A Review of Recent Research on Catalytic Biomass Pyrolysis and Low-Pressure Hydropyrolysis

Direct catalytic upgrading of biomass-derived fast pyrolysis vapors can occur in different process configurations, under either inert or hydrogen-containing atmospheres. This review summarizes the myriad of different catalysts studied and benchmarks their deoxygenation performance by also taking into account the resulting decrease in bio-oil yield compared to a thermal pyrolysis oil. Generally, catalyst modifications aim at improving the initial selectivity of the catalyst to more desirable oxygen-free hydrocarbons and/or improving the catalysts’ stability against deactivation by coking. Optimizing the pore structure and acid site density/distribution of solid acid catalysts can slow down deactivation and prolong activity. Basic catalysts such as MgO and Na₂O/γ-Al₂O₃ are excellent ketonization catalysts favoring oxygen removal via decarboxylation, whereas solid acid catalysts such as zeolites primarily favor decarbonylation and dehydration. Basic catalysts can therefore produce bio-oils with higher H/C ratios. However, since their coke formation per surface area is higher, compared to a microporous HZSM-5 zeolite, precoking (or imperfect regeneration) of these basic catalysts and operating for longer time-on-stream can be approaches to improve the oil yield. In-line vapor-phase upgrading with a dual bed comprising a solid acid catalyst followed by a basic catalyst active in ketonization and aldol condensation further improves deoxygenation while maintaining high bio-oil carbon recovery. Also low-cost catalysts such as iron-rich red mud have deoxygenation activity. An improved bio-oil carbon recovery─compared at a similar level of oxygen removal─can be obtained when changing from an inert atmosphere to a hydrogen-containing atmosphere and using an effective hydrodeoxygenation (HDO) catalyst. To keep costs low, this can be conducted at near-atmospheric pressure conditions. Pt/TiO₂ and MoO₃/TiO₂ showed high activity and reduced coke formation. Stable performance has been demonstrated using Pt/TiO₂ for 100+ reaction/regeneration cycles with woody biomass feedstock. If future works can demonstrate the same durability for lower-cost biomass containing higher contents of ash, N, and S, this would considerably boost the commercial viability of near-atmospheric pressure HDO. Further research should be directed to testing the durability of lower-cost HDO catalysts such as MoO₃/TiO₂ and further improving the activity and stability of lower-cost catalysts.