Hydrothermal liquefaction of Salix psammophila in the presence of ionic liquid and water

Li, C., Wang, Y., Bian, Y., Ren, J., Zhang, J., Tang, Y., Yan, X., and Zhang, X. (2023). “Hydrothermal liquefaction of Salix psammophila in the presence of ionic liquid and water,” BioResources 18(4), 7341-7352.

Abstract

Salix psammophila was converted to value-added chemicals in the presence of 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) and water by the hydrothermal liquefaction method. [Bmim]Cl acted as both liquefier and catalyst, while water served as both co-solvent and reactant. The optimal reaction parameters for the highest liquefaction yield were determined by an orthogonal experiment. The possibility of application of the recycled [Bmim]Cl was investigated. FT-IR spectra results of the fresh used, the firstly reused, and the secondary reused IL [Bmim]Cl confirmed the high stability of [Bmim]Cl. Moreover, an explanation of the possible liquefaction mechanism for S. psammophila was also provided. This study may provide fundamental data for S. psammophila conversion and application.

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Hydrothermal Liquefaction of Salix psammophila in the Presence of Ionic Liquid and Water

Chun Li,^a Youwen Wang,^a Yongsheng Bian,^b Jianguo Ren,^c Jingjing Zhang,^aYunong Tang,^a Xiaolin Yan,^a,* and Xiaotao Zhang^a,*

DOI: 10.15376/biores.18.4.7341-7352

Keywords: Salix psammophila; [Bmim]Cl; Hydrothermal liquefaction; Orthogonal experiment; Recyclability

Contact information: a: College of Science, Inner Mongolia Agriculture University, Hohhot 010018, China; b: CHN Energy Yulin Chemical Co., Ltd., Yulin, China; c: Inner Mongolia Donghua Energy Co., Ltd., Ordos, China; *Corresponding authors: 13947113956@163.com; lianzixiaotao@163.com

GRAPHICAL ABSTRACT

INTRODUCTION

The desert shrub Salix psammophila is one of the typical sand-fixing plants in northwestern China. It is a natural, abundant, and renewable biomass feedstock. It is planted to fix wind-blown sand on the ground surface and control desertification. However, it is necessary to cut the parts of S. psammophila above ground every 3 to 5 years to allow it to regenerate and maintain a sustainable ecological system, producing a large amount of S. psammophila branch residues (Ji et al. 2019). Harvested S. psammophila has traditionally been used as a fuel, animal fodder, raw materials of boards and pulp, and herbal medicines, etc. (Kubo et al. 2013). Recently, researchers have found that the potential of liquefied S. psammophila can be exploited as chemicals (Li et al. 2013). Therefore, the full exploitation of S. psammophila plants for high-value alternative fuels and chemicals is meaningful and feasible.

As a lignocellulosic biomass, cellulose, hemicellulose, and lignin are the main components of S. psammophila. In recent years, the liquefaction of this plant under sulfuric acid, nitric acid, or hydrochloric acid catalysis have been studied (Zhong et al. 2020, 2022). However, it is well established that the highly hydrogen-bonded cellulose component cannot be easily dissolved in any conventional solvents (Maldas and Shiraishi 1997).

Ionic liquids (ILs) are environmentally friendly molten salts with advantages of high polarities, low melting point, non-volatility, and they are identified as promising green solvents for lignocellulosic biomass dissolution (Yoo et al. 2017). The solubility of cellulose varies to an extreme extent with the chosen IL, because it depends on the ability of IL to disrupt the inter-chain hydrogen bonds in cellulose (Schenzel et al. 2014). Among various ILs, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) has been first studied for cellulose dissolution by Swatloski et al. (2002). Numbers of reports about [Bmim]Cl have been published for cellulose pretreatment and utilization (Vitz et al. 2009; Schenzel et al. 2014; Reina et al. 2016) since then, but few have discussed S. psammophila liquefaction.

The hydrothermal liquefaction process is a transformation process that involves very complex reactions and takes place at very high temperatures (220 to 380 °C) and pressures (5 to 25 MPa) in the water solvent environment with and without catalyst (Durak et al. 2019, 2023). Though hydrothermal liquefaction of biomass to obtain bio-based chemicals has been investigated extensively (Gollakota et al. 2018), reports on hydrothermal liquefaction of S. psammophila with IL [Bmim]Cl are scare.

Herein, hydrothermal liquefaction of S. psammophila in the presence of ([Bmim]Cl) and water were studied. The effects of temperature, reaction time, and solid-liquid ratio (mass ratio of S. psammophila to ([Bmim]Cl)) on liquefaction process were investigated. Liquefied products were examined by using GC analysis. Recyclability of ([Bmim]Cl) were also tested.

EXPERIMENTAL

Materials and Reagent

Salix psammophila plants were harvested from Ordos, Inner Mongolia, China. Samples used in the liquefaction were dried at 105 °C for 24 h, grounded, and sieved through a 40-mesh sieve. 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl) was purchased from Aolike Chemical Reagents Co., Ltd., Lanzhou, China. Ethyl acetate and ethanol used were of analytical grade obtained from Guangfu Reagents Co., Ltd. (Tianjin, China).

Liquefaction reaction and product separation procedure

[Bmim]Cl and S. psammophila with various solid-liquid ratios (1:0.5, 1:1, 1:1.5) were charged into a reaction kettle (100 mL, WHF-0.1, Weihai control the reaction kettle Co., Ltd.), and mixed with 50.0 mL distilled water. The reaction kettle was set at varying temperatures (180, 200, and 240 °C). When the temperature reached the set value, it was maintained for the definite reaction time (30, 60, or 150 min). After the desired reaction time, the reaction kettle was cooled to room temperature and the total reaction products in the autoclave were fully collected.

The separation procedure of the reaction products is illustrated in Fig. 1. As shown, the collected brown reaction products were first washed by 25.0 mL ethyl acetate, then the solid phase and filtrate were obtained after centrifugation. 25.0 mL ethanol was used to wash the solid phase and to remove the [Bmim]Cl, the final solid residue was dried at 105 °C to a constant weight. The liquefaction ratio (%) of the S. psammophila was calculated by the following equation:

(1)

The filtrate was separated using a separatory funnel. The obtained supernatant was used for gas chromatography (GC) analysis as oil phase products with ethyl acetate and the water phase with [Bmim]Cl was separated. The aqueous solution was then evaporated in a rotary evaporator to recycle [Bmim]Cl.

Fig. 1. The procedure for liquefied products separation

The analysis of liquefied products

The liquefied products were verified using GC (Agilent 7820A, Santa Clara, CA, USA) equipped with a DB-1 column by external standard method. The injector temperature was set at 230 °C, and the injection volume was 1.0 μL. The working conditions were set as follows: the column temperature increased to 40 °C and held for 2 min, then ramped to 200 °C at a heating rate of 10 °C∙min^-1 and held for 3 min. Subsequently, the column temperature decreased to 40 °C at a heating rate of 30 ^oC∙min^-1; the whole running time was 25 min. The chromatograph used a split/splitless injector and a ﬂame ionization detector (FID) with a 1:10 hydrogen (30 mL∙min^-1)/air mixture (300 mL∙min^-1) as the ﬂame fuel. The carrier gas used was dry, high-purity nitrogen. The detector temperature was adjusted to 260 °C.

Range analysis

In a range analysis, there are two parameters, K_i and R. K_i is defined as the sum of the indexes of the same level in each factor. The quantity k_iis the average value of K_i. A higher value k_idemonstrates that the corresponding level has a bigger effect; R is defined as the range between the maximum and minimum value of k_i. R=max(k_i)-min(k_i), such that a large R value means a greater importance of the factor.

For example, consider Table 3, where K₁ to K₃ values refer to the sum of the liquefaction ratio of all levels in factor A, factor B, and factor C. Meanwhile, k₁ to k₃ values represent the average values corresponding to K₁ to K₃ values. The calculation processes for K₁=164.9, k₁=55.0, and R=15.0 are shown below:

K₁=55.4+53.8+55.7=164.9; k₁= K₁/3=55.0; R=66.8-51.8=15.0 (2)

And so on, for K₂, K₃, k₂ and k₃ calculations.

Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared (FT-IR) absorption spectra ranged from 4000 to 600 cm^-1. The spectra were recorded by Perkin Elmer Spectrum 65, and the resolution was 1 cm^-1.

RESULTS AND DISCUSSION

Compositional Analysis of Salix psammophila

The composition analysis of S. psammophila was performed by Changsha Willsun Technology Co., Ltd., China. The air-dried carbon and hydrogen were determined using an automatic carbon hydrogen analyzer (WS-CH100). The air-dried moisture, ash, and volatile matter were determined with an automatic industrial analyzer (WS-G808), and the air-dried higher heating value (HHV) was determined with an automatic calorimeter (WS-C502).

As shown in Table 1, the carbon and hydrogen contents of S. psammophila were 13.3% and 4.98%, respectively. Salix psammophila had a low content of moisture (3.07%) and ash (0.87%), but a high content of volatiles (82.8%). The HHV was 4684 cal·g^-1, which was 33.1% lower than that of standard coal (7000 cal·g^-1). The main components of S. psammophila were cellulose, hemicellulose, and lignin contents, which had been analyzed and verified by many researchers (Li et al. 2013).

Table 1. Proximate Analysis and Compositions of Salix psammophila ^a

^aAll measured on air-dried basis

Orthogonal Experiment Design and Results

The main factors affecting the hydrothermal liquefaction of S. psammophila were reaction temperature, reaction time, and solid-liquid ratio (Li et al. 2013).

In all chemical transformations involving the interaction of two or more species, initiation of the reaction is triggered by collision events of the reacting species. The frequency and strength of such collisions are certainly very dependent on the temperature of the reaction, since molecular motion is strongly dependent on temperature (Masa et al. 2019). Arrhenius formulated an equation relating the dependence of the rate constant (k) of a chemical reaction on the reaction temperature (T), (Masa et al. 2019), which is widely applied for studying chemical kinetics. The pre-exponential factor or frequency factor A is a measure of the rate at which collisions occur, E_a is the activation energy, the minimum kinetic energy that the reactants must have in order to form a product(s), while R is the universal gas constant. If we consider the conversion of reactants S. psammophila into the products, the molecules of S. psammophila must come into contact with each other, resulting in breakage, distortion, or re-arrangement of bonds and atoms. Thereby, extensive depolymerization occurs in S. psammophila. This increases both the concentration of free radicals and probability of repolymerization of fragmented species. The competition among hydrolysis, fragmentation, and repolymerization reactions defines the role of temperature during pyrolysis. Depolymerization of biomass is a dominant reaction during initial stages of pyrolysis. Repolymerization and gaseous reactions become active at high temperatures, resulting in the formation of gases and the free radical reactions and thus leading to char formation (Akhtar and Amin 2011). Therefore, a suitable temperature usually yields higher amounts of liquefaction products. Moreover, the thermal decomposition temperature of IL ([Bmim]Cl) was approximately 257 °C. Thus, the temperature set in this study ranged from 180 to 240 °C.

Table 2. Levels and Factors Affecting the Hydrothermal Liquefaction of S. psammophila

Table 3. Orthogonal Experiment List, Liquefaction Ratio Results, and the Range Analysis Data

Optimization of the reaction time is necessary for effective destruction of organic compounds in S. psammophila. Many studies have reported that a shorter reaction time might usually be favored, but that a longer reaction time might suppress the liquid yields, which could be explained by the cracking of bio-oil or intermediate products to gases and the formation of chars by condensation, cyclization, and repolymerization (Jindal and Jha 2016). The optimal reaction time here was determined by the condition with the maximum liquefaction ratio. Reaction time was set at 30, 60, and 150 min in this study.

To determine the optimal reaction parameters for the highest liquefaction yield of S. psammophila, an L₉(3⁴) was employed to assign the considered factors as shown in Table 2, which was an orthogonal array with three factors and three levels. The following three factors were analyzed: temperature (factor A), reaction time (factor B), and solid-liquid ratio (factor C). Nine experiments were carried out to achieve the optimization process according to Table 3. Based on Table 3, the range of the liquefaction ratio ranged from 48.4% to 75.6%. As mentioned, a higher average value for k_i demonstrates that the corresponding level has a bigger effect on liquefaction ratio. Therefore, the best levels for the three factors were as follows: temperature was 240 °C, reaction time was 90 min, and solid-liquid ratio was 1:0.5. Meanwhile, the R value demonstrates the significance of the factor’s influence and a larger R means that the factor has a bigger impact on the liquefaction ratio (Gao et al. 2015; Shen et al. 2016). In Table 3, the decreasing order R_A > R_B> R_C indicates that the difference in temperature influenced the liquefaction ratio the most, followed by the reaction time and solid-liquid ratio. Moreover, the appropriate solid-liquid ratio could not only facilitate liquefaction reaction, but it also reduced the occurrence of repolymerization during later stage. In Table 3, it could be observed that different solid-liquid ratios (1:0.5; 1:1; 1:1.5) seemed to have less influence on the S. psammophila liquefaction in this study.

Recyclability of the Ionic Liquid [Bmim]Cl

Ionic liquids are effective solvents/media for biomass utilization. The unique properties of IL enable them to effectively dissolve and/or convert the biomass into various types of products (Yoo et al. 2017). Generally, the prices of ILs are much higher than commercial solvents. Therefore, the efficiency with which ILs are recycled and reused is very important. To make this S. psammophila liquefaction process sustainable and cost-friendly, two cycles recycling-reusability of the IL [Bmim]Cl were carried out. The water phase with [Bmim]Cl derived from the first liquefaction reaction was evaporated in a rotary evaporator, the obtained [Bmim]Cl was reused for orthogonal experiment, and the liquefaction ratio results were presented in Table 4. Similarly, [Bmim]Cl recycled from the second liquefaction reaction was reused for another orthogonal experiment, and the liquefaction ratio results were demonstrated in Table 5. Compared with the S. psammophila liquefaction ratio in Table 3, Table 4, and Table 5, it was observed that the liquefaction ratios displayed minimal difference under the same reaction condition. Moreover, it should be noted that under the optimized liquefaction conditions (temperature was 240 °C, the reaction time was 90 min, and solid-liquid ratio was 1:0.5), the corresponding liquefaction ratios (75.6%, 74.8%, and 73.8%) remained the highest, with nearly identical values. Consequently, the fresh, the first reused and the second reused [Bmim]Cl exhibited high stability.

Fourier transform infrared (FT-IR) absorption spectra were measured. Among the orthogonal experiments listed in Tables 3 to 5, a random FT-IR spectrum of the freshly used, firstly reused, and secondarily reused IL [Bmim]Cl were measured and shown in Fig. 2. The peak at 2112 cm⁻¹ is attributed to C=N stretching vibration in imidazole ring. The peak at 1644 cm⁻¹ corresponds to imidazole ring skeleton vibration. The peak at 1174 cm⁻¹ is assigned to alkyl carbon C-N stretching vibration. The peak at 647 cm⁻¹ is ascribed to C-Cl stretching vibration. The FT-IR spectra conﬁrmed that there were no signiﬁcant peak changes after the first and the second reuse compared with that of the fresh used [Bmim]Cl, further indicating the high stability IL [Bmim]Cl.

Fig. 2 FT-IR spectra of the fresh used, the firstly reused and the secondary reused IL [Bmim]Cl

Table 4. The First Reuse of [Bmim]Cl for Orthogonal Experiment and the Liquefaction Ratio Results

Table 5. The Second Reuse of [Bmim]Cl for Orthogonal Experiment and Liquefaction Ratio Results

GC Analysis Results of Oil Phase Products

The chemical components in the oil phase were examined through GC analysis.

Table 6. GC Results of the Liquefied Products Based on Different States of [Bmim]Cl at the Optimal Liquefaction Conditions

^a Retention time; ^bThe compounds obtained by GC based on the fresh used [Bmim]Cl in experiment No. 8 experiment; ^c The compounds obtained by GC based on the firstly reused [Bmim]Cl in experiment No.8 experiment; ^d The compounds obtained by GC based on the secondary reused [Bmim]Cl in experiment No. 8 experiment; and ^e undetectable

As a representative, the compounds identified by GC at the optimal liquefaction conditions (temperature – 240 °C, reaction time – 90 min, and solid-liquid ratio – 1:0.5) are listed in Table 6. Table 6 indicated that the compounds obtained could be classified into several groups, including alcohols, esters, aldehydes, ketones, long-chain alkanes, and furfurals. It was noticed that the compounds acquired based on the nature of IL, such as fresh IL, the firstly reused IL, and secondary reused IL [in experiment No.8 experiment (at the optimal hydrothermal liquefaction conditions) were of the same types and their concentration (mg·L^-1) displayed little differences. It could be suggested that the optimal liquefaction conditions determined by the orthogonal experiments were reliable. To some extent, it was confirmed that the recycling and reusing of [Bmim]Cl displayed high stability with nearly identical liquefaction products and concentration. Moreover, it was found that the dominant liquefied compounds were ethanol, ethyl propionate, 15 hydrocarbon, 1-octane, furfural, and isopentyl alcohol.

Brief Explanation on Salix psammophila Decomposition Mechanism in Hydrothermal Liquefaction

Hydrothermal liquefaction is a thermo-chemical conversion process that uses water as a reaction medium at elevated pressure and temperature (Kumar 2022). Generally, the pathway of hydrothermal liquefaction comprises three major steps, depolymerization, followed by decomposition, and recombination (Gollakota et al. 2018).

Depolymerization of S. psammophila involves a sequential dissolving of macromolecules. Adequate dissolution was achieved by the utilization of [Bmim]Cl. Known as one of the optimal solvents for lignocellulosic material, the IL [Bmim]Cl was selected here. The dissolution mechanism of cellulose component of S. psammophila in the presence of [Bmim]Cl could be attributed to the nature of the imidazolium cation and the relatively strong electronegativity and small size of the chloride ion (Dadi et al. 2006). The imidazolium cation with its electron-rich aromatic π system interacts with cellulose hydroxyl oxygen atoms via non-bonding or π electrons and in addition prevents crosslinking of the cellulose molecules. The chloride ion attacks the free hydroxyl groups and deprotonates cellulose. The activity of [Bmim]Cl was effective in breaking down the H-bond network of cellulose (Zhang et al. 2017). Moreover, in depolymerization process, the decisive parameters temperature and pressure in this work changed the structure of the long chain polymers to shorter chain products.

Decomposition of the S. psammophila monomers was accomplished by cleavage, dehydration (the loss of water molecule), and decarboxylation (loss of CO₂ molecule) steps (Gollakota et al. 2018). Salix psammophila was hydrolyzed to form polar oligomers and monomers. Water is present in all biomass, including S. psammophila. Hydrothermal liquefaction could handle and liquefy biomass with any level of moisture contents. Here the addition of water served as both solvent and reactant in the hydrothermal liquefaction. Under the condition of hydrothermal liquefaction, the structure and properties of water changed dramatically (Kruse and Dinjus 2007) when its temperature was raised, resulting in the rapid drop of dielectric constant. Consequently, the ion product decreased and hydrogen bonding became weak. Additionally, water behaved like a nonpolar component, resulting in an increase in solubility of organic molecules. Deconstruction of cellulose and hemicellulose carbohydrate polymers by water into their constituent C5 and C6 sugars occurred, leading to subsequent catalytic reforming into other chemical intermediates and products by different types of reactions (Zhang et al. 2016). Decomposition and deoxygenation were the major reactions, which produced degradation products containing acids, aldehydes, and aromatic compounds (Akhtar and Amin 2011; Gollakota et al. 2018). In addition, the hydrolyzed and depolymerized products usually undergo secondary reactions. Major reactions among these were condensation, dehydrations, and isomerization. Alcohols and aldehydes were produced by the results of the secondary reactions (Xu and Lancaster 2008; Zou et al. 2011), while furfurals were formed as secondary intermediates.

Under certain conditions, the unavailability of the hydrogen or the concentration of the free radicals were excessively large, such that the fragments were recombined or repolymerized. Recombination and repolymerization process usually generate acids, alcohols, aldehydes, esters, ketones, and other aromatic compounds (Chornet and Overend 1985). The liquefied products detected and listed in Table 6 were in accordance with the above brief mechanism.

CONCLUSIONS

The optimal reaction parameters (temperature was 240 °C, reaction time was 90 min, and solid-liquid ratio was 1:0.5) were determined by orthogonal experiments. Under the optimized liquefaction condition, the liquefaction ratio of Salix psammophila reached 75.6%.
The FT-IR spectra conﬁrmed that there were no signiﬁcant peak changes after the first and the second reuse compared with that of the freshly used [Bmim]Cl, indicating the stability of IL [Bmim]Cl.
The IL [Bmim]Cl used for three consecutive liquefaction reactions displayed high liquefaction ratios (75.6%, 74.8%, and 73.8%). GC results of the liquefied products exhibited nearly identical liquefaction products and concentration.
The liquefied products obtained by GC confirmed the presence of value-added chemicals, such as acids, alcohols, aldehydes, esters, ketones, and furfural.

The hydrothermal liquefaction is one of the promising technologies to produce highly valuable chemicals from S. psammophila. The desired compounds C7 to C8 that are desirable for transportation fuels were not found due to the limited operating conditions. In the near future, there is a need for scale-up experiments and further studies about S. psammophila liquefaction mechanism are required.

ACKNOWLEDGEMENTS

This work was financially supported by the Interdisciplinary Research Fund of Inner Mongolia Agricultural University (BR22-15-02), the Central Government Guides Local Science and Technology Development Fund Projects (2022ZY0066), the Innovative Entrepreneurship Training Program of Inner Mongolia (202310129016, 202210129017, 202210129028)

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Article submitted: March 3, 2023; Peer review completed: June 24, 2023; Revised version received and accepted: July 16, 2023; Published: September 7, 2023.

DOI: 10.15376/biores.18.4.7341-7352