Development of nickel catalysts supported on silica for green diesel production

Two series of nickel catalysts supported on silica were prepared and evaluated for the selective deoxygenation of sunflower oil into green diesel, under conditions without solvent. The catalysts were characterized with various techniques (N2 physisorption, CO chemisorption, XRD, SEM-EDS, TEM, H2-TPR, NH3-TPD). The first series involves catalysts with Ni content ranging between 10 and 60 wt%, synthesized by successive dry impregnation. The green diesel in the liquid product seems to depend mainly on the active nickel surface and the moderate acidity, following a volcano trend which is maximized over the sample with 50 wt% Ni loading. The second series concerns catalysts with 50 wt% Ni, synthesized by the use of four different techniques: Successive Dry Impregnation (SDI), Wet Impregnation (WI), using Ni(en)3(ΝΟ3)2 (en: ethylene diamine) as the Ni precursor and Deposition – Precipitation at room temperature (DP-NH3), Deposition – Precipitation at high temperature (DP-Urea), using Ni(ΝΟ3)2 as the Ni precursor. SDI or WI results mainly to granular nickel supported nanoparticles, whereas DP-NH3 and mainly DP-Urea to the formation of filamentous structures of a nickel phyllosilicate phase. The performance of the catalysts towards the production of green diesel follows the order WI>SDI> DP-NH3 >DP-Urea. The catalyst synthesized by WI exhibits the higher active surface in combination with high population of sites of moderate acidity. The catalysts prepared by DP, although exhibiting a very high surface area and a very high nickel dispersion, did not appear more effective in the n-alkanes production, due to the formation of a very well dispersed nickel phyllosilicate phase, but not fully reducible even at very high temperatures and thus inactive in the selective deoxygenation of sunflower oil.