Initial Carbon?Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization

Article abstract of DOI:10.1002/anie.202007283

Methane dehydroaromatization (MDA) is among the most challenging processes in catalysis science owing to the inherent harsh reaction conditions and fast catalyst deactivation. To improve this process, understanding the mechanism of the initial C?C bond formation is essential. However, consensus about the actual reaction mechanism is still to be achieved. In this work, using advanced magic-angle spinning (MAS) solid-state NMR spectroscopy, we study in detail the early stages of the reaction over a well-dispersed Mo/H-ZSM-5 catalyst. Simultaneous detection of acetylene (i.e., presumably the direct C?C bond-forming product from methane), methylidene, allenes, acetal, and surface-formate species, along with the typical olefinic/aromatic species, allow us to conclude the existence of at least two independent C?H activation pathways. Moreover, this study emphasizes the significance of mobility-dependent host–guest chemistry between an inorganic zeolite and its trapped organic species during heterogeneous catalysis.

Full text of DOI:10.1002/anie.202007283

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Illuminating Initial Carbon-Carbon Bond Formation during the
Early Stages of Methane Dehydroaromatization
Authors: Mustafa Çağlayan, Alessandra Lucini Paioni, Edy Abou-
Hamad, Genrikh Shterk, Alexey Pustovarenko, Marc Baldus,
Abhishek Dutta Chowdhury, and Jorge Gascon
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202007283
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Illuminating Initial Carbon-Carbon Bond Formation during the
Early Stages of Methane Dehydroaromatization
Mustafa Çağlayan[a], Alessandra Lucini Paioni[c], Edy Abou-Hamad[b], Genrikh Shterk[a], Alexey
Pustovarenko[a], Marc Baldus[c], Abhishek Dutta Chowdhury*[a][d] and Jorge Gascon*[a]
[a]
M. Çağlayan, Dr. A. D. Chowdhury, G. Shterk, A. Pustovarenko, Prof. Dr. J. Gascon
KAUST Catalysis Center (KCC), Advanced Catalytic Materials
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955 (Saudi Arabia)
E-mail: jorge.gascon@kaust.edu.sa
[b]
[c]
[d]
Dr. E. Abou-Hamad
Imaging and Characterization Department, Core Labs
King Abdullah University of Science and Technology
Thuwal 23955 (Saudi Arabia)
A. Lucini Paioni, Prof. Dr. M. Baldus
NMR Spectroscopy group, Bijvoet Centre for Biomolecular Research
Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
Dr. A. D. Chowdhury
The Institute for Advanced Studies (IAS)
Wuhan University
Wuhan 430072, Hubei (China)
E-mail: abhishek@whu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: Still in 2020, methane dehydroaromatization (MDA) is
among the most challenging processes in catalysis science due to the
inherent harsh reaction conditions and fast catalyst deactivation. To
improve it further, understanding the initial C-C bond formation
mechanism is sine qua non. However, consensus about the actual
reaction mechanism is still to be achieved. In this work, using
reaction mechanism, particularly during the early/induction period,
of this potential industrially relevant reaction should be prioritized,
as such information often plays a critical role to upgrade both
catalysts and the process itself.[5,6]
Based on the growing agreement, active Mo species for methane
activation are originated from mostly monomeric and/or dimeric
advanced magic angle spinning (MAS) solid-state NMR spectroscopy, MoxOy complexes attached to the bridging oxygens of the zeolite,
we study in detail the early stages of the reaction over a well-
dispersed Mo/H-ZSM-5 catalyst. Simultaneous detection of acetylene
(i.e., presumably the direct C-C bond forming product from methane),
methylidene, allenes, acetal and surface-formate species along with
the typical olefinic/aromatic species allow us to conclude the
existence of two independent C-H activation pathways. Moreover, this
study emphasizes the significance of mobility-dependent host-guest
chemistry between inorganic zeolite and its organic trapped species
during heterogeneous catalysis.
especially the ones between framework Al3+ and Si4+.[7–9]
However; a strong debate (Section S2.1) about the structure of
active Mo sites (MoOxCy, MoxCy etc.), location (pore surface vs.
external surface) and their role in the reaction mechanism is still
to be resolved.[10–15] Besides the ongoing debates regarding the
active sites, gaining insight into the initial C-C bond formation
mechanism and further oligomerization-cyclization of the
intermediates is critical. In many earlier studies, Mo sites and
Brønsted acid sites (BAS) of ZSM-5 were considered as two
‘independent’ reaction centers.[16–21] Herein, active molybdenum
The direct conversion of methane, the main component of natural
gas and a byproduct of oil refining, into more value-added liquid
fuels and chemicals, keeps attracting a great deal of attention.
Although the non-oxidative methane dehydroaromatization
(MDA) was first reported on Mo loaded ZSM-5 catalyst by Wang
et al. in 1993[1], it is still an attractive research field, not only due
to its inherent challenges with respect to the initial C-H bond
activation (first bond dissociation energy: 439.3 kJ/mol)[2], but
also its reaction mechanism and the exact role of the popular Mo-
ZSM-5 catalysts are yet to achieve consensus within the scientific
community. Since the success of Mo/ZSM-5 system, several
different metals (W, Fe, V, Cr etc.) and supports (ZSM-8, MCM-
22, TNU-9 etc.) have already been screened.[3,4] However, none
of these catalysts has been able to overcome the inevitable
deactivation due to coking. Therefore; the understanding of
Scheme 1. MDA mechanisms proposed by (a) Wang et al.[20], (b) Chen et
al.[17] and (c) Mériaudeau et al.[23]
1
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sites would form ethylene that would be further oligomerized-
cyclized to aromatics on BAS. Wang et al.[1,20] proposed that CH4
is polarized (CH3δ+-Hδ-) by partially reduced MoO(3-x) forming a
carbene-like “CH2=MoO(3-x)” species, which led to ethylene
(Scheme 1a) However; Chen et al.[17] suggested that methane
activation occurs with the formation of free CH3· radicals formed
at the interphase of MoOx and BAS (Scheme 1b). Then, the
system proceeds with dehydrodimerization of the methyl radicals
and aromatization of the ethylene produced. In contrast to the
abovementioned mechanisms based on bifunctionality in which
ethylene is the main intermediate, Mériaudeau et al. proposed an
alternative ‘acetylene-based’ monofunctional mechanism
(Scheme 1c).[22,23] Initially they detected low-concentrations of
acetylene (compared to ethylene) that increased upon reducing
contact time.[22] In light of these results, the following two
mechanisms were proposed: (i) a monofunctional pathway, where
acetylene, the main intermediate, is oligomerized on molybdenum
sites (Mo2C) into polyene and cyclized into aromatics; and (ii) a
bifunctional pathway, where acetylene (and possibly ethylene
Figure 1. Catalytic activity of 2 wt% Mo/H-ZSM-5 at 725 °C under non-oxidative
conditions (11.765 mol% N2 and 88.235 mol% CH4 in the inlet, WHSV: 2.58 h-1)
(a) Conversion and Total Yield (without CO and CO2), (b) Yields of CO,CO2 and
H2 Flow (c) Yields of major aromatic products, (d) Yields of major aliphatic
products
+
also) form high-weight polyenes through vinyl cations (C2H3 ) on
BAS.[23] The latter pathway was considered as extremely fast and
requires free acid sites. However, the spectroscopic identification
on both solid-state (i.e., zeolite-trapped) and gas-phase of any
abovementioned intermediates has not been achieved yet. In
recent literature as well, the mechanism is still ambiguous. In
2018, Kosinov and his colleagues[11] brought a new perspective
to the induction period first defined by Rosynek et al.[18] by
distinguishing it from the reduction of Mo species. After the
formation of active Mo sites, they observed a buildup of
hydrocarbon species inside the pores and proposed that these
confined polyaromatic molecules have a similar role to that of the
organocatalytic intermediates found in the well-known methanol-
to-hydrocarbon (MTH) reactions. Vollmer et al. also investigated
the mechanism with 13CH4 after showing the active molybdenum
site formation without any coke via CO treatment.[12,24] They
observed the incorporation of carbidic carbons in Mo sites into the
products (i.e. benzene) and stated that it is very similar to the role
of oxygen in the Mars-Van-Krevelen mechanism. Moreover; in a
recent study, Vollmer and her colleagues stated that C2H4 is likely
not being the main intermediate of MDA.[25] Therefore, the nature
of the formation of the first C-C bond containing species, is still
unclear and ambiguous. With the aim to illuminate this issue,
herein, we establish a fundamental understanding on the early
stages of DHA reaction; primarily through the identification of
numerous reactive intermediates (acetylene, surface-formate,
acetal, allene etc.) as well as mono- and bi-functionality of the
catalytic materials. As a result, the ‘mobility-dependent’ host-
guest chemistry between Mo-ZSM-5 and its trapped organics has
also been evaluated.
but only at the beginning and in a very small amounts (푌
=
퐶푂2
0.03 % in the first injection). After this period, the typical MDA
activity of Mo/ZSM-5 catalyst is observed. Total product yield
(푌푡표푡푎푙) and conversion (푋퐶퐻4) reach maximum values and then
decay with time. In Figure 1c and d, the dominant aromatic and
aliphatic products in the outlet stream are presented. As expected,
the major contribution to total product yields comes from the
aromatics. When the catalytic system reaches the maximum
hydrocarbon yield (not including CO and CO2), some of the major
products (i.e. benzene, toluene, naphthalene, ethane) also reach
their peak values. However; products like ethylene, propylene, 1-
butene (Figure S8b) and o-xylene (Figure S8c) show their
maximum production rate after the maximum total product yield.
More details about the synthesis, characterization of the
fresh/spent catalyst, and catalysis can be found in the Supporting
Information.
In the next-phase of our study, in order to provide more in-depth
mechanistic information, advanced magic angle spinning (MAS)
solid-state NMR spectroscopy was performed on post-reacted 2
wt. % Mo/H-ZSM-5 material after the MDA reaction at 725 °C
using fully 13C-enriched methane (13CH4) as reactant (Figures 2-
3, Section S2.5-S2.6 and Figures S11-S19 in the ESI). We
prepared two sets of samples, after 50 min and 2 hours of reaction
time, i.e., (i) MoZSM5_50 min, and (ii) MoZSM5_2 hr, to identify
the catalytically influential species during the early stages of
reaction by multidimensional solid-state NMR correlation
experiments. The 1H−13C cross-polarization (CP)[26]
,
1H−13C
insensitive nuclei enhanced by polarization transfer (INEPT)[27]
,
In the first phase of our study, to prevent the formation of
molybdenum clusters, we prepared a Mo loading of ca. 2 wt. %
on a nano-sized ZSM-5. The characterization of the fresh catalyst
(Raman, XRD, N2 physisorption and TEM-EDX mapping) proved
the good dispersion of Mo (see Section S2.2). In Figure 1, the
performance of the catalyst under non-oxidative conditions is
shown. As it can be clearly observed (Figure 1a-b); during the
initial two GC injections, a low total product yield accompanied by
the high formation rate of CO and H2 indicate the reduction of Mo
sites and activation of the catalyst. Although CO formation after
the initial Mo reduction period decreases significantly; it continues
in low amounts during the whole process. CO2 was also detected
and 13C direct excitation (DE) solid-state NMR spectra of the
spent catalyst are presented in Figure S11. In general, the
following four features were primarily observed in the 1D 13C
spectra: (i) -5 − -10 ppm zeolite trapped multiple adsorbed forms
of unreacted methane, (ii) 20−52 ppm aliphatic moieties (e.g.
2
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olefinic/(poly)aromatic species (Figure 2c-e). For
example in 13C-1H correlation experiments, the
resonances at ~121 (13C) / ~7.6 (1H) ppm and ~129
(13C) / ~8 (1H) ppm in MoZSM5_2hr sample could be
attributed to olefinic and aromatic species, respectively
(Figure 2e).[29] Moreover, in the MoZSM5_50 sample,
the cross-peaks between ~81 (13C) and ~2 (1H) ppm are
possibly the most significant observation in this work,
which is a characteristic spectroscopic signature of
acetylene (Figure 2d).[38] Acetylene has long been
hypothesized in MDA reaction as the most influential
direct C-C bond containing intermediate from
methane,[22,23] but its spectroscopic detection was
elusive till now. Not only on solid-state, acetylenes have
also been detected in the gas-phase (along with other
alkenes and allenes), through cold-trapping control
experiments (Section S2.6 & Figure S20-21).[39] The
slight downfield nature of signals could be attributed to
the bi-functional nature of the zeolite environment,
which is also paramagnetic in nature.[40] In addition, two
more important oxygenate-based intermediates have
been identified particularly in the MoZSM5_50 sample:
(i) acetal (~104 (13C) and ~6.5 (1H) ppm in Figure
2c[28,41] and (ii) surface-formate (~174 (13C) and ~10
(1H) ppm in Figure S13a) species.[28,42]
Figure 2. Identification of post-reacted Mo/H-ZSM-5 zeolite-trapped mobile molecules in J-
coupling based 2D MAS solid-state NMR 13C-1H correlations experiment(s) (*:spinning side
bands). The spectra were obtained after (13C-)methane dehydroaromatization reaction over
Mo-ZSM-5 at 725 °C for 50 min (in blue) and 2 hour (in red): (a) identified molecular keys,
zooms of (b) different adsorbed forms of methane, (c) alkylated unsaturated hydrocarbon
(olefins/aromatics) regions, including acetal and (d) acetylene/alkyne (in 50 min sample only)
as well as (e) alkylated unsaturated hydrocarbon (olefins/aromatics) regions in 2 hour sample.
The respective full-range spectra, as well as experimental details, were included in the
supporting information.
To probe rigid/immobilized molecules, both 2D 13C–1H and 13C–
13C dipolar-based correlation spectra were acquired (Figure 3,
see also Figure S12, S14-S16, S18-S19). Characteristically, rigid
species are very similar to their mobile counterparts (Figure 3a).
Again, the multiple rigid-version of adsorbed unreacted methane
was identified (Figure 3a).[33–37] The presence of numerous
zeolite-trapped alkylated olefinic/(poly)aromatic species were
also detected through both 2D 13C–1H (Figure 3c,e) correlations,
which is consistent with our GC analysis as well. Interestingly, a
higher degree of alkylation was observed among rigid molecules
alkyl/methoxy groups), (iii) 110−150 ppm aromatic/olefinic groups,
and (iv) 170−175 ppm surface-formate species.
A complementary set of solid-state NMR magnetization transfer
sequences was deliberately employed for the spectral separation
of zeolite-trapped species based on mobility.[28–30] This strategy,
that was originally developed for the analysis of
biomolecules,[31,32] has recently gained attention in the elucidation
of reaction mechanisms in zeolite-chemistry and to evaluate host-
guest chemistry. As a result, we could independently distinguish
both mobile (i.e., with fast tumbling) and rigid (i.e.,chemisorbed)
versions of zeolite-trapped organics (cf. Figure S11).
Such spectral separation of mobile and rigid species is
possible by invoking through-bond (scalar interactions
such as in INEPT)[27] and through-space (dipolar
transfer such as in CP)[26] magnetization transfer
sequences, respectively. For instance, through-bond
mediated magnetization transfer schemes will be able
to detect molecules that exhibit fast overall tumbling or
contain locally mobile groups, such as seen in methyl
groups that display fast rotation around the C–C axis
(see the individual INEPT spectrum in Figure S11).
Similarly, the (CP-like) dipolar-based magnetization
transfer mechanisms primarily could detect either
immobilized/rigid (such as physisorbed in or on the
catalyst material) or limited mobility (such as in the
case of
a molecule trapped within the zeolite
structures) species. Notably, all chemical species
(irrespective of mobility) could be identified using direct
excitation experiments.
In the solid-state NMR spectra probing mobile
molecules (Figure 2, see also Figure S13 & S17),
signals corresponding to multiple adsorbed forms of
unreacted methane were readily identified in the upfield
region (Figure 2b).[33–37] However, the predominant
zeolite-trapped hydrocarbon species are (expectedly)
Figure 3. Identification of post-reacted Mo/H-ZSM-5 zeolite-trapped rigid molecules in dipolar-
coupling based 2D MAS solid-state NMR 13C-1H correlation experiments. The spectra were
obtained after (13C-)methane dehydroaromatization reaction over Mo-ZSM-5 at 725 °C under
dry conditions for 50 min (in blue) and 2 hour (in red): (a) identified molecular keys, zooms of
(b) different adsorbed forms of methane, (c) degree of alkylation (methyl/
methylene/methylidene groups), as well as (d) alkylated unsaturated hydrocarbon
(olefins/aromatics) regions, acetal and formate species . The respective full-range spectra, as
unsaturated
in
nature,
such
as
alkylated well as experimental details, were included in the supporting information.
3
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compared to their mobile counterparts, as both methylene (-CH2-,
e.g., ~36 (13C) and ~2.8 (1H) ppm in MoZSM5_2hr sample) and
surface-formate species, and Mo-methylidene as well as
acetylene, which could easily be correlated with bi- and mono-
methyl
individually
distinguishable
3c).[29] Contrary, only
groups
were
functionality
catalytic
of
the
material,
(Figure
respectively.[10]
Almost two decades ago,
Mériaudeau et al. first
proposed activation of
methane to acetylene,
methyl
groups
were
observed among mobile
molecules (Figure 2c,e).
Interestingly, methylene
signals were only visible
and
aromatization over Mo-
carbide into
followed
by
after
2
hours, which
indicates a progression of
the degree of alkylation
benzene.[22,23] In this work,
the spectral identification
of acetylene provides
support to their proposal.
Hence, in this route,
during
Moreover,
the
reaction.
a few
methylene-based
spin systems
correlations
rigid
have
between
bifunctionality is not a
pre-requisite criterion, as
the Mo-methylidene could
be the precursor of Mo-
Scheme 2. Plausible reaction pathways during the MDA process evaluating the (a) mono- and
(b) bi-functional features of the involved catalytic materials (CHA: C-H activation, COI: CO-
insertion, MNA: methoxy nucleophilic attack). All intermediates proposed in this scheme are
experimentally/spectroscopically verified in this work.
~42-45 ppm (13C) and ~5-
6 ppm (1H), which could
be
attributed
to
carbide (pathway a in
methylidene groups (CH2=X, X ≠ C or O, see Figure 3c). Herein,
we hypothesize that such methylidene groups should be attached
to Mo, inducing carbene character on the catalyst (Mo=CH2), and
could be treated as a precursor of the well-known molybdenum
carbide species. It should be mentioned that similar “Mo=CH2”
groups are recognized as the active sites during Mo/W-zeolite
catalysed metathesis reactions.[43–45]
Next, both rigid-version of acetals and surface-formate species
(Figure 3e) were identified. Overall, signals of rigid molecules
have always demonstrated more than one peak for the same
resonance, which means that the same molecule exists in
Scheme 2).[52,53] Simultaneously, it is important to recognize that
methane activation upon molybdenum is also associated with
deoxygenation (XPS spectrum of spent catalyst Figure S5b, and
additional discussion in Section S2.3), which eventually leads to
the formation of CO (Figure 1b) during the process. The Brønsted
acid sites of zeolite then undergo CO-insertion to form the
corresponding surface-formate species, which then lead to the
formation of acetal upon nucleophilic attack from the in-situ
formed surface-methoxy species (pathway b in Scheme 2).[28,41]
All three key intermediates, i.e., surface-formate, surface-
methoxy, and acetal, were experimentally verified in this work
different molecular environments inside the zeolite framework, i.e., (Figures 2,3). Unlike in the acetylene/methylidene-route,
heterogeneity in the molecular environment of the entrapped
species. This same observation was also observed for unreacted
methane (Figure 2b & 3b), as its multiple adsorbed forms were
identified too and hence, further advocates for the local
heterogeneity within the material. The existence of surface-
formate species, i.e., the 13C correlations observed at 170-173
ppm correlating with a 1H-signal at ~8 ppm (Figure 3e), is
indicative of the occurrence of CO-insertion[46–51], which is known
to generate hydrocarbon pool species during the zeolite-
catalysed MTH process (vide-infra).[28,41,42] However, it should
also be mentioned that these species can only be formed in the
very early stages of the reaction making use of O present in the
zeolite and metal oxide Mo precursor (cf. Figure 1b).[11,12,24]
On the basis of the above-mentioned results, different reaction
pathways for the MDA process are proposed in Scheme 2. Due
to the controversial status of the MDA reaction mechanism, we
have only considered the concrete experimental/spectroscopic
evidence observed in this work to construct this mechanistic
overview, particularly during early stages of reaction. Although it
has been long hypothesized that the reaction mechanisms of both
MTH and MDA processes have similarities,[11] the extent of
resembles has never properly been illustrated. Our study reveals
that the at-least two C-H activation pathways were simultaneously
operational during the initial period of MDA reaction; one of them
bifunctionality is a mandatory criterion in the carbonyl-route
(pathway b in Scheme 2). The later route is indeed analogous to
MTH chemistry and extensively verified by both spectroscopic
and theoretical means in recent times.[6,28,29,54–56] Finally, all these
intermediates lead to the formation of the classical hydrocarbon
pool (HCP) type mechanism during the auto-catalytic period of the
reaction. Similar to MTH catalysis on ZSM-5, hydrocabons
trapped in the framework affect the unit cell dimensions, so the
difference between the lengths of a- and b- vectors decreases
with deactivation (XRD refinement analysis in Section S2.3-
2.4).[57]
In summary; beyond identifying initial C-C bond containing
reactive intermediates not mentioned before in MDA literature and
initial C-H activation pathways during the early stages of reaction,
probably the most prominent outcome of this work is the utilization
of ‘mobility-dependent’ advanced MAS solid-state NMR
spectroscopic methodologies to unravel the nature of zeolite-
trapped organic species during MDA. This information is
extremely difficult to obtain by operando techniques (or other
spectroscopic/analytic means) due to the harsh reaction
conditions inherent to MDA process. We anticipate that such
precise information would allow the catalysis community to
develop technologically and economically viable MDA processes
in the near future. In a broader perspective, this work also
has a conceptual resemblance with the MTH reaction mechanism. highlights the significance of host-guest chemistry during zeolite-
In our work, three distinct and significant findings (compared to
the current state-of-the-art) are the spectroscopic identification of
catalyzed hydrocarbon conversions.
4
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18814−18824.
Acknowledgements
[25] I. Vollmer, E. Abou-Hamad, J. Gascon, F. Kapteijn, ChemCatChem
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Funding for this work was provided by King Abdullah University of
Science and Technology (KAUST). ADC also thanks the starting
grant support from IAS, Wuhan University. ALP was supported by
a TOP‐PUNT grant (no. 718.015.001) to M.B. from Netherlands
Organization of Scientific Research (NWO).
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Keywords: methane dehydroaromatization • reaction
mechanism • zeolites • solid-state NMR • bifunctional catalyst
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5
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10.1002/anie.202007283
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The initial carbon-carbon (C-C) bond formation during the early stages of methane dehydroaromatization was investigated by employing
‘mobility-dependent’ solid-state NMR. Based on the detected species (like acetylene, acetal etc.); two different mechanisms (mono-
functional vs bi-functional) to the formation of direct C-C bonds were identified.
6
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