Recent advances in heterogeneous catalysts for bio-oil upgrading via “ex situ catalytic fast pyrolysis”: catalyst development through the study of model compounds

Advances in heterogeneous catalysis are driven by the structure–function relationships that define catalyst performance (i.e., activity, selectivity, lifetime). To understand these relationships, cooperative research is required: prediction and analysis using computational models, development of new synthetic methods to prepare specific solid-state compositions and structures, and identification of catalytically active site(s), surface-bound intermediates, and mechanistic pathways. In the application of deoxygenating and upgrading biomass pyrolysis vapors, a fundamental understanding of the factors that favor C–O bond cleavage and C–C bond formation is still needed. In this review, we focus on recent advances in heterogeneous catalysts for hydrodeoxygenation of biomass pyrolysis products. Focus is placed on studies that made use of model compounds for comparisons of catalysts and the reaction networks they promote. Applications of transition metal sulfide catalysts for deoxygenation processes are highlighted, and compared to the performances of noble metal and metal carbide, nitride, and phosphide catalysts. In general, it is found that bifunctional catalysts are required for deoxygenation in a single reactor, with bifunctionality achieved on the catalyst or in conjunction with the catalyst support. Catalysts that activate hydrogen well will be preferred for ex situ catalytic pyrolysis conditions (upgrading downstream of pyrolysis reactor prior to condensation of bio-oil, pressures near atmospheric, temperatures between 350–500 °C). Supports that limit chemisorption of large reactants (leading to blockage of catalyst sites) should be employed. Finally, the stability of the catalyst and support in high-steam and low hydrogen-to-carbon environments will be critical.