The Synthesis of Monodisperse MFI Nanozeolites: Controlling the Crystallization Process
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e Synthesis of Monodisperse MFI Nanozeolites: Controlling the Crystallization Process K. A. Sashkinaa, b, *, D. O. Shestakovaa, b, and E. V. Parkhomchuka, b aBoreskov
Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia bNovosibirsk State University, Novosibirsk, 630090 Russia *e-mail: [email protected] Received February 25, 2020; revised March 25, 2020; accepted April 10, 2020
Abstract—The sinthesis conditions of a series of monodisperse crystals of ZSM-5 zeolite, silicalite-1, and Fesilicalite-1 with an average particle size in the range of 30 to 450 nm have been determined. The kinetic features of the silicalite-1 crystallization at low temperatures (60−90°C) and atmospheric pressure have been studied. The effective activation energies of silicalite-1 nucliation and crystal growth have been determined to be 95 ± 7 and 100 ± 7 kJ/mol, respectively. The influence of the synthesis time and temperature as well as the silicon source concentration on the size of the MFI type nanocrystals has been studied. Keywords: nanozeolites, crystallization, silicalite-1, Fe-silicalite-1, ZSM-5 DOI: 10.1134/S0965544120080137
Due to the zeolite microporous structure there are diffusion limitations resulting in the low utilization degree and rapid deactivation of classical zeolites with a crystal size of >1 μm in a number of processes, such as cracking, oxidation, isomerization, alkylation, and esterificetion [1]. To increase the utilization degree two main approaches are known: to decrease the diffusion path by decreasing the crystal size; to increase the effective diffusion coefficient by introducing additional mesopores into the structure of the crystals [2]. The combination of these two approaches makes it possible to synthesize materials that are built of zeolite nanocrystals and have a hierarchical pore system [3]. Catalysts based on nanosized zeolites turned out to be significantly more active and stable than the classical microcrystalline catalysts in various processes, such as cracking of n-hexane (ZSM-5) [4], synthesis of lower C2–C4 olefins from acetone (ZSM-5) [5], synthesis of propylene from methanol (ZSM-5) [6], selective liquid-phase oxidation of phenol with hydrogen peroxide (TS-1) [7], deep liquid-phase oxidation of organic substances with hydrogen peroxide (Fe-silicalite-1) [8, 9]. To synthesise crystals with a certain size, it is important to know the ways of controlling the nucleation and crystal growth processes, namely, of choosing the suitable components ratio, synthesis time and temperature. The influence of the synthesis temperature and time and the concentration silicon source concentration in the reaction mixture on the average size of nanocrystals is considered in this work by the
example of the materials with the MFI structure (silicalite-1, Fe-silicalite-1, ZSM-5 zeolite). EXPERIMENTAL Chemicals The following chemicals were used for the synthesis of zeolites: tetraethyl orthosilicate (TEOS, AngaraReaktiv), tetrapropylammonium hydroxide (TPAOH, 25 wt %
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