Benzene co-reaction with methanol and dimethyl ether over zeolite and zeotype catalysts: Evidence of parallel reaction paths to toluene and diphenylmethane

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.

Operando Modeling of Zeolite-Catalyzed Reactions Using First-Principles Molecular Dynamics Simulations

Within this Perspective, we critically reflect on the role of first-principles molecular dynamics (MD) simulations in unraveling the catalytic function within zeolites under operating conditions. First-principles MD simulations refer to methods where the dynamics of the nuclei is followed in time by integrating the Newtonian equations of motion on a potential energy surface that is determined by solving the quantum-mechanical many-body problem for the electrons. Catalytic solids used in industrial applications show an intriguing high degree of complexity, with phenomena taking place at a broad range of length and time scales. Additionally, the state and function of a catalyst critically depend on the operating conditions, such as temperature, moisture, presence of water, etc. Herein we show by means of a series of exemplary cases how first-principles MD simulations are instrumental to unravel the catalyst complexity at the molecular scale. Examples show how the nature of reactive species at higher catalytic temperatures may drastically change compared to species at lower temperatures and how the nature of active sites may dynamically change upon exposure to water. To simulate rare events, first-principles MD simulations need to be used in combination with enhanced sampling techniques to efficiently sample low-probability regions of phase space. Using these techniques, it is shown how competitive pathways at operating conditions can be discovered and how broad transition state regions can be explored. Interestingly, such simulations can also be used to study hindered diffusion under operating conditions. The cases shown clearly illustrate how first-principles MD simulations reveal insights into the catalytic function at operating conditions, which could not be discovered using static or local approaches where only a few points are considered on the potential energy surface (PES). Despite these advantages, some major hurdles still exist to fully integrate first-principles MD methods in a standard computational catalytic workflow or to use the output of MD simulations as input for multiple length/time scale methods that aim to bridge to the reactor scale. First of all, methods are needed that allow us to evaluate the interatomic forces with quantum-mechanical accuracy, albeit at a much lower computational cost compared to currently used density functional theory (DFT) methods. The use of DFT limits the currently attainable length/time scales to hundreds of picoseconds and a few nanometers, which are much smaller than realistic catalyst particle dimensions and time scales encountered in the catalysis process. One solution could be to construct machine learning potentials (MLPs), where a numerical potential is derived from underlying quantum-mechanical data, which could be used in subsequent MD simulations. As such, much longer length and time scales could be reached; however, quite some research is still necessary to construct MLPs for the complex systems encountered in industrially used catalysts. Second, most currently used enhanced sampling techniques in catalysis make use of collective variables (CVs), which are mostly determined based on chemical intuition. To explore complex reactive networks with MD simulations, methods are needed that allow the automatic discovery of CVs or methods that do not rely on a priori definition of CVs. Recently, various data-driven methods have been proposed, which could be explored for complex catalytic systems. Lastly, first-principles MD methods are currently mostly used to investigate local reactive events. We hope that with the rise of data-driven methods and more efficient methods to describe the PES, first-principles MD methods will in the future also be able to describe longer length/time scale processes in catalysis. This might lead to a consistent dynamic description of all steps-diffusion, adsorption, and reaction-as they take place at the catalyst particle level.

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.

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.

Insight into Carbocation-Induced Noncovalent Interactions in the Methanol-to-Olefins Reaction over ZSM-5 Zeolite by Solid-State NMR Spectroscopy

Carbocations such as cyclic carbenium ions are important intermediates in the zeolite-catalyzed methanol-to-olefins (MTO) reaction. The MTO reaction propagates through a complex hydrocarbon pool process. Understanding the carbocation-involved hydrocarbon pool reaction on a molecular level still remains challenging. Here we show that electron-deficient cyclopentenyl cations stabilized in ZSM-5 zeolite are able to capture the alkanes, methanol, and olefins produced during MTO reaction via noncovalent interactions. Intermolecular spatial proximities/interactions are identified by using two-dimensional ¹³ C-¹³ C correlation solid-state NMR spectroscopy. Combined NMR experiments and theoretical analysis suggests that in addition to the dispersion and CH/π interactions, the multiple functional groups in the cyclopentenyl cations produce strong attractive force via cation-induced dipole, cation-dipole and cation-π interactions. These carbocation-induced noncovalent interactions modulate the product selectivity of hydrocarbon pool reaction.

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.

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.

A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion

A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Brønsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Brønsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]⁺ and double protonated binuclear metal clusters [M(μ-OH)₂M]²⁺ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Brønsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Brønsted and Lewis acid sites.

Critical role of formaldehyde during methanol conversion to hydrocarbons

Formaldehyde is an important intermediate product in the catalytic conversion of methanol to olefins (MTO). Here we show that formaldehyde is present during MTO with an average concentration of ~0.2 C% across the ZSM-5 catalyst bed up to a MeOH conversion of 70%. It condenses with acetic acid or methyl acetate, the carbonylation product of MeOH and DME, into unsaturated carboxylate or carboxylic acid, which decarboxylates into the first olefin. By tracing its reaction pathways of 13C-labeled formaldehyde, it is shown that formaldehyde reacts with alkenes via Prins reaction into dienes and finally to aromatics. Because its rate is one order of magnitude higher than that of hydrogen transfer between alkenes on ZSM-5, the Prins reaction is concluded to be the major reaction route from formaldehyde to produce dienes and aromatics. In consequence, formaldehyde increases the yield of ethene by enhancing the contribution of aromatic cycle.

Kinetic Modeling of Catalytic Olefin Cracking and Methanol-to-Olefins (MTO) over Zeolites: A Review

The increasing demand for lower olefins requires new production routes besides steam cracking and fluid catalytic cracking (FCC). Furthermore, less energy consumption, more flexibility in feed and a higher influence on the product distribution are necessary. In this context, catalytic olefin cracking and methanol-to-olefins (MTO) gain in importance. Here, the undesired higher olefins can be catalytically converted and, for methanol, the possibility of a green synthesis route exists. Kinetic modeling of these processes is a helpful tool in understanding the reactivity and finding optimum operating points; however, it is also challenging because reaction networks for hydrocarbon interconversion are rather complex. This review analyzes different deterministic kinetic models published in the literature since 2000. After a presentation of the underlying chemistry and thermodynamics, the models are compared in terms of catalysts, reaction setups and operating conditions. Furthermore, the modeling methodology is shown; both lumped and microkinetic approaches can be found. Despite ZSM-5 being the most widely used catalyst for these processes, other catalysts such as SAPO-34, SAPO-18 and ZSM-23 are also discussed here. Finally, some general as well as reaction-specific recommendations for future work on modeling of complex reaction networks are given.

Co-Aromatization of n-Butane and Methanol over PtSnK-Mo/ZSM-5 Zeolite Catalysts: The Promotion Effect of Ball-Milling

The ball-milling (BM) method benefits the stabilization and dispersion of metallic particles for the preparation of the PtSnK–Mo/ZSM-5 catalyst. Based on the TPR, H2-TPD, XPS, and CO-FTIR results, the Pt–SnOx and MoOx species were formed separately on the BM sample. During the aromatization of cofeeding the n-butane with methanol, the yield of the aromatics is 59 wt.% at a n-butane conversion of 86% at 475 °C over the Pt Mo BM catalyst. The more weak acid sites also contribute to the aromatics formation with the less light alkanes formation. For the Pt Ga catalysts, the slow loss of activity suggests that the BM method can restrain the coke deposition on the Pt-SnOx species, because of a certain distance between the Pt–SnOx and GaOx species on the surface of ZSM-5.

A Systematic Study of Isomorphically Substituted H-MAlPO-5 Materials for the Methanol-to-Hydrocarbons Reaction

Substituting metals for either aluminum or phosphorus in crystalline, microporous aluminophosphates creates Brønsted acid sites, which are well known to catalyze several key reactions, including the methanol to hydrocarbons (MTH) reaction. In this work, we synthesized a series of metal-substituted aluminophosphates with AFI topology that differed primarily in their acid strength and that spanned a predicted range from high Brønsted acidity (H-MgAlPO-5, H-CoAlPO-5, and H-ZnAlPO-5) to medium acidity (H-SAPO-5) and low acidity (H-TiAlPO-5 and H-ZrAlPO-5). The synthesis was aimed to produce materials with homogenous properties (e.g. morphology, crystallite size, acid-site density, and surface area) to isolate the influence of metal substitution. This was verified by extensive characterization. The materials were tested in the MTH reaction at 450 °C by using dimethyl ether (DME) as feed. A clear activity difference was found, for which the predicted stronger acids converted DME significantly faster than the medium and weak Brønsted acidic materials. Furthermore, the stronger Brønsted acids (Mg, Co and Zn) produced more light alkenes than the weaker acids. The weaker acids, especially H-SAPO-5, produced more aromatics and alkanes, which indicates that the relative rates of competing reactions change upon decreasing the acid strength.

New insights into catalyst deactivation and product distribution of zeolites in the methanol-to-hydrocarbons (MTH) reaction with methanol and dimethyl ether feeds

The ability of a zeolitic catalyst to dehydrate methanol to dimethyl ether affects catalyst deactivation and product distribution during the methanol-to-hydrocarbons (MTH) reaction.

Investigation of methanol conversion over high-Si beta zeolites and the reaction mechanism of their high propene selectivity

Large pore high-Si beta zeolites (Si/Al = 136 to 340) were synthesized by a HF-assisted method, and their catalytic performance for the conversion of methanol to propene was explored.