Lignin Valorization: A Novel in Situ Catalytic Hydrogenolysis Method

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Biofuels and Biomass

Lignin valorization: a novel in situ catalytic hydrogenolysis method in alkaline aqueous solution Da Wang, Yuyang Wang, Xiaoyu Li, Lei Chen, Guangci Li, and Xuebing Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01032 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Lignin valorization: a novel in situ catalytic hydrogenolysis method in alkaline aqueous solution

Da Wang, Yuyang Wang, Xiaoyu Li, Lei Chen, Guangci Li, Xuebing Li* Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, PR China *Corresponding author: E-mail: [email protected], Tel: +86 0532 80662759

1

ACS Paragon Plus Environment

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Abstract In situ catalytic hydrogenolysis of lignin is an efficient method for lignin valorization. Herein, a novel in situ lignin catalytic hydrogenolysis technique in alkaline aqueous solution over NiAl alloy catalyst under mild conditions was systematically suited. Ni was served as the active phase and could be exposed by etching Al atoms with alkaline aqueous solution. Meanwhile, H2 resources could be in situ generated and easily arrived at the adjacent exposed Ni sites to be activated. The in situ lignin hydrogenolysis system exhibits attractive depolymerization results at mild condition, with 86.8% conversion degree and 18.9 wt% yield of aromatic monomers at 220 °C, much higher than the traditional hydrogenolysis method using commercial Raney Ni catalyst under high pressure external H2. The mechanism studies with lignin model compounds suggests that alkaline aqueous solution could promote the cleavage of C-O-C bond and hinder the hydrogenation of benzene ring. In addition, a moderate amount of exposed Ni sites as well as rich active hydrogen species are determinant for efficient lignin depolymerization.

Keywords: Biomass; Lignin valorization; In situ hydrogenolysis; Aromatic chemicals; NiAl alloy

2


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Introduction Lignin is the most abundant renewable source composed of aromatic units in nature and has received great attention as a sustainable resource for production of aromatic chemicals, which are mainly obtained from traditional fossil-based feedstock1. Lignin is primarily a complex cross-linked structure consists of C-O-C ether bond or C-C bond. Unfortunately, the robust spatial structure and stable bond cause the difficulty to depolymerize lignin into valuable chemicals2-3. More than 95% of lignin is burned directly as low value fuel or drained into rivers, resulting in the significant deterioration of resource waste4-5. A

variety

of

technologies

include

pyrolysis,

base-catalyzed,

acid-catalyzed, hydrogenolysis and oxidation have focused on the valorization of lignin6-11. In these technologies, catalytic hydrogenolysis of lignin, where H2 is utilized to cleavage the C-O-C bond in the presence of catalyst, is the frequent and effective method. Catalysts based on noble metals (Pt, Pd, Ru)12-14 as well as non-noble metals (Ni, Mo, Fe) have been deeply investigated15-19. More importantly, it is believed that the in situ hydrogenolysis method could promote the lignin conversion and phenolic monomer yield

20-22

. Xu et al. used alcohols as in situ hydrogen

source and Ni/C as catalyst for lignin hydrogenolysis, the result showed that lignin can be depolymerized to propylguaiacol and propylsyringol with total selectivity >90% at a conversion of about 50%20. Jae et al. used formic acid as in situ hydrogen source and Ru/C as catalyst to depolymerize lignin, they found that the bio-oil yield achieved 66.3 wt% 3


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and the aromatic monomer content was 6.1 wt%22. It is noted that the reported hydrogen donor are organic, which are a certain toxic and expensive compared with the aqueous solution. Therefore, utilizing aqueous solution as hydrogen donor would be of great significance. As we all know, alkaline aqueous solution could easily react with aluminum to produce H2 (Eq.1). Furthermore, alkaline aqueous solution is in favor of the cleavage of C-O-C linkages between lignin units23. Therefore, choosing alkaline aqueous solution as solvent and using the hydrogen resource in situ produced by the reaction of aluminum and alkaline aqueous solution for the catalytic hydrogenolysis of lignin might be very interesting. Al + NaOH + H2O → NaAlO2 + H2

Eq. 1

In this study, we constructed a novel system for catalytic hydrogenolysis of lignin at mild condition, that is, dilute alkaline aqueous solution was used as solvent to dissolve lignin and NiAl alloy was utilized as catalyst. In this system, Ni was served as the active phase and could be exposed by etching Al atoms with alkaline aqueous solution. Meanwhile, H2 could be in situ generated and easily arrived at the nearby exposed Ni sites to be activated for lignin hydrogenolysis. The traditional hydrogenolysis method, which using commercial Raney Ni and high pressure external H2 was also carefully investigate to demonstrate the advantages of the in situ hydrogenolysis system. Lignin model compound was also employed to further investigate the mechanism of the novel in situ lignin catalytic hydrogenolysis system. 4


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Experimental section Materials All commercial reagents in this study were analytical grade and used without further purification. Raney Ni, matellic Al powder, 5 wt% Pd/C, 5

wt%

Pt/C,

NiAl

alloy

(Ni

contents:

50

wt%),

NaBH4,

2-bromoacetophenone, phenol were purchased from Aladdin (Shanghai, China). NaOH, HCl, NaCl, NH4Cl, MgSO4, K2CO3, ethyl acetate, diethyl ether, acetone, methanol THF, n-hexane were supplied by Sinopharm Chemical Reagent Co. Ltd. (CD3)2SO was purchased from Shanghai adama reagent Co. Ltd. Deionized water was prepared by our laboratory. Poplar wood was purchased from a local manufactory and shattered into wood sawdust. Lignin extraction and purification The lignin was extracted according to a reported procedure24. Poplar wood sawdust (200 g) and 1,4-dioxane (1440 mL) were added to a 2 L round-bottom flask. Then 160 mL solution of HCl (2 mol L−1) was slowly added to the mixture. Under Ar atmosphere, the reaction solution was heated to a reflux temperature for 1 h. After the reaction mixture was cooled to RT, the residue was filtered and concentrated. The resulting concentrate was dissolved in 250 mL solvent (acetone : water = 9 : 1) and then precipitated with 2.5 L ice water. The precipitate was filtered and washed. Then the precipitate was again dissolved and precipitated. Finally, the precipitate was freeze-dried overnight under vacuum and the obtained solid was 21 g (Mw = 14800 g/mol). 5


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Catalytic hydrogenolysis of lignin The catalytic hydrogenolysis of lignin was conducted in a 100 mL stainless batch autoclave reactor (Yantai shuguang precision instrument factory). For the in situ catalytic hydrogenolysis of lignin reaction, 0.20 g lignin was added into 30 mL 0.2 M NaOH solution and stirred for 30 min. Then the above lignin solution and 0.20 g NiAl alloy (the content of active Ni phase was 50 wt%) were added into the autoclave. The reactor was purged with Ar for three times and pressurized to 2.0 MPa. The reactions were conducted at a stated temperature for a certain time under the stirring of 600 rpm. After reaction, the autoclave was cooled to room temperature using electron fan. For the traditional catalytic hydrogenolysis of lignin reaction, 0.20 g lignin was added into 30 mL 0.2 M NaOH solution and stirred for 30 min. Then the above lignin solution and 0.10 g Raney Ni (or Pd/C, Pt/C) were added into the autoclave. The reactor was purged with H2 for three times and pressurized to 2.0 MPa. The reactions were conducted at a stated temperature for a certain time under the stirring of 600 rpm. After reaction, the autoclave was cooled to room temperature using electron fan. Products separation and analysis Too few gaseous products were generated during the lignin hydrogenolysis process thus were not investigated in the work. The liquid and solid products were separated by a method (Fig.S1) based on the 6


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reported work 6. First, the mixture products were filtered, and the filter cake was washed with 15 mL THF for three times. The solid fraction was considered as catalyst and char. The filtrate and THF solution were combined and the mixture was acidified to pH 7 with HCl to precipitate the unconverted lignin. The unconverted lignin was filtered and freeze-dried overnight under vacuum. For the in situ hydrogenolisis system, Al(OH)3 was also formed during the precipitate process and the weight of recovered lignin could be regarded as the lost weight of calcinating the mixture. The filtrate was then extracted with diethyl ether many times to obtain the aromatic monomers and thymol was added to the the monomers as internal standard. After extraction, the remaining chemicals in the solution were aromatic oligomers. The aromatic monomers extracted by diethyl ether were analyzed by GC-MS (Agilent 7890 GC with an Agilent 5975 mass-selective detector) equipped with a HP-5 capillary column (30 m × 0.32 mm × 0.25 mm). He was used as carrier gas and split injection ratio was 50. The GC heating ramp was as follows: (1) holding at 60 °C for 2 min (2) heating to 280 °C at 10 °C min−1 (3) holding at 280 °C for 6 min. The quantitative analysis of these aromatic monomers was carried out on an Agilent 7890 GC with a FID using thymol as internal standard at the same capillary column and temperature program as the GC–MS analysis. In order to estimate the yield of monomers, the effective carbon number (ECN)25 was used to determine the relative response factors of the compounds related to thymol as the internal standard. All the results were based on the average 7


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of three reactions. The conversion of lignin was measured by the weight of the unconverted and the original lignin as shown in Eq. (2). The yield of cyclohexanol and aromatic monomers was measured by the following Eq. (3) and Eq. (4) according to the GC results. The yield of the formed char during reaction was calculated by Eq. (5). According to the separation procedure, the products were consist of gas (could be neglect), cyclohexanol, aromatic monomers, aromatic oligomers and char. Therefore, the yield of the aromatic oligomers could be calculated by the weight subtraction method (Eq. (6)). Cl = (Wl – Wu)/Wl × 100%

Eq.2

Yc = Wc/Wl × 100%

Eq.3

Ym = Wm/Wl × 100%

Eq.4

Ych = Wch/Wl × 100%

Eq.5

Yo = (Cl – Ym - Yc- Ych) × 100%

Eq.6

Cl: conversion of lignin; Yc: yield of cyclohexanol; Ym: yield of aromatic monomer; Ych: yield of char; Yo: yield of oligomer; Wl: the weight of raw lignin; Wu: the weight of unconverted lignin; Wc: the weight of cyclohexanol; Wm: the weight of aromatic monomer; Wch: the weight of char; Wo: the weight of oligomer. Characterization of original and unconverted lignin The morphologies of raw and recovered lignin were obtained by cold field emission scanning electron microscope (SEM) with spraying gold on the samples. The molecular weights of original and unconverted lignin 8


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were determined by gel permeation chromatography (GPC). The characteristics of the aromatic oligomers products were carefully investigated by FT-IR, 1H NMR and GPC. The FT-IR spectra of the samples were obtained on a Nicolet 6700 FT-IR spectrometer with MCT-A detector by coating samples on KBr. 1H NMR experiment was performed on a Bruker Advance III 600 spectrometer, where D2O was used as solvent. GPC analysis was performed by using Wyatt HELEOS System equipped with a Waters 2489 detector. Analysis was carried out at 60 °C using THF as the eluent with a flow rate of 1 mL min−1.

Results and discussion Characterization of raw and recovered lignin Table 1. The average molecular weight of raw lignin, recovered lignin and aromatic oligomer. Samples

a

Mn

Mp

Mw

Mz

Raw lignin

12500

9380

14800

25200

Recovered lignin aa

12800

12600

19700

34500

Recovered lignin bb

4450

4130

7290

15700

Oligomer

343

282

480

895

Recovered after depolymerized with Raney Ni under 2 Mpa H2 at

225 °C for 3 h. b

Recovered after depolymerized at the in situ hydorgenolysis system at

225 °C for 3 h. The average molecular weight (Mw) of raw and recovered lignin was shown in Table 1. As can be seen, the average molecular weight of raw 9


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lignin was 14800 g/mol. After depolymerized with Raney Ni under external H2 (recovered lignin a) the molecular weight of recovered lignin increased to 19700 g/mol, which might be attributed to the serious repolymerization. The lignin depolymerization intermediates contained plenty of active carbonyl groups, which could easily react with lignin by aldol condensation, resulting in the enhanced molecular weight6. However, for the in situ catalytic hydrogenolysis system (recovered lignin b) the average molecular weight decreased to 7290 g/mol. It is believed that the in situ catalytic hydrogenolysis system contained plenty of active hydrogen species, which could hinder the repolymerization of the products, thus leading to the decrease of the molecular weight.

Fig. 1. Partial 2D HSQC NMR spectra of lignin fractions. The relative volume intervals of β-O-4(A), β-5(B), β-β(C), syringyl(S) and guaiacyl (G) units are given based on QQ-HSQC. Cross peaks are color-coded according to the structure or linkage they are assigned to. 2D HSQC NMR spectra of the original lignin was displayed in Fig. S2. Three regions corresponding to side chain (δC/δH 10-45/0.5-2.5), 10


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linkages (δC/δH 50-95/2.5-6.0) and aromatic (δC/δH 95-135/5.5-8.0) 13

C-1H correlations could be observed, indicating the perfect full structure

of the extracted lignin26-27. The side chain region signal showed no structural information and therefore could be negligible. The relative amounts of β-O-4, β-5, β-β, syringyl and guaiacyl units could be calculated from linkages and aromatic regions28-29 by QQ-HSQC and the detailed results were given in Fig. 1. The 2D HSQC NMR spectra of the recovered lignin were shown in Fig. S3. As can be seen, the signals at linkages region reduced obviously. This suggests that the main chemical bonds among each units of the lignin were cleaved during the depolymerization process. SEM images of raw and recovered lignin were shown in Fig. 2. For the raw lignin (Fig. 2a), loose and uniform “globes” were observed on the surface. However, after depolymerizing with Raney Ni under 2 MPa external H2 the surface morphology changed remarkably (Fig. 2b). It could be clearly observed that the surface became irregular and some “smooth lumps” formed, which might be char species. This suggests that the repolymerization of oligomer followed by dehydration to form char occurred simultaneously with the lignin depolymerization. For the in situ hydrogenolysis system (Fig. 2c), the morphology of the recovered lignin changed slightly and no obvious char species was observed. This could be attributed to the high concentration of active hydrogen species in the in situ hydrogenolysis system, which could retard the repolymerization reactions of reactive intermediate species30. 11


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Fig. 2. SEM images of: a) raw lignin; b) recovered lignin of traditional hydrogenolysis system (Raney Ni + H2); c) recovered lignin of in situ catalytic hygrogenolysis system.

Fig. 3. FT-IR spectra of: a) raw lignin; b) recovered lignin of traditional hydrogenolysis system (Raney Ni + H2); c) recovered lignin of in situ hygrogenolysis system. FT-IR was used for the study of raw and recovered lignin structure and the peaks were identified by previous report31-34. For the raw lignin (Fig. 3a), the peak at 3438 cm-1 was regarded as the stretching vibration of -OH group. The peak at 2992 cm-1 was assigned to the stretching vibration of 12


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Ar-H. Peaks at 2930 and 2850 cm-1 were considered as the characteristic absorption of –CH3 and –CH2 respectively. The peak at 1710 cm-1 was regarded as the characteristic absorption of the carbonyl. Peaks at 1425, 1507 and 1610 cm-1 could be recognized as the characteristic vibration of benzene structure. The peak at 1330 and 1122 cm-1 was assigned to the characteristic absorption of syringyl and guaiacyl unit. The peak at 1270 cm-1 was considered as the characteristic absorption of methoxy group in guaiacyl. And the peak at 855 cm-1 could be regarded as the vibrations of lignin-carbohydrate complex. Interestingly, the FT-IR spectra changed a lot for the recovered lignin (Fig. 3b and Fig.3c). As can be seen, the peak strength of 1330, 1270 and 1122 cm-1 decreased obviously, indicating the partly destroy of the lignin structure. This result was also in accordance with the 2D HSQC NMR experiment, which the signals of linkages region reduced obviously for the recovered lignin, suggesting the cleavage of main chemical bonds during hydrogenolysis. Chemical structure of the oligomer products The chemical structure and functional group of the aromatic oligomers were carefully characterized by FT-IR, 1H NMR and GPC. As can be seen in Fig. S4, peaks represented the characteristic absorption of –OH (3439 cm-1), Ar-H (2992 cm-1) and benzene structure (1618, 1514, 1423, 1060, 950, 844 cm-1) could be clearly observed. The peak at 1710 cm-1 could be regarded as the characteristic absorption of the carbonyl. The peak at 1458 cm-1 could be assigned to the deformation vibration of –CH3 and -CH2. And the peak at 1230 cm-1 was recognized as the stretching 13


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vibration of C-C bond. In brief, the IR results indicated that the oligomer possessed similar functional group to the raw lignin and might be composed of partly decomposed lignin. The

1

H NMR spectra was displayed in Fig. S5 and the peak

assignments were based on previous reports33, 35. As can be seen, the chemical shift at 0.81-1.24 ppm could be ascribed to aliphatic or alicyclic side chain of lignin. A strong absorption peak at 1.83 ppm could be observed, suggesting that plenty of CH3 directly attached to the aromatic ring existed in the oligomer. Chemical shifts at 2.14 and 2.31 ppm was assigned to the characteristic absorption of alkyl group on the benzene ring. The observed chemical shift at 2.45 ppm was considered as the aliphatic H connected on the carbonyl group. Furthermore, the chemical shift at 3.27 ppm could be assigned to the Hβ in the phenylcoumarane substructure and the strong absorption peak at the chemical shift of 3.57 ppm indicated the abundant β-O-4’ substructure. The chemical shift at 3.73 ppm was also observed because of the existence of methoxyl group in the oligomer. The relative weak signal at 4.02 ppm was recognized as Hγ in p-hydroxycinnamyl alcohol end group. The detected Hβ and Hγ signal suggested that the hydroxyl-phenyl structure was easily to be depoymerized. In addition, the chemical shifts at 8.36 and 7.06-7.13 ppm could be designated as the aromatic ring structure. And the chemical shifts at 6.49-6.63 illustrated the existence of guaiacyl and syringyl units in the oligomer. In conclusion, the 1H NMR result confirmed that the basic aromatic ring skeleton had not been broken in the oligomer, which 14


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was in line with the IR result that the oligomer are composed of partly depolymerized lignin structure. The GPC result of the oligomer was displayed in Table 1. As can be seen, the average molecular weight of the oligomer was 480 g/mol. Combing with the FT-IR and 1H NMR result and assuming that the oligomer was the linkage of the traditional sinapyl, coniferyl and p-coumaryl units, the estimate calculation result showed that the oligomer was dimer, trimer and tetramer derived from the depolymerization of lignin and the repolymerization of unsaturated aromatic monomers. Catalytic hydrogenolysis of lignin Table 2. Catalytic performance of various catalysts for lignin hydrogenolysis at 220°C for 3h. Entry Catalyst

Gas

Conv. (%)

Weight yield (%) Cyclohexanol Monomer

Oligomer

Char

1

None

Ar

-

-

-

-

-

2

None

H2

25.8

-

1.8

10.3

13.7

3

Raney Ni

H2

75.4

2.3

10.2

40.3

22.3

4

NiAl alloy

Ar

86.8

0.7

18.9

55.5

11.7

5

Al

Ar

28.1

-

1.1

12.6

14.4

6

Raney Ni+Al Ar

81.3

1.8

11.6

52.7

15.2

7

Pd/C

H2

41.0

0.8

2.7

17.9

19.6

8

Pt/C

H2

45.6

0.5

2.1

19.7

23.3

The lignin depolymerization performances with and without using catalysts under different atmospheres obtained from GC results are shown 15


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in Table 2. As can be observed, in Ar atmosphere and without using catalyst no obvious aromatic monomer was detected, indicating the negligible

lignin

depolymerization

behavior

(entry

1).

Slight

improvement of the depolymerization was achieved under H2 atmosphere, that is, 25.8% lignin was converted and 1.8% aromatic monomer was produced meanwhile (entry 2). This tells us that slightly lignin hydrogenolysis occurred under the reaction condition. With the addition of Raney Ni, both the conversion rate and aromatic monomer yield increased significantly (entry 3). This could be ascribed to the strong ability of cleavage C-O-C bonds for Ni-based catalyst36. In addition, 2.3 wt% cyclohexanol was detected, which could be attributed to the hydrogenation of the produced phenols. Remarkably, for the novel in situ lignin catalytic hydrogenolysis system (entry 4), the lignin conversion degree increased to 86.8% and the aromatic monomers yield increased to 18.9%, much higher than the traditional hydrogenolysis method using commercial Raney Ni under 2 MPa external H2 condition (entry 3). The images of the products for entry 3 and entry 4 were shown in Fig. S6. A control experiment was also performed, namely the aluminum powders was added into the lignin alkaline aqueous solution separately to eliminate the possible catalytic effect of metallic Al phase (entry 5). A similar lignin depolymerization result was observed compared with the blank experiment without using catalyst under H2 atmosphere experiment (entry 2). Furthermore, because of the low surface area of NiAl alloy powders, a very small amount of Ni 16


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and Al atoms were exposed to the solution simultaneously, and the synergistic effect of Ni and Al could also be excluded. Therefore, it convinced us that the Al phase in NiAl alloy was not the active phase for catalyzing lignin depolymerizatin. Actually, for the in situ lignin hydrogenolysis system over NiAl alloy in alkaline aqueous solution, we believed that Al atoms could react with alkaline aqueous solution to generate H2 in situ. Meanwhile, Ni active sites were gradually exposed after Al atoms were etched by alkaline aqueous solution. In addition, because of the short diffusion distance the in situ generated H2 could easily contact with the adjacent exposed Ni sites and produce a large amounts of active hydrogen species. It should be mentioned that in the novel system NiAl alloy was put into the dilute alkaline aqueous solution for a short time (only a few hours) thus the exposed Ni active sites could be much less than the commercial Raney Ni, which was obtained by removing almost all Al elements of NiAl alloy with very high concentration of alkali liquor. Actually, the number of exposed Ni sites as well as active hydrogen species were determinant for efficient lignin hydrogenolysis. The in situ produced rich active hydrogen species in combination with the continually exposed Ni sites of entry 4 rendered the lignin depolymerization more efficient than entry 3. Many studies have also shown that hydrogenation and hydrogenolysis using hydrogen donor molecules are more efficient than using hydrogen gas20, 37. The in situ produced hydrogen species contact with the active sites more easily because the external gaseous H2 should diffuse from gas to liquid and 17


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then from liquid to solid in order to be activated. In addition, the entry 4 exhibited lower char yield than entry 3 using 2 MPa external H2. The formed char during lignin depolymerization process was considered caused by the repolymerization and condensation of oligomer, which contains plenty of unsaturated chemical bond and followed by dehydration on catalytic active sites with thermal effect38. The high concentrations of active hydrogen species in entry 4 could retard the oligomer repolymerization via hydrogenation, thus leading to the decrease yield of char materials. The effect of NiAl alloy dosage was shown in Fig.S7. The lignin conversion showed unobvious change but the char yield decreased with the increase of NiAl usage, which could also be attributed to the higher active hydrogen species. Table 3. GC-MS analysis of the depolymerization products for the in situ H2 production system.

18


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RT(min)

Compound

Structure

4.87

1-hexanol

HO

5.57

cyclohexanol

5.78

cyclohexanone

7.12

phenol

8.97

2-methoxy-phenol

RT(min)

Compound

Structure OH

11.82

4-ethyl-2-methoxyphenol

12.26

4-methylsyringol

12.78

4-propylguaiacol

O

OH

OH O

O

OH

O

O

OH

OH

O

14.04

1,2,3-trimethoxybenzene

O

14.38

3,4,5-trimethoxybenzaldehyde

O

O

O O

O O

O

10.12

acetic

acid

OH

O

15.01

syringylpropane

16.05

1-(3,4,5-trihydroxyphenyl)ethanone

O

O

4-ethylphenylester 10.57

2-methoxy-4-methylphenol

OH OH

HO

O

OH

O

The aromatic monomers of entry 4 obtained by GC-MS were displayed in Table 3. And the major aromatic monomers selectivity according to the GC (Fig. S8) was also displayed on Table S1. The major aromatic monomers almost the same for entry 3 and entry 4, which might be attributed to the same lignin material and alkaline aqueous solution solvent. Another control experiment using commercial Raney Ni and metallic 19


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Al powders in alkaline aqueous solution (entry 6) was used to further investigate the in situ hydrogenolysis of lignin. In this system, Al powders could react with alkaline aqueous solution and produce H2 in situ. But the generated H2 should diffuse from the liquid phase to arrive at the solid Raney Ni for activation. Compared with entry 4, the decrease of conversion rate of entry 6 might be attributed to the relative stronger H2 mass transfer resistance. Nobel metals were widely studied and exhibited excellent performance for lignin catalytic hydrogenolysis in previous report. Herein, commercial Pd/C and Ru/C catalysts have also been tested under 2 MPa external H2 for comparison. Interestingly, the performances of these noble metal catalysts (entry 7 and entry 8) were not attractive, with lower lignin conversion rate and aromatic monomer yields compared with entry 4. It was worth noting that the lignin catalytic hydrogenolysis was mainly performed in organic solvents. The alkaline aqueous solution here might lead to the catalyst deactivation thus resulting in the bad hydrogenolysis result. In a word, the in situ lignin catalytic hydrogenolysis system (entry 4) exhibited better performance than the traditional hydrogenolysis method using precious metals in alkaline aqueous solution. Fig. 4 shows the effect of reaction temperature on catalytic hydrogenolysis of lignin. The conversion rate of lignin was increased with the rise of temperature and almost 100% of it was arrived at 245°C. With regard to the aromatic monomer yield, it was observed that with the increase of reaction temperature from 200 to 230°C it increased from 6.7 20


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to 13.4%. Further rising the reaction temperature to 245°C leaded to a decrease of aromatic monomer yield. It is considered that the depolymerization and monomers repolymerization are in existence simultaneously in lignin depolymerization39. At high temperature, the repolymerization reaction becomes seriously thus leading to the falling yield of aromatic monomer. The char yield also exhibited a positive correlation with reaction temperature. As talked above, the generated char during lignin depolymerization derived from the repolymerization and condensation of oligomer and followed by dehydration on catalytic active sites by thermal effects38. Obviously, high temperature is beneficial to the dehydration of intermediate polymers, resulting in the increase of char yield.

Fig. 4. The depolymerization performances at different reaction temperatures for in situ hydrogenolysis system. Reaction time: 2 h. 21


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Fig. 5 shows the effect of reaction time on catalytic hydrogenolysis of lignin. The conversion rate of lignin was increased with the extension of reaction time. Moreover, the results showed that the aromatic monomer yield increased to a maximum value at 3 hours and then decreased with the prolonging of reaction time. This could be attributed to the fact that monomer

repolymerization

coexisted

simultaneously

with

the

depolymerization reaction and longer reaction time contributed to the monomer repolymerization, leading to the decrease of monomer yield. The char yield exhibited no obvious change within 3 hours but increased sharply to above 20% for longer reaction time. We believed that the serious oligomer condensation and carbonization for longer reaction time should be responsible for that. The oligomer yield first increased within 3 hours and then remained almost the same further extending the reaction time to 5 hours also demonstrated the above conclusion.

22


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Fig. 5. The depolymerization performances at different reaction times for in situ hydrogenolysis system. Reaction temperature: 225°C The recyclability of the in situ catalytic hydrogenolysis system was displayed in Fig. 6. The conversion degree and bio-oil (monomer + oligomer) yield remained above 90% during all the three consecutive cycles. A measurement of aluminum content in NiAl before and after utilization was performed with ICP-OES analysis (Table S2), showing that a large amount of aluminum (reduced from 50 wt % to 28 wt %) were dissolved in the solution to produce H2 during all the three cycles. However, the dissolved aluminum elements could be easily separated by adjusting the pH value to neutral (Al(OH)3 formation, Fig. S6 c)), avoiding the environmental pollution of waste water.

Fig. 6. The recycle experiment of the in situ hydrogenolysis system In short, even without using external H2 the in situ hydrogenolysis 23


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system exhibited attractive lignin depolymerization result with higher aromatic monomers and lower char yields than traditional method. In addition, the novel system also allowed easy products separation with extraction method, owing to the organic products and alkaline aqueous solution. Furthermore, the introduced aluminum element in the solvents could be easily separated by adjusting the pH value. Mechanistic studies 2-phenoxy-1-phenylethanol was employed as β-O-4 lignin model compound to investigate the mechanism of the in situ lignin catalytic hydrogenolysis system. The model compound was synthesized according to a previous report40. And the structure of the model compound was carefully characterized via FT-IR and 1H NMR (Fig. S9 and Fig.S10).

Scheme

1.

Reaction

scheme

of

the

depolyemrization

of

2-phenoxy-1-phenylethanol with 8 detected products. According to the GC-MS and GC results, the depolymerization 24


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products of the model compound were listed in scheme 1. 2-phenylethyl phenyl ether (5) could be considered formed by dehydroxylation process. Phenol (3) and 1-phenylethanol (4) result from the depolymerization of the

model

compound.

Ethylbenzene

(2)

is

produced

by

the

depolymerization of (5) with the cleavage of ether bond or dehydroxylation of (4). Ethylcyclohexane (6), cyclohexanol (7) and 1-cyclohexylethanol (8) derived from the hydrogenation of (2), (3) and (4) and could be regarded as the over hydrogenation products. Table 4. Hydrogenolysis of 2-phenoxy-1-phenylethanol over different catalytic system at 110°C for 1 h. Entry

Catalyst

Gas

Conv.

Selectivity (%)

(%)

2

3

4

5

6

7

8

-

-

-

-

-

-

15.3

-

1

None

Ar

-

-

-

2

None

H2

4.2

5.4

1.6

3

Raney Ni

H2

98.2

1.9 34.4

42.3

0.8

1.3 10.6

4

Raney Ni (n-hexane)

H2

45.9

4.3 30.4

35.2

2.4

2.8 14.7 10.2

5

NiAl alloy

Ar

96.3

2.2 41.5

47.9

1.6

-

5.7

3.1

6

Raney Ni +Al

Ar

96.7

1.0 34.9

47.1

0.9

1.6

8.1

6.4

10.3 67.4

8.7

As can be seen in Table 4, for entry 1 without using catalyst under Ar condition no depolymerization product was detected. However, 4.2 % of model compound was converted in H2 atmosphere (entry 2). Interestingly, product (5) was the major product, suggesting that the dehydroxylation was the main reaction path. Remarkably, for Raney Ni catalyst in H2 atmosphere (entry 3) the conversion rate arrived at 98.2% and (3) and (4) 25


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Page 26 of 35

turned into the major products. This confirmed us that the cleavage of ether bond main followed a metal-catalyzed route41. In addition, the cyclohexanol could be detected, indicating the over hydrogenation effect. Entry 3 and entry 4 were used to make comparisons for investigating the solvent effect on depolymerization. As can be seen, the conversion rate of entry 4 decreased sharply in the n-hexane solvent. The apparent activation energy of the two system was calculated using the Arrhenius plot (Fig. 7) to further investigate that phenomenon. It could be observed that the apparent activation energy for entry 3 was 61.9 kJ/mol, lower than that of entry 4. It suggests us that the alkaline aqueous solution could promote the cleavage of ether bond in combination with the Raney Ni catalyst. In addition, for entry 4 the selectivity of over hydrogenation products ((6), (7), (8)) was higher than entry 3, indicating that the alkaline aqueous solution could hinder the benzene hydrogenation. Compared with entry 3, the in situ catalytic hydrogenolysis system (entry 5) possessed lower over hydrogenation products selectivity, which could be ascribed to the large amounts of Ni active sites in commercial Raney Ni catalyst. We believed that the amount of catalyst active sites as well as active hydrogen species were determinant for lignin catalytic hydrogenolysis. For the NiAl alloy catalyst in alkaline aqueous solution, active Ni sites could be exposed by the Al atoms etched with aqueous alkali and H2 resource could be generated meanwhile. Plenty of active hydrogen species could be formed because of the short diffusion distance. Noticeably, dilute aqueous alkali (0.2 M) was used and the reaction was carried out under low temperature 26


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Energy & Fuels

(110°C), therefore, a small amount of Ni active sites were created, avoiding the over hydrogenation of the substrate. The enhanced over hydrogenation products selectivity of entry 6 could also be ascribed to the large amount of Ni sites, which further verified our above conclusion.

Fig.

7.

Arrhenius

plot

of

the

hydrogenolysis

of

2-phenoxy-1-phenylethanol with Raney Ni catalyst in alkaline aqueous solution and n-hexane. It was reported that the lignin catalytic hydrogenolysis was first fragmented into smaller lignin species and then the smaller fragments were converted into monomeric phenols over catalyst20. We believed that the in situ lignin hydrogenolysis in alkaline aqueous solution with NiAl alloy catalyst could also follow the route (scheme 2). Just as we demonstrated, the alkaline aqueous solution could promote the cleavage of ether bond and hinder the over hydrogenation of reactants. On the 27


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other hand, a moderate amount of active Ni sites were created and rich active hydrogen species were produced during the Al atoms etching process. These factors could be responsible for the attractive result of lignin in situ hydrogenolysis in alkaline aqueous solution over NiAl alloy catalyst.

Scheme 2. The proposed reaction scheme of the in situ catalytic hydrogenolysis of lignin. Conclusion A novel in situ lignin catalytic hydrogenolysis method was performed in alkaline aqueous solution over NiAl alloy catalyst. Ni was the active phase and could be exposed by the Al atoms etched with aqueous alkali. And the H2 resource could be generated meanwhile. The produced H2 could easily contact with the adjacent exposed Ni sites to be activated for short diffusion distance. The in situ lignin hydrogenolysis system exhibited much higher aromatic monomer yield and lower char yield compared with the traditional hydrogenolysis method using commercial Raney Ni under high pressure external H2. The lignin model compound test indicated that the alkaline aqueous solution could promote the cleavage of ether bond and hinder the over hydrogenation of reactants. 28


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Furthermore, a moderate amount of active Ni sites might be created and rich active hydrogen species were produced during the Al atoms etching process. These might be the reasons for the attractive depolymerization results for the in situ lignin catalytic hydrogenolysis system.

Associated content Supporting information The

synthesis

of

lignin

model

compound,

NMR

and

FT-IR

characterization of lignin model compound, lignin depolymerization products separation method, 2D HSQC NMR spectra, FT-IR and NMR characterization of aromatic oligomer, images of the products and GC spectra.

Author information Corresponding author Xuebing Li, E-mail: [email protected], Tel: +86 0532 80662759 Notes There are no conflicts to declare.

Acknowledgments We acknowledge the financial support from the National Natural Science Foundation of China (Grant No.21676287, 21761132006).

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Lignin Valorization: A Novel in Situ Catalytic Hydrogenolysis Method

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