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Emerging catalytic applications of transition metal oxide nanomaterials under microwave and conventional heating

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Emerging Catalytic Applications of Transition Metal Oxide
Nanomaterials under Microwave and Conventional Heating
Shanthakumar Sithambaram, Ph.D.
University of Connecticut, 2010
Heterogeneous transition metal oxide catalysts have advantages over
homogeneous catalysts, such as easy separations and efficient recycling and
minimization of metal traces in the products. Transition metal oxide
nanomaterials with different properties such as shapes and particle size were
synthesized by hydrothermal, solvothermal, solvent-free and by energy efficient
microwave heating methods and characterized using X-Ray and microscopic
techniques. The synthesized catalysts were tested for tandem reactions to form
quinoxalines, oxidations of hydrocarbons to form alcohols, aldehydes and
ketones, epoxidation, epoxide ring opening, and jV-aryl coupling reactions. The
kinetics and energy consumption associated with these reactions were compared
for both microwave and conventionally heated reactions. Further, Synchrotron
radiation-based time-resolved XRD experiments under a wide variety of
temperature and pressure conditions were conducted to study the reactions under
working conditions. EXAFS and XANES data collections were performed to
determine inter-atomic distances and oxidation states of the catalysts.
Emerging Catalytic Applications of Transition Metal Oxide
Nanomaterials under Microwave and Conventional Heating
Shanthakumar Sithambaram
MS, University of Connecticut, USA, 2006
BS, University of Peradeniya, Sri Lanka, 1996
A Dissertation
Submitted in Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
at the
University of Connecticut
2010
UMI Number: 3415573
All rights reserved
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APPROVAL PAGE
Doctor of Philosophy Dissertation
Emerging Catalytic Applications of Transition Metal Oxide
Nanomaterials under Microwave and Conventional Heating
Presented by
Shanthakumar Sithambaram
Major Adviser
Dr. Steven L. Suib
Associate Adviser
Dr. John Tanaka
Associate Adviser
Dr. Ronald Wikholm
Associate Adviser
University of Connecticut
2010
11
DEDICATION
To my parents, Muniyandi and Pathmavathy Sithambaram, my wife Poorani and
my daughter Keerthana for the amazing support you have always provided
iii
Acknowledgements
I would first like to express my gratitude to my advisor, Dr. Steven L.
Suib, for his guidance and support. Not only has his influence been critical to my
development as a research scientist, but he has afforded me other wonderful
opportunities to grow in the areas of communication, administration, and
leadership. I would also like to thank the other committee members, Dr. John
Tanaka, Dr. Ronald J. Wikholm, Dr. Raymond Joesten, and Dr. Xudong Yao for
their help. I would also like to extend my gratitude to Dr. Francis Galasso for
continuous support throughout my stay at Uconn.
I would like to express additional thanks to Dr. Edward Nyutu, he has
been the most influential person in my research development. I want to express
my deepest gratitude for being a wonderful friend and mentor. I would also like to
thank each group member, past and present, that I have worked with during my
time in the Suib Laboratory. I have learned something from every one of you,
from science to culture, but most importantly, I have appreciated your friendship.
Current group members include, Chun-Hu Chen, Aparna Iyer, Hui Huang,
Saminda Dharmarathne. Former group members include Linping Xu, Xiongfei
Shen, Yunshuan Ding, Weina Li, Young-Chan Son, Vinit Makwana, and Ruma
Ghosh. I would like to give a special thanks to Drs. Jonathan C. Hanson and Wen
Wen from the Brookhaven National Laboratory. Finally, I would to thank my
wife, Dr. Poorani Shanthakumar and her advisor Dr. Douglas Pease for the
collaborative work in synchrotron radiation.
iv
TABLE OF CONTENTS
CHAPTER 1
INTRODUCTION
1
1.1
Overview
1
1.2
The purpose, importance, and novelty of the research
2
1.3
Microwaves in Synthesis and Catalysis
3
1.4
Tandem Single-pot Reactions
5
1.5
Sustainability through Catalysis
6
CHAPTER 2
OMS-2 CATALYZED OXIDATION OF
TETRALIN: A COMPARATIVE STUDY OF MICROWAVE
AND CONVENTIONAL HEATING CONDITIONS
7
2.1.
Introduction
7
2.2.
Experimental Section
10
2.2.1.
Catalyst preparation
10
2.2.2.
Catalyst characterization
12
2.2.3
Catalytic reactions
13
2.3.
Results
16
2.3.1. Effect of catalysts
19
2.3.2
Effect of Solvents
19
2.3.3
Catalyst Reusability
21
2.3.4. Effect of Micro wave power
21
2.4.
23
Discussion
v
2.4.1
Oxidation of tetralin
23
2.4.2. Microwave versus conventional heating
25
2.4.3. Characterization of catalysts
30
2.5.
Conclusion
33
2.6.
Reference
34
CHAPTER 3
MICROWAVE-PROMOTED COPPER-
CATALYZED ARYLATION OF 7V-HETEROCYCLES
37
3.1
Introduction
37
3.2
Experimental Section
39
3.2.1
CuO Catalyst Preparation
39
3.2.2
Catalyst Characterization
39
3.2.3
Catalytic Reactions
43
3.3
Results
43
3.3.1. Screening and optimization
43
3.3.2. Effect of aryl halides and N-heterocylces
47
3.3.3. Scale Up
47
3.4
Discussion
49
3.5
Conclusion
50
3.6
Reference
51
vi
CHAPTER 4
MANGANESE OCTAHEDRAL
MOLECULAR SIEVES CATALYZED TANDEM PROCESS
FOR SYNTHESIS OF QUINOXALINES
53
4.1
Introduction
53
4.2
Experimental Section
55
4.2.1. Catalyst preparation and characterization
55
4.2.2. Catalytic reactions
55
4.3
55
Results
4.3.1. Synthesis of quinoxalines with K-OMS-2 catalysts.
56
4.3.2. Effect of various substrates in quinoxaline synthesis
60
4.4.
63
Discussion
4.4.1. Reusability of OMS-2 catalysts
63
4.4
The proposed mechanism
65
4.5
Conclusion
67
4.6
Reference
68
CHAPTER 5
MANGANESE OCTAHEDRAL
MOLECULAR SIEVE CATALYSTS FOR SELECTIVE
STYRENE OXIDE RING OPENING
70
5.1
Introduction
70
5.2
Experimental Section
72
5.2.1
Preparation of catalysts
72
vn
5.2.2
Catalyst Characterization
72
5.2.3
Catalytic testing
72
5.3
Results
73
5.3.1. Catalytic activity
73
5.3.2. Effect of nucleophiles
77
5.3.3. Effect of solvents
77
5.4.
Discussion
81
5.4.1. Catalytic reactions
81
5.4.2. Effect of nucleophiles
85
5.4.3. Effect of solvents
86
5.4.4. Catalyst Reusability
86
5.4.5. Proposed Mechanism of the reaction
87
5.5.
Conclusion
90
5.6.
Reference
91
CHAPTER 6
H2 PRODUCTION THROUGH THE WATER-
GAS-SHIFT REACTION: AN IN SITU TIME-RESOLVED XRAY DIFFRACTION INVESTIGATION OF MANGANESE
OMS-2 CATALYST
93
6.1.
Introduction
93
6.2.
Experimental Section
96
6.2.1. Catalyst preparation and characterization
96
6.2.2.
97
Temperature Programmed Reduction
viii
6.2.3. Catalytic Experiments
97
6.3.
98
Results
6.3.1. Characterization of OMS-2 catalysts
98
6.3.2. Time-resolved X-Ray diffraction studies
99
6.4.
Discussion
102
6.5.
Conclusion
110
6.6.
Reference
111
CHAPTER 7
EXAFS/XANES ANALYSIS OF NANO-SIZED
MANGANESE OCTAHEDRAL MOLECULAR SIEVES AND
THEIR CATALYTIC ACTIVITIES IN THE SYNTHESIS OF
PYRAZINES
113
7.1
Introduction
113
7.2
Experimental Section
115
7.2.1. Catalyst Preparation and characterization
115
7.2.2. XANES/EXAFS Measurements
116
7.2.3. Data Reduction
118
7.2.4. Catalytic Reactions
119
7.3.
120
Results
7.3.1. X-ray absorption Near-edge Spectroscopy (XANES)
120
7.3.2. Extended Absorption Fine Structure (EXAFS).
120
7.3.3. Catalytic Activity of K-OMS-2 Nano Materials.
124
7.4.
126
Discussion
IX
7.4.1. XANES.
126
7.4.2. EXAFS.
127
7.4.3. Catalytic Activity of K-OMS-2Nano Materials.
129
7.5.
Conclusion
129
7.6.
Reference
131
CHAPTER 8
FUTURE WORK
APPENDIX
133
137
x
LIST OF FIGURES
Figure 2.1: Crystal structure of the OMS-2 catalyst. Mn06 octahedra are shown
in blue; potassium atoms are shown as brown spheres.
9
Figure 2.2: CEM MARS 5 Microwave reactor set up.
15
Figure 2.3: XRD patterns of catalysts (a) K-OMS-2R - reflux, (b) K-OMS-2SF solvent free, (c) K - O M S - 2 H Y - hydrothermal and (d) K - O M S - 2 H T - high
temperature.
16
Figure 2.4: Effect of microwave power on conversion and selectivities
22
Figure 2.5: Oxidation of tetralin using K-OMS-2.
23
Figure 2.6: (a) Conversion of tetralin (b) selectivities of products from tetralin
oxidation with time under microwave and conventional heating.
25
Figure 2.7: FE-SEM images of K-OMS-2 catalysts after microwave and
conventional heating.
30
Figure 2.8: XRD patterns of K-OMS-2 catalyst in oxidation tetralin (a) before
reaction (b) after conventional heating (c) after microwave irradiation.
30
Figure 2.9: Raman spectra of K-OMS-2 catalyst; (a) before reaction (b) after
conventional heating and (c) after microwave irradiation.
31
Figure 3.1: Reaction scheme for CuO catalyzed N-arylation
38
Figure 3.2: X-Ray Diffraction pattern of urchin-like CuO.
40
Figure 3.3: FE-SEM image of urchin-like CuO
41
Figure 3.4: Biotage Initiator single-mode microwave reactor set-up.
42
Figure 4.1: Schematic representation of quinoxaline synthesis.
54
Figure 4.2: XRD patterns of K-OMS-2 catalyst in synthesis of quinoxalines (a)
"fresh" catalyst before reaction (b) "spent" catalyst after 2nd cycle.
64
Figure 4.3: Proposed Catalytic Cycle for quinoxaline formation
66
Figure 5.1 (a) Conversion and (b) Selectivities of products with time for ring
opening of styrene oxide.
80
Figure 5.2: X-Ray diffraction patterns of V, Mo and W doped OMS-2 materials.
84
xii
Figure 5.3: Scheme for OMS Catalyzed epoxide ring opening
85
Figure 5.4: Formation of monoimine by (a) oxidative cleavage and diimine by
(b) oxidation-condensation processes.
87
Figure 5.5: Mechanism of imine formation from amine.
88
Figure 6.1: A 3-D plot of in situ TR-XRD patterns of OMS-2 catalyst during the
water-gas shift reaction. The catalyst was heated from room temperature to 350°C
and WGS reaction was carried out at 350, 400 and 500°C.
100
Figure 6.2: a) H2 and CO2 concentrations measured during the WGS reaction
with OMS-2 at various temperatures for 1st and 2nd pass, b) TR-XRD patterns of
OMS-2 catalyst after 1st and before and after 2nd passes.
101
Figure 6.3: TPR of OMS-2 catalyst, (a) Reduction of catalyst to MnO.
103
Figure 6.4: XRD patterns of catalyst before and after the WGS reaction under
laboratory conditions.
104
Figure 6.5: WGS reactivity of OMS-2 catalyst under laboratory conditions.
Conversion of CO with temperatures from 50°C to 350°C.
xiii
106
Figure 6.6: TR-XRD patterns of the OMS-2 catalyst acquired during the flow of
H2. The final form of the catalyst observed was MnO.
108
Figure 6.7: Time-resolved XRD patterns for the oxidation of reduced MnO
catalyst with oxygen.
109
Figure 7.1: Schematic representation of XAS phenomenon.
114
Figure 7.2: Experimental set-up for XAS measurements.
117
Figure 7.3: (a) Mn K-edge XANES spectra of K-OMS-2 (HT) catalyst and
reference compounds, (b) Oxidation state determination of K-OMS-2 (HT)
catalyst using the Mn K-edge energy shift of the Mn reference compounds.
122
Figure 7.4: EXAFS refinement using theoretical phases and amplitudes to
experimental K-OMS-2.
123
Figure 7.5: Proposed catalytic cycle for OMS-2 catalyzed synthesis of pyrazines.
130
Figure 8.1: Synthesis of Esters from Alcohols.
134
Figure 8.2: Synthesis of Amides from Alcohols.
134
xiv
Figure 8.3: C-C Bond formation with Alcohols.
135
Figure 8.4: Synthesis of Nitriles from Alcohols.
135
Figure 8.5: Synthesis of Oximes from Alcohols.
135
xv
LIST OF TABLES
Table 2.1: Effect of different types of K-OMS-2 in microwave (MW) and
conventional (A) heating.
18
Table 2.2: Effect of different solvents in microwave and conventional heating
20
Table 2.3: The catalysts reusability in microwave and conventional heating.
20
Table 3.1: Optimization of Cu-catalyzed C-N coupling.
44
Table 3.2: Arylation Scope with Respect to Aryl halides.
45
Table 3.3: Arylation Scope with Respect to N-heterocycles
46
Table 3.4: Scale-up of C-N coupling
48
Table 4.1: Synthesis of quinoxalines with K-OMS-2 catalysts
58
Table 4.2: K-OMS-2 catalyzed synthesis of quinoxalines with various substrates.
59
Table 4.3: Catalyst reusability
62
xvi
Table 5.1: Conversion of Styrene Epoxide by OMS-2 Catalysts
75
Table 5.2: Styrene Oxide Ring Opening Catalyzed by Doped OMS-2 (reflux)
76
Table 5.3: Styrene oxide ring opening with different nucleophiles.
78
Table 5.4: Ring opening of styrene oxide with solvents over K-
79
OMS-2R
Table 5.5: Lewis acidity/ Lewis basicity of K-OMS-2 catalysts.
82
Table 7.1: Energy shifts and oxidation states of reference compounds and
samples.
121
Table 7.2: EXAFS fit results of theoretical model calculations to the experimental
K-OMS-2 catalyst.
124
Table 7.3: Synthesis of dihydropyrazines using K-OMS-2 catalysts.
125
Table 7.4: Properties of K-OMS-2 catalysts.
126
xvii
CHAPTER 1
INTRODUCTION
1.1
Overview
This research centers on novel catalytic applications of transition metal
oxide catalysts to produce value added chemicals which are useful in fine
chemicals, pharmaceutical intermediates, and energy industry. The research
consists of two sections. The first section describes novel catalytic applications of
transition metal oxide catalysts to produce value added chemicals. The second
section deals with synchrotron radiation based in situ and ex situ characterization
of catalysts using Time-resolved X-ray diffraction
and Extended X-ray
Absorption Fine Structure techniques.
Heterogeneous catalysis is of vital importance to the world's economy,
allowing us to convert raw materials into valuable chemicals and fuels in an
economical, efficient, and environmentally benign manner. For example,
heterogeneous catalysts have numerous industrial applications in the chemical,
food, pharmaceutical, automobile and petrochemical industries, and it has been
estimated that 90% of all chemical processes use heterogeneous catalysts.
Heterogeneous catalysis is also finding new applications in emerging areas such
as fuel cells, green chemistry, nanotechnology, and biorefining/biotechnology.
Indeed, continued research into heterogeneous catalysis is required to allow us to
address increasingly complex environmental and energy issues facing our
industrialized society.
1
1.2
The purpose, importance, and novelty of the research
Historically, non-catalytic routes were used for the syntheses of fine
chemicals. The pressure on production cost (cutting of processing steps, batch
versus continuous), the need for waste minimization, safety aspects, changes in
raw materials and many other things have led fine chemical producers to look at
transition metal-based catalytic processes. The choice of the industrial
manufacturing process for a chemical product is determined primarily by
economic and environmental considerations. Low price, high quality (e.g., purity),
and secure availability are important for the buyer of a fine chemical. Transition
metals may lead to new reaction pathways that are not found in conventional
organic chemistry, thus leading to a reduction in the number of reaction steps. Of
course, they may also provide routes to novel products. In addition, the use of
transition metals can reduce the severity of the reaction conditions, which can
lead to substantial cost savings (energy efficiency). One of the greatest virtues of
applying transition metals rests in the improvement of all kinds of selectivity:
chemoselectivity, regioselectivity, and stereoselectivity.
Concerns about our environment are already forcing us to modify many
older technologies in order to reduce atmospheric pollution or to eliminate
hazardous waste. Government legislation will force changes in the way
companies produce chemicals. The process with the highest yield may not always
be the appropriate choice. Instead, there will be a balance between the selectivity
2
of the products and the yield. Even small quantities of by-products in fine
chemical synthesis will no longer be acceptable if they create unacceptable
environmental hazards. It may be more desirable to suffer a slight yield loss than
to have to dispose of a highly toxic by-product associated with a higher-yield
process. In this respect, Sheldon introducing the concept of "E-factor", has
reported that in fine chemicals manufacture 5-50 kg by-products per kg products
(for pharmaceuticals 25-100 kg) can be expected [1]. In fine chemicals synthesis,
the goals of "green chemistry" and sustainable chemistry must be aimed at.
Transition metals are used in the synthesis of fine chemicals either catalytically or
stoichiometrically. As a general rule in industry, catalytic routes are preferred
over stoichiometric ones whenever possible. However, this often does not hold for
fine chemicals, which are manufactured predominantly by multi-step syntheses,
and the added value can be very high.
1.3.
Microwaves in Synthesis and Catalysis
Improving research and development (R&D) productivity is one of the biggest
tasks facing the chemical industry. Consequently, there is increased interest in
technologies and concepts that facilitate more rapid synthesis and screening of
chemical substances to identify compounds with appropriate qualities. One such
high-speed technology is microwave-assisted synthesis (MAS). Traditionally,
many reactions are heated using an external heat source (such as an oil bath), and
therefore heat is transferred by conductance. This is a comparatively slow and
inefficient method for transferring energy into the system because it depends on
3
the thermal conductivity of the various materials that must be penetrated, and
results in the temperature of the reaction vessel being higher than that of the
reaction mixture. By contrast, microwave irradiation produces efficient internal
heating by direct coupling of microwave energy with the polar molecules such as,
solvents, reagents and catalysts that are present in the reaction mixture. Although
most of the early pioneering experiments in MAS were performed in domestic,
sometimes modified, kitchen microwave ovens, the current trend is to use
dedicated instruments which have only become available in the last few years for
chemical synthesis. Microwave-assisted heating under controlled conditions is an
invaluable technology for medicinal chemistry and drug discovery applications
because it often dramatically reduces reaction times, typically from days or hours
to minutes or even seconds.
Many reaction parameters, such as reaction temperature and time, variations
in solvents, additives and catalysts, or the molar ratios of the substrates, can be
evaluated in a few hours to optimize the desired chemistry. In addition, MAS
technology often facilitates the discovery of novel reaction pathways, because the
extreme reaction conditions attainable by microwave heating sometimes lead to
unusual reactivity that cannot always be duplicated by conventional heating. The
monitoring mechanisms for temperature and pressure in modern microwave
reactors allow for an excellent control of reaction parameters, which generally
leads to more reproducible reaction conditions. Microwave heating can rapidly be
adapted to a parallel or automatic sequential processing format. In particular, the
4
latter technique allows for the rapid testing of new ideas and high-speed
optimization of reaction conditions. The fact that a "yes or no answer" for a
particular chemical transformation can often be obtained within 5 to 10 minutes
(as opposed to several hours in a conventional method) has contributed
significantly to the acceptance of microwave chemistry both in industry and
academia. Therefore, many academic and industrial research groups are already
using MAS as a forefront technology for rapid optimization of reactions, for the
efficient synthesis of new chemical entities, and for discovering and probing new
chemical reactivity.
1.4.
Tandem Single-pot Reactions
Environmental concerns and regulations have increased in the public, political
and economical world over the last 15 years because quality of life is strongly
connected to a clean environment. The impulse for developing new, more
efficient and selective catalysts and for the realization of new process technology
is strongly related to environmental compatibility. One way to achieve those
targets can be described by 'one-pot' reaction procedure that means chemical
conversions consisting of a number of individual reactions are brought about in
one reaction step by applying multifunctional catalysis. Such direct synthesis
routes help to avoid side product formation and loss of starting material as well as
to reduce capital investment and operation costs.
5
1.5.
Sustainability through Catalysis
To reach the goals that the community of scientists focusing on selective
oxidation has nowadays identified, one must recognize a number of important
needs. These are enumerated below:
- the desirability of effecting single-step and/or solvent-free processes;
- to maximize the use of mild conditions, ideally involving benign reagents and
benign catalysts;
- to replace stoichiometric oxidation processes by catalytic ones;
- to design durable heterogeneous (solid) catalysts of very high activity and
selectivity that are amenable to recycling.
Finally, as well as seeking cheap, readily preparable catalysts, they should also be
capable of using oxygen (preferably) or H2O2 (or alkyl hydroperoxides) - in order
of decreasing preference - as the oxidizing agent.
6
CHAPTER 2
OMS-2 CATALYZED OXIDATION OF TETRALIN:
A COMPARATIVE STUDY
2.1.
Introduction
Microwave irradiation is becoming a popular method to heat materials in
chemical laboratories, offering a clean, inexpensive, convenient, and selective
alternative heating method [1]. Microwaves differ from conventional heat sources,
in that the solvents or reactants are selectively heated without heating the
microwave transparent reaction vessel. In contrast, in conventional heating
methods, the vessel is heated and this heat is then transferred by convection. Thus,
since the solvents or reactants are heated directly, microwave heating is more
efficient in terms of the energy used, and is considerably more rapid than
conventional heating [2]. Microwave heating in catalysis has gained greater
attention in recent years [3] and has been used in gas phase heterogeneous
catalysis to attain rate enhancements, higher yields and improved selectivities [4].
"Hot spot" formation is believed to be responsible for the observed enhanced
results in gas phase reactions. Liquid phase heterogeneous catalysis under
microwave irradiation has been studied to a lesser extent. The majority of these
liquid phase reactions reported was performed using microwave heating and was
carried out in sealed vessels at higher temperatures [5]. The use of sealed vessels
at high temperatures can pose safety problems; changing from sealed to an open
reaction vessel eliminates such safety concerns in microwave and allows us to
compare reactions carried out conventionally. In general, microwave heating has
7
been shown to enhance reaction rates in the literature. However, most of these
reports are based on inaccurate comparisons with conventional heating conditions
which do not allow conclusions to be made on the effects of microwave heating.
Hence, strict comparisons between reactions carried out under similar conditions
(reaction medium, temperature, pressure, time, reactor type (open/closed vessel),
etc.) for microwave and conventional heating are needed [6].
Oxidation of alkyl aromatics using peroxide oxidants has attracted interest
in the scientific community because the products are commercially important [7].
Traditionally, stoichiometric oxidants such as permanganate and dichromate have
been used [8] and recently, a commercial tetravalent chromium dioxide
(Magtrieve) has been employed to transform hydrocarbons to ketones under
microwave irradiation [9]. These reactions however require excessive amounts of
stoichiometric oxidants and produce large amounts of toxic waste when they are
scaled up in industrial processes. Therefore, the discovery and utilization of
environmentally benign catalysts is a promising area of research.
Manganese octahedral molecular sieves are well known for their catalytic
activity and the most recent examples have been efficient synthesis of imines [10]
and synthesis of 2-aminodiphenylamine [11]. Various types of OMS have been
synthesized in our laboratory and their characterization are well established [12,
13]. For the present study, K-OMS-2 (cryptomelane) type materials synthesized
by reflux, solvent-free, hydrothermal and high temperature methods were
8
employed as catalysts. They contain one-dimensional tunnel structures formed by
2 x 2
edge shared MnQ6 octahedra.
The composition of K-OMS-2 is
KMn80i6-nH20. The average oxidation state of Mn in K-OMS-2 is -3.8 due to
the presence of Mn4+, Mn3+, and Mn2+ions in the framework [14, 15]. OMS-2
materials have been reported to be environmentally friendly and can be reused
without loss of activity. Environmentally friendly catalytic oxidation reactions
have tremendous advantages over those oxidation processes that use a
stoichiometric reagent [16].
^D
Fngmire 2.1: Crystal structure of the OMS-2 catalyst. Mn©6 octahedra are shown
in blue; potassium atoms are shown as brown spheres.
9
Microwave irradiation as an alternative source of heating has advantages
over conventional heating by reducing the reaction time, enhancing conversion,
and selectivity. Many manganese Octahedral Molecular Sieve catalyzed reactions
involving conventional heating have been reported in our laboratory. Some of
these reactions required long reaction times (4-48 h). The objective of this study
was to use microwave heating as an alternative to conventional heating in open
vessel batch reactors and to study the effects of microwave irradiation on the
conversion, selectivity and the stability of the catalysts. The work focuses on the
oxidation of tetralin catalyzed by manganese octahedral molecular sieves using
ter/-butyl hydroperoxide (TBHP) as an oxidant in the liquid phase. Tetralin
oxidation can lead to a range of commercially valuable products, such as atetralone [17].
Comparative experiments were carried out under similar conditions using
microwave and conventional heating. XRD, SEM and Raman spectroscopy
methods were utilized to characterize the catalysts before and after reactions. The
catalysts were reused in both microwave and conventional heating employing a
simple regeneration procedure.
2.2
Experimental Section
2.2.1. Catalyst preparation
A solvent free method was used for
K-OMS-2 S F
to prepare in the
following manner [18]. A mixture of 9.48 g of KMn04 and 22.05 g of
10
Mn(Ac)2.4H 2 0 was ground homogeneously in an agate mortar. The mixed
powders were then placed in a capped glass bottle and maintained at 80°C for 4
hrs. The resulting black powder was thoroughly washed with deionized water
several times to remove any ions that might be present and was finally dried
overnight at 80°C.
A combination of sol-gel and combustion methods [19] were used for
preparation of the high temperature K - O M S - 2 H T . KNO3 and Mn(N03)2 in a molar
ratio of 1:5 were dissolved in distilled deionized water (solution A). Glycerol and
KNO3 were mixed in a 1:10 ratio (solution B). Solutions A and B were then
mixed in deionized water with vigorous stirring to form a clear solution and then
heated to 120°C to form a gel (usually 5 h). The gel was heated to 250°C for 2 h
to complete the combustion reaction. The black powder was then calcined at
600°C for 3 h to obtain the final product.
A reflux method was employed for the preparation of K- O M S - 2 R [20]. 225
mL of 0.4 M potassium permanganate solution was added to a mixture of 1.75 M
manganese sulfate hydrate solution (67.5 mL) and concentrated nitric acid (6.8 mL).
The dark brown slurry was refluxed for 24 h, then filtered and washed with deionized
water several times. The catalyst was dried at 120°C overnight before use.
A hydrothermal method was used to prepare K - O M S - 2 H Y [21]. The synthesis
mixture was composed of a total of 32 mmol of K2SO4, K2S2O8, and MnSC^.thO
in a 3:3:2 ratios dissolved in 70 mL of doubly distilled water. The mixture was
11
transferred to a 125 mL Teflon vessel held in a stainless-steel vessel. The vessel
was sealed and placed in an oven and heated at 250°C for four days.
2.2.2. Catalyst characterization
2.2.1. X-ray powder diffraction studies
X-ray powder diffraction (XRD) experiments were carried out using a
Scintag Model PDS 2000 diffractometer in a continuous scan mode. Samples
were loaded on glass slides, and Cu Ka radiation [k = 1.5418 A] was used at a
beam voltage of 45 kV and 40 mA beam current. The X-ray patterns of the
catalysts were comparable to that of the standard OMS-2 materials (Figure 2.3).
2.2.2
Scanning Electron Microscopy
Scanning Electron Micrographs were taken on a Zeiss DSM 982 Gemini field
emission scanning microscope with a Schottky Emitter at an accelerating voltage
of 2 kV with a beam current of 1 uA (Figure 2.4).
2.2.4
Raman Spectroscopy
Raman spectra were taken at room temperature in the range of 100-2000
cm-1 with a Renishaw 2000 Raman microscope setup, which includes an optical
microscope and a CCD camera for multichannel detection. A 514 nm argon ion
laser was used to record the spectra for the K-OMS-2 materials.
12
2.2.3
Catalytic Reactions
The conventional reactions were carried out in a batch reactor operating
under reflux conditions at 70°C for a period of 2 hours. To a 25 mL round
bottomed flask, 1 mmol of tetralin, 5 mL of acetonitrile (dichloroethane or
dimethyl formamide), and 50 mg of K-OMS-2 catalyst were added. Finally, 5
mmol of TBHP was added drop wise to the mixture. The reaction mixture was
continuously stirred using a magnetic stirrer. The reaction progress was monitored
by TLC (thin-layer chromatography).
Comparative experiments were performed using microwave irradiation in
an open batch reactor operating under reflux. Identical quantities of catalyst and
reactant were loaded into the reactor as in the conventionally heated reactions. To
a 25 mL 3-necked round bottomed flask, 1 mmol of tetralin, 5 mL of acetonitrile
(dichloroethane or dimethyl formamide), and 50 mg of K-OMS-2 catalyst were
added. Finally, 5 mmol of TBHP was added drop wise to the mixture. The
reaction mixture was placed in the microwave cavity of a MARS 5 (CEM)
microwave system. The reaction temperature was monitored by an infrared
pyrometer and a feedback loop provided the required variations in microwave
power necessary to maintain the predetermined temperature. The reaction mixture
was stirred using a magnetic stirrer. The progress of the reaction was monitored
using TLC as they were in the conventionally heated reactions.
13
2.4
Analytical procedure
By the end of each reaction, aliquots were taken and diluted to constant
concentration. Gas chromatography-mass spectroscopy (GC-MS) methods were
used for the identification. GC-MS analyses were done using an HP 5890 series II
chromatograph with a thermal conductivity detector coupled with an HP 5970
mass selective detector. An HP-1 column (non polar cross linked siloxane) with
dimensions of 12.5 m x 0.2 mm x 0.33 um was used in the gas chromatograph.
14
Figure 2„2: CEM MARS 5 Microwave reactor set up.
2.3.
Results
(310)
(211)
(110) (200)
20
40
Two-theta (degree)
80
60
Figure 2.3: XRD patterns of catalysts (a) K-OMS-2R - reflux, (b) K-OMS-2SF
solvent free, (c) K-OMS-2HY - hydrothermal and (d) K-OMS-2HT - high
temperature.
16
Figure 2.4: FESEM images of K-(J
Conventional (c) Hydrothermal and
catalysts, (a) Solvent free (b)
High Temperature.
17
Table 2.1: Effect of K-OMS-2 in microwave (MW) and conventional (A) heatinga
OH
CO — C p Cg Op CQ i
OH
1
OH
O
2
3
O
4
Product selectivity (%)c
catalyst
b
method
Conversion
(%)
K-OMS-2R
MW
A
83.4
70.9
30.1
18.0
59.8
70.1
5.6
9.8
3.7
2.1
0
0
K-OMS-2SF
MW
A
87.9
80.3
8.9
16.2
65.1
65.8
22.5
16.2
1.3
1.8
2.0
0
K-OMS-2HT
MW
A
52.5
41.8
41.2
32.2
48.5
61.8
4.1
5.2
6.2
0
0
0
K-OMS-2HY
MW
A
79.9
68.2
13.0
16.3
67.4
64.4
13.2
11.7
4.8
7.6
1.3
0
a.
Reaction conditions:
MW: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 2 h, 150W power.
A: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 2 h.
b.
K-OMS-2R - reflux, K-OMS-2SF - solvent free, K-OMS-2HT - high temp., K-OMS-2HY hydrothermal.
c.
(1) 1-tetralol, (2) 1-tetralone, (3) 1, 4-dihydroxytetralin, (4) 1, 4-naphthaquinone, (5)
naphthalene.
18
2.3.1
Effect of catalysts
First, the oxidation of tetralin using TBHP as the oxidant was carried out
with K-OMS-2 catalysts prepared by reflux, solvent free, high temperature and
hydrothermal methods. The results are listed in Table 2.1. The reaction produced
several oxidative products of tetralin; the major products were 1-tetralone and 1tetralol (>75%), followed by small amounts (0-22%) of 1, 4-dihydroxytetralin, 1,
4-naphthaquinone and naphthalene. The reactions were performed using
microwave and conventional heating while keeping the reaction conditions
similar for comparison purposes. In all cases, the conversion was higher when
microwave heating was used instead of conventional oil bath heating. Among the
catalysts used, K-OMS-2 catalysts prepared by the solvent free methods gave the
highest conversions in both heating methods; 87.9% conversion with microwave
heating and 80.3% conversion with conventional heating. K-OMS-2 prepared by
high temperature methods gave the lowest conversions for microwave and
conventional heating, 52.5% and 41.8% respectively.
2.3.2
Effect of Solvents
Acetonitrile (MeCN), Dichloroethane (DCE) and dimethylformamide
(DMF) were evaluated as solvents for the oxidation of tetralin (Table 2.2).
Conversions and selectivities were higher when acetonitrile was used as the
solvent for reactions conducted in microwave and conventional heating. The
reason for the low conversion of tetralin [10-56%] with DCE and DMF could be
19
that competitive binding of polar solvent molecules to the active sites of the
catalysts prevented the reaction process in the active sites.
Table 2.2: Effect of different solvents in microwave and conventional heating3.
Conversion/%
Solvent"
Dielectric
constant
Tan 8°
MeCN
36.6
0.062
83.4
70.9
DCE
10.3
0.127
56.5
42.6
DMF
38.3
0.161
12.2
10.8
MW
a.
Reaction conditions:
MW: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 2 h, 150W power.
A: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 2 h.
b.
MeCN: acetonitrile, DCE: dichloroethane, DMF: dimethylformamide.
c.
Tan 8: e'Vs', e"- loss factor, e'- dielectric constant.
Table 2.3: The catalysts reusability in microwave and conventional heating.
cycle
1
2
3
4
5
Catalyst amount
(mg)
100
75
50
25
10
Conversion (%)
MW
A
54.1
45.2
57.6
48.9
60.3
52.3
57.2
50.8
55.2
48.3
Reaction conditions:
MW: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 30 min, 150W power.
A: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 30 min.
20
2.3.3
Catalyst Reusability
In order to test the reusability of the K-OMS-2 catalyst in both microwave
and conventionally heated reactions, initial runs were done with 100 mg of the
catalysts in both cases. After each subsequent reaction cycle, the catalyst was
recovered from the reaction mixture by filtration, regenerated by washing with
acetone and water to remove any organics adsorbed and then dried at 300°C for 6
h. Due to the loss of catalysts during the filtration process, a reduced amount of
catalysts was used in the subsequent cycles. The results of conversions in each
cycle are listed in Table 2.3. The regenerated catalyst was reused without any
appreciable loss of activity.
Moreover, a turnover frequency up to 20 was
achieved with 50 mg of catalyst.
2.3.4. Effect of Microwave power
The microwave power was varied from 100 W to 300 W during the
oxidation of tetralin. Effects of irradiation during a reaction should reflect the
magnitude of the radiation. Different microwave powers were applied and the
resulting graphs are shown in Figure 2.4. Altering microwave power should bring
about changes in selectivity. However, no significant changes in both conversions
and selectivities were observed.
21
0J
75
,
125
,
175
,
225
,
275
1
325
Power (Watts)
Figure 2.4. Effect of microwave power on conversion and selectivities; Reaction
conditions:
MW: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 30 min, 150W power.
A: 1 mmol tetralin, 5 mmol TBHP, 50 mg K-OMS-2, 5 mL ACN, 70° C, 30 min.
22
2.4.
Discussion
2.4.1
Oxidation of tetralin
o
Figure 2.5: Oxidation of tetralin using K-OMS-2.
In the catalytic oxidation of hydrocarbons, the catalytic oxygen activation
process uses a metal catalyst and an oxygen donor to form either a peroxometal or
an oxometal intermediate [22]. In the peroxometal pathway, the oxidation state of
the metal remains constant and the metal ion merely acts as a Lewis acid to
promote the oxidizing ability of the peroxo group. The oxometal pathway on the
other hand, involves a two electron reduction of the metal ion, which is then
reoxidized by the oxygen donor [23]. The oxidation of tetralin with K-OMS-2
catalysts is believed to proceed via the peroxometal pathway. Therefore, in the
oxidation of tetralin, TBHP functions as the oxygen donor and the OMS-2 metal
catalyst act as a Lewis acid. The catalyst interacts with TBHP to form a peroxocomplex, which then interacts with tetralin to give an unstable intermediate tert-
23
butylperoxytetralin, which was not detected by GC-MS [24]. The results obtained
for the oxidation of tetralin, listed in Table 2.1, indicated that increased Lewis
acidity of the catalyst increases conversion. Ghosh et al. have reported that the
Lewis acidity of catalysts used as K-OMS-2SF >K-OMSR>
OMS-2 H T
K-OMS-2 H Y>
K-
[25]. The ratio of Mn4+, Mn3+, and Mn2+ present on the K-OMS-2
catalysts may influence the strength of Lewis acid sites. However, it is not certain
that this factor solely contributes to the measured Lewis acidity of these catalysts.
K-OMS-2SF
prepared by the solvent free method which has the highest Lewis
acidity shows the highest conversion, while
K-OMS-2 H T
prepared by the high
temperature method shows the lowest conversion (43-52%) for the oxidation of
tetralin for both microwave and conventional heating. The products obtained in
the oxidation of tetralin over K-OMS-2 catalysts are similar to those obtained by a
homogeneous pathway using manganese (III) porphyrins as the catalyst and H2O2
as the oxidant [26]. Garcia et al. have reported that the oxidation of tetralin with
oxygen using Cu catalysts leads to the formation of tetralol, tetralone, and low
molecular weight "polyoxygenates" [27]. In the oxidation of tetralin with KOMS-2 catalysts, the major products are tetralol and tetralone by C-H bond
oxidation. The other products are obtained due to further oxidation of tetralol. The
highest selectivity (-70%) was obtained for tetralone for both in microwave and
conventional heating.
24
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The oxidation of tetralin with K-OMS-2 catalysts showed enhanced
conversion (88%) when microwave heating was used as compared to
conventional heating results under open vessel conditions (Figure 2.6). The
reaction profiles indicate that the initial rates of reactions for both microwave and
conventional heating are higher and later the rates plateau. In the case of the
oxidation tetralin, 88% conversion with microwave heating in 4 hours can only be
achieved after 10.5 hours with conventional heating. Earlier, Bogdal et al. have
shown that enhanced conversions can be achieved for the oxidation of aromatics
under microwave irradiation in comparison to conversions obtained in
conventional processes [28]. Significant rate enhancements have been reported in
microwave assisted heterogeneous catalysis in the gas phase and this has been
accounted for non-uniformity in heating of the materials.
However, similar
phenomena are not present in liquid phase heterogeneous catalysis because the
temperature gradient between the catalyst particle and the liquid surrounding is
significantly lower due to efficient heat transfer by the liquid while stirring as
compared to the gradient in gas phase. Therefore, the rate enhancements observed
in the liquid phase oxidation of tetralin under microwave irradiation are not as
dramatic as would be expected. Furthermore, stirring or agitation of microwave
heated solutions has been shown to prevent superheating of the reaction mixture
[29].
The enhancement in reaction rate however corresponds to differences in
the reaction temperatures. For both microwave and conventional heated reaction
26
systems, the bulk temperature is equal, but, the temperature at the reaction site, i.e.
the catalytic surface in this case, is higher than the surroundings. This is because
the heat transfer by microwaves depends on the specific loss factors (tan 8) of
different materials present in the reaction mixture. Selective heating may create
elevated temperatures at the surface of the catalyst in heterogeneous catalytic
reactions involving montmorillonite and zeolites under microwave irradiation [30].
Many transition metal oxide catalysts are semiconductors and will readily absorb
microwave energy. K-OMS-2 catalysts which consist of mixed valent manganese
oxides have been reported to have very high dielectric constants; therefore, good
coupling can be expected with microwave irradiation [31, 32]. The apparent
temperature of the catalytic site under microwave irradiation can be estimated
using the Arrhenius equation k = A exp (-Eact/RT). In the steady state, the heat
transfer to the catalyst due to the microwaves is equal to the heat loss of the
catalyst to its surroundings. The temperature gradient will depend on the loss
factor of catalyst, solvent, and the radius of the catalyst [33].
Specific microwave effects can also be anticipated when the polarity is
increased during the reaction profile from the ground state to the transition state
[34]. A similar effect should be observed for polar reaction mechanisms at the
catalytic surfaces, where the polarity is increased going from the ground state to
the transition state, thus resulting in an enhancement of the reactivity by lowering
the activation energy. During the oxidation of tetralin, starting from a neutral
hydrocarbon, polar alcohols and ketones are formed as products, involving
27
polarized transition states. Therefore, the stabilization energies of the transition
states are more effective than that of the ground state under microwave irradiation,
which in turn leads to enhanced reactivity by decreasing the activation energy
(AG A >AGMW)-
The oxidation of tetralin under microwave irradiation yields more
products due to decreases in the activation energies. Fig. 2.6(b) shows that at 15
and 30 minutes, apart from tetralol and tetralone as major products, other products
were also observed under microwave conditions, whereas for conventional
heating, only the major products were observed.
The. selectivity of the products obtained in both microwave and
conventional heating were different in the oxidation of tetralin catalyzed by OMS2. Selectivities of reactions can be altered under microwave irradiation when
compared selectivities for conventional heating [35]. This microwave effect was
prominent in the selectivities in the oxidation of tetralin with all of the K-OMS-2
catalysts (Table 2.1). Even though, the different K-OMS-2 types used for this
study have the same composition, their intrinsic properties, such as, average
oxidation number, surface area, and pore volume vary [36]. The interaction of
microwaves at the active sites of the catalysts also varies leading to a difference in
selectivity.
In order to investigate the effect of solvents in the oxidation of tetralin,
dichloroethane and DMF were also studied. Dichloroethane and DMF absorb
microwaves more efficiently compared to acetonitrile due to their higher
28
dielectric loss factors (tan 8). However, the observed rate enhancement does not
correlate due to selective absorption in microwave heating as compared to that in
conventional heating in the oxidation of tetralin (Table 2.2). The results obtained
also suggest that the superheating of solvents did not enhance conversions. If
superheating was one of the reasons for the enhanced conversion in microwave
heating, the reactions done in solvents that have higher tan 5, such as DMF,
should have resulted in higher conversions [37]. In addition, when the solvents are
heated by microwave irradiation at atmospheric pressure in an open vessel, the
boiling point of the solvent (as in an oil-bath experiment) typically limits the
reaction temperature that can be achieved. In the absence of any specific or
nonthermal microwave effects (such as the superheating effect at atmospheric
pressure) the expected rate enhancements should be comparatively small. This is
another reason why rate enhancements obtained in an open vessel liquid phase
reaction are not comparable to the rate enhancements obtained in closed vessel
liquid phase (under pressure) or gas phase reactions under microwave conditions.
29
2.4.3
Characterization of catalysts
Figure 2.7: FE-SEM images of K-OMS-2 catalysts after microwave and
conventional heating.
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(110)
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30
40
50
60
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Figure 2.8: XRD patterns of K-OMS-2 catalyst in oxidation tetralin (a) before
reaction (b) after conventional heating (c) after microwave irradiation.
30
300
400
500
600
700
800
Raman Shift (cm-1)
Figure 2.9: Raman spectra of K-OMS-2 catalyst; (a) before reaction (b) after
conventional heating and (c) after microwave irradiation.
900
31
The X-ray diffraction (XRD) patterns of K-OMS-2 catalysts prepared by
reflux, solvent-free, hydrothermal, and high temperature methods are in good
agreement with the standard pattern of the pure tetragonal cryptomelane phase
(Figure 2.3). However, diffraction peak broadening effects were observed among
the catalysts, implying that K-OMS-2 catalysts prepared by different methods
have varying crystallite size. The crystallite sizes calculated from the (211)
diffraction lines using Scherrer's equation for K-OMS-2R, K-OMS-2SF, K-OMS2 H T,
and K-OMS-2 H Y are 18, 10, 20, and 17 nm, respectively. The data imply that
the lower the particle size the higher the catalytic activity and vice versa.
The X-ray diffraction (XRD) patterns of K-OMS-2R catalysts before and
after the reactions remain the same indicating that the catalysts were not affected
by microwave heating. The intensity and width of the peaks in used catalysts were
similar to those of the K-OMS-2R catalysts before the reactions (Figure 2.8).
Hence, there was neither a change in particle size nor a change in the oxidation
numbers of the Mn metal in the catalyst upon exposure to microwave irradiation.
The morphology of K-OMS-2R catalyst prepared by reflux methods used
during the reaction exposed to conventional and microwave heating was studied
by scanning electron microscopy (SEM). Some catalysts can be affected by
microwave irradiation and hence their morphology can be altered during reactions
[38]. In the oxidation of tetralin, the original fibrous morphology of K-OMS-2R
was retained before and after the reactions (Figure 2.7). There was no crack
32
formation or melting of the catalysts in the microwave and conventional heating
experiments.
The catalysts were also characterized using Raman spectroscopy. Raman
spectra of
K-OMSR
catalysts before and after reactions with conventional and
microwave heating are represented in Figure 2.9. The spectra were identical and
indicated that there were no structural changes in the catalysts during the reactions.
The peaks that appear at 570 and 650 cm"1 are characteristic of Mn-0 lattice
vibrations [39].
2.5.
Conclusions
In summary, an efficient tetralin oxidation process catalyzed by K-OMS-2
has been described. The microwave induced oxidation of tetralin catalyzed by KOMS-2 shows enhanced conversion (52-88%) in comparison to conversions in
conventionally heated reactions (42-80%) under similar conditions. Selective
heating of the polar transition states of the substrate at the active sites of the
catalysts can be attributed for this ~ 10% enhanced conversion. K-OMS-2
prepared by solvent free methods showed the highest activity (80-88%)
conversion) among the catalysts used. Characterization of the catalysts after the
reactions suggests that the catalysts were not affected by microwave irradiation.
Moreover, the K-OMS-2 catalysts can be reused without loss of any activity.
33
2.6.
References
[1]. C. O. Kappe, D. Dallinger, Nature Rev. 5 (2006) 51.
[2] C. O. Kappe, Angew. Chem. Int. Ed. 43 (2004) 6250.
[3] X. Zhang, D. O. Hayward, M. P. Mingos, Cat. Lett. 88 (2003) 33.
[4]. H. Will, P. Scholz, B. Ondruschka, Top. Catal. 2004, 29, 175.
[5] N. Kunhert, Angew. Chem. Int. Ed. 41 (2002) 1863.
[6] P.Goncalo, C. Roussel, J. M. Melot, J. Vebrel, J. Chem. Soc. Perkin. Trans. 2
(1999)2111.
[7] A. K. Suresh, M. M. Sharma, T. Sridhar, Ind. Eng. Chem. Res. 39 (2000) 3958.
[8] A. Shaabani, P. Mirzaei, S. Naderi, D. G. Lee, Tetrahedron, 60 (2004) 11415.
[9] M. Lukasiewicz, D. Bogdal, J. Pielichowski, Adv. Synth. Catal. 345 (2003)
1269.
[10] S. Sithambaram, R. Kumar, Y. C. Son, S. L. Suib, J. Catal. 253 (2008) 269.
[11] R. Kumar, L. J. Garces, Y-C. Son, S. L. Suib, R. E. Malz, J. Catal. 236
(2005) 387.
[12] S. L. Suib, J. Mater. Chem. 18 (2008) 1623.
[13] Y. C. Son, V. D. Makwana, A. R. Howell, S. L. Suib, Angew. Chem. Int. Ed.
40(2001)4280.
[14] X-F. Shen. Y-S. Ding, J. Liu, Z-H. Han, J. I. Budnick, W. A. Hines, S. L.
Suib, J. Am. Chem Soc. 127 (2005) 6166.
[15] W-N. Li, J. Yuan, X-F. Shen, S. Gomez, L-P. Xu, S. Sithambaram, M.
Aindow, S. Suib, Adv. Func. Mater, 16 (2006) 1247.
[16] R. A. Sheldon, J. Chem. Tech. Biotechnol. 68 (1997) 381.
34
[17] W. F. Taylor, US Patent 4723963 (1988).
[18] Y-S. Ding, X-F. Shen, S. Sithambaram, S. Gomez, R. Kumar, M. B. Vincent,
S. L. Suib, Chem. Mater. 17 (2005) 5382.
[19] X. F. Shen, Ph. D. Thesis, University of Connecticut, 2007.
[20] R. N. DeGuzman, Y-F. Shen, E. J. Neth, S. L. Suib, C. L. O'Young, S.
Levine, J. M. Newman, Chem. Mater. 6 (1994) 815.
[21] J. Yuan, K. Laubernds, J. Villegas, S. Gomez, S. L. Suib, Adv. Mater. 16
(2004)1729.
[22] Y. Moro-oka, Catal. Today 45 (1998) 3.
[23] R. A. Sheldon, J. K. Kochi, Metal Catalyzed Oxidations of Organic
Compounds; Academic Press: New York, 1981.
[24] M. Martan, J. Manassen, D. Vofsi, Tetrahedron 26 (1970) 3815.
[25] R. Ghosh, X-F. Shen, J. C. Villegas, Y. Ding, K. Malinger, S. L. Suib, J.
Phys. Chem. B 110 (2006) 7592.
[26] S. L. H. Rebelo, M. M. Q. Simoes, P. M. S. Neves, A. M. S. Silva, P.
Tagliatesta, J. A. S. Cavaleiro, J. Mol. Catal. A. 232 (2005) 135.
[27] R. G. Tailleur, C. J. G. Garcia, J. Catal. 250 (2007) 110.
[28] M. Lukasiewicz, D. Bogdal, J. Pielichowski, Mol. Diver. 10 (2006) 491.
[29] M. A. Herrero, J. M. Kremsner, C. O. Kappe, J. Org. Chem. 73 (2008) 36.
[30] R. N. Gedye, J. B. Wei, Can. J. Chem. 76 (1998) 525.
[31] D. R. Baghurst, D. M. P. Mingos, J. Chem. Soc. Chem. Commun. 12 (1988)
829.
[32] K. A. Malinger, Y-S. Ding, S. Sithambaram, L. Espinal, S. Gomez, S. L.
Suib, J. Catal. 239 (2006) 290.
35
[33] Z. C. Djenni, B. Hamada, F. Chemat, Molecules, 12 (2007) 1399.
[34] L. Perreux, A. Loupy, Tetrahedron, 57 (2001) 9199.
[35] I. Almena, A. Diaz-Ortiz, E. Diez-Barra, A. de la Hoz, A. Loupy, Chem.
Lett. 37 (1996) 333.
[36] S. L. Suib, Ace. Chem. Re. 41 (2008) 479.
[37] F. Chemat, E. Esveld, Chem. Eng. Technol. 24 (2001) 735.
[38] B. Toukoniitty, J. P. Mikkola, K. Eranen, T. Salmi, D. Y. Murzin, Catal.
Today 100(2005)431.
[39] M. Polverejan, J. C. Villegas, S. L. Suib, J. Am. Chem. Soc. 126 (2006) 7774.
36
CHAPTER 3
MICROWAVE-PROMOTED COPPER-CATALYZED
EFFICIENT ARYLATION OF 7V-HETEROCYCLES
3.1.
Introduction
Methods of C-N bond formation are key building blocks for the synthesis
of pharmaceuticals, natural product analogues, and heterocyclic molecules [1]. In
recognition of their widespread importance, many synthetic methods have
emerged over the years for the formation of C-N bonds. The discovery of efficient
Pd-catalyzed C-N formation reactions pioneered by Buchwald and Hartwig has
been a major breakthrough in this field [2]. Despite significant improvements
since, limitations still exist for Pd-catalyzed C-N bond formation, such as air and
moisture sensitivity, or the high cost of palladium. Another serious drawback of
the Pd-catalyzed amination protocol has been the necessity to use strong bases,
such as /-BuONa, which essentially limited the overall tolerance of the method to
base-insensitive functional groups.
These limitations have forced chemists to reconsider other transition metal
catalysts. In this respect Cu-catalyzed procedures requiring milder bases are an
improvement, particularly for base sensitive reagents. There have been an
impressive number of Cu-mediated or Cu-catalyzed methods published for the Narylation of ^--excessive heterocycles [3]. However, these Cu-catalyzed reactions
are predominantly homogeneous in nature with usage of ligands, and often
required stoichiometric quantities of Cu compounds. Although homogeneous
37
catalysts show high activities and selectivities, the separation of homogeneous
catalysts from the reaction mixture and the recovery of the catalysts are
cumbersome. Consequently, heterogeneous catalysts have advantages, such as
easy separations and efficient recycling and minimization of metal traces in the
products. Recently, heterogeneous copper oxide has been used to catalyze ligand
free C-N coupling reactions [4]. Despite the fact that these methods are efficient,
many of these reactions required longer reaction times and inert atmospheric
conditions and sometimes ligands. Herein we report a very efficient microwave
promoted expedited C-N coupling reactions catalyzed by urchin-like copper
oxides synthesized in our laboratory. The reaction process was found to be very
efficient, fast, and did not require inert conditions.
1 equiv
1.2 equiv
Figure 3.1: Reaction scheme for CuO catalyzed N-arylation
The use of microwave energy to directly heat chemical reactions has
gained popularity among the scientific community. More often than not,
controlled microwave heating under sealed-vessel conditions has resulted in a
dramatic reduction in reaction times, increased product yields and enhanced
product purity due to reduction of undesired side reactions compared to
conventional heating methods [5]. Many successful microwave assisted transition
metal catalyzed coupling reactions have been reported. Under sealed vessel
conditions many of these types of rapid transformations can be carried out without
38
an inert atmosphere. The direct heating of the reaction mixture rather than of the
reaction vessel is associated with microwave irradiation and the lifetime of the
transition metal catalysts can be extended through the minimization of wall
effects [1].
3.2.
Experimental
3.2.1. CuO Catalyst Preparation
Urchin-like copper oxide was prepared by the following method.
Cu(NOj)2-3H20 (10 mmol) and urea (50 mmol) were dissolved in the solvent
with 20 mL distilled deionized water and 50 mL ethylene glycol monomethyl
ether in a round bottom flask, after the clear blue solution was obtained, the
magnetically stirred round bottom flask with cooling water condenser was
transferred to the oil bath and the temperature of oil bath was kept at 100°C for 6
hours.
After reaction, the products were transferred to a centrifuge tube
immediately, and washed with a centrifugation-redispersion cycle with distilled
deionized water and ethanol several times, and dried in an oven at 80°C overnight.
3.2.2. Catalyst Characterization
The crystal structure of the sample was determined by X-ray diffraction
(XRD) using a Scintag XDS 2000 diffractometer with Cu KR radiation with a
1.5418 A wavelength as described in the previous chapter. Morphologies of the
samples were investigated using a Zeiss DSM 982 Gemini field-emission
scanning electron microscope (FE-SEM) with a Schottky emitter as described in
39
the previous chapter. Powder samples were dispersed in ethanol and dropped onto
a gold-coated silicon wafer, and the wafer was then mounted onto a stainless steel
sample holder using silver conductive paint.
14x10 -i
JCPDS 41-0254
12108c
64-
CM
^-v
°
CO ' • " ^
*~ 1
CM O ^
'->
4c\J
20-
1
10
I
20
I
I
I
30
40
50
2 theta / degree
I
60
70
Figure 3.2: X-Ray Diffraction pattern of urchin-like CuO.
40
*-,.
%%M'I,.
*•
Figure 3.3: FE-SEM image of urchin-like CuO
41
a)
Figure 3.4: Biotage Initiator single-mode microwave reactor set-up.
3.2.3. Catalytic Reactions
A mixture of aryl halide (1 mmol), nitrogen-containing heterocycle (1.2
mmol), CuO (0.025 mmol), K2CO3 (1 mmol) and DMF (2 mL) was sealed in
glass vessel. The reaction was then irradiated in a Biotage Initiator™ at 200° C
for 10 min. After cooling to room temperature in the microwave cavity, the
catalyst was filtered from the reaction mixture and solvent evaporated in vacuo.
The mixture was partitioned between CHCI3 (10 mL) and water (3 mL). The
organic layer was separated, and the aqueous layer was extracted with CHCI3 (5
mL). The combined organic layers were washed with brine (5 mL), dried over
Na2SC>4, and GC analysis was done. Gas chromatography-mass spectroscopy
(GC-MS) methods were used for the identification as described in the previous
chapter.
3.3.
Results
3.3.1. Screening and optimization
CuO catalysts and solvents were first screened for C-N coupling reactions
with l-chloro-4-nitro benzene, imidazole and K2CO3 as the base (Table 3.1).
Initial results in a sealed microwave tube indicated that the reactions were very
effective at 200° C in DMF as solvent in the presence of 2.5 mol % CuO catalyst
in 10 minutes. Urchin-like copper oxide showed superior catalytic activity with a
TOF of 198 h"1 (entry 1). The use of CsC0 3 as the base instead of K 2 C0 3 did not
have much impact on the yield (entry 4). Hence, inexpensive K2CO3 was used as
the base for all the reactions. Alternative solvents, DMSO, and water were also
43
tried under the same conditions, but they gave 80% and 49% yield, respectively
(entries 5 and 6).
Table 3.1: Optimization of Cu-catalyzed C-N couplinga
2.5 mol % CuO
+ HN
0,N
1
2
3
4
5
6
7
8
02N
1.2 equiv
1 equiv
Entry
»•
1 equiv Base
solvent, MW, 10 min
Catalyst
D
CuO (1)
CuO (2)c
CuO (3)d
CuO (1)
CuO (1)
CuO (1)
CuO (1)
None
Base
Solvent
Yield, %
K 2 C0 3
K 2 C0 3
K 2 C0 3
CsC03
K 2 C0 3
K 2 C0 3
none
K 2 C0 3
DMF
DMF
DMF
DMF
DMSO
water
DMF
DMF
84.1
78.4
65.7
85.2
80.4
49.4
0
0
"Reaction conditions : 1 mmol 1 -chloro-4-nitro benzene, 1.2 mmol imidazole, 1
mmol base, 2 mL solvent, 200°C, 10 min MW heating. Yields determined by GC and
confirmed by GC-MS.b Urchin-like CuO. c CuO nanoparticles (commercial). d 200
mesh CuO (commercial).
44
Table 3.2: Arylation Scope with Respect to Aryl halides.
2.5 mol % CuO
^
n
X + HN
1 equiv K 2 C0 3
v ^
Entry
X
Y
1
I
4-H
2
I
4-OCH.
Product
Yield r%l
rN
/
\
/
H3CO-
62.3
-N
\
56.6
-N
\
3
I
4
Br
4-NO?
N
0,N-
/
\
-N
N
4-H
87.8
61.7
-N
NO,
a
5
Br
2,4-N0 2
6
CI
4-H
7
CI
4-NO2
8
F
4-H
0,N-
/
78.0
\
/ V.
:N
14.2
N
0,N-
/
\
-N
84.1
N
/
\
Reaction conditions : 1 mmol aryl halide, 1.2 mmol imidazole, 1 mmol K2CO3, 2
mL DMF solvent, 200°C, 10 min MW heating. Yields determined by GC and
Table 3.3: Arylation Scope with Respect to N-heterocycles'
./V-heterocyle
Product
HN
0,N-
/
\
Yield, %
-N
34.3
:
N/
HN
OoN-
HN
07N-
r^/
63.8
N
/
\
-N
N ^
84.1
N-^
HN
OpN-
N/
HN
OoN-
^N
\
100
r\/-
100
0,N
48.0
H
-N
0,N
/
•N
84.8
a
Reaction conditions : 1 mmol l-chloro-4-nitro benzene, 1.2 mmol azole, 1 mmol
K 2 C0 3 , 2 mL DMF solvent, 200°C, 10 min MW heating. Yields determined by GC and
confirmed by GC-MS.
46
3.3.2. Effect in aryl halides and N-heterocylces
Another set of experiments was carried out under optimized reaction
conditions to evaluate the scope of this process with a variety of substituted aryl
halides and imidazole as the nucleophile (Table 3.2, entries 1-8). Various
heterocycles such as pyrrole, pyrazole, imidazole, triazole, indole, and
benzimidazole in the A/-arylation of heterocycles were also studied under
optimized reactions and the results are summarized in Table 3.3 (entries 1-7). In
all the cases the coupling heterocycles with 4-nitro-l-chlorobenzene gave their
corresponding A^-phenyl heterocycles in moderate to excellent yields (34-100%).
The yield of corresponding products increased with the number of heteroatoms
present in the heterocycles. Pyrrole as heterocycle gave only a 34% yield while
both 1,2,3-triazole and 1,2,4-traizole gave a 100% yield to corresponding products.
Interestingly, even the benzylic heterocyles such as benzimidazole and indole
gave good yields.
3.3.3. Scale Up
Finally, a scale-up of the arylation of TV-heterocycles from 1 mmol to 100
mmol was attempted. Both 1 mmol and 10 mmol reactions were carried out under
sealed-vessel conditions. For 100 mmol scale up reactions, a translation from
sealed to open-vessel was required, thus eliminating the need to work at elevated
pressures (Table 3.4).
47
Table 3.4: Scale-up of C-N coupling
Scale
Time, min
Temp, °C
Yield, %
1 mmol
10
200
84.2
10 mmol
10
200
72.1
100 mmol
30
150
59.2
3.4.
Discussion
Under sealed-vessel microwave conditions, it is possible to attain higher
reaction temperatures above the boiling points. Loss tangent (tan 8) values of
solvents govern the ability of a solvent to convert microwave energy into heat at
given frequencies and temperatures. Hence, solvents with higher tan 8, such as
DMF, DMSO, and others are preferable for microwave heated reactions [5]. The
results obtained with urchin-like CuO as catalysts were also compared with
commercially available CuO. However, the urchin-like CuO showed the highest
activity and the unique morphology of the catalyst may be the cause of the
enhanced activity [6]. A blank experiment confirmed that in the absence of CuO
no N-arylation could occur.
The nature of the substiruents in the aryl halides played a key role in the
yield to their corresponding products. In the case of aryl iodides, the observed
decrease in reactivity was in the order of l-iodo-4-nitrobenzene > iodobenzene >
l-iodo-4-methoxybenzene, indicating electron withdrawing substitution increased
the yield whereas electron donating substitution increased the yield in the
iodobenzenes. A similar trend was observed with other aryl halides as well. The
attempt to use unsubstituted aryl halides was successful with iodo- and bromobenzenes whereas, the use of chloro and flouro benzenes as aryl halides for
coupling failed. However, as we have seen in our first example of our coupling
reactions, the activated nitro-chloro-benzene gave a 84% yield. The coupling
49
between l-bromo-2,4-dinitrobenzene and imidazole (entry 5) gave a 78% yield,
demonstrating that or/Ao-substitution did not hamper the 7V-arylation process.
The vast majority of the reactions reported in the literature are carried out
in sealed-vessels at elevated temperatures. When these reactions are scaled up, the
sealed vessels can pose safety issues as the volume of the vessels increase. Hence,
for 100 mmol reactions, a translation from sealed to open-vessel was required,
thus eliminating the need to work at elevated pressures [7]. When moving from
sealed to open-vessel conditions, it was necessary to increase the reaction time
from 10-30 min and the catalyst amount from 2.5-10% because of the difference
in volume. The bulk temperature was 150°C in the open-vessel in comparison to
200°C in the sealed-vessel.
3.5.
Conclusions
In conclusion, a highly efficient microwave-promoted rapid method for
the direct C-N coupling of azoles with aryl halides has been developed. High
yields up to 85% for JV-arylated products with TOF of 198 h"1 were obtained in 10
minutes. The use of microwave heating as opposed to conventional heating for
these coupling reactions allowed convenient access to the temperatures for
completion of reactions thus eliminating the inert atmospheric conditions
associated with sealed tubes used for other coupling methods. The generality and
rapid accessibility of products should make this method useful for C-N bond
formation in organic synthesis.
50
3.6.
References
[1] (a) I. P. Beletskaya, A. V. Cheprakov, Coord. Chem. Rev. 248 (2004) 2337
(b)P. Appukuttan, E. Van der Eycken, Eur. J. Org. Chem. (2008) 1133.
[2} (a) A. S. Guram, S. L. Buchwald, J. Am. Chem. Soc. 116 (1994) 7901, (b) F.
Paul, J. Patt, J. F. Hartwig, J. Am. Chem. Soc. 116 (1994) 5969.
[3] (a) G. Evano, N. Blanchard, M. Toumi, Chem. Rev. 108 (2008) 3054. (b) S. V.
Ley, A. W. Thomas, Angew. Chem. Int. Ed. 42 (2003) 5400. (c) M. Taillefer, N.
Xia, A. Ouali, Angew. Chem. Int. Ed. 46 (2007) 934.
[4] (a) A. Correa, C. Bolm, Adv. Synth. Catal. 349 (2007) 2673. (b) M. L.
Kantam, J. Yadav, S. Laha, B. Sreedhar, S. Jha, Adv. Synth. Catal. 349 (2007)
1378. (c) L. Rout, S. Jammi, T. Punniyamurthy, Org. Lett. 17 (2007) 3397. (d) S.
Liu, J. P. C. Pestano, C. Wolf, Synthesis, 22 (2007) 3519. (e) D. S. Yadav, B. S.
Yadav, V. K. Rai, Synthesis (2006) 1868. (f) X. Zhu, Y. Ma, L. Su, H. Song, G.
Chen, D. Liang, Y. Wan, Synthesis (2006) 3955. (g) H. Xu, C. Wolf, Chem.
Commun. (2009) 1715.
[5] CO. Kappe, Angew. Chem. Int. Ed. 43 (2004) 6250.
[6] (a) CO. Kappe, Chem. Soc. Rev. 37 (2008) 1127. (b) C. O. Kappe, D.
Dallinger, Mol. Divers, 13 (2009) 71.
[7] L. Xu, S. Sithambaram, Y. Zhang, C-H, Chen, L. Jin, R. Joesten, S. L. Suib,
Chem. Mater. 21 (2009) 1253.
[8] (a) A. Stadler, B. H. Yousefi, D. Dallinger, P. Walla, E. V. Eycken, N. Kaval,
C. O. Kappe, Org. Process Res. Dev. 7 (2003) 707. (b) M. D. Bowman, J. L.
Holcomb, C. M. Kormos, N. E. Leadbeater, V. A. Williams, Org. Process Res.
51
Dev. 12 (2008) 41. (c) K. T. J. Loones, B. U. W. Maes, G. Rombouts, S. Hostyn,
G. Diels, Tetrahedron, 61 (2005) 10338.
CHAPTER 4
MANGANESE OCTAHEDRAL MOLECULAR SIEVES
CATALYZED TANDEM PROCESS FOR SYNTHESIS OF
QUINOXALINES
4.1.
Introduction
Single-pot tandem reactions involving catalysis have recently become an
important methodology in chemistry [1]. Multi-step organic syntheses are
common in the fine chemical industry, and they suffer from several disadvantages.
Such reactions are often carried out non-catalytically using relatively large
amounts of reagents that produce many kilograms of waste per kilogram of final
product. In addition, the separation and purification steps needed after each
conversion step produce waste heat. Going from traditional step-by-step methods
to a one-pot coupled conversion saves raw materials and energy and reduces
waste [2].
Quinoxalines are a versatile class of nitrogen containing heterocyclic
compounds and they constitute useful intermediates in organic synthesis [3].
Quinoxalines have been reported to be biocides, pharmaceuticals, and organic
semiconductors [4-6]. Conventionally, quinoxalines are synthesized by a double
condensation reaction involving a dicarbonyl and orf/jo-phenylenediamine [7].
Due to the highly reactive nature of the dicarbonyls, alternative routes have been
proposed recently. Antoniotti et al. have reported one of these methods to
synthesize quinoxalines from epoxides and ene-l,2-diamines [8]. Active
53
manganese oxide and molecular sieves in combination or manganese oxides in
combination with microwaves have also been used in producing quinoxalines [9].
These processes however, require excessive amounts of manganese oxide as
stoichiometric oxidants and scaling them up for industrial processes can lead to
the formation of large amounts of toxic waste leading to environmental issues. In
additional studies, Robinson et al. reported a homogeneous catalytic process
utilizing Pd(OAc)2, RuCl2(PPh3)2 to synthesize quinoxalines from hydroxy
ketones 10], and recently a copper catalyzed oxidative cyclization process has
been reported [11].
An improved ruthenium catalyzed direct approach to
synthesize quinoxalines from diols and ortho-diamines has also been reported [12].
These processes are efficient, but suffer from the major drawback that the
catalysts cannot be recovered and reused.
R = aryl, R' = aryl, H
Figure 4.1: Schematic representation of quinoxaline synthesis
The objective of this project is to design a highly efficient catalytic single
pot tandem synthetic route (Figure 4.1) to form quinoxalines (3) from hydroxy
ketones (1) and diamines (2) using manganese oxide octahedral molecular sieves
(K-OMS-2). OMS-2 materials have been used as catalysts for the oxidation and
54
condensation reactions in our laboratory [13]. K-OMS-2 is a cryptomelane-type
manganese oxide with the composition KMn8Oi6.nH20 and consists of Mn06
octahedral units, which are edge and corner shared to form a 2 x 2 tunnel structure
[14]. This alternative process to synthesize quinoxalines with K-OMS-2 does not
require any additives or promoters as in the processes described above for this
transformation and the reaction times are shorter. Moreover, the K-OMS-2
catalysts are relatively inexpensive, easy to prepare and can be reused many times
without loss of activity. The reaction proceeds via two steps, i.e. the oxidation of
the hydroxy ketones to their diketones and then the condensation of the diketones
with a diamine to form the final product, quinoxaline.
4.2.
Experimental
4.2.1. Catalyst preparation and characterization
K-OMS-2 catalysts were prepared by reflux, solvent-free and high
temperature methods as described in the previous chapter. They were
characterized using XRD, SEM and BET. The characterization procedures have
been detailed in the previous chapters.
4.2.2. Catalytic reactions
The catalytic reactions were carried out in batch reactors. A typical
reaction procedure as follows; to a round-bottomed flask (50 mL), furoin (1 mmol,
192 mg), toluene (10 mL) as solvent, 4-methoxy-o-phenylenediamine (2 mmol,
276 mg), and K-OMS-2 catalyst (50.0 mg) were added. The mixture was stirred
55
under reflux for 1 h at 110°C in air. After the reaction time, the mixture was
cooled; the catalyst was removed by filtration. A gas chromatography-mass
spectroscopy (GC-MS) method was used for the identification and quantification
of the product mixtures. GC-MS analyses were done using an HP 5890 series II
chromatograph with a thermal conductivity detector coupled with an HP 5970
mass selective detector. An HP-1 column (non polar cross linked siloxane) with
dimensions of 12.5 m x 0.2 mm x 0.33 um was used in the gas chromatograph.
The products were also confirmed by !H and
13
C NMR data collected on a
Brucker DRX-400 (400.144 MHz ! H, 100.65 MHz
chromatography
after
concentration
afforded
13
C). Silica-gel column
2,3-bis(2-furyl)-6-methoxy
quinoxaline (Table 2, Entry 7). m/z = 292, 'H NMR (400 MHz, CDC13): 5 = 3.83
(s, 3H), 6.43 (m, 3H), 6.55 (d, 1H, J= 4 Hz), 7.26 (dd, 1H, J= 4, 8 Hz), 7.51 (m,
2H), 7.87 (d, l H , J = 8 H z ) ; 13C NMR (400 MHz, CDC13): 5 = 55.9, 106.5, 111.9,
112.0, 113.1, 123.8, 130.1, 136.7, 140.2, 142.4, 142.7, 143.7, 144.3, 149.5, 150.9,
151.1, 161.4.
4.3.
Results
4.3.1. Synthesis of quinoxalines with K-OMS-2 catalysts.
The synthesis of a quinoxaline with a-pyridoin and 1,2-phenylenediamine
was attempted with K-OMS-2 catalysts prepared by different methods (Table 4.1).
K-OMS-2 prepared by solvent free (S.F) methods is known as
K-OMS-2 S F,
K-
OMS-2 prepared by reflux (R) methods is known as K-OMS-2R and K-OMS-2
synthesized by high temperature (HT) methods is known as
K-OMS-2HT-
The
56
main criterion for selecting these catalysts was based on their surface areas. The
K-OMS-2 S F
m /g), while
prepared by the solvent-free method had the highest surface area (156
K-OMS-2 H T
prepared by a high temperature method had the lowest
surface area (13 m2/g) determined by BET. Interestingly, all the catalysts
produced quinoxaline in high to moderate yields. The use of
in a 98% quinoxaline yield, while the use of
K-OMS-2SF
K-OMS-2R
and
resulted
K-OMS-2HT
gave
74% and 49% yield respectively. In all cases, the selectivity for quinoxaline was
100% and no other side products were formed. The difference in the yields of
quinoxalines can be related to their intrinsic properties. The surface area and the
average oxidation number of the catalysts in combination seem to play a key role
in the process. The surface area is related to crystallite size of the catalysts. The
average oxidation number that represent the ratio of Mn2+, Mn3+, and Mn4+ which
influence the strength of Lewis acidity of the catalysts may also play a role.
However, a detailed investigation to relate the catalytic activity to their properties
is still in progress.
57
Table 4.1: Synthesis of quinoxalines with K-OMS-2 catalysts.
Entry
Catalyst"
AOSc
Surface Area
crystallite size
infM
(Dm)
Yield (%)d
TOFe
(h'1)
1
K-OMS-2SF
3.72
156
10
74
11.8
2
K-OMS-2R
3.90
90
18
98
15.7
3
K-OMS-2HT
3.85
13
20
49
7.8
" 1 mmol a-pyridoin, 2 mmol 1,2-phenylenediamine and 50.0mg catalyst stirred in 10 mL toluene under
reflux for 1 h.
* Catalyst preparation methods, SF-solvent free, R-reflux, HT- high temperature.c Average oxidation
number determined by potentiometric titrations. Ref. 15. d Determined by GC-MS and NMR (Yield =
conversion x selectivity).e Turnover frequency= moles of converted substrate/(moles of catalyst x reaction
time in h).
f
50 mg = 0.0625 mmol K-OMS-2.
Table 4.2; K-OMS-2 catalyzed synthesis of quinoxalines with various substrates."
Entry
Hydroxy ketone
Diamine
Ph^O
1
2
Quinoxaline
to
Ph^N
NH,
Ph^O
Pri
NH22a
OH l b
100
I
NH2 2a
NH2
OH l a
Yield (%)
~N
IC
47 (>99)*
Ph-^N'
H3CO.
3a
3b
H3CO.
24
NH2
( "^r^oH
NH
H3C0' -
l c
^
2 2a
:
H3C0'
3c
84
NH2
NH22a
OH
<sr^
Id
^NH
3d
98
2
NH2 2a
OH
3e
le
0,N
OH
NH
Id
H,C0
OH
<n
NH 2
XJ
2 2b
NH 2
NH
Id
3f
0
89(82)'
OCH-,
VrY
N
2 2C
3g
100
Ph^N
ilv^s^NH
Ph^O
20
NO,
I
S
OH l a
PhyD
2 2d
CI
Ph^OH 15
"0
c
3h
xxy \.
NH 2
^ ^ N H .
N
Ph
N
Ph^N
2 2d
37
H3CO.
CI
10
NH
lc
OH
11
le
<?1
CI
OH
Id
^S&AYN.^^^CI
NH 2
"V"OH
H3CO'
CI
12
51
IS
2 2d
NH 2
NH
2 2d
NH2
NH22d
H3CO'
xo?
86
3k
100
xr°
31
"1 mmol hydroxyketone, 2 mmol diamine and 50mg catalyst were stirred in lOmL toluene under
reflux for l h . b Yield in 8h. c Isolated yield of product.
59
4.3.2. Effect of various substrates in quinoxaline synthesis
This protocol was then extended to a range of substrates using K-OMS-2
as catalysts prepared by reflux methods and the results are listed in Table 4.2.
Quinoxaline
synthesis
with
2-hydroxyacetophenone
(la)
and
1,2-
phenylenediamine (2a) gave 100% yield to the corresponding quinoxaline (3a).
Benzoin (lb) gave a moderate yield to its quinoxaline (3b), while heterocyclic
substrates pyridoin (le) and furoin (Id) showed good yields to their
corresponding quinoxalines, 98% (3e) and 84% (3d) respectively. Hetero-atoms
in the cyclic structures also seem to play a role in the formation of the
quinoxalines. Anisoin (lc) gave the lowest yield (24%) of all the hydroxy ketone
substrates tried. The presence of the electron donating OCH3 group in the benzene
ring may retard the nucleophilic attack on the in situ formed dicarbonyl leading to
a lower yield (24%). The effects of electron withdrawing and electron donating
substituents in the nucleophile diamine substrate were also studied. The reaction
between furoin (Id) and 4-methoxy-l,2-phenylenediamine (2c) gave an 89%
yield for 3g, indicating that the presence of an electron donating methoxy group in
the
diamine
enhances
its
nucleophilicity.
On
the
other
hand,
4-
nitrophenylenediamine (2b) with furoin (Id) yielded only 20% quinoxaline 3f.
The other product obtained was the intermediate diketone, furil in 22% yield.
These results show that electron withdrawing nitro groups in the diamine retard its
nucleophilicity. The use of chloro- substituted diamines (2d) led to enhanced
yields of up to 100% for their quinoxalines (entries 8-12).
60
Alternative solvent systems such as acetonitrile, THF, toluene, and
benzene were evaluated in the formation of quinoxalines at their reflux
temperatures. However, toluene gave the highest conversion (98%) among the
solvents tried.
The main reason for this could be that the highest reflux
temperature (110°C) attained with toluene aids in the oxidation step of
quinoxaline. In consideration of its abundance, economic, and environmental
attractiveness, water was also tried as a solvent for the reaction. The reaction in
water afforded only a 9% yield.
Finally, the quinoxaline synthesis was studied on a gram scale. One gram
of pyridoin was reacted with one gram of 1,2-phenylenediamine under the same
reaction conditions. The reaction occurred to produce the corresponding
quinoxaline in 81% yield in one hour. This promising result suggests that this
catalytic protocol can be extended to a larger scale, such as one millimol to gram
scale reactions.
61
Table 4.3: Catalyst reusability"
Cycle
Catalyst amount, mg
Yield (%)
1
5O0
96 (98)*
2
37.5
85(84) 6
3
25.0
62(64)fe
4
12.5
39(43) 6
" 1 mmol pyridoin, 2 mmol 1,2-phenylenediamine and K-OMS-2 were stirred in 10 mL toluene under reflux
for lh.
6
Yield with fresh catalyst.
62
4.4.
Discussion
4.4.1. Reusability of OMS-2 catalysts
Additional studies were performed to test the reusability of the catalyst.
The reaction with a-pyridoin and 1,2-phenylenediamine was carried out over four
cycles with the same catalyst which was regenerated after each use by simply
washing with acetone/methanol and water and heating to 250° C. Due to the loss
of catalysts during the filtration process after each reaction, a reduced amount of
catalysts was used in the subsequent cycles. The yields of the "spent" catalysts
used in all the cycles were comparable to yields for "fresh" catalysts (Table 4.3).
The X-ray diffraction patterns of the regenerated catalysts indicated that the
structure of K-OMS-2 was not altered during the reaction (Figure 4.2). In order to
prove that the reaction is heterogeneous, a standard leaching experiment was
conducted. The catalyst was filtered at the reaction temperature, and the reaction
was allowed to proceed without the catalyst. There was no change in yield
observed even after 8 hrs indicating that no homogeneous catalysis was involved.
Turnover frequency (TOF) for this catalytic process which is defined as, moles of
converted substrate per mole of catalyst per hour has been listed in Table 4.1.
TOF of 15 h"1 have been achieved for this reaction with K-OMS-2 as catalyst and
this value is very high compared to the similar process which requires 10-15
equivalents of active Mn0 2 in 20 h (TOF = 1.4 x 10"3 h"1) [16].
63
2500 n
2000
(b)
£• 1500 °3>
c 1000
500 -
i
5
25
45
65
85
2 Theta
Figure 4.2: XRD patterns of K-OMS-2 catalyst in synthesis of quinoxalines (a)
"fresh" catalyst before reaction (b) "spent" catalyst after 2nd cycle.
64
4.4.2. The proposed mechanism
A proposed mechanism for the formation quinoxalines from hydroxyl
ketones and ortho-diamines catalyzed by K-OMS-2 is shown in Figure 4.3. There
are two steps involved in the process; the oxidation of hydroxyl ketone to a
diketone followed by a double condensation to form the quinoxaline ring. This
proposed mechanism is consistent with the products reported (Table 4.2, entry 6).
K-OMS-2 catalyst plays a prominent role in the oxidation of hydroxy ketone to its
diketone. A detailed mechanism of the oxidation of alcohols to carbonyls by KOMS-2 has been reported by Makwana et al. [17]. A Mars-Krevelen type of
mechanism has been proposed for the oxidation of alcohols to carbonyls. The
second step of the synthesis, the double condensation between diketone and
diamine may require a catalyst or a water removal agent. Recently, many catalytic
methods have been reported for the double condensation using metal precursors,
molecular iodine, and zeolites [18]. However, the role of K-OMS-2 in the second
step of this reaction process is not yet clear.
A series of reactions starting from diketone to quinoxaline were carried
out with and without the catalyst under the same conditions. The results indicated
that the reaction between the diketones and diamine may not require a catalyst,
under the conditions used. However, the porous structure of the OMS-2 materials
may facilitate the removal of water formed during the double condensation. In an
alternative mechanism, where the condensation of hydroxy ketones and diamines
could occur in the first step, followed by an oxidation and a cyclization in the
65
presence of K-OMS-2. However, the hydroxy-imine formation via this route was
not detected by GC or NMR. Hence, the possibility of condensation-oxidationcyclization route could be eliminated in the proposed mechanism.
Figure 4.3: Proposed catalytic cycle for quinoxaline formation
R' = H, aryl
R" = aryl
R'
66
4.5.
Conclusions
In summary, an efficient environmentally benign tandem synthetic route to
prepare quinoxalines leading to 100% yields using reusable manganese oxide
octahedral molecular sieves (OMS-2) is described. Manganese octahedral
molecular sieves efficiently catalyze the single-pot synthesis of quinoxalines from
hydroxyl ketones and diamines. The reactions require only a catalytic amount of
K-OMS-2 and do not require any additives or promoters for the reaction. The KOMS-2 catalysts are environmentally benign and after a simple regeneration
process can be reused without loss of activity.
67
4.6.
References
[I] a) J. C. Wasilke, S. J. Obrey, R. T. Baker, G. C. Bazan, Chem. Rev. 105
(2005) 1001.; b) J. L. Notre, D. V. Mele, C. G. Frost, Adv. Synth. Catal. 349
(2007) 432.;
c) A. Ajamian, J. L. Gleason, Angew. Chem. Int. Ed. 43 (2004) 3754.
[2] a) T. L. Ho, Tandem Organic Reactions, Wiley, New York, 1992; b) R.
Shoevaart, T. Kieboon, Chem. Innov. 31 (2001) 33.
[3] D. J. Brown, The Chemistry of Heterocyclic Compounds, Vol 61,
Quinoxalines: Supplement II, Wiley, New York, 2004.
[4] R. Sarges, H. R. Howard, R. G. Browne, L. A. Lebel, P. A. Seymour, J. Med.
Chem. 33(1990)2240.
[5] L. E. Seitz, W. J. Suling, R. C. Reynolds, J. Med. Chem. 45 (2002) 5605.
[6] S. Dailey, J. W. Feast, R. J. Peace, I. C. Sage, S. Till, E. L. Wood, J. Mater.
Chem. 11 (2001)2238.
[7] a) Z. Zhao, D. D. Wisnoski, S. E. Wolkenberg, W. H. Leister, Y. Wang, C. W.
Lindsley, Tetrahedron Lett. 45 (2004) 4873. b) S. V. More, M. N. V. Sastry, C-F.
Yao, Green Chem., 8 (2006) 91.
[8] S. Antoniotti, E. Donach, Tetrahedron Lett. 43 (2002) 3971.
[9] a) S. A. Raw, C. D. Wilfred, R. J. K. Taylor, Chem. Comm. (2003) 2286. b) S.
Y. Kim, K. H. Park, Y. K. Chung, Chem. Commun. (2005) 1321.
[10] R. S. Robinson, R. J. K. Taylor, Synlett. 6 (2005) 1003.
[II] C. S. Cho, S. G. Oh, J. Mol. Catal. A: Chem. 276 (2007) 205.
[12] C. K. Cho, S. G. Oh, Tetrahedron Lett. 47 (2006) 5633.
68
[13] a) Y. C. Son, V. D. Makwana, A. R. Howell, S. L. Suib, Angew. Chem. Int.
Ed. 40 (2001) 4280; b) R. Kumar, L. J. Garces, Y-C. Son, S. L. Suib, R. E. Malz,
J. Catal. 236 (2005) 387; c) S. Sithambaram, R. Kumar, Y. C. Son, S. L. Suib, J.
Catal. 253 (2008) 269.
[14] a) Y. F. Shen, R. P. Zerger, R. N. DeGuzman, S. L. Suib, L. McCurdy, D. I.
Potter, C. L. O'Young, Science 260 (1993) 511; b) R. N. DeGuzman, Y. F. Shen,
E. J. Neth, S. L. Suib, C. L. O'Young, S. Levine, J. M. Newsam, Chem. Mater. 6
(1994) 815; c) S. L. Suib, Curr. Opin. Solid State Mater. Sci. 3 (1988) 63; d) S. L.
Brock, N. Duan, Z. R. Tian, O. Giraldo, H. Zhou, S. L. Suib, Chem. Mater. 10
(1998)2619.
[15] G.-G. Xia, W. Tong, E. N. Tolentino, N. -G. Duan, S. L. Brock, J.-Y. Wang,
S.L. Suib, T. Ressler, Chem. Mater. 13 (2001) 1585.
[16] S. A. Raw, C. D. Wilfred, R. J. K. Taylor, Org. Biomol. Chem., 2 (2004)
788.
[17] R. N. DeGuzman, Y-F. Shen, E. J. Neth, S. L. Suib, C. L. O'Young, S.
Levine, J. M. Newman, Chem. Mater. 6 (1994) 815.
[18] Y-S. Ding, X. F. Shen, S. Sithambaram, S. Gomez, R. Kumar, M. B. Vincent,
S. L. Suib, Chem. Mater. 17 (2005) 5382.
[19] X. F. Shen, Ph. D. Thesis, University of Connecticut, (2007).
69
CHAPTER 5
MANGANESE OCTAHEDRAL MOLECULAR SIEVE
CATALYSTS FOR SELECTIVE STYRENE OXIDE RING
OPENING
5.1.
Introduction
p-amino alcohols are versatile building blocks in the synthesis of a wide range
of biologically active natural and synthetic products [1], artificial amino acids [2],
and chiral auxiliaries [3]. The classical synthesis of P-amino alcohols involves the
ring opening of epoxides with amines [4]. However, these reactions, which are
generally carried out with a large excess of amines at elevated temperatures, often
fail when poorly nucleophilic amines or sterically crowded amines or epoxides
are used. In addition, these reactions are accompanied by the poor regioselectivity of ring opening. Transition metal based catalysts are widely used for
successful ring opening reactions. C0CI2, VCI3, and Zn(OAc)2 were used as Lewis
acids to catalyze the reaction [5-7]. Onaka et al. have used zeolites for the ring
opening of unsymmetrical epoxides with aniline [8]. Recently, Chakraborti et al.
have reported silica gel catalysis for ring opening under solvent free conditions
[9]. The use of transition metal based catalysts or zeolites for the ring opening of
epoxides reduces the reaction time and enhances regio-selectivity. However, these
processes which require expensive and stoichiometric amounts of reagents, suffer
from poor regio-selectivity and most of the times the catalyst can not be reused.
70
Here, we describe a route to styrene oxide ring opening to produce paminoalcohols in an environment friendly manner using inexpensive, reusable
Manganese Octahedral Molecular Sieve (OMS) catalysts. In this reaction, KOMS-2 acts as a Lewis acid to catalyze the reaction. K-OMS-2 materials have
been shown to possess excellent catalytic properties in oxidation [10, 11] and
condensation reactions [12]. These materials can be synthesized by various
methods and their characteristics are well documented [13]. K-OMS-2 materials
have a one-dimensional tunnel structure formed by 2 x 2 edge shared Mn06
octahedra.
The composition of K-OMS-2 is KMngOi6-nH20. The average
oxidation state of Mn in K-OMS-2 is -3.8 with the presence of Mn4+, Mn3+, and
Mn2+ ions in the framework. Doping other metals can enhance the catalytic
properties of K-OMS-2 materials. Chen et al. have successfully doped Cu2+, Zn2+,
Ni2+, Co2+ into OMS-2 materials [14]. The catalytic activities of these OMS-2
materials were evaluated for the decomposition of 2-propanol [15]. Cu-OMS-2
showed significantly enhanced activity towards decomposition of propanol. In the
present study, OMS-2 materials prepared by different methods from our
laboratory were evaluated for catalytic activity for the ring opening of epoxides.
Furthermore, V, Mo, and W doped OMS-2 prepared by reflux methods were
studied for terminal ring opening of styrene oxide. This environmentally friendly
process produces either expensive or commercially unavailable amino alcohols
from cheaper starting materials.
71
5.2.
Experimental Section
5.2.1
Preparation of catalysts
Catalysts syntheses were performed using methods described in the
previous chapters.
K-OMS-2SF
was prepared using a solvent free method which
was reported by Ding et al. [16]. The high temperature
K-OMS-2 H T
was prepared
by a combination of sol-gel and combustion methods by Shen et al. [17]. KOMS-2R
was prepared by a reflux method according to the literature [18]. K-
OMS-2HY
was prepared by a hydro thermal method according to the literature [19].
5.2.2. Catalyst Characterization
The synthesized catalysts were characterized using XRD, SEM and BET.
The characterization procedures have been detailed in the previous chapters. Xray powder diffraction (XRD) experiments were carried out using a Scintag
Model PDS 2000 diffractometer. The surface areas of the OMS-2 materials were
measured using the Braunuer-Emmet-Teller (BET) method on a Micrometrics
ASAP 2010 instrument. Scanning Electron Micrographs were taken on a Zeiss
DSM 982 Gemini field emission scanning microscope with a Schottky Emitter at
an accelerating voltage of 2 kV with a beam current of 1 juA.
5.2.3. Catalytic testing
Reactions in a stirred glass reactor were carried out in a 50 ml round-bottomed
flask connected to a reflux condenser. About 50.0 mg of the catalyst was
suspended in a solution of 1 mmol of styrene oxide and 2 mmol of aniline and 10
72
ml toluene as the solvent. The reaction mixture was stirred under reflux for 24
hours. Analyses of products in the reaction were carried out using gas
chromatography-mass spectrometry (GC-MS) and NMR. An HP 5890 series gas
chromatograph coupled with an HP 5971 mass detector was used for the
identification and quantification of the reaction products using an internal
standard. An HP-1 column (non polar cross linked siloxane) with dimensions of
12.5 m x 0.2 mm x 0.33 um was used for the gas chromatograph. ! H and 13C
NMR were collected on Brucker DRX-400 (400.144 MHz 'H, 100.65 MHz ,3C).
5.3.
Results
5.3.1
Catalytic activity
Styrene oxide and aniline were reacted in toluene with OMS-2 catalysts
prepared by hydrothermal, reflux, solvent free, and high temperature methods in
our laboratory. According to Table 5.1, K-OMS-2 prepared by reflux and
hydrothermal methods gave higher conversions for the reaction than K-OMS-2
prepared by other methods. Apart from the anticipated p-amino alcohol, imines
were also formed as side products in the reactions. Moreover, metal doped MOMS-2 catalysts [M=V, Mo and W] prepared by the reflux method were used for
the styrene epoxide ring opening under the same conditions. P-amino alcohol 1
resulting from the ring opening, a monoamine 2, and a diimine 3 were formed in
most cases. However, the selectivity for P-amino alcohol was the highest in all
cases. Table 5.2 shows that doping transition metals in the K-OMS-2 catalysts
73
prepared by reflux methods significantly enhanced the conversion of styrene
oxide in the reaction.
74
Table 5.1: Conversion of Styrene Epoxide by OMS-2 Catalysts'
N-Ph
N-Ph
OMS-2
Catalyst
TON"
'0/„
Conversion/%'
c
Selectivity/%d
P-amino
Imine
alcohol
K-OMS-2 R
12.2
76
K-OMS-2 HY
12.8
80
69
K-OMS-2 s
7.5
47
95
K-OMS-2 HT
2.2
14
Diimine
12
28
100
No catalyst
"Reaction conditions: 50.0mg of catalyst; 1 mmol of styrene oxide; 2 mmol of aniline; t=24 h and 10
ml of toluene as solvent; T=l 10°C.
b
TON - [(molsubsuXmolcaiaiys,)"1] where molsubst, - moles of styrene oxide converted.
c
Conversion (%) based on substrate = [1 - (concentration of substrate left after reaction/initial
concentration of substrate)] * 100.
Selectivities for 1,2 and 3.
75
Table 5.2: Styrene Oxide Ring Opening Catalyzed by Doped OMS-2 (reflux)1
Selectivity /%
Catalyst
OMS-2R
l%V-OMS-2Rb
2%V-OMS-2R
10%V-OMS-2R
l%Mo-OMS-2R
2%Mo-OMS-2R
10%Mo-OMS-2R
1%W-0MS-2R
Reused OMS-2R
Conversion
-
/%
p-amino
alcohol
imine
diimine
76
83
88
100
91
95
100
98
79
88
86
83
73
81
84
62
81
88
12
8
14
20
13
9
31
13
11
0
6
3
7
6
6
8
4
1
"Reaction conditions: 50.0mg of catalyst; 1 mmol of styrene oxide; 2 mmol of aniline;
t = 24 h and 10 ml of toluene as solvent; T=l 10°C.
b
Initial ratio of V:Mn = 1:100 in the catalyst
76
3.2
Effect of nucleophiles
In a further set of experiments, styrene oxide was reacted with different
nucleophiles to study their effect in ring opening. The results are summarized in
Table 5.3. Anilines and phenols with different substituents in the benzene rings
were used as nucleophiles. In some cases, two isomers of the P-amino alcohol
were observed among the products.
3.3
Effect of solvents
The influence of solvents in the catalytic ring opening of styrene oxide
was investigated at 75° C for the comparison of activity. The results of the effect
of solvents are given in Table 5.4. Both polar and non polar solvents were
evaluated for the reaction and toluene was found to be the best solvent among
MeCN, DCM, THF and hexane resulting 72% conversion for this reaction system.
The other solvents gave conversions between 6-45%.
77
Table 5.3: Styrene oxide ring opening with different nucleophiles3
\
+
Nu
OMS-2
Ph'
Entry
Nucleophile
A:BC
Conversion,
Selectivity
%
,%
Q^NH2
76
88
100:0
H 3 CO-{)—NH 2
80
59
66:34
70
94:6
84
77
77:23
49
45
97:3
35
69
76:24
19
46
53:47
1
2
H3C—<\
0
2
h— NH2
N-^^-NH
O"
H3CO^
2
0H
I
OH
OH
OCH,
* Reaction conditions: 50.0mg of K- OMS-2R catalyst; styrene oxide and nucleophile mmol ratio :
1: 2; t=24 h and 10 ml of toluene as solvent; T=l 10°C.
Ph-phenyl, Nu- nucleophile
b
Selectivity for both A and B isomers.
c
Ratio between A and B.
78
Table 5.4; Ring opening of styrene oxide with solvents over K- OMS-2Ra
Solvent
//
£
polarity Conversion, % Selectivity, %c
MeCN
3.2
37.5
46
45
92
DCM
1.8
9.1
30.1
8
100
THF
1.75
7.6
21
6
100
Toluene
0.4
2.38
9.9
72
93
hexane
0
1.9
0.9
44
89
^Reaction conditions: 50.0 mg of K- OMS-2R catalyst; 1 mmol of styrene oxide; 2 mmol of
aniline;
t = 24 h and 10 ml of solvent; T = 75°C.
V-dipole moment, e-dielectric constant.
c
Selectivity for |3-amino alcohol.
79
(a)
30
40
Time, Hr
(b)
1 40
Amino alcohol
M o n o im in e
D i im i n e
J)
60
CO
40
0
0.5
1
Figure 5.1 (a) Conversion and (b) Selectivities of products with time for ring
opening of styrene oxide.
80
5.4.
Discussion
5. 4.1 Catalytic reactions
Epoxide ring opening reactions can either proceed along acid or base
catalyzed pathways [20]. Transition metals can play the role of Lewis acids to
facilitate the ring opening by nucleophilic attack. OMS-2 materials have
predominant Lewis acidity arising due to defects, oxygen vacancies, and positive
charges in the metals. The Lewis acidity of OMS-2 is believed to primarily
catalyze the ring opening reaction of styrene epoxide. Doping other transition
metal ions such as V5+, Mo6+, and W6+ into mixed valent (Mn4+, Mn3+, and Mn2+)
manganese oxide octahedral molecular sieves may alter their Lewis acidity and
hence has significant impact on their catalytic activity. Our initial set of
experiments investigated the K-OMS-2 mediated epoxide ring opening reaction
with styrene oxide and aniline using toluene as the solvent (Table 5.1). The
method of preparation of K-OMS-2 had significant impact on conversions and
selectivities. K-OMS-2 catalysts function as Lewis acids in ring opening reactions
and the conversions in the reactions can be primarily related to their Lewis acidity.
81
Table 5.5: Lewis acidity/ Lewis basicity of K-OMS-2 catalysts
Catalyst
Total Lewis acidity
(mmol g"1)
Lewis acidity due
to chemisorption
(mmol g"1)
Total Lewis
basicity
(mmol g"1)
K-OMS-2R
0.7
0.4
0.05
K-OMS-2HY
0.63
0.3
0.07
K-OMS-2s
0.98
0.4
0.2
K-OMS-2HT
0.06
0.02
0.02
Table 5.5 shows Lewis acidity/basicity of K-OMS-2 catalysts determined
by NH3 and CO2 chemisorption that was reported recently by Ghosh et al. [21].
Furthermore, the temperature programmed desorption (TPD) of ammonia with
these catalysts show that the OMS-2 materials have at least one strong
Lewis/Bronsted acid site. These acidic properties are crucial in the ring opening
reaction. The Lewis acidity of K-OMS-2 R and K - O M S - 2 H Y are close and they
show comparable conversions and selectivities (Table 5.1) for the epoxide ring
opening reaction. The lowest conversion observed with K - O M S - 2 H T can be
attributed to poor Lewis acidity of the material. Although K-OMS-2s has the
highest Lewis acidity and surface area, the conversion was found to be lower
(47%). Nevertheless, the selectivity for Lewis acid catalyzed generation of the Pamino alcohol was the highest when K-OMS-2s was used. A detailed
82
investigation of other properties of catalysts governing this reaction is now under
investigation. Turnovers of up to 13 h"1 were achieved in a single batch with KOMS-2 catalysts.
Vanadium, molybdenum, and tungsten based materials have been known
to have excellent catalytic properties [22]. V, Mo and W metals were doped into
OMS-2 catalysts in different ratios and their detailed characterization properties
will be discussed elsewhere [23]. The doped materials available for this study
were 1%, 2% and 10% V-OMS-2, 1%, 2% and 10% Mo-OMS-2 and 1% WOMS-2 (Table 2). As the vanadium loading increased from 1% to 10% in the KOMS-2R the conversion of styrene oxide was increased from 83% to 100%. On
the other hand, the selectivity for P-amino alcohol decreased from 86% to 73%
and the selectivity for the imine increased to 20% from 8%. A 1% loading of
molybdenum in K-OMS-2R raised the conversion from 76% to a significantly
higher 91%. Doubling the amount of molybdenum in the catalyst to 2% had only
a slight enhancement of conversion to 95%. As the loading of molybdenum
increased to 10% the conversion reached 100%. However, the selectivity for the
ring opening product decreased dramatically from 88% to 62%. Tungsten showed
the highest conversion enhancement to 98% from 76% for just 1% of the tungsten
loading into K-OMS-2R. In general, as the amount of doping increased in OMS-2,
there was a significant increase in the conversion of styrene oxide.
83
The XRD patterns of doped OMS-2 are given in Figure 5.2. With the
increase in amount of dopant in the OMS-2 catalyst, the crystallinity of OMS-2
decreases. Doping transition metals into OMS-2 may lead to tunnel or framework
occupancy. Previous XRD studies with OMS-2 have shown that in V-OMS-2, the
vanadium is in the framework [24]. However, the determination of the exact
position of vanadium, tungsten, and molybdenum in the M-OMS-2 mixed
materials using EXAFS is still in progress. This difference in occupancies of the
framework or tunnel sites of OMS-2 may play a crucial role in the reaction.
10
20
30
40
50
60
Two-theta (degree)
Figure 5.2: X-Ray diffraction patterns of V, Mo and W doped OMS-2 materials.
84
5.4.2
Effect of nucleophiles
According to the results listed in Table 5.3, apart from aniline, both p-
toluidine and /?-anisidine exhibited the same conversion of 81%, but selectivities
were 70% and 59% respectively. /?-nitroaniline showed 84% conversion for
styrene oxide and 77% selectivity for the corresponding ring opened product.
Phenol gave 49% conversion and 45% selectivity. /?-methoxy phenol showed 35%
conversion with an anticipated enhancement in selectivity (69%) for the ring
opened product, omethoxy phenol gave only a 19% conversion and 40%
selectivity. In a typical situation, the presence of electron donating groups
enhances the nucleophilicity of nucleophile and electron withdrawing groups
retard the nucleophilicity. Surprisingly, this trend was not observed in these
systems and the other regioisomer of the (3-amino alcohol also appeared when
substituted nucleophiles were used. These observations could be attributed to
steric interactions at the nucleophilic centers and Lewis acid sites of the catalyst.
OMS
i
66+
o
Ar
Ar
1
ArNH 2
Figure 5.3: Scheme for OMS-2 catalyzed epoxide ring opening
85
5.4.3
Effect of solvents
For heterogeneous catalysis in the liquid phase, the solvent can influence
the rates of reaction by the solvation of reactants and intermediates in solution. On
the other hand, the solvent can also affect the rate by competing with reactant
molecules for active sites on the surface of a heterogeneous catalyst [25]. Solvents
may stabilize or destabilize transition states and intermediates formed on the
catalyst surface. However, in the reaction between styrene epoxide and aniline
catalyzed by K-OMS-2 there was no significant difference in conversions when
polar (acetonitrile, toluene) or non-polar (n-hexane) solvents were used (Table
5.4). Toluene showed the highest conversion (72%) followed by acetonitrile
(45%) and hexane (44%). For dichloromethane (8%) and tetrahydrofuran (THF)
(6%), a dramatic lowering of the conversion was observed. The reason for the low
conversion could be that competitive binding of solvent molecules to the active
sites (Lewis acid sites) of the catalysts, prevented the reaction process.
5.4.4
Catalyst Reusability
K-OMS-2R
was reused after a simple regeneration procedure by washing
with water and acetone and drying at 250°C. The catalyst was used at least for 4
cycles without any loss in activity. Furthermore, the XRD pattern of the reused KOMS-2R catalyst was identical to that of the original, indicating that the
crystallinity of the catalyst was not affected during the reaction. Catalyst
deactivation may occur due to chemical, mechanical and thermal reasons [26].
The chemical deactivation is common in catalysts and caused by strong
86
chemisorption of species on the catalytic sites, thereby blocking sites for catalytic
reaction. The regeneration process used for K-OMS-2, removes the species that
cause the deactivation of the catalyst.
Ph
N
II
+
HCHO
Figure 5.4: Formation of monoimine by (a) oxidative cleavage and diimine by
(b) oxidation-condensation processes.
5.4.5
Proposed mechanism of the reaction
The products obtained by reacting styrene oxide and aniline in the
presence of OMS-2 indicate that a Lewis acid catalyzed ring opening and an
oxidative cleavage process are taking place (Figure 5.5). The major product, (3amino alcohol, is formed by OMS-2 Lewis acid catalyzed ring opening via a
nucleophilic attack by aniline (Scheme 1). OMS-2 coordinates with the oxirane
oxygen to promote nucleophilic attack of the nucleophile leading to two
regioisomers A and/or B. Attack of the nucleophile is governed by the nature of
the oxirane and stability of the incipient carbonium ion. In the case of the OMS-2
coordinated styrene oxide, the positive charge on the oxygen appears to be
87
localized on the more highly substituted benzylic carbon. The nucleophile attacks
the benzylic carbon of the styrene oxide leading to amino alcohol A as the major
product. With the use of aniline as a nucleophile, amino alcohol A is exclusively
produced. However, when substituted anilines and phenols were used as
nucleophiles both regioisomers were observed, predominantly resulting in isomer
A. A reasonable explanation for this observation is that steric factors predominate
over electronic factors under these reaction conditions [27].
Ph-CHR-NHPh + Mn4+
H
r>
+
Ph-RC-N
^
^ Ph-CR-NHPh + H+ + Mn2+
Ph-CR=N-Ph + H+
Ph
Figure 5.5: Mechanism of imine formation from amine.
The appearance of monoimines and dimines in the reaction mixture may
be due to the oxidation of the p-amino alcohol (Figure 5.5). N-phenyl
benzylamine, the monoimine is formed by an oxidative cleavage of the P-amino
alcohol [28]. Benzalanilines can be oxidized to imines by a variety of oxidants,
including manganese oxide [29]. Pratt et al. have successfully transformed a
series of benzalanilines to imines using manganese oxide [30]. Their mechanism
suggests that Mn4+ changes to Mn2+ in the manganese, oxide catalyst used in the
oxidation process (Figure 5.5). However, Makwana et al. suggested that in an
oxidative process with OMS-2 materials, lattice oxygen of OMS-2 participates in
88
the oxidation reaction and is later replenished by fluid-phase oxygen. Hence the
structure and catalytic properties of OMS-2 are retained (A Mars-van Krevelen
type mechanism) [31]. The formation of diimines in the reaction can be explained
in terms of a double oxidation. Along with C-N bond oxidation, the terminal
hydroxy 1 group in the P-amino alcohol is also oxidized to form a carbonyl group.
OMS-2 catalysts have been known to oxidize alcohols to aldehydes or ketones
[32]. The P-amino alcohol is also oxidized by OMS-2 to form a carbonyl and
subsequently the in situ formed carbonyl reacts with aniline to form an imine [33].
The formaldehyde believed to be produced during this process was not detected
by GC-MS.
To test the above proposed oxidation mechanism a substrate similar to Pamino alcohol was reacted with OMS-2. Oxidation of 2-amino-2-phenylethanol
with K-OMS-2 catalyst in the absence of aniline gave rise to 2-amino-2phenylactaldehyde and phenylmethanamine. The products obtained in this
reaction support the reaction mechanism proposed for the formation of
monoimine and diimines from P-aminoalcohols. XRD studies on the catalyst after
the reaction show that the structure of K-OMS-2 is retained; hence lattice oxygen
is not consumed in the reaction.
89
5.5.
Conclusions
In summary, an efficient method to produce p-amino alcohols from
styrene epoxide using OMS-2 materials as catalysts has been proposed. OMS-2
acts as a Lewis acid to catalyze the reaction between styrene oxide and aniline. KOMS-2 materials prepared by different synthetic methods possess different
chemical and physical properties. Hence, the conversions and selectivities for the
ring opening reaction vary depending primarily on the Lewis acid strengths of the
catalysts. Moreover, conversions and selectivities of products can be enhanced or
altered by doping other transition metals into manganese oxide based OMS-2.
Among the metals doped into OMS-2, tungsten gave the highest conversion
(100%). However, as the conversion increased, the selectivity for the P-amino
alcohol diminished. An oxidation process clearly occurs, producing imines from
P-amino alcohol. The results suggest that the catalytic activity of doped M-OMS2 are in the decreasing order of W>Mo>V. Aniline gives the best regioselectivity
for P-amino alcohol of all the nucleophiles used for reaction with styrene oxide.
OMS-2 catalysts can be tuned by doping metals into the structure to obtain
desirable amounts of selective products. OMS-2 catalysts are cheap and easy to
prepare, can be reused, and they are environmentally benign.
90
5.6.
References
[I] S. C. Bergmeier, Tetrahedron 56 (2000) 2561 and references therein.
[2] P. O'Brien, Angew. Chem., Int. Ed. 38 (1999) 326.
[3] D. J. Ager, I. Prakash, D. R. Schaad, Chem. Rev. 96 (1996) 835.
[4] R. M. Hanson, Chem. Rev. 91 (1991) 437.
[5] G. Sundararajan, K. Vijayakrishna, B. Varghese, Tetrahedron Lett. 45 (2004) 8253.
[6] G. Sabitha, G. Reddy, J. S. Yadav, Synthesis 15 (2003) 2298.
[7] H. Eshghi, M. Rahimizadeh, A. Shoryabi, Synth. Commun. 35 (2005) 791.
[8] M. Onaka, M. Kawai, Y. Izumi, Chem. Lett. (1985) 779.
[9] A. K. Chakraborti, S. Rudrawar, A. Kondaskar, Org. Biomol. Chem. 2 (2004) 1277.
[10] Y. C. Son, V. D. Makwana, A. R. Howell, S. L. Suib, Angew. Chem. Int. Ed. 40
(2001)4280.
[II] R. Ghosh, Y. C. Son, V. D. Makwana, S. L. Suib, J. Catal. 224 (2004) 288.
[12] R. Kumar, L. J. Garces, Y-C. Son,S. L. Suib, R. E. Malz J. Catal. 236 (2005) 387.
[13] S. L. Suib, Curr. Opin. Solid State Mater. Sci. 3 (1998) 63.
[14] X. Chen,Y-F. Shen, S. L. Suib, C. L. O'Young, Chem. Mater. 14 (2002) 940.
[15] X. Chen, Y-F. Shen, S. L. Suib, C. L. O'Young, J. Catal. 197 (2001) 292.
[16] Y-S. Ding, X. Shen, S. Sithambaram, S. Gomez, R. Kumar,M. B. Vincent, S. L. Suib,
Chem. Mater. 17 (2005) 5382.
[17] X. F. Shen, Ph. D. Thesis, University of Connecticut, 2007.
[18] R. N. DeGuzman, Y-F. Shen, E. J. Neth, S. L. Suib, C. L. O'Young, S. Levine, J. M.
Newman, Chem. Mater. 6 (1994) 815.
91
[19] J. Yuan, K. Laubernds, J. Villegas, S. Gomez, S. L. Suib, Adv. Mater. 16 (2004)
1729.
[20] K. Daasbjerg, H. Svith, S. Grimme, M. Gerenkamp, C. Muck-Lichtenfeld, A.
Gansauer, A. Barchuk, Top. Curr. Chem. 263 (2006) 39.
[21] R. Ghosh, X-F. Shen, J. C. Villegas, Y. Ding, K. Malinger, S. L. Suib, J. Phys.
Chem. B 110(2006)7592.
[22] L. Giebeler, P. Kampe, A. Wirth, A. H. Adams, J. Kunert, H. Fuess, H. Vogel, J.
Mol. Catal., A Chem 259 (2006) 309.
[23] C. Chen, C. A. Calvert, L. Xu,. S. Sithambaram, S. L. Suib, manuscript in
preparation.
[24] M. Polverejan, J. C. Villegas, S. L. Suib, J. Am. Chem. Soc. 126 (2004) 7774.
[25] R. L. Augustine, R. W. Warner, M. J. Melnick, J. Org. Chem. 49 (1984) 4853.
[26] C. H. Bartholomew, Appli. Catal. 212 (2001) 17.
[27] S. K. De, R. A. Gibbs, Synth. Commun. 35 (2005) 2675.
[28] M. Shimizu, H. Makino, Tetrahedron Lett. 42 (2001) 8865.
[29] T. Mukiyama, A. Kawana, Y. Fukuda, J. Matsuo, Chem. Lett. (2001) 390.
[30] E. F. Pratt, T. P. McGovern, J. Org. Chem. 29 (1964) 1540.
[31] V. D. Makwana, Y. C. Son, A. R. Howell, S. L. Suib, J. Catal. 210 (2002) 46.
[32] Y. C. Son, V. D. Makwana, A. R. Howell, S. L. Suib, Angew. Chem. Int. Ed.
40(2001)4280.
[33] S. Sithambaram, Y-C. Son, S. L. Suib, US Patent Appli. 0027344 (2007).
92
CHAPTER 6
H2 PRODUCTION THROUGH THE
REACTION:
AN
IN
SITU
WATER-GAS-SHIFT
TIME-RESOLVED
X-RAY
DIFFRACTION INVESTIGATION OF MANGANESE OMS-2
CATALYST
6.1.
Introduction
There is a growing demand for energy around the world and researchers in
both industry and academia are striving to develop sustainable energy sources and
renewable alternatives to petroleum. Hydrogen has been identified as a very
promising renewable clean energy source to satisfy energy needs while protecting
the environment. Hydrogen is obtained industrially using the water-gas shift
(WGS) reaction, which involves the reaction between carbon monoxide and water
to give hydrogen and carbon dioxide as products. (CO(g) + H20(g) —> H2(g) +
C02(g)). Pt-group metals, such as Au are used as effective catalysts for the WGS
reaction because of their high levels of activity and stability [1]. However, Pt- and
Au-based catalysts can often display more activity than desired for WGS
reactions than Cu-based catalysts [2]. In addition, the noble metals are recognized
as a scarce resource as well as a limiting factor in the development of viable
energy alternatives to petroleum. Because of the high cost of precious metals,
some transition metals with high levels of catalytic activity for the WGS reaction
have been evaluated as alternatives.
93
Since the early 1940s the WGS reaction has represented an important step
in the industrial production of hydrogen. At the present time, mixtures of Fe-Cr
and Zn-Al-Cu oxides are the commercially used catalysts for the water-gas shift
reaction at temperatures between 350-500°C and 180-250°C, respectively.
However, these oxide catalysts are pyrophoric and normally require lengthy and
complex activation steps before usage [3]. Therefore, better and inexpensive
catalysts are being sought for the generation of hydrogen using WGS reactions [4].
Transition metal oxides have been extensively studied for water-gas shift
reactions. Hutchings et al. have studied various mixed manganese oxide catalysts
for WGS reactions and found that copper and cobalt manganese oxides are very
active at temperatures above 300°C [5]. In addition, the kinetics of the water-gas
shift reaction on manganese oxides under atmospheric pressure have been
reported by Krupay et al. [6].
In the recent past, in situ spectroscopy in catalysis has gained a lot of
attention among scientists because such methods are powerful tools in probing
events taking place in a heterogeneous catalyst under reaction conditions. This is
crucial for understanding reaction mechanisms of many important chemical
processes and would allow the rational design of new or better catalysts [7].
Synchrotron radiation-based in situ spectroscopy has become a very powerful
method to study catalysts under working conditions. Synchrotron radiation in
combination with modern data collection devices makes it possible to conduct
sub-minute, time-resolved XRD experiments under a wide variety of temperatures
94
and pressures [8]. Recently, Shen et al. from our laboratory have successfully
used synchrotron X-ray diffraction to study the in situ formation of mixed valent
manganese oxide nanocrystals [9].
Manganese oxide octahedral molecular sieves (OMS) are well known and
these materials have applications in cation-exchange, ion and molecule separation
procedures, and chemical sensor, battery, and catalysis applications [10]. OMS-2
catalysts have a 2 x 2 one-dimensional tunnel structure. Their structures are
constructed from edge-shared double chains of [Mn06] octahedra, which are
corner-connected to form one-dimensional (ID) tunnel structures (Figure 1) [11].
Recently, these OMS materials are the subject of intense research as low cost,
efficient, and environmentally friendly catalysts [12]. Some of the catalytic
applications include the synthesis of imines [13], quinoxalines [14], aminoalcohols [15], epoxides [16], and the oxidation of hydrocarbons [17]. The
catalytic activities of the OMS-2 materials vary with their synthesizing methods
[18]. In the present study, we describe the use of synchrotron based in situ TimeResolved X-ray Diffraction (TR-XRD) to study the behavior of Cryptomelanetype manganese oxide (OMS-2) catalyst under water-gas shift reaction conditions
for hydrogen production. The study shows that inexpensive and environmental
friendly OMS-2 catalysts can be used to generate hydrogen through the water gas
shift reaction. Further, the structural changes that occur in the catalysts during the
reaction process can be monitored using TR-XRD. Optimization of water-gas
shift reaction conditions and thorough mechanistic studies is ongoing.
95
6.2.
Experimental Section
6.2.1. Catalyst preparation and characterization
A reflux method described previously as well as in the literature was
employed for the preparation of OMS-2 catalysts [19]. The catalyst was
characterized by techniques described in previous chapters. A Scintag Model PDS
2000 diffractometer in a continuous scan mode was used for X-ray powder
diffraction (XRD) experiments. Samples were loaded on glass slides, and Cu Ka
radiation [X, = 1.5418 A] was used at a beam voltage of 45 kV and 40 mA beam
current. The Joint Committee on Powder Diffraction Society (JCPDS) database
was used to index the peaks in the XRD patterns. A Zeiss DSM 982 Gemini field
emission scanning microscope with a Schottky Emitter at an accelerating voltage
of 2 kV with a beam current of 1 uA was used to obtain Scanning Electron
Micrographs of the OMS-2 catalyst. The surface area of the OMS-2 catalyst was
determined using the Brunauer-Emmet-Teller (BET) method on a Micrometrics
ASAP 2010 instrument. Using N2 as the absorbent and a multipoint method the
area of OMS-2 was determined. Potentiometric titration was used to measure the
average oxidation state (AOS) of the K-OMS-2 catalysts. The catalyst was
dissolved in hydrochloric acid so as to convert all the manganese to Mn
and
titrated to a Mn3+ complex with sodium pyrophosphate versus potassium
permanganate. This gives total Mn content, based on which the AOS is
determined by reducing the solid to Mn2+ using ferrous ammonium sulfate and
back-titrating the excess Fe2+ with a permanganate standard.
96
6.2.2. Temperature Programmed Reduction
Temperature-programmed reduction (TPR) was carried out by feeding
10% H2 in Ar to 25 mg of OMS-2 catalyst in a conventional flow reactor without
oxidation treatment prior to measurements. The flow rate of the reducing gas was
set at 40 mL/min. The temperature of the reactor was raised from room
temperature to 800°C at a rate of lOK/min. The rate of H2 consumption was
determined using a thermal conductivity detector and recorded on an online
personal computer.
6.2.3. Catalytic Experiments
In situ time-resolved X-ray diffraction (TR-XRD) experiments were
carried out on beam line X7B of the National Synchrotron Light Source (NSLS)
at Brookhaven National Laboratory. The experimental set up is similar to that
described by Rodriguez et al. [20]. The sample (~ 1 mg) was loaded into a
sapphire capillary cell which was attached to a flow system. A small resistance
heater was wrapped around the capillary, and the temperature was monitored with
a 0.1 mm chromel-alumel thermocouple that was placed straight into the capillary
near the sample. The WGS reaction was carried out isothermally at several
temperatures (350, 400, and 500° C) with a 5% CO and 95% He gas mixture at a
flow rate of ~10 mL/min. This gas mixture passed through a water bubbler before
entering the reactor. Two dimensional powder patterns were collected with a
Mar345 image plate detector and the powder rings were integrated using the
FIT2D code.
97
The water-gas shift reaction using OMS-2 catalyst was conducted in our
laboratory. Under laboratory conditions, the catalytic activity of OMS-2 was
examined in a conventional flow reactor at atmospheric pressure in the
temperature range 50 to 350°C and at a constant flow rate of 10 mL/min., using
100 mg of catalyst. The gas composition before and after the reaction was
analyzed by an on-line gas chromatograph, (SRI 86IOC, TCD, He (UHP 99.999%,
Airgas) carrier) with a packed silica gel and molecular sieve column.
6.3.
Results
6.3.1. Characterization of OMS-2 catalysts
The synthesized OMS-2 catalysts were characterized by XRD, FE-SEM,
BET, and average oxidation state determination. The X-ray patterns of the OMS-2
catalysts in Figure 2 were comparable to that of standard cryptomelane (JCPDS
29-1020) and no other phases were present. An SEM micrograph of OMS-2
catalyst in Figure 3 shows the typical fibrous morphology of the catalyst. The
specific surface area of the OMS-2 sample was determined by the BrunauerEmmett-Teller (BET) method. The surface area was found to be -90 m g" .
Potentiometric titration method to measure the average oxidation state (AOS)
revealed that the AOS of the K-OMS-2 catalyst was 3.84.
98
6.3.2. Time-resolved X-Ray diffraction studies
Time-resolved XRD experimental results are shown in Figures 6.1, 6.2,
and 6.6. The OMS-2 catalyst underwent several structural changes during the
water-gas shift reaction as shown in Figure 6.1. The final form of the catalyst was
MnO, which was consistent with data obtained in comparative experiments in our
laboratory (Figure 6.5). Figure 6.6 shows structural changes of OMS-2 catalyst
when H2 was passed over the catalyst. The OMS-2 catalyst was reduced to MnO
and further reduction to elemental Mn was not observed. When oxygen was
passed over the reduced form of the catalyst, the catalyst was oxidized to Mn02 as
shown in Figure 6.7.
99
H
18
24
30
36
Two Theta (degrees)
Figure 6.1: A 3-D plot of in situ TR-XRD patterns of OMS-2 catalyst during the
water-gas shift reaction. The catalyst was heated from room temperature to 350°C
and WGS reaction was carried out at 350, 400 and 500°C.
100
(a)
-1
^1.8x10
O
—I
1
1
ri
1 pass H
4—>
1 pass CO,
i—
|
r——Tf—
500 C
2" passH^
1.2x10"*
2"4 Pass CO
°-6.0x10"
o
3
T3
O
0.0
,„i
1
*,„
A
tin,
6
9
Time (hours)
(b)
""f
t
»
T'"
""1
t
|
T
f
|
T
|
1
<"'
1.2x10" h
After 2 t.a Pass
J.
JL!
=$ 8.0x10
cd
MnO
,IK!
c/3
>§ 4.0x1 04
Before 2
1
A
Pass
L_JU\
Mn02
After 1 Pass
0.0
10
_A k.
20
30
MnO
40
Two Theta (degrees)
Figure 6.2: a) H2 and CO2 concentrations measured during the WGS reaction
with OMS-2 at various temperatures for 1st and 2 nd pass, b) TR-XRD patterns of
OMS-2 catalyst after 1st and before and after 2 nd passes.
101
6.4.
Discussion
Hydrogen has emerged has an alternative and important source of energy
because of its cleanliness. The water-gas shift reaction is widely used in the
industrial hydrogen production. This reaction is relatively controllable due to the
moderate exothermic nature {Mi2n
= -41.1 kJ/mol) as opposed to large
exothermic heat for the oxidation of CO to C 0 2 (A/f29g = -283 kJ/mol). Therefore,
the water-gas shift reaction still remains the best means to remove high
concentrations of CO from reformed fuels [21]. The kinetics and mechanisms of
the WGS reaction with various catalyst systems have been studied in the past by
many researchers. Based on kinetic results two types of mechanisms were
proposed. The first one is the Rideal-Elay type oxidation-reduction, or
regenerative mechanism, in which water oxidizes the surface and CO re-reduces
the oxidized surface.
H 2 0 ( g ) + * - H2(g) + O*
CO (g) + 0 * ^ C 0 2 ( g ) + *
Where * is an active metal site.
The second mechanism describes a bi-functional process where the
adsorbed CO on the precious metal or mixed metal oxide is oxidized by the
support and then water fills oxygen vacancies of the support [22, 23].
CO (g) + * -> CO*
M203(S) + H 2 0 ( g ) -» 2M02(s) + H2(g)
2M02(g) + CO* - • M 2 0 3(g) + C0 2 ( g )
102
Earlier reports by Krupay et al. kinetic and mechanistic studies with
manganese oxides suggest a similar mechanism [6].
CO(ads) + [O] -+ C0 2(ads) + [ ]
[ ] +H 2 0 (g) -
H2(g) + [O]
Where [O] and [ ] represent an oxygen atom or ion and an oxide vacancy
on the surface of the catalyst, respectively. The focus of our present study was to
investigate the structural changes that take place on the OMS-2 catalyst during the
water-gas shift reaction. The optimization of reaction parameters, a detailed
kinetic and mechanistic study for this OMS-2 catalyzed water-gas shift reaction is
underway.
150
200
250
300
350
400
450
500
550
600
650
700
750
800
Temperature (°C)
Figure 6.3: TPR of OMS-2 catalyst, (a) Reduction of catalyst to MnO.
The TR-XRD patterns were collected under water-gas shift (WGS)
reaction conditions from room temperature to 350°C and then holding the
temperature at 350, 400 and 500°C. Figure 4 shows the structural changes
103
observed on the OMS-2 catalyst during reaction. A sequential reduction of OMS2 (OMS-2 —• Mn 2 0 3 —* MnO) occurred during the WGS reaction process. OMS-2
catalyst, which has a cryptomelane structure, transformed to Mn203 around 350°C.
Further increases in temperature in the reaction system led to the formation of
MnO as the final form of the catalyst. The formation of H2 during the WGS
reaction gradually reduces the mixed valent Mn in the OMS-2 catalyst (average
oxidation state ~3.8) to Mn2+ in MnO.
7000
- MnO - after reaction
-6000
- OMS-2 - before reaction
5000
.•2 4000 -I
-2-3000
ud
K~)
"g 2000
i 1000
15
25
35
45
55
65
75
2 theta (degrees)
Figure 6.4: XRD patterns of catalyst before and after the WGS reaction under
laboratory conditions.
The concentrations of products of the water-gas shift reaction with OMS-2,
H2 and CO2, are shown in Figure 6.2a. According to Figure 6.2a, the catalytic
activity increased with increasing temperature in both the 1st and 2" pass.
However, the catalytic activity in the 2nd pass at 500°C is less that that in the 1st
104
pass. The structural changes in OMS-2 after the 1st pass due to water-gas shift
activity can be attributed to the difference in reactivity. Using the in situ
experimental set-up at the BNL for this experiment, we were able to detect the
products at temperatures only above 300°C due to the amount of the catalyst used
in the micro-reactor. In a separate investigation, OMS-2 catalysts have been found
to be active even below 300°C under laboratory reactor conditions. Figure 6.5
shows the results of the water-gas shift reaction activity under laboratory
conditions. The activity was monitored at temperatures ranging from 50-350°C.
The activity started above 100°C and continued to increase with increasing
temperature. Temperatures below 100°C are not desirable because water
molecules from the reactants are adsorbed on the active sites of the OMS-2
catalyst thus preventing catalytic activity.
105
50
100
150
200
250
300
350
400
Temperature, (°C)
Figure 6.5: WGS reactivity of OMS-2 catalyst under laboratory conditions.
Conversion of CO2 with temperatures from 50°C to 350°C.
As described before, the OMS-2 catalyst is reduced to MnO during the
WGS reaction process. However, it is not clear if the reactant carbon monoxide or
the product hydrogen induces the reduction of OMS-2 to MnO. CO oxidations
with OMS-2 catalysts have been reported previously and the catalysts have been
found to be very stable in the presence of CO and oxygen environments. Luo et al.
have shown that the activity of OMS-2 for the oxidation of CO is comparable to
that found in the Pt/Al203 system. They also conducted the reaction with CO in
helium (no oxygen) to study the function of lattice oxygen. Pt/AUOs system did
not show any activity, however, OMS-2 showed the same initial activity and the
activity diminished with the consumption of lattice oxygen [24].
106
To illustrate the role of H2 in the reduction, a systematic in situ reduction
of OMS-2 with hydrogen was carried out. H2 was passed over the OMS-2 catalyst
while ramping the temperature from room temperature to 640°C. TR-XRD were
recorded and the results are shown in Figure 6.3. Around 400°C, the diffraction
peaks of the OMS-2 catalyst started to disappear and the final diffraction pattern
observed corresponded to MnO. This observation is consistent with the TPR
experiments conducted with the OMS-2 catalyst. Figure 8 shows pronounced
hydrogen consumption peaks in the temperature range 250-550°C. The TPR
experiment resulted in a greenish powder which was identified as MnO from
XRD studies. Furthermore, XRD patterns of the catalyst before and after the
water-gas shift reaction under our laboratory conditions are shown in Figure 6.4.
This further illustrates that the reduced form of the catalyst after the WGS
reaction is MnO. In a previous report, Te et al. have studied the structural changes
in various manganese oxides (MnO, Mn304, Mn02, etc.) feeding carbon
monoxide and hydrogen (CO:H2 = 1:1). Using X-ray diffraction studies on the
used catalysts, they have found that these manganese oxides were converted to
MnO [25]. This study reveals that MnO is the final form of the reduced catalyst
and further reduction to Mn metal is not possible.
107
u
o
/
£
CO
5—<
°°
CO
^
?—I
ON
S a
H
15
20
25
30
35
Two Theta (degrees)
Figure 6.6: TR-XRD patterns of the OMS-2 catalyst acquired during the flow of
H2. The final form of the catalyst observed was MnO.
Finally, another experiment was carried out to investigate the structural
changes during the reoxidation of the reduced catalyst during the WGS reaction
using time-resolved XRD. As we have seen earlier, the OMS-2 catalyst is reduced
to MnO in the WGS reaction process. Figure 10 shows the TR-XRD patterns
recorded during the oxidation of MnO in a stream of 5% 02/95% He at 300700°C. The reoxidation resulted in many phases from MnO—>Mn304 and then to
Mn02. The rapid formation of Mn304 from MnO is in agreement with reported
work by Stobbe et al. [26]. They investigated the utilization of manganese oxide
as an oxygen storage compound and reported the various phases observed at
108
different temperatures during the reoxidation of MnO. Their studies showed that
the formation of MU3O4 at 673 K and Mn203 at the same temperature occurred
upon prolonged exposure to oxygen. However, they indicated Mn02 can only be
formed at an elevated temperature which was also observed in our studies.
Two Theta (degrees)
Figure 6.7: Time-resolved XRD patterns for the oxidation of reduced MnO
catalyst with oxygen.
109
6.6.
Conclusions
In summary, manganese oxide octahedral molecular sieve catalysts have
been used for hydrogen generation using the water gas shift reaction. OMS-2
catalysts show very good catalytic activity in the water-gas shift reaction above
150°C. In situ TR-XRD shows that the OMS-2 catalyst is transformed to MnO
during the WGS reaction. This investigation essentially reveals the possibility of
using an inexpensive tunnel structured manganese oxide material as new catalysts
for the water-gas shift reaction to generate hydrogen, which is a significant
development in clean energy research.
110
6.6.
References
[I] X. Wang, J. A. Rodriguez, J. C. Hanson, D. Gammara, A. Martinez-Arias, M.
Fernadez-Garcia, Top. Catal. 49 (2008) 81.
[2] A. A. Phatak, N. Koryabkina, S. Rai, J. L. Ratts, W. Ruettinger, R. J. Farrauto,
G. E. Blau, W. N. Delgass, F. H. Ribeiro, Catal. Today 123 (2007) 224.
[3] V. Palma, E. Palo, P. Ciambelli, Catal. Today 147S (2009) S107.
[4] J.A. Rodriguez, P. Liu, J. Hrbek, M. Perez, J. Evans, J. Mol. Catal. A 281
(2008) 59.
[5] (a) G. J. Hutchings, F. Gottschalk, R. Hunter, S. W. Orchard, Faraday Trans. 1
85(1989)363.
[6] B. W. Krupay, R. A. Ross, Can. J. Chem. 51 (1973) 3520.
[7] B. M. Weckhuysen, Chem. Commun. (2002) 97.
[8] J. A. Rodriguez, J. Y. Kim, J. C. Hanson, M. Perez, A. I. Frenkel, Catal. Lett.
85 (2003) 247.
[9] X-F. Shen, Y-S. Ding, J. C. Hanson, M. Aindow, S. L. Suib, J. Am. Chem.
Soc. 128(2006)4570.
[10] S. L. Suib, J. Mater. Chem. 18 (2008) 1623.
[II] Y-S. Ding, X-F. Shen, S. Sithambaram, S. Gomez, R. Kumar, M. B. Vincent,
S. L. Suib, Chem. Mater. 17 (2005) 5382.
[12] K. A. Malinger, Y-S. Ding, S. Sithambaram, L. Espinal, S. Gomez, S. L.
Suib, J. Catal. 239 (2006) 290.
[13] S. Sithambaram, R. Kumar, Y. C. Son, S. L. Suib, J. Catal. 253 (2008) 269.
Ill
[14] S. Sithambaram, Y-S. Ding,W-N. Li, X-F. Shen, S. L. Suib, Green Chem. 10
(2008)1029.
[15] S. Sithambaram, L-P. Xu, C-H. Chen, Y-S. Ding,R. Kumar, C. A. Calvert, S.
L. Suib, Catal. Today 140 (2009)162.
[16] R. Ghosh, X-F. Shen, J. C. Villegas, Y. Ding, K. Malinger, S. L. Suib, J.
Phys. Chem. B 110 (2006) 7592.
[17] S. Sithambaram, E. K. Nyutu, S. L. Suib, J. Appl. Catal. A, 348 (2008) 214.
[18] S. L. Suib, Ace. Chem. Re. 41 (2008) 479.
[19] R. N. DeGuzman, Y-F. Shen, E. J. Neth, S. L. Suib, C. L. O'Young, S.
Levine, J. M. Newman, Chem. Mater. 6 (1994) 815.
[20] J. A. Rodriguez, J. C. Hanson, W. Wen, X. Wang, J. L. Brito, A. MatinezArias, M. Fernadez-Garcia, Catal. Today 145 (2009) 188.
[21] Y. Tanaka, T. Utaka, R. Kikuchi, T. Takeguchi, K. Sasaki, K. Eguchi, J.
Catal. 215(2003)271.
[22] J. Barbier Jr, D. Duprez, Appl. Catal.,B: Environmental, 4 (1994) 105.
[23] Y. Li, Q. Fu, M. Flytzani-Stephanopoulos, Appl. Catal. B: Environmental, 27
(2000) 179.
[24] J. Luo, Q. Zhang, J. Garcia-Matinez, S. L. Suib, J. Am. Chem. Soc. 130
(2008)3198.
[25] M. Te, H. C. Foley, Appl. Catal. A, 119 (1994) 97.
[26] E. R. Stobbe, B. A. De Boer, J. W. Geus, Catal. Today 47 (1999) 161.
112
CHAPTER 7
EXAFS/XANES ANALYSIS OF MANGANESE OCTAHEDRAL
MOLECULAR
SIEVES
AND
THEIR
CATALYTIC
ACTIVITIES IN THE SYNTHESIS OF PYRAZINES
7.1.
Introduction
X-ray absorption spectroscopy (XAS) is a versatile technique that is atom
specific and capable of probing the structure around an atom of interest. The Xray absorption spectroscopic structure normally consists of two regions: (1) the Xray absorption near-edge structure (XANES) and (2) the extended X-ray
absorption fine structure (EXAFS). XANES can provide useful structural
information on the oxidation state of an atom of interest as well as the site
symmetry. Analysis of the EXAFS can provide a significant amount of
quantitative structural information on the atom of interest including coordination
number, interatomic distance, and Debye-Waller factor of the coordinated atoms
In this chapter Mn K-edge XAS study of nanosized manganese octahedral
molecular sieves (OMS-2) prepared by solvent free method and high temperature
methods have been reported. OMS-2 materials have a one-dimensional tunnel
structure formed by 2 x 2 edge shared MnC>6 octahedra. The composition of KOMS-2 is KMn80i6nH20.. The average oxidation state of Mn in OMS-2 is -3.8
with the presence ofMn 4+ ,Mn 3+ ,andMn 2+ ions in the framework [2].
113
Continuum
Photo-electron
/WW
X-tov
K
Figure 7.1: Schematic representation of XAS phenomenon
114
7.2.
Experimental Section
7.2.1. Catalyst Preparation and characterization
K-OMS-2SF
was prepared using a solvent free method [3] by mixing 9.48
g of KMO4 and 22.05 g Mn(Ac)2'4 H2O and was ground homogeneously in an
agate mortar. The mixed powders were then placed in a capped glass bottle and
maintained at 80°C for 4 hrs. The resulting black powder was thoroughly washed
with deionized water several times to remove any ions that may be present and
was finally dried overnight at 80°C.
The high temperature
K-OMS-2HT
was prepared by a combination of sol-
gel and combustion methods [4] with the Mn source being Mn(N03)2- KNO3 and
Mn(N03)2 in a molar ratio of 1:5 were dissolved in distilled deionized water
(solution A). Glycerol and KNO3 were mixed in a 1:10 ratio (solution B).
Solutions A and B were mixed in deionized water with vigorous stirring to form a
clear solution and then heated to 120°C to form a gel (usually 5h). The gel was
then heated to 250°C for 2 hours to complete the combustion reaction. The black
powder was then calcined at 600°C for 3 hours to obtain the final product.
7.2.2. Sample Characterization
X-ray powder diffraction (XRD) experiments were carried out using a Scintag
Model PDS 2000 diffractometer. Samples were loaded on glass slides, and Cu
Ka radiation was used at a beam voltage of 45 kV and 40 raA beam current. The
115
X-ray patterns of the catalysts were compared to that of the standard OMS-2
materials (JCPDS file # 29-1020).
Scanning Electron Micrographs were taken on a Zeiss DSM 982 Gemini field
emission scanning microscope with a Schottky Emitter at an accelerating voltage
of 2 kV with a beam current of 1 juA. The images showed a characteristic fibrous
morphology of OMS-2 materials.
7.2.2. XANES/EXAFS Measurements
The X-ray absorption spectra were measured at the X19A beamline of the
NSLS, National Synchrotron Light Source, Brookhaven National Laboratory,
Upton, New York. XANES spectra at the Cr, Mn, Fe, and Co K-edge (5989, 6539,
7112, and 7709 eV, respectively) were recorded in air at room temperature in
transmission mode with two ion chambers as detectors: one before the sample to
measure the incident X-ray intensity, IQ, one after the sample and before the
corresponding reference metal foil to measure the intensity after the sample, I\,
and one after the metal foil, I2. The sample and the metal foil spectra were
expressed as log(/0/7i) and log(/i/2), respectively. The corresponding spectrum
from the metal foil was used to calibrate the absolute energy scale for the
corresponding sample spectrum, by positioning the absorption edge at the first
inflection point. Monochromators on the beamlines were equipped with Si(lll)
crystals. The 0.3 mm vertical aperture of the beam definition slit in the hutch
provided a resolution of about 2.5 eV at the K absorption edge of Mn atoms.
116
Sampls
I
Optics
T
St its
:
h
\
Source
Amplifiers &
Computer
Io - Incident ion chamber
IT - Transmitted ion chamber
IF - Fluorescence chamber
IR - Reference ion chamber
Transmission data is collected when the sample concentration is high. Sample has
to be uniform.
Fluorescence data is collected when the sample concentration is low.
Figure 7.2: Experimental set-up for XAS measurements
7.2.3. Data Reduction
The EXAFS data were extracted from the measured absorption spectra
following standard methods [5, 6]. Raw data files were averaged and the pre-edge
region was approximated using a polynomial curve. The normalization procedure
was completed by dividing by the height of the absorption edge; the background
was subtracted using cubic spline routines. The main contributions to the spectra
were isolated on the Fourier transforms of the final EXAFS functions. The phase
shifts and the amplitude functions for, Mn—O, Mn—Mn and Mn—Kr pairs were
calculated using the FEFF [7] program from the metallic Mn and manganese
oxide structures, respectively.
To obtain the mean oxidation state of the metal in each sample, the edge shift
relative to the metal reference compound was determined following the method
proposed by Capehart et al. [8]. This method is based on the integration of the
normalized absorption spectra for an interval extending from well below the edge
up to the first value of energy for which the normalized absorption of the
reference is equal to one (ER). Thus, the integrated absorption of the sample is
obtained by integrating up to the energy ES, whose value is such that the
integrated area of the sample and reference absorption are equal (the white line
contribution to the normalized absorption area was eliminated taking 80% of the
integrated absorption area of the reference). Therefore, the edge shift in the
XANES spectrum is given by DE = ES - ER. This method is independent of the
fine structure; namely, the occurrence of certain peaks or shoulders in the raising
118
edge, which is opposite to what happens with the simple determination of the
inflection points. The position of the edge is related to the oxidation state of the
elements in such a way that the edge energy shifts vary monotonically with the
valence of the metal atom [9]. In a set of chemically similar compounds, it has
been shown a linear relationship between the energy shift and the oxidation state
[10-12]. This is true in a model where the energy shift of the core level is mainly
due to coulombic effects [13, 14], therefore, it is possible to determine the
oxidation state of the metal atom from the absorption edge position.
7.2.4. Catalytic Reactions
The conventional reactions were carried out in a batch reactor operating under
reflux conditions at 70° C for a period of 2 hours. To a 25 mL round bottomed
flask, 1 mmol of tetralin, 5 mL of solvent, and 50 mg of catalyst were added.
Finally, 5 mmol of TBHP was added drop wise to the mixture. The reaction
mixture was continuously stirred using a magnetic stirrer. The reaction progress
was monitored by TLC. After each of the reactions was performed, aliquots were
removed, diluted to constant concentration and a gas chromatography-mass
spectroscopy (GC-MS) method was used for the identification and quantification
of the product mixtures. GC-MS analyses were done using an HP 5890 series II
chromatograph with a thermal conductivity detector coupled with an HP 5970
mass selective detector. An HP-1 column (non polar cross linked siloxane) with
dimensions of 12.5 m x 0.2 mm * 0.33 /an was used in the gas chromatograph.
119
The products were also confirmed by ! H (400.144 MHz) and 13C (100.65 MHz)
NMR data collected on a Brucker DRX-400.
7.3.
Results
7.3.1. X-ray absorption Near-edge Spectroscopy (XANES)
Normalized Mn K-edge XANES spectra of the K-OMS-2 samples and
reference compounds Mn0 2 , Mn203, Mn304 and MnO are presented in Figure 7.3.
XANES spectra of the K-OMS-2 samples and four reference compounds we
measured were used for subsequent determination of average oxidation state of
Mn in each sample.
7.3.2. Extended Absorption Fine Structure (EXAFS)
Figure 7.4 show the XRD Rietweld refinement and Fourier Transforms
(FT) of the Mn K-edge k3x(k) spectra of the K-OMS-2 catalysts prepared high
temperature methods. K-OMS-2 structures are constructed from edge-shared
double chains of [Mn06] octahedra, which are corner-connected to form onedimensional (ID) tunnel structures. K+ ions are located in the tunnels. The first
two distinct peaks between 1 and 3 A in the diagram results from simple
backscattering of the photoelectron from the first coordination shell of six oxygen
atoms (Mn-O) and the second coordination shell manganese atoms (Mn-Mn). The
peaks between 3 and 4 A result from the coordination of K atoms (Mn-K).
120
Table 7.1: Energy shifts and oxidation states of reference compounds and
samples.
Samples
Energy
Energy shift
E-Eo (eV)
Mn oxidation state
Metallic Mn
6539
0
0
MnO
6545
6
2
M113O4
6547.5
8.5
2.67
Mn 2 0 3
6548.6
9.6
3
Mn0 2
6553
14
4
K-OMS-2 (SF)
6551.8
12.8
3.7
K-OMS-2 (HT)
6552.5
13.5
3.8
121
(a)
1.8-r
1.61.41.21.00.80.60.4-
Mn 2 0 3
Mn 3 0 4
K-OMS-2 (HT)
K-OMS-2 (SF)
0.20.0-0.2
6500
6550
6600
6650
6700
Energy (eV)
(b)
K-OMS-2HT =
K-OMS-2SF =
Energy Shift (E-E„) (eV)
Figure 7.3: (a) Mn K-edge XANES spectra of K-OMS-2 catalysts and reference
compounds, (b) Oxidation state determination of K-OMS-2 catalysts using the
Mn K-edge energy shift of the Mn reference compounds.
122
(a)
a=9.8545 A c=2.8623 A
X
z
y
Mi 0.3514 0.1692 0.0
01 0.5522 0.1739 0.0
02 0.1548 0.2025 0.0
K
I:
A*
W
0.0
0.5
.JU/V-A^jyw
A
2b
0.0
¥^^ V^^VJ^W-^
A ^ Y - * " ^80
20
(b)
2.<H
°S
1.5-
Mn-O
Mn-Mnl
•a
3
C
(0
E
1.0 J
Mn-Mn2
0.5 J
Mn-Kl
Figure 7.4: (a) XRD and (b) EXAFS refinements using theoretical phases and
amplitudes to experimental K-OMS-2.
123
Table 7.2: EXAFS fit results of theoretical model calculations to the experimental
K-OMS-2 catalyst.
CN
^model ( A )
Rm (A)
a2(A2)
2
4
1.854
1.911
1.896
1.914
0.0029
0.0008
Second shell Mn-Mn
4
2.862
2.902
0.0034
Third shell
4
1
2
3.449
3.573
4.101
3.432
3.655
4.110
0.0021
0.0154
0.0236
Shell
First shell
Pair
Mn-0
Mn-0
Mn-Mn
Mn-K
Mn-K
7.3.3. Catalytic Activity of K-OMS-2 Nano Materials.
OMS-2 materials have been shown to have outstanding catalytic activities
for oxidation reactions with interest to fine chemical industry and academia. The
catalytic activities for K-OMS-2 prepared by both solvent free methods and high
temperature
methods
were
evaluated
for
the
one-pot
synthesis
of
dihydropyrazines from benzoin and 1,2-diaminocyclohexane under microwave
heating conditions. The conversion, selectivity and yield results are summarized
in table 7.3. Both the catalysts showed 100% selectivity for the dihydropyrazine.
K-OMS-2 prepared by the solvent free method gave a yield of 67% while KOMS-2 prepared by the high temperature method gave 44% yield for the pyrazine.
124
Table 7.3: Synthesis of dihydropyrazines using K-OMS-2 catalysts.
Ph.
^
C
I
Ph'
OH
Catalyst
Conversion, %
Selectivity, %
Yield, %
TOF, h"1
K-OMS-2
(SF)
66.7
100
66.7
21.1
K-OMS-2
(HT)
43.8
100
43.8
14.1
" 1 mmol benzoin, 2 mmol 1,2-hexanediamine and 50.0 mg catalyst stirred in 10 mL benzonitrile under
microwave reflux for 1/2 h.
* Catalyst preparation methods, SF-solvent free, HT- high temperature. d Yield determined by GC-MS
(Yield = conversion x selectivity).' Turnover frequency = moles of converted substrate/(moles of catalyst x
reaction time in h).
7
50 mg = 0.0625 mmol K-OMS-2.
125
Table 7.4: Properties of K-OMS-2 catalysts1
Catalyst
AOS
Surface area,
m2/g
Crystallite
size, nm
Lewis acidity,
mmol/g
K-OMS-2 (SF)
3.72
156
10
0.98
K-OMS-2 (HT)
3.85
13
20
0.06
7.4.
Discussion
7.4.1. XANES
Recently, Farges have conducted pre-edge investigations of Mn K-edge
XANES of oxide materials and shown that the pre-edge feature is related to
electronic transitions from the Is core levels to the empty 3d levels.[15] A single
pre-edge peak around 6542 eV in the XANES of Mn-compounds is an indication
that Mn atoms occupy sites without center of inversion. The peak intensity of
XANES spectra of samples in Fig. is much lower than in systems with regular
M n 0 4 species. This implies that Mn in our samples is preferably located on
octahedral sites [16]. XANES spectra of K-OMS-2 materials were compared with
reference spectra arising from known Mn compounds. Figure 7.3 shows the
normalized XANES spectra Mn reference compounds. It shows spectra of KOMS-2 (SF) and K-OMS-2 (HT), respectively.
126
In principle, it is possible to obtain a quantitative estimation, averaged for
all the Mn atoms in the sample, for the Mn oxidation state. Each different
chemical species of Mn contribute to the experimental spectrum with its specific
weight (relative concentration). The energy shifts on the absorption edge are
directly related with the average oxidation state of the absorbent atom [17]. The
position of the Mn edges shown in Figure 7.3 is related to the oxidation state of
the element. The energy shift of the edge varies linearly with valence state of the
metal atom. Hence, it is possible to directly determine the oxidation state of Mn in
K-OMS-2 from the corresponding energy shifts derived from the XANES spectra.
The energy shifts of K-OMS-2 (SF) and K-OMS-2 (HT) are 12.8 eV and 13.5 eV,
thus these values are lower than the energy shift of Mn02 (14 eV) and higher than
the energy shift of Mn203 (9.6 eV) implying that Mn in K-OMS-2 in a mixed
valent state (Table 7.1). The obtained average oxidation state for Mn in the KOMS-2 samples is shown in Figure 7.3b and Table 7.1. The average oxidation
states determined by XANES analysis are in good agreement with the average
oxidation states determined by potentiometric titrations. This implies that XANES
is alternative method to determine the oxidation states of mixed valent transition
metal oxides.
7.4.2. EXAFS
EXAFS data at the Mn K-edge was collected for K-OMS-2 sample. KOMS-2 structures are constructed from edge-shared double chains of [Mn06]
octahedra, which are corner-connected to form one-dimensional (ID) tunnel
127
structures. K+ ions are located in the tunnels. The Fourier transform of EXAFS
data is shown in Figure 7.4. The first two distinct peaks between 1 and 3 A in the
diagram results from simple backscattering of the photoelectron from the first
coordination shell of six oxygen atoms (Mn-O) and the second coordination shell
manganese atoms (Mn-Mn). The peaks between 3 and 4 A result from the
coordination of K atoms (Mn-K). The curve fitting analysis of the EXAFS spectra
provides the parameters found for each coordination shell. Table 7.2 summarizes
the results of the curve fitting and provides quantitative distances of Mn-O, MnMn and Mn-K. The fit values differ from the model values suggesting that the KOMS-2 consisting of distorted Mn06. Furthermore, K-OMS-2 has a 2 * 2 tunnel
structure. The curve fitting values for Mn-K obtained are 3.6 and 4.1 and this
strongly suggests that the K+ ions are in the tunnel [18].
128
7.4.3. Catalytic Activity of K-OMS-2 Nano Materials.
The synthesis of dihydropyrazine was evaluated using K-OMS prepared
by solvent free and high temperature methods under microwave heating. The
reaction goes through a two step oxidation-condensation process to form the
pyrazine ring. The proposed catalytic cycle is given in Figure 7.5. This single pot
synthesis is advantageous over multi-step reactions in terms of 'atom economy'
[19]. Furthermore, the K-OMS-2 catalysts used for this reaction are
environmental friendly and inexpensive as opposed to active manganese oxides
used as stoichiometric oxidants for similar reactions reported in the literature. The
use of stoichiometric oxidants in industrial process could result in an enormous
amount of waste [20].
7.5.
Conclusions
The XANES and EXAFS studies and analyses of K-OMS-2 samples were
successfully performed using synchrotron radiation. XANES analysis was used as
an alternative method to determine average oxidation state of Mn in K-OMS-2.
The average oxidation state for Mn in K-OMS-2 (HT) and K-OMS-2 (SF) were
determined to be 3.8 and 3.7, respectively. The EXAFS analysis provided
distances between Mn-O, Mn-Mn and Mn-K atoms. However, the fit distances
were different than the model distances suggesting that the structure is distorted.
The distances obtained for Mn-K suggest that K+ ions are in the tunnels.
129
Figure 7.5: Proposed catalytic cycle for OMS-2 catalyzed synthesis of pyrazines.
130
7.6.
References
[I] K. J. Chao, A. C. Wei, J. Elect. Spectro. 119 (2001) 175.
[2] K. A. Malinger, Y-S. Ding, S. Sithambaram, L. Espinal, S. Gomez, S. L. Suib,
J. Catal. 239 (2006) 290.
[3] Y-S. Ding, X-F. Shen, S. Sithambaram, S. Gomez, R. Kumar, M. B. Vincent,
S. L. Suib, Chem. Mater. 17 (2005) 5382.
[4] X. F. Shen, Ph. D. Thesis, University of Connecticut, 2007.
[5] T. Ressler, S.L. Brock, J. Wong, S.L. Suib, J. Phys. Chem. B 103 (1999) 6407.
[6] T. Ressler, J. Synch. Rad. 5 (1998) 118.
[7] S.I. Zabinsky, J.J. Rehr, A. Ankudinov, R.C. Albers, M.J. Eller, Phys. Rev. B
52(1995)2995.
[8] T.W. Capehart, J.F. Herbst, F.E. Pinkerton, Phys. Rev. B 52 (1995) 7907.
[9] I. Arcon, B. Mirtic, A. Kodre, J. Am. Ceram. Soc. 81 (1998) 222.
[10] M. Ferna'ndez-Garci'a, Catal. Rev. 44 (2002) 59.
[II] J. Wong, F.W. Lytle, R.P. Messmer, D.H. Maylotte, Phys. Rev. B 30 (1984)
5596.
[12] C. Pak, G.L. Haller, Micropor. Mesopor. Mater. 48 (2001) 165.
[13] R.E. Watson, M.L. Perlman, J.F. Herbst, Phys. Rev. B 13 (1976) 2358.
[14] P.S. Bagus, G. Pacchioni, C. Sousa, T. Minerva, F. Parmigiani, Chem. Phys.
Lett. 196(1992)641.
[15] F. Farges, Phys. Rev. B (2005) 71, 155109.
[16] S. J.A. Figueroa, F. G. Requejo, E. J. Lede, L. Lamaita, M. A. Peluso, J. E.
Sambeth, Catal. Today 107-108 (2005) 849.
131
[17] J. M. Ramallo-Lopez, E. J. Lede, F. G. Requejo, J. A. Rodriguez, J-Y. Kim,
R. Rosas-Salas, J. J. Dominiguez, J. Phys. Chem. B (2004) 108, 20005.
[18] R. N. DeGuzman, Y-F. Shen, E. J. Neth, S. L. Suib, C. L. O'Young, S.
Levine, J. M. Newman, Chem. Mater. (1994) 6, 815.
[19] S. L. Suib, Ace. Chem. Re. 41 (2008) 479.
[20] S. A. Raw, C. D. Wilfred, R. J. K. Taylor, Org. Biomol. Chem., (2004) 2,
788.
132
CHAPTER 8
FUTURE WORK
Catalysts and catalysis are fundamental to being able to produce the fuels,
polymers, medicines, plant growth regulators and herbicides, paints, lubricants,
fibers, adhesives and a vast array of other consumer products. Based on the
findings of this research, the following areas have been identified for future work.
Modeling Catalytic Structures and Their Reaction Environment - there have
been significant growth of theoretical understanding and modeling of surface
catalytic structures. As a result, it is now possible to calculate activation energies
of elementary surface reactions for various reactions and catalysts and to
understand the trends in reactivity from one catalyst to the next.
Synthesis of Metal Organic Frameworks with Nanostructured Metal Oxides During the past few years, catalysis scientists have dramatically improved their
ability to design and synthesize inorganic sites with controlled size, atomic
connectivity, and hybridization with either organic or other inorganic
superstructures. The resulting materials contain chemical functions and physical
properties that can be tuned for energy conversion, petrochemical synthesis, and
environmental reactions.
OMS-2 materials have been found to exhibit excellent catalytic activities
in several applications with interest to fine chemical industry and academia.
133
Based on the findings of this research with respect to catalytic activity of OMS-2
materials, the following reactions have been identified for future work
8.1.
Synthesis of esters
Esters have been synthesized via a single-pot method from alcohols and
NaCN in the presence of active manganese oxide. OMS-2 can replace the active
manganese oxide to make the process more environmentally benign.
OH
^
20MnO 2 5 NaCN
ROH
^
^
78%
Figure 8.1. Synthesis of Esters from Alcohols.
8.2
Synthesis of amides
Carboxylic amides can be prepared by oxidation of aromatic and a,|3-
unsaturated aldehydes with manganese dioxide in the presence of NaCN and an
amine. OMS-2 can be utilized as a catalyst to produce carboxylic amides from
alcohols via an in-situ oxidation amide-formation process.
OMS-2
NHR
NaCN, RNH 2
Figure 8.2. Synthesis of Amides from Alcohols.
134
8.3
Oxidation-Wittig reactions/ C-C bond formation
Very successful in-situ alcohol oxidation-Wittig reactions using transition
metal oxides have been reported. This C-C bond formation method can be
extended to OMS-2 catalysts.
OMS-2
Ph 3 PCH 3 Br
Base
o2N'
Figure 8.3. C-C Bond formation with Alcohols.
8.4
Conversion of alcohols into nitriles using ammonia
Nitriles are widely used as important intermediates in organic synthesis.
For instance, they can be readily converted to heterocylic compounds. Efficient
synthesis aromatic nitriles from aldehydes have been reported using ammonia,
MgS04j and manganese dioxide.
OH
OMS-2
NH*
Figure 8.4. Synthesis of Nitriles from Alcohols.
8.5
Synthesis of oximes
OMe
OMS-2
MeONH,
Figure 8.5. Synthesis of Oximes from Alcohols.
135
Oximes have been synthesized by manganese oxide mediated oxidation of
alcohol followed by in situ trapping of the resulting aldehyde by alkoxylamines.
OMS-2 could be used as catalysts for these types of reactions. The process can be
extended to synthesize pharmaceutically important Citaldoxime.
136
Appendix
REFEREED JOURNAL PUBLICATIONS
1. Sithambaram, S.; Xu, L. P.; Chen, C-H.; Ding, Y.; Kumar, R.; Calvert, C. A.; Suib, S.
L., "Manganese Octahedral Molecular Sieve Catalysts for Selective Styrene Oxide Ring
Opening." Catal. Today,2009, 140, 162-168.
2. Sithambaram, S.; Nyutu, E. K.; Suib, S. L. "OMS-2 Catalyzed Oxidation of Tetralin: A
Comparative study of Microwave and Conventional Heating under Open Vessel
conditions." J. Appl. Catal. A, 2008, 348, 214-220.
3. Sithambaram, S.: Ding, Y.; Shen, S-F.; Li, W-N.; Gaenzler, F.; Suib, S. L, "Manganese
Octahedral Molecular Sieves Catalyzed Tandem Process for Synthesis of Quinoxalines."
Green Chem. 2008, 10, 1029-1032.
4. Sithambaram, S.; Kumar, R.; Son, Y-C; Suib, S. L., "Tandem Catalysis: Direct
Catalytic Synthesis of Imines from Alcohols Using Manganese Octahedral Molecular
Sieves."/. Catal. 2008, 253, 269-277.
5. Iyer, A.; Galindo, H.; Sithambaram, S.; King'ondu, C; Chen, C-H.; Suib, "Nanoscale
Manganese Oxide Octahedral Molecular Sieves (OMS-2) as Efficient Photocatalysts in
2-propanol Oxidation." J. Appl. Catal. A. 2010, 375, 295-302.
6. Xu, L.; Sithambaram, S.; Zhang, Y.; Chen, C-H.; Jin, L.; Joesten, R.; Suib, S. L.,
"Novel Urchin-like CuO Synthesized by a Facile Reflux Method with Efficient Olefin
Epoxidation Catalytic Performance" Chem. Mater. 2009, 21, 1253-1259.
7. Kumar, R.; Sithambaram, S.; Suib, S. L., "Cyclohexane oxidation catalyzed by
manganese oxide octahedral molecular sieves-Effect of acidity of the catalyst" J. Catal.
2009,262,304-313.
8. Karunanithi, A. T.; Acquah, C; Achenie, L. E. K.; Sithambaram, S.; Suib, S. L.,
"Solvent design for crystallization of carboxylic acids" Comp. Chem. Eng., 2009, 33,
1014-1021.
9. Chen, C-H.; Abbas, S. F.; Morey, A.; Sithambaram, S.; Xu, L-P.; Garces, H. F.; Hines,
W. A.; Suib, S. L. "Controlled synthesis of self-assembled metal oxide hollow spheres
via tuning redox potentials: versatile nanostructured cobalt oxides" Adv. Mater. 2008, 20,
1205-1209.
10. Nyutu, E. K.; Chen, C-H.; Sithambaram, S.; Crisostomo, V. M. B.; Suib, S. L.
"Systematic Control of Particle Size in Rapid Open-Vessel Microwave Synthesis of KOMS-2 Nanofibers" J. Phy. Chem. C 2008, 112, 6786-6793.
11. Li, W-N.; Zhang, L.; Sithambaram, S.; Yuan, J.; Shen, X-F.; Aindow, M.; Suib, S L.
"Shape Evolution of Single-Crystalline Mn 2 0 3 Using a Solvothermal Approach" J. Phy.
Chem. C 2007, 111, 14694-14697.
137
12. Karunanithi, A. T.; Acquah, C; Achenie, L. E. K.; Sithambaram, S.; Suib, S. L.; Gani,
R., "An experimental verification of morphology of ibuprofen crystals from CAMD
designed solvent" Chem. Eng. Sci. 2007, 62, 3276-3281.
13. Li, W-N; Yuan, J.; Shen, X-F.; Gomez-Mower, S.; Xu, L.-P.; Sithambaram, S.; Aindow,
M.; Suib, S. L., "Hydrothermal synthesis of structure- and shape-controlled manganese
oxide octahedral molecular sieve nanomaterials" Adv. Funct. Mater. 2006, 16, 12471253.
14. Malinger, K. A.; Ding, Y-S.; Sithambaram, S.; Espinal, L.; Gomez, S.; Suib, S. L.,
"Microwave frequency effects on synthesis of cryptomelane-type manganese oxide and
catalytic activity of cryptomelane precursor" J. Catal. 2006, 239, 290-298.
15. Li, W-N; Yuan, J.; Gomez-Mower, S.; Sithambaram, S.; Suib, S. L., "Synthesis of
Single Crystal Manganese Oxide Octahedral Molecular Sieve (OMS) Nanostructures
with Tunable Tunnels and Shapes" J. Phy. Chem. B 2006, 110, 3066-3070.
16. Ding, Y-S.; Shen, X-F.; Sithambaram, S.; Gomez, S.; Kumar, R.; Crisostomo, V. M. B.;
Suib, S. L.; Aindow, M.. "Synthesis and Catalytic Activity of Cryptomelane-Type
Manganese Dioxide Nanomaterials Produced by a Novel Solvent-Free Method" Chem.
Mater. 2005, 17, 5382-5389
MANUSCRIPTS IN PROCESS
17. Sithambaram, S.; Xu, L. P.; Suib, S. L. "Microwave-Promoted Copper-Catalyzed
Efficient Arylation of A/-Heterocycles." Submitted to Chem. Comm.
18. Sithambaram, S.; Wen, W.; Shen, X-F.; Hanson, J. C ; Suib, S. L. "H2 Production
through Water-gas-shift Reaction: an in situ Time-resolved X-ray Diffraction
investigation of manganese OMS-2 Catalyst." Invited Paper. Submitted to Catal. Today
Special Issue "Renewable Alternatives to Petroleum.
19. Huang, H.; Sithambaram, S.; Chen, C-H.; Xu, L. P.; King'ondu, C; Suib, S. L.
"Microwave-assisted Hydrothermal Synthesis of Cryptomelane-type Manganese Oxide
Octahedral Molecular Sieves (OMS-2) and Their Catalytic Studies." Submitted to Chem.
Mater.
20. Sithambaram, S.; Shanthakumar, P.; Nyutu, E. K.; Ngala, K.; Pease, D. M.; Suib, S. L.
"EXAFS/XANES Analysis of Nano-sized Manganese Octahedral Molecular Sieves and
their Catalytic activities in the synthesis of Pyrazines." To be submitted to J. Phys. Chem.
C.
138
CONFERENCE PROCEEDINGS/PREPRINTS
1. Sithambaram, S.; Li, W-N.; Ding, Y.; Kumar, R.; Suib, S. L., "K-OMS-2 catalyzed
liquid phase synthesis of quinoxalines" Preprints - American Chemical Society, Division
of Petroleum Chemistry 2007, 52,.
2.
Sithambaram, S.; Calvert, C ; Opembe, N.; Suib, S. L., Novel Ag-MnOx catalysts for
selective epoxidation of olefins" Preprints - American Chemical Society, Division of
Petroleum Chemistry 2007, 52, 243-245.
3.
Sithambaram, S.; Ding, Y.; Shen, X-F.; Xu, L.; Suib, S. L. "Manganese octahedral
molecular sieves catalyzed preparation of P-amino alcohols from epoxides". Preprints American Chemical Society, Division of Petroleum Chemistry 2007', 52, 59-61.
4.
Sithambaram, S.; Wen, W.; Hanson, J. C ; Suib, S. L. "OMS-2 catalyst for H2
production: an in situ time-resolved X-ray diffraction investigation". Preprints American Chemical Society, Division of Petroleum Chemistry 2009, 54, 218-220.
CONFERENCE PRESENTATIONS
1. OMS-2 catalyst for H2 production: an in situ time-resolved X-ray diffraction
investigation.
Sithambaram. S.; Wen. W.; Hanson, J. C ; Suib, S. L. ACS National Meeting, Washington
D. C, August - 2009.
2. Transition metal oxide nanomaterials for emerging catalytic applications.
Sithambaram, S.; Suib, S. L. ACS National Meeting, Washington D. C, August - 2009.
3. Selective oxidation of benzylamines over manganese oxide octahedral molecular
sieves.
Dharmarathne. S.; Sithambaram. S.; Nyutu, E. K.; Suib, S. L. ACS National Meeting,
Washington D. C, August - 2009.
4. In situ time-resolved characterization of manganese oxide octahedral molecular
sieves catalysts during the water-gas shift reaction.
Sithambaram. S.; Wen, W.; Hanson, J. C ; Suib, S. L. ACS National Meeting,
Philadelphia, August - 2008.
5. EXAFS and XANES analysis of porous manganese octahedral molecular sieves and
their catalytic activities under microwave heating.
Sithambaram, S.; Shanthakumar, P.; Nyutu, E. K.; Pease, D. M.; Suib, S.L.ACS
National Meeting, Philadelphia, August — 2008.
6. Open Vessel Microwave Assisted Catalytic Oxidation of Alkyl Aromatics.
Sithambaram. S.; Nyutu, E. K.; Suib, S. L. 6th International Microwave Chemistry
Conference, Boston, May - 2008.
7. Novel Ag-MnOx catalysts for selective epoxidation of olefins.
Sithambaram, S.; Calvert, C. A.; Opembe, N.; Suib. S. L. ACS National Meeting, Boston,
August - 2007.
139
8.
Metal-doped cryptomelane manganese oxides in catalysis: Structure-activity
correlation.
Sithambaram. S.; Xu, L.; Ding, Y.; Chen, C-H.; Suib, S. L. ACS National Meeting,
Boston, August — 2007.
9.
Manganese octahedral molecular sieves catalyzed preparation of amino alcohols
from epoxides.
Sithambaram, S.; Ding, Y.; Shen, X.; Xu, L.; Suib, S. L. ACS National Meeting,
Boston, August - 2007.
10.
Analysis of solvent-solute interactions and its effect on crystal morphology.
Acquah. C : Karunanithi, A. T.; Achenie, L. E. K; Gascon, J. A.; Sithambaram, S.;
Suib, S. L. ACS National Meeting, Boston, August - 2007.
11.
K-OMS-2 Catalyzed liquid phase synthesis of quinoxalines.
Sithambaram, S.; Li, W-N.; Ding, Y.; Kumar, R.; Suib. S. L. ACS National Meeting.
Chicago, March — 2007.
12.
High-boiling solvents and morphology of crystals.
Acquah. C; Achenie, L. E. K.; Karunanithi, A. T.; Sithambaram, S.; Suib, S. L. ACS
National Meeting, Chicago, March - 2007.
13.
Hydrothermal Synthesis of Structure and Shape Controlled Manganese Oxide
Octahedral Molecular Sieve (OMS) Nanomaterials.
Li. W-N.; Yuan, J.; Chen, X, Shen, X-F.; Gomez-Mower, S.; Xu, L.; Sithambaram,
S.; Aindow, M.; Suib, S. L.; Ding, Y.; Chen, C-H.; MRS Spring meeting, San
Francisco, April - 2007.
14.
Direct catalytic conversion of alcohols into imines using octahedral molecular
sieves.
Sithambaram. S.: Son, Y-C; Suib, S. L. ACS National Meeting, Washington, August 2005.
15.
Solvent design for pharmaceutical process: A combined experimental,
computer-aided molecular design and database search approach.
Karunanithi. A.T.; Acquah, C; Sithambaram, S.; Achenie, L.E.K.; Suib, S. L. AIChE
Annual Meeting, Salt Lake City, November - 2007.
16.
Quantitative analysis of H-bonding interactions: Relation to crystal shape and
size.
Acquah C ; Achenie, L.E.K.; Karunanithi, A.T.; Sithambaram, S.; Suib, S. L.;
Gascon, J. A. AIChE Annual Meeting, Salt Lake City, November - 2007.
17.
Morphological considerations in solvent design for ibuprofen.
Karunanithi. A.T.; Sithambaram, S.; Achenie, L.E.K.; Suib, S. L. Gani. R. AIChE
Annual Meeting, Cincinnati, November- 2005.
18.
A computer-aided molecular design approach for the design of organic
molecules (solvents) for crystallization.
Karunanithi. A.T.; Sithambaram, S.; Achenie, L.E.K.; Suib, S. L. ACS National
Meeting, San Diego, March- 2005.
PATENT
• Sithambaram, S.; Son, Y-C; Suib, S. L.; Method of producing imines, US Patent
7355075, April 8, 2008.
141
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