Fractionation of lignin using organic solvents: A combined experimental and theoretical study

Refining of industrial lignin to produce homogeneous fractions is essential for high-value applications. However, the understanding of key interactions between a variety of solvents with lignin polymer is still uncertain. In this work, single-step fractionation of industrial hardwood kraft lignin (HKL) using organic solvents of different polarities - ethanol, acetone, diethyl ether and hexane - was investigated by combining an experimental and theoretical approach. Experimental results revealed that higher polarity solvents (ethanol and acetone) exhibited higher solubility yield compared to moderate and low polarity solvents. The chemical differences between lignin fractions were proven by pyrolysis gas chromatography mass spectrometry and near infrared spectroscopy. Density functional theory (DFT) results indicated that ethanol presented higher interaction energy followed by acetone, diethyl ether and hexane, which was consistent with experimental findings. Hydrogen bond and non-covalent interaction results from DFT demonstrated that the predominant interaction was found for high polarity of ethanol over other solvents and gamma-OH in the lignin model is the key site.The authors gratefully acknowledge the European Commission for funding the InnoRenew project [Grant Agreement #739574] under the Horizon2020 Widespread-Teaming program, the Republic of Slovenia (investment funding from the Republic of Slovenia and the European Union European Regional Development Funds) and infrastructural ARRS program IO-0035. Additionally, O.G. is grateful for the financial support received from the University of the Basque Country (post-doctoral grant DOCREC18/29) and R.H. acknowledges to the Department of Education of the Basque Government (post-doctoral grant INGVTCL4-D00112-7)