Homogeneous and heterogeneous rhenium catalysts for sustainable transformations of polyols

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Homogeneous and heterogeneous rheniumcatalysts for sustainable transformations of polyols,alcohols, and aminesJing YiPurdue University

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Thesis/Dissertation Acceptance

To the best of my knowledge and as understood by the student in the Thesis/Dissertation Agreement, Publication Delay, and Certification/Disclaimer (Graduate School Form 32), this thesis/dissertation adheres to the provisions of Purdue University’s “Policy on Integrity in Research” and the use of copyrighted material.

Jing Yi

HOMOGENEOUS AND HETEROGENEOUS RHENIUM CATALYSTS FORSUSTAINABLE TRANSFORMATIONS OF POLYOLS, ALCOHOLS, AND AMINES

Doctor of Philosophy

Mahdi Abu-Omar

Ei-ichi Negishi

Mark A. Lipton

Mahdi Abu-Omar

Lyudmila Slipchenko

R. E. Wild 04/02/2015

i

HOMOGENEOUS AND HETEROGENEOUS RHENIUM CATALYSTS FOR

SUSTAINABLE TRANSFORMATIONS OF POLYOLS, ALCOHOLS, AND AMINES

A Dissertation

Submitted to the Faculty

of

Purdue University

by

Jing Yi

In Partial Fulfillment of the

Requirements for the Degree

of

Doctor of Philosophy

May 2015

Purdue University

West Lafayette, Indiana

ii

For My Family

iii

ACKNOWLEDGEMENTS

First and foremost I wish to thank my advisor, Dr. Mahdi Abu-Omar. Mahdi is more

than a professor; he is a teacher, a life mentor, an inspiring researcher, and a warm friend.

I have cherished the opportunity to work in his group, and enjoyed working under his

enthusiastic and creative supervision. This thesis would not have been possible without

the guidance, encouragement, inspiration, and support from him. I hope that I could be as

energetic as Mahdi, and to someday activate, encourage, and inspire people.

Many thanks go to my committee members, Dr. Mark Lipton, Dr. Lyudmila

Slipchenko, and Dr. Ei-ichi Negishi for their valuable discussions. Special thanks also go

to Dr. Jeffrey Miller, Dr. Fabio Ribeiro, Dr. Kothanda Rama Pichaandi, and Dr. Bruce

Cooper, for their insightful discussions and suggestions. Thanks to all the Abu-Omar

group members, especially Dr. Shuo Liu, Dr. Benjamin Wegenhart, Thilina Gunasekara,

Dr. Keith Steelman, Yuan Jiang, Dr. Basudeb Saha, Paul Pletcher, Ian Klein, and Dr.

Trenton Parsell, as well as other groups’ research fellows Ruihong Zhang, SungHwan

Hwang, Xin Yan, Dr. Shiqing Xu, Yu Bai, Dr. Paul Dietrich, Dr. Dmitry Zemlyanov, and

John Di Iorio.

I would like to express special gratitude to my previous advisors, Dr. Wenfang Sun,

Dr. Zixing Shan, Dr. Jiong Chen, Dr. Liangmin Yu and my committee Dr. Sanku Mallik,

as well as my undergraduate organic chemistry teacher Huanzhi Xu, for starting my

iv

organic discovery and adventure. I also wish to thank previous colleagues Dr. Bingguang

Zhang, Dr. Jing Zhang, Dr. Yunjing Li, and Dr. Zhongjing Li.

Finally, but most importantly, I would like to thank my whole family and my best

friends. I love you all dearly.

v

TABLE OF CONTENTS

Page

LIST OF TABLES ........................................................................................................... viii

LIST OF FIGURES ........................................................................................................... ix

LIST OF SCHEMES......................................................................................................... xii

ABSTRACT ..................................................................................................................... xiv

CHAPTER 1. CURRENT SUSTAINABLE CHEMICAL TRANSFORMATIONS FOR POLYOLS, ALCOHOLS, AND AMINES ................................................................1

1.1 Deoxydehydration of Biomass-Derived Polyols ....................................................... 1 1.1.1 Rhenium-Catalyzed Deoxydehydration of Diols and Polyols ......................... 2 1.1.2 Heterogeneous Catalysis of Deoxydehydration ............................................... 6 1.1.3 Other Metal Catalyzed Deoxydehydration ...................................................... 7

1.2 Acceptorless Dehydrogenation of Alcohols and Amines.......................................... 7 1.2.1 Homogeneous Acceptorless Dehydrogenation of Alcohols and Amines ........ 8 1.2.2 Heterogeneous Acceptorless Dehydrogenation of Alcohols and Amines ..... 10

1.3 Current Hydrogen Storage in Liquid Organic Heterocycles ................................... 10 1.4 Chapter Overview ................................................................................................... 11 1.5 References ............................................................................................................... 13

CHAPTER 2. RHENIUM-CATALYZED TRANSFER HYDROGENATION AND DEOXYGENATION OF BIOMASS-DERIVED POLYOLS TO SMALL AND USEFUL ORGANICS .............................................................................................17

2.1 Introduction ............................................................................................................. 17 2.2 Experimental ........................................................................................................... 18

2.2.1 Chemical Preparations ................................................................................... 18 2.2.2 Instrumentation .............................................................................................. 19

2.3 Results and Discussion ............................................................................................ 20 2.3.1 Catalysts Usage .............................................................................................. 20 2.3.2 Additives Usage ............................................................................................. 24 2.3.3 Substrates Scope ............................................................................................ 27 2.3.4 Kinetics and Intermediates Study .................................................................. 28

vi

Page

2.3.5 Isotopic Labeling Experiment ........................................................................ 32 2.3.6 Mechanism ..................................................................................................... 33

2.4 Conclusion ............................................................................................................... 35 2.5 References ............................................................................................................... 37

CHAPTER 3. REUSABLE UNSUPPORTED RHENIUM NANOCRYSTALLINE CATALYST FOR ACCEPTORLESS DEHYDROGENATION OF ALCOHOLS ........ 38



3.3 Results and Discussion ............................................................................................ 41 3.3.1 Catalyst Characterization ............................................................................... 41

3.3.1.1 Transmission Electron Microscopy (TEM) .............................................. 41 3.3.1.2 X-Ray Photoelectron Spectroscopy (XPS) ............................................... 43 3.3.1.3 X-Ray Absorption Spectroscopy (XAS) .................................................. 46 3.3.1.4 X-Ray Diffraction (XRD) ......................................................................... 48 3.3.1.5 Fourier Transform Infrared Spectroscopy (FT-IR)................................... 48

3.3.2 Catalysts Usage .............................................................................................. 50 3.3.3 Substrates Scope ............................................................................................ 51 3.3.4 Kinetic Study ................................................................................................. 53 3.3.5 Temperature Effect ........................................................................................ 55 3.3.6 Catalyst Recycling ......................................................................................... 55 3.3.7 Hydrogen Characterization and Quantification ............................................. 57 3.3.8 Kinetic Isotopic Effect ................................................................................... 58 3.3.9 Mechanism ..................................................................................................... 58

3.4 Conclusion ............................................................................................................... 61 3.5 References ............................................................................................................... 62 CHAPTER 4.RHENIUM-CATALYZED ACCEPTORLESS- DEHYDROGENATIVE AMINE-COUPLING REACTION FOR THE SYNTHESIS OF IMINES ........................................................................................................................63



4.3 Results and Discussion ............................................................................................ 66 4.3.1 Reaction Optimization ................................................................................... 66 4.3.2 Catalyst Recycling ......................................................................................... 69 4.3.3 Substrate Scope .............................................................................................. 70 4.3.4 Kinetic Isotope Effect .................................................................................... 86

vii

Page

4.3.5 Mechanism ..................................................................................................... 88 4.4 Conclusion ............................................................................................................... 90 4.5 References ............................................................................................................... 91 CHAPTER 5. HYDROGEN STORAGE IN LIQUID ORGANIC HETEROCYCLES BY USING RHENIUM NANOCRYSTALLINE CATALYST .......92 5.1 Introduction ............................................................................................................. 92 5.2 Experimental ........................................................................................................... 93


5.3 Results and Discussion ............................................................................................ 95 5.3.1 Hydrogenation Scope ..................................................................................... 95 5.3.2 Dehydrogenation Scope ................................................................................. 97 5.3.3 Catalyst Recycle ............................................................................................ 99 5.3.4 Kinetic Study ............................................................................................... 100

5.3.4.1 Hydrogen Dependence ........................................................................... 100 5.3.4.2 Catalyst Dependence .............................................................................. 101 5.3.4.3 Temperature Dependence ....................................................................... 101

5.3.5 Catalyst Characterization ............................................................................. 102 5.3.5.1 Transmission Electron Microscopy (TEM) ............................................ 102 5.3.5.2 Energy-Dispersive X-Ray Spectroscopy (EDX) .................................... 103 5.3.5.3 X-Ray Diffraction (XRD) ....................................................................... 104

5.4 Conclusion ............................................................................................................. 105 5.5 References ............................................................................................................. 106

VITA ............................................................................................................................... 107

PUBLICATIONS ............................................................................................................ 108

viii

LIST OF TABLES

Table .............................................................................................................................. Page

2.1 Oxo-rhenium catalysts for glycerol deoxygenation .................................................... 20

2.2 Effect of salt additive in oxorhenium-catalyzed deoxygenation of glycerol .............. 25

2.3 Effect of alcohol additive/solvent in oxorhenium-catalyzed deoxygenation of glycerol .....................................................................................................................26

2.4 Different substrates with 2 mol% MTO at 165 oC in 1 h ............................................ 27

3.1 Atomic concentrations of the elements obtained from Ar-protected (Ar), air-dried (Air), and a Re foil (Foil). The fractions of the different Re oxidation states were obtained by the curve fitting of the Re 4f spectra .................................................... 45

3.2 Dehydrogenation of alcohols by NH4ReO4 ................................................................ 52

3.3 Dehydrogenation of 3-octanol over recycled Re NPs catalyst ................................... 56

4.1 Dehydrogenation of 3-octanol over different Re catalysts ......................................... 67

4.2 AD amine-coupling of benzylamine over recycled Re NPs catalyst .......................... 70

4.3 AD reaction of secondary amines ............................................................................... 85

5.1 Rates for different quinoline derivatives ..................................................................... 97

5.2 Hydrogenation rates comparison between Re NPs in situ and regular Re/C ............ 100

5.3 Different initial hydrogen pressure influences the rate of hydrogenation ................ 101

5.4 Catalyst concentration influences the rate of hydrogenation .................................... 101

5.5 The rates of hydrogenation in different temperatures ............................................... 102

ix

LIST OF FIGURES

Figure ............................................................................................................................. Page

2.1 Reaction distillation apparatus .................................................................................... 21

2.2 1H NMR in d6-DMSO of the volatile products from glycerol .................................... 21

2.3 13C NMR in d6-DMSO of the volatile products from glycerol ................................... 22

2.4 GC-MS of volatile products from glycerol ................................................................. 23

2.5 Reaction profile for glycerol conversion to allyl alcohol, acrolein, and propanal. In condition with 2 mol% MTO at 165 oC ................................................................... 29

2.6 MTO-Dependence kinetic fitting ................................................................................ 29

2.7 1H NMR of Re-diolate in CDCl3. Condition: glycerol:MTO = 2:1 in molar ratio at 165 oC for 30 s, cooled, and added CDCl3 ............................................................... 31

2.8 1H NMR of Re-diolate in d6-DMSO. Condition: glycerol:MTO = 1:1 in molar ratio at 165 oC for 50 s, cooled, and added d6-DMSO ..............................................32

3.1 (a) TEM image of Re NPs (b) Enlarged HRTEM image of hexagonal Re NPs. The inset in (b) shows the FFT pattern of a single Re nanoparticle ............................... 42

3.2 Re NPs particle size distribution histogram ................................................................ 43

3.3 Re 4f spectra obtained from (a) Re polycrystalline foil and (b) Re NP handled under Ar (Ar sample) ................................................................................................45

3.4 Re L3 XANES from 10.50 to 10.57 keV .................................................................... 46

3.5 EXAFS Fourier transforms of Re NPs ........................................................................ 47

3.6 Powder-XRD of Re NPs ............................................................................................. 48

3.7 FT-IR for (a) fresh NH4ReO4, (b) 1st-recycled NH4ReO4, and (c) 10th-recycled NH4ReO4 .................................................................................................................. 49

x

Figure ............................................................................................................................. Page

3.8 Different Re pre-catalyst at 180 oC for 10 h ............................................................... 50

3.9 Time profile from aliquots for different NH4ReO4 concentrations ............................ 53

3.10 Continuous reaction of 2 mol% NH4ReO4 with neat 3-octanol reaction .................. 54

3.11 Temperature effect in 3-octanol reaction .................................................................. 55

3.12 Residual gas analysis (RGA) of compounds 1, 2, 3 ................................................. 60

4.1 Entry 1 of Table 4.1 reaction profile for benzylamine ................................................ 68

4.2 Eyring plot .................................................................................................................. 69

4.3 Hammett plot .............................................................................................................. 72

4.4 1H and 13C NMR of 1 ................................................................................................ 73

4.5 1H and 13C NMR of 2 ................................................................................................ 74

4.6 1H and 13C NMR of 3 ................................................................................................ 75

4.7 1H and 13C NMR of 4 ................................................................................................ 76

4.8 1H and 13C NMR of 5 ................................................................................................ 77

4.9 1H and 13C NMR of 6 ................................................................................................ 78

4.10 1H and 13C NMR of 7 .............................................................................................. 79

4.11 1H and 13C NMR of 8 .............................................................................................. 80

4.12 1H and 13C NMR of 9 .............................................................................................. 81

4.13 1H and 13C NMR of 10 ............................................................................................ 82

4.14 1H and 13C NMR of 11 ............................................................................................ 83

4.15 1H and 13C NMR of 12 ............................................................................................ 84

4.16 2D NMR at 9 h of deuterated benzylamine (C6H5CD2NH2) ..................................... 87

4.17 RGA gas products from deuterated benzylamine (C6H5CD2NH2) ........................... 87

4.18 1H NMR of benzhydrylamine ((C6H5)2CHNH2) reaction in sealed system. The characteristic peaks 10.89 and 11.27 ppm are cis- and trans-ketimine ................... 89

xi

Figure ............................................................................................................................. Page

5.1 Different position substituted quinolones ................................................................... 97

5.2 Hydrogen comsumptions of 8-time recycled Re NPs ................................................. 99

5.3 5-nm, 2-nm and 10 nm of TEM images and Re NPs size distribution ..................... 102

5.4 EDX spectrum of Re NPs ......................................................................................... 103

5.5 XRD comparisons between in situ Re and regular Re/C .......................................... 104

xii

LIST OF SCHEMES

Scheme ........................................................................................................................... Page

1.1 Deoxygenation of diols and epoxides with methyltrioxorhenium catalyst and hydrogen reductant..................................................................................................... 2

1.2 1,2-Deoxygenation of diols with methyltrioxorhenium catalyst and sulfite reductant .....................................................................................................................3

1.3 1,3-Deoxygenation of diols with rhenium carbonyl catalysts and scrificial alcohol .... 4

1.4 Deoxygenation of C3 – C6 sugar polyols with methyltrioxorhenium catalyst and scrificial alcohols ....................................................................................................... 5

1.5 1,4-Deoxygenation and 1,6-deoxygenation reactions ................................................... 6

1.6 Milstein catalysts for alcohol dehydrogenation ............................................................ 8

2.1 Oxo-Rhenium-Catalyzed transfer hydrogenation and deoxygenation of glycerol ..... 18

2.2 Oxo-Rhenium-Catalyzed deoxygenation of 1,2-propanediol ..................................... 26

2.3 MTO-Catalyzed deoxygenation of cis-1,2-cyclohexanediol ...................................... 27

2.4 Kinetic steps in MTO-catalyzed deoxygenation of glycerol ...................................... 30

2.5 Isotopic labeling experiements ................................................................................... 33

2.6 Proposed mechanism for the formation of allyl alcohol, propanal, and acrolein ....... 34

2.7 MTO-Catalyzed glycerol reaction by using 1,3-propanediol or 1,2-propanediol ...... 35

3.1 Deuterated alcohol substrates for KIE experiments .................................................... 58

3.2 Proposed acceptorless dehydrogenation (AD) of 3-octanol ....................................... 61

4.1 Rhenium-Catalyzed acceptorless dehydrogenation (AD) reaction of alcohols and amines ...................................................................................................................... 65

4.2 Benzylamine coupling reaction in Table 4.1 .............................................................. 67

xiii

Scheme ........................................................................................................................... Page

4.3 Substrate scope for AD amine-coupling ..................................................................... 71

4.4 Proposed mechanism for AD amine-coupling ............................................................ 89

5.1 Re NPs catalyzed reversible hydrogenation/dehydrogenation ................................... 93

5.2 Hydrogenation of N-heterocycles ............................................................................... 96

5.3 Dehydrogenation of N-heterocycles ........................................................................... 98

xiv

ABSTRACT

Yi, Jing. Ph.D., Purdue University, May 2015. Homogeneous and Heterogeneous Rhenium Catalysts for Sustainable Transformations of Polyols, Alcohols, and Amines. Major Professor: Mahdi Abu-Omar.

Biomass-derived molecules, such as sugar polyols and lignin, are promising

feedstock for making small and useful organics (SUO) and high value-added organics

(HVO). However, the high oxygen to carbon ratio (C:O ≈ 1) prohibits the direct use of

biomass-derived molecules as energy carriers and chemicals. Deoxygenation of polyols

has become an important challenge in the utilization of biomass resources. Oxorhenium

complexes show excellent oxophilicity to transfer oxygen and deoxygenate alcohols.

Oxorhenium(VII) complexes can efficiently catalyze the transformation of glycerol to

allyl alcohol, acrolein, and propanal. The volatile SUO products were easily separated

from the nonvolatile residues via simple distillation. Based on kinetic studies and

isotope labeling experiments, an oxo-Re(V) diolate intermediate was proposed as both a

proton acceptor and a hydride acceptor to facilitate hydrogen transfer and deoxygenation

reactions.

In contrast to high valent oxorhenium complexes, a nanoparticulate Re0 metallic

catalyst shows great ability to activate C-H bonds in alcohols and amines.

Dehydrogenation of alcohols into carbonyl groups is an important reaction. Acceptorless

dehydrogenation is a novel, green, and atom-economical reaction, which has attracted

xv

attention recently. Metallic Re0 nanoparticles catalyze acceptorless dehydrogenation in

the neat alcohol to give the corresponding ketone with concurrent release of hydrogen

gas. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS),

powder X-ray diffraction (XRD), high resolution transmission electron microscopy

(HRTEM/TEM) demonstrated that the Re0 catalyst is a well-structured 2 nm

nanocrystalline particles (Re NPs) covered by ReIVO2 oxide. The Re NPs can also be

utilized to amine acceptorless dehydrogenation reactions. Unlike alcohols, amines

dehydrogenation was followed by homo-coupling to give imine as the final product.

Another difference between amine and alcohol acceptorless dehydrogenation is -C-H

activation for alcohols versus -C-H activation for amines, which might be caused by

the different electronegativities of oxygen and nitrogen.

Similar to other metal hydrogenation catalysts, Re NPs catalyze hydrogenation of

N-heterocycles. When combined with its dehydrogenation activity, Re NPs supported on

carbon (Re/C) acts as a catalyst for reversible hydrogenation/dehydrogenation that has

applications in hydrogen storage. This reaction was demonstrated with various N-

heterocyclic substrates, which provide stable and recyclable liquids. Five cycles of

hydrogenation/dehydrogenation was achieved with Re/C catalyst without significant loss

of activity accounting for 1962 turnovers.

1

CHAPTER 1. CURRENT SUSTAINABLE CHEMICAL TRANSFORMATIONS FOR POLYOLS, ALCOHOLS, AND AMINES

1.1 Deoxydehydration of Biomass-Derived Polyols

Deoxygenation of vincinal diols and polyols, common moieties in biomass-derived

compounds, is an important challenge in utilizing biomass resources. Catalytic

deoxydehydration (DODH) is a promising deoxygenation method, which can remove

two adjacent hydroxyl groups from vicinal diols to generate alkenes. In 1996, Andrew

and Cooks reported on the use Cp*Re(O)3 as a DODH catalyst.1 Subsequently, DODH

has received growing interest in biomass research. Applications of different metal

complexes in DODH have been documented. These include rhenium, ruthenium,

vanadium, and molybdenum. High-valent oxo-rhenium complexes are the most efficient

catalysts in DODH reactions, in combination with other reductants like phosphines, H2,

sulfites, and alcohols. These complexes exhibit intriguing oxophilic performance, which

facilitates C-O bond cleavage of biomass-derived polyols. Several groups have

investigated the reaction mechanism via kinetics or DFT calculations, and proposed

different mechanisms that involved different intermediates and postulated transition

states. In this chapter, rhenium-catalyzed DODH will be the main focus, as well as

heterogeneous DODH catalysis. Brief comparisons and description of other metal

catalyzed DODH will also be provided.

2

1.1.1 Rhenium-Catalyzed Deoxydehydration of Diols and Polyols

Generally speaking, in deoxydehydration (DODH) reaction, the substrates (vicinal

diols or polyols) are reduced to alkenes or allylic alcohols, while the oxygen transfers to

a reducing agent. Among all the catalysts, rhenium catalysts are efficient and well-

studied. In 2009 our group reported methyltrioxorhenium (MTO) catalyzed DODH of

vicinal diols/epoxides to alkene or alkanes with H2 as reductant under mild conditions

(Scheme 1.1).2 Under lower H2 pressure, the products were dominated by alkenes,

whereas under higher pressure the major products were alkanes. Several biomass-

derived substrates were also tested. Erythritol afforded significant char, while 1,4-

anhydroerytheritol produced 2,5-dihydrofuran (25% yield) and tetrahydrofuran (5%

yield). The authors also found that cis-1,2-cyclohexanediol generated the desired

cyclohexene product, but no reaction was observed for trans-1,2-cyclohexanediol, which

is consistent with the previous report.1 The DODH stereoselectivity for cis-vicinal

hydroxyls indicates a rhenium diolate intermediate.

Scheme 1.1 Deoxygenation of diols and epoxides with methyltrioxorhenium catalyst and hydrogen reductant.

Soon the Nicholas group reported on MTO and perrhenate salts catalyzed DODH

of diols using sulfite as reductant (Scheme 1.2).3,4 Both aromatic diols and aliphatic

3

diols were converted to the corresponding alkenes with moderate yields. However,

aliphatic diols took longer reaction time than aromatic diols. Addition of the crown ether

15-crown-5 as a phase transfer reagent could significantly shorten the reaction time and

increase the rate. NBu4ReO4 was a better catalyst than MTO in the conversion of

erythritol under the same conditions. Small and useful organics, such as, 1,3-butadiene

(27% yield), 2,5-dihydrofuran (6% yield), and cis-2-butene-1,4-diol (3% yield) were

obtained by using NBu4ReO4 as the catalyst, but substantial charring was observed when

MTO was used.

Scheme 1.2-Deoxygenation of diols with methyltrioxorhenium catalyst and sulfite reductant.

The Bergman group used rhenium carbonyl Re2(CO)10 and BrRe(CO)5 as catalysts,

and secondary alcohols as reductant/solvent (Scheme 1.3).5 In the presence of a

sacrificial alcohol, 3-octanol or 5-nonanol, both terminal and internal vincinal diols were

deoxygenated to olefins with good yields, while the sacrificial alcohol was oxidized to

the corresponding ketone. In the presence of p-toluenesulfonic acid (TsOH), erythritol

could be converted to 2,5-dihydrofuran with very high yield (62%). The system was

demonstrated to need air and high temperature for activation, which indicated that the

4

real catalyst might be an oxidized rhenium species rather than Re0 or ReI in the

precatalyst form.

Scheme 1,3-Deoxygenation of diols with rhenium carbonyl catalysts and sacrificial alcohol.

In 2012 Toste and his group extended the substrate scope to C-5 and C-6 sugar

polyols, and demonstrated high efficiency and selectivity of DODH for sugar polyols

(Scheme 1.4).6 C3 – C6 sugar polyols can be readily obtained from the hydrogenation,

fermentation, and decarbonylation of biomass carbohydrates. In the presence of

secondary or primary sacrificial alcohols, C3 – C6 sugar polyols were efficiently

converted to the corresponding olefinic products that are useful precusoors for polymers

and liquid fuels. Comparing with previously reported research by Bergman,5 high-valent

oxorhenium catalysts (MTO and NH4ReO4) are more efficient than low-valent rhenium

catalysts (Re2(CO)10 and BrRe(CO)5).

5

Scheme 1.4 Deoxygenation of C3 – C6 sugar polyols with methyltrioxorhenium catalyst and sacrificial alcohols.

Based on the 1,2-DODH, the Toste group further reported on 1,4-DODH and 1,6-

DODH reactions via tandem [1,3]-OH shift-DODH process (Scheme 1.5).7 This new

DODH strategy improve the efficiencies and selectivities of biomass polyol conversions.

This method was applied to the conversion from sugar acids to unsaturated esters, in

which perrhenic acid HOReO3 acted as both DODH catalyst and Brønsted acid.

6

Scheme 1.5 1,4-Deoxygenation and 1,6-deoxygenation reactions.

The reaction mechanism of the MTO-catalyzed DODH of diols was investigated

by density functional theory (DFT) calculations,8,9 which supported the original

mechanism proposed by Gable and Cook.1,10-12 Part of my research on the applications

and mechanism of the oxorhenium-catalyzed DODH is detailed in Chapter 2.13,14

1.1.2 Heterogeneous Catalysis of Deoxydehydration

There are a few reports about heterogeneous DODH reactions. Jentoft and

Nicholas reported the first heterogeneous polyol-into-olefin DODH reactions catalyzed

by carbon-supported perrhenate, employing both H2 and hydrogen-transfer reductants

with moderate yield.15 Interestingly, in 2011 the Schlaf group has reported that stainless

steel reactors could catalyze the deoxygenation of glycerol and levulinic acid in aqueous

acidic medium.16 Ferdi Schuth reported an iron oxide-catalyzed conversion of glycerol

to allylic alcohol, and proposed a mechanism through dehydration and consecutive

hydrogen transfer.17 Andreas Martin also reported glycerol deoxygenation reaction in

the gas phase using a series of heteropolyacid catalysts.18 Most recently, Nicholas

7

reported on deoxydehydration of glycols using heterogeneous elemental reductants, such

as zinc, iron, manganese and carbon.19

1.1.3 Other Metal Catalyzed Deoxydehydration

In addition to rhenium, there has been a number of DODH reactions catalyzed by

other transition metal complexes, such as vanadium,20 molybdenum,21,22 and

ruthenium.23-25 Nicholas reported a DODH reaction of diols to olefins, catalyzed by

inexpensive metavandate (VO3-) and chelated dioxovanadium derivatives, with

phosphine or sulfite as reductants.20 Dioxomolybdenum(VI) complexes with

acylpyrazolonate ligands were synthesized by the Claudio Pettinari group, and showed

moderate activity towards diol DODH reactions, with PPh3 as reductant.21 The Fristrup

group also reported another DODH reaction catalyzed by a series of Mo-oxo complexes

under neat conditions.22 In addition, Schlaf and Bullock have pioneered the use of

organometallic ruthenium catalysts for the deoxygenation of alcohols.23 Two other Ru-

catalyzed DODH were also reported by Srivastava and Nicholas independently, using

[Cp*Ru(CO)2]2 or Ru(II)-sulfoxides as catalysts, for hydrodeoxygenation (HDO) and

hydrocracking of diols and epoxides.24,25

1.2 Acceptorless Dehydrogenation of Alcohols and Amines

The transformations of hydroxyl or amino groups into the corresponding carbonyl

or imine functional groups is one of the most important reactions in organic

synthesis.26,27 Traditionally, stoichiometric or even over-stoichiometric amounts of

8

oxidant is needed. Recently, acceptorless dehydrogenation (AD) reactions provided an

environmental-friendly and highly atom-economy methods to produce target carbonyl

and imine molecules with concurrent release of dihydrogen gas.28 Additionally, the

hydrogen gas is a valuable green energy molecule.

1.2.1 Homogeneous Acceptorless Dehydrogenation of Alcohols and Amines

In 2004, the Milstein group reported on PNP (2,6-bis-(di-tert-

butylphosphinomethyl)pyridine) ligated Ru complexes that can effectively catalyze the

dehydrogention of secondary alcohols to the corresponding ketones with the evolution

of dihydrogen.29 Subsequently they developed a series of pincer ligated Ru complexes

(Scheme 1.6)30 which could catalyze many acceptorless dehydrogenation reactions with

heteroatom molecules, such as, alcohol dehydrogenation,29,30 dehydrogenative coupling

of alcohols to form esters,31 dehydrogenative coupling of alcohols and amines to form

amides.32 Besides Ru, Ir33 and Os34 were also found to catalyze alcohol dehydrogenation.

The interesting aromatization-dearomatization dehydrogenation mechanism has been

postulated for these PNP pincer ligated complexes.35

Scheme 1.6 Milstein catalysts for alcohol dehydrogenation.

9

During the development of pincer ligated metal complexes, Fujita and Yamaguchi

group found that Cp*Ir complexes can catalyze acceptorless dehydrogenation of

alcohols with the assistance of 2-hydroxypyridine ligand36 or 2-hydroxypyridyl

fragment(s) in bipyridyl37 or phenylpyridyl ligands.38 Hence, ligand-promoted

dehydrogenation of alcohols became a major mechanistic theme in homogeneous

systems.

In comparison to acceptorless dehydrogenation of alcohols, acceptorless

dehydrogenation of amines has been reported only on a few examples. The product

imines are important building blocks for chemical and pharmaceutical synthesis.39 Since

the products aldimine from primary amine and ketamine from secondary amine are quite

reactive, the acceptorless dehydrogenation is accompanied by subsequent coupling

reactions. Nevertheless, this type of dehydrogenative coupling can provide an innovative

strategy to combine N-H activation with C-N formation.40 Intramolecular hydrogen

transfer that consumes H2 produced from dehydrogenative coupling in the first step can

facilitate N-alkylation reactions.41 Huang’s group reported acceptorless dehydrogenative

coupling of amines to produce imine by using PNN (2-(di-tert-butylphosphinomethyl)-

6-(diethylaminomethyl)pyridine) Ru complexes as catalyst.42 Hartwig’s group

successfully extended the substrate scope to hydrazines with PCP Ir complexes.43

In sum, homogeneous acceptorless dehydrogenation of alcohols and amines

requires often the ligand’s assistance, in which pincer ligands have been widely used in

this field. For metal options, researchers were inclined to choose transition metals like

Ru and Ir.

10

1.2.2 Heterogeneous Acceptorless Dehydrogenation of Alcohols and Amines

In 2004, Park heterogenized a Shvo-type diruthenium complex in sol gel and

showed its catalytic activity for acceptorless dehydrogenation of alcohols.44 However,

not many heterogeneous catalysts were well-defined. For example, Pd/Al2O3,45

RuCl3·xH2O,46 Ag/hydrotalcite,47 Cu/hydrotalcite,48 Ag/alumina,49 Au/hydrotalcites,50

Co/TiO2,51 Ni/alumina,52 and Pt/Al2O3

53 share similar advantages like recyclability of

catalysts, and high TON (15 ~ 4050 h-1), but the disadvantage of high reaction

temperatures (100 ~ 500 oC) make them less favorable in comparison with the mild

conditions (< 100 oC) of the homogeneous system.

There were a few reports about heterogeneous amine acceptorless dehydrogenation.

In 2005, Chihara documented that a molybdenum halide cluster can dehydrogenate

aliphatic amines to nitriles, imines, or vinylamines at 300 oC.54 In 2013, the Mizuno

group reported Cu/Al2O3 as a catalyst for N-alkylation of primary amines at 170 oC.55

1.3 Current Hydrogen Storage in Liquid Organic Heterocycles

Hydrogen gas is a green and promising energy carrier for future energy systems.

As the development and availability of hydrogen vehicles, hydrogen storage has become

an important challenge in the energy landscape.56 Besides high-pressure, cryogenic-

liquid, and adsorptive storage, chemical solutions have attracted more and more

attention.57 In comparison to hydrides, amine-borane adducts, and amides, organic

heterocyclic molecules are safe, stable, and easily recyclable, which show obvious

advantages for hydrogen storage.58 Hydrogen storage in organic heterocyclic was first

proposed by Guido Pez at Air Products.59,60 The release/storage steps involve catalytic

11

dehydrogenation/hydrogenation in heterocycle rings. The heterocyclic molecules N-

ethyl carbazole can be hydrogenated with 72 atm H2 at 160 oC with Pd catalyst and

dehydrogenated at 50 ~ 197 oC with Ru catalyst for 5 cycles.60

Interestingly, Cp*Ir complexes were reported by Fujita and Yamaguchi group for

acceptorless dehydrogenation of alcohols,36 which were also found effective in

dehydrogenation/hydrogenation of heterocycles.61 As an expert in hydrogen storage,

Professor Crabtree performed careful mechanistic research and proposed an outer-sphere

pathway for Ir-catalyzed homogeneous systems.62 Jones and co-workers used PNP

pincer ligated Fe complexes to dehydrogenate/hydrogenate N-heterocycles under mild

conditions.63

1.4 Chapter Overview

Presented above is a brief introduction regarding the current art in catalysis on

deoxydehydration of polyols, acceptorless dehydrogenation of alcohols and amines, as

well as reversible dehydrogenation/hydrogenation of N-heterocyclic compounds.

Chapter 2 gives details on homogeneous oxo-rhenium catalytic application in transfer

hydrogenation and deoxygenation of biomass-derived polyols. Under neat conditions,

polyols such as glycerol produce small and useful organic (SUO) molecules that can be

efficiently separated from the nonvolatile residue via distillation. Based on kinetic

profiles and isotope labeling experiments, a mechanism in which rhenium diolate acts as

hydride and proton acceptor is proposed.

Chapters 3 and 4 describe recyclable heterogeneous rhenium (Re) nanocrystalline

particles (NPs) and their catalytic applications in acceptorless dehydrogenation of

12

alcohols and amines. The novel Re NPs were generated from NH4ReO4 in situ under

neat alcohol conditions. Various spectroscopic techniques including X-ray absorption

spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction

(XRD), high resolution transmission electron microscopy (HRTEM/TEM) were

performed to confirm the Re NPs structure and composition. The mechanisms of alcohol

dehydrogenation via -C-H activation and amine dehydrogenation via -C-H activation

have been elucidated and the difference in mechanism is attributed to the different

electronegativities of oxygen and nitrogen.

Chapter 5 focuses on reversible dehydrogenation/hydrogenation of N-heterocycles

with reusable C-supported Re NPs catalyst, which has the promising application in

hydrogen storage. Quinoline substrates and derivatives are safe liquids that can be easily

recycled from the reaction. Five dehydrogenation/hydrogenation cycles has been

achieved without loss in activity.

13

1.5 References

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(2) Ziegler, J. E.; Zdilla, M. J.; Evans, A. J.; Abu-Omar, M. M. Inorg. Chem. 2009, 48, 9998.

(3) Vkuturi, S.; Chapman, G.; Ahmad, I.; Nicholas, K. M. Inorg. Chem. 2010, 49, 4744.

(4) Ahmad, I.; Chapman, G.; Nicholas, K. M. Organometallics 2011, 30, 2810.

(5) Arceo, E.; Ellman, J. A.; Bergman, R. G. J. Am. Chem. Soc. 2010, 132, 11408.

(6) Shiramizu, M.; Toste, F. D. Angew. Chem. Int. Ed. 2012, 51, 8082.

(7) Shiramizu, M.; Toste, F. D. Angew. Chem. Int. Ed. 2013, 52, 12905.

(8) Bi, S.; Wang, J.; Liu, L.; Li, P.; Lin, Z. Organometallics 2012, 31, 6139.

(9) Qu, S.; Dang, Y.; Wen, M.; Wang, Z. X. Chem. Eur. J. 2014, 19, 3827.

(10) Gable, K. P.; Phan, T. N. J. Am. Chem. Soc. 1994, 116, 833.

(11) Gable, K. P. Juliette, J. J. J. J. Am. Chem. Soc. 1995, 117, 955.

(12) Gable, K. P.; Juliette, J. J. J. J. Am. Chem. Soc. 1996, 118, 2625.

(13) Yi, J.; Liu, S.; Abu-Omar M. M. ChemSusChem 2012, 5, 1401.

(14) Liu, S.; Senocak, A.; Smeltz, J. L.; Yang, L.; Wegenhart, B.; Yi, J.; Kenttamaa, H. I.; Ison, E. A.; Abu-Omar, M. M. Organometallics 2013, 32, 3210.

(15) Amada, Y.; Watanabe, H.; Hirai, Y.; Kajikawa, Y.; Nakagawa, Y.; Tomishige, K. ChemSusChem 2012, 5, 1991.

(16) Di Mondo, D.; Ashok, D.; Waldie, F.; Schrier, N.; Morrison, M.; Schlaf, M. ACS Catal. 2011, 1, 355.

(17) Liu, Y.; Tuysuz, H.; Jia, C. J.; Schwickardi, M.; Rinaldi, R.; Lu, A. H.; Schmidt, W.; Schuth, F. Chem. Comm. 2010, 46, 1238.

(18) Atia, H.; Armbruster, U.; Martin, A. J. Catal. 2008, 258, 71.

(19) McClain, J. M.; Nicholas, K. M. ACS Catal. 2014, 4, 2109.

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14

(21) Hills, L.; Moyano, R.; Montilla, F.; Pastor, A.; Galindo, A.; Alvarez, E.; Marchetti, F.; Pettinari, C. Eur. J. Inorg. Chem. 2013, 19, 3352.

(22) Dethlefsen, J. R.; Lupp, D.; Oh, B. C.; Fristrup, P. ChemSusChem 2014, 7, 425.

(23) Schlaf, M.; Ghosh, P.; Fagan, P. J.; Hauptman, E.; Bullock, M. R. Angew. Chem. Int. Ed. 2001, 40, 3887.

(24) Stanowski, S.; Nicholas, K. M.; Srivastava, R. S. Organometallics 2012, 31, 515.

(25) Murru, S.; Nicholas, K. M.; Srivastava, R. S. J. Mol. Catal. A: Chem. 2012, 363-364, 460.

(26) Tojo, G.; Fernandez, M. Oxidation of Alcohols to Aldehydes and Ketones, Springer, New York, 2006.

(27) Yamaguchi, K.; Mizuno, N. Angew. Chem. Int. Ed. 2003, 42, 1480.

(28) Gunanathan, C.; Milstein, D. Science 2013, 341, 249.

(29) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. Organometallics 2004, 23, 4026.

(30) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Milstein, D. Dalton Trans. 2007, 107.

(31) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2005, 127, 10840.

(32) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790.

(33) Musa, S.; Shaposhnikov, I.; Cohen, S.; Gelman, D. Angew. Chem. 2011, 123, 3595.

(34) Bertoli, M.; Choualeb, A.; Lough, A. J.; Moore, B.; Spasyuk, D.; Gusev, D. G. Organometallics 2011, 30, 3479.

(35) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011, 44, 588.

(36) Fujita, K.; Tanino, N.; Yamaguchi, R. Org. Lett. 2007, 9, 109.

(37) Kawahara, R.; Fujita, K.; Yamaguchi, R. J. Am. Chem. Soc. 2012, 134, 3643.

(38) Fujita, K.; Yoshida, T.; Imori, Y.; Yamaguchi, R. Org. Lett. 2011, 13, 2278.

(39) Largeron, M. Eur. J. Org. Chem. 2013, 5225.

(40) Kruger, K.; Tillack, A.; Beller, M. ChemSusChem 2009, 2, 715.

15

(41) Hollmann, D.; Bahn, S.; Tillack, A.; Beller, M. Angew. Chem. Int. Ed. 2007, 46, 8291.

(42) He, L.-P.; Chen, T.; Gong, D.; Lai, Z.; Huang, K.-W. Organometallics 2012, 31, 5208.

(43) Huang, Z.; Zhou, J.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 11458.

(44) Choi, J. H.; Kim, N.; Shin, Y. J.; Park, J. H.; Park, J. Tetrahedron Lett. 2004, 45, 4607.

(45) Burgener, M.; Mallat, T.; Baiker, A. J. Mol. Catal. A: Chem. 2005, 225, 21.

(46) Kim, W.-H.; Park, I. S.; Park, J. Org. Lett. 2006, 8, 2543.

(47) Mitsudome, T.; Mikami, Y.; Funai, H.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Angew. Chem. 2008, 120, 144.

(48) Mitsudome, T.; Mikami, Y.; Ebata, K.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Chem. Commun. 2008, 4804.

(49) Shimizu, K.; Sugino, K.; Sawabe, K.; Satsuma, A. Chem. Eur. J. 2009, 15, 2341.

(50) Fang, W.; Zhang, Q.; Chen, J.; Deng, W.; Wang, Y. Chem. Commun. 2010, 46, 1547.

(51) Shimizu, K.; Kon, K.; Seto, M.; Shimura, K.; Yamazaki, H.; Kondo, J. N. Green Chem. 2013, 15, 418.

(52) Shimizu, K.; Kon, K.; Shimura, K.; Hakim, S. S. M. A. J. Catal. 2013, 300, 242.

(53) Kon, K.; Hakim, S. S. M. A.; Shimizu, K. J. Catal. 2013, 304, 63.

(54) Kamiguchi, S.; Nakamura, A.; Suzuki, A.; Kodomari, M.; Nomura, M.; Iwasawa, Y.; Chihara, T. J. Catal. 2005, 230, 204.

(55) Kim, I.; Itagaki, S.; Jin, X.; Yamaguchi, K.; Mizuno, N. Catal. Sci. Technol. 2013, 3, 2397.

(56) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353.

(57) Felderhoff, M.; Schuth, F. Angew. Chem. Int. Ed. 2009, 48, 6608.

(58) Crabtree, R. H. Energy Environ. Sci. 2008, 1, 134.

(59) Pez, G. P.; Scott, A. R.; Cooper, A. C.; Cheng, H. US Pat. 7101530, 2006.

16

(60) Pez, G. P.; Scott, A. R.; Cooper, A. C.; Cheng, H.; Wilhelm, F. C.; Abdourazak, A. H. US Pat. 7351395, 2008.

(61) Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K. J. Am. Chem. Soc. 2009, 131, 8410.

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(63) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2014, 136, 8564.

17

CHAPTER 2. RHENIUM-CATALYZED TRANSFER HYDROGENATION AND DEOXYGENATION OF BIOMASS-DERIVED POLYOLS TO SMALL AND

USEFUL ORGANICS

2.1 Introduction

As the depletion of fossil fuels, more attention has been paid to efficient utilization

of biomass resources. Promotion of biodiesel causes the oversupplied glycerol in market

since glycerol is the only byproduct of biodiesel production.1,2 Hence efficient and

environmentally responsible conversions from glycerol to small and useful organics

(SUO) are needed.

Myriad methods have been reported for the preparation of allyl alcohol (one of SUO)

from biomass substrates.3 Bergman and Ellman groups used formic acid as the catalyst to

deoxygenate glycerol.4,5 They have also reported the deoxygenation of polyols with 3-

octanol as the reductant in the presence of rhenium carbonyl catalyst and an acid

additive.6 Nicholas group have employed oxorhenium catalysts with sulfite as the oxygen

atom acceptor to deoxydehydrate vicinal diols.7 Our group has previously reported H2-

driven deoxygenation of vicinal diols and epoxides.8 Besides rhenium catalysts, Schlaf

and Bullock have advanced the ruthenium catalysts for the deoxygenation of alcohols.9-

11Heterogeneous iron oxide has been documented to deoxygenate glycerol via

dehydration and hydrogen transfer.12

18

In this chapter, we focus on a catalytic method that can transform neat glycerol to

SUO by using stable oxo-rhenium catalysts, such as, methyltrioxorhenium (MTO) and

ammonium perrhenate (NH4ReO4). Glycerol goes through transfer hydrogenation and

deoxygenation to give SUO allyl alcohol, propanal, acrolein, and non-volatile

dihydroxyacetone (DHA) by-product in the residue (Scheme 2.1).

Scheme 2.1 Oxo-Rhenium-Catalyzed transfer hydrogenation and deoxygenation of glycerol.

2.2 Experimental

2.2.1 Chemical Preparations

All commercial materials were used as received without further purification unless

specified otherwise. Glycerol, 3-octanol, 1-heptanol, cyclohexanol, cis-1,2-

cyclohexanediol, trans-1,2-cyclohexanediol, meso-erythritol, meso-threitol, NH4ReO4,

1,2-propanediol, 1,3-propanediol were purchased from Aldrich. MTO, NaReO4, KReO4

were purchased from Strem Chemicals. MTO was purified by sublimation at 40 oC (1

19

mm Hg). 1,1,2,3,3-D5-Glycerol and (OD)3-glycerol were purchased from Cambridge

Isotope Laboratories.

General procedure for glycerol reaction is: in a 15 mL two neck round-bottom

flask were placed 5.10 g (55 mmol) of glycerol and 0.274 g (1.1 mmol, 2 mol%) MTO.

The flask was connected to a distillation set (including thermometer, distillation column,

and collecting flask). The temperature in the reaction mixture was measured by an

immersed thermometer. The system was heated using a preheated 175 oC oil bath. 10

min later first drop was collected at 75 oC. Then 50 min later almost nothing remained in

the distillation column, at which point the reaction was presumed to have reached

completion. The volatile products ~2.82 g (with water) were collected over an ice bath.

Without any further purification, an aliquot of the volatile fraction was added to D6-

DMSO and analyzed by NMR and GC-MS.

2.2.2 Instrumentation

1H and 13C NMR spectra were recorded on Bruker Avance ARX-400

spectrometers. 2H NMR spectra were recorded on Varian INOV A300-1. NMR data was

plotted by using MestReNova. GC-MS analysis was performed using a Pegasus 4D

GCxGC/TOF-MS (LECO Corporation). The GC column was a non-polar DB-5

capillary column (J&W Scientific, 30 m x 0.25 mm x 0.25 m). The electron impact (EI)

ion source was held at 200 oC, with a filament bias of -70 V. Mass spectra were

collected from 15 to 400 m/z at 100 spectra/sec.

20

2.3 Results and Discussion

2.3.1 Catalysts Usage

MTO and NH4ReO4 are both efficient catalysts that could be at the low loading of

2 mol% relative to glycerol (Table 2.1). The reactions were effective at 165 oC to

produce allyl alcohol as the major product. With 2 mol% MTO, 74% volatile yield was

achieved within one hour. The volatile products were easily separated from non-volatile

DHA by-product via distillation apparatus (Figure 2.1). The distilled fraction is

composed of allyl alcohol, propanal, and acrolein in the ratio of 1.0:0.22:0.15, which has

been characterized by 1H, 13C NMR, and GC-MS (Figure 2.2, 2.3, and 2.4). Besides

MTO and NH4ReO4, NaReO4 and KReO4 were also tested as catalysts under the same

conditions (Table 2.1), but both gave lower conversions and yields. All catalysts gave

the major product allyl alcohol, and basically follow the fraction ratio mentioned before.

The cation effect will explained in details in next 2.3.2 session.

Table 2.1 Oxo-rhenium catalysts for glycerol deoxygenation.

Catalyst Catalyst Amount (mol%)

Time (h) Conversion (%) Volatile Products Yield (%)

MTO 1.0 1.7 100 74 MTO 2.0 1 100 74

NH4ReO4 2.0 1 100 78 NH4ReO4 5.0 1 100 80 NaReO4 5.0 18 9 6 KReO4 5.0 10 27 24

8.59.09.5

0.22

0.14

9.53

9.55

9.67

Figu

F

7.7.58.05

6.49

6.59

6.61

ure 2.2 1H NM

Figure 2.1 Re

56.06.50

1.00

0.21

0.27

0.25

586

5.89

5.90

5.93

6.28

6.30

6.32

6.34

6.35

6.36

6.45

MR in d6-DM

eaction distil

44.55.05.5f1 (ppm)

0.97

1.08

393

3.94

4.99

5.00

5.02

5.02

5.15

5.16

5.19

5.20

5.86

MSO of the

llation appar

3.03.54.0

12.5

5

2.31

2.50

3.58

3.92

3.92

3.93

3.93

H2O

volatile prod

ratus.

1.52.02.5

0.36

096

2.41

2.43

2.44

2.46

ducts from g

0.00.51.0

-

0

5

1

1

2

2

3

3

4

0.53

0.80

0.82

0.84

0.93

0.95

0.96

glycerol.

21

-5

0

5

10

15

20

25

30

35

40

22

Figure 2.3 13C NMR in d6-DMSO of the volatile products from glycerol.

Figure 2.4 GC-MS of volatile pproducts fromm glycerol.

23

24

2.3.2 Additives Usage

The obvious difference between NH4ReO4 and NaReO4 merited further

investigation to understand the role of the cation. Several experiments with different

catalysts and salt additives were investigated and the results were summarized in Table

2.2. For MTO, addition of sodium or chloride has no effect on the amount of volatile

products produced but appears to reduce the ratio of propanal in the product mixture.

Potassium, on the other hand, appears to reduce the productivity of the catalyst

somewhat. The addition of ammonium salt or HCl exerts moderate enhancement in the

rate and somewhat reduced propanal production. This observation is consistent with the

ammonium ion acting as an acid (proton) source at high temperature. Indeed, when

NH4Cl or HCl is used as an additive with NaReO4 it behaved similarly to NH4ReO4,

high yields of volatile products with selectivity for allyl alcohol. The NH4Cl additive

exhibited higher selectivity than HCl. Interestingly addition of NaCl to NH4ReO4

reduced the catalyst’s productivity while KCl showed no adverse effect. While our

additive studies results are cannot be fully rationalized at this time, they point to the

need for proton or an activating cation to be associated with the perrhenate anion to

make it catalytically viable. Similar observations with different cations have been noted

recently for the sulfite driven deoxygenation of glycols.7

25

Table 2.2 Effect of salt additive in oxorhenium-catalyzed deoxygenation of glycerol.

Catalyst Salt Additive Time (h) Conversion

(%)

Volatile Products Yield

(%)

MTO

NaCl 2 100 74 KCl 1.5 N/A 61 HCl 1 100 78

NH4Cl 1 100 78

NaReO4 NH4Cl 1 100 96

HCl 1 100 93

NH4ReO4 NaCl 2 N/A 42 KCl 1.5 100 91

Since 3-octanol has been used as a reducing agent,6 we tested its use and other

high boiling alcohols (1-heptanol, 1-cyclohexanol, 1,3-propanediol, 1,2-propanediol) as

solvents and potential hydrogen transfer agents in our reaction. The yields of volatile

products from glycerol with MTO as the catalyst after 1 h varied but in a narrow range

of 50-55% (Table 2.3). The exception was 1,3-propanediol, which gave lower

conversion and yield. 3-Octanol with NH4ReO4 as a catalyst afforded 74% of volatile

products with the ratio allyl alcohol : acrolein : propanal = 1.0 : 0.02 : 0.01. The

reactions with alcohol solvents yielded ketone and aldehyde that is less than would be

expected from the amount of allyl alcohol produced. Therefore, even in 3-octanol as a

solvent, transfer hydrogenation from glycerol was competitive with that from 3-octanol

(the sacrificial alcohol). In general, the use of alcohols improved the ratio of allyl

alcohol to the other two volatile products acrolein and propanal. It should also be noted

that a significant amount of alkene from the sacrificial alcohol was produced. This is

attributed to oxorhenium catalyzing alcohol dehydration under our reaction condition.13

26

Table 2.3 Effect of alcohol additive/solvent in oxorhenium-catalyzed deoxygenation of glycerol.

Catalyst amount

Alcohol additive

Glycerol conversion

(%)

Volatile products

yield from glycerol

(%)

Alkenes yield from

alcohol additive (%)

Carbonyl product

yield From alcohol (%)

NH4ReO4 5 mol%

3-Octanol 100 74 26 24

MTO 2 mol%

3-Octanol 100 52 26 24

MTO 2 mol%

1-Heptanol 100 55 0 38

MTO 2 mol%

1-Cyclo-hexanol

100 50 70 N/A

MTO 2 mol%

1,3-Propanediol

30 15 N/A N/A

MTO 2 mol%

1,2-Propanediol

100 55 N/A 58

The background tests for these sacrificial diols or alcohols brought some

interesting phenomena. 1,2-Propanediol with MTO would give 1,2-propanediol-

coordinated hemiacetal and hemiketal products (Scheme 2.2). Secondary alcohol like 3-

octanol with oxorhenium will be explained in Chapter 3, which involved Re0 and Re(IV)

oxide heterogeneous catalysis.

HO

OH MTOO

OO

OOH+ +

Ratio of Volatile Productsin Distillation Fraction

gas

1 : 0.3

Scheme 2.2 Oxo-Rhenium-Catalyzed deoxygenation of 1,2-propanediol.

27

2.3.3 Substrates Scope

MTO-Catalyzed transfer hydrogenation and deoxygenation was investigated for

other biomass-derived polyols meso-erythritol and meso-threitol (Table 2.4). The

reaction was found to be stereospecific. meso-Erythritol gives a reasonably high yield of

dihydrofuran (58%), which is higher than previously reported for the reaction of meso-

erythritol with formic acid (39%).4 meso-Threitol, on the other hand, gives low yields of

deoxygenated volatile product and is converted to (3S,4S)-tetrahydrofuran-3,4-diol.

Similarly, cis-1,2-cyclohexanediol is converted under solvent free conditions with MTO

to cyclohexene (91% yield) (Table 2.4 and Scheme 2.3), while trans-1,2-

cyclohexanediol did not react.

Table 2.4 Different substrates with 2 mol% MTO at 165 oC in 1 h.

Substrate Solvent Products (Yield)

meso-Erythritol None 2,5-Dihydrofuran (46%)

1-Heptanol 2,5-Dihydrofuran (58%)

meso-Threitol None (3S,4S)-Tetrahydrofuran-3,4-diol (1%)

1-Heptanol (3S,4S)-Tetrahydrofuran-3,4-diol (7%)

cis-1,2-Cyclohexanediol None Cyclohexene and 1,2-cyclohexanedion

(91%) trans-1,2-Cyclohexanediol None No reaction

Scheme 2.3 MTO-Catalyzed deoxygenation of cis-1,2-cyclohexanediol.

28

2.3.4 Kinetics and Intermediates Study

The reaction with glycerol was followed by quantifying yields at different time

intervals. A typical reaction profile is shown in Figure 2.5. It is evident that acrolein and

propanal are formed in parallel to allyl alcohol. The rate of formation of allyl alcohol

follows first-order kinetics and the observed rate constant showed depends on MTO: (at

215 ± 5 oC) k = 2.3 x 10-3 s-1 for 1 mol% MTO, and 3.5 x 10-3 s-1 for 2 mol% MTO

(Figure 2.6). Therefore, the experimentally determined rate law is d[allyl alcohol]/dt = -

d[glycerol]/dt = k [glycerol][MTO]T. Based on previous studies by the groups of

Andrews,14 Espenson,15 and Nicholas,7 and our results herein, we propose allyl alcohol

formation from rhenium diolate via hydride transfer from a second molecule of glycerol

according to the kinetic Scheme 2.4. The corresponding steady-state rate law is given in

Eq. 1 below:

21

21

211

221 ][[Re]

][][

][[Re]][

kk

glycerolkk

glycerolkglycerolkk

glycerolkk

dt

olallylalcohd TT

(Eq. 1)

In the limit of (k1 + k2) [glycerol] >> k-1, the rate simplifies to that observed

experimentally. While it would be useful to demonstrate second-order dependence on

[glycerol] at lower concentrations, such experiments have been difficult as the reaction

is significantly retarded in inert solvents.

in

Figure 2.

Both Es

n our hands t

.5 Reaction ppropan

ally

lalc

ohol

/mm

ol

spenson15 an

the reaction

profile for gnal. In condit

0.0

1.0

2.0

3.0

4.0

5.0

5 10

ally

l alc

ohol

/ mm

ol

2 mol%

kobs

=

Figure 2.6 M

nd Nicholas7

of MTO wit

lycerol convtion with 2 m

0 15 20

t/ m

% MTO

3.5 x 10-3 s-1

MTO-Depen

7 have obser

th glycerol a

version to allmol% MTO

25 30

min

1 mol% MTO

kobs

= 2.3 x 10-3 s

ndence kinet

rved rhenium

after heating

lyl alcohol, aat 165 oC.

35

s-1

tic fitting.

m diolate wi

g for a brief 5

acrolein, and

ith glycols. A

50 s followe

29

d

Also

d by

30

rapid cooling and addition of CDCl3 (Figure 2.7) or d6-DMSO (Figure 2.8) shows

rhenium diolate formation by 1H NMR. Furthermore, heating the diolate in d6-DMSO to

elevated temperature produces allylic alcohol. Another point of interest is to investigate

the effect of water on the reaction as it is a by-product of diolate formation. When the

glycerol reaction is conducted in the presence of 4 Å molecular sieves the volatile

products yield was increased to 99% with enhanced selectivity for allylic alcohol (allylic

alcohol : acrolein : propanal = 1:0.06:0). The reaction time, however, in the presence of

molecular sieves was longer (2.5 versus 1 h to reach 100% conversion). All of these

observations are consistent with the proposed reaction mechanism in session 2.3.6.

Scheme 2.4 Kinetic steps in MTO-catalyzed deoxygenation of glycerol.

31

012345678910111213f1 (ppm)

0

10

20

30

40

50

60

70

80

90

100

Re - diolate

Glycerol

MTO

Methyl groups in Ia & Ib

Figure 2.7 1H NMR of Re-diolate in CDCl3. Condition: glycerol:MTO = 2:1 in molar ratio at 165 oC for 30 s, cooled, and added CDCl3.

32

-1012345678910111213f1 (ppm)

-5

0

5

10

15

20

25

30

35

40

45

GlycerolMTO

Re - diolateMethyl groupin Ia & Ib

Figure 2.8 1H NMR of Re-diolate in d6-DMSO. Condition: glycerol:MTO = 1:1 in molar ratio at 165 oC for 50 s, cooled, and added d6-DMSO.

2.3.5 Isotopic Labeling Experiment

Glycerol-(OD)3 and d5-glycerol-(OH)3 were used and their reaction rates were

compared with normal H5-glycerol-(OH)3 to gain insight into the mechanism and the

rate determining step. MTO was employed as the catalyst in these isotopic labeling

experiments (Scheme 2.5). The times it took the reaction to reach completion for each of

the labelled glycerol substrates were compared to obtain kinetic isotope effects (KIE).

Deterium labeling on the alcohol groups in glycerol showed no KIE while d5-glycerol-

(OH)3 took 3.5 h to reach completion (100% conversion), corresponding to a KIE of ca.

33

2.4. Furthermore, the product distributions for glycerol-(OD)3 and d5-glycerol-(OH)3

were comparable giving allyl alcohol : acrolein : propoanal = 1 : 0.26 : 0.07. In both

deuterium labelled substrates, the amount of propanal was less than that observed with

normal H5-glycerol-(OH)3 that mentioned in session 2.3.1. Nevertheless, the KIE

observed for d5-glycerol-(OH)3 is reproducible, so the primary KIE indicates that the C-

H/D bond of glycerol is involved in the rate determining step.

Scheme 2.5 Isotopic labeling experiments.

2.3.6 Mechanism

An important mechanistic question is the parallel pathway in which acrolein and

propanal are produced. It is feasible that acrolein results from the oxidation of allyl

alcohol. There were two control experiments can rule out this possibility: 1) When the

reaction is run under Ar instead of open air the amount of acrolein and propanal did not

change; 2) Allyl alcohol and MTO does not produce acrolein at the same conditions. In

Figure 2.5 the first-order kinetic profiles for allyl alcohol formation and the KIE are

consistent with the formation of a Re diolate (Ia) followed by transfer hydrogenation as

34

detailed in Scheme 2.6. Intermediate Ia acts as a bifunctional catalyst with the oxo ligand

as a hydride acceptor and the alkoxide as a proton acceptor.

Scheme 2.6 Proposed mechanism for the formation of allyl alcohol, propanal, and acrolein.

Bullock showed that 1,2-propanediol can produce propanal.16 We hypothesize that

propanal and acrolein are produced from side reactions involving intermediate Ia to give

1,2-propanediol and 1,3-propanediol. Indeed, control experiments with these diols

35

afforded 2-ethyl-4-methyl-1,3-dioxolane (resulting from the condensation of propanal

and 1,2-propanediol) and 2-vinyl-1,3-dioxane (resulting from the condensation of

acrolein and 1,3-propanediol) (Scheme 2.7). In the case of the glycerol reaction 1,2- and

1,3-propanediol do not accumulate to give the condensation products. Furthermore, any

small condensation products from aldehyde and glycerol remain in the residue as non-

volatiles.

Scheme 2.7 MTO-Catalyzed glycerol reaction by using 1,3-propanediol or 1,2-propanediol.

2.4 Conclusion

In sum, MTO and NH4ReO4 catalyzed the conversion of glycerol to allyl alcohol

under neat conditions at 165 oC. Propanal and acrolein are the minor volatile products.

Dihydroxyacetone is the oxidation product and it remains in the reaction flask because

of its low vapor pressure. Kinetic isotope effect in conjunction with control experiments

provided insights with regards to the reaction mechanism with MTO. Allyl alcohol is

formed via transfer hydrogention from glycerol to a rhenium-diolate complex followed

36

by rapid alkene extrusion from the resulting rhenium(V) diolate, a reaction with ample

literature precedence.14,15 Furthermore, our investigations with different perrhenate salts

point to the requirement of an activating “proton or cation,” which can be hypothesized

to afford ZO-ReO3 (where Z = H+, Na+, K+, or NH4+) as the viable catalytic core. The

use of high boiling point alcohol facilitates the reaction and the alcohol can serve as a

hydrogen transfer agent. Nevertheless, the use of alcohol does not drastically increase

the yields of allylic alcohol as transfer hydrogenation from glycerol itself remains

kinetically competitive. This reaction can be employed successfully with other biomass-

derived polyols such as meso-erythritol.

37

2.5 References

(1) Wilson, E. K. Chem. Eng. News 2002, 80, 46.

(2) Christoph, R.; Schmidt, B.; Steinberner, U.; Dilla, W.; Karinen, R. Glycerol, in Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH, Weinheim, 2006, pp. 1.

(3) Krahling, L.; Krey, J.; Jakobson, G.; Grolig, J.; Miksche, L. Allyl Compounds, in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002, pp. 447.

(4) Arceo, E.; Marsden, P.; Bergman, R. G.; Ellman, J. A. Chem. Commun. 2009, 3357.

(5) Arceo, E.; Ellman, J. A.; Bergman, R. G. ChemSusChem 2010, 3, 811.

(6) Arceo, E.; Ellman, J. A.; Bergman, R. G. J. Am. Chem. Soc. 2010, 132, 11408.

(7) Ahmad, I.; Chapman, G.; Nicholas, K. M. Organometallics 2011, 30, 2810.

(8) Ziegler, J. E.; Zdilla, M. J.; Evans, A. J.; Abu-Omar, M. M. Inorg. Chem. 2009, 48, 9998.

(9) Thibault, M. E.; DiMondo, D. V.; Jennings, M.; Abdelnur, P. V.; Eberlin, M. N.; Schlaf, M. Green Chem. 2011, 13, 357.

(10) Ghosh, P.; Fagan, P. J.; Marshall, W. J.; Hauptman, E.; Bullock, R. M. Inorg. Chem. 2009, 48, 6490.

(11) Di Mondo, D.; Ashok, D.; Waldie, F.; Schrier, N.; Morrison, M.; Schlaf, M. ACS Catal. 2011, 1, 355.

(12) Liu, Y.; Tuysuz, H.; Jia, C. J.; Schwickardi, M.; Rinaldi, R.; Lu, A. H.; Schmidt, W.; Schuth, F. Chem. Commun. 2010, 46, 1238.

(13) Korstanje, T. J.; Jastrzebski, J. T. B. H.; Gebbink, R. J. M. K. ChemSusChem 2010, 3, 695.

(14) Cook, G. K.; Andrews, M. A. J. Am. Chem. Soc. 1996, 118, 9448.

(15) Zou, Z.; Espenson, J. H. J. Org. Chem. 1996, 61, 324.

(16) Schlaf, M.; Ghosh, P. J.; Fagan, E.; Hauptman, E.; Bullock R. M. Angew. Chem. 2001, 113, 4005.

38

CHAPTER 3. REUSABLE UNSUPPORTED RHENIUM NANOCRYSTALLINE CATALYST FOR ACCEPTORLESS DEHYDROGENATION OF ALCOHOLS

3.1 Introduction

Acceptorless dehydrogenation (AD) of alcohols to produces hydrogen gas and

carbonyl products (aldehydes or ketones), which is an atom-economy and efficient

reaction. Catalytic AD reactions have been attracted significant attention recently, since

they can avoid stoichiometric in/organic wastes and the hydrogen by-product is a green

gas that can be easily separated and recycled.1 There were many reports about

homogeneous catalytic AD,2-4 but a few papers focus on heterogeneous catalytic AD.5

Comparing with homogeneous system that usually requires acid or base additives, well-

designed ligands, and difficult separation, heterogeneous catalysts bring an easier way

for both laboratory and industrial reactions. Heterogeneous AD reactions are dependent

on the properties of metal and support,6-8 which needs sophisticated techniques to

characterize the metal catalyst. In this chapter, one rhenium nanocrystalline particles (Re

NPs) that can efficiently catalyze AD of alcohols were synthesized and thoroughly

characterized by transmission electron microscopy (TEM), Re K-edge X-ray absorption

near-edge structure (XANES), X-ray absorption fine structure (EXAFS), X-ray

photoelectron spectroscopy (XPS), and powder X-ray diffraction (PXRD).

39

3.2 Experimental


All commercial materials were used as received without further purification unless

specified otherwise. Methyltrioxorhenium(VII) (MTO), NH4ReO4, ReO3, Re2O7 were

purchased from Strem Chemicals. MTO was purified by sublimation at 40 °C (1 mm

Hg). LiAlD4 was purchased from Cambridge Isotope Laboratories. (Ph3SiO)ReO39 and

different deuterated 3-octanols were synthesized according to literature methods10-12. All

alcohols and other standard reagents were purchased from Aldrich or Alfa Aesar.

General catalytic procedure for secondary aliphatic alcohols dehydrogenation is: 3-

Octanol (15 mmol) and NH4ReO4 (0.3 mmol, 2 mol%) were mixed and refluxed in 15-

ml flask at room temperature with stir bar, Teflon sleeve, and a reflux condenser open to

air. The temperature was measured by a thermometer immersed in the reaction mixture.

The reaction system was heated at 180 °C over a preheated Armor beads bath for 10 h. If

the reaction is run in neat alcohol, for 1H NMR and 13C NMR analysis aliquots were

directly collected, cooled-down and filtered from precipitated catalyst. If

hexamethylbenzene was used as a solvent, the catalyst was filtered with a small amount

of silica gel, and the products and solvent were run on a silica chromatography column

with 1:20 = acetyl acetate:hexane eluent to separate the ketone product.

40


1H, 2H, and 13C-NMR spectra were recorded on Bruker Avance ARX-400

spectrometers, Bruker Avance DRX-500, or Varian Inova-300. NMR data was plotted

using MestReNova software. GC analysis was performed on Agilent 6890 Series gas

chromatograph system. GC-MS analysis was performed using a Pegasus 4D gas

chromatography/gas chromatography time-of-flight mass spectrometer (GCxGC/TOF-

MS, LECO Corporation, St. Joseph, MI). IR was carried on Thermo-Nicolet Nexus 470

FTIR. H2 gas analysis was performed on a residual gas analyzer (RGA) model RGA 100

(Stanford Research Systems). XRD analysis was performed by H&M Analytical

Services, Inc. on Panalytical X’pert MPD diffractometer using Cu radiation at

45KV/40ma. Scans were run over the range of 10o – 90o with a step size of 0.0157o and

a counting time of 1,500 sec/step. HRTEM/TEM were done on a Titan 80-300 keV

Field-Emission Environmental Transmission Electron Microscope/Scanning

Transmission Electron Microscope from the FEI Corporation at Purdue University Birck

Nanotechnology Center.

X-ray Photoelectron Spectroscopy (XPS) data was collected on a Kratos Axis

Ultra DLD spectrometer using an Al Kα monochromic X-ray radiation. The survey

spectra and the high-resolution spectra of the Re 4f, C 1s, N 1s, and O 1s core levels

were collected at photoemission angle of 0o, with respect to the surface normal, at fixed

analyzer pass energy of 160 and 20 eV, respectively. A build in Kratos charge-

neutralizer was used for charge compensation. The charge correction was performed

setting the C 1s peak at 285.1 eV. The resolution of the spectrometer at the 20 eV pass

energy was approximately 0.4 eV and it was measured as full width at half maximum

41

(FWHM) of the Ag 3d5/2 peak. The CasaXPS software package, version 2.3.16dev85,

was used for data analysis. Curve-fitting was done after subtraction of the Linear,

Shirley, or Tougaard type background assuming asymmetric Gaussian/Lorenzian line

shape. A glove box (Innovative Technology, Inc.) is attached directly to a load-lock of

the Kratos spectrometer. This allowed us to perform the final washing under argon

atmosphere and transfer to vacuum without contact with air.

XAS was performed in Advanced Photon Source in Argonne National Laboratory.

MRCAT 10ID beamline provides the bright X-ray source that can get through the thick

reactor to get sufficient signal at Re L3 metal K edge with low signal to noise ratio.


3.3.1 Catalyst Characterization

The black solid powder was filtered out after NH4ReO4 reacted with 3-octanol.

The black precipitate and filtrate were both added new fresh 3-octanol. Filtrate had no

reaction, while the black solid still obtain comparably catalytic ability with previous

reaction. The black solid has been characterized by TEM, XPS, XAS, XRD, and FT-IR

like below.

3.3.1.1 Transmission Electron Microscopy (TEM)

Open to air at 180 oC, Re NP of 2 nm size (Figure 3.1a) were prepared from

NH4ReO4 in neat 3-octanol. Re NP TEM samples were prepared without any support,

an

su

[0

cr

as

sh

T

(2

b

F

nd diluted i

upports. Fig

0001] zone

rystallograph

s well as th

hows this Re

TEM images

2 nm) make

ecause they

Figure 3.1 (aT

in acetone

gure 3.1b sh

axis. An

hic planes in

he FFT (fast

e NP size di

(5 nm and

up the nano

are suscepti

a) TEM imaghe inset in (b

and evapora

hows enlarge

inter-planar

n hexagonal

t Fourier tra

istribution is

20 nm resol

clusters. Th

ble to oxidat

ge of Re NPsb) shows the

ated over C

ed HRTEM

spacing of

structure is

ansform) pat

s mainly 2 nm

lution TEM

he surface or

tion upon ex

s (b) Enlargee FFT pattern

Cu grids wi

image of a

f 2.39 Å w

measured b

ttern (inset

m, which ba

images). Ap

r outside Re

xposure to ai

ed HRTEM n of a single

ithout interf

a Re nanocr

which belon

based on the

of Figure 3

ased on 100

pproximatel

atoms are ir

ir to give rhe

image of hee Re nanopar

ference of o

rystal aligne

ngs to {1-1

HRTEM im

.1b). Figure

Re NP parti

ly 200 Re at

rregular in sh

enium(IV) o

xagonal Re rticle.

42

other

ed to

100}

mage

e 3.2

icles

toms

hape

oxide.

NPs.

43

Figure 3.2 Re NPs particle size distribution histogram.

3.3.1.2 X-Ray Photoelectron Spectroscopy (XPS)

The XPS spectrum of the Re NP is shown in Figure 3.3. Only, the photoemission

peaks of rhenium, oxygen, carbon, and nitrogen were detected. The atomic

concentrations of the elements are shown in Table 3.1. In order to identify the chemical

state of rhenium, the high-resolution Re 4f spectra were collected. Figure 3.3 (b) shows

the Re 4f spectra obtained from the Re NP washed in an Ar filled glove box with

acetone and dried over vacuum. The spectrum from a Re polycrystalline foil is provided

as a reference. The spectra obtained from the Re NP samples have a complex shape and

were curve-fitted with four spin-orbital doublets. The spin-orbital splitting and the

0 2 4 6 8 10

Fre

qu

en

cy

(%

)

Size (nm)

0

5

10

15

20

25

44

intensity ratio were fixed to be 2.4 eV and 4:3, respectively. The curve-fitting analysis

showed the sample manipulated under argon contains Re(0) as characterized by the

sharp Re 4f7/2 peak at 40.8 eV. This value is 0.5 eV higher than the corresponding

number obtained from Re foil. The 0.5 eV shift is consistent with the particle size effect,

since TEM data before shows the average size of the Re NP is approximately 2 nm. On

the other hand, this high binding energy shift cannot be assigned to oxidation because of

the peak width. Thus, the FWHM of the Re 4f7/2 peak at 40.8 eV is less than 0.7 eV,

whereas any oxidation state shows the FWHM over 1.6 eV. Two oxidation states are

detected that show the Re 4f7/2 peaks at 41.9 and 43.1 eV. The former 41.9 eV is the

ReO2 surface oxide that covered metallic Re NP. The Re 4f7/2 peak at 43.1 eV is the

bulky ReO2 phase between the ReO2 oxide and Re NP. The photoemission peaks of

rhenium, oxygen, carbon, and nitrogen were detected. The atomic concentrations of the

elements are shown in Table 3.1. In comparison, the Re NP that has been exposed to air

did not show a Re 4f7/2 peak at 40.8 eV and only ReO4 oxide is observed on the surface.

FFigure 3.3 R

Table 3.1 Adried (Air),

Sam

AAFo

e 4f spectra

Atomic conceand a Re fo

obtai

mple C 1s (%)

Ar 50.9 ir 30.7

oil 54.3

obtained fround

entrations ofil (Foil). Theined by the c

N 1s (%)

O 1(%

6.4 25.6.0 41.3.0 27.

om (a) Re poder Ar (Ar sa

f the elemente fractions ocurve fitting

1s %)

Re 4f(%)

1 17.6 2 22.1 3 15.4

olycrystallineample).

ts obtained fof the differe

of the Re 4f

Re0 (%)

Re4+

(%)

15 310 48

77 3

e foil and (b

from Ar-protent Re oxidatf spectra.

+ )

Re4x+ (%)

50 30 20

) Re NP han

tected (Ar), tion states w

Re7+ (%)

4 21 0

45

ndled

air-were

3

sa

d

pr

th

so

st

a

in

fi

R

th

.3.1.3 X-R

The ex

ample loadin

iluting the p

ressed into a

he sample in

X-ray a

oftware. X

tandard meth

sample of N

n Figure 3.4

irst peak in

ReO2. Howev

he XANES s

Ray Absorptio

situ XAS

ng was calc

pure sample

a self-suppor

n air at room

absorption s

X-ray absorp

hods for bac

NH4ReO4 fo

. The edge e

the first der

ver, the shap

shows 45% o

Figure

on Spectrosc

experiment

culated to y

with an ap

rting wafer i

temperature

spectra were

ption near

ckground sub

r Re7+ (pink

energy was

rivative. The

pe is slightly

of the sampl

e 3.4 Re L3 X

copy (XAS)

was conduc

yield an abs

ppropriate am

in a 6 well sa

e.

e fit using

edge (XAN

btraction. Th

k), Re foil fo

determined

e K-edge en

different illu

e being ReO

XANES from

cted in a qu

sorbance (µx

mount of Si

ample holde

standard pr

NES) spectr

he XANES

or Re0 (red),

by the posit

nergy of the

ustrating add

O2 and 55% R

m 10.50 to 1

uartz tube r

x) of approx

O2. The mix

er. Scans we

ractices with

a were obt

were energy

, and ReIVO2

tion of the m

Re NP is si

ditional type

Re0.

0.57 keV.

reactor. The

ximately 2.0

xed sample

ere conducte

h WINXAS

tained by u

y calibrated w

2 for Re4+ (b

maximum of

imilar to tha

es of Re. A f

46

e Re

0 by

was

d on

3.1

using

with

blue)

f the

at of

fit of

(E

am

R

in

th

Å

n

R

In

S

ox

o

Fourier

EXAFS) dat

mplitudes fr

Re7+, 4 at 1.7

ndicates besi

he oxidation

Å) is longer

anoparticles

ReO2 and 2.8

n summary,

ince many

xidation-stat

f surface Re

transforms

ta were fit u

rom referenc

74 Å) and Re

ides Re-O b

takes place

than that in

can be esti

8 in Re NP. T

Re NP are

metallic na

te stable me

to rhenium(

Figur

(FT) of k2-w

using experi

ce compoun

e-Re (Re foi

bonds (2.05

at the surfac

n bulk ReO

imated from

Thus the per

ca. 2 nm in

anoparticles

etal-oxide,14

(IV) oxide.

re 3.5 EXAF

weighted ex

imentally ob

nds for Re-O

il, 12 at 2.75

Å) metallic

ce of the Re

O2 (1.98 Å).

m the numbe

rcentage of R

n size comp

are known

it is not sur

FS Fourier tr

xtended X-ra

btained phas

O (ReO2, Re

5 Å) and sho

Re-Re bon

NP, the Re-

.13 The frac

er of Re-O b

ReO2 in Re N

posed of Re0

to oxidize

rprising that

ransforms of

ay absorptio

se shifts and

e4+, 6 at 1.9

owed in Fig

nds (2.70 Å)

-O in the nan

ction of oxid

bonds, whic

NP is 2.8/6 =

0 core with

in air to g

t our Re NP

f Re NPs.

on fine struc

d backscatte

98 Å, NH4R

ure 3.5. EXA

. Assuming

noparticles (

dized Re in

ch are 6 in b

= 0.47 (i.e. 4

surface ReIV

give the low

show oxida

47

cture

ering

ReO4,

AFS

that

(2.05

n the

bulk

47%).

VO2.

west

ation

3

th

d

3

8

N

R

m

.3.1.4 X-R

Recycle

he powder-X

omain sizes

.3.1.5 Four

The Re

0% convers

NH4ReO4 and

Re NP show

multiple bond

Ray Diffractio

ed Re cataly

XRD (Figure

is 293 Å. Th

rier Transfor

NP can be r

ion of 3-oct

d 1st and 10

ws no Re=O

ds 800-1000

on (XRD)

st from 3-oc

e 3.6), in whi

his confirms

Figure 3.6

rm Infrared S

recycled up t

tanol substra

0th recycled

stretching

cm-1.

ctanol reactio

ich quantitat

s hexagonal R

Powder-XR

Spectroscop

to 10 times,

ate. The com

d Re NP cata

frequencies

on showed th

tive phase an

Re crystals (

RD of Re NP

py (FT-IR)

and previou

mparison of

alyst (Figure

in the char

he reflection

nalysis (wt %

(P63/mmc).

Ps.

us 8 times ca

f FT-IR spec

e 3.7) show

racteristic re

n in 2-, 37.6

%) is 92.5%

an obtain aro

ctra of the f

wed that recy

egion for R

48

6o in

% and

ound

fresh

ycled

Re=O

49

Figure 3.7 FT-IR for (a) fresh NH4ReO4, (b) 1st-recycled NH4ReO4, and (c) 10th-recycled NH4ReO4.

wavenumbers3500 3000 2500 2000 1500 1000

0

0.2

0.4

0.6

871.

7

1396

2819

3186

fresh NH4ReO4

wavenumbers3500 3000 2500 2000 1500 1000

0

0.2

0.4

0.6 837

1294

1377

1390

1408

1423

1st recycled NH4ReO4

wavenumbers3500 3000 2500 2000 1500 1000

0

0.25

0.5

0.75

868

962

1292

1369

1402

10th recycled NH4ReO4

(a)

(b)

(c)

g

kn

pr

as

w

rh

ca

st

Several

ave high co

nown activit

roduct with

s a pre-catal

when starting

henium oxid

atalyst but a

tarting pre-c

precatalysts

nversion rat

ty of homog

MTO is 3-o

lyst, which a

g with (Ph3S

des such Re

at a lower eff

atalyst to pro

Figure 3

3.3.2

s have been

te, its slightl

geneous MT

octene (17%)

also gave deh

SiO)ReO3 w

e2O7 and Re

ficacy than N

oduce Re NP

3.8 Different

2 Catalyst

tested. Alth

ly lower sele

TO for alcoh

). Similar be

hydration by

was also sig

eO3 are suita

NH4ReO4 or

P, which is c

t Re pre-cata

s Usage

hough methy

ectivity (83%

hol dehydrati

ehavior was

y-product. H

gnificantly lo

able precurs

r MTO. NH4

consistent w

alyst at 180 o

yltrioxorheni

% 3-octanon

ion reaction

observed fo

However, the

ower (28%

sors for mak

4ReO4 appea

with a previou

oC for 10 h.

ium (MTO)

ne) is due to

n. The major

or (Ph3SiO)R

e conversion

in 10 h). O

king the Re

ars to be the

us report.15

50

also

o the

r by-

ReO3

n rate

Other

e NP

best

51

3.3.3 Substrates Scope

Dehydrogenation of various alcohols was investigated and the results summarized

in Table 3.2. The reaction condition is: NH4ReO4 0.1 mmol and 5 mmol alcohol were

mixed and refluxed at 180 oC for 10 h. The conversion and selectivity were confirmed by

GC, GC-MS, and NMR. Secondary aliphatic alcohols (entries 1-3) all afforded

quantitative 100% conversion and high > 99% selectivity. Primary alcohols such as 1-

heptanol (entry 4a) did not convert to aliphatic aldehydes. Using 2-heptanol as an

additive (1:1 molar ratio to 1-heptanol, entry 4b) also gave low conversion (5%) of 1-

heptanal and even low conversion (35%) of 2-heptanol to 2-heptanone. This result is

indicative of primary alcohols and their aldehyde product as inhibitors of the AD reaction

of secondary alcohols. Indeed mixing 1-heptanal and 2-heptanol in 1:1 molar ratio (entry

3b), gave low conversion (33%) to 2-heptanone. Dehydrogenation of cyclohexanol (entry

5, the molar ratio of additive hexamethylbenzene to substrate alcohol is 2:1, same to entry

6) gave lower conversion (45%) than other C7- or C8-cyclo-alcohols (entry 7 & 8, 100%).

The reason might be due to the difference in the C-H bond strength between these

molecules. Dilution of cyclohexanol with hexamethylbenzene as a solvent reduced the

Aldol side product. While hexamethylbenzene is a solid at room temperature, it is a liquid

at the reaction temperature of 180 oC. Upon completion of the reaction and cooling, the

mixture was dissolved in CH2Cl2 for GC and GC-MS analyses, then concentrated and run

through a silica column to separate cyclohexanone and recycle hexamethylbenzene.

Dehydrogenation of the unconjugated alkene alcohol 6-methyl-5-hepten-2-ol (entry 9)

gave 100% conversion and moderate selectivity for 6-methyl-5-hepten-2-one (80% yield).

Other side products included 6-methyl-2-heptanone (15%) and 6-methyl-6-en-2-heptone

52

Table 3.2 Dehydrogenation of alcohols by NH4ReO4.

Entry Substrate Additives Product Conversion

(%)

Selectivity (%)

(isolated)

1 - 100 >99

2 - 100 >99

3(a)

-

100 >99

3(b) 1-Heptanal 33 33

4(a)

-

0 0

4(b) 2-Heptanol 5 5

5 Hexamethylbenzene

45 44 (42)

6

Hexamethylbenzene

40 40 (39)

7

- 100 99

8

-

100 99

9 - 100 80 (78)

10

-

25 2

11 Hexamethylbenzene

100 61

(5%). Conjugated systems like benzyl alcohol (entry 11) must be proceeding through a

different pathway/mechanism because the reaction is not strictly AD. The reaction of the

53

Re NP catalyst with benzyl alcohol within 3 hours produces only ~20% H2 and a large

amount of toluene (38%) from complete deoxygenation of the alcohol.

3.3.4 Kinetic Study

The catalyst NH4ReO4 concentration dependence experiments were demonstrated

in Figure 3.9. The reaction conditions were: NH4ReO4 (0.2, or 0.4, or 0.6 mmol)

respectively and neat 20 mmol 3-octanol were mixed and refluxed at 180 oC for 10 h.

The results were based on 1H NMR. The reaction rates were not perfectly linear based

on NH4ReO4 concentration, but the rates were still dependent on catalyst concentrations.

Figure 3.9 Time profile from aliquots for different NH4ReO4 concentrations.

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

0 2 4 6 8 10

Different Concentration NH 4ReO4 in 3-Octanol Reaction

2 mol% (0.125846 M) NH4ReO44 mol% (0.251692 M) NH4ReO46 mol% (0.377538 M) NH4ReO4

Time [h]

54

The continuous reaction had done after 10-hour 3-octanol reaction in Figure 3.10.

In 10-hour point, reaction conversion is 100% and yield is >99%, the addition of fresh 3-

octanol to the system continues to convert 3-octanol to 3-octanone. Although the

substrate 3-octanol was diluted in the second cycle, the reaction rate was increased,

which indicated that more active catalyst was formed in the first batch reaction.

Figure 3.10 Continuous reaction of 2 mol% NH4ReO4 with neat 3-octanol reaction.

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

0 5 10 15

2 mol% NH4ReO4 w/ Two Portion 3-Octanol

3-Octanol Conversion

Time [h]

3

co

th

re

0

an

N

The effe

-octanone is

onversion ra

Since th

he alcohol m

eduction of r

.276 V vs. N

nd do not ag

NP is isolated

fect of tempe

s shown in F

ate while reta

Figure

he Re NP are

must be suff

rhenium oxid

NHE). Becau

ggregate, m

d by evapora

3.3.5

erature on th

Figure 3.11.

aining produ

3.11 Tempe

3.3.6

e prepared in

ficiently red

des to the m

use of their

aking their

ation of the 3

Temperatu

he conversio

Temperatur

uct selectivity

erature effect

Catalyst R

n neat 3-octa

ducing. Inde

metal is exerg

small size, t

separation b

3-octanone p

ure Effect

on rate for 3-

res of 180 o

y for 3-octan

t in 3-octano

Recycling

anol solution

ed under ac

gonic (ReO4-

the Re NP d

by filtration

product unde

-octanol deh

oC or higher

none.

ol reaction.

n starting fro

cidic and ne

-/Re = 0.375

disperse eve

difficult. Th

er vacuum. R

hydrogenatio

r give maxim

om rhenium(

eutral condit

5 and ReO2/R

enly in 3-oct

herefore, the

Remarkably

55

on to

mum

(VII),

tions

Re =

tanol

e Re

y, the

56

recovered Re NP catalyst can be recycled several times without major loss in activity

and selectivity (Table 3.3). Experimental procedure for the reuse of NH4ReO4 in 3-

octanol acceptorless dehydrogenation is: in a 15 ml round bottom flask, 3-octanol (15

mmol) and NH4ReO4 (0.3 mmol) were mixed and refluxed at 180 oC for 10 h. To make

sure the accurate of conversion and yield calculation, measuring the total weight

difference of flask and recycled catalyst is necessary. The product was removed through

reduced-pressure distillation, and the conversion and yield were determined by 1H NMR

and weight measurement with internal standard 1,3,5-trimethoxybenzene. The black

residue was dried under vacuum. A new reaction is started by adding same amount of 3-

octanol (15 mmol) to the flask, the mixture was stirred and heated at 180 oC for 10 h,

and repeated as described above for each subsequent cycle.

Table 3.3 Dehydrogenation of 3-octanol over recycled Re NPs catalyst.

Reuse 1st 2nd 3rd 4th 5th 6th 7th 8th

Conversion (%)

100 100 97 96 90 83 82 79

Filtering out the recycled catalyst without mass loss is difficult since the Re

nanoparticle (NP) is 2 nm, so direct evaporation of 3-octanone and any unreacted 3-

octanol is the best way to fully recycle Re NP catalyst. Two experiments were also

performed to test whether the active catalyst is homogeneous or heterogeneous:

57

1. Mercury poisoning reaction: 3-octanol (10 mmol), NH4ReO4 (0.2 mmol)

and mercury (2 mmol) were mixed and refluxed under an Ar balloon at 180 oC for 10 h.

The reaction was cooled down to room temperature and product yields were determined

by 1H NMR. The yield was comparable to reactions without mercury.

2. Filtration experiment: 3-octanol (20 mmol), NH4ReO4 (0.4 mmol) were

mixed and refluxed at 180 oC for 8 h. The hot reaction solution was filtered through a

warmed up 1-cm-celite glass frit. Mixing the recycled filtrate with a fresh sample of 3-

octanol (10 mmol) followed by refluxing for 48 h gave no significant amount 3-

octanone demonstrating no catalytic activity from the homogeneous filtrate solution. So

these two experiments confirmed that the AD reaction catalyzed by Re NP is

heterogeneous.

3.3.7 Hydrogen Characterization and Quantification

Under argon atmosphere, a 5 ml flask was charged with 3-octanol (0.65 g, 5.0

mmol) and NH4ReO4 (26.8 mg, 0.1 mmol). Another flask under argon atmosphere was

charged with 1-decene (0.70 g, 5.0 mmol), Wilkinson’s catalyst (92.5 mg, 0.1 mmol)

and 8 ml benzene. The two flasks were connected through a rubber tube and a needle.

The yield of decane was determined by GC analysis through calibration curve for

decane. The yield of 3-octanone was determined by 1H NMR by using 1,3,5-

trimethoxybenzene as an internal standard. After conversion of 3-octanol to 3-octanone

(10 h), the decane yield in the second flask was 95%. This demonstrates quantitative H2

gas production from 3-octanol AD reaction.

58

3.3.8 Kinetic Isotopic Effect

The Re NPs quantitatively convert secondary aliphatic alcohols to ketones through

catalytic acceptorless dehydrogenation (AD). To explore the mechanism of Re NP

catalyzed AD reactions of alcohols, we used deuterated 3-octanol substrates as shown in

Scheme 3.1. The reaction conditions were: NH4ReO4 0.2 mmol and pure 20 mmol

alcohol or deuterated alcohols were mixed and refluxed at 180 oC for 180 min. Result

based on 1H NMR or 2D NMR is the average of two runs. Deuteration of the alcohol

hydrogen –OH versus –OD showed no kinetic isotope effect (KIE). Surprisingly,

deuteration of the -CH also showed no KIE. However, when -CH was replaced with -

CD, an inverse KIE of 0.38 was observed (Scheme 3.1). This result indicates that the -

CH is involved and since the KIE is an inverse effect, it must be resulting from a

combination of a prior-equilibrium complex formation and a kinetic C-H cleavage step.

OH

D

OH

D D D D

0.38

3-D-3-Octanol 2,2,4,4-D4-3-Octanol

1.10KIE

OD

D

3-D-(OD)-Octanol

1.05

Scheme 3.1 Deuterated alcohol substrates for KIE experiments.

3.3.9 Mechanism

According to above experiments, we can assumed that an alkane complex with -

CH is formed with rhenium(0) NP followed by C-H activation (Scheme 3.2). This

mechanistic proposal is corroborated by the observation of C-H/D scrambling in the 3-

59

octanone product as well as a previously suggested agostic interaction in heterogeneous

rhenium catalysts.16 Further support for C-H scrambling and involvement of the -CH

was obtained by detecting HD and D2 products in mass spectrometry analyses in RGA

(Figure 3.12). In Scheme 3.2, a Re0 is used to represent the active atoms on the Re

nanocluster. The generated proton (in grey circle) during the course of the reaction is

most likely accommodated on an oxo-Re site or picked up by Re hydride to release

dihydrogen. Additionally the -abstraction mechanism was tested with 2,2,4,4-

tetramethyl-3-pentanol, which has no -CH (entry 10 in Table 3.2). The conversion was

low with the corresponding ketone product as the minor product (2%), and the major

product 2,2-dimethyl-4-methylhexene (22%), which must be resulting from a

sigmatropic methyl shift in association with alcohol dehydration.

60

Figure 3.12 Residual gas analysis (RGA) of compounds 1, 2, and 3.

0.0 100

2.0 10-6

4.0 10-6

6.0 10-6

8.0 10-6

1.0 10-5

1.2 10-5

1.4 10-5

0 50 100 150 200 250 300

H2HDD2Ar

Time (s)

0.0 100

2.0 10-6

4.0 10-6

6.0 10-6

8.0 10-6

1.0 10-5

1.2 10-5

0 50 100 150 200 250 300

H2 HD D2 Argon

Time (s)

0.0 100

2.0 10-6

4.0 10-6

6.0 10-6

8.0 10-6

1.0 10-5

1.2 10-5

0 50 100 150 200 250 300

H2HDD2Argon

Time (s)

61

Re0

ReH

O

HH

II ReH

O

HH

II

ReH

O

HII

-

H

ReH

H+

ReH

O

HIV

H

OH

H2

OHO

Scheme 3.2 Proposed acceptroless dehydrogenation (AD) of 3-octanol.

3.4 Conclusion

An unsupported crystalline Re NP (~ 2 nm) was generated under very mild

conditions (180 °C and neat 3-octanol). The resulting Re NP are Re0 core covered with

ReO2 oxide on the surface. The Re NP is an excellent and selective catalyst for AD

reactions of different alcohols. Based on KIE experiments and substrates study the

dehydrogenation involves -CH bond activation.

62

3.5 References


(2) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2010, 110, 681.

(3) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761.

(4) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. Organometallics 2011, 30, 5716.

(5) Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2004, 126, 10657.

(6) Fang, W.; Chen, J.; Zhang, Q.; Deng, W.; Wang, Y. Chem. Eur. J. 2011, 17, 1247.

(7) Mitsudome, T.; Mikami, Y.; Funai, H.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Angew. Chem. Int. Ed. 2008, 47, 138.

(8) Kon, K.; Hakim Siddiki, S. M. A.; Shimizu, K.-I. J. Catal. 2013, 304, 63.

(9) Schoop, T.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H. Organometallics 1993, 12, 571.

(10) Yeo, A. N. H.; Williams, D. H. J. Am. Chem. Soc. 1969, 91, 3582.

(11) Lambert, J. B.; Greifenstein, L. G. J. Am. Chem. Soc. 1974, 96, 5120.

(12) Carpenter, W.; Duffield, A. M.; Djerassi, C. J. Am. Chem. Soc. 1968, 90, 160.

(13) Ivanovskii, A. L.; Chupakhina, T. I.; Zubkov, V. G.; Tyutyunnik, A. P.; Krasilnikov, V. N.; Bazuev, G. V.; Okatov, S. V.; Lichtenstein, A. I. Phys. Lett. 2005, 348, 66.

(14) Commereuc, D. J. Chem. Soc., Chem. Commun. 1995, 791.

(15) Jurewicz, J. W.; Guo, J. U.S. Patent 7494527B2, February 24, 2009.

(16) Lesage, A.; Emsley, L.; Chabanas, M.; Coperet, C.; Basset, J.-M. Angew. Chem. Int. Ed. 2002, 41, 4535.

63

CHAPTER 4. RHENIUM-CATALYZED ACCEPTORLESS-DEHYDROGENATIVE AMINE-COUPLING REACTION FOR THE

SYNTHESIS OF IMINES

4.1 Introduction

Imines are highly desirable intermediates that have wide applications in

synthetic organic chemistry.1 For example, enantioselective hydrogenation to chiral

amines,2 cross-coupling to produce multi-functional molecules,3 and cyclization to

prepare heterocycles.4 However, it is difficult to prepare imines in good selectivities

since reactions yield amine products through rapid subsequent hydrogen-borrowing

reactions.5 Traditional methods to produce imines include amine oxidations by using

stoichiometric metals or bases, which make enormous amounts of waste.6 Recently,

molecular oxygen has been used as a “green” stoichiometric oxidant to produce

imines, but the water coproduct can diminish the yields due to rapid hydrolysis of

imines.7 In the same manner, coupling between alcohols and amines produces water

that must be removed from the reaction system.5-8 In this study, we demonstrate the

use of pure amines to generate imines with release of molecular ammonia, which can

be removed easily. Our system is heterogeneous combined with acid solution

absorption of NH3, Re0 catalyst can be prevented from poison or decomposition due

to the dissolution of ammonia.9 The straightforward synthesis of the polar functional

group (C=N) is needed because it can eliminate the use of precursor aldehyde or

64

ketone, facilitate water-sensitive reactions, as well as minimize the use of protective

groups and production of chemical waste unnecessarily. Catalytic acceptorless

dehydrogenation (AD) reactions have emerged as greener methods to obtain

oxidized molecules containing C=O or C=N polar functional groups.10 Until now

there have been only a few reports on the use of AD reactions to synthesize imines.11

PNN or PNP ligated complexes are dominant in the homogeneous AD reactions with

the C-H and N-H activations.9-11 Meanwhile, Re0 nanoparticles (NPs) are

underdeveloped because of the harsh conditions required to generate them and their

facile oxidation in air. Recently, we have found that Re NPs can be produced in situ

and they catalyze AD reactions of alcohols to give ketones and dihydrogen.12 In this

chapter, heterogeneous Re NPs-catalyzed acceptorless dehydrogenative (AD)

coupling of amines to generate imines, a type of green and environmentally friendly

reaction, is developed (Scheme 4.1). Without any oxidants or acceptors, Re NPs

catalyze amine-coupling to produce imines, which is intriguing because of its

potential versatility. Hydrogen and ammonia can be easily separated from imine

products. Meanwhile Re NPs can be recycled for important industrial uses. Here we

expand the substrates to amines and investigate the kinetic and mechanistic aspects

of this novel AD reaction of amines that involves -C-H activation. In addition,

heterogeneous Re NPs catalysts produced in situ are environmentally and

technologically advantageous due to their facile recyclability, which has been

demonstrated over five cycles.

65

Scheme 4.1 Rhenium-Catalyzed acceptorless dehydrogenation (AD) reactions of alcohols and amines.

4.2 Experimental


Reactant amines and other standard reagents were purchased from Sigma-

Aldrich (U.S.A.) and Alfa Aesar (U.S.A.). All commercial materials were used as

received without further purification unless specified otherwise.

Methyltrioxorhenium(VII) (MTO), NH4ReO4, ReO3, Re2O7 were purchased from

Strem Chemicals (U.S.A.). -Deuterated benzylamine (C6H5CD2NH2) was

synthesized according to literature method13 by using LiAlD4 that was purchased

from Cambridge Isotope Laboratories (U.S.A.).

The acceptorless dehydrogenation can be applied to 12 different amines at the

reaction temperature range from 120 oC to 200 oC, and the reaction time ranges from

10 to 24 hours. The amines (5 mmol) were dissolved in n-decene (50 mmol). The

solution was supplemented with internal standard 1,3,5-trimethoxybenzene (2 mmol)

for the kinetic study. The conversion and yield profiles in real time were determined

66

by 1H NMR. The mixture was cooled down after completion of reaction for further

purification. n-Decane was removed under vacuum overnight at 30 oC, flash column

chromatography was performed to separate the imine product from catalyst and

other impurities. The final imine products are either white solid, colorless clear oil or

pale yellow oil. The isolated pure products were identified by 1H and 13C NMR.


1H, 2H, and 13C NMR spectra were collected on Bruker Avance DRX-500.

NMR data was plotted by using MestReNova software. GC analysis was performed

on Agilent 6890 Series with DB-5 column gas chromatography system. H2 gas

analysis was performed on a residual gas analyzer (RGA) model RGA 100 (Stanford

Research Systems).


4.3.1 Reaction Optimization

The scope of the optimized Re NPs catalytic system for AD of amines was

investigated at different temperatures, pre-catalysts, and catalyst loadings (Scheme

4.2 and Table 4.1). Without Re catalysts, no reaction occurs. Within 10 hours 4 mol%

NH4ReO4 affected 100% conversion of benzylamine to quantitative yield (> 99%) of

N-benzylidene-1-phenylmethanamine (1) and small amount (< 1%) of

dibenzylamine (Entry 1). Monitoring the reaction through 1H NMR confirmed a

first-order dependence on benzylamine (Figure 4.1). In entries 3 and 4,

67

methyltrioxorhenium (MTO) and decacarbonyl dirhenium (Re2(CO)10) both gave

high yields of 1, 98% and 89%, respectively, as well as small yields of dibenzyl-

NH2NRe catal.

1

2+ H2 + NH3

Decane

Scheme 4.2 Benzylamine coupling reaction in Table 4.1.

Table 4.1 Dehydrogenation of 3-octanol over different Re catalysts.

Entry Re Catalyst Temperature

(oC) Time (h)

Yield of 1 (%)

1 none 180 24 0

2 4 mol% NH4ReO4 180 10 > 99

3 4 mol% MTO 180 10 98

4 4 mol% Re2(CO)10 180 10 89

5 4 mol% ReO3 180 10 98

6 4 mol% Re2O7 180 10 95

7 4 mol% ReO2·H2O 180 10 97

8 2 mol% NH4ReO4 180 10 83

9 8 mol% NH4ReO4 180 5 98

10 4 mol% NH4ReO4 140 5 69

11 4 mol% NH4ReO4 160 5 79

12 4 mol% NH4ReO4 200 5 91

13 4 mol% recycled Re

NPs 180 10 98

am

R

(E

th

N

N

te

eq

g

J·

tr

mine (2% a

Re(VI), Re(V

Entries 5, 6 a

he AD amine

In Entr

NH4ReO4 (at

NH4ReO4, a

emperatures,

quation 2:

ave the acti

·mol-1·K-1 (F

ransition stat

Fig

and 1%). No

VII) and Re

and 7). More

e-coupling w

ries 8 and

t 180 oC, k

and 0.761

, from 140

ivation param

Figure 4.2).

te mechanism

gure 4.1 Ent

0

0

0

0

0

0

o observable

(IV) oxides

eover, recyc

with compara

9 of Table

=0.179 h-1

h-1 for 8

oC to 200 o

meters H≠

The large ne

m.

ry 1 of Tabl

0.0

0.1

0.2

0.3

0.4

0.5

0 2

First-Orde

e dibenzylam

catalysts, w

cled Re NPs

able yield (9

4.1, the o

for 2 mol%

mol% NH4

oC (Entries

=15.4 ± 0.

egative entro

e 4.1 reactio

4 6

er Dependence on

Time (h)

y = m1*exp(

V

0.4m1

0.3m2

-0.003m3

9.2185Chisq

0.9R2

mine peaks

which all ga

from alcoho

98%) per Ent

observed rat

% NH4ReO4

4ReO4, Fig

1 and 10-12

6 kJ·mol-1 a

opy is consi

on profile for

8

Benzylamine

-m2*m0)+m3

ErrorValue

0.003416743747

0.007921638539

0.00244397905

NA5e-05

NA99952

in the NMR

ave high yie

ol AD reacti

try 13.

te constant

4, 0.385 h-1

gure 4.1). A

2), the fittin

Eq. 2

and S≠ = -

istent with a

r benzylamin

10

6

R or GC for

elds (>95%)

on catalyzed

depends on

for 4 mol%

At differen

ng of Eyring

-290.4 ± 1.4

bimolecular

ne.

68

r

)

d

n

%

t

g

4

r

69

Figure 4.2 Eyring plot.

4.3.2 Catalyst Recycling

Immediate filtration the reaction mixture while hot causes a large amount of Re

NPs to remain in the solution. Therefore, only after cooling down, the heterogeneous

Re NPs catalyst can be separated by filtration (without much loss of Re according to

ICP-MS analysis). The recovered catalyst is washed with acetone and

dichloromethane each 3 times and dried overnight under vacuum. Since Re NPs is

fine black powder, it can stick onto filter paper and after each reaction batch

separation 5-15% weight loss is noted. Based on the recycled Re NPs catalyst weight,

fresh benzylamine was added to the system (Table 4.2). The reaction conditions for

the Table 4.2 is as follows: NH4ReO4 (1 mmol), benzylamine (25 mmol), and 50 mL

decane were mixed and refluxed at 180 oC for 10 h, followed by filtration, wash, and

vacuum drying of Re NPs for the next catalytic cycle. The fresh benzylamine added

to recycled system is scaled relative to the moles of recycled Re NPs (weight/186) /

-15.7

-15.6

-15.5

-15.4

-15.3

-15.2

-15.1

-15.0

0.0021 0.0022 0.0023 0.0024 0.0025

1/T

y = m1 + m2 * M0

ErrorValue

0.17348-11.173m1

76.557-1851.9m2

NA0.00061487Chisq

NA0.99659R2

70

4 % × 107 g/mol. Solvent n-decane also can be recycled. The yield calculation was

based on GC. Total TON of six cycles is 145.

Table 4.2 AD amine-coupling of benzylamine over recycled Re NPs catalyst.

Cycle 1st 2nd 3rd 4th 5th 6th

Yield (%) > 99 99 96 97 94 92

4.3.3 Substrate Scope

Various amines were examined to explore the substrate scope of this AD

reaction (Scheme 4.3). Heating the decane solution of benzylamine with 4 mol%

NH4ReO4 at reflux for 10 h resulted in 100% conversion and over 99% yield of 1.

Only trace amount of hydrogenation product dibenzylamine was detected. Imine 1

was isolated in 95% yield through 3-cm vacuum flash neutral aluminum

chromatography, after solvent decane removed under vacuum overnight. Formation

of ammonia was confirmed Nessler’s reagent and quantified through the weight

change of absorbent 10% HCl solution. Dihydrogen gas was collected and analyzed

by residue gas analysis (RGA).

71

Scheme 4.3 Substrate scope for AD amine-coupling.

Different electron-withdrawing (2 and 3) and –donating (4 and 5) functional

groups didn’t influence the yields (Scheme 4.3) nor rate constants (Figure 4.3).

Nonlinearity and small value (Hammett plot) indicate that AD amine-coupling is

not sensitive to electronic properties of substituents. So the intermediate(s) involved

in the rate determine step (RDS) is neutral, or have weak electronic interaction with

Re NPs.

72

Figure 4.3 Hammett plot.

This AD method also can be applied to aliphatic primary amines. For example,

secondary-C amines (6 and 7) and primary-C long-chain amines (8 and 9) all gave

quantitative yields according to GC analysis. However, aliphatic imines are more

vulnerable to hydrolysis than aromatic imines, so the isolated yields were quite low

(8~25%). For imine 10, we also get isolated 40% yield. Using chiral amines can

produce chiral imines like (R)-11 and (S)-11, over 95 ee% were obtained. meso-1-

(1-Naphthyl)ethylamine gave axial-chiral diastereomers meso-12 that can be

differentiated from (S)-12 (by using (S)-(-)-1-(1-naphthyl)ethylamine as starting

material) through 1H and 13C NMR spectra. NMR spectra from 1 to 12 are shown in

from Figure 4.4 to 4.15.

0

0.01

0.02

0.03

0.04

0.05

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

Substituent Constant

73

Figure 4.4 1H and 13C NMR of 1.

74


75


76


77


78


79


80


N

N

81


82


83


84


85

Besides primary amines, several secondary amines were also used to test the

broad AD applications of Re NPs (Table 4.3). For non-conjugated molecules, like

dioctylamine, dihexylamine, there is no reaction. Even though using dibenzylamine,

after 24 hours, only 4% yield of 1 were observed (Entry 3, Table 4.3). However,

indoline gave 100% yield of indole after 10 hours with 4 mol% loading NH4ReO4,

which might be contributed to aromaticity. For 1,2,3,4-tetrahydroquinoline after 10

hours (4 mol% NH4ReO4), we observed the sole perdehydrogenation14 product

quinoline (33% yield) without other dehydrogenated products.

Table 4.3 AD reaction of secondary amines

Entry Substrate Product Yield (%)

1

- 0

2

- 0

3

4

4

100

5 NH

33

86

4.3.4 Kinetic Isotope Effect

To investigate the mechanism of this AD amine-coupling reaction, we used

deuterated benzylamine (C6H5CD2NH2). Equation 1 was confirmed by both NMR

(Figure 4.4) and RGA (Figure 4.5). The reaction conditions is as follows: NH4ReO4

(0.2 mmol), C6H5CD2NH2 (5 mmol), and internal standard cyclohexane-d12 (4 mmol)

were mixed in solvent dodecane (11 mL). The mixture was heated at 180 oC for 24 h.

And the rate constants kH/kD = 1.22 indicate -C-H activation.

Eq. 3

87

Figure 4.16 2D NMR at 9 h of deuterated benzylamine (C6H5CD2NH2).

Figure 4.17 RGA gas products from deuterated benzylamine (C6H5CD2NH2).

0.0 100

2.0 10-8

4.0 10-8

6.0 10-8

8.0 10-8

1.0 10-7

1.2 10-7

1.4 10-7

0 50 100 150 200 250 300

HDH2

Time (s)

(a)

0.0 100

2.0 10-8

4.0 10-8

6.0 10-8

8.0 10-8

1.0 10-7

1.2 10-7

1.4 10-7

1 2 3 4 5 6 7 8 9 10 11

Molecular Mass (g/mol)

(b)

88

4.3.5 Mechanism

Based on the above evidence, we propose the mechanism for AD amine-

coupling reaction in Scheme 4.4. First, benzylamine has the dehydrogenation to

produce H2 and benzylaldimine via -C-H activation. Previous results have shown

an interesting -C-H activation,13 and the difference between -C-H activation of

amines and -C-H activation of alcohols may be attributed to the strong Lewis

basicity of amines. Benzylaldimine that is very reactive might have weak interaction

with Re NPs to form 14. It is very difficult for us to capture aldimine 14, which is

consistent with previous report.15 We tried to capture the ketimine from

benzhydrylamine (Figure 4.6). After adding triethylborane, the ketimine 10.89 ppm

characteristic peak didn’t give an observable shift. This phenomenon indicated the

weak interaction between imine and catalyst Re NPs that is in lower oxidation state

than that of literature.15 The different electronic and structural properties of

intermediate 14 lead to different final products of imines and amides. Second,

another benzylamine can nucleophilically attack 14 to form 15. Finally, Re NPs

facilitate the hydrogen transfer, then the deamination to form product 1. The last step

is a typical SN2 reaction, which is the reason why the reaction has good chiral

selectivities toward chiral imines like (R)-, (S)-11 and 12.

89

Scheme 4.4 Proposed mechanism for AD amine-coupling.

Figure 4.18 1H NMR of benzhydrylamine ((C6H5)2CHNH2) reaction in sealed system. The characteristic peaks 10.89 and 11.27 ppm are cis- and trans-ketimine.

90

4.4 Conclusion

In summary, we have developed the convenient method to generate rhenium

nanoparticles (Re NPs) that can catalyze acceptorless dehydrogenation (AD) amine-

coupling reaction. The product aliphatic and aromatic imines have the pro-chiral

C=N bond that can assist wide applications. A mechanism is proposed to be -C-H

activation and to form the intermediate aldimine, subsequently to couple with amine

to form imine product.

91

4.5 References

(1) Gawronski, J.; Wascinska, J.; Gajewy, J. Chem. Rev. 2008, 108, 5227.

(2) Noyori, R.; Hashiguchi S. Acc. Chem. Res. 1997, 30, 97.

(3) Mennen, S. M.; Gipson, J. D.; Kim, Y. R.; Miller, S. J. J. Am. Chem. Soc. 2005, 127, 1654.

(4) Michlik, S.; Kempe, R. Nature Chem. 2013, 5, 140.

(5) Watson, A. J. A.; Williams, J. M. J. Science 2010, 329, 635.

(6) Blackburn, L.; Taylor, R. J. K. Org. Lett. 2001, 3, 1637.

(7) Sun, H.; Su, F.-Z.; Ni, J.; Cao, Y.; He, H.-Y.; Fan, K.-N. Angew. Chem. Int. Ed. 2009, 48, 4390.

(8) Kegnas, S.; Mielby, J.; Mentzel, U. V.; Christensen, C. H.; Riisager, A. Green Chem. 2010, 12, 1437.

(9) Myers, T. W.; Berben, L. A. J. Am. Chem. Soc. 2013, 135, 9988.


(11) Gnanaprakasam, B.; Zhang, J.; Milstein, D. Angew. Chem. Int. Ed. 2010, 49, 1468.

(12) Yi, J.; Miller, J. T.; Zemlyanov, D. Y.; Zhang, R.; Dietrich, P. J.; Ribeiro, F. H.; Suslov, S.; Abu-Omar, M. M. Angew. Chem. Int. Ed. 2014, 53, 833.

(13) Lang, X.; Ji, H.; Chen, C.; Ma, W.; Zhao, J. Angew. Chem. Int. Ed. 2011, 50, 3934.

(14) Fujita, K.; Tanaka, Y.; Kobayashi, M.; Yamaguchi, R. J. Am. Chem. Soc. 2014, 136, 4829.

(15) Kang, B.; Fu, Z.; Hong, S. H. J. Am. Chem. Soc. 2013, 135, 11704.

92

CHAPTER 5. HYDROGEN STORAGE IN LIQUID ORGANIC HETEROCYCLES BY USING RHENIUM NANOCRYSTALLINE

CATALYST

5.1 Introduction

As the depletion of fossil fuels and the increasing concern about green-house

gas emissions intensity, hydrogen becomes a promising sustainable energy

alternative for transportation.1 More and more technologies have been developed to

use hydrogen in automotive vehicles. However, hydrogen storage remains a

significant challenge in this field. Conventional hydrogen storage focused on

hydrogen compression, cryotechniques, large surface materials, and metal hydride.2

Pez proposed hydrogen storage in liquid organic heterocycles (LOH), which

suggested that a heteroatom in the ring can lower the dehydrogenation enthalpy.3,4

Dehydrogenation is attracting research since most metal catalysts are well-known for

their hydrogenation activities. Crabtree reported that Pd/C and Rh/C achieved 100%

conversion in refluxing toluene.5 Jessop et al. found that electron-donating or

conjugated substituents on a piperidine ring could increase the rate of

dehydrogenation.6 However, mechanism of heterogeneous catalysis for the

reversible hydrogenation/dehydrogenation is still obscure.

For homogeneous systems, some of the acceptorless dehydrogenation catalysts

mentioned in the previous chapters can also be used in reversible

hydrogenation/dehydrogenation. Jones and co-workers used bis(phosphino)amine

93

pincer ligated Fe complexes to catalyze acceptorless dehydrogenation and

hydrogenation of N-heterocycles.7 The Fujita group used 2-hydroxypyridine ligated

Cp*Ir complexes to catalyzed the same reversible hydrogenation/dehydrogenation of

N-heterocycles.8 Crabtree conducted detailed mechanistic study, and proposed an

outer-sphere pathway.9

In this chapter, Re NPs are shown to catalyze reversible

hydrogenation/dehydrogenation (Scheme 5.1). Different electron-donating and –

withdrawing groups influence the yields of the reaction, which is consistent with

Jessop’s results.6 Interestingly, 2- and 4-methyl quinoline exhibited distinctly

different kinetics, which might share similarity of homogeneous outer-sphere

mechanism proposed by Crabtree.9

Scheme 5.1 Re NPs catalyzed reversible hydrogenation/dehydrogenation.

5.2 Experimental


All quinoline or its derivative compounds were purchased from Sigma-Aldrich,

and were used as received without further purification unless otherwise noted.

94

NH4ReO4 was purchased from Alfa Aesar. Activated charcoal was used the

DARCO® (100 mesh particle size, powder) from Sigma-Aldrich. Activated charcoal

was pretreated with 10 M HNO3 for 4 hours, then was washed with distilled water to

neutralize, then dried at 120 oC for 12 hours.

The reversible hydrogenation and dehydrogenation can be applied to 12

different N-heterocycle molecules. The general hydrogenation process is: quinoline

(1.83 g, 14.22 mmol, 1 equiv.), NH4ReO4 (19.0 mg, 0.071 mmol, 0.5 mmol), and

activated carbon (142.0 mg) were mixed and heated at 180 oC for 4 hours under 18.5

bar H2. The general dehydrogenation process is the same comparative amount

reagents that showed before. The mixture was heated at 180 oC open to air.


Hydrogenation reactions were run in Parr 5000 Multiple Reactor System.

Elemental analysis results were obtained from Galbraith Laboratories, Inc. 1H, 2H,

and 13C NMR spectra were collected on Bruker Avance DRX-500. NMR data was

plotted by using MestReNova software. GC analysis was performed on Agilent 6890

Series with DB-5 column gas chromatography system. H2 gas analysis was

performed on a residual gas analyzer (RGA) model RGA 100 (Stanford Research

Systems). XRD analysis was performed on Bruker D8 Focus X-Ray Diffractometer

with a Cu K source at 40 KV/40 ma.

95


5.3.1 Hydrogenation Scope

Quinoline and its derivatives in Scheme 5.2 were evaluated. The general

condition is quinoline (1.834 g 14.22 mmol), NH4ReO4 (19.0 mg, 0.071 mmol), and

activated carbon (362.0 mg) were mixed and heated at 180 oC for 4 hours under 18

bar of H2 atmosphere. After cooling, aliquoting and diluting the mixture in

dichloromethane got GC yields were obtained. The mixture can be directly used for

dehydrogenation reaction, or filtered to remove the heterogeneous catalyst and purify

the product. The yields in parenthesis of Scheme 5.2 are isolated yields after column

chromatography. Quinoline (1) and isoquinoline (2) hydrogenated to 1’ (99%) and 2’

(98%) in quantitative yields. Comparing with more steric ally hindered compounds

atom in the ring 2-methylquinoline (3), 4-methylquinoline (4) generated moderate

yield of 4’ (70%). 6-Methylquinoline (5) also gave moderate yield of 5’ (70%), but

in Scheme 5.3 later the dehydrogenation yield is not much lower. 8-Methylquinoline

(6) or 2-phenylquinoline (7), theoretically steric to the pyridine ring, both gave ideal

yields (93% and 99% respectively). Electron-withdrawing 2-

trifluoromethylquinoline (8) gave much lower yield of (8’), which is consistent with

previous report.6 1,5-Naphthyridine (9), two nitrogen atoms in two different

conjugated pyridine rings, only one pyridine was hydrogenated to produce 9’ in

moderate yield (85%). One aromatic ring assists the adsorption on Re NPs, so fully

hydrogenation is not achievable. However, quinoxaline has two nitrogen atoms in

the same ring, which had side decomposition reaction due to hydrogenolytic

96

instability.6 Other five-member conjugated cyclic molecules like indole (10) and

benzofuran (11) produce ideal hydrogenated products 10’ (92%) and 11’ (90%).

Naphthalene hydrogenation enthalpy to tetralin is -125.2 kJ/mol,10 whereas 1,2-

dihydronaphthalene is -100.8 kJ/mol.11 So there is no reaction for naphthalene with

Re NPs, but Re NPs catalyzed the hydrogenation of 1,2-hydronaphthalene to

generate tetralin 12’ with low yield 61%.

The slow hydrogenation 4-methyl-quinoline raised a concern. Therefore, we

investigated the kinetic profiles of hydrogenation of different position-substituted

methyl group (Figure 5.1). Table 5.1 shows the rate is: 4-methylquinoline << 2-

methylquinoline < 8-methylquinoline ≈ 6-methylquinoline ≈ quinoline, which

indicates that the steric group close to nitrogen does not influence the rate of

hydrogenation. Interestingly, para-position relative to the nitrogen atom, like 4-

methylquinoline, has the slowest rate.

Scheme 5.2 Hydrogenation of N-heterocycles.

97

Figure 5.1 Different position substituted quinolines.

Table 5.1 Rates for different quinoline derivatives.

Substrate Quinoline 2-Methyl-quinoline

4-Methyl-quinoline

6-Methyl-quinoline

8-Methyl-quinoline

k (X 10-4 s-1)

2.9 1.8 0.7 2.5 2.4

5.3.2 Dehydrogenation Scope

Dehydrogenation is the reverse reaction of hydrogenation, and usually the

activation energy of dehydrogenation is higher than that of hydrogenation. In this

research, reaction time is much longer than hydrogenation. The typical conditions in

Scheme 5.3 is as follows: 1,2,3,4-tetrahydroquinoline (1.891 g, 14.22 mmol),

NH4ReO4 (19.0 mg, 0.071 mmol), and activated carbon (362.0 mg) were mixed and

heated at 180 oC for 24 hours open to air. The yields in parenthesis of Scheme 5.3

are column-purified yields, otherwise they are GC yields. Dehydrogenation yield

(90%) of 2-methyl-1,2,3,4-tetrahydroquinoline (3’) is ideally similar to that of

1,2,3,4-tetrahydroquinoline (1’) (99%) and 1,2,3,4-tetrahydroisoquinoline (2’) (94%).

14.0

16.0

18.0

20.0

22.0

24.0

26.0

0 1 104

2 104

3 104

4 104

5 104

4-MethylquinolineQuinoline8-Methylquinoline2-Methylquinoline6-Methylquinoline

Time (s)

98

4-Methyl-1,2,3,4-tetrahydroquinoline (4’) still gave low yield (60%) that is similar to

the yield of hydrogenation. 6- and 8-Methyl-1,2,3,4-tetrahydroquinoline both gave

moderate yields (73%). 2-Phenyl-1,2,3,4-tetrahydroquinoline dehydrogenated with

ideal yield 89%. Electron-withdrawing 2-trifluoromethyl-1,2,3,4-tetrahydroquinoline

gave low yield 38% as expected. 1,2,3,4-Tetrahydro-1,5-naphthyridine (9’) was

recycled and purified from hydrogenation reaction, and dehydrogenated back to 1,5-

naphthyridine (9) with 85% yield. Indoline (10) and 2,3-dihydrobenzofuran (11)

produced indole (10’, 92%) and benzofuran (11’, 90%), respectively. 1,2-

Dihydronaphthalene can be dehydrogenated to form naphthalene (12) with

quantitative (99%) yield.

Scheme 5.3 Dehydrogenation of N-heterocycles.

99

5.3.3 Catalyst Recycling

Based on our previous dehydrogenation research, we assumed that NH4ReO4

can produce Re NPs in situ, which is ca. 2 nm Re nanocrystalline particles. This type

of heterogeneous Re NPs can be recycled for several times. Figure 5.2 shows 8-time

recycling of the Re NPs catalyst, in which 1st is the initial, 2nd is the 1st recycle.

This figure indicates after 2nd, continuous 3th, 4th, and 5th reached the fastest rates.

Then the Re NPs catalyst slightly decayed, but remained better than 1st and 2nd.

We used two different types of Re/C in Table 5.2 to compare our Re NPs in

situ with the regular Re/C produced from incipient wetness impregnation method.12

The regular Re/C obtained the trend to increase the rate after recycling, but can’t

reach the rate of Re NPs in situ, which indicates that the size or the nanocrystalline

structure influences the reaction.

Elemental analysis was performed to testify Re loss during recycles, and Re

NPs lost 1.24 wt% for 8th, and the regular Re/C lost 1.13 wt%.

Figure 5.2 Hydrogen consumptions of 8-time recycled Re NPs.

14.0

16.0

18.0

20.0

22.0

24.0

0 5000 1 104

1.5 104

1st2nd3rd4th5th6th7th8th

Time (s)

100

Table 5.2 Hydrogenation rates comparison between Re NPs in situ and regular Re/C.

Selected cycle

1st Re NPs in situ

3rd Re NPs in situ

1st regular Re/C

8th regular Re/C

k (X 10-4 s-1) 2.9 7.4 5.0 3.1

5.3.4 Kinetic Study

To understand the mechanistic pathway of the reaction, a kinetic study was

conducted.

5.3.4.1 Hydrogen Dependence

Hydrogen dependence is shown in Table 5.3. The reaction condition is:

quinoline (1.832 g, 14.20 mmol), NH4ReO4 (19.0 mg, 0.071 mmol), and activated

carbon (362.0 mg) were mixed and heated at 180 oC for 4 to 10 hours. Since the

hydrogenation reaction was carried out in a par reactor, the volume (75 mL) of the

reactor is fixed. Thus, the hydrogen molar amount can be calculated based on ideal

gas law. The lowest pressure (9.38 bar) corresponds to a molar ratio between

H2:quinoline = 2:1. As the initial hydrogen pressure increases, the rate of

hydrogenation increased but nonlinearly, which can attribute to the complicated

mass and hear transfer among solid catalyst, liquid substrate, and hydrogen gas

(three phases).

101

Table 5.3 Different initial hydrogen pressure influences the rate of hydrogenation.

Initial H2 pressure (bar) at 25 oC k (X 10-4 s-1) Reaction time (h) 18.33 2.9 4 12.43 1.5 6 9.38 1.0 10

5.3.4.2 Catalyst Dependence

The catalyst concentration dependence is shown in Table 5.4. The

hydrogenation condition is as follows: quinoline (1.832 g, 14.20 mmol), NH4ReO4

and activated carbon (362.0 mg) were mixed and heated at 180 oC for 4 hours in 18

bar H2 atmosphere. The linearity demonstrated the first-order dependence in Re NPs

catalyst.

Table 5.4 Catalyst concentration influences the rate of hydrogenation.

Re molar percentage to quinoline (mol%) k (X 10-4 s-1) 0.75 5.4 0.50 3.6 0.25 1.8

5.3.4.3 Temperature Dependence

The rate of the hydrogenation varied on reaction temperature (Table 5.5).

Through the Erying equation 2, the activation parameters were found to be H≠ =

45.3 ± 0.1 kJ·mol-1; S≠ = -216.6 ± 0.7 J·K-1·mol-1, which is consistent with the Ru

homogeneous catalytic system measured before (H≠ = 42 ± 6 kJ·mol-1; S≠ = -115

± 2 J·K-1·mol-1).13

5

av

Ta

T 4444

.3.5.1 Tran

The TE

verage Re N

Figure 5.3

able 5.5 The

(K) k43 53 63 73

nsmission El

M images cl

NPs size is ca

5-nm, 2-nm

rates of hyd

k (X 10-4, s-1

2.1 2.9 3.6 4.7

5.3.5 Cat

lectron Micr

learly showe

a. 2.07 nm. R

m and 10-nm

drogenation i

1) 1/T 0.000.000.000.00

talyst Charac

oscopy (TEM

ed Re nanocr

Re NPs on ca

of TEM ima

in different t

(K-1) 02257 02208 02160 02114

cterization

M)

rystalline str

arbon still k

ages and Re

temperatures

ln(k/T) -14.6 -14.3 -14.1 -13.8

ructure (Figu

keep the smal

NPs size dis

10

s.

ure 5.3). The

ll 2 nm size.

stribution.

02

e

.

5

p

5

d

C

th

5

.3.5.2 Ener

The ED

eaks are due

.3.5.3 X-R

The XR

iffraction pe

Comparison w

he second hi

.6.

rgy-Dispersi

DX spectrum

e to the Cu m

F

Ray Diffractio

RD was used

eaks, both XR

with Re0 cry

ighest intens

ive X-Ray Sp

m in Figure 5

membrane us

Figure 5.4 ED

on (XRD)

d to confirm R

RD of in-situ

ystalline diff

sities at 37.6

pectroscopy

5.4 shows Re

sed as a supp

DX spectrum

Re NPs. Sin

u Re NPs an

fraction patt

6o, 40.4o, an

y (EDX)

e is the only

port.

m of Re NPs

nce Re size i

nd regular R

tern,14 the hi

nd 67.9o all

y metal teste

.

s too small t

Re/C showed

ighest intens

show good

10

ed. Other Cu

to give sharp

broad peaks

sity at 42.9o

fit in Figure

03

u

p

s.

o,

e

Figuure 5.5 XRDD comparisonns between in

n situ Re and regular Re

10

e/C.

04

105

5.4 Conclusion

In this chapter, we explored catalysis of Re nanocrystalline particles (NPs) on

carbon support for the reversible hydrogenation/dehydrogenation of N-heterocyclic

compounds, which potentially can be used in hydrogen storage. Catalyst

characterization shows Re NPs size is ca. 2 nm. The interesting phenomenon: 4-

methylquinoline is the slowest among quinoline and other derivatives, indicates the

outer-sphere mechanism.

106

5.5 References

(1) Sartbaeva, A.; Kuznetsov, V. L.; Wells, S. A.; Edwards, P. P. Energy Environ. Sci. 2008, 1, 79.

(2) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353.

(3) Pez, G. P.; Scott, A. R.; Cooper, A. C.; Cheng, H. US Pat. 7101530, 2006.

(4) Pez, G. P.; Scott, A. R.; Cooper, A. C.; Cheng, H.; Wilhelm, F. C.; Abdourazak, A. H. US Pat. 7351395, 2008.

(5) Moores, A.; Poyatos, M.; Luo, Y.; Crabtree, R. H. New J. Chem. 2006, 30, 1675.

(6) Cui, Y.; Kwok, S.; Bucholtz, A.; Davis, B.; Whitney, R. A.; Jessop, P. G. New J. Chem. 2008, 32, 1027.

(7) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2014, 136, 8564.

(8) Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K. J. Am. Chem. Soc. 2009, 131, 8410.

(9) Dobereiner, G. E.; Nova, A.; Schley, N. D.; Hazari, N.; Miller, S. J.; Eisenstein, O.; Crabtree R. H. J. Am. Chem. Soc. 2011, 133, 7547.

(10) Hargittai, M.; Hargittai, I. Advances in Molecular Structure Research, Vol. 4, Elsevier, 1998, pp. 361.

(11) Williams, R. B. J. Am. Chem. Soc. 1942, 64, 1395.

(12) Shao, Z.; Li, C.; Di, X.; Xiao, Z.; Liang, C. Ind. Eng. Chem. Res. 2014, 53, 9638.

(13) Rosales, M.; Alvarado, Y.; Boves, M.; Rubio, R.; Soscun, H. Transition Met. Chem. 1995, 20, 246.

(14) Sims, C. T.; Craighea, C. M.; Jaffee, R. I. J. Metals 1955, 7, 168.

107

VITA

Jing Yi was born in August 1985 in Nanchang, P. R. China. She graduated from

Ocean University of China with a BS in 2006. During her undergraduate program, Jing

worked on anti-fouling marine paint polymer synthesis in Dr. Liangmin Yu’s Lab. In

2006, Jing began graduate study at Wuhan University under the direction of Dr. Zixing

Shan. She synthesized asymmetric borate to catalyze asymmetric reactions, and used

crystallization to enrich enantiomeric BINOL. After earning a MS from Wuhan

University in 2008, Jing started to work on synthesis and photophysics of Pt(II)

complexes at North Dakota State University (NDSU) in Dr. Wenfang Sun’s lab. In 2010,

Jing finished MS at NDSU and joined Purdue University. From March to July 2011, she

worked as a Scientist II at Sundia Meditech Company in Shanghai, China. In August

2011, Jing began PhD work under the supervision of Dr. Mahdi Abu-Omar. In Mahdi’s

lab, Jing studied multidisciplinary catalysis research in homogeneous oxorhenium

catalysts and heterogeneous rhenium nanocrystalline catalysts as well as their

application in biomass utilization, renewable energy, and sustainable transformations.

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