Sequential Fractionation of Palm Empty Fruit Bunch and Microwave-Assisted Depolymerization of Lignin for Producing Monophenolic Compounds

We develop a sequential fractionation of palm empty fruit bunches (EFB) and microwave-assisted depolymerization of lignin for producing monophenolic compounds with high yields. EFB has been known as a low-priced and abandoned residue from the palm oil milling process due to its low heating value and generation of toxic gases upon burning; therefore, valorization of EFB has been an important task for circular economy. In this study, a subsequent alkaline hot compressed water fractionation to separate hemicellulose and lignin from cellulose has been studied. The lignin separation by lignin precipitation and a filtration process followed by microwave-assisted lignin oxidative depolymerization to high value phenolic compounds was proposed. We systematically screened many catalysts as well as investigated reaction conditions (such as the concentration of hydrogen peroxide, reaction time, etc.) to achieve the highest yield of lignin-derived phenolic compounds. It was found that the optimal conditions for depolymerization of pretreated EFB lignin were (i) NaOH solution with 2.5% (w/w) hydrogen peroxide, (ii) microwave (300 W) for 15 min, and (iii) bimetallic Cu(OH)(2) and Fe2O3 catalysts. The highest percentage of the total phenolic compound concentration peak area is 91.78%, including 42.84% of syringol, 5.42% of vanillin, 8.71% of acetovanillone, 6.65% of syringaldehyde, and 28.16% of acetosyringone. The proposed sequential fractionation and microwave-assisted treatment would be promising for converting other lignocellulosic raw biomass to useful phenolic compounds.