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Factors affecting the catalytic oligomerization of methane via microwave heating

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FACTORS AFFECTING THE CATALYTIC
OLIGOMERIZATION OF METHANE VIA
MICROWAVE HEATING
Luis Daniel Conde, Ph.D.
University o f Connecticut, 2003
Catalytic microwave heating has been used as a method for the oligomerization
o f methane to higher hydrocarbons. Many catalysts were tested in this reaction. Nickel
powder, raney nickel, iron powder and activated carbon were the most active and
efficient catalysts for the production o f higher hydrocarbons. When helium was used as
a diluent gas and the applied power was optimized, the selectivities were controlled to
the most desired products. In general, the most abundant products for all the
experiments were C ^. Iron powder was active only at high power (1130 W). At these
conditions acetylene was avoided and ethylene and ethane were produced in the same
proportion. Activated carbon catalysts with helium as diluent led to a selectivity
towards benzene up to 33%.
Some manganese oxides such as OMS-1, OMS-2 and Mn02 (dielectric constant,
e = 104) were not active in these reactions. These data suggest that the dielectric
constant is not foe most important factor in foe oligomerization o f methane via
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Luis Daniel Conde - University o f Connecticut, 2003
microwave heating. Conversion and activities o f these materials are not proportionally
related to the surface area o f the catalysts.
Higher catalytic activity was observed for Raney nickel than for regular nickel
powder. The maximum conversion obtained was 24 % at 400 W and 10 min o f
irradiation time. For regular nickel powder that conversion can be achieved only after
700 W o f power and more than 20 min o f reaction. BET surface area, Scanning
Electron
Microscopy,
X-ray
Photoelectron
Spectroscopy,
and
Temperature-
Programmed Desorption and Reduction analysis were performed to characterize the
catalyst before and after reaction. Deactivation o f Raney nickel by fouling and sintering
was observed after 500 W and/or 15 min o f reaction.
The effect o f microwave radiation frequency on activity and product distribution
for methane oligomerization has been studied. Nickel, iron, and activated carbon
catalysts were used in these studies. Experiments were done w ith pure methane and
using He as diluent Changes in product distribution due to changes in frequency have
been observed, and might be related to different transverse magnetic modes at different
frequencies. Different transient heating may occur at different values o f frequency.
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FACTORS AFFECTING THE CATALYTIC
OLIGOMERIZATION OF METHANE VIA
MICROWAVE HEATING
Luis Daniel Conde
B.S., Universidad del Valle, 1992
A Dissertation
Submitted in Partial Fulfillment o f the
Requirements for the Degree o f
Doctor o f Philosophy
at
The University o f Connecticut
2003
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APPROVAL PAGE
Doctor o f Philosophy Dissertation
FACTORS AFFECTING THE CATALYTIC
OLIGOMERIZATION OF METHANE VIA MICROWAVE
HEATING
Presented by
Luis Daniel Conde, B.S.
Major Advisor:__
Steven L. Suib
Associate Advisor
Associate A dvisor
Robert Coi
in
University o f Connecticut
2003
it
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DEDICATION
To my loving parents, brothers, and sister, m y caring wife, and the motive o f my
happiness, my sons for their unconditional support, love, and encouragement.
iii
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ACKNOWLEDGEMENTS
I wish to thank m y major advisor, Dr. Steven L. Suib, for his support, advice,
guidance, and understanding. Dr. Suib has been there to listen, guide, and encourage me
all these years. I thank my committee members, Dr. John Tanaka, and Dr. Robert
Coughlin, for their time and help to complete m y graduate studies.
I acknowledge the support o f the Texaco Research Center, the National Science
Foundation, and the Electrical Power Research Institute under Grant CTS-9413394 o f the
joint NSF/EPRI Initiative on Microwave Induced Reactions.
I am really grateful to Carolina Marun for all her tune, guidance, knowledge, and
friendship throughout all this time. I thank Professor Jeffrey Wan o f Queen’s University
for helpful discussions on microwave absorption, catalysis, and detection o f catalytic
intermediates.
I thank Lambda Technology Inc. for support o f this research. I also thank Todd
Ellis and Denise Tucker o f Lambda Technology Inc. for their advice and technical
support I acknowledge Dr. Daniel Scola and Chris Simone at the Institute o f Materials
Science, University o f Connecticut for support on this research.
I sincerely appreciate the help o f Bill W illis, Guang-Guang Xia, and Aimin Xia
for their guidance in characterization techniques. Also, I would like to acknowledge the
help o f every member o f Dr. Suib’s group.
Finally, I want to thank m y family for their support and love.
iv
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TABLE OF CONTENTS
CHAPTER I. INTRODUCTION........................................................................1
CHAPTER H. EXPERIMENTAL SECTION.................................................. 11
2.1. Microwave apparatus and gas system......................................................... 11
2.2. Catalysts........................................................................................................17
2.3. Product analysis...........................................................................................26
2.4. Characterization methods........................................................................... 27
2.4.1. BET surface area analysis...................................................................... 27
2.4.2. Scanning Electron Microscopy...............................................................28
2.4.3. X-ray photoelectron spectroscopy..........................................................28
2.4.4. Temperature-programmed desorption and reduction............................ 28
CHAPTER m . RESULTS.............................................................................. 30
3.1. Catalytic oligomerization o f methane using fix frequency
microwave radiation............................................................................... JO
3.1.1. General observations............................................................................. 30
3.1.2. Nickel powder.........................................................................................33
3.1 J . Activated carbon.................................................................................... 39
3.1.4. Iron powder............................................................................................-44
32. Fixed frequency oligomerization w ith Raney nickel
and its characterization........................................................................... -47
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3.2.1. Non-reduced Raney nickel....................................................................47
3.2.2. Reduced Raney nickel...........................................................................50
3.3. Variable frequency oligomerization with different catalysts...............54
3.3.1. Frequency effect on nickel catalyst......................................................58
3.3.2. Frequency effect on iron catalyst......................................................... 60
3.3.3. Frequency effect on activated carbon catalyst..................................... 66
3.4. Factors affecting the variable frequency oligomerization
with nickel catalyst.................................................................................70
3.4.1. Pure methane as reactant.......................................................................72
3.4.2. Methane diluted in 25% helium............................................................72
CHAPTER IV. DISCUSSION..................................................................... 80
4.1. Catalytic oligomerization o f methane using fixed
frequency microwave radiation............................................................. 80
4.1.1. General comments and mechanistic ideas...........................................80
4.1.2. Catalyst, applied power, and diluent effect on
product distribution.............................................................................83
4.2. Fixed frequency oligomerization with Raney nickel
and its characterization.......................................................................... 85
4 3 . Variable frequency oligomerization with different catalysts................95
4.4. Factors affecting the variable frequency oligomerization
with nickel catalyst.............................................................................. -98
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CHAPTER V. CONCLUSION................................................................. 103
APPENDIX. LIST OF PUBLICATIONS................................................ 106
REFERENCES........................................................................................... 108
vii
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LIST OF FIGURES
Figure 2.1 Fixed frequency microwave apparatus, applicator,
and reactor............................................................................................... 12
Figure 2.2 Resonant cavity, and variable frequency microwave apparatus............ 13
Figure 2.3 Transverse magnetic modes (TMoim n= 2 to 8) for the
variable frequency microwave cavity.....................................................15
Figure 2.4 Gas system............................................................................................... 16
Figure 3.1 Reflected power vs. applied power, and effect o f
applied power on temperature................................................................31
Figure 3.2 Effect o f applied power on conversion using nickel
powder (Conditions: series A)...............................................................35
Figure 3.3 Selectivity vs. applied power and effect o f applied
power on product distribution o f C& using nickel
powder (Conditions: series B)..............................................................36
Figure 3.4 Selectivities vs. time at 628 W using nickel
powder (Conditions: series D)............................................................ 37
Figure 3.5 Product distribution o f C2s vs. time at 754 W
using nickel powder (Conditions: series F)..................................... .40
Figure 3.6 Selectivities vs. tune at 503 W using nickel
powder (Conditions: series F)............................................................41
Figure 3.7 Applied power and tim e o f reaction effect on
conversion (Conditions: series I).................................................... .42
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Figure 3.8 Product distribution vs. tim e at 378 W using
AC-1 (Conditions: series I)............................................................ .43
Figure 3.9 Product distribution vs. time at 754 W using
AC-1 (Conditions: series K )............................................................ 45
Figure 3.10 Product distribution o f C2* vs. time at 754 W
using AC-1 (Conditions: series K)............................................... .46
Figure 3.11 Effect o f irradiation time on conversion and selectivity
at 250 W using non-reduced Raney nickel...................................48
Figure 3.12 Effect o f applied power and irradiation
time on conversion after activation stage
using non-reduced Raney nickel..................................................49
Figure 3.13 Effect o f applied power on selectivity at 5 min
o f irradiation tune using non-reduced Raney nickel....................51
Figure 3.14 Effect o f applied power on C ^ selectivities at 5 min
o f irradiation time using non-reduced Raney nickel....................52
Figure 3.15 Effect o f applied power on selectivity at 10 min
o f irradiation tune using non-reduced Raney nickel....................53
Figure 3.16 Effect o f applied power and irradiation tune on
conversion using reduced Raney nickel...................................... 55
Figure3.17 Effect o f applied power on selectivity at 5 min
o f irradiation tim e using reduced Raney nickel.......................... 56
Figure 3.18 Effect o f irradiation tim e on conversion and
selectivity w ith nickel powder catalyst at 500 W
ix
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using 2.4 GHz Fix Frequency........................................................59
Figure 3.19 Effect o f irradiation tim e on conversion and
selectivity with nickel powder catalyst at 500 W
using 4.6 GHz Fix Frequency...................................................... 60
Figure 3.20 Effect o f irradiation tune on conversion and
selectivity with nickel powder catalyst at 500 W
using 2.4 - 7.0 GHz Variable Frequency with
a sweep rate o f 0.5 sec.................................................................61
Figure 3.21 Effect o f irradiation tune on conversion and
selectivity with iron powder catalyst at 1130 W
using 2.4 GHz Fix Frequency.....................................................63
Figure 3.22 Effect o f irradiation tim e on conversion and
selectivity with iron powder catalyst at 500 W
using 4.6 GHz Fix Frequency..................................................... 64
Figure 3.23 Effect o f irradiation tune on conversion and
selectivity with iron powder catalyst at 500 W
using 2.4 - 7.0 GHz Variable Frequency with
a sweep rate o f 0.5 sec.................................................................65
Figure 3.24 Effect o f irradiation tim e on conversion and
selectivity with activated carbon catalyst at 500 W
using 2.4 GHz Fix Frequency..................................................... 67
Figure 3.25 Effect o f irradiation tim e on conversion and
selectivity w ith activated carbon catalyst at 500 W
x
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using 4.6 GHz Fix Frequency........................................................ 68
Figure 3.26 Effect o f irradiation time on conversion and
selectivity with activated carbon catalyst at 500 W
using 2.4 - 7.0 GHz Variable Frequency with
a sweep rate o f 0.5 sec.............................................................. 69
Figure 3.27 Effect o f applied power on product distribution
at 4.60 GHz using pure methane as a feed and
nickel powder catalyst................................................................ 73
Figure 3.28 Frequency effect on product distribution at 370 W
using pure methane as a feed and
nickel powder catalyst................................................................74
Figure 3.29 Effect o f applied power on product distribution
at 4.60 GHz using methane diluted in helium (25%)
and nickel powder catalyst..........................................................77
Figure 3.30 Frequency effect on product distribution at 370 W
using methane diluted in helium (25%) and
nickel powder catalyst................................................................ 78
Figure 3.31 Fresh nickel catalyst Temperature-Programmed
Desorption analysis...................................................................... 79
Figure 4.1 Temperature-Programmed Desorption analysis o f
fresh Raney nickel......................................................................... 87
Figure 4.2 Scanning Electron Microscopy photograph o f
Raney nickel before reaction........................................................89
xi
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Figure 4.3 Scanning Electron Microscopy photograph o f
Raney nickel after reaction........................................................... 90
Figure 4.4 Temperature-Programmed Reduction analysis o f
fresh Raney n ick el....................................................................... 93
xii
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LIST OF TABLES
Table 2.1 Different catalysts used for the oligomerization o f
methane via microwave heating........................................................... 18
Table 2.2 Conditions for the oligomerization o f methane using
fixed frequency microwave................................................................. 19
Table 2.3 Conditions for the oligomerization o f methane using
Raney nickel at fixed frequency.........................................................21
Table 2.4 Conditions for the oligomerization o f methane at variable
frequency using different catalyst.......................................................23
Table 2.5 Conditions for the oligomerization o f methane using nickel
powder at variable frequency under different parameters.................24
Table 3.1 Summary o f results for the oligomerization at fixed frequency........34
Table 3.2 Summary o f results for the oligomerization o f methane
at variable frequency using different catalyst................................... 57
Table 3.3 Summary o f results for the oligomerization o f methane
at variable frequency using nickel powder under
different conditions............................................................................. 71
Table 4.1 X-Ray Photoelectron Spectroscopy analysis lor Raney nickel..........92
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CHAPTER I
INTRODUCTION
Natural gas is essentially methane (83-97 vol % depending on the origin)
and therefore difficult to liquefy and quite chemically unreactive. Methane is
thermodynamically stable with respect to its elements. The reactions to make
other hydrocarbons, all o f which are less stable than methane around 1000°C,
have unfavorable free energies o f reaction and are strongly limited by
equilibrium. They need a considerable energy input, and therefore temperatures
above 1400°C are required to transform CH4 into benzene, acetylene, and
ethylene.
The products are unsaturated and unstable hydrocarbons such as
acetylene. Consequently, appropriate temperature, residence time control, and
rapid reaction quenching are necessary to achieve selectivity to desired products.
Otherwise, carbon and hydrogen will be the only products.
Methane is a very valuable fuel from the environmental point o f view and
the production o f energy is so far its main use. However, a more rational use o f
this resource is that it should also be used to make either petrochemical or
gasoline components, so natural gas can be used as an oil substitute. This goal
has been actively pursued by many groups around the world.1'7
Many efforts have been focused on the activation o f methane to more
valuable products in the last two decades.1’6,8 Thermal,9’10 homogeneous,11’12 and
heterogeneous13’14 catalytic conversion o f methane to higher hydrocarbons have
1
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2
been studied by many authors. Most recent efforts have been focused on the
development o f direct methane conversion by means o f oxidative coupling into
ethane and ethylene.15’16 Other processes involve electrochemical activation o f
methane, photocatalytic oxidation o f methane, electrical discharges,17 partial
oxidation o f methane over redox catalysts, catalytic reactions involving transition
metal complexes,3*5’13’17*23 microwave plasmas,24*27 and microwave heating.6,28*31
The use o f microwave heating to induce oligomerization o f methane has
been studied for several years, primarily by Wan and coworkers.32 Recently there
has been considerable interest in determining whether or not there are inherent
advantages to using microwave heating over more classical heating methods. A
review o f the potential o f this area has been recently prepared,33 which focuses on
fundamental mechanistic aspects, hardware, system design and applications o f
microwave heating.
In addition, a symposium 34 has addressed the use o f
microwave heating in waste management, synthesis, advanced dielectric
processing, medical, food technology, and other areas.
In organic chemistry, microwave ovens have been used by Majetich and
coworkers35 in Diels-Alder, Claisen and olefin reactions. Similar methods were
used to produce short lived radio-pharmaceuticals,36 to induce hydrolysis o f
nucleotide triphosphates,37 to selectively irradiate oxygenates trapped in
zeolites,38 to produce heterocycles,39 to synthesize anthraquinone,40 to effect
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3
hydrocarbon rearrangements,41’42 and for synthesis o f lactams 43 These are just
some o f the reactions that have been driven by microwave heating in ovens.
Microwave assisted synthesis o f inorganic materials has been actively
pursued by Mingos and coworkers.44"52 An excellent review o f this area is
available.44
oxides,46
Superconducting ceramics,45 solid state metal and mixed metal
inorganic
and
organometallic
intercalates,47
coordination
compounds,48*49 organometallics,50 and metal powders51-52 have been produced via
microwave heating in modified microwave ovens. Much o f the emphasis o f this
work has focused on new ways to produce materials and ways to modify specific
equipment to efficiently heat reactants in various states o f matter.
Applications o f microwaves in biomass conversion,53 for NOx and SO2
decompositon,54 synthesis o f HCN ,55 and the development o f fluidized bed reactor
systems56 have been reported. The engineering and scale-up considerations o f
flow systems have also been investigated.57
Microwave-induced oligomerization o f methane to higher hydrocarbons
has been the focus o f very few fundamental studies. Although it is not well
established how microwave pulses or continuous microwave heating induce
oligomerization to occur, microwave energy interacts with materials at the
molecular level. During microwave heating o f a dielectric, internal electric fields
are generated within the material.
Such fields can produce translations o f
electrons and ions, and cause rotations o f charged species. This movement can be
opposed by friction, inertia, and other forces that can lead to attenuation o f the
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4
electric fields, and to volumetric heating o f the material.58 The ability o f the
microwave electric field to polarize molecules and the ability o f these molecules
to follow the rapid reversal o f the electric field results in the conversion o f
electromagnetic energy into heat within the irradiated material.
Microwave radiation has a range o f frequency from about 0.3 to 300
GHz, which corresponds to a range o f wavelengths from I m to 1 mm.
Interactions o f microwaves are highly dependent on the nature o f materials. The
permitivity o f the material controls the degree o f absorption o f microwaves.58
The permitivity e* is composed o f two parts, a real part e’ related to the dielectric
constant and an imaginary part e”, related to dielectric loss as described in
Equation 1:
£* = e’ - je” = £o (e’r - je”eff)
[I]
In Equation I, So is the permitivity o f a vacuum, s ’ is the dielectric constant
relative to So, s”eff is an effective relative dielectric loss factor, and j= (- 1)1/2.58
During microwave heating o f a dielectric, internal electric fields are
generated within the material. Such fields can produce translations o f electrons
and ions and cause rotations o f charged species. This movement can be opposed
by friction, inertia, and other forces, which can lead to attenuation o f these electric
fields, and to volumetric heating o f the material, hi practice, it is possible to
measure the loss tangent (tan 5) as shown in Equation 2:
tan 5 = e’W / e’r = <y/ 27cfE0£>r
[2 ]
CO
where <y is a total effective conductivity and f is the frequency in GHz.
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The propagation o f radiowaves as plane waves in unbounded loss-free
space is the well documented and simplest solution o f the Maxwell equation .59*61
Plane waves are very useful in helping to visualize practical field configurations.
For example, they can be used, by addition o f waves propagating along inclined
axes, to synthesize the fields existing in a waveguide or cavity, illustrating and
quantifying some o f their important features.62
The solution o f Maxwell’s
equations for plane waves shows they consist o f an electric field vector and a
complementary magnetic field vector, orthogonal to each other. Moreover, there
is no field component in the direction o f the propagation. The wave propagating
under these conditions is called transverse electromagnetic (TEM) because all the
field components are transverse to its direction o f travel.
Two types o f waveguide can be used to channel and transmit
microwaves, namely, guides with simply connected cross sections, or coaxial
guides with dimensions depending on the wavelength o f the propagating field.
Multiple reflections o f the propagating wave at the walls o f the waveguide
produce a certain distribution o f the fields inside the guide and o f conduction
currents on the walls, giving rise to a mode o f guided propagation.63
The plane wave corresponds to the TEM mode.
This mode will
propagate in the coaxial guide, but cannot propagate in the simple connected
(cavity) guide. In the latter it is either the electric or the magnetic field that is
perpendicular to the direction o f propagation. Two modes namely, transverse
electric (TE) and transverse magnetic (TM) are found to propagate. The way
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these two modes are distributed within the cavity affects directly the absorption o f
microwave energy by the material.
The power absorbed by the material also varies linearly with frequency.
Thermal equilibrium is often assumed, but often this is not true .63 The heating o f
a material will generally depend on a number o f factors including the size and
shape o f the material, and the exact location o f the material in the microwave
field. The location o f the material depends on the type o f microwave cavity that
will be used. A cavity is defined as a volume enclosed by a conducting wall.
Depending on the dimensions as compared to the wavelength, cavities are said to
be resonant or oversized, respectively. Such cavities play a vital role in the
synthesis o f materials as well as in microwave catalyzed reactions.58
The
electromagnetic energy trapped in a cavity is reflected by its walls and takes the
form o f stationary waves. In waveguides, the possible frequencies constitute a
discrete series, or characteristic frequencies o f the cavity. The corresponding
special configurations, TE and TM, are the cavity modes. The cavity behaves as a
filter that transmits only certain frequencies.
Attenuation o f microwaves occurs as they propagate through a material.
This results in specific penetration depths o f microwave, which is related to the
wavelength with greater wavelengths leading to more penetration. Only certain
frequencies such as 0.915 and 2.45 GHz are often used due to regulations o f the
Federal Communications Commission. The degree o f heating can be dependent
on the frequency; however, this is highly dependent on the nature o f the material.
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Both the dielectric constant e’r and the loss tangent tan 5 are often
measured or known since these are related to the power absorbed by the material
and the penetration depth o f microwave radiation. The dielectric constant is often
related to the degree o f polarizability o f the material in an electric field. The loss
tangent is related to the absorption o f microwave energy by the material.58
The dielectric constant and loss factor vary with temperature.
Unfortunately the temperature dependence o f such parameters is often unknown.
Several factors can influence the temperature dependence o f the dielectric
constant including the composition, density, and coefficient o f thermal expansion
o f the material.
The loss factor is even more influenced by an increase in
temperature. There is a rapid rise in loss factor at some temperature, Tcru- Above
Tent, the loss factor increases exponentially due to thermal runway. Thermal
runway is highly dependent on the nature o f the material, impurities, and can lead
to hot spots. In order to control thermal runway, several methods can be used
including the use o f a pulsed microwave power supply and via wise choice o f a
microwave cavity.
Microwave cavities o r applicators are the heart o f a microwave heating
system. Many types o f cavities are available, although it may be necessary to
produce specific cavities for specific microwave heating or microwave catalytic
reactions. Cavities are made o f metal and are o f different shapes. Cavities can
handle various loads; however, inhomogeneous heating and hot spots can result.
Microwaves are reflected o ff the walls o f the cavity to form standing waves o f
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8
variable intensity. The location o f a sample in a cavity therefore is critical.
Various techniques can be used to try to modify the fields in order to provide
more uniform heating. Both multimode and single mode cavities are available,
with the latter providing more precise control o f the electromagnetic fields in the
cavity for simple material geometries.
However, the multimode applicator
(cavity) has the capability o f forcing electromagnetic energy onto large and
complex shaped materials. Thus, the multimode applicator is compatible with the
production o f large and complex functional material components.
Multimode applicators can be powered with fixed and or variable
frequency sources. When a fixed frequency microwave signal is launched within
the microwave cavity, it suffers multiple reflections and results in the
establishment o f several modal patterns.
The overall distribution o f
electromagnetic energy is not uniform throughout the microwave cavity resulting
in high and low energy field areas, i.e., hot and cold spots. The thermal gradients
established due to non-uniform electromagnetic energy distribution can be
tolerated for some materials, but this is not always the case for advanced
materials.64 When a variable frequency microwave signal is launched within the
microwave cavity, the electric field distribution, on a time average basis, is
uniform throughout the entire cavity volume leading to uniform exposure o f the
processed materials to the microwave energy.65,66 Furthermore, the incident
microwave frequencies can be changed to optimized the microwave energy
absorption by the materials o f interest.
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9
Particularly relevant to our research are developments in the microwave
catalysis, organic synthesis, and inorganic synthesis.
Microwave-induced
catalytic reactions have been pursued largely by Wan.67 Pulsed microwave
catalysis o f methane coupling,30 hydrocarbon oxidation,30 destruction o f
environmental pollutants,30 decomposition o f organic halides,68 decomposition o f
olefins,29 production o f acetylene,28 and other reactions have been reported.
Much o f this work has focused on the use o f sensitizers that can absorb
microwave energy and transfer heat efficiently in order to selectively activate
certain chemical bonds. Other groups such as Roussy et al.69 have used similar
approaches to study hydrocarbon rearrangements.
Our group has previously studied the oligomerization o f methane to
higher hydrocarbons using microwave plasma.70 Microwave plasmas can activate
C-H bonds in methane molecules to yield ethylene, ethane and acetylene at 200
W.
Conversions and selectivities for the oligomerization o f methane via
microwave plasmas are highly sensitive to pressure, flow rate, and type o f
resonant cavities used in the system. Microwave heating offers the alternative o f
working at atmospheric pressure and with a wide range o f flow rates. Controlling
these factors in the reaction would make the system more stable; so more
systematic studies can be conducted.
The objectives o f this research are: 1) to activate C-H bonds in methane
molecules to produce more valuable and higher molecular weight hydrocarbon
molecules with the use o f microwave radiation. Previous studies31,32 indicated
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that microwave-induced catalysis involve absorption o f microwaves by a catalyst
followed by energy transfer to reactant molecules at the solid-gas interface; 2) to
study fundamental aspects o f reactor configuration, additives (chain propagators,
dielectrics), temperature measurements, magnetic field effects, and reaction
conditions to understand the factors that control the microwave-induced
oligomerization processes; 3) to study the microwave-induced oligomerization o f
methane to elucidate mechanistic details o f such reactions; 4) to systematically
control the variables affecting the processes in 1 in order to optimize the
formation o f desirable products.
Some catalysts are characterized before and after reaction by a variety o f
techniques includidng powder X-ray diffraction (XRD), and photoelectron
spectroscopy (XPS) for structural characterization, elemental composition, and
chemical state o f the elements. Scanning electron microscopy (SEM) is utilized
for studying the morphology and topography. The surface area o f the materials is
measured by gas sorption (BET method). Identification o f species absorbed in the
catalysts after reaction is done by temperature-programmed desorption (TPD).
All the reactants, solvents and materials that are used for this study are
inexpensive and commercially available.
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CHAPTER n
EXPERIMENTAL SECTION
2.1. Microwave Apparatus and Gas System
Microwave-Induced Oligomerization (MIO) o f methane to higher
hydrocarbons will be carried out in a straight 3/8 in-quartz reactor, which is
mounted vertically inside the applicator o f the microwave apparatus as shown in
Figure 2.1. An ASTEX microwave power source model GL139 with a magnetron
type GL 130WC, a 3 stub tuner model AX304I and an ASTEX applicator model
AX7020 will be used for these experiments. The applicator corresponds to a
fixed single mode resonant cavity. The power supply provides up to 1250 W o f
power at an operating frequency o f 2.54 GHz.
The power is emitted in pulses
with a periodicity o f 8.3 ms (120 Hz).
In order to achieve a desired average power the pulse duration or the
pulse width and the amplitude was changed. Thus, varying the power exposure o f
the catalyst by varying the emitted power from the source essentially results in
different duty cycles at different power levels.
A Lambda Technologies variable frequency unit as seen in Figure 2.2
(Vari-Wave model LT-502Xb) will be used for experiments at different
frequencies. The operational power range for this unit is 0 - 500 W. The incident
center frequency can be varied from 2.4 to 7.0 GHz with a bandwidth sweep rate
o f 100 to 0.1 sec.
Various frequencies and modes can be produced in the
11
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
i.WMoudi
Modal o u ao w c
POMTfrUSOWMto
Pidcuoner 2N 0*2N 0 MM*
Figure 2.1 Fixed frequency microwave apparatus, applicator, and reactor
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
13
McnNMSMm
MocMVW-2750
Pmwr0400 Watts
Fraquancy 2.4-6.0 GHz
Tuning knob
Figure 2.2 Resonant cavity, and variable frequency microwave apparatus
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14
microwave cavity. The cavity for variable frequency experiments allows TMot„
modes (n = 2 to 9) over the whole frequency range. The cavity was designed by
Lambda Technologies to have different modes for each frequency. In Figure 2.3
we show how these transverse magnetic modes were distributed in the cavity and
the value o f frequency that corresponds to each TM mode.
Our variable
frequency experiments were done with and without diluent at 2.40 GHz at TM012
and also at 4.60 GHz at TMotg for comparison, and at a variable frequency within
a range o f 2.4 - 7.0 GHz with a sweep rate o f 0.5 sec. Thermal paper was
inserted into the microwave cavity and used to map the heating patterns o f the
various modes.
For variable frequency experiments, the length o f the aluminum
multimode resonant cavity was 29.5 cm and the radius was 5.15 cm. There was a
tuning knob and antenna perpendicular to the length o f the cavity. In order to
ensure microwave absorption for a specific frequency value and power level, the
reflected power had to be minimized first by means o f the tuning knob and the
frequency tuning control on the main panel. Therefore, the working frequency
was a result o f the tuning process. Reflected power higher than 60 W makes the
safety feature o f the unit shut off. Thus, not all frequencies were possible for a
particular system.
The oligomerization reactions was run under a continuous flow. Gases
w ill be mixed on line using a panel as seen in Figure 2.4. High Purity (HP) grade
methane obtained from Matheson and Ultra High Purity (UHP) helium
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
15
2.43TCH*
M
—
►
X
2.688 GHz
3.002GHz
3.780CHZ
•
•
2 1 4 -0 ________
aua ^
214
0
•
•
•
U W C IIi
•
•
•
•
•
•
.
•
•
III
T w i3
4.1MGHZ
T mOIS
•
•
•
•
•
•
•
•
•
T «017
Figure 2.3 Transverse magnetic modes (TMoin, n= 2 to 8) for the variable
frequency microwave cavity.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
16
2 1*
—i
Q»
I
•
»
a
a
“i i
J
GUQ
r # V i.
1
2
3 .4 ,9 ,1 0
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7,8
II
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tS
16,17
18,19
20
Methane tank
Helium tank
Shut off valves
Shut off valves
Metering valves
Rotameters
3-way valve
Microwave applicator
Electronic flowmeter
Gas sampling valve (4-port)
Regulators
Pressure gauges
Transducer
Figure 2.4 Gas system
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
■
f t
17
purchased from Airgas will be used.
An electronic J&W Scientific Gas
Flowmeter will be utilized to measure the flow rate o f the gases, and also a set o f
rotameters (Omega FL-3545-HRV and FL-3541-HRV Series) coupled with fine
metering valves to control the flow rates.
2.2. Catalysts
Several different materials, including some manganese oxide materials,
were tested as catalysts for the oligomerization o f methane using fixed frequency
microwave radiation.
Most o f the materials were purchased from different
companies (see Table 2.1 for more information) and they were used as received.
In Table 2.1, the literature values o f the dielectric constants for each material in
reference to a vacuum and the temperature and the frequency at which they were
measured are given.71 Synthetic todorokite [an octahedral molecular sieve (OMS)
designated as OMS-1 having a 6.9 A pore size], and synthetic cryptomelane [(a K.+
hollandite) another OMS structure having a 4.6 A pore size which is designated
as OMS-2 made by conventional methods72'74 doped with nickel and cobalt] were
tested as catalysts for methane oligomerization.
The conditions at which the first set o f experiments was run include a 35 mL/min flow rate o f methane, a 1-10 mL/min flow rate o f helium, and
atmospheric pressure. Catalyst activities were tested at maximum power using 3
mL/min o f methane and 9 mL/min o f helium. For those catalysts that were active
runs were made at low, medium, and high power and analyses were performed at
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
18
N/Iaterial
Source
D. Constant30’*
Diamond Shamrock Chemicals
MnOa
TiQ>
Fisher Scientific
Mallinkrodt Chemical Works
SnQ>
Si
Atlantic Equipment Co.
CaO
Baker and Adanson
SiC
Aldrich
S i0 2
Silica gel, Davisil Aldrich
S1TI2O3
American Potash & Chemical Co.
Alfa Products
Pt/Ai20 3
Aldrich
C-60
Co
Inco Co.
Fe
Inco Co.
Ni
Inco Co.
Activated Carbon*
OMS-1 Ni(I% ) Synthesized at Uconn 54-56
OMS-2 Ni(0.1 %)Synthesized at Uconn54*56
OMS-Co (1 %) Synthesized at Uconn 54-56
10,000
170
14
12.1
11.8
9.72
4.6
*
*
*
*
*
*
*
T(K ) v(Hz)
298
300
298
4.2
283
298
298
*
*
*
*
*
*
*
104
104- 106
104- 101°
107- 109
2x l 06
10 I2- 1014
9.4xlOl°
*
*
*
*
*
*
*
*referred to vacuum.
* not available.
*2200 m2/g and 3100 m 2/g.
Table 2.1 Different catalysts used for the oligomerization o f methane via
microwave heating
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
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Table 2.2 Conditions for the oligomerization o f methane using fixed frequency
microwave
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20
different sampling times (from 1 to 30 min). Table 2.2 summarizes these
conditions.
Since nickel powder showed to be the most stable and reactive catalyst,
a more reactive form o f nickel, Raney nickel, was used in a second set o f
experiments with the purpose o f characterizing the catalyst before and after
reaction. Raney nickel was purchased from Acros Organics, and was supplied as
50% slurry in water, 10.5% aluminum-balance content. Particle size ranged from
5 to 128 pm. The slurry was desiccated using silica gel under 2.67xl0'3 bar and
room temperature for 6 hours. Handling o f the catalyst was always carried out in
a nitrogen atmosphere glove bag. After drying, 100 mg Raney nickel powder was
placed inside the reactor with high purity quartz wool on either side o f the catalyst
bed.
Although, Raney nickel was handled under a nitrogen atmosphere, some
exposure to air might had occurred when transferring the reactor from the glove
bag to the microwave apparatus and while connecting fittings.
For activity
comparison, additional experiments were done with a reduced catalyst. Raney
nickel was reduced in situ with UHP hydrogen at 300°C for 2 h. Feed was 30%
hydrogen in helium and the total flow rate was 15 mL/min. All experiments were
run with a mixture o f 3 mL/min pure methane and 9 mL/min helium at
atmospheric pressure. Samples for product analyses were taken at 5, 10 and 15
min o f reaction tim e over a power range o f 250 to 1125 W. Table 2.3 summarizes
the conditions for this set o f experiments.
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21
Catalyst
Details
Raney-Ni Powder
Dried under vacuum
Raney-Ni Powder
Dried under vacuum
Reduced with Hydrogen
CH4: He Total (low rata
(mL/min)
Power
(W)
Time
(min)
3 :9
12
503
10
3 :9
12
251
5
Table 2.3 Conditions for the oligomerization o f methane using Raney nickel at
fixed frequency
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22
For microwave variable frequency experiments, three different catalysts
were tested in the reaction. Materials were purchased from different companies
and they were used as received. Nickel powder with morphology o f branched
filaments was used for these experiments. The diameters o f the filaments were
2.5 pm as measured by Scanning Electron Microscopy (SEM). Clusters o f nickel
metal particles were aggregated to form the filaments. Activated carbon
MAXSORB produced by Kansai Coke and Chemicals Co was also used.
MAXSORB is a very high surface area (2200 m2/g) activated carbon, produced
from petroleum coke and coconut shell char, which results in a low ash content
material (less than 2% ).75 XRD analysis suggested activated carbon is entirely
amorphous. The other catalyst tested was iron powder. As reported by Marun et
al.70 the dielectric properties63 o f these materials do not seem to be an important
factor in the oligomerization o f methane via microwave heating. Approximately
100 mg o f catalyst were placed inside the quartz reactor. The catalyst bed was
plugged at both ends with high purity quartz wool. The catalytic quartz reactor
was placed inside o f the applicator so that the material was positioned in the
center the waveguide cavity.
The experiments were run at a flow rate o f 12 mL/min o f a 3:1 mixture
o f helium and pure methane respectively at atmospheric pressure. Samples for
analyses were taken after 3 and 20 min o f irradiation time at 500 W.
Each
experiment at different frequencies was carried out with a fresh catalyst load.
Table 2.4 summarizes these conditions.
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23
Catalyst
Ni
Feed
CH4:He
Flow rate
Power
Frequency
Time
(mL/min)
(W)
(GHz)
(min)
2.4
3
20
3
20
3
20
3
20
3
20
3
20
3
20
3
20
3
20
3:9
500
4.6
V.F.'
2.4
Fe
CH4:He
3:9
500"
4.6
V.F/
2.4
A.C.
CH4:He
3:9
500
4.6
V.F.*
* Variable Frequency from 2.4 to 7.0 GHz. Sweep = 0.5 sec
** For 2.4 GHz power was 1130 W. Conversion below 700 W was zero.
Table 2.4 Conditions for the oligomerization o f methane at variable frequency
using different catalysts
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24
Feed
CH«
CH4:He
Flow rate
(mL/min)
Time
(min)
Power***
Astex Unit
<W)
2.45 GHz
2.40 GHz
4.60 GHz
3
3
130
3
370
130
370
N.R.
Y
N.R.
Y
N.R
N.R
N.R
Y
Y
Y
Y
Y
3:9
Var. Freq. Unitw
N.R. = no reaction; Y = see Rg. 4 - 7 for product distribution.
<a) 100,130,170,210,300,370 Watts of power level were used. Those not shown
in the table were neither relevant nor reaction detected at any frequency.
(b> It was not possible to set other frequency values with successful microwave absorption.
Table 2.S Conditions for the oligomerization o f methane using nickel powder at
variable frequency
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25
Finally, nickel powder with morphology o f branched filaments was used
to study the factors affecting the catalytic oligomerization o f methane using
microwave radiation with variable frequency.
Table 2.5 summarizes the
conditions for this 4th set o f experiments. The experiments were run at two
different flow rates: 3 mL/min o f pure methane; and 12 mL/min o f a 3:1 mixture
o f helium and pure methane respectively at atmospheric pressure. Samples for
analyses were taken after 3 min o f irradiation time for each power level (100, 130,
170,210,300, and 370 W).
For all the experiments, approximately 0.1 g o f catalyst was placed
inside the quartz reactor which was plugged at both ends with high purity quartz
wool. The catalyst was placed in the quartz reactor which was inside o f the
applicator so that the material was positioned in the center o f the waveguide
cavity (see the expanded portion o f Figure 2.1). In order to purge the atmosphere,
helium gas was flowed through the reactor for 30 min prior to starting the
microwave absorption.
Temperature measurements o f the catalyst bed were made using a
Microprocessor Thermometer model HH21 with a type K. thermocouple
purchased from Omega. The temperature was not measured in situ since the
thermocouple will interfere with the microwave field.
Therefore, after we
achieved steady state under reaction conditions we shut o ff the microwave power
and rapidly inserted the thermocouple in the catalyst bed to obtain information
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26
about the temperatures reached at different applied power for the different
materials used as catalysts.
2.3. Product Analysis
Products were trapped with a gas sampling valve and analyzed by
connecting it between a helium tank and an HP5890 Series II Chromatograph
which was equipped with a mass detector. The GC has a sampling valve and
another six-port valve for column switching. Three analytical columns were used
for the separation. A precolumn (HP Porapack Q 1/8 in x 6 ft) was used to
separate the permanent gases from the rest o f the sample. The permanent gases
are further separated individually on a molecular sieve column (HP Molsieve 25
m x 0.53 mm 50 mm) while the rest o f the sample mixture is back flushed from
the precolumn and passed through a split to a Poraplot Q column (HP Poraplot Q
25m x 0.32 mm). The splitting ratio is typically 30:1. The permanent gases after
passing through the Molsieve column, re-enter the precolumn and then enter the
split and the Poraplot Q column.
The total conversion o f methane was estimated with a carbon balance.
All the compounds multiplied by the number o f carbons present in that molecule
were summed. We report selectivities (Os) as the selectivities toward the sum o f
the compounds with specific "i" numbers o f carbon atoms.
The compounds analyzed using gas chromatography were: methane,
carbon dioxide, carbon monoxide; C*, ethylene, acetylene and ethane; C 3S,
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27
propene, propane, 1,2-propadiene, propyne; C4s, 2-methylpropane, 2-butene, 1buten-3-yne, 1,2-butadiene, butadiyne, l-butyne, 2-butyne; Css, 3-pentene-l-yne;
C6s,
benzene;
C 7S> toluene,
Cgs,
ethylbenzene,
ethynylbenzene,
and
ethenylbenzene.
The conversion in percentage is calculated as follows:
X(%)cH4 = (Ct - C CH4«it)/Cr x 100
where C t is the sum o f the concentration o f each compound (Q ) multiplied by the
number o f carbons present in the molecule i (n i), i.e.:
c^
Z Hj Cj
The selectivities in percentage toward compounds with number o f
carbons “i” (SO were calculated by:
S i ( % ) = ( I n s C i ) / ( C T - C CH4 exit) x 100
2.4. Characterization Methods
2.4.1. BET surface area analysis
Surface area o f Raney nickel samples before and after reaction was
determined by Brunauer-Emmett—Teller measurements (BET). The instrument
used was a NOVA 1000 Gas Sorption BET unit, purchased from Quantachrome.
A five-point BET method for nitrogen adsorption was used.
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28
2.4.2. Scanning Electron Microscopy
SEM photographs o f Raney nickel samples before and after reaction
were taken on an AMRAY 1810D scanning electronic microscope, using a
tungsten filament and an acceleration potential o f 16 and 17 kV.
2.43. X-ray photoelectron spectroscopy
The XPS data were acquired using a Leybold-Heraeus LHS-10
instrument equipped with an A1 KR X-ray source and a SPECS EA 10MCD
energy analyzer. Data for all detailed spectra were obtained using a Mg X-ray
anode (1253.6 eV) and a constant energy analyzer pass o f 59.21 eV. The ion gun
was oriented 30° above the sample plane. Raney nickel samples before and after
reaction were analyzed.
2.4.4. Temperature-Programmed Desorption and Reduction
TPD and TPR experiments were used to study the nature o f the adsorbed
species on the catalyst surface. The reactor used for TPD studies was home made
with a thermocouple inserted as close to the catalyst as possible. Eighty
milligrams o f catalyst was used for each analysis. Ultrahigh purity helium was
used as the carrier gas at 30 mL/min. For TPR analysis the feed was I mL/min o f
ultrahigh purity hydrogen and the carrier gas was 29 mL/min o f helium. In each
run, the catalyst was purged with helium gas for more than 2 h at room
temperature until the baseline was flat to eliminate weakly physisorbed species
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29
and then heated linearly from 25 to 825 °C at 10 °C /min. The species evolving
from the catalysts during TPD were analyzed with a mass spectrometer with a
quadrupole ionizing detector (MKS Instrument Inc.).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER HI
RESULTS
3.1. Catalytic Oligomerization of Methane Using Fixed Frequency
Microwave radiation
3.1.1. General observations
Oligomerization o f methane to higher hydrocarbons was performed by
means o f catalytic microwave heating in a controlled manner. Microwaves were
coupled to the catalyst bed and reflected power was minimized using the 3-stub
tuner o f the microwave system. The reflected power was measured at different
applied powers for all the catalytic materials we used and the values for nickel
and activated carbon are shown in Figure 3.1. The reflected power was found to
be very low with respect to the applied power (less than 0.03%). When arcing
was formed the reflected power was increased instantaneously up to 0.06% which
is still very low in comparison with the applied power.
The temperatures reached in the bulk o f the catalyst bed as a function o f
the applied power for nickel and activated carbon (3100 n r/g ) are shown in
Figure 3.1. Activated carbon absorbs microwave energy more efficiently than
nickel which results in much higher temperatures for carbon than nickel at the
same applied power. Very high temperatures were observed (up to 1250 °C) and
30
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31
0.6
1500
Power (Watts)
-1 2 0 0
-
1200
900
-9 0 0
500
-6 0 0
300
-3 0 0
0
0.0
0
I - 1500
200
400
600
500
1000
1200
1400
Forward Powar (Watts)
o
A.
■
•
Ni (Reflected power)
Activated Carbon (Reflected power)
Ni (Temperature)
Activated Carbon (Temperature)
Figure 3.1 Reflected power vs. applied power, and effect o f applied power on
temperature
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32
in some cases the quartz reactor was melted. These data suggest that temperatures
reached in the reactor were higher than the ones detected with the thermocouple.
Different materials were tested as catalysts for the oligomerization o f
methane (Table 2.1). Results suggest that other systems like activated carbon,
nickel and iron powder were considerably more active than manganese oxide
materials. All other catalysts were not at all active for this reaction under these
experimental conditions. The parameters o f all experiments such as: catalyst,
flow rates o f methane and helium, average applied power, sampling tim e as well
as relevant comments regarding the reaction are shown in Table 2.2.
Typical product distributions consist o f ethane, ethylene, and acetylene
for C2s, propane, propene for C 3$, methylpropane, benzene and Cgs were also
detected.
The major products were the C2s.
Selectivities toward different
products were controllable by changing and optimizing the reaction conditions
and the catalyst that was used.
In some cases arc formation was observed accompanied by minor
explosions. Catalysts which have produced discharges in the microwave field
could not be heated again without the production o f discharges. Catalysts may
undergo a change in dielectric properties upon heating in the microwave field and
this may dramatically alter their ability to interact with the applied field.76
Usually the nickel was melted if arcing was present during reaction. Iron did not
form arcs o r m elt during the oligomerization o f methane. Coke was deposited on
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33
the walls o f the quartz reactor in most cases. Coke was not present for iron and
nickel at very short reaction times. Activated carbon glowed at very low power
(190 W) and it formed a carbon-like coating on the walls o f the reactor at the end
o f the reaction. In Table 3.1 we show the most relevant results obtained for the
oligomerization o f methane via microwave heating using nickel powder, activated
carbon, and iron powder as catalysts. The details for these runs will be discussed
in the following sections.
3.1.2. Nickel powder
For nickel powder runs were made with and without helium as diluent at
different power levels which are in most cases 378, 755 and 1130 W. A plot o f
conversion versus power is shown in Figure 3.2. In general, conversion increases
as applied power increases. In some cases, when using the same catalyst bed for
long periods o f time at different power levels, the contrary was observed,
suggesting catalyst deactivation.
When the experiment was carried out without diluent and a period o f 3
minutes was used, the observed products were carbon monoxide, carbon dioxide,
and Cz hydrocarbons (Figure 3.4). Selectivity toward Czs increases as applied
power increases unless arc formation occurs, which promoted formation o f
acetylene (C 2H2) and carbon monoxide (Figure 3.3). At low power (power £ 378
W) with no diluent the major product among the Czs was ethane (C2H6). When
arc formation is noticeable at 503 W the proportion o f C2 products changed from
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34
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Table 3.1 Summary o f results for the oligomerization at fixed frequency
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
35
Conversion (%)
60.0
40.0
20.0
0.0
378
754
1130
Power (Watts)
Figure 3.2 Effect o f applied power on conversion using nickel powder
(Conditions: series A)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
36
Figure 3.3 Selectivity vs. applied power and effect o f applied power on product
distribution o f C2s using nickel powder (Conditions: series B)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
S*toctivNy(%)
37
Figure 3.4 Selectivities vs. time at 628 W using nickel powder (Conditions: series
D)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
38
ethane > ethylene > acetylene to ethylene > acetylene > ethane. At high power
(1130 W) the amount o f C2H2 increases (Figure 3.3).
When the reaction was run for longer times (> 30 min) the general
tendencies remained the same. Conversion and selectivity values were not
completely reproducible possibly due to changes in the catalyst due to arcing.
When arcs were not observed (series C in Table 2.2), as time increases from 2 to
20 min at 378 W, total conversion toward C2s hydrocarbon decreased from 54 to
13 %, and selectivities toward CO increase from 44 to 85%. At 378 and 755 W
the selectivities toward Qzs were ethylene (28%) > acetylene (21%) > ethane (5%)
at 378 W and 2 min o f reaction. At very high power (1130 W) methane was
converted to CO (Sco = 99%). At low power (315-378 W) with no diluent,
although conversions were very low (= 2%), the oligomerization o f methane to
hydrocarbons ranged from compounds with one carbon atom (CO and CO2) up to
compounds with 8 carbon atoms. Selectivities toward CO2 were higher than
selectivities toward CO during the reaction (15% vs. less than 1% at 315 W and
10 min o f reaction). Under these conditions the m ajor product o f the C^s is ethane
(Sethane 73%, Sethyiene and Saccate = 1% at 315 W and 10 min o f reaction).
W ith pure methane feeds, although the m ajor product is CO (not shown
in Figure 3.4), the selectivities to C 3Scan be as high as 5% at 628 W (Figure 3.4).
The selectivity toward benzene ( C ^ ) can be as high as 25% at 1130 W. The
selectivities toward benzene can be as high as 20% and 10% at 378 W and 755 W
respectively when arcing is observed.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
39
The selectivity toward C2 hydrocarbons increased as power increased
when a ratio o f methane/helium o f 1:3 was used. After 10 min at any power
setting the only C2 present is ethane (Figure 3.5). At low power (378 W), the
selectivities to C3S were as high as 18% when helium is used as a diluent. The
selectivities toward C 3Sand C4s were as high as 16 and 18% respectively when the
power is increased to 503 W (Figure 3.6). The major product among the C3S was
propane and for the C4s was methylpropane.
3.1.3. Activated carbon
When the catalyst is changed to activated carbon, the conversion again
increases as applied power increases. High power levels (greater than 754 W)
lead to conversions o f almost 100% o f the methane feed (Figure 3.7). At high
power (1130 W) with helium diluent the only product is CO (Sco = 100%). At
low power (between 3 15 and 378 W) when times on stream are increased (from I
to 20 min) the selectivities toward Cj*and toward CO increase (Figure 3.8). In
this case, selectivities o f ethylene are greater than ethane and no acetylene is
formed. When the power is increased to 378 W, acetylene is generated and the
selectivities are in the order ethylene > acetylene > ethane (49, 11 and 6%
respectively at 20 min o f reaction).
The major C3 hydrocarbon product is
propene.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
40
EthylMw
Figure 3.5 Product distribution o f Cjs vs. time at 754 W using nickel powder
(Conditions: series F)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.6 Selectivities vs. tim e at 503 W using nickel powder (Conditions: series
F)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
42
Figure 3.7 Applied power and time o f reaction effect on conversion (Conditions:
series I)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C2'S
C3'»
C S 'S
CO
Figure 3.8 Product distribution vs. time at 378 W using AC-1 (Conditions: series
I)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
44
When no diluent is used and at very low power (190 W) conversions are
low, around 7%. As time on stream increases (from 15 to 85 min) the selectivities
toward C^s increase from 30 to 88% and selectivity toward CO decreases from 62
to 7%. No acetylene is formed during these conditions and the selectivity towards
ethylene is greater than that o f ethane. At an intermediate power o f 378 W, as
time increases from 3 to 20 min, selectivities to CO decrease to 4% and C ^ and
C3S increase to 90 and 6 % respectively. Propene is again the major C3 product
detected. The amount o f C2 product decreases when the power is increased to 754
W; as tim e increases and the selectivity toward C
&
s increases up to 33% (Figure
3.9). The selectivities o f C& change to ethylene > ethane > acetylene (Figure
3.10) when medium (566 W) and high power (754 W) levels are used.
3.1.4. Iron powder
Iron powder was active at high power values (1130 W). The conversion
decreases from 10 to 3% as time increases from 3 to 20 min. Selectivity to CO
was enhanced to 70% at high power (1130 W) and longer times (20 min).
Selectivity to C2 hydrocarbons decreases from 21 to 12% as time increases from 3
to 20 min. No acetylene was produced during reaction. Ethane was the major
product among the C 2*. Ethylene was also produced only at 3 minutes o f reaction.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Selectivity (%)
45
Tim* (min)
Figure 3.9 Product distribution vs. time at 754 W using AC-1 (Conditions: series
K)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
46
H im (min)
ehm m
Figure 3.10 Product distribution o f Cas vs. time at 754 W using AC-l (Conditions:
series K.)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission
47
3.2. Fixed frequency oligomerization with Raney nickel and its
characterization
The effects o f power and irradiation time on the oligomerization o f
methane via microwave heating were studied using Raney nickel as catalyst and
helium as diluent. Experiments were done with two different sets o f Raney
nickel: 1. Non-reduced Raney nickel, i.e., after drying slurry; 2. Reduced Raney
nickel, i.e., after drying and treating with hydrogen at 300°C for 2h. Conversion
o f methane decreases from a maximum o f 24% when using non-reduced catalyst
to a maximum o f 14% when using reduced catalyst.
When non-reduced Raney Nickel was used, the reaction required an
“activation” stage at low power (250 W) for 30 min where oxygen in the system
was consumed. Figure 3.11 shows conversion o f methane during this stage. At
these conditions, only CO was produced and no other hydrocarbon. After this
period, higher hydrocarbons were detected as seen in Figure 3.11. When reduced
Raney nickel was used, the “activation” stage was also required at a higher power
(600 W) for 80 min and there was no product detected during this period.
3.2.1. Non-reduced Raney nickel
When using non-reduced Raney nickel catalyst, and 25% methane
diluted in helium as reactant, as power increased from 280 to 750 W at short
irradiation time (5 min), conversion went through a maximum o f 13% at around
400 W (Figure 3.12). At this irradiation time and at all powers, the major
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
48
Selectivity (%)
Figure 3.11 Effect o f irradiation time on conversion and selectivity at 250 W
using non-reduced Raney nickel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
49
time (min)
Figure 3.12 Effect o f applied power and irradiation time on conversion after
activation stage using non-reduced Raney nickel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
products were C^s, C6 and C^srespectively (Figure 3.13). Furthermore, at 400 W
Cg reached its maximum selectivity o f 12%, while C2s reached a minimum
selectivity o f 73%. Figure 3.14 shows that when the inflection occurred, the
major product among C& was ethylene with a selectivity o f 37%, then acetylene
(34%), and finally ethane (2%). During the maximum conversion range i.e. 340 470 W, and 5 min irradiation time, the three most abundant products were
ethylene, acetylene, and benzene respectively. Meanwhile, for the other powers at
the same irradiation time, the most significant products were ethylene and ethane.
For a longer irradiation time o f 10 min, the conversion increased up to
24% at about 400 W, and the major products were C 2S as observable in Figure
3.15, with an average selectivity o f 86% for all powers. The foremost products
were ethylene and acetylene. The later became important as power increased as
seen in Figure 3.15. When power was increased above 400 W, the reactor melted
after 8 min. Samples were collected only up to 300 W for 15 min o f irradiation
time. The most abundant products for these conditions were ethylene with a
selectivity o f 50%, acetylene (24%), and benzene (13%). Beyond 300 W the
reactor melted after 10 min.
3.2.2. Reduced Raney nickel
Raney nickel was pre-treated with hydrogen for 2 h at 300°C. After
reduction, the catalyst was purged with helium for 3 h to eliminate excess o f
hydrogen. For this reaction the feed was 25% o f methane in helium. Conversion
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
51
Power (W)
Figure 3.13 Effect o f applied power on selectivity at 5 m in o f irradiation time
using non-reduced Raney nickel
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission
Selectivity (%)
52
40B
4 3 9
Power (W)
Figure 3.14 Effect o f applied power on C2s selectivities at 5 min o f irradiation
tim e using non-reduced Raney nickel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Selectivity (%)
53
20*
314
Power (W)
Figure 3.15 Effect o f applied power on selectivity at 10 min o f irradiation time
using non-reduced Raney nickel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
54
to products was higher than that o f non-reduced catalyst at lower powers.
However, the maximum power that could be applied was 280 W. After 280 W,
the reactor and catalyst melted. Figure 3.16 shows that the maximum conversion
(14%) was reached at 280 W after 5 min o f irradiation time. As time increased,
deactivation occurred and conversion decreased from 14 to about 4%.
Selectivities toward Cts decreased from 80 to about 70% compared to nonreduced catalyst at short and medium irradiation time. The major change was the
product distribution among C2s. Figure 3.17 shows that the main product for the
reduced catalyst was ethane (54%) followed by ethylene (13%). C 3S (mostly
propane) were comparatively more abundant than acetylene. Benzene was not
detected.
33. Variable frequency oligomerization with different catalysts
Microwave Induced-Oligomerization (MIO) o f methane was performed
at two different frequency levels (2.40 and 4.60 GHz). Variable frequency
reactions were also done varying the frequency from 2.4 to 7.0 GHz with a sweep
rate o f 0.5 sec. The effects o f microwave frequency, irradiation time, and nature
o f catalyst on the oligomerization o f methane were studied. A summary o f the
results can be seen in Table 3.3.
No visible arc formation was detected during these runs and the catalyst
bed remained the same after reaction (checked by SEM photographs). In general,
C2s were the most abundant products for the m a jo rity o f the cases. For iron
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
tim* (min)
Power (W)
2S1
Figure 3.16 Effect o f applied power and irradiation time on conversion using
reduced Raney nickel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
56
282
Powar(W)
Figure 3.17 Effect o f applied power on selectivity at 5 min o f irradiation time
using reduced Raney nickel
Reproduced with
permission of the copyright owner. Further reproduction prohibited without permission.
57
OMyet
Feed
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•VarieHeFrequencyfrom2.4 to 70 GHz Swnp.QSsec
~For24»te powervwe 1130W. Oraeoicnbetaw700Wwb zero.
Table 3.2 Summary o f results for the oligomerization o f methane at variable
frequency using different catalyst
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
58
powder, the amounts o f CO and CO? were also important. Among C2* the product
distribution regarding abundance was ethane, ethylene; and acetylene, where
acetylene was the least abundant Benzene was also produced with a maximum
selectivity o f 28%. As we reported77, and will discuss in the next section, there
are some absorbed species on the surface o f the catalyst such as water; carbon
monoxide; carbon dioxide; and molecular oxygen.
3.3.1. Frequency effect on nickel catalyst
When irradiating nickel powder with low frequency microwave energy
at 500 W, the conversion increased slightly from 4 to 5 % as time increases from
3 to 20 min. Ethane is the m ajor and only C? product with a maximum selectivity
o f 81 %. After 20 min at 500 W, the amount o f produced ethane decreases to 55
%, while some C 3S and C4S started forming. Figure 3.18 shows the detailed
product distribution for these conditions.
When increasing the microwave frequency to 4.6 GHz, the methane
converted into products is higher than that o f low frequency experiment, with an 8
% conversion at 20 min o f irradiation time. Figure 3.19 shows the product
distribution for high frequency experiments.
Ethylene is formed at high
microwave frequency with a selectivity o f 49 %. Acetylene and benzene also
became important after 20 m in o f irradiation tim e with a selectivity o f 16 and 10
% respectively.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
59
100.0
20 min
Figure 3.18 Effect o f irradiation time on conversion and selectivity with nickel
powder catalyst at 500 W using 2.4 GHz Fix Frequency
Reproduced with
permission o tth e c o p * * , owner. Further reproduction prohibited whhou, permission.
60
Figure 3.19 Effect o f irradiation time on conversion and selectivity with nickel
powder catalyst at 500 W using 4.6 GHz Fix Frequency
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
61
Figure 3.20 Effect o f irradiation time on conversion and selectivity with nickel
powder catalyst at 500 W using 2.4 - 7.0 GHz Variable Frequency with a sweep
rate o f 0.5 sec.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
62
As seen in Figure 3.20, the conversion o f methane doubles when
performing the reaction at variable frequency. Frequency was set to vary from
2.4 to 7.0 GHz within a 0.5 sec range at 500 W. The reaction was very selective
to ethane with a maximum selectivity o f 83 %. CO2 was the second major
product w ith a selectivity o f 15 % at 20 min o f irradiation time.
3.3.2. Frequency effect on iron catalyst
Figure 3.21, 3.22, and 3.23 show the product distribution o f the
oligomerization o f methane using iron powder for low frequency, high frequency,
and variable frequency experiments respectively. At low frequency and 500 W
iron catalysts were not active for the investigated reaction. However, after 700 W
some CO and CO2 started forming with a conversion o f less than 1 %. To obtain
a reasonable conversion o f about 10 %, it was necessary to increase the incident
power to 1130 W. Although CO and CO2 were still the major products at these
conditions, Cjs were also produced. The maximum selectivity reached for Ct*
was 21 % at 3 min o f irradiation time with ethane as major C2 product.
Interestingly, when microwave frequency was increased to 4.6 GHz iron
catalyst became active for the reaction at 500 W with a conversion o f 15 %.
While tim e increased to 20 min, the conversion decreased to 9 %. At 3 min o f
irradiation time the selectivity toward C2s reached 35 %, with ethane being the
major product. A considerable amount o f C& were formed with a maximum
selectivity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
63
Figure 3.21 Effect o f irradiation time on conversion and selectivity with iron
powder catalyst at 1130 W using 2.4 GHz Fix Frequency
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
64
Figure 3.22 Effect o f irradiation time on conversion and selectivity with iron
powder catalyst at 500 W using 4.6 GHz Fix Frequency
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
65
3mfn
Figure 3.23 Effect o f irradiation tim e on conversion and selectivity with iron
powder catalyst at 500 W using 2.4 —7.0 GHz Variable Frequency with a sweep
rate o f 0.5 sec.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
66
o f 18 %. At 20 min, selectivity toward C t* decreased to 23 %, which is almost
double than that o f low frequency experiments.
When experiments were carried out at variable frequency and 500 W,
conversion was comparable with the one at low frequency (10 %). However,
selectivity toward Cis more than triples the value o f those C2s at low frequency
(68 %). The major product was ethane with a maximum selectivity o f 65%.
Benzene was also formed with a selectivity o f 12 % at 3 min. Selectivity toward
CO and COi considerably decreased to 11 and 6 % respectively.
3 3 3 . Frequency effect on activated carbon catalyst
Unlike the other catalyst, activated carbon was very active for the
oligomerization o f methane at low power (500 W). Figure 3.24 shows the results
for the reaction at low frequency. Conversion o f methane to products increased
from 24 to 30 % as time increased from 3 to 20 min o f irradiation. The major
products were C ^ with 60 % selectivity. Distinctively from the other catalyst, the
product distribution among Ct* was ethylene, acetylene, and ethane, where
ethylene was the most abundant and ethane the least abundant. The second major
product was benzene with a maximum selectivity o f 28 % at 3 min o f irradiation
time.
When frequency increased to 4.6 GHz, conversion expectedly increased
to 31 %. Differently from the other experiments, conversion increased while
irradiation time increased as seen in Figure 3.25. Although selectivity toward C2s
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
67
3 min
Figure 3.24 Effect o f irradiation time on conversion and selectivity with activated
carbon catalyst at 500 W using 2.4 GHz Fix Frequency
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
68
Figure 3.25 Effect o f irradiation tune on conversion and selectivity with activated
carbon catalyst at 500 W using 4.6 GHz Fix Frequency
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
69
Figure 3.26 Effect o f irradiation tim e on conversion and selectivity with activated
carbon catalyst at 500 W using 2.4 - 7.0 GHz Variable Frequency with a sweep
rate o f 0.5 sec.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
increased to 72 %, product distribution remained fairly sim ilar to the one at low
frequency. Selectivity toward benzene slightly decreased from 25 to 19 % as
irradiation time increased from 3 to 20 %.
Figure 3.26 shows the product distribution for the reaction at variable
frequency.
Conversion o f methane was between the ones at low and high
frequency experiments with a maximum o f 39 %. Again, product distribution
remains rather sim ilar to the ones in previous experiments. The major product
was ethylene with a selectivity o f 51 %, followed by benzene with a selectivity o f
26 %.
3.4. Factors affecting the variable frequency oligomerization with nickel
catalyst
Microwave Induced-Oligomerization (MIO) o f methane was performed
at two different frequency levels (2.40 and 4.60 GHz). It was not possible to set
the unit to operate under other frequencies. The effect o f power and the presence
o f helium as diluent gas on conversion and product distribution were also studied
at these two different frequency values. A summary o f the results can be seen in
Table 3.4.
No visible arc formation was detected during these runs and the catalyst
bed remained the same after reaction (checked by SEM photographs). Ethylene,
acetylene, and ethane were the m ajor Ci products observed for the reaction o f
methane via microwave heating using Ni powder as a catalyst Benzene was
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
71
Feed
ch 4
CH4:He
Flow rate Time Power4*’
(mL/min) (min)
<W)
3
3:9
3
q
AstexUnit
Var. Freq. Unit46’
2.45 GHz
2.40 GHz
4.60 GHz
130
N.R.
N.R
Y
370
Y
N.R
Y
130
N.R.
N.R
Y
370
Y
Y
Y
N.R. = no reaction; Y = se e Fig. 4 - 7 for product distribution.
(a> too. 130.170,210.300.370 Watts of power level were used.
Those not shown
in the table were neither relevant nor reaction detected at any frequency.
m It was not possible to set other frequency values with successful microwave absorption.
Table 3.3 Summary o f results for the oligomerization o f methane at variable
frequency using nickel powder under different conditions
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
72
also formed with selectivities that ranged from 10 to 29%. In the case o f 25%
diluted methane as a feed and low frequencies (2.45 GHz), low selectivities o f
benzene were detected (2 to 5%).
3.4.1. Pure methane as reactant
When using pure methane as reactant, as power increased from 130 to
370 W at 4.60 GHz, conversion increased from 8 to 24%. Selectivities towards
Czs decreased from 60 to 45% and selectivities to C6$ increased from 6 to 29%
(Figure 3.27). Acetylene was not detected at low power (130 W). At 370 W, the
relative proportions o f C?products were ethylene > acetylene > ethane. At a low
frequency (2.45GHz), when using pure methane as reactant, oligomerization was
not achievable using the variable frequency unit. Therefore, for pure methane, the
effect o f frequency was compared using an ASTEX unit at the same conditions
except at a fixed 2.45 GHz frequency. The selectivities towards C2s decreased
from 71 to 45% and the selectivities towards benzene (C ^) increased from 20 to
29% when the frequency was increased from 2.45 to 4.60 GHz at 370 W. The
selectivities towards ethylene, acetylene, and ethane decreased as frequency
increased, maintaining their relative proportions (Figure 3.28).
3.4.2. Methane diluted in 25% He
When the feed was changed to 25% methane in helium, as power was
increased from 130 to 370 W at 4.60 GHz, conversion increased from 6 to 8%,
selectivities towards Czsdecreased from 83 to 72%, and selectivity towards
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
73
Selectivity (%)
XCH« (170 W): 8%
XCH4 (170 W): 24%
Figure 3.27 Effect o f applied power on product distribution at 4.60 GHz using
pure methane as a feed and nickel powder catalyst
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
Selectivty (%)
XCH«(VF): 24%
XCM«(Attn): 20%
2.45 GHz (Astex)
4.60 GHz (VF unit)
Figure 3.28 Frequency effect on product distribution at 370 W using pure
methane as a feed and nickel powder catalyst
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75
benzene increased from I to 10%. Conversion levels were lower than those in the
case o f using pure methane as a feed (6 and 8% compared to 8 and 24%). Also,
while increasing power, less drastic changes in selectivity towards C& and C 6S
were observed than those when using pure methane (See Figures 3.27 and 3.29).
Acetylene was not detected at 130 W and 4.60 GHz frequency. The
relative proportion o f Cz*changed from ethylene > ethane to ethane > ethylene >
acetylene as power was increased from 130 to 370 W (Figure 3.29), which was
different from the proportion o f
in the case o f pure methane as a feed
(ethylene > acetylene > ethane). The systematic effect o f frequency was studied
using the Lambda variable frequency unit and comparisons were also made when
using the fixed frequency ASTEX unit in order to check for reproducibility o f
results at low frequency (2.45 GHz).
The data here show that both the ASTEX fixed frequency and the
Lambda variable frequency systems behave almost the same when comparing
data at low frequency (2.45 GHz) with respect to conversion levels (about 7%)
and product distributions (Figure 3.30) at 370 W. As frequency was increased
from 2.45 to 4.60 GHz, selectivities towards C2s decreased from 81 to 72% and
selectivity towards benzene increased from 5 to 10% (Figure 3.30). When using
He as diluent at 370 W, the only C2 detected was ethane at a low frequency value
(2.45 GHz). As frequency was increased from 2.45 to 4.60 GHz, the proportion
o f C2 products changed from ethane to ethane > ethylene > acetylene (Figure
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76
3.30). The effect o f frequency on product distribution among C2s was different
with respect to the case o f pure methane in the feed as seen in Figures 3.28 and
3.30. The proportion o f C2s changed from ethylene > acetylene > ethane to ethane
> ethylene > acetylene at 4.60 GHz. At 4.60 GHz and pure methane, selectivity
o f benzene was higher than that at 4.60 GHz and diluted methane (29 us. 10%
respectively).
There are different sources o f oxygen that account for the formation o f
carbon monoxide and carbon dioxide detected in the products. Nickel catalyst
was used as received for the experiments. Subsequent temperature-programmed
desorption analysis revealed the presence o f oxygenated species in the catalyst as
seen in Figure 3.31.
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77
Selectivity (%)
XCH«(130 W): «%
XCM4(370W): 8%
Figure 3.29 Effect o f applied power on product distribution at 4.60 GHz using
methane diluted in helium (25%) and nickel powder catalyst
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78
XctU (AtlU)*
6%
XctU(VFTM012)>7%
Selectivity (%)
XCKU(VFTM018W8%
5.0 7.JO
2.46 (Astex)
2.40 (VFuntt. TM012)
C2’e m i, 1-8 * 10.0*
®** C4'e rg’s
/
“ * C0 2
,'
120 -^ — /
c o
__ ___
* *
^
..
4.60 (VFuntt. TM018)
M H 4 C2H2 C2H6
Frequency (GHz)
Figure 3 3 0 Frequency effect on product distribution at 370 W using methane
diluted in helium (25%) and nickel powder catalyst
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79
TPD-MS
*D 2e-8 w
3.
E
^ 2e-8 C
D
(O
CO
CO
c
s
5e-9
Cq
0e+0 -
0
100
200
300
400
500
600
700
Temperature C
Figure 3.31 Fresh nickel catalyst Temperature-Programmed Desorption analysis
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CHAPTER IV
DISCUSSION
4.1. Catalytic Oligomerization of Methane Using Fixed Frequency
Microwave radiation
4.1.1. General comments and mechanistic ideas
Minimization o f the reflected power was achieved by proper use o f the
3-stub tuner, therefore, most o f the applied power was delivered to the catalytic
bed. In order to achieve an average desired power the pulse duration in these
experiments also changed. The pulse width and amplitude are increased as the
average power increases. Greater pulse width and amplitude might explain the
trend o f increased bulk measured temperature with increase in the applied power.
Since temperatures could not be measured insituthe exsitu measurements appear
to be much lower than real temperatures based on physical evidence o f m elting or
fusing processes o f nickel catalysts and the quartz reactor. Ethylene, ethane, and
acetylene were the major Cz products for the oligomerization o f methane via
microwave heating using nickel powder, activate carbon, and iron powder as
catalytic materials.
Proposed reaction mechanisms which account for the detected Cz
products have been proposed by other authors.1,6,28’30,32 Equations (1-6)
summarize
some
of
the
proposed
80
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reactions:
81
ch
4
—>
:C H
0)
CH4
—
»
:C H 2 +
H2
(2)
2CH 4
—
>
C2H4 + 2 H2
( 3)
C2H2 + 3 H 2
(4 )
c 2h 6
( 5)
2CH 4
C2H4 + h 2
—
>
3 C 2H2 - >
C6H6
(6)
The presence o f H2 from reaction (2) leads to hydrogenation o f olefins
and production o f ethane as shown in reaction (S). The highly reactive methylene
(:CH2> intermediate can be used to elucidate the formation o f ethylene and
acetylene as indicated in reactions (3) and (4). The formation o f benzene may be
explained by the well-known cyclization reaction given in Equation (6 ). Another
viable mechanism for ethylene and acetylene formation could involve the
dehydrogenation o f secondary hydrocarbons:
C2H6
-»
C2H4 +
H2
(7)
C 2H4
->
C2H2 +
H2
( 8)
The reaction paths m ost likely proceed via formation o f free radical
intermediates.
The formation o f propane and propene may be explained by
reactions given in Equations (7-8):
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82
c 2h 6 + c h 2 -»
C3H8
->
c 2h 6
CjH6
(9)
+
h2
( 10)
The mechanism by which G»s to Cgs are formed could be very
complicated and many different pathways could be involved for the production o f
these compounds. Metal samples or continuous metal films in high microwave
fields result in large electric field gradients and may cause visible and at times
dramatic electric discharges.78
Arcs are short lived, localized plasmas caused by the build-up o f an
intense electrical field.
The electrical field attempts to remain continuous
throughout the volume o f the cavity but will be zero at the metal surface. Arcs
are ionized gas which can be destructive to the metal surfaces at which they form.
Once an arc occurs and damages a surface, the tendency is for that surface to
more easily produce more arcs. The effect o f an arc on a reaction depends
strongly on the reaction itself.
High energy excited state atoms, ions, or
molecules produced during arc formation can lead to explosions. Variable
frequency applicators can eliminate the arcing problems experienced in
microwaves ovens when a metal or a semiconducting material is irradiated .64
4.1.2. Catalyst, applied power and diluent effects on product distribution
Ni powder, activated carbon, and iron powder are active catalysts for the
oligomerization o f methane. Our results for N i are in agreement with studies by
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83
Wan and coworkers.29’30,67,68-79 Conversion increases as power increases. This
trend might be explained by the increase in the duration o f the pulse in order to
achieve a certain average power, therefore an increase in the bulk temperature o f
the catalyst occurs. There is evidence that catalysts are changing during reaction
when conversion decreases as time and power increase.
The presence o f CQ> and CO as products could come from oxygen
adsorbed on the nickel, nickel oxide (since we did not reduced the nickel before
reaction), as well as from sputtering o f the walls o f the reactor. Sources o f
oxygen for the reaction will be discussed in subsequent sections. Results from
activated carbon are novel and difficult to compare to data for nickel and iron
powders. One reason for this difficult comparison is that the surface area o f the
activated carbon samples is as high as 3100 m2/g whereas the nickel and iron
powder are considerably smaller. As seen in the next section, BET surface area o f
both nickel and iron powder is approximately 5 m2/g.
The most fascinating data are for nickel and activated carbon systems.
Using nickel as a catalyst and no diluent, selectivity to benzene was 24%. The
proportions o f ethylene, acetylene, and ethane could be controlled depending on
the conditions used. Acetylene increases when arcing is present and is in accord
with data for oligomerization o f methane via microwave plasmas.70 Selectivities
are highly dependent on the nature and concentration o f diluents like helium and
average applied power. Oligomer products like C3S and C4s were enhanced up to
16 and 18%, respectively when helium is used as a diluent for the nickel system
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84
and at medium power (503 W).
The diluent effect may be due to partial
absorption o f microwave energy by the helium atoms and collisional interactions
o f such species with methane molecules.
When activated carbon was used as a catalyst the overall selectivity to
benzene can be as high as 33%. Selectivities to ethane and ethylene were reached
at high power (1130 W) with no production o f acetylene when using iron powder
as catalyst.
Nickel, iron powder, and activated carbon are the most active catalytic
materials for the oligomerization o f methane using microwave heating.
Manganese oxide systems such as OMS-1, OMS-2, and OL-l are not very active
for this reaction. The dielectric constant o f manganese oxide is on the order o f
10,000; i.e., one o f the highest values reported for any material. The data reported
here clearly show that dielectric constants are not the most important parameters
in the catalytic oligomerization o f methane induced via microwave heating.
The surface area o f activated carbon is two orders o f magnitude larger
than the surface area o f Ni powder. The data reported here also show that
conversion and activity o f these materials is not proportionally related to the
surface area o f the catalysts we used. One possible explanation for the lack o f
direct relationship between surface area and conversion might be the effect o f the
interaction o f microwaves with surface functional groups such as OH groups and
other catalytic intermediates like: CH 3, CH2, CH, and other species. Such
functional groups may also influence the adsorption o f the reactant species.
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85
4.2.
Fixed frequency oligomerization with
Raney nickel and
its
characterization
In general, the most noticeable products were ethylene, acetylene, and
benzene for non-reduced Raney nickel.
For reduced Raney nickel the most
important products were ethane and ethylene. TPD studies indicate two forms o f
adsorbed hydrogen, one o f which is a weakly bound molecular form, and the
other is a strongly bound atomic form .80 Reduction o f Raney nickel increases the
amount o f chemisorbed hydrogen on the surface o f the catalyst.81 This higher
concentration o f reactive hydrogen on the surface favors the formation o f
saturated hydrocarbons such as ethane, explaining the product distribution
observed in the reduced Raney nickel experiments.
Comparing the results o f this study with the results o f the same reaction
using nickel power, it is unambiguous that the lifetime o f Raney nickel is about
one third o f the lifetime o f nickel powder. The activity o f Raney nickel is also
remarkable higher than that o f nickel powder.
Raney nickel gives higher
conversions at the same moderate power (24% vs. less than 10% at about 400W ).
Moreover, products are detected at a lower power (300 W) than that o f the nickel
powder experiments (700 W).82
Even though handling o f Raney nickel was carried out in a glove bag
with an inert atmosphere, some oxidation might have occurred on the surface o f
the catalyst while transferring the reactor to the microwave unit. TPD studies
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86
revealed the presence o f a small amount o f oxygen on the surface as well as a
larger amount o f CO2 and an even larger amount o f H2O (Figure 4.1.). Clearly
some water remains in the Raney nickel after the drying process. These absorbed
species on the surface supply the source o f oxygen to form CO, which was
detected in the “activation” stage o f non-reduced catalyst.
Desorption o f
hydrogen from the catalyst confirmed the presence o f two forms o f hydrogen.
Weakly adsorbed hydrogen evolved at 110°C is the reactive hydrogen (Figure
4 .1.), as already mention by Barbier et al.83 The second hydrogen peak is a
strongly bound dissociative form, which desorbs at around 165°C. On the other
hand, the reaction temperatures for the 200 - 400 W range are in the order or 800
- 1000°C, according to previous work published elsewhere.82 Therefore, all
hydrogen from the catalyst is quickly consumed to produce ethane as major
product. Once the highly reactive hydrogen is consumed, the major products are
acetylene and benzene respectively, at about 400 W. Thereafter the catalyst
deactivates as observed in Figure 3.12.
Mikhailenko et a /.81 reported that redox treatment on Raney nickel
causes a redistribution o f the energy inhomogeneity o f the surface, shifting the
desorption peaks o f hydrogen to a lower temperature, and the total amount o f
hydrogen desorbed decreases. As discussed later, the BET surface area decreases,
leading to a decrease in the amount o f hydrogen adsorbed, mainly at the expense
o f strongly bound form.
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87
2e-7
2e-7
•p 2e-7
1e-7
-g
5
8
e
16-7
3
3
6e-8
1e-7
3
x
to
4e-8
2e-8
0e +0 ■
0
100
200
300
400
500
600
700
800
Temperature C
Figure 4.1 Temperature-Programmed Desorption analysis o f fresh Raney nickel
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88
This more readily available reactive hydrogen explains the formation o f ethane at
lower power for the case o f reduced Raney nickel. W hile hydrogen is being
consumed Raney nickel is losing its activity most probably due to sintering. Once
hydrogen is finished the catalyst also has sintered, significantly plummeting
conversion.
BET studies on the catalyst, performed before and after reaction, showed
a considerable loss o f surface area. Fresh dried Raney nickel had a surface area o f
60.42 n r/g . After reaction the surface area o f the catalyst decreased by 65% o f
the initial value, i.e. 21.15 m2/g. This surface area loss suggested deactivation o f
the catalyst by sintering, which is confirmed by scanning electron microscopy.
SEM photographs in Figure 4.2 revealed fresh Raney nickel consisting o f large
particles (about 10 pm and above) with a platelike morphology. Some o f the
particles have agglomerated to form larger units (size up to 200 pm). Figure 4.3
shows sintered Raney nickel after the reaction, giving a bulk-like appearance.
Samples for after reaction photographs were taken at intermediate power (around
400 W) and 5 min o f irradiation time.
Beyond this point, most o f the catalyst
was melted. Michailenko et al. found out that partial sintering o f Raney nickel
occurred during the reduction process explaining the decrease in activity with
respect to non-reduced catalyst.
General observations from XPS studies can be seen in Table 4.1. The
amount o f zero-valent metal on Raney nickel surface was not enough to resolve
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89
¥
*
-
t ■
Figure 4.2 Scanning Electron Microscopy photograph o f Raney nickel before
reaction
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
90
i
. -i* U 1 -
Figure 4.3 Scanning Electron Microscopy photograph o f Raney nickel after
reaction
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91
the contribution to the spectra and differentiate the metal phase from the
contribution o f nickel oxide. Despite the small amount o f nickel metal, the
catalyst was still very active compared to bulk nickel.82 In addition to high
surface area o f Raney nickel there are other species on the surface o f the catalyst
that might be responsible for high activity. Residual metallic aluminum and its
corresponding oxides are also present on the surface and can play an important
role in the activity o f the catalyst as suggested by Birkenstock et a /.84
In addition, Diskin et alss observed that nickel oxides on the surface o f
the catalyst can be active sites for the activation o f methane as well as nickel
metal. XPS also detected different forms o f oxygen on the surface o f the fresh
catalyst. Sources o f oxygen might be assigned to H2O, Ni(OH>2, NiO, and AI2O3
which would be in agreement with Yoshino et a /.86 and with the data obtained
from TPR analysis (Figure 4.4). Reduction o f NiO generally proceeds within the
280-300°C range.87
The first hydrogen peak o f the TPR plot most probably correspond to
consumption o f hydrogen to reduce Ni(OH)2, which is present in large amounts
and requires lower reduction activation energy than that o f the more stable NiO .87
After reaction catalyst analysis showed a very small amount o f nickel on the
surface and almost 90% o f carbonaceous species. This finding confirms that
deactivation o f the catalyst also is due to blocking o f active sites with carbon.
Deactivation mechanisms responsible for loss o f Raney nickel catalysts in
methanation reactions are sintering and fouling (series and parallel).88 Series
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92
Ni
Fresh
After rxn
29.64*
0.14
C
7.86“
88.15***
o
57.97
11.71
AT*
2.27
Atom ratios
•Nl^andNI0
** CO and 0 0 2
*** Alfph+Olef
Table 4.1 X-Ray Photoelectron Spectroscopy analysis for Raney nickel
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Al°
2.27
93
1.4e-6
Partial Pressure mmHg
1.2e-6
1.Oe-6
8.0e-7
6.0e~7
4.0e-7
2.06-7
0.0e+0
T"
0
"
t
1 " ' " I................................■■■■■■ —
100
200
300
400
500
600
Temperature C
Figure 4.4 Temperature-Programmed Reduction analysis o f fresh Raney nickel
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94
fouling occurs when the produced CO generates a side reaction to form carbon.
The process can be illustrated by the equation 11, where s is the catalytically
active site.
2 CO + s
*-►
sC
+
CO»
(H)
CO is produced mainly during the initial stage o f the oligomerization o f
methane as seen in Figure 3.11. Besides, low temperatures at low power favor the
decomposition o f carbon monoxide.88 When power increases during the reaction,
high temperatures favor the decomposition o f methane deactivating the catalyst
by a parallel fouling mechanism. Equation 12 describes this process, where s is
the catalytic active site.
CH4 + s
«-►
s*C
+
2H 2
( 12)
Finally, at the end o f the reaction the temperature is high enough to
favor sintering o f the catalyst as seen in Figure 4.3, rapidly ceasing the catalytic
activity o f Raney nickel.
4 3. V ariable frequency oligomerization with different catalysts
Three different catalysts were tested. Nickel powder, iron powder, and
activated carbon were used for the activation o f methane via microwave heating at
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95
500 W. Interestingly, activated carbon was the most active catalyst under our
system conditions. The maximum conversion was reached using activated carbon
(45 %) at a fixed 4.6 GHz microwave frequency after 20 min o f irradiation time.
The amount o f benzene was also comparatively high (28%) when using activated
carbon as catalyst. While ethane is the major C2 product for both nickel and iron
catalyst, ethylene is the major product for activated carbon. Acetylene is only
produced when using activated carbon as catalyst. Formation o f benzene may be
explained by the well-known cyclization reaction. Bamwenda observed similar
reactions as reported in a previous publication.28 This is in agreement with the fact
that for our system selectivity to benzene increases as selectivity to acetylene also
increases.
Activated carbon absorbs microwave radiation more efficiently than
nickel powder.89 Consequently, the temperature reached by activated carbon is
higher than that reached by nickel and iron.82
This condition o f higher
temperature on the active sites would favor the conversion o f methane on
activated carbon.
On the other hand, the surface area o f activated carbon is two orders o f
magnitude larger than the surface area o f nickel and iron powder. Conversion and
activity o f these materials are not proportionally related to the surface area o f the
catalysts we used. As mentioned in previous sections, one possible explanation
for the lack o f direct relationship between surface area and conversion might be
the effect o f the interaction o f microwaves with surface functional groups such as
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96
OH groups and other catalytic intermediates like CH 3, CH2, CH, and other
species. Such functional groups may also influence the adsorption o f the reactant
species.
The reaction pathway for the catalytic oligomerization o f methane using
microwave heating most likely proceeds via formation o f free radical
intermediates as shown by Wan et. al.90 The primary decomposition fragments
from methane are CH? and CH radicals, which recombine on very hot surfaces to
yield acetylene, whereas on cooler surfaces methane leads to the formation o f
ethylene. Ethane is favored with even cooler surfaces than those o f ethylene.
This finding is in agreement with our results as seen in Figures 3.18 to 3.26.
Figures show formation o f acetylene and ethylene for reactions with activated
carbon, the most active catalyst.
Ethylene was also formed using the other
catalysts when working at the most energetic frequency (4.6 GHz).
As discussed on a previous publication,77 the effects o f frequency on
product distribution for the oligomerization o f methane via microwave heating
might be related to the diverse transient heating patterns that are generated by
transverse magnetic modes at different frequencies.
As quenching o f
intermediates has an important role in ethylene formation, different transient
heating patterns would affect the environment in which the reaction is taking
place, consequently affecting the selectivities o f the final products.
The general trend found was an increase in activity as frequency
increases. Previous studies77 correlate the microwave radiation frequency with the
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97
activity o f nickel catalyst. As radiation frequency increases, the particles trying to
align with the electromagnetic field are going to move more rapidly, generating
more heat than that at lower frequency.
Therefore, at the same power and
irradiation time, the energy density in the material is higher for high frequency
radiation. The higher energy density in the material produces different and more
energetic transient heating patterns. When the temperature reached by the active
sites is too high, the reaction leads to undesirable products such as carbon
monoxide and carbon dioxide.
The two parameters that define the dielectric properties o f materials and
concomitant heating patterns are dielectric constant and dielectric loss.
The
dielectric constant, e \ describes the ability o f the molecules to be polarized by the
electric field.
At low frequencies this value will reach a maximum as the
maximum amount o f energy that can be stored in the material. As frequency
increases e’ decreases as shown by Mingos et al.91 The dielectric loss, e”,
measures the efficiency at which microwave energy can be converted into heat
and goes through a maximum as frequency increases 91 The relationship between
these two parameters e”/e’ defines the dielectric loss tangent, tan 5, which
measures the ability o f a material to convert electromagnetic energy into heat at a
given frequency and temperature. The dielectric loss also goes through a
maximum as frequency increases. Therefore, higher formation o f benzene and
acetylene at higher frequency in our experiments can be explained by the increase
o f the dielectric loss as frequency increases before going through the maximum.
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98
Greater dielectric loss is directly related to greater temperatures, and consequently
related to ethylene and acetylene formation.90
When the frequency o f the microwave radiation varies over a period o f
time, the transient heating patterns vary constantly, not allowing the formation o f
hot spots. Therefore, heating is completely uniform throughout the material.
Although the microwave variable frequency radiation can be highly energetic for
activating methane, this does not lead to a large amount o f undesirable products.
Even though conversions are comparable for high and variable frequency
experiments, selectivities toward CO> and CO are somewhat higher for the
experiments at 4.6 GHz (Figures 3.18 to 3.26). There are different sources o f
oxygen (O2, CO 2, and H2O) that account for the formation o f carbon monoxide
and carbon dioxide detected in the products that supply the source o f oxygen for
undesirable products (as mention in the previous section.).
4,4. Factors affecting the variable frequency oligomerization with nickel
catalyst
Microwave radiation does not have sufficient energy to cause any
chemical changes, such as breaking bonds and transferring electrons. Nickel
catalyst strongly absorbs microwave energy providing the necessary kinetic
energy for the surface electrons to enhance the surface chemical reaction. The
energy absorbed by the metallic sites is transformed from a rapid oscillating
electric field into thermal energy. Since only the catalyst surface site is heated,
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99
there is a large temperature gradient (about 500" C) between the surface and the
bulk o f the gas phase.
The primary decomposition fragment from methane is the methylene
radical which recombines on very hot surfaces (above 1000aC) to yield acetylene,
whereas on cooler surfaces it leads to the formation o f ethylene. This finding is in
agreement with our results as seen in Figures 3.27 and 3.29, which show
formation o f acetylene as power increased.
The increase in the duration o f the pulse in order to achieve a certain
average power and therefore an increase in the surface temperature o f the catalyst
explains the general trend where conversion increases as power increases.
Conversion levels were smaller when using He as a diluent in the feed due to the
fact that the residence time was about four times smaller than that when using
pure methane in the feed (0.08 vs. 0.33 min' 1 respectively). Allowing the reactant
to spend more tim e in the reaction zone will permit the methane molecules to find
adequate sites for reaction to occur. Since the irradiation time o f the catalyst was
small (3 min), the bulk temperature was not completely homogeneous. That is,
there were hot spots, cold spots, and intermediate temperature spots. Methane
will only react on those sites that have reached the proper temperature.
Thermodynamic analysis has indicated that a catalyst particle surface temperature
on the order o f 1400°C - t600°C is required to yield C? products from methane.92
The proportion o f C 2 products changed from ethylene > acetylene > ethane when
using pure methane to ethylene > ethane > acetylene when using methane and He
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100
in the feed. More benzene was also formed when using pure methane. These
observable facts can be explained by both the difference in reagent residence time
and the difference in concentration o f methane. Wan found that sudden cooling o f
products would favor the formation o f ethylene and ethane.92 For the case o f
CH4:He feed, low concentration o f active intermediates and products, relatively
cold He molecules combined with low residence time, would give the molecules
more likelihood for rapid quenching. As mentioned previously, benzene very
likely comes from the trimerization o f acetylene. High concentrations o f methane
will statistically favor the formation o f acetylene and benzene. Including a diluent
in the feed will help prevent the coupling o f intermediates to these products.93'96
At present, the issue o f microwave effects is very controversial.
Unfortunately, many o f the expected results from microwave processing such as
rapid and uniform heating, more uniform microstructures, inverse temperature
profiles, and selective heating are included in the general category o f microwave
effects.
However, only those anomalies that cannot be predicted or easily
explained based on our present understanding o f differences between thermal and
microwave heating should be referred to as microwave effects.97,98 “Microwave
specific” activation has been a debated concept which actually refers to a unique
interaction, reaction, or activation, specific to the microwave radiation. Two
models o f the chemical mechanisms in such reactions have emerged from
research work done so far on microwave induced reaction chemistry. One model
assumes that rate enhancement is simply due to thermal dielectric heating and the
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101
other assumes that there is a specific activation due to microwave radiation that
occurs in addition to the dielectric heating mentioned earlier."
Our work is unique in the sense that systematic frequency effects have
been found to markedly influence conversion and selectivity under microwave
radiation in the oligomerization o f methane. The effect o f frequency on product
distribution for the oligomerization o f methane was studied with and without He
as a diluent. It is a fact that when increasing microwave frequency, selectivities
towards Cts decreased and selectivity towards C&increased for both cases (with
and without diluent).
All Ci hydrocarbons decreased as frequency increased when using pure
methane feed. When using He as a diluent, ethylene and acetylene increased as
frequency increased while ethane decreased (see Figures 3.27 and 3.30). These
effects on product distribution are related to the transverse magnetic modes at
different frequencies that would generate diverse transient heating patterns. As
mentioned in the previous section, the two parameters that define the dielectric
properties o f materials and associated heating patterns are dielectric constant and
dielectric loss.
As discussed previously, the quenching o f intermediates has an
important role in ethylene formation and affects the selectivities o f the final
products. As pointed out, the dielectric loss also goes through a maximum as
frequency increases. Therefore, higher formation o f benzene and acetylene at
higher frequency in our experiments can be explained by the increase o f the
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102
dielectric loss as frequency increases before going through the maximum. Greater
dielectric loss is directly related to greater temperatures and consequently related
to acetylene formation. Acetylene is later consumed in the trimerization reaction
to obtain benzene, which is detected in the final products.
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CHAPTER V
CONCLUSION
The microwave heating oligomerization experiments have shown that
nickel, iron powder, and activated carbon can act as selective catalysts for
oligomerized products o f methane.
Oligomers ranging from C2 to C
&
hydrocarbons have been prepared in high selectivity (from 20 to 80 %) depending
on the nature o f diluent and the power levels used in the microwave reactor. The
use o f He as diluent gas favors the oligomerization o f methane via microwave
heating. When using He as a diluent and Ni powder as a catalyst, selectivities
toward C3* and C4S were enhanced up to 16 and 18%, respectively. Although He
might be expensive, the presence or absence o f He is an additional parameter that
should be considered. Selectivities toward benzene as high as 33% were achieved
by using activated carbon as a catalyst
Raney nickel has been shown to be a selective catalyst in the
oligomerization o f methane via microwave heating. The maximum conversion
achieved in this reaction was 24 % at 400 W and 10 min o f microwave irradiation
time.
With nickel powder, comparable conversions and selectivities were
obtained only at high power (above 700 W) and long microwave irradiation time
(above 20 min). Selectivities as high as 71, 13 and 54% have been obtained for
ethylene, benzene, and ethane, respectively.
Short microwave irradiation
103
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104
times and high power (above 530 W) enhance the formation o f ethylene, whereas
intermediate power (400 W) favors the formation o f benzene.
Pre-treatment o f Raney nickel catalyst with hydrogen reduction
increases the formation o f ethane at low and intermediate power.
Several
mechanisms are responsible for deactivation o f the catalyst. Parallel fouling most
probably is present at the beginning o f the reaction when CO is formed. Later in
the reaction, while temperature increases, decomposition o f methane might cause
deactivation by series fouling. Finally, sintering o f Raney nickel occurs at higher
temperatures terminating the reaction.
Our work is unique in the sense that systematic frequency effects have
been found to markedly influence conversion and selectivity under microwave
radiation in the oligomerization o f methane. No other heterogeneous catalytic
studies using microwave radiation where either direct or indirect frequency
effects, nature o f the catalyst, effects o f applied power, and irradiation tim e have
been published. Systematic effects o f power level, He as diluent, and microwave
frequency were studied for the microwave oligomerization o f methane using Ni
powder catalyst with branched filament morphology. W e are not aware o f other
heterogeneous catalysis studies where either direct o r indirect frequency effects
have been observed under microwave radiation.
As frequency increased, selectivity towards benzene also increased.
When using He as a diluent, as frequency increased, selectivities towards ethylene
and acetylene also increased. When pure methane was used, the opposite result
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105
was observed. These changes in product distribution are most likely due to
different transverse modes that generate different transient heating patterns and
changes in dielectric properties o f the catalyst (dielectric constant and dielectric
loss o f the catalyst).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX
LIST OF PUBLICATIONS
Marun,
Carolina;
Conde,
Daniel;
Suib,
Steven.
“Catalytic
Oligomerization o f Methane via Microwave Heating.’* J. Phys. Chem. A.
1999, 103,4332-4340
Conde, Daniel; Marun, Carolina; Suib, Steven; Fathi, Zak. “Frequency
Effects in the Catalytic Oligomeization o f Methane via Microwave
Heating.” J. ofCatal 2001,204,324-332
Conde, Daniel; Suib, Steven. “Factors Affecting the Catalytic Activation
o f Methane via Microwave Heating.” Utilization of Greenhouse Gases
Symposium Proceedings, 223th ACS National Meeting, Orlando, Florida,
April 7-11 (2002), UGG27R. Publisher. American Chemical Society,
Washington, D.C.
Conde, Daniel;
Suib, Steven.
“Effects o f Frequency on Different
Catalysts for the Oligomerization o f Methane via Microwave Heating.”
106
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107
Paper presented at the American Chemical Society National Meeting,
Orlando, Florida, April 7-11 (2002)
Conde, Daniel;
Marun, Carolina; Suib, Steven.
“Oligomerization o f
Methane via Microwave Heating Using Raney Nickel Catalyst.”
Submitted to J. ofCataL, November 2002
Conde, Daniel; Suib, Steven. “Catalyst Nature and Frequency Effects on
the Oligomerization o f Methane via Microwave Heating.” Submitted to J.
Phys. Chem., November 2002
Suib, Steven;
Vileno, Elizabeth;
Zhang, Qiuhua;
Marun, Carolina;
Conde, Daniel. “Microwave Induced Chemical Reactions in Synthesis
and Catalysis.” Ceramic Transactions, 1997,80,331-339
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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