Importance of pore size and Lewis acidity of $Pt/Al_2O_3$ for mitigating mass transfer limitation and catalyst fouling in triglyceride deoxygenation

Abstract

The deoxygenation of triglycerides via hydrotreating is a promising route for producing hydrocarbon fuels from renewable biomass. Because triglycerides are bulky esters possessing multiple C=C bonds, mass transfer limitation and coke deposition within the pore space of catalysts are significant problems. In this study, the catalytic effects of porous structure and surface acidity of $Pt/Al_2O_3$ were rigorously investigated in triglyceride deoxygenation. Pt was supported on a series of $Al_2O_3$ materials with various crystal structures (γ, θ, and α), pore sizes (3.3–28.6 nm), and Lewis acid amounts (0–337 µmol g-1), which were prepared by sol–gel synthesis. The γ- and θ-$Al_2O_3$ phases containing appreciable pentacoordinated Al sites effectively stabilized highly dispersed Pt particles (1.3–1.5 nm) via strong metal–support interactions and exhibited high catalytic activities. Conversely, Pt on α-$Al_2O_3$ without pentacoordinated Al sites exhibited very poor dispersion (> 12.6 nm), resulting in low catalytic activity. The results also showed that large mesopores (pore diameter > ~12 nm) were essential to obtain maximum catalytic activity without mass transfer limitation and to suppress catalyst deactivation by fouling. When comparing the γ- and θ-$Al_2O_3$ phases, the latter generally showed inhibited formation of heavy products and coke owing to its larger pore size and smaller amount of Lewis acid sites that can catalyze undesirable oligomerization of unsaturated fatty acids. Consequently, Pt on θ-$Al_2O_3$ with extra-large mesopores (pore diameter: 28.6 nm) simultaneously exhibited the highest deoxygenation activity, selectivity toward diesel-range paraffins, and long-term stability.