Selective catalytic transformation of lignin with guaiacol as the only liquid product

Abstract

Guaiacol is an important feedstock for producing various high-value chemicals. However, the current production route of guaiacol relies heavily on fossil resources. Using lignin as a cheap and renewable feedstock to selectively produce guaiacol has great potential, but it is a challenge because of its heterogeneity and inert reactivity. Herein, we discovered that La(OTf)₃ could catalyze the transformation of lignin with guaiacol as the only liquid product. In the reaction, La(OTf)₃ catalyzed the hydrolysis of lignin ether linkages to form alkyl-syringol and alkyl-guaiacol, which further underwent decarbonization and demethoxylation to produce guaiacol with a yield of up to 25.5 wt%, and the remaining residue was solid. In the scale-up experiment, the isolated yield of guaiacol reached up to 21.2 wt%. To our knowledge, this is the first work to produce pure guaiacol selectively from lignin. The bio-guaiacol may be considered as a platform to promote lignin utilization.

Fig. 1. Production route to guaiacol: (A) traditional route, (B) new route in this work.

Fig. 2. The 2D-HSQC NMR spectra of the organosolv lignin from Eucalyptus (top) and reaction residue (bottom) transformation (reaction conditions: Table 1, entry 9).

Fig. 3. (A) The time-course of the product distributions for the C–C bond cleavage of the ethyl group in 4-ethyl guaiacol with the La(OTf)₃ catalyst. Reaction conditions: 4-ethyl guaiacol (50 mg), La(OTf)₃ (20 mg), methanol 4 mL, 10 μL water, 270 °C, 0.1 MPa Ar, and stirring at 800 rpm. (B) Reaction pathway for C–C bond cleavage of the ethyl group in 4-ethyl guaiacol.

Fig. 4. Scale-up production of guaiacol from lignin. Reaction conditions: 1.50 g organosolv lignin, 0.6 g La(OTf)₃, 120 mL methanol, 0.3 mL water, 270 °C, 0.1 MPa Ar, and 24 h.