Efficient and Mild Transfer Hydrogenolytic Cleavage of Aromatic Ether

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Efficient and Mild Transfer Hydrogenolytic Cleavage of Aromatic Ether Bonds in Lignin-Derived Compounds Over Ru/C Haoran Wu, Jinliang Song, Chao Xie, Congyi Wu, Chunjun Chen, and Buxing Han ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02993 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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ACS Sustainable Chemistry & Engineering

Efficient and Mild Transfer Hydrogenolytic Cleavage of Aromatic Ether Bonds in Lignin-Derived Compounds Over Ru/C Haoran Wu,†,‡ Jinliang Song,*,† Chao Xie,†,‡ Congyi Wu,† Chunjun Chen,†,‡ and Buxing Han*,†,‡ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and

Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, No. 2 Zhongguancun North First Street, Haidian District, Beijing 100190, P.R.China ‡

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, 19A Yuquan Road,

Shijingshan District, Beijing 100049, P.R.China E-mails: [email protected]; [email protected]

ABSTRACT: Cleavage of aromatic ether bonds is crucial for the valorization of lignin and its fragments, which is challenging under mild condition because the bonds are very stable. Herein, we found that Ru/C could efficiently catalyze the cleavage of the aromatic ether bonds in various lignin-derived compounds via a transfer hydrogenolytic route using isopropanol as the hydrogen resource. Various lignin-derived compounds could be efficiently cleaved over commercial Ru/C to generate the corresponding aliphatic alkanes, aliphatic alcohols and aromatic derivatives under milder condition. Mechanism study indicated that the reaction occurred through the direct cleavage of aromatic ether bonds or the formation of the reaction intermediate cyclohexyl phenyl ether. KEYWORDS: Lignin-derived compounds, aromatic ether bonds, transfer hydrogenolytic cleavage, biomass conversion, isopropanol INTRODUCTION Transformation of lignin and its derived compounds into valuable chemicals and fuels is of great importance for sustainable future.1-6 As well-known, lignin and its derived compounds contain various

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kinds of aromatic ether bonds in their molecular structures.7 Therefore, efficient cleavage of aromatic ether bonds is a key step for the valorization of lignin and its derived compounds.8-10 However, cleavage of these aromatic ether bonds is difficult due to their high strength and stability.11 Among various strategies, hydrogenolysis is one of the most promising routes because of its high atom-economy of the overall reaction.12 In this aspect, some homogeneous or heterogeneous catalytic systems have been developed for the hydrogenolytic cleavage of aromatic ether bonds in the presence of H2, including Ni(COD)2 complex,13-15 Ni/SiO2,16 W-based catalyst,17 Cu-based porous metal oxide,18 and Pd-Fe bimetallic catalyst,19 etc. However, there are still some limitations, such as the separation and recycle of catalyst for homogeneous systems, harsh conditions (high temperature and H2 pressure) and low activity for heterogeneous systems. Therefore, exploration of efficient catalytic routes for hydrogenolytic cleavage of aromatic ether bonds under milder condition is highly desirable. Catalytic transfer hydrogenation (CTH) has been considered as an attractive alternative choice for avoiding the use of high-pressure H2 in biomass conversion.20 However, there have been only a few examples for the cleavage of aromatic ether bonds through CTH reaction. For example, Pd/C could catalyze transfer hydrogenolysis of lignin model compounds and organosolv lignin using formic acid or hemicellulose as a hydrogen source.21, 22 Paone and co-workers reported that transfer hydrogenolysis of benzyl phenyl ether could be efficiently conducted over Pd/Fe3O4 with a reaction temperature of 240 oC.23 Organosolv lignin could be transformed over Cu-based catalysts using supercritical methanol as the hydrogen resource.24, 25 Rinaldi and co-workers found that lignin could be converted into non-pyrolytic bio-oils over Pd/C with isopropanol as the hydrogen resource.26 Despite these achievements, development of highly efficient and milder catalytic systems for transfer hydrogenolytic cleavage of aromatic ether bonds in lignin-derived compounds is still very attractive.


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In the last few years, Ru-based catalysts have shown good performance in hydrogenolysis the aromatic ether bonds of lignin.27,28 However, harsh conditions such as high temperature or high H2 pressure still limits this reaction. Herein, we report the first work on the transfer hydrogenolytic cleavage of aromatic ether bonds over Ru/C using isopropanol as the hydrogen resource, and it was found that commercial Ru/C was an efficient catalyst for the transformation of various lignin-derived compounds to produce corresponding aliphatic alkanes, aliphatic alcohols and aromatic derivatives under milder condition.

RESULTS AND DISCUSSIONS Catalytic cleavage of aromatic ether bonds in diaryl ether, a model molecule of the 4-O-5 type linkage in lignin, is an important but challenging step for the valorization of lignin-derived compounds due to the high stability of the diaryl ether bonds. Therefore, diphenyl ether was selected as a model compound to study the transfer hydrogenolytic cleavage of aromatic C-O bonds with isopropanol as the hydrogen resource. Table 1 presents the catalytic activity of various catalysts for the transfer hydrogenolytic cleavage of diphenyl ether at 120 oC with a reaction time of 10 h. No reaction occurred in the absence of catalysts (Table 1, entry 1). Among the three examined carbon-supported metal catalysts (commercial Ru/C, Pd/C and Pt/C), Ru/C showed the best performance for the hydrogenolytic cleavage of diphenyl ether with a conversion of >99% to generate the corresponding cyclohexane, benzene, and cyclohexanol (Table 1, entry 2), and the total percentage of benzene and cyclohenxanol reached around 80%. In contrast, Pt/C (Table 1, entry 3) only provided 77.6% conversion of diphenyl ether, while only 4.2% conversion of diphenyl ether was achieved over Pd/C (Table 1, entry 4). Furthermore, a series of Ru-based catalyst (Ru/MoS2, Ru/TiO2 and Ru/SiO2) were also prepared to investigate the effect of catalyst support. The results showed that the conversion of diphenyl ether were 72.1%, 91.4%, and 63.5% ACS Paragon Plus Environment


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over Ru/MoS2, Ru/TiO2 and Ru/SiO2, respectively (Table 1, entries 5-7), indicating the key role of supports on the activity of Ru-based catalysts. In addition, Raney Ni was also examined for the cleavage of diphenyl ether (Table 1, entry 8). Although good conversion of diphenyl ether was obtained, Raney Ni easily lost its activity29 and was flammable when it touched the air, and these drawbacks limited the application of Raney Ni to some extent. Overall consideration, commercial Ru/C was selected for the transfer hydrogenolytic cleavage of diphenyl ether.

Table 1. Catalytic activity of different catalysts for the transfer hydrogenolytic cleavage of diphenyl ether.a O

+

Isopropanol 1a

1

Entry

Catalyst

OH

OH

Catalyst

Conversion (%)

+ 1b

+ 1d

1c

Yield (%)b

b

1a

1b

1c

1d

1

None

0

0

0

0

0

2

Ru/C

>99.0

16.3

33.2

49.5

0

3

Pt/C

77.6

21.6

14.9

32.7

4.3

4

Pd/C

4.2

0

2.0

0

2.1

5

Ru/MoS2

72.1

9.9

24.6

31.2

3.5

6

Ru/TiO2

91.4

17.2

27.4

45.3

0

7

Ru/SiO2

63.5

6.4

25.1

29.6

1.5

8

Raney Ni

94.7

12.7

32.5

46.3

0

a

Reaction conditions: diphenyl ether, 1 mmol; isopropanol, 4.0 g (66 mmol); reaction temperature, 120 oC (referred to

the constant-temperature air bath temperature); reaction time, 10 h; amount of catalyst, 0.05 g for Pt/C (10 wt% Pt), and 0.1 g for other catalysts. bThe conversion and yields were determined by GC using n-dodecane as a standard.

Reaction temperature was an important parameter for the overall efficiency of the reaction. As


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shown in Figure 1a, the major resultants were benzene and cyclohexanol. Besides, a small amount of cyclohexane and phenol were also generated in the reaction process. The conversion of diphenyl ether gradually increased from 80 to 120 oC. Meanwhile, the yield of benzene, cyclohexanol and cyclohexane also increased with the rise of temperature while the yield of phenol gradually decreased until disappeared. Full conversion (>99.0%) of diphenyl ether could be observed at 120 oC with a reaction time of 10 h, and benzene, cyclohexane and cyclohexanol were the final products. Additionally, when the temperature was increased to 160 oC, the final products were cyclohexane and cyclohexanol under the same reaction conditions except for the change of temperature. Furthermore, we also examined the influence of reaction time on the reaction efficiency at 120 oC (Figure 1b). Obviously, the conversion of diphenyl ether increased with the increase of time within 12 h. Meanwhile, the yield of cyclohexane and cyclohexanol also increased in the process while the change trend of phenol was opposite, which was attributed to that the generated phenol was converted into cyclohexanol. This was proved by our control experiment for the phenol transfer hydrogenation, which gave 99% yield of cyclohexanol under the reaction conditions shown in entry 2 of Table 1. Besides, the yield of benzene first increased and then gradually decreased with the rise of time. This phenomenon was due to that benzene was transformed into cyclohexane, which could be verified by the control experiment of the hydrogenation of benzene. In addition, Ru/C could be reused at least four cycles without decrease in conversion of diphenyl ether and the product yields (Figure 2).



Figure 1. Effect of reaction temperature (a) and reaction time (b) on the conversion of diphenyl ether. Reaction conditions: Ru/C, 0.1 g; diphenyl ether, 1 mmol; isopropanol, 4.0 g (66 mmol); reaction time, 10 h for (a). Reaction temperature for (b) was 120 oC (referred to the constant-temperature air bath temperature) and other conditions were the same as for (a). The conversion and yield were determined by GC using n-dodecane as a standard.

Figure 2. The reusability of Ru/C for the transfer hydrogenolytic cleavage of diphenyl ether. Reaction conditions: Ru/C, 0.1 g; isopropanol, 4.0 g (66 mmol); diphenyl ether, 1 mmol; reaction time, 10 h; reaction temperature, 120 oC (referred to the constant-temperature air bath temperature).

Encouraged by the excellent results of the transfer hydrogenolytic cleavage of diphenyl ether, we explored the possibility of transfer hydrogenolytic cleavage of other aromatic lignin-derived fragments


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over Ru/C with isopropanol as the hydrogen resource (Table 2). The results indicated that this transfer hydrogenolytic route showed satisfactory performance for the cleavage of aromatic C-O bonds in the examined chemicals. 4,4'-Ditert-butyl diphenyl ether could be completely transformed to afford the corresponding 4-tert-butyl cyclohexanol, 4-tert-butyl cyclohexane, tert-butyl benzene, and a small amount of tert-butyl phenol at 160 oC with a reaction time of 10 h (Table 2, entry 1). The cleavage of 4,4'-diethyl diphenyl ether occurred readily to form ethylbenzene, 4-ethylcyclohexanol and ethylcyclohexane with good yields at 180 °C (Table 2, entry 2). However, when the diphenyl ethers were substituted by methoxy group, the reactivity was affected significantly by the substituted position. Di-2-methoxyphenyl ether (Table 2, entry 4) had a higher reactivity than di-4-methoxyphenyl ether (Table 2, entry 3), and di-2-methoxyphenyl ether could even be almost fully converted without using sodium tert-butoxide. This result indicated that the ortho-substituted substrates had higher reactivity than the para-substituted ones. 4,4’-Dihydroxy diphenyl ether could also be converted to form cyclohexane and cyclohexanol, but the conversion was only 65.6% at 200 oC (Table 2, entry 5), indicating that the phenolic hydroxyl group decreased the reactivity.30 These results indicated significant influence of the substitute group on the reactivity of the diphenyl ether derivatives. Except for the diphenyl ether derivatives, this Ru/C-catalyzed transfer hydrogenolytic route was also suitable for phenyl methyl ether, which could be converted to form cyclohexanol and cyclohexane with 90.3% conversion at 180 oC (Table 2, entry 6). In our catalytic route, benzyl phenyl ether could be fully converted into cyclohexanol and methylbenzene with high yields at 120 °C with a reaction time of 10 h (Table 2, entry 7). Moreover, we found that the cleavage of phenethyl phenyl ether could readily occur in the presence of sodium tert-butoxide at 180 °C (Table 2, entry 8), while almost no reaction was observed in the absence of sodium tert-butoxide, indicating that base (sodium tert-butoxide) acted co-catalyst in the reaction process. Interestingly,



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α-phenethyl alcohol phenyl ether could be cleaved to form some corresponding chemicals with a conversion of 66.7% at 150

o

C in our catalytic system (Table 2, entry 9). Furthermore,

2-(2-methoxyphenoxy)-1-phenylethanol (a methoxy group ortho-substituted β-ether bond) could also be highly converted at 150 oC with a reaction time of 10 h (Table 2, entry 10). The results in entries 8-10 suggested that the reactivity of methoxy ortho-substituted β-O-4 compounds was higher than that of non-phenolic methoxy-substituted compounds, further indicating ortho-substituted methoxy group could assist

the

cleavage

of

β-ether

bond.

More

importantly,

more

complex

compound,

2-phenoxy-1-phenylpropane-1,3-diol, could also be converted with a conversion of 83.5% at 200 oC in the presence of sodium tert-butoxide as a promoter (Table 2, entry 11). These results in Table 2 proved that transfer hydrogenolytic cleavage of the C-O bonds was an efficient method for valorization of lignin-derived compounds to generate the corresponding aliphatic alkanes, aliphatic alcohols and aromatic derivatives.

Table 2. Cleavage of different aromatic ether substrates over Ru/C using isopropanol as the hydrogen resource.a Entry

Substrate (mmol)

t (h)

T (oC)

Con. (%)b

10

160

>99.0

Yields of major products (mmol)b

O

0.18

1

0.33

HO

(0.5 mmol)

0.47

O

0.24 12

2

180

0.28

>99.0 OH

(0.5 mmol)

0.44

H3CO

3

O

c

OH

OCH3

0.40 26

200

84.7 0.37

0.5 mmol

O

OH

0.04


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OCH3

OCH3

OH O

0.38

4

26

200

95.3

OH

0.41 OCH3

0.5 mmol

HO

O

0.13

OH

24

5

200

OH

65.6

0.5 mmol

OH

OCH 3

10

6

0.32

0.29

180

0.53

90.3

0.14 OCH3

(1mmol)

0.24

O

7

10

120

>99.0

OH

0.98

0.93

(1 mmol) O

8d

24

180

>99.0

OH

0.41

0.49

(0.5 mmol)

0.04 OH

0.13

O

9

10

150

66.7

0.07

OH

0.15

OH

OH

0.34

(1 mmol)

0.11 0.05

OCH3

OH

OH

0.16

O

0.39

10

10

150

91.4

OCH3

OCH3 OH

0.11

OH

(0.5 mmol)

0.15

0.19

HO

0.11 OH

11c

0.21

O

24

200

83.5

0.08

OH

O

0.03

0.5 mmol

0.08


0.07


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OH

0.04 a

Reaction conditions: isopropanol, 2.0 g (33 mmol) for entries 1, 2, 5, 8, 10 and 11, 3.0 g (50 mmol) for entry 3, and 4.

4.0 g (66 mmol) for other entries; Ru/C (5 wt% Ru), 0.05 g for entries 1, 2, 8 and 10, 0.1 g for other entries. bt=reaction time; T=reaction temperature (referred to the constant-temperature air bath temperature); Con.=Conversion. The conversion and yields were determined by GC using n-dodecane as a standard. c0.1 mmol Sodium tert-butoxide was added. d0.05 mmol Sodium tert-butoxide was added.

Understanding the reaction pathway is a key point for the valorization of lignin-derived chemicals. According to the above research, cyclohexane, cyclohexanol, benzene, and phenol were detected for the conversion of diphenyl ether. Therefore, we can propose possible reaction pathway based on the above results and the reported knowledge.31-34 Initially, a weak coordinative bond could be generated between the phenoxide group and the Ru nanoparticles through the free electron pairs of the oxygen atom,31 and subsequently, the aryl-O bond homolytically dissociated to form phenoxide and phenyl radicals,31,34 which could be hydrogenated to phenol and benzene by the active hydrogen from isopropanol on the surface of the Ru nanoparticles (Figure 3, Pathway I). The formed phenol and benzene were then converted into the final product (cyclohexane and cyclohexanol) by the formed active hydrogen from isopropanol (Figure 3, Pathway I). Except for the direct cleavage of aromatic ether bond, the active hydrogen could hydrogenate the benzene ring to form cyclohexyl phenyl ether (Figure 3, Pathway II) examined by GC-MS (Figure S1). Although the formed cyclohexyl phenyl ether could be converted through the cleavage of the C(sp2)-O bond or the C(sp3)-O bond, the cleavage of C(sp3)-O bond may be the reasonable route considering the higher bonding energy of aromatic C(sp2)-O bond than the alkyl C(sp3)-O bond.32,33 Therefore, cyclohexyl phenyl ether was converted to generate cyclohexane and phenol through the cleavage of the C(sp3)-O bond (Figure 3, Pathway II) on the surface of the Ru


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nanoparticles. Additionally, from the results in Table 1, the amount of benzene was much higher than cyclohexane, and thus, we could deduce that Pathway I may be the main reaction pathway. Based on the above discussion, a possible reaction mechanism for the transfer hydrogenolytic cleavage of aromatic C-O bonds in diphenyl ether over Ru/C was proposed as follows. Diphenyl ether was converted by direct cleavage of aromatic ether bonds to form benzene and phenol through the formation of a weak coordinative bond between the phenoxide group and the Ru nanoparticles and their subsequent hydrogenation31,34 (Figure 3, Pathway I, the main reaction pathway) by the active hydrogen from isopropanol or the formation of the reaction intermediate cyclohexyl phenyl ether (Figure 3, Pathway II), which was subsequently converted to cyclohexane and phenol through the cleavage of the C(sp3)-O bond on the surface of the Ru nanoparticles. Except for diphenyl ether, this reaction mechanism could be suitable for diphenyl ether derivatives (Table 2, entries 1-8) on the basis of the results in Table 2. However, more work should be done to explore the reaction mechanism for more complex substrates (Table 2, entries 9-11).

Figure 3. The possible reaction pathway for the transfer hydrogenolytic cleavage of diphenyl ether.

CONCLUSIONS In summary, we found that commercial Ru/C could effectively promote the transfer hydrogenolytic



cleavage of aromatic ether bonds in lignin-derived compounds with isopropanol as the hydrogen resource under relatively mild conditions. By tuning the reaction conditions, various lignin-derived compounds were efficiently transformed into corresponding aliphatic alkanes, aliphatic alcohols and aromatic derivatives. Mechanism study indicated that the reactions occurred through the direct cleavage of aromatic ether bonds or the formation of the reaction intermediate cyclohexyl phenyl ether. We believe that transfer hydrogenolytic reaction over Ru/C provides the new opportunity for the valorization of lignin and its fragements. More works should be conducted to design new catalysts (introducing supplemental metal components, using functional supports, and designing new catalysts with special structures) with high efficiency for transfer hydrogenolytic cleavage of the aromatic ether bonds in order to further make the reaction conditions milder.

ASSOCIATED CONTENT Supporting Information Experimental Section, GC-MS spectra of cyclohexyl phenyl ether. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mails: [email protected], [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21673249), the National Key Research and Development Program of China (2017YFA0403003), and Youth Innovation Promotion Association of Chinese Academy of Sciences (2017043).


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Synopsis: Commercial Ru/C could efficiently catalyze transfer hydrogenolytic cleavage of aromatic ether bonds in lignin-derived compounds under milder condition.


Efficient and Mild Transfer Hydrogenolytic Cleavage of Aromatic Ether

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