Glycerol Acetylation into Acetins over SnO₂-Based Bimetallic Oxide Catalyst

Tin oxide-based bimetallic oxide catalysts, MO_x (MO_x = Al₂O₃, TiO₂, Y₂O₃) with a molar ratio of MO_x:SnO₂ of 1:1 was prepared by the solid-state method. X-ray diffraction (XRD) analysis showed that tin oxide, SnO₂, was present in all samples as a cassiterite phase with a tetragonal structure. The presence of other oxides, aluminum oxide (Al₂O₃), titanium oxide (TiO₂), yttrium oxide (Y₂O₃) in the respective Al₂O₃/SnO₂, TiO₂/SnO₂, Y₂O₃/SnO₂ were also identified and confirmed by X-ray fluorescence (XRF) analysis. A small exothermic peak at 480 °C and 750 °C in the differential thermal gravimetric (DTG) analysis revealed the oxidation of sub-oxide defects, took place which are very common in SnO₂-based samples. Further Fourier transform infrared spectroscopy (FTIR) analysis indicated the presence of Sn-O bonds, reflecting the abundance of tin-oxygen bonds in the materials. Acidity measurements by titration showed that both TiO₂/ SnO₂ and Y₂O₃/SnO₂ have the same acidity value of 0.23 mmol/g whereas Al₂O₃/SnO₂ and monoxide SnO₂ at 0.20 mmol/g. These catalysts were tested for the acetylation of glycerol to monoacetin (MA), diacetin (DA) and triacetin (TA) in a batch reactor at a molar ratio of glycerol: acetic acid of 1:6 w/v, a temperature of 100 °C, a reaction time of 2 h and a catalyst loading of 0.5 g. The tests showed complete conversion of glycerol by all catalysts, with the selective product being mainly MA, which was 64%, 62%, 57% and 43% for Al₂O₃/SnO₂, Y₂O₃/SnO₂, SnO₂ and TiO₂/SnO₂, respectively. The percentage selectivity of the more valuable products, DA + TA, was produced in the following order, TiO₂/SnO₂> SnO₂ > Y₂O₃/SnO₂ > Al₂O₃/SnO₂ with 57%, 43%, 37%, and 35%, respectively. Further analysis of TiO₂/SnO₂ revealed that the catalyst not only had a high acidity, but also an increased BET surface area of 9.8 m²/g compared to 8.6 m²/g for SnO₂ when TiO₂ was incorporated into SnO₂. Analysis of the surface elements by energy dispersive X-ray spectroscopy (EDX) revealed a high surface oxygen content, which is consistent with the high acidity of the sample. (author)