Detailed insights into lignin stabilisation and carbohydrate chemical nature in the H-transfer RCF process: On the role of solvent fractionation in the structural profiling of lignin oil

Abstract

Reductive Catalytic Fractionation (RCF) of lignocellulosic materials produces lignin oil rich in monomer products and high-quality cellulosic pulps. RCF lignin oil also contains lignin oligomers/polymers and hemicellulose-derived carbohydrates. The variety of components makes lignin oil a complex matrix for analytical methods. As a result, the signals are often convoluted and overlapped, making detecting and quantifying key intermediates challenging. Therefore, to investigate the mechanisms underlining lignin stabilisation and elucidate the structural features of carbohydrates occurring in the RCF lignin oil, fractionation methods reducing the RCF lignin oil complexity are required. This report examines the solvent fractionation of RCF lignin oil as a facile method for producing lignin oil fractions for advanced characterisation. We demonstrate solvent fractionation using small volumes of environmentally benign solvents (methanol, acetone and ethyl acetate) to produce multigram lignin fractions comprising products in different molecular weight ranges. This feature allowed for determining structural heterogeneity across the entire molecular weight distribution of the RCF lignin oil via high-resolution HSQC NMR spectroscopy. This study provides detailed insight into the role of the hydrogenation catalyst (Raney Ni) in stabilising lignin fragments and defining the chemical nature of hemicellulose-derived carbohydrates in lignin oil obtained by the H transfer RCF process.