Mesopore modification of beta zeolites by sequential alkali and acid treatments: Narrowing mesopore size distribution featuring unimodality and mesoporous texture properties estimated upon a mesoporous volumetric model

•Mesopore modification of H-Beta zeolite by sequential alkali and acid treatments.•Disordered mesoporous structure featuring a borrowing pore size distribution.•Model equation on the volumetric fraction of intracrystalline mesopores.

For mesopore modification of zeolite beta prepared in protonic form (H-Beta, Si/Al = ca.18.7) to concentrate pore size distributions, the framework T-atoms of H-Beta crystals were extracted according to the sequential schemes: hydrothermal pre-etching using incipiently wetted alkaline solutions, bulk alkaline treatment, and acid leaching coupling with ammonium exchange. Based on XRD, FTIR, N2 adsorption at 77 K, SEM, TEM, EDX, ICP–AES, and immersion porosimetry, structure properties of the hierarchical materials obtained were correlated with the sequential treatment processes. In contrast to single bulk alkaline treatment, the hydrothermal alkali pre-etching enables to impair or eliminate broadly secondary pore–distributed peaks, the bulk alkaline treatment before acid leaching facilitates textural mesopore formation, and the final acid leaching plus ammonium exchange leads to moderate increases in mesoporosity and mesopore diameters, and also to a suitable enhancement of Si/Al ratios. FTIR spectra provided evidence for the short range ordering of modified zeolites and for the presence of zeolitic ring sub-units in the basic filtrate. However, the microporosity, crystallinity and yield relative to the parent are reduced by increased alkaline concentrations. A novel model was built for estimating a volumetric fraction of intracrystalline-to N2 adsorbed mesopores. Calculation results show that this volumetric fractional model promises to identify the mesoporous textural property, explaining how many volumetric fractions of mesopores are to locate in beta crystallite entities. In addition, the hierarchical beta possesses a mesopore network with higher hydrothermal stability at 873 K.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide