Experimental and mechanistic insights into the delignification of oil palm empty fruit bunch using choline amino acid ionic liquids

Choline amino acid ionic liquids ([Cho][AA]) represent promising green solvents for biomass pretreatment, yet their lignin extraction mechanisms remain insufficiently understood. In this study, the delignification efficiencies of choline glycinate ([Cho][Gly]), choline β-alaninate ([Cho][Ala]), and choline phenylalaninate ([Cho][Phe]) were investigated using oil palm empty fruit bunch as the lignocellulosic substrate. Experimental investigations were complemented by molecular dynamics (MD) simulations to elucidate the molecular-level interactions governing lignin dissolution. Experimental findings showed that [Cho][Gly] exhibited the highest lignin extraction efficiency, following the trend [Cho][Gly] > [Cho][Ala] > [Cho][Phe]. This behaviour was primarily driven by the strength of electrostatic interactions between lignin and amino acid anions ([AA] - ). Glycine anions formed stronger electrostatic interactions with lignin than bulkier anions such as phenylalaninate anions, while choline cations contributed mainly through van der Waals interactions with the aromatic regions of lignin. Radial and spatial distribution function analyses revealed that [AA] - preferentially interacts in the vicinity of the α-hydroxyl groups and β-O-4 linkages of lignin. Concentration-dependent studies further identified 70 wt% [Cho][Gly] as the optimal concentration, with reduced efficiency at 90 wt% due to prolonged hydrogen-bond lifetimes. Overall, this work provides valuable insights for the rational design of amino acid-based ionic liquids for sustainable biomass valorization.