Carbazole hydrodenitrogenation over nickel phosphide and Ni-rich bimetallic phosphide catalysts

•Nickel-based phosphide catalysts out-performed a commercial Ni–Mo catalyst for carbazole HDN.•Nickel phosphide catalysts exhibited excellent stability in a carbazole/benzothiophene feed.•Nickel-based phosphide catalysts favored ring-opened and ring-contracted products.•Ni2P produced more of the partially hydrogenated hydrocarbon cyclohexylbenzene than a Ni–Mo catalyst. Silica-supported nickel phosphide (Ni2P) and Ni-rich bimetallic phosphide catalysts were investigated for the hydrodenitrogenation (HDN) of carbazole with and without a benzothiophene co-feed. The Ni2P/SiO2, Co0.1Ni1.9P/SiO2, and Fe0.03Ni1.97P/SiO2 catalysts exhibited high carbazole HDN activities and out-performed a commercial sulfided Ni–Mo/Al2O3 catalyst under the testing conditions employed. Co-feeding of benzothiophene inhibited carbazole HDN over the metal phosphides, but the Ni2P/SiO2 and Ni-rich bimetallic phosphide catalysts maintained higher activities than the sulfided Ni–Mo/Al2O3 catalyst. The product selectivities of the metal phosphide and sulfide catalysts were similar, except that the sulfided Ni–Mo/Al2O3 catalyst yielded more hydrogenated carbazoles while the metal phosphide catalysts produced more of the partially hydrogenated hydrocarbon cyclohexylbenzene. The metal phosphide catalysts favored ring-opened (hexylcyclohexane) and ring-contracted ((2-methylcyclopentyl)cyclohexane) products, indicating the presence of metal and Brönsted acid sites on the catalyst surfaces. The Ni2P/SiO2 and bimetallic phosphide catalysts exhibited excellent stabilities in the HDN conditions employed (carbazole-only and mixed feeds), as indicated by phase purity, average crystallite size and resistance to sulfur incorporation of the supported metal phosphide particles.