Novel catalysts for advanced hydroprocessing: transition metal phosphides

Abstract

The field of catalysis is continuously invigorated by the discovery of new catalytic materials. Transition metal phosphides represent one such group of compounds that have recently been shown to have excellent activity for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). This is an important area, as continuous decline in the quality of petroleum feedstocks and increasingly stringent environmental regulations have made the removal of sulfur a paramount problem in the refining industry. The structure of the metal-rich phosphides is based on trigonal prisms, which can well accommodate the relatively large phosphorus atoms. The prisms are similar to those in sulfides, but phosphides do not take on layered structures and are metal conductors, not insulators or semiconductors. Lack of layers leads to a more isotropic crystal morphology and potentially better exposure of surface metal atoms to fluid phase reactants. Phosphides can be prepared readily in bulk and supported forms by reduction of phosphate precursors. The catalytic activity of the phosphides for dibenzothiophene HDS and quinoline HDN at 643 K and 3.1 MPa followed the order Fe2P<CoP<MoP<WP<Ni2P. The best catalyst, Ni2P/SiO2, had better activity in hydroprocessing than a commercial Ni–Mo–S/Al2O3 (HDS 98% vs 78% and HDN 80% vs 43%), based on equal sites loaded in the reactor. The sites were titrated by CO chemisorption at room temperature for the phosphides and by pulse O2 chemisorption at dry-ice acetone temperatures for the sulfide. The Ni2P/SiO2 also compared favorably with a commercial Co–Mo–S/Al2O3 catalyst (Ketjenfine 756) using a real feed at 593 K and 3.9 MPa. On the basis of equal weights loaded in the reactor, the Ni2P was again found to have higher performance than the sulfide (HDS 85% vs 80%). Studies of Ni2P/SiO2 catalysts with varying Ni/P ratios indicated that the principal phase remained Ni2P, but that the crystallite size decreased with increasing P content as a result of facilitated contact on the surface between the Ni component and the phosphorus reagent. Activity measurements on these samples indicated that HDS was structure-insensitive, but that HDN was structure-sensitive. Extended X-ray absorption fine structure measurements indicated that the surface formed a phospho sulfide after reaction. The mechanism of HDN was investigated using a series of molecular probes of varying structure. The reactivity order for pentylamines indicated that CNH2 bond scission proceeded by a β-hydride elimination mechanism, similar to that occurring on sulfides. The reactivity order of substituted piperidines indicated that ring-opening proceeded by CN bond scission and involved an α-carbon, and thus was different from the pathway on sulfides, which again appeared to involve a β-hydride elimination. This difference may account for the higher activity of phosphides over sulfides.