Analysis of methyl substitution in phenolic compounds as potential biofuel additives: from solid-state description to antioxidant potential

Abstract

Background: The growing demand for renewable energy sources has positioned biodiesel as a promising alternative to fossil fuels. However, its limited oxidative stability poses significant challenges for storage and performance, particularly at elevated temperatures. Objectives: This study aimed to investigate the influence of a methyl substituent group on the structural, electronic, and antioxidant properties of hydroquinone-derived phenolic compoundsto evaluate their potential as biofuel additives. Methods: A combination ofsupramolecular and theoretical approaches was employed to examine the intermolecular interaction patternsin the crystals of 2,5-dimethylhybenzene-1,4-diol and 2-methylhybenzene-1,4-diol through QTAIM and natural bond orbital (NBO) analyses. Additionally, density functional theory (DFT) calculations at the M06-2X/6-311++G(d,p) level of theory were performed to optimize the molecularstructures and evaluate the corresponding chemical reactivity descriptors. The antioxidant mechanisms and the stability of the radicals generated during free-radical scavenging were further investigated using thermodynamic descriptors. Results: Electronic-structure analysis showed that methyl substitution slightly lowers the bond-dissociation enthalpy and ionization potential of the phenolic compounds, thereby enhancing their antioxidant capacity. Compared with commercial additives, the studied compounds exhibited competitive thermodynamic antioxidant profiles. Conclusions: Both compounds demonstrated antioxidant potential as multifunctional biofuel additives, with dimethyl substituted hydroquinone standing out as a promising candidate.