Zeolites have been extensively studied for years in different areas of chemical industry, such as shape selective catalysis, ion-exchange, and gas adsorption and separation. Generally, zeolites are prepared from solvothermal synthesis in the presence of a large amounts of solvents such as water and alcohols in sealed autoclaves under autogenous pressure. Water has been regarded as essential to synthesize zeolites for fast mass transfer of reactants, but it occupies a large space in autoclaves, which greatly reduces the yield of zeolite products. Furthermore, polluted wastes and relatively high pressure due to the presence of water solvent in the synthesis also leads to environmental and safety issues. Recently, inspired by great benefits of solvent-free synthesis, including the environmental concerns, energy consumption, safety, and economic cost, researchers continually challenge the rationale of the solvent and reconsider the age-old question "Do we actually need solvents at all in zeolite synthesis?" In this Account, we briefly summarize our efforts to rationally synthesize zeolites via a solvent-free route. Our research demonstrates that a series of silica, aluminosilicate, and aluminophosphate-based zeolites can be successfully prepared by mixing, grinding, and heating starting solid materials under solvent-free conditions. Combining an organotemplate-free synthesis with a solvent-free approach maximizes the advantages resulting in a more sustainable synthetic route, which avoids using toxic and costly organic templates and the formation of harmful gases by calcination of organic templates at high temperature. Furthermore, new insights into the solvent-free crystallization process of zeolites have been provided by modern techniques such as NMR and UV-Raman spectroscopy, which should be helpful in designing new zeolite structures and developing novel routes for synthesis of zeolites. The role of water and the vital intermediates during the crystallization of zeolites have been proposed and verified. In addition to a significant reduction in liquid wastes and a remarkable increase in zeolite yields, the solvent-free synthesis of zeolites exhibits more unprecedented benefits, including (i) the formation of hierarchical micro-, meso-, and macrostructures, which benefit the mass transfer in the reactions, (ii) rapid synthesis at higher temperatures, which greatly improve the space-time yields of zeolites, and (iii) construction of a novel catalytic system for encapsulation of metal nanoparticles and metal oxide particles within zeolite crystals synergistically combining the advantages of catalytic metal nanoparticles and metal oxide particles (high activity) and zeolites (shape selectivity). We believe that the concept of "solvent-free synthesis of zeolites" would open a door for deep understanding of zeolite crystallization and the design of efficient zeolitic catalysts.

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The preparation of hierarchical zeolites with reduced diffusion limitation and enhanced catalyst efficiency has become a vital focus in the field of zeolites and porous materials chemistry within the past decades. This review will focus on the diffusion and catalyst efficiency of hierarchical zeolites and industrial catalysts. The benefits of diffusion and catalyst efficiency at two levels of hierarchy (zeolitic component level and industrial catalyst level) from chemical reaction engineering point of view will be analysed. At zeolitic component level, three types of mesopores based on the strategies applied toward enhancing the catalyst effectiveness factor (η) are presented: (1) ‘functional mesopores’ (raising effective diffusivity); (2) ‘auxiliary mesopores’ (decreasing diffusion length); and (3) ‘integrated mesopores’ (a combination thereof). At industrial catalyst level, location and interconnectivity among the constitutive components are revealed. The hierarchical pore interconnectivity in multi-component zeolite based industrial catalysts is exemplified by fluid catalytic cracking and bi-functional hydroisomerization catalysts. The rational design of industrial zeolite catalysts at both hierarchical zeolitic component and catalyst body levels can be fully comprehended using the advanced in-situ and/or operando spectroscopic, microscopic and diffraction techniques.

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ZSM‐5 zeolite crystals with controllable b‐axis length (sheet‐like, S‐ZSM‐5; chain‐like, C‐ZSM‐5) have been synthesized by using urea and starch as additives in the starting aluminosilicate gels. X‐ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images show that these zeolite samples have good crystallinity. Transmission electron microscopy (TEM) images show that there is a strong chemical interaction between the stacked crystals in C‐ZSM‐5 samples. N2 sorption isotherms indicate that C‐ZSM‐5 crystals are mesoporous. Catalytic tests for the formation of p‐xylene from m‐xylene isomerization show that, compared with other zeolite catalysts, C‐ZSM‐5 catalysts give both high conversion and improved p‐xylene selectivity, which are attributed to the combination of relatively long b‐axis length and the present mesoporosity in the crystals. The improvement of p‐xylene selectivity in catalytic m‐xylene isomerisation is of great importance for selective industrial production of p‐xylene in the future.

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