Reductive Catalytic Fractionation of Microwave-Pretreated Rice Straw and Sugar Cane Bagasse for Monomeric Phenols

Reductive catalytic fractionation (RCF) is an emerging biorefinery concept that enables simultaneous valorization of lignin to phenols and carbohydrates to bioethanol from lignocellulosic biomass. While herbaceous agriculture residue biomasses like rice straw (RS) and sugar cane bagasse (SCB) are potential feedstocks for the biorefinery, the presence of inorganic ash in them poses a challenge in achieving a high degree of delignification and optimum phenolic monomer yields. This study investigates the effects of microwave-assisted mild pretreatment of RS and SCB to reduce the ash content (AC) and key elements while improving the efficacy of RCF using Pt-loaded activated carbon (Pt/ACC) and HZSM-5 (Pt/HZSM-5) catalysts. The catalysts were synthesized and characterized for their structure, texture, pore size distribution, and acidity. At the optimal pretreatment condition of 80 °C and 40 min, the reduction in AC achieved in the case of RS was substantial, i.e., 16.8 wt % to 4.2 wt %. Particularly, the amount of Na decreased from 0.5% (raw RS) to 0.07% (pretreated RS), K decreased from 1.1% to 0.16%, and Si from 9.2% to 3.16%. As a result, the degree of delignification increased from 70.1% for untreated RS to 82.9% with pretreated RS, with a concomitant increase in monomer yield from 10.9 wt % to 23.5 wt % at 220 °C and 3 h using Pt/ACC. At similar conditions, Pt/HZSM-5 demonstrated better delignification compared to Pt/ACC, but the phenolic monomer yields were lower. Even in the case of SCB, pretreatment promoted the yield of monomeric phenols from RCF to 30.3% at 220 °C for 1 h. Ethyl homovanillate, propyl guaiacol, and eugenol were the major G-type products, methoxy eugenol and syringol were the S-type products, and methyl coumarate was the major H-type product from both RS and SCB.