Modification of the preparation procedure for increasing the hydrodesulfurisation activity of the CoMo/γ-alumina catalysts

In this work we have examined whether the re-impregnation of CoMo/γ-alumina catalysts or the replacement of the conventional non-dry impregnation step by “equilibrium deposition filtration” (EDF) may be used for improving their surface characteristics and thus their catalytic activity. Two samples were prepared. In the first sample (EDF) the molybdenum species were mounted by “equilibrium deposition filtration” whereas in the second sample these species were mounted by non-dry impregnation (NDI). In both cases the Co was deposited on the calcined Mo/γ-Al2O3 precursor solid by simple dry impregnation. An aliquot of each sample was impregnated again with an amount of pure water equal to its pore volume and then it underwent drying and calcination. The catalysts prepared were characterized using N2 adsorption measurements (BET), UV–vis diffuse reflectance spectroscopy (DRS), laser Raman spectroscopy (LRS) and NO chemisorption. The hydrodesulfurization (HDS) activities over the catalysts studied were determined using a continuous-flow tubular fixed-bed microreactor operating in a differential mode at atmospheric pressure. It was confirmed that the replacement of the conventional impregnation by equilibrium deposition filtration results to catalysts with relatively high active surface and high portion of the well-dispersed octahedral cobalt and thus, to catalysts with 30% higher HDS activity. The re-impregnation resulted to partial dissolution and re-dispersion of the Mo and Co supported oxidic phases. Concerning the NDI catalyst re-impregnation resulted to an increase of the active surface and of the portion of the well-dispersed octahedral cobalt and thus to 25% higher catalytic activity. The opposite effects were observed for the EDF catalyst which exhibited almost 7% lower activity after re-impregnation.