Multinuclear MAS NMR Monitoring of the Effect of Silicate Speciation on the Mechanism of Zeolite BEA Formation: Toward Engineering of Crystal Size and Morphology

Time-resolved in situ 13C, 27Al, and 29Si MAS NMR and ex situ XRD, X-ray fluorescence, and TG studies have been applied for the elucidation of the mechanisms of hydrothermal synthesis of zeolite BEA from a clear solution and a dense hydrogel. Isotopic labeling with 29Si and 13C isotopes has been used to follow the fate of siliceous species and structure-directing agent ((13CH3-CH3)4NOH)). The results point to different mechanistic pathways depending on silicate speciation in the initial reaction mixture: (i) an aggregation/nucleation process for the reaction mixture prepared from a colloidal silica source and (ii) solution-mediated crystallization for the reaction mixture prepared from a dense silica hydrogel. The products obtained have been characterized using a set of physicochemical methods including XRD, X-ray fluorescence analysis, TEM, low-temperature nitrogen adsorption–desorption, and NH3-TPD. The results demonstrate that the crystallization mechanism affects the crystal size and morphology: the aggregation/nucleation process leads to very tiny crystals with a size of 5–15 nm, whereas solution-mediated crystallization results in dense polycrystals with a size of 300–700 nm with the core enriched with Al. The evaluation of the catalysts in liquid-phase alkylation of benzene with propylene points to the higher activity of zeolite BEA obtained from a colloidal silica source.