Recent Progress on the Involvement of Formaldehyde in the Methanol-To-Hydrocarbons Reaction

This review summarized the recent research progress on the crucial role of formaldehyde during the methanol-to-hydrocarbons (MTH) reaction. As a reaction intermediate, formaldehyde participates in the formation of carbon-carbon, the establish of dual-cycle, and the coking process of MTH reaction. Different techniques for formaldehyde detection in the study of MTH are also introduced.The conversion of methanol-to-hydrocarbons (MTH) over zeolite catalysts has been the subject of intense research since its discovery. Great effort has been devoted to the investigation of four key topics: the initiation of C-C bonds, the establishment of hydrocarbon pool (HCP), the adjustment of product selectivity, and the deactivation process of catalysts. Despite 50 years of study, some mechanisms remain controversial. However, an intermediate species, formaldehyde (HCHO), has recently garnered considerable attention for its influence on the entire MTH process. The discovery of HCHO and its significant role in the MTH process has been facilitated by the application of in situ analytical techniques, such as synchrotron radiation photoionization mass spectrometry (SR-PIMS) and photoelectron photoion coincidence spectroscopy (PEPICO). It is now revealed that HCHO is involved in the initiation, propagation, and termination process of MTH reaction. Such mechanistic understanding of HCHO's involvement has provided valuable insights for optimizing the MTH process.

Recent advances in bifunctional synthesis gas conversion to chemicals and fuels with a comparison to monofunctional processes

In order to meet the climate goals of the Paris Agreement and limit the potentially catastrophic consequences of climate change, we must move away from the use of fossil feedstocks for the production of chemicals and fuels. The conversion of synthesis gas (a mixture of hydrogen, carbon monoxide and/or carbon dioxide) can contribute to this. Several reactions allow to convert synthesis gas to oxygenates (such as methanol), olefins or waxes. In a consecutive step, these products can be further converted into chemicals, such as dimethyl ether, short olefins, or aromatics. Alternatively, fuels like gasoline, diesel, or kerosene can be produced. These two different steps can be combined using bifunctional catalysis for direct conversion of synthesis gas to chemicals and fuels. The synergistic effects of combining two different catalysts are discussed in terms of activity and selectivity and compared to processes based on consecutive reaction with single conversion steps. We found that bifunctional catalysis can be a strong tool for the highly selective production of dimethyl ether and gasoline with high octane numbers. In terms of selectivity bifunctional catalysis for short olefins or aromatics struggles to compete with processes consisting of single catalytic conversion steps.

The Oxygenate-Mediated Conversion of COx to Hydrocarbons─On the Role of Zeolites in Tandem Catalysis

Decentralized chemical plants close to circular carbon sources will play an important role in shaping the postfossil society. This scenario calls for carbon technologies which valorize CO2 and CO with renewable H2 and utilize process intensification approaches. The single-reactor tandem reaction approach to convert COx to hydrocarbons via oxygenate intermediates offers clear benefits in terms of improved thermodynamics and energy efficiency. Simultaneously, challenges and complexity in terms of catalyst material and mechanism, reactor, and process gaps have to be addressed. While the separate processes, namely methanol synthesis and methanol to hydrocarbons, are commercialized and extensively discussed, this review focuses on the zeolite/zeotype function in the oxygenate-mediated conversion of COx to hydrocarbons. Use of shape-selective zeolite/zeotype catalysts enables the selective production of fuel components as well as key intermediates for the chemical industry, such as BTX, gasoline, light olefins, and C3+ alkanes. In contrast to the separate processes which use methanol as a platform, this review examines the potential of methanol, dimethyl ether, and ketene as possible oxygenate intermediates in separate chapters. We explore the connection between literature on the individual reactions for converting oxygenates and the tandem reaction, so as to identify transferable knowledge from the individual processes which could drive progress in the intensification of the tandem process. This encompasses a multiscale approach, from molecule (mechanism, oxygenate molecule), to catalyst, to reactor configuration, and finally to process level. Finally, we present our perspectives on related emerging technologies, outstanding challenges, and potential directions for future research.

Liquid-phase benzylation of toluene by benzyl alcohol over micro-mesoporous hierarchical mordenite zeolite

In this work, post-synthetic effective acid (HNO₃) and base (NaOH) etching technique are used to create hierarchical mordenite having different pore structure. The powder X-ray diffraction (P-XRD) technique was used to confirm the crystalline structure of the base-modified and acid-modified mordenite. Field emission-scanning electron microscope (FE-SEM) was employed to confirm the structural morphology of the materials. The modified mordenite was further characterised by inductive coupled plasma-optical emission spectrometry (ICP-OES), N₂ adsorption-desorption isotherms, thermogravimetric analysis (TGA), and acid-base titration, to confirm the structural integrity, presence of active acidic sites, and other vital parameters. The structure was well conserved after the change, as evidenced by the characterisation. The toluene benzylation with benzyl alcohol using hierarchical mordenite and H-mordenite produced mono-benzylated toluene. Comparison between acid treated, base treated, and H-mordenite was done. All samples were catalytically active as proved by the catalytic result in the benzylation reaction. The results show that the base alteration dramatically enhances the mesoporous surface area of H-mordenite. Furthermore, the acid-treated mordenite had the highest benzyl alcohol conversion (75%), but the base-modified mordenite had benzyl alcohol conversion of 73% with the highest mono-benzylated toluene selectivity (61%). The process was further optimised by varying the reaction temperature, duration, and catalyst quantity. Gas chromatography (GC) was used to evaluate the reaction products and gas chromatography-mass spectrometry (GC-MS) was used to confirm them. Introduction of mesoporosity in the microporous mordenite was found to have significant effect on their catalytic activity.

Role of formaldehyde in promoting aromatic selectivity during methanol conversion over gallium-modified zeolites

Gallium-modified HZSM-5 zeolites are known to increase aromatic selectivity in methanol conversion. However, there are still disputes about the exact active sites and the aromatic formation mechanisms over Ga-modified zeolites. In this work, in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) experiments were carried out to study the behaviors of intermediates and products during methanol conversion over Ga-modified HZSM-5. The increased formaldehyde (HCHO) yield over Ga-modified HZSM-5 was found to play a key role in the increase in aromatic yields. More HCHO was deemed to be generated from the direct dehydrogenation of methanol, and Ga₂O₃ in Ga-modified HZSM-5 was found to be the active phase. The larger increase in aromatic production over Ga-modified HZSM-5 after reduction‒oxidation treatment was found to be the result of redispersed Ga₂O₃ with smaller size generating a larger amount of HCHO. This study provides some new insights into the internal driving force for promoting the production of aromatics over Ga-modified HZSM-5.

Multi-scale dynamical cross-talk in zeolite-catalyzed methanol and dimethyl ether conversions

Establishing a comprehensive understanding of the dynamical multiscale diffusion and reaction process is crucial for zeolite shape-selective catalysis and is urgently demanded in academia and industry. So far, diffusion and reaction for methanol and dimethyl ether (DME) conversions have usually been studied separately and focused on a single scale. Herein, we decipher the dynamical molecular diffusion and reaction process for methanol and DME conversions within the zeolite material evolving with time, at multiple scales, from the scale of molecules to single catalyst crystal and catalyst ensemble. Microscopic intracrystalline diffusivity is successfully decoupled from the macroscopic experiments and verified by molecular dynamics simulation. Spatiotemporal analyses of the confined carbonaceous species allow us to track the migratory reaction fronts in a single catalyst crystal and the catalyst ensemble. The constrained diffusion of DME relative to methanol alleviates the high local chemical potential of the reactant by attenuating its local enrichment, enhancing the utilization efficiency of the inner active sites of the catalyst crystal. In this context, the dynamical cross-talk behaviors of material, diffusion and reaction occurring at multiple scales is uncovered. Zeolite catalysis not only reflects the reaction characteristics of heterogeneous catalysis, but also provides enhanced, moderate or suppressed local reaction kinetics through the special catalytic micro-environment, which leads to the heterogeneity of diffusion and reaction at multiple scales, thereby realizing efficient and shape-selective catalysis.

Hydrogenation of Carbon Dioxide to Value-Added Liquid Fuels and Aromatics over Fe-Based Catalysts Based on the Fischer–Tropsch Synthesis Route

Hydrogenation of CO2 to value-added chemicals and fuels not only effectively alleviates climate change but also reduces over-dependence on fossil fuels. Therefore, much attention has been paid to the chemical conversion of CO2 to value-added products, such as liquid fuels and aromatics. Recently, efficient catalysts have been developed to face the challenge of the chemical inertness of CO2 and the difficulty of C–C coupling. Considering the lack of a detailed summary on hydrogenation of CO2 to liquid fuels and aromatics via the Fischer–Tropsch synthesis (FTS) route, we conducted a comprehensive and systematic review of the research progress on the development of efficient catalysts for hydrogenation of CO2 to liquid fuels and aromatics. In this work, we summarized the factors influencing the catalytic activity and stability of various catalysts, the strategies for optimizing catalytic performance and product distribution, the effects of reaction conditions on catalytic performance, and possible reaction mechanisms for CO2 hydrogenation via the FTS route. Furthermore, we also provided an overview of the challenges and opportunities for future research associated with hydrogenation of CO2 to liquid fuels and aromatics.

Elucidation of radical- and oxygenate-driven paths in zeolite-catalysed conversion of methanol and methyl chloride to hydrocarbons

Understanding hydrocarbon generation in the zeolite-catalysed conversions of methanol and methyl chloride requires advanced spectroscopic approaches to distinguish the complex mechanisms governing C-C bond formation, chain growth and the deposition of carbonaceous species. Here operando photoelectron photoion coincidence (PEPICO) spectroscopy enables the isomer-selective identification of pathways to hydrocarbons of up to C₁₄ in size, providing direct experimental evidence of methyl radicals in both reactions and ketene in the methanol-to-hydrocarbons reaction. Both routes converge to C₅ molecules that transform into aromatics. Operando PEPICO highlights distinctions in the prevalence of coke precursors, which is supported by electron paramagnetic resonance measurements, providing evidence of differences in the representative molecular structure, density and distribution of accumulated carbonaceous species. Radical-driven pathways in the methyl chloride-to-hydrocarbons reaction(s) accelerate the formation of extended aromatic systems, leading to fast deactivation. By contrast, the generation of alkylated species through oxygenate-driven pathways in the methanol-to-hydrocarbons reaction extends the catalyst lifetime. The findings demonstrate the potential of the presented methods to provide valuable mechanistic insights into complex reaction networks.

Two-Step Dry Gel Method Produces MgAPO-11 with Low Aspect Ratio and Improved Catalytic Performance in the Conversion of Methanol to Hydrocarbons

In this article, the synthesis, characterization and catalytic performance of three MgAPO-11 catalysts with distinct crystal morphologies (sunflower, ball and candy) are presented. Among the three samples, the candy-like MgAPO-11-C, with high crystallinity and uniform particle size (of about 1 µm), was synthesized for the first time by using a unique two-step dry gel method. Despite the similar acid strength of the three samples, the different and distinct morphologies of the catalysts resulted in very different methanol-to-hydrocarbons (MTH) performances. In particular, the candy-like MgAPO-11-C presented the best MTH performance with the highest total conversion capacity (4.4 gMeOH·gcatalyst−1 h−1) and the best selectivity to C5+ aliphatics (64%).

Mechanistic differences between methanol and dimethyl ether in zeolite-catalyzed hydrocarbon synthesis

Methanol conversion to hydrocarbons has emerged as a key reaction for synthetic energy carriers and light alkenes. The autocatalytic nature and complex reaction network make a mechanistic understanding very challenging and widely debated. Water is not only part of the overall conversion, it is also frequently used as diluent, influencing, in turn, activity, selectivity, and stability of the catalysts. Water directly and indirectly influences the processes that initiate the C–C formation via adjusting the chemical potential of methanol and dimethyl ether, with the latter being more efficient to generate highly reactive C₁ species via hydride transfer. The insight shows paths to optimize the stability of catalysts and to tailor the product distribution for H-ZSM-5–based catalysts.

Water influences critically the kinetics of the autocatalytic conversion of methanol to hydrocarbons in acid zeolites. At very low conversions but otherwise typical reaction conditions, the initiation of the reaction is delayed in presence of H₂O. In absence of hydrocarbons, the main reactions are the methanol and dimethyl ether (DME) interconversion and the formation of a C₁ reactive mixture—which in turn initiates the formation of first hydrocarbons in the zeolite pores. We conclude that the dominant reactions for the formation of a reactive C₁ pool at this stage involve hydrogen transfer from both MeOH and DME to surface methoxy groups, leading to methane and formaldehyde in a 1:1 stoichiometry. While formaldehyde reacts further to other C₁ intermediates and initiates the formation of first C–C bonds, CH₄ is not reacting. The hydride transfer to methoxy groups is the rate-determining step in the initiation of the conversion of methanol and DME to hydrocarbons. Thus, CH₄ formation rates at very low conversions, i.e., in the initiation stage before autocatalysis starts, are used to gauge the formation rates of first hydrocarbons. Kinetics, in good agreement with theoretical calculations, show surprisingly that hydrogen transfer from DME to methoxy species is 10 times faster than hydrogen transfer from methanol. This difference in reactivity causes the observed faster formation of hydrocarbons in dry feeds, when the concentration of methanol is lower than in presence of water. Importantly, the kinetic analysis of CH₄ formation rates provides a unique quantitative parameter to characterize the activity of catalysts in the methanol-to-hydrocarbon process.

A Kinetic Model Considering Catalyst Deactivation for Methanol-to-Dimethyl Ether on a Biomass-Derived Zr/P-Carbon Catalyst

A Zr-loaded P-containing biomass-derived activated carbon (ACPZr) has been tested for methanol dehydration between 450 and 550 °C. At earlier stages, methanol conversion was complete, and the reaction product was mainly dimethyl ether (DME), although coke, methane, hydrogen and CO were also observed to a lesser extent. The catalyst was slowly deactivated with time-on-stream (TOS), but maintained a high selectivity to DME (>80%), with a higher yield to this product than 20% for more than 24 h at 500 °C. A kinetic model was developed for methanol dehydration reaction, which included the effect of the inhibition of water and the deactivation of the catalyst by coke. The study of stoichiometric rates pointed out that coke could be produced through a formaldehyde intermediate, which might, alternatively, decompose into CO and H₂. On the other hand, the presence of 10% water in the feed did not affect the rate of coke formation, but produced a reduction of 50% in the DME yield, suggesting a reversible competitive adsorption of water. A Langmuir–Hinshelwood reaction mechanism was used to develop a kinetic model that considered the deactivation of the catalyst. Activation energy values of 65 and 51 kJ/mol were obtained for DME and methane production in the temperature range from 450 °C to 550 °C. On the other hand, coke formation as a function of time on stream (TOS) was also modelled and used as the input for the deactivation function of the model, which allowed for the successful prediction of the DME, CH₄ and CO yields in the whole evaluated TOS interval.

The intricacies of the “steady-state” regime in methanol-to-hydrocarbon experimentation over H-ZSM-5

The operating conditions window and experimental procedures ensuring “steady-state” operation in methanol to hydrocarbon conversion have been experimentally determined over an H-ZSM-5 zeolite with considerable acidity (Si/Al = 40).

Trimethyloxonium ion – a zeolite confined mobile and efficient methyl carrier at low temperatures: a DFT study coupled with microkinetic analysis

The reaction network of ethene methylation over H-ZSM-5, including methanol dehydration, ethene methylation, and C3H7+ conversion, is investigated by employing a multiscale approach combining DFT calculations and microkinetic modeling.

Mechanistic kinetic modeling for catalytic conversion of DME to gasoline-range hydrocarbons over nanostructured ZSM-5

A new kinetic model for the synthesis of gasoline-range hydrocarbons from dimethyl ether over a nanostructured ZSM-5 catalyst was developed based on the dual-cycle reaction mechanism.

Molecular Routes of Dynamic Autocatalysis for Methanol-to-Hydrocarbons Reaction

The industrially important methanol-to-hydrocarbons (MTH) reaction is driven and sustained by autocatalysis in a dynamic and complex manner. Hitherto, the entire molecular routes and chemical nature of the autocatalytic network have not been well understood. Herein, with a multitechnique approach and multiscale analysis, we have obtained a full theoretical picture of the domino cascade of autocatalytic reaction network taking place on HZSM-5 zeolite. The autocatalytic reaction is demonstrated to be plausibly initiated by reacting dimethyl ether (DME) with the surface methoxy species (SMS) to generate the initial olefins, as evidenced by combining the kinetic analysis, in situ DRIFT spectroscopy, 2D ¹³C-¹³C MAS NMR, electronic states, and projected density of state (PDOS) analysis. This process is operando tracked and visualized at the picosecond time scale by advanced ab initio molecular dynamics (AIMD) simulations. The initial olefins ignite autocatalysis by building the first autocatalytic cycle-olefins-based cycle-followed by the speciation of methylcyclopentenyl (MCP) and aromatic cyclic active species. In doing so, the active sites accomplish the dynamic evolution from proton acid sites to supramolecular active centers that are experimentally identified with an ever-evolving and fluid feature. The olefins-guided and cyclic-species-guided catalytic cycles are interdependently linked to forge a previously unidentified hypercycle, being composed of one "selfish" autocatalytic cycle (i.e., olefins-based cycle with lighter olefins as autocatalysts for catalyzing the formation of olefins) and three cross-catalysis cycles (with olefinic, MCP, and aromatic species as autocatalysts for catalyzing each other's formation). The unraveled dynamic autocatalytic cycles/network would facilitate the catalyst design and process control for MTH technology.

Ultrasound-assisted rapid hydrothermal design of efficient nanostructured MFI-Type aluminosilicate catalyst for methanol to propylene reaction

The influence of ultrasound-assisted rapid hydrothermal synthesis of aluminosilicate ZSM-5 catalysts was examined in this work. A series of MFI-type nanostructured materials with sonochemical approach and conventionalheating were synthesized and evaluated for conversion of methanol to propylene reaction. The prepared samples were tested by characterization analyses such as XRD, FESEM, BET-BJH, FTIR, TPD-NH₃ and TG/DTG. The obtained results confirmed that ultrasound treatment enhanced the nucleation process and crystal growth for ZSM-5 sample synthesized at moderate temperature of 250 °C. Therefore, it was found the formation of pure MFI zeolite with high crystallinity and improved textural, structural and acidic properties for ZSM-5(UH-250) sample compared with the other zeolites. This observation was attributed to the relationship between the perfect crystallization mechanism and catalytic properties, which led to producing an efficient MFI zeolite toward the optimal catalytic performance. In this manner, the methanol conversion and products selectivity of prepared materials were carried out in MTP reaction at 460 °C and atmospheric pressure. The ZSM-5(UH-250) zeolite with slower deactivation regime exhibited the constant level of methanol conversion (84%) and high propylene selectivity (78%) after 2100 min time on stream. Moreover, the synthesis pathway for MFI zeolite at moderate temperature and also deactivation mechanism of improved sample were proposed.

CO2 hydrogenation to methanol and hydrocarbons over bifunctional Zn-doped ZrO2/zeolite catalysts

The tandem process of carbon dioxide hydrogenation to methanol and its conversion to hydrocarbons over mixed metal/metal oxide-zeotype catalysts is a promising path to CO₂ valorization.

Direct conversion of syngas into aromatics over a bifunctional catalyst: inhibiting net CO2 release

Tandem catalysis via methanol intermediate is a promising route for the direct conversion of syngas into aromatics. However, the simultaneous formation of CO2 is a serious problem. Here, we demonstrate that CO2 was formed by the water-gas shift (WGS) reaction (CO + H2O → CO2 + H2) over a ZnO-ZrO2/H-ZSM-5 catalyst, and the net CO2 formation could be inhibited without affecting the formation of aromatics by co-feeding CO2.

Lower olefins from methane: recent advances

Modern methods for methane conversion to lower olefins having from 2 to 4 carbon atoms per molecule are generalized. Multistage processing of methane into ethylene and propylene via syngas or methyl chloride and methods for direct conversion of CH4 to ethylene are described. Direct conversion of syngas to olefins as well as indirect routes of the process via methanol or dimethyl ether are considered. Particular attention is paid to innovative methods of olefin synthesis. Recent achievements in the design of catalysts and development of new techniques for efficient implementation of oxidative coupling of methane and methanol conversion to olefins are analyzed and systematized. Advances in commercializing these processes are pointed out. Novel catalysts for Fischer – Tropsch synthesis of lower olefins from syngas and for innovative technique using oxide – zeolite hybrid catalytic systems are described. The promise of a new route to lower olefins by methane conversion via dimethyl ether is shown. Prospects for the synthesis of lower olefins via methyl chloride and using non-oxidative coupling of methane are discussed. The most efficient processes used for processing of methane to lower olefins are compared on the basis of degree of conversion of carbonaceous feed, possibility to integrate with available full-scale production, number of reaction stages and thermal load distribution.

The bibliography includes 346 references.

On the conversion of CO2 to value added products over composite PdZn and H-ZSM-5 catalysts: excess Zn over Pd, a compromise or a penalty?

Zn was found to possess a dual role in composite PdZn–H-ZSM-5 catalysts for CO₂ hydrogenation reactions: it promotes methanol formation when alloyed with Pd, but inhibits hydrocarbon formation by ion exchange with Brønsted acid sites in H-ZSM-5.

Deactivation of Zeolites and Zeotypes in Methanol-to-Hydrocarbons Catalysis: Mechanisms and Circumvention

Solid catalysts deployed in industrial processes often undergo deactivation, requiring frequent replacement or regeneration to recover the loss in activity. Regeneration occurs under conditions distinct from, and typically more harsh than, the catalysis, placing strict requirements on physicochemical material properties that divert catalyst optimization toward addressing regenerability over high activity and selectivity. Deactivation arises from mechanical, structural, or chemical modifications to active sites, promoters, and their surrounding matrices, and the prevailing mechanism for deactivation varies with the reaction, the catalyst, and the reaction conditions. Methanol-to-hydrocarbons processes utilize zeolites and zeotypes-crystalline, microporous oxides widely deployed as catalysts in the refining and petrochemical industries-as solid acid catalysts. Deposition and growth of highly unsaturated carbonaceous residues within the micropores congest molecular transport and block active sites, resulting in deactivation. In this Account, we describe studies probing the underlying mechanisms of deactivation in methanol-to-hydrocarbons catalysis and discuss examples of leveraging the acquired mechanistic insights to mitigate deactivation and prolong catalyst lifetime. These fundamental principles governing carbon deposition within zeolites and zeotypes provide opportunity to broaden versatility of processes for C₁ valorization and to relax constraints imposed by hydrothermal catalyst stability considerations to achieve more active and more selective catalysis. Methanol-to-hydrocarbons catalysis occurs via a chain carrier mechanism. A zeolite/zeotype cavity hosts an unsaturated hydrocarbon guest to together constitute the supramolecular chain carrier that engages in a complex network of reactions for chain carrier propagation. Productive propagation reactions include olefin methylation, aromatic methylation, and aromatic dealkylation. Methanol undergoes unproductive dehydrogenation to formaldehyde via methanol disproportionation and olefin transfer hydrogenation. Subsequent alkylation reactions between formaldehyde and active olefinic/aromatic cocatalysts instigate cascades for dehydrocyclization, resulting in the formation of inactive polycyclic aromatic hydrocarbons and termination of the chain carrier. Addition of a distinct catalytic function that selectively decomposes formaldehyde mitigates chain carrier termination without disrupting the high selectivity to ethylene and propylene in methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes. The efficacy of this bifunctional strategy to prolong catalyst lifetime increases with increasing proximity between the active sites for formaldehyde decomposition and the H⁺ sites of the zeolite/zeotype. Coprocessing sacrifical hydrogen donors mitigates chain carrier termination by intercepting, via saturation, intermediates along dehydrocyclization cascades. This strategy increases in efficacy with increasing concentration of the hydrogen donor and provides opportunity to realize steady-state methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes.