Greeen Chemistry

8/14/2019 Greeen Chemistry

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ZEOLITECATALYSTS IN

GREEN CHEMISTRYDipak Kumar Chakrabarty

Professor EmeritusINDIAN INSTUTUTE OF TECHNOLOGY,

BOMBAY

MUMBAI 400076


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In the year 1987, the World Environment Commission publisheda report Our Environmentthat emphasized the need for sustainable

development. Sustainable development means development to fulfill

the needs of the present generation without endangering the needs

of the future generation. The concept includes many aspects of which

utilization of renewable resources and protection of the environment isprimary.

The traditional concept of process efficiency that considered product yield

as the main criteria is being replaced by including such considerations

as elimination of waste and protection of environment.

A positive development of the twenty first century is that we have woken up to

the danger to our future - the danger to our environment created my mindless

industrial expansion of the last century.

This has brought the concept of GREEN CHEMISTRYproducing chemicals

With minimumdamage to the environment .


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Green Chemistry

Chemical Industry has its share in polluting the environment and today

it has realized that the time has come to make the industry more and more

efficient not only in terms of profit and yield, but more accountable in terms

of pollution abatement and eco-friendliness.

This has led to the term GREEN CHEMISTRY.

Traditionally, organic synthesis is highly logical, but highly inefficient. This

situation was allowed to continue mainly because

1. The scale of production was not very large,

2. Organic chemists were not much concerned with catalysis.

3. Short life cycle of the products and the processes.

Situation has changed since then and more and more organic chemists

are taking the catalytic route. Our concern here will be zeolite catalysts.


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Briefly, Green Chemistry is

Efficient use of (preferablyrenewable) raw materials,

Elimination of wasteful byproducts,

Avoiding use of toxic/hazardousreagents and solvents,

Use of safer final (biodegradable)products, and

Increasing energy efficiency.


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Two considerations that dominated

green chemistry are:

A.Maximum atom utilization,

B.The minimum waste produced (E Factor).

The waste includes byproducts, reagents,

solvent loss and even fuel.


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In simple form, it defined as:

(Chemicals in (kg) - Desired product (kg) ) / Total product (kg)

The enormous waste in different segments of industry are

shown in the table below.

Industry

segment

Product,

tonn/anum

kg waste/ kg

productOil

refining106-108


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Another important parameter is the extent of harmfulness

of the waste. For example, sodium sulphate as a waste is

certainly far less harmful than a cyanide waste. A new term

environment quotient (Q)has been coined to emphasize

this difference and some number has been arbitrarily

assigned to different wastes according to their extent of

their harmful effect.

Q FACTOR

R. A. Sheldon, Chemtech., 38, (1994).


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Some examples of atom utilization

3PhCH(OH)CH3 + 2CrO3 + 3H2SO4 3PhCOCH3 + Cr2(SO4)3 + 6H2O

atom efficiency = 360/860 or 42%

catalyst

3PhCH(OH)CH3 + O2 PhCOCH3 + H2O

atom efficiency = 120/138 or 87%

C2H2 + 1/2O2 C2H2Oatom efficiency = 100%

The idea was first introduced by Trost. See B. M. Trost, Science, 254, 147 (19910.


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Zeolite Structure

Zeolites are crsytalline alumino-silicates withexchangeable cations

The cations can be exchanged with protons thatmake them acidic solids.

Si:Al ratio can vary in zeolites from 1 to infinity. Number of acid sites decreases with increase in

the Si:Al ratio

Strength of the acid sites increases with the Si:Al

ratio. There are also Lewis acid sites (tri-coordinated

Al).

They have micro-pores of entry port size varying

from about 3 to 12 A.


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Zeolite Pore Structure

Zeolites are formed by oxygen-sharing T (Si orAl) atoms, each T atom linked to four oxygenmaking tetrahedra.

These tetrahedra are linked to form ringscontaining equal number of oxygen and silicon

atoms. The rings share common oxygen (or silicon)

forming different polyhedra. The polyhedra then join by sharing common

oxygen (or silicon) forming the three dimensional

network structure giving rise to pores. The pores may be all in one direction or may run

in several directions.


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Each corner is a Si atom and oxygen atoms are at the middle of each edge.

a: sodalite polyhedron; b: zeolite sodalite; c: zeolite A; d: faujasite


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Five-member silicon-oxygen ring. Oxygen sharing between rings

formation chain.


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a: Chains joining to form 10-Oxygen pores in ZSM-5; b: three-dimensional

pore structure of ZSM-5 showing zigzag structure


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Pore openings in a: Ferrierite and b: ZSM-23


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8-Oxygen, 10-Oxygen and 12-Oxygen rings found in erionite, ZSM-5

and faujasite.


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Table 2. Pore size of zeolites.

Pore typeNo. of O

atoms in the

ring

Framework

structurePore size, A Dimensionalit

y

Small 8 ZEOLITE A 4.1 3

medium 10 ZSM-5ZSM-11

SAPO-11

1.x 5.4

5.4 x 5.3

6.3 x 3.9

3

3

1

Large 12 X, YMORDENITE

BETA

SAPO-5

L

7.4

7.0 x 6.5

7.5 x 5.7

7.3

7.1

3

2

3

1

1

Extralarge 1418

20

SAPO-8

VPI-5

CLOVERITE

8.7 x 7.9

12.1

13.2 x 6.0

1

3

3


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A major application of the zeolites in catalysis

is in acid catalyzed reactions such as alkylation,

acylation, electrophilic aromatic substitution,

cyclization, isomerization and condensation. Weshall take some examples here.


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+ C2H2HZSM-5

Catalysis with acidic zeolites1. ALKYLATION

Mobil-Badger Process

(Polyalkylation is suppressed)

Similarly, propylbenzene could be manufactured

using a 3-dimensional dealuminated mordenite

(3-DDM) catalyst

+

dealuminated

mordenite

Si/Al = 100-1000

Dealumination enabled to obtain veryhigh Si:Al ratio (up to 1000). In these form,

the micropores of mordenite were connected

through mesopores (5-10 nm).

K.Tanabe and W.F. Holderich, Appl.Catal.A General, 181,399 (1999).


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Naphthalene is dialkylated with propene over 3-DDM.

The product is used in making the carboxylic acid that

is an important monomer for making plastics.

+

AcylationAcylation with heterogeneous catalysis ismuch more

difficult because of the polarity difference of the substrate

and the acylating reagent that makes it difficult to achieve

Favourable adsorption ratio of the two. This could be

achieved by the use of H-Beta.

+ (CH3CO)2O

O

G.R. Meima, G.S. Lee and J.M. Garces, in Fine Chemicals Through heterogeneousCatalysis (Ed. R.A. Sheldon, Wile-VCH, Weinheim, 2001.

A. Vogt and A. Pfenninger, EP0701987A1, 1996 to Uetikon AG.


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.

Hydroxyalkylaton using zeolites is difficult because

of unfavourable adsorption ratio of the reagent and the

substrate. This difficulty is avoided by having the aromatic

and the epoxide functions in the same molecule

Ce3+exchanged Y zeolite could catalyze toluene and

xylenes using with higher carboxylic acids showing that

free carboxylic acids can be used in acylation.

+ RCO2H

O

R

O

H-ZSM-5

or H-Beta

J.A. Elings, R.S. Downing and R.A. Sheldon, Stud.Surf.Sci.Catal, 105, 1125 (1997).

B. Chiche, A. Finiels, C. Gauthier, P. Geneste, J. Graille and D. Pioch,J. Org. Chem., 51, 2128 (1986).


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Formation of N-heterocycles by intermolecular cyclization

is catalyzed by acidic zeolites. Synthesis of pyridine and

picoline from a mixture of acetaldehyde, formaldehyde and

ammonia in presence of H-ZSM-5 is an example.

N N+

NH2H2N N

H-ZSM-5 Pd

NH

CH 3CHO + HCHO + NH 3

NOH

sulfuric acid

H-ZSM-5

Si : Al > 1000, 350oCv ap. phase

O

NH

produces 2 kg 0f ammon. sulfateper kg of product

conventional

Sumitomo

Beckmann rearrangement

W. F. Holderich et al. in Fine Chemicals through Catalysis, pp.217 -231, Wiley-VCH, (2001)

M.J.Burk et. Al., J. Org. Chem., 64, 3290 (1999)


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O

TS-1 or

ZSM-5 O

H TS-1 ZSM-5

conv. 100% 99%select. 98% 97-100%

Epoxide reaarangement ( key step in the manufacture of many intermediates ithe fine chemical industry. Traditional method used stoichiometric Lewis acidsor bases.

G. P. Heitmann, G. Dahlhoff and W.F.Holderich, J.Catal, 186,, 12 (1999)

Traditional method for preparing 2,6-dichlorobenzonitrile uses stoichiometricAmonts of chlorine, HCN and POCl3 with atom efficiency 31%. The new process

Was developed uses zeollite catalysts Eur.Pat.Appl. EP948988 (1999)

+ Cl 2Ag-H-M or

350 o

Cl Cl

Cl

Cl

Cl

Cl Cl Cl

Cl ClCl

CN

adsorption in

faujasite

NH 3+ O 2

catalyst

350-450 oC

C

K. Iwayama, S. Yamakawa, M. Kato and H. Okino, Eur. Pat. Appl. EP948988 (1999) to Toray


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Bayer-Villiger Oxidation (cinversion of ketone or an aldehyde to the esterThe peracid undergoes a nucleophylic attack on the carbonyl group givimg an inetrmediate.

In the next step, a concerted migration of one of the alkyl groups takes place releasing the

Carboxylate anion. The reaction is widely used in organic chemistry.

Zeolite beta containing 1.4 wt% of tin is a good catalyst using hydrogen peroxide.

O

O O

1 eq. H2O2

0.66 mol% Sn-Beta

dioxane, 90 oC

A lrge number of bulk chemicals are produced by using molecular oxygen either in the liquidor in the vapour phase reaction. S0me of these are:Benzene/ethene to styrene, p-xylene to terephthalic acid, formaldehyde to methanol,Ethene ot ethene oxide, n-butane to acetic acid, propene to acrylonitrile, n-butane toMalic anhydride, o-xylene to phthalic anhydride, isobutene to methyl methacrylate etc.

Molecular oxuygen is a spin triplet and its direct reaction to a organic singlet compoundis spin forbidden. To overcome this, the triplet is allowed to react with paramegneticMetal ions forming a superoxo-metal complex that forms a variety of metal-oxygen species.Use of catalytic route to selective oxidation in presence of several functional groups isA big challenge.


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Basic zeolitesComarattively much less attention has been paid to basic zeolites. Zeolites can be made basic by

1.Exchange of protons with alkali or rare earth ions or 2. by depositing nano-particles of alkali or

alkali earth oxides in the pores. The basic sites are weak. They can be used to generate C-C bond

in the side chains of substituted aromatics.

CHO

+ NCCH2CO2Et

COOE

CN

Na-X

NH2

+ (MeO)2CO

NHCH3

+ MeOH + CO2

CN+ MeOH

CN

+ H2O

K-Y

Cs-X

K.R. Kloestra, H. van Bekkum, Chem.Soc.Chem.Commun., 1005 (1995).

Chemicals through Hetero. Catal.,Cormma and S. Iborra in Fine309 (2001).

SIDE CHAIN ALKYLATIONS

Greeen Chemistry

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