Pd-loaded mesoporous silica as a robust adsorbent in adsorption/desorption desulfurization cycles

Desulfurization of benzothiophene/isooctane mixtures and gasoline (42.5 and 1000 ppmw S) was studied by batch and fixed bed experiments using PdCl2 supported on mesoporous silica of SBA-15 type (PdCl2/SBA-15). The Pd-loaded silica was prepared by solid-phase impregnation of the metal halide on the mesoporous siliceous matrix (SBA-15). SBA-15 and PdCl2/SBA-15 were characterized with N2 adsorption/desorption isotherms, small angle X-ray diffraction and transmission electron microscopy (TEM). Then desulfurization of benzothiophene/isooctane mixtures and gasoline (42.5 ppmw S) with these sorbents was investigated by batch and fixed bed experiments. PdCl2/SBA-15 showed much higher capacity for sulfur adsorption than both the starting material and PdCl2 itself. A mathematical model was used to represent the breakthrough curves for adsorption and desorption runs in fixed bed, providing very good agreement between numerical simulations and adsorption data. The spent PdCl2/SBA-15 was regenerated using isooctane as eluent, and the regenerated PdCl2/SBA-15 was tested again for the desulfurization of 1000 ppm S gasoline (down to 50 ppm). The results showed that the sulfur adsorption capacity at the break point (C = 50 ppm) remained nearly unchanged for three cycles, even though XPS elemental analysis for the fresh and spent adsorbent revealed some degree of leaching of the metal from the external surface, particularly as Pd(II). In conclusion, PdCl2/SBA-15 appears to be a robust adsorbent for desulfurization of liquid fuels and the model proposed herein was able to predict deep desulfurization of gasoline in cyclic mode.