Expanding the phase space for crystallization of boron and aluminum containing Beta zeolites

Abstract

Inclusion of boron in growth mixtures for aluminosilicate Beta zeolites has been used previously as a strategy to vary zeolite crystallite sizes, yet prior reports describe limited compositional regimes of silicon-to-aluminum and silicon-to-boron molar ratios in which the Beta topology will crystallize. This study demonstrates that manipulation of growth mixture compositions can enable crystallization of Beta zeolites across a broader range of silicon-to-aluminum and silicon-to-boron molar ratios in boron-and aluminum-containing Beta zeolites (B-Al-Beta) than previously reported. B-Al-Beta samples are characterized by X-ray diffraction, solid-state ¹¹B magic angle spinning nuclear magnetic resonance (MAS NMR), nitrogen physisorption, elemental analysis, scanning electron microscopy, and ammonia temperature programmed desorption (TPD). Silicon-to-aluminum molar ratios in the crystallized B-Al-Beta solids were directly proportional to those present in growth mixtures, reflecting more equal extents of Si and Al incorporation than is typical when aluminosilicate Beta is synthesized in hydroxide media. Beta crystallite sizes were found to increase with increasing B content at constant Si/Al ratio. Ammonia TPD protocols developed previously to selectively quantify protons at Al-O-Si linkages, but not at B-O-Si linkages, in B-Al-MFI zeolites were demonstrated here to also do so for B-Al-Beta zeolites. These synthesis and characterization protocols enable synthesis of Beta zeolites with constant silicon-to-aluminum ratios but varied crystallite sizes, achieved via variation in the silicon-to-boron molar ratio, and selective quantification of the Brønsted acid sites associated with Al centers in the framework.