Functionality and molecular weight distribution of red oak lignin before and after pyrolysis and hydrogenation

Three red oak derived lignin samples: 1. lignin extracted from red oak chips using gamma -valerolactone (GVL lignin), 2. lignin extracted from the pyrolysis oil of red oak chips by fractionation and water extraction (pyrolytic lignin) and 3. pyrolytic lignin hydrogenated over Ru/C (hydrogenated pyrolytic lignin), were analyzed by FT-ICR MS, NMR, and GPC. More than 1100 distinct molecular weights were observed by FT-ICR MS of the lignin streams while changes in the O/C and H/C ratios suggested the dehydration of hydroxylated sidechains from pyrolysis and partial saturation of the compounds from hydrogenation. The relative average molecular weight of the lignin determined by GPC decreased five-fold after pyrolysis. Quantitative 13C, HSQC, and HMBC NMR revealed a decrease in the C-O aliphatics from pyrolysis potentially forming alkane, alkene, and carbonyl functionalities. The aldehydes and ketones were highly reactive during hydrogenation and may potentially be responsible for coke formation.