Nanocrystalline zeolites beta: Features of synthesis and properties

Аннотация

The effect of synthesis conditions for BEA zeolite nanocrystals (BEA-ZNC) on the combination of their morphological, textural, and acidic properties has been studied. BEA-ZNC have been synthesized by the hydrothermal or vapor-phase crystallization of reaction mixtures with a high template concentration using the following variable synthesis parameters: the SiO₂/Al₂O₃ ratio the presence of mineralizing additives, the SiO₂ source and template type, and the synthesis pH. It has been found that an increase in the aluminum content in BEA-ZNC from 2.7 to 7.8 Al atoms per unit cell is accompanied by a decrease in the size of the ZNC from 250–350 to 100 nm, an increase in the acid site concentration, and an increase in the fraction of weak acid sites in the acidity range of the samples. It has been shown that the specific feature of the synthesis of BEA-ZNC is a low degree of inclusion of silica and the template into the zeolite composition, which is 28–62 and 3.4–5.5%, respectively, of the amounts initially introduced into the reaction mixture. It has been proposed that silica gel should be used as the most promising SiO₂ source for the preparation of BEA-ZNC; the use of silica gel provides formation of a material in the form of isolated nanocrystals with a size of 150–300 nm, a developed pore structure, and a high concentration of acid sites. It has been shown that the BEA-ZNC synthesized by vapor-phase crystallization are inferior to the samples prepared by conventional hydrothermal crystallization with respect to the pore structure characteristics and acidic properties.