The feasibility of electro-assisted organosolv pretreatment of lignocellulosic materials for delignification and reducing sugar production
Creators: name: SUN, Wangqiyue
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401102
subject: Environmentally sustainable engineering
classificationCode: 401106
subject: Waste management, reduction, reuse and recycling
classificationCode: 401199
subject: Environmental engineering not elsewhere classified

description: Decreasing supplies of fossil fuels and climate change have been the main driver for the increased interest in producing renewable fuels and chemicals. Of all potential renewable carbon resources, lignocellulosic materials (LCMs) are gaining increased attention due to their high abundance. Currently, 10-50×109 ton of LCMs is produced worldwide each year, providing a vast, underutilised resource (Taylor and Warnken, 2008).

LCMs can be utilised as a resource for biofuel or green chemical production, while a pretreatment process is required. Pretreatment aims to break down the LCM microstructure and transform it into a more accessible form. Of all the existing pretreatment processes, organosolv pretreatment showed high delignification efficiency (&gt;90 wt.%), suggesting its position as a promising pretreatment method. However, organosolv pretreatment needs to be conducted at harsh conditions (especially high temperature &gt;150 °C), which causes some issues, including side-reactions, carbohydrate dissolution, and costly reactors.
 
This study aims to develop a pretreatment method that uses electric energy instead of thermal energy and pretreats LCM effectively at mild operation conditions. This method is referred to as electro-assisted organosolv pretreatment (EAOP). The scope of this study involves the solvent selection, condition development, process validation and verification, mechanism investigation and optimisation of EAOP. In terms of desktop work, Chapter 1 included the introduction of this project, and Chapter 2 provided a detailed literature review of the relevant studies. The experimental work involved in this project are represented in Chapter 3, 4, 5 and 6.

Based on literature review, Chapter 3 aims to investigate the feasibility of replacing thermo energy with electric energy during organosolv pretreatment. A novel binary solution consisting of gamma-valerolactone (as solvent) and 1-butyl-3-methylimidazolium acetate ([Bmim]OAc, as electrolyte and co-solvent) was specially designed (also known as one of the ionic liquid promoted organosolv, i.e. IL/O, in the following text) for EAOP and results revealed that the use of electric energy has the potential to replace thermo energy and decreases the reaction temperature of organosolv pretreatment methodologies.
 
To further enhance the delignification efficiency, solvent selection was conducted in Chapter 4. There were 12 IL/O subjected to physicochemical and electrochemical characterisation for initial screening. To validate the process, selected IL/O were assessed for their performance towards the delignification of Eucalyptus sawdust, rice straw, and wheat straw at ambient temperature (25 °C) through EAOP. Results support that EAOP is a viable delignification method at ambient temperature compared to conventional organosolv pretreatment.

With EAOP proven viable and effective, in Chapter 5, the mechanism investigation and optimisation of EAOP were investigated. Results in Chapter 5 showed that the electrochemically generated hydroxyl radical (OH*) during EAOP is key to the delignification efficiency. To enhance the OH* formation during EAOP, inorganic salts (NaOH, NaCl, FeCl3) and water were used as additives into the solvent system of EAOP to enhance the OH* formation during EAOP. Results indicated that NaCl/water enhanced EAOP could be done rapidly (in 30 mins) and at high biomass loading (25%) in the presence of NaCl/water.
 
Chapter 6 investigated the regeneration of spent liquor of pretreatment, which played an important role before the scaling-up of EAOP. This stage presented a new method, referred to as lignin-incompatible extraction, developed for the simultaneous recovery of GVL, lignin and monosaccharides from liquor. Through the lignin-incompatible extraction, for liquor from organosolv pretreatment, 87.6% of GVL was regenerated from liquor and regenerated GVL can be reuse without significant performance decrease (10%) after recycling the solvent four times.

In summary, EAOP is viable at room temperature (25 °C) and could be done quickly (in 30 mins) even at high biomass loading (25 wt.%). Notably, the energy requirement of EAOP was low (2.0 V is required), and the residual biomass produced from EAOP showed a high reducing sugars yield (204.4 mg/g). This method solved the drawbacks of conventional thermal-based organosolv pretreatments which were mentioned in previous paragraphs.