Optimization of microwave-expanding pretreatment and microwave-assisted extraction of hemicellulose from bagasse cells with the exploration of the extracting mechanism

Hemicellulose is mainly distributed in the tightly packed S₂ layer of the plant cell wall and the middle lamella. This rigid microstructure of wood and interactions among hemicellulose, lignin, and cellulose jointly restrict the separation and transformation of hemicellulose in the wood matrix. To address this issue, a method combined with microwave-expanding pretreatment (MEP) and microwave-assisted extraction (MAE) with a NaOH solution was carried out. We found that the MEP could effectively create new pathways for bagasse cells in mass transferring. More specifically, 195 % of the specific surface area (m²/g) with 193 % of the pores (>50 nm) increased after MEP; the SEM images also confirmed that the microstructure of bagasse was modified. MAE could considerably exfoliate hemicellulose from cellulose fiber and accelerate mass transfer. Additionally, we optimized MEP and MAE by using response surface methodology (RSM). The optimal parameters were 370 K, 3.7 min, 1081 W microwave power, and 9.9 wt% NH₄HCO₃ consumption for the MEP and 1100 W microwave power, 2.5 wt% NaOH concentration, 34.6 min reaction time for MAE, respectively. Moreover, molecular dynamics (MD) simulation suggests that NaOH could significantly lower the work needed to peel off the xylan chain from cellulose nanofibril.