Hydrothermal Synthesis and Catalytic Assessment of High-Silica (B,Fe)-beta Zeolites

The synthesis of high-silica BEA zeolite has attracted great attention from the zeolite scientific community, and several approaches have been proposed for the preparation of crystalline materials with a very low aluminum content. In this work, high-silica crystalline BEA zeolites were prepared starting from an Al-free synthesis gel, by using boron and iron as trivalent atoms. A Si/Fe molar ratio equal to 100, 200 or ∞ was adopted, and crystallization was successful in the presence of tetraethylammonium (TEA+), with a low Si/B starting synthesis gel molar ratio, i.e. 8 or 12, and with a hydrothermal synthesis time of 6 days maximum at 150 °C. The Si/Fe ratio affects TEA+-zeolite interactions, crystallographic patterns, and surface acidity. In particular, the proposed synthesis procedure allowed obtaining BEA zeolites with a very low acidity, with a tunable Lewis/Brønsted acids site distribution. Liquid-phase etherification of 5-hydroxymethyl furfural (HMF) with ethanol highlighted the suitability of the obtained materials for the production of 5-(ethoxymethyl)furan-2-carbaldeyde (EMF), a green fuel additive. Particularly, a high EMF selectivity (>97%) was obtained in the presence of both iron and boron. As the main outcome, the presented results indicated that high-silica BEA zeolite can be easily obtained starting from a low Si/B ratio in the synthesis gel, with conventional tetraethylammonium as a template, and iron can be also incorporated for catalytic purposes.