Surface-Dependent Activation of Model α-Al2 O3 -Supported P-Doped Hydrotreating Catalysts Prepared by Spin Coating

Requirements for improved catalytic formulations is continuously driving research in hydrotreating (HDT) catalysis for biomass upgrading and heteroatom removal for cleaner fuels. The present work proposes a surface-science approach for the understanding of the genesis of the active (sulfide) phase in model P-doped MoS₂ hydrotreating catalysts supported on α-Al₂ O₃ single crystals. This approach allows one to obtain a surface-dependent insight by varying the crystal orientations of the support. Model phosphorus-doped catalysts are prepared via spin-coating of Mo-P precursor solutions onto four α-Al₂ O₃ crystal orientations, C(0001), A(11 2 ‾ 0), M(10 1 ‾ 0) and R(1 1 ‾ 02) that exhibit different speciations of surface -OH. ³¹ P and ⁹⁵ Mo liquid-state NMR are used to give a comprehensive description of the Mo and P speciation of the phospho-molybdic precursor solution. The speciation of the deposition solution is then correlated with the genesis of the active MoS₂ phase. XPS quantification of the surface P/Mo ratio reveal a surface-dependent phosphate aggregation driven by the amount of free phosphates in solution. Phosphates aggregation decreases in the following order C(0001)≫M(10 1 ‾ 0)>A(11 2 ‾ 0), R(1 1 ‾ 02). This evolution can be rationalized by an increasing strength of phosphate/surface interactions on the different α-Al₂ O₃ surface orientations from the C(0001) to the R(1 1 ‾ 02) plane. Retardation of the sulfidation with temperature is observed for model catalysts with the highest phosphate dispersion on the surface (A(11 2 ‾ 0), R(1 1 ‾ 02)), suggesting that phosphorus strongly intervene in the genesis of the active phase through a close intimacy between phosphates and molybdates. The surface P/Mo ratio appears as a key descriptor to quantify this retarding effect. It is proposed that retardation of sulfidation is driven by two effects: i) a chemical inhibition through formation of hardly reducible mixed molybdo-phosphate structures and ii) a physical inhibition with phosphate clusters inhibiting the growth of MoS₂ . The surface-dependent phosphorus doping on model α-Al₂ O₃ supports can be used as a guide for the rational design of more efficient HDT catalysts on industrial γ-Al₂ O₃ carrier.

Effect of Mono-, Di-, and Triethylene Glycol on the Activity of Phosphate-Doped NiMo/Al2O3 Hydrotreating Catalysts

The effect of glycols on the catalytic properties of phosphate-doped NiMo/Al2O3 catalysts in the hydrotreating of straight-run gas oil (SRGO) was studied. The NiMo(P)/Al2O3 catalysts were prepared using ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG) as additives. The organic agent was introduced into the aqueous impregnation solution obtained by the dissolving of MoO3 in H3PO4 solution, followed by Ni(OH)2 addition. The Raman and UV–Vis studies show that the impregnation solution contains diphosphopentamolybdate HxP2Mo5O23(6−x)− and Ni(H2O)62+, and that these ions are not affected by the presence of glycols. When the impregnation solution comes in contact with the γ-Al2O3 surface, HxP2Mo5O23(6−x)− is decomposed completely. The catalysts were characterized by Raman spectroscopy, low-temperature N2 adsorption, X-ray photoelectron spectroscopy, and transmission electron microscopy. It is shown that the sulfide catalysts prepared with glycols display higher activity in the hydrotreating of straight-run gas oil than the NiMoP/Al2O3 catalyst prepared without the additive. The hydrodesulfurization and hydrodenitrogenation activities depend on the glycol type and are decreased in the following order: NiMoP-DEG/Al2O3 > NiMoP-EG/Al2O3 > NiMoP-TEG/Al2O3 > NiMoP/Al2O3. The higher activity of NiMoP-DEG/Al2O3 can be explained with the higher dispersion of molybdenum on the surface of the catalyst in the sulfide state.