Sustainable Desulfurization Processes Catalyzed by Titanium-Polyoxometalate@TM-SBA-15

A titanium-polyoxometalate with a µ-hydroxo dimeric structure [(PW₁₁O₃₉Ti)₂OH]⁷⁻ ((PW₁₁Ti)₂OH) was used efficiently for the desulfurization of a model diesel containing a mixture of various refractory sulfur compounds present in real fuels. The catalytic performance of the µ-hydroxo dimeric compound was compared in its homogeneous form and also when immobilized in the trimethylammonium-functionalized SBA-15 ((PW₁₁Ti)₂OH@TM-SBA-15). An optimization study was performed using the homogeneous and heterogeneous catalysts to obtain high catalytic efficiency, sustainability and cost-effectiveness of the system. Different optimized conditions were found using the homogeneous and heterogeneous catalysts. Lower amounts of solvent extraction (MeCN, 175 µL), catalyst (0.5 µmol of active center) and oxidant (50 µL) were used to produce sulfur-free model diesel after 2 h at 70 °C, using the heterogeneous (PW₁₁Ti)₂OH@TM-SBA-15 catalyst. On the other hand, complete desulfurization was achieved with homogeneous (PW₁₁Ti)₂OH catalyst after only 40 min, although higher amounts of MeCN (750 µL), catalyst (1.5 µmol) and oxidant (75 µL) were used. Both systems combined liquid–liquid extraction and catalytic oxidation. Using the heterogeneous catalyst, adsorption of oxidized-sulfur compound was also observed. Both homogeneous and heterogeneous desulfurization systems presented a high capacity to be reused/recycled for consecutive desulfurization cycles.