Reversible Nature of Coke Formation on Mo/ZSM-5 Methane Dehydroaromatization Catalysts

Article abstract of DOI:10.1002/anie.201902730

Non-oxidative dehydroaromatization of methane over Mo/ZSM-5 zeolite catalysts is a promising reaction for the direct conversion of abundant natural gas into liquid aromatics. Rapid coking deactivation hinders the practical implementation of this technology. Herein, we show that catalyst productivity can be improved by nearly an order of magnitude by raising the reaction pressure to 15 bar. The beneficial effect of pressure was found for different Mo/ZSM-5 catalysts and a wide range of reaction temperatures and space velocities. High-pressure operando X-ray absorption spectroscopy demonstrated that the structure of the active Mo-phase was not affected by operation at elevated pressure. Isotope labeling experiments, supported by mass-spectrometry and ¹³C nuclear magnetic resonance spectroscopy, indicated the reversible nature of coke formation. The improved performance can be attributed to faster coke hydrogenation at increased pressure, overall resulting in a lower coke selectivity and better utilization of the zeolite micropore space.

Full text of DOI:10.1002/anie.201902730

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Accepted Article
Title: Reversible Nature of Coke Formation on Mo/ZSM-5 Methane
Dehydroaromatization Catalysts
Authors: Nikolay Kosinov, Evgeny A. Uslamin, Lingqian Meng,
Alexander Parastaev, Yujie Liu, and Emiel J. M. Hensen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902730
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10.1002/anie.201902730
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Reversible Nature of Coke Formation on Mo/ZSM-5 Methane
Dehydroaromatization Catalysts
Nikolay Kosinov*, Evgeny A. Uslamin, Lingqian Meng, Alexander Parastaev, Yujie Liu, Emiel J. M.
Hensen*
Abstract: Non-oxidative dehydroaromatization of methane over
Mo/ZSM-5 zeolite catalysts is a promising reaction for the direct
conversion of abundant natural gas into liquid aromatics. Rapid
coking deactivation hinders the practical implementation of this
technology. Here, we show that catalyst productivity can be
improved by nearly an order of magnitude by raising the reaction
pressure to 15 bar. The beneficial effect of pressure was found for
workers in the early 2000s [17] and, more recently, by Fila et al.
[18]. It was found that applying reaction pressures in the 3 – 6
bar range results in a more stable catalytic performance at the
expense of the maximum attainable benzene yield. The results
of a recent computational modelling study performed by Kee et
al. demonstrated that increasing pressure should result in a
higher benzene selectivity over naphthalene, the latter being
generally considered a coke precursor [19].
different Mo/ZSM-5 catalysts and
a wide range of reaction
temperatures and space velocities. High-pressure operando X-ray
absorption spectroscopy demonstrated that the structure of the
active Mo-phase was not affected by operation at elevated pressure.
Isotope labeling experiments, supported by mass-spectrometry
and 13C nuclear magnetic resonance spectroscopy, indicated the
reversible nature of coke formation. The improved performance can
be attributed to faster coke hydrogenation at increased pressure,
overall resulting in a lower coke selectivity and better utilization of
the zeolite micropore space.
According to Le Chatelier's principle, the equilibrium of
methane dehydroaromatization reaction is shifted towards
methane at increasing pressure:
6 ꢀꢁ4 ⇄ ꢀ6ꢁ6 + 9 ꢁ2
We also note that coke formation should be suppressed stronger
by increasing pressure (as nearly two times more molecules are
produced than converted):
ꢀꢁ4 ⇄ ꢀꢁꢂ
) + ꢃ( ) ꢁ2
ꢄ→2
(
ꢂ→0
Methane dehydroaromatization (MDA) over Mo-containing
zeolite catalysts is a promising reaction for the direct conversion
of natural gas to a mixture of liquid BTX aromatics (mainly
benzene) and hydrogen [1]. This thermodynamically limited non-
oxidative reaction requires temperatures as high as 600 –
800 °C to achieve a significant methane conversion. A major
challenge in the realization of a practical MDA process is the
rapid deactivation of Mo/ZSM-5 catalysts. Recent advances in
our understanding how these catalysts work at the atomic level
can aid in strategies to reduce the coking deactivation. It has
been demonstrated that the mechanism involves a pool of
(radical) hydrocarbon reaction intermediates relevant to the
formation of aromatics [2]. The structure of the active
molybdenum centers has been thoroughly studied by means
of 95Mo NMR [3], operando Raman [4] and XAS spectroscopy [5],
and other techniques [6]. There is a growing consensus that the
active centers are monomeric or dimeric molybdenum
(oxy)carbidic species which are stabilized inside the zeolite
pores, while larger Mo2C species on the external surface are
mere spectators [7]. Various methods to enhance the activity
and stability of Mo/ZSM-5 catalysts have been developed as
well and they include: (i) application of hydrogen-selective
membranes to remove hydrogen from the reaction zone and
shift the equilibrium towards aromatic products [8]; (ii) adsorptive
or oxidative scavenging of hydrogen for the same purpose [9];
(iii) periodic pulsing of small amounts of oxygen for the selective
coke combustion [10]; (iv) use of oxygen-permeable membranes
for a controlled supply of a small amount of oxygen for removal
of coke species [11]; and (v) reaction-regeneration cycling via
combustion or reduction of coke [12]. Furthermore, the influence
of such parameters as temperature and space velocity of
methane [13] and continuous co-feeding of hydrogen [14],
oxygen [15] and oxygenates [16] has been studied in great detail.
The reaction pressure remains a poorly explored parameter.
Nearly all laboratory MDA studies have been performed at
atmospheric pressure. In fact, to the best of our knowledge the
only reports on the effect of pressure were by Ichikawa and co-
To illustrate this point, Fig. S1 shows the results of
a
thermodynamic analysis. The maximum thermodynamic yields
of the main MDA products, except for ethane, decrease with
increasing pressure. The effect of pressure on product formation
increases in the order: ethane (no effect) << ethylene = xylene <
toluene
<
benzene
<
naphthalene << graphite (coke).
Accordingly, this thermodynamic analysis suggests that
performing the reaction at elevated pressure might lead to a
decreased coke selectivity and, therefore, higher catalyst
productivity [20].
In this work, we show that operation of the MDA reaction at
elevated pressure is an efficient way to increase catalyst lifetime
and total hydrocarbon productivity, by decreasing the coke
selectivity. Transient kinetic measurements using 13C-labelled
methane evidence the reversible nature of coke formation during
the MDA reaction. At elevated pressure, coke hydrogenation
becomes faster, effectively resulting in a slower build-up of
carbonaceous deposits. We also present operando high-
pressure X-ray absorption near-edge structure (XANES)
spectroscopy results, demonstrating that the speciation of the
active Mo species is not affected by high-pressure operation.
For the experiments, we used a 2%Mo/ZSM-5 (Si/Al 13, see
Table S1 for physico-chemical properties). Earlier, we have
shown that in such a catalyst nearly all Mo atoms are involved in
the MDA reaction [2a]. MDA activity measurements were
performed at 700 °C in a quartz reactor (i.d. 4 mm, o.d. 8 mm),
which can be safely operated up to a pressure of 20 bar. The
catalytic results (Fig. 1 and Table S2) demonstrate that raising
the reaction pressure results in a higher catalyst productivity.
The cumulative yield of aromatic products is substantially higher
at elevated reaction pressure. For instance, the cumulative
amount of benzene, toluene and xylenes obtained at 15 bar is
about one order of magnitude higher than that at 1 bar. About
four times more methane could be converted at 15 bar than at
atmospheric pressure, mainly due to a lower selectivity to
carbonaceous deposits that deactivate the catalyst. We observe
that with increasing pressure the formation of ethane is
promoted due to the fact that its production is least influenced by
pressure. The main reason for the enhanced hydrocarbon
product yields is a decreased total coke selectivity (from 32.6%
Laboratory of Inorganic Materials and Catalysis
Eindhoven University of Technology
PO Box 513, 5600 MB Eindhoven, The Netherlands
E-mail: N.A.Kosinov@tue.nl, E.J.M. Hensen@tue.nl
1
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10.1002/anie.201902730
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to 10.6%) when the pressure is increased from 1 to 15 bar. Next,
we investigated how the Mo weight loading, the reaction
temperature and the space velocity influence the catalytic
performance by carrying out reaction experiments at 1 bar and
10 bar. Fig. S2 and Table S3 emphasize that for all Mo loadings
a higher total productivity is obtained at 10 bar. At low Mo
loading a significantly higher coke selectivity is observed in line
with previous studies [12c]. Also, a higher productivity is
observed at all temperatures in the 600 – 800 °C range (Fig. S2
and Table S4). While the coke selectivity gradually increased
with temperature at 1 bar; at 10 bar the coke selectivity was
around 10% between 600 °C and 700 °C and then significantly
increased when the temperature was raised to above 750°C.
Overall, the beneficial effect of higher reaction pressure on
(lower) coke selectivity became smaller at higher reaction
temperature. Given that the total amount of coke formed at
temperatures above 700 °C was much larger than the maximum
amount of coke that can be accommodated inside the pores (Fig.
S3), we infer that radical reactions of methane decomposition to
carbon on the external surface are likely to play a significant role
at higher temperature. Finally, we established that operation at
10 bar is beneficial in a wide GHSV range from 3000 h-1 to
22500 h-1 (Fig. S2 and Table S5). This finding is particularly
important, because for an equilibrium-limited reaction such as
MDA it is favorable to operate at a high space velocity to
maximize the space-time yield of products.
Figure 1. Effect of pressure on catalytic performance of 2%Mo/ZSM-5 catalyst. (a) Yields of benzene and naphthalene (with inset highlighting the induction
period), (b) ethane and ethylene, and (c) toluene and xylene. (d) Overall product distributions and total amounts of converted methane. (e) Equilibrium and
maximum observed yields of benzene. (f) Equilibrium yield of graphite plotted with observed total coke selectivity. Reaction conditions: 700 °C, GHSV 15000 h-1,
900 min.
To investigate the effect of pressure on the structure of Mo-
species, we carried out a XANES study. The catalyst samples
were exposed to the methane feed at different pressures (from
0.5 to 5 bar), whilst increasing the temperature from ambient to
700 °C (ramp rate of 5°/min) followed by an isothermal period of
1 h. The XANES spectra recorded at 1 bar and 5 bar and
represented as heat maps as a function of temperature (Figs.
2a,b) display several important features. Expectedly, Mo
reduction starts at significantly lower temperature at higher
methane pressure (~500 °C at 5 bar and ~600 °C at 1 bar), as
manifested by the disappearance of the pre-edge feature and
structural changes in the Mo species before and after the
catalyst activation with varying pressure.
Fig. 2d compares the trends in the edge positions derived
from XANES and simultaneously recorded mass spectrometry
traces of benzene (m/z = 78). Although the reduction of the Mo-
phase occurs faster at elevated pressure, the induction (time for
benzene yield to reach the maximum) is significantly slower. In
our previous work, we distinguished two processes: (i) activation
involving the reduction of Mo-oxo species to active Mo-centers
and (ii) induction involving the formation of proton-deficient
aromatic intermediates stabilized inside the pores [2a]. These
intermediates are involved in the catalytic cycle and necessary
for the MDA reaction to occur. The longer induction period at
elevated pressure suggests that, similar to the coke species, the
polyaromatic hydrocarbon pool precursors are formed slower.
the shift of the rising edge feature to
a lower energy.
Furthermore, we note that neither spectra before reduction
started (i.e., at 400 °C) nor those at the final reduction
temperature of 700 °C changed as a function of the pressure
(Fig. 2c). This finding indicates that there are no substantial
2
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Figure 2. Operando XANES analysis of the effect of pressure on the reduction of initial Mo(VI) phase. Intensity countur maps constructed from spectra recorded
during programmed CH4 reduction at (a) 1 bar and (b) 5 bar. Indicated are pre-edge region, rising edge region related to the reduction of Mo(VI)-oxo precursors
to active Mo-species and region at 20075 eV related to a gradual agglomeration of the Mo-species. (c) XANES spectra of 2%Mo catalysts recorded at different
preccure during CH4 TPR at 400 °C and 700 °C. (d) Intensity of the detected benzene m/z=78 MS signal during the operando experiments together with the edge
energies (defined as energies at half-edge) in 1 bar and 5 bar experiments.
To probe the processes occurring on the surface of the
catalyst during MDA reaction we performed an isotope labelling
study as illustrated by Fig. 3. In these experiments we first pre-
heated the catalyst till 700 °C in a flow of 12CH4 and then
switched the feed to labelled 13CH4 to form 13C-enriched carbon
species inside the pores. After 20 min in 13CH4 flow we either
quickly cooled the catalyst (Fig. 3a) or switched to 12CH4 and
kept the catalyst at 700 °C for additional 100 min to study the
removal of surface 13C atoms upon exchange with gas-
phase 12CH4 (Fig. 3b).
During the reaction the gas phase composition was monitored
by GC and MS analyses (Fig. S4). After the reaction the spent
catalysts were collected and characterized by 13C NMR
spectroscopy and TPO-MS to determine the surface 13C content
before and after exchange with 12CH4. Figs. 3c,d demonstrate
that it is possible to observe the interaction of surface and
gaseous carbon atoms by analyzing the ratio between m/z = 78
and 79 MS signals, corresponding to benzene molecules with 0
and 1 13C labels, respectively. Initially, in the 12CH4 flow the m/z
78/79 ratio of 16 is consistent with the natural abundance of 13C
(1.1%). After the switch to 13CH4, the m/z 78/79 ratio decreased,
indicating formation of 13C-rich benzene molecules in the
absence of gaseous 12CH4. When the flow was switched back
to 12CH4, a slow increase of the m/z 78/79 ratio was observed.
The transient increase corresponds to the exchange of gas-
phase 12C atoms with surface 13C atoms. Based on these data,
we can estimate the rate of removal of 13C atoms from the
catalyst surface via benzene molecules. We note here that the
formation of labelled benzene molecules can occur via two main
pathways: (i) direct interaction of labeled hydrocarbon pool
molecules with initial products of methane activation and (ii)
hydrogenation of labelled surface species to 13C methane
followed by its aromatization. Both these processes contribute to
the observed isotope exchange.
Fig. 4a shows that the initial rate of isotope exchange at 5
bar is significantly higher than that at 1 bar (ca. 6 times). This
difference points to a higher rate of coke hydrogenation at
elevated pressure. The nature and the initial amount of the 13C
carbon species were similar at both pressures (see Fig. S5 for
detailed HETCOR 1H-13C NMR spectra). Furthermore,
quantitative 13C NMR spectroscopy (Figs. 4b,c) analysis
demonstrated that extensive exchange of sp2 carbon atoms
occurs, that is to say that more than 40% of 13C atoms were
removed from the surface after 12CH4 treatment at 5 bar. In line
with MS results, a much lower value of 19% was obtained for the
1 bar experiments.
Figure 3. Scheme of isotope labelling experiments when 2%Mo catalyst was
preheated in a flow of 12CH4 to 700°C, then kept under 13CH4 flow for 20 min
and finally was either quickly cooled down (a) or kept under 12CH4 flow for 100
min before cooling down. Moment when the temperature of 700°C was
reached and the switch from 12CH4 to 13CH4 was performed is taken as time =
0. MS-derived intensity of 13CH4 m/z =17 signal and transient response of
benzene isotopologues with masses 78 and 79, demonstrating gradual
removal of 13C from the catalyst surface during 1 bar (c) and 5 bar (d) labelling
experiments.
3
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catalyzed by Mo, also increases with temperature. We explain
these observations by (i) stronger diffusion limitations,
experienced by oxygen and combustion products, with
increasing occupancy of the zeolite pores with coke species
[22]; and (ii) a higher efficiency of Mo-catalyzed combustion at
increased temperature. The dependence of total methane
conversion capacity on coke selectivity rules out the extensive
formation of coke species on the external surface at 700 °C (Fig.
S3). Clearly, reversible growth of carbonaceous species at
elevated pressure results in the formation of denser carbon
structures and higher pore occupancy.
Figure 5. TG analysis of spent samples after MDA reaction at different
pressure. (a) total coke content and (b) TG and DTG (inset) profiles. Reaction
conditions: 700 °C, GHSV 15000 h-1, 900 min.
Figure 4. (a) Rates of removal of 13C surface atoms with benzene molecules
after the switch from 13CH4 to 12CH4 methane feed. Quantitative 13C NMR
spectra of 2%Mo catalysts preheated in a flow of 12CH4 to 700°C, then kept
under 13CH4 flow for 20 min and immediately cooled down or kept
under 12CH4 flow for 100 min before cooling at (b) 1 bar and (c) 5 bar.
To summarize, rapid deactivation due to coke deposition is a
common feature of Mo/ZSM-5 catalysts used in the
dehydroaromatization of methane to aromatics. We demonstrate
that performing the MDA reaction at elevated pressure leads to
an increased rate of surface species hydrogenation, lower
overall coke selectivity and higher methane conversion capacity.
A more than 8-fold increase in the aromatics productivity of
2%Mo/ZSM-5 catalyst was observed by increasing the pressure
from 1 bar to 15 bar. The improvement of the catalytic
performance at increased pressure is independent of the Mo
loading, reaction temperature and methane space velocity. It is
also noted that higher pressure results in higher selectivity
towards more valuable products such as toluene and xylene.
Operando XAS results evidence that, although the Mo-oxide
precursor is reduced easier under elevated pressure, the
structure of the active sites during the actual reaction is
independent of pressure. Notably, the total amount of coke
deposits is increased at elevated pressure, showing how
reversible growth of coke deposits results in a higher pore
utilization. The current finding that elevated pressure operation
has such a strong positive effect on the performance of
Mo/ZSM-5 is also important for practical methane valorization,
as it eliminates the need to depressurize the natural gas stream
before aromatization.
It is noteworthy that the reversible formation of coke at higher
pressure does not result in a lower amount of coke formed. Fig.
5a shows that, although the coke selectivity significantly
decreases with increasing pressure, the actual amount of coke
accumulated inside the zeolite pores after complete deactivation
increases. At a pressure of 15 bar, nearly 60% more coke was
formed as compared to the experiment carried out at 1 bar.
Earlier, we found that, during the MDA reaction at 700 °C, coke
species grow predominantly inside the zeolite pores [2]. Based
on detailed characterization, we proposed that the coke deposits
with an overall stoichiometry of about CH0.4 are most likely
acenes, occluded in the straight channels (Fig. S9). With a
structural coke model outlined in the Fig. S9, we estimate the
maximum coke capacity of ZSM-5 to be 270 mgcoke/gzeolite
.
Carrying out the MDA reaction at 1 bar does not lead to a
complete filling of the zeolite micropores: both our previous work
and other literature showed that the typical coke content of spent
Mo/ZSM-5 catalysts was limited to 80 – 180 mgcoke/gcatalyst at
atmospheric pressure [21]. With increasing pressure, however, a
higher coke content of the pores is attained. Furthermore, both
combustion peaks in DTG profiles (Fig. 5b), corresponding to
coke species in close proximity (low temperature peak) and
distant from Mo-centers (high temperature peak), shift to higher
temperature. The fraction of coke species, whose combustion is
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Entry for the Table of Contents
COMMUNICATION
Under pressure Mo/ZSM-5 methane
dehydroaromatization catalysts display
Nikolay Kosinov*, Evgeny A.
Uslamin, Lingqian Meng,
Alexander Parastaev, Yujie
Liu , Emiel J.M. Hensen*
much
better
methane
dehydro-
aromatization performance in a wide range
of conditions. An eightfold increase in
aromatic productivity is obtained by
increasing the reaction pressure from 1 to
15 bar. These findings are rationalized by
reversible growth of coke species and
better utilization of the zeolite micropores.
Reversible Nature of Coke
Formation on Mo/ZSM-5
Methane
Dehydroaromatization
Catalysts
6
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