Effective B2O3 modified Ni/Al2O3 co precipitated catalysts for waste cooking oil transformation into green diesel

The replacement of fossil fuels with biofuels produced from renewable sources is imperative nowadays, as the latter reduce some undesirable aspects of fossil fuels production and use. More precisely, biofuels don't contain S, N, or metal substances, which lead to the production of conventional pollutants, and contribute almost zero to the emission of greenhouse gases causing global warming and climate change. Biodiesel, as a biofuel, is already widely used despite the disadvantages it presents due to the presence of oxygen in its composition. On the other hand, green diesel or hydrogenated vegetable oil (HVO) is an oxygen free drop in fuel, produced via hydrotreatment of triglyceride feedstock. Conventional hydrotreatment catalysts (NiMoS and CoMos) used for oil upgrading have been extensively studied also for HVO production. However, their performance is not stable in case that a sulfur containing compound is not added in the feed. To overcome this difficulty, research has turned towards metallic catalysts for the aforementioned process. Noble metal catalysts have proved very active, but their cost is prohibited for industrial application. Some pioneer groups have tried to develop such catalysts based on base transition metals (e.g. Ni, Co, Fe, Cu, etc.). In the frame of this effort, our group has focused its recent research on the development of Ni based catalysts. The simultaneous achievement of high activity, selectivity for hydrocarbons in the diesel range with small amounts of branched hydrocarbons for obtaining good pour properties together with high catalyst stability and low hydrogen demand were our main targets. Considerable progress has been pointed out towards these ambitious goals. It seems that catalysts with high surface/active metal area, medium acidity and mesoporous structure and suitable formulation are very promising. The role of metallic and metal oxide promoters in metallic nickel catalysts used for HVO production has been recently reviewed by our group. Boron is a non-metallic promoter usually used for transition metal-based catalysts (Ni, Cu etc.) as it alters the electronic environment of metal ions enhancing metal dispersion and coking resistance. In this work a series of Ni/Al2O3 catalysts with constant Ni loading (57.5 wt %) doped with various amounts of B2O3 (0-5 wt. %) were studied for the transformation of waste cooking oil (wco) into HVO. The catalysts were thoroughly characterized by XRD, XPS, N2-physisorption, H2- TPR, NH3-TPD, CO-chemisorption, TEM, TGA, and evaluated in a semi-batch high pressure reactor. A catalyst containing 2.5 % B2O3 proved the most effective one among the studied catalysts. This catalyst exhibited the highest active surface (metallic Ni), pore volume and acidity.
Acknowledgment: This paper has been financed by the funding program “MEDICUS”, of the University of Patras