pH‐Controlled Efficient Conversion of Hemicellulose to Furfural Using Choline‐Based Deep Eutectic Solvents as Catalysts

Abstract

The valorization of hemicellulose isolated from lignocellulosic biomass (wheat straw, rice husk, and bagasse) to furfural was achieved by pH‐controlled acid catalysis using choline‐based Brønsted acidic (BA) and natural acidic (NA) deep eutectic solvents (DES) serving both as catalyst and solvent. The effect of pH variation on the catalytic activity of various BADES and NADES prepared in 1 : 1 molar ratio was observed, and choline chloride/p‐toluene sulfonic acid (ChCl/p‐TSA) was found to be the best with lower pH value of 1.0. The yield of furfural decreased from 85 to 51 % with increase in pH from 1.0 to 3.0. The molar ratio of hydrogen bond donor to acceptor components was varied from 1 : 1 to 1 : 9 to achieve the lowest possible pH values of the DESs and to increase the furfural yield. Further optimization of reaction conditions was also done in terms of DES loading, time of reaction, and temperature using the model DES to achieve higher furfural yield. The best results were obtained using 5 mmol DES at pH 1.0 in 1.5 h at 120 °C. ChCl/p‐TSA and ChCl/oxalic acid among BADES and ChCl/levulinic acid among NADES investigated in this work yielding 85 % furfural were found to be most efficient. The reported methodology is advantageous in terms of using bio‐based green solvents, mild reaction conditions, and efficient scale‐up of the reaction. The DESs were found to be efficiently recyclable up to five consecutive runs for the process.

Recent Advances in the Catalytic Conversion of Biomass to Furfural in Deep Eutectic Solvents

“…Two methods are generally used to produce furfural in the DES system. In the “two-step method” in which the lignin carbohydrate complex is destroyed by pretreatment, the hemicellulose-derived sugar is selectively dissolved from the biomass, and then the acidified aqueous DES solution is used to produce furfural from the xylose-rich pretreatment solution ( Arora et al, 2021 ), and the “one-step method” in which the acidified DES is used to produce furfural by directly treating pentosan or biomass ( Lee et al, 2019 ; Rusanen et al, 2021 ). The reaction can be further enhanced to promote furfural synthesis if Brønsted acid or Lewis acid catalyst is added to the DES system to some extent ( Wang et al, 2018 ; Chen and Wan, 2019 ).…”

“…The acidic strength of the acid catalyst/solvent is well known to play a critical role in the hydrolysis of hemicellulose ( Choudhary et al, 2012 ; Wang et al, 2015 ; Delbecq et al, 2018 ), and it is thus critical to optimize and mediate ADES acidity ( Kore and Srivastava, 2011 ). The higher the pH grew closer to 1, the larger the productivity of furfural, according to Arora et al (2021) . The yield of furfural was 85.4% after treatment of hemicellulose with ChCl: p-toluene sulfonic acid (p-TSA) = 1:1 and pH = 1 at 120°C for 1.5 h ( Table 2 , entry 1).…”

“…The yield of furfural was 85.4% after treatment of hemicellulose with ChCl: p-toluene sulfonic acid (p-TSA) = 1:1 and pH = 1 at 120°C for 1.5 h ( Table 2 , entry 1). When the pH was increased from 1 to 3, the yield of furfural reduced to 51.4% ( Table 2 , entry 2) ( Arora et al, 2021 ). The effect of acidity on the yield of furfural was also confirmed by Cornelius et al when comparing choline chloride-dicarboxylate-based low eutectic solvents.…”

Synthesis of furfural from xylose using a choline chloride-based deep eutectic solvent and mechanistic insights

“…20,21 DES have been reported in the literature in processes for evaluating biomass and the carbohydrates derived from them, such as for the synthesis of 5-hydroxymethylfurfural, ethyl levulinate and furfural. [22][23][24][25] DES were studied for the first time in 2001 by Abbott A. P. with the aim of seeking alternatives to conventional organic solvents and ionic liquids used in laboratories and industry. 26,27 They are a mixture of two or more substances-one of which is a hydrogen bond acceptor and the other a donor--which, in certain molar ratios, has a lower melting temperature than the pure components.…”

Separation of Lignocellulose and Preparation of Xylose from Miscanthus lutarioriparius with a Formic Acid Method

“…Lignocellulose is mainly composed of cellulose (30~45%), hemicellulose (20~25%), and lignin (15~25%). Hemicellulose can be used to produce products such as xylose, furfural, xylitol, and xylo-oligosaccharides, and it can be further derived into biofuels, bio-based materials, and chemicals [2][3][4]. In nature, cellulose, hemicellulose, and lignin are combined with each other and closely linked to form complex supramolecular complexes, which makes lignocellulose difficult to effectively degrade and utilize; meanwhile, the enzymatic hydrolysis of cellulose largely depends on the adsorption of enzymes on the substrate, while lignin can irreversibly adsorb enzymes, leading to enzyme inactivation [5].…”