The Influence of NiMoS Catalyst Composition on the Hydrogenation and Dimerization Kinetics of Conjugated Olefins

The selective hydrogenation of olefinic species in hydrotreated streams is often limited by concurrent acid-catalyzed oligomerization. To clarify the interplay between metallic and acidic functions, the hydrogenation and dimerization of 4-methylstyrene were investigated over sulfided NiMo catalysts supported on silica–alumina (10 wt % SiO2/Al2O3) under conditions representative of Canadian oil sands naphtha hydrotreating (200 °C, 3.4 MPa H2, LHSV = 4 h–1). The effects of molybdenum (3–15 wt %), nickel (1.1–3.3 wt %), and phosphorus (0–4 wt %) loadings were systematically examined. Catalysts were characterized by N2 physisorption, XRD, HRTEM, HAADF-STEM with multielement mapping (Ni, Mo, S, and P), and NH3-TPD. XRD showed no bulk crystalline MoS2 or phosphate phases, while HRTEM revealed few-layer MoS2 nanoslabs (2–3 layers) with composition-dependent slab length and edge site density. Elemental mapping confirmed the homogeneous spatial distribution of Ni, Mo, S, and P, indicating the formation of a uniformly sulfided Ni-promoted MoS2 phase without segregation. Catalysts with comparable acidity but varying Mo content demonstrated that 7 wt % Mo maximized hydrogenation activity, whereas 10 wt % Mo increased dimer formation. Phosphorus addition up to 2 wt % enhanced acidity and MoS2 dispersion, improving hydrogenation efficiency, while higher P loadings reduced dispersion and favored dimerization. Kinetic modeling using power-law expressions with a deactivation term enabled normalization of initial rates by active site density, revealing that hydrogenation scales with the MoS2 edge density, whereas dimerization correlates with accessible acid sites. The strong correlation between both rates demonstrates that the two pathways are intrinsically coupled within the investigated catalyst design space.