Catalytic Deoxygenation of Bio-Oil Model Compounds over Acid–Base Bifunctional Catalysts

An acid–base bifunctional catalyst was synthesized by treating a natural mixed-metal oxide, serpentine, with sulfuric acid. Catalyst characterization revealed that the number of acidic and basic sites increased after the acid treatment largely due to an increase in surface area. However, stronger acid sites were also introduced by the formation of bridged hydroxyl groups between a Si atom and a heteroatom, as inferred by H NMR and NH3-TPD analysis. Results from SEM-EDS and 1H NMR suggested that the acid and base sites were in close proximity. Catalytic conversions of carbohydrate-derived bio-oil model compounds were performed over different acid/base catalysts. Eight single bio-oil model compounds and two binary mixtures were used. The reactivity of the model compounds was found to be strongly correlated to the number of oxygen-containing functional groups in the reactant. The results from the binary mixtures showed that the acid–base bifunctional catalyst had the highest activity in aldol condensation reactions. The best deoxygenation performance was also observed with the bifunctional catalyst for the model compounds. Reaction pathways were proposed on the basis of an isotope labeling study. Deoxygenation reactions were found to be promoted by the cooperative catalysis between closely located acid and base sites.