Polar solvents enhance the efficiency of microwave pre-treatment of woody biomass

Microwave pre-treatment of woody biomass may be the key to unlocking an energy efficient process for the production of renewable chemicals and fuels. Here we investigated the dipolar polarisation mechanism of polar solvents, ranging from dilute aqueous solutions of ethylene glycol, isopropyl alcohol, formaldehyde and water-only, to improve the efficiency of MW pre-treatment of woody biomass. The optimal conditions were 3.7 wt% FA solution for 10 min at 180 °C, where heating rate was accelerated by 50%, saccharification of biomass increased by 56% and stabilised lignin monomers were produced compared to water-only. Autoclave pre-treatment of woody biomass released 3 to 6 times less sugar yield in the presence of water-only and 3.7 wt% FA compared to MW irradiation. Our efficient method can potentially produce three streams of building blocks – cellulose fibres, solvated hemicellulose and stabilised lignin – for transformation into renewable products. Microwave irradiation of wood in dilute formalin solution breaks apart the wood matrix in record time (10 min) compared with other commonly used polar solvents. The lignin monomers in the pre-treated wood were found to be stabilised in the presence of formaldehyde. In its absence, lignin monomers tend to polymerise together. This novel approach gives three streams of products that can be further refined into value added products and fuels. [Display omitted] •Microwave dipolar polarisation mechanism was exploited for efficient woody biomass depolymerisation.•Microwave heating to 180 °C using 10 wt% formaldehyde improved the efficiency by 52% compared to water.•Stabilization of lignin monomers was achieved using formaldehyde under microwave irradiation.