Synthesis of hierarchical zeolite Beta with low organic template content via the steam-assisted conversion method

摘要

We report the synthesis of hierarchiczal zeolite Beta with low organic template (tetraethylammonium hydroxide, TEAOH) content by the steam-assisted conversion (SAC) method. Neither zeolite seeds nor the secondary mesoporogen were required. Only a bit amount of organic template (denoted as TEAOH/SiO2 ratio = 0.06) was used to obtain highly crystalline zeolite Beta, which was remarkably lower than the reported lowest value (0.10). Physicochemical properties of the resulting samples were investigated by comprehensive characterizations including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, solid state Al-27 MAS NMR spectra, nitrogen adsorption-desorption, X-ray fluorescence, temperature-programmed desorption of ammonia and infrared spectra of pyridine adsorption. These characterization results showed that the SAC method led to hierarchical crystalline zeolite Beta aggregates assembled by 20-40 nm nanosized crystals with controllable SiO2/Al2O3 ratios among 29-47 by varying the compositions of synthetic gels with the Na2O/SiO2 ratio of 0.12 and the H2O/dry gel ratios at 0.20-030. These zeolite Beta aggregates possessed large external surface area (158-180 m(2)/g) and mesoporous volume (0.27-0.30 cm(3)/g) resulted from the formation of intercrystalline mesopores and of the small zeolite crystal sizes. The obtained hierarchical zeolite Beta showed 41.2% conversion in the esterification reaction of levulinic acid with ethanol and four catalytic runs without obvious decrease in conversion, which were better than those on the zeolite Beta samples from the conventional hydrothermal synthesis. This was due to the promotion of the accessibility of reactants to the acid sites distributed on the external surface of zeolite nanocrystals and of the prominent diffusion in intercrystalline mesopores. (C) 2016 Elsevier B.V. All rights reserved.