Fig. 3: Development of different DES systems by model compound study.

a Reaction pathways during treatment of β-O-4 lignin model compound 1a in CS-DES or DP-DES: the sequence starts with rapid dehydration of 1a. In CS-DES: intermediate 3a is formed; rehydration and cleavage lead to the formation of guaiacol and 5a, unstable 5a is trapped in the form of cyclic acetal 6aa by the ethylene glycol present in CS-DES. In DP-DES: cleavage is suppressed by a controlled amount of OA, 4aa is formed by the attack of ethylene glycol on the carbocation intermediate originating from 1a. Optimization of 1a (25 mg) reaction in CS-DES (ChCl, 4.2 g, 30 mmol: OA, 30 mmol, 3.8 g: EG, 60 mmol, 3.6 g) by b. The temperature of 100–140 °C for 20 min; c EG amount (10–90 mmol) in CS-DES at 120 °C for 20 min; d Reaction time from 5 to 30 min at 120 °C. Optimization of 1a (50 mg) reaction in DP-DES (ChCl, 4.2 g, 30 mmol: OA, 0.6 mmol, 0.08 g, 1 wt%: EG, 60 mol, 3.6 g). e Temperature of 80–100 °C for 18 h; f OA amount (0.6–60 mmol, 0.08–0.8 g, 1–10 wt%) in DP-DES at 80 °C for 6 h; g Reaction time of 1–24 h at 80 °C. Conversion of 1a and selectivity of the products were calculated by internal standard calibration with octadecane based on GC-FID analysis; products were identified by GC/MS; DCM and water were used to extract the products from DESs.