Highly Promising Sulfate Ion Promoted M-ZrO 2 (M=Al 2 O 3 and CeO 2 ) Heterogeneous Solid Acids for Biodiesel-Derived Glycerol Esterification

Esterification of biodiesel-derived glycerol with acetic acid was investigated over Al2O3-ZrO2, CeO2-ZrO2, SO4 2- /Al2O3- ZrO2 and SO4 2- /CeO2-ZrO2 solid acid catalysts. The obtained glycerol esters, namely mono-, di- and tri-acylglycerols are versatile chemicals to improve both physicochemical properties of biodiesel as well as its economic profitability. The prepared solid acids were characterized by means of XRD, BET surface area, and ammonia-TPD techniques. It was found that the addition of sulfate ions significantly enhances the surface acidity of mixed oxides. Interestingly, the SO4 2- /CeO2-ZrO2 solid acid showed higher amount of super acidic sites, whereas only medium and strong acidic sites were observed in the case of SO4 2- /Al2O3-ZrO2 sample. Among various catalysts investigated, the SO4 2- /CeO2-ZrO2 catalyst exhibited excellent catalytic activity (~100% glycerol conversion) with better reusability up to three catalytic runs. The results showed that promoted zirconia-based solid acids are the promising heterogeneous catalysts for esterification of glycerol.