Phosphorus promotion in nickel-molybdenum/alumina catalysts: model compound reactions and gas oil hydroprocessing

Abstract

Three different preparation methods for Ni-Mo/Al 2O 3 catalysts containing between 0 and 7.0 wt.-% phosphorus are compared in thiophene hydrodesulphurization (HDS) and diisopropylbenzene (DIPB) hydrocracking reactions. A promotional effect of phosphorus is seen in thiophene HDS regardless of preparation method, with maximum activity at around 1 wt.-% phosphorus. P/Al 2O 3 catalysts (P = 0–7 wt.-% ) have little activity in DIPB hydrocracking indicating that phosphorus has very little effect on the Brønsted acidity of alumina. An increase in DIPB hydrocracking is observed for P-Ni-Mo catalysts with low phosphorus loadings, however, and so phosphorus does have an indirect effect on the Brønsted acidity of these catalysts. Impregnation of phosphorus prior to the metal species leads to the most active catalysts for both thiophene HDS and DIPB hydrocracking and this is explained in terms of a dual promotional effect of phosphorus; phosphorus increases both the dispersion and reducibility/sulphidibility of molybdate and increases the amount of nickel available to form the Ni-Mo-S active HDS phase by impeding nickel aluminate formation. One series of catalysts was investigated in the hydroprocessing of a Syncrude combined gas oil-fuel oil, and the same gas oil spiked with 5 vol.-% quinoline. The promotional effect of phosphorus in HDS was in good agreement with the model compound studies: 1 wt.-% phosphorus was the most active HDS catalyst in all cases. Furthermore, phosphorus loadings of between 1 and 3 wt.-% gave a product with lowest nitrogen content, and aromaticity. These loadings also gave the greatest sulphur removal and maintained good hydrogenation in the presence of quinoline, and gave the best quinoline hydrodenitrogenation, suggesting optimum surface acidity and hydrogenation properties. Optimum loadings of phosphorus (around 3 wt.-%) also improve the longevity of the catalyst by maintaining high surface area during the hydroprocessing reaction.