Alcohol Mediated Liquefaction of Lignocellulosic Materials: A Mini Review

Acid catalysed liquefaction of lignocellulosic materials in the presence of various alcohols is an efficient way to prepare alkoxy-glycosides and levulinic esters. In this mini review, the mechanism of the liquefaction process and the applications of liquefied products, such as upgrading into polyurethane and phenolic resol resin, are highlighted. INTRODUCTION The coming decades will be marked with the gradual depletion of fossil fuel resources [1], necessitating sustainable technologies to emerge and to replace those in current use. Lignocellulosic biomass, the most accessible and abundant biomass, receives a remarkably increased interest from the scientific community [2,3]. Nevertheless, significant fundamental research are still required [4] to enable the manufacturing of second generation biofuels and biomaterials from woody biomass industrially viable. Woody biomass has three main components, namely lignin, hemicellulose and cellulose [5]. Lignin, a branched polymer composed mainly of phenyl derived alcohols, confers to cell walls structural reinforcement and water proofing. Three main subunits, including guaiacyl, syringyl and p-hydroxyphenyl are found in lignin. These moieties are further crosslinked either by C-O or C-C bonds. Hemicellulose, a carbohydrate polymer mainly composed of hexose and pentose sugars, is composed of five subunits, including D-glucose, D-Mannose, D-Galactose, D-Xylose and L-arabinose [6] (Figure 1, B). Different moieties such as methyl and acetyl groups can be incorporated on the sugar monomers [7]. The hemicellulose polymer chains can be branched and interestingly, can be crosslinked with lignin [8,9]. While being non-covalently bound to cellulose, this copolymeric structure creates an efficient sheath to protect the cellulose fibres and strengthens lignocellulosic cell walls. Finally, cellulose is a linear polysaccharide composed solely of D-glucose units (Figure 1, C) linked together by β-1-4 glycosidic bonds. It is the most abundant organic polymer on earth [10]. Proportion of cellulose in hardwoods usually ranges from 40 to 55 wt%. Due to its organized structure and the strong stabilizing effect of intra and intermolecular hydrogen bonding, [11,12] cellulose is relatively resistant to physical and chemical treatments. Chemical liquefaction of woody biomass has been studied for decades. By liquefaction process, the biomass components are depolymerized to liquid products, which are potential intermediates to produce various value-added polymers or chemicals. It is an economically feasible method for the transformation of biomass because it can convert biomass into materials that could be easily utilized or further upgraded under relatively low temperatures with a short reaction time. Among other methods, alcohol mediated liquefaction of lignocellulosic materials is particularly interesting and have been extensively carried out. A wide array of alcohols such as ethylene glycol (EG) [13,14], propylene glycol (PG) [15], polyethylene glycol (PEG), glycerol (Gly) [16] and phenol [17,18] were screened and they exhibited remarkable abilities to liquefy cellulose. In addition, two cyclic carbonates—ethyl carbonate (EC) and propyl carbonate (PC) [19] — appeared to be suitable candidates as liquefaction reagents. This mini review will highlight most representative liquefaction process in the presence of these reagents. The reaction mechanism, the product characteristics, the effect of different acids, and the influence of acid-alcohol ratios [20] on the liquefaction are discussed. The purpose of using polyhydoxyl alcohols or phenols to liquefy woody substances is not for the production of biofuels as more efficient methods already exist. Polyhdroxyl compounds are often too expensive to produce cost efficient biofuels. Instead, glycosides and levulinic esters are obtained after woody biomass liquefaction in the presence of polyols and phenols. Alkyl glycosides can be used as detergent, surfactant, emulsifier as well as solubilizer, some widespread cosmetic chemicals. In addition, the diverse glycosides being produced through this method might be employed to a wide array of applications which are partially reviewed in the last section of the article. For example, glycosides can be further utilized to manufacture biodegradable polyurethane (PU) resins and phenolic resin foams [21]. Central Pierson et al. (2013) Email: [email protected] Chem Eng Process Tech 1(2): 1014 (2013) 2/5 Levulinic esters are important platform molecules and different derivatives can be used for multiple purposes. Methyl, ethyl and butyl levulinates are useful components in the fuel industry. They can be used as oxygenated fuel additive in regular diesel engines [22]. Ethyl levulinate is also used nowadays as a food additive. The general liquefaction methodology is straightforward. The process is simply achieved by adding an excess of solvent with a catalytic amount of acid with dried cellulose. The solution is stirred at a set temperature for an appropriate time. Afterwards the reaction is generally quenched by adding either 1,4-dioxane or distilled water and neutralised with a base (KOH or NaOH are most common). The reaction parameters in various reports are compiled in (Table 1.) Note that this table does not intend to be exhaustive but to provide representative examples. Other experimental conditions might be found in the literature. The time in the table is the duration required to achieve > 90% yield, which ranges from 10 min to 4 hours. The main methods for product analysis are HPLC, 1H-NMR, 13C-NMR, FT-IR, and size exclusion chromatography. To thoroughly understand the general mechanism of cellulose depolymerisation, cellobiose, a D-glucose disaccharide, has been used as a model compound. The study of its glycosidic bond hydrolysis mechanism provides an insight on the cellulose hydrolysis mechanism. It was confirmed later that cellulose and cellobiose follow the same liquefaction pathway [23,24]. In addition, linear alcohols such as PG, EG and PEG do not affect the reaction mechanism. As such, a general mechanism for cellulose liquefaction can be proposed, as shown in (Figure 2.) Cellulose hydrolysis follows two possible mechanisms. One is the closed chain path and other is the opened chain path. In the closed chain path the oxonium formed expresses a strong electrophilic character. It is believed that the alcohol hydroxy group acts as a nucleophile consequently leading to the formation of glycosides. In the opened chain path, the protonation of the ring oxygen leads to an increased electrophilicity of the C-1. This is consequently followed by a nucleophilic attack from the hydroxy group on C-1. These two mechanisms have been studied through the stability of methyl glycosides and evidence has pointed to the higher stability of the oxonium intermediate [25]. The presence of alkoxy glycosides in such conditions was reported [18,26] but as water is part of the reaction mixture it is impossible to exclude the possibility that cellulose hydrolysis is faster or that water acts as a co-catalyst. A reasonable mechanical pathway describing the transformation of glycoside to levulinate esters would logically ensue from the previous assumption. Indeed, transformation from a glycoside to a 5-(alkoxymethyl) furfural without hydrolysis seems unlikely. Mechanistically, in order to isomerise to fructose the opening of the D-glucose ring proceeds through the formation of an aldehyde. In the case of a glycoside this formation seems unlikely as the aldehyde formation would be hindered by the alkoxy group. Glycoside hydrolysis to D-glucose was reported to be an equilibrium process through the formation of an oxonium [27], providing a strong argument that cellulose depolymerisation proceeds through the closed chain pathway. In the case of a solvolysis, the high alcohol concentration and acidic conditions would shift the equilibrium towards the glucose alkoxylation. In order to fully convert the glycosides, the alkoxy group needs to follow hydrolysis under acidic condition releasing a D-glucose O O Guaiacyl subunit O O Syringyl subunit