The introduction of synthetic zeolites has led to a paradigm shift in catalysis, separations, and adsorption processes, due to their unique properties such as crystallinity, high-surface area, acidity, ion-exchange capacity, and shape-selective character. However, the sole presence of micropores in these materials often imposes intracrystalline diffusion limitations, rendering low utilisation of the zeolite active volume in catalysed reactions. This critical review examines recent advances in the rapidly evolving area of zeolites with improved accessibility and molecular transport. Strategies to enhance catalyst effectiveness essentially comprise the synthesis of zeolites with wide pores and/or with short diffusion length. Available approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality for practical implementation, establishing a firm link between the properties of the resulting materials and the catalytic function. We particularly dwell on the exciting field of hierarchical zeolites, which couple in a single material the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. The carbon templating and desilication routes as examples of bottom-up and top-down methods, respectively, are reviewed in more detail to illustrate the benefits of hierarchical zeolites. Despite encircling the zeolite field, this review stimulates intuition into the design of related porous solids (116 references).

Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design.

Liquid hydrocarbon fuels play an essential part in the global energy chain, owing to their high energy density and easy transportability. Olefins play a similar role in the production of consumer goods. In a post-oil society, fuel and olefin production will rely on alternative carbon sources, such as biomass, coal, natural gas, and CO(2). The methanol-to-hydrocarbons (MTH) process is a key step in such routes, and can be tuned into production of gasoline-rich (methanol to gasoline; MTG) or olefin-rich (methanol to olefins; MTO) product mixtures by proper choice of catalyst and reaction conditions. This Review presents several commercial MTH projects that have recently been realized, and also fundamental research into the synthesis of microporous materials for the targeted variation of selectivity and lifetime of the catalysts.

Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity

Catalytic transformations of syngas (a mixture of H2 and CO), which is one of the most important C1-chemistry platforms, and CO2, a greenhouse gas released from human industrial activities but also a candidate of abundant carbon feedstock, into chemicals and fuels have attracted much attention in recent years. Fischer-Tropsch (FT) synthesis is a classic route of syngas chemistry, but the product selectivity of FT synthesis is limited by the Anderson-Schulz-Flory (ASF) distribution. The hydrogenation of CO2 into C2+ hydrocarbons involving C-C bond formation encounters similar selectivity limitation. The present article focuses on recent advances in breaking the selectivity limitation by using a reaction coupling strategy for hydrogenation of both CO and CO2 into C2+ hydrocarbons, which include key building-block chemicals, such as lower (C2-C4) olefins and aromatics, and liquid fuels, such as gasoline (C5-C11 hydrocarbons), jet fuel (C8-C16 hydrocarbons) and diesel fuel (C10-C20 hydrocarbons). The design and development of novel bifunctional or multifunctional catalysts, which are composed of metal, metal carbide or metal oxide nanoparticles and zeolites, for hydrogenation of CO and CO2 to C2+ hydrocarbons beyond FT synthesis will be reviewed. The key factors in controlling catalytic performances, such as the catalyst component, the acidity and mesoporosity of the zeolite and the proximity between the metal/metal carbide/metal oxide and zeolite, will be analysed to provide insights for designing efficient bifunctional or multifunctional catalysts. The reaction mechanism, in particular the activation of CO and CO2, the reaction pathway and the reaction intermediate, will be discussed to provide a deep understanding of the chemistry of the new C1 chemistry routes beyond FT synthesis.

New horizon in C1 chemistry: breaking the selectivity limitation in transformation of syngas and hydrogenation of CO2 into hydrocarbon chemicals and fuels

While experts in various fields discuss the potential of carbon capture and storage (CCS) technologies, the utilization of carbon dioxide as chemical feedstock is also attracting renewed and rapidly growing interest. These approaches do not compete; rather, they are complementary: CCS aims to capture and store huge quantities of carbon dioxide, while the chemical exploitation of carbon dioxide aims to generate value and develop better and more-efficient processes from a limited part of the waste stream. Provided that the overall carbon footprint for the carbon dioxide-based process chain is competitive with conventional chemical production and that the reaction with the carbon dioxide molecule is enabled by the use of appropriate catalysts, carbon dioxide can be a promising carbon source with practically unlimited availability for a range of industrially relevant products. In addition, it can be used as a versatile processing fluid based on its remarkable physicochemical properties.

Chemical Technologies for Exploiting and Recycling Carbon Dioxide into the Value Chain

Green Routes for Synthesis of Zeolites

Selective production of propylene from methanol: Mesoporosity development in high silica HZSM-5

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. Developing robust catalysts for direct synthesis of light olefins from syngas remains a challenge. Here, the authors report a novel bifunctional catalyst composed of ZnCr binary oxide and low-Si AlPO-18 zeolite which exhibits enhanced CO conversion and selectivity to C2-C4 olefins under mild reaction conditions.

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

Catalytic activity and selectivity of methylbenzenes in HSAPO-34 catalyst for the methanol-to-olefins conversion from first principles

Theoretical insight into the minor role of paring mechanism in the methanol-to-olefins conversion within HSAPO-34 catalyst

SAPO‐34 is an important member in the family of zeolites, which shows excellent catalytic activity in MTO reaction, but suffers from the diffusion bottleneck. To overcome the diffusion problem, porogen was added to generate meso/macro pores in SAPO‐34 zeolite. In this work, amphipathic molecules were designed as CSDA to synthesize a tri‐level hierarchically porous SAPO‐34, and the introduction of CSDA could promote the Si atoms incorporating into the AlPO4 framework, change the morphology of particles and decrease the acidity of prepared SAPO‐34 zeolite. The prepared SAPO‐34 catalyst exhibits higher selectivity of C2H4 and C3H6 in the MTO reaction than the conventional SAPO‐34 zeolite. The morphology and the microstructure of the prepared samples vary with the carbon chain of the amphipathic molecules due to the matching between amphipathic molecules and topological structure of CHA.

Hongxing Liu

Papers

Selective production of propylene from methanol: Mesoporosity development in high silica HZSM-5

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

Catalytic activity and selectivity of methylbenzenes in HSAPO-34 catalyst for the methanol-to-olefins conversion from first principles

Theoretical insight into the minor role of paring mechanism in the methanol-to-olefins conversion within HSAPO-34 catalyst

Affecting the Formation of the Micro-structure and Meso/macro-structure of SAPO-34 zeolite by Amphipathic Molecules