A series of supported NiMo catalysts were prepared by impregnating ammonium heptamolybdate and nickel nitrate (at 12 and 3 wt% of Mo and Ni, respectively) over chitosan (Chi) modified Al2O3 carrier. Alumina substrate was first impregnated with the organic additive (in acidic aqueous HNO3 solution, given the insolubility of chitosan at neutral pH) at concentrations corresponding to Chi/Ni mol ratios of 0.5, 1 and 2. Lower Mo dispersion in materials prepared over Chi-modified alumina was observed by Raman spectroscopy. Also, in those samples formation of hardly reducible aluminum molybdates (due to acidic conditions used during chitosan deposition) was evidenced by thermal analysis. Sulfided (under H2S/H2 flow, at 400 °C) catalysts were tested in dibenzothiophene hydrodesulfurization (HDS, at 72.4 kg/cm2, 320 °C, batch reactor, n-hexadecane as solvent) where no improved activity (in pseudo first order kinetic constant basis) was registered for chitosan-modified samples. However, increased selectivity to the product from direct desulfurization route (biphenyl) suggested enhanced promotion of MoS2 phase (by Ni) in catalysts prepared with organic additive. CO adsorption at -173 °C (followed by infrared spectroscopy) showed lower concentration of NiMoS active sites over catalysts prepared over chitosan-impregnated alumina carrier pointing out to the existence of highly efficient sites, in spite of their lower surface concentration. The present investigation opens the possibility of using chitosan, a sub-product from seafood industry, as efficient HDS catalyst additive.

Chitosan-added NiMoP catalysts supported on alumina and alumina-titania were studied in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The preparation of catalysts containing Mo (12 wt%), Ni (3 wt%), P (1.6 wt%), and chitosan/nickel = 2 (mol ratio) was accomplished by sequential pore-filling impregnation varying the order of chitosan integration. Materials were characterized by DRIFTS, TPR, TG-DTA, and XPS techniques. The TG-DTA study showed that the nature of the support influences the degradation of chitosan onto the catalytic materials and also influences the HDS of DBT and the product distribution as well. The series of catalysts supported on alumina presented the most remarkable effect of chitosan, in which the OH and NH groups of the organic molecule interact with acid sites of the support weakening the interaction between alumina and deposited metal phases. In all cases, DBT was converted mainly through direct sulfur removal. The catalysts ChP3/A (alumina support impregnated with chitosan in phosphoric acid solution, prior to NiMoP deposition) and ChP4/AT (alumina-titania support impregnated with NiMoP solution, prior to contacting with a solution comprising chitosan and phosphorus) exhibited the best performance in HDS reactions and also showed the highest selectivity in biphenyl formation. Presence of carbonaceous residua on the catalyst’s surface, as shown by XPS, could enhance the HDS activity over the ChP4/AT sample.