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Mild hydrogenation of lignin depolymerization products over Ni/SiO2 catalyst Riyang Shu, Ying Xu, Pengru Chen, Longlong Ma, Qi Zhang, Lie Zhou, and Chenguang Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00934 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Mild hydrogenation of lignin depolymerization products over Ni/SiO2 catalyst Riyang Shua,b,c,d, Ying Xua,b,c,*, Pengru Chena,b,c,d, Longlong Maa,b,c, Qi Zhanga,b,c,∗, Lie Zhoua,b,c, Chenguang Wanga,b,c a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China

b

CAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China

c

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development,

Guangzhou 510640, PR China d

University of Chinese Academy of Sciences, Beijing 100049, PR China

Abstract: Many efficient methods have been proposed to realize lignin depolymerization, while effective usage of lignin depolymerization products at mild conditions is still a big challenge. Conversion of them to stable products in thermal and chemical properties is a necessary step. Herein, a mild hydrogenation process of lignin depolymerization products over Ni/SiO2 catalyst was proposed. Model compound of 2,3-dihydrobenzofuran exhibited a good reaction result. Nearly 100% conversion was obtained at 130 °C, and the selectivity of 2-ethylcyclohexanol product reached 94.6%. The lignin depolymerization products also had a good hydrogenation result, in which not only the hydrogenation of unsaturated groups, but also the cleavage of β-O-4 bonds occurred. Stable products in thermal and chemical properties were formed,

∗

Corresponding author. Tel.: +86 020 87057789; fax: +86 020 87057789. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Y. Xu). 1


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which possessed high heated value and low molecular weight. This treatment is conducive to suppress the occurrence of condensation and favorable for the further catalytic conversion.

Keywords: Lignin depolymerization products, hydrogenation, hydrogenolysis, Ni/SiO2, 2,3-dihydrobenzofuran.

1.

Introduction Lignin, as the abundant byproduct from biomass refinery, is the richest source of

renewable aromatic ingredient on the earth.1 With the special 4-propenyl-phenol units, lignin harbors great potential to produce precursors of aromatics and alkane derivatives for liquid fuels.2 Recently, a lot of approaches have been proposed for lignin depolymerization, especially the catalytic oxidization and reduction methods.3-6 The liquid product mainly consists of phenolic monomers and oligomers. Both of them cannot be directly used as transportation fuels because of the high oxygen content, the large molecular weight and the poor thermal stability. The lignin depolymerization products usually contain many unsaturated groups, such as carbonyl, which are prone to condense and cause the severe carbon deposition.7,8 This is a big challenge to the further catalytic upgrading at high temperature. Hydrogenation is a very promising approach to achieve this goal.9,10 It can not only stabilize the unsaturated products, but also increase the fuel value by adding H2 2


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equivalents. Meanwhile, lower reaction temperature is very conducive to suppress the condensation of unsaturated groups and avoid the formation of undesired char. Ni-based catalysts have been very popular for hydrogenation, due to their high catalytic activity and cost-efficient property.11 For example, the sol-gel nickel-silica catalysts showed a great performance on the benzene hydrogenation.12 Ni/HZSM-5 catalyst exhibited high activity on the hydrogenolysis and hydrogenation of substituted phenols, and the synergistic effects of Ni and acid sites on the cleavage of ether bonds were also confirmed.13 Efficient hydrodeoxygenation of phenolic model compounds was also able to realize over Ni/SiO2-ZrO2. Wherein, the guaiacol was converted to hydrocarbons completely.14 In our previous study, Ni/SiO2 catalyst prepared by step by step precipitation method exhibited a good effect on guaiacol hydrogenation.15 Based on this, a mild hydrogenation strategy for lignin depolymerization products over Ni/SiO2 catalyst was proposed in this work. 2,3-dihydrobenzofuran was selected as the model compound for its similar properties with phenolics. The catalyst showed high activity on the hydrogenation of 2,3-dihydrobenzofuran at low temperature, and the reaction mechanism was revealed. The lignin depolymerization products was also used for hydrogenation test. This study could be in favour to making effective use of lignin depolymerization products.

3


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2.

Experimental Section

2.1.

Materials All chemicals were of analytical grade and used directly without any treatment.

Ni(NO3)26H2O, C2H5OH, ZnCl2 and NH3H2O was purchased from Tianjin Fu Chen chemical reagent company. Poly-vinylpyrrolidone (PVP) was supplied from Shanghai Yuan Ju biotechnology Co., Ltd. Cetylhexadecyl trimethyl ammonium bromide (CTAB) and Tetraethylorthosilicate (TEOS) were provided by Da Mao chemical reagent company. 2,3-dihydrobenzofuran and 5 wt% Pd/C was supplied from J&K Chemical Co., Ltd. and Aladdin (Shanghai, China) respectively. The alkaline lignin was provided by Sigma Aldrich. Other chemicals were supplied by Jinhuada Chemical Reagent Co., Ltd. Ni/SiO2 catalysts were prepared using step by step precipitation method following our previous study.15 Briefly, 7.0 g Ni(NO3)26H2O and 1.0 g PVP were weighed and added to a 700 mL ethanol aqueous solution (ethanol 400 mL). Subsequently, 120 mL 25 wt% NH3H2O was added dropwise. 6.0 g CTAB was put in and 20 mL TEOS was injected in dropwise. After 72 h stirring, the precipitation was filtrated, washed with ethanol and distilled water, and dried at 60 °C overnight. Then the sample was calcined at 500 °C for 6 h. After reduction at 550 °C with a slow H2 flow, 20% Ni/SiO2 catalyst sample was obtained. As for 5%, 10% and 15% Ni/SiO2 catalyst, the reagents with corresponding weights were mixed in an appropriate proportion as above.

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2.2.

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Hydrogenation of 2,3-dihydrobenzofuran The reaction was running in a 50 ml stainless autoclave. 0.3 g

2,3-dihydrobenzofuran, 0.1 g catalyst and 20 mL decalin were put into the reactor. After purging with hydrogen three times, the reactor was pressurized to 3 MPa. Then heat up to 130 °C (with rate 5 °C/min) and start the reaction under vigorous stirring of 400 rpm. When the specific time was reached, circulation water was used for cooling. The solid catalyst was separated out by filtration and washed by decalin and ethanol. After dried at 50 °C under vacuum, the recovered catalyst was collected for the next term to investigate the catalyst recyclability without further treatment.

2.3.

Hydrogenation of lignin depolymerization products The lignin depolymerization products were prepared according to our previous

research.16 In order to achieve high products selectivity and less by-product, ethanol, Pd/C and ZnCl2 were selected as the solvent and the synergistic catalysts, respectively. Typically, 1.0 g alkali lignin, 0.1 g Pd/C, 1 mmol ZnCl2 and 40 mL ethanol were charged into a 100 mL 316L stainless autoclave (Weihai Chemical Machinery Co., Ltd.). After air change with H2, the reactor was heated to 280 °C. When the specific reaction time was arrived, the reactor was cooled to room temperature within 30 min by circulation water. The product mixture was filtered. 20 mL decalin was added into the filtrate and the solution was rotary evaporated at 30 °C under reduced pressure to remove the ethanol.

After

ultrasonic

treatment

to

disperse

the

5


solution,

the

lignin

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depolymerization products used for feedstock was obtained. The hydrogenation process was the same as above hydrogenation of 2,3-dihydrobenzofuran.

2.4.

Products separation and analysis The product mixtures were filtered and the filtrate was collected. The obtained

liquid phase was divided into two parts. One was diluted for the qualitative and quantitative analysis of the volatile product. The other part was concentrated by evaporation and freeze dried under vacuum. This part was the oligomer, namely the nonvolatile product. Gas chromatography/mass spectrometry (GC-MS, Agilent 5890) was used to identify the components of volatile product, with a column of HP-INNOWAX, 30 m × 0.25 mm × 0.25 um. The temperature program was designed as 60 °C hold 2 min, then heated up to 260 °C with 10 °C min-1 and hold for another 10 min. The injector was maintained at 280 °C in split mode (5:1) with helium carrier gas. SHIMADZU GC 2014C was used for quantitative analysis, with a flame ionization detector (FID) and a HP-INNOWAX column. The temperature program was the same as the GC-MS analysis. Gel permeation chromatography (GPC) was used to measure the molecular weights of products on Agilent 1260 Infinity with two 7.8 × 300 mm columns (HR 4E THF). The used mobile phase was tetrahydrofuran. The average molecular weight was measured out on the basis of external standard method. The element measurement of nonvolatile product was carried out on a vario EL III element analyzer. 1H Nuclear 6


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Magnetic Resonance (1H NMR) result was recorded on a Bruker Advance III 400 WB spectrometer (7.05 T) using d6-DMSO solvent.

3.

Results and Discussion

3.1.

Catalytic hydrogenation of 2,3-dihydrobenzofuran The model compound of 2,3-dihydrobenzofuran was used to test the catalytic

activity of Ni/SiO2 catalyst. Table 1 showed the hydrogenation results. The single support SiO2 had not any effect on the conversion of 2,3-dihydrobenzofuran. With the loading of Ni, the occurrence of hydrogenation was promoted. The main products included ethylbenzene, 2-ethylphenol, ethylcyclohexane and 2-ethylcyclohexanol. Only 68.6% conversion was obtained over 5% Ni/SiO2 catalyst. 2-ethylcyclohexanol was the main product, and a small amount of ethylbenzene, 2-ethylphenol and ethylcyclohexane were also formed at the same time. While with the increase of Ni loadings, the conversion rose up, as well as the yield of 2-ethylcyclohexanol. The substrate was able to completely transform to 2-ethylcyclohexanol over 20% Ni/SiO2 at 130 °C. It indicated that the active metal Ni played an important role on the catalytic hydrogenation. Compared with other reported results, the performance received by the Ni/SiO2 catalyst in this study was relative better. The hydrogenation process of 2,3-dihydrobenzofuran in methanol at 300 °C over Cu-PMO catalyst was investigated.17 Wherein, the conversion and the selectivity of 2-ethylcyclohexanol reached 90% and 70% respectively after 4 h. The same reaction was also carried out 7


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over reduced Ni-Mo/Al2O3 catalyst.18 The conversion could reach about 90% at 280 °C, but the ethylphenol was the major product, probably due to the relative low catalytic hydrogenation activity of the catalyst. In this study, the complete conversion of 2,3-dihydrobenzofuran was realized at low temperature of 130 °C. The large specific surface area and high nickel dispersion of Ni/SiO2 were the main reason for its high activity.15 Moreover, the recyclability of 20% Ni/SiO2 was also studied. Slight deactivation was exhibited after 3 runs. With the third cycle, the conversion dropped from 94.5% to 91.3% at 120 °C. According to our previous study,15 the repolymerization of phenolics was prone to occur and the char or tar formed, causing the slight deactivation of catalyst. But through simply calcining and reducing, the catalyst can be refreshed.15 Therefore, Ni/SiO2 is demonstrated to be an efficient catalyst for hydrogenation under mild condition.

3.2.

Effects of reaction temperature and time on the hydrogenation of 2,3-dihydrobenzofuran The effects of reaction temperature and time was further studied. As shown in

Figure 1a, this hydrogenation process was highly dependent on temperature. At the temperature of 100 °C, the conversion of 2,3-dihydrobenzofuran and the yield of 2-ethylcyclohexanol were all around 50%. With the elevation of reaction temperature, the conversion sharply increased. When the temperature got to 130 °C, the conversion rate reached 100% and the 2-ethylcyclohexanol yield was up to 90%. Meanwhile, the 8


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oxygen-bearing product 2-ethylphenol was disappearing and the oxygen-free product ethylcyclohexane was forming. It indicated that higher temperature was conducive to the occurrence of deoxygenation. The acid sites existed in Ni/SiO2 catalyst was also contributed to the deoxygenation.15 Moreover, the yield of ethylcyclohexane was decreasing, demonstrating the hydrogenation of benzene ring was also promoted. Higher temperature meant more energy, which was favourable to active the benzene ring and realize the hydrogenation.9 As for the effect of reaction time, a similar trend was exhibited in Figure 1b. After only 0.5 h, 2,3-dihydrobenzofuran could convert to 2-ethylcyclohexanol with approximate 80% yield. It indicated that the hydrogenation could occur in short time in the presence of Ni/SiO2. With the extension of reaction time, the selectivity of 2-ethylphenol and ethylbenzene decreased, and that of ethylcyclohexane increased. All the product selectivities reached the summit when it came to 3 h.

3.3.

Proposed reaction pathway Based on the detailed analysis of this hydrogenation process, a brief reaction

pathway of 2,3-dihydrobenzofuran conversion over Ni/SiO2 catalyst was proposed. As shown in Figure 2, the ether bond was cleaved through hydrogenolysis firstly, forming the product of 2-ethylphenol. Ni active sites catalyzed this step9,13,19,20 and high amount of Ni had a promoting effect on it (Table 1). Afterwards, the hydrogenation of aromatic ring occurred over Ni sites, with 2-ethylcyclohexanol produced as the eventual product. These two steps accounted for the major reaction pathway. 9


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Otherwise, the side reactions were taking place meanwhile. With the small number of acid sites in Ni/SiO2 catalyst,15 the deoxygenation reaction occurred,13,21,22 forming a handful of ethylcyclohexane. Moreover, the deoxygenation of 2-ethylphenol into ethylbenzene also happened slightly, which could be further hydrogenated to form ethylcyclohexane. These were proved by the existence of ethylbenzene and ethylcyclohexane in the products list.

3.4.

Hydrogenation of lignin depolymerization products

3.4.1. Analysis of the volatile product The hydrogenation of lignin depolymerization products were carried out over 20% Ni/SiO2 at 130 °C. The volatile chemicals of lignin depolymerization products before and after reaction were comparatively measured by GC-MS (Figure 3). The products were mainly composed of guaiacol and its derivatives, especially multiple methyl and ethyl guaiacols.16,23 Other aromatic compounds with hydroxyl, carbonyl, carboxyl and ester groups were also identified. After hydrogenated, the species of the volatile product did not change much, but some certain compounds had great variation in content. Also, their images were exactly different. The lignin depolymerization products was light brown liquid. After hydrogenation, the products became colorless and clear. It was probably contributed to the saturation of color-developing groups, such as carbonyl.10 Table S1 compared the detailed component of volatile fraction from lignin depolymerization with its hydrogenated product. The results demonstrated that the 10


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comparative contents (detected by the area percentage of GC) of some components changed a lot. For example, the amount of guaiacol and phenol decreased, meanwhile cyclohexanol was formed, as well as a small amount of cyclohexane. This was in good accordance with our previous study,15 where guaiacol and phenol could be converted to cyclohexanol at 120 °C efficiently. Moreover, the hydrogenation occurred significantly. Some aromatic products such as toluene and 4-ethyl-phenol were transformed to cyclohexanes and cyclohexanols. Besides, the C=C and carbonyl groups in the side-chain were also saturated. This confirmed the good catalytic hydrogenation capacity of Ni/SiO2, which was correlated well with the hydrogenation of 2,3-dihydrobenzofuran. But overall speaking, the hydrogenation efficiency in lignin depolymerization products was not high compared to the model reaction. As for the reason, the phenolics from lignin depolymerization contained more complex structures than the model compounds, and a big part of them possessed electron denoting groups, which brought about the difficulty for hydrogenation.24 3.4.2. Analysis of the nonvolatile product The molecular weight distributions of the nonvolatile product from lignin depolymerization before and after the hydrogenation were measured by GPC. Results listed in Figure 4 and Table 2 showed that after the treatment with Ni/SiO2 catalyst, the average molecular weight of nonvolatile product was remarkably turned down, from 795 g/mol to 362 g/mol. And the degree of dispersity also decreased from 2.82 to 2.37. The hydrogenated product was corresponded to approximately 2 phenylpropane units (hydroxypropyl-guaiacol M = 178 g mol−1 was chosen for the 11


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standard unit),25 which meant that the polymerization degree was really low. Moreover, the peak intensity became stronger after hydrogenation, which implied that the content of low molecular weight compound increased. These phenomena indicated the depolymerization of phenolic oligomers also occurred over Ni/SiO2 catalyst. As mentioned above, Ni/SiO2 had a good catalytic ability of ether bonds hydrogenolysis

on

2,3-dihydrobenzofuran

(Figure

2).

Therefore,

this

depolymerization of phenolic oligomers was probably due to the cleavage of ether bonds through hydrogenolysis as well. The main element composition of original nonvolatile product and its hydrogenated product was also investigated. Results in Table 3 showed that the carbon and hydrogen element contents were significantly increased, which was contributed to the occurrence of hydrogenation. Meanwhile the oxygen element was removed a lot, which implied that the deoxygenation occurred. The high heated value (HHV) of the product also increased a lot, from 26.43 MJ kg-1 to 32.60 MJ kg-1, presenting a great potential to produce alternative fuels. Moreover, after hydrogenation, the unsaturation degree of the oligomer decreased significantly from 4.76 to 3.29, confirming the occurrence of hydrogenation again, which was in good accordance with the GC-MS results (Table S1). NMR technologies are widely used for elucidating lignin structure, in this work 1

H NMR was also employed for the characterization of lignin depolymerization

products. Based on the reported articles,25-28 the peak at 3.73-3.59 ppm is for the protons in β-O-4 substructures. The peak at 3.54-3.36 ppm is assigned to the protons in 12


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the phenylcoumarane substructures. The chemical shifts at 3.10-3.00 and 2.90-2.80 ppm are respective assigned to the protons in β-β substructure and the aliphatic H connected on the C=O. The appearance of peaks at 1.21-1.10 ppm are due to the presence of the aliphatic methyl and methylene. Figure 5 indicated that structure of the lignin depolymerization product was significantly changed after hydrogenation. Although the β-O-4 characteristic peak was still existed, the decrease of peak intensity demonstrated that the cleavage of β-O-4 bonds occurred.7 This was correlated well with the GPC results (Figure 4 and Table 2). The intensity of phenylcoumarane peak became weaker after hydrogenation, indicating that a part of this substructure had been transformed. The signal at 3.05 ppm assigned to β-β structure was not changed much, probably due to the high stability. Moreover, the disappearance of the peak at 2.85 ppm implied the significant saturation of the C=O bond in nonvolatile products, which was in accordance with GC-MS result of volatile product (Table S1). The increase of the signal intensity at 1.15 ppm suggested the occurrence of methylation during the hydrogenation, which produced a large amount of multiple methyl aromatic chemicals (Table S1). Therefore, the comparative analysis demonstrated that the characteristic chemical bonds of phenolic oligomers such as β-O-4 were ruptured. Meanwhile, the unsaturated groups such as C=O bond were hydrogenated to form saturated chemicals, which was considered to be the reason for the colour change of volatile fraction (Figure 3). In addition, this saturation was also conducive to suppress the condensation. In summary, after hydrogenation, the lignin depolymerization products possessed 13


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a low molecular weights and a small unsaturation degree. More stable products in thermal and chemical properties were formed. This treatment was helpful to suppress the occurrence of condensation and avoid the carbon deposition on catalysts in the next catalytic process (for example hydrodeoxygenation). The high heated value of lignin depolymerization products also increased significantly, which facilitated the biofuels production as well.

4. Conclusions A promising strategy for lignin depolymerization products hydrogenation was proposed over Ni/SiO2 catalyst in mild condition. Model compound of 2,3-dihydrobenzofuran exhibited a good hydrogenation result at only 130 °C. Nearly 100% conversion of model compound was obtained, with 94.6% product selectivity of

2-ethylcyclohexanol.

Analysis

showed

that

the

ether

bond

in

2,3-dihydrobenzofuran was cleaved through hydrogenolysis, followed by the hydrogenation of benzene ring to form the eventual product 2-ethylcyclohexanol. The hydrogenation of lignin depolymerization products was also efficient, where not only the hydrogenation of unsaturated groups, but also the cleavage of β-O-4 bonds occurred and resulted in a significant decrease of molecular weight. Thermal and chemical stable products were formed, which was favorable for further catalytic conversion. This process will be a valuable reference for lignin depolymerization products utilization, due to its simplicity, and technical and economical advantages.

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Acknowledgments The authors gratefully acknowledge the financial support of Natural Science Foundation of China (No. 51476178 & 51676191), the Youth Science and Technology Innovation Talents of Guangdong Province (No.2015TQ01N652) and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2016313).

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2017, 91, 1-5. [22] Lin, Y.C.; Li, C.L.; Wan, H.P.; Lee, H.T.; Liu, C.F. Energy Fuels 2011, 25, 890-896. [23] Shu, R.Y.; Long, J.X.; Xu, Y.; Ma, L.L.; Zhang, Q.; Wang, T.J.; Wang, C.G.; Yuan, Z.Q.; Wu, Q.Y. Bioresour. Technol. 2016, 200, 14-22. [24] Long, J.X.; Shu, R.Y.; Yuan, Z.Q.; Wang, T.J.; Xu, Y.; Zhang, X.H.; Zhang, Q.; Ma, L.L. Appl. Energy 2015, 157, 540-545. [25] Joffres, B.; Lorentz, C.; Vidalie, M.; Laurenti, D. Appl. Catal. B: Environ. 2014, 145, 167-176. [26] Sun, S.N.; Li, H.Y.; Cao, X.F.; Xu, F.; Sun, R.C. Bioresour. Technol. 2015, 176, 296-299. [27] Huang, X.M.; Koranyi, T.I.; Boot, M.D.; Hensen, E.J.M. ChemSusChem 2014, 7, 2276-2288. [28] Long, J.X.; Xu, Y.; Wang, T.J.; Shu, R.Y.; Zhang, Q.; Zhang, X.H.; Fu, J.; Ma, L.L. Bioresources 2014, 9, 7162-7175.

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Energy & Fuels

Figure Captions Figure 1. Effects of (a) reaction temperature and (b) time on the hydrogenation of 2,3-dihydrobenzofuran. Condition: (a) 0.3 g substrate, 0.1 g 20% Ni/SiO2, 20 mL decalin, 3 MPa H2, 4 h; (b) 0.3 g substrate, 0.1 g 20% Ni/SiO2, 20 mL decalin, 4 MPa H2, 130 °C. Figure 2. Proposed reaction pathways of the hydrogenation of 2,3-dihydrobenzofuran. Figure 3. GC-MS of lignin depolymerization products before and after hydrogenation. Figure 4. GPC analysis of the nonvolatile product (a) before and (b) after hydrogenation. Figure 5. 1H NMR analysis of the nonvolatile product (a) before and (b) after hydrogenation.

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Energy & Fuels

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Table 1 The catalytic performance of Ni/SiO2 catalysts on 2,3-dihydrobenzofuran hydrogenation. Catalyst

Products (selectivity %)

Temperature

Conversion

(°C)

(%)

130

0

-

-

-

-

5% Ni/SiO2

130

68.6

1.3

1.2

3.5

94.0

10% Ni/SiO2

130

88.3

0.9

0.4

4.2

94.5

15% Ni/SiO2

130

98.9

0.4

-

4.8

94.8

20% Ni/SiO2

130

99.9

-

-

5.1

94.9

20% Ni/SiO2a

130

95.8

-

-

3.9

96.1

20% Ni/SiO2

120

94.5

3.7

2.3

5.0

89.0

20% Ni/SiO2b

120

91.3

3.2

4.8

3.9

88.1

SiO2

a

Recycled for 3 runs at 130 °C; b Recycled for 3 runs at 120 °C.

Condition: 2,3-dihydrobenzofuran 0.3 g, catalyst 0.1 g, decalin 20 mL, H2 3 MPa, 4 h.

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Energy & Fuels

Table 2 Molecular weight distribution of the nonvolatile product before and after hydrogenation. Before

After

Change (%)

Mn

282

152

-46.1

Mw

795

362

-54.5

Mz

1731

791

-54.3

D

2.82

2.37

-16.0

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Energy & Fuels

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Table 3 Elemental analysis of the nonvolatile product before and after hydrogenation. Samples

Elemental content (wt%)

Experimental

HHVb

Degree

of

C

H

Oa

N

S

molecular formula

(MJ kg-1 )

unsaturation

Before (a)

65.45

6.40

27.10

0.63

0.42

C9H10.56O2.79N0.07S0.02

26.43

4.76

After (b)

70.26

8.76

20.42

0.42

0.14

C9H13.47O1.96N0.05S0.01

32.60

3.29

a

The oxygen content was estimated by the conservation of mass based on the assumption that the samples only contain C, H, N, S and O.

b

Evaluated by Dulong formula: HHV (MJ kg-1 ) = 0.3383 × C + 1.422 × (H - O/8).

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Energy & Fuels

Figure 1. Effects of (a) reaction temperature and (b) time on the hydrogenation of 2,3-dihydrobenzofuran. Condition: (a) 0.3 g substrate, 0.1 g 20% Ni/SiO2, 20 mL decalin, 3 MPa H2, 4 h; (b) 0.3 g substrate, 0.1 g 20% Ni/SiO2, 20 mL decalin, 4 MPa H2, 130 °C.

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Energy & Fuels

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Figure 2. Proposed reaction pathways of the hydrogenation of 2,3-dihydrobenzofuran.

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Figure 3. GC-MS of lignin depolymerization products before and after hydrogenation.

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Energy & Fuels

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Figure 4. GPC analysis of the nonvolatile product (a) before and (b) after hydrogenation.

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Energy & Fuels

Figure 5. 1H NMR analysis of the nonvolatile product (a) before and (b) after hydrogenation.

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SiO2

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