Quantifying Lignin Hydroxyl Groups

Hydroxyl groups of lignin samples were measured using quantitative ³¹P NMR spectroscopy after derivatization of lignin with 100 µL of 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) [34 (link),35 (link)]. The derivatized samples (30 mg) were dissolved in 0.75 mL of pyridine and deuterated chloroform (1.6:1 v/v) and mixed with 100 μL of a solution of N-hydroxy-5-norbornene-2,3-dicarboxylic acid imide (10 mg mL⁻¹) and chromium(III) acetylacetonate (5 mg mL⁻¹) as internal standard and relaxation agent, respectively. ³¹P NMR spectra were acquired using an inverse-gated decoupling pulse sequence with a 90° pulse angle, 25 s relaxation delay, and 256 scans.