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Chem Synth 2022;2:21. 10.20517/cs.2022.26 © The Author(s) 2022.
Open Access Review

Fundamentals of the catalytic conversion of methanol to hydrocarbons

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Institute of Advanced Interdisciplinary Studies, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China.

*Correspondence to: Prof. Jianfeng Huang, State Key Laboratory of Coal Mine Disaster Dynamics and Control, Institute of Advanced Interdisciplinary Studies, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China. E-mail: jianfeng.huang@cqu.edu.cn

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    Academic Editor: Ying Wan | Copy Editor: Peng-Juan Wen | Production Editor: Peng-Juan Wen

    © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

    Abstract

    For more than four decades, the methanol-to-hydrocarbons (MTH) reaction has been a successful route to producing valuable fuels and chemicals from non-petroleum feedstocks. This review provides the most comprehensive summary to date of recent research concerning the mechanistic fundamentals of this important reaction, covering different reaction stages. Mechanisms that have been proposed to explain the initial C-C bond formation during the induction stage of the MTH reaction are introduced, including the methoxymethyl cation, Koch carbonylation, carbene and methane-formaldehyde processes. At present, there is no consensus regarding these hypothetical mechanisms as a consequence of the limited amount of conclusive experimental evidence. The steady state of the MTH reaction is also examined with a focus on the widely accepted indirect hydrocarbon pool mechanism and the dual cycle concept that provides a mechanistic basis for the effects of zeolite structures and reaction conditions on product distribution. In the following section, advanced characterization techniques capable of providing new insights into the formation of coke species during the MTH reaction and innovative approaches effectively inhibiting coke formation are introduced. Finally, a summary is provided and perspectives on current challenges and the future development of this area are presented.

    INTRODUCTION

    The methanol-to-hydrocarbons (MTH) reaction catalyzed by zeolites was first discovered by Mobil scientists in 1977[1] and subsequently commercialized for the first time in New Zealand in 1985[2]. This process then became an important platform for the production of various high-demand chemical commodities, such as via methanol to gasoline (MTG)[3,4], methanol to olefins (MTO)[5-9], methanol to propene (MTP)[10-12] and methanol to aromatics (MTA) systems[13-15]. Since the MTH reaction uses methanol or dimethyl ether (DME) as the feedstock, both of which can be readily produced from nonpetrochemical resources such as natural gas, coal and biomass[16-18], this process provides an alternative to current petroleum-based synthetic routes and has thus attracted extensive attention[19-21].

    In addition to obvious economic benefits[22,23], understanding the complex MTH reaction mechanism as a means of improving efficiency represents an important fundamental research goal. Over the past several decades, numerous studies of MTH have been reported and various reaction mechanisms have been proposed. There have also been several review articles in the literature that focus on different aspects of the MTH reaction, including the effects of zeolite pores on product selectivity[4,22], the sub-reactions associated with the dual cycle mechanism[20], the analysis of chemical kinetics using molecular modeling[24] and principles for the design of catalysts[21].

    This review aims to provide a comprehensive summary of the MTH reaction mechanisms reported to date. To this end, the mechanisms associated with the different reaction stages are separately elucidated. We first discuss four direct mechanisms explaining the formation of the first C-C bond during the induction period. These comprise the methoxymethyl cation, Koch carbonylation, carbene and methane-formaldehyde mechanisms. These discussions primarily focus on the differences between the proposed mechanisms as well as areas where there is a lack of consensus. Advances in characterization techniques that have provided new experimental evidence and an improved understanding of these long-standing issues are also assessed. The next section examines the MTH process when in the steady state, which primarily determines product selectivity, with a focus on the widely accepted indirect hydrocarbon pool mechanism and the dual cycle concept. On the basis of the well-developed dual cycle mechanism, the effects of zeolite structures and reaction conditions on product distribution are explained. In particular, the enhanced production of propene based on optimizing the extent to which the aromatic and olefin cycles proceed is discussed. Subsequently, the deactivation of MTH catalysts is assessed. This section presents the most recent works investigating the mechanisms by which coke species [comprising primarily polycyclic aromatic hydrocarbons (PAHs) and oxygen-containing species] are formed and introduces some effective approaches to reducing coke formation. Finally, we conclude this review with a summary and share our vision for the future of MTH research.

    FORMATION OF THE INITIAL C-C BOND

    Since the 1970s, numerous scientists have devoted considerable effort to determining the mechanism of the MTH reaction and more than twenty different theories have been proposed. However, the precise route to the formation of the first C-C during the induction stage of the MTH reaction is still being debated. Initially, it was believed that trace impurities (potentially originating from the methanol feed, incomplete combustion of organics in the zeolite, the carrier gas or other sources) were responsible for the conversion of methanol in any direct mechanism that converts methanol/DME to hydrocarbons via a coupling process[25]. This concept was dominant for a long time, primarily because of a lack of convincing experimental data to support a direct mechanism. More recently, there has been much research devoted to the analysis of the first C-C bond formed in the initial reaction stage. Several years ago, Yarulina et al. and Olsbye et al. discussed potential mechanisms in high-quality reviews[21,26]. The present review provides more recent discoveries based on the use of advanced characterization techniques, with a particular focus on elaborating on the reasons for the ongoing lack of consensus. Four firmly supported direct mechanisms are discussed in this section.

    Methoxymethyl cation mechanism

    In 2003, Wang et al. found that methanol or DME molecules are adsorbed on Brønsted acid sites to produce surface methoxy species (SMSs) that are highly active with regard to hydrocarbon generation[27]. This prior work indicated that, rather than organic impurities affecting product distribution or the hydrocarbon pool species, these SMS sites could promote the initial C-C bond formation during the MTH reaction[28]. With the ongoing development of spectroscopic techniques, many investigations of the first C-C bond formation have been performed and the facile generation of SMSs as active intermediates on catalytic sites has often been confirmed. As an example, Li et al. observed the formation of methoxy groups on the zeolite SAPO-34 using infrared (IR) spectroscopy and proposed that methoxymethyl cations (CH3OCH2+) generated by the reaction of SMSs with DME served as intermediates[29]. These cations are coupled with additional DME/methanol molecules to produce various compounds incorporating C-C bonds (such as 1,2-dimethoxyethane and 2-methoxyethanol)[29]. In 2017, Wu et al. successfully observed a surface metheneoxy analog species on ZSM-5 with the aid of a solid-state nuclear magnetic resonance (NMR) technique and hypothesized that the strong interaction of the active C-H bond in DME/methanol adsorbed on the zeolite with SMSs played a crucial role in the initial C-C bond formation process[30]. In 2018, Wang et al. observed the bonding of SMSs to extra-framework Al atoms on ZSM-5 using 13C-{27Al} solid-state NMR, thus establishing the possibility of interactions between these Al atoms and SMSs[31]. In addition, SMSs adsorbed on the 8-MR pore windows of the zeolite SAPO-34 were detected by Lo et al. using synchrotron X-ray powder diffraction-mass spectrometry (MS) with Rietveld refinement of the data[32]. This prior work demonstrated that the presence of SMSs limited the diffusion of C4 species through the pore openings of the SAPO-34 and thus facilitated the production of light olefins during the MTO reaction[32]. More recently, Sun et al. reported the first observation of surface ethoxy species serving as ethene precursor on a CHA zeolite during an MTH reaction using in situ solid-state NMR[33]. Ab initio molecular dynamics simulations were also applied to simulate the dynamics of the first C-C bond generation. As shown in Figure 1, the presence of a reactive SMS intermediate on an active site was determined by NMR analysis and a short-range collision between a DME molecule and this SMS was found to activate the DME. Subsequently, the first C-C bond was formed via an electrophilic attack to produce a C2 precursor[33]. These previous studies confirmed the formation of methoxymethyl cations as a consequence of the reactions between SMSs and methanol/DME, representing the methoxymethyl cation mechanism.

    Figure 1. An ab initio molecular dynamics simulation of C-C bond formation subsequent to the collision of DME with an SMS. (A) The two-dimensional free energy surface and minimal energy path associated with the first C-C bond formation. The images show five (meta-) stable states: (I) reactant basin, (II) approaching state, (III) activation state, (IV) product state and (V) product basin. (B) Changes in the C-O, C-C, O-H and C-H bond distances in DME and the SMS over the SSZ-13 zeolite. Reproduced with permission from ref. 33[33]. Copyright 2021, Elsevier. DME: Dimethyl ether; SMS: surface methoxy specie.

    Koch carbonylation mechanism

    The electrophilic C in methanol/DME and the nucleophilic C in CO readily undergo an addition reaction to form a C-C bond. In 2016, Liu et al. proposed that the carbonylation of SMSs contributes to the first C-C bond formation to generate an acetyl species that then dissociates to produce methyl acetate or acetic acid, which can be detected as intermediates [Figure 2][34]. While monitoring the MTH reaction over ZSM-5 at 400 °C, this same group detected an increased conversion rate following the addition of CO to the methanol feedstock. Since here the involved carbonylation is similar to the Koch-type carbonylation by which carboxylic acids can be produced from the reaction of high-pressure CO with alcohols/olefins and H2O, the mechanism for the C-C bond formation was recognized as the Koch carbonylation mechanism. The feasibility of the Koch carbonylation mechanism has also been supported by theoretical calculations that have shown a relatively low energy barrier of 80 kJmol-1 compared with the value of 330 kJmol-1 for the ylide-type mechanism[34,35]. Using solid-state NMR and operando UV-visible diffuse reflectance spectroscopy, Chowdhury et al. identified a direct C-C bond-containing species, i.e., surface-bound acetate, a known Koch carbonylation product that can be obtained from SMSs[36]. The surface-bound acetate species reacted with methanol to produce methyl acetate, as confirmed by the 13C NMR signals at 55.2 and 178.5 ppm[36]. Very recently, using in situ Fourier transform IR spectroscopy, Airi et al. tracked the roto-vibrational CO signal throughout a methanol conversion reaction over the catalyst CoAlPO-18 and the resulting data provided support for a Koch carbonylation process involving SMSs and CO[37]. Plessow et al. also provided theoretical evidence for the Koch carbonylation mechanism using a model in which CO was formed from the dehydrogenation of methanol via a formaldehyde intermediate[38]. In 2019, Yang et al. confirmed the formation of acetaldehyde as a result of the reaction of SMSs, CO and H2 based on a combination of online MS and NMR spectroscopy[39]. In addition, using the advanced operando synchrotron photoelectron-photoion coincidence technique, Wu et al. observed the production of a ketene from methyl acetate, further confirming the occurrence of carbonylation during the MTH reaction[40].

    Figure 2. A diagram showing the initial C-C bond formation via the Koch carbonylation mechanism. Reproduced with permission from ref. 34[34]. Copyright 2016, Wiley-VCH. MTH: Methanol-to-hydrocarbons.

    Carbene mechanism

    In addition to reacting with cations (such as methoxymethyl cation) and CO, SMSs can also serve as methylation agents via the formation of carbene-like species. In 2008, Wang et al. reported that the C-H bonds of SMSs can be activated at high reaction temperatures to generate active compounds possessing carbene/ylide-like properties[41]. Indirect evidence for this phenomenon was also obtained from a carbene trapping experiment in which cyclohexane was used as the probe molecule and a typical carbene insertion reaction generated a methylcyclohexane indicator[41]. Following this early work, Yamazaki et al. obtained direct evidence of a carbene-like intermediate using IR spectroscopy. Monitoring the IR spectra of samples obtained during the reaction of surface-bound deuterated methoxy (OCD3) with ethene at 250 °C over ZSM-5 indicated that the intensities of the C-D stretching bands decreased while a peak related to an acidic O-D group appeared at 2655 cm-1 and increased in intensity, demonstrating the attachment of a D atom to a neighboring O[42]. Additional evidence for this process was obtained by reacting OCD3 with DME at 200 °C[43]. In 2016, Chowdhury et al. provided further support for the carbene mechanism on the basis of analyses by two-dimensional magic angle spinning 1H-13C and 13C-13 C solid-state NMR spectroscopy[36]. As shown in Figure 3A and B, the peaks at 57.7 ppm in the 13C NMR spectrum and at 3.54 ppm in the 1H NMR spectrum were assigned to SMSs and the strong cross-peak between 13C NMR signals revealed the proximity of the SMSs to adsorbed methanol molecules. The close proximity of the 57.7 and 52.2 ppm peaks in the 13C NMR spectrum was attributed to interactions of the polarized C-H bonds in the SMSs with neighboring O atoms [Figure 3C]. This result indicated the insertion of carbenes into the sp3 C-H bonds of methanol molecules. Dimethoxymethane and its hydrolysis product methanediol were also detected in this prior work [Figure 3D and E][36]. More recently, Minova et al. employed Fourier transform IR spectroscopy with a mass spectrometer to directly observe the deprotonation of SMSs in large single crystals of SAPO-34[44]. This process was thought to initiate the first C-C bond formation via the insertion of a carbene into an adjacent methoxy group[44].

    Figure 3. NMR spectra of methanol, SMSs and acetal species on SAPO-34. (A) Magnifications of two-dimensional 13C-13C (blue) and 13C-1H (red) solid-state NMR spectra acquired with a mixing time of 150 ms and a cross-polarization contact time of 500 µs. (B) Structures associated with peaks related to different surface species including methanol, DME and SMSs. (C) Identification of a surface adduct between an SMS and methanol (solid arrows: electron flows, dotted arrows: 13C-13C NMR correlation). (D) Chemical exchange of anomeric conformations of DMM as evident from the 13C-13C NMR data. (E) Identification of methanediol from the 13C-1H NMR data with a cross-polarization contact time of 50 µs. Reproduced with permission from ref. 36[36]. Copyright 2016, Wiley-VCH. NMR: Nuclear magnetic resonance; DME: dimethyl ether; SMSs: surface methoxy species.

    Methane-formaldehyde mechanism

    In 1984, Kubelková et al. demonstrated the formation of formaldehyde and methane as a result of methanol disproportionation on ZSM-5 at 400 °C, working at low methanol pressures[45]. In 1987, Hutchings et al. observed the formation of methane prior to C2+ hydrocarbons in conjunction with low methanol coverage of the catalyst[46]. Based on these findings, the methane-formaldehyde mechanism has been postulated as yet another theory regarding the initial C-C bond formation process during the MTH reaction[46,47]. In this mechanism, methanol and SMS first react to produce formaldehyde and methane, after which the C-C coupling of the formaldehyde with a CH3- anion generated by the donation of an H+ cation from CH4 to ZO- (i.e., deprotonated zeolite) occurs to produce ethanol [Figure 4][26]. Several recent studies using advanced analytical techniques have also confirmed the formation of methane and formaldehyde. As an example, Liu et al. identified the formation of olefins following the reaction of CH4, HCHO and CO using in situ IR spectroscopy in conjunction with MS[48]. Wen et al. employed in situ synchrotron radiation photoionization MS to quantify the amount of formaldehyde generated during the reaction[49]. Data such as these provide solid evidence to support the methane-formaldehyde mechanism.

    Figure 4. A diagram summarizing the methane-formaldehyde mechanism. Reproduced with permission from ref. 26[26]. Copyright 2015, Royal Society of Chemistry.

    HYDROCARBON POOL MECHANISM AND THE DUAL CYCLE CONCEPT

    An autocatalytic process occurring during the induction stage of the MTH reaction and related to the direct mechanisms has attracted much attention from researchers. Several excellent reviews have fully summarized the development of these mechanisms[21,26]. Although a general agreement has been reached regarding the formation of the initial C-C bond via a direct mechanism involving the direct coupling of methanol with its derivatives (e.g., methoxymethyl cations, CO, carbene, formaldehyde), the exact mechanistic process for each direct mechanism has not yet been fully established. Moreover, computational results have shown that the energy barrier to the direct coupling mechanism is very high and the reaction intermediates are labile, making direct C-C bond formation difficult to achieve[35,50]. While there are ongoing debates regarding the initial C-C bond formation mechanism, an indirect hydrocarbon pool mechanism which is responsible for the later stages after the initial C-C bond formation in MTH reactions has now been widely accepted with abundant supporting evidences.

    The proposed hydrocarbon pool mechanism and the dual cycle concept

    In the 1990s, Dahl et al. first proposed the hydrocarbon pool mechanism for the MTH reaction[51,52]. In this process [Figure 5A], methanol molecules first generate a pool of hydrocarbon species [(CH2)n]. These species are active and further react with the reactant methanol to generate multiple products, including olefins, alkanes and aromatics. Simultaneously, coke is produced from the hydrocarbon pool species[52]. This indirect hydrocarbon pool mechanism has been widely studied and the associated theory has evolved considerably over time. In 2000, Song et al. found that the use of pretreated SAPO-34 containing methylbenzenes greatly increased the extent of methanol conversion from just 14% in the case of fresh catalyst to approximately 100%, suggesting that methylbenzenes were the active intermediates in the hydrocarbon pool[5]. Based on the co-reaction of 13C-methanol with 12C-benzene, which yields methylbenzenes, in a beta zeolite, Bjørgen et al. demonstrated the insertion of 12C atoms in both the reaction effluent and identified the compounds retained in the catalyst, providing concrete evidence for the active nature of methylbenzenes[53].

    Figure 5. (A) The hydrocarbon pool mechanism for MTH conversion. Adapted with permission from ref. 52[52]. Copyright 1996, Elsevier. (B) The total 13C concentrations in the reaction products (left) and the proposed dual cycle concept for the MTH reaction on ZSM-5 (right). Reproduced with permission from ref. 55[55]. Copyright 2007, Elsevier. MTH: Methanol-to-hydrocarbons.

    Another important theory that evolved from the hydrocarbon pool mechanism is the dual cycle concept. Isotopic switching experiments in which a 12C-methanol feedstock was switched to 13C-methanol during the steady state of the MTH process by Bjorgen et al. indicated that the amount of 13C inserted into ethene closely matched that in the methylbenzenes while the amounts of 13C in propene and higher olefins were highly similar [Figure 5B][54,55]. These results demonstrated that there were two different groups of active intermediates in the MTH hydrocarbon pool, leading to the dual cycle concept. In this mechanism, aromatic intermediates and olefins simultaneously react to generate complex products, representing a modification of the hydrocarbon pool mechanism[54,55]. Notably, this isotopic switching technique is commonly accepted and widely applied to the study of dual cycle reaction routes.

    Elementary reactions

    The proposed dual cycle mechanism has made a significant contribution to our understanding of the effect of the hydrocarbon pool on product selectivity. However, many different elementary reactions comprise the overall MTH process and an assessment of the kinetics of the pool species and the associated sub-reactions can help to elucidate links between the dual cycle mechanism and various products. In this regard, Ilias et al. produced an excellent review focusing on the chemistry of the MTH reaction[20]. Here, we provide a brief introduction to the sub-reactions and primarily concentrate on the effects of these sub-reactions on product distribution. Based on the dual cycle mechanism, six important elementary reactions may be associated with the MTH process, as discussed in the following paragraphs.

    Olefin methylation, generating higher olefins by incorporating methyl groups into light olefins, is the main way for the C-C chain growth in MTH reactions. A prior kinetic study established that such reactions are primarily affected by the olefin pressure with only a minimal effect from methanol/DME[56-61], on the basis of the relationship between olefin methylation rate and the partial pressure of reactants (i.e., light olefin and methanol/DME) which implies that, during this process, the active sites on the catalyst are fully saturated by methyl groups obtained from methanol or DME[20]. Additionally, with increases in olefin size, the activation energy barrier decreases and the methylation rate increases, such that highly branched aliphatic compounds are favored as intermediates during MTH conversion[56-61]. Interestingly, the oligomerization of light olefins is also an effective source of the C-C chain growth of hydrocarbons and can be catalyzed by the Brønsted acid sites of zeolites. Similar to the olefin methylation reaction, the formation of heavy olefins via this route is promoted by moderate temperatures and high pressures[62,63].

    Olefin cracking was first reported by Dessau and LaPierre[64,65] and was proposed as an important means of increasing the yield of propene in Lurgi’s MTP reaction on ZSM-5[20]. During this process, alkoxides formed by the protonation of long-chain olefins generate short-chain olefins and small alkoxides through β-scission. The small alkoxides subsequently desorb from the catalyst and acquire protons. The competition between methylation and cracking is a critical factor that decides the resulting product distribution. Specifically, more rapid olefin cracking tends to produce lower molecular weight olefins, while a slower reaction gives higher molecular weight olefins that eventually generate aromatics via cyclization reactions. The rates of β-scission also affect the proportion of olefins in the product mixture[66-68]. Simonetti et al. found that the rate of C5 methylation was more than 100 times that of β-scission, but the C8 olefin (i.e., 3,4,4-trimethyl-2-pentene) had a higher rate of β-scission[66]. Buchanan et al. also revealed that the olefin cracking process was enhanced by elongating the olefin chains[68].

    Hydrogen transfer and cyclization reactions can function as bridges between olefin- and aromatic-based cycles in MTH reactions. Higher olefins tend to produce aromatic compounds via hydrogen transfer together with the formation of stoichiometric amounts of alkanes. This process can be quantified using the hydrogen transfer index[69-72]. Specifically, this index can be used as an indicator for the formation of aromatics and the participation extent of the associated catalytic cycle (i.e., aromatic-based cycle)[73]. It should also be noted that cycloalkanes and cycloolefins are unstable products that quickly dehydrogenate to produce aromatics, so the cyclization process also contributes to the propagation of the aromatic cycle. Fan et al. found that dienes and monoenes can also form methylcyclohexenes via the Diels-Alder reaction based on assessments using solid-state NMR and gas chromatography (GC)-MS[74]. The same group also demonstrated that the Diels-Alder process exhibited much lower energy barriers than the oligomerization and cyclization reactions along the route to the construction of large molecules[74].

    The methylation of aromatics is similar to that of olefins, but the dealkylation of the former can follow two different routes: the paring and side-chain mechanisms [Figure 6][75]. These two mechanisms both begin with the gem-methylation of methylbenzene. In the former, this step results in a ring contraction to generate a five-membered ring with an alkyl substituent that can produce either ethene or propene via dealkylation[76-78]. The participation of cyclopentenyl cations in this mechanism has previously been confirmed by IR and NMR analyses[77-79]. In the case of the side-chain mechanism, following the release of a proton, an exocyclic double bond is formed that then undergoes stepwise side chain methylation and dealkylation[75,80]. Hexamethenecyclohexadiene and a heptamethylbenzenium cation have also been identified using GC-MS and NMR, providing direct evidence for the side-chain mechanism[81].

    Figure 6. A diagram summarizing the paring and side-chain mechanisms. Reproduced with permission from ref. 75[75]. Copyright 2009, Wiley-VCH.

    Contribution of catalytic cycles to product formation

    In-depth investigations of the dual cycle mechanism have been performed in attempts to assess the relative contributions of the two cycles to the product distribution generated by the MTH reaction. As discussed above, each catalytic cycle contains different elementary sub-reactions that will vary with the reaction conditions and catalysts employed. Careful kinetic studies of these different sub-reactions have provided much knowledge regarding the effects of these elementary steps on product distribution and the dominant catalytic cycle.

    Hill et al. recently carried out a detailed study of the dual cycle mechanism[59-61,82]. They first systematically investigated the rate constants for ethene and propene methylation over four zeolites (MFI, BEA, MOR and FER) and found that propene methylation had higher rate constants (by an order of magnitude) and lower activation energies compared with ethene methylation[59]. On the basis of isotopic switching experiments that showed that ethene was a product of the aromatic dealkylation process[55], this group proposed that ethene was an end product and could be used as an indicator of the progression of the aromatic cycle. Similarly, because alkanes formed during the MTH reaction are inactive, these compounds can serve as indicators of the occurrence of the olefin catalytic cycle. However, alkanes are also formed via a hydrogen transfer process that is related to aromatic formation. To exclude this effect, the same group proposed that the total yield of 2-methylbutane and 2-methyl-2-butene could serve as an indicator of the progression of the olefin-based cycle, while the ethylene/(2-methylbutane + 2-methyl-2-butene) ratio could be used to assess the extent to which aromatic-/olefin-based cycles occurred during the MTH reaction over ZSM-5[82].

    Building on Bhan’s work, Sun et al. suggested slightly different indicators for the dual cycles[83,84]. In the case of the aromatic-based cycle, they concluded that methane can also be formed via the dealkylation of methylbenzene and represents a terminal product of this catalytic cycle, meaning that the formation of this compound should exhibit the same trend as ethene. In contrast, the selectivity for C4-C7 aliphatics, which readily diffuse out of ZSM-5, can be employed as an indicator for the olefin-based cycle[83,84]. It is known that propene can be produced in both catalytic cycles and is also able to react with methanol as an olefin intermediate. Another important feature of the dual cycle mechanism is that the two catalytic cycles compete for acidic sites. Based on such fundamental information concerning the dual-cycle mechanism, researchers are trying many different approaches to achieving the selective propagation of one specific cycle as a means of obtaining product selectivity from the MTH reaction[20,82,83].

    PRODUCT SELECTIVITY

    The product distribution obtained from the MTH reaction during its steady state is an important aspect of the process and has therefore been extensively studied. Many factors, including the raw material feed, reaction conditions and zeolite structure, can have significant effects on the reaction routes and subsequently on product distribution. In the following section, we elaborate on the influences of these factors on product formation during the MTH reaction with a particular focus on the effects of zeolite structure.

    Active intermediates in the raw material feed

    According to the dual cycle concept, increasing the relative amounts of specific active species in the feedstock should enhance certain catalytic cycles to promote the formation of desired products. As an example, the addition of toluene or p-xylene to the methanol feedstock has been found to increase the formation of methane, ethene and aromatic products while decreasing the amounts of C4-C7 aliphatics [Figure 7A]. These results can be attributed to promotion of the aromatic-based cycle and inhibition of the olefin catalytic cycle upon adding active aromatic compounds. Interestingly, the addition of a low concentration of C3-6 olefins cannot effectively restrain the aromatic-based catalytic cycle as a result of the rapid transformation of these compounds to aromatics[83]. Bhan’s group also found that the addition of small amounts of acetaldehyde (1-4 C%) to the methanol stream led to a monotonic enhancement of the ethene selectivity, ranging from 9.3 C% to 15 C% in trials with bulk ZSM-5 and from 1.4 C% to 6.4 C% in trials with diffusion-free ZSM-5 nanosheets. In addition, methylbenzene selectivities from 4.9 C% to 7.8 C% with bulk ZSM-5 and from 2.6 C% to 5.3 C% with ZSM-5 nanosheets were observed. Although acetaldehyde is not a direct active hydrocarbon pool species, it can produce aromatics through a multistep aldol-condensation reaction[85]. Zhang et al. also recently reported a kinetics study showing that the addition of olefins (such as in the case of olefin recycling) to the feedstock enhanced the olefin methylation and cracking processes, facilitating the formation of propylene during the MTP reaction[86].

    Figure 7. (A) The effect of co-feeding p-xylene on product distribution. Reproduced with permission from ref. 83[83]. Copyright 2014, Elsevier. (B) Product selectivity and methanol conversion (left), selectivities for ethene and 2-methyl-2-butene + methylbutane (right). Reproduced with permission from ref. 73[73]. Copyright 2017, Elsevier.

    Reaction conditions

    Elementary reactions have varied kinetic orders and active energies, so the reaction temperature and pressure will affect the rates of these sub-reactions differently and thus modify the product distribution. For example, Goetze et al. found that 1-methylnaphthalene deactivated the MTH reaction at 350 °C on the zeolite DDR but functioned as a catalytically active intermediate at 450 °C[87]. The same group established that the primary active intermediates during the MTO reaction over CHA zeolites varied at different temperatures. Specifically, methylbenzenes were the main active species at low temperatures (300-350 °C), whereas methylated naphthalenes were the primary active intermediates at higher temperatures (350-500 °C)[88,89]. However, Qi et al. reported that naphthalene species can also serve as active intermediates at a lower temperature of 290 °C in trials using ZSM-5[90]. In addition, Yarulina et al. found that adjusting the temperature from 380 to 475 °C changed the ethene: propene ratio obtained from the MTO reaction over a Sigma-1 zeolite from 0.5 to 1.6[91]. This same reaction gave a butene yield of 14 C% at 380 °C but yields of less than 5 C% at a higher temperature of 475 °C[91].

    Arora et al. demonstrated the effect of the methanol partial pressure on the product distribution [Figure 7B][73]. As a typical example, increasing the methanol pressure from 0.6 to 52.5 kPa during the MTH process over ZSM-5 at 400 °C increased the selectivities for methylbenzenes and ethene from 0.1 C% to 5.9 C% and from 1.5 C% to 9.8 C%, respectively. In contrast, the propene selectivity decreased from 41.8 C% to 28.7 C% such that the propene/ethene (P/E) ratio was decreased by a factor of 9. It was concluded that a higher methanol pressure facilitated the hydrogen transfer reaction which, in turn, generated large amounts of aromatics during the MTH reaction[73].

    Zeolite structure

    As a result of the rapid development of crystalline zeolites with ordered micropores having sizes in the range of 0.3-1 nm, such materials have been widely used as catalysts or sorbents. These inorganic substances include aluminosilicates, aluminophosphates and silicates, and the International Zeolite Association database currently contains over 200 different types of zeolites. Their topologies (based on the presence of cavities and channels), morphologies (that is, the crystal dimensions and mesoporosities) and compositions (the relative amounts of Si, Al, P, Ti and other elements) of these materials vary greatly and all these factors can affect catalytic performance in the MTH reaction.

    The first factor assessed herein is topology. As an example, ZSM-22 has one-dimensional 10-MR channels that are too narrow to allow the formation of methylbenzene intermediates, thus inhibiting the aromatic catalytic cycle but promoting the olefin-based cycle to produce a high proportion of branched C5+ paraffins[92,93]. The ZSM-5 zeolite, which has an MFI structure containing three-dimensional 10-MR pores approximately 5.6 Å in size, produces a mixture of olefins, paraffins and aromatics, while the small-pore CHA-structured SAPO-34 containing pore opening of only approximately 3.8 Å mainly produces C2-C4 alkenes under similar reaction conditions[19]. Moreover, the primary active hydrocarbons during the MTH reaction over zeolites also vary in morphology. Hexa- and penta-methylbenzenes were found to be the main active hydrocarbon pool intermediates in trials using a beta zeolite, facilitating the production of propene and butenes[53]. In contrast, the reaction intermediates were restricted to xylenes and tri-methylbenzenes in work with ZSM-5, which produced ethene and propene[54].

    The texture of the zeolite is another important factor determining the MTH reaction mechanism. In particular, the transport of reactants and products across the channels in the zeolite plays a vital role[94]. A Fickian diffusion model in which the zeolite is assumed to be a spherical crystal without any other transport restrictions can be used to calculate the adsorption properties of a zeolite under isothermal conditions[95,96]. This method allows a quantitative comparison of the difference between zeolite sizes, which can have a remarkable effect on product selectivity during the MTH reaction[97,98]. Studies have shown that increasing the ZSM-5 particle size increases the selectivity for ethene while reducing that for C4-C7 aliphatics. The same trend in product selectivity was also observed in trials in which access to ZSM-5@SiO2 was blocked by the formation of a SiO2 shell. These results show that a large effective crystallite size prolongs the residence time of large active methylbenzenes in the catalyst particles, facilitating the aromatic-based cycle to produce light olefins[97]. Work with the self-pillared pentasil ZSM-5 having a diffusion length of only 1 nm indicated that the ethene selectivity resulting solely from olefin interconversion was limited to 1.1% because the aromatic cycle essentially did not proceed in the zeolite[98]. The effects of using a mesoporous beta zeolite as the catalyst in the MTH reaction were previously examined. The results indicated that intracrystalline mesoporosity provided much shorter diffusion lengths than those in the standard material, leading to better catalytic performance in terms of conversion capacity, reaction rate and lifetime. More importantly, methylbenzenes could easily diffuse out of the mesoporous catalyst so that less ethylene but more long-chain aliphatics were obtained compared with the conventional beta zeolite[99].

    In addition to the diffusion effect, the product distribution is also sensitive to the acidity of the zeolite. In 2011, Wei et al. reported that ZSM-5 zeolites with different SiO2/Al2O3 ratios all showed high MTH activity, but the propene selectivity was 45.9% greater on a Na-ZSM-5 specimen having a high SiO2/Al2O3 ratio of 220[100]. In 2017, Zhao et al. achieved an elevated propene selectivity of 58.3% using a high-Si structured beta zeolite-catalyzed MTP process because the olefin-based catalytic cycle was the dominant reaction route[101]. In 2018, Yarulina et al. determined that ZSM-5 modified with alkaline earth metals had a lower density of Brønsted acidic sites[102]. This suppressed the accumulation of aromatics but allowed the olefin methylation/cracking process to proceed, leading to the generation of more propene and less ethene than were obtained using pure ZSM-5[102]. In cooperation with Bhan’s group, the authors also investigated the effects of the SiO2/Al2O3 ratio in ZSM-5 zeolites on the formation of ethene during DME conversion. This work indicated that a high aluminum content increased the interactions between aromatics and catalytic sites before these compounds exited the crystalline particles, providing higher selectivity for ethene. As noted above, increasing the catalyst particle size can also increase the interactions between aromatics and catalytic sites by prolonging the residence time of the methylbenzenes. These two factors were therefore combined to produce a single descriptor (ΝH+) for ethene selectivity. Increasing the NH+ value was found to promote both catalytic cycles, although the aromatic cycle was promoted to a larger extent than the olefin cycle, meaning that ethene formation was enhanced[103]. Very recently, Liang et al. proposed that the relationship between the acid site density and the aromatic-based cycle is more complicated than is presently thought[104]. Isotopic tracing experiments by this group showed that the acid site density had a negligible impact on the reactivity of active aromatics that were preferentially transformed into coke precursors and thus caused deactivation of the catalytic sites[104]. Moreover, Yuan et al. showed that active Al atoms located on the external surfaces of ZSM-11 promoted rapid coke formation, while the uniformly distributed Al species in the micropores inhibited the aromatic-based cycle during the MTO reaction[105].

    Zeolites having small pores tend to exhibit high selectivity for light olefins because they are able to confine larger organic compounds produced during the MTO reaction. The MTO process is an important reaction in C1 chemistry and is capable of producing valuable olefin-based chemicals via non-petroleum routes. Many fundamental investigations of this process have also recently been performed. Initially, aromatic-based species were widely believed to function as active intermediates during the MTO reaction because of the shape-selective effect of the small pores in the catalysts. Isotopic studies have also demonstrated that methylbenzenes make the main contribution to the production of olefins[106]. Analyses by GC-MS and solid-state NMR have produced evidence for the presence of active hexamethenecyclohexadiene and the heptamethylbenzenium cation[81]. In addition, operando UV-visible spectra have indicated that methylated aromatic species are the major active species[88,89]. However, variations in the specific aromatic compounds in the hydrocarbon pool can have a significant effect on the MTO products. Kang et al. systematically established the correlations between the cage sizes of small-pore zeolites and the MTO product distribution[107]. This work compared 30 different zeolites having 14 different cage-type topologies and found that the MTO reaction proceeded in a similar manner on specific topologies independent of the composition[107]. As shown in Figure 8, an ellipsoidal model was initially defined based on three axes, with ab denoting the cage-defining ring and b being the ring size. As the ellipsoidal cage size was increased from 12-ring to 16-ring, the primary light olefin product changed from ethene to butene, so a relationship between ring size and product selectivity could be established. These results suggest that the ellipsoidal cages in small-pore zeolites determine the sizes of methylated aromatic intermediates during the MTO process[107].

    Figure 8. (A) The ellipsoidal model of a cage-defining ring. (B) Correlations between cage-defining ring sizes and olefin distributions. Reproduced with permission from ref. 107[107]. Copyright 2019, American Chemical Society.

    In addition to aromatic active species, olefin-based catalytic intermediates formed during the MTO process are also of interest. Based on an analysis of 1H and 13C NMR spectra, Dai et al. identified several olefin-like species in trials using SAPO-34 during the induction period of the MTO reaction, and suggested that these compounds evolved into aromatic intermediates when the reaction was in the steady state[8]. However, the functioning of these olefin-like species as active intermediates was not assessed[8]. Hwang et al. showed that active olefin intermediates appeared during the MTO reaction in an early stage but also established that accumulated aromatics dominated the catalytic process and increased the turnover number[108]. More recently, the authors found that propene selectivity underwent a monotonic increase with decreases in the aluminum content or crystal size of DDR zeolites[109]. As shown in Figure 9A, 12C/13C methanol switch results indicated that the formation of propene was closely associated with the propagation of the olefin-based cycle, an effect that has been neglected in most studies. The authors have proposed that catalytic behavior during this process is related to the presence of small pores in the zeolite that restrict the diffusion of both aromatics and long-chain olefin intermediates. The effect of olefin-related β-scission reactions during the entire process was also determined to be significant[109]. Yang et al. prepared a SAPO-14 zeolite with ultrasmall cages (5.3 × 10.05 Å) and an AFN topology and reported 77.3% propene selectivity from a one-pass methanol conversion process with a time-on-stream of 3 min, representing the highest value up to that point [Figure 9B][110]. Using 12C/13C-methanol switch experiments, the same group established that the olefin-based route, rather than a mechanism based on aromatics, dominated the conversion of methanol [Figure 9C] and thus maximized the selectivity for propene based on the effect of limited diffusion[110]. Zhou et al. fabricated a SAPO-14 zeolite with a high Si content and achieved a P/E ratio of 4.17 during the MTO reaction at 450 °C[111]. These studies served to fill gaps in the body of knowledge regarding the effect of the olefin-based cycle on MTO reactions.

    Figure 9. (A) The 13C concentrations in ethene and trans-2-butene over time during 12C/13C-methanol switch experiments using various DDR zeolites. Reproduced with permission from ref. 109[109]. Copyright 2020, American Chemical Society. (B) Product distribution and (C) 13C concentrations in effluents and residual methylbenzenes during the MTO reaction over SAPO-14. Reproduced with permission from ref. 110[110]. Copyright 2020, American Chemical Society. MTO: Methanol to olefins.

    MTP reaction

    Ethene, propene and aromatics are the main value-added products that can be obtained from the MTH process. The dual cycle mechanism indicates that the formation of ethene and aromatics is largely associated with the aromatic-based cycle, while the production of propene is connected with either the olefin-based or aromatic-based cycles. Because of the complexity of the MTH reaction network, adjusting the single-pass selectivity for propene to achieve a high selectivity value is highly desirable. Based on the preceding discussions of the reaction mechanism, effective means of enhancing the propene yield are discussed below.

    In general, the MTH process can take place only in the cages and interconnected channels of zeolites. Therefore, the structures of zeolites have a substantial effect on the catalytic route and affect the final product selectivity. Dyballa et al. compared three different 10-MR zeolites having varying structures (ZSM-5, ZSM-11 and ZSM-22) in MTP reactions and demonstrated that propene selectivity, coke formation and catalyst lifetime were highly dependent on the acid density of the material[112]. As indicated by the data in Table 1, a propene selectivity greater than 50% could be achieved over ZSM-5 (130) (here, the number 130 in the bracket indicates the Si/Al ratio of the zeolite) and ZSM-11 (200). Moreover, regardless of the morphology, the propene selectivity and the P/E ratio increased with increasing Si/Al ratio[112]. Adding a binder[113] and heteroatoms such as B[114], Mn[12] and Ca[102] had a similar effect to that of increasing the Si/Al ratio to dilute the acid density, and greatly enhanced propene selectivity. This phenomenon was especially noticeable in trials using the catalysts B-CON[115] (with a propene selectivity of 60%) and Mn-ZSM-5 (200)[12] (with a propene selectivity of 58.4%). This research showed that decreasing the density of Brønsted acid sites hindered the hydrogen transfer process and changed the main catalytic route during the MTH process. As a result, the olefin-based cycle became dominant, leading to a high propene selectivity.

    Table 1

    Propene selectivities over different zeolites

    CatalystaTemperature ()Conversion (%)Propene Sel. (C%)Ethene Sel. (C%)P/E ratioRefs. (year)
    ZSM-5 (20)45010027151.8[112] (2016)
    ZSM-5 (130)45010051510.2[112] (2016)
    ZSM-11 (30)4509414101.4[112] (2016)
    ZSM-11 (200)45010051317[112] (2016)
    ZSM-22 (30)4501004394.8[112] (2016)
    ZSM-22 (50)4501004585.6[112] (2016)
    Binder-ZSM-5 (40)45010041.95.28[113] (2010)
    B-ZSM-5 (162)460> 8043//[114] (2014)
    Mn-ZSM-5 (200)48010058.410.65.5[12] (2015)
    Ca-ZSM-5 (46)500100517.56.8[102] (2018)
    B-CONb500100604.513.3[115] (2015)
    Meso-ZSM-5 (78)47099.642.24.1810.1[10] (2008)
    Meso-beta (277)550> 9955.55.610[116] (2018)
    2 nm ZSM-5 (87.5)350~5020.51.513.7[97] (2015)
    17 µm ZSM-5 (38.0)350~5026.220.31.3[97] (2015)
    100 nm ZSM-22 (46)45010053212.6[117] (2014)
    300 nm ZSM-22 (46)45010050212.4[117] (2014)
    Beta (125)45010046.03.314.1[118] (2017)
    SAPO-17 (6)425> 9827550.5[107] (2019)
    SAPO-34 (0.123)400> 9839301.3[107] (2019)
    SAPO-18 (0.063)400> 9846241.9[107] (2019)
    DDR (22)400~5038.136.61.0[109] (2020)
    DDR (172)400~5048.327.81.7[109] (2020)
    Meso DDR (172)400~5050.626.41.9[109] (2020)
    SAPO-14c40089.277.38.09.7[110] (2020)
    SAPO-14c45099.365.715.94.1[110] (2020)

    Hierarchically structured zeolites (also referred to as mesoporous zeolites) combine the advantages of micropores and mesopores to provide an enhanced molecular diffusion rate. As discussed above in Section “Zeolite structure”, mesoporous zeolites have very short diffusion lengths of only a few nanometers and show promise as catalysts when applied to the MTP process. As reported previously, meso-ZSM-5[10] and meso-beta[116] zeolites produce more propene than their counterparts because the short diffusion lengths in these materials favor the olefin-based cycle and limit the aggregation of aromatics[97]. However, Khare et al. achieved a propene selectivity of only 20.5% with 2 nm ZSM-5 (87.5) particles at 350 °C, albeit with a high P/E ratio of 13.7[97]. These results were attributed to the relatively low reaction temperature, at which the methylation and oligomerization of low alkenes proceeded rather than the cracking process that would be expected to produce light olefins. Therefore, a high temperature (typically above 400 °C)[117,118] is needed for the MTP reaction, as indicated in Table 1.

    Small-pore zeolites are highly selective for both ethene and propene because the smaller openings in these catalysts easily trap large products. As the aromatic-based cycle proceeds, ethene is inevitably produced via the dealkylation of branched aromatics and subsequently competes with the formation of propene. As such, the P/E ratio is usually less than 2 [Table 1][107]. However, many groups have recently made efforts to tune the P/E ratio obtained during the MTO reaction. Kang et al. established that the topology of a small-pore zeolite can determine the branching of aromatics in the cages and release different olefins through dealkylation[107]. Tuning the acid content and crystallization size of small-pore zeolites is also an effective strategy to regulate the relative extents to which the two catalytic cycles proceed[109,110] and this approach has previously been used in work with medium-/large-pore zeolites. Notably, Liu’s group achieved the highest-ever propene selectivity of 77.3% using a newly developed small-pore SAPO-14 zeolite[110]. Compared with the complex outflows from medium- or large-pore zeolites, the main products of the MTO process over small-pore zeolites are limited to light olefins. As a consequence, it is easier to adjust the selectivity for propene when using small-pore zeolites. The data discussed in this paragraph therefore suggest innovative approaches to designing and engineering catalysts for the MTP process.

    CATALYST DEACTIVATION

    Pore blocking by polycyclic aromatic hydrocarbons

    The deactivation of catalysts is another issue with equal importance to the product selectivity of the MTH reaction. As the MTH reaction proceeds over molecular sieve catalysts, many cross-linked PAHs featuring graphene-like structures are formed via the cage-passing growth of confined hydrocarbon pool species. This phenomenon suppresses mass transfer, blocks the catalytic centers and deactivates the catalyst. According to the mechanism described in Section “FORMATION OF THE FIRST C-C BOND”, the MTH reaction proceeds based on a pool of hydrocarbons that produce needed light olefins but also undesirable heavy molecules that cause catalyst deactivation[119-124]. Much effort has been devoted to understanding this deactivation phenomenon. In 2020, Lezcano-Gonzalez et al. tracked the formation of carbonaceous species during the MTO reaction using a newly developed operando Kerr-gated Raman spectroscopic technique that discriminated between Raman signals and fluorescence[125]. This work examined the formation of heavy hydrocarbons at different stages of the MTO reaction and showed that small branched polyenes with reduced mobility [Figure 10A-C] eventually underwent a cyclization process to form PAHs within large cages [Figure 10D]. However, an MFI topology that provided steric constraint inhibited the cyclization of polyenes and thus the generation of PAHs, increasing the catalyst lifetime[125]. Using a single-crystal electron diffraction technique, Wennmacher et al. found that the nucleation of coke began at channel intersections while large PAHs were preferentially formed in straight pores[119].

    Figure 10. (A-C) The mobility of polyene species and (D) hydrocarbon species evolution during the MTO reaction. Reproduced with permission from ref. 125[125]. Copyright 2020, Nature Publishing Group. MTO: Methanol to olefins.

    Similar to the outcomes of many other reactions, the coke generated during the MTH process is a complex mixture of various polycyclic aromatics with broad mass distributions[126-129]. However, in the case of the MTH reaction, the deactivating species are uncertain in terms of their exact compositions and chemical structures and are largely related to the pore structure of the zeolite. For example, hexamethylbenzene is an active pool species in reactions over beta and SAPO-34 zeolites but can also function as a deactivation species in conjunction with other topologies such as ZSM-22 (with a pore size of 4.6 × 5.7 Å)[26]. Interestingly, the hydrocarbon pool species undergo spatial interactions with the zeolite framework and thus cause the zeolite cells to expand[130-132]. Yang et al. investigated the dynamic structural evolution of SAPO-34 induced by organic compounds using 27Al and 31P NMR spectroscopy[132]. The 31P NMR signal was found to shift from -30.1 to -30.8 ppm with the accumulation of deposits, indicating an increase in the P-O-Al bond angle[132]. In addition to expanding the unit cells in the zeolite, coke can grow by traversing the windows of zeolitic cages to form cross-links with neighboring coked cells and therefore generate large interconnected multicore PAHs[133]. Based on analyses by matrix-assisted laser desorption ionization-Fourier transform ion cyclotron resonance (MALDI-FT-ICR) with MS and isotope labeling, Wang et al. developed a so-called aromatic cage-passing deactivation mechanism occurring in MTO reactions over different small-pore zeolites[133]. In these materials, aromatics with three or four rings are initially formed in the cages and then cross the opening windows and link together as the reaction proceeds. As shown in Figure 11, the chemical composition of the coke will vary with changes in the zeolite cavity shape, suggesting that the coke formation/deactivating processes in zeolite-type catalysts are also associated with a shape-selective effect similar to product selectivity[133].

    Figure 11. Molecular structures of coke species in different cage-structured zeolites and the associated MALDI FT-ICR mass spectra. Reproduced with permission from ref. 133[133]. Copyright 2020, Nature Publishing Group. MALDI FT-ICR: Matrix-assisted laser desorption ionization-Fourier transform ion cyclotron resonance.

    Conventional zeolites with ordered cavities and microporous channels can serve as molecular sieves, explaining the unique shape selectivity function within the MTH reaction. However, micropores also impose restrictions on molecular diffusion throughout the zeolite crystals. Hierarchical zeolites containing mesopores provide enhanced mass transfer and exhibit prolonged lifetimes during the MTH reaction[120,134]. Choi et al. prepared thin ZSM-5 nanosheets that showed reaction times five times that of bulk ZSM-5 particles as a consequence of the rapid mass diffusion and high coke tolerance of the sheets[135]. Liu et al. fabricated single-crystalline hierarchical ZSM-5 zeolites via a protozeolite seeding method[136]. These materials contained faceted mesopores and possessed high hydrothermal stability, exhibiting a remarkable lifetime of 18 h and an impressive propylene selectivity of up to 52.7%[136]. Using high-resolution electron energy loss spectroscopy, the authors probed the distribution of coke species in a mesoporous beta zeolite (Beta-MS) and a conventional beta zeolite (Beta-C) following the MTH reaction [Figure 12]. A significant difference observed in elemental carbon mapping data is that coke species were uniformly distributed throughout each entire Beta-MS crystal, while the coke in Beta-C crystals was mainly concentrated close to the particle exteriors [Figure 12B and E]. This same work examined the chemical compositions of carbon-based coke compounds at different locations. These analyses showed that H-rich coke with low molecular weights was deposited in the micropores but the intracrystalline mesopores in Beta-MS accommodated heavy graphite-like species [Figure 12C and F][99]. The data also suggested that micropores containing methylbenzenes and long-chain aliphatics may remain active during the MTH reaction even if the channels are blocked by large coke molecules. Liang et al. reported that the high density of surface acid sites on the H-MCM-22 zeolite promoted coke deposition on the external surfaces of the material[137]. In contrast, the acid sites in the sinusoidal channels in this zeolite were highly resistant to coke formation, providing additional evidence for the effects of acid site density and distribution on deactivation[137].

    Figure 12. HAADF-STEM images (left), carbon mapping data (middle) and line profiles (right) for Beta-MS (A-C) and Beta-C (D-F). The carbon concentrations in (B) and (E) are indicated by the color bar and the red arrows show the locations at which line profiles were acquired. Reproduced with permission from ref. 99[99]. Copyright 2015, American Chemical Society.

    Oxygen-containing species promoting coke formation

    In 2015, Müller et al. investigated the deactivation of a catalyst in a plug-flow reactor (PFR) and a fully back-mixed reactor (CSTR) and found more rapid deactivation of ZSM-5 in the PFR [Figure 13A][124]. This work established that the local methanol pressure in the top layer of the PFR was high, while the mixing of products with feedstock in the CSTR resulted in a more moderate methanol pressure. A mechanism was proposed to explain this phenomenon and is summarized in Figure 13B. At the start of the reaction, the relatively high methanol pressure leads to the formation of oxygen-containing compounds that strongly bind with active Brønsted acid sites on the catalyst, resulting in rapid deactivation. As the reaction continues, the O-containing surface compounds evolve into aromatics and eventually grow into typical coke molecules such as PAHs. This pioneering work established the important effect of O-containing compounds on catalyst deactivation throughout the MTH reaction[124]. In 2016, our own group observed the formation of various O-containing compounds comprising cyclopentenone derivatives during the MTH reaction by isotopic tracing with 13C-methanol. As shown in Figure 13C, co-feeding this labeled compound along with the methanol feedstock showed that the presence of certain species inhibited methanol conversion by competing for adsorption on catalytic acid sites, where they were readily converted to heavy aromatics and coke species[138]. Notably, many subsequent reports following this pioneering work confirmed the appearance of the same O-containing compounds [Figure 13D][39,139].

    Figure 13. (A) Deactivation characteristics of a catalyst in PFR and CSTR trials and (B) proposed pathway for O-containing coke formation. Reproduced with permission from ref. 124[124]. Copyright 2015, Elsevier. (C) Mass spectra of O-containing coke compounds. Reproduced with permission from ref. 138[138]. Copyright 2016, Royal Society of Chemistry. (D) Mass spectra of organic materials retained in ZSM-5 catalysts during methanol conversion under N2 and CO. Reproduced with permission from ref. 139[139]. Copyright 2018, Wiley-VCH. PFR: Plug-flow reactor; CSTR: continuous stirred-tank reactor.

    Formaldehyde has also attracted attention as another O-containing compound that could be related to catalyst deactivation during the MTH reaction[49,140]. Because formaldehyde readily undergoes nucleophilic addition, either the Prins or aldol-condensation reactions could produce coke precursors from this molecule[48,85]. In 2017, Martinez-espin et al. compared the use of methanol and DME as feedstocks for MTH reactions on ZSM-5, SSZ-24 and SAPO-5 zeolites and found that a methanol feed caused faster deactivation, which they attributed to the formation of formaldehyde[141]. More recently, Shi et al. reported that SSZ-13 had a shorter lifetime than SAPO-34, even though both materials have the same CHA framework[140]. The greater acid strength of the former was thought to promote the generation of formaldehyde, which subsequently transformed active intermediates into inactive polycyclic species[140,142]. This same work investigated means of circumventing formaldehyde-mediated chain carrier termination by adding Y2O3 to the zeolite to decompose formaldehyde[143] or combining high-pressure H2 with the feedstock to hydrogenate the formaldehyde to methanol[144,145].

    Transformation of coke to active species

    Methylbenzenes, which promote the formation of light olefins in the hydrocarbon pool, can also evolve into PAHs through cyclization and cross-linking reactions. In many reactions that are known to be accompanied by coke deposition, air calcination or steam gasification is used to remove coke species as an approach to catalyst regeneration. Zhou et al. demonstrated that coke species confined in SAPO-34 can be directly transformed to active naphthalenic species by steam cracking, which ultimately enhances the light olefin selectivity of the process[146]. As shown in Figure 14A, increasing the treatment time applied to a coked ZEOS specimen gradually increased the concentration of naphthalene even though the total coke amount was reduced. These results suggest the stability of naphthalene under steam cracking conditions at 680 °C. More importantly, the average molecular weight of the carbonaceous species was decreased after steam treatment, indicating that H2O could serve as an active agent to hydrogenate coke species [Figure 14B]. The Raman spectra in Figure 14C contain bands at 1360, 1415 and 1630 cm-1 assigned to PAHs and at 1600 cm-1 corresponding to the G band of amorphous carbon, all of which decreased in intensity during the early stage of steam cracking. Simultaneously, bands at 1240 and 1125 cm-1 assigned to branched exocyclic aromatics appeared. These results confirm that coke compounds underwent ring-opening as the catalyst was regenerated. Steam treatment has also been found to regenerate blocked micropores such that they become accessible again [Figure 14D][146]. In another study, Wang et al. created active naphthalenic species in SAPO-34 by ethylene pre-coking and reported that subsequent steaming of the material significantly promoted the selectivity for lower olefins and prolonged the catalyst lifetime during the MTO reaction[147].

    Figure 14. The formation of coke species with molecular masses (A) less than 200 Da and (B) greater than 200 Da under various conditions. (C) Operando UV-Raman spectra of a coked ZEOS specimen during steam treatment. (D) Micropore volume and coke mass of a ZEOS specimen over time during steam treatment. Reproduced with permission from ref. 146[146]. Copyright 2021, Nature Publishing Group.

    CONCLUSION AND OUTLOOK

    Over the 40-year time span during which the MTH process has been developed, this method has been recognized as an extremely important research area in the catalysis community. Several industrial plants using this process are already operated successfully, especially in China, attracting much more interest from both academia and industry. However, the fundamentals of the initial C-C bond formation mechanism are still debated. As discussed in the first part of this review, many active species have recently been discovered using advanced analytical techniques. Even so, details regarding the transformation of active species in this process remain elusive. More work is needed to investigate the mechanism responsible for the initial C-C bond formation during the MTH reaction using these advanced methods.

    Understanding the mechanism of the MTH reaction and the effect of the zeolite structure will facilitate the design and synthesis of new high-performance catalysts. Based on the proposed indirect hydrocarbon pool mechanism, the direct mechanism seems unnecessary because the product distribution obtained from the MTH process can be clearly explained with the indirect mechanism, especially the dual cycle mechanism. As discussed in the second part of this review, assessments of the complexity of the catalyst compositions and the multiple elementary sub-reactions occurring within the MTH system can provide basic knowledge that allows control of the reaction routes and product distributions. With much effort, the MTG and MTO reactions have been commercialized but not the MTP and MTA reactions. Studies of the MTP process have shown that small-pore zeolites with weak acidity show promise as catalysts for the olefin-based catalytic cycle and can increase the propene yield. The MTA process can also efficiently produce aromatics[148,149] that are very important platform molecules for high-value commodity chemicals[150]. Although the MTA reaction was not addressed in detail in this review, highly acidic zeolites having large pore channels may be able to promote this reaction based on prior research regarding the dual cycle mechanism.

    Coke formation is another severe problem that cannot be avoided in the MTH reaction. Currently, the commercially available catalyst SAPO-34 is used for the MTO process with only a very short time for the one-pass reaction, after which coke deposits must be burned off by heating at high temperatures in air. This process, unfortunately, generates a considerable amount of CO2 that is released into the atmosphere. A number of advanced techniques, such as operando Kerr-gated Raman and NMR spectroscopy, MALDI FT-ICR MS and electron energy loss spectroscopy, have provided detailed insights into the formation of coke species. Further work, already being performed by many groups (including Olsbye, Bhan and Lercher), is required to find effective protocols to slow coke formation as a prerequisite for industrial applications.

    Ever-increasing CO2 emissions have resulted in many environmental challenges and the catalytic conversion of CO2 to produce valuable fuels and chemicals, such as via CO2 hydrogenation to methanol, has become one of the most effective approaches to addressing this issue. Commercial CO2-to-renewable-methanol plants have been established in China, Iceland, Japan and elsewhere[151]. Considering the well-developed methanol-to-hydrocarbon technologies that are available, the catalytic hydrogenation of CO2 to C2+ chemicals (such as olefins and aromatics) via methanol-mediated routes is especially desirable because these compounds possess higher energy densities and greater value than C1 products[152-154]. It is our hope that this review will inspire new research intended to mitigate existing challenges associated with CO2 hydrogenation and methanol conversion.

    DECLARATIONS

    Authors’ contributions

    Manuscript preparation and correction: Liu Z, Huang J

    Availability of data and materials

    Not applicable.

    Financial support and sponsorship

    This work was supported by the Fundamental Research Funds for the Central Universities (2020CDJQY-A072), the Venture and Innovation Support Program for Chongqing Overseas Returnees (cx2020107), the Thousand Talents Program for Distinguished Young Scholars, Natural Science Foundation of Chongqing (cstc2021jcyj-msxmX0945), and the Postdoctoral Science Foundation of China (2021M700579).

    Conflicts of interest

    Both authors declared that there are no conflicts of interest.

    Ethical approval and consent to participate

    Not applicable.

    Consent for publication

    Not applicable.

    Copyright

    © The Author(s) 2022.

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    Cite This Article

    OAE Style

    Liu Z, Huang J. Fundamentals of the catalytic conversion of methanol to hydrocarbons. Chem Synth 2022;2:21. http://dx.doi.org/10.20517/cs.2022.26

    AMA Style

    Liu Z, Huang J. Fundamentals of the catalytic conversion of methanol to hydrocarbons. Chemical Synthesis. 2022; 2(4):21. http://dx.doi.org/10.20517/cs.2022.26

    Chicago/Turabian Style

    Liu, Zhaohui, Jianfeng Huang. 2022. "Fundamentals of the catalytic conversion of methanol to hydrocarbons" Chemical Synthesis. 2, no.4: 21. http://dx.doi.org/10.20517/cs.2022.26

    ACS Style

    Liu, Z.; Huang J. Fundamentals of the catalytic conversion of methanol to hydrocarbons. Chem. Synth. 20222, 21. http://dx.doi.org/10.20517/cs.2022.26

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    Author Biographies

    • Zhaohui Liu
      Dr. Zhaohui Liu has been a Research Assistant at Chongqing University since 2020. He received his Ph.D. degree in Chemical Science under Prof. Yu Han from King Abdullah University of Science and Technology (KAUST) in 2018. After that, he moved to the University of Delaware and worked there with Prof. Dionisios (Dion) G. Vlachos as a Postdoctoral Researcher from 2018 to 2020. His main research interests focus on porous materials, heterogeneous catalysis and methanol conversion.
      Zhaohui Liu
      Dr. Zhaohui Liu has been a Research Assistant at Chongqing University since 2020. He received his Ph.D. degree in Chemical Science under Prof. Yu Han from King Abdullah University of Science and Technology (KAUST) in 2018. After that, he moved to the University of Delaware and worked there with Prof. Dionisios (Dion) G. Vlachos as a Postdoctoral Researcher from 2018 to 2020. His main research interests focus on porous materials, heterogeneous catalysis and methanol conversion.
    • Jianfeng Huang
      Prof. Jianfeng Huang has been a Professor of Chemistry at Chongqing University since 2019. He received his Ph.D. degree in Chemical Science under Prof. Yu Han from King Abdullah University of Science and Technology (KAUST) in 2015. After that, he moved to École Polytechnique Fédérale de Lausanne (EPFL) and worked there with Prof. Raffaella Buonsanti as a Postdoctoral Researcher from 2016 to 2019, before joining Chongqing University via the National Overseas High-level Talent Recruitment Programs of China. His current research interest focuses on the energy-concerned design of nanoscale materials, covering fields including colloidal nanochemistry, nanoplasmonics, photo/electro/thermocatalysis, etc.
      Jianfeng Huang
      Prof. Jianfeng Huang has been a Professor of Chemistry at Chongqing University since 2019. He received his Ph.D. degree in Chemical Science under Prof. Yu Han from King Abdullah University of Science and Technology (KAUST) in 2015. After that, he moved to École Polytechnique Fédérale de Lausanne (EPFL) and worked there with Prof. Raffaella Buonsanti as a Postdoctoral Researcher from 2016 to 2019, before joining Chongqing University via the National Overseas High-level Talent Recruitment Programs of China. His current research interest focuses on the energy-concerned design of nanoscale materials, covering fields including colloidal nanochemistry, nanoplasmonics, photo/electro/thermocatalysis, etc.

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