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

Abstract

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.