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Solvent-enhanced Depolymerization of Lignin under Microwave Irradiation

- ORCID：
Wenliang Wang ^1,2
✉
- ORCID：
Jiale Huang ¹
- ORCID：
Xingjin Zhao ¹
- ORCID：
Xiaoxiao Ren ¹
- ORCID：
Hui Miao ¹

1. National Demonstration Center for Experimental Light Chemistry Engineering Education (Shaanxi University of Science & Technology), Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty Paper Development, Key Laboratory of Paper based Functional Materials of China National Light Industry, College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi'an, Shaanxi Province, 710021, China； 2. Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education, Qilu University of Technology (Shandong Academy of Sciences), Ji'nan, Shandong Province, 250353, China

Updated：2021-07-14

DOI：10.1213/j.issn.2096-2355.2021.03.004

OUTLINE

Abstract

Lignin is considered an ideal natural material for the production of sustainable monophenols. In this study, a microwave-assisted depolymerization (MAD) strategy was developed. The introduction of solvent vapors in the dynamic vapor flow reaction system was performed to enhance the lignin conversion efficiency. The results showed that no liquid products were generated from the MAD of lignin without solvent vapors. With the introduction of solvents (CH₃OH, HCHO, HCOOH, and CH₂Cl₂), liquid products appeared (especially with CH₂Cl₂, which had the highest yield of 41.9 wt%). Results from gas chromatography/mass spectrometry of liquid products showed that seven kinds of compounds, including guaiacols, phenols, syringols, methoxyphenyls, heterocycles, esters, and aromatics were identified. CH₂Cl₂ can significantly enhance the production of monophenols (guaiacols, phenols, and syringols). The introduction of these solvent vapors can also facilitate the generation of porous char with high Brunauer-Emmett-Teller specific surface areas. Some carbon nanospheres deposited on the surface of the char were obtained with the assistance of CH₂Cl₂. This study provides a facile method for the utilization of lignin in the field of bio-based fine chemicals.

Keywords

lignin; microwave; solvent; monophenols; porous char

1 Introduction

With the depletion of fossil resources and the threat of global warming, the global community has focused on exploring renewable sources instead of fossil-based fuels and chemicals ^[

1]. As an important component in the abundant lignocellulosic biomass, lignin has a high carbon source and intrinsic bio-aromatic structure; therefore, it possesses huge potential for use as a renewable material for producing fine chemicals ^{[Reference 2-5}2-5]. The majority of technical lignin (kraft lignin, lignosulfonate, enzymatic hydrolysis lignin, etc.) usually comes from the biorefinery process of pulp and bioethanol production ^{[Reference 6-7}6-7]. The utilization of lignin remains a major challenge because of its complexity and recalcitrance.Unfortunately, a large amount of lignin has been commonly used as a low-grade fuel for heat and power generation. For example, kraft lignin in black liquor is usually concentrated and burned by pulp mills to recover alkali salts and energy. In view of the unique structure of lignin with natural phenylpropane units, the development of effective methods for the conversion of lignin into value-added chemicals is of great importance.

Pyrolysis is one of the most promising technologies for the conversion of lignin into renewable chemicals, such as monophenols. As an important and effective pyrolysis method, microwave-assisted depolymerization (MAD) has attracted extensive attention owing to its ultra-fast heating, flexible control (no thermal inertia), high thermal efficiency, and special selectivity in chemical reactions ^[

8-10]. However, there are still serious challenges in the conversion of lignin to the target products of monophenol: (1) high heterogeneity and natural recalcitrance of lignin ^{[Reference 11-12}11-12] and (2) competition between depolymerization and fragment re-condensation reactions ^{[Reference 13

Baidu Scholar}13]. Hence, process control is essential for lignin depolymerization. Carrier gas is a vital factor for the smooth degradation of lignin in a flow-through system. In most previous studies, N₂ ^{[Reference 14-16}14-16], Ar ^{[Reference 12

Baidu Scholar}12, 14], and H₂ ^{[Reference 14

Baidu Scholar}14, 17] were most commonly used as carrier gases for the depolymerization process. Zhang et al ^{[Reference 14

Baidu Scholar}14] found that N₂ and Ar had slightly different effects on the properties of liquid products from lignin pyrolysis. However, the presence of H₂ (H₂∶Ar = 1:9, V/V) facilitated the degradation of oligomers to monophenols by inhibiting the occurrence of secondary reactions. N₂ and Ar are usually used as inert gases to remove air in the reactor and carry the products to the collecting system. The presence of H₂ can be used to provide a reduction environment and effectively react with the intermediates from lignin, but it also causes a security risk in the MAD process. Furthermore, conventional carrier gases, such as N₂, Ar, and H₂, do not help prevent the target products from adhering to the tube wall, which is a universal problem in flow-through systems.

Based on the above discussion, a new strategy involving the introduction of solvent vapors as the carrier gas for the MAD system was developed for the lignin depolymerization process. This dynamic vapor flow reaction system was designed to draw support from solvent vapors to reform the intermediates (pyrolysis vapor) and restrain the lignin fragment condensation, finally obtaining the target products of monophenols from lignin. In a previous study on the cleavage of 5-5′ C (phenyl)—C (phenyl) bonds in the kraft lignin model ^[

18], it was found that methanol vapor as a carrier gas can produce hydrogen and methyl radicals under microwave irradiation and reduce the activation barrier to facilitate C—C bond cleavage. Consequently, typical solvent vapors, including CH₃OH, HCHO, HCOOH, and CH₂Cl₂, were selected and applied for lignin depolymerization to elucidate their effects on the formation of monophenols.

2 Materials and methods

2.1　Materials

Lignin (CAS: 9005-53-2) was purchased from TCI (Shanghai) Development Co., Ltd. Methanol (CH₃OH, AR), formaldehyde (HCHO, AR), formic acid (HCOOH, AR), dichloromethane (CH₂Cl₂, AR), and silicon carbide (SiC, CAS: 409-21-2) were purchased from Aladdin Industrial Corp.

2.2　Microwave-assisted depolymerization (MAD) method

The experiments were performed in a self-developed MAD reactor equipped with a device for solvent-vapor input. SiC particles are good microwave absorbents for the rapid heating of lignin materials (lignin has poor microwave absorptivity). In this study, 10 g of lignin mixed well with 20 g of SiC particles was placed in the MAD reactor. N₂ (600 mL/min) flowed continuously through the reactor for 10 min to remove air prior to experiments. Subsequently, different solvents (CH₃OH, HCHO, HCOOH, CH₂Cl₂) were added to a gas-washing bottle. N₂ flowed through the gas-washing bottle and then went to the reactor. The microwave reactor was operated with a constant microwave power of 1000 W (2.45 GHz). The reactor was maintained for 15 min after reaching 600℃ . The volatile vapor produced by depolymerization was cooled by rapid condensation at (-35±1)℃ with a cooling medium of ethanol and collected as liquid products. The solid products were obtained in the MAD reactor and collected after cooling.

2.3　Liquid product analysis

The chemical composition of the liquid products was determined by gas chromatography/mass spectrometry (GC-MS, 6892N/5975I, Agilent). The liquid samples obtained at 600℃ with different solvents (CH₃OH, HCHO, HCOOH, and CH₂Cl₂) were automatically injected into the GC/MS for analysis. The peaks were identified using the NIST11 library. The conditions for GC-MS were as follows: inlet temperature of 280℃, He gas flow rate of 1.0 mL/min, and split-flow ratio of 30∶1. The heating schedule was preset such that the temperature remained at 50℃ for 5 min, increased to 280℃ at a rate of 5℃/min, and then was maintained for 7 min. A junction temperature of 280℃ , ion temperature of 230℃ , electron ionization source electron energy of 70 eV, and scanning range of 18-500 μm were used for MS.

The functional groups of the liquid products were characterized by a Fourier transform infrared (FT-IR) analyzer (VERTEX 70, Bruker). Hydrogen nuclear magnetic resonance (¹H NMR) analysis was conducted on a Bruker ADVANCE III 400 MHz spectrometer to understand the structural information of the liquid products. The liquid products were dried in vacuum at 60℃ , dissolved in dimethyl sulfoxide-d6, and transferred to ¹H NMR sample tubes for ¹H NMR analysis.

2.4　Solid product analysis

The Brunauer-Emmett-Teller (BET) specific surface area and pore structures of the solid products were evaluated by N₂ adsorption-desorption isotherms at 77 K using a Micromeritics ASAP2460 analyzer. The surface morphology was investigated using field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi).

3 Results and discussion

3.1　Yields of the products and FT-IR of liquid products

The experiments were performed in the MAD reactor using a dynamic vapor flow reaction system (Fig. 1(a)). The solvent vapors were carried by N₂ to flow through the microwave reactor and were in direct contact with lignin/lignin vapor. The function of solvent vapor is as follows: (1) reforming reactions of lignin pyrolysis vapor; (2) constant removal of target products to avoid side reactions and minimize undesirable secondary reactions ^[

19]; and (3) collecting liquid products by inhibiting adhesion to the tube wall.

Fig. 1 (a) Process for MAD of lignin. (b) Yields of the liquid, gas, and solid products, and (c) FT-IR spectra of liquid products using different solvents

As shown in Fig. 1(b), no liquid products were generated from the MAD of lignin without solvent vapors (control). The results showed the highest yield of char (64.2 wt%) resulting from the rapid benzenoid polycondensation ^[

20]. Conversely, liquid products were produced by the introduction of these solvents. The yields of the liquid products produced by CH₃OH, HCHO, and HCOOH were less than 10 wt% . Interestingly, the MAD of lignin with CH₂Cl₂ significantly increased the liquid production and showed the highest yield (41.9 wt%). It was about 5-8 times that of CH₃OH, HCHO, or HCOOH, indicating the positive effect of CH₂Cl₂ on lignin degradation while minimizing the production of solid and gas products. HCHO vapor facilitated the formation of gas products with a yield of over 50 wt%.

FT-IR (Fig. 1(c)) analysis showed that there was a strong —OH stretching peak at 3437 cm^-1, indicating abundant phenolic hydroxyl groups in the liquid products, which can be confirmed by Fig. 2. The in-plane bending vibration of —OH was also observed at 1394 cm^-1. The peaks at 2990, 1601, and 786 cm^-1 corresponded to the aromatic ring ^[

21], especially the sharp and strong C=C skeleton vibration at 1601 cm^-1. The C=O stretching vibration at 1721 cm^-1 observed in the liquid samples with HCOOH and CH₂Cl₂ indicated the existence of carboxyl or carbonyl groups ^{[Reference 22

Baidu Scholar}22]. The C—O stretching vibrations of esters and phenols were observed at peaks of 1178, 1024, and 1001 cm^-1. The FT-IR results demonstrated the rich functional groups and complex compounds in these liquid products.

Fig. 2 (a) Total ion chromatograms of liquid products from MAD of lignin using different solvents; (b) component distribution of liquid products

3.2　Component distribution of liquid products

To further understand the role of these solvents during lignin depolymerization under microwave irradiation, the chemical composition of the liquid products was investigated by GC-MS (Fig. 2(a)). More-abundant peaks appeared with the introduction of CH₂Cl₂ and HCOOH. Especially with CH₂Cl₂, a large amount of small molecular components was released in the first 30 min of the retention time, indicating the accumulation of monomeric products. After analysis using the NIST11 library, the qualitative analysis of seven kinds of compound (guaiacols, phenols, syringols, methoxyphenyls, heterocycles, esters, and aromatics) was performed, and the results are summarized in Fig. 2(b). Phenolic compounds, including guaiacols (G-type), phenols (H-type), and syringols (S-type), were the main products in the liquid products. In the liquid products of MAD with HCOOH, guaiacols accounted for over 80% (relative content). The relative content of phenols was approximately 35% in the liquid products of MAD with HCHO. It should be noted that CH₂Cl₂ facilitated the generation of aromatics, including benzene, toluene, and xylene, indicating the occurrence of deoxygenation or demethoxylation ^[

23]. As demonstrated in our previous work, HCl in situ dissociated from CH₂Cl₂ plays a key role in the conversion of phenolic compounds to aromatics because of the decrease in the activation barrier and Gibbs free energy with the assistance of HCl.

3.3　Monophenols of liquid products

To further analyze the component distribution quantitatively, peak areas were used to reflect the content variation of the corresponding compounds. As shown in Fig. 3(a), guaiacol and vanillin were the main G-type monophenols. The production of guaiacols with different solvent vapors was CH₂Cl₂ > HCOOH > CH₃OH > HCHO. The highest production of guaiacols with CH₂Cl₂ is indicative the cleavage of ether bonds (β-O-4) in lignin, leading to the release of the etherified phenolic hydroxyl groups ^[

24]. HCHO is a good protectant for preventing lignin condensation by forming 1,3-dioxane structures with lignin side-chain hydroxyl groups ^{[Reference 25

Baidu Scholar}25]. However, HCHO had a weak effect on the depolymerization of lignin in the vapor phase. Syringol was the main S-type monophenol (Fig. 3(b)). The amount of syringols with different solvent vapors was CH₂Cl₂ > HCOOH > CH₃OH > HCHO. Syringols are usually present in lignin from hardwood or non-wood materials. Although the introduction of CH₂Cl₂ facilitated the formation of syringols, the total peak area of syringols was much smaller than that of guaiacols and phenols (Fig. 3(a) and Fig. 3(c)). This is because it is difficult to regenerate syringols from other phenols owing to steric effects. Phenol and salicylaldehyde were the main H-type phenols, as shown in Fig. 3(c). The influence of solvent vapor on the phenol content was as follows: CH₂Cl₂ > CH₃OH > HCHO > HCOOH. Similarly, CH₂Cl₂ presented the highest content of H-type phenols, indicating a positive effect on the occurrence of demethoxylation. The relatively high content of phenols obtained with the introduction of CH₃OH resulted from the reduction environment provided by the hydrogen radicals from the dissociation of CH₃OH ^{[Reference 18

Baidu Scholar}18].

Fig. 3 Peak area of guaiacols (a), syringols (b), and phenols (c) in liquid products from MAD of lignin using different solvents

Overall, the contents of guaiacols, phenols, and syringols with CH₂Cl₂ were much higher than those with CH₃OH, HCHO, and HCOOH, respectively, indicating the enhanced effect of the introduction of lignin depolymerization with CH₂Cl₂. In this case, CH₂Cl₂ vapor may provide a strong reductive environment by the generation of hydrogen radicals from the electromagnetic field effect of microwave irradiation ^[

23, 26-27]. Subsequently, hydrogen radicals in situ dissociated from CH₂Cl₂ can significantly decrease the activation barrier of the scission of C—O—C bonds and demethoxylation significantly. As a result, CH₂Cl₂ can reform lignin pyrolysis vapors into monophenols in gas-phase reactions under reductive conditions.

3.4　¹H NMR analysis of liquid products

The structural characteristics of the liquid products were determined by ¹H NMR spectroscopy (Fig. 4). All liquid samples from the MAD of lignin using solvents of CH₂Cl₂, CH₃OH, HCHO, and HCOOH showed chemical shifts of 6.6-8.3 ppm for aromatic nuclei and conjugated H, which agreed with the FT-IR results (Fig. 1(c)). The peak at 9.77 ppm was attributed to the presence of aldehyde groups in the aromatic ring, such as vanillin, salicylaldehyde, and syringaldehyde. The peak of methoxy groups appeared at 3.84 ppm, demonstrating the formation of guaiacols and syringols. The strong peaks in the region less than 1.5 ppm corresponded to aliphatic carbons (primary, secondary, and tertiary alkyl groups) on the side chain of the aromatic ring ^[

22]. A relatively strong signal intensity in the region of 6.6-8.3 ppm for the aromatic nuclei was clearly presented with the introduction of CH₂Cl₂, indicating the enhancement of the production of phenolic compounds.

Fig. 4 ¹H NMR spectra of liquid products from MAD of lignin using different solvents

3.5　Analysis of solid products

Solid products are usually produced from the intramolecular and intermolecular rearrangement of intermediates during the lignin decomposition process with the formation of an aromatic polycyclic struc-ture ^[

11, 28]. Meanwhile, phenolic hydroxyl can also accelerate the repolymerization during lignin pyrolysis because of the increase in the electron cloud density of the benzene ring ^{[Reference 29

Baidu Scholar}29]. As Fig. 5 shows, different surface morphologies and surface areas of solid products (solid chars) were observed using different solvents. The surface of the solid char from the MAD of lignin without solvents (Fig. 5(b)) became smooth compared with that of the raw material of lignin (Fig. 5(a)). After the use of solvents, such as CH₃OH, HCHO, and HCOOH, the resultant solid chars were porous, with BET specific surface areas of 164, 9, and 152 m²/g, respectively (Fig. 5(c)-Fig. 5(e) and Fig. 5(g)). The higher surface area of the solid char using CH₃OH and HCOOH than that of HCHO was mainly caused by the better solvent effects of CH₃OH and HCOOH, resulting in surface erosion during the change from lignin to solid char.

Fig. 5 (a-f) FE-SEM images of solid products from MAD of lignin using different solvents. (a) Lignin material; (b) control; (c) CH₃OH; (d) HCHO; (e) HCOOH; (f) CH₂Cl₂; (g) N₂ adsorption-desorption isotherms of solid products

The solid char from the MAD of lignin with CH₂Cl₂ presented significantly different shapes, with some carbon nanospheres inlayed on the surface of the char (Fig. 5(f)). The formation of carbon nanospheres can increase the BET specific surface area, as shown in Fig. 5(g) (231 m²/g). The adsorption isotherms exhibited a sharp increase at a relative pressure below 0.1, corresponding to type I and demonstrating the microporous and mesoporous properties ^[

30]. The formation of carbon nanospheres is attributed to the coupling effect of CH₂Cl₂ and microwave irradiation. As a good lignin solvent, CH₂Cl₂ vapor continuously flows through the lignin particles, leading to constant surface erosion and a reduction in surface energy. Meanwhile, microwave "hot spots" are generated during the conversion of lignin to carbon precursors owing to the increase in the dielectric loss tangent ^{[Reference 31

Baidu Scholar}31]. As shown in our previous study, the "hot spots", combined with the surface erosion and the reduction in the surface energy of carbon precursors by CH₂Cl₂ vapor, are the most probable driving force for the appearance of carbon nanospheres on the surface of solid char.

4 Conclusions

Based on the microwave-assisted dynamic vapor flow reaction system, solvent vapors were injected to successfully depolymerize lignin into renewable monophenols. The introduction of solvents (CH₃OH, HCHO, HCOOH, and CH₂Cl₂) facilitated the production of liquid products. Seven kinds of compound such as guaiacols, phenols, syringols, methoxyphenyls, heterocycles, esters, and aromatics were identified in the liquid products. The solvent vapor of CH₂Cl₂ significantly enhanced the conversion efficiency of lignin for monophenols (guaiacols, syringols, and phenols). In addition, the introduction of these solvents can facilitate the generation of porous char with high Brunauer-Emmett-Teller specific surface areas. This work demonstrated that solvent vapor (especially CH₂Cl₂) has a positive effect on the production of monophenols. The results provide an efficient and facile approach to enhance the value of lignin as an alternative to petroleum for fine chemicals.

Acknowledgements

This work was supported by the Foundation of the Key Laboratory of Pulp and Paper Science and Technology of the Ministry of Education of China (No. KF201917) and the National Natural Science Foundation of China (31800497).

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Solvent-enhanced Depolymerization of Lignin under Microwave Irradiation

Abstract

Keywords

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Microwave-assisted depolymerization (MAD) method

2.3 Liquid product analysis

2.4 Solid product analysis