumption that the data are

normal distributed by:

3-11

This will be used to estimate the uncertainty of the experimental data. This is done for the

BET surface area, total pore volume, NH3 TPD surface area, conversion and product

selectivities. In all experiments are multiple tests repetition experiments performed to

determine the uncertainity. It is assumed that the procentuel error is constant for all the data.

3.7 References

1. Xu, R., Preparation, Secondary Synthesis and Modification of Zeolites in Chemistry of

Zeolites and Related Porous Materials: Synthesis and Structure, R. Xu, Editor 2010.

2. Brunauer, S., Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc.,

1938. 60(2): p. 309–319.

3. Barrett, P., The Determination of Pore Volume and Area Distributions in Porous

Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc, 1951. 73(1):

p. 373–380.

4. Toch, K., Calculation of outlet composition, flow rates, conversions and selectivities

in continuous flow (multiphase) reactors. (internal document Ugent), 2010.

5. Dietz, W., Response Factors for Gas Chromatographic Analysis Journal of Gas

chromatography, 1967.

Chapter 4

Experimental study of the

ethanol conversion on HZSM5

In this chapter, ethanol conversion experiments are performed on a lab scale fixed bed reactor

to acquire further insight in the effect of process conditions and the reaction mechanism.

The reference catalyst used throughout the following chapters is HZSM5 with a Si/Al molar

ratio of 30. Application of this catalyst will, according to the literature study, lead to a good

performance in the ethanol to hydrocarbon reaction.

The effect of reaction time, temperature, space time and addition of water to the feed will be

investigated. The obtained experimental data led to further comprehension in the reaction

mechanism, especially in the paraffin formation. Finally, the effect of temperature and

cofeeding water on the selectivity is determined.

4.1 Introduction

It is necessary to determine a temperature zone were the formation of hydrocarbons is present.

Hence preliminary experiments were performed over a broad temperature range from 170°C-

400°C. the yield of the product(s) (lumps) over a broad temperature range is shown In Figure

4-1 . From this figure, 4 distinct zones can be identified.

Chapter 4: Experimental study of the ethanol conversion on HZSM5

54

Figure 4-1: Yield in function of temperature (reaction conditions: Wcat/FEtOH= 16.6 kgcat.s/molEtOH,

EtOH/He=1/10,Ptot=1bar, TOS=4-7h catalyst:HZSM5Si/Al=30).

At a temperature lower than 220°C (zone I) ethanol results in intramolecular dehydration of

ethanol to diethylether. In this zone, there is no significant formation of ethylene and higher

hydrocarbons. In zone II (220-260°C), ethylene will be formed which is accompanied by the

decrease of diethylether. Characteristic to this zone is the presence of both ethylene as

diethylether. Starting from 260°C (zone III) ethanol will be completely converted to primarily

ethylene and, in very small quantities, further to higher hydrocarbons (primarily propene and

butenes). Under these conditions a maximum ethylene yield of more than 93% can be

achieved. Above 300°C (zone IV), the ethylene yield will decrease, leading to a strong

increase of higher hydrocarbons.

Further catalytic testing is done at a temperature higher than 300°C (350°C). At lower

temperatures it is less interesting due to the potential beneficial effect of catalyst

modifications, due to the already high selectivity towards ethylene.

In this thesis the primarily aim was to do an extensive catalytic screening nevertheless it is

validated that plugflow &intrinsic kinetic data is present. However an experimental approach

to investigate potential internal transport limitations is advisable. This can be done through

variation of the catalyst weight with a constant space time. This was not the main goal of the

thesis and therefore not yet being performed.

0

10

20

30

40

50

60

70

80

90

100

150 200 250 300 350 400

Yie

ld (

%)

Temperature (°C)

ethanol diethylether ethene C2-C5 hydrocarbons C5+ hydrocarbons

I II III IV


55

4.2 Product distribution

In this paragraph the product distributionat the reference process conditions (Table 4-1) is

discussed.

Table 4-1:Reference process conditions.

Process conditions

Temperature 350°C

16.6 kgcat.s/molEtOH

0.1atm

1 atm

Space time

PEtOH

Ptot

The yield of the different products at a reaction temperature of 350°C in plotted in Figure 4-2.

Ethylene is the most prominent product responsible for about 50% of the total product

mixture. The other gaseous olefins make out for about 30% of the organic yield fraction. The

remaining products are primarily liquid hydrocarbons, consisting of about 20% of BTXE,

and C4-5 paraffins. Ethane and propane have a very small contribution to the total product

distribution. Ethanol, diethylether and methane are negligible under these reaction conditions

(<0.3%). A more detailed product distribution is added in Appendix C. It has to be noted that

in the C5+ product group, benzene, toluene, xylenes and ethyl benzene are not included.


56

Figure 4-2: A typical product distribution (reaction conditions: T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH,

EtOH/He=1/10, Ptot=1atm catalyst: HZSM5 (Si/Al=30). Detailed values are added in Appendix C.)

4.3 Stability

Before the influence of the process parameters can be studied further studied, verification is

necessary whether the experimental data is reproducible and whether deactivation has to be

considered.

The conversion of ethanol and ethylene and the selectivity towards different product lumps

are plotted in function of time on stream in Figure 4-3.

0

10

20

30

40

50

60

ethene ethane propene propane C4 olefins C4 paraffins C5 olefins C5 paraffins C5+ BTXE

Yie

ld (

%)


57

Figure 4-3: Conversion/selectivity (mol%) in function of time on stream TOS (h). (reaction conditions:

T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, catalyst: HZSM5(Si/Al=30).

Detailed values can be found in appendix C.)

For the first 4 hours’ time on stream, it can be observed that, relative high fluctuations ( both

the ethylene conversion as the selectivities) are present. This can be attributed to startup

fluctuations in the flow and the stabilization of the process. Hereafter no significant changes

of the conversion/selectivity were noticed. Hence, more as 4 hour time on stream is necessary

to insure steady state behavior.

4.4 Effect of process conditions

4.4.1 Space time

The conversion of ethanol and ethylene and the selectivity towards different products in

function of the space time is presented in Figure 4-4. The ethanol conversion remained, at

every tested space-time, close to 100%. Increase of the space-time led to an increase of the

0

10

20

30

40

50

60

70

80

90

100

1 3 5 7 9 11

Co

nve

rsio

n /

sel

ecti

vity

(mo

l%)

TOS (h)

ethanol conversion

ethylene conversion

C1-C5 paraffin selectivity

C3-C5 olefins selectivity

C5+ selectivity

BTXE selectivity


58

ethylene conversion. An increase of the paraffin and the total C5+ liquid hydrocarbon have

been observed as well as results of a lower C3-C5 olefin selectivity. These trends are in

correspondence with literature. literature[1]

Figure 4-4: Conversion of ethanol and ethylene (mol%) in function of W/F [kgcat.s/molEtOH]. (Reaction

conditions: T=350°C, EtOH/He=1/10,Ptot=1bar, catalyst:HZSM5 Si/Al=30)).

4.4.2 Temperature

Temperature has a major effect on the product distribution, which is already illustrated in

Figure 4-1 over a broad temperature range. A more detailed analysis is here done specific at

temperatures where C2+ hydrocarbons are significantly formed. In Figure 4-5 is for the

temperature range of 300-400°C, the conversion of ethanol/ethylene and the selectivities of

the others products, presented.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

Co

nve

rsio

n /

sel

ecti

vity

(mo

l%)

W/F [kgcat.s/molEtOH]

ethanol conversion

ethylene conversion

C2-C5 olefin selectivity


C5+ selectivity

BTXE selectivity


59

Figure 4-5: Conversion/selectivity (mol%) in function of temperature. (Reaction conditions: Wcat/FEtOH=

16.6 kgcat.s/molEtOH , Ptot=1bar, TOS=3-5hr, catalyst: HZSM5 (Si/Al=30).)

The ethanol conversion was, within this temperature range, always complete. Increase in

temperature led to an higher ethylene conversion . The general trends, regarding product

selectivity are: increase towards gaseous olefins and decrease of the selectivity towards the

other product lumps.

At high temperatures (T>370°C) a higher selectivity towards gaseous olefins and a lower

selectivity towards C5+ and gaseous paraffin’s is noticeable.

Despite the lack of data at low ethylene conversion, it is clear that at low temperature a great

shift in product distribution is present . At a temperature of 300°C, the C5+ and gaseous

paraffin fraction is much higher than at higher temperatures as a result of the lower C3-C5

liquid selectivities.

The effect of temperature on selectivity an hence on the reaction mechanism will be examined

in 4.5 through comparison with data obtained with a variable space time.

0

10

20

30

40

50

60

70

80

90

100

300 320 340 360 380 400

Co

nve

rsio

n/s

elec

tivi

ty (

mo

l%)

Temperature (°C)

ethanol conversion

ethylene conversion

C2-C5 olefin selectivity


C5+ selectivity

BTXE selectivity


60

4.4.3 Effect of water

The effect of the addition of water to the feed on the activity and selectivity is investigated.

An ethanol water mixture (50%-50%) was used. The ethylene selectivity is shown with and

without the addition of water In Figure 4-6.

Figure 4-6:Ethylene conversion in function of feed composition. reaction conditions: T=350°C, ,Ptot=1bar,

TOS=3-5hr, catalyst: HZSM5 (Si/Al=30). feedstock:50wt% ethanol-50wt% water, EtOH/(He+H2O)=1/10

Wcat/FEtOH=16.6 kgcat.s/molEtOH

It is suggested by Phillips et al.[2] that water may have a stabilizing effect of the adsorpted

intermediates which can explain the higher ethylene conversion.

4.5 Reaction mechanism elucidation

As shown in the literature survey, the reaction mechanism is still unclear. Attempts have been

made to model the reaction by means of the following reaction mechanism:[1]

0

20

40

60

80

100

Eth

yle

ne

co

nve

rsio

n (

mo

l%)

ethanol ethanol-water(50wt%) mixture


61

Figure 4-7: Lumped reaction mechanism.

Only a limited amount of lumped reactions are utilized here and thus don’t reflect the reality.

Based on the previously gathered experimental results, a closer look at the reaction

mechanism will be made based on elementary steps. The potential influence of temperature

and addition of water to the feed on the reaction mechanism will then be examined.

4.5.1 Reaction mechanism proposal

Through variation of the space time, it is illustrated (cf. 4.4.1) that both the ethylene

conversion and the selectivities towards the different product lumps, change. In Figure 4-8 are

the corresponding product selectivities plotted in function of the ethylene conversion. A good

relationship is found between all the different product selectivities and the ethylene

conversion. (The linear trend lines are only added to make the correlations more clear ). It is

observed that an increase of the ethylene conversion led to an increase of the gaseous paraffin,

BTXE and C5+ hydrocarbons as a result of the decrease of C3-C5 olefins.


62

Figure 4-8: Selectivity (mol%) in function of ethylene conversion(mol%) (reaction conditions: T=350°C,

EtOH/He=1/10,Ptot=1atm, TOS=3-8h, variable W/F [8.3-55 kgcat.s/molEtOH]. catalyst: HZSM5 Si/Al=30).

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100

Sel

ecti

vity

(mo

l%)

Ethylene conversion (mol%)

C2-C5 paraffins C3-C5 olefins C5+ BTXE


63

A more detailed representation of the product selectivities is done in Figure 4-9.

Figure 4-9: (a) selectivity of C3-C5 olefins, (b) selectivity of C1-C5 paraffin’s, (c) BTXE selectivity d)

selectivity of C5+ in function of ethylene conversion. (reaction conditions: T=350°C, EtOH/He=1/10,

Ptot=1bar, TOS=4-8hr, variable W/F [8.3-55 kgcat.s/molEtOH] catalyst: HZSM5 Si/Al=30)

In Figure 4-9 (a), a systematical decrease is noticeable of the selectivity of propene with

increasing ethylene conversion. The selectivity of C4-C5 olefins reach a maximum. In Figure

4-9 (b) is a strong increase of paraffin formation noticeable except ethane which is decreasing.

In Figure 4-9 (c) are the C6,C7,C8 and BTXE selectivities plotted in function of the ethylene

conversion. The C6-C8 reach a maximum and the C8+ and BTXE increase strongly. In Figure

4-9(d), are the selectivities of benzene, toluene , the xylene group and ethyl benzene shown.

The most abundant species are the xylenes. The selectivity towards xylenes and toluene

strongly increase with ethylene conversion. Benzene and ethyl benzene however, are not

strongly affected by the increase of ethylene conversion.

Combination of the previously summarized experimental results reveal information about the

reaction path. It is observed that while the BTX and paraffin selectivity increase, a decrease


64

is noticeable (or a maximum) of all olefins (<C8). The same trends are observed by

Viswanadham et.al.[3]

A possible explanation here for is that cyclic olefins (formed through oligomerization &

cyclization steps) will be converted into aromatics and hydrides. These hydrides can then be

used for the transformation of olefins to the corresponding paraffin. This explains the

correlation of the selectivities between BTX and paraffins.

A maximum of the selectivity towards C4-C5 olefins and C6-C8 olefins is noticeable. This is

a possible indication of their intermediate character. The former group could be an

intermediate between primarily lower (C2-C3) and higher olefins (C5+). The latter between

primarily between lower olefins and aromatics. The systematic increase of the BTXE

selectivity indicates that this group is an “end product”.

In previous discussion is the hydrocarbon pool mechanism not mentioned. No explicit

evidence is found for this mechanism: for a clarification of this, isotopic labeling experiments

should be performed as done with the MTO process.[4] In a recent review, the same authors

mention that the ethanol conversion does not need to go through the same hydrocarbon pool

mechanism since ethylene is present with the ethanol to hydrocarbon conversion reaction[5]

Hence, the steady state character of this process can be explained be acid catalyzed

elementary steps only.

A classification based on types of acid elementary steps is done in Table 4-2. In A, are α

olefins formed through oligomerization reactions. Branched olefins can be formed through the

elementary steps presented


65

Table 4-2 C. Elementary steps which can explain the further oligomerization of these

branched olefins is shown in Table 4-2D.Cyclization and aromatization (D and E) reaction

steps can explain the presence of aromatics which can explain the formation of paraffins (F).

cracking reactions are necessary in the reaction mechanism as well in the reaction scheme to

elucidate the presence of not even olefins. (Some secondary reactions steps disproportionation

and (de)alkylation of aromatics are not included in this table)

A-Oligomers of ethylene are being formed

here which all have even carbon numbers

(4,6,8):

B-Oligomerization and alkylation of C2-C4

olefins with the formation of higher olefins:

C-Branching and isomers are obtained with

the following elementary steps:

D-Cyclization of olefinic carbenium ions

form cyclic carbenium ions. This forms the

bridge between acyclic and cyclic species

E-Hydride transfers between cyclic and

carbenium ions give rise to the formation of

aromatics:

F-Paraffin formation is possible through

hydride transfer

G- β-scission

Table 4-2: Reaction mechanism proposal.

A-Oligomers of ethylene are being formed

here which all have even carbon numbers

(4,6,8):

B-Oligomerization and alkylation of C2-C4

olefins with the formation of higher olefins:

C-Branching and isomers are obtained with

the following elementary steps:

D-Cyclization of olefinic carbenium ions

form cyclic carbenium ions. This forms the

bridge between acyclic and cyclic species

E-Hydride transfers between cyclic and

carbenium ions give rise to the formation of

aromatics:

F-Paraffin formation is possible through

hydride transfer

G- β-scission

+

+

++

+ ++

+ ++

+ + +

+++

++

+

+

+

+

+

+

+

+ +

+ +++

++

++

+

4.5.2 Effect of temperature on reaction mechanism

A significant change in the reaction path can be perceived through a shift of product

selectivities (at isoconversion). Hence, the influence of temperature on selectivity is examined

through comparison with the data obtained at variable space time and a fixed reaction

temperature of 350°C.

Over a temperature interval of 300-400°C, the selectivities of different product lumps are

plotted (Figure 4-10) in function of the ethylene conversion . The data obtained with a fixed

space time and a variable temperature are represented by the solid data points. The data

obtained with variable space time and at a reference temperature of 350°C is represented by

the not filled data points.

Figure 4-10: Selectivity (mol%) in function of conversion (mol%). reaction conditions: Solid data points:

variable temperature( T=300-400°C). Not filled data points: variable space time: Wcat/FEtOH=8.8-55

kgcat.s/molEtOH (EtOH/He=1/10,Ptot=1bar, TOS=4-6hr, catalyst: HZSM5 Si/Al=30)

It can be concluded that temperature has a similar effect as a variable space time on

selectivity. However small deviations are noticeable.

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90

Se

lect

ivit

y (m

ol%

)


C2-C5 paraffins C3-C5 olefins C5+ BTXE


67

Additional data at low ethylene conversion with a variable space time and temperature are

necessary to make a more thoroughly study of the effect of temperature on the product

selectivities.

It has to be emphasized that at temperatures lower than 330°C the conversion and thereby the

yield of these products is much lower than at a higher temperature despite the higher product

selectivities towards higher hydrocarbons and paraffins.

4.5.3 . Effect of water on the reaction mechanism

In Figure 4-11, the data of feeding only ethanol with a variable space time and feeding an

ethanol water mixture are presented. The solid data points are attributed to the ethanol water

mixture at reference conditions (cf.4.4.3). The not filled data points represent feeding only

ethanol and variation of the space time, the dotted data points belong to feeding ethanol at

exactly the same conditions (same space time as in the case of the water ethanol feed). It can

be noticed that only activity (higher ethylene conversion) and not selectivity is significantly

altered through addition of water to the feed.

Figure 4-11: selectivity (mol%) in function of ethylene conversion. No filling data points: feed: ethanol,

Wcat/FEtOH=8.8-55 kgcat.s/molEtOH EtOH/(He)=1/10. Dotted data points: feed: ethanol, Wcat/FEtOH=16.6

kgcat.s/molEtOH EtOH/(He)=1/10 solid data points: feedstock:50wt% ethanol-50wt% water,

EtOH/(He+H2O)=1/10 Wcat/FEtOH=16.6 kgcat.s/molEtOH.

0

10

20

30

40

50

60

70

80

20 30 40 50 60 70 80 90

Sele

ctiv

ity

(mo

l%)


C3-C5 olefins C2-C5 paraffins C5+ BTXE


68

4.6 Conclusion

Ethanol is successfully converted to hydrocarbons over a non-modified HZSM5 catalyst . It

has been illustrated that for formation of C5+ hydrocarbons a reaction temperature higher than

320°C is necessary. Hence, the following experiments were performed at 350°C leading to a

significant liquid hydrocarbon fraction. At these conditions, reproducible results were

obtained and a no significant deactivation, for at least 10 hours’ time on stream, was present.

Then, the effect of temperature, space time and co feeding water has been studied. First

focusing on primarily the effect on product distribution and especially on the ethylene

conversion (activity). Increase in temperature, decrease in space time and addition of 50% of

water to the feed (at the same ethanol partial pressure) has led to an increase in activity.

Then A relationship is found between the product selectivities and the ethylene conversion,

obtained through altering the space time at 350°C. A more thoroughly look, at less lumped

product selectivities, provided more insight in the reaction mechanism especially in the

paraffin formation. No direct indications for the hydrocarbon pool model have been detected.

For a clarification of this, isotopic labeling experiments should be performed.

The potential effect of temperature and addition of water to the feed on the product

selectivities, which could indicate a change in the previously discussed essential elementary

steps, is examined. No large change in product selectivities through both variation of the

temperature or co feeding water have been observed. Hence, the same elementary steps will

be dominating at other temperatures (within the tested temperature range) and with co

feeding water. Further experiments (both with variable temperature and space time ) are

necessary to make any solid statement of the effect of temperature at low temperatures

(T<320°C) and at low (<20mol%) ethylene conversion.


69

4.7 References

1. Gayubo, A.G., et al., Kinetic Model for the Transformation of Bioethanol into Olefins

over a HZSM-5 Zeolite Treated with Alkali. Industrial & Engineering Chemistry

Research, 2010. 49(21): p. 10836-10844.

2. Phillips, C.B. and R. Datta, Production of ethylene from hydrous ethanol on H-ZSM-5

under mild conditions. Industrial & Engineering Chemistry Research, 1997. 36(11): p.

4466-4475.

3. Viswanadham, N., et al., Catalytic performance of nano crystalline H-ZSM-5 in

ethanol to gasoline (ETG) reaction. Fuel, 2012. 95(1): p. 298-304.

4. Dahl, I.M. and S. Kolboe, On the Reaction-Mechanism for Hydrocarbon Formation

from Methanol over Sapo-34 .1. Isotopic Labeling Studies of the Co-Reaction of

Ethene and Methanol. Journal of Catalysis, 1994. 149(2): p. 458-464.

5. Li, J.Z., et al., Comparative study of MTO conversion over SAPO-34, H-ZSM-5 and

H-ZSM-22: Correlating catalytic performance and reaction mechanism to zeolite

topology. Catalysis Today, 2011. 171(1): p. 221-228.

Chapter 5

Ethanol conversion over

modified HZSM5

5.1 Introduction

The potential benefits of modifying HZSM5 has been illustrated in the literature study.

Modification can be done without or with the insertion of additives and both during as after

synthesis. In are some important modification techniques are presented in Table 5- 1 . In this

thesis will the Si/Al ratio be varied and will metals be introduced through both incipient

wetness impregnation and ion exchange (bold in table). Multiple HZSM5 with different Si/Al

are tested to investigate the effect of acidity. The addition of metals is done for investigate

additional functionalities.

These three techniques are commonly applied in the modification of HZSM5 in the ethanol to

hydrocarbon conversion reaction (cf. chapter 2). Commercially H-ZSM5 (Zeolyte) zeolites

are used.

Table 5- 1:Classification with examples of modification techniques.

Modification techniques Examples

Synthesis Si/Al ratio

Isomorphous substitution

Post synthesis Dealumination

Incipient Wetness Impregnation (IMP)

Ion Exchange (EX)

Chemical Vapor Deposition (CVD)

Chapter 5: Ethanol conversion over modified HZSM5

72

The following approach will be systematically followed:

(i) Detailed characterization (NH3 TPD, H2 TPR, BET surface area and total pore

volume ): physical chemical properties

(ii) Catalytic testing: activity (ethylene conversion) and product selectivities.

(iii) Linking (i) and (ii)

5.2 Tuning the acidity by changing the Si/Al ratio

5.2.1 Physical and acid properties

For HZSM5 (Si/Al=30), HZSM5 (Si/Al=50) and HZSM5 (Si/Al=80), the corresponding BET

surface areas, total pore volume and concentration of strong acid sites (based on NH3 TPD

profiles, cf. chapter 3) are determined. The results are listed in Table 5- 2.

Table 5- 2: BET surface area, total pore volume and concentration of strong acid sites for HZSM5 catalysts

(Si/Al=30,50 and 80).

Si/Al

ratio

BET surface area

[103

m2

/kg]± 3.88%

Total pore volume

[10-6

m3

/kg]±7.22%

Concentration Strong acid sites

[mol NH3 /kg]±6.65%

30 336 0,096 0,23

50 363 0,098 0,21

80 385 0,116 0,14

It can be observed that an increase of the Si/Al ratio leads to an increase of the BET surface

and total pore volume. The same trend is observed by Shirazi et al.[1]. The same authors

performed scanning electron graphs of HZSM5 with a variety of Si/Al ratios. A reduction of

the crystal size was detected with increasing Si/Al ratio. Hence, the decrease of BET surface

area and total pore volume can be ascribed to the increase of the crystal size. The NH3 TPD

profiles for the different HZSM5 with a Si/Al ratio of 30,50 and 80 are depicted in Figure 5-1.

The maximum desorption temperature of the high temperature peak are added.


73

Figure 5-1: NH3 TPD profiles of HZSM5 (Si/Al=30,Si/Al=50,Si/Al=80) (β=5 °C/min).

Two effects can be observed with decreasing Si/Al ratio: (i) increase of the concentration of

high temperature acid sites and (ii) increase of maximum desorption temperature. The former

can be ascribed to a relatively increase of aluminum species in the framework, hence Brönsted

acid sites.

The latter which is observed by Lin et al. [2] is harder to explain. According to the next

nearest neigbors theorem (NNN theorem); isolated Brönsted sites possess a higher individual

strength that less isolated Brönsted acid sites it is expected that a decrease of the Si/Al ratio

leads to a lower desorption temperature. However another additional effect has to be

considered which is based on the evolution of Extra Framework Aluminum species (EFAL)

species in function of Si/Al ratio. According to Barthomeuf et. al.[3], EFAL species will

interact with framework Brönsted sites leading to an increased acid strength. According to

Guisnet et al. will EFAL species lead to increase of activity in the cracking of n heptane The

evolution of the concentration of EFAL species in function of the Si/Al ratio is studied by

Gonzalez et.al. [4] . Here, through Al-MAS MR spectroscopy it has been concluded that

alumina more present in EFAL species in HZSM5 with a Si/Al ratio of 30 then for a Si/Al

ratio of 80. Hence, decrease of the Si/Al ratio cause on increase of acid strength.

382°C

368°C

365°C


74

5.2.2 Catalytic testing

Catalytic testing over HZSM5 catalysts with a Si/Al ratio of respectively 30, 50 and 80 have

been performed at reference conditions. The conversion of ethanol was for all the catalysts

approximately 100% . In Figure 5-2, is the ethylene conversion shown for HZSM5

(Si/Al=30), HZSM5(Si/Al=50) and HZSM5 (Si/Al=80).

Figure 5-2: ethylene conversion for HZSM5 (Si/Al=30), HZSM5 (Si/Al=50) and HZSM5 (Si/Al=80). (reaction

conditions: T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, TOS=4-6hr.)

Increase of the Si/Al ratio leads to a decrease of the ethylene conversion according to the

trend in Table 5- 2.Similar trends are observed by Bandermann et al.[5] and others [2] as well.

In Figure 5-3 are the product selectivities in function of the ethylene conversion shown. The

solid data points represent at reference conditions HZSM5 Si/Al=30, 50 and 80. The not filled

data points are obtained with a variable space time for a HZSM5 Si/Al=30 catalyst.

0

10

20

30

40

50

60

Eth

yle

ne

co

nve

rsio

n (

mo

l%)

30 50 80


75

Figure 5-3: Selectivity (mol%) in function of ethylene conversion(mol%) (Solid data points :T=350°C, W/F= 16.6

kgcat.s/molEtOH HZSM5 Si/Al: 30, 50 and 80). No filled data points T=350°C W/F=8.8-55 kgcat.s/molEtOH, HZSM5

Si/Al=30).

The selectivity is not significantly affected by altering the Si/Al ratio. This can intuitively be

explained because altering the acid sites concentration and the space time has a similar effect;

the ratio of ethanol flow/number acid sites is altered.

5.2.3 Linking properties and testing

The ethylene conversion in function of the concentration of strong acid sites is shown In

Figure 5-4. Based on this figure, an increase of the concentration of strong acid sites will lead

to an increase of the ethylene conversion which is found by Lin et al.[2] as well (in the

ethylene to hydrocarbon reaction over HZSM5 at 450°C). According to FT-IR experiments

performed by the same authors the Brönsted acid sites were identified as active sites for

ethylene oligomerization.

The activity per active site (AAS) is plotted in function of the activity (ethylene conversion)

in figure Figure 5-4 B. A good correlation is found which means that the activity is not only

dependent of the concentration of strong acid sites. It can be observed that a decrease of the

Si/Al ratio leads to an higher ethylene yield and a higher AAS. In combination with Figure

5-1 where it was illustrated that increase of the Si/Al ratio decreases the acid strength, it can

be concluded that increase of the acid strength will promote the conversion of ethylene as

well.

.


76

Figure 5-4: A) Ethylene conversion in function of concentration strong acid sites [mol NH3/kgcat] B) Activity per active

center(=AAS) in function of ethylene conversion ( Reaction conditions : cf.5.2.2).

5.3 Metal introduction

Different metal modified HZSM5 (M HZSM5 with M=Fe, Ga or Ni) will be prepared through

(i) ion exchange (EX) and (ii) incipient wetness impregnation (IMP). The aimed and actual

metal loadings are listed in Table 5- 3 for all the made catalysts listed. (The metal loadings

analysis of the ion exchanged catalysts are not yet finished).

Table 5- 3: Aimed and actual (ICP) metal loading (%).

Aimed/Actual metal loading (%)

Ni HZSM5 Ga HZSM5 Fe HZSM5 Technique

1 - - - EX

1 0,71 0,79 0,57 IMP

2 1,42 1,58 1,14 IMP

5 3,18 3,87 2,65 IMP

10 5,79 8,11 5,94 IMP

A possible explanation for the deviation of the aimed metal loadings is the loss of metal

preparation during the procedure.


77

5.3.1 Physical and acid properties

In Table 5- 4 are the BET surface areas and total pore volumes of the different catalysts

reported.

Table 5- 4: BET surface area, total pore volume and concentration of strong acid sites for HZSM (Si/Al=30) and M-

ZSM5 catalysts.

Catalyst (aimed loading/technique)

BET surface area [10

3m

2/kg] 3.88% Total pore volume

[mol NH3 /kg] 7.22%

Fe Ga Ni Fe Ga Ni

no loading 336 0,096

1 EX - 325 328 - 0,14 0,105

1 IMP 327 320 302 0,092 0,095 0,092

2 IMP 325 306 295 0,101 0,11 0,09

5 IMP 305 304 274 0,096 0,082 0,081

10 IMP 277 271 252 0,092 0,078 0,073

According to the presented results in Table 5- 4 an increase in metal content will lead to a

decrease of the BET surface area and the total pore volume. An explanation will be further

formulated through combination with the H2-TPR profiles.

In the following paragraphs are for every metal type introduced the H2 TPR profiles and NH3

TPD profiles presented and discussed.

5.3.1.1 Fe-ZSM5

In Figure 5-5 are the H2 TPR and NH3 TPD profiles plotted for the Fe-ZSM5 catalysts with

different loadings.


78

100 300 500 700 900

H2

con

sum

pti

on

(a.

u)

Temperature (

C)

100 200 300 400 500

TCD

sig

nal

(a.

u)

Temperature (°C)

According to the presented H2 TPR profiles, for low iron loadings (EX, 0.27% and 1.14%)

no clear peaks can be distinguished. Hence, no significant amount of reducible species were

detected. Two possible explanations exist the sensitivity of the technique is too low or/and

iron is present as cation compensating cations partially replacing H+. According to Lobree et

al. [6] introduction of iron through ion exchange for low iron will content, lead to iron

present as compensation cations.

Increase of the iron content to 2.65%, leads to two additional peaks. The first peak (380°C),

which is clear visible for the (2.65 and 5.9% samples) , can be ascribed to the reduction of

Fe2O3 to Fe3O4. Peaks between 400-700°C represent further reduction of Fe3O4 to Fe0 with

different reduction difficulties.[7] A shift of the maximum temperature, is noticeable when

comparing the 2.65% and 5.9% iron loaded ZSM5 catalysts . The maximum of the first peak

shifts to a lower temperature. This might due to different migration effects caused by the

different metal loadings during calcination.[7] On the contrary, the second reduction peak

shifts towards higher temperature. Thus indicating a more difficult reduction which might

due to oligomerization of these metal species leading to increased crystal size and a lower

dispersive character as well. [8]

For an iron loading of 5.9% iron an additional high temperature peak (T=900°C) is found. A

similar trend of the Fe-ZSM5 of different loadings (EX modified method) is noticed in

literature as well.[6] A possible explanation is the agglomeration of iron particles[8] which

are due to their larger “stabilizing environment” harder to reduce.

Figure 5-5: a H2 TPR of Fe-ZSM5 b NH3 TPD of Fe-ZSM5.

5.9%

2.65%

1.14%

0.57%

EX

5.9%

2.65%

1.14%

0.57%

0%


79

The NH3-TPD profile of Fe-ZSM5 with different metal loadings is presented in Figure 5-5b .

After introduction of iron, a strong reduction of the second (high temperature peak) on the

NH3-TPD profile is noticeable even at introduction of a very small iron loading. No

significant effect on the position of the maximum desorption temperature is present. At a

loading of 5.9% Fe, an additional peak is observed. A possible explanation is formulated in

5.3.1.3.


80

100 300 500 700 900

H2

con

sum

pti

on

(a.

u)

Temperature (°C) 0 100 200 300 400 500 600

TCD

sig

nal

(a.

u)

Temperature (°C)

5.3.1.2 Ga-ZSM5

In Figure 5-6 a en b are the H2 TPR and NH3 TPD profiles plotted for the Ga-ZSM5 catalysts

with different loadings.

According to the H2 TPR profiles of Ga-ZSM5, at even at low gallium loadings, one clear

peak is visible which can be ascribed to the reduction of Ga2O3 to Ga2O [9] .

For the 3.87% and 8.11%, gallium loaded samples an additional peak arises.[10] A shift of

this peak with further increase of gallium content (3.87→8.11) is observed. This additional

peak and the change of the strength of this peak can be ascribed to the formation of less

dispersive oligomeric Ga2O3 species which are harder to reduce.[11]

A reduction of the concentration of strong acid sites after introduction of gallium becomes

clear based on the NH3 TPD profiles.

8.11%

3.87%

1.58%

0.79%

EX

8.11%

3.87%

1.58%

0.79%

EX

0%

Figure 5-6: a H2 TPR of Ga ZSM5 b NH3 TPD of Ga ZSM5.


81

100 200 300 400 500

TCD

sig

nal

(a.

u)

Temperature (°C)

5.3.1.3 Ni-ZSM5

In Figure 5-7 a en b are the H2 TPR and NH3 TPD profiles plotted for the Ni-ZSM5 catalysts

with different loadings.

Figure 5-7: H2 TPR of Ni-HZSM5 b NH3 TPD of Ni-HZSM5.

At low nickel loadings, no significant amount of reducible species are detected which is

similar as for Fe HZSM5 samples. The same possible explanation is therefore suggested [12]

(cf.5.3.1.1).

Increasing the nickel content to 1.42% nickel leads to a peak at 500°C. Further increase gives

rise to two additional peaks at 200-240°C and 340°C-370°C. The low temperature peak can

be ascribed to the reduction of NiO species, the two higher peaks are assumed to be small

nickel species and oligomeric nickel species.[12]

Based on the NH3 TPD profiles, a strong reduction of the concentration of strong acid sites is

noticeable, after introduction of a small nickel content (EX,0.71% and 1.42%). Further

increase of the nickel content leads to an additional peak. In appendix F, several MS spectra

of the corresponding NH3 TPD for nickel loadings 0.71% and 5.79% are added. For the

100 300 500 700 900

H2 c

on

sum

pti

on

(a.

u)

Temperature (°C)

5.79%

3.18%

1.42%

0.71%

EX

5.79%

3.18%

1.42%

0.71%

EX

0%


82

5.79% Ni sample a clear peak of water is observed which is not present in the 0.71% sample.

A possible explanation is that nickel has an effect of the stability of the catalyst and enhances

dehydroxylation.

For Fe HZSM5 at high loading a similar effect is probably valid. However further

confirmation is necessary .

5.3.1.4 Acid concentration in function of metal content

In Figure 5-8 is de concentration of acid sites plotted in function of the metal content. (The

experimental error is determined based on repetition experiments on HZSM5).

Figure 5-8: Concentration of high temperature acid sites[molNH3/kgcat] in function of metal content (%).

Increase of the metal content leads to a decrease of the concentration of acid sites. The same

trend is seemingly present for all the different types of metals. However, the height of the

error bars illustrates that, a relatively large error exists on the values for the concentration of

strong acid sites. Hence, possible subtle differences are not detectable. It is described in

literature it is observed for iron [8], nickel [13] and gallium [14] increase of the metal content

will lead to a reduction of the concentration of (strong) acid sites.

5.3.1.5 Conclusion

The addition of gallium, nickel and iron results in a decrease of the concentration of strong

acid sites, surface area and total pore volume.

0,1

0,15

0,2

0,25

0,3

0 1 2 3 4 5 6 7 8 9

Co

nce

ntr

atio

n a

cid

sit

es

[m

ol N

H3/k

g]

Metal content (%)

HZSM5 Ni HZSM5 Ga HZSM5 Fe HZSM5


83

No distinguishable peaks are present on the H2 TPR profiles For Ni HZSM5 and Fe HZSM5

are at low metal loadings (<1%). However for similar gallium loadings (prepared through

both EX as IMP) clear peaks are visible which indicates that nickel and iron are (partially

present) as compensating cations whereas gallium as an oxide, even at small metal contents is

present as oxides. The type of metal species present at higher loadings are primarily metal

oxides and agglomerates. These species will lead to pore blocking, reducing the specific

surface area and the concentration of available acid sites.

5.3.2 Catalytic performance

The different prepared catalysts are then all applied for the conversion of ethanol. In Figure

5-9 are respectively for Ga/HZSM-5 , Fe/HZSM5 and Ni/HZSM5 the yields towards C3-5

olefins, C1-5 paraffin’s, C5+ hydrocarbons and BTXE plotted for all the different loadings.

The total yield, as shown in this figure is not 100%. This is because the ethylene yield is not

directly incorporated into these figures. The addition of the ethylene yield to this figure would

lead to a total yield of 100%. Hence, the height of the accumulative presented yield is directly

related to the activity of the catalyst.


84

Figure 5-9: Yield (%) for Fe HZSM5 (0.57%,1.14%,2.65%,5.94%), Ga HZSM5(EX,0.79%,1.58%,3.87%,8.11%) and

Ni HZSM5 (EX,0.71%,1.42%,3.18%,5.79%) (T=350°C,W/F= 16.6 kgcat.s/molEtOH.)

The introduction of iron (0.57%), gallium (EX,0.79%) and Ni (EX,0.71,1.42%, 3.18%) led to

an increase of the activity compared to the not loaded HZSM5.The top three catalysts where

the highest ethylene conversion and yield towards the different product groups is achieved,

are presented in Table 5- 5.

Table 5- 5: Catalysts with highest ethylene conversion/ product yield.

“Best” performance ethylene conversion Ga(0.79%)>Ni EX> Ni(0.71%)

C1-C5 paraffin yield Ga EX>Ga(0.79%)>Ni EX

C2-C5 olefin yield Ga(0.79%) > Fe(0.57%) > NiEX

BTXE yield Ga EX> Ni(0.71%) > Ni EX

C5+ yield Ni(0.71%)> Ni EX > Fe(0.57%)


85

The highest paraffin and BTXE yield is obtained with Ga (EX) HZSM5. The highest ethylene

conversion and yield of gaseous olefin yield is reached with the Ga (0.79%) HZSM5 catalyst.

After impregnation of ZSM5 with 0.71% Nickel, the highest liquid hydrocarbon yield is

achieved.

Yield is a good quantity for choosing the “best” catalyst. However, it offers no direct

information about the enhancing/inhibiting effect of a metal to certain steps in the reaction

pathway. Hence, further analysis will be done based on combination of selectivity and

conversion of ethylene. In Figure 5-10 are the selectivity of the C3-C5 olefin fraction, C2-C5

paraffin fraction, C5+ fraction and BTXE fraction plotted versus the ethylene conversion for

both all the different M HZSM5 and for HZSM5 as well. The solid data point corresponds

with a metal modified HZSM5. Each data point belongs to a different metal loading. The ion

exchanged catalysts (Ni and Ga) are represented through arrayed data points of the color

corresponding to the metal type. The not filled data points correspond to HZSM5 (Si/Al=30)

catalyst obtained through a variable space time.

Figure 5-10: Selectivity (mol%) in function of ethylene conversion(mol%). (Solid data points=M (IMP) HZSM5

arrayed data point= M (EX) HZSM5 W/F= 16.6 kgcat.s/molEtOH not filled data points = HZSM5 W/F=5.5- 55

kgcat.s/molEtOH).

It can be observed in figure 5-10 that the introduction of metals will not lead to a large shift of

the selectivities of the different product lumps (C3-C5 olefins,C1-C5 paraffins,C5+

hydrocarbons and BTXE lump) compared to the non-modified HZSM5.


86

5.3.3 Linking properties and testing

In this paragraph the results obtained through characterization of the acidity and through

catalytic testing are combined. More specific, a relationship between the concentration of

strong acid sites and corresponding activity (ethylene conversion) is found. In Figure 5-11 is

the ethylene conversion plotted in function of concentration of strong acid sites. Every point

indicates a different metal loading at the same reference reaction conditions. Ni-ZSM5 is not

incorporated in the data, due to the inaccurate NH3 TPD profiles (cf.5.3.1.3). As mentioned

before, a relatively large error is present on the concentration of strong acid sites.

Nevertheless, some trends remain clear and the following conclusions are still valid:

(i) Metal introduction leads to a strong decrease of the concentration of strong acid

sites despite the higher ethylene conversion for the (<1%) samples compared to the

non-modified HZSM5 (Si/Al=30)

(ii) Increase in metal content leads to a further decrease of the concentration of strong

acid sites and the ethylene conversion

(iii) The slope of the metal modified HZSM5 is more steep compared to the not metal

modified HZSM5.

Figure 5-11: Ethylene conversion (mol%) versus concentration strong acid sites[mol NH3/kg]. Reference reaction

conditions.

is the activity per active center is expressed as the ethylene conversion normalized by the

active sites plotted in function of the ethylene conversion In Figure 5-12. This figure

illustrates more clearly the promoting effect of gallium and nickel.


87

For a specific conversion of ethylene a higher activity per active center is noticed comparing

M HZSM5 with HZSM5. An activity increase is noticed as well by Calsavara et al.[15]

however no explanation is suggested.

It is suggested that gallium species could have a Lewis acid site character which has a

synergetic effect in combination with the Brönsted acid sites, explaining the higher

activity.[14]

Increase in comprehension of the effect of the metals is only possible through application of

other characterization techniques which can give more information about the type of acid sites

(FT-IR [16], Al MNR[17],…) and the effect of the introduction of metals to the lattice

parameters/crystallinity (XRD [1]).

Figure 5-12: Activity per active center(=AAS) (ethylene conversion (mol%)/concentration of acid sites(molNH3/kgcat in

function of activity (ethylene conversion(mol%)) Reaction conditions= standard reference reaction conditions

(cf.5.2.2)

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70

Act

ivit

y p

er a

ctiv

e si

te

[m

ol%

eth

.kg c

at/m

ol N

H3]


HZSM5 Fe HZSM5 Ga HZSM5


88

5.4 Conclusion

HZSM5 (Si/Al=30) is successfully modified through altering the Si/Al and introduction of

metals by incipient wetness impregnation and ion exchange.

It is observed that increasing the Si/Al ratio led to a lower concentration of acid sites with a

lower average strength. A good correlation between both, the concentration and the strength

of the acid sites and the activity (ethylene conversion) is found.

Iron, gallium and nickel are introduced through both ion exchange and incipient wetness

impregnation on HZSM5 (Si/Al=30).

A strong reduction of the concentration of acid sites was noticeable. Increase in metal content

led to a systematic decrease of the BET surface area and the total pore volume as well. In

combination with the corresponding H2 TPR profiles, it could be generally concluded that

increasing the metal content (>1%) led to metal species primarily present as metal oxides and

agglomerates responsible for pore blocking and thus decreasing the concentration of

(ammonia available) acid sites. However, after introduction of a small metal content (<1%)

for Ni-HZSM5 and Fe-HZSM5 are no significant amount of reducible species present

detected (based to the corresponding H2 TPR profiles). Possible indicating that nickel and

iron are primarily present at these low concentration as compensations cations whereas for

Ga HZSM5 where at even low concentration a clear peak is observed, is present as oxide.

All the different metal modified HZSM5 catalyst were tested in the ethanol conversion

reaction. An increase of activity (ethylene conversion) was noticeable for all the modified

zeolites with a metal loading smaller than 1%. Further increase of metal content reduced the

activity. No significant changes in product selectivities were detected for all the metal loaded

catalysts.

A correlation is found between the concentration of strong acid sites and the activity for the

metal modified zeolites as well. Ga HZSM5 and Fe HZSM5 showed a higher activity per acid

site.


89

Further insight in the exact effect of the metal is only possible through further characterization

by methods which can better identify the type and location of the acid sites (FT-IR, Al

NMR,…).


90

5.5 References

1. Shirazi, L., E. Jamshidi, and M.R. Ghasemi, The effect of Si/Al ratio of ZSM-5 zeolite

on its morphology, acidity and crystal size. Crystal Research and Technology, 2008.

43(12): p. 1300-1306.

2. Lin, B.M., Q.H. Zhang, and Y. Wang, Catalytic Conversion of Ethylene to Propylene

and Butenes over H-ZSM-5. Industrial & Engineering Chemistry Research, 2009.

48(24): p. 10788-10795.

3. Barthomeuf, D., Zeolite Acidity Dependence on Structure and Chemical Environment

- Correlations with Catalysis. Materials Chemistry and Physics, 1987. 17(1-2): p. 49-

71.

4. Simon, U., et al., The acid properties of H-ZSM-5 as studied by NH3-TPD and Al-27-

MAS-NMR spectroscopy. Applied Catalysis a-General, 2007. 328(2): p. 174-182.

5. Schulz, J. and F. Bandermann, Conversion of Ethanol over Zeolite H-Zsm-5. Chemical

Engineering & Technology, 1994. 17(3): p. 179-186.

6. Lobree, L.J., et al., Investigations of the state of Fe in H-ZSM-5. Journal of Catalysis,

1999. 186(2): p. 242-253.

7. Machado, N.R.C.F., et al., Obtaining hydrocarbons from ethanol over iron-modified

ZSM-5 zeolites. Fuel, 2005. 84(16): p. 2064-2070.

8. Lu, J.Y., Y.C. Liu, and N. Li, Fe-modified HZSM-5 catalysts for ethanol conversion

into light olefins. Journal of Natural Gas Chemistry, 2011. 20(4): p. 423-427.

9. Meitzner, G.D., et al., The Chemical-State of Gallium in Working Alkane

Dehydrocyclodimerization Catalysts - Insitu Gallium K-Edge X-Ray Absorption-

Spectroscopy. Journal of Catalysis, 1993. 140(1): p. 209-225.

10. Gheno, S.M. and E.A. Urquieta-Gonzalez, Conversion of n-butane to iso-butene on

gallium/HZSM-5 catalysts. Brazilian Journal of Chemical Engineering, 2002. 19(3): p.

335-342.

11. Machado, F.J., et al., The transformation of n-butane over Ga/SAPO-11 - The role of

extra-framework gallium species. Applied Catalysis a-General, 2002. 226(1-2): p.

241-252.

12. Pawelec, B., et al., Simultaneous 1-pentene hydroisomerisation and thiophene

hydrodesulphurisation over sulphided Ni/FAU and Ni/ZSM-5 catalysts. Applied

Catalysis a-General, 2004. 262(2): p. 155-166.

13. Pereira, M.M., et al., Ni-ZSM-5 catalysts: Detailed characterization of metal sites for

proper catalyst design. Journal of Catalysis, 2010. 269(1): p. 103-109.


91

14. Rane, N., et al., Cracking of n-heptane over Bronsted acid sites and Lewis acid Ga

sites in ZSM-5 zeolite. Microporous and Mesoporous Materials, 2008. 110(2-3): p.

279-291.

15. Calsavara, V., M.L. Baesso, and N.R.C. Fernandes-Machado, Transformation of

ethanol into hydrocarbons on ZSM-5 zeolites modified with iron in different ways.

Fuel, 2008. 87(8-9): p. 1628-1636.

16. Seddigi, Z.S., Acidic properties of HZSM-5 using acetonylacetone, TPD of ammonia,

and FTIR of adsorbed pyridine. Arabian Journal for Science and Engineering, 2002.

27(2A): p. 149-156.

17. Faro, A.C., V.D. Rodrigues, and J.G. Eon, Correlations between Dispersion, Acidity,

Reducibility, and Propane Aromatization Activity of Gallium Species Supported on

HZSM5 Zeolites. Journal of Physical Chemistry C, 2010. 114(10): p. 4557-4567.

Chapter 6

Conclusions and future work

In this work, the conversion of ethanol to hydrocarbons over non modified and modified

HZSM5 catalysts is studied which is systematically done through linking physical-chemical

properties with the corresponding performance in the ethanol conversion reaction.

The experimental data were found to be reproducible and present in the intrinsic kinetic

reaction regime. No deactivation has been observed. The effect of temperature, space time

and water have been investigated. Increase in temperature, decrease in space time and

addition of water to the feed increased the conversion of ethylene. ;A reaction mechanism, in

line with previous gathered experimental data, is suggested based on acid elementary steps.

No significant effect of temperature (330-400°C) and addition of water on the significance of

the proposed elementary steps was noticed. Further testing at lower temperature is necessary

to make solid statements at lower temperature.

Commercially available HZSM5 catalysts were applied with different Si/Al ratios. An

increase of the Si/Al ratio led to a lower concentration of acid sites with a lower average

strength based on the corresponding NH3 TPD profiles. A relationship between concentration

and the strength of the strong acid sites and the ethylene conversion is found.

Different iron, gallium and nickel loadings are introduced through incipient wetness

impregnation and ion exchange. For all the different metals ( and different loadings) a strong

reduction of the acid concentration is noticed which can be ascribed to metals present as

primarily oxides and clusters blocking the pores.

All the metal modified HZSM5 catalyst were applied for the ethanol conversion. Introduction

of a small metal content (<1%) led to an increase of the activity of the catalyst whereas for

higher metal loadings a decrease is observed. The product selectivities were not strongly

affected by introduction of metals.

94

Other modification techniques like chemical vapor deposition or atomic layer deposition may

lead to other results.

A correlation between the concentration of strong acid sites and the ethylene conversion is

detected for the metal modified HZSM5 catalysts. The activity per acid site was found higher

after metal modifications zeolites compared to not metal loaded HZSM5 catalysts. Hence a

synergetic effect is present between metal species and acid sites.

Further identification of this promoting effect is only possible through characterization

techniques which are able to distinguish between the type of acid sites, location of acid sites

and effect of modification on crystallinity and lattice parameters (pyridine adsorption FT IR,

Al MAS NMR,XRD,…).

Appendix A:

Progressing GC data

In this appendix, an example of a GC chromatogram is shown. These retention times apply to

the temperature program of the GC as described in Chapter 3. The identification of the peaks

was done based on several injections of hydrocarbon mixtures and data from literature.

Figure C-1: GC profile, signal (a.u) in function of retention time (min).

In Table C-1 is the complete list of retention times added. Due to the complexity of the

chromatogram with increasing carbon number, the identification of the individual peaks

C2

C3

C4

C5 C5+

Appendix B: Progressing GC data

92

becomes more difficult. In some cases, a time range is given rather than an single retention

time indicating the location of a certain component group. This is done for C6,C7,C8,C9 and

C9+ . The used calibration factors are used as well.

Table C-1: Expected retention times.

Compound Retention time (min) Calibration factors

methane 3,4 0,976

ethene 3,6 0,865

ethane 3,83 0,9

propene 5,57 0,873

propane 5,8 0,88

isobutane 8,8 0,93

isobutene 9,99 0,818

1-butene 10,02 0,954

cyclopentane 10,36 1,15

n-butane 10,47 0,916

trans-2-butene 11,1 0,903

cis-2-butene 11,85 0,871

3methyl-1-butene 13,54 1,218

isopentane 14,44 1,2

ethanol 14,61 1,5625

1-penteen 15,24 1,187

2-methyl-1-buteen 15,61 1,205

n-pentane 15,81 1,13

trans-2-pentene 16,24 1,13

cis-2-pentene 16,61 1,13

2-methyl-2-butene 16,84 1,146

cyclopentene 18,33 1,079

benzene 22,95 1

toluene 27,52 1,2

ethylbenzene 31,15 1,2

p/m xylene 31,48 1,2

C6 group 18.5-23.1 1,2

C7 group 23.1-29.2 1,2

C8 group 29.2-32.8 1,2

C9 group 32.8-36.8 1,15

C9+ group >36.8 1,15

Appendix B:

Data used for estimated

standard deviation calculation

Error bars on ethylene conversion

Table D- 1: Conversion/Selectivities (T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar,

TOS=5h catalyst: HZSM5(Si/Al=30)).

Conversion (mol%)

Ethanol 100,0 99,9 100,0

Ethylene 51,8 49,1 52,9

Selectivity (mol%)

C1-C5 paraffins 15,8 16,6 16,0

C3-C5 olefins 67,0 68,7 65,8

C5+ hydrocarbons 13,2 10,7 14,4

BTXE 3,8 3,8 3,6

Appendix A: Chromatogram

90

Error bars on concentration strong acid sites

Table D- 2: Concentration of strong acid sites calculation through manual integration of the

corresponding NH3 TPD profiles of HZSM (Si/Al=30)

Filename Concentration of strong acid sites

[mol NH3 /kg]

HZ30A14 0,232

30/mei/11 0,214

Si/Al30 0,22

Table D-3: Data used for calculation Error bars on BET surface are and total pore volume.

Filename BET surface area

[103m

2/kg]±

Total pore volume

[10-6

m3/kg

HZ30J7

HZSM-30

332

340

0,0939

0,09785

Appendix C:

Detailed product distributions

In table H-1 is the detailed product distribution listed corresponding to figure 4-2.

Table H 1:Detailed product distribution. Product distribution reaction conditions: T=350°C,Wcat/FEtOH=

16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar catalyst: HZSM5 (Si/Al=30)

product yield(%)

ethanol 0,10

Methane 0,01

Ethene 50,92

Ethane 0,59

Propene 10,15

Propane 0,61

butane 3,68

Isobutene 5,59

1-buteen 1,58

n-butane 0,33

Trans-2-butene 2,80

Cis-2-butene 1,82

3-methyl-1-butene 0,20

Isopentane 2,09

1 pentene 0,23


n-pentane 0,17

pentene(cis/trans) 1,36


Cyclopentene 0,09

benzene 0,55

C6 paraffines/olefins 1,19

toluene 0,68

C7 paraffins/olefins 3,37

C8 aromatics (EB,Xylene) 1,72

C8 paraffins/olefins 1,19

Appendix A: Chromatogram

90

C8+ 3,61

In Table H 2 are the detailed selectivities (mol%) plotted corresponding to figure 4-3.

Table H 2: detailed selectivities (mol%) in function of time on stream. reaction conditions:

T=350°C,Wcat/FEtOH= 16.6 kgcat.s/molEtOH, EtOH/He=1/10,Ptot=1bar, catalyst: HZSM5(Si/Al=30).

TOS (hr)

product 5 7 9 11 5 2

methane 0,03 0,04 0,04 0,03 0,04 0,03

ethane 1,20 1,28 1,09 1,37 2,43 0,70

propene 28,53 29,36 29,87 29,23 29,85 29,90

propane 1,76 1,73 1,70 1,66 1,71 2,68

C4 olefins 25,43 26,37 26,45 26,29 25,99 26,11

C4 paraffins 8,78 8,79 8,72 8,46 8,54 8,66

C5 olefins 13,05 13,45 13,42 13,57 12,83 12,91

C5 paraffins 3,98 4,00 3,94 4,01 3,87 3,88

BTXE 3,80 3,85 3,87 3,70 3,79 4,31

C6 2,29 1,84 1,81 1,92 1,70 1,74

C7 5,49 3,91 3,83 4,12 4,24 4,37

C8 1,76 1,42 1,39 1,47 1,31 1,34

C8+ 3,68 3,74 3,61 3,81 3,44 3,12

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