Dual Active Sites on Molybdenum/ZSM-5 Catalyst for Methane Dehydroaromatization: Insights from Solid-State NMR Spectroscopy

Article abstract of DOI:10.1002/anie.202017074

Methane dehydroaromatization (MDA) on Mo/ZSM-5 zeolite catalyst is promising for direct transformation of natural gas. Understanding the nature of active sites on Mo/ZSM-5 is a challenge for applications. Herein, using ¹H{⁹⁵Mo} double-resonance solid-state NMR spectroscopy, we identify proximate dual active sites on Mo/ZSM-5 catalyst by direct observation of internuclear spatial interaction between Br?nsted acid site and Mo species in zeolite channels. The acidic proton–Mo spatial interaction is correlated with methane conversion and aromatics formation in the MDA process, an important factor in determining the catalyst activity and lifetime. The evolution of olefins and aromatics in Mo/ZSM-5 channels is monitored by detecting their host–guest interactions with both active Mo sites and Br?nsted acid sites via ¹H{⁹⁵Mo} double-resonance and two-dimensional ¹H–¹H correlation NMR spectroscopy, revealing the intermediate role of olefins in hydrocarbon pool process during the MDA reaction.

Full text of DOI:10.1002/anie.202017074

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 10709–10715
International Edition: doi.org/10.1002/anie.202017074
German Edition: doi.org/10.1002/ange.202017074
Zeolites
Dual Active Sites on Molybdenum/ZSM-5 Catalyst for Methane
Dehydroaromatization: Insights from Solid-State NMR Spectroscopy
Wei Gao, Guodong Qi, Qiang Wang, Weiyu Wang, Shenhui Li, Ivan Hung, Zhehong Gan,
Jun Xu,* and Feng Deng
Abstract: Methane dehydroaromatization (MDA) on Mo/
ZSM-5 zeolite catalyst is promising for direct transformation
of natural gas. Understanding the nature of active sites on Mo/
typically between 873 and 1073 K, to achieve equilibrium with
[
5,6]
ca. 10% conversion and up to 80% benzene selectivity.
The commercial application of the MDA process is hampered
1
95
[7–9]
ZSM-5 is a challenge for applications. Herein, using H{ Mo}
double-resonance solid-state NMR spectroscopy, we identify
proximate dual active sites on Mo/ZSM-5 catalyst by direct
observation of internuclear spatial interaction between Brønst-
ed acid site and Mo species in zeolite channels. The acidic
proton–Mo spatial interaction is correlated with methane
conversion and aromatics formation in the MDA process, an
important factor in determining the catalyst activity and
lifetime. The evolution of olefins and aromatics in Mo/ZSM-
by the rapid deactivation of catalysts.
Considerable
research effort has been invested in improving the MDA
catalyst and process with focus on the understanding of the
[10,11]
active sites and reaction mechanism.
Various spectroscopic techniques including NMR, EX-
AFS and Raman and theoretical calculations as well as kinetic
analysis have been employed to reveal the active Mo-phase
[12–21]
on Mo/ZSM-5.
It is generally accepted partially reduced
and/or carburized Mo-oxo species in zeolite channels such as
Mo C and oxycarbidic Mo (MoO C ) serve as the active sites
5
channels is monitored by detecting their host–guest inter-
2
x
y
actions with both active Mo sites and Brønsted acid sites via
H{ Mo} double-resonance and two-dimensional H– H cor-
relation NMR spectroscopy, revealing the intermediate role of
olefins in hydrocarbon pool process during the MDA reaction.
while the Mo carbides on the external surface are less active.
Although the exact structure of the active Mo species is still in
debate, previous reports underlined the importance of the
1
95
1
1
[22–25]
bifunctionality of Mo/ZMS-5 for the MDA reaction:
dehydrogenation and oligomerization of methane occur on
Introduction
active Mo sites forming C intermediates such as ethene and
2
acetylene, followed by cyclization producing aromatics and
naphthalene on the Brønsted acid sites (BAS) in zeolite. The
subsequent transformation of hydrocarbons can be under-
stood by the hydrocarbon pool mechanism that resembles the
well-established organocatalytic process in methanol-to-hy-
Owing to the depleting of crude oil and increasing
availability of natural gas, direct conversion of methane, the
main component of the natural gas, into value-added chem-
icals is of great academic and industrial interest. Methane
dehydromatization (MDA) under non-oxidative condition on
Mo and other transition metal (W, Fe, V, Cr, etc.) modified
ZSM-5 zeolites has attracted increasing attention owing to the
high selectivity to hydrocarbons particularly aromatics.
However, the MDA reaction is a thermodynamically unfa-
vored process, which necessitates high reaction temperature,
[26–28]
drocarbon reaction.
The hydrocarbon pool can be simply
considered as a catalytic scaffold in zeolite, constituted by the
organics trapped in zeolite channels or cages, to which the
reactant is added and products are removed in a catalytic
cycle. The confined aromatics and their derivatives were
proposed to play an important role in the hydrocarbon pool
mechanism of the MDA reaction. The oxycarbidic Mo species
[1–4]
[18,26]
initiates the hydrocarbon pool in the early stage,
while
[
*] W. Gao, Dr. G. D. Qi, Dr. Q. Wang, Dr. W. Y. Wang, Dr. S. H. Li,
Prof. Dr. J. Xu, Prof. Dr. F. Deng
a close correlation between the benzene formation and the
[
22]
Brønsted acidity was found in the MDA reaction. Never-
theless, how the Mo species and BAS function together to
achieve a full activity is still unclear.
National Center for Magnetic Resonance in Wuhan, State Key
Laboratory of Magnetic Resonance and Atomic and Molecular
Physics, Wuhan Institute of Physics and Mathematics, Innovation
Academy for Precision Measurement Science and Technology,
Chinese Academy of Sciences
Concerning the acidity of zeolite, it was found that Mo-
[29]
modified silicalite-1 exhibited low MDA activity,
while
Wuhan 430071 (P. R. China)
E-mail: xujun@wipm.ac.cn
methane activation by the BAS solely is negligible and the Mo
[22]
species is necessary to assist in the formation of benzene.
W. Gao
Comparing the component alone, the mixture of Mo carbides
with H-ZSM-5 zeolite exhibits higher methane conversion,
but the yield of benzene is an order of magnitude less than on
University of Chinese Academy of Sciences
Beijing 100049 (China)
Dr. I. Hung, Dr. Z. H. Gan
National High Magnetic Field Laboratory
[22,30]
Mo/ZSM-5 with both exchanged Mo sites and BAS.
The
previous investigations manifested the crucial role of co-
existence of Mo sites and BAS and their synergistic promo-
1
800 East Paul Dirac Drive, Tallahassee, FL 32310-3706 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202017074.
[
31]
tion in the reaction.
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Apart from structure, the location/distribution of active
TEM and N physisorption experiments indicate that Mo
2
[32]
sites define the catalytic performance of zeolite.
It was
species exist both inside and outside of the channels of the
fresh Mo/ZSM-5 (Supporting Information, Table S1 and
Figures S4,S5). The Brønsted acidity of the fresh Mo/ZSM-5
supposed that only the BAS that interacts with the active Mo
sites is required for the MDA reaction and the free BAS
exerts detrimental effect on the reaction by causing carbona-
1
and the reacted samples was probed by H MAS NMR
[33]
ceous deposits. The interaction between the two species
would thus be a key factor in modulating their catalytic
experiments. Three signals at 4.0, 2.6, and 1.7 ppm are
observed on the parent H-ZSM-5 and fresh 5Mo/ZSM-5,
ascribed to Brønsted acidic protons (SiOHAl, bridging
hydroxyl group), extra-framework AlOH, and nonacidic
[
30]
function.
Since the formation of different types of Mo
species on the zeolite external surface is often unavoidable,
identification of active sites inside zeolite channels is a chal-
lenge for spectroscopic characterization.
[39]
SiOH,
respectively (Figure 1, left panel). The acidic
protons are significantly decreased on 5Mo/ZSM-5 as com-
pared to the parent zeolite, with ca. 70% of them being
replaced (Supporting Information, Figure S6). This can be
accounted for by the formation of exchanged Mo-oxo species
on the conjugated base site of BAS inside the channels of
ZSM-5. Slight decreasing density of the acidic protons
(4.0 ppm) is observed in the reaction before 30 min, corre-
sponding to the early stage of MDA (Supporting Information,
Figure S3). In this period, MoO species is reduced by CH
Solid-state NMR spectroscopy has become a critical tool
for characterizing the active sites on heterogeneous catalysts,
capable of providing detailed information on correlation and
[34–38]
connectivity of distinct nuclei in a local structure.
In this
work, we established a fundamental understanding of the
active sites on Mo/ZSM-5 for MDA reaction. The formation
of active Mo species is correlated with proximate Brønsted
1
95
acid site on zeolite. By using H{ Mo} double-resonance
solid-state NMR spectroscopy, we provide first experimental
evidence on the existence of strongly interacted acidic proton-
Mo dual site on Mo/ZSM-5. The spatially proximate acidic
proton-Mo sites correlate with catalyst activity for the MDA
reaction. By detecting the interactions between hydrocarbons
and active Mo sites/Brønsted acid sites, the intermediate role
of olefins in the hydrocarbon pool process is revealed.
x
4
and transformed into active Mo C and/or MoO C species,
2
x
y
95
which is confirmed by the following Mo MAS NMR
experiments. The high reaction temperature of 973 K further
promotes the migration of the Mo species from the external
surface into the channels of zeolite to replace more acidic
[40,41]
protons
(Supporting Information, Figures S5 and S6). In
the meantime, two new signals appear at 6.4 and 8.1 ppm,
which can be ascribed to the formed olefins and aromatics
1
13
with the assistance of two-dimensional (2D) H- C HET-
COR NMR experiment (Supporting Information, Figure S7).
The prevailing formation of aromatics after 30 min of reaction
time is evidenced by the evident increase in the intensity of
signal at 8.1 ppm. This is accompanied with the further
decreasing of acid protons (4.0 ppm; Supporting Information,
Results and Discussion
Mo/ZSM-5 catalysts with Mo loading of 1–10 wt.%
(
denoted as 1Mo/ZSM-5–10Mo/ZSM-5) were prepared by
incipient wetness impregnation of H-ZSM-5 zeolites (Si/Al =
1
5; see details in the Supporting Information).
Mo (94.8%) enriched 5Mo/ZSM-5 samples
9
5
95
were prepared for Mo MAS NMR and
1
95
H{ Mo} S-RESPDOR NMR experiments.
X-ray diffraction (XRD) analysis (Supporting
27
Information, Figure S1a) and Al MAS NMR
spectroscopy (Supporting Information, Fig-
ure S2) show that the crystal structures of
ZSM-5 (MFI type) zeolites are well main-
tained for all the Mo/ZSM-5 samples and
reacted 5Mo/ZSM-5 (Supporting Information,
Figure S1b). Crystalline MoO is detected at
3
Mo loading higher than 5 wt% (4.7 wt% from
ICP analysis), while the molybdenum species
are well dispersed in ZSM-5 at lower Mo
loadings. The typical methane conversion and
product distribution were observed on the
Mo/ZSM-5 samples for MDA reaction at
9
73 K (Supporting Information, Figure S3).
The relatively higher aromatics yields (ben-
zene, toluene, and naphthalene) indicates the
better activity and stability of the 5Mo/ZSM-5
sample. All the following measurements were
performed on this sample unless otherwise
mentioned specifically.
1
1
1
Figure 1. H MAS NMR (left) and H- H DQ-SQ (right) MAS NMR spectra (acquired at
.4 T) of H-ZSM-5, fresh 5Mo/ZSM-5 and reacted 5Mo/ZSM-5 samples for 10–120 min
9
at 973 K.
1
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Figure S6), indicating the participation of BAS in methane
[42]
1
1
aromatization. 2D H– H double-quantum-single-quantum
DQ-SQ) MAS NMR was used to characterize the proximity
(
of protons on Mo/ZSM-5, which provides insight into the
distribution of the protons of zeolites as well as the hydro-
carbon products in zeolite channels. The two auto-correlation
peaks at (1.7, 2 ” 1.7) and (4.0, 2 ” 4.0) ppm indicate the
proximate silanols and BAS respectively in zeolite. The
significant reduction in the peak intensity at 4.0 ppm on the
Mo samples manifests the substitution of acidic protons by
Mo species, which leads to decrease and isolation of the BAS.
The proceeding of MDA reaction results in further decline of
the proximate BAS due to either the formation of more
exchanged Mo species on BAS or the consumption of acidic
protons in the MDA. In the meantime, the close proximities
of aromatics were observed with reaction, reflected by the
stronger intensity in the autocorrelation peak at (8.1, 2 ”
8
.1) ppm. This indicates the accumulation of the bulk organic
molecules in the confined space of zeolite, which can be
correlated with the rapid catalyst deactivation (Supporting
Information, Figure S3). Our previous investigation on cata-
lyst deactivation showed that the intermolecular spatial
proximities/interactions in zeolite channels favor the coupling
of aromatics and derivatives to form naphthalene or polyaro-
9
5
Figure 2. Experimental and deconvoluted Mo WURST-QCPMG MAS
NMR spectra (acquired at 35.2 T) of fresh 5Mo/ZSM-5 and 5Mo/ZSM-
reacted for 60 min at 973 K. Solid (black c): experimental, solid
(red c): simulated, dash (purple c, green c): deconvoluted.
5
[43]
matics as precursor to coke.
Active Mo species on Mo/ZSM-5 was studied by solid-
9
5
Table 1: NMR parameters extracted from Mo WURST-QCPMG NMR
95
spectra (acquired at 35.2 T) of 5Mo/ZSM-5 and 5Mo/ZSM-5 reacted for
state NMR experiments. Both NMR active Mo nuclei Mo
[a]
9
7
60 min of MDA reaction.
and Mo are quadrupolar nuclei of spin 5/2. Due to the low
9
5
7
À1 À1
gyromagnetic ratio nature, g( Mo) = À1.751 ” 10 radT
s
Sample
Component
d
iso [ppm]
C
Q
[MHz]
h
9
7
7
À1 À1
and g( Mo) = À1.78 ” 10 radT s , sensitivity-enhanced
WURST-QCPMG method was employed in combination
with ultra-high magnetic field of 35.2 T (1.5 GHz) which
shows advantages for high-resolution NMR spectroscopy of
5
6
Mo/ZSM-5
0 min
Mo-oxo
Mo-oxo
À35Æ5
À35Æ5
800Æ50
5.5
5.5
32
0.7–1.0
0.7–1.0
0.9
Mo C/MoO C
x y
2
[
a] diso: isotropic chemical shift, C : quadrupolar coupling constant, h:
Q
[44–48]
95
half-integer quadrupolar nuclei.
The Mo WURST-
asymmetry factor.
QCPMG MAS NMR spectrum acquired at 35.2 T of the
fresh 5Mo/ZSM-5 exhibits a broad quadrupolar peak (CQ
ꢀ
5.5 MHz) with an isotropic chemical shift (d ) of ca.
À35 ppm and an anisotropic chemical shift (CSA) of ca.
To provide direct experimental evidence on the spatial
proximity between protons and active molybdenum species,
H{ Mo} S-RESPDOR solid-state NMR experiments were
iso
1
95
[50]
3
20 ppm obtained by fitting the spectrum with consideration
of both quadrupolar interaction and CSA (Figure 2, Table 1).
performed to measure internuclear dipolar interaction/dis-
tances on 5Mo/ZSM-5 reacted for different reaction time at
973 K. The signal of protons in close proximity to Mo atoms
According to the d , this Mo site could be assigned to MoO -
iso
3
[49]
like species,
while the larger CQ compared to the bulk
1
95
95
MoO in orthorhombic phase is indicative of asymmetric local
will be modulated by H- Mo dipolar interaction upon Mo
3
1
structure, which can be accounted for by the high dispersion
of the Mo-oxo species in zeolite channels and on the external
surface. After reacting for 60 min, a new and dramatically
broad resonance with C ꢀ 32 MHz and d ꢀ 800 ppm (Æ
irradiation (S spectrum), resulting in decreased H signal
0
1
intensity (S spectrum). The H signal from Brønsted acidic
protons (4.0 ppm) is observed in the difference spectrum
(DS = S ÀS) of the fresh sample (Figure 3a). This evidences
Q
iso
0
95
5
5
0 ppm) is detectable in the Mo MAS NMR spectrum of
Mo/ZSM-5-60 sample. The broad Mo NMR peak with
the dipolar interactions and spatial proximity between the
acidic protons and Mo species (Mo C or MoO C species) in
95
2
x
y
similar d to that of Mo C compound in orthorhombic
zeolite channels. The AlOH (2.6 ppm) is also observed, while
the SiOH (1.7 ppm) group is invisible in the difference
spectrum, due to the absence of close proximities between
this species with Mo sites. On the reacted samples (30 and
120 min), the proximity between acidic protons (4.0 ppm) and
iso
2
[
49]
phase is most likely due to the formation of the active Mo C
2
or MoO C species in zeolite channels in the MDA. However,
x
y
the large second-order quadrupolar broadening of the line
shape indicates the extreme heterogeneity and structural
distortion in the local Mo site. Although the exact Mo
specification is not determined, this demonstrates the strong
interactions of the active Mo species with the anchoring BAS,
which produces large electric field gradient and correspond-
1
95
Mo species is still present as demonstrated by the H- Mo
1
dipolar dephasing effect (Figure 3b,c). Interestingly, the H
signals from aromatic (8.1 ppm) and olefins (6.4 ppm) also
appear in the difference spectra. This observation demon-
strates that these hydrocarbons are formed near the active Mo
ing C on the Mo nuclei.
Q
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The spatial interaction/proximity be-
tween Mo and acidic protons reflected by
the decay of DS is decreasing at longer
reaction time for two reasons. First, the
acidic protons are consuming with reac-
tion (Supporting Information, Fig-
ure S6), leading to the decrease of the
amount of the proximate acidic proton-
Mo sites. Second, the active Mo species
may detach from the BAS and form bulk
Mo C on the external surface of zeo-
2
[53]
lite
(Supporting Information, Fig-
ure S5). This would make the Mo species
and the acidic protons distantly separat-
ed. Note that the DS of olefins shows
a similar trend (Figure 3e). The larger
DS is produced by the strong interaction
between these species with Mo in the
initial reaction stage, a clear indication of
forming hydrocarbons around the acidic
proton-Mo sites. The subsequent diffu-
sion and consumption of olefins lead to
1
95
Figure 3. a)–c) H{ Mo} S-RESPDOR NMR spectra (acquired at 18.8 T) of a) fresh 5Mo/ZSM-
, b) 5Mo/ZSM-5 reacted for 30 min and c) for 120 min of MDA reaction at 973 K. Recoupling
5
time is 5.12 ms. d)–f) Normalized DS of d) Brønsted acid, e) olefins, and f) aromatics versus
MDA reaction time. DS=S ÀS.
0
sites in zeolite channels, indicating the catalytic role of the
proximate acidic proton-Mo species in the MDA reaction.
Since the concentration of acidic protons shown on the S0
spectrum is decreasing with reaction (Supporting Informa-
tion, Figure S6), we use the dephasing effect DS expressed as
the decreasing interactions with the active Mo sites, reflected
by the declining DS. On the contrary, aromatics exhibit
different DS variation behavior, increasing with reaction time
(Figure 3 f), indicative of the closer proximity and stronger
interactions between aromatics and Mo sites. The formation
and accumulation of aromatics and their derivatives (partic-
ularly polyaromatics nearby the Mo sites in zeolite channels)
should be the main reason for this trend, which eventually
results in catalyst deactivation.
By analogy to methanol conversion on acidic zeolite,
olefins and aromatics and derivatives in the MDA reaction
may complex with the active sites propagating the catalytic
cycles where carbocations or radicals would be involved as
(
S ÀS) (where S and S represent the signal intensity with and
0
0
95
without Mo irradiation, respectively) to show the evolution
of the proximate acidic proton-Mo sites with reaction time in
the MDA. In principle, for a fixed recoupling time in the
upward trend of the buildup curve, DS reflects the degree of
1
95
the H- Mo dipolar interaction/spatial proximity between the
two species. A significant increase of DS for acidic protons on
the reacted samples in the initial period (before 30 min)
compared to the fresh sample indicates their stronger
interactions with Mo species in zeolite. The carburization
process promotes the migration of MoO from the external
3
surface into zeolite channels (Figure 2) and formation of
more proximate acidic proton-Mo sites. We further measured
1
95
the H– Mo internuclear distance for the acidic protons by
1
95
simulating the H{ Mo} S-RESPDOR dephasing built-up
[50]
curve using analytical formula
(Figure 4). The yielded
internuclear distance is 4.0 Æ 0.3 Š, which is slightly smaller
[51]
than that of the proximate BAS (ca. 4.5 Š) in ZSM-5. This
close acidic proton-Mo proximity results in the strong spatial
interactions between the two species. The generation of
proximate acidic proton-Mo site also explains the disappear-
1
1
ance of the neighboring BAS in the H- H DQ-SQ NMR
Figure 1). The spatially interacted acidic proton-Mo species
(
endows the Mo/ZSM-5 catalyst with synergistic bifunction-
ality, which correlates well with the MDA performance
1
95
(
Supporting Information, Figure S3). For example, the yield
Figure 4. Experimental H{ Mo} S-RESPDOR dephasing fraction of
Brønsted acidic protons in 5Mo/ZSM-5 as a function of recoupling
time. The best fit of the experimental results was determined to be
of aromatics (for example, benzene) follows a similar trend to
the evolution of acidic proton-Mo interaction that is built up
in the initial period (30 min time-on-stream) and decreasing
at longer reaction time. It is plausible that the MDA proceeds
via a concerted pathway enabled by the co-existence of BAS
1
95
f=0.15 and D =120 Hz, corresponding to a H- Mo internuclear
IS
distance of 4.0 Š. f denotes the pre-factor parameter and DIS is the
heteronuclear dipolar interaction between two spins used in the
analytical formula. Sim. and exp. represent simulated and experimental
curves, respectively.
[52]
and metal sites within molecular distances.
1
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[
26,54]
1
1
intermediates.
2D H– H spin diffusion experiments were
prevailing formation of aromatics leads to the accumulation
performed on the reacted 5Mo/ZSM-5 samples, which allow
to correlate the development of the organic compounds with
active sites in catalyst, thus giving information on their role in
the MDA reaction. Only cross peaks between Brønsted acidic
protons (4.0 ppm) and olefins (6.4 ppm) can be observed on
the sample reacted for 10 min (Figure 5a), demonstrating the
short-range spatial proximity between these protons. Along
with the concurrent formation of aromatics (Figure 1; Sup-
porting Information, Figure S3), this experiment indicates
that olefins might be firstly produced in the initial stage. The
analysis of the gas product shows that ethene dominates the
light hydrocarbons before 60 min of reaction time (Support-
ing Information, Table S2). Although the initial CÀC bond
of aromatics and other organics in zeolite channels, which is
demonstrated by the signal broadening. More aromatics
produced and distributed around the active acidic proton-Mo
sites promote spin diffusion between proximate protons,
enabling the observation of stronger cross peak for aromatics,
1
95
which is also supported by the above H{ Mo} S-RESPDOR
NMR experiment (Figure 3 f). On the other hand, the
correlation of olefins with acidic protons becomes weakened
as evidenced by the reduced cross peak intensity (Figure 5b–
d). This can be explained by the consumption of olefins in
zeolite channels on catalyst deactivation, in agreement with
1
95
the H{ Mo} S-RESPDOR result (Figure 3e). Taken togeth-
er with the product analysis, these data suggest the inter-
mediate role of olefins in the hydrocarbon pool process
during the MDA reaction: olefins produced in the initial
reaction period are expected to link methane reactant and
aromatics product. Although the proximate aromatics and
acidic protons would facilitate olefins production via the
aromatics-based reaction cycle as established in the hydro-
[
55]
formation in the MDA is still unclear, the observation of
the proximity between olefins and acidic protons provides
direct evidence for the further dehydrocyclization of olefins
to aromatics on BAS in close proximity to Mo sites. New cross
peak between Brønsted acidic protons (4.0 ppm) and aro-
matics (8.1 ppm) appears at 30 min of reaction time (Fig-
ure 5b), which is absent in the spectrum with short mixing
time (50 ms; Supporting Information, Figure S8). This sug-
gests the location of aromatics in a medium-range spatial
proximity to the BAS, resulting in the unfavorable polar-
ization-transfer between these species by spin diffusion. Their
correlation is enhanced with reaction, reflected by the
increase of the cross peak intensity (Figure 5c and d). The
[27,28]
carbon pool mechanism,
the formation of polyaromatics
would be favored on the BAS, leading to the formation of
catalyst deactivating species such as naphthalene.
The determination of the active sites and their functions
benefits a better understanding of the MDA reaction
mechanism. Probing the spatial proximity between the acidic
proton-Mo sites allows for observation and determination of
the complex reaction center, constitut-
ed by the strongly interacted Mo C/
2
MoO C and Brønsted acid sites in Mo/
x
y
ZSM-5 channels. The catalytic activity
for methane conversion and the pro-
duction of aromatics are well correlated
with the interaction strength/spatial
proximity between the acidic proton-
Mo sites. This suggests that a concerted
reaction route for methane activation
and transformation would likely occur
in the MDA due to the synergistic effect
generated by the proximate acidic-Mo
sites. Moreover, the proximity of the
acidic proton-Mo sites is shown to be
a key factor in determining the catalyst
lifetime as the migration of Mo species
to the zeolite external surface leads to
quick catalyst deactivation. In the
meantime, the remained BAS in zeolite
channels is deactivated by polyaromat-
ics. Light olefins such as ethene mostly
generated in the initial stage of the
MDA are found to strongly interact
with the proximate acidic proton-Mo
sites and subsequently transform into
aromatics. From the host-guest point of
view, the intermediate role of olefins
for the formation of aromatics in the
proposed hydrocarbon pool mechanism
is ascertained.
1
1
Figure 5. 2D H- H spin diffusion NMR spectra (acquired at 9.4 T) of 5Mo/ZSM-5 reacted at
73 K for a) 10 min, b) 30 min, c) 60 min, and d) 120 min. Mixing time is 300 ms.
9
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Conclusion
[7] Z. Cao, H. Jiang, H. Luo, S. Baumann, W. A. Meulenberg, J.
Assmann, L. Mleczko, Y. Liu, J. Caro, Angew. Chem. Int. Ed.
2
013, 52, 13794 – 13797; Angew. Chem. 2013, 125, 14039 – 14042.
The active sites on Mo/ZSM-5 zeolite for methane
[
[
8] N. Kosinov, F. J. Coumans, E. Uslamin, F. Kapteijn, E. J. Hensen,
dehydroaromatization reaction were investigated by solid
state NMR spectroscopy. The spatial interaction between the
Brønsted acid sites and active Mo sites in zeolite channels has
been identified on Mo/ZSM-5 catalyst, providing direct
evidence on the synergistic bifunctionality of the catalyst for
the MDA reaction. The strongly interacted acidic proton-Mo
sites correlate with the products formation and catalyst
lifetime, which shows the important implication in modulat-
ing the catalytic MDA performance by taking advantage of
the proximity of the acidic proton-Mo sites at atomic-level.
The hydrocarbon pool process in the MDA is revealed by the
analysis of the host-guest interactions between the trapped
olefins/aromatics and Brønsted acid sites associated with Mo
sites. Light olefins dominant in the early reaction stage and
having strong interactions with the proximate acidic proton-
Mo sites are connected with the formation of aromatics. The
reduction of the acidic proton-Mo proximity in zeolite
channels and increasing interactions between aromatics/their
derivatives and Brønsted acid sites evidence catalyst deacti-
vation. The results presented herein provide new insights into
the understanding of the active sites on Mo/ZSM-5 catalyst as
well as the hydrocarbon pool chemistry in MDA reaction.
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Manuscript received: December 23, 2020
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