Brønsted/Lewis acid sites synergistically promote the initial C-C bond formation in the MTO reaction

Methanol to Olefins (MTO): Understanding and Regulating Dynamic Complex Catalysis

The research and development of methanol conversion into hydrocarbons have spanned more than 40 years. The past four decades have witnessed mutual promotion and successive breakthroughs in both fundamental research and industrial process development of methanol to olefins (MTO), demonstrating that MTO is an extremely dynamic, complex catalytic system. This Perspective summarizes the endeavors and achievements of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in the continuous study of reaction mechanisms and process engineering of the dynamic, complex MTO reaction system. It elucidates fundamental chemical issues concerning the essence of the dynamic evolution of the MTO reaction and the cross-talk mechanisms among diffusion, reaction, and catalyst (coke modification), which are crucial for technology development and process optimization. By integrating the chemical principles, the reaction-diffusion model, and coke formation kinetics of MTO, a mechanism- and model-driven modulation of industrial processes has been achieved. The acquisition of a deepening understanding in chemistry and engineering has facilitated the continuous optimization and upgrading of MTO catalysts and processes.

Identification and Quantification of Al Pairs and Their Impact on Dealumination in Zeolites

Understanding the precise quantity and spatial distribution of paired aluminum (Al) sites in zeolite catalysts is crucial, as they significantly impact the catalytic performance via synergistic effects and long-term stability. In this study, a novel strategy by employing divalent cation titration with varying cation sizes, in combination with advanced quantitative 1H NMR and 1H-1H homonuclear correlation techniques, has been developed to accurately identify and classify three distinct types of Al pairs. These include two types of Al pairs aligned along six-membered rings (6-MRs) and 10-membered rings (10-MRs), the latter of which are essentially composed of Al atoms located in different 6-MR or 5-MR. The third type comprises two Al atoms located in different channels. The second and third types had been challenging to probe in the past, yet they may be critical for catalysis, particularly the second type demonstrating proximity close enough to accommodate Ba2+ (with a radius of 1.49 Å). Our strategy for quantifying each type of Al pair marks a significant advancement in the understanding of the zeolite framework. Furthermore, controlled hydrothermal treatments using stepwise steaming reveal that a higher concentration of Al pairs accelerates dealumination, primarily for dynamic reasons of water molecules but not intrinsic structural instability induced by Al pairs. To address this, we propose a "bi-Al" vs "mono-Al" hydrolysis model, offering fresh insights into the pivotal role of Al pairs on zeolite stability. This work opens new avenues for optimizing zeolite-based catalysts for enhanced performance and longevity.

How the Pretreatment Temperature of Zeolitic Catalysts Can Affect the Reaction Temperature of Methanol to Olefins and Gasoline Processes

The dehydration of methanol to produce light olefins and gasoline, known as MTO (methanol-to-olefins) process requires acidic catalysts that maintain their acidity at reaction temperatures. Zeolites, such as SAPOs and ZSM-5, are commonly used for this purpose due to their acidic centers. The initial step in these experiments involves the activation or pretreatment of these solids to remove physically adsorbed water from their pores. Inadequate pretreatment can lead to the destruction of the existing Brönsted sites through the dihydroxylation of surface -OH groups. Therefore, it is crucial to pretreat the zeolites properly to preserve the Brönsted sites. One method is to subject the fresh catalyst to programmed dehydration, which involves desorption at a controlled temperature while monitoring the appearance of water that results from Brönsted site dihydroxylation. The temperature at which the dehydration peak appears determines the optimal reaction temperature. The results presented in this work will demonstrate the progressive deactivation of the catalysts when the reaction temperature exceeds 400 °C.

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.

Electrospinning and Partial Etching Behaviors of Core-Shell Nanofibers Directly Electrospun on Mesh Substrates for Application in a Cover-Free Compact Air Filter

The exposure of workers to propylene glycol monomethyl ether acetate (PGMEA) in manufacturing environments can result in potential health risks. Therefore, systems for PGMEA removal are required for indoor air quality control. In this study, core-shell zeolite socony mobil-5 (ZSM-5)/polyvinylpyrrolidone-polyvinylidene fluoride nanofibers were directly electrospun and partially wet-etched on a mesh substrate to develop a cover-free compact PGMEA air filter. The electrospinning behaviors of the core-shell nanofibers were investigated to optimize the electrospinning time and humidity and to enable the manufacture of thin and light air-filter layers. The partial wet etching of the nanofibers was undertaken using different etching solvents and times to ensure the exposure of the active sites of ZSM-5. The performances of the ZSM-5/PVDF nanofiber air filters were assessed by measuring five consecutive PGMEA adsorption-desorption cycles at different desorption temperatures. The synthesized material remained stable upon repeated adsorption-desorption cycles and could be regenerated at a low desorption temperature (80 °C), demonstrating a consistent adsorption performance upon prolonged adsorption-desorption cycling and low energy consumption during regeneration. The results of this study provide new insights into the design of industrial air filters using functional ceramic/polymer nanofibers and the application of these filters.

Water structures on acidic zeolites and their roles in catalysis

The local reaction environment of catalytic active sites can be manipulated to modify the kinetics and thermodynamic properties of heterogeneous catalysis. Because of the unique physical-chemical nature of water, heterogeneously catalyzed reactions involving specific interactions between water molecules and active sites on catalysts exhibit distinct outcomes that are different from those performed in the absence of water. Zeolitic materials are being applied with the presence of water for heterogeneous catalytic reactions in the chemical industry and our transition to sustainable energy. Mechanistic investigation and in-depth understanding about the behaviors and the roles of water are essentially required for zeolite chemistry and catalysis. In this review, we focus on the discussions of the nature and structures of water adsorbed/stabilized on Brønsted and Lewis acidic zeolites based on experimental observations as well as theoretical calculation results. The unveiled functions of water structures in determining the catalytic efficacy of zeolite-catalyzed reactions have been overviewed and the strategies frequently developed for enhancing the stabilization of zeolite catalysts are highlighted. Recent advancement will contribute to the development of innovative catalytic reactions and the rationalization of catalytic performances in terms of activity, selectivity and stability with the presence of water vapor or in condensed aqueous phase.

A density functional theory study on the mechanism of toluene from dimethylcyclopentane catalyzed by the [GaH]2+ active site of Ga-ZSM-5

The ONIOM (ωb97xd/6-31G(d,p):pm6) method was used to study the reaction mechanism of dimethylcyclopentane to toluene by the [GaH]2+ active site of Ga-ZSM-5. The results showed that the rate-determining step in the dimethylcyclopentane aromatization process is the ring expansion process. Compared to those of methylcyclopentane to benzene (D. D. Zhang, H. Y. Liu, L. X. Ling, H. R. Zhang, R. G. Zhang, P. Liu and B. J. Wang, Phys. Chem. Chem. Phys., 2021, 23, 10988-11003.), the free energy barriers of dimethylcyclopentane to toluene are significantly decreased, indicating that toluene is easier to produce than benzene, which confirmed the experimental results that a higher proportion of toluene than benzene is produced in the MTA process.

Atomic Layer Deposition of the Geometry Separated Lewis and Brønsted Acid Sites for Cascade Glucose Conversion

Solid acid catalysts with bi-acidity are promising as workhouse catalysts in biorefining to produce high-quality chemicals and fuels. Herein, we report a new strategy to develop bi-acidic cascade catalysts by separating both acid sites in geometry via the atomic layer deposition (ALD) of Lewis acidic alumina on Brønsted acidic supports. Visualized by transmission electron microscopy and electron energy loss spectroscopy mapping, the ALD-deposited alumina forms a conformal alumina domain with a thickness of around 3 nm on the outermost surface of mesoporous silica-alumina. Solid state nuclear magnetic resonance investigation shows that the dominant Lewis acid sites distribute on the outermost surface, whereas intrinsic Brønsted acid sites locate inside the nanopores within the silica-rich substrate. In comparison to other bi-acidic solid catalyst counterparts, the special geometric distance of Lewis and Brønsted acid sites minimized the synergetic effect, leading to a cascade reaction environment. For cascade glucose conversion, the designed ALD catalyst showed a highly enhanced catalytic performance.

Dynamic Catalytic Mechanism of the Methanol-to-Hydrocarbons Reaction over Zeolites

ConspectusThe methanol-to-hydrocarbons (MTH) process has provided a new route to obtaining basic chemicals without relying on an oil resource. Acidity and shape selectivity endow the zeolite with a decisive role in MTH catalysis. However, the inherent reaction characteristics of the MTH reaction over zeolites, such as the complexity of catalytic reaction kinetics, the diversity of catalytic reaction modes, and even the limitations of catalytic and diffusive decoupling, have all confused people with respect to obtaining a comprehensive mechanistic understanding. By examining the zeolite-catalyzed MTH reaction from the perspective of chemical bonding, one would realize that this reaction reflects the dynamic assembly process of C-C bonds from C1 components to multicarbon products. The key to understanding the MTH reaction lies in the mechanism by which C-C bonds are formed and rearranged in the confined microenvironment of the channel or cage structures of zeolite catalysts to achieve shape-selective production.The applications of advanced in situ spectroscopy as well as computational chemistry provide tremendous opportunities for capturing and identifying the details of the structure and properties of reactants, intermediates, and products in the confined reaction space of zeolite channels or cages, observing the real-time dynamic evolution of the catalytic surface, and modeling the elementary reaction steps at the molecular and atomic levels.In this Account, the dynamic catalytic mechanism of the zeolite-catalyzed MTH reaction will be outlined based on decades of continuous research and in-depth understanding. The combination of advanced in situ spectroscopy and theoretical methods allowed us to observe and simulate the formation, growth, and aging process on the working catalyst surface and thus map the dynamical evolution of active sites from a Brønsted acid site (BAS) to an organic-inorganic hybrid supramolecule (OIHS) in the MTH reaction. Moreover, the ever-evolving dynamic succession of the OIHS from surface methoxy species (SMS) to active ion-pair complexes (AIPC) to inert complexes (IC) guided the dynamic autocatalytic process from initiation to sustaining and then to termination, resulting in a complex interlaced hypercycle reaction network. The concept of dynamic catalysis will provide deep insight into the complex catalytic mechanisms as well as the structure-activity relationships in MTH chemistry. More importantly, we are now getting closer to the nature of zeolite catalysis beyond the traditional view of BAS catalysis.

Unraveling the capture mechanism of gaseous As2O3 over H-zsm-5 zeolite from coal-fired flue gas: Experimental and theoretical insights

Gaseous As₂O₃ discharged from coal-fired power plants results in severe detriments to the ecological environment. It is of great urgency to develop highly efficient As₂O₃ capture technology for reducing atmospheric arsenic contamination. Utilizing solid sorbents for gaseous As₂O₃ capture is a promising treatment for As₂O₃ capture. The zeolite of H-ZSM-5 was applied for As₂O₃ capture at high temperatures of 500-900 °C. Special attention was paid to clarifying its capture mechanism and identifying the influence of flue gas components via density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. Results revealed that due to high thermal stability with large specific areas, H-ZSM-5 demonstrated excellent arsenic capture at 500-900 °C. The captured arsenic consisted of As³⁺ and As⁵⁺ speciations, ascribed to As₂O₃ adsorption and oxidation. Moreover, As³⁺ and As⁵⁺ compounds were both through physisorption or chemisorption at 500-600 °C while dominant chemisorption at 700-900 °C. In particular, As³⁺ compounds were much more steadily fixed in products at all operating temperatures. Combining the characterization analysis and DFT calculations, it further verified that both Si-OH-Al groups and external Al species of H-ZSM-5 could chemisorb As₂O₃, and the latter exhibited much stronger affinities via orbital hybridization and electron transfer. The introduced O₂ could facilitate As₂O₃ oxidation and fixation in H-ZSM-5, especially at a lower concentration of 2%. Additionally, H-ZSM-5 possessed great acid gas resistance for As₂O₃ capture under the concentration of NO or SO₂ less than 500 ppm. AIMD simulations further identified that compared to NO and SO₂, As₂O₃ was far more competitive and occupied the active sites of the Si-OH-Al groups and external Al species of H-ZSM-5. Overall, it demonstrated that H-ZSM-5 is a promising sorbent for As₂O₃ capture from coal-fired flue gas.

Effective synergies in indium oxide loaded with zirconia mixed with silicoaluminophosphate molecular sieve number 34 catalysts for carbon dioxide hydrogenation to lower olefins

Tandem catalysts consisting of metal oxides and zeolites have been widely studied for catalytic carbon dioxide (CO₂) hydrogenation to lower olefins, while the synergies of two components and their influence on the catalytic performance are still unclear. In this study, the composite catalysts composed of indium oxide loaded with zirconia (In₂O₃/ZrO₂) and silicoaluminophosphate molecular sieve number 34 (SAPO-34) are developed. Performance results indicate that the synergies between these two components can promote CO₂ hydrogenation. Further characterizations reveal that the chabazite (CHA) structure and acid sites in the SAPO-34 are destroyed when preparing In-Zr/SAPO by powder milling (In-Zr/SAPO-M) because of the excessive proximity of two components, which inhibits the activation of CO₂ and hydrogen (H₂), thus resulting in much higher methane selectivity than the catalysts prepared by granule stacking (In-Zr/SAPO-G). Proper granule integration manner promotes tandem reaction, thus enhancing CO₂ hydrogenation to lower olefins, which can provide a practicable strategy to improve catalytic performance and the selectivity of the target products.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Progressive steps and catalytic cycles in methanol-to-hydrocarbons reaction over acidic zeolites

Understanding the complete reaction network and mechanism of methanol-to-hydrocarbons remains a key challenge in the field of zeolite catalysis and C1 chemistry. Inspired by the identification of the reactive surface methoxy species on solid acids, several direct mechanisms associated with the formation of the first C-C bond in methanol conversion have been recently disclosed. Identifying the stepwise involvement of the initial intermediates containing the first C-C bond in the whole reaction process of methanol-to-hydrocarbons conversion becomes possible and attractive for the further development of this important reaction. Herein, several initial unsaturated aldehydes/ketones containing the C-C bond are identified via complementary spectroscopic techniques. With the combination of kinetic and spectroscopic analyses, a complete roadmap of the zeolite-catalyzed methanol-to-hydrocarbons conversion from the initial C-C bonds to the hydrocarbon pool species via the oxygen-containing unsaturated intermediates is clearly illustrated. With the participation of both Brønsted and Lewis acid sites in H-ZSM-5 zeolite, an initial aldol-cycle is proposed, which can be closely connected to the well-known dual-cycle mechanism in the methanol-to-hydrocarbons conversion.

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.

A quantitative multiscale perspective on primary olefin formation from methanol

The formation of the first C-C bond and primary olefins from methanol over zeolite and zeotype catalysts has been studied for over 40 years. Over 20 mechanisms have been proposed for the formation of the first C-C bond. In this quantitative multiscale perspective, we decouple the adsorption, desorption, mobility, and surface reactions of early species through a combination of vacuum and sub-vacuum studies using temporal analysis of products (TAP) reactor systems, and through studies with atmospheric fixed bed reactors. These results are supplemented with density functional theory calculations and data-driven physical models, using partial differential equations, that describe the temporal and spatial evolution of species. We consider the effects of steam, early degradation species, and product masking due to the inherent autocatalytic nature of the process, which all complicate the observation of the primary olefin(s). Although quantitative spectroscopic determination of the lifetimes, surface mobility, and reactivity of adspecies is still lacking in the literature, we observe that reaction barriers are competitive with adsorption enthalpies and/or activation energies of desorption, while facile diffusion occurs in the porous structures of the zeolite/zeotype catalysts. Understanding the various processes allows for quantitative evaluation of their competing energetics, which leads to molecular insights as to what governs the catalytic activity during the conversion of methanol to primary olefins over zeolite/zeotype catalysts.

Accurate heteronuclear distance measurements at all magic-angle spinning frequencies in solid-state NMR spectroscopy

Heteronuclear dipolar coupling is indispensable in revealing vital information related to the molecular structure and dynamics, as well as intermolecular interactions in various solid materials. Although numerous approaches have been developed to selectively reintroduce heteronuclear dipolar coupling under MAS, most of them lack universality and can only be applied to limited spin systems. Herein, we introduce a new and robust technique dubbed phase modulated rotary resonance (PMRR) for reintroducing heteronuclear dipolar couplings while suppressing all other interactions under a broad range of MAS conditions. The standard PMRR requires the radiofrequency (RF) field strength of only twice the MAS frequency, can efficiently recouple the dipolar couplings with a large scaling factor of 0.50, and is robust to experimental imperfections. Moreover, the adjustable window modification of PMRR, dubbed wPMRR, can improve its performance remarkably, making it well suited for the accurate determination of dipolar couplings in various spin systems. The robust performance of such pulse sequences has been verified theoretically and experimentally via model compounds, at different MAS frequencies. The application of the PMRR technique was demonstrated on the H-ZSM-5 zeolite, where the interaction between the Brønsted acidic hydroxyl groups of H-ZSM-5 and the absorbed trimethylphosphine oxide (TMPO) were probed, revealing the detailed configuration of super acid sites.

QM/MM study of the reactivity of zeolite bound methoxy and carbene groups

The conversion of methanol-to-hydrocarbons (MTH) is known to occur via an autocatalytic process in zeolites, where framework-bound methoxy species play a pivotal role, especially during catalyst induction. Recent NMR and FT-IR experimental studies suggest that methoxylated zeolites are able to produce hydrocarbons by a mechanism involving carbene migration and association. In order to understand these observations, we have performed QM/MM computational investigations on a range of reaction mechanisms for the reaction of zeolite bound methoxy and carbene groups, which are proposed to initiate hydrocarbon formation in the MTH process. Our simulations demonstrate that it is kinetically unfavourable for methyl species to form on the framework away from the zeolite acid site, and both kinetically and thermodynamically unfavourable for methyl groups to migrate through the framework and aggregate around an acid site. Formation of carbene moieties was considered as an alternative pathway to the formation of C-C bonds; however, the reaction energy for conversion of a methyl to a carbene is unfavourable. Metadynamics simulations help confirm further that methyl species at the framework acid sites would be more reactive towards formed C₂₊ species, rather than inter-framework migration, and that the role of carbenes in the formation of the first C-C bond will be via a concerted type of mechanism rather than stepwise.

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.

The effect of introducing Ga on the ZSM-5-catalyzed methanol to aromatics reaction: taking methylcyclopentane to benzene as an example

Naphthenes are key intermediates in the formation of aromatic compounds during the methanol to aromatics (MTA) reaction, and the dehydrogenation process is more important than the hydrogen transfer process. Theoretical studies were performed to investigate the methylcyclopentane, which represents a naphthene, to benzene MTA process catalyzed by ZSM-5 before and after introducing Ga, showing that Ga-ZSM-5 was more favorable for carrying out the reaction than two H-type ZSM-5 (H-Z₁ and H-Z₂) models. H-Z₁ and H-Z₂ are favorable for the transfer of H during ring expansion reactions and the reformation of Brønsted acids, but the dehydrogenation reactions involving H-Z₁ and H-Z₂ require high free-energy barriers to be overcome. Although introducing Ga to ZSM-5 is not conducive to the transfer of H after dehydrogenation, it can reduce the extremely high dehydrogenation free-energy barrier compared with H-Z₁ and H-Z₂; this is mainly because Ga at dehydrogenation active centers, [GaH]²⁺, can accept electrons and donate them to the H atoms of [GaH]²⁺, giving H negative charge and making it easy to combine with positive B-acid H atoms that come from methylcyclopentane, cyclohexene, and cyclohexadiene to produce H₂. Also, analysis of the transition state structures of all DH processes shows that Ga-ZSM-5 is more favorable for promoting the combination of H to produce H₂ than H-Z₁ and H-Z₂.

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

Abstract

The relevance of a selection of organic impurities for the initiation of the MTO process was quantified in a kinetic model comprising 107 elementary steps with ab initio computed reaction barriers (MP2:DFT). This model includes a representative part of the autocatalytic olefin cycle as well as a direct initiation mechanism starting from methanol through CO-mediated direct C–C bond formation. We find that the effect of different impurities on the olefin evolution varies with the type of impurity and their partial pressures. The reactivity of the considered impurities for initiating the olefin cycle increases in the order formaldehyde < di-methoxy methane < CO < methyl acetate < ethanol < ethene < propene. In our kinetic model, already extremely low quantities of impurities such as ethanol lead to faster initiation than through direct C–C bond formation which only matters in complete absence of impurities.

Graphic Abstract

Decoupling nucleation from crystal-growth for the synthesis of nanocrystalline zeolites

The increasing demands for catalysts with improved accessibility to reactants call for a rational synthesis of nanosized zeolites. Herein we developed a facile approach for highly crystalline MFI-type (Silicalite-1 and ZSM-5) and beta zeolites with variable Si/Al ratios and high yield. This was achieved by kinetically decoupling the nucleation from the growth process of the zeolites in a highly concentrated gel system via a temperature-staged treatment. The carefully controlled low-water environment ensures only nucleation in stage I (aging), and hence the generation of abundant nuclei for the subsequent rapid crystallization within a self-confined space at stage II (growth). The method, without using expensive templates or additives, allows the syntheses of nanosized, well-isolated ZSM-5 (Si/Al = 100-∞, 36-88 nm, yield >85%) and Beta (Si/Al = 25-100, 21-66 nm, yield >95%). The ultra-small crystal size endows ZSM-5 zeolites with good catalytic activity, product selectivity and remarkably longer lifetime in methanol conversion reactions.

Two-way desorption coupling to enhance the conversion of syngas into aromatics by MnO/H-ZSM-5

We herein report a composite catalyst containing partially reducible and highly active manganese oxide and nano-size H-ZSM-5 with short b-axis, prepared for the direct conversion of syngas into aromatics.

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.

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.

Identification of the Reaction Sequence of the MTO Initiation Mechanism Using Ab Initio-Based Kinetics

The initiation of the methanol-to-olefins (MTO) process is investigated using a multiscale modeling approach where more than 100 ab initio computed (MP2:DFT) rate constants for H-SSZ-13 are used in a batch reactor model. The investigated reaction network includes the mechanism for initiation (42 steps) and a representative part of the autocatalytic olefin cycle (63 steps). The simulations unravel the dominant initiation pathway for H-SSZ-13: dehydrogenation of methanol to CO is followed by CO-methylation leading to the formation of the first C-C bond in methyl acetate despite high barriers of >200 kJ/mol. Our multiscale approach is able to shed light on the reaction sequence that ultimately leads to olefin formation and strikingly demonstrates that only with a full reactor model that includes autocatalysis with olefins as cocatalysts is one able to understand the initiation mechanism on the atomic scale. Importantly, the model also shows that autocatalysis takes over long before significant amounts of olefins are formed, thus guiding the interpretation of experimental results.