Enhanced oxidative desulfurization over silica-gel-immobilized heteropoly acids

The development of efficient oxidative desulfurization catalysts is essential for producing clean fuels under mild conditions. In this work, phosphomolybdic (PMo) and phosphotungstic (PW) heteropoly acids were immobilized onto silica gel supports by ion-exchange methods and compared with analogous Silochrom-supported systems. The catalysts were characterized using SALDI mass spectrometry, SEM, and EDX spectroscopy to assess the structural integrity of the heteropoly anions and the uniformity of their distribution on the support surface. Catalytic activity was evaluated in the oxidation of model sulfur compounds — thiophene, dibenzothiophene (DBT), and methyl phenyl sulfide — as well as in a straight-run diesel fraction (1080 ppm S) using hydrogen peroxide as the oxidant. Both catalysts achieved maximum conversion of model fuel sulfur compounds (MFSCs) within the first hour of reaction, with thiophene conversions of 69–70% and DBT conversions of 76–78%. Optimal results were obtained at an H₂O2:S ratio of 20:1 and 60°C, and sequential oxidant addition further improved thiophene conversion. Importantly, silica gel–supported catalysts displayed greater stability over five consecutive cycles compared to Silochrom analogues, with consistent MFSC conversions of 100%.