Synthesis of polymorph A-enriched beta zeolites in a HF-concentrated system

Abstract

Polymorph A-enriched beta zeolites were synthesized by employing high HF concentrations in the synthesis medium. The polymorphic compositions of the synthesized beta zeolites were determined by the complementary characterization methods ¹⁹F NMR analysis and PXRD simulation. With a variety of SDAs, a high HF concentration (HF/SDA > 1.0) in the synthesis medium results in the A-rich feature (55–65% A) of beta zeolites, while a moderate HF concentration only results in typical beta zeolites. A systematic study on the synthesis conditions reveals the existence of a buffered system of H⁺ and F⁻ formed in the highly HF-concentrated medium. This buffer results in a small but continuous supply of F⁻ during zeolite crystallization, in contrast to the conventional fluoride route where all F⁻ are discharged all-at-once at the initial stage.