Variation of the Si/Al ratio in nanosized zeolite Beta crystals

Abstract

Zeolite Beta nanocrystals were prepared from basic aluminosilicate precursor solutions upon hydrothermal treatment at 100 °C. The Si/Al ratio of the initial system was systematically changed from 25 to infinity in order to study the limits in the framework composition of BEA-type crystallites synthesized from clear basic solutions. Furthermore, the effect of the Si/Al ratio on the precursor species, ultimate crystal size, morphology and yield was investigated. The results revealed that the crystallization kinetics of nanosized Beta are dependent on the amount of Al in the precursor solutions, that is, the nucleation and growth processes are faster in Al-rich systems. The crystallization process of zeolite Beta with Si/Al ratios in the initial solutions of 14, 23 and 32 was accomplished within 72 h, whereas longer crystallization times, 140 and 264 h, were necessary to obtain crystalline products with Si/Al ratios of 42 and infinity, respectively. The intermediates and final products were investigated by complementary techniques such as XRD, HRTEM, DLS, IR, NMR spectroscopy and chemical analysis. Low temperature (77 K) CO adsorption infrared spectroscopy was used to study the Brønsted acidity of zeolite Beta samples with different Si/Al ratios. The properties of Beta nanocrystals important for the design of catalysts and selective separation materials are provided based on the results obtained from the detailed characterization.