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Review

Recent Advances of Solvent Effects in Biomass Liquefaction Conversion

by
Hui Ming
*,†,
Xin Yang
†,
Pu Zheng
,
Yifan Zhang
,
Haoxin Jiang
and
Libo Zhang
State Key Laboratory of Heavy Oil Processing, College of Engineering, China University of Petroleum-Beijing at Karamay, Karamay 834000, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Energies 2024, 17(12), 2814; https://doi.org/10.3390/en17122814
Submission received: 4 April 2024 / Revised: 28 May 2024 / Accepted: 3 June 2024 / Published: 7 June 2024
(This article belongs to the Topic Advances in Biomass Conversion)
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Abstract

:
Liquefaction conversion technology has become one of the hottest biomass conversion methods due to its flexible material selection and extensive product applications. Exploring biomass liquefaction conversion focuses on catalysts, biomass/water ratio, and reaction temperature. However, it is found that solvents are crucial in the biomass liquefaction process and significantly impact the type of liquefied products and bio-oil yield. Given the current rapid development trend, timely sorting and summary of the solvent effect in the biomass liquefaction process can promote the subsequent development and industrialization of more efficient and cleaner biomass liquefaction technology. Therefore, this review first introduces the characteristics of water as the liquefaction solvent, then summarizes the effects of organic solvents on liquefaction, and finally elaborates on the synergistic effect of co-solvents, which provides a more systematic overview of solvent effects in the liquefaction process. Meanwhile, prospects are put forward for the future development of biomass liquefaction conversion.

Graphical Abstract

1. Introduction

Currently, approximately 81.8% of global energy demand comes from non-renewable fossil fuels [1]. However, on the one hand, fossil fuels are increasingly depleted due to their non-renewability [2]. On the other hand, large amounts of greenhouse gases are emitted during the conversion and utilization of fossil fuels [3], causing severe environmental pollution. To come to grips with the potential energy crisis and mitigate the environmental harm caused by the combustion of traditional fossil fuels, researchers are actively expanding the scope of renewable energy. Compared with other renewable energy sources, bioenergy has the following characteristics and advantages: (1) Bioenergy is cheap and has a wide range of sources [4,5]. (2) Total bioenergy is abundant and currently occupies about 14% of global primary energy consumption [6]. (3) Bioenergy can be converted into heat, electricity, and transport fuel. Converting biomass into low-carbon energy is an essential goal for sustainable development [7]. (4) Bioenergy has the characteristics of good environmental friendliness and carbon neutrality [8].
Biomass, as the carrier of bioenergy, has a low calorific value for direct use and usually needs to be converted into high-grade products to improve performance before use. Standard conversion methods of biomass include liquefaction, pyrolysis, and gasification [9,10,11]. Among them, liquefaction or solvent liquefaction is the process by which the biomass is processed for a period of time to decompose the solid biopolymer structure into the main liquid components [12]. Solvent liquefaction in water is called hydrothermal treatment, hydrothermal liquefaction, or hydrothermolysis (HTL). The predicted path of the HTL mechanism is shown in Figure 1 [13]. As a promising biomass conversion method, liquefaction conversion has advantages that are incomparable to other conversion methods: (1) Most biomass contains a large amount of inherent and processed moisture, and additional pretreatment and drying steps are required before other conversion methods are used to produce biofuels [14]. In contrast, liquefaction conversion usually does not require dehydration treatment, is universal for a considerable range of moisture content biomass feedstocks, and is more flexible in feedstock selection, while reducing the process flow, lowering costs and energy consumption, and achieving more excellent economic benefits. (2) Liquefaction conversion technology offers lower reaction temperatures and more robust safety than pyrolysis technology. Meanwhile, the liquefaction conversion technology capitalizes on the characteristics of the superheated fluid to reduce the mass transfer resistance. It utilizes high pressure to facilitate the breakdown of biomass molecules, which increases bio-oil yield [15,16,17]. (3) Biomass liquefaction conversion technology has the advantages of a short reaction cycle and wide adaptability of raw materials [18].
Most recent studies on biomass liquefaction technology concentrate on the influences of reaction conditions such as catalysts and rapid hydrothermal liquefaction on production efficiency and product physical and chemical properties. Yan et al. [19] explored the influence of catalyst sodium hydroxide on bagasse liquefaction and found that adding sodium hydroxide reduced bio-oil acid and furfural content. Ni et al. [20] showed that the bio-oil yield obtained from rapid hydrothermal liquefaction was 3.3 wt% higher than that from isothermal hydrothermal liquefaction. Hao et al. [21] investigated the influences of different reaction conditions on the liquefaction of different types of biomass. As shown in Figure 2a–d, bio-oil production generally increases and then decreases with increasing temperature, and the same result was also found by De Caprariis et al. [22] Also, the solid product content falls as the temperature rises, and more material is decomposed and converted to other phases. From Figure 2e–h, it can be seen that prolonged residence time favors biomass conversion due to the reduction in the content of solid products. Meanwhile, according to Nakason [23], prolonging the residence time within a specific range can increase bio-oil yield, and the products can obtain better physical properties. As shown in Figure 2i–l, bio-oil production decreases rapidly with increasing biomass-to-water ratio, while solids production increases significantly.
Similarly, the properties of the solvent will also significantly affect the process of biomass liquefaction conversion. During the liquefaction process, the solvent can act as a chemical component and participate in the reaction, greatly enhancing the dissolution and dispersion processes of the reactants and the chemical reactivity, thereby enabling the reaction to occur at a lower temperature. On the other hand, the solvent could act as an expansion accelerator and a pressure transmission medium to affect the formation of the product by accelerating the osmotic reaction and controlling the physical and chemical factors of the conversion process. Protic solvents such as water, ethanol, and methanol can also provide hydrogen sources during the reaction process, thus inhibiting side reactions and increasing bio-oil yield [24,25,26]. Therefore, this review mainly focused on the solvent effect of biomass liquefaction conversion and systematically summarized the current liquefaction solvents, which will help to deepen the understanding of the solvent effect on the liquefaction process to select suitable solvents for biomass conversion.
This paper first introduced the distribution of world energy consumption by type of energy source and the hazards brought by traditional fossil fuel combustion; summarized the advantages of bioenergy compared to other renewable energy sources; generalized the benefits of biomass liquefaction conversion methods; and expounded the research hotspots of the current biomass liquefaction conversion technology. Secondly, two typical biomass raw materials were introduced. Thirdly, the common solvents used in the liquefaction process were systematically classified, the liquefaction characteristics of the water, organic solvent, and co-solvent systems were analyzed, and the solvent effect on the liquefaction conversion process of each solvent system was expounded. Finally, this paper summarized the biomass liquefaction conversion technology and its prospects.

2. Typical Biomass Raw Materials

Biomass mainly includes organic matter generated by plants through photosynthesis (such as plants, animals, and their excrement), garbage, and organic wastewater. Before the formal review of the solvent effect in the liquefaction process, three typical biomass feedstocks will be introduced first to give readers a general understanding of biomass feedstocks.

2.1. Lignocellulose

Lignocellulose, chiefly composed of cellulose, hemicellulose, and lignin, has become a popular feedstock for liquefaction conversion because of its high annual yield, wide range of sources, and low nitrogen and sulfur oxide emissions during conversion [27]. Cellulose is a macromolecular polysaccharide composed of D-glucose. Due to the dense intermolecular and intramolecular hydrogen bonds, cellulose has high crystallinity, can resist the attack of enzymes, and is insoluble in water. However, under subcritical and supercritical conditions, cellulose will decompose and dissolve rapidly [28]. Hemicellulose is a heteropolymer composed of several monosaccharides and is linked to cellulose through hydrogen bonding and lignin through covalent bonding, respectively [29]. Lignin, which consists of coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol, improves cell wall strength by filling the space between cellulose and hemicellulose and combining the lignocellulose matrix [30]. The relationship among the three is shown in Figure 3b [31].
During the liquefaction process, the cellulose and hemicellulose degradation pathway is mainly hydrolyzed to monosaccharides and further generates acids, aldehydes, ketones, and so on. Lignin has a more complex structure and contains a large number of aromatic compounds in the liquefaction products. The hydrolysis process of lignocellulosic biomass in hydrothermal liquefaction is shown in Figure 3a [32]. Guedes et al. [34] revealed that the bio-oil yield is usually proportional to the cellulose and hemicellulose content of the biomass, which paves the way to improving the bio-oil yield concerning material selection. However, because of its complicated structure, lignocellulose typically needs to be pretreated in order to maximize conversion efficiency. Furthermore, the conversion process of lignocellulose typically necessitates elevated temperatures and pressures, potentially resulting in escalated energy consumption.

2.2. Algae

Algal biomass, as the main raw material for biodiesel, has shown enormous potential in certain application fields. Compared with biofuels of the first and second generation, algae have the advantages of fast growth rate, strong CO2 fixation capacity, growth in non-arable land or saline soil and wastewater, non-competition with food and feed crops, and high-fat content potential, thus becoming the ideal third generation biofuel. Algae are mainly composed of lipids, proteins, and carbohydrates [35], of which the lipid is a crucial metabolite of microalgae and is also an essential source of biodiesel and high-value-added products [36], usually accounting for 1–70 wt% of microalgal stem cells. In the past few decades, the research on lipid production in microalgae cells has become the focus of attention to increase microalgae’s lipid content. The synthesis of fatty acids and triglycerides (TAGs) are the two main processes of lipid accumulation in microalgae, as shown in Figure 3c [33]. Malonyl–CoA is the substrate of fatty acid synthase (FAS) polymerization [37,38], which catalyzes fatty acid synthesis. The formed free fatty acids are converted with diacylglycerol (DAG) to synthesize triglycerides (TAGs), which are finally transported to the lipid synthesis site to synthesize lipids.
The major drawbacks of algal biomass typically include the following: (1) high levels of alkali and halogen elements, ash (inorganic matter), N, O, S, Si, and many trace elements (including deleterious elements); (2) low levels of organic matter, carbohydrates, cellulose, volatiles, cellulosic crystallinity index, calorific value, and reactivity [39]. Lipids are the main component of the microalgae liquefaction process converted to bio-oil. The lipid decomposition pathway is shown in Figure 3d [21]. Under HTL circumstances, lipids can undergo rapid non-catalytic hydrolysis to create glycerol and higher fatty acids due to their low dielectric constant. The fatty acids produced are partly converted into alkanes by decarboxylation and hydrogenation, partly formed into fatty acid esters by deoxygenation and esterification with fatty acids, and partly reacted with ammonia produced during amino acid conversion to form fatty acid amides [21]. In general, the above components constitute the bio-oil produced by hydrothermal liquefaction of microalgal biomass.

2.3. Sludge

The treatment of wastewater not only helps to alleviate water scarcity and environmental challenges but also produces a by-product called municipal sludge. The composition of municipal sludge is complex, so most researchers use model compounds to study its hydrothermal liquefaction reaction pathways. The three main components include lipids (15.4–20.6 wt%), proteins (6.8–10.6 wt%), and carbohydrates (16.1–22.7 wt%) [40]. Lipids are the main components of biocrude produced from municipal sludge and produce glycerol and fatty acids through hydrolysis. Glycerol undergoes degradation and condensation to form water-soluble substances and gaseous products. Fatty acids, on the other hand, are converted to long-chain alkanes, fatty acid esters, and fatty acid amides. Proteins are the main source of nitrogenous compounds in the high-temperature hydrothermal reaction products of municipal sludge. In the hydrothermal environment, proteins undergo hydrolysis to produce amino acids, which then undergo parallel deamination and decarboxylation reactions to produce amides, carboxylic acids, ammonia, and carbon dioxide [41]. Non-cellulosic carbohydrates produce glucose and fructose upon hydrolysis. Of these, the degradation products of fructose include 5-hydroxymethylfurfural, while glucose undergoes a Meladic reaction with amino acids produced by protein hydrolysis to produce nitrogen-containing heterocyclic compounds [21]. Lignocellulose includes cellulose, hemicellulose, and lignin, and each component undergoes a different reaction process in HTL. Cellulose and hemicellulose are first decomposed into glucose, pentose, and hexose in the reaction, and these monosaccharides are further decomposed into aldehydes, ketones, and 5-HMF [42]. Lignin, on the other hand, forms different phenolic compounds and methylbenzene by breaking bonds. Some of these compounds may also be hydrolyzed to form aromatic hydrocarbons, aromatic polymers, etc. [43].

3. Liquefaction Conversion Solvent

In the process of hydrothermal liquefaction of biomass for bio-oil production, the physical properties (dielectric constant, density, and boiling point) of the solvent exert significant influence on the bio-oil yield.
Solvents with a low dielectric constant have the ability to enhance the dissolution of high-molecular-weight compounds. Thereby preventing the formation of solid char [44,45]. This is attributed to their ability to enhance the thermal decomposition of high-molecular-weight intermediates [46]. By promoting this decomposition, they contribute significantly to the increased bio-oil yield during the hydrothermal liquefaction of biomass. Organic solvents, due to their relatively lower dielectric constant and surface tension, are more prone to reaching the critical point during the liquefaction process [47]. This facilitates the improved dissolution of high molecular intermediates compared to water solvents. Consequently, the low dielectric constant of these solvents prevents the formation of char and ultimately enhances the bio-oil yield.
The density of the solvent affects its interaction with the biomass feedstock. Solvents with lower density and viscosity should penetrate into the biomass substrate more easily, enabling closer contact and interaction with the biomass [48]. This intimate contact facilitates more effective solubilization and dispersion of organic substances within the biomass, increasing the reaction surface area between the biomass and the solvent, thereby promoting biomass degradation and bio-oil formation.
The boiling point of the solvent, which is closely related to its saturated vapor pressure, has a profound impact on the yield of bio-oil. Specifically, an elevated vapor pressure favors the occurrence of secondary reactions, such as thermal cracking, repolymerization, and recondensation, that lead to the formation of a more coalified and stable solid product [49]. However, this high vapor pressure hinders the critical degradation reactions of biopolymers [50,51] and hemicellulose [52], which are the primary sources of bio-oil. These degradation reactions, which include hydrolysis and decomposition, are essential for converting the biomass into liquid fuels and chemicals. By limiting the availability of reactive intermediates and shifting the equilibrium towards more condensed products, a high boiling point solvent reduces the efficiency of these degradation reactions, thereby decreasing the overall yield of bio-oil. In other words, the boiling point of the solvent in the hydrothermal liquefaction process reduces the conversion efficiency to bio-oil.
Liquefaction reactions using water, organic solvents, and co-solvents as liquefaction solvents are current research hotspots. Table 1 lists the progress of research on the solvent effects of some of the solvents used in biomass liquefaction.

3.1. Water

Water is vital as the catalyst, solvent, and reactant in biomass hydrothermal liquefaction conversion [53,85]. During hydrothermal liquefaction, the presence of water helps to enhance the dissolution of intermediates. Also, water stabilizes free radicals and improves bio-oil quality [86]. Therefore, studying the changes like water under different conditions plays a crucial role in hydrothermal conversion reactions. As shown in Figure 4 [87], water’s physical and chemical properties would change significantly with the increase in temperature. The significant decrease in the amount of hydrogen bonding in water leads to a sharp reduction in the density of water with increasing temperature at the critical point (~374 °C). At the same time, the decrease in the number of hydrogen bonds and the dielectric constant makes supercritical water (SCW) fully compatible with small organic molecules, producing a homogeneous reaction environment. In addition, the decrease in viscosity of SCW further reduces the resistance of the transfer process.
According to Li et al. [88], water can play the role of a catalyst in biomass liquefaction. Under subcritical conditions, water molecules dissociate into H+ and OH−, promoting acid- and alkali-catalyzed reactions under hydrothermal conditions and increasing bio-oil yield. SCW is an excellent solvent for biomass liquefaction conversion due to its high miscibility, lack of phase boundary, and strong mass transfer capacity [89]. As shown in Figure 5a, overall, the higher the heteroatom content in the biomass, the lower the bio-oil yield and calorific value [90]. Xu et al. [54] found that even without the use of catalysts, the upgrading treatment of supercritical water (400 °C, 1 h, H2 addition) could produce a positive change in the composition of bio-oils while almost halving the heteroatom content of the feedstock. Moreover, the bio-oil C content increased by 14.1 wt% and the calorific value by 6.1 MJ/kg, while the total N, S, and O content reduced by 14 wt%. Isa et al. [91] stated that supercritical water can contribute 0.5% hydrogen during liquefaction. The generated hydrogen can be used as a deoxidizer to remove oxygen to increase bio-oil yield on the one hand and as a hydrogenolytic agent to decompose biomass on the other hand.
Tian et al. [55] investigated the influence of Spirulina/water ratio on the product yield distribution, as shown in Figure 5b. Bio-oil production is highest when the Spirulina /water ratio is 1/4. This increase may be because water participates in the hydrothermal liquefaction reaction as a solvent and a hydrogen donor, promoting reactions such as pyrolysis and hydration and facilitating intermediate product dissolution. Meanwhile, as the water content in the constant volume container increases, water density increases and the decomposition and extraction of biomass are also naturally enhanced, thereby increasing the bio-oil yield [92], as shown in Figure 5c. Furthermore, the decrease may be that the excess water dilutes the concentration of reactants, resulting in insufficient collisions between reactant molecules. At the same time, the transfer of water-soluble oils to water-insoluble oils and solid phases cannot be ignored.
The most commonly used solvent in biomass liquefaction is water, which has the lowest cost and is environmentally friendly. Water can act as a solvent, hydrogen donor, and catalyst, facilitating reactions such as pyrolysis and hydration, which help to decompose and extract biomass. However, when only water is used as the solvent of biomass liquefaction reaction, the obtained bio-oil has a low calorific value and high viscosity, and some organic compounds produced in the liquefaction process may be transferred from the aqueous phase to the other phases, decreasing the total bio-oil production [93]. Simultaneously, material corrosion is a problem that cannot be ignored when using water as a solvent, especially for liquefaction reactions in subcritical and supercritical water. To solve the above-mentioned problems when water is used as a solvent, the solvent effects of organic solvents and co-solvents have been studied.

3.2. Organic Solvents

In recent years, many reports have used organic solvents as liquefaction solvents. Organic solvents generally have the following advantages in biomass liquefaction: (1) Organic solvents can facilitate alkylation and esterification reactions and help dissolve and stabilize intermediates [94]. (2) Compared with water, the reaction conditions of organic solvents are mild, and the liquefaction products have a high energy density and low acidity [56,95]. (3) The protic solvent in the organic solvent can donate hydrogen, inhibit side reactions, and increase bio-oil yield [57].
Organic solvents used in biomass liquefaction reactions are generally divided into polar solvents, non-polar solvents, and ionic liquids. In this context, polar solvents refer to fluids with electric dipoles or multipolar moments and are divided into polar protic and polar aprotic solvents based on the donating ability of the hydrogen atom. Polar protic solvents have lone pairs of electrons and hydrogen atoms bound to strongly electronegative atoms that can accept hydrogen bonds and provide protons or participate in forming hydrogen bonds [96]. The polar proton solvents commonly used in the liquefaction conversion of biomass include water, methanol, and ethanol. Polar aprotic solvents can accept hydrogen bonds via lone pairs of electrons but cannot provide protons or form hydrogen bonds [97]. Liquefaction transformation of biomass in polar aprotic solvents (e.g., acetone) has been widely reported. Non-polar solvents have insignificant electric dipoles or multipolar moments, cannot give or receive acidic protons, and do not form hydrogen bonds with other molecules [98]. Ionic liquids have difficulty forming stable crystal lattices and are poorly coordinated liquid organic salts [99]. Ionic liquids could dissolve various compounds and show high solubility for insoluble cellulose and lignin [100]. Some standard biomass conversion ionic liquid solvents include 1-ethyl-3-methylimidazolium chloride and choline acetate.

3.2.1. Polar Aprotic Solvents

During biomass liquefaction conversion, polar aprotic solvents, although usually unable to decompose biomass directly, may affect the kinetic and thermodynamic equilibrium of the reaction or promote mass and heat transfer during the reaction. Mellmer et al. [58] found that γ-valerolactone could decrease the activation energy of the reaction and increase the reaction rate of liquefaction and the selectivity of the product. Song et al. [59] studied the influences of polar aprotic solvents and polar protic solvents on the primary decomposition reactions of glucose and fructose. The study found that adding polar aprotic solvents could reduce raw materials’ decomposition activation energy, but the liquefaction process’s reaction rate constants were lower than those in polar protic solvents. At the same time, aprotic solvents can significantly increase 5-HMF yield from sugar products, which increases the quality of the liquefied conversion product. During the breakdown of glucose and fructose, aprotic solvents promote dehydration processes and inhibit isomerization reactions. In particular, 5-hydroxymethyl-furfural (5-HMF), which in protic solvent/water mixes is a secondary result for the breakdown of both glucose and fructose, turns into a primary product in aprotic solvent/water mixtures. The possible mechanism can be summarized from three aspects.
(1)
Aprotic solvents boost the proportion of furanose forms of sugar tautomer by forming intramolecular hydrogen bonds [101,102]. The significance of furanose forms in augmenting 5-HMF production during fructose decomposition in acidic circumstances was validated by recent in situ NMR analyses [103]. Due to the fact that 5-HMF can be easily generated from the breakdown of fructose, it appears that the furanose forms are crucial for the synthesis of 5-HMF from the breakdown of sucrose.
(2)
It was reported in a molecular dynamic (MD) simulations study that the direct dehydration of glucose to 5-HMF begins with protonation of the C2-OH and breakdown of the C2−O2 link, followed by the creation of the C2−O5 bond [104]. Nevertheless, it is challenging for these processes to occur in non-acidic environments. An aprotic solvent may alter the local solvent arrangement surrounding the glucose molecules, which would enable the acid-catalyzed processes that lead to the synthesis of 5-HMF [105].
(3)
Aprotic solvents increase the stability of 5-HMF formed from sugar decomposition. Aprotic solvents such as DMSO prefer to coordinate around 5-HMF, as shown by earlier simulation studies [106,107]. This creates a shielding effect that stops further rehydration to levulinic acid and formic acid or condensation to produce humins.

3.2.2. Polar Protic Solvents

In biomass liquefaction, polar protic solvents as hydrogen-donating solvents can significantly enhance product distribution, and the bio-oil quality is also improved because of the hydrocracking reaction, which inhibits polycondensation. Hu et al. [60] studied the liquefaction properties of microalgae in ethanol, ethyl acetate, acetone, and methanol. It was shown that the bio-oil yields obtained after the liquefaction of polar protic solvents (methanol or ethanol) were significantly higher than those obtained after the liquefaction of polar aprotic solvents (ethyl acetate or acetone), as shown in Figure 6a. The possible reason is that methanol and ethanol, as hydrogen donor solvents, release hydrogen free radicals (H·) to promote the hydrocracking of large molecules into small molecules. Meanwhile, the fragments/intermediates produced by liquefaction are stabilized under the action of the hydrogen-donating solvent, which could prevent char from forming and improve the bio-oil yield.
Many solvents can be used as hydrogen donor solvents (such as ethanol, glycerol, and cyclohexane), which can supply hydrogen to unstable biomass fragments because they have mobile C-H bonds. Figure 6b reveals the possible hydrogen-donating mechanism of ethanol [108]. The first mechanism suggests that the reaction frees α-hydrogen as hydrides to form electron-deficient hydroxyl alkylation products. The second mechanism is similar to Meerwein–Pondorff–Verley reduction, with ethanol reduction leading to alkoxy ions and protons transfer, followed by α-hydrogen supply and aldehyde formation. The third mechanism shows that cleavage of hydroxide ions and protons may occur in ethanol, leading to the formation of alkenes, except for the donation of α-hydrogens and protons.

3.2.3. Non-Polar Solvents

The permeation and dissolution effects of non-polar solvents on cellulose and hemicellulose are poor. Li et al. [61] explored the effects of methanol, acetone, isopropanol, propanol, heptane, and cyclohexane on cellulose liquefaction, as shown in Figure 7a. Previous studies have shown that the ionic and radical reactions occurring in the system are mainly related to the polarity of the solvent [62]. Therefore, solvents with strong polarity may be beneficial to converting raw materials. Non-polar solvents such as n-heptane and cyclohexane cannot provide hydrogen atoms during liquefaction. Also, they cannot weaken and destroy the hydrogen bonds between and within the molecules of cellulose, which is not conducive to the transformation of cellulose. Fan et al. [63] liquefied oil palm empty fruit bunch (EFB) fibers using water, ethanol, toluene, acetone, and ethylene glycol. As shown in Figure 7b, polar solvents have higher liquefaction yields and polar solvents are more suitable for dissolving EFB fibers.

3.2.4. Ionic Liquids

Ionic liquids can break the hydrogen bond network between biomass macromolecules, effectively dissolving biomass at low temperatures and improving the biomass conversion rate [109]. Jablonski et al. [64] efficiently and selectively converted biopolymer poly-(3-hydroxybutyrate) to crotonic acid at 140 °C using alkaline ionic liquids containing imidazolium cations and the acetate anions, and its yield reached 97%. The conversion process is shown in Figure 8a. Li et al. [65] converted glucose to 5-hydroxymethylfurfural in ionic liquid ([EMIM]Br) at 140 °C using heterogeneous sulfonated poly(phenylene sulfide) (SPPS) as a catalyst with up to 87.2% yield, and a possible conversion mechanism is shown in Figure 8b.
The solvent’s heat and mass transfer ability will affect the biomass’s conversion rate and liquid production. Song et al. [66] used different solvents (ethanol, isopropanol, cyclohexane, cyclohexanol, and tetrahydronaphthalene) to liquefy sawdust at 320 °C for 30 min. They found that cyclohexanol’s product conversion efficiency and liquid yield were the highest, as depicted in Figure 9a. The reasons for the above results can be viewed from the following two main perspectives: (1) From the point of view of heat transfer. Intense evaporation of low-boiling solvents (e.g., ethanol) at 320 °C weakens the heat transfer efficiency between the solvent and the feedstock. On the contrary, cyclohexanol has a more robust heat transfer ability, promoting sawdust’s cleavage and improving the yield. (2) From the perspective of mass transfer. According to the similarity and compatibility principle, cyclohexanol has more substantial solubility to sawdust and dispersion ability to oxygen-containing intermediates, facilitating mass transfer of the reaction. In addition, other characteristics of cyclohexanol, such as high hydrogen supply capacity and good solubility, also promote the mass transfer reaction of cyclohexanol. The liquid phase products are predominantly hydrocarbons when using tetralin as the liquefaction solvent. However, when alcohols are used as solvents, the outcomes are dominated by esters or ketones, as shown in Figure 9b.
To sum up, organic solvents dramatically affect the distribution of bio-oils. In general, the more polar the solvent, the higher the conversion rate of the liquefaction process [67]. Compared with the aprotic solvents, the yield of biological components and the calorific value of bio-oil in proton solvents are usually higher. Ionic liquids easily break the hydrogen bond network between biomass macromolecules and effectively dissolve biomass at low temperatures. The solvent’s heat transfer and mass transfer capabilities could also affect the biomass’s conversion rate and the amount of liquid produced. Compared with water, the liquefaction products of organic solvents have a higher calorific value. However, organic solvents face the problems of high solvent cost, difficulty in recycling, and increased requirements for drying biomass before liquefaction, which is inconducive to the liquefaction reaction. Meanwhile, Leng et al. [110] pointed out that another easily overlooked disadvantage of using organic solvents is that the N content in bio-oils may increase significantly, as shown in Figure 10a. The high nitrogen content in bio-oil increases the amount of nitrogen oxides emitted during combustion and pollutes the atmosphere. In addition, the extraction solvents’ polarity seems to be positively correlated with N content, with more polar solvents extracting more polar components into the bio-oil phase, including N-containing compounds, as shown in Figure 10b [111,112]. Therefore, it is not an excellent choice to use only organic solvents for the liquefaction conversion of biomass.

3.3. Co-Solvent

There are strengths and weaknesses to using only water or organic solvents as the liquefaction solvent. In response to the inadequacy of water and organic solvents alone in the biomass liquefaction process, researchers have developed many co-solvent systems of water/organic solvents and mixtures of different organic solvents. Studies have found that some mixed solvents show synergistic effects on biomass liquefaction [68,69,70,113], which can increase the bio-oil yield.

3.3.1. Organic Solvent/Water Co-Solvent

Walker et al. [71] showed generalized dynamic solvent effects in acid-catalyzed biomass-derived oxygenate reactions. As shown in Figure 11b, the reaction in this study consists of two steps that may be affected by the solvent: (1) Proton transfer from the bulk domain to the reactant. In a polar aprotic co-solvent, the water-enriched local solvent domain would be formed around the hydroxyl group [72]. As shown in Figure 11a, the favorable interaction environment between the co-solvent and the hydrophilic reactants can promote the generation of water-rich regions near the reactants, resulting in a more significant water molecule density near the reactant than in the bulk solvent [114]. Moreover, because the protons have a higher appetency for water than organic phases, they are unstable in the bulk solvent, creating a thermodynamic driving force to transfer protons to the reactants. Classical molecular dynamics simulations can characterize the solvent environment near reactant molecules. Figure 11c,d show simulated snapshots of xylitol in water and 90 wt% of 1,4-dioxane, respectively. (2) Formation of carbocation-like transition states. Since the water molecules tightly bound to reactants are pre-organized into the stable charged transition state configurations, it is inferred that the stability of reactants, protons, and transition states might be relevant in these local solvent domains [115,116].
Ji et al. [73] found a synergistic effect in the liquefaction of Spirulina using a co-solvent (ethanol/water). As can be seen from Figure 12a, with the increase in ethanol content in the co-solvent, the bio-oil production first increased and then decreased. Hu et al. [74] also observed the same phenomenon when using a co-solvent (ethanol/water) to co-liquefy algal biomass and sawdust, as shown in Figure 12b, which the following reasons can explain. On the one hand, subcritical and supercritical water is more acidic than ethanol, which favors ionic and radical reactions [75]. On the other hand, ethanol has a lower dielectric constant than water, which facilitates the decomposition of high-molecular-weight products and prevents coke formation. Therefore, an appropriate increase in ethanol content will increase bio-oil production and minimize the creation of solid residues. When the ethanol content is too high, the free radical reaction will be diminished, leading to decreased bio-oil yield and some bio-oil repolymerization to form solid residues. In addition, compared with hydrothermal liquefaction, the compounds with the highest bio-oil content from ethanol/water co-solvents and pure ethanol liquefaction are esters, suggesting that ethanol can increase bio-oil production by promoting ester formation.
Wu et al. [76] converted lignocellulose to bio-oil using a subcritical ethanol/water co-solvent system. The results showed that the addition of ethanol enhanced the penetration of the solvent into the rigid structure of lignocellulose. The highest bio-oil yield of 36.62% was achieved when 60% content of ethanol was used to liquefy lignocellulose. And with the increase in ethanol concentration, the HHV value of heavy oil decreased, while the HHV value of light oil increased. The heavy oil product contained more esters, ethers, and alcohols and less aldehydes due to the transesterification reaction that occurred with the addition of ethanol. Chen et al. [77] investigated the liquefaction reaction of mulberry bark in subcritical ethanol/water co-solvent (50:50, v/v) and subcritical water. The results showed that the bio-oil yield of mulberry bark in subcritical ethanol/water co-solvent (30.32 wt%) was slightly higher than that in subcritical water (28. 81 wt%), which indicated that ethanol could promote the hydrothermal liquefaction of mulberry bark and increase the liquefaction yield. Moreover, the phenolic content of bio-oil derived from subcritical water was higher than that of bio-oil derived from subcritical ethanol/water co-solvent, while the ester content of bio-oil derived from subcritical ethanol/water co-solvent was higher.
Feng et al. [78] explored the influence of ethanol, water/ethanol co-solvent, and water on the liquefaction process of cellulose, lignin, cellulose/lignin mixtures, and acetone-extracted birch bark (EWBB), as shown in Figure 13d. The high water/ethanol co-solvent yield for biomass liquefaction can be attributed to two aspects: (1) Hot pressurized water accelerates biomass depolymerization by hydrolyzing cellulose and hemicellulose. From the SEM images, it can be observed that the rough surface of the EWBB and the natural cellulose cellular structure shown in Figure 13a are almost missing in Figure 13b,c, suggesting that degradation of the cellulose structure occurs in hot pressurized water or water/ethanol mixtures. (2) Ethanol promotes lignin degradation by solubilizing depolymerized lignin products. Moreover, the ethanol/water co-solvent increases solvent penetration into the lignocellulosic biomass structure, thereby increasing bio-oil yield from lignocellulosic liquefaction, as shown in Figure 13c. Cui et al. [79] studied the influences of glycerol, ethanol, and isopropanol as co-solvents on the liquefaction of microalgae. It has been shown that bio-oils are mainly produced due to the reaction of alcohols and algal fragments through esterification, Maillard, and alkylation, respectively. The alcohol co-solvent improves the bio-oil yield by facilitating the transfer of nitrogen-containing compounds to the oil phase.
Jiang et al. [47] studied the liquefaction characteristics of Amaranthus hypochondriacus L. in water, 60% ethanol, and aqueous phase recycling in 60% ethanol systems, and the distribution of liquefied products is shown in Figure 14a. The outcomes manifested that bio-oil production increases with the growth in water phase recirculation times. Ethanol/water co-solvents can transfer organic nitrogen to the aqueous phase, reducing the bio-oil’s nitrogen content. With the increase in water phase recirculation times of 60% ethanol system, the content of nitrogen compounds in bio-oil is decreasing, as shown in Figure 14b,c. Li et al. [80] used different solvents (water, n-hexane/water co-solvent, methanol/water co-solvent) to liquefy sewage sludge and found that the synergistic effect between organic solvents and water could help to increase the bio-oil production while reducing the bio-oil nitrogen content, as shown in Figure 14d. Compared with water, the content of nitrogen compounds in the liquefaction product bio-oil of organic solvent/water mixture is significantly reduced, and the nitrogen compounds are transferred from the oil phase to other phases.

3.3.2. Organic Solvent/Organic Solvent Co-Solvent

Arturi et al. [81] explored the influence of a co-solvent (acetone/THF) on the liquefaction conversion process using Fallopia Japonica as biomass. The findings indicated that tetralin has both dissolution and scavenging effects, which could increase the abundance of monomeric aromatics in the product. Acetone promotes the cracking of retro-aldol to produce low-molecular-weight oxygenates. Zhou et al. [82] utilized four different solvents (C4H10O2/C6H15NO3 co-solvent, C6H15NO3, C4H10O2, H2O) to liquefy pine trees, and the corresponding results are presented in Figure 15a. The bio-oil yield obtained by C4H10O2/C6H15NO3 co-solvent liquefaction was the highest, while the biochar yield was the lowest. There was a synergistic effect between the solvent C4H10O2 and C6H15NO3.
Zhang et al. [83] used a polyethylene glycol/glycerol co-solvent to liquefy sugarcane bagasse, and it was found that the number of hydroxyl groups in the liquefied product was reduced and the acid value was enhanced as the liquefaction reaction proceeded, as illustrated in Figure 15c. The reduction in the number of hydroxyl groups of the liquefaction products (as shown in Figure 15b) may be because of dehydration and thermal oxidation reactions of the liquefied solvents and reactions with bagasse components. The increase in acid value could be caused by depolymerized bagasse components or oxidation of sugars. Ding et al. [84] investigated the liquefaction characteristics of lignocellulose in ethanol, ethylene glycol, and ethanol/ethylene glycol co-solvents and showed that ethanol/ethylene glycol co-solvents have a synergistic effect on the liquefaction process, which is manifested in higher oil production rates and lower biochar yields. In addition, oil production from ethanol/ ethylene glycol co-solvents gradually increases with decreasing ethanol percentage over a range. The highest bio-oil yield was achieved when the ratio of ethanol to glycol was 1:3, as shown in Figure 15d.
To sum up, the excellent interaction environment between the co-solvent and hydrophilic reactants could promote the generation of water-rich regions near the reactants in the acid-catalyzed biomass-derived oxygenated compounds reaction, in which the stability of protons and transition states could reduce the surface free energy barrier formed in the mixed solvent environment. When using co-solvents as biomass liquefaction media, different solvent ratios affect the product compositions of the liquefaction reaction and there is often a synergistic effect between solvents, which leads to an increase in the yield of bio-oil, but the reaction mechanism of this synergistic effect needs further in-depth study. Moreover, using organic solvent/water as a co-solvent to liquefy biomass can reduce bio-oil’s nitrogen content and improve its quality. Table 2 shows a summary of the advantages and disadvantages of each of the above three solvent systems during liquefaction.

4. Conclusions

Liquefaction technology is crucial for converting biomass into liquid fuels or platform chemicals. Although certain research advancements have been achieved, issues such as high liquefaction costs and stringent reaction conditions have hindered its large-scale industrial application. The liquefaction solvent is an essential factor affecting the liquefaction process of biomass. When used alone as liquefaction solvents, water and organic solvents have shortcomings. Water efficiently corrodes materials, and the obtained bio-oil has a low calorific value. When organic solvents are used for liquefaction conversion of biomass, they face problems such as high solvent costs and difficulties in recovery. The synergistic effect between the co-solvents can significantly increase the yield of bio-oil, which is a good choice of solvent in the liquefaction process, but the reaction mechanism needs to be further investigated. Furthermore, the biomass liquefaction conversion process’s reaction network is extremely complex, usually composed of multiple series and parallel reactions. Any reaction may produce a series of undesirable products (such as typical humus), affecting the results of biomass conversion reactions [117]. Aside from understanding the effect of solvent composition on the liquefaction, researchers also need to master the selectivity of the reaction to control the reaction process better and improve the efficiency and product quality of biomass liquefaction conversion. More importantly, these insights should be translated into broadly applicable design rules to enhance liquefaction process yields.

Author Contributions

H.M.: Writing—original draft, Validation and Supervision; X.Y.: Investigation, Writing—original draft, Writing—review and editing; P.Z.: Investigation, Writing—review and editing; Y.Z.: Investigation; H.J.: Writing—review and editing; L.Z.: Supervision; H.M. and X.Y. contributed equally to this work. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of Xinjiang Uygur Autonomous Region (2022D01F60), Tianshan Talents Plan in Xinjiang Uygur Autonomous Region (2022TSYCJC0001), the National Science Foundation of China (22368051), and the Science and Technology Plan Project of Karamay (20232023hjcxrc0038).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Predictive pathways for HTL mechanisms. Reproduced with permission [13]. Copyright 2020, Elsevier.
Figure 1. Predictive pathways for HTL mechanisms. Reproduced with permission [13]. Copyright 2020, Elsevier.
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Figure 2. (a–l) Effect of different reaction conditions on the distribution of different types of biomass liquefaction products. (The symbol Energies 17 02814 i001 represents the biomass: Cyanophyta; the symbol Energies 17 02814 i002 represents the biomass: L. sacharina (Marcoalgae); the symbol Energies 17 02814 i003 represents the biomass: Spirulina; the symbol Energies 17 02814 i004 represents the biomass: S. patens C. Agardh; the symbol Energies 17 02814 i005 represents the biomass: Cattle Manure; the symbol Energies 17 02814 i006 represents the biomass: Cellulose; the symbol Energies 17 02814 i007 represents the biomass: Chitin) Reproduced with permission [21]. Copyright 2021, The Royal Society of Chemistry.
Figure 2. (a–l) Effect of different reaction conditions on the distribution of different types of biomass liquefaction products. (The symbol Energies 17 02814 i001 represents the biomass: Cyanophyta; the symbol Energies 17 02814 i002 represents the biomass: L. sacharina (Marcoalgae); the symbol Energies 17 02814 i003 represents the biomass: Spirulina; the symbol Energies 17 02814 i004 represents the biomass: S. patens C. Agardh; the symbol Energies 17 02814 i005 represents the biomass: Cattle Manure; the symbol Energies 17 02814 i006 represents the biomass: Cellulose; the symbol Energies 17 02814 i007 represents the biomass: Chitin) Reproduced with permission [21]. Copyright 2021, The Royal Society of Chemistry.
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Figure 3. (a) Hydrolysis of lignocellulosic biomass. Reproduced with permission [32]. Copyright 2021, The Royal Society of Chemistry. (b) Schematic diagram of lignocellulose structure. Reproduced with permission [31]. Copyright 2020, The Royal Society of Chemistry. (c) Lipid synthesis pathway. Reproduced with permission [33]. Copyright 2022, Elsevier. (d) Lipid decomposition reaction. Reproduced with permission [21]. Copyright 2021, The Royal Society of Chemistry.
Figure 3. (a) Hydrolysis of lignocellulosic biomass. Reproduced with permission [32]. Copyright 2021, The Royal Society of Chemistry. (b) Schematic diagram of lignocellulose structure. Reproduced with permission [31]. Copyright 2020, The Royal Society of Chemistry. (c) Lipid synthesis pathway. Reproduced with permission [33]. Copyright 2022, Elsevier. (d) Lipid decomposition reaction. Reproduced with permission [21]. Copyright 2021, The Royal Society of Chemistry.
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Figure 4. Relationship between properties of water and temperature at 25 MPa. Reproduced with permission [87]. Copyright 2023, Elsevier.
Figure 4. Relationship between properties of water and temperature at 25 MPa. Reproduced with permission [87]. Copyright 2023, Elsevier.
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Figure 5. (a) Effect of heteroatom content on bio-oil yield and calorific value. Reproduced with permission [90]. Copyright 2022, Elsevier. (b) Influence of Spirulina/water ratio on product yield distribution (BO represents bio-oil, SR represents solid residue, GA represents gaseous and aqueous products). Reproduced with permission [55]. Copyright 2018, Elsevier. (c) Effect of water volume percentage on product fraction yield (VT, BC, SD, and AQ represent volatiles, bio-oil, solids, and aqueous phase products in liquefied products, respectively). The first row of data in the horizontal coordinate represents the reactor pressure, and the second row in the horizontal coordinate represents the volume percent of water. Reproduced with permission [92]. Copyright 2017, Elsevier.
Figure 5. (a) Effect of heteroatom content on bio-oil yield and calorific value. Reproduced with permission [90]. Copyright 2022, Elsevier. (b) Influence of Spirulina/water ratio on product yield distribution (BO represents bio-oil, SR represents solid residue, GA represents gaseous and aqueous products). Reproduced with permission [55]. Copyright 2018, Elsevier. (c) Effect of water volume percentage on product fraction yield (VT, BC, SD, and AQ represent volatiles, bio-oil, solids, and aqueous phase products in liquefied products, respectively). The first row of data in the horizontal coordinate represents the reactor pressure, and the second row in the horizontal coordinate represents the volume percent of water. Reproduced with permission [92]. Copyright 2017, Elsevier.
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Figure 6. (a) Product distribution of microalgae after liquefaction in different solvents. Reproduced with permission [60]. Copyright 2019, Elsevier. (b) Main hydrogen supply mechanism of ethanol. Reproduced with permission [108]. Copyright 2015, Elsevier.
Figure 6. (a) Product distribution of microalgae after liquefaction in different solvents. Reproduced with permission [60]. Copyright 2019, Elsevier. (b) Main hydrogen supply mechanism of ethanol. Reproduced with permission [108]. Copyright 2015, Elsevier.
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Figure 7. (a) Effect of solvent on cellulose liquefaction process. Reproduced with permission [61]. Copyright 2016, Elsevier. (b) Influences of different solvents on the distribution of EFB fiber liquefaction products ((O + G) for oil and gas, (PA + A) for preasphaltene and asphaltene). Reproduced with permission [63]. Copyright 2011, Elsevier.
Figure 7. (a) Effect of solvent on cellulose liquefaction process. Reproduced with permission [61]. Copyright 2016, Elsevier. (b) Influences of different solvents on the distribution of EFB fiber liquefaction products ((O + G) for oil and gas, (PA + A) for preasphaltene and asphaltene). Reproduced with permission [63]. Copyright 2011, Elsevier.
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Figure 8. (a) The conversion of poly-(3-hydroxybutyrate) to crotonic acid. Reproduced with permission [64]. Copyright 2022, The Royal Society of Chemistry. (b) Mechanism of glucose conversion to 5-hydroxymethylfurfural in the presence of SPPS. Reproduced with permission [65]. Copyright 2018, The Royal Society of Chemistry.
Figure 8. (a) The conversion of poly-(3-hydroxybutyrate) to crotonic acid. Reproduced with permission [64]. Copyright 2022, The Royal Society of Chemistry. (b) Mechanism of glucose conversion to 5-hydroxymethylfurfural in the presence of SPPS. Reproduced with permission [65]. Copyright 2018, The Royal Society of Chemistry.
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Figure 9. (a) Conversion and yield of sawdust after liquefaction in different solvents (C represents the conversion rate. Yg, Ys, and Yl represent the yield of gas, solid, and liquid products, respectively). Reproduced with permission [66]. Copyright 2022, Elsevier. (b) Liquid phase product distribution after sawdust liquefaction. Reproduced with permission [66]. Copyright 2022, Elsevier.
Figure 9. (a) Conversion and yield of sawdust after liquefaction in different solvents (C represents the conversion rate. Yg, Ys, and Yl represent the yield of gas, solid, and liquid products, respectively). Reproduced with permission [66]. Copyright 2022, Elsevier. (b) Liquid phase product distribution after sawdust liquefaction. Reproduced with permission [66]. Copyright 2022, Elsevier.
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Figure 10. (a) Effect of reaction solvent on nitrogen content in bio-oil. The number in the bracelet is the solvent polarity. (Energies 17 02814 i008 represents the result of liquefaction of biomass (Sewage sludge) at 340 °C for 20 min; Energies 17 02814 i002 represents the results of liquefaction of biomass (Spirulina) at 350 °C for 20 min; Energies 17 02814 i009 represents the results of liquefaction of biomass (Chlorella pyrenoidosa) at 280 °C for 120 min; Energies 17 02814 i010 represents the results of liquefaction of biomass (Algae) at 295 °C; Energies 17 02814 i011 represents the results of liquefaction of biomass (Rice husk) at 260 °C for 20 min; Energies 17 02814 i012 represents the results of liquefaction of biomass (Chlorella pyrenoidosa) at 300 °C for 60 min; Energies 17 02814 i013 represents the liquefaction results of biomass (Sewage sludge) at 280~360 °C; Energies 17 02814 i014 represents the results of liquefaction of biomass (Sewage sludge) at 400 °C for 30 min; Energies 17 02814 i015 represents the results of liquefaction of biomass (Low-lipid microalgae) at 225 °C for 60 min; Energies 17 02814 i016 represents the results of liquefaction of biomass (Laminaria Saccharina) at 330 °C for 15 min) (b) Influence of different solvents on biomass liquefaction conversion of extraction solvent on bio-oil N content [111,112]. Reproduced with permission [110]. Copyright 2020, Elsevier.
Figure 10. (a) Effect of reaction solvent on nitrogen content in bio-oil. The number in the bracelet is the solvent polarity. (Energies 17 02814 i008 represents the result of liquefaction of biomass (Sewage sludge) at 340 °C for 20 min; Energies 17 02814 i002 represents the results of liquefaction of biomass (Spirulina) at 350 °C for 20 min; Energies 17 02814 i009 represents the results of liquefaction of biomass (Chlorella pyrenoidosa) at 280 °C for 120 min; Energies 17 02814 i010 represents the results of liquefaction of biomass (Algae) at 295 °C; Energies 17 02814 i011 represents the results of liquefaction of biomass (Rice husk) at 260 °C for 20 min; Energies 17 02814 i012 represents the results of liquefaction of biomass (Chlorella pyrenoidosa) at 300 °C for 60 min; Energies 17 02814 i013 represents the liquefaction results of biomass (Sewage sludge) at 280~360 °C; Energies 17 02814 i014 represents the results of liquefaction of biomass (Sewage sludge) at 400 °C for 30 min; Energies 17 02814 i015 represents the results of liquefaction of biomass (Low-lipid microalgae) at 225 °C for 60 min; Energies 17 02814 i016 represents the results of liquefaction of biomass (Laminaria Saccharina) at 330 °C for 15 min) (b) Influence of different solvents on biomass liquefaction conversion of extraction solvent on bio-oil N content [111,112]. Reproduced with permission [110]. Copyright 2020, Elsevier.
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Figure 11. (a) Influence of co-solvent molecules on the distribution of solvent molecules. The excellent interaction environment between co-solvent and hydrophilic reactants promotes the generation of water-rich regions near the reactants. (b) Influence of co-solvent molecules on non-reactive energy cases. Stabilizing protons and transition states in water-rich localized domains reduces the surface free energy barriers for reactions in mixed solvent environments relative to bulk parts. (c,d) Snapshots of molecular dynamics simulations of xylitol in water (c) and 90 wt% 1,4-dioxane (d). Reproduced with permission [71]. Copyright 2018, The Royal Society of Chemistry.
Figure 11. (a) Influence of co-solvent molecules on the distribution of solvent molecules. The excellent interaction environment between co-solvent and hydrophilic reactants promotes the generation of water-rich regions near the reactants. (b) Influence of co-solvent molecules on non-reactive energy cases. Stabilizing protons and transition states in water-rich localized domains reduces the surface free energy barriers for reactions in mixed solvent environments relative to bulk parts. (c,d) Snapshots of molecular dynamics simulations of xylitol in water (c) and 90 wt% 1,4-dioxane (d). Reproduced with permission [71]. Copyright 2018, The Royal Society of Chemistry.
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Figure 12. (a) Effect of ethanol content on Spirulina liquefaction yield and conversion. Reproduced with permission [73]. Copyright 2017, Elsevier. (b) Influence of solvent composition of the ethanol/water mixture on the distribution of algal biomass and sawdust (50/50, w/w) co-liquefaction products. Reproduced with permission [74]. Copyright 2018, Elsevier.
Figure 12. (a) Effect of ethanol content on Spirulina liquefaction yield and conversion. Reproduced with permission [73]. Copyright 2017, Elsevier. (b) Influence of solvent composition of the ethanol/water mixture on the distribution of algal biomass and sawdust (50/50, w/w) co-liquefaction products. Reproduced with permission [74]. Copyright 2018, Elsevier.
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Figure 13. (a) SEM image of EWBB before liquefaction (A). (b) SEM image of the solid residues produced by EWBB after liquefaction in water (B). (c) SEM image of solid residue produced by liquefaction of EWBB in water/ethanol co-solvent (C). (d) Distribution of products from liquefaction of different biomasses. Reproduced with permission [78]. Copyright 2018, Elsevier.
Figure 13. (a) SEM image of EWBB before liquefaction (A). (b) SEM image of the solid residues produced by EWBB after liquefaction in water (B). (c) SEM image of solid residue produced by liquefaction of EWBB in water/ethanol co-solvent (C). (d) Distribution of products from liquefaction of different biomasses. Reproduced with permission [78]. Copyright 2018, Elsevier.
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Figure 14. (a) Distribution of the HTL product of Amaranthus hypochondriacus L. in water and 60% ethanol with or without aqueous phase recycling (In the Fure, 0%, 60% represents liquefaction experiments performed in water, 60% ethanol (first run), respectively. In aqueous phase cycling experiments using 60% ethanol, the aqueous phase obtained after the anterior experiment is used as the reaction solvent for subsequent experiments (60% ethanol is kept in the reaction solvent). The second through fourth runs of the aqueous phase recirculation experiments are labeled 60% + 1, 60% + 2, and 60% + 3, respectively). Reproduced with permission [47]. Copyright 2021, Elsevier. (b) Nitrogen equilibrium during liquefaction of Amaranthus hypochondriacus L. in water or 60% ethanol (with or without aqueous phase circulation). Reproduced with permission [47]. Copyright 2021, Elsevier. (c) Volatile chemical composition of bio-oils obtained by liquefying Amaranthus hypochondriacus L. in water or 60% ethanol (with or without aqueous phase circulation). Reproduced with permission [47]. Copyright 2021, Elsevier. (d) Classification of bio-oils obtained in water and mixed solvents. (HEX-H2O: n-hexane/water co-solvent; MeOH-H2O: methanol/water co-solvent; the symbol Energies 17 02814 i017 represents the compound: Fatty acids; the symbol Energies 17 02814 i018 represents the compound: Fatty acid esters; the symbol Energies 17 02814 i019 represents the compound: Nitrogenated compounds; the symbol Energies 17 02814 i020 represents the compound: Hydrocarbons; the symbol Energies 17 02814 i021 represents the compound: Ketones; the symbol Energies 17 02814 i022 represents the compound: Alcohols; the symbol Energies 17 02814 i023 represents the compound: Aldehydes; the symbol Energies 17 02814 i024 represents other compounds.) Reproduced with permission [80]. Copyright 2018, Elsevier.
Figure 14. (a) Distribution of the HTL product of Amaranthus hypochondriacus L. in water and 60% ethanol with or without aqueous phase recycling (In the Fure, 0%, 60% represents liquefaction experiments performed in water, 60% ethanol (first run), respectively. In aqueous phase cycling experiments using 60% ethanol, the aqueous phase obtained after the anterior experiment is used as the reaction solvent for subsequent experiments (60% ethanol is kept in the reaction solvent). The second through fourth runs of the aqueous phase recirculation experiments are labeled 60% + 1, 60% + 2, and 60% + 3, respectively). Reproduced with permission [47]. Copyright 2021, Elsevier. (b) Nitrogen equilibrium during liquefaction of Amaranthus hypochondriacus L. in water or 60% ethanol (with or without aqueous phase circulation). Reproduced with permission [47]. Copyright 2021, Elsevier. (c) Volatile chemical composition of bio-oils obtained by liquefying Amaranthus hypochondriacus L. in water or 60% ethanol (with or without aqueous phase circulation). Reproduced with permission [47]. Copyright 2021, Elsevier. (d) Classification of bio-oils obtained in water and mixed solvents. (HEX-H2O: n-hexane/water co-solvent; MeOH-H2O: methanol/water co-solvent; the symbol Energies 17 02814 i017 represents the compound: Fatty acids; the symbol Energies 17 02814 i018 represents the compound: Fatty acid esters; the symbol Energies 17 02814 i019 represents the compound: Nitrogenated compounds; the symbol Energies 17 02814 i020 represents the compound: Hydrocarbons; the symbol Energies 17 02814 i021 represents the compound: Ketones; the symbol Energies 17 02814 i022 represents the compound: Alcohols; the symbol Energies 17 02814 i023 represents the compound: Aldehydes; the symbol Energies 17 02814 i024 represents other compounds.) Reproduced with permission [80]. Copyright 2018, Elsevier.
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Figure 15. (a) Effect of solvent on bio-oil and biochar yields. Reproduced with permission [82]. Copyright 2022, Elsevier. (b) Change in the liquefied solvent’s hydroxyl number and acid number with reaction time (hollow squares are hydroxyl values, and solid circles are acid values). Reproduced with permission [83]. Copyright 2013, Springer. (c) Effect of liquid ratio on hydroxyl number and acid value of liquefaction products (hollow symbols represent hydroxyl number, solid symbols represent acid value). Reproduced with permission [83]. Copyright 2013, Springer. (d) Effect of ethanol/ethylene glycol ratio on bio-oil and biochar yield. Reproduced with permission [84]. Copyright 2021, Elsevier.
Figure 15. (a) Effect of solvent on bio-oil and biochar yields. Reproduced with permission [82]. Copyright 2022, Elsevier. (b) Change in the liquefied solvent’s hydroxyl number and acid number with reaction time (hollow squares are hydroxyl values, and solid circles are acid values). Reproduced with permission [83]. Copyright 2013, Springer. (c) Effect of liquid ratio on hydroxyl number and acid value of liquefaction products (hollow symbols represent hydroxyl number, solid symbols represent acid value). Reproduced with permission [83]. Copyright 2013, Springer. (d) Effect of ethanol/ethylene glycol ratio on bio-oil and biochar yield. Reproduced with permission [84]. Copyright 2021, Elsevier.
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Table 1. Progress in the study of solvent effects of some solvents used in biomass liquefaction processes.
Table 1. Progress in the study of solvent effects of some solvents used in biomass liquefaction processes.
SolventsType of Biomass FeedstockReaction ConditionsResultsBio-Oil YieldRef.
ethanol/water co-solventCd-enriched Amaranthus hypochondriacus L. (AHL)275 °C, 30 mincompared with pure water, ethanol/water co-solvent can transfer organic nitrogen to the aqueous phase, thereby reducing the nitrogen content in bio-oilthe yield of bio-oil obtained after liquefying 60% ethanol content (32.07%) is higher than that obtained after liquefying water (15.93%)[47]
waterAzolla filiculoides260 °C, 280 °C and 300 °C, 15~60 minusing water as the solvent, the concentration of total organic carbon is higherthe yield is 13.6–21.5 wt%[53]
waterNannochloropsis microalgae400 °C, 1 h, adding H2upgraded treatment with supercritical water almost halves the heteroatom content of the feedstockthe yield of water-insoluble biocrude is 37.6 ± 2.1 wt%, the yield of water-soluble biocrude is 8.2 ± 0.6 wt%, and the total yield of biocrude oil is 45.8 ± 2.9 wt%[54]
waterSpirulina260 °C, 30 minbio-oil production is highest when the ratio of Spirulina to water is 1/4the highest bio-oil yield is 43.05 wt%[55]
ethanol or methanolKariba weed300 °C, 60 minorganic solvents usually have lower dielectric constants than water, which improves the solubility of solventsthe optimal yield is 20.86 wt%[56]
ethanolsewage sludge200~280 °C, 0~60 minethanol as a hydrogen-donating solvent can improve the bio-oil yieldthe yield is 26.8 wt%[57]
γ-valerolactonexyloseacid catalysisγ-valerolactone can decrease the activation energy of the liquefaction reaction and increase the reaction rate and product selectivity/[58]
γ-valerolactone, acetone, 1,4-dioxane, methanol and ethanolglucose and fructose175~225 °C, 150~200 °C, 5~120 minpolar aprotic solvents decrease raw materials’ decomposition activation energy, but the reaction rate constant for liquefaction is lower than that in polar proton solvents/[59]
methanol, ethanol,
ethyl acetate, acetone, and water		low-lipid microalgae275 °C, 60 minbio-oil yields from polar proton solvents are significantly higher than those from polar aprotic solventswhen the biomass/solvent mass ratio is 1:5, the highest yield of biocrude oil is 85.5 wt%[60]
methanol, acetone, isopropanol, propanol, heptane, and cyclohexanecellulose320 °C, 30 minmore polar solvents may favor the transformation of raw materials; non-polar solvents are unable to break the hydrogen bonds of cellulose during liquefaction, which is not conducive to the conversion of cellulosethe most efficient solvent for liquefaction of cellulose is methanol, and the yield of bio-oil obtained by liquefaction in methanol is up to 32.21%[61]
water, tetrahydrofuran (THF), and tolueneC6 sugar monomer/polymer180 °C, 120 minthe polarity of the solvent is critical to the formation of the product/[62]
acetone, ethylene glycol, ethanol, water, and tolueneoil palm empty fruit bunch (EFB) fibers275 °C, 60 minthe polar solvents are more suitable for the dissolution of EFB fibers, resulting in higher yields/[63]
[EMIM][AcO]poly 3-hydroxybutyrate (PHB)140 °C, 90 minefficient and selective conversion of poly-3-hydroxybutyrate to crotonic acid with 97% yield by ionic liquids/[64]
[EMIM]Brglucose and celluloseheterogeneous sulfonated poly(phenylene sulfide) (SPPS), 140 °Cthe yield of glucose into 5-hydroxymethylfurfural is 87.2%, and the yield of cellulose directly into 5-hydroxymethylfurfural can reach 68.2%/[65]
ethanol, isopropanol, cyclohexane, cyclohexanol, and tetralinsawdust320 °C, 30 minthe heat and mass transfer capabilities of the solvent will affect the conversion of biomass the highest liquid yield of cyclohexanol is 79.1%, followed by tetralin (72.0%), ethanol (57.0%), isopropyl alcohol (47.7%), and cyclohexane (44.5%)[66]
water, methanol, ethanol, acetonered macroalga Gracilaria corticata260 °C, 280 °C and 300 °C, 15 minthe maximum bio-oil yield is obtained by acetone liquefaction at 300 °C the maximum bio-oil yield is 16.16 wt%[67]
ethanol/water co-solventsewage sludge220 °C, 250 °C, 280 °C, 310 °C, 340 °C and 370 °C, 30 minthe highest bio-oil yield is achieved when water is mixed with ethanol at a ratio of 1:1the highest yield reached 40.69 wt%[68]
isopropanol/water co-solventmarine microalgae350 °C, 30 minbio-oil yield is enhanced by about 14% when using isopropanol as co-solventthe maximum bio-oil yield (35.4%) is achieved when IPA is added as a co-solvent[69]
ethanol/water co-solventbagasse, high-density polyethylene280 °C, 90 minthere is a significant synergy between water and ethanola high bio-oil yield at the water-to-ethanol volume ratio of 60%:40%[70]
1,4-dioxane/water co-solvent, γ-valerolactone/water co-solvent, tetrahydrofuran/water co-solventethyl tert-butyl ether, tert-butanol, levoglucosan, 1,2-propanediol, fructose, cellobiose, and xylitolacid catalysts the excellent interaction environment between co-solvent and hydrophilic reactants could promote the generation of water-rich regions near the reactants in the acid-catalyzed biomass-derived oxygenated compounds reaction, in which the stability of protons and transition states could reduce the surface free energy barrier formed in the mixed solvent environment/[71]
dioxane/water co-solvent, dimethyl sulfoxide/water co-solvent, tetrahydrofuran/water co-solvent, γ-valerolactone/water co-solvent, acetonitrile/water co-solventfructose, tert-butanol, and 1,2-propanediolacid catalysisthe degree of solvation increases the rate of acid-catalyzed reactions /[72]
ethanol/water co-solventlow-lipid microalgae250 °C, 275 °C, 300 °C, 325 °C and 350 °C, 15 min, 30 min, 45 min, 60 min, and 75 minas the ethanol content in the solvent mixture increases, the bio-oil production first increases and then decreaseswhen the reaction temperature is 300 °C, the reaction time is 45 min, the ethanol content is 50 vol%, and the bio-oil yield is 59.5%[73]
ethanol/water co-solventalgal, sawdust200~300 °C, 30~120 minethanol/water co-solvent exhibits a synergy, and when the solvent mixture (ethanol/water = 75 wt%:25 wt%) is liquefied at 250 °C, the bio-oil yield is the highestthe highest bio-oil yield of 58 wt% is obtained[74]
ethanol/water co-solventrice husk533 K, 573 K and 613 K, 20 minethanol/water co-solvent combines the advantages of water and ethanol and shows a synergistic effect, with the highest bio-oil yield at 533 Kin ethanol/water co-solvent v/v( 5:5), the highest bio-oil yield of 21.15% is obtained at 533 K[75]
ethanol/water co-solventlignocellulose260 °Cthe heavy oil product contained more esters, ethers, and alcohols and less aldehydes due to the transesterification reaction that occurred with the addition of ethanolthe highest bio-oil yield of 36.62% is achieved when 60% content of ethanol is used to liquefy ligno-cellulose[76]
ethanol/water co-solventmulberry bark300 °C, 60 minthe phenolic content of bio-oil derived from subcritical water is higher than that of bio-oil derived from subcritical ethanol/water co-solvent, while the ester content of bio-oil derived from subcritical ethanol/water co-solvent is higherthe bio-oil yield of mulberry bark in subcritical ethanol/water co-solvent (30.32 wt%) is slightly higher than that in sub-critical water (28.81 wt%)[77]
ethanol/water co-solventcellulose, lignin, cellulose/lignin blend, white birch bark300 °C, 15 minethanol/water co-solvent enhances solvent penetration into lignocellulosic biomass structures and improves the solubility of liquefaction intermediates, thereby increasing bio-crude yields from lignocellulosic liquefactionthe bio-oil yield in water/ethanol (50:50, v/v) co-solvent is as high as 58.8 wt%[78]
ethanol/water co-solvent, isopropanol/water co-solvent, glycerin/water co-solventlow-lipid, high-protein algae310 °C, 330 °C and 350 °C, 30 minbio-oils are mainly produced by the reaction of alcohol with algae fragments through Maillard, alkylation, and esterificationas the temperature increases from 310 °C to 350 °C, the yield of light bio-oil from dry algae using glycerol as the liquefaction solvent increases from 24.9 wt% to 73.2 wt%[79]
hexane/water co-solvent, methanol/water co-solventsewage sludge300~380 °C, 0~60 minsynergistic effect between organic solvent/water can help reduce bio-oil nitrogen content while increasing bio-oil yieldthe highest yield of bio-oil in methanol/water co-solvent is 46.5 wt%[80]
acetone/tetrahydrofuran co-solventFallopia Japonica280 °C, 300 °C and 320 °Ctetralin has both dissolution and scavenging effects, increasing the abundance of monomeric aromatics in the product; acetone facilitates the cleavage of retro-aldol to produce low-molecular-weight oxygenates/[81]
H2O, C4H10O2/C6H15NO3 co-solvent, C6H15NO3, C4H10O2pine tree200~300 °C, 10 min, 30 min, 60 min, 90 min and 100 minC4H10O2/C6H15NO3 co-solvent liquefaction produces the highest bio-oil yield with synergistic effectsthe maximum bio-oil yield is 65.0%[82]
polyethylene glycol/glycerin co-solventbagasse130~170 °Cas the liquefaction reaction proceeds, the acid value of the liquefaction product increases, and the number of hydroxyl groups decreases/[83]
ethanol, ethylene glycol, ethanol/glycol co-solvent 1lignocellulose170~290 °Cethanol/glycol co-solvent exerts a synergistic effect on the liquefaction process, manifesting as higher oil yield and lower biochar yieldthe highest bio-oil yield is 52.3% when the solvent ratio is 1:1[84]
1 Terms such as A/B co-solvent appear in the text to indicate a mixture of two different solvents, A and B. “/” means that there is no relevant content.
Table 2. Summary of the advantages and disadvantages of each of the three solvent systems in the liquefaction process.
Table 2. Summary of the advantages and disadvantages of each of the three solvent systems in the liquefaction process.
WaterOrganic SolventCo-Solvent
advantagesIt is environmentally friendly, inexpensive, and can act as a solvent, hydrogenator, and catalyst, facilitating reactions such as pyrolysis and hydration and contributing to the decomposition and extraction of biomass.(1) Organic solvents can facilitate alkylation and esterification reactions and help dissolve and stabilize intermediates. (2) Compared with water, the reaction conditions of organic solvents are mild, and the liquefaction products have high energy density and low acidity. (3) The protic solvent in the organic solvent can donate hydrogen, inhibit side reactions, and increase bio-oil yield.There is often a synergistic effect between the co-solvents, which leads to an increase in bio-oil yield. In addition, the use of organic solvents/water as co-solvents to liquefy biomass reduces the nitrogen content of the bio-oil and improves the quality of the bio-oil.
disadvantagesIt may corrode materials, the bio-oil obtained from its liquefaction has a low calorific value and high viscosity, and some of the organic compounds produced during the liquefaction process may be transferred from the aqueous phase to other phases, thus reducing the total bio-oil yield.Organic solvents face high solvent costs, difficulties in recovery, and increased biomass drying requirements prior to liquefaction.The synergistic reaction mechanism between the co-solvents needs to be further investigated in depth.
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Ming, H.; Yang, X.; Zheng, P.; Zhang, Y.; Jiang, H.; Zhang, L. Recent Advances of Solvent Effects in Biomass Liquefaction Conversion. Energies 2024, 17, 2814. https://doi.org/10.3390/en17122814

AMA Style

Ming H, Yang X, Zheng P, Zhang Y, Jiang H, Zhang L. Recent Advances of Solvent Effects in Biomass Liquefaction Conversion. Energies. 2024; 17(12):2814. https://doi.org/10.3390/en17122814

Chicago/Turabian Style

Ming, Hui, Xin Yang, Pu Zheng, Yifan Zhang, Haoxin Jiang, and Libo Zhang. 2024. "Recent Advances of Solvent Effects in Biomass Liquefaction Conversion" Energies 17, no. 12: 2814. https://doi.org/10.3390/en17122814

APA Style

Ming, H., Yang, X., Zheng, P., Zhang, Y., Jiang, H., & Zhang, L. (2024). Recent Advances of Solvent Effects in Biomass Liquefaction Conversion. Energies, 17(12), 2814. https://doi.org/10.3390/en17122814

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