One-Pot Reaction Conversion of Delignified Sorghum Bicolor Biomass into Levulinic Acid using a Manganese Metal Based Catalyst

Sorghum stems, an agricultural biomass waste, can be used as a raw-material carbon source for platform chemicals, such as levulinic acid. Levulinic acid can be produced with high percentage yields using delignified sorghum stems as starting materials. The purpose of this study was to evaluate manganese-based catalysts (Mn²⁺ and Mn₃O₄) as Fenton-like reagents for the production of levulinic acid from sorghum stems. A mixture of finely powdered delignified sorghum stems (containing 76.66% cellulose) dispersed in a mixture of phosphoric acid (40%), H₂O₂ (30%), and either 2% Mn²⁺ or 2% Mn₃O₄ as a catalyst in a one-pot mini reactor was observed at 130°C for 10 h. The yield of the conversion products was quantitatively analyzed for levulinic acid using high-performance liquid chromatography. The reaction using the Mn₃O₄ catalyst yielded a higher percentage of levulinic acid (26.57%) than the Mn²⁺ catalyst (25.59 %) reaction after 8 hours. This study points to the opportunity of the one-pot synthesis of levulinic acid using renewable biomass waste resources.

Cellulose; Delignification; Fenton-like system; Levulinic Acid; Sorghum

    Sorghum bicolor biomass can be used as a raw-material carbon source for industrial chemicals (platform chemicals), such as levulinic acid. Levulinic acid is one of the top ten US DOE 2004 chemicals derived from carbohydrates (Bozell et al., 2000). Levulinic acid is a short-chain fatty acid compound containing ketone and carbonyl functional groups, which makes it a potential source for the synthesis of various chemical compounds, such as polymers, resins, plastics, textiles, solvents, herbicides, and fuel additives (Rackemann and Doherty, 2011). As well as being able to be converted into chemicals with high economic value, levulinic acid is also a non-toxic material with an LD₅₀ of 1,850 mg/kg. Levulinic acid can be obtained by mixing biomass with acids and heating at high temperatures (>100°C) to produce sugar, which is then converted into intermediary hydroxyl methyl furfural to produce levulinic and formic acid (Girisuta, 2007). Kang et al. (2018) reported heating between 140°C and 200°C for biomass hydrolysis reactions originating from biomass feedstocks, such as sugar cane bagasse, rice husks, sorghum flour, wheat straw, and corn stalks. However, before hydrolysis, the biomass must be pretreated to make the conversion into levulinic acid easier. Pretreatment occurs in three ways: chemically (Harahap et al., 2019; Hermansyah et al., 2019), physically (Ruksathamcharoen et al., 2018), and biologically (Hossain et al., 2017). Biomass is usually delignified to weaken the binding between the lignin and the cellulose to remove the lignin from the substrate (Krisnandi et al., 2019). Delignification using 10% NaOH is the optimal concentration for the delignification process, and this condition was used in this study.

    The conversion reaction of biomass to levulinic acid usually uses either a homogeneous catalyst with acids (Girisuta et al., 2007; Van et al., 2011) or a heterogeneous catalyst (Ya’aini et al., 2013; Ramli and Amin, 2016). In the current study, semi-heterogeneous manganese catalysts (Mn²⁺ and Mn₃O₄) were used in a Fenton-like system to produce levulinic acid from delignified sorghum stems heated to only 130°C. Fenton systems that use catalysts, including the homogeneous catalysts of iron (Fe²⁺ and Fe³⁺) (Eckenfelder, 2000), are called Fenton-like systems. Fenton’s reagent can produce hydroxyl radicals, which with a transition metal have a high H₂O₂ oxidation capability, making it suitable for difficult to degrade organic materials (Catalkaya and Kargi, 2009). Fenton oxidation using mangan not only degrades cellulose and but also functions in conversion to levulinic acid (Chen et al., 2011a; Chen et al., 2011b), which is why the current study used a manganese base catalyst. The purpose of this study was to demonstrate the conversion of delignified stem sorghum, a lignocellulosic biomass waste, into high economic value levulinic acid using a semi-heterogenous manganese catalyst via a Fenton-like system with a low reaction temperature.

Levulinic acid was successfully produced by the conversion reaction of delignified sorghum stems using manganese-based catalysts that produced HO• radicals via a Fenton-like mechanism. The reaction result of sorghum stem conversion using the Mn₃O₄ catalyst gave a higher percentage yield of levulinic acid due to the presence of both Mn²⁺ and Mn³⁺. These results point to an opportunity for the one-pot synthesis of levulinic acid from renewable biomass waste resources.

            This research was funded by The Indonesia Endowment Fund for Education, Ministry of Finance, Republik Indonesia (LPDP Kementerian Keuangan RI), 2018 for a PhD student.

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