Catalytic vapor phase upgrading of sawdust pyrolysis using metal oxide catalysts: The support effect

Abstract

Biocrude derived from biomass is unstable and has limited applications and vapor-phase upgrading of biomass pyrolysis over a catalyst can enhance bio-oil quality and yield, yet catalyst deactivation due to coke formation remains a major challenge. This study explores the effects of different magnesium oxide (MgO) catalysts supported on ZSM-5, Al₂O₃, ZrO₂, TiO₂ on the pyrolysis product yield, composition, and the extent of coke formation. Studies were conducted in a fixed-bed reactor at 550 °C at two different catalyst to biomass loadings (C/B) of 1:6, and 1:1. Non catalytic pyrolysis of sawdust resulted in the biocrude, gas, and char yields of ∼32 wt%, ∼29 wt%, and ∼39 wt% respectively. At a low C/B ratio, pyrolysis product yields were approximately comparable to those under non-catalytic conditions. However, at a high C/B ratio, biocrude yield decreased significantly with increase in gas yield, while char yield showed only minor changes. Using ZSM-5 and Al₂O₃ supported catalysts, low C/B ratios significantly increased phenolic content in the biocrude (∼58%). In contrast, higher C/B ratios enhanced the hydrocarbons (∼10%) and aromatics (∼32%) content, reducing phenolics and thereby lowering the biocrude's overall oxygen content, enhancing its stability. TiO₂ and ZrO₂ supports produced higher proportions of lower carbon chain length compounds, though with reduced hydrocarbon content compared to ZSM-5. ZSM-5 supported catalysts yield a high proportion of aromatic compounds, attributed to the synergy between the acidic sites of ZSM-5 and the basic sites of MgO that favored conversion of phenolics to aromatics through dehydroxylation and demethoxylation. Overall, this study highlights the influence of different catalyst supports in catalytic vapor phase upgrading of sawdust derived biocrude to selectively optimize bio-oil composition, increasing fuel or chemical potential while addressing coke formation.