Molecular transport in zeolite catalysts: depicting an integrated picture from macroscopic to microscopic scales.

Increasing social sustainability triggers the persistent progress of industrial catalysis in energy transformation and chemical production. Zeolites have been demonstrated to be pivotal catalysts in chemical industries due to their moderate acidity and versatile well-defined pore structures. However, in the context of enhancing the performances of zeolite catalysts, the perspectives on the diffusion regulations within the pores and channels in the bulk phases or external surfaces of the zeolites are often overlooked. Establishing the structure-transport-reactivity relationships in heterogeneous catalysis can provide rational guidelines to design high-performance catalysts. Herein, this tutorial review attempts to systematically depict an integrated picture of molecular transport behaviors in zeolite catalysts from macroscopic to microscopic perspectives. The advances in the accurate diffusion measurements employing both macroscopic and microscopic techniques are briefly introduced. The diffusion characteristics in zeolite catalysts under working conditions (e.g., high temperature, multi-components, and reaction coupling) are then addressed. The macroscopic internal diffusion and the microscopic diffusion occurring in the micro-zones of zeolite crystals (e.g., surface diffusion, diffusion anisotropy, and confined diffusion) are reviewed and discussed in more detail. These diffusion behaviors highly impact the underlying reaction mechanism, catalytic performances, and catalyst optimization strategies. Finally, the multi-type pore systems of practical zeolite catalysts in industrial reactors and their transport behaviors are analyzed. The fully-crystalline monolithic zeolites in the absence of binders are highlighted as rising-star catalytic materials for industrial applications. The research challenges in this field and the potential future development directions are summarized.

Insights into the Effect of the Adsorption Preference of Additives on the Anisotropic Growth of ZSM-5 Zeolite.

Invited for the cover of this issue are Zaiku Xie, Jiawei Teng, Chuanming Wang, and co-workers at the SINOPEC Shanghai Research Institute of Petrochemical Technology, Liaoning Petrochemical University and University of Science and Technology of China. The image depicts the effect of eight organic additives on the anisotropic growth of classical ZSM-5 zeolite at the molecular level. Read the full text of the article at 10.1002/chem.202201781.

Insights into the Effect of the Adsorption Preference of Additives on the Anisotropic Growth of ZSM-5 Zeolite.

Zeolite morphology plays a crucial role in affecting catalytic performance, while is persistently challenging to tailor through crystal anisotropic growth. It has been recognized that specific additives can be introduced into the synthesis of zeolites to achieve anisotropic growth, however their role and the underlying mechanism are not well understood. Herein, the effect of eight specific additives on the anisotropic growth of the ZSM-5 zeolite is unveiled within the framework of crystallization engineering. Either an inhibition effect or a promotion effect is revealed for each additive according to the crystallization kinetics. The adsorption preference of typical additives on different surfaces was demonstrated by total internal reflection fluorescence microscopy (TIRFM) and transmission X-ray microscopy (TXM) together with 3D reconstruction. The calculated adsorption energy difference between MFI [100]/[101] and [010] surfaces was proposed as a key descriptor to estimate the possible morphology induced by additive. ZSM-5 zeolites varying from sphere-like, plate-like to noodle-like morphology could be synthesized by employing specific additives with increasing adsorption strength difference on distinct surfaces.

Hierarchically Structured Zeolites: From Design to Application.

Hierarchical zeolites combine the intrinsic catalytic properties of microporous zeolites and the enhanced access and transport of the additional meso- and/or macroporous system. These materials are the most desirable catalysts and sorbents for industry and become a highly evolving field of important current interests. In addition to the enhanced mass transfer leading to high activity, selectivity, and cycle time, another essential merit of the hierarchical structure in zeolite materials is that it can significantly improve the utilization effectiveness of zeolite materials resulting in the minimum energy, time, and raw materials consumption. Substantial progress has been made in the synthesis, characterization, and application of hierarchical zeolites. Herein, we provide an overview of recent achievements in the field, highlighting the significant progress in the past decade on the development of novel and remarkable strategies to create an additional pore system in zeolites. The most innovative synthesis approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality while establishing a firm link between the preparation route and the resultant hierarchical pore quality in zeolites. Zeolites with different hierarchically porous structures, i.e., micro-mesoporous structure, micro-macroporous structure, and micro-meso-macroporous structure, are then analyzed in detail with concrete examples to illustrate their benefits and their fabrications. The significantly improved performances in catalytic, environmental, and biological applications resulting from enhanced mass transport properties are discussed through a series of representative cases. In the concluding part, we envision the emergence of "material-properties-by-quantitative and real rational design" based on the "generalized Murray's Law" that enables the predictable and controlled productions of bioinspired hierarchically structured zeolites. This Review is expected to attract important interests from catalysis, separation, environment, advanced materials, and chemical engineering fields as well as biomedicine for artificial organ and drug delivery systems.

Acetylene-Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite.

The selective hydrogenation of alkynes to alkenes is an important type of organic transformation with large-scale industrial applications. This transformation requires efficient catalysts with precise selectivity control, and palladium-based metallic catalysts are currently employed. Here we show that four-coordinated cationic nickel(II) confined in zeolite can efficiently catalyze the selective hydrogenation of acetylene to ethylene, a key process for trace acetylene removal prior to the polymerization process. Under optimized conditions, 100% acetylene conversion and an ethylene selectivity up to 97% are simultaneously achieved. This catalyst system also exhibits good stability and recyclability for potential applications. Spectroscopy investigations and density functional theory calculations reveal the heterolytic dissociation of hydrogen molecules and the importance of hydride and protons in the selective hydrogenation of acetylene to ethylene. This work provides an efficient strategy toward active and selective zeolite catalysts by utilizing the local electrostatic field within the zeolite confined space for small-molecule activation and by linking heterogeneous and homogeneous catalysis.

Recent advances of pore system construction in zeolite-catalyzed chemical industry processes.

The kaleidoscopic applications of zeolite catalysts (zeo-catalysts) in petrochemical processes has been considered as one of the major accomplishments in recent decades. About twenty types of zeolite have been industrially applied so far, and their versatile porous architectures have contributed their most essential features to affect the catalytic efficiency. This review depicts the evolution of pore models in zeolite catalysts accompanied by the increase in industrial and environmental demands. The indispensable roles of modulating pore models are outlined for zeo-catalysts for the enhancement of their catalytic performances in various industrial processes. The zeolites and related industrial processes discussed range from the uni-modal micropore system of zeolite Y (12-ring micropore, 12-R) in fluid catalytic cracking (FCC), zeolite ZSM-5 (10-R) in xylene isomerization and SAPO-34 (8-R) in olefin production to the multi-modal micropore system of MCM-22 (10-R and 12-R pocket) in aromatic alkylation and the hierarchical pores in FCC and catalytic cracking of C4 olefins. The rational construction of pore models, especially hierarchical features, is highlighted with a careful classification from an industrial perspective accompanied by a detailed analysis of the theoretical mechanisms.

Identification of tert-Butyl Cations in Zeolite H-ZSM-5: Evidence from NMR Spectroscopy and DFT Calculations.

Experimental evidence for the presence of tert-butyl cations, which are important intermediates in acid-catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with (1)H/(13)C magic-angle-spinning NMR spectroscopy, the tert-butyl cation was successfully identified on zeolite H-ZSM-5 upon conversion of isobutene by capturing this intermediate with ammonia.

Hydrogen-based industry: a prospective transition pathway toward a low-carbon future.

The hydrogen-based industrial systems are key enablers that can help save fossil energy, reduce pollution, and achieve high-quality development goals for the process industry in the future.

Steering interface effect of H-ZSM-5 zeolites with tailored surface barriers to improve their catalytic performances.

An efficient zeolite interface with optimized surface barriers was tailored by passivating the hydroxyl-group defects at surfaces or near pore mouths. The surface permeability of the modified zeolite was almost 90% greater than that of the pristine one, leading to remarkable improvements in C=2-3 selectivity and an anti-inactivation rate of 75% for the catalytic cracking reaction.

Pyridinic nitrogen dominated doping on Pd/carbon catalysts for enhanced hydrogenation performance.

The hydrogenation of 4-carboxylbenzaldehyde over Pd catalysts is a crucial process during the production of pure terephthalic acid. Herein, ZIF-8 derived carbon materials (NC) with adjustable N types were synthesized and used as the supports of Pd catalysts. Pd supported on NC with 50.5% of pyridinic N exhibited best hydrogenation activity with a TOF value of 4.1 min-1. The microstructures of NC support and electronic structures of Pd in Pd/NC were investigated by techniques such as XRD, N2 physisorption, XPS, H2-O2 titration and TEM. The nitrogen species in CN surface not only can adjust chemical state and dispersion of Pd nanoparticles (NPs), but also promote the adsorption and activation capability of H2 molecular. Besides, the ratio of Pd0/Pd2+ and Pd dispersion were closely correlated with pyridinic nitrogen content. The improvement in hydrogenation activity and stability of Pd/CN catalyst in relative to Pd/C were ascribed to the synergistic effect of pyridinic nitrogen and active site Pd0.

Nitrogen doped carbon for Pd-catalyzed hydropurification of crude terephthalic acid: roles of nitrogen species.

The purification of crude terephthalic acid was performed by the hydrogenation of 4-carboxybenzaldehyde (4-CBA) over activated carbon (AC) supported Pd catalysts in industry. However, traditional Pd/AC catalysts usually suffer from low hydrogenation activity and poor thermal stability. Herein, nitrogen was incorporated into AC via a simple hydrothermal treatment of AC with urea as the nitrogen resource. The N doped AC contained pyridinic N, pyrrolic N, graphitic N and oxidized N. Wide characterizations revealed that N doping not only effectively improved the dispersion of Pd NPs but also increased the proportion of Pd0. In addition, N doping also enhanced the dissociative adsorption capacity of molecular hydrogen. More importantly, the resistance to sintering of Pd NPs was efficiently suppressed after N doping. As a result, N doped AC supported Pd showed both higher activity and better thermal stability than the N-free one.

A diffusion anisotropy descriptor links morphology effects of H-ZSM-5 zeolites to their catalytic cracking performance.

Zeolite morphology is crucial in determining their catalytic activity, selectivity and stability, but quantitative descriptors of such a morphology effect are challenging to define. Here we introduce a descriptor that accounts for the morphology effect in the catalytic performances of H-ZSM-5 zeolite for C4 olefin catalytic cracking. A series of H-ZSM-5 zeolites with similar sheet-like morphology but different c-axis lengths were synthesized. We found that the catalytic activity and stability is improved in samples with longer c-axis. Combining time-resolved in-situ FT-IR spectroscopy with molecular dynamics simulations, we show that the difference in catalytic performance can be attributed to the anisotropy of the intracrystalline diffusive propensity of the olefins in different channels. Our descriptor offers mechanistic insight for the design of highly effective zeolite catalysts for olefin cracking.

An overview of recent development in composite catalysts from porous materials for various reactions and processes.

Catalysts are important to the chemical industry and environmental remediation due to their effective conversion of one chemical into another. Among them, composite catalysts have attracted continuous attention during the past decades. Nowadays, composite catalysts are being used more and more to meet the practical catalytic performance requirements in the chemical industry of high activity, high selectivity and good stability. In this paper, we reviewed our recent work on development of composite catalysts, mainly focusing on the composite catalysts obtained from porous materials such as zeolites, mesoporous materials, carbon nanotubes (CNT), etc. Six types of porous composite catalysts are discussed, including amorphous oxide modified zeolite composite catalysts, zeolite composites prepared by co-crystallization or overgrowth, hierarchical porous catalysts, host-guest porous composites, inorganic and organic mesoporous composite catalysts, and polymer/CNT composite catalysts.

Editorial for the Special Issue of ChemSusChem on Green Carbon Science: CO2 Capture and Conversion.

In their Editorial to the Special Issue on Green Carbon Science: CO2 Capture and Conversion, Guest Editors Bao-Lian Su, Buxing Han, Haichao Liu, Licheng Sun, and Zai-Ku Xie introduce the concept of green carbon science and discuss some of the Special Issue's contents.

Syngas to light olefins conversion with high olefin/paraffin ratio using ZnCrOx/AlPO-18 bifunctional catalysts.

Direct synthesis of light olefins from syngas (STO) using a bifunctional catalyst composed of oxide and zeolite has attracted extensive attention in both academia and industry. It is highly desirable to develop robust catalysts that could enhance the CO conversion while simultaneously maintain high selectivity to C2-C4 olefins. Herein, we report a bifunctional catalyst consisting of ZnCr binary oxide (ZnCrOx) and low-Si AlPO-18 zeolite, showing both satisfying selectivity to C2-C4 olefins of 45.0% (86.7%, CO2 free) and high olefin/paraffin ratio of 29.9 at the CO conversion of 25.2% under mild reaction conditions (4.0 MPa, 390 °C). By optimizing the reaction conditions, the CO conversion could be markedly increased to 49.3% with a slight drop in selectivity. CD3CN/CO-FTIR characterizations and theoretical calculations demonstrate that low-Si AlPO-18 zeolite has lower acid strength, and is therefore less reactive toward the hydride transfer in the STO reaction, leading to a higher olefin/paraffin ratio.

One-step controllable synthesis for high-quality ultrafine metal oxide semiconductor nanocrystals via a separated two-phase hydrolysis reaction.

A one-step synthesis method is described to prepare high-quality ultrafine inorganic semiconductor nanocrystals via a two-phase interface hydrolysis reaction under hydrothermal conditions. With the synthesis of ZrO2 quantum dots as an example, we show that the prepared nanocrystals have good monodispersity and high crystallinity, as well as other related superior properties, e.g., strong photoluminescence and excellent photocatalytic activities. Also the crystal size can be conveniently adjusted in the range below 10 nm through controlling the reaction temperature. Besides that, this method also shows other distinct advantages compared with other methods reported previously. First, the preparation process is simple and cheap and does not contain any complicated posttreatment procedure. Second, products (without coating) can be collected from the organic phase which effectively avoids grain aggregation induced by the capillary concentration in the water environment. Third, the production yield is very high (almost 100%) and the organic and water phases after reaction can be easily recycled for next reaction. Therefore, it provides a promising strategy for the large-scale industrial production of different kinds of high-quality inorganic nanocrystals.

High performance catalytic distillation using CNTs-based holistic catalyst for production of high quality biodiesel.

For production of biodiesel from bio oils by heterogeneous catalysis, high performance catalysts of transesterification and the further utilization of glycerol have been the two points of research. The process seemed easy, however, has never been well established. Here we report a novel design of catalytic distillation using hierachically integrated CNTs-based holistic catalyst to figure out the two points in one process, which shows high performance both for the conversion of bio oils to biodiesel and, unexpectedly, for the conversion of glycerol to more valuable chemicals at the same time. The method, with integration of nano, meso to macro reactor, has overwhelming advantages over common technologies using liquid acids or bases to catalyze the reactions, which suffer from the high cost of separation and unsolved utilization of glycerol.

Mesostructural Bi-Mo-O catalyst: correct structure leading to high performance.

Structure-activity relationship has been one of the main topics of research on catalysts all the time. Component and structure are the two moieties governing the performance of solid materials as catalysts. Multicomponent bismuth molybdates are well known catalysts for propene oxidation but pure crystalline phases of bismuth molybdate are inactive for the reaction. We have designed mesostructural Bi-Mo-O catalyst with pure bismuth molybdate nanocrystals attached to molybdenum oxide nanobelts and found it is a high performance catalyst for the reaction, though the two domains themselves are inactive. The strongly expitaxial interaction between the two domains causes the lattice shrinkage and distortion of the bismuth molybdate nanocrystals and extremely promotes their catalytic activity toward propene oxidation while keeping high selectivity at the same time. The results are instructive for design of nano oxide catalysts with mesostructures leading to high performance.