Impact of Impregnation pH on NiMo Surface Species in Al2O3-Supported Catalysts for Green Diesel Production

Green diesel is a high-quality biofuel obtained through the transformation of triglycerides into linear alkanes. In order to obtain green diesel, this study investigates the impact of impregnation pH on the surface species of NiMo/Al2O3 catalysts in the hydroprocessing of soybean oil. NiMo catalysts supported on Al2O3 were synthesized at different pH values (pH = 7 and 9). In the oxide state, solids were characterized by UV-Vis diffuse reflectance, Raman, and FT-IR spectroscopies, and, in the sulfide state, they were characterized by HR-TEM. The results show that the pH of impregnation significantly determines the surface species formed. An impregnation at pH = 7 favors the formation of Ni2+(Oh) and Ni2+(Oh-dis) interacting with non-crystalline molybdenum trioxide, while the formation of Ni2+/Al2O3, Ni2+(Oh-dis), and MoO3 species is favored at pH = 9. These surface species play a fundamental role in the hydrogenolysis and deoxygenation steps. Catalyst impregnated at pH = 7 shows higher activity due to the formation of shorter MoS2 slabs. This study emphasized the importance of controlling impregnation conditions for optimizing catalyst performance.