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 C-13, Al-27, and Si-29 MAS NMR and ex situ XRD, X-rayfluorescence, and TG studies have been appliedfor the elucidation of the mechanisms of hydrothermal synthesis ofzeolite BEA from a clear solution and a dense hydrogel. Isotopic labelingwith Si-29 and C-13 isotopes has been used tofollow the fate of siliceous species and structure-directing agent(((CH3)-C-13-CH3)(4)NOH)). Theresults point to different mechanistic pathways depending on silicatespeciation in the initial reaction mixture: (i) an aggregation/nucleationprocess for the reaction mixture prepared from a colloidal silicasource and (ii) solution-mediated crystallization for the reactionmixture prepared from a dense silica hydrogel. The products obtainedhave been characterized using a set of physicochemical methods includingXRD, X-ray fluorescence analysis, TEM, low-temperature nitrogen adsorption-desorption,and NH3-TPD. The results demonstrate that the crystallizationmechanism affects the crystal size and morphology: the aggregation/nucleationprocess leads to very tiny crystals with a size of 5-15 nm,whereas solution-mediated crystallization results in dense polycrystalswith a size of 300-700 nm with the core enriched with Al. Theevaluation of the catalysts in liquid-phase alkylation of benzenewith propylene points to the higher activity of zeolite BEA obtainedfrom a colloidal silica source. Colloidal silicaleads to an aggregation/nucleation mechanismof BEA synthesis providing tiny crystals. Silica hydrogel initiatessolution-mediated crystallization leading to dense polycrystals.