The catalytic liquefaction of cellulose under atmospheric pressure mainly uses Lewis acids (molecule, group or ionic acid that can accept foreign electron pairs), Bronsted acids (molecule, group or ion that can release protons), or certain alkali and ionic liquids as catalysts. And the different catalysts affect the liquefaction efficiency, liquefaction path and products distribution. In addition, a comparative analysis has indicated that both the Lewis acid and the Bronsted acid show better performance and have been more widely applied than alkali in catalytic liquefaction of lignocellulosic raw materials to prepare platform compounds (Kumar et al., 2015). Some studies have been conducted on the catalytic liquefaction of lignocellulosic biomass, and various liquefaction methods have been proposed to improve liquefaction yield (Isa et al., 2018) or to reveal related reaction kinetics and degradation mechanisms (Chang et al., 2006; Shi et al., 2016a; Shi et al., 2016b; Walker et al., 2018). In addition, the distinct structures of lignin, cellulose and hemicellulose result in different distributions of liquefaction products: the main liquefaction products of lignin are phenolic derivatives, whereas the counterparts of cellulose/ hemicellulose are furfural and fatty acids and their derivatives (Fig. 3), thus making the separation and purification of liquefied products challenging (Capunitan and Capareda, 2013). To solve this problem, researchers usually use the following two methods: 1) raw materials with low lignin content (such as bamboo or corncob) or low holocellulose (cellulose and hemicellulose) content (such as bagasse) are selected to be liquefied (Li et al., 2014; Liu et al., 2018); or 2) the three major components of lignocellulosic raw materials are separated before use (Liu et al., 2018; Jiang et al., 2018; Yang et al., 2018).
Many studies have reported methods for the separation of the three components of lignocellulosic raw materials and the efficient utilization of lignin (Ma et al., 2014; Rahimi et al., 2014; Shuai et al., 2016; Arni, 2018), but this paper mainly describes the efficient liquefaction of lignocellulosic raw materials and cellulose. In the liquefaction process, the main factors affecting product distribution are reaction time, temperature, pressure, catalyst and catalyst content, solvent and solvent content, among which the catalyst and solvent play prominent roles in the liquefaction process. Xu et al. (2010; 2012) have reported that methanol not only improved biomass conversion but also effectively prevented the further decomposition of liquefied products (sugar derivatives) into carbonyl compounds, thus enhancing the stability of the degradation products of cellulose and hemicellulose (sugar derivatives), and increasing the yield of products. Feng et al. (2015) have studied the effects of methanol and the dimethoxymethane/methanol composite solvent system on the liquefaction behavior of cellulose by changing the system of the liquefaction solvent, and the results showed that since dimethoxymethane/methanol composite solvent system has better protonation ability than methanol single solvent system, the platform compounds including methyl glycoside and levulinic acid ester could be prepared more efficiently. Ma et al. (2017) have analyzed the optimum liquefaction conditions for microcrystalline cellulose, xylose and bamboo by using response surface methodology. This study yielded a set of optimum technological parameters for bamboo powder: a ratio of 0.99:1 and 18.75:1 for phenol-solid and alcohol-solid, respectively, reaction time of 66.94 min, reaction temperature of 204.56℃ and catalyst dosage of 5.09% (according to the mass fraction of bamboo powder). Under these conditions, the liquefaction rate of bamboo powder reached 99.902%. In the final experiment, when a 1:1 ratio of phenol to solid, 20:1 ratio of alcohol to solid and 5% dosage of catalyst were used, the actual conversion rate of bamboo powder reached 98.5%, and the yield of levulinic acid ester reached 65% after reaction of 60 min at 200℃.
Furthermore, Li et al. (2016) have found an increased content of heavy components in the liquefied products of straw cellulose with increasing methanol dosage in subcritical/supercritical methanol. Their further experiments on the influence of the hydrogen radical and hydroxyl radical on the distribution of liquefied products have revealed that increasing the content of free radicals in the reaction system resulted in a tendency of the light components in liquefied products to be polymerized to heavy components, thereby decreasing the liquefaction rate. The results of above studies showed that biomass liquefaction technology is an effective means of converting solid biomass into high value-added platform compounds and liquid products. And the types and dosages of solvents also play important roles in the distribution of liquefied products. Especially for inert solvents, their stability not only provides a favorable reaction medium but also prevents side effects. In addition, under the action of an acid catalyst on proton solvents with strong proton donor capacity, polymers such as cellulose and hemicellulose can be readily protonated in the initial stage of liquefaction, and open-chain depolymerization and open-ring reaction of depolymerized monomers subsequently occur (Isa et al., 2018; Mellmer et al., 2018).
We also summarized the parameters of the liquefaction process for several typical platform compounds, such as alkyl glycosides, furfural, 5-hydroxymethyl furfural, and levulinic acid and its esters, prepared from lignocellulosic biomass liquefaction (Table 1, Table 2 and Table 3). These data showed that, to some extent, the different catalysts could provide milder reaction conditions, and target chemicals could be prepared efficiently by using lower reaction temperature and less reaction time than before (Pileidis and Titirici, 2016). This phenomenon occurred during the preparation of either low-grade chemicals (e.g., alkyl glycosides) or high value-added chemicals (furfural, 5-hydroxymethyl furfural, levulinic acid and its esters) (Pileidis and Titirici, 2016). Moreover, because of their disadvantages of strong corrosiveness, poor or absent recyclability, and environmental unfriendliness, typical protonic acids such as inorganic strong acids (such as sulfuric acid and hydrochloric acid) are difficult to industrialize, although they exhibit high selectivity and conversion efficiency in catalytic liquefaction. Therefore, a growing number of studies have been focused on heterogeneous catalysts such as solid acids, zeolite- supported and Lewis acid catalysts. These catalysts have the advantages of high selectivity, easy recovery and reusability in the conversion of cellulose model compounds (Pritchard et al., 2015). However, the conversion efficiency and selectivity of liquefied products decrease when biomass materials are liquefied. On the basis of characterization of the catalysts before and after use, this problem could be further explained by the deactivation of the catalysts as a result of continuously reduced active sites, due to the varying degrees of carbon deposition on the surfaces of the solid catalysts. Further, scientists have attempted to compound two different types of catalysts to prepare bifunctional catalysts (Li et al., 2017) to achieve efficient catalytic liquefaction of lignocellulosic materials.
No. Substrate Solvent Catalyst T (℃) T (min) Yield (%) Reference 1 α-cellulose Ethanol H2SO4 200 30 48.0 Zheng et al., 2018 2 α-cellulose Ethanol H3PW12O40 200 30 53.0 Zheng et al., 2018 3 Cellulose Ethanol Lig-SO3H-17% 200 120 61.0 Zheng et al., 2018 4 Microcrystalline cellulose Ethanol H4SiW12O40 180 15 63.0 Deng et al., 2010 5 Bamboo Methanol H2SO4 200 10 40.6 Feng et al., 2015 6 Microcrystalline cellulose Methanol [Amim]Cl-H3PW12O40 170 165 70.2 Dora et al., 2012
Table 1. Preparation of alkyl glycoside by catalytic liquefaction of lignin cellulosic
No. Substrate Solvent Catalyst T (℃) T (min) Yield (%) Reference 1 Grass 4 mL water-(0.35 g) NaCl/THF (1:3, V/V) AlCl3-6H2O 180 30 66 Yu and Tsang, 2017 2 Maple wood Water H2SO4 170 40 62 Yu and Tsang, 2017 3 Microcrystalline cellulose Ionic liquid SPPS 140 240 68 Li et al., 2018 4 Cornstalk Water-THF (1:4, V/V) FeCl3 170 80 42 Yu and Tsang, 2017 5 Barley shell Water-DMSO (0.3:0.7) Sulphani-licacid 150 60 41 Yu and Tsang, 2017 6 Maple wood Water-THF (1:1, V/V) FeCl3 170 60 51 Yu and Tsang, 2017 Notes: ionic liquid, 1-methyl-3-ethyl imidazolium bromide; SPPS, heterogeneous sulfonated poly (phenylene sulfide); THF, tetrahydrofuran; DMSO, dimethyl sulfoxide.
Table 2. Preparation of furfural or 5-HMF (5-hydroxymethyl furfural) by catalytic liquefaction of lignin cellulosic
No. Substrate Solvent Catalyst T (℃) T (min) Yield (%) Reference 1 Cornstalk Water AlCl3-NaCl 180 120 35.1 Zuo et al., 2014 2 Microcrystalline cellulose Water SA-SO3H 180 720 51.5 Shen et al., 2017 3 Bamboo Water HCl 160 180 9.16 Sweygers et al., 2018 4 Microcrystalline cellulose Water Sulfated TiO2 240 15 27.2 Wang et al., 2010 5 Pine wood Ethanol H2SO4 145 120 44.4 Li et al., 2014 6 Wheat straw Ethanol H2SO4 183 36 51.0 Tang et al., 2014
Table 3. Preparation of levulinic acid/ethyl levulinate (LA/EL) by catalytic liquefaction of lignin cellulosic