Solvent-free microwave-assisted synthesis of solketal from glycerol using transition metal ions promoted mordenite solid acid catalysts

The present study demonstrates a clean and green approach for the production of bio-fuel additives from renewable glycerol acetalization using microwave irradiation as heating method. Various transition metal ion (M = Fe-, Co-, Ni-, Cu- & Zn-) promoted mordenite solid acid catalysts were prepared by wet impregnation method and examined for the acetalization of glycerol with acetone. Investigation of the structural and chemical properties of the catalysts was achieved using XRD, FT-IR, TGA-TPD, SEM/EDX and XPS techniques. Comparative studies using conventional heating were performed in order to study the efficiency of microwave-assisted acetalization reactions. The effect of various operating parameters on the catalytic activity studies was examined. Amongst the metal-promoted mordenite catalysts tested, Cu-Mor had the best performance due to the presence of a large amount of acidic sites and the synergetic effects of metal particles interacting with mordenite. Microwave-assisted acetalization of glycerol into Solketal was found to be an energy efficient route for glycerol valorization achieving 98% selectivity to solketal at 95% glycerol conversion during a reaction time of 15 min over Cu-Mor catalyst and acetone/glycerol at a molar ratio of 3:1. Reuse of spent catalyst showed good repeatability for up to four reaction cycles with a marginal decrease in conversion.