Novel Strategy for the Synthesis of Ultra-Stable Single-Site Mo-ZSM-5 Zeolite Nanocrystals

Article abstract of DOI:10.1002/anie.202006524

The current energy transition presents many technological challenges, such as the development of highly stable catalysts. Herein, we report a novel “top-down” synthesis approach for preparation of a single-site Mo-containing nanosized ZSM-5 zeolite which has atomically dispersed framework-molybdenum homogenously distributed through the zeolite crystals. The introduction of Mo heals most of the native point defects in the zeolite structure resulting in an extremely stable material. The important features of this single-site Mo-containing ZSM-5 zeolite are provided by an in-depth spectroscopic and microscopic analysis. The material demonstrates superior thermal (up to 1000 °C), hydrothermal (steaming), and catalytic (converting methane to hydrogen and higher hydrocarbons) stability, maintaining the atomically disperse Mo, structural integrity of the zeolite, and preventing the formation of silanols.

Full text of DOI:10.1002/anie.202006524

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: Novel strategy for synthesis of ultra-stable single-site Mo-ZSM-5
zeolite nanocrystals
Authors: Svetlana Mintova, Stanislav V. Konnov, Florent Dubray,
Edwin B. Clatworthy, Cassandre Kouvatas, Jean-Pierre
Gilson, Jean-Pierre Dath, Delphine Minoux, Cindy Aquino,
Valentin Valtchev, Simona Moldovan, Nikolai Nesterenko, and
Siddardha Koneti
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006524
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10.1002/anie.202006524
Angewandte Chemie International Edition
RESEARCH ARTICLE
Novel strategy for synthesis of ultra-stable single-site Mo-ZSM-5
zeolite nanocrystals
Stanislav V. Konnov,a† Florent Dubray,a† Edwin B. Clatworthy,a Cassandre Kouvatas,a Jean-Pierre
Gilson,a Jean-Pierre Dath,b Delphine Minoux,b Cindy Aquino,b Valentin Valtchev,a Simona Moldovan,c
Siddardha Koneti,c Nikolai Nesterenko,b Svetlana Mintova*,a
[a]
Dr. Stanislav V. Konnov, Florent Dubray, Dr. Edwin B. Clatworthy, Dr. Cassandre Kouvatas, Prof. Jean-Pierre Gilson, Prof. Valentin Valtchev, Dr. Svetlana
Mintova
Normandie Université, ENSICAEN, UNICAEN, CNRS
Laboratoire Catalyse et Spectrochimie (LCS)
14050 Caen, France
E-mail: svetlana.mintova@ensicaen.fr
Prof. Jean-Pierre Dath, Dr. Delphine Minoux, Dr. Cindy Aquino, Dr. Nikolai Nesterenko
Total Research and Technologies Feluy (TRTF)
B-7181 Seneffe, Belgium
Dr. Simona Moldovan, Dr. Siddardha Koneti
Institut des Sciences Appliquées de Rouen, Rouen University
Groupe de Physique des Matériaux (GPM)
76801 Rouen, France
[b]
[c]
[†]
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of the document.
Abstract: The current energy transition presents many technological
challenges, such as the development of highly stable catalysts.
Such scenarios would benefit from the development of novel
and stable metal-containing zeolite catalysts.
Herein, we report
preparation of a single-site Mo-containing nanosized ZSM-5 zeolite
possessing atomically dispersed framework-molybdenum
a
novel “top-down” synthesis approach for
For example, the dehydrogenation of propane to propylene
employs alumina-supported catalysts,[4] however, state of the art
synthetic procedures have demonstrated several new strategies
to improve the activity and stability of zeolite-supported catalysts
such as the encapsulation of metal atoms and clusters during
the zeolite synthesis,[5] stabilisation of the metal phase in the
zeolite channels by alkali metals,[2] and the use of framework-
metal sites to stabilise extra-framework active sites.[6] Similar to
the dehydrogenation of propane, there are no commercial
zeolite-based catalysts for the non-oxidative conversion of
methane to hydrogen and industrially important chemicals
(olefins, aromatics). This process is a direct route to decarbonize
methane to petrochemicals while co-producing a clean energy
vector - hydrogen. A big competitive advantage relative to the
other routes to produce hydrogen from methane (e.g. steam
reforming process) is in no direct CO2 emission and, as a
consequence, no requirement for CO2 capture. The route is also
more carbon efficient and requires lower capital investments
relative to syngas routes, which are the only industrial processes
currently available to transform methane to petrochemicals. This
reaction faces two critical issues: (1) low conversion due to
unfavorable thermodynamics and (2) catalyst deactivation due to
coke deposition at high temperatures (> 700 °C) causing
irreversible damage to the zeolite structure. Recent studies on
metal-containing zeolites for the non-oxidative conversion of
methane, such as Mo-impregnated ZSM-5, have focussed on
the optimization of the metal loading, reaction process, and
catalyst regeneration.[7]
homogenously distributed through the zeolite crystals. The
introduction of Mo heals most of the native point defects in the
zeolite structure resulting in an extremely stable material. The
important features of this single-site Mo-containing ZSM-5 zeolite
are provided by an in-depth spectroscopic and microscopic analysis.
The material demonstrates superior thermal (up to 1000 °C),
hydrothermal (steaming), and catalytic (converting methane to
hydrogen and higher hydrocarbons) stability, maintaining the
atomically disperse Mo, structural integrity of the zeolite, and
preventing the formation of silanols.
Introduction
As the world undergoes a decades-long transformation of
the energy industry to low or zero carbon-based resources and
fuels, natural gas (mainly CH4) and biomass will play
increasingly important roles in transitioning away from traditional
fossil fuel feedstocks such as coal, light and heavy oils.[1] The
shift in focus to other energy sources will require the
modification of existing and the development of emerging
industrial processes. Zeolites, already a key player in catalysis
and separations in oil refining and petrochemistry[1a] are poised
to play an important role in such technologies if the stability of
metal sites in zeolite-based catalysts can be insured.[2] Moreover,
dedicated crude-to-chemicals technologies will start to boost the
production of chemicals from oil at refineries from ≈ 10–15% at
For this process to be industrially viable, the catalysts
envisioned will have to possess, among other features, a low
density of similar active sites (single sites), short diffusion path
lengths of guest molecules (nanosized crystalline domains) to
minimize secondary reactions (e.g. coking), shape selectivity
(selective production of high-value aromatics such as p-xylene)
present to
≈ 30–80% in the coming decades and many
traditional facilities are already considering changing their
product slate from transportation fuels to chemical feedstocks.[3]
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10.1002/anie.202006524
Angewandte Chemie International Edition
and superior hydrothermal stability. It is clear that such catalysts
will have to operate outside the current “coomfort zone” of
conventional industrial catalytic processes. At present, metal-
reported earlier by our group,[13] the current method allows
stabilization of single metal species in already synthesized
aluminium containing MFI type (ZSM-5)) zeolites (Scheme S1).
containing zeolites operating at high tem
p
p
erature in the
ages including:
(1) the loss of dispersion of the metal phasee, (2) structural
This new procedure can be applied to aany calcined pure silica
and aluminosilicate zeolites including commercially available
materials.
presence of steam suffer from irreversible da
m
m
collapse, (3) strong dealumination due to the reaction between
volatile metals and framework Al, and (4) hydroolysis of Al–O–Si
The structural properties of the noveel single-site [Mo]-ZSM-5
zeolite sample were characterized by in-depth diffraction and
spectroscopic analyses. The XRD pattterns of the single-site
[Mo]-ZSM-5 and reference R-ZSM-5 zeolite samples are
depicted in Figure 1A. The [Mo]-ZSM-5 sample is monoclinic
(P21/n) while the R-ZSM-5 is orthorhombic (Pnma). Pristine
untreated calcined Mo-free ZSM-5 was orthorhombic and the
bonds. Such challenges are seldom addresse
literature. Thermal and hydrothermal stability of zeolites and the
currently available methodologies for their sta ilization remain
dd in the current
b
b
important issues for the industrial scale appliication of novel
materials or even known zeolite catalysts. In addition to the
recent examples for propane dehydrogenation mmentioned earlier, XRD pattern is provided in Figure S1 for comparison. In addition,
reported methods for the stabilization of zeollites include the
addition of phosphorous in MFI,[8] ion-exchangee with rare-earth
metals in FAU,[9] dealumination/silication by (NH4)2SiF6
treatments and/or partial dealumination combineed with extraction
orthorhombic symmetry was maintained for the pristine ZSM-5
that was subjected to the same treatmeent as that used for the
preparation of the [Mo]-ZSM-5 sample, however, without the
presence of sodium molybdate (Figure S1). The characteristic
of Al extra-framework species (USY).[10]
stabilization strategies of zeolites are based o
M
ost of these
splitting of the diffraction peaks at 23.3, 223.8, and 24.5° 2
θ in the
nn defect healing,
XRD pattern of the single-site [Mo]-ZSM-5 zeolite sample is
or the creation of substantially “defect free” sstructures. Very
recently, Iyoki et al. demonstrated the stabilizat on of high-silica
zeolites (SiO2/Al2O3 > 240, BEA, MFI and MOOR type) using a
liquid-mediated treatment consisting of hy roxide anions,
i
d
d
fluoride anions and organic structure directing cations, without
requiring the addition of silylating agents. The treated zeolites
were able to withstand high temperature steam
iing conditions of
900–1150 °C.[11]
In this work we report for the first time the “top-down”
synthesis of novel single-site Mo-containing Z M-5 nanozeolite
with superior stability (up to 1000 °C) by treat ent of calcined
nanosized zeolite with sodium molybdate undder autogenous
S
mm
pressure. Mo is atomically dispersed in the f
r
r
amework of an
aluminium containing MFI zeolite ([M
oo]-ZSM-5) and
homogeneously distributed through the crystalss. Mo effectively
heals silanol defects and the resulting cataly t demonstrates
s
s
stable activity over consecutive catalytic reactions. The
exceptional stability and performance of the single-site Mo-
containing nanozeolite for H2 and highe
r
hydrocarbons
production from CH4 under methane dehy
d
roaromatization
(MDA) conditions was demonstrated. The single-site Mo-
containing ZSM-5 nanozeolite is stable
reductive, and steaming conditions.
uunder oxidative,
Results and Discussion
1. Synthesis and comprehensive characterization of Mo-
containing zeolites
The single-site [Mo]-ZSM-5 zeolite sample wwas synthesized
following the novel procedure outlined in thhe experimental
section and presented in Scheme S1. The synt esis approach is
based on the treatment of calcined Mo-free Z M-5 nanosized
h
h
S
zeolite with sodium molybdate at 90 °C forr 5 days under
autogenous pressure, followed by purification and calcination
(550 °C for 6 h). The prepared [Mo]-ZSM-5 conttains 0.5 wt.% of
Mo according to EDS-TEM and ICP analysess. The reference
sample, R-ZSM-5, was prepared by incipient wetness
impregnation of the parent Mo-free ZSM-5 nannozeolite with 1
wt.% of Mo according to standard procedures reported in the
MDA literature.[12] In contrast to the direct synnthesis approach
Figure 1. XRD patterns of [Mo]-ZSM-5 and R-ZSM-5 zeolite samples in the
range of (A) 5–45 °2θ and (B) 22–26.5 °2θ
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10.1002/anie.202006524
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clearly seen (Figure 1B); stars marked the splitting of the single
peaks typical for orthorhombic R-ZSM-5. The monoclinic
symmetry is an indication for the isomorphous substitution of a
metal with an ionic radius larger than Si in the MFI framework,
however, the specific T-site locations occupied by Mo in the
ZSM-5 framework were not determined.[13] The Le Bail profile
refinements of the XRD patterns are summarized in Table S1.
The larger unit-cell volume of the single-site [Mo]-ZSM-5 zeolite
sample further indicates the substitution of SiIV (ionic radius:
0.26 Å) for MoVI (ionic radius: 0.59 Å) in the MFI framework.[13]
The morphology, size and elemental composition of both
samples [Mo]-ZSM-5 and R-ZSM-5 are studied by STEM-
HAADF (Figure 2). The statistical analysis of the particle size
(measured for about 200 particles) is presented as inserts in the
Figure 2A and 2B. They reveal the average particles size is
about 150 (+/- 20) nm for both Mo-ZSM-5 and R-ZSM-5 samples.
The most striking differences between the samples are found in
the morphology of the particles. The crystals of [Mo]-ZSM-5
exhibit smooth surfaces and sharp edges, whereas in the case
of R-ZSM-5 crystals, the outer surface is less regular and the
inner crystals morphology is marked by the presence of
mesopores (darker circular areas). The sharpness of the edges
in the case of specimen [Mo]-ZSM-5 can be interpreted as a
signature of its monocrystalline nature, whilst the irregular outer
edges of the R-ZSM-5 crystals are associated with their
polycrystallinity. The polycrystalline nature of the R-ZSM-5
crystals is also associated with the presence of an abundant
number of defects (notably the mesopores), which is further
confirmed by NMR (1H and 29Si MAS NMR), and IR
spectroscopy (see below). The atomically dispersed Mo in [Mo]-
ZSM-5 (seen as light points in the micrographs) are highlighted
with circles in the STEM-HAADF micrographs, and confirm the
presence of homogeneously distributed Mo atoms within the
zeolite crystals (Figure 2A). The size of Mo light points in STEM-
HAADF micrographs is similar in the two samples, however, they
differ substantially in their stability under high temperature
treatment (up to 1000 ºC) and steaming. The chemical
composition of the single-site [Mo]-ZSM-5 crystals is determined
by ICP and confirmed by EDS-TEM. The elemental distributions
in the [Mo]-ZSM-5 and R-ZSM-5 samples are shown in Figure
S2. The shape and morphology of the pristine ZSM-5 was
examined by STEM-EDS (Figure S3). The agglomerated
crystals have irregular shapes, and a mean size of 170 nm. The
crystals in the reference sample (R-ZSM-5) have a similar shape
and size (Figure S2); the distribution of both Si and Al in the
pristine and reference sample is homogeneous suggesting that
no changes under impregnation occurred.
Further structural information of the single-site [Mo]-ZSM-5
sample is provided by 29Si MAS NMR spectroscopy (Figure 3A).
The spectrum of the calcined [Mo]-ZSM-5 sample reveals well-
resolved Q4 ((SiO)4Si) and no Q3 ((SiO)3Si-OH) species,
indicating an almost total absence of silanols. In comparison, the
reference R-ZSM-5 sample displays unresolved Q4 and Q3
species, indicating the presence of silanol defects. These results
are consistent with the HRTEM observations revealing the
formation of [Mo]-ZSM-5 crystals with smooth surface, while the
Figure 2. STEM-HAADF micrographs of (a) [Mo]-ZSM-5: left scale bar = 100 nm, right scale bar = 10 nm, and (B) R-ZSM-5: left scale bar = 100 nm, right scale
bar = 10 nm. Inset: particle size distribution.
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10.1002/anie.202006524
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R-ZSM-5 particles contain additional mesopores and surface
defects. The presence of Al in the framework of both samples is
confirmed by 27Al MAS NMR spectroscopy (Figure 3B). The
spectra for both the [Mo]-ZSM-5 and R-ZSM-5 samples contain
a sharp peak at 60 ppm corresponding to Al in tetrahedral
coordination, and no octahedral species are detected at 0 ppm.
The results from the 29Si and 27Al NMR analyses are
consistent with the IR observations performed in the silanol
region for the two samples (Figure 4A). Sample [Mo]-ZSM-5
contains a negligible amount of isolated silanols represented by
the peak at 3745 cm-1, and no silanol nests. In comparison,
sample R-ZSM-5 contains a significant amount of isolated
silanols and silanol nests represented by the highly intense
peaks at 3745 cm-1 and 3510 cm-1, respectively. The peak
corresponding to bridging silanol species at 3610 cm-1 in both
samples remains unchanged, confirming the preservation of
Brönsted acid sites (Figure 4A). Additionally, the low amount of
silanol species is present in the spectrum of [Mo]-ZSM-5, while
no peaks in the range 4–5.5 ppm are observed which is
attributed to the absence of silanol nests. For the R-ZSM-5
reference sample, broad peaks at 2.3, and 4.0–5.5 ppm related
to isolated silanols, and silanol nests and/or bridging hydroxyl
groups respectively are observed (Figure 4B).[14]
The location and state of the Mo are further studied by 31P
MAS NMR spectroscopy. The spectra of adsorbed
trimethylphosphine oxide (TMPO) on the single-site [Mo]-ZSM-5
and reference R-ZSM-5 samples are presented in Figure 5. The
presence of Mo in the framework positions of the [Mo]-ZSM-5
zeolite is confirmed by the resonance at 45 ppm which is
consistent with previous work,[13] while a small amount of
silanols are also observed (50 ppm).[15] Two Brønsted acid sites
(64 and 74 ppm) as well as crystalline (43 ppm) and physisorbed
(29 ppm) TMPO are present, indicating that TMPO accesses all
16]
acidic and metallic sites.[15c,
In contrast, for the Mo
1
silanol defects in the [Mo]-ZSM-5 is confirmed by H MAS NMR
impregnated R-ZSM-5 sample, Brønsted acid sites (64 and 74
ppm) and silanols (50 ppm) are present, however, no framework
Mo is detected as the resonance at 45 ppm is not observed
(Figure 5).
spectroscopy (Figure 4B); the samples were dehydrated at
200 °C overnight prior to the measurements to remove the
zeolitic water. One peak at 2.3 ppm corresponding to isolated
Figure 4. (A) FTIR spectra and (B) 1H MAS NMR spectra of dehydrated [Mo]-
ZSM-5 and R-ZSM-5 samples.
Figure 3. (A) 29Si MAS NMR and (B) 27Al MAS NMR spectra of samples [Mo]-
ZSM-5 and R-ZSM-5.
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10.1002/anie.202006524
Angewandte Chemie International Edition
carbides, resulting in irreversible deactivation of the catalyst.[15a,
18]
In contrast, the single-site [Mo]-ZSM-5 nanozeolite shows
exceptional stability for the conversion of CH4 after three
reactions at 850 °C and regeneration cycles at 750 °C (Figure
6A). Moreover, the [Mo]-ZSM-5 catalyst produces relatively
stable amounts of hydrogen and aliphatics throughout the cycles
in comparison with the R-ZSM-5 sample (Figure S4). Such a
stable catalytic performance of the [Mo]-ZSM-5 over three
reaction-regeneration cycles clearly demonstrates that the
catalytically active sites attributed to the single-site Mo in the
MFI structure are maintained, in strong contrast with the
reference R-ZSM-5 sample. That the catalyst demonstrates
stable activity at a high reaction temperature (850 °C) allows for
the targeted production of ethylene which requires more severe
conditions. The [Mo]-ZSM-5 catalyst can operate over a greater
temperature range without significant changes in its catalytic
performance, which is one of the critical points for process
design. The significant improvement in the stability of the metal
Figure 5. 31P MAS NMR spectra of TMPO adsorbed on samples [Mo]-ZSM-5
and R-ZSM-5.
2. Catalytic test of Mo-containing ZSM-5 zeolite samples: non-
oxidative high temperature CH4 conversion to H2 and higher
hydrocarbons
The catalytic performances of the Mo single-site [Mo]-ZSM-5
and reference R-ZSM-5 samples were assessed for the high
temperature non-oxidative conversion of CH4 (850 °C,
atmospheric pressure, 0.3 g of catalyst, WHSV 1.2 h-1) over 12 h
(Figure 6). The reference sample, R-ZSM-5, rapidly deactivates
until the conversion of CH4 plateaus after 7 h. In contrast, the
single-site [Mo]-ZSM-5 zeolite sample with only 0.5 % Mo shows
exceptional stability after three consecutive reaction-
regeneration cycles. It is worth noting that for [Mo]-ZSM-5, we
deal with Mo-sites which remain within the zeolite framework
allowing for the presence of either non-carburized or partially
carburized Mo-sites. The location and retention of Mo in the
zeolite framework results in the high stability of the catalyst,
allowing it to operate under severe conditions over multiple
reaction-regeneration cycles. However, further in-depth study is
required to determine the exact structure and evolution of the
actives sites. Mo remains in the framework structure of the [Mo]-
ZSM-5 zeolite and does not undergo any discernible changes
over the three reaction-regeneration cycles, vide infra. The total
methane conversion to hydrogen and the detailed product yields
of the two catalysts are compared in Figure 6B. The [Mo]-ZSM-5
sample affords a significantly higher yield of hydrogen and
ethylene in comparison to the R-ZSM-5 reference sample. It is
also worth noting that the [Mo]-ZSM-5 sample produces a
relatively high proportion of ethylene in the C2 fraction (> 90%).
The production of C3-C4 hydrocarbons is negligible for both
samples.
To assess the stability of the Mo in the single-site [Mo]-ZSM-
5 zeolite sample, the catalysts were subjected to multiple cycles
of reaction (MDA, i.e. reducing conditions) and regeneration
(calcination, i.e. oxidative conditions). Under typical MDA
operating conditions, the classical Mo containing zeolite (R-
ZSM-5) deactivates due to the sintering and migration of
Figure 6. (A) CH4 conversion after the first and third consecutive cycles of
reaction-regeneration as a function of time on stream for the [Mo]-ZSM-5 and
reference R-ZSM-5 catalysts and (B) total CH4 conversion (X: milligrams per
gram of catalyst or %), products yield (milligrams per gram of catalyst)
corresponding to 2–12 h time gap of calculation over the [Mo]-ZSM-5 and R-
ZSM-5. Reaction conditions: T = 850 °C, atmospheric pressure, WHSV = 1.2
h-1, time on stream window = 2–12 h.
molybdenum carbide species as
a consequence of weak
interactions with the zeolite.[17] During the exothermic catalyst
regeneration, the molybdenum carbide is oxidized, and steam is
generated from the oxidation of coke. Under such conditions the
Mo species become mobile and react with framework Al to form
aluminium molybdates. These new Mo species cannot form
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10.1002/anie.202006524
Angewandte Chemie International Edition
phase is highly beneficial, in particular for catalysts working
under harsh conditions (e.g. high temperatures and continuous
oxidation/reduction cycles).
steamed [Mo]-ZSM-5 (S) samples, while the diffraction pattern of
the R-ZSM-5 (S) sample reveals the appearance of several
Bragg peaks (Figure 7A). In addition, the 29Si MAS NMR spectra
of the [Mo]-ZSM-5 (S) sample (Figure 7B) displays highly
As shown above, the single-site metal species incorporated
into the zeolite framework results in stable catalytic performance. resolved Q4 species and no Q3 species, demonstrating that no
Further investigation of the thermal and hydrothermal stability of
the single-site Mo-containing [Mo]-ZSM-5 zeolite and the
reference sample is provided below. Emphasis is placed on the
characterization of the samples in an attempt to describe the
structural evolution of the catalysts under harsh reaction and
regeneration conditions.
silanol species were generated after the combined reaction-
regeneration cycles and steaming treatments despite the
simultaneous occurrence of dealumination. These observations
further support the conclusion that the Mo atoms are retained in
the MFI framework. This is the first example of a zeolite structure
that contains heteroelements, such as Mo, able to withstand
such harsh treatment (3 high temperature reaction/regeneration
cycles in MDA, followed by steaming at 550 °C). In contrast, the
R-ZSM-5 (S) sample displays both unresolved Q4 and Q3
species attributed to the presence of silanol species. The
formation of aluminium molybdates and the
3. Stability and Mo-containing zeolite samples.
The stability of the single-site [Mo]-ZSM-5 and the reference [R]-
ZSM-5 zeolite samples was evaluated after the catalytic reaction
and subsequent recycling treatments. The samples subjected to
3 catalytic (reaction-regeneration) cycles are denoted with “(3
cycles)”, while the samples that are subjected to 3 catalytic
cycles followed by steaming are denoted with “(S)”. The XRD
patterns of the calcined ([Mo]-ZSM-5) and used ([Mo]-ZSM-5 (3
cycles)) samples after 3 catalytic cycles (methane conversion
and regeneration at 750 °C) clearly show that all the Bragg
peaks are identical, confirming the preservation of the crystalline
structure (Figure S5). A marginal decrease of the unit cell
volume for [Mo]-ZSM-5 (3 cycles) is observed, suggesting that
the sample underwent a degree of dealumination (Table S1). In
contrast, the unit cell volume of R-ZSM-5 sample decreased
substantially from 5359.283 to 5344.653 Å3, which is indicative
of significant structural change occurring in the sample R-ZSM-5
(3 cycles). Remarkably, the 29Si MAS NMR spectrum of the
single-site [Mo]-ZSM-5 (3 cycles) sample is almost identical to
that of the calcined [Mo]-ZSM-5 sample (Figure S6). The 29Si
MAS NMR spectra of [Mo]-ZSM-5 (3 cycles) still displays highly
resolved Q4 species and no Q3 species, in stark contrast to R-
ZSM-5 (3 cycles) where both Q3 and Q4 species are observed.
This further demonstrates the exceptional stability of the
framework Mo in the [Mo]-ZSM-5 zeolite under reductive
(catalytic reaction) and oxidative (regeneration) conditions. The
intensity and high resolution of the Q4 peaks in the spectrum of
the [Mo]-ZSM-5 (3 cycles) sample is related to the degree of
dealumination, the conservation of Mo in the structure, and the
absence of silanols. The occurrence of dealumination in both the
[Mo]-ZSM-5 (3 cycles) and R-ZSM-5 (3 cycles) samples was
revealed in the 27Al MAS NMR spectra by the presence of
octahedral aluminum species at 0 ppm (Figure S7). Based on
27Al MAS NMR spectroscopy, dealumination occurred to a
slightly greater degree for the [Mo]-ZSM-5 (3 cycles) sample in
comparison to the R-ZSM-5 (3 cycles) sample. This suggests
that the Brønsted sites due to framework Al in the Mo
impregnated ZSM-5 (R-ZSM-5 sample) are more resilient to
dealumination than in pure ZSM-5. This discrepancy could be
attributed to the presence of a protective molybdenum oxide
shell around the framework Al species in the R-ZSM-5 sample
consistent with recent work reporting a preferential location of
Mo near framework Al species in ZSM-5.[19]
Additionally, the stability of the samples after 3 catalytic
cycles was studied after a subsequent steaming treatment
performed at 550 °C. The XRD patterns and 29Si MAS NMR
spectra of the [Mo]-ZSM-5 (S) and R-ZSM-5 (S) samples (Figure
7) establishes the outstanding hydrothermal stability of the
framework Mo species in [Mo]-ZSM-5 zeolite. All the Bragg
peaks are identical for the calcined [Mo]-ZSM-5 and the
Figure 7. (A) XRD patterns, and (B) 29Si MAS NMR spectra of samples [Mo]-
ZSM-5 (S) and R-ZSM-5 (S). Diffraction peaks marked by “*” at 20.8, 26.6,
36.5, 40.2, and 42.4 ° 2
θ on sample R-ZSM-5 (S) are attributed to the
formation of catalytically inactive aluminium molybdate species.
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10.1002/anie.202006524
Angewandte Chemie International Edition
occurrence of dealumination of the R-ZSM-5 (S) sample were
further supported by 27Al MAS NMR analysis (Figure 8A) due to
the clear appearance of a peak at 14 ppm, which was not
observed for the [Mo]-ZSM-5 (S) sample. Indeed, in the case of
the [Mo]-ZSM-5 (S) sample, a portion of the tetrahedral Al (53
ppm) is converted to distorted framework Al and/or penta-
coordinated Al species (27 ppm), and octahedral Al. Critically,
no aluminum molybdate is formed, in stark contrast with the R-
ZSM-5 (S) sample. Indeed, it has been reported that MoO3
reacts with framework Al above 550 °C to form aluminum
molybdate and possibly surface AlMo6 Anderson entities.[7a]
Consequently, the absence of aluminum molybdate in [Mo]-
ZSM-5 (S) further indicates that all Mo atoms remain atomically
dispersed in the framework of [Mo]-ZSM-5 after steam treatment.
31P MAS NMR spectra (using TMPO) of the steamed samples
are depicted in Figure 8B. The number of Brønsted acid sites in
the [Mo]-ZSM-5 (S) sample decreased substantially after
steaming due to dealumination and the subsequent formation of
extra-framework aluminium, evidenced from both 31P MAS NMR
(Figure 8B) and 27Al MAS NMR (Figure 8A) spectroscopy, vide
supra. Importantly, no silanols are observed after the reaction-
regeneration cycles and subsequent steaming of the [Mo]-ZSM-
5 sample according to the 31P MAS NMR spectra (Figure 8B),
even though dealumination occurs. This demonstrates that
atomically dispersed Mo introduced into the MFI zeolite
framework stabilizes the crystalline structure and prevents the
formation of silanols. This is further supported by the fact that,
despite the occurrence of dealumination, the resonance at 45
ppm remains unchanged, suggesting that Mo is retained in the
framework.
Conclusion
Here we report the synthesis of a novel single-site [Mo]-
ZSM-5 zeolite using
a Mo-free ZSM-5 nanosized zeolite
subjected to treatment with sodium molybdate under
autogenous pressure. The single-site [Mo]-ZSM-5 zeolite
contains highly disperse Mo (0.5 wt.%) according to EDS-TEM
and ICP analyses; the Mo]-ZSM-5 zeolite was compared with a
reference sample prepared with 1 wt.% Mo by the classical
impregnation method (R-ZSM-5). The properties of the single-
site Mo containing ZSM-5 zeolite were characterized at the
atomistic and molecular level using diffraction, microscopy and
spectroscopy methods.
The novel single-site Mo-containing nanozeolite possesses
atomically
dispersed
framework-molybdenum
atoms
homogenously distributed through the zeolite crystals. The
introduction of atomically dispersed Mo in the zeolite framework
heals silanol defects resulting in an extremely stable material,
resistant to severe structural degradation. The novel single-site
Mo-containing ZSM-5 nanozeolite exhibits superior performance
for CH4 conversion to H2 and higher hydrocarbons operating
under successive reductive and oxidative atmospheres at high
temperature (850 and 750 °C), as well as demonstrating
exceptional structural stability under hydrothermal steaming
conditions.
In summary, zeolites have so far been used successfully and
extensively in oil refining and petrochemistry, and will continue
to play an important role by meeting the challenges in these
fields. However, the operating conditions (e.g. temperatures in
the 750–850 °C range) of some emerging industrial processes
will push zeolites outside this “comfort zone” and successful
zeolitic catalysts will require exceptional thermal and
hydrothermal resilience.
The single-site Mo-containing ZSM-5 represents a strong
example of a zeolite material able to operate “outside” of the
comfort zone. This novel synthesis approach can be applied for
the introduction of other metals (e.g. V, W, Cu, Zn) in other
calcined zeolites including pure silica and aluminosilicates.
Acknowledgements
Financial support from TOTAL and Industrial Chair ANR-TOTAL
“Nanoclean Energy” is acknowledged as well as from the
Normandy Region through his RIN Recherche Program. Authors
declare no competing interest in this work. GENESIS is
supported by the Région Haute-Normandie, the Métropole
Rouen Normandie, the CNRS via LABEX EMC and the French
Figure 8. (A) 27Al MAS NMR spectra of dehydrated [Mo]-ZSM-5 (S) and R-
ZSM-5 (S) samples, and (B) 31P MAS NMR spectra of TMPO adsorbed on
dehydrated [Mo]-ZSM-5 (S) and R-ZSM-5 (S) samples.
National Research Agency as
a
part of the program
This article is protected by copyright. All rights reserved.

10.1002/anie.202006524
Angewandte Chemie International Edition
“Investissements d’avenir” with the reference ANR-11-EQPX-
0020. The 111 Project (Grant No. B17020) providing
opportunities for future collaboration with Jilin University, China
is acknowledged.
Keywords: Mo single-sites, zeolite, synthesis, methane
conversion, hydrogen production, thermal stability, regenerability.
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This article is protected by copyright. All rights reserved.

10.1002/anie.202006524
Angewandte Chemie International Edition
Entry for the Table of Contents
A single-site Mo-containing nanosized ZSM-5 zeolite with atomically dispersed framework-molybdenum is synthesized. The single-
site Mo-containing nanosized ZSM-5 zeolite displays superior thermal (1000 °C), hydrothermal (steaming), and catalytic (converting
methane to hydrogen and higher hydrocarbons) stability, maintaining the atomically disperse Mo, structural integrity of the zeolite,
and preventing the formation of silanols.
.
This article is protected by copyright. All rights reserved.