Sooting tendency of substituted aromatic oxygenates: The role of functional groups and positional isomerism in vanillin isomers

Substituted aromatics are commonly observed in lignin-based biofuel; however, their high sooting tendency prevents direct utilization in commercial combustors. Recent studies have revealed that oxygenated functional group substitution could effectively suppress the soot emission from aromatic biofuels. This study aims to enhance the understanding of sooting tendencies in aromatic oxygenates with mono-, di-, and tri-substitutions, focusing on various functional groups and their positional isomerism. We established a yield sooting index (YSI) database of 42 single-ring aromatic compounds, including 30 new measurements from the present study. The constructed database was utilized to develop a multivariate linear regression (MLR) model to predict the YSI of substituted aromatic oxygenates based on their structural features. The fitted coefficients of the MLR model indicate vastly different impacts of hydroxyl, formyl, and methoxy functional group, as well as the importance of positional isomerism. To understand the role of oxygenated functional groups, we used substituted vanillin isomers containing hydroxyl, methoxy, and formyl groups as a model system. Comparing the sooting tendencies of these compounds revealed a high sensitivity of YSI to positional isomerism. A further mechanistic study using quantum-mechanical calculations showed that subtle interactions between three oxygenated functional groups in vanillin isomers can alter their thermal decomposition pathway, affecting the sooting tendencies of these aromatic fuels. The present study provides a novel statistical and theoretical explanation of how oxygenated substitution and its positional isomerism influence sooting behaviors, facilitating the rational design of lignin-based biofuels with minimal soot emission.