Catalytic in-situ hydrogenolysis of lignin in supercritical ethanol: Effect

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Catalytic in-situ hydrogenolysis of lignin in supercritical ethanol: Effect of phenol, catalysts and reaction temperature Minghao Zhou, Brajendra K. Sharma, Peng Liu, Jun Ye, Junming Xu, and Jian-Chun Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00701 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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ACS Sustainable Chemistry & Engineering

Catalytic in-situ hydrogenolysis of lignin in supercritical ethanol: Effect of phenol, catalysts and reaction temperature Minghao Zhouab, Brajendra K Sharmab, Peng Liua, Jun Yea, Junming Xua*, Jian-Chun Jianga a. Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry (CAF), No.16, Suojin Five Village, Nanjing 210042, China b. Illinois Sustainable Technology Center, Prairie Research Institute, one Hazelwood Dr. Champaign, University of Illinois at Urbana-Champaign, IL 61820, USA * Corresponding Author: E-mail: [email protected]

Abstract This study aimed to explore the in-situ hydrogenolysis of alkali lignin into bio-oil over three kinds of heterogeneous catalysts with varied catalytic properties (Ru/C, Ni/ZSM-5, CuNiAl hydrotalcite-based catalyst) in supercritical ethanol. Phenol was firstly introduced to the in-situ hydrogenolysis system to form complex solvent to improve the lignin depolymerization over heterogeneous catalysts. The promotion effect of phenol was obviously observed during the hydrogenolysis process, leading to improved bio-oil yield and decreased solid residue yield, due to the unique dissolution and diffusion properties of phenol-containing solvent. The synergistic effect of basic sites and complex solvents were observed; so herein, the effect of catalysts, reaction temperature and time on the hydrogenolysis, repolymerization and coking on catalysts was investigated in detail, considering the molecular weight, elemental composition

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and HHV of bio-oil. The highest bio-oil yield was up to 81.8%, with an improved HHV of 30.09 MJ/kg, when the hydrogenolysis reaction was carried out at 290℃ for 3h over CuNiAl catalyst, in ethanol-phenol solvent (phenol/lignin ratio of 0.8). This study could provide a beneficial reference for the hydrogenolysis of lignin over heterogenous catalyzed systems in complex solvent. Keywords: lignin; in-situ hydrogenolysis; complex solvent; bio-oil

Introduction Lignin is an abundant aromatic biopolymer in nature, accounting for 15-30 wt.% of the lignocellulosic biomass and about 40% of its energy content, which is regarded as a promising, renewable and sustainable source of aromatic chemicals

1-3

. Lignin

has already been applied as biomass-based starting materials in the field of manufacture of dispersants, polyurethane foams, and so on 4. Apart from those above applications, lignin is also a promising source of aromatic compounds. However, lignin is a nonlinearly and randomly linked polymer, containing ether linkages such as β-O-4 (aryl ether), α-O-4 (aryl ether), and 4-O-5 (diaryl ether), etc., which is highly recalcitrant toward depolymerization due to its compact network structure, making it difficult to the effective conversion to aromatic chemicals and liquid fuels

5-7

.

Eventually, lignin is always left as residual waste after the utilization of cellulose and hemicelluloses of biomass. However, lignin is a potential alternative resource for fossil energy with higher sustainability 8, which could be valorized into high-valued biofuels and/or chemicals under suitable catalytic systems. Various thermochemical methods for the effective depolymerization of lignin


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into liquid fuel (named bio-oil) or chemicals have been investigated, including pyrolysis

9-11

, hydrolysis

12-16

, and hydrogenolysis

14, 17-20

. Among those methods,

pyrolysis process has been successfully confirmed to be uncontrollable, leading to inevitable recondensation as it always involved different kinds of free-radical reactions

9-11

. Hydrolysis process, mainly catalyzed over either acidic or basic

catalysts (e.g. H2SO4, NaOH ), was always accomplished through the hydrolytic cleavage of the aryl-alkyl ether bonds in lignin, which would also lead to repolymerization into either oligomers or char due to the presence of reactive phenolic monomers 15-16. Hydrogenolysis of lignin has exhibited to be effective and feasible for the conversion of lignin to liquid fuel or other high-valued chemicals. However, the hydrogenolysis process is always in need of supplying external hydrogen and recycling and/or purification of those unreacted hydrogen gas, which would add to the cost for the conversion 18-20. The in-situ hydrogenolysis of lignin has been extensively studied recently using hydrogen-donor solvents (such as ethanol, 2-propanol, formic acid, etc.) as internal hydrogen source, with the combination of heterogeneous catalysts and acidic/basic catalysts (e.g. Pd/C-CrCl3, Ni/ZSM-5-NaOH, Ru/C-NaOH) 21-23

. The in-situ hydrogenolysis process does not need the supply of extra hydrogen to

the reaction system, the hydrogenolysis could take place utilizing the hydrogen generated from hydrogen-donor solvents. Although the synergistic effect between the metal and acid and/or base sites could improve the lignin depolymerization, the application of homogeneous catalysts was not sustainable due to the separation, recycling and environmental problem. Recently, the combination of supported metal



and solid-acid/base catalysts was reported to be possible and could provide promising alternatives for those homogeneous catalysts

24-26

. In comparison with solid-acid and

solid-base catalysts, previous studies indicated that acidic support would suppress the lignin depolymerization and lead to the condensation of reactive intermediate products

22

. In comparison, solid-base catalysts exhibited better catalytic activity

during the lignin hydrogenolysis process. Ethanol was reported to be suitable solvent for the hydrogenolysis of lignin

26-28

, and better depolymerization activity of

ZrO2-based solid-base catalysts was observed during the hydrogenolysis of lignin in supercritical ethanol 26. In this work, in-situ hydrogenolysis of alkali lignin was investigated in supercritical ethanol over two non-precious metal based catalysts (CuNiAl hydrotalcite-based catalyst and Ni/ZSM-5), in comparison with expensive Ru/C catalyst. As lignin is a complex biopolymer mainly consisting of syringyl, guaiacyl, and p-hydroxyphenyl units, so herein, phenol was chosen as the simplest structural unit and firstly introduced to the hydrogenolysis system. Catalysts properties exhibited to have great effect during the lignin depolymerization process. Besides the catalysts, the promotion effect of phenol and the reaction parameters, such as reaction temperature and reaction time, were also studied in detail. Promotion effect of phenol and reaction temperature were found to have significant effect on the hydrogenolysis of alkali lignin, and the properties of obtained bio-oil (including molecular weight, elemental composition and its HHV) improved with the addition of phenol during the hydrogenolysis process.


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Experimental Materials and catalysts preparation In this study, alkali lignin was purchased from TCI Chemical Reagent. Other reagents were all analytic purity grade from local Sinopharm Chemical Reagent, which were directly used as received. HZSM-5 (SiO2/Al2O3=38) powder was purchased from Nanjing refinery Co., Ltd., and was calcined at 823 K for 4 h before use. 10wt%Ni/ZSM-5 (SiO2/Al2O3=38) and CuNiAl hydrotalcite-based catalyst (metal ratio of 1.2:4.8:2) were prepared by impregnation and coprecipitation methods respectively, which were reported in our previous studies 29, 30. All catalysts were loaded in a tubular reactor and reduced under H2 before use. 5% Ru/C was purchased (Sigma-Aldrich, China) and used as received. In-situ catalytic hydrogenolysis of alkali lignin In-situ catalytic hydrogenolysis of alkali lignin was conducted in a 100 mL high-pressure reactor. In a typical experiment, 1.0 g lignin, 0.5 g catalyst and 30 g phenol-containing ethanol (lignin-to-solvent mass ratio of 1:30) were loaded into the reactor. Varied amount of phenol was added to the hydrogenolysis system to study the promotion effect of phenol. The hydrogenolysis reaction was carried out between a temperature range of 230-310℃ to evaluate the effect of reaction temperature, for a reaction time of 1-10h to evaluate the effect of reaction time. The reaction systems were almost all under supercritical conditions in ethanol (temperature> 240 ℃ and pressure> 6.2 MPa), apart from the reaction carried out at 230℃. In addition, different reaction atmosphere (such as nitrogen, hydrogen) were also taken into consideration.



Page 6 of 32

The air in the reactor was removed by purging hydrogen/nitrogen into the reactor three times or more, and then the pressure in the reactor was released to normal pressure. Then the reactor was heated to the desired temperature using an electrical furnace and the stirring speed was set to 600 rpm. After the reaction, the reactor was quickly cooled down. The gaseous products were vented to the atmosphere, and the liquid and solid products were collected. The solid residue stuck on the reactor was washed with acetone and then the mixture was separated by filtration. After separation, the filtrate was subjected to a rotary evaporator at about 60℃ to remove ethanol and acetone, followed by drying in a vacuum oven at 80℃. The dried liquid product was named as bio-oil (BO). The filter cake was dried at 100℃, and weighted. The weight of solid product residue, which was denoted as solid residue (SR), was estimated by subtracting the weight of catalyst charged from the weight of the dried filter cake. The BO and SR yields were calculated by the weight percentage of the weights of BO and SR products in relation to the dry weight of lignin, respectively. The results reported in this work were all average values of two or three runs. Standard deviation values were shown as errors in Tables. The product yields were calculated using the following equations: YBO ( wt %) =

WBO × 100% Wlignin

(1)

YSR ( wt %) =

WSR × 100% Wlignin

(2)

Where WBO, WSR and Wlignin, depict the weight of bio-oil, solid residue, feed lignin. The mass balance was evaluated by the sum of bio-oil and residual solid in comparison with feed lignin, which was all above 91.8%, which was calculated using the following equations:


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Mass balance (%) =

W BO + WSR × 100% Wlignin

(3)

Characterization of lignin, lignin-derived products and catalysts GPC analysis was carried out on a Waters 1515 instrument to determine the molecular weights of lignin and bio-oil, where tetrahydrofuran and polystyrene were used as solvent and internal standard, respectively. Thermo Fischer Flash EA 1112 series CHNS-O elemental analyzer was used to determine the C, H, O, N and S composition of bio-oil. BET surface area and pore size measurements were conducted by N2 adsorption/desorption isotherms using a Micromeritics ASAP-2020 instrument. The XPS measurements were performed on an ESCALAB-250 (Thermo-VG Scientific, USA) spectrometer with Al Kα (1486.6 eV) irradiation source. Temperature programmed desorption studies (NH3-TPD and CO2-TPD) were carried out using a Micromeritics 2920 instrument with a thermal conductivity detector (TCD). SEM analysis was carried out to measure the surface morphology of catalysts using Hitachi S-4800 field emission instrument. The spectrometer microstructure of the catalyst was examined with TEM (Tecnai G2 20). The metal compositions of fresh and spent catalysts were detected by ICP-AES with a Perkin-Elmer OPTIMA 3300 DV spectrometer (Norwalk, CT, U.S.A.). The fresh and used catalysts were characterized by XRD on a Bruker D8 Advance X-ray powder diffractometer. TGA experiments of spent catalysts were performed on a PerkinElmer’s analyzer under air atmosphere.

Results and discussion



Page 8 of 32

Feedstock characterization The elemental composition of alkali lignin used for the following depolymerization reaction was presented in Table 1. The C, H, O, N and S contents of alkali lignin were 46.42, 4.71, 45.09, 0.08 and 3.70%, respectively. As can be seen in Table 1 that, the alkali lignin used herein had a much higher O content with an O/C ratio of 0.73, and a low higher heating value (HHV) of 17.94 MJ/kg. Additionally, this kind of lignin contained a relatively large amount of sulfur impurities (up to 3.70%). All the above indicated that some necessary upgrading processes were needed for the higher value application of lignin resource. Table 1 Elemental composition and HHV of alkali lignin Elemental composition (wt.%) HHVa (MJ/kg)

Feedstock

Kraft lignin

C

H

O

N

S

H/C

O/C

46.42

4.71

44.57

0.6

3.7

1.22

0.72

18.02

a-HHV (MJ/kg) = (34C+124.3H+6.3N+19.3S-9.8O)/100, where C, H, N, S, and O are the weight percentages of carbon, hydrogen, nitrogen, sulfur, and oxygen [13]

Screening of catalysts on catalytic hydrogenolysis of lignin in ethanol The effect of catalysts on the products distribution during the lignin hydrogenolysis process in ethanol was investigated over three kinds of catalysts, with a reaction time of 4 h at 250, 270, 290 ℃, respectively. The results exhibited that both reaction temperature and catalysts had significant effect on the yields of bio-oil and solid residue. It could be seen in Table 2 that the reaction would give lower bio-oil yield and higher solid residue amount, when the depolymerization of lignin was carried out without catalysts; and the increased reaction temperature would lead to a little higher bio-oil yield, which confirmed that the depolymerization of lignin ACS Paragon Plus Environment

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might be closely related with temperature. Further, it could be found that different catalysts

exhibited

varied

catalytic

hydrogenolysis

activities

during

the

depolymerization process. The solid residue yields were much higher when the hydrogenolysis reaction was carried out over either Ru/C or Ni/ZSM-5, as it was reported that acidic support (such as zeolite and C) would lead to the condensation of reactive intermediate products or lignin on the catalyst surface during the depolymerization process

31-33

. As hydrotalcite-based catalysts possessed stronger

alkalinity than the other two catalysts, so it could suppress the condensation reaction on the catalyst surface, and then give much lower solid residue amount. The results suggested that base sites could inhibit the recondensation of those reactive compounds in lignin-based bio-oil, and help to promote the hydrogenolysis of lignin. This phenomenon was also confirmed by Jae’s research, which indicated that MgO/ZrO2 could effectively improve the catalytic activity of Ru/C during the depolymerization of lignin

24, 26, 32

. Furthermore, the molecular weight of bio-oil in Table 2 decreased

obviously with the rising temperature and the application of varied catalysts, which also confirmed that both temperature and catalysts could significantly affect the depolymerization

of

lignin.

The

results

indicated

promise

of

CuNiAl

hydrotalcite-based catalysts as possible alternatives to those unrecoverable homogeneous catalysts (eg. H2SO4, NaOH) for lignin hydrogenolysis. So herein, CuNiAl hydrotalcite-based catalysts (metal ratio: 1.2:4.8:2)

was chosen for the

further study in the following experiment as it exhibited much better catalytic activity during the hydrogenolysis process.



Page 10 of 32

Table 2 Product yields and molecular weights of bio-oil obtained from in-situ hydrogenolysis of alkali lignin over different catalysts in ethanol Reaction

Product yield (wt.%)

Molecular weight

Catalyst conditions

BO

SR

Mn(g/mol)

Mw(g/mol)

/

/

/

~5500

~10000

None

30.8(±2.9)

65.6(±3.5)

1170

6230

5% Ru/C

35.4(±2.3)

59.4(±2.8)

950

5680

20% Ni/ZSM-5

37.5(±1.9)

55.6(±3.6)

970

5470

CuNiAl-HT

49.5(±2.8)

45.8(±2.1)

940

5080

None

32.5(±2.2)

60.3(±3.1)

980

5580

5% Ru/C

37.9(±3.1)

57.1(±3.0)

890

4890

20% Ni/ZSM-5

38.8(±1.3)

54.2(±2.3)

930

4580

CuNiAl-HT

54.5(±3.1)

42.5(±3.1)

905

4050

None

34.5(±1.9)

57.3(±3.4)

850

4310

39.4(±3.5)

55.4(±2.8)

780

3770

20% Ni/ZSM-5

39.5(±4.1)

56.5(±3.8)

810

3680

CuNiAl-HT

62.2(±3.5)

37.4(±2.6)

790

3250

Lignin

250℃,4h

270℃,4h

5% Ru/C 290℃,4h

Reaction conditions: 1g lignin, 30g solvent, 4h

Effect of the proportion of phenol on the depolymerization of lignin Lignin was a complex polymer of phenolic groups, mainly including syringyl, guaiacyl, p-hydroxy-phenyl, so herein, phenol was chosen as the simplest phenolic compound, to study the promotion effect during lignin depolymerization. The effect of phenol mass fraction in the lignin in-situ hydrogenolysis process was studied and the results were exhibited in Table 3. It could be seen that the depolymerization ACS Paragon Plus Environment

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process was not very effective in the absence of phenol (with a bio-oil yield of 62.2%). There was an obvious improvement in bio-oil yield and reduction of solid residue yield with the increase of phenol mass fraction during the reaction, and the following two reasons mainly contributed to the above improvements: for one thing, phenol could promote the solubilization of both lignin and reaction products in the phenol-containing

reaction

system,

which

would

definitely

improve

the

depolymerization efficiency. For another thing, phenol would act as capping agent during the hydrogenolysis of lignin, which would both favor the formation of mono-phenol compounds and suppress the re-polymerization of those reactive intermediates formed in the depolymerization process. As can be seen in Table 3 that, increased introduce of phenol into the lignin hydrogenolysis system could help to increase the bio-oil yield and decrease the solid residue yield. Furthermore, the Mn and Mw molecular weights of bio-oil both decreased obviously from 790 and 3250 to 640 and 2280 g/mol, respectively. And the optimal phenol to lignin ratio was inferred to be 0.8, because that phenol/lignin ratio could give both highest bio-oil yield and lowest solid residue yield. However, excessive phenol could also lead to condensation with other reactive aromatic intermediates, which could explain why the bio-oil yield did not exhibit a continuous increase at much higher phenol mass composition. As can be seen in Table 3 that, the solid residue yield increased with both reduced bio-oil yield and increased molecular weights in bio-oil, indicating recondensation happened in the reaction system. Similar results were also found in Maschietti’s research, indicating that



Page 12 of 32

phenol might react with 1-ring aromatic to form phenolic dimers or polymers 15. The results in Table 3 also confirmed the above phenomenon, indicating that molecular weights first decreased with small addition of phenol in the hydrogenolysis system (e.g. from a phenol/lignin ratio of 0.2 to 0.8), whereas at higher levels of phenol amount in the reaction system, the molecular weights increased (e.g. from a phenol/lignin ratio of 0.8 to 1.2). This suggested that the secondary re-polymerization reactions did occur between excessive phenol and other smaller reactive intermediates during the hydrogenolysis process in a phenol-containing system. Table 3 The effect of phenol proportion on the product yields and molecular weights Product yield (wt.%)

Molecular weight

P/L ratio BO

SR

Mn

Mw

0

62.2(±3.5)

37.4(±2.6)

790

3250

0.2

64.2(±3.0)

35.8(±2.2)

750

2910

0.4

66.0(±2.9)

33.4(±2.0)

690

2750

0.6

68.5(±2.8)

31.2(±2.3)

660

2580

0.8

72.3(±2.5)

25.4(±2.7)

640

2280

1.0

69.5(±2.3)

28.8(±2.8)

670

2480

1.2

66.3(±2.7)

30.4(±2.6)

720

2590

Reaction conditions: 1g lignin, 30g solvent, 290℃, 4h

The elemental composition and HHV of bio-oil obtained during the hydrogenolysis in ethanol with varied phenol amount might improve to different extent. Herein, due to varied phenol/lignin ratio, the improvement of elemental composition and HHV were not discussed in detail in Table 3. However, in the


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following study, it was found that phenol containing ethanol would not only decrease the solid residue yield, but also improve the hydrogen content and HHV of bio-oil (see Table 4, Table 5 and Table 6), where the hydrogenolysis reactions were carried out with a fixed phenol/lignin ratio of 0.8. The results confirmed that the hydrogenolysis would be promoted when phenol added in the reaction system. Finally, an optimal phenol/lignin ratio of 0.8 was proposed in the hydrogenolysis process aimed at decreasing the solid residue yield and improving bio-oil yield.

Effect of reaction atmosphere, reaction temperature and reaction time The lignin hydrogenolysis was also investigated under hydrogen and nitrogen atmosphere. The reaction results (including bio-oil yield, molecular weights, elemental composition and HHV) were exhibited in Table 4 over H2 and N2 atmosphere, in comparison of those results obtained from in-situ hydrogenolysis of lignin. The addition of external gas (either hydrogen or nitrogen) was found to have quite marginal promoting effect on the lignin hydrogenolysis, considering the bio-oil yield and HHV. Both bio-oil yield and HHV decreased when the hydrogenolysis carried out under external gaseous atmosphere, which indicated that the addition of external gas would not improve the lignin depolymerization reaction. Because ethanol could supply sufficient hydrogen during the lignin hydrogenolysis process under higher reaction temperature 26. When external gas was added into the reactor, then the reactive hydrogen amount from the ethanol would decrease due to the presence of other gases (either nitrogen or hydrogen). So it exhibited that in-situ hydrogenolysis of lignin could provide an alternative pathway for the utilization of lignin.



Page 14 of 32

Table 4 The effect of phenol proportion on the product yields, molecular weights and HHV Product yield (wt.%)

Molecular weight

Elemental composition (wt.%)

HHV

Bio-oil

SR

Mn

Mw

C

H

O

N

S

In-situ

72.3(±2.5)

25.4(±2.7)

640

2280

49.47

12.92

37.53

0.08

0

29.21

N2

66.1(±1.9)

31.6(±2.1)

700

2580

47.26

11.24

41.02

0.28

0.2

26.08

H2

70.3(±2.3)

26.8(±2.0)

670

2470

47.5

11.9

40.25

0.17

0.18

27.04

Gas

Reaction conditions: 1g lignin, 30g solvent (phenol/lignin ratio of 0.8), 4h

The influence of reaction temperature on the in-situ hydrogenolysis of alkali lignin was investigated between a temperature range from 230 to 310℃. It could be concluded from Table 2 and Table 5 that lignin hydrogenolysis process was highly dependent on reaction temperature. Wherein, the bio-oil yield increased from 41.2% to 71.3% with a gradually decreased molecular weight. As can be seen in Table 5 that, the bio-oil yield increased obviously with the elevated temperature, a sharp increase was observed between the temperature of 250 to 290℃. Simultaneously, the amount of solid residues decreased from 51.4% to 25.4%, which was probably promoted by the change of solvent status from normal to supercritical

33, 34

. The reaction system

was of better diffusion, dissolution and activity in supercritical state, which would promote the cleavage of ether bonds during the lignin depolymerization process, due to the acidity and basicity of solvents in its supercritical state

33-35

. To be specific,

when ethanol used as solvent under normal temperature, it always possessed a certain number of hydrogen bonds. However, when it was under a supercritical state, the amount would decrease sharply and most of ethanol molecules would exist as single molecules in solvent, which would then improve the diffusion and the occurrence of


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depolymerization reaction. The interaction between supercritical alcohol solvents and those polar groups of lignin could also improve the solubility of lignin, then facilitate the hydrogenolysis over the catalysts 33, 36. Similar results were also observed in Xu’s research with methanol as solvent, which also indicated that supercritical solvent could act as a reactant to realize the alcoholysis and alkylation during the lignin depolymerization

33

. Apart from reaction temperature, catalyst also played an

important role during the depolymerization process. In comparison with reaction results of blank experiment (without catalyst) in Table 5, there was an obvious increase in solid residue yield with much lower bio-oil yield. Additionally, the molecular weights of bio-oil obtained without catalyst were also relatively higher in comparison with the results catalyzed by CuNiAl. The above results both indicated that CuNiAl catalyst could not only improve the bio-oil yield, but also improve the depolymerization of those macromolecular compounds to small molecule compounds. Furthermore, when the hydrogenolysis was carried out at higher temperature, much larger amount of ethanol would reform into hydrogen-rich gaseous products, which was previously confirmed in Hensen’s research by analyzing the gas of the autoclave after reaction, indicating that higher hydrogen pressure facilitated the hydrogenolysis process and improved the bio-oil yield

31, 37

. Higher temperature

would also lead to higher reaction pressure in the system, due to ethanol and the increasing amount of hydrogen-rich gaseous products from its reforming during the reaction, which could be observed that the reaction pressure was up to 12MPa at 290℃. Higher hydrogen pressure would definitely facilitate the hydrogenolysis of



Page 16 of 32

lignin, as effective thermocatalytic and/or thermal cracking of those most recalcitrant fraction of lignin would be promoted. A higher bio-oil yield of 72.3% was obtained at 290℃ during the in-situ hydrogenolysis process, the temperature of which was much lower than Hensen’s research (up to 380℃) 37. Table 5 The effect of reaction temperature on the product yields, molecular weights and HHV Product yield (wt.%)

Molecular weight


Temp.

HHV BO

SR

Mw

C

H

O

N

S

Mn

230

41.2(±2.4)

58.8(±2.7)

750

2510

47.46

11.64

40.52

0.22

0.16

26.68

250

46.7(±2.1)

51.4(±2.4)

690

2450

47.79

11.85

40.09

0.15

0.12

27.08

270

62.8(±2.8)

36.6(±1.9)

660

2470

48.52

12.3

39.05

0.13

0

27.97

290

72.3(±2.5)

25.4(±2.7)

640

2280

49.47

12.92

37.53

0.08

0

29.21

310

63.5(±2.6)

33.8(±2.8)

670

2580

49.22

11.85

38.81

0.12

0

27.67

250a

34.8(±2.4)

58.4(±2.1)

980

4820

/

/

/

/

/

/

270 a

39.5(±2.2)

55.7(±2.3)

930

4230

/

/

/

/

/

/

290 a

43.8(±2.5)

51.4(±2.7)

840

3890

/

/

/

/

/

/

Reaction conditions: 1g lignin, 30g solvent (phenol/lignin ratio of 0.8), 4h; a-blank experiment (without catalyst)

Table 6 exhibited the effect of reaction time on the in-situ hydrogenolysis of lignin in ethanol with a phenol/lignin ratio of 0.8. The hydrogenolysis reactions were carried out over CuNiAl hydrotalcite based catalysts at a time range from 1h to 10h. It could be inferred that reaction time was also of great importance on both bio-oil and solid residue yield. An obvious promoting effect was observed during the lignin hydrogenolysis, when the reaction was carried out within three hours. Bio-oil yield increased from 61.2% to 81.8% with decreased solid residue yield and molecular ACS Paragon Plus Environment

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weight of bio-oil, within a prolonged reaction time from 1h to 3h. This result was in good accordance with Jae’s research

26

, whose studies also observed similar

phenomenon during the lignin depolymerization process. Apart from bio-oil and solid residue yield, molecular weight, elemental composition also showed a subtle improvement with increased HHV from 28.43 to 30.09 MJ/kg, indicating that the hydrogenation and/or hydrodeoxygenation during the lignin hydrogenolysis process would be enhanced with prolonged reaction time (herein from 1h to 3h). However, the molecular weight and solid residue yield tended to increase when the hydrogenolysis reaction was carried out over 3h. As it was observed during the reaction that the pressure of the reaction system steadily kept at around 10MPa, when the reaction time was over 3h, which might be inferred that the ethanol reforming reactions have already reached an equilibrium during the hydrogenolysis process, and no more ethanol could be reformed to generate hydrogen in the system. Therefore, there would be not sufficient reactive hydrogen in the reactor after 3h of hydrogenolysis. As a result, the re-condensation reactions would dominate over the in-situ hydrogenolysis reaction due to the lack of reactive hydrogen and the prolonged reaction time. When there was sufficient reactive hydrogen, those degraded products could be stabilized though hydrogenation and/or hydrodeoxygenation, then the repolymerization could be also prevented

33, 38

. Otherwise, the repolymerization

reactions would be dominant. The above phenomenon was also observed during the lignin depolymerization with formic acid using inexpensive supported Ni-based catalysts 31. Since there was no sufficient reactive hydrogen in the reactor after 3h of



Page 18 of 32

depolymerization, those re-condensation reactions happened for a prolonged reaction time (>3h), indicating that the lignin hydrogenolysis reaction might complete within 3h. Overall, the highest bio-oil yield of 81.8% with higher HHV and lower solid residue yield (30.09MJ/kg and 17.6%, respectively) could be achieved during this hydrogenolysis system at a reaction time of 3h. Table 6 The effect of reaction time on the product yields, molecular weights and HHV Product yield (wt.%)

Molecular weight


Time

HHV BO

SR

Mn

Mw

C

H

O

N

S

1h

61.2(±2.2)

33.8(±2.5)

680

2510

47.46

12.93

39.16

0.25

0.21

28.43

2h

73.2(±2.3)

24.4(±2.1)

630

2150

48.68

13.05

37.98

0.14

0.15

29.09

3h

81.8(±2.0)

17.6(±1.9)

610

1880

49.66

13.52

36.76

0.06

0

30.09

4h

72.3(±2.5)

25.4(±2.7)

640

2280

49.47

12.92

37.53

0.08

0

29.21

6h

63.5(±3.0)

34.9(±2.2)

690

2580

49.62

11.96

38.32

0.1

0

27.99

10h

55.2(±2.6)

42.4(±2.5)

770

2770

49.31

12.73

37.84

0.12

0

28.89

Reaction conditions: 1g lignin, 30g solvent (phenol/lignin ratio of 0.8), 290℃

Characterization of catalysts The XRD patterns of fresh and spent CuNiAl hydrotalcite-based catalysts (1.2:4.8:2) were comparatively presented in Fig. 1. In the XRD patterns, strong diffraction lines were detected at 37.4°, 43.4° and 62.3°, with weak diffraction at 75.2°and 76.4°. The characteristic diffraction peaks at 43.4° and 75.2° ascribed to the (111) and (220) crystal planes of CuNi alloy, respectively 39, 40, and the peaks at about 43.4° and 76.4° could be assigned to the diffraction of the (111) and (220) planes of metallic Ni. The diffraction lines at 37.4° and 62.3°corresponded to the spinel phases


Page 19 of 32 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


of NiAl2O4 41. And no obvious phase changes were observed after the hydrogenolysis reaction in the used catalyst, indicating that CuNiAl hydrotalcite-based catalyst was of good stability during the reaction. There was a very subtle decrease in diffraction intensity, which might be caused by the masking effect of the formation of carbon on the spent catalyst 31.

Fig. 1 XRD patterns of fresh and used CuNiAl catalysts

The surface element compositions of CuNiAl catalyst were analyzed by XPS. The survey scan and XPS patterns of Cu2p and Ni2p were presented in Fig. 2. The general spectra (Fig. 2-a) exhibited the presence of respective metals. As can be seen in Fig. 2-b, binding energies at 933.4 eV and 954.2 eV were corresponding to Cu0(2p3/2) and Cu0(2p1/2) respectively, indicating the presence of metallic Cu, and very subtle peaks corresponded to Cu2+ at about 936.2 eV and 956.1 eV were observed, indicating almost no presence of CuO on the catalyst surface. In Fig. 2-c, binding energies at 854.5 eV and 873.6 eV might ascribe to Ni0(2p3/2) and Ni0(2p1/2) respectively, and binding energies at 862.2 eV and 880.9 eV corresponded to the main



line of Ni2+(2p3/2), and Ni2+(2p1/2) , suggesting the presence of both metallic Ni and NiO on the surface.

Fig. 2 XPS patterns of CuNiAl catalyst: a-General spectra; b-XPS pattern of Cu2p; c- XPS pattern of Ni2p; d-Recyclability for CuNiAl catalyst

Herein, the CuNiAl catalyst was used repeatedly for the hydrogenolysis reaction at 290℃ for 3h, to investigate the recyclability of catalyst. After each run, the catalyst was separated by centrifugation, washed two times with ethanol, then centrifugated, dried at 80℃ and finally reused for the next run. The results in Fig. 2-d demonstrated that there was no obvious catalyst deactivation after the catalysts reused twice, as there was very subtle decrease of bio-oil yield. The XRD spectra of fresh and used catalysts in Fig. 1 also confirmed the good stability of catalyst, without the presence of obvious phases change.


Page 20 of 32

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The BET specific surface areas, and catalysts acidity and basicity of the three kinds of catalysts, fresh and used CuNiAl catalyst were measured and presented in Table 7. As exhibited in the table, CuNiAl catalyst possessed relatively larger mean pore size, in comparison with the other two catalysts from the BET results, which might lead

to some

improvements in catalytic

performance during the

depolymerization process. Herein, it was only a speculation that larger pore size might facilitate the depolymerization of lignin, and still deserved further study. To our knowledge, very few studies reported the diffusion effect during the lignin depolymerization. It was reported that MgO/ZrO2 promoted Ru/C was of larger pore size than Ru/C (19.3 and 5.6 nm, respectively), which also exhibited better activity 26. This might be an evidence of the effect of diffusion effect for the lignin depolymerization. Apart from the diffusion effect, the acidity might of much greater influence on the catalytic activity. As exhibited in Table 7, Ru/C and Ni/ZSM-5 catalysts possessed certain amount of acidity, while CuNiAl catalyst was basic, which have led to great influence during the lignin depolymerization. Acidic catalysts exhibited much worse catalytic activity due to the formation of more coke and led to the condensation of reactive intermediate products 22, 26. In comparison, basic CuNiAl catalyst showed better depolymeriztaion activity, due to the combination effect of metal promoted hydrogenolysis and basic sites in the catalyst. The synergistic effect of metal-catalyzed hydrogenolysis and base-catalyzed solvolysis during the depolymerization of lignin in supercritical ethanol was also confirmed in Joe’s research

26

, which stated that Ru/C showed lower catalytic activity during the



Page 22 of 32

hydrogenolysis process, and also confirmed that MgO/ZrO2 could effectively improve the catalytic activity of Ru/C 24, 26, 32. So herein, it was reasonable to see the improved catalytic activity in CuNiAl catalyst in comparison with the other two catalysts, due to its stronger alkalinity. Table 7 Chemical and physical properties of different catalysts Ru/C

Ni/ZSM-5(38)

CuNiAl1

CuNiAl2

548.2

169.6

188.3

172.8

Mean pore size (nm)

4.8

5.9

17.2

16.1

Average metal sizeb(nm)

6.8

22.5

14.4

15.3

Catalysts SBETa (m2/g) a

a-Evaluated from N2 adsorption-desorption isotherms; b-Calculated by TEM; 1-fresh catalyst; 2-reused catalyst

The characterization results of fresh and used CuNiAl in Table 7 showed a decrease in both surface area, average pore size (from 188.3 to 172.8 m2/g, from 17.2 to 16.1 nm, respectively) and mean particle size (from 14.4 to 15.3 nm), which might lead to the decrease of those catalytic activity sites in catalysts. The SEM images of fresh and used CuNiAl were shown in Fig. S1. The recycled catalyst showed relatively slight agglomeration to form spherical particles, which would lead to the decrease of surface area. The CO2-TPD analysis (Table S1) was carried out to further study the basic site change in the fresh and used CuNiAl catalysts. The results indicated that there was a certain loss of basic sites in used catalyst, with a CO2-uptake decreasing from 108 to 94 µmol/g. ICP analysis results (Table S2) showed that there were subtle decrease of both Cu/Al and Ni/Al ratio, from 0.59 and 2.38 in the fresh catalyst to 0.53 and 1.96 in the reused catalyst, indicating metal loss happened during the reuse. All these changes might lead to the decrease in catalytic


Page 23 of 32 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


activity during the lignin hydrogenolysis process, and further studies were still in need in the future. As coke deposition was one of the main reasons for the deactivation of catalyst and always occurred on the catalyst surface. Herein, the carbon content in the used catalysts was evaluated after the reaction. As exhibited in Fig. S2, Ni/ZSM-5 and Ru/C catalysts processed much more coke deposition after the hydrogenolysis reaction, in comparison with CuNiAl hydrotalcite-based catalyst. Those two catalysts exhibited much lower catalytic activity due to those coke formed and covered the active sites on the catalyst surface. The results indicated that acidic support/catalysts (herein Ni/ZSM-5 and Ru/C) would lead to the formation of coke deposition on the catalysts, which was also confirmed by previous studies

31-33

. Also, the results

suggested that CuNiAl hydrotalcite-based catalyst could be a good alternative for homogenous catalysts used in the depolymerization of lignin.

Conclusion In this work, in-situ hydrogenolysis of alkali lignin was carried out over Ru/C, Ni/ZSM-5, CuNiAl hydrotalcite-based catalyst at relatively mild conditions. Catalysts acid-base properties exhibited to have great effect on the hydrogenolysis activity. Among those catalysts, CuNiAl hydrotalcite-based catalyst exhibited better catalytic hydrogenolysis activity with higher bio-oil yield (up to 81.8%) and HHV (about 30 MJ/kg) in ethanol-phenol solvent, due to its stronger alkalinity. The promotion effect of phenol was obviously confirmed during the hydrogenolysis process, leading to improved bio-oil yield, HHV and decreased solid residue yield. Synergistic effects of



base-catalyzed

depolymerization,

metal-catalyzed

hydrogenolysis,

Page 24 of 32

and

phenol-promoted solvolysis were observed during the hydrogenolysis catalyzed by CuNiAl hydrotalcite-based catalyst in phenol-containing solvent. The obtained bio-oil (yield up to 81.8%) was of much lower molecular weight and higher HHV, when the reaction was conducted at 290℃ for 3h over CuNiAl catalyst, in ethanol-phenol solvent (phenol/lignin ratio of 0.8). The CuNiAl catalyst showed better catalytic activity and good application prospect for the utilization of lignin, which would provide possible alternatives for the depolymerization of lignin catalyzed by those homogenous catalysts (such as H2SO4, NaOH, etc.).

Acknowledgments Authors are grateful for the financial support from Fundamental Research Funds of CAF (No. CAFYBB2018QB007), National Natural Science Foundation of China (31700645), and the Natural Science Foundation of Jiangsu Province (BK20170159). Minghao Zhou (201703270013) would like to acknowledge the fellowship from the China Scholarship Council (CSC).

Supporting Information List of SEM images, acidity data of catalysts and so on. (PDF)

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Table of contents use only

Synopsis Promotion effect of phenol was confirmed during the in-situ hydrogenolysis of lignin catalyzed by CuNiAl catalyst in supercritical ethanol.


Page 32 of 32

Catalytic in-situ hydrogenolysis of lignin in supercritical ethanol: Effect

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