Tunable Acidity of Sulfated Zr-Sn Mixed Oxides for Solvent-Free Sorbitol Dehydration to Isosorbide

One of the most important processes in creating sustainable monomers for high-performance polymers is the catalytic dehydration of sorbitol obtained from biomass to isosorbide. This study investigated a series of sulfated Zr_xSn_{1 - x}O₂ (x = 0.1-0.9) mixed oxides prepared by coprecipitation method for sorbitol dehydration under solvent-free conditions. Comprehensive evaluation of different characterization techniques, i.e., X-ray diffraction, Fourier transform infrared, transmission electron microscope, N₂ physisorption, and NH₃-TPD demonstrated that surface acidity, textural characteristics, and catalytic activity were greatly affected by catalyst composition. Catalyst with intermediate Zr content (0.4-0.6) had an ideal balance of weak, medium, and strong Brønsted acid sites, which resulted in complete conversion of sorbitol and higher isosorbide selectivity. 100% sorbitol conversion and 87% isosorbide selectivity were achieved with sulfated (S)-Zr_0.5Sn_0.5O₂ at 220 °C within 3 h, exceeding many reported heterogeneous solid acids catalyst. A minor loss in catalyst activity was observed after four cycles, showing its potential for long-term isosorbide production.