Comparison of Morphology and Physicochemical Properties of Embryonic and Nanosized ZSM-5 Zeolites and Their Use in the D
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mparison of Morphology and Physicochemical Properties of Embryonic and Nanosized ZSM-5 Zeolites and Their Use in the Dealkylation Reaction of Aromatic Hydrocarbons (a Review) V. A. Ostroumovaa, *, V. A. Severinaa, and A. L. Maksimova aTopchiev
Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia *e-mail: [email protected] Received March 13, 2020; revised April 9, 2020; accepted April 10, 2020
Abstract—This review presents the analysis of the characteristic features of the structure, synthesis methods, and physicochemical properties of embryos that form the basis of ZSM-5 zeolites. Various physicochemical characteristics of the embryos and nanocrystals of ZSM-5 (X-ray diffraction, textural characteristics, morphology, infrared and nuclear magnetic resonance spectroscopy) are discussed and compared, and their similarities and differences are analyzed. Keywords: embryonic zeolites, ZSM-5 nuclei, nanosized ZSM-5, tetrapropylammonium hydroxide, template, physicochemical properties, structural features, zeolite synthesis DOI: 10.1134/S0965544120080083
Zeolites are used as heterogeneous catalysts in multiple industrial processes [1, 2]. They possess a high specific surface area, which mostly concentrates in micropores, and active sites providing high product selectivity [3–5]. However, in view of the limited pore size (0.25–1.0 nm), zeolites slightly participate in the transport of large molecules, the kinetic diameter of which exceeds 1 nm [5]. The active sites accessible for such molecules are located only on the surface of zeolite crystals and in the pore openings. The presence of such diffusion constraints in zeolites does not allow their use in the transformations of heavy petroleum fractions and biomass feedstock. Currently, one of the priority tasks in the zeolites catalysis is the search for alternative materials combining high activity with the accessibility of the active sites. To solve this problem, multiple studies are devoted to the development of hierarchical and mesoporous materials [6–8]. Although mesoporous structure created on the basis of initial zeolite indeed facilitates mass transport, but it does not completely remove the diffusion limitations for bulky molecules. Another solution for the problem is a decrease in the size of zeolite crystals, resulting in materials with a high external surface accessible for bulky molecules, which shortens the diffusion path length. Thus, the external surface area of nanosized ZSM-5, the average crystal size of which is 100 nm, reaches 100 m2/g [9]. As a result of a decrease in the crystal size to 10– 20 nm, the external surface area of the zeolite can be increased up to 250 m2/g. Such nanosized zeolites
containing five to ten unit cells in a crystal substantially improve the mass transport of small molecules; however, a large part of the active sites remains inaccessible for molecules that have a size larger than the pore mouth. This review considers a special type of ultrasmall particles called embryonic zeolites [10–13] fo
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