Methanol To Hydrocarbons Research Articles

Evaluating pore characteristics and acid site locations in hierarchical SAPO-11 by catalytic model reactions

Hierarchical SAPO-11 molecular sieves were synthesized with three different mesopore structure directing agents (meso-SDAs): cetyltrimethylammonium bromide (CTAB), polyvinyl alcohol (PVA) and [3-(trimethoxysilyl) propyl] dimethyloctadecylammonium chloride (TPOAC). Two model reactions, methanol-to-hydrocarbons (MTH) and the Beckmann rearrangement (BMR) of cyclohexanone oxime, were employed to evaluate the pore topology and acid site locations of the hydrothermally synthesized hierarchical SAPO-11s. Initially, the modified porosity of the hierarchical SAPO-11s was thoroughly probed by employing a set of general characterization methods and by comparing the results to the conventional microporous C-SAPO-11. The nitrogen physisorption results revealed that CTAB-11 had a uniform distribution of mesopores centered at 2.8 nm, whereas the presence of mesopores in PVA-11 could not be convincingly resolved through conventional methods. Instead, the pore topology of PVA-11 was determined by utilizing model reactions, where the shape selective MTH model reaction revealed that the sample had mesopores present through an increased production of large products compared to the conventional C-SAPO-11. Additionally, the MTH model reaction showed that while PVA shifted the location of the Brønsted acid sites (BAS) towards the mesopores, CTAB did not affect the BAS location of SAPO-11. Finally, the BMR model reaction elucidated the excellent intrapore connectivity of the hierarchical SAPO-11s through an increased lifetime compared to the conventional C-SAPO-11. • Hierarchical SAPO-11 (H-S11) with interconnected micro- and mesopores was synthesized with CTAB and PVA as meso-SDAs. • Model reactions are crucial for understanding the topological differences between microporous SAPO-11 (µ-S11) and H-S11. • CTAB significantly alters the textural properties of the resulting H-S11, while PVA shows similar properties to µ-S11. • PVA directs formation of Brønsted acid sites to mesopores, whereas CTAB forms silanols in mesopores. • Superior lifetime for the isomerization of cyclohexanone oxime revealed the intraconnected pore topology for H-S11.

Fundamentals of the catalytic conversion of methanol to hydrocarbons

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.

Methanol to hydrocarbons conversion: Why dienes and monoenes contribute differently to catalyst deactivation?

Dienes, formed at much lower concentrations than monoenes via alkylation of monoenes with formaldehyde (HCHO), have much higher propensity than monoenes to foment catalyst deactivation in methanol-to-hydrocarbons (MTH) conversion over zeolites via HCHO-mediated deactivation pathways. The mechanistic basis for why monoenes and dienes contribute differently to cause catalyst deactivation during MTH is investigated using independent kinetic studies of HCHO reactions with representative monoenes and dienes, i.e., propylene and 1,3-butadiene, over zeolites. Despite 3,6-dihydro-2H-pyran being the predominant product for both propylene/HCHO and 1,3-butadiene/HCHO reactions, a combination of transient and steady-state rate measurements and stoichiometric experiments reveals distinct persistent intermediates being involved in the addition of HCHO to different surface-bound alkyl species derived from propylene and butadiene. These persistent intermediates desorb only via HCHO- and water-mediated proton transfer steps and as such can be sequestered on the catalyst. A nonlinear mixed-effects kinetic model reveals that the intermediate formed during HCHO-diene reactions has a higher steady-state surface coverage and its desorption via HCHO- and water-mediated routes exhibit > 8× and > 3×, respectively, higher rate constants than those during HCHO-monoene reactions. These results evidence the distinct identity, surface coverage, and reactivities of surface-bound alkyl intermediates as critical factors in the much more pernicious effects of dienes compared to monoenes on catalyst lifetime during MTH.

Synthesis of Mesoporous Zeolites and Their Opportunities in Heterogeneous Catalysis

Currently, zeolites are one of the most important classes of heterogeneous catalysts in chemical industries owing to their unique structural characteristics such as molecular-scale size/shape-selectivity, heterogenized single catalytic sites in the framework, and excellent stability in harsh industrial processes. However, the microporous structure of conventional zeolite materials limits their applications to small-molecule reactions. To alleviate this problem, mesoporous zeolitic frameworks were developed. In the last few decades, several methods have been developed for the synthesis of mesoporous zeolites; these zeolites have demonstrated greater lifetime and better performance than their bulk microporous counterparts in many catalytic processes, which can be explained by the rapid diffusion of reactant species into the zeolite framework and facile accessibility to bulky molecules through the mesopores. Mesoporous zeolites provide versatile opportunities not only in conventional chemical industries but also in emerging catalysis fields. This review presents many state-of-the-art mesoporous zeolites, discusses various strategies for their synthesis, and details their contributions to catalytic reactions including catalytic cracking, isomerization, alkylation and acylation, alternative fuel synthesis via methanol-to-hydrocarbon (MTH) and Fischer–Tropsch synthesis (FTS) routes, and different fine-chemical syntheses.

Catalyst Deactivation Probed by Positron Annihilation Spectroscopy

There is a widespread need to understand and improve the aging characteristics and properties of catalysts that support essential chemical reactions, including methanol-to-hydrocarbons (MTH) conver...

Core-shell and egg-shell zeolite catalysts for enhanced hydrocarbon processing

Developing structure-performance relationships with the underlying goal of optimizing known zeolite catalysts involves the manipulation of their physicochemical properties. Here, we systematically assessed the impact of mesoscopic gradients in acid site concentration, which has generally received little attention in the design of zeolite catalysts for hydrocarbon upgrading. A series of core–shell MEL-type zeolites were synthesized with catalytically active ZSM-11 cores and passivated silicalite-2 shells of varying thickness. Our findings revealed that ZSM-11@silicalite-2 particles with ultrathin shells (<10 nm) have enhanced mass transport, characteristic of relatively smaller particles, compared to the corresponding ZSM-11 core. Catalytic testing using the methanol-to-hydrocarbon (MTH) reaction showed that core–shell zeolites exhibit longer lifetimes, higher total turnovers, and an unexpected promotion of the aromatic cycle in the hydrocarbon pool mechanism. Time-resolved acid titration of core and core–shell catalysts confirmed that the siliceous shell introduces a hydrophobic exterior that impacts molecular diffusion. In comparison, prepared MFI core-shells (ZSM-5@silicalite-1) showed similar enhancement in catalyst performance. Moreover, we prepared egg-shell configurations of each zeolite, silicalite-2@ZSM-11 and silicalite-1@ZSM-5, comprised of an inert core and catalytically active shell. This inverse design of the egg-shell created pseudo nanosheets with total turnovers that were markedly higher than their homogeneous counterparts. Collectively, this study demonstrated that mesoscopic gradients in acid concentration via the design of core–shell and egg-shell zeolites significantly improve catalyst performance over conventional analogues for hydrocarbon upgrading.

How Many Molecules Can Fit in a Zeolite Pore? Implications for the Hydrocarbon Pool Mechanism of the Methanol-to-Hydrocarbons Process

The methanol-to-hydrocarbons (MTH) process is a very advantageous way to upgrade methanol to more valuable commodity chemicals such as light alkenes and gasoline. There is general agreement that, at steady state, the process operates via a dual cycle “hydrocarbon pool” mechanism. This mechanism defines a minimum number of reactants, intermediates, and products that must be present for the reaction to occur. In this paper, we calculate (by three independent methods) the volume required for a range of compounds that must be present in a working catalyst. These are compared to the available volume in a range of zeolites that have been used, or tested, for MTH. We show that this straightforward comparison provides a means to rationalize the product slate and the deactivation pathways in zeotype materials used for the MTH reaction.

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 first carbon-carbon bond formation mechanism in methanol-to-hydrocarbons process over chabazite zeolite

<h2>Summary</h2> The first C–C bond formation mechanism in the methanol-to-hydrocarbons (MTH) process has always been a highly controversial issue. Using solid-state NMR spectroscopy, the initial C1 reactants and C1 intermediates were observed on the HSSZ-13 zeolite surface. In particular, for the first time, the highly reactive ethene precursor, surface ethoxy species, was directly captured under a real MTH reaction. These findings and <i>in situ</i> captured activated C1 reactants linked C1 species and initially generated ethene, in which C1 intermediates activate methanol/DME to form the first C–C bond. For such a difficult-to-detect and extremely initial reaction process, the advanced <i>ab initio</i> molecular dynamics technology simulated the complete process from C1 reactants to product. The visualized reaction process vividly shows methanol/DME in the gas phase approaching SMS/TMO on zeolite, being activated via effective collision and methylation to generate an ethene precursor. The zeolite catalyst and active C1 intermediates contribute together to the first C–C bond formation in a synergistic manner.

Dynamic Activation of C1 Molecules Evoked by Zeolite Catalysis.

Direct observation of the activation and transformation of reactant molecules is extremely attractive but very challenging in the study of most chemical processes. Here is reported the first case of dynamic activation of C1 molecules in zeolite-catalyzed chemistry. During the methanol conversion over the HZSM-5 zeolite, a sequence of progressive activation states of dimethyl ether (DME) evoked by the special catalysis from CH3-Zeo, a hybrid supramolecular catalytic system formed by the organic methylic species growing on the inorganic silico-aluminate zeolite framework, has been directly observed by in situ ssNMR spectroscopy at programmed temperatures. Operando simulations visually display the variability of this hybrid supramolecular system of which the C–O bond property goes through a dynamic transition from covalent to ionic with the temperature increase, and thus the gradually enhanced electrophilicity of CH3δ+ and nucleophilicity of Zeoδ− lead to the dynamic activation of DME. This dynamic transition is generally reflected in the alkyl-Zeo system with other alkoxy groups, which linked the alkoxy species and carbocations in zeolite catalysis. Consequently, this work not only sheds light on the key issue of the first carbon–carbon (C–C) bond formation in the methanol to hydrocarbons (MTH) process but also brings a new awareness on the essence of acid catalysis in zeolite mediated chemical processes.

Kinetic Evaluation of Deactivation Pathways in Methanol-to-Hydrocarbon Catalysis on HZSM-5 with Formaldehyde, Olefinic, Dieneic, and Aromatic Co-Feeds

Formaldehyde (HCHO), formed in situ by transfer dehydrogenation of methanol in methanol-to-hydrocarbon (MTH) conversion, reacts with other organic species including olefins, dienes, and aromatics t...

Unravelling the effect of impurities on the methanol-to-olefins process in waste-derived zeolites ZSM-5

One of the main challenges in the next decades for the chemical industry is the valorization of low-grade industrial waste streams into valuable materials, as their increasing amount can have a major environmental impact. For example, in copper mining industry, waste flotation tailings can be valorized into solid acid catalysts, such as zeolites. Numerous studies have shown that modification of zeolites by metal ions has a positive effect in their catalytic performance. However, the presence of impurities in waste-derived materials is usually disregarded. In this work, the influence of impurities in waste-derived zeolites ZSM-5 on the physicochemical properties and the catalytic performance in methanol-to-hydrocarbons (MTH) reaction was investigated. Flotation tailings from copper industry, mainly consist of SiO2 as quartz, next to Ca, Mg and Fe species. By altering the pH in the SiO2 precipitation step, the impurities content in the waste-derived zeolites ZSM-5 could be controlled. Chemical analysis confirmed the presence of Fe, Ca and Mg in low concentrations in the waste-derived samples. UV–Vis Diffuse reflectance spectroscopy (DRS) and electron paramagnetic resonance (EPR) showed the presence of different Fe species in the waste-derived samples in the form of isolated Fe3+, oligomeric FexOy species and some extended Fe2O3-like clusters. Temperature programmed desorption (TPD) of NH3 and infrared (IR) spectroscopy measurements after CO and pyridine adsorption were used in order to probe the effect of the cations present in the acidity of the waste-derived materials. It was found that the zeolite material with a higher amount of metal impurities contained less Brønsted acid sites and more Lewis acid sites. Catalytic testing showed that the metal impurities had a positive influence on the overall catalyst stability and the yields towards ethylene and propylene were respectively 8% and 6% up. Meanwhile, operando UV–Vis DRS showed the influence of impurities on the catalytic mechanism, as its increase led to the formation of less hydrocarbon pool species. This study not only show that waste can be transformed into value-added materials, namely a zeolite-based catalyst material, but also that the impurities present in the waste can further improve the catalytic performance of the solid catalyst.

Platelike MFI Crystals with Controlled Crystal Faces Aspect Ratio.

Zeolite crystals offering a short diffusion pathway through the pore network are highly desired for a number of catalytic and molecule separation applications. Herein, we develop a simple synthetic strategy toward reducing the thickness along the b-axis of MFI-type crystals, thus providing a short diffusion path along the straight channel. Our approach combines preliminary aging and a fluoride-assisted low-temperature crystallization. The synthesized MFI crystals are in the micrometer-size range along the a- and c-axis, while the thickness along the b-axis is a few tens of nanometers. The synthesis parameters controlling the formation of platelike zeolite are studied, and the factors controlling the zeolite growth are identified. The synthesis strategy works equally well with all-silica MFI (silicalite-1) and its Al- and Ga-containing derivatives. The catalytic activity of platelike ZSM-5 in the methanol-to-hydrocarbons (MTH) reaction is compared with a commercial nanosized ZSM-5 sample, as the platelike ZSM-5 exhibits a substantially extended lifetime. The synthesis of platelike MFI crystals is successfully scaled up to a kilogram scale.

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.

Two-in-One Catalyst Turns Carbon Dioxide in Base Chemicals

Cascading chemical reactions can be realized when the local structure and composition of two catalytically active phases are fine-tuned and combined in one material. In this issue of Chem, Fan et al. report an innovative approach for the conversion of carbon dioxide into light olefins with a metal oxide-zeolite tandem catalyst.

Effect of Gadolinium Introduced HZSM-5 Zeolite on the Products Distribution of MTH Reaction

A series of Gd/HZSM-5 catalysts were synthesized by incipient-wetness impregnation method and their surface properties and catalytic performances in methanol to hydrocarbons (MTH) reaction were studied in detail. The addition of gadolinium has little effect on the structure of zeolite and the specific surface area of catalyst. However, with the increase of Gd content, the strong acid density of catalysts shows a monotonously increasing trend and the Gd/HZSM-5 catalysts exhibit high C2–C5 aliphatic hydrocarbon selectivity in MTH reaction which can reach 71.6% over the catalysts with the gadolinium content of 7%. In addition, the Gd/HZSM-5 catalysts exhibit much lower carbon deposition rates than the parent HZSM-5 catalyst in MTH reaction due to the low yield of C9+ aromatics. Thus, the introduction of Gd into the HZSM-5 catalysts can significantly affect the amount of acid on the surface of catalyst, the distribution of reaction products and anticoking ability in MTH reaction.

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

Detailed insight into molecular diffusion in zeolite frameworks is crucial for the analysis of the factors governing their catalytic performance in methanol-to-hydrocarbons (MTH) reactions. In this work, we present a molecular dynamics study of the diffusion of methanol in all-silica and acidic zeolite MFI and Beta frameworks over the range of temperatures 373–473 K. Owing to the difference in pore dimensions, methanol diffusion is more hindered in H-MFI, with diffusion coefficients that do not exceed 10 × 10−10 m2s−1. In comparison, H-Beta shows diffusivities that are one to two orders of magnitude larger. Consequently, the activation energy of translational diffusion can reach 16 kJ·mol−1 in H-MFI, depending on the molecular loading, against a value for H-Beta that remains between 6 and 8 kJ·mol−1. The analysis of the radial distribution functions and the residence time at the Brønsted acid sites shows a greater probability for methylation of the framework in the MFI structure compared to zeolite Beta, with the latter displaying a higher prevalence for methanol clustering. These results contribute to the understanding of the differences in catalytic performance of zeolites with varying micropore dimensions in MTH reactions.

Controllable synthesis of ultra-tiny nano-ZSM-5 catalyst based on the control of crystal growth for methanol to hydrocarbon reaction

A series of aggregated nano-ZSM-5 with controlled crystal size (<100 nm) were prepared via low temperature hydrothermal synthesis and utilized for the study of particle size effect of ZSM-5 on methanol to hydrocarbon (MTH) reaction for the first time. The textural and acidic properties of ZSM-5 at different crystal growth process were characterized and analyzed by N2-sorption, XRD, NH3-TPD, TEM, SEM, TG, Py-IR and XPS. Two stages of crystal growth were found during the synthesis which could be used to guide the controlled synthesis of ultra-tiny nano-ZSM-5. According to catalytic performance, it was found that compared to external acid sites, internal acid sites play a crucial role on the synthesis of gasoline products. A positive relationship was found between the acidity and liquid hydrocarbon yield. In nanometer scale (<100 nm) the crystal size and acidity of ZSM-5 still strongly influenced the catalytic performance and a balance existed between the crystal size control and acidity regulation for the catalytic stability. This work could attract more attention on the structure fine control of nanoscale ZSM-5 catalyst.

Elucidating Zeolite Channel Geometry–Reaction Intermediate Relationships for the Methanol‐to‐Hydrocarbon Process

AbstractThe chemical industry has exploited zeolite shape selectivity for more than 50 years, yet our fundamental understanding remains incomplete. Herein, the zeolite channel geometry–reactive intermediate relationships are studied in detail using anisotropic zeolite ZSM‐5 crystals for the methanol‐to‐hydrocarbon (MTH) process, and advanced magic‐angle spinning solid‐state NMR (ssNMR) spectroscopy. The utilization of anisotropic ZSM‐5 crystals enabled the preferential formation of reaction intermediates in single‐orientation zeolite channels, as revealed by molecular dynamics simulations and in situ UV/Vis diffuse‐reflectance spectroscopy. The ssNMR results show that the slightly more constrained sinusoidal zeolite channels favor the olefin cycle by promoting the homologation of alkanes, whereas the more extended straight zeolite channels facilitate the aromatic cycle with a higher degree of alkylation of aromatics. Dynamic nuclear polarization experiments further indicate the preferential formation of heavy aromatics at the zeolite surface dominated by the sinusoidal channels, providing further insight into catalyst deactivation.

Elucidating Zeolite Channel Geometry-Reaction Intermediate Relationships for the Methanol-to-Hydrocarbon Process.

The chemical industry has exploited zeolite shape selectivity for more than 50 years, yet our fundamental understanding remains incomplete. Herein, the zeolite channel geometry–reactive intermediate relationships are studied in detail using anisotropic zeolite ZSM‐5 crystals for the methanol‐to‐hydrocarbon (MTH) process, and advanced magic‐angle spinning solid‐state NMR (ssNMR) spectroscopy. The utilization of anisotropic ZSM‐5 crystals enabled the preferential formation of reaction intermediates in single‐orientation zeolite channels, as revealed by molecular dynamics simulations and in situ UV/Vis diffuse‐reflectance spectroscopy. The ssNMR results show that the slightly more constrained sinusoidal zeolite channels favor the olefin cycle by promoting the homologation of alkanes, whereas the more extended straight zeolite channels facilitate the aromatic cycle with a higher degree of alkylation of aromatics. Dynamic nuclear polarization experiments further indicate the preferential formation of heavy aromatics at the zeolite surface dominated by the sinusoidal channels, providing further insight into catalyst deactivation.