Catalytic direct conversion of ethane to value-added chemicals under microwave irradiation

Article abstract of DOI:10.1016/j.cattod.2020.03.001

The reaction mechanism of ethane conversion over 4 % Mo-0.5 % Fe/ZSM-5 catalyst was investigated using two different reactors with different heating modes. A microwave fixed-bed reactor (MWFB) was used and its performance was compared to a conventional thermal fixed-bed reactor (CTFB). Over 80 % ethane conversion was achieved under microwave irradiation mode at 375 °C. To achieve the same ethane conversion conventionally, the reaction temperature had to be elevated to 660 °C in the CTFB. The formation rates and selectivity of the products were compared under these two heating methods. The CTFB products had a higher selectivity towards aromatic compounds; whereas, the MWFB products had a higher selectivity towards ethylene; 26.5 % higher than the CTFB. Carbon analysis over spent catalysts of CTFB showed three distinct types of coke, whereas the microwave resulted in a single higher temperature coke type.

Full text of DOI:10.1016/j.cattod.2020.03.001

Journal Pre-proof
Catalytic Direct Conversion of Ethane to Value-added Chemicals Under
Microwave Irradiation
Brandon Robinson (Methodology) (Investigation) (Writing - original
draft), Ashley Caiola (Writing - review and editing) (Visualization)
(Investigation), Xinwei Bai (Writing - review and editing), Victor
Abdelsayed (Supervision) (Writing - review and editing), Dushyant
Shekhawat (Supervision) (Writing - review and editing), Jianli Hu
(Project administration) (Conceptualization) (Supervision)
PII:
S0920-5861(20)30105-X
DOI:
https://doi.org/10.1016/j.cattod.2020.03.001
CATTOD 12714
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
2 October 2019
9 January 2020
2 March 2020
Please cite this article as: Robinson B, Caiola A, Bai X, Abdelsayed V, Shekhawat D, Hu J,
Catalytic Direct Conversion of Ethane to Value-added Chemicals Under Microwave
Irradiation, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.03.001

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Catalytic Direct Conversion of Ethane to Value-added Chemicals Under Microwave
Irradiation
Brandon Robinson a, Ashley Caiola a, Xinwei Bai a, Victor Abdelsayed b,c, Dushyant Shekhawat b, Jianli
Hu* a
a Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV, 26505
b National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Rd. Morgantown, WV,
26505
c Leidos Research Support Team, 3610 Collins Ferry Road, Morgantown, WV 26507
*Corresponding Author, Email: john.hu@mail.wvu.edu
Graphical abstract
MoFe/ZSM-5
Microwave
Metal Site
T = 660 C
Microwave
Bulk
ΔT 285 C
T = 375 C
Selective Heating
Metal Site
T = 660 C
Thermal
Thermal
Uniform
T = 660 C
Convective Heating
Bulk Temperature

Highlights
 Over 80% of ethane conversion was achieved under microwave irradiation at 375°C
 The conventional reactor required a temperature of 660°C to achieve 80% conversion
 The microwave selectivity to ethylene was higher than the conventional reactor
 The conventional fixed bed had a higher aromatic yield then the microwave reactor
 The spent microwave sample resulted in less total coke formation
Abstract
The reaction mechanism of ethane conversion over 4% Mo-0.5% Fe/ZSM-5 catalyst was
investigated using two different reactors with different heating modes. A microwave fixed-bed
reactor (MWFB) was used and its performance was compared to a conventional thermal fixed-
bed reactor (CTFB). Over 80 % ethane conversion was achieved under microwave irradiation
mode at 375 °C. To achieve the same ethane conversion conventionally, the reaction temperature
had to be elevated to 660 °C in the CTFB. The formation rates and selectivity of the products
were compared under these two heating methods. The CTFB products had a higher selectivity
towards aromatic compounds; whereas, the MWFB products had a higher selectivity towards
ethylene; 26.5% higher than the CTFB. Carbon analysis overspent catalysts of CTFB showed
three distinct types of coke, whereas the microwave resulted in a single higher temperature coke
type.
Keywords: Ethane, Aromatization, Microwave, Mo-Fe/ZSM-5
Introduction

Ethane is the second-largest component in natural gas and is typically separated to minimize
condensation in natural gas pipelines. The technical difficulties and cost associated with the
transportation and the storage of ethane has led companies to start burning natural gas to produce
power or flaring it. In 2012 approximately 3.5 % of natural gas produced went to flaring, which
certainly would add to the global CO2 emission problem [1,2]. Therefore, developing low-cost
technologies would be needed to utilize these waste fuels. The conversion of ethane to higher-
value liquid products is of increasing importance so that the natural gas can be safely utilized and
cost-effectively transported. Ethane converted to liquid products can be used as chemical
intermediates, in the polymer industry, and liquid fuels thus reducing our nation’s dependence on
crude oil. Existing commercial technologies used to convert ethane to liquid chemical products
are typically based on the indirect syngas route, such as dry and steam reforming. This is where a
complete oxidation of ethane is required to produce syngas followed by Fischer Tropsch reaction
to produce new hydrocarbons with higher molecular weight such as diesel [3,4]. In general,
indirect conversion of light alkanes by the syngas route is capital-intensive accounting for up to
50% of the capital cost incurred and energy-inefficient due to several intermediate production
steps [5]. Thus, direct catalytic conversion of light alkanes to higher-value products could be a
cost-effective solution in the future.
Direct conversion of light hydrocarbons (methane, ethane, etc.) is typically carried out over a
zeolite-supported metal catalyst that has a three-dimensional structure. The zeolite HZSM-5 has
a pore size and structure that allows for the formation and transportation of benzene and toluene
aromatic hydrocarbons, while the size of the pores is small enough to exclude larger highly
polyaromatics compounds [6]. Molybdenum supported on HZSM-5 is a well-studied catalyst and
has shown good activity towards the aromatization of methane and ethane [7-9]. When the

molybdenum is calcined on the HZSM-5 support, the MoO3 started to migrate and form a chelate
with the Bronsted acid sites on the surface and inside the pore structure of HZSM-5 zeolite
support [10]. When the catalyst is reduced in the presence of ethane, it forms Mo2C, which is
proposed to be the active phase of Mo that catalyzes the dehydrogenation step of ethane to
ethylene [11-15].
Different metal promoters are added and tested on Mo/ZSM-5 [5,16-18]. The addition of Fe to
Mo/ZSM-5 was found to improve the activity and stability of Mo/ZSM-5 [10,19-22]. The
addition of Fe suppressed the formation of low-temperature amorphous coke by the formation of
carbon nanotubes (CNT) which acted as a competitor for the coke precursors. The selectivity to
CNT formation was believed to increase gas diffusion into the pores due to the decreased
formation of amorphous surface coke that can cause pore blockage and thus rapid deactivation.
Our previous study on methane dehydroaromatization reaction suggested that the addition of Fe
to Mo/ZSM-5 catalyst acted as a stabilizer where little to no decrease in both aromatic selectivity
and yield were observed over five reaction and regeneration cycles [19]. Although, the direct
conversion of ethane to aromatic products by non-oxidative dehydroaromatization has been
studied for several decades, no commercialization yet for this technology due to technical
challenges. The direct transformation of ethane into aromatic hydrocarbons requires high
activation energies and typically yields low conversion to aromatics even with increasing
reaction temperatures [23,24]. The high reaction temperatures (500-700°C) which are needed to
facilitate the endothermic reaction, lead to low selectivity to aromatic products, coking, and
catalyst deactivation [25]. Conventional catalytic technologies have resulted in little or no
progress towards the commercialization of this technology. Thus, novel reaction engineering
concepts are required, not only to develop a robust catalyst but also to revolutionize the entire

process by applying nonconventional heating processes that reduce catalyst deactivation.
Microwave heating is a rapidly emerging tool in chemical reactions due to microwave’s unique
characteristics of selective material heating, rapid non-contact heating, and quick start-up and
shut-down operation [26,27].
Microwaves induced thermal effects within solids are generated due to the oscillating electric
and magnetic fields of microwaves. The thermal effects of the electric field are typically
generated under high frequencies when the dipoles are not able to align with the oscillating
electric field sufficiently. This causes friction and collisions between the dipoles that results in
the generation of heat [28,29]. Depending on the material, heating can be caused by electrical
resistance due to the collisions of charges being transported by the oscillating electrical current.
Thermal effects from the magnetic field contribute to microwave heating especially in metal
oxides, such as ferrites and other magnetic materials, by including eddy currents, hysteresis
losses, and magnetic resonance [30-32].
In heterogeneous catalysis, of gas-solid phases, microwave heating has demonstrated the ability
to be energy efficient by lowering bulk catalyst temperatures and achieving higher selectivity
due to suppressing further side reactions of undesired products [33-35]. The enhanced activity of
heterogenous reaction, observed under microwave irradiation, is attributed to microwave thermal
and non-thermal effects as mentioned in literature [36]. Microwave thermal effects can be
attributed to the localization of heat in a material when placed in the microwave field, also
known as a hot spot. [36-38] Zhang et al. reported the formation of hot-spots as large as 90 -
1000 micrometers and temperatures of 100 - 200°C higher than the bulk temperature of the
catalyst bed [39]. The formation of a hot spot can explain most of the catalytic effects generated
in a microwave-assisted reaction. However, microwave non-thermal effects have been reported

by Stiegman et al where the activation energy for the Boudouard reaction was lowered under
microwave reaction of carbon and CO2 [40], NO decomposition [41], ammonia synthesis
[42,43], and DHA [44] are also other reaction examples where non-thermal effects of microwave
energy were observed. A microwave field acting on a metal particle can generate the transfer of
free electrons from the conduction band within the metal particle to its surface. It is suggested,
that the sharing of these free electrons at the surface of the metal can facilitate dipole-dipole like
interactions between the catalyst metal sites and the gas phase molecules [42-44].
In this work, the direct ethane conversion to value-added products was studied under different
heating methods using the MWFB and CTFB reactor systems under the same ethane conversion.
A comparison between these two systems helped to elucidate the thermal effects (selective
heating) of microwave irradiation that result in mechanistic differences in product formation and
factors causing catalyst deactivation. The product formation rate and hydrocarbon selectivity of
the catalyst were compared. The catalyst was characterized post reaction to explain deactivation
differences in MWFB compared to that of a CTFB.
Materials and method:
2.1 Catalyst Preparation
The ZSM-5 zeolite was purchased from Zeolyst, Inc with a silica/alumina ratio of 23. The zeolite
was first calcined at 550 °C for four hours in air to convert the zeolite from the ammonium form
to its protonated form (HZSM-5). 4 wt% Mo-0.5 wt% Fe-ZSM-5 catalyst was prepared by
conventional incipient wetness co-impregnation with ammonium molybdate tetrahydrate and
iron (III) nitrate nonahydrate that were purchased from Acros Organics. The catalyst was then

dried at 105°C for five hours, the dried powder was then calcined in air at 550 °C for ten hours.
Catalyst powder were pressed and sieved between 70 and100 mesh particle size.
2.2 Catalyst pre-characterization
The XRD patterns of the powdered samples were captured using a PANalytical X’Pert Pro set to
45kV and 40mA. Samples were scanned from 5 ° to 45 ° (2) using CuK radiation.
Surface area measurements were carried out using a Micromeritics ASAP 2020+. The samples
were degassed for ten hours at 300 °C under vacuum. Nitrogen was used as an absorption gas to
calculate the Brunauer-Emmett-Teller (BET) model, and the t-plot method for micropore
volumes, external surface, area and micropore area.
Ammonia temperature programmed desorption (NH3-TPD) experiments were carried out using a
Micromeritics Autochem 2950 equipped with a thermal conductivity detector. The samples were
heated to 300 °C at 5°C/min for 60 min under an inert flow of He to remove moisture and then
cooled to 150 °C. The catalyst was then dosed with 15% ammonia in for 30 min at 30 mL/min. A
baseline was determined by flowing helium over the sample for 30 min at 30 mL/min to remove
weekly bounded ammonia from the catalyst surface. The samples were then heated to 700 °C at
5 °C/min in 30 mL/min He.
2.3. Catalyst Evaluation
2.3.1. Reactor Configuration

The catalytic reaction of ethane dehydroaromatization were evaluated under two different
heating modes; using microwave irradiation (MWFB) and conventional thermal (CTFB) heating.
The catalyst was evaluated at 375 °C (bulk catalyst temperature) in a Lambada MC1330-200
microwave reactor operated at a fixed frequency of 6.650 GHz. The only heat supplied to the
reaction was from the microwave heating of the catalyst. The bulk catalyst temperature and
quartz tube reactor temperature were measured by two different IR pyrometers. The reaction
temperature was maintained by controlling the forward microwave power window (between 0 to
180 W). The quartz tube with the catalyst penetrated through the H-plane axis of the waveguide
in a horizontal fixed bed setup. The catalyst was also evaluated under the same ethane
conversion using a conventional thermal horizontal furnace at 660 °C. The reaction temperature
was monitored using a K-type thermocouple in the catalyst bed. The reactor temperature
remained constant during the reaction time in both heating methods.
2.3.2. Reaction conditions and product analysis
Catalyst evaluations were carried out in a fixed bed setup with continuous flow in quartz tubes (8
mm-ID, 12mm-OD, and 381 mm-L). 0.4800g of pelletized MoFe/ZSM-5 was loaded into the
center of the quartz tube and held in place by quartz wool. The catalyst was pre-reduced at 550°C
for one hour in a 10% H2 – 90% N2 environment in a tubular furnace. The catalyst was then
heated by the designated heating method at 5 °C/min under an inert flow of N2 to reaction
temperature. The reaction was carried out in an atmosphere of 36% ethane balance of N2 at 50
ml/min. After 36 minutes of reaction time, the catalyst was allowed to cool to room temperature
in an inert flow of N2.

The reactant product gases were analyzed by an Agilent 3000A 4 column micro gas
chromatograph (micro-GC). The micro-GC was equipped with four columns consisting of a
Molecular sieve, Plot-U, Aluminum, and an OV-1. The four columns allowed for the complete
analysis of all reported products in under 3 mins per data point. The exit of each reactor was heat
traced at 150°C to the micro-GC inlet. Xylene and naphthalene were not tracked in this
experimental setup. Ethane conversion and product selectivity were calculated by equation (1)
and (2). Nitrogen was used as an internal standard.
(
)
푒푡ℎ푎ꢁ푒 푓푒푑
% −푒푡ℎ푎ꢁ푒 표푢푡 (%) × 100
(1)
( )
퐶표푛푣푒푟푠푖표푛 % =
푒푡ℎ푎ꢁ푒 푓푒푑 (%)
(
)
푆푒푙푒푐푡푖푣푖푡푦 표푓 푃푟표푑푢푐푡 % = 푝푟표푑푢푐푡 표푢푡 % × # 표푓 푐푎푟푏표ꢁ푠 ꢀꢁ 푝푟표푑푢푐푡 × 100
(2)
( )
(
)
[푒푡ℎ푎ꢁ푒 푓푒푑 % −푒푡ℎ푎ꢁ푒 표푢푡 (%)]×2
2.4. Post catalytic characterization
Temperature Programmed Oxidation (TPO) was performed on a Micrometrics Autochem 2950
HP equipped with a thermal conductivity detector was used to determine the TPO profile.
0.2000g of coked catalyst was loaded into the tubular reactor supported by quartz wool. The
coked catalyst was first heated to 300°C with a ramp rate of 5°C/min in a flow of N2 at 30
ml/min for two hours to remove moisture. The sample was cooled to ambient temperature where
it was then heated in the presence of 10% oxygen at a ramp rate of 5°C/min 700°C.
The thermogravimetric analysis (TGA) of the coked catalyst was carried out using a TA
Instruments SDT 650. The sample was loaded to 90-µL cups and placed into the balance and
allowed to purge with N2 for 1 hour. The sample was then heated to 300°C at 5°C/min and held
for 120 minutes under inert flow of N2 to dry the sample. The sample was cooled to 150°C and

held at that temperature for ten minutes. The gas flow was switched to 10% O2 and after 10
minutes the temperature was ramped at 5°C/min to 700°C and held for 10 minutes.
Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100. The
energy-dispersive spectroscopy (EDS) was obtained with an Oxford EDS. The samples were
taken from the center of the catalyst bed in both the microwave and the conventional fixed bed.
The samples were prepared by sonication in acetone for 15 minutes. The sample was then
loaded via a glass dropper onto a formvar coated copper grid. The sample was allowed to dry
suspended in air for one hour then placed in a grid storage container.
3. Results and Discussion
3.1. Catalyst Characterization
3.1.1. Surface Area and Acidity
Table 1 shows a summary of the surface area and NH3-TPD results of HZSM-5 support and the
freshly prepared MoFe/ZSM-5 catalyst. The addition of Mo and Fe to the unpromoted zeolite
resulted in a decrease of total surface area, including the micropore and the external surface area
of the catalyst. It has been shown previously that Mo is located mainly on the external surface
but also can diffuses into the pores. [45] Iron particles are expected to be located external of the
pores structure of the zeolite with this impregnation method [19].
The acidity of the HZSM-5 as seen in Table 1 has two desorption peaks at temperatures of 229oC
and 408oC. The higher desorption temperature peaks corresponds to the strong acid sites (mostly
Bronsted acid sites) and the lower desorption temperature corresponds to the weak acid sites
(mostly Lewis acid sites) [18,46]. The addition of Mo and Fe to the HZSM-5 support resulted in

a reduction in the temperature and the amount of ammonia desorbed. This suggests that the metal
promotors are anchored to the Bronsted acid sites located on the zeolite support.
3.1.2. XRD
Figure 1 illustrates the x-ray diffraction patterns for the fresh catalyst and compares the spectra
to the spent MWFB and spent CTFB catalysts. By comparing the diffraction patterns of the
fresh and spent catalysts at diffraction angles between 22 o and 25 o, almost no changes were
observed. No crystalline carbon peaks were observed which suggests the formation of mostly
amorphous carbon or that the amount of carbon was too little to detect. The carbon build-up also
contributed to the decrease in peak intensity after reaction which could be due to the formation of
coke on the surface of the zeolite. This indicates that the ZSM-5 maintained its crystallinity in
both the conventional thermal and microwave reactors. Metal oxide peaks were not observed in
the fresh and spent catalysts, this could suggest that the metal loadings were evenly distributed,
and the metal particles were of small particle size that could not be detectable under the XRD
instrument.
3.2. Catalyst Performance in MWFB vs CTFB towards ethane DHA
The catalyst performance in MWFB and CTFB is compared in Figures 2-4. In Figure 2 (a) the
ethane conversion is plotted as a function of time on stream during the reaction. The catalyst
was tested over different reaction temperature thermally (CTFB) and the conversion at which the
two methods coincide was determined. The CTFB required a bulk temperature of 660 °C to
obtain the same conversion of ethane as in the MWFB at 375 °C. This temperature difference is
attributed to the mechanism of selective microwave heating. The zeolite support is an inefficient

absorber of microwaves due to high SiO2 content, therefore most of the heat generated comes
from the selective heated metal sites of Mo and Fe species on the zeolite surface. For direct
ethane conversion on ZSM-5 zeolite, ethane is firstly converted to ethylene on metal sites
(Mo2C), and the aromatics are produced from the oligomerization of ethylene intermediate [20].
In other words, due to the selective heating feature of microwave irradiation, the microscopic
“hot spot” temperature on metal sites can reach around 660 °C while the bulk catalyst
temperature is at 375 °C in the MWFB reactor. The deactivation trend of the MWFB and the
CTFB was very similar over the course of the 36 minutes of the reaction. The initial absorbed
power required by the microwave system was 38 watts to maintain the bulk catalyst temperature
at 375 °C under inert flow. After the start of the reaction, only 11 watts were needed to maintain
the bulk catalyst temperature at 375 °C. This can be attributed to the formation of coke, given
that carbon is a good absorber of microwaves.
As seen in Figures 2-4, the formation rate and selectivity of methane, benzene, and toluene were
higher in the CTFB than in the MWFB reactor throughout the reaction. However, the formation
rate and selectivity of ethylene was much higher in the MWFB than in the CTFB reactor.
Ethylene acts as the major intermediate to aromatic formation [5]. It is suggested that the
microwaves induce dipoles on the metal sites located on the surface of the catalyst [41-44].
These dipoles could facilitate electron transfer between ethane and the metal site to accelerate the
formation of ethylene via the dehydrogenation process. At 10 minutes’ time on stream, Figure 4
(b) shows that the CTFB had an 11.5 % carbon selectivity to ethylene, whereas the MWFB had a
higher (38%) carbon selectivity to ethylene. Although the ethane deactivation curve for MWFB
and CTFB appeared to be similar, the coke formation mechanism and type of coke are different
between the two method, coke formation is further discussed below. Different coke would affect

active sites (dehydrogenation sites and aromatization sites) differently, which could affect
temperature distribution on the catalyst surface differently for the MWFB, therefore we observed
different product selectivity. The low aromatic selectivity could be due to the inefficient
dehydroaromatization of ethylene. Y. kuo et al. [47] reported different surface temperature affect
selectivity differently for ethylene dehydroaromatization. It was reported that the benzene
selectivity for ethylene dehydroaromatization over a Ga/ZSM-5 catalyst sharply decreased from
20% to 0%, between 400-300oC, while maintaining a high ethylene conversion. The low bulk
temperature of 375oC along with possible uneven heat distribution throughout the catalyst bed
could allow only parts of the catalyst bed to be fully utilized throughout the reaction.
3.3. Investigation of Catalyst Deactivation
3.3.1. TEM
TEM images of the MWFB spent catalyst are shown in Figure 5, where the sample was taken
from the center of the catalytic bed. The EDS of the 4 spots shown in Figure 5 confirms that the
dark spots are made up of Mo and Fe agglomerated metal particles on the catalyst surfaces.
Several large agglomerations of metal particles up to 60 nm in diameter can be distinguished.
The TEM images of CTFB spent catalyst and EDS spots can be seen in Figure 6. The EDS
confirms the dark spots are agglomerated metal Mo and Fe particles of much smaller diameter.
The largest metal agglomerations on the CTFB spent sample are of only between 10 and 20 nm.
The MWFB spent sample’s larger particle agglomerations are attributed to the selective heating
nature of the microwave. The results imply that anchoring Mo and Fe in the structure of ZSM-5
is necessary to prevent metal agglomeration on the surface due to the mobility of Mo and Fe on

the zeolite surface. The fresh catalyst TEM images are not given since the fresh catalyst had little
to no metal agglomerations visible.
Furthermore, the formation of carbon structures were observed in both the MWFB and CTFB
spent samples, agreeing with literature on Fe-promoted catalysts under thermal conventional
reaction conditions [10,19,22,44]. The formation of CNTs with distinct tubular walls and
diameters up to 20-nm can be seen in CTFB samples in Figure 6. In comparison, the MWFB
spent sample resulted in the formation of unordered CNT-like structures with diameters as large
as 60-nm. The CNTs formed have little to no open voids with several encapsulations within the
tube structure. CNTs are very good microwave absorbers as well as electric conductors [48, 49].
The results suggest that the coke formations themselves can be altered by electron restructuring
due to the induced electron transfer by the MWFB fields at the agglomerated metal sites or by
rapid thermal gradients at the metal particle site.
3.3.2 Carbon analysis by TPO and TGA
The TPO and TGA profiles of the spent catalyst obtained are shown in Figure 7. TPO profiles
are used for qualitative purpose to determine how structurally ordered coke is based on peak
shape and oxidation burn-off temperature. The TPO and TGA profiles of spent CTFB sample
suggest that at least three types of coke, were formed due to the appearance of three temperature
peaks at 459 °C, 542 °C, and 601 °C. The MWFB spent sample temperature profile only had one
distinct peak at 562 °C. This temperature is between the middle and the higher temperature peak
of the CTFB and is believed to contain two of the three CTFB peaks. The microwave was more
selective to forming the higher burn-off temperature coke species and less selective towards the

formation of the lower temperature burning amorphous carbon. Amorphous carbon has been
shown to lead to faster deactivation due to pore and acid site blockage [10,50]. The decreased
selectivity to amorphous carbon suggests that the catalyst could remain active for longer periods
if the MW power reaching the metal active sites remain constant. The total weight loss in the
spent MWFB was only 4.04% compared to a total weight loss of 9.839% for the spent CTFB.
Furthermore, the total weight loss suggest that the fixed bed was more selective to overall coke
formation throughout the 36 minutes of reaction time.
4. Conclusion
In this study, it was experimentally determined that the microscopic “hot spot” temperature of
metal particles on the zeolite could reach about 660 °C while the bulk catalyst temperature was
only 375 °C due to the selective heating feature of microwave. The CTFB had higher formation
rates and selectivity’s to hydrogen, methane, benzene and toluene than the MWFB. However, the
MWFB had higher formation rates and selectivity’s to ethylene. The low selectivity to aromatics
was not due to the lack of ethylene in the stream but, was attributed to low bulk temperature and
uneven heat distribution resulting in the inability of ethylene to diffuse into the catalyst pore
structure efficiently. Metal particle agglomeration is a key factor of catalyst deactivation, and
more severe agglomeration was observed in MWFB. TEM analysis also revealed that the MWFB
induced the formation of nonuniform CNT with little to no tubular structure up to 60 nm in
diameter, while the CTFB had very uniform CNT formation with distinct wall structures up to 20
nm in diameter. The presence of multiple types of coke in the CTFB was observed, whereas the
MWFB only contained one type of higher burn-off temperature coke. TGA confirmed that the
CTFB resulted in more carbon formation than the MWFB sample which suggests that the lower

bulk temperature prevented the formation of amorphous carbon which is one of the leading
causes of zeolite deactivation. Future work will be focused on the non-thermal effects of
microwave irradiation, and the reaction kinetics of microwave-assisted reactions.
CRediT author statement
Brandon Robinson: Methodology, Investigation, Writing - Original Draft. Ashley Caiola:
Writing - Review & Editing, Visualization, Investigation. Xinwei Bai: Writing - Review &
Editing. Victor Abdelsayed: Supervision, Writing - Review & Editing. Dushyant Shekhawat:
Supervision, Writing - Review & Editing. Jianli hu: Project administration, Conceptualization,
Supervision.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge financial support from West Virginia Higher Education Policy
Commission under the grant number HEPC.dsr.18.7. The authors also acknowledge financial
support from the National Energy Technology Laboratory, Morgantown, WV 26507, and the
Oak Ridge Institute for Science and Education, 1299 Bethel Valley Road, Oak Ridge, TN 37830

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20000
15000
10000
5000
0
(a) H/ZSM-5
(b) MWFB
(c) CTFB
(a)
(b)
(c)
5
10
15
20
25
30
35
2
Figure 1. XRD patterns of (a) Fresh MoFe/ZSM-5, (b) MWFB spent catalyst, and (c)CTFB
spent catalyst.

Figure 2. Comparison of the conversion and formation rates for the MoFe/ZSM-5 catalyst in
MWFB at 375oC and CTFB at 660oC. (a) percent ethane conversion, (b) hydrogen formation, (c)
benzene formation, and (d) toluene formation.

3.0E-04
2.5E-04
2.0E-04
1.5E-04
1.0E-04
5.0E-05
0.0E+00
2.0E-04
1.6E-04
1.2E-04
8.0E-05
4.0E-05
0.0E+00
(a)
(b)
4
8
12 16 20 24 28 32 36
Time (min)
4
8
12 16 20 24 28 32 36
Time (min)
MWFB
CTFB
Figure 3. Comparison of the formation rates for the MoFe/ZSM-5 catalyst in MWFB at 375oC
and CTFB at 660oC. (a) methane formation and (b) ethylene formation.

30
25
20
15
10
5
60
50
40
30
20
10
0
(a)
(b)
0
4
8
12 16 20 24 28 32 36
4
8
12 16 20 24 28 32 36
(c)
25
20
15
10
5
8
7
6
5
4
3
2
1
0
(d)
0
4
8
12 16 20 24 28 32 36
4
8
12 16 20 24 28 32 36
Time (min)
Time (min)
MWFB
CTFB
Figure 4. Compares the hydrocarbon selectivity’s of the major carbon products for the MWFB at
375oC and CTFB at 660oC. (a) methane selectivity, (b) ethylene selectivity, (c) benzene
selectivity, and (d) toluene selectivity.