Catalytic deoxygenation waste cooking oil into green diesel range hydrocarbons in a continuous processing over monolayered CoS-MnS supported Al₂S₃ catalyst

The drive towards using carbon neutral fuel sources is based on the need for improved catalytic processes to convert the hydrocarbons in waste derived lipids into quality hydrocarbon products. In this paper, we have developed a CoS-MnS/Al catalyst capable of performing a hydrogen-free deoxygenation process on waste cooking oil (WCO) at exceptionally high productivities, selectivities and stabilities. Structural characterization of the catalyst has been performed through a range of methods including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N₂ adsorption/desorption (BET), ammonia temperature-programmed desorption (NH₃-TDP) and electron paramagnetic resonance (EPR). These analyses demonstrated that the catalyst forms a unique bifunctional heterostructure composed of three distinct phases; Co3S4, MnS and an interface phase composed of Al2S3. The optimized acid-base properties of the surface along with electronic strain effects associated with the sulfur edge sites allow modulation of all possible C-O bond activation pathways. Optimization of the composition of Co and Mn created a balance of acidity across the catalyst's surface that directed the reactivity to both cooperative hydrodeoxygenation and decarboxylation-decarbonylation reactions resulting in highly effective conversion of triglycerides and free fatty acids to long chain n-paraffins. Under continuous flow conditions and mild temperatures (approximately 300 °C), the CoS0.5MnS0.5/Al catalyst achieved approximately 89% hydrocarbon yield with approximately 93% selectivity to C₁₅-C₁₇ diesel-range n-paraffin molecules while demonstrating structural integrity after several reaction cycles. Kinetics were also examined as they relate to the competitive nature of the decarbonylation of ester vs. decarboxylation of carboxylic acid, illustrating how reaction kinetics are influenced by temperature, space velocity and gas dynamics, and thus highlighting the impact of residence time, diffusion and access to active site.