Use of zirconium phosphate-sulphate as acid catalyst for synthesis of glycerol-based fuel additives

In the present work, zirconium phosphates and mixed zirconium phosphate-sulphateacid catalysts have been investigated in the acetylation of glycerol in order to obtain acetins as fueladditives. The following catalysts with chemical composition, Zr3(PO4)4, Zr(SO4)2, Zr2(PO4)2SO4,Zr3(PO4)2(SO4)3 and Zr4(PO4)2(SO4)5 have been prepared and characterized by acid capacitymeasurements, BET, XRD, FT-IR, XPS. The surface chemical composition in terms of P/Zr and S/Zratomic ratios was monitored in the fresh and used catalysts. Zr3(PO4)2(SO4)3 and Zr4(PO4)2(SO4)5showed the highest acidity associated with the synergic effect of two main crystalline phases,Zr2(PO4)2SO4 and Zr(SO4)24H2O. The reactions of glycerol acetylation were carried out by using amass ratio of catalyst/glycerol equal to 5 wt% and molar ratio acetic acid/glycerol equal to 3:1. Theglycerol conversion versus time was investigated over all the prepared samples in order to identifythe best performing catalysts. Over Zr3(PO4)2(SO4)3 and Zr4(PO4)2(SO4)5 full glycerol conversionwas achieved in 1 h only. Slightly lower conversion values were registered for Zr3(PO4)4 andZr2(PO4)2SO4, while Zr(SO4)2 was the worst catalyst. Zr4(PO4)2(SO4)5 was the most selective catalystand was used for recycling experiments up to five cycles. Despite a modest loss of activity, a drasticdecrease of selectivity to tri- and diacetin was observed already after the first cycle. This finding wasattributed to the leaching of sulphate groups as detected by XPS analysis of the spent catalyst.