Improving the Catalytic Performance of Keggin [PW₁₂O₄₀]^3- for Oxidative Desulfurization: Ionic Liquids versus SBA-15...

Different methodologies were used to increase the oxidative desulfurization efficiency of the Keggin phosphotungstate [PW₁₂O₄₀]^3- (PW₁₂). One possibility was to replace the acid proton by three different ionic liquid cations, forming the novel hybrid polyoxometalates: [BMIM]₃PW₁₂ (BMIM as 1-butyl-3-methylimidazolium), [BPy]₃PW₁₂ (BPy as 1-butylpyridinium) and [HDPy]₃PW₁₂ (HDPy as hexadecylpyridinium. These hybrid Keggin compounds showed high oxidative desulfurization efficiency in the presence of [BMIM]PF₆ solvent, achieving complete desulfurization of multicomponent model diesel (2000 ppm of S) after only 1 h, using a low excess of oxidant (H₂O₂/S = 8) at 70 °C. However, their stability and activity showed some weakness in continuous reused oxidative desulfurization cycles. An improvement of stability in continuous reused cycles was reached by the immobilization of the Keggin polyanion in a strategic positively-charged functionalized-SBA-15 support. The PW₁₂@TM⁻SBA-15 composite (TM is the trimethylammonium functional group) presented similar oxidative desulfurization efficiency to the homogeneous IL⁻PW₁₂ compounds, having the advantage of a high recycling capability in continuous cycles, increasing its activity from the first to the consecutive cycles. Therefore, the oxidative desulfurization system catalyzed by the Keggin-type composite has high performance under sustainable operational conditions, avoids waste production during recycling and allows catalyst recovery.