Synthesis of Sulfated Sn-Zr Mesoporous Catalysts for the Selective Dehydration of Hexose-Type Monosaccharides

Synthesis of combined Brønsted and Lewis acid sites in SBA-15 by Zr and Sn incorporation onto the mesoporous support, followed by a sulfation procedure to get enhanced acid catalysts for fructose-to-5-hydroxymethylfurfural (HMF) selective dehydration, is here studied. Different preparation methods are used to attain the Sulfated Sn-Zr-SBA-15 material with adequate combination of Brönsted and Lewis acidity. Samples are characterized by using different analytical and spectroscopic techniques, results indicating a correlation between the preparation procedure and the acid characteristics of the materials. The latter are evaluated in the catalytic conversion of concentrated fructose solutions (30 wt%) into HMF in a biphasic system (water + iso-butylketone/2-butanol). The most efficient catalyst, SO₄/Sn-Zr-SBA-15 (Zr/Sn molar ratio = 1.98), yields ≥ 70% HMF at a substrate conversion of ≈90% under optimal reaction conditions (150 °C for 120 min.), these results being particularly relevant when considering the higher initial fructose concentrations here used compared to those typically employed in previous reports. This catalyst exhibits a higher ratio of Bronsted/Lewis (B//L) acid sites of moderate strength and a lower density of acid sites compared to the other catalytic samples evaluated, mainly due to the presence of well-dispersed tin and zirconium species, as well as to the enhanced acidic properties provided by sulfate groups in the material.