Integrated two-stage chemically processing of rice straw cellulose to butyl levulinate

Highlights

• Integrated two-stage reaction system was developed to synthesize butyl levulinate from rice straw cellulose.

• A novel co-solvent system consisting of dilute H₃PO₄ and THF was used for the preparation of levulinic acid.

• Significant contribution of residual 5-HMF was observed for the direct synthesis of butyl levulinate.

• Effectiveness of co-solvent treatment on rice straw for fermentable sugar release was investigated to the post-reaction solid residues through enzymatic digestion.

Abstract

A two-stage reaction system was developed to synthesize butyl levulinate (BL), a derivative chemical of levulinic acid, from agricultural residue (rice straw). A single reactor was employed during the first processing stage for the conversion of rice straw cellulose to levulinic acid (LA) in a novel co-solvent system consisting of dilute phosphoric acid and tetrahydrofuran. The highest yield of 10.8% wt. LA concentration (i.e., ∼42% of theoretical LA yield) with intermediate residuals concentration of 1.5% wt. glucose and 0.5% wt. 5-hydroxymethylfurfural (5-HMF) on dry weight basis of biomass was obtained at modest reaction conditions. During subsequent esterification reaction, approximately 7.8% wt. BL yield (at 89% conversion yield) was achieved from the solvent extracted precipitate containing majorly LA and residual 5-HMF in the presence of 0.5 M sulfuric acid using n-butanol. Based on comparative esterification results obtained using commercial chemicals (LA and 5-HMF), apparently 5-HMF exhibited ∼8% wt. BL yield through direct synthesis in the presence of sulfuric acid using n-butanol under the same specified reaction conditions. Alongside, effectiveness of co-solvent treatment on rice straw for potential fermentable sugar release (glucose) was investigated by subjecting the respective post-reaction solid residues to enzymatic digestion using cellulase and yielded highest of 11% wt. per wt. solids (27% wt. glucose conversion efficiency), amongst solid residues underwent different processing conditions.

Introduction

In recent years, substantial research interest has been garnered towards chemically or biologically processing of lignocellulosic biomass to chemical products as a more sustainable system than fossil fuel derived chemicals (Jäger & Büchs, 2012). Currently more than 90% of the chemical industry relies on fossil fuel of petroleum and natural gas as feedstock to produce their products. Many researchers have proposed that lignocellulosic biomass substrates such as agricultural residues including rice, wheat, and sorghum straw, could be a potential resource in the near future for sustainable production of fuels (bio-ethanol and -butanol) and chemicals because of their abundant availability, and relatively low-cost, and renewable nature (Berndes, Hoogwijk, & van den Broek, 2003; Field, Campbell, & Lobell, 2008). Among crop residues, rice straw accounts for the largest portion of available biomass feedstock globally with annual production of ∼731 million tons of which about 90% comes from Asia alone (Belal, 2013; Wi, Choi, Kim, Kim, & Bae, 2013). Rice straw, like other agriculture residues, is primary composed of three biopolymers including cellulose (35–40%), hemicellulose (20–25%) and lignin (20–27%) polymers, and also considerable ash matter (10–15% wt. basis). Unlike some other lignocellulosic material such as grasses and legumes, rice straw is considered a low quality dietary fiber because of its higher silica level (or content) that results in poor digestion in animals (Wi, Choi, Kim, Kim, & Bae, 2013) and also reduces its ability to biodegrade in fields. Therefore, the major practice followed in many producing countries is in-field burning as the most cost-effective method of elimination to enable nutrient release and disposal to enable new plantings. However, this creates serious environmental, safety and health issues due to the atmospheric emissions of fine particulate matter and other toxic compounds from the low-temperature combustion (Gadde, Bonnet, Menke, & Garivait, 2009).

Levulinic acid (LA), which may be obtained directly from lignocellulosic materials, is a promising platform chemical that in turn is able to be converted into a wide variety of chemical and fuel products including gamma valerolactone, methyl tetrahydrofuran and others. LA contains a ketone and carboxylic acid functional groups that makes it a versatile building block for the synthesis of various bulk chemicals (Bozell et al., 2000; Galletti, Antonetti, De Luise, & Valentini, 2011). LA is generally prepared from a hexose sugar such as glucose through isomerization to fructose which undergoes dehydration to lose three H₂O molecules to form 5-hydroxymethylfurfural (5-HMF) which then further degrades to levulinic and formic acid. Chemical approaches for LA production involves high temperatures under acidic conditions, using either mineral acids, such as sulfuric, hydrochloric and phosphoric acids (Runge & Zhang, 2012) or solid acids such as acid functionalized zeolites, heteropoly acids, or acid functionalized resins (Mukherjee, Dumont, & Raghavan, 2015). The final product yield is dependent upon the extent of the degradation reactions involved, which are significantly influenced by various reaction parameters including acid concentration, temperature and residence time (Chang, Cen, & Ma, 2007). Typically, this reaction yields less than 50% wt. (maximum theoretical yield is 64.5% wt.) due to the formation of undesired solid co-products typically referred to as humins, from the polymerization of HMF (de Souza, Yu, Rataboul, & Essayem, 2012).

Beyond the yield challenges from the reaction conditions, using biomass cellulose introduces additionally challenges to be effectively hydrolyzed to monomeric sugar molecules through either chemical or biological catalyst strategies because of its crystalline structure (Chundawat et al., 2011; Laureano-Perez, Teymouri, Alizadeh, & Dale, 2005). Extensive research has already been conducted for LA preparation from many possible biomass feedstocks including agricultural residues, energy crops, forestry residues, municipal wastes, algal marine biomass and biomass derived monomeric sugars (Jeong & Park, 2010; Mukherjee, Dumont, & Raghavan, 2015; Rackemann & Doherty, 2011). The majority of these studies have used sulfuric acid as the catalyst because of its low cost and effectiveness under severe conditions (maximum 250 °C and 10% wt. for 7 h), with yields approaching 33% wt. (Galletti, Antonetti, De Luise, & Valentini, 2011; Mukherjee, Dumont, & Raghavan, 2015).

Co-solvent systems, either as a mono- or bi-phasic systems, have been recently developed as a processing strategy to create furanics and their derivatives in higher yields. These systems typically consist of an aqueous solvent, containing an acid catalyst to perform the hydrolysis and dehydration reaction, and an organic solvent, serving as an extraction medium. Several detailed mechanistic studies have proposed that solvent plays a key role by offering continuous extraction of chemicals as soon as they are produced thereby exhibiting improved product recovery by protecting them from possible occurrence of side reactions that generates humic substances. Commonly used extraction solvent agents are methyl isobutyl ketone (MIBK), methyl-tetrahydrofuran (MTHF), dichloromethane (DCM), 2-butanol and tetrahydrofuran (THF) (Cao, Schwartz, McClelland, Krishna, Dumesic, & Huber, 2015; Saha & Abu-Omar, 2014). Among extraction solvents, THF is considered one of the most promising because it is biorenewable and has suitable physical properties for the reaction conditions. Recently, Cai, Zhang, Kumar, and Wyman (2013) have reported improved furanics yield using different biomass substrates in a dilute sulfuric acid-THF co-solvent system under high severity conditions. The researchers proposed that molecular similarities between THF and 5-HMF might be responsible for enhanced furan production by enabling increased extractability. In general, classical LA preparation methods, either through homogeneous or heterogeneous catalysis, encounter corrosion issues due to the reaction media’s corrosive nature of strong acids under severe conditions. Additionally, heterogeneous catalysis must also overcome expensive solid acid catalyst synthesis and its rapid deactivation (Mukherjee, Dumont, & Raghavan, 2015; Rackemann & Doherty, 2011).

Alkyl levulinates (denoted as BioEs) like ethyl, methyl, propyl and butyl levulinates, are derivatives of levulinic acid, (Chang, Xu, & Jiang, 2012; Chen, Zhao, & Chen, 2014; Hu & Li, 2011) and have recently gained attention due to their numerous potential industrial applications as solvent, plasticizer, flavour additive (Mascal & Nikitin, 2010) and oxygenated energy fuel additive to gasoline and diesel fuels (Gürbüz, Alonso, Bond, & Dumesic, 2011). These alkyl esters are generally obtained through Fischer esterification reaction of LA in the presence of strong mineral acids, either homogenous or heterogeneous, and alcohol (Nandiwale, Niphadkar, Deshpande, & Bokade, 2014). Numerous studies have also demonstrated for direct synthesis of alkyl levulinates from cellulose and lignocellulosic substrates through one-pot acid-catalysed reaction with alcohol (Le Van Mao, Zhao, Dima, & Petraccone, 2011; Li, Peng, Lin, Chen, & Zhang, 2013; Peng, Lin, Li, & Chen, 2013; Tominaga, Mori, Fukushima, Shimada, & Sato, 2011). For instance, recently, Chang et al. have reported ∼18% wt. levulinic acid ethyl ester yield from wheat straw using sulfuric acid and ethyl alcohol under elevated reaction conditions (200 °C; 3% wt. H₂SO₄ conc.) (Chang, Xu, & Jiang, 2012). However, most of these chemically processing strategies involve corrosive chemicals such as sulfuric acid, require more energy consuming techniques like distillation, and use environmentally unfriendly solvent chemicals for downstream product processing (Olson, Kjelden, Schlag, & Sharma, 2001). The present study was developed to produce butyl levulinate (BL) from rice straw cellulose via an integrated two-stage chemical transformation reactions with levulinic acid produced in a co-solvent system and then subjecting it to esterification reaction for BL synthesis. A novel combination of dilute phosphoric acid and tetrahydrofuran, as extraction solvent was evaluated for LA production in order to reduce corrosiveness (as compared to strong mineral acids; 150–250 °C; 2–6 h) and improved product yields. An attempt was also made to improve the BL yield by subjecting both LA and residual 5-HMF, which were selectively isolated from the post-reaction liquid medium by employing typical liquid–liquid solvent extraction protocol using ethyl acetate. Lastly, the post-reaction solid residues were subjected to enzymatic digestibility in order to investigate the effectiveness of the co-solvent dilute acid treatment on fermentable sugar release.