Influence of chain length and saturation on carboxylic acid pyrolysis mechanisms

This study investigates radical-driven deoxygenation mechanisms during the non-catalytic pyrolysis of saturated and unsaturated carboxylic acids. Pyrolysis experiments were conducted at 410 °C, between 0.5 and 2 h, using carboxylic acids of varying carbon chain lengths (C6 to C18) and saturation levels, along with ketones. Feedstock conversion and deoxygenation products were quantified using GC–MS/FID for liquids and GC-TCD/FID for gases. Results demonstrated that carboxylic acid chain length significantly influences pyrolysis behavior, with significant differences in deoxygenation pathway linked to acid chain length and saturation level. Decarboxylation was the predominant pathway for short-chain carboxylic acids, whereas long-chain acids showed increased tendency towards decarbonylation. Short-chain saturated carboxylic acids favoured ketonic decarboxylation during the initial stages of pyrolysis, resulting in notable amounts of ketones and carbon dioxide. Subsequent decarbonylation of these ketones contributed to further deoxygenation, generating hydrocarbons and shorter-chain ketones via radical-driven mechanisms. In contrast unsaturated carboxylic acids underwent extensive cracking, which suppressed ketonic decarboxylation and led to reduced overall hydrocarbon yields. Additionally, mixed carboxylic acid feedstocks showed decreased conversion efficiencies, primarily due to limited intermolecular interactions necessary for effective ketonic decarboxylation. This work explores radical-driven, non-catalytic pyrolysis of fatty acids, providing a detailed mechanistic understanding of how chain length and saturation influence reaction pathway. The findings highlight key determinants of product selectivity and deoxygenation efficiency, providing valuable insights for optimizing feedstock compositions in pyrolysis-based sustainable biofuel production. • Radical-driven non-catalytic fatty acids deoxygenation was investigated. • Carboxylic acid chain length influences pyrolysis and deoxygenation pathways. • Ketonic decarboxylation drives early deoxygenation in short-chain carboxylic acids. • Unsaturated carboxylic acids do not undergo ketonic decarboxylation. • Unsaturation increases cracking, reducing hydrocarbon yield in pyrolysis products.