Synthesis of zeolite Beta from nonalkaline fluoride aqueous aluminosilicate gels

Abstract

Zeolite Beta is usually prepared from basic media in presence of Na⁺ and TEA⁺ cations as the structuring agents. By using an original synthesis procedure, where the hydroxide ions are replaced by fluoride anions as the mineralizing agent, we have been able to prepare zeolite Beta from near-neutral nonalkaline reaction mixtures containing diaza-1,4-bicyclo [2.2.2] octane and methylamine. Full transformation of the gel into Beta zeolite requires the presence of seed crystals. This Beta “F⁻” zeolite can be directly converted by calcination into its acid form. The size of the crystals obtained lies typically between 2 and 3 μm with a Si to Al molar ratio of the framework ranging from ca. 9 to 22. The samples were characterized through the usual techniques, e.g., STEM, TEM, TED, d.t.a. and t.g.a., and¹³CMAS n.m.r. spectroscopy. The latter technique reveals a partial transformation of the organic molecules originally present; indeed, the zeolitic precursor contains beside the “dabconium” cation (monoprotonated form) a polymeric compound identified as polyethylene piperazine. Although the methylamine (or the corresponding protonated form) is not found unchanged inside the framework, its presence is necessary for the complete crystallization of the gel to be achieved; in fact, it is probably involved in the previously mentioned polymerization process.