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.

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Biomass valorization to building block chemicals in food and pharmaceutical industries has tremendously gained attention. To produce monophenolic compounds from palm empty fruit bunch (EFB), EFB was subjected to alkaline hydrothermal extraction using NaOH or K2CO3 as a promotor. Subsequently, EFB-derived lignin was subjected to an oxidative depolymerization using Cu(II) and Fe(III) mixed metal oxides catalyst supported on γ-Al2O3 or SiO2 as the catalyst in the presence of hydrogen peroxide. The highest percentage of total phenolic compounds of 63.87 wt% was obtained from microwave-induced oxidative degradation of K2CO3 extracted lignin catalyzed by Cu-Fe/SiO2 catalyst. Main products from the aforementioned condition included 27.29 wt% of 2,4-di-tert-butylphenol, 19.21 wt% of syringol, 9.36 wt% of acetosyringone, 3.69 wt% of acetovanillone, 2.16 wt% of syringaldehyde, and 2.16 wt% of vanillin. Although the total phenolic compound from Cu-Fe/Al2O3 catalyst was lower (49.52 wt%) compared with that from Cu-Fe/SiO2 catalyst (63.87 wt%), Cu-Fe/Al2O3 catalyst provided the greater selectivity of main two value-added products, syringol and acetosyrigone, at 54.64% and 23.65%, respectively (78.29% total selectivity of two products) from the NaOH extracted lignin. The findings suggested a promising method for syringol and acetosyringone production from the oxidative heterogeneous lignin depolymerization under low power intensity microwave heating within a short reaction time of 30 min.

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