Comparative study on various alcohols solvolysis of organosolv lignin using microwave energy: Physicochemical and morphological properties

Accepted Manuscript Title: Comparative study on various alcohols solvolysis of organosolv lignin using microwave energy: Physicochemical and morphological properties Authors: Dengle Duan, Yunpu Wang, Roger Ruan, Maimaitiaili Tayier, Leilei Dai, Yunfeng Zhao, Yue Zhou, Yuhuan Liu PII: DOI: Reference:

S0255-2701(17)30803-6 https://doi.org/10.1016/j.cep.2017.10.023 CEP 7107

To appear in:

Chemical Engineering and Processing

Received date: Revised date: Accepted date:

14-8-2017 26-9-2017 29-10-2017

Please cite this article as: Dengle Duan, Yunpu Wang, Roger Ruan, Maimaitiaili Tayier, Leilei Dai, Yunfeng Zhao, Yue Zhou, Yuhuan Liu, Comparative study on various alcohols solvolysis of organosolv lignin using microwave energy: Physicochemical and morphological properties, Chemical Engineering and Processing https://doi.org/10.1016/j.cep.2017.10.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Comparative study on various alcohols solvolysis of organosolv lignin using microwave energy: physicochemical and morphological properties Dengle Duan a,b, Yunpu Wang a,b,c, Roger Ruan a,b,d, Maimaitiaili Tayier a,b, Leilei Dai a,b

a

, Yunfeng Zhao a,b, Yue Zhou a,b, Yuhuan Liu a,b*

Nanchang University, State Key Laboratory of Food Science and Technology, Nanchang 330047,

China b

Nanchang University, Engineering Research Center for Biomass Conversion, Ministry of

Education, Nanchang 330047, China c Guangdong Provincial

Key Laboratory of New and Renewable Energy Research and Development,

Guangzhou 510640, China d Center

for Biorefining and Department of Bioproducts and Biosystems Engineering University of

Minnesota, 1390 Eckles Ave., St. Paul, MN 55108, USA

Highlights 

Lignin solvolysis was conducted in microwave device with sulfuric acid as catalyst.



The effects of various alcohols on depolymerized products was determined.



Methanol was the most effective solvent and the conversion rate reached 84.86%.



The degraded solid residue from methanol had higher stability and char yield.

*Corresponding author at: Nanchang University, State Key Laboratory of Food Science and Technology, Nanchang 330047, China. E-mail address: [email protected] (Yuhuan Liu)

Abstract: Solvolysis approach is an efficient pathway to covert solid biomass into different end products since it can provide a mild environment and a single-phase condition. A comparative study on various alcohols-assisted liquefaction of lignin has been carried out using sulfuric acid as catalyst under microwave condition. Methanol and ethanol showed a higher conversion rate compared to other alcohols. Both liquefied products and residues have been investigated using gel permeation chromatography– high-performance liquid chromatography (GPC–HPLC), Fourier transform infrared (FT-IR), Scanning electron microscopy (SEM) and Thermogravimetric (TG) analysis. The results showed that methanol and ethanol as solvents achieved a higher conversion rate, meantime, the molecular weight of liquefied products was dramatically lower than those of other alcohols. The FT-IR indicated that there were obvious differences in the contents of carbonyl, methyl and hydroxyl group in the liquefied products with various alcohols as solvents. The solid residue exhibited a big difference in microstructure morphology after depolymerization, in addition, the resulted solid residue from methanol solvents showed a higher thermal stability and char yield, which can be used as a fire retardant in composite materials. Keywords: Lignin; alcohols; liquefied products; solid residue; depolymerization 1. Introduction Lignin is the second most abundant renewable raw materials available on earth,

which always as s a kind of by-products of paper and pulp industries and bioethanol industries [1, 2]. It is a polyphenolic amorphous material containing three typical phenylpropane structural units that can provide valuable low molecular weight aromatic compounds through depolymerization [3-5]. However, the major obstacle for the efficient utilization of lignin is its highly complex structure of lignin, technological barriers, and adverse economic consideration [6, 7]. Therefore, it is very difficult to achieve the successful valorization of lignin into value-added products. Nowadays, the various technologies for the production of platform chemicals and renewable materials have received increasing attention, such as hydrogenolysis, supercritical solvents, oxidation and solvolysis, etc [8-12]. Nowadays, lignin is converted to aromatic compounds under mild conditions is a promising technology for its degradation [13]. As an important thermochemical method, solvolysis approach is an efficient pathway to convert solid biomass into different end products [14-16]. The biggest advantage of solvolysis approach is that it can provide a mild environment and a single-phase condition because of the miscibility of the organic products in the solvent [17]. In addition, the use of catalysis is also conducive to improve the conversion rate of biomass into monophenolics [18, 19]. Acid-catalyzed is one of the common types to depolymerize lignin [20, 21]. Xie et al. reported that when bamboo residues were degraded in a mixture of glycerol and methanol as solvent and sulfuric acid as catalyst, the maximum conversion yield was 96.7% [14]. Forchheim et al. conducted that the depolymerization of wheat straw lignin in different proportion of ethanol/formic acid solution. The results found that the yield of methoxyphenols can

achieve 2.9 wt.% when the concentration of formic acid and ethanol were 10 wt.% and 81 wt.%. respectively [22]. Microwave-assisted biomass conversion has been demonstrated a potential technology. Compared with traditional heating, microwave irradiation leads to more specific and selective reactions due to its higher speed and efficiency [23, 24]. In addition, microwave energy also provides many advantages, such as uniform fast internal heating, ease of operation and maintenance, energy cost saving, and etc [25]. Nowadays, microwave energy has combined with solvents to promote the biomass transformation due to the fact that microwave energy can be converted directly into heat at the molecular level [9, 26]. Due to the fact that solvolysis approach is always affected by solvent types, and there are different reaction pathways between various alcohols and biomass materials [27]. Therefore, different alcohols are expected to achieve the maximum liquefaction degree from lignin depolymerization. In experiment, alcohols, which with various hydroxyl groups, were selected to serve as solvents during lignin depolymerization. The effects of the solvent types on the products from ethanol organosolv lignin depolymerization were studied in detail. The whole experimental was carried out in a microwave using sulfuric acid as catalyst. 2. Materials and methods 2.1. Materials Ethanol organosolv lignin was prepared from bamboo culms according to the method reported previously [28, 29].

The lignin powder contains 60.1 wt% C, 5.9 wt%

H, 32.8 wt% O, 0.5 wt% N, 0.3 wt% ash, 85.39 wt% volatile and 14.31 wt% fixed carbon, respectively. The elemental analyzer Vario EL III was used to analyze the C, H, O, and N contents. The content of ash, fixed carbon and volatile were analyzed according to ASTM E870-82 (2013). Methanol, Ethanol, Butanol, Ethanediol and Isopropanol were purchased from Xi Long Scientific Co. (Guangdong, China). Sulfuric acid was obtained from Da Mao Reagent Co. (Tianjin, China). Tetrahydrofuran (THF, HPLC grade) was purchased from Solarbio (Beijing, China). 2.2. Methods 2.2.1.

Microwave depolymerization

Microwave depolymerization of ethanol organoslv lignin was carried out in a CEM Explorer microwave reactor (CEM Corporation, USA). Briefly, lignin (0.5 g) was mixed with 11.4 mL of alcohol and 0.6 mL of sulfuric acid were placed in a 35 mL tube. Then the tube was placed in CEM microwave instrument, which was equipped with an APM pressure controller. Before the experiment, the reaction system achieved a sealed state when the APM was locked, and then sample was stirred with a magnetic rotor for 2 min. The depolymerization process was performed at 160 oC for 30 min by maintaining a pressure and power below 220 psi and 100 W, respectively. After the reaction, the degraded products were cooled down to 25 oC before opening and subjected to subsequent studies. 2.2.2.

Measurement of conversion rate

The measurement of conversion rate was determined by liquefaction extent [30]. The liquid and solid products were filtered. The solid product was washed two times

with 20 mL alcohol, and dried at 105 oC for 12 h in vacuum oven then weighed. The yield of gas product was negligible, then conversion rate was expressed by the yield of liquid products and calculated as follows[16, 30]: Conversion rate (%)=(1- residue content⁄total raw content)×100 2.2.3.

(1)

Molecular weight determination

The average molecular weight of liquid product was determined by a gel permeation chromatography–high-performance liquid chromatography (GPC–HPLC) instrument (G1312B binary pump, Agilent Technologies, State of California, USA, UV detector at 280 nm; GPC column at a column temperature of 30 °C). The distilled water (5/2, v/v) was added into liquid product, then the phenolic compounds were obtained from precipitate. These products were filtered and dried at 30 °C in a vacuum oven for 12 h. The obtained dry liquefied precipitate dissolved in tetrahydrofuran (THF) and then filtered through a 0.45-μm syringe filter prior to analysis. Using THF as the mobile phase with a flow rate at 0.5 mL/min. In addition, polystyrene was used as the standard for calibration. 2.2.4.

Scanning electron microscopy

A JSM-6701F field emission scanning electron microscopy (SEM, JEOL, Japan) (0.5–30 kV accelerated voltage) was used to test the morphology of the raw samples and solid residue in order to compare the effect of different alcohols on lignin depolymerization. Before the experiment, specimens for SEM inspection were goldplated. 2.2.5.

FTIR analysis

Characterization of the functional groups of liquid products were analyzed by Fourier transform infrared (FT–IR) spectroscopy (Nicolet iS5, Thermo, USA). The samples were ground with KBr, and the data collection was recorded with

4 cm −1

spectral resolution per sample. In addition, the spectra were in the range of 4000 to 600 cm−1 with 32 scans. 2.2.6.

Thermogravimetric (TG) analysis TG analysis of solid residue and fresh lignin was conducted using a differential

thermal analyzer (TGA 4000, PerkinElmer company, USA). Before the experiment, all samples were dried at 105 oC overnight in the absence of oxygen. Samples (4 mg) were used to conduct TG analysis and DTG experiment. The corresponding curve was collected by heating the sample from 30 °C to 800 °C at a constant heating rate of 10 °C/min, a high-purity nitrogen gas flow of 100 mL/min was used as the purge gas prior to experiment. 3. Results and discussion 3.1. Conversion rate determination In order to explore the pathway of lignin depolymerization under various solvents, alcohols with different hydroxyl position were used as solvent. Fig. 1 shows that the influence of different alcohols on lignin conversion rate. It can be seen that alcohols with different hydroxyl position exerted a considerable influence on the lignin depolymerization. Methanol and ethanol showed higher conversion rates of 84.86% and 84.22%, respectively, when compared with butanol, ethanediol and isopropanol. This is due to the fact that the molecular weight of methanol and ethanol is lower, which is

conducive to provide higher permeability and fluidity. In the process of lignin depolymerization, simple alcohols are usually incorporated into the lignin-derived products within acidic medium, which is helpful to prevent repolymerization between lignin-degraded products by competitive reactions with intermediate carbonium ions, and further promote the conversion of lignin into low molecular weight liquid products [27, 31]. However, the conversion rate of butanol was only 49.34%, this is because of that when increased the length of carbon chain, alcohols will become more hydrophobic, which is not beneficial for the conversion of biomass [27]. It can be seen from the Fig.1, the conversion rate in a mixture of ethanol and water give worse results than in pure ethanol solvent. The similar result was reported by Song et al., who found that the conversion rate of lignin was related to the alcohol concentration, and higher concentration of alcohol was beneficial for the conversion of lignin [13]. In addition, ethanediol and isopropanol gave the lower conversion rate of lignin, only about 38% and 27%, respectively. These is because of that the low lignin dissolution capacity when ethanediol and isopropanol as solvents. The result was not in accordance with that reported by Song et al. [13], who found that ethanediol provided a higher lignin conversion, this may due to the fact that the different source of lignin. 3.2.

Molecular weight analysis for the liquefied precipitate

The Mw and Mn values, as well as the molecular weight distribution (Mw/Mn) of the liquefied products are shown in Table 1. Liquefied precipitate mainly consists of hydrophobic phenolic, which could be separated from aqueous solutions [27]. Different alcohols as solvents have different effects on molecular weight distribution of liquefied

precipitated. As shown in Table 1, when compared with other alcohols, reaction conducted in isopropanol is less effective, which achieves a higher Mw and Mw/Mn of 1919 and 4.95, respectively (entry 6). Reaction in ethanediol and butanol give a similar value of Mw, however, the Mw/Mn for ethanediol is remarkably narrower than that for butanol, this will significantly influence the application of the liquefied products (entry 4-5). Ethanol as solvent give a lower Mw and Mn than that of ethanol/water mixture as solvent, indicating reaction in ethanol is more effective than in ethanol/water mixture (entry 2-3). In addition, methanol as solvent has a similar pattern of molecular weight distribution (Mw=1164, Mn=364, Mw/Mn=3.20) compared with the ethanol as solvent (entry 1). This may be due to the fact that both methanol and ethanol are lower molecular weight alcohols, which show higher nucleophilic activity for conversion lignin and promote the C-O-C cleavage of lignin in solvolytic reaction, leading to the lignin of a very high molecular weight was successfully degraded in to oligomers [13, 17, 27]. 3.3.

FT-IR Spectra of the liquefied products

The FT-IR spectra of liquefied products from depolymerization under various alcohols as solvents are shown in Fig. 2. The spectra patterns of liquefied products from different alcohols solvents were not exactly same, especially when the solvents were the methanol, ethanol and ethanol/water mixtures, the spectra in the range of 2000 to 600 cm-1 were significantly different from those when the butanol, ethanediol and isopropanol as solvents. The results indicating that the main chemical components were diverse [14]. It can be seen from the Fig. 2, reaction conducted in butanol, ethanediol

and isopropanol achieve a higher relative intensities of band around 3396 cm-1 (-OH groups), indicating that higher hydroxyl content in liquefied products under the condition of butanol, ethanediol and isopropanol as solvents. For the spectra of liquefied products from methanol, ethanol and ethanol/water mixtures as solvents, the peak around 3396 cm-1 were weaker while around 1711 cm-1 (C=O groups) were clearer, which indicated that a lower hydroxyl content and a higher carbonyl content. This was because of that oxidation of hydroxyl groups into carbonyl groups was promoted when methanol, ethanol and ethanol/water mixtures as solvents [30]. In addition, reaction in methanol, ethanol and ethanol/water mixtures give weaker peak around 2922 cm-1 (CH stretching), which indicates a decrease in the methyl groups and suggests the formation of monophenolic compounds and the breakage of long chain hydrocarbon [29, 32]. This was accordance with the aforementioned results of molecular weight analysis. 3.4.

Microstructure morphology of solid residue

SEM micrograph (500×, 1000 × and 5000 ×) analysis were conducted to examine the surface morphology of raw lignin powder and the solid residue powder by different solvents. It can be seen from Fig. 3A, B and C, smooth and continuous surface structures are observed for the raw lignin powder. After depolymerization by different solvents, the surface of the solid residue was destroyed in different extent. Reaction in isopropanol solvent leaded a porous surface for solid residue (Fig. 3S, T and U). For ethanediol, the surface of the solid residue was rough (Fig. 3P, Q and R). In the presence of butanol, the surface of the solid residue was severely damaged and became

disordered (Fig. 3M, N and O). After depolymerization in ethanol/water mixture solvent, the solid residue comprised of many small globular shapes deposits, which had a smooth surface, simultaneously, the size of the solid residue tended to be smaller (Fig. 3J, K and L). Reaction in methanol and ethanol solvents, the tendency for size of the solid residue became more observably, conversely, lignin degraded into small irregular fluffy products (Fig. 3D~I). This may be related to the ability of lignin solubility, which can be promoted in methanol and ethanol solvents due to their low molecular size and molecular weight [27]. These images clearly exhibit significant depolymerization and transformation of lignin in methanol, ethanol and ethanol/water mixture, indicating that the low molecular weight alcohols provide a more efficient influence on lignin depolymerization when compared with butanol, ethanediol and isopropanol. The results were consistent with the conclusion of conversion rate determination in section 3.1. 3.5.

TG analysis for solid residue

Thermogravimetric (TG) and differential TG (DTG) curves obtained for the raw lignin and solid residue from lignin depolymerization are shown in Fig. 4. Since TG analysis provides a theoretical basis of thermal stability and different decomposition temperatures, which can help understand the future value added applications for lignin depolymerization products [33]. It can be seen from Fig. 4, both TG and DTG curves of solid residue turned to lower temperature when compared with raw lignin, which indicated that the thermal stability of lignin after depolymerization was decreased. In addition, different alcohols as solvents have a significantly influence on the thermal degradation of lignin depolymerization products. In the TG curve, both raw lignin and

solid residues have a continuous weight loss from the initial to the end, which is related with the unique structures of lignin [34]. The peaks in the DTG curve (below 450 oC) of solid residues were lower than that of raw lignin, which could be due to the cleavage of methyl-aryl ether bond and extensive depolymerization of lignin could have already taken place during microwave treatment with various alcohols [17, 33]. The characteristic parameters of TG and DTG curves were listed in Table 2. As shown in Table 2, the onset temperature (Ti, the temperature at which the weight loss is 5%) and DTGmax (the maximum weight loss rate) of solid residue was lower than that of raw lignin. It should be noted that the solid residue from lignin depolymerization led to higher values of char yields (Ychar) than that of raw lignin. This was because that in the process of lignin depolymerization in various alcohols, the side chain structure of the benzene ring was interrupted, and leading to the formation of condensed aromatic structures [17, 35]. Among of various alcohols, the butanol solvent showed the highest char yields of 44.43%, the next was methanol solvent. In the meantime, the methanol solvents showed a higher Ti and DTGmax of 210.06 oC and 3.810%/min, respectively. The results indicate that the thermal stability of solid residue from methanol solvents depolymerization was better than that from other alcohols. In addition, methanol can be used as a kind of pretreatment solvent to improve the char content of lignin, and resulted solid residue can be further used as a fire retardant in composite materials [33]. It can be seen from the whole paper, microwave-assisted solvolysis lignin is significantly affected by the type of solvents. Unlike traditional heating mechanism, the effects of microwave heating are non-thermal and the coherent topological excitation

can exist in feedstocks. Microwave heating can penetrate throughout the volume of material, and the volumetric heating is conducive to reduce processing times and save energy, which rests upon the additives or fillers in the feedstocks [36]. Therefore, fast heating and effective depolymerization can be achieved with a befitting utilization of solvents that absorb microwave radiation. Conclusion Organosolv lignin extracted from bamboo was subjected to solvolysis with various alcohols as solvents and sulfuric acid as catalyst under microwave assisted. The main purpose of this study was to understand the effects of various alcohols on the physicochemical and morphological properties liquefied products and solid residues from lignin solvolysis. The solvolysis of lignin in alcohols with low molecular weight showed a higher conversion rate. Liquefied products were characterized by GPC and FT-IR and the results showed that reaction in solvents of methanol and ethanol induced lower molecular weights for liquefied products, simultaneously, more oxidation cleavage and C-O-C cleavage of lignin than other alcohols. The SEM indicated that solid residue exhibited a big difference in microstructure morphology after depolymerization with various alcohols as solvents. In addition, solid residues from methanol showed higher thermal stability and char yields, so that it can be used as a fire retardant in composite materials. Our future work will focus on the enhancement the yield of monophenolics from liquefied products for fine chemicals production.

Acknowledgements This work was supported by Natural Science Foundation of China (project 21466022) and the Key project (20161BBF60057) and (GCXZ2014-124) of Jiangxi Provincial Department of Science and Technology for financial support. References [1] S. Nanayakkara, A.F. Patti, K. Saito, Chemical depolymerization of lignin involving the redistribution mechanism with phenols and repolymerization of depolymerized products, Green Chem. 16 (2014) 18971903, https://doi.org/10.1039/C3GC41708E. [2] R. Gadioli, J.A. Morais, W.R. Waldman, M.-A. De Paoli, The role of lignin in polypropylene composites with semi-bleached cellulose fibers: Mechanical properties and its activity as antioxidant, Polymer

Degradation

and

Stability

108

(2014)

23-34,

https://doi.org/10.1016/j.polymdegradstab.2014.06.005. [3] L.B. Davin, N.G. Lewis, Lignin primary structures and dirigent sites, Curr. Opin. Biotechnol. 16 (2005) 407-415, https://doi.org/10.1016/j.copbio.2005.06.011. [4] L.C. Barbosa, C.R. Maltha, V.L. Silva, J.L. Colodette, Determinação da relação siringila/guaiacila da lignina em madeiras de eucalipto por pirólise acoplada à cromatografia gasosa e espectrometria de massas (PI CG/EM), Química Nova 31 (2008) 2035-2041, http://dx.doi.org/10.1590/S010040422008000800023. [5] X. Zhang, Q. Zhang, T. Wang, B. Li, Y. Xu, L. Ma, Efficient upgrading process for production of low quality fuel from bio-oil, Fuel 179 (2016) 312-321, https://doi.org/10.1016/j.fuel.2016.03.103. [6] S.K. Badamali, J.H. Clark, S.W. Breeden, Microwave assisted selective oxidation of lignin model phenolic

monomer

over

SBA-15,

Catalysis

Communications

9

(2008)

2168-2170,

https://doi.org/10.1016/j.catcom.2008.04.012. [7] G. Jiang, D.J. Nowakowski, A.V. Bridgwater, Effect of the temperature on the composition of lignin pyrolysis products, Energy Fuels 24 (2010) 4470-4475, DOI: 10.1021/ef100363c. [8] M. Sasaki, M. Goto, Recovery of phenolic compounds through the decomposition of lignin in near and supercritical water, Chemical Engineering and Processing: Process Intensification 47 (2008) 16091619, https://doi.org/10.1016/j.cep.2007.09.001. [9] A. Toledano, L. Serrano, A. Pineda, A.A. Romero, R. Luque, J. Labidi, Microwave-assisted depolymerisation of organosolv lignin via mild hydrogen-free hydrogenolysis: catalyst screening, Applied Catalysis B: Environmental 145 (2014) 43-55, https://doi.org/10.1016/j.apcatb.2012.10.015. [10] V. Roberts, V. Stein, T. Reiner, A. Lemonidou, X. Li, J.A. Lercher, Towards quantitative catalytic lignin

depolymerization,

Chemistry-A

European

Journal

17

(2011)

5939-5948,

http://onlinelibrary.wiley.com/enhanced/exportCitation/doi/10.1002/chem.201002438. [11] Q. Liu, P. Li, N. Liu, D. Shen, Lignin depolymerization to aromatic monomers and oligomers in isopropanol assisted by microwave heating, Polymer Degradation and Stability 135 (2017) 54-60, https://doi.org/10.1016/j.polymdegradstab.2016.11.016. [12] X. Zhang, Q. Zhang, T. Wang, L. Ma, Y. Yu, L. Chen, Hydrodeoxygenation of lignin-derived phenolic compounds to hydrocarbons over Ni/SiO2-ZrO2 catalysts, Bioresource technology 134 (2013) 73-80, https://doi.org/10.1016/j.biortech.2013.02.039. [13] Q. Song, F. Wang, J. Cai, Y. Wang, J. Zhang, W. Yu, J. Xu, Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation–hydrogenolysis process, Energy Environ. Sci. 6 (2013) 994-1007, https://doi.org/10.1039/C2EE23741E. [14] J. Xie, J. Qi, C. Hse, T.F. Shupe, Optimization for microwave-assisted direct liquefaction of bamboo

residue in glycerol/methanol mixtures, Journal of forestry research 26 (2015) 261-265, DOI 10.1007/s11676-015-0032-1. [15] G. Mishra, S. Saka, Effects of water in water/phenol mixtures on liquefaction of Japanese beech as treated under subcritical conditions, Holzforschung 67 (2013) 241-247, https://doi.org/10.1515/hf-20120050. [16] X. Zhang, Q. Zhang, J. Long, Y. Xu, T. Wang, L. Ma, Y. Li, Phenolics production through catalytic depolymerization of alkali lignin with metal chlorides, BioResources 9 (2014) 3347-3360. [17] J.R. Gasson, D. Forchheim, T. Sutter, U. Hornung, A. Kruse, T. Barth, Modeling the lignin degradation kinetics in an ethanol/formic acid solvolysis approach. Part 1. Kinetic model development, Ind. Eng. Chem. Res. 51 (2012) 10595-10606, DOI: 10.1021/ie301487v. [18] M.P. Pandey, C.S. Kim, Lignin depolymerization and conversion: a review of thermochemical methods,

Chemical

Engineering

&

Technology

34

(2011)

29-41,

http://onlinelibrary.wiley.com/enhanced/exportCitation/doi/10.1002/ceat.201000270. [19] D. Shen, N. Liu, C. Dong, R. Xiao, S. Gu, Catalytic solvolysis of lignin with the modified HUSYs in formic acid assisted by microwave heating, Chemical Engineering Journal 270 (2015) 641-647, https://doi.org/10.1016/j.cej.2015.02.003. [20] H. Wang, M. Tucker, Y. Ji, Recent development in chemical depolymerization of lignin: a review, Journal of Applied Chemistry 2013 (2013), http://dx.doi.org/10.1155/2013/838645. [21] W. Xu, S.J. Miller, P.K. Agrawal, C.W. Jones, Depolymerization and hydrodeoxygenation of switchgrass

lignin

with

formic

acid,

ChemSusChem

5

(2012)

667-675,

http://onlinelibrary.wiley.com/enhanced/exportCitation/doi/10.1002/cssc.201100695. [22] D. Forchheim, J.R. Gasson, U. Hornung, A. Kruse, T. Barth, Modeling the Lignin Degradation

Kinetics in a Ethanol/Formic Acid Solvolysis Approach. Part 2. Validation and Transfer to Variable Conditions, Ind. Eng. Chem. Res. 51 (2012) 15053-15063, DOI: 10.1021/ie3026407. [23] F. Monteil-Rivera, G.H. Huang, L. Paquet, S. Deschamps, C. Beaulieu, J. Hawari, Microwaveassisted extraction of lignin from triticale straw: Optimization and microwave effects, Bioresource technology 104 (2012) 775-782, https://doi.org/10.1016/j.biortech.2011.11.079. [24] B. Zhang, Z. Zhong, P. Chen, R. Ruan, Microwave-assisted catalytic fast pyrolysis of biomass for bio-oil production using chemical vapor deposition modified HZSM-5 catalyst, Bioresource technology 197 (2015) 79-84, https://doi.org/10.1016/j.biortech.2015.08.063. [25] Q. Bu, H. Lei, L. Wang, Y. Wei, L. Zhu, X. Zhang, Y. Liu, G. Yadavalli, J. Tang, Bio-based phenols and fuel production from catalytic microwave pyrolysis of lignin by activated carbons, Bioresource technology 162 (2014) 142-147, https://doi.org/10.1016/j.biortech.2014.03.103. [26] X. Zou, T. Qin, Y. Wang, L. Huang, Mechanisms and product specialties of the alcoholysis processes of poplar components, Energy Fuels 25 (2011) 3786-3792, DOI: 10.1021/ef200726w. [27] J. Xu, J. Jiang, C. Hse, T.F. Shupe, Renewable chemical feedstocks from integrated liquefaction processing of lignocellulosic materials using microwave energy, Green Chem. 14 (2012) 2821-2830, https://doi.org/10.1039/C2GC35805K. [28] L. Fan, R. Ruan, Y. Liu, Y. Wang, C. Tu, Effects of extraction conditions on the characteristics of ethanol organosolv lignin from bamboo (Phyllostachys pubescens Mazel), BioResources 10 (2015) 7998-8013, DOI: 10.15376/biores.10.4.7998-8013. [29] D. Duan, Y. Zhao, L. Fan, L. Dai, J. Lv, R. Ruan, Y. Wang, Y. Liu, Low-Power Microwave Radiationassisted Depolymerization of Ethanol Organosolv Lignin in Ethanol/Formic Acid Mixtures, BioResources 12 (2017) 5308-5320, DOI: 10. 15376/biores.12.3.5308-5320.

[30] W. Xiao, L. Han, Y. Zhao, Comparative study of conventional and microwave-assisted liquefaction of corn stover in ethylene glycol, Industrial Crops and Products 34 (2011) 1602-1606, https://doi.org/10.1016/j.indcrop.2011.05.024. [31] T. Voitl, P. Rudolf von Rohr, Oxidation of lignin using aqueous polyoxometalates in the presence of alcohols,

ChemSusChem

1

(2008)

763-769,

http://onlinelibrary.wiley.com/enhanced/exportCitation/doi/10.1002/cssc.200800050. [32] X.-p. OUYANG, Y.-d. TAN, X.-q. QIU, Oxidative degradation of lignin for producing monophenolic compounds, Journal of Fuel Chemistry and Technology 42 (2014) 677-682, https://doi.org/10.1016/S1872-5813(14)60030-X. [33] S. Sahoo, M. Seydibeyoğlu, A. Mohanty, M. Misra, Characterization of industrial lignins for their utilization in future value added applications, Biomass and bioenergy 35 (2011) 4230-4237, https://doi.org/10.1016/j.biombioe.2011.07.009. [34] Q. Liu, S. Wang, Y. Zheng, Z. Luo, K. Cen, Mechanism study of wood lignin pyrolysis by using TG–FTIR

analysis,

Journal

of

Analytical

and

Applied

Pyrolysis

82

(2008)

170-177,

https://doi.org/10.1016/j.jaap.2008.03.007. [35] R. Sun, J. Tomkinson, G.L. Jones, Fractional characterization of ash-AQ lignin by successive extraction with organic solvents from oil palm EFB fibre, Polymer Degradation and Stability 68 (2000) 111-119, https://doi.org/10.1016/S0141-3910(99)00174-3. [36] E.T. Thostenson, T.-W. Chou, Microwave processing: fundamentals and applications, Composites Part A: Applied Science and Manufacturing 30 (1999) 1055-1071, https://doi.org/10.1016/S1359835X(99)00020-2.

Fig. 1. Results of conversion rate of lignin depolymerization under various alcohols. Fig. 2. FT-IR spectra of liquefied precipitate from lignin depolymerization under various solvents

Fig. 3. SEM of fresh lignin and residue from lignin depolymerization under various solvents. A, B and C: raw lignin 500×, 1000 × and 5000 ×；D, E and F：Methanol solvent residue 500×, 1000 × and 5000 ×; G, H and I：Ethanol solvent residue 500×, 1000 × and 5000 ×; J, K and L：Ethanol-50% solvent residue 500×, 1000 × and 5000 ×; M, N and O：Butanol solvent residue 500×, 1000 × and 5000 ×; P, Q and R： Ethanediol solvent residue 500×, 1000 × and 5000 ×; S, T and U：Isopropanol solvent residue 500×, 1000 × and 5000 ×. Fig. 4. TG/DTG curves of raw lignin and solid residue from lignin depolymerization in various alcohol solvents.

Fig. 1. Results of conversion rate of lignin depolymerization under various alcohols.

Fig. 2. FT-IR spectra of liquefied precipitate from lignin depolymerization under various solvents

Fig. 3. SEM of fresh lignin and residue from lignin depolymerization under various solvents. A, B and C: raw lignin 500×, 1000 × and 5000 ×；D, E and F：Methanol solvent residue 500×, 1000 × and 5000 ×; G, H and I：Ethanol solvent residue 500×, 1000 × and 5000 ×; J, K and L：Ethanol-50% solvent residue 500×, 1000 × and 5000 ×; M, N and O：Butanol solvent residue 500×, 1000 × and 5000 ×; P, Q and R： Ethanediol solvent residue 500×, 1000 × and 5000 ×; S, T and U：Isopropanol solvent residue 500×, 1000 × and 5000 ×.

Fig. 4. TG/DTG curves of raw lignin and solid residue from lignin depolymerization in various alcohol solvents.

Table 1 Molecular weight-average (Mw), number-average (Mn) and distributions (Mw/Mn) of liquefied precipitate Entry 1 2 3 4 5 6

Solvent Methanol Ethanol Ethanol-50% Butanol Ethanediol Isopropanol

Mw 1164 1162 1296 1522 1517 1919

Mn 364 315 378 498 609 388

Mw/Mn 3.20 3.69 3.43 3.06 2.49 4.95

Table 2 TG and DTG analysis for raw lignin and solid residue from lignin depolymerization Solvents

Ti (oC) DTGmax (%/min) Ychar (%)

Raw lignin

210.70

4.018

34.05

Methanol

210.06

3.810

42.22

Ethanol

145.84

2.896

34.27

Ethanol-50% 178.51

3.677

41.90

Butanol

111.12

2.252

44.43

Ethanediol

145.04

3.224

34.38

Isopropanol

159.81

3.513

38.73