Renewable methane and liquid hydrocarbon production through catalytic hydrothermal decarboxylation of waste cooking oil

Hydrothermal decarboxylation is an effective method for converting triglycerides, such as waste cooking oil (WCO), into renewable liquid hydrocarbons. However, one of the major challenges in producing liquid hydrocarbons from WCO is catalyst deactivation due to coke deposition, which prevents the catalyst from actively participating in the reaction. Additionally, CO2, a greenhouse gas, is evolved during the decarboxylation reaction (s). Converting CO2 into a reusable fuel, such as CH4, can help to reduce the environmental impact and can be used for power generation, appliances, industrial applications, and transportation. Ru metal is a well-known methanation catalyst that enhances methanation reactions of carbon oxides to minimize CO2 or CO production. In this study, we proposed a one-step process for producing both liquid hydrocarbons and CH4 during hydrothermal decarboxylation of WCO using Ru-Ni-theta Al2O3 catalysts in a batch reactor system. We hypothesized that the addition of Ru (0.2 wt% and 0.5 wt%) onto a 11wt%Ni/theta Al2O3 catalyst would significantly reduce coke deposition and convert CO2 and CO into renewable CH4. Experimental results showed that 11wt%Ni/theta Al2O3 catalyst was more acidic and displayed a higher degree of decarboxylation (97 %) compared to 0.2 wt%Ru/11wt %Ni/theta Al2O3 (94 %) and 0.5wt%Ru/11wt%Ni/theta Al2O3 (92 %) catalysts. Meanwhile, 0.2wt%Ru/11wt%Ni/theta Al2O3 and 0.5wt%Ru/11wt%Ni/theta Al2O3 catalysts yielded 87.64 and 96.31 mol% of CH4 compared to 11wt%Ni-theta Al2O3 catalyst (35.32 mol%). Ru reduced acidity and increased the resistance to carbon deposition on the catalyst's surface by significantly enhancing the methanation reaction.