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Reactivity, Selectivity, and Stability of Zeolite-Based
Catalysts for Methane Dehydroaromatization
Nikolay Kosinov* and Emiel J. M. Hensen
Dr. N. Kosinov, Prof. E. J. M. Hensen
Laboratory of Inorganic Materials and Catalysis Eindhoven University of Technology
P. O. Box 513, Eindhoven, MB 5600, The Netherlands E-mail: [email protected]
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202002565.
DOI: 10.1002/adma.202002565
strong C–H bonds complicates the selective conversion of methane to higher hydrocar-bons and oxygenates. Currently, the main industrial method to use methane as a mate-rial feedstock involves the transformation to syngas by steam reforming, followed by processes such as methanol synthesis and Fischer–Tropsch synthesis to obtain liquid fuels and other useful chemicals. A disad-vantage of this indirect technology is the high capital cost of reforming plants, making it only economically attractive at a very large scale. An efficient direct process for the conversion of methane to higher hydrocar-bons or oxygenates remains one of the “holy grails” of chemistry.[6,7] The approaches to
directly convert methane can be categorized into oxidative and non-oxidative methods. Oxidative processes are often met with low product selectivity because of the higher reactivity of targeted products as compared to methane itself, resulting in overoxidation to CO2. Non-oxidative methods are generally more atom-efficient,
because, in addition to valuable hydrocarbon products, pure COx
-free hydrogen gas (H2) is obtained, which can be used for instance
in fuel cells.[8] Among several alternatives, including non-oxidative
coupling of methane to ethylene and deep methane dehydrogena-tion to solid carbon and hydrogen,[9–12] methane
dehydroaromati-zation (MDA) remains one of the most promising non-oxidative reactions for the direct valorization of natural gas. MDA involves the selective conversion of methane to a mixture of easily trans-portable aromatics, that is, benzene (predominantly), naphthalene, and toluene. As all other non-oxidative methane conversion reac-tions, MDA is a highly endothermic process:
6CH4→C H6 6+9H2 ∆Hr0=532 kJ mol−1 (1)
The strong endothermicity dictates that high temperature is required to obtain a significant benzene yield (Figure 1). For example at a typical reaction temperature of 700 °C, a benzene yield of 11.7% can be achieved. Further, in the whole tempera-ture range of 300–1100 °C graphitic carbon (coke) is the ther-modynamically preferred product. These limitations make it necessary to use catalysts for performing the MDA reaction. An efficient MDA catalyst should i) effectively activate stable C–H bonds in CH4 molecules, ii) allow selective production of light
aromatics, minimizing the formation of unwanted graphitic carbon (coke), and iii) remain stable at high reaction temper-ature. Because of these strict catalyst requirements, materials chemistry is at the center of the MDA research.
Non-oxidative dehydroaromatization is arguably the most promising process for the direct upgrading of cheap and abundant methane to liquid hydrocarbons. This reaction has not been commercialized yet because of the suboptimal activity and swift deactivation of benchmark Mo-zeolite catalysts. This progress report represents an elaboration on the recent developments in understanding of zeolite-based catalytic materials for high-temperature non-oxidative dehydroaromatization of methane. It is specifically focused on recent studies, relevant to the materials chemistry and elucidating i) the structure of active species in working catalysts; ii) the complex molecular pathways underlying the mechanism of selective conversion of methane to benzene; iii) structure, evolution and role of coke species; and iv) process intensification strategies to improve the deactivation resistance and overall performance of the catalysts. Finally, unsolved challenges in this field of research are outlined and an outlook is provided on promising directions toward improving the activity, stability, and selectivity of methane dehydroaromatization catalysts.
1. Introduction
Methane is the main component of natural gas (≈95%) and the most abundant source of fossil carbon on Earth.[1] Currently,
nat-ural gas is an important source of energy and the primary feed-stock for the production of hydrogen and synthesis gas (syngas, CO + H2). Although natural gas is not sustainable in the strict
sense,[2] it remains the cleanest among all fossil fuels. In the
transi-tion to a renewable chemical industry, natural gas will increasingly become a major source of carbon. Further, in a sustainable fossil-free scenario of low-carbon economy, methane will remain an important and abundant molecule within the chemical industry. Methane is, for example, a major by-product of Fischer–Tropsch synthesis, CO2 hydrogenation, and biomass conversion as well as
the main component of biogas.[3–5] The chemical inertness of its
© 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and re-production in any medium, provided the original work is properly cited.
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The first example of suitable catalytic materials was reported in the 1980s. Minachev and co-workers demonstrated that, similar to aromatization of ethylene, ethane, and propane,[13–15]
it is possible to achieve benzene yields as high as 14% from methane at 750 °C in the presence of ZSM-5 zeolites, modi-fied by transition metals (Re, Cr, and Pt among others).[16,17]
In 1993 Wang et al. reported that Mo-modified ZSM-5 is a par-ticularly efficient MDA catalyst.[18] Following these pioneering
works, the MDA reaction has been attracting increasing atten-tion from both industry and academia. Besides its obvious com-mercial potential, MDA represents an intriguing conundrum from the perspectives of catalysis and reactor engineering. Although the reaction mechanism and the nature of the active sites have been actively studied, a complete chemical picture of the reaction is lacking. In recent years, several review papers dedicated to the non-oxidative conversion of methane and MDA in particular have been published.[19–25] Among others,
Ismagilov et al. reviewed the effects of preparation methods, composition, and pretreatment of Mo/ZSM-5 catalysts and the influence of reaction conditions on MDA performance.[26]
Schwach et al. provided a detailed analysis of the reaction path-ways in direct methane conversion processes.[27] Vollmer et al.
discussed developments in mechanistic understanding of the MDA reaction.[28] In this contribution, we focus on recent
lit-erature, relevant to the design of improved catalytic materials and eventually development of a commercially attractive MDA process. Special attention is attributed to materials chemistry aspects of MDA catalysts. We start by discussing the struc-ture of active Mo-centers and their evolution during activation, induction, and deactivation stages of the reaction. We continue with the mechanistic pathways underlying the conversion of methane to benzene and concurrent formation of coke species. Next, approaches to improve the efficiency of the MDA pro-cess, involving catalytic membrane reactors, chemical looping,
catalyst regeneration and optimal process parameters are briefly reviewed. Finally, we provide an analysis of possibilities and limitations of zeolite-based catalysts in direct non-oxidative methane conversion and an outlook on the most promising research directions in this field.
2. Structure of the Active Mo-Species
Mo/ZSM-5 is the most studied catalyst for the MDA reaction, which has been at the center of attention for several decades. With Mo/ZSM-5, it is possible to achieve benzene yields close to the thermodynamic limits (5–15% at 600–800 °C) at high benzene selectivity (70–90%) in a broad range of space veloci-ties and methane partial pressures. Despite hundreds of reports dedicated to this catalytic system, a complete understanding of the active sites and the roles of the different catalyst compo-nents remains lacking. Several important aspects are neverthe-less broadly accepted in literature. First, as-prepared Mo/ZSM-5
Nikolay Kosinov received a master degree from Novosibirsk State University, Russia, in 2010 and
obtained a Ph.D. degree from Eindhoven University of Technology (TU/e), The Netherlands, in 2014. After several projects as a research associate, he is an assistant professor at TU/e since 2018. His research is aimed at the synthesis, operando characterization, and application of heterogeneous catalytic materials as well as mechanistic understanding of their catalytic action. The main areas of interest include unconventional transformations of natural gas (methane) and CO2 to liquid fuels and chemicals.
Emiel Hensen obtained a Ph.D. degree from Eindhoven University of Technology in 2000. He was an assistant professor at the University of Amsterdam (2000–2001) and at Eindhoven from 2001 onwards. He was appointed associated professor in 2008 and full professor in 2009. He is chairman of the Netherlands Institute for Catalysis Research (NIOK) and board member of the European Research Institute on Catalysis (ERIC). His research interests include mechanism of heterogeneous catalysis for natural and synthesis gas conversion, biomass conversion, as well as topics related to synthesis of porous catalysts and catalysis for solar fuels.
Figure1. Maximum thermodynamically allowed levels of non-oxidative methane conversion to hydrogen and ethylene, benzene, coronene, and graphite. Calculated with Aspen Plus V8.6 software.
catalysts contain highly dispersed Mo(VI)-oxo centers, which are reduced and/or carburized under the reaction conditions giving rise to the active Mo-sites.[29] Second, the Si/Al ratio (and
hence Brønsted acidity) of ZSM-5 zeolite plays a critical role in the catalytic activity. Although non-acidic Mo/Silicalite-1 pre-sents low MDA activity,[30] excessive acidity (i.e., low Si/Al ratio)
leads to severe coking and catalyst deactivation.[31] Third, zeolite
shape selectivity is critical for obtaining high selectivity to light aromatics. While Mo dispersed on non-zeolitic acidic sulfated zirconia displays some MDA activity, such materials are much more selective toward naphthalene and coke than optimal zeo-lite-based catalysts.[32] Further, only ten-membered-ring (10MR)
zeolite topologies such as MFI (ZSM-5), MWW (MCM-22), MEL (ZSM-11), IMF (IM-5), TUN (TNU-9) with pore sizes of about 5.5 Å demonstrate high benzene selectivity.[33–36] Zeolites
with either smaller 8MR pores or larger 12MR pores are not selective and yield coke as the predominant product.[30,37]
Fourth, Mo-based systems are usually significantly more active and selective as compared to other transition metals.[38] Fifth,
there are several distinct stages during the MDA reaction over zeolite-based catalysts: induction, quasi-steady-state production of benzene, and deactivation. With this knowledge in mind, we will focus on the structure and evolution of Mo-centers in Mo/ ZSM-5 catalyst during its lifetime.
A typical synthesis of Mo/ZSM-5 involves the dispersion of a Mo(VI)-oxo precursor (most often molybdenum trioxide or ammonium heptamolybdate) on the external surface of the zeo-lite followed by a thermal treatment to enable the diffusion of Mo-species inside the zeolite pores and their interaction with Brønsted acid sites. Dong et al. were the first to demonstrate the migration of MoO3 into the pores of ZSM-5, a process that starts
already at 450 °C.[39] The nature of the precursor Mo(VI)-oxo
spe-cies has been a topic of debate. Figure 2a depicts the proposed structures, a common motive being the exchange of highly dis-persed Mo-oxo species with Brønsted acid sites. A recent work by Liu et al., who used integrated differential phase-contrast scan-ning transmission electron microscopy (iDPC-STEM) to study an as-prepared 4 wt% Mo/ZSM-5 catalyst (Figure 2c) directly illustrates the isolated nature of Mo-oxo complexes inside the zeolite pores.[40] Some of the most frequently
spectroscopi-cally observed species are MoO22+ monomers[41] and Mo2O52+
dimers,[42] stabilized on two proximate Brønsted acid sites. Also,
MoO2(OH)+ monomers stabilized on Brønsted acid sites and
silanols[43] as well as larger Mo(VI)-oxo oligomers on the external
surface have been observed.[44] The exact nature of the Mo-oxo
precursor in practical catalysts depends on several parameters, including the preparation method, the Si/Al ratio, and the Mo loading. Increasing the Si/Al ratio and/or the Mo content leads to the formation of more Mo2O52+ dimers, while lowering the
Si/Al ratio and/or the Mo content results in the predominant formation of MoO22+ monomers.[45] Even the Al distribution
within the zeolite framework can have an effect on the structure of the Mo-oxo precursors. Vollmer et al. showed that by varying the siting of Al atoms from mainly Al pairs to mainly isolated Al sites on can shift the distribution from MoO22+ to MoO2(OH)+.[46]
Although the structure of Mo(VI)-oxo species is an interesting and relevant topic, the actual active sites of methane activation are reduced/carburized Mo-centers. The location of the active Mo sites in the zeolite pores has been firmly established. Bao and
co-workers used electron paramagnetic resonance (EPR)[47]
spec-troscopy and 95Mo nuclear magnetic resonance (NMR)[48,49]
spectroscopy to correlate the aromatic formation rate with the amount of partially reduced Mo atoms originating from initially exchanged Mo-sites (Figure 2d). Many other kinetic, synthetic, and spectroscopic studies confirmed this finding.[50–53] The
struc-ture and evolution of the active Mo-centers during induction, quasi-steady-state reaction and deactivation periods is a much more complicated phenomenon. Initial TPSR studies demon-strated the formation of significant amounts of CO, CO2, and
H2O before benzene was observed.[54] Clearly, Mo(VI)-oxo species
are not the active MDA sites and reduced and/or carburized Mo-centers are responsible for the activation of methane. The exact structure of these centers, however, has been a topic of debate for several decades already. Based on many spectroscopic investiga-tions, proposed active Mo-species include MoO3, partially reduced
MoO3–x, α-Mo2C1–x, β-Mo2C, η-Mo3C2, and MoOxCy nanoparticles,
clusters of MoCx and MoCxOy dispersed inside the zeolite pores,
smaller MoxCy clusters consisting of several Mo atoms, and
par-tially reduced monomeric and dimeric Mo species stabilized by the zeolite framework (Figure 2b).[55–59] This inconsistency in
lit-erature is caused by the difficulties associated with the characteri-zation of Mo/ZSM-5. First of all, at typical Mo loadings >2 wt%, the system is rather heterogeneous. Fresh Mo/ZSM-5 can contain different types of Mo-oxo clusters, stabilized inside the pores, and larger agglomerates on the external surface. During the reaction the agglomerated species transform into larger Mo2C
nanoparti-cles and the dispersed species remain inside the pores even after the reduction as either small (oxy)carbidic particles, clusters, or even single-atom species.[60] To make it even more complicated
the distribution of Mo between all these structures depends on Mo loading, Si/Al ratio, crystal size of the zeolite support, catalyst preparation and pretreatment methods, and many other parame-ters. Second, the harsh reaction conditions of MDA reaction make the operando characterization difficult, because not many oper-ando spectroscopy cells can reach such high temperature (≥650 °C). Finally, quick accumulation of polyaromatic carbon species com-plicates UV–vis, IR, and to some extent NMR analysis.
In the last two decades, in situ and operando spectroscopic characterization of heterogeneous catalysts has seen a sig-nificant progress.[61] Operando X-ray absorption spectroscopy
has become an essential technique to study nanoscale active sites, including those in Mo/ZSM-5. Using a combination of operando high-energy resolution fluorescence detection X-ray absorption near-edge spectroscopy (HERFD-XANES) and X-ray emission spectroscopy (XES), Lezcano-González et al. fol-lowed the evolution of Mo-centers during the lifetime of 4 wt% Mo/ZSM-5 catalyst at 677 °C.[62] The authors concluded that
the initially isolated Mo(VI)O22+ species attached to the
zeo-lite framework were converted to metastable Mo(V), Mo(IV), and Mo(III)-oxycarbide species and then detached from the zeolite framework forming Mo(II)C3 clusters. These clusters
can diffuse out of the pores and eventually form larger Mo2C
agglomerates on the external surface. By using a pulse reaction technique combined with operando XANES analysis at 700 °C (Figure 2e), we showed that the structure and evolution of the Mo-species strongly depends on the Mo loading.[63]
Com-paring Mo/ZSM-5 catalysts with Mo loadings of 1 wt% (ini-tially fully dispersed as MoO22+ monomers), 2 wt% (initially
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mainly dispersed as MoO22+ monomers with some larger
MoxCy agglomerates on the external surface) and 5 wt%
(sim-ilar fractions of dispersed species and surface agglomerates) demonstrated a strong dependence of the performance on the Mo speciation. While the 1 wt% Mo/ZSM-5 sample was only partially reduced at the moment benzene formation was observed, the 5 wt% Mo/ZSM-5 sample started pro-ducing benzene after the Mo reduction was complete. Two stages of Mo reduction were identified: an initial fast reduc-tion step followed by a much slower one, which were respec-tively assigned to removal of oxygen atoms from Mo(VI)-oxo species and detachment of the Mo species from the zeolite framework after complete reduction. Extended X-ray absorp-tion fine structure (EXAFS) and high-angle annular dark-field (HAADF) STEM measurements showed that the Mo phase in 1 wt% Mo/ZSM-5 catalyst did not agglomerate even after 16 h
of reaction at 700 °C. Most likely, the Mo-species remain iso-lated during the whole process of induction, activation, and deactivation. The samples with a higher Mo content showed formation of Mo2C nanoparticles on the external surface.
Agote-Arán et al. combined operando XANES, EXAFS, and powder X-ray diffraction characterization of a 3.8%Mo/ZSM-5 catalyst to demonstrate that as much as 70% of the Mo-species, initially dispersed inside the pores, can be detached from the zeolite framework and agglomerate into larger MoCx species
during the reaction.[64] Although these recent operando
char-acterization studies have greatly advanced our understanding of the Mo/ZSM-5 catalysts, the detailed structure of the most active dispersed Mo-species or MoxCy clusters responsible for
the actual MDA reaction remains to be elucidated.
To summarize, many active site configurations have been proposed to operate in the MDA catalytic cycle. It is likely that Figure2. a) Proposed structures of Mo(VI)-oxo species dispersed in zeolites; b) proposed structures of active reduced Mo-centers; c) representa-tive iDPC-STEM image of Mo/ZSM-5, showing the presence of off-center contrast in many 10-MR channels. Insets show zoomed-in areas 1, 2, and 3 respectively and corresponding intensity line profiles. Adapted with permission.[40] Copyright 2020, Wiley-VCH. d) 95Mo NMR spectra of as-prepared Mo/HZSM-5 with varying Mo loadings and MoO3 as well as correlation of the aromatic formation rate with concentration of different molybdenum species. Adapted with permission.[48] Copyright 2008, American Chemical Society. e) Operando Mo K-edge XANES spectra recorded while pulsing methane over a 5 wt% Mo/ZSM-5 catalyst at 700 °C. Corresponding reduction degree of Mo and the benzene yield values obtained simultaneously by MS analysis. Adapted with permission.[63] Copyright 2018 Wiley-VCH.
practical catalysts, usually characterized by a relatively high Mo loading, will contain Mo in a diverse range of structures discussed above, each contributing to the catalytic activity.[65]
Based on the shape-selective nature of benzene formation and microscopic and spectroscopic observations the MDA-active Mo-species are most likely located inside the zeolite pores, while Mo-species on the external surface predominantly form coke.[30] Nevertheless, the structure of the most active
Mo-species remains one of the main open questions in the MDA field. Fast development of operando spectro scopy characteriza-tion makes us confident that this quescharacteriza-tion will be answered sooner than later. It will definitely lead to the development of better catalysts by designing of materials with the maximum number of the most active Mo-sites. Knowing the active site requirements, however, will likely not be enough to design the optimal MDA catalysts. There is a second crucial aspect: the actual reaction mechanism. The complete understanding of the reaction pathway from methane to benzene should allow designing catalysts that, in addition to high activity in methane conversion, would demonstrate low coke selectivity and slow rate of deactivation. Next section is dedicated to discussing and evalu-ating the existing mechanistic proposals.
3. Reaction Mechanism
Initially in analogy to other zeolite-catalyzed processes carbe-nium ion and radical mechanisms were proposed.[18,66] After
three decades of MDA research, there are two main mecha-nistic proposals remaining: i) a bifunctional mechanism in which C–H bond activation and C–C coupling take place on the Mo active sites, while further aromatization of C2 intermediate
fragments occurs on Brønsted acid sites; and ii) a hydrocarbon pool-like pathway where activation of C–H bonds on Mo sites is followed by the transformation of (hydro)carbon intermediates confined inside the zeolite pores (Figure 3).
The bifunctional mechanism has been widely discussed in the literature and follows conventional high-temperature chemistry of C2 hydrocarbons, generated by C–C coupling, on Brønsted acid
sites. Ethylene is commonly considered as the initial product of C–C coupling and main reaction intermediate, mainly because it is a significant product of the reaction and its selectivity increases during the catalyst deactivation.[67] In a recent kinetic study Razdan
et al., however, proposed that ethane is the initial product of C–C coupling and acetylene is the key reactive intermediate.[68] In
addi-tion, Vollmer et al. showed that significantly different reactivities of ethylene and methane over Mo/ZSM-5 at MDA conditions might
indicate that ethylene is not the actual intermediate.[69] Regardless
of the nature of C2 intermediates, the bifunctional mechanism
explains many of the observations, particularly with respect to the superior activity of acidic zeolites toward formation of aro-matics.[70] There is, however, a significant amount of experimental
data contradicting this model. First, several recent studies showed that Mo/Silicalite-1 can also catalyze the MDA reaction.[30,71] As
Mo-oxo species bind much stronger to Brønsted acid sites than to silanol groups, another role of zeolite acidity is to improve the dispersion of the Mo sites, which are involved in methane activa-tion. Indeed, upon comparing Mo/ZSM-5 and Mo/Silicalite-1 cata-lysts Agote-Arán et al. found that the distance between Mo centers and zeolite framework oxygen atoms is significantly longer in Mo/ Silicalite-1.[71] Thus, the inferior activity of Mo/Silicalite-1 can be
explained by the weaker stabilization of highly dispersed Mo and thus the lower activity in methane activation.
Induction period at the early stage of the reaction is likely the key to understanding the MDA mechanism. The induction period has been for a long time exclusively attributed to the reduction and/or carburization of Mo-oxide precursor phase. A proper kinetic analysis of the MDA reaction is hampered by the absence of a steady-state operating regime, as induction, reaction and deactivation quickly succeed each other.[72] Pulsed
reaction techniques, which allow titration of surface species are powerful tools for studying such transient processes.[73] By
lim-iting the amount of methane supplied to the catalyst, fast tran-sient processes can be resolved, which is often not possible in a continuous flow regime. Jiang et al. applied a pulse reaction technique to show the complex autocatalytic nature of processes taking place during the induction period.[74] Partial reduction of
Mo6+ and carbon deposition were proposed to take place at the
beginning of reaction. A further detailed analysis of several Mo/ ZSM-5 catalysts varying in Mo loading by pulsing reaction tech-nique combined with operando and quasi-in-situ spectroscopic characterization led to the identification of three main reaction stages (Figure 4a): i) activation (autocatalytic reduction of Mo(VI) with CO as main carbon-containing product); ii) induction (for-mation of surface carbon species), and iii) autocatalytic forma-tion of benzene.[63,75] We note that rapid carbon deposition at the
beginning of the reaction has been observed in continuous flow experiments as well.[76] In pulse experiments, the duration of the
induction period was nearly independent of the Mo loading. The carbon deposited on the catalyst during the induction period and the resulting zeolite/carbon composites were investigated by a combination of techniques such as Ar adsorption, 13C, and 1H
NMR spectroscopy, and EPR spectroscopy. Together these data point to the predominant formation of polyaromatics with a rad-ical character confined in the zeolite micropores (Figure 4b). Iso-tope experiments with 13CH
4/12CH4 switches demonstrated that
the deposited carbon species are involved into the catalytic cycle. As much as 70% of benzene molecules produced directly after an isotope switch contained at least one carbon atom derived from the surface carbon (Figure 4c). Figure 4d shows that this involvement of surface carbon is not related to the reversible nature of the overall reaction.[63,75] Although this mechanism
shares similarities with the well-established carbocation-based hydrocarbon pool mechanism as commonly assumed for the methanol-to-hydrocarbons reaction,[77] the nature of the reactive
intermediates and the overall chemical pathway seem different. Figure3. Two proposed reaction mechanisms for the MDA reaction.
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Recently, a similar hydrocarbon pool type mechanism was pro-posed for the conversion of ethane to benzene over Mo/ZSM-5, whereas ethane aromatization over Ga/ZSM-5 proceeded via a different mechanism and metal-free HZSM-5 was not active for the reaction.[78] This finding indicates that even in case of C
2
compounds, being the main initial products of methane activa-tion, the hydrocarbon pool-like mechanism would still be a valid explanation for the reactivity of Mo-zeolite catalysts.
Despite the significant progress, the complete mechanism of the MDA reaction remains unclear. For example, the possibility of both bifunctional and hydrocarbon pool-like mechanisms to co-exist during the MDA reaction cannot yet be ruled out. The structure of the confined (hydro)carbon pool intermediates is not known either and the next section is dedicated to the recent progress in this direction.
4. Structure and Evolution of Deposited
Carbon Species
While coke is a widely studied subject in the field of zeolite chemistry,[79,80] the nature and structure of carbon species
formed during MDA has not been resolved yet. Mainly temper-ature-programmed oxidation (TPO) and thermal gravimetric
analysis (TGA) have been used for the characterization of coke deposits formed over Mo/ZSM-5. A common distinction is made between oxidation of Mo-species to MoO3 (corresponding
to a combustion peak at 400–450 °C and weight gain in TGA profiles), “soft coke” (combustion peak at 450–550 °C overlap-ping with Mo2C oxidation and corresponding to a weight loss
in TGA profiles) and “hard coke” (separate combustion peak at 550–650 °C and weight loss).[81] The two types of coke are
usually considered to differ in chemical nature, being formed respectively on Mo and Brønsted acid sites.[82] Such
distinc-tion might not be appropriate as, for instance, comparison of DTG profiles and 13C NMR spectra for a series of deactivated
Mo/ZSM-5 catalysts showed that all coke species exhibit very similar sp2 nature of formed polyaromatic carbon, despite
com-pletely different combustion behavior (Figure 5a,b). An expla-nation for differences in the combustion can be the amount of coke formed in the zeolite micropores. It has been indeed shown that a significant fraction of coke species is formed inside the zeolite micropores.[83] Furthermore, the
combus-tion behavior of coke strongly depends on the size of zeolite crystals[84] and the nature and location of metal species which
catalyze the oxidation of coke.[85] All these findings point at the
diffusion-limited nature of coke combustion during TPO and TG characterization, while the chemical specificity of these Figure4. a) Per-pulse conversion of methane and yields of surface carbon species (estimated by the carbon balance), carbon monoxide, and benzene during pulsing methane over 2 wt% Mo/ZSM-5 catalyst. b) Structure of surface carbon species that might form inside the straight ZSM-5 channels during the induction period as derived from 13C NMR, EPR and modeling studies. Adapted with permission.[75] Copyright 2018, American Chemical Society. c) Results of pulsing isotopic exchange experiments where 12CH4 was pulsed after pulsing 13CH4 over Mo/ZSM-5 at 700 °C. Complete profiles demonstrating the distribution of benzene isotopologues; a zoom in at the first pulse after the switch to 12CH
4 is shown in the inset. d) Development of 13C concentration in benzene and methane molecules with the number of 12CH
4 pulses demonstrating a non-equilibrium nature of the observed isotope exchange in benzene molecules. Adapted with permission.[63] Copyright 2018, Wiley-VCH.
methods is questionable. The dissolution of zeolite framework in completely spent Mo/ZSM-5 (Figure 5c) revealed that at late stages of the reaction the carbonaceous deposits form essen-tially a zeolite-templated carbon material (Figure 5d),[75] that is,
particulate microporous carbon framework consisting of curved graphene fragments and featuring short-range order.[86] This
result demonstrates that MDA coke is predominantly formed inside the zeolite pores, and that the polyaromatic carbon forms a (semi)connected structure, completely blocking the access to the active sites. The model of zeolite-templated carbon (ZTC) nature of MDA coke species together with the hydrocarbon pool mechanism can explain, for example, the extremely low MDA selectivity of Mo/MOR catalysts (virtually zero ben-zene yield was observed over Mo/MOR).[30] The structure and
connectivity of ZTC materials strongly depends on the topology of the zeolite template,[87] the inability of Mo/MOR to catalyze
the MDA reaction might be related to the unsuitable structure of the hydrocarbon pool intermediates formed inside its pores.
At the moment, it is not yet clear how the growth of carbon structures inside the zeolite pores proceeds, how it depends on the zeolite topology, what is the fraction of reactive intermediates in surface carbon during the whole course of the MDA reaction and how to avoid the transformation of the (hydro)carbon inter-mediates into the extended ZTC-like structures. These aspects are important for designing more deactivation-resistant MDA catalysts and will require further research efforts. The already developed strategies to decrease the deactivation rate of the existing MDA catalysts are discussed in the next section.
Figure5. a) DTG profiles and b) 13C MAS NMR spectra of Mo/ZSM-5 catalyst exposed to methane flow at 700 °C for different periods of time. Labeled 13CH
4 methane was used to prepare the samples, except for the samples exposed for 960 min. c) A process of coke liberation from the spent Mo/ZSM-5 catalyst by dissolving the zeolite in HF; d) Scanning electron microscopy (SEM) image of spent Mo/ZSM-5 catalysts with carbon content of 13 wt% and carbon product remained after dissolving the inorganic fraction of spent Mo/ZSM-5 in HF with carbon content of 96 wt%. Ar adsorption isotherm of the obtained carbon material is shown in inset. Adapted with permission.[75] Copyright 2018, American Chemical Society.
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5. Process Intensification
The formation of surface carbon, as we outlined above, pre-cedes the formation of light aromatics in the MDA reaction. The further growth of these carbon species into large polyaro-matic ZTC-like structures lead to complete catalyst deactivation.
Many approaches to improve the stability of MDA catalysts and overall efficiency of the MDA process have been developed. As shown by Figure 6, all these approaches can be categorized as aiming to mitigate the rate of coke deactivation, to increase the single-pass methane conversion or to achieve both these targets simultaneously. To increase the single-pass methane Figure6. Process intensification strategies developed for improving the catalytic performance of MDA catalysts, including approaches directed toward i) mitigating the formation of coke; ii) increasing single-pass methane conversion; and iii) both these aspects simultaneously. a) Scheme of a current-controlled co-ionic catalytic membrane reactor (CMR), where methane is converted to benzene and hydrogen over a Mo/zeolite catalyst, H2 is trans-ported as protons to the sweep side and oxide ions are transtrans-ported to the reaction medium to react with H2 and form steam as an intermediate before reacting with coke to form CO and H2; b) comparison of aromatic yield versus time of the CMR and a fixed-bed reactor (FBR). Gray-shaded areas indi-cate when hydrogen is extracted; c) methane conversion and selectivity to main products after 5 h (FBR) and 9 h (CMR). d) Coke deposition amounts and cumulative aromatic production. Reaction conditions: 710 °C, 1.5 L g−1 h−1, 1 bar, and current density of 40 mA cm−2. Adapted with permission.[100] Copyright 2016, AAAS. Effect of pressure on the MDA performance of Mo/ZSM-5 catalyst: e) benzene yields at 1 and 10 bar and f) pressure depend-ence of cumulative product distributions and total amounts of converted methane. Reaction conditions: 700 °C, 10 L g−1 h−1, total reaction time 15 h. Adapted with permission.[103] Copyright 2017, Wiley-VCH.
conversion of equilibrium-limited (Figure 1) MDA reaction it is necessary to shift the equilibrium toward the products. Hydrogen-permeable reactors that allow selective removal of hydrogen from the reaction zone have been studied for this purpose. Conventional hydrogen-selective membranes are Pd-based and both pure Pd[88,89] and Pd–Ag[90] membranes have
shown a significant improvement of methane conversion and the initial aromatic yield. Since the stability of Pd-based mem-branes under harsh MDA reaction conditions might become an issue, alternative membrane materials such as mixed oxide La5.5W0.6Mo0.4O11.25−δ have been studied and demonstrated
an increased methane conversion as well.[91] In addition to
hydrogen permeable membranes, passive hydrogen-absorbing materials can be used to shift the equilibrium. Kumar et al. found that under MDA conditions Zr reacts with H2 to yield
ZrH1.75 and that the absorbing function of Zr pellets can be
efficiently regenerated by treatment in He flow at 700 °C. It was not possible, however, to regenerate the catalytic Mo/ ZSM-5 function under these conditions.[92] A common
disad-vantage of passive hydrogen removal from the reaction zone is the increased rate of coke formation. The reactions leading to the formation of hydrogen-poor coke species, in accordance with the Le Chatelier’s principle, are more sensitive to the hydrogen concentration than the aromatization itself.[93]
6CH4↔1C H6 6+9H2 Aromatization (2)
6CH4↔CH (solid)x x→0+12H2 Coke formation (3)
As a result, in majority of studies dedicated to passive hydrogen removal, an increased selectivity to coke and naphthalene and faster catalyst deactivation were observed. Alternative approaches of active hydrogen removal can be used to over-come this problem. Simple continuous co-feeding of oxidants (O2, CO2, CO2 + O2, H2O) have been shown to have a
promo-tional effect on the deactivation stability of Mo/zeolite catalysts but in a narrow range of oxidant concentration.[94] There exist a
fine balance between the amount of surface carbon that can be removed and the re-oxidation of the active Mo-phase to the inac-tive Mo(VI)-species. Another concern, related to the continuous co-feeding of oxidants in fixed-bed reactors, is inhomogeneous distribution of the oxidant along the bed. As experimentally demonstrated by Bhan and co-workers, at 700 °C the reactivity of even mild oxidants such as CO2 is so high that majority of
oxidant molecules are converted with methane in the top zone of the reactor, resulting in the formation of syngas. In the bottom zone of the reactor a conventional MDA reaction in presence of CO and extra H2 takes place.[95,96] In view of this
effect, non-continuous strategies of oxidant supply have been developed and showed a significant potential. We developed a pulsing protocol for the selective combustion of coke species, where instead of continuous co-feeding of small concentration of O2 together with methane, short and concentrated pulses of O2
were periodically fed to the reactor. This strategy allowed over-coming the problem of inhomogeneous distribution of oxygen through the catalyst bed and resulted in a significant improve-ment of cumulative aromatic yields.[97] Zhang et al. showed
that mechanical mixing of Mo/ZSM-5 with a reducible oxide (Ce0.9Gd0.1Oy) for the chemical combustion of the formed
hydrogen is a promising strategy to shift the MDA equilibrium toward aromatic products without excessive formation of car-bonaceous deposits.[98] In this process, the steam formed from
hydrogen can oxidize coke precursors and reduce the deacti-vation rate. The oxide nature of Ce0.9Gd0.1Oy component also
makes the combustion regeneration of the hybrid catalysts feasible. Cao et al. demonstrated that using an oxygen-selec-tive membrane, based on Ba0.5Sr0.5Co0.8Fe0.2O3-δ perovskite, it
is possible to uniformly distribute oxygen within the catalyst bed and significantly improve the MDA performance of Mo/ MCM-22 catalyst.[99] Application of catalytic membrane reactors,
capable of simultaneously extract hydrogen and distribute oxide ions within the catalyst bed is particularly promising. Morejudo et al. demonstrated that electrochemical co-ionic BaZrO3-based
membrane reactors (Figure 6a) are effective for this purpose. Comparison of BaZrO3 membrane reactor with a conventional
fixed bed reactor (Figure 6b–d) showed a significantly slower deactivation rate and an increased maximum aromatic produc-tivity at the same time.[100]
The second group of MDA process intensification strategies is based on shifting the equilibrium toward the reactants to minimize the coke formation rate. As discussed above the reac-tions of coke formation are more sensitive to approaching the equilibrium and therefore it is possible to selectively mitigate the growth of coke by shifting the equilibrium toward the reac-tants. Increasing the reaction pressure, first described by Ichi-kawa and co-workers,[101,102] is a simple and effective method to
retard the deposition of coke and enhance the cumulative yield of hydrocarbon products at the expense of maximum methane conversion. We recently observed an order of magnitude higher benzene productivity of 2%Mo/ZSM-5 catalyst after increasing the pressure of methane from 1 to 15 bar (Figure 6e). Interest-ingly the improvement of catalyst stability is also accompanied by a higher selectivity toward valuable alkylated aromatics: tol-uene and xylene (Figure 6f). Elevated pressure operation was found efficient for catalysts with different Mo loading and in a broad range of temperatures and space velocities.[103] Using
isotope labeling, TG-MS and 13C NMR we found that the
for-mation of coke in MDA reaction is a highly reversible process. In fact, as much as 40% of 13C coke species formed after first
20 min of MDA reaction in 13CH
4 flow can be exchanged
fol-lowing 100 min of reaction in 12CH
4 at 5 bar. At elevated
pres-sure, a larger fraction of surface carbon participates in the MDA reaction and it results in a much higher degree of pore utilization and aromatic productivity.[103]
Other related process intensification strategies include CH4-H2 switch operation and continuous co-feeding of H2 and
CO. Continuous co-feeding of H2[104] can improve the aromatic
productivity in the same way as the elevated pressure opera-tion, although the thermodynamic penalty on methane conver-sion is much higher in case of added H2. Co-feeding of CO is
another efficient method to improve the catalyst stability in a non-oxidative manner and without decreasing the methane conversion.[105,106] It is not yet completely clear why adding CO
leads to a significantly slower coking deactivation of Mo/ZSM-5 catalysts, but a viable hypothesis involves dissociation of CO on Mo-sites and removal of a certain amount of coke precursors as CO2, while the “extra” carbon atoms participate in the
forma-tion of aromatics.[107] Finally, periodic CH
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the most efficient methods to stabilize the performance of Mo/ ZSM-5 catalysts.[108] In this mode, it is possible to minimize the
formation of coke by its intermediate hydrogenation without suppressing the methane conversion level and overoxidation of the active Mo-phase. Using CH4 (15 min)–H2 (45 min) periodic
switch operation Sun et al. achieved a stable MDA performance of 5.5%Mo/ZSM-5 catalysts for over 1000 h.[109] Disadvantages
of CH4-H2 switch strategy include the use of excess of hydrogen
gas and suboptimal catalyst productivity related to the long regeneration time.
6. Conclusions and Outlook
As demonstrated in this progress report, MDA field has expe-rienced a significant development in the last years. The level of mechanistic and structural understanding has significantly increased; synthetic advances enabled fabrication of more efficient catalytic materials, while novel process intensifica-tion techniques allowed improved catalysts stability and aro-matic yields. Further developments are nevertheless necessary to bring the MDA process to the level of commercialization.
Figure 7 presents our view on the main development directions
within the MDA field. 6.1. Catalyst Engineering
The main directions of further MDA development are related to the materials chemistry. As discussed in this progress report the active site requirements for designing active, selective, and
stable MDA catalysts are generally known and future advances in high-temperature operando characterization of catalysts will definitely complete the picture. The actual synthetic approaches to meet these requirements, however, have not been developed yet. First of all it is still impossible to achieve a high concen-tration of dispersed Mo-sites (>0.2 mmol g−1) while minimizing
the concentration of Brønsted acid sites in order to obtain active, aromatic-selective, and coking-resistant catalytic mate-rials. Several promising synthesis strategies to achieve this target can be identified. First of all, it is now established that Mo(VI)-oxo precursors preferentially interact with two acid sites to form either MoO2+ monomers or Mo
2O52+ dimers. Also,
engi-neering of siting and distribution of Al atoms within the ZSM-5 framework in such a way that preferably Al pairs are formed is a known technique.[110] Using ZSM-5 materials with Al centers
distributed in pairs followed by a well-defined deposition of Mo-oxo precursors (using for instance CVD approaches[111]) on
the paired Brønsted acid sites to exchange them completely might lead to a high loading of the active Mo-species and min-imum residual Brønsted acidity. Second, the exchange of the residual Brønsted acid sites with catalytically inert cations (Ca2+,
Na+) can be an effective method to minimize the rate of coke
formation.[112] Finally, Mintova and co-workers have recently
demonstrated that it is possible to isomorphously substitute framework Si atoms with Mo and W atoms in Silicalite-1 zeo-lite.[113,114] Such materials are potentially promising MDA
cata-lysts because of the ultimate dispersion of the active transition metal species and complete absence of Brønsted acidity.
Furthermore, other transition metals such as Co,[115] Fe,[116]
Mn,[117] Ni,[118] Re,[119] and W[120] are known to exhibit a
sub-stantial MDA activity. These metals are present as reduced metallic, carbidic, or cationic centers in working catalysts, but the structure of the active species have not been resolved in the same detail as for Mo. It has also been established that disper-sion of these metals in zeolite pores is required for obtaining aromatics, similar with the Mo/zeolite case. Nevertheless, all these metals are significantly less active than Mo and it would be important to establish the reason why Mo is a much better MDA catalyst. This phenomenon can relate to intrinsic activity differences among the transition metals but may also depend on the structure of active sites and their location, leaving room for improvements. Therefore, answering the question about the superior activity of Mo-zeolite catalysts will not only shed light on the chemistry of non-oxidative methane activation but also allow designing better and potentially cheaper catalytic mate-rials. In our opinion, synthesis of different well-defined tran-sition metal sites, confined in zeolite pores, detailed operando characterization, and accurate comparison of their intrinsic C–H activation and C–C coupling activity (as well as hydro-carbon selectivity) combined with DFT modeling of the catalytic pathways is a research direction well worth exploring.[121–123]
Promotion of Mo/ZSM-5 with a second catalytic metal func-tion is another viable opfunc-tion for designing better MDA cata-lysts. The majority of transition metals have been evaluated as dopants to Mo/ZSM-5.[124–126] Significant improvements
in stability, activity, and selectivity of Mo/ZSM-5 catalysts were observed with Fe, Co, and Ni.[127–129] As these metals
demonstrate high methane conversion activity (for instance for the decomposition of methane to carbon materials and Figure7. Key future directions in the field of MDA development, with
focus on materials chemistry aspects: engineering of active, selective, and stable catalysts; understanding the structure and mechanism of coke formation; development of materials and processes for efficient catalyst regeneration; and process intensification technologies.
hydrogen gas), it is likely that their promotional role is related to increasing the rate of C–H bond activation. Another expla-nation for the observed improved performance can be the exchange of remaining Brønsted acid sites with metal (oxo) cations, effectively decreasing coking propensity. The influence of a second metal on the performance of Mo/ZSM-5 catalysts is only understood at the phenomenological level and therefore deserves more attention.
Shape selectivity is a critical descriptor of activity and espe-cially selectivity of zeolite-based catalysts. So far, exclusively 10MR zeolite topologies have shown reasonable aromatic selec-tivity in MDA reaction. One may even draw a parallel with methanol-to-hydrocarbons reaction in which the 10MR zeo-lites are known to produce a significant amount of BTX aro-matics as well.[130] Another explanation for such a significant
topology effect should be considered here: different stabiliza-tion of Mo-oxo species in different coordinastabiliza-tion environments. Unlike for many other transition metal–zeolite composites, for instance, Cu/zeolite systems,[131] this aspect has not been
studied in Mo/zeolite materials. The zeolite topology can likely have an effect on at least the reducibility of Mo-species, stabi-lized inside the zeolite pores. Detailed analysis of the effect of zeolite topology on the structure and chemical properties of dis-persed Mo-species remains a promising direction in the field of MDA as well.
The textural, morphological and surface structuring of the zeolite component is a promising (and generally underexplored in the context of MDA) strategy to improve the performance of Mo/ZSM-5 catalysts. First of all, external Brønsted acidity strongly affects the performance of zeolite catalysts and leads to the enhancement of unselective reaction pathways, increased coke selectivity, and faster catalyst deactivation.[132] In MDA
reaction, the issue of coke formation on the external surface is amplified by the high reaction temperature and accordingly swift graphitization of hydrocarbon fragments in a not shape-selective environment. The inertization of the external surface via i) silylation of the external active sites[133]; ii) removal of acid
sites by NH4F treatment[134]; and iii) core–shell structuring with
a layer of Al-free Silicalite-1 on top of the ZSM-5[135] have been
reported to significantly improve the hydrocarbon selectivity of MDA catalysts.
As with any other reaction of zeolite-catalyzed hydrocarbon conversion, the MDA activity suffers from the slow diffusion of molecules inside the micropores and suboptimal utilization of the zeolite crystal.[136,137] Conventional methods of decreasing the
cat-alyst particle size and hierarchical pore structuring influence the performance of MDA catalysts in a non-linear manner. While shortening the effective diffusion pathway inside the micropores is beneficial for the MDA performance, the excessive external surface area leads to the enhanced rate of coke deposition. This effect results in the existence of optimal zeolite particle size (≈2 µm) with larger particles being too diffusion-limited and smaller particles too coke-selective.[138] In case of hierarchical
pore structuring, a similar phenomenon results in the presence of optimal proportion between micro- and mesoporosity. To pre-pare active and hydrocarbon selective catalysts it is necessary to achieve the balance between the effective length of diffusion pathways in micropores and the non-shape-selective conver-sion of methane on the external surface.[139,140] In addition to
conventional hierarchically porous zeolites, in recent years, sev-eral novel hollow zeolite structures have been synthesized, used as MDA catalysts and displayed significant deactivation stability and high extent of crystal utilization.[141–143] The promising
per-formance of hollow zeolite catalysts is likely related to a combi-nation of shortened diffusion pathway within the micropores and limited external surface area. Further systematic studies are necessary to determine the optimal (hierarchical and/or hollow) porous structure and identification of textural descriptors similar to the hierarchy factor[144] will likely be useful in this regard.
6.2. Structure and Role of Surface Carbon
The surface carbon plays a key role in both the activity and deac-tivation of Mo/ZSM-5 catalysts. Provided that the hydrocarbon pool mechanism offers a more suitable model for explaining the reactivity of MDA catalysts, understanding the structure of the surface intermediates is of significant importance. Once the structure of the intermediates is known a rational design of the optimal transition metal–zeolite topology pairs will become much more feasible. Stabilization of organic intermediates in confined space of zeolite micropores is sensitive to the reac-tion condireac-tions. It means that operando or quasi-in situ spec-troscopy efforts will be required to solve the structure of the hydrocarbon pool components.[145] Isotope labeling techniques
will be required to distinguish the reactive intermediates from the spectator coke-like species. Finally, impressive recent devel-opments in NMR[146–148] and STEM[149–151] characterization of
organic molecules, confined inside the zeolite pores, will defi-nitely be useful for unraveling the fine structural details of MDA intermediates.
6.3. Process Intensification and Regeneration Stability
After three decades of MDA research, it has become clear that industrial application of this reaction would take a cer-tain degree of chemical engineering creativity. Low single-pass methane conversion and quick and inevitable accumulation of heavy carbonaceous deposits require significant process inten-sification efforts to turn the MDA reaction commercially attrac-tive. Several particularly efficient approaches, including CH4-H2
switch strategy, elevated pressure operation, and use of electro-chemical co-ionic membrane reactors have been developed and optimized at the lab-scale. Further progress in this direction, especially related to the long-term stability and up-scaling, is necessary. Although the number of long-term reaction-regener-ation studies in MDA field is limited, it is known that oxidative regeneration (combustion of coke in air) is much more harmful toward Mo/zeolite catalysts than reductive regeneration (metha-nation of coke in hydrogen):
x+ → + +
CH O2 CO CO2 H O Combustion2 (4)
x+ →
CH H2 CH4 Methanation (5)
Nevertheless, air combustion of coke is a much more effective and economically viable strategy[152] and it is the most likely
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future commercial choice. The stability of Mo-zeolite compos-ites upon oxidative regeneration is, therefore, one of the key development directions in the field of MDA. There are several factors, determining the instability of Mo/ZSM-5 during oxida-tive regeneration: exothermic nature of coke combustion results in a significant overheating during regeneration; evolving water leads to the hydrothermal degradation; and finally the reduced Mo-centers are oxidized to MoO3 during the air regeneration.
The strong negative effect of excessive loading of MoO3 on the
thermal stability of aluminosilicate zeolite materials is well-documented.[153] Too high concentration of MoO
3 results in a
chemical reaction with framework aluminum atoms, formation of Al2(MoO4)3 phase, dealumination and eventually collapse of
the crystalline zeolite structure. Decreasing the Mo loading to avoid the formation of the Al2(MoO4)3 is an effective strategy
to improve the stability of Mo/ZSM-5 composites during high-temperature reaction–oxidative regeneration cycling.[154] In the
future, exploring other active transition metals, not aggressive or even capable of stabilizing the zeolite framework,[155] might
also be a promising approach for enhancing the regeneration stability of MDA catalysts. Further, improving the thermal and hydrothermal stability of the zeolite matrix itself is another important development line. Indeed, the ideal MDA catalyst should work for thousands of high-temperature reaction and regeneration cycles while retaining the initial structure. Typical degradation mechanisms of zeolitic materials under such con-ditions are related to the cleavage of Si–O–Si bonds, this process being accelerated by the presence of Al atoms and framework defects.[156,157] Recent synthetic investigations in this direction
are therefore focused on decreasing the Si/Al ratio and low-ering the number of silanol defects. Several reports have shown that controlled hydrothermal treatment in ammonium fluoride solution is a versatile strategy to lower both the surface Si/Al ratio and number of framework defects.[158,159] The NH
4F
treat-ment is particularly efficient for removing Al from the external surface. As a result, the zeolite materials treated with NH4F
dis-play a significant improvement of the framework stability.[160]
To summarize, methane dehydroaromatization to a mix-ture of benzene, naphthalene, and toluene is one of the most promising direct gas-to-liquid technologies. Swift coking deac-tivation and low single-pass methane conversion present a significant hurdle for the commercialization of this process. Further developments in synthesis and operando characteriza-tion of MDA catalysts will definitely bring this process closer to the industrial application. For this purpose a number of challenges still need to be solved, including improvement of the long-term reaction-regeneration stability of MDA cata-lysts, decrease of naphthalene and coke selectivity, increase of single-pass methane conversion, development of effective sep-aration technologies to recover H2 from methane-rich stream
prior to its recycling, etc. Apart from the future commercial value, MDA reaction is a fundamentally important chemical problem. The very process of understanding the structure and catalytic action of Mo-sites and complex molecular pathways, underlying the direct conversion of methane to benzene in zeolite pores, will teach us important lessons about the reac-tions of C–H bond activation and C–C coupling, catalysis in confined spaces, and design of advanced microporous materials.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
catalyst characterization, methane conversion, shape selectivity, zeolite catalysts
Received: April 15, 2020 Revised: June 3, 2020 Published online: July 12, 2020
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