Insights into the Pyridine-Modified MOR Zeolite Catalysts for DME Carbonylation

Confinement-Driven Dimethyl Ether Carbonylation in Mordenite Zeolite as an Ultramicroscopic Reactor

ConspectusThe conversion of C1 molecules to methyl acetate through the carbonylation of dimethyl ether in mordenite zeolite is an appealing reaction and a crucial step in the industrial coal-to-ethanol process. Mordenite zeolite has large 12-membered-ring (12MR) channels (7.0 × 6.5 Å2) and small 8MR channels (5.7 × 2.6 Å2) connected by a side pocket (4.8 × 3.4 Å2), and this unique pore architecture supplies its high catalytic activity to the key step of carbonylation. However, the reaction mechanism of carbonylation in mordenite zeolite is not thoroughly established in that it is able to explain all experimental phenomena and improve its industrial applications, and the classical potential energy surface exerted by static density function theory calculations cannot reflect the reaction kinetics under realistic conditions because the diffusion kinetics of bulk DME (kinetic dimeter: 4.5 Å) and methyl acetate (MA, kinetic dimeter: 5.5 Å) were not well considered and their restricted diffusion in the narrow side pocket and 8MR channels may greatly alter the integrated kinetics of DME carbonylation in mordenite zeolite. Moreover, the precise illustration of the dynamic behaviors of the ketene intermediate and its derivatives (surface acetate and acylium ion) confined within various voids in mordenite has not been effectively portrayed.Advanced ab initio molecular dynamics (AIMD) simulations with or without the acceleration of enhanced sampling methods provide tremendous opportunities for operando modeling of both reaction and diffusion processes and further identify the geometrical structure and chemical properties of the reactants, intermediates, and products in the different confined voids of mordenite under realistic reaction conditions, which enables high consistency between computations and experiments.In this Account, the carbonylation process in mordenite is comprehensively described by the results of decades of continuous research and newly acquired knowledge from both multiscale simulations and in-(ex-)situ spectroscopic experiments. Three primary steps (DME demethylation to surface methoxy species (SMS), carbon-carbon bond coupling between SMS and CO to acetyl species, and methyl acetate formation by acetyl species and methanol/DME) have been respectively studied with a careful consideration of different molecular factors (reactant distribution, concentration, and attack mode). By utilizing the free-energy surface of diffusion and reaction obtained from AIMD simulations, a comprehensive reaction/diffusion kinetic model was formulated for the first time, illustrating the entire zeolite catalytic process. In this context, a comprehensive and informative analysis of the reaction kinetics of carbonylation in mordenite, including the function of the 12MR channels, 8MR channels, and side pockets in the adsorption, diffusion, and reaction of DME carbonylation, was performed. The different channels of mordenite play different roles in all ordered reaction steps, illustrating a highly organized ultramicroscopic reactor that is encompassed.

Selective catalytic oxidation of DMF over Cu-Ce/H-MOR by modulating the surface active sites

Selective catalytic oxidation of the hazardous DMF exhaust gas presents a significant challenge in balancing oxidation activity and products selectivity (CO, NOx, N2, etc.). It is found that Cu/H-MOR demonstrates superior performance for DMF oxidation compared to CuO on other supports (γ-Al2O3, HY, ZSM-5) in terms of product selectivity and stability. The geometric and electronic structures of CuO active sites in Cu/H-MOR have been regulated by CeO2 promoter, leading to an increase in the ratio of active CuO (highly dispersed CuO and Cu+ specie). As a result, the oxidation activity and stability of the Cu/H-MOR catalyst were enhanced for DMF selective catalytic oxidation. However, excessive CuO or CeO2 content led to decreased N2 selectivity due to over-high oxidation activity. It is also revealed that Ce3+ species, active CuO species, and surface acid sites play a critical role in internal selective catalytic reduction reaction during DMF oxidation. The 10Cu-Ce/H-MOR (1/4) catalyst exhibited both high oxidation activity and internal selective catalytic reduction activity due to its abundance of active CuO specie as well as Ce3+ species and surface acid sites. Consequently, the 10Cu-Ce/H-MOR (1/4) catalyst demonstrated the widest temperature window for DMF oxidation with high N2 selectivity. These findings emphasize the importance of surface active sites modification for DMF selective catalytic oxidation.

Boron-incorporated nanosized SUZ-4 zeolite for DME carbonylation

Boron-incorporated nanosized HB-SUZ-4 showcased a noteworthy 24% boost in dimethyl ether carbonylation, with an elevation in methyl acetate selectivity from 91.8% to 96.0%. The improved performance is attributed to shortened diffusion lengths along the 8-member ring channels, decreased Brønsted acidity in the 10-member ring channels, and Lewis acid sites stabilizing CO.

H2-Promoted Benign Coke Strategy for Dimethyl Ether Carbonylation with Long-Term Stability and High Activity

Zeolite-catalyzed dimethyl ether (DME) carbonylation provides a novel route to producing methyl acetate (MeOAc). Mordenite (MOR) has drawn significant interest because of its remarkable MeOAc selectivity in DME carbonylation, albeit with limited catalytic stability. Herein, novel MOR-based DME carbonylation catalysts, distinguished by long-term stability and high activity were successfully developed, based on an H2-promoted benign coke strategy. Both the H2 cofeeds and the presence of metal species with hydrogenation capability are demonstrated to be crucial for the regulation of coke depositions. The coke deposits can potentially cover the acid sites in the 12-MR main channels, thereby mitigating the occurrence of undesirable methanol-to-hydrocarbon side reactions. Meanwhile, the elimination of ultralarge coke species under the assistance of H2 and Cu species could ensure smooth mass transfer within the catalyst, contributing to its remarkable catalytic performance. The most highlighted DME carbonylation performance was achieved on coke-mediated CuZn-HMOR with a high MeOAc yield of 0.4-0.5 g·gcat-1·h-1 for over 520 h (over 50× enhancement versus HMOR), exhibiting promising industrial application potential. The current strategy is expected to inspire further research into zeolite-catalyzed reactions, which could be potentially improved by the presence of benign coke.

Enhanced Stability of Dimethyl Ether Carbonylation through Pyrazole Tartrate on Tartaric Acid-Complexed Cobalt-Iron-Modified Hydrogen-Type Mordenite

In this study, pyrazole tartrate (Pya·DL) and tartaric acid (DL) complexed with cobalt-iron bimetallic modified hydrogen-type mordenite (HMOR) were prepared using the ion exchange method. The results demonstrate that the stability of the dimethyl ether (DME) carbonylation reaction to methyl acetate (MA) was significantly improved after the introduction of Pya·DL to HMOR. The Co∙Fe∙DL-Pya·DL-HMOR (0.8) sample exhibited sustainable stability within 400 h DME carbonylation, exhibiting a DME conversion rate of about 70% and MA selectivity of above 99%. Through modification with the DL-complexed cobalt-iron bimetal, the dispersion of cobalt-iron was greatly enhanced, leading to the formation of new metal Lewis acidic sites (LAS) and thus a significant improvement in catalysis activity. Pya·DL effectively eliminated non-framework aluminum in HMOR, enlarged its pore size, and created channels for carbon deposition diffusion, thereby preventing carbon accumulation and pore blockage. Additionally, Pya·DL shielded the Bronsted acid sites (BAS) in the 12 MR channel, effectively suppressing the side reactions of carbon deposition and reducing the formation of hard carbon deposits. These improvements collectively contribute to the enhanced stability of the DME carbonylation reaction.

Improved catalytic performance in gas-phase dimethyl ether carbonylation over facile NH4F etched ferrierite

Gas-phase dimethyl ether (DME) carbonylation to methyl acetate (MA) initiates a promising route for producing ethanol from syngas. Ferrierite (FER, ZSM-35) has received considerable attention as it displays excellent stability in the carbonylation reaction and its modification strategy is to improve its catalytic activity on the premise of maintaining its stability as much as possible. However, conventional post-treatment methods such as dealumination and desilication usually selectively remove framework Al or Si atoms, ultimately altering the intrinsic composition, crystallinity, and acidity of zeolites inevitably. In this study, we successfully prepared a series of hierarchical ZSM-35 materials through post-treatment with NH4F etching, which dissolved framework Al and Si at similar rates and preferentially attacked the defective sites. Interestingly, the produced pore systems effectively penetrated the [100] plane, offering elevated access to both the 8-membered ring (8-MR) and 10-membered ring (10-MR) channels. The physicochemical and acid properties of the pristine and NH4F etched ZSM-35 samples were comprehensively characterized using various techniques, including XRD, XRF, FESEM, HRTEM, Nitrogen adsorption-desorption, NH3-TPD, Py-IR, 27Al MAS NMR, and 29Si MAS NMR. Under moderate treatment conditions, the intrinsic microporous structure, acid properties, and crystallinity of zeolite were retained, leading to superior catalytic activity and stability with respect to the pristine sample. Nonetheless, severe NH4F etching disrupted the crystalline framework and created additional defective sites, bringing about faster deposition of coke precursors on the interior Brønsted acid sites (BAS) and decreased catalytic performance. This technique provides a novel and efficient method to slightly enhance the micropore and mesopore volume of industrially pertinent zeolites through a straightforward post-treatment, thus elevating the catalytic performance of these zeolites.

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.

Shape selectivity of zeolite for hydroisomerization of long-chain alkanes

The matching degree between the zeolite channel size and the isomer size determines the product distribution of dodecane hydroisomerization.

Charge-separation driven mechanism via acylium ion intermediate migration during catalytic carbonylation in mordenite zeolite

By employing ab initio molecular dynamic simulations, solid-state NMR spectroscopy, and two-dimensional correlation analysis of rapid scan Fourier transform infrared spectroscopy data, a new pathway is proposed for the formation of methyl acetate (MA) via the acylium ion (i.e.,CH3 − C ≡ O+) in 12-membered ring (MR) channel of mordenite by an integrated reaction/diffusion kinetics model, and this route is kinetically and thermodynamically more favorable than the traditional viewpoint in 8MR channel. From perspective of the complete catalytic cycle, the separation of these two reaction zones, i.e., the C-C bond coupling in 8MR channel and MA formation in 12MR channel, effectively avoids aggregation of highly active acetyl species or ketene, thereby reducing undesired carbon deposit production. The synergistic effect of different channels appears to account for the high carbonylation activity in mordenite that has thus far not been fully explained, and this paradigm may rationalize the observed catalytic activity of other reactions.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Self-Assembled Nano-Filamentous Zeolite Catalyst to Realize Efficient One-Step Ethanol Synthesis

The non-petroleum synthesis route of ethanol from syngas (H₂ +CO) with methyl acetate (MA) as the core intermediate product has been confirmed as an excellent industrialization route for high purity ethanol production. However, as the central part of this tandem-catalysis path, the carbonylation of dimethyl ether (DME) to MA is limited by the undesirable catalytic activity and stability of zeolite catalysts. Herein, a facile inhibitor-assisted strategy was developed for constructing self-assembled nano-Mordenite (nano-MOR) zeolites without using any expensive or complex template. A nano-filamentous MOR zeolite with only 70 nm crystal diameter was successfully synthesized by selectively controlling the crystal growth orientation with a specific inhibitor. The catalytic performance of self-assembled nano-MOR catalysts was remarkably outstanding in DME carbonylation reaction. The highest Space-Time Yield (STY) of MA was achieved over Nanofilament MOR (NF-MOR), which was significantly improved comparing with that of the traditional Ellipsoid-MOR (ES-MOR) [3780 mmol/(kg ⋅ h) vs. 1368 mmol/(kg ⋅ h)]. One-step ethanol synthesis was realized by combining the MOR catalyst and an innovative self-reduced Cu-ZnO/SiO₂ (CZ/SiO₂ ) catalyst in a rationally designed dual-bed catalysis system. Adopting the tailor-made NF-MOR&CZ/SiO₂ combination, it obtained the highest STY of ethanol, about 4 times of the conventional ES-MOR&CZ combination [1800 mmol/(kg ⋅ h) vs. 476 mmol/(kg ⋅ h)]. The present self-assembled nano-MOR zeolites synthetic strategy opens a new way for the fabrication of high-performance zeolites for practical industrial applications in catalytic conversions of one-carbon (C1) small molecules to high value-added chemicals.

Recent advances in the routes and catalysts for ethanol synthesis from syngas

Ethanol, as one of the important bulk chemicals, is widely used in modern society. It can be produced by fermentation of sugar, petroleum refining, or conversion of syngas (CO/H₂). Among these approaches, conversion of syngas to ethanol (STE) is the most environmentally friendly and economical process. Although considerable progress has been made in STE conversion, control of CO activation and C-C growth remains a great challenge. This review highlights recent advances in the routes and catalysts employed in STE technology. The catalyst designs and pathway designs are summarized and analysed for the direct and indirect STE routes, respectively. In the direct STE routes (i.e., one-step synthesis of ethanol from syngas), modified catalysts of methanol synthesis, modified catalysts of Fischer-Tropsch synthesis, Mo-based catalysts, noble metal catalysts and multifunctional catalysts are systematically reviewed based on their catalyst designs. Further, in the indirect STE routes (i.e., multi-step processes for ethanol synthesis from syngas via methanol/dimethyl ether as intermediates), carbonylation of methanol/dimethyl ether followed by hydrogenation, and coupling of methanol with CO to form dimethyl oxalate followed by hydrogenation, are outlined according to their pathway designs. The goal of this review is to provide a comprehensive perspective on STE technology and inspire the invention of new catalysts and pathway designs in the near future.

Quantitatively Mapping the Distribution of Intrinsic Acid Sites in Mordenite Zeolite by High-Field 23Na Solid-State Nuclear Magnetic Resonance

It is of great significance to accurately quantify the Brønsted acid sites (BASs) at different positions of mordenite (MOR) zeolite. However, H-MOR obtained from Na-MOR can hardly avoid dealumination under hydrothermal conditions, which causes difficulty in the acid characterization. Herein, 23Na-27Al D-HMQC was performed combined with high-field 23Na MQ MAS NMR and DFT calculation, which provided an unambiguous attribution of the 23Na chemical shifts and further helped to improve the resolution of 27Al MAS NMR. By fitting the 23Na and 1H MAS NMR spectra of Na/H-MOR, the intrinsic BAS contents in different T-sites were measured by characterizing the location and content of sodium ions. These Na/H-MOR zeolites with various acid distributions were used for DME carbonylation and showed that the amount of BASs in the T3 site was proportional to the activity of carbonylation. This study provides a new method for investigating the intrinsic acid properties of zeolites.

Assessment of total concentrations of heavy metals in industrial sludges from the North of Vietnam and their potential impact on the ecosystem

Industrial sludges from wastewater treatment plants of industrial parks and a drinking water treatment plant in northern Vietnam were investigated in this study. The total concentrations of heavy metals (As, Cd, Cu, Cr, Ni, Hg, Pb, Zn) and other elements (Mn, Pd, Sb, V) in the sludges were measured using the ICP-MS method. In addition, the surface characteristics of the samples were analyzed using SEM-EDS and FTIR techniques. According to Vietnam's current waste management regulation, the investigated industrial sludges belonged to the hazardous waste category (with Pb concentration > 300 µg/g). In contrast, the sludge from the drinking water treatment plant had a low content of heavy metals and toxic elements. The sequential extraction method revealed that the heavy metals in the industrial sludges exhibited higher mobilization forms (exchangeable and reduceable fractions) than those in the drinking water sludges. The mobilization ability of heavy metals is probably related to the surface function groups of the sludges, which were dominated by (-COOH) and (-OH) groups. The potential ecological risk assessment calculations indicated that the industrial sludges had high potential risk (with the RI values ranging from 229.7 to 605.4), mainly due to the content of Cd in the sludge samples. Further studies about the fate and transport of Cd and other toxic metals in the sludges are highly recommended to better understand their risk to the surrounding environment, such as groundwater and agricultural soil.

Increasing the Number of Aluminum Atoms in T3 Sites of a Mordenite Zeolite by Low-Pressure SiCl4 Treatment to Catalyze Dimethyl Ether Carbonylation

Controlling the location of aluminum atoms in a zeolite framework is critical for understanding structure-performance relationships of catalytic reaction systems and tailoring catalyst design. Herein, we report a strategy to preferentially relocate mordenite (MOR) framework Al atoms into the desired T₃ sites by low-pressure SiCl₄ treatment (LPST). High-field ²⁷ Al NMR was used to identify the exact location of framework Al for the MOR samples. The results indicate that 73 % of the framework Al atoms were at the T₃ sites after LPST under optimal conditions, which leads to controllably generating and intensifying active sites in MOR zeolite for the dimethyl ether (DME) carbonylation reaction with higher methyl acetate (MA) selectivity and much longer lifetime (25 times). Further research reveals that the Al relocation mechanism involves simultaneous extraction, migration, and reinsertion of Al atoms from and into the parent MOR framework. This unique method is potentially applicable to other zeolites to control Al location.

Selective oxidation conversion of methanol/dimethyl ether

As important platform compounds, methanol and dimethyl ether (DME) are vital bridges between the coal chemical, petrochemical and fine chemical industries. At present, the synthesis of methanol/DME has been industrialized, and the production capacity is much larger than the market demand. Therefore, the conversion of methanol/DME into more valuable chemicals is an important and significant topic. The synthesis of high value-added oxygenated chemicals and diesel oil additives from methanol/DME by an oxidation method has attracted substantial attention due to it being green and environmentally friendly and having good atom economy. In this feature article, we have summarized the recent advances in the synthesis of formaldehyde, methyl formate, dimethoxymethane, and polyoxymethylene dimethyl ethers, from the selective oxidation of methanol/DME, and further discussed the adsorption and activation of reactant molecules, selective cleavage of C-O, C-H or O-H bonds in methanol/DME molecules and the C-O chain growth in the target products. In the end, major challenges and future prospects are proposed from the viewpoint of catalyst design and application. It is expected that this feature article will provide theoretical guidance for the activation and cleavage of C-O, C-H, or O-H bonds in other small molecules of alcohol/ether as well as low-carbon alkanes, so as to synthesize high value-added chemicals.

Insight into Crystallization Features of MOR Zeolite Synthesized via Ice-Templating Method

Hydrothermal, solvothermal or ionothermal routes are usually employed for the synthesis of zeolite, which is often accompanied by a high energy consumption, high cost and low efficiency. We have developed a novel route for the rapid and high yield synthesis of mordenite (MOR) zeolite via an ice-templating method. In comparison with traditional hydrothermal synthesis, not only the high yield, but also the superior crystallinity, large reduction in water level and reaction pressure, simple device and conventional silica sources by this route can have great potential for the commercial production of pure MOR zeolite. Moreover, the changed bonding environment of silicon atoms in MOR zeolite, that is, a relative decrease in the tetrahedrally coordinated Si–O–Si bond, and accordingly, an increase in the T–OH (T = Si, Al) groups and Si–O–Al sites, remarkably enhances its acid strength.

Ammonia pools in zeolites for direct fabrication of catalytic centers

Reduction process is a key step to fabricate metal-zeolite catalysts in catalytic synthesis. However, because of the strong interaction force, metal oxides in zeolites are very difficult to be reduced. Existing reduction technologies are always energy-intensive, and inevitably cause the agglomeration of metallic particles in metal-zeolite catalysts or destroy zeolite structure in severe cases. Herein, we disclose that zeolites after ion exchange of ammonium have an interesting and unexpected self-reducing feature. It can accurately control the reduction of metal-zeolite catalysts, via in situ ammonia production from 'ammonia pools', meanwhile, restrains the growth of the size of metals. Such new and reliable ammonia pool effect is not influenced by topological structures of zeolites, and works well on reducible metals. The ammonia pool effect is ultimately attributed to an atmosphere-confined self-regulation mechanism. This methodology will significantly promote the fabrication for metal-zeolite catalysts, and further facilitate design and development of low-cost and high-activity catalysts.

A Carbonylation Zeolite with Specific Nanosheet Structure for Efficient Catalysis

Up to now, the member of zeolite family has expanded to more than 230. However, only little part of them have been reported as catalysts used in reactions. Discovering potential zeolites for reactions is significantly important, especially in industrial applications. A carbonylation zeolite catalyst Al-RUB-41 has special morphology and channel orientation. The 8-MR channel of Al-RUB-41 is just perpendicular to its thin sheet, making a very short mass-transfer distance along 8-MR. This specific nature endows Al-RUB-41 with efficient catalytic ability to dimethyl ether carbonylation reaction with beyond 95% methyl acetate selectivity. Compared with the most widely accepted carbonylation zeolite catalysts, Al-RUB-41 behaves a much better catalytic stability than H-MOR and a greatly enhanced catalytic activity than H-ZSM-35. A space-confined deactivation mechanism over Al-RUB-41 is proposed. By erasing the acid sites on outer surface, Al-RUB-41@SiO₂ catalyst achieves a long-time and high-efficiency activity without any deactivation trend.

Silver-Modified Nano Mordenite for Carbonylation of Dimethyl Ether

Mordenite (H-MOR) catalysts were synthesized by a hydrothermal method, and silver-modified mordenite (Ag-MOR) catalysts were prepared by ion exchange with AgNO3 at different concentrations. The performance of these catalysts in the carbonylation of dimethyl ether (DME) to methyl acetate (MA) was also evaluated. The catalysts were characterized by Ar adsorption/desorption, XRD, ICP-AES, SEM, HRTEM, 27Al NMR, H2-TPR, NH3-TPD, Py-IR, and CO-TPD. According to the characterization results, Ag ion exchange sites were mainly located in the 8-membered ring (8-MR) channels of Ag-MOR; evenly dispersed Ag2O particles were also present. The acid site distribution was changed by the modification of Ag, and the amount of Brønsted acid sites increased in 8-MR and decreased in 12-MR. The CO adsorption performance of the catalyst significantly increased with the modification of Ag. These changes improved the conversion and selectivity of the carbonylation of DME. Over 4Ag-MOR in particular, DME conversion and MA selectivity reached 94% and 100%, respectively.

Regulating the location of framework aluminium in mordenite for the carbonylation of dimethyl ether

The relationship between the specific location of framework Al atoms and the reactivity for DME carbonylation was established over mordenite.