Acidic V-MCM-41 catalysts for the liquid-phase ketalization of glycerol with acetone
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SiMCM-41 and V-MCM-41 were hydrothermally synthesized with different quantities of vanadium, characterized by small angle X-ray scattering, nitrogen physisorption, and the insertion of vanadium was assessed by temperature-programed desorption of ammonia, pyridine chemisorption followed by infrared spectroscopy and H2 temperature-programed reduction. Vanadium-based materials are notable oxidation catalysts, however acid sites can be developed, making them potential bifunctional catalysts combining redox and Lewis acid sites. Herein, mesoporous vanadosilicates were used as acidic catalysts in the Ketalization of glycerol with acetone for solketal formation. The catalytic activity was dependent on the amount of acid sites, based on three types of vanadium oxide species: (i) isolated or (ii) oligomeric vanadium species inserted in the silica framework, i.e. -Si-O-(V-O-V)n, where n = 1 and n > 1, respectively, and (iii) surface vanadium oxides highly dispersed or interacting with surface hydroxyl groups. By performing recycling experiments it was possible to conclude that the surface vanadium oxides species, despite of being more active, are leached by water molecules formed in the course of the reaction, decreasing the activity. On the other hand, framework vanadium are more stable Lewis acid sites for liquid-phase Ketalization reaction.