Functions of local structural surrounding in activity of Ca-containing catalysts for vapor upgrading during biomass thermal decomposition

The catalytic activity of calcium-containing complex oxides with perovskite structure (CaTiO₃), brownmillerite structure (Ca₂Fe₂O₅ and Ca₂FeAlO₅), spinel structure (CaAl₂O₄), and pseudowollastonite structure (CaSiO₃) was compared to CaO for the upgrading of Oakwood fast pyrolysis vapors. Initial catalyst particle sizes range between 0.61 and 3.21 μm, while the BET surface area is between 0.2 and 8.7 m²/g. In contrast to CaO, the complex oxides compounds demonstrated negligible CO₂ and H₂O chemisorption during catalytic fast pyrolysis, thereby a low deactivation of catalysts, good structural and morphological stability and thus high reusage feasibility. Due to their unique features, Ca-containing catalysts were found to promote specific reaction pathways, such as the conversion of guaiacol to 3-methyl phenol, 4-ethenyl-2-methoxyphenol to 4-ethyl-2-methoxyphenol, 1-(4-hydroxy-3,5-dimethoxyphenyl)ethanone to 3,5-dimethoxy acetophenone and long chain carboxylic acids to acetic acid through a multitude of reaction routes including demethylation/demethoxylation, hydrogenation and hydrodeoxygenation, oxidative cleavage, and dehydration reactions. CaO and Ca₂Fe₂O₅ promoted the alkylation reactions within the methoxy phenolics, whilst CaSiO₃ promoted hydrogenation reactions. The formation of acetic acid was promoted over Ca₂FeAlO₅, CaAl₂O₄, and CaSiO₃, while acetone, 2-butanone, new cyclic C5 ketones were revealed over CaO. The resulting ketonic fraction was noticeably affected by the use of Ca₂Fe₂O₅ and CaAl₂O₄ catalysts.