Selective Deoxygenation of Waste Cooking Oil to Diesel-Like Hydrocarbons Using Supported and Unsupported NiMoS2 Catalysts

Abstract

This work aimed to study the deoxygenation of two different waste cooking oils (WCOs; palm oil and soybean oil) using alumina (γ-Al2O3)-supported and unsupported NiMoS2 catalysts prepared by the hydrothermal method. The variables evaluated in this study were the reactant concentration, reaction time, and nickel (Ni)/[Ni + molybdenum (Mo)] atomic ratio (0.2 and 0.3) affecting the yield and selectivity of alkane products. The supported NiMo sulfide (NiMoS2)/γ-Al2O3 catalyst prepared by impregnation had the drawback of a lack of layers and stacks, so combining the γ-Al2O3 with unsupported NiMoS2 catalysts using a hydrothermal method was evaluated. The main products obtained from the deoxygenation of the two WCOs were normal (n-)alkane compounds (C15, C16, C17, and C18). The catalyst efficiency was ranked as 0.2-NiMoS2/γ-Al2O3 ≈ 0.2-NiMoS2 > 0.3-NiMoS2/γ-Al2O3 ≈ 0.3-NiMoS2. The catalyst that gave the high n-C15-C18 yield was 0.2-NiMoS2/γ-Al2O3 under a reaction condition of 300 °C, 40 bar initial H2 pressure, and oil concentration of 5 wt %. For the hydrodeoxygenation (HDO) of waste palm oil, the n-C14-C18 yield was 56.4% (C14, C15, C16, C17, and C18 at 1.3, 6.7, 14.5, 11.8, and 22.1%, respectively), while that for the waste soybean oil was 58% (C14, C15, C16, C17, and C18 at 1.1, 3.8, 6.7, 17.2, and 29.2%, respectively). The n-C18/n-C17 and n-C16/n-C15 ratios were both greater than 1 for both types of WCO, revealing that the deoxygenation mainly proceeded via HDO rather than decarbonylation and decarboxylation. The 5-10% lower n-C14-C18 yield from the waste oil compared with the fresh oil was acceptable, implying the effective oil treatment and some impurity removal.