(397e) Polar Solvents Facilitate Efficient Microwave Pretreatment of Woody Biomass

Microwave pretreatment of woody biomass may be the key to unlocking an energy efficient process for recalcitrant waste stream for the production of renewable chemicals and fuels. Sugars derived from non-food sources such as woody biomass have the potential to be converted into a wide range of chemicals. However, woody biomass requires pretreatment to open the structure to access cellulose. Steam explosion, organosolv and sulphite pretreatment are currently used to remove lignin from wood [1]. However, these methods require substantial energy to be effective [1]. Development of an alternative, more efficient pretreatment processes for the release of sugars and other useful fractions from renewable biomass sources is necessary for a sustainable future.

Ionic liquids have been extensively researched for woody biomass pretreatment as they have proven to yield exceptional results via ionic conduction mechanism. However, limited research has been reported on the intentional exploitation of polar solvents, which increase the microwave absorption capacity via dipolar polarisation mechanism, for the pretreatment of recalcitrant biomass. Microwave absorption can vary significantly depending upon the ratio of effective dielectric loss factor (𝜀") to the dielectric constant (𝜀), of which the ratio is commonly known as the loss tangent (tan 𝛿).

In this research, we investigated the use of solvents with high tan 𝛿 value to increase the efficiency of microwave-assisted solvolysis of woody biomass through an increase in the heating rates, and hence reduction of the energy consumption. To the best of our knowledge formaldehyde pretreatment of wood has not yet been reported under microwave irradiation. We studied the microwave irradiation of Eucalyptus Regnans and Pinus Radiata sawdust (~0.5 mm) in the presence of formalin solution containing 1.8, 3.7 and 10 wt% formaldehyde and water as the control at 180°C for 10 minutes.

It was found that 3.7 wt% formaldehyde (FA) solution is more efficient than water only for the pretreatment of wood sawdust at 180 °C and 10 min microwave exposure. The presence of formaldehyde in the solution accelerated the heating rate by 50% in the microwave. Furthermore, >75% of the hemicellulose sugars were released under mild microwave treatment conditions for both the wood species. Whereas in water-only pretreatment 32% and 55% of the hemicellulose was released from E. regnans and P. radiata, respectively.

A proposed mechanism for formaldehyde-mediated deconstruction of hemicellulose was developed. The lignin in the remaining sawdust from the FA pretreatment was analysed by 2D NMR, which detected the presence of stabilised lignin monomers. Whereas, with water-only pretreatment this does not occur. This stabilised platform may open-up a more advanced continuous process which could allow near theoretical lignin monomer yields through the subsequent hydorgenolysis.