Theoretical Investigation About Catalytic Depolymerization Mechanism Of The Lignin Model Compounds Over Metal-Based Ionic Liquids

Ionic liquids?ILs?are a kind of novel green solvent and efficient catalyst whose structure and properties are mainly affected by the hydrogen-bonding interactions between cations and anions.Particularly,metal ionic liquids?MILs?exhibit excellent catalytic properties in the lignin conversions.Nevertheless,the structural properties of MILs and the mechanism of lignin utilization by MILs are scarcely understood.Therefore,this thesis has discussed the hydrogen-bonding interactions between methanol and MILs,particularly the mechanism of lignin depolymerization under MILs investigated by simulation calculations in combination with excess infrared spectrum.?1?Firstly,the hydrogen-bonding interactions between[Bmim][FeCl₄]and methanol at different concentration are studied by using excess infrared spectroscopy,density functional theory?DFT?and molecular dynamics simulations.The results show that methanol can simultaneously interact with the cations and anions of[Bmim][FeCl₄],and has priority to the[FeCl₄]^-anions.Hydrogen-bonding interactions between cations and anions of[Bmim][FeCl₄]are gradually weakened by methanol.It is found that[Bmim][FeCl₄]ion clusters,[Bmim][FeCl₄]-methanol and the free ions are the main aggregation forms in the mixing system.When the molar concentration of methanol is less than 0.2,the anions and cations of[Bmim][FeCl₄]tend to bind together because of the original steady hydrogen-bonds network.While the molar ratio of methanol and[Bmim][FeCl₄]exceeds 2,[Bmim][FeCl₄]is dissociated into individual ions.The-CD₃ group in the methanol is electron-withdrawing,making positive contributions to the stability of the O-H···Cl hydrogen-bond.?2?Secondly,the HOMO-LUMO energy gap between different MILs and reactants are studied by DFT calculations.Compared to other MILs,iron-based ILs perform excellent catalytic activity.Therefore,phenyl p-hydroxycinnamate?PPC?,as a?-O ester lignin,was selected to investigate the decomposition mechanism over MILs?[Bmim][FeCl₄]?.This work has put forward three potential pathways and calculate the energy barrier of each element reaction.In this case,combined with experiments,the calculation results have verified that the nature of PPC depolymerization under[Bmim][FeCl₄]is the combination of acid catalysis and acyl chlorination,and the formation of Lewis acid FeCl₃ and free Cl^-anion are the necessary steps for obtaining MPC.Furthermore,the energy barrier of pathway c is lower than the other two transformation processes,which has the highest chemical reactivity and selectivity.Further investigation of the solvent effect reveals that methanol as a solvent can facilitate the PPC depolymerization.The results of this paper will provide theoretical guidances of the lignin depolymerization over MILs.