Selective Conversion of Enzymatic Hydrolysis Lignin into Alkylphenols

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Kinetics, Catalysis, and Reaction Engineering

Selective conversion of enzymatic hydrolysis lignin into alkylphenols in supercritical ethanol over a WO3/#-Al2O3 catalyst Fuhang Mai, Zhe Wen, Yunfei Bai, Zewei Ma, Kai Cui, Kai Wu, Fei Yan, Hong Chen, and Yongdan Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01593 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on June 1, 2019

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Industrial & Engineering Chemistry Research

Selective conversion of enzymatic hydrolysis lignin into alkylphenols in supercritical ethanol over a WO3/γ-Al2O3 catalyst Fuhang Maib‡, Zhe Wenb‡, Yunfei Baib, Zewei Mab, Kai Cuib, Kai Wub, Fei Yanb, Hong Chena,*, Yongdan Lib,c a b

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin Key

Laboratory of Applied Catalysis Science and Technology, State Key Laboratory of Chemical Engineering (Tianjin University), School of Chemical Engineering, Tianjin University, Tianjin 300072, China c

Department of Chemical and Metallurgical Engineering, School of Chemical Engineering, Aalto

University, Espoo, 02150, Finland * Corresponding authors E-mail address: [email protected] ‡

These authors contributed equally to this work.

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: A WO3/γ-Al2O3 catalyst prepared by an incipient-wetness impregnation method is employed in the selective conversion of enzymatic hydrolysis lignin (EHL) into various alkylphenols in supercritical ethanol. In the reaction, EHL is completely dissolved and converted into aliphatic and aromatic compounds with a total yield of 363.4 mg/g lignin at 320 oC for 8 h. The yield of aromatic compounds is 315.8 mg/g lignin accounting for 86.9% of the total yield of the products, and the overall selectivity of alkylphenols reaches 67.5%. Compared to WO3 and H2WO4, the WO3/γ-Al2O3 shows a higher activity for the conversion of EHL in supercritical ethanol. Besides, ethanol is found to be the most effective solvent for the depolymerization of EHL with the WO3/γ-Al2O3 catalyst. The WO3/γ-Al2O3 keeps the high catalytic activity in three reuses. Key words: Enzymatic hydrolysis lignin (EHL), Supercritical ethanol, WO3/γ-Al2O3, Alkylphenols

2


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1. Introduction Lignin is a complex polymer consisting of various methoxylated phenylpropanoid units, which make it a sustainable resource for the synthesis of aromatic chemicals and fuels

1, 2.

However, its highly branched structure and the strong connections

between lignin monomers are the main challenges for its depolymerization 3. Lignin depolymerization for the production of chemicals has been extensively explored and a number

of

pathways

have

been

reported,

such

as

pyrolysis

4,

hydrogenolysis/hydrogenation 5, 6, oxidation 7, hydrolysis 8, and reforming 9. Supercritical alcohols such as methanol, ethanol, isopropanol and ethylene glycol have been utilized as the solvents for lignin depolymerization and the alcohol molecules were regarded as the hydrogen donors in the reaction

10.

Ma et al.11

reported that Kraft lignin was depolymerized into C6-C10 aliphatic alcohols, esters, phenols, benzyl alcohols and arenes over a supported molybdenum-based catalyst in supercritical ethanol. The overall yield was as high as 1.64 g/g lignin over an α-MoC1-x/AC catalyst. Notably, no char or tar formed in the process. Huang et al.12 examined the CuMgAlOx catalyst in the one-step conversion of soda lignin in supercritical ethanol and the overall yield of monomers, including furans, hydrogenated cyclics and aromatics, were as high as 23 wt% of the original lignin without char and tar formation. Yan et al.

13

further examined the composite of Ma’s

and Huang’s catalyst, i.e. the MoC1-x/Cu-MgAlOz catalyst in Kraft lignin conversion, and demonstrated a substantial improvement over both the two catalysts in working individually. The yield of aromatic compounds as high as 575 mg/g lignin was obtained at 330 oC for 6 h. Li and co-workers first reported the direct catalytic conversion of raw woody biomass over a Ni-W2C/AC catalyst and found it exhibited high activity for the selective cleavage of β-O-4 linkages in lignin 14. The lignin component in birch was selectively depolymerized

into

monophenols

mainly

including

guaiacylpropanol,

syringylpropanol, guaiacylpropane and syringylpropane with a total yield of 46.5% (based on lignin) over a 4%Ni-30%W2C/AC catalyst at 235 oC and 6 MPa H2 for 4 h 3



in ethylene glycol. The enhancement of lignin depolymerization in small molecular alcohols might be contributed to the increased solubility of lignin and hydrogen. The group further investigated the catalytic transformation of lignin feedstocks isolated from woody biomass over a W2C/AC catalyst in methanol 15-17. Results showed lignin materials were selectively converted into monophenols and aromatic liquid oils in high yields. W2C/AC showed potential in replacing noble metals for the highly selective cleavage of the major aryl ether bonds of lignins without destroying the aromatic rings, leading to the high yield of aromatics liquid oil without any cycloalkane. Li et al.

18

also investigated the deconstruction of organosolv lignin

materials into aromatic liquid oils with high yields of 42.7 to 66.7 wt% over tungsten-based bimetallic catalysts M-W/AC (M= Ru, Pt, and Pd). The noble-metal species improved the hydrogenolysis activity of W and suppressed over hydrogenation that avoided the formation of cycloalkanes, which enhances the selectivity of the catalysts

19.

Jongerius et al.

20

found that W2C/CNF exhibited an

excellent gaseous hydrodeoxygenation (HDO) activity in the conversion of guaiacol at 55 bar hydrogen pressure over a temperature range of 300-375 °C in dodecane. They found that the W2C/CNF experienced a partial oxidation process in the reaction and the oxide (WOX) may facilitate the guaiacol conversion. Furthermore, a WO3 and Ru/C composite catalyst showed an excellent gaseous HDO activity for the conversion of cellulose-derived glucose to ethylene glycol and propylene glycol with selectivities of 51.5% and 6.7% at 6 MPa H2 in water, respectively 21. Herein, a WO3/γ-Al2O3 catalyst is prepared and utilized in the conversion of enzymatic hydrolysis lignin (EHL) in supercritical ethanol. The EHL is selectively depolymerized into aromatic compounds, including aromatic esters and alkylphenols in the supercritical ethanol system. The depolymerization of EHL in supercritical methanol and isopropanol is also examined, and the results show that the supercritical ethanol is the most effective solvent.

2. Experimental 4


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2.1. Materials The analytical grade solvents and chemicals, including phenol, catechol, methanol, ethanol and isopropanol were all purchased from Tianjin Guangfu Technology Development Co. Ltd. The EHL was provided by Shandong Long Li biological technology Co., Ltd., and its main components are shown in Table 1, and the results of organic elemental analysis of EHL sample are shown in Table 2. Ammonium tungstate ((NH4)6H2W12O40.xH2O, AMT), H2WO4 and γ-Al2O3 were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All the materials are used as received.

2.2. Catalyst preparation and activity measurement The WO3 catalyst was obtained by calcining AMT at 500 oC for 4 h in static air. 10 wt% WO3/γ-Al2O3 was synthesized with an incipient-wetness impregnation method. The sample after impregnation step was dried at 120 °C for 12 h and then calcined at 500 oC for 4 h. All the reactions were carried out in a 300 mL batch reactor (MS 250, Anhui Kemi Machinery Technology Co., LTD.). A certain amount of EHL, WO3/γ-Al2O3 catalyst and 80 mL solvent were loaded into the reactor. Then, the reactor was purged 5 times with ultrapure N2 to remove the remaining air. Finally, the reactor was heated to desired temperature and kept for a prescribed reaction time with a stirring rate of 600 rpm. After reaction, the reactor was fast cooled to room temperature in a cold water bath. The liquid products and the spent catalyst were separated and recovered with a filtration technique.

2.3. Catalyst characterization The scanning electron microscope (SEM) micrographs were taken with A S4800 microscope with acceleration voltage of 3 kV. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEM-2000F electron microscope. Samples were ground to a fine powder and dispersed in analytical grade ethanol. The dispersed sample was then placed in an ultrasonic bath for 15 min, before a drop of 5



the suspension was placed on a grid. The grid was positioned in the microscope specimen holder. The energy dispersive X-ray spectroscopy (EDX) test was performed on a Genesis XM2 instrument to detect the dispersion of the WO3 active sites on the γ-Al2O3 surface. X-ray diffraction patterns (XRD) of the catalysts were recorded with a D8 Focus diffractometer (Bruker Corp., Germany) with a Cu-Kα radiation source at 40 kV and 40 mA between 2θ of 10 o and 80 o at a scanning rate of 8 o/min. The specific surface area and pore size distribution of the catalyst were measured using the BET method with a Quantachrome Autosorb-1 equipment. The samples were degassed under vacuum at 250 oC for 3 h and followed by adsorption and desorption of N2 in liquid nitrogen cold trap at -196 oC. The specific surface area of the sample was calculated according to the BET formula and the total pore volume was calculated from the adsorption amount at the relative pressure of 0.99. Thermogravimetric analysis (TG) of the catalyst was carried out with a PerkinElmer thermal analyzer at a rate of 10 oC/min from 100 oC to 900 oC in air. NH3 temperature programmed desorption (NH3-TPD) was performed in a fixed bed reactor using a Thermo Nicolet IS10 FT-IR as the NH3 detector. N2 physisorption measurements at -196 oC were performed on an automatic surface analyzer (QuadraSorb Station 3). The X-ray photoelectron spectroscopy (XPS) analysis was performed with a PHI-1600 ESCA system spectrometer using Mg Kα as the X-ray source with the binding energy calibrated using C1s at 285 eV. The NMR spectra were recorded using an Ascend 400 spectrometer. For liquid product from EHL depolymerization, the ethanol solvent was firstly evaporated, then obtained oily product was dissolved in DMSO before the NMR spectrum was recorded. Data processing was performed using the MestReNova software. The gel permeation chromatography (GPC) spectra were obtained with using an HLG-8320GPC equipped with two columns connected in series (TSK gel super HZM-M 6.0*150 mm and TSK gel SuperHZ3000 6.0*150 mm) and a differential refraction detector. The column was calibrated with Polystyrene standards. The detail was carried out at 40 °C using THF as eluent with a flow rate of 0.6 mL/min. 6


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2.4. Analysis of products The liquid products were qualitatively analyzed with a GC-MS (Agilent 6890N-5973) and quantitatively analyzed using an internal standard method with a GC-FID (Agilent 6890). Compounds were identified by comparing the mass spectra with NIST 02 library database. The GC-MS and GC-FID were both equipped with an Agilent HP-5MS (30 m × 0.2 mm × 0.33 μm) capillary column. The typical parameters used for the analysis were as follows: injector temperature 280 oC, detector temperature 300 oC, split ratio 50:1. The column temperature program was set from an initial temperature of 45 °C to a final temperature of 250 °C at a heating rate of 10 °C/min, and then kept at 250 °C for 7 min. The product yield from lignin depolymerization was calculated as the following:

Yield (mg/g lignin) 

m( product ) m( EHL)initial

During the reaction, only a part of the EHL is depolymerized into the downstream products although it was completely dissolved in supercritical ethanol. Hence, specific aromatic product selectivity (S (%)) is based on an individual product and overall detected aromatics after the reaction.

S (%) =

n ( individual product ) n (overall aromatics ) deteced

 100%

3. Results 3.1. Catalyst characterization The morphologies of fresh and spent catalysts were characterized by SEM and the results are displayed in Figure 1. The γ-Al2O3 sample is mainly composed of micron-sized irregular particles with sizes of 1-10 um. Similarly, the fresh WO3/γ-Al2O3 catalyst is loosely stacked in bulk. After reaction, the stacking of the 7



spent WO3/γ-Al2O3 catalyst becomes more compact, while the particle sizes have no significant change. The X-ray diffraction (XRD) patterns of the fresh and spent WO3/γ-Al2O3 catalysts are shown in Figure 2. The fresh WO3/γ-Al2O3 catalyst represents quasi-amorphous pattern of γ-Al2O3 with no diffraction peaks of WO3 observable. Besides, the recovered catalysts after each of the three runs show similar patterns to that of the fresh catalyst, indicating that the crystalline phase of WO3/γ-Al2O3 remains stable during the reuses. HRTEM and the EDX elemental mapping were performed to confirm the dispersion of the WO3 active sites on the γ-Al2O3 surface and the results are given in Figure 3. It can be seen from the elemental mapping that the WO3 active sites are highly dispersed on the γ-Al2O3 support. The dispersion of WO3 has no significant change after the reaction. The XPS analysis was performed to analyze the valence state of the WO3 active sites, and the results are shown in Figure 4. The XPS spectrum of the bulk WO3 shows only the existence of W6+ with two well-resolved peaks at binding energy of 36.7 and 34.6 eV assigned to the W 4f5/2 and 4f7/2 orbits, respectively. For the fresh WO3/γ-Al2O3 catalyst, W remains as W6+ state with binding energy shifting to 37.6 and 35.5 eV, respectively. The shift of the binding energy indicates the electron transfer from γ-Al2O3 to the WO3 sites. In addition, the binding energy of W 4f5/2 and 4f7/2 orbits attributed to W6+ in the spent WO3/γ-Al2O3 catalyst after the first run decrease to 37.1 and 35.0 eV, respectively, which means the interaction between the γ-Al2O3 carrier and the WO3 sites is weakened. Besides, XPS analysis of the spent WO3/γ-Al2O3 catalyst shows that W6+ was partly reduced to W5+ and W4+ with W6+ proportion decreasing to 87% and W5+ and W4+ proportions increasing to 9% and 4%, respectively. The texture data of γ-Al2O3, WO3/γ-Al2O3 and the spent WO3/γ-Al2O3 samples after three reuses are listed in Table 3. After loading 10 wt% WO3 onto γ-Al2O3 support, the total pore volume decreased from 0.260 cm3/g to 0.214 cm3/g, meanwhile, the average pore diameter decreased from 7.19 nm to 6.98 nm, but the specific surface area increased from 144.8 m2/g to 173.5 m2/g. After three reuses, the specific surface area and the average pore diameter of the spent WO3/γ-Al2O3 catalyst 8


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decreased slightly meanwhile the total pore volume fell significantly to 0.119 cm3/g. The surface acidity of the fresh and spent catalysts were further characterized with an NH3-TPD method in a temperature rage 100-700 °C, and the results are depicted in Figure 5. The position of the desorption peaks in the TPD curves are correlated to the acid strength 22, 23. For the γ-Al2O3 sample, a distinct desorption peak was observed at 150 oC, ascribing to the NH3 desorption from the weak acid sites. After loading 10 wt% WO3, the amount of weak acid sites has a significant increase although the NH3 desorption temperature has a slight decrease to 130 oC. Compared with the fresh WO3/γ-Al2O3 catalyst, the amount of weak acid sites on the spent WO3/γ-Al2O3 catalyst underwent a slight decrease but still higher than that of γ-Al2O3 sample. TG and DTG curves of the recovered WO3/γ-Al2O3 catalyst after three reuses are showed in Figure 6. It indicated that a certain amount of carbon species, accounting for 15 wt% of the catalyst itself, were accumulated on the surface of WO3/γ-Al2O3 during reuses. The carbon species were proved to be graphite carbon according to the previously reported work about the gaseous HDO of biomass-derived oxygenates on MoO3 catalyst 24.

3.2. Catalytic conversion of enzymatic hydrolysis lignin 3.2.1. Product distribution from lignin depolymerization Figure 7 (a) shows the total-ion chromatogram (TIC) of the liquid products from EHL depolymerization in supercritical ethanol over the WO3/γ-Al2O3 catalyst at 320 oC for 8 h. A total of 24 compounds are identified and listed in Figure 7 (b). No tar or char was observed. In brief, the products consist of aliphatic compounds and aromatic compounds and the total yield is up to 363.4 mg/g lignin. Although the product yield is lower than that of the results obtained with the α-MoC1-x/AC catalyst and Kraft lignin in our previous work 11, the yield of aromatic compounds is 315.8 mg/g lignin, which occupies 86.9% of the overall yield from the reaction. Two aliphatic compounds, including propionic ether, 3-butenylethyl ether (Peak 1 and 2, respectively), were identified with total yield of 47.6 mg and were confirmed to be 9



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formed from ethanol with a blank reaction. The other 21 aromatic compounds were only obtained in the reaction of EHL depolymerization, which were further classified into aromatic ethers (Peak 5, 11 and 19), aromatic esters (Peak 23) and alkylphenols (Peak 6-10, 12-18, 20-22, and 24). Notably, alkylphenols were the most abundant aromatics with a total yield of 245.2 mg/g lignin. According to the substituting alkyls, alkylphenols were divided into methylphenols, ethylphenols, isopropylphenols and tert-butylphenols. The yield of methylphenols, ethylphenols, isopropylphenols and tert-butylphenols are 112.4, 24.5, 14.7 and 93.6 mg/g lignin, respectively. It is noteworthy that methylphenols were the most abundant products from the EHL depolymerization

with

a

selectivity

of

45.8%.

Interestingly,

5-methoxy-2,3,4-trimethylphenol (Peak 10) is obtained as the dominant product with a yield of 89.3 mg/g lignin, which occupies 30.9% of the overall product yield from the EHL depolymerization. Notably, one key point that restricts the efficient depolymerization of lignin is that the downstream products are complicated and their selectivities are low in general. In this work, we have achieved the selective depolymerization of EHL into aromatics, especial alkylphenols. As we all know, the alkylphenols are the potential precursor for production of some high-valued chemicals, and intermediate formulas in the antioxidant and medicine

25, 26.

Besides,

the alkylphenols can be further upgraded as fuel by catalytic hydrodeoxygenation to replace traditional fossil fuels 27. A blank experiment was carried out under the same reaction condition but without a catalyst. The overall yield of the alcoholysis products was as high as 214.7 mg/g lignin but that of the aromatic compounds was only 45.7 mg/g lignin, and the main products were aromatic dimers. Besides, no sugar product was detected, probably because the amount of residual sugar in the EHL was originally very low (0.12%, Table 1), or the residual sugar may be converted into other products 21.

In addition, we observed that the residual pressure in the batch reactor after the

reaction usually exceeded 2 MPa. Hence, we have qualitatively analyzed the gas element. The results showed that the gas mainly consists of hydrogen and low hydrocarbons (such as methane, ethylene and ethane). The result was consistent with 10


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the phenomenon that the volume of ethanol solvent was significantly reduced after the reaction.

3.2.2. Effect of the catalyst For comparison, the catalytic depolymerization of EHL over WO3, H2WO4 and γ-Al2O3 were also carried out under the same conditions, and the results are listed in Table 4. For H2WO4 catalyst, the yield of aromatic products reached 169.1 mg/g lignin, among which more than three fourths were alkylphenols. Interestingly, tert-butylphenols

were

the

most

abundant

alkylphenols

and

2,6-di-tert-butyl-4-ethylphenol was obtained with the highest yield of 48.8 mg/g lignin. The yield of aromatic products reached 218.0 mg/g lignin with WO3 as the catalyst. Compared to the result obtained with H2WO4, the yield of aromatic ethers over WO3 had an obvious increase to 51.1 mg/g lignin, leading to a higher overall yield of aromatic compounds obtained with WO3 as catalyst than that with H2WO4. However, both WO3 and WO3/γ-Al2O3 exhibited lower selectivity of tert-butylphenols than that of H2WO4 even though WO3 and WO3/γ-Al2O3 gives higher overall yield of all the products. For γ-Al2O3 as catalyst, the total product yield was up to 360.9 mg/g lignin, but the yield of aromatic compounds was only 65.5 mg/g lignin. Overall, WO3/γ-Al2O3 gave the highest overall product yield and the highest selectivity of alkylphenols, especially methylphenols and tert-butylphenols. The results showed that there existed a synergistic effect between WO3 and γ-Al2O3.

3.2.3. Effect of solvent To investigate the effect of solvent on the reaction, methanol and isopropanol were also employed as the solvents and the results are also listed in Table 4. In supercritical methanol, lignin was completely dissolved and the yield of aromatic compounds was up to 179.6 mg/g lignin. Besides, the product distribution in supercritical methanol was obviously different from that obtained in the supercritical ethanol. No isopropylphenol was detected and the yields of alkylphenols followed the order: 11



methylphenols>ethylphenols>tert-butylphenols. For the reaction in supercritical isopropanol, no methylphenol was detected and the yields of alkylphenols followed the order: tert-butylphenols>ethylphenols>isopropylphenols. Besides, in supercritical isopropanol the yield of aromatic products was only 128.2 mg/g lignin, which was the lowest value, obtained with the three solvents. It was observed that the highest selectivity of alkylphenols was obtained over the WO3/γ-Al2O3 catalyst, and the supercritical ethanol system was proved to be the most efficient solvent for lignin depolymerization.

3.3. Recycle tests The catalyst was recovered with filtration and then directly reused in the successive runs after drying. Figure 8 shows the result of the recycle test in supercritical ethanol. The catalyst underwent a slight deactivation and the total yield of products decreased by 10.3% after the second run. The yields of aliphatic compounds remained essentially unchanged but the yields of aromatic compounds showed a slight decrease. Compared to the result of the second run, the catalytic activity in the third run had no significant changes. Overall, after three runs the yields of aromatic compounds and alkylphenols were still up to 283.3 and 221.8 mg/g lignin, respectively. More importantly, the yields of alkylphenols accounted for more than 77% that of the aromatic compounds in each run, indicating that the catalyst kept a good catalytic activity for deoxygenation and alkylation during reuses.

4. Discussion Ma et al.11 reported that the depolymerization of Kraft lignin over an α-MoC1-x/AC catalyst in supercritical ethanol, and their results proved that the solid Kraft lignin is converted into ethanol-soluble lignin fragments under supercritical conditions without a catalyst. The short lignin fragments were further converted into monomers with the involvement of the α-MoC1-x/AC catalyst. We performed a blank experiment that the depolymerization of EHL was conducted in supercritical ethanol without any catalyst 12


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involvement. Although 45.7 mg/g lignin aromatic compounds were obtained, the products were almost aromatic dimers. Besides, no tar or char was observed in the reactor and on the catalyst surface. The results here indicated that EHL was completely dissolved in supercritical ethanol. Meanwhile, it is likely true from the yield of aromatic products that only a small portion of dissolved EHL fragments were depolymerized into oligomers. Hence, the molecular weight distribution of the products was analyzed through a gel permeation chromatography and the results are presented in Table 5. Compared with the EHL fragments, the average MZ of liquid products slightly decreases while the ratio of MZ to MW obviously increase. Based on the total yield of liquid products, about one third of EHL was converted into monomers while the rest was dissolved in ethanol in the form of small fragments. Hence, the average MZ and MZ/MW shows the aforementioned changes compared to EHL, which means that the liquid products had a broader molecular weight distribution. The broader molecular weight distribution is contributed to the existence of small molecules such as aromatic monomer and macromolecules (polymer) formed through the depolymerization of EHL fragments and the macromolecule products produced by the repolymerization of the highly active fragments during the reaction. Based on the results, we concluded that the solid EHL was firstly depolymerized into intermediate sized ethanol-soluble fragments, which were further converted into aromatic monomers on WO3/γ-Al2O3 catalyst 13. We have also performed the 1H NMR measurement of the EHL sample and the liquid products from EHL depolymerization, and the results are shown in Figure 9. Compared with the origin EHL sample, the spectra of the liquid product from EHL depolymerization had some significant changes. The chemical shift between 0 and 2 ppm arise many new peaks, corresponding to the alky groups 3. The results were consistent with the product distribution from EHL depolymerization. Besides, the peaks located between 3-4 ppm had a noticeable increase, which may be due to the breakage of ether linkage in the EHL and the appearance of many methoxy group 3. Overall, those results indicated that WO3/γ-Al2O3 effectively catalyzed the 13



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depolymerization of EHL into aromatic monomers. We speculated that the WO3 sites play the key role in the EHL depolymerization and the γ-Al2O3 carrier promotes the steps. Firstly, a far lower yield of aromatics (65.5 mg/g lignin) was obtained when the γ-Al2O3 was used as catalyst. The results showed that the WO3 catalyst was more efficient than the γ-Al2O3 carrier for EHL depolymerization. Besides, as shown in Figure 5, the acid sites on the spent WO3/γ-Al2O3 catalyst obviously decreases, while the

catalytic

activity

doesn’t

decrease

significantly.

During

the

EHL

depolymerization, we speculated that the ethanol-soluble fragments were firstly absorbed on the support γ-Al2O3 surface and the linkages (such as ether bond) were weaken under the effect of the γ-Al2O3 during the reaction. Subsequently, the activated fragments were converted into aromatics under the catalysis of the WO3 sites. The reaction pathways were similar with the reaction that the model compound was converted on the H2WO4 catalyst

28.

However, there are some differences

between lignin and the model compounds. We are not clear about the detailed mechanism about EHL depolymerization currently, but we are trying to figure out the depolymerization steps and some breakthroughs are obtained 29. No tar or char formed here during the depolymerization of EHL in supercritical

ethanol over WO3/γ-Al2O3 at 320 oC for 8 h. The volume of ethanol solvent was 80 mL before the reaction, but the amount was reduced to less than half of the initial amount after the reaction. This proved that ethanol not only played the role of the solvent, but also participated in the reaction during the depolymerization of EHL. Yang et al.30 and Huang et al.12 proposed that ethanol not only acts as a hydrogen-donor, but also as a capping agent which stabilizes the highly reactive phenolic intermediates by O-alkylating the hydroxyl groups and by C-alkylating of the aromatic rings. Hence, ethanol plays a similar function during the depolymerization of EHL, which could be proved from the product structures. There are many alkyl groups, including methyl group, ethyl group, isopropyl group and tert-butyl group, substituent on the benzene ring of the aromatics. Those alkyl groups were derived from the ethanol conversion 31. With these alkyl group substitutions on 14


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the phenolic hydroxyl group and on the aromatic ring, the repolymerization reactions were suppressed obviously and a high yield of alkylphenols were obtained due to the steric inhibition. Hence, the WO3/γ-Al2O3 catalyst kept the high catalytic activity after three reuses. XPS analysis showed that about 13% of W6+ in the WO3/γ-Al2O3 catalyst was reduced to W5+ and W4+ after the first run, leading to the decrease of W6+ sites in the recovered catalyst. TG curves showed that there was only about 15 wt% of carbon accumulated on the surface of WO3/γ-Al2O3 during reuses. The dispersion of the WO3 sites on the γ-Al2O3 support had no significant change after the reaction. However, after three reuses the specific surface area, the total pore volume and the average pore diameter of the spent WO3/γ-Al2O3 catalyst decreased significantly. Besides, the amount of weak acid sites on the spent WO3/γ-Al2O3 catalyst underwent an obvious decrease. Overall, the partial reduction of W6+, the accumulation of carbon on the catalyst surface, the changes in pore structure and the decrease of acid sites acidity on the catalyst may be responsible for the decay in catalyst activity.

5. Conclusions A WO3/γ-Al2O3 catalyst exhibited a good catalytic performance in the selective depolymerization of EHL into alkylphenols in supercritical ethanol. In supercritical ethanol, EHL was firstly dissolved completely and depolymerized into small lignin fragments which were further converted into the aromatics during the reaction. The total yield of small molecular products was up to 363.4 mg/g lignin, among which aromatic compounds occupied 86.9%. Notably, the alkylphenols were the main aromatic products with the total yield of 245.2 mg/g lignin. Besides, 5-methoxy-2,3,4-trimethylphenol was the dominant aromatic product with a yield of 89.3 mg/g lignin. The recyclability of the WO3/γ-Al2O3 catalyst was tested, and the results showed it kept the high catalytic activity after three reuses. After three runs, the yields of aromatic compounds and alkylphenols were still up to 283.3 and 221.8 mg/g lignin, respectively. Ethanol was found to be the most effective solvent for EHL depolymerization. During the reaction, ethanol not only acted as the solvent, but also 15



Page 16 of 34

played the role of stabilizing agent by alkylating the active lignin fragments. The characterization showed that the partial reduction of W6+, the accumulation of carbon species on the catalyst surface, the change in acidity on the catalyst surface and the change in pore structure may be responsible for the slight decay in catalytic activity.

Acknowledgements

National Natural Science Foundation of China (21808163 and 21690083) and Financial supports from the Ministry of Science and Technology of China (2011DFA41000) are gratefully acknowledged. This research was also supported in part by the Program of Introducing Talents to the University Disciplines (B06006) and the Program for Changjiang Scholars and Innovative Research Teams in Universities (IRT 0641).

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19



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Table 1. The main components of enzymatic hydrolysis lignin Lignin

Phenolic hydroxy

Moisture

Ash

Residual sugar

91.2%

13.2%

7.9%

0.59%

0.12%

20


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Table 2. The result of organic elemental analysis of EHL sample. Element

C

H

N

S

O

Content

61.75%

6.48%

0.90%

0.58%

30.29%

21



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Table 3. The texture data of γ-Al2O3, fresh WO3/γ-Al2O3 and the spent catalyst. Catalyst

Specific surface area (m2/g)

Total pore volume (cm3/g)

Average pore diameter (nm)

γ-Al2O3

144.8

0.260

7.19

WO3/γ-Al2O3

173.5

0.214

6.98

Third reuse

155.2

0.119

6.54

22


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Table 4. Depolymerization of enzymatic hydrolysis lignin using different catalysts in alcohols. [a] Yield[b] (mg/g lignin) Catalyst

Solvent 1

2

3

4

5

6

7

8

9

10

WO3

Ethanol

274.0

56.0

218.0

51.1

123.3

43.6

33.0

12.1

16.9

52.7

H2WO4

Ethanol

261.5

92.4

169.1

23.1

128.3

11.5

4.6

8.4

17.1

92.1

γ-Al2O3

Ethanol

360.9

295.4

65.5

19.3

43.7

2.4

6.3

7.3

7.6

17.5

WO3/γ-Al2O3

Ethanol

363.4

47.6

315.8

32.9

245.2

26.3

108.8

24.5

14.7

93.6

WO3/γ-Al2O3

Methanol

179.6

n.d.

179.6

16.2

160.2

3.5

77.7

42.1

-

30.4

WO3/γ-Al2O3

Isopropanol

217.3

88.8

128.2

26.8

96.0

5.7

-

30.6

15.6

49.9

[a] Reaction conditions: enzymatic hydrolysis lignin (1.0 g), catalyst (1.0 g), ethanol (80 mL), initial 0 MPa N2 pressure, 600 rpm, at 320 oC for 8 h. [b] Yield: the yield of each product from enzymatic hydrolysis lignin (1: total yield, 2: aliphatic compounds, 3: aromatic compounds, 4: aromatic ethers, 5: alkylphenols, 6: aromatic esters, 7: methylphenols, 8: ethylphenols, 9: isopropylphenols, 10: tert-butylphenols).

23



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Table 5. The average MW and MZ of EHL and liquid products from EHL depolymerization. MZ

MW

MZ/MW

EHL

1231

805

1.530

Liquid products

1115

621

1.795

24


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Figure 1. The SEM micrographs of (a and d) γ-Al2O3, (b and e) fresh WO3/γ-Al2O3 catalyst and (c and f) spent WO3/γ-Al2O3 catalysts (c and f were recovered after first run)

25



Figure 2. XRD patterns of the WO3/γ-Al2O3 catalysts.

26


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Figure 3. The TEM micrographs of (a, c) fresh WO3/γ-Al2O3 catalyst and (b) spent WO3/γ-Al2O3 catalysts (which was recovered after first run) and the corresponding EDX elemental mapping of fresh WO3/γ-Al2O3 catalyst: (d) Al, (e) O, (f) W.

27



Figure 4. XPS spectra of WO3, the fresh WO3/γ-Al2O3 catalyst and the spent WO3/γ-Al2O3 catalyst.

28


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Figure 5. NH3-TPD profiles of the γ-Al2O3, fresh WO3/γ-Al2O3 catalyst and spent WO3/γ-Al2O3 catalysts (which was recovered after first run).

29



Figure 6. TG and DTG curves of the spent WO3/γ-Al2O3 catalyst after three reuses.

30


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(a)

(b) No .

Yiel d

No .

Yiel d

No .

Yiel d

No .

1

39.4

7

3.1

13

5.3

19

5.4

2

8.2

8

6.5

14

89.3

20

12.7

3

-

9

13.0

15

11.6

21

5.7

4

4.9

10

7.7

16

10.3

22

13.0

5

13.8

11

13.7

17

4.6

23

26.3

6

6.6

12

9.3

18

15.3

24

37.6

Structure

Structure

Structur e

Structure

Yiel d

Figure 7. (a) The total-ion chromatogram (TIC) of the liquid products obtained from lignin depolymerization over WO3/γ-Al2O3. (b) Structures of identified molecules in the liquid product. Reaction conditions: lignin (1.0 g), catalyst (1.0 g), ethanol (80 mL), initial 0 MPa N2 pressure, 600 rpm, at 320 oC for 8 h. Numbers in the figure and the table are the same. Anisole is the internal standard marked as 3 in the figure. 31



Figure 8. Recyclability of WO3/γ-Al2O3 catalyst for lignin depolymerization. Reaction conditions: lignin (1.0 g), catalyst (1.0 g), ethanol (80 mL), 600 rpm, at 320 oC for 8 h.

32


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Figure 9. The 1H NMR spectrum of the EHL sample (a) and the liquid products from EHL depolymerization (b).

33



Graphical abstract

34


Page 34 of 34

Selective Conversion of Enzymatic Hydrolysis Lignin into Alkylphenols

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