Determination of carbonyl functional groups in lignin-derived fraction using infrared spectroscopy

[Display omitted] •Infrared (IR) absorption of carbonyl in the lignin-derived fraction with and without dimethyl sulfoxide-d6 (DMSO‑d6) was observed at 1707 cm−1.•The absorption at 1707 cm−1 captured aliphatic ketones, aliphatic carboxylic acids, or aromatic esters.•The IR area ratio of carbonyl compounds (aliphatic and aromatic ketones, carboxylic acids, and esters) to carbon-deuterium of DMSO‑d6 increased proportionally to their mole ratios.•Estimation of the amount of carbonyl revealed that approximately 20% of oxygen in the lignin-derived fraction was from carbonyl functional groups. Carbonyl functional groups contained within lignin-derived fractions were examined using Fourier-transform infrared spectroscopy (FT-IR). The absorption of carbonyl in lignin-derived fractions with and without dimethyl sulfoxide-d6 (DMSO‑d6) was observed at 1707 cm−1. This absorption captured aliphatic ketones, aliphatic carboxylic acids, or aromatic esters because the absorption range of cyclohexanone, heptanoic acid, ethyl benzoate, and butyl benzoate with and without DMSO‑d6 is 1704–1721 cm−1. To estimate the amount of carbonyl in the lignin-derived fraction, DMSO‑d6 solutions of various model carbonyl compounds were analyzed by FT-IR. The FT-IR area ratios of these compounds to carbon-deuterium in DMSO‑d6 at 2250 cm−1 increased proportionally to their mole ratios. The carbonyl oxygen to carbon atomic ratio in the lignin-derived fraction was estimated to be 0.05, indicating that approximately 20% of oxygen in the lignin-derived fraction was from carbonyl functional groups.