Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for Acetalization and Ketalization of Glycerol towards Biofuel Additives

WOx-promoted TiO2–ZrO2 solid acid catalyst was prepared and applied in the catalytic acetalization and ketalization of glycerol with carbonyl compounds to produce biofuel additives. The presence of WOx promoter and TiO2 remarkably improved the catalytic activity of ZrO2. Approximately, 100% glycerol conversion was evidenced with non-bulky aliphatic aldehydes and ketones like, propanol and cyclohexanone. The physical characterization of WOx-promoted TiO2–ZrO2, revealed a higher formation of tetragonal crystalline phase of ZrO2, over monoclinic. The total surface acidity and the ratio of Bronsted to Lewis acidic site concentrations were determined by NH3-TPD and pyridine-chemisorbed FTIR spectroscopy, respectively. A considerably higher concentration of Lewis acidic sites, ~ 213.29 μmol/gm, was evidenced on the WOx-promoted TiO2–ZrO2 catalyst surface. Catalytic activity study revealed a direct correlation between the surface Lewis acidic site concentration and the activity of catalyst. This significant observation indicated the key role of Lewis acidic sites in this catalytic process. The WOx-promoted TiO2–ZrO2 catalyst was also considerably stable and showed good performance in the acetalization/ketalization of glycerol with other substituted carbonyl compounds. The WOx-promoted TiO2–ZrO2 solid acid catalyst exhibits superior catalytic performance for acetalization and ketalization of glycerol with carbonyl compounds to produce biofuel additives.