Fragmentation of Woody Lignocellulose into Primary Monolignols and

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Fragmentation of Woody Lignocellulose into Primary Monolignols and their Derivatives Jiankui Sun, Helong Li, Ling-Ping Xiao, Xuan Guo, Yunming Fang, Run-Cang Sun, and Guoyong Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Sustainable Chemistry & Engineering

Fragmentation of Woody Lignocellulose into Primary Monolignols and their Derivatives

Jiankui Sun,† Helong Li,† Ling-Ping Xiao, § Xuan Guo,‡ Yunming Fang,‡, Run-Cang Sun§ and Guoyong Song†,*

†

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, No.

35, Qinghua East Road, Beijing, 100083, People’s Republic of China § Center

for Lignocellulose Science and Engineering, Liaoning Key Laboratory of Pulp

and Paper Engineering, Dalian Polytechnic University, No. 1, Qinggongyuan, Dalian 116034, China ‡

National Energy R&D Center of Biorefinery, Beijing University of Chemical

Technology, No. 15, East Beisanhuan Road, Beijing 100029, People’s Republic of China

ACS Paragon Plus Environment

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Corresponding Author Email: [email protected]

KEYWORDS: lignin; molybdenum; reductive catalytic fractionation; monolignol; biomass

ABSTRACT: Lignin, which is biosynthesized through oxidative radical polymerization from primary monolignols during plant growth, represents the most abundant source of renewable aromatic resources. The search for efficient and selective catalysts for the production of monolignols and their corresponding unsaturated derivatives from the direct depolymerization of lignin is of great interest and importance, as such products are important platform chemicals for the synthesis of natural products, pharmaceuticals, and functional materials. We report herein the first case of a supported molybdenum catalyst that functions as an efficient and selective catalyst for the fragmentation of woody lignocelluloses, leading to monolignols and ethers in high yields with high selectivity. Hydrogenation of the side-chain and recondensation were not observed, suggesting that etherification acts as a new stabilization mechanism in the current Mo catalytic system. The (hemi)cellulose components were well preserved and are amenable to valorization via enzymatic hydrolysis and chemocatalytic conversion. This


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method constitutes an economically responsible pathway for lignin valorization as well as fractionation and sequential utilization of all of the biomass components.

Introduction Lignin represents a major component of lignocellulosic biomass (15-30% by weight and 40% by energy). Lignin is biosynthesized in the plant cell wall through oxidative radical polymerization from primary monolignols, i.e., p-coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S) (Figure 1).1,2 In most plants, the β-O-4 alkyl aryl ether linkages are predominant; these linkages result from the addition of the phenol group from one monolignol to the propyl chain of the second monolignol.3,4 In addition to possessing anti-inflammatory and antinociceptive activities,5 these monolignols bear a functional allylic alcohol specie, which has been evaluated as a lignin-derived platform chemical for the synthesis of natural products, pharmaceuticals, and functional materials.6-12 Depolymerization of lignin, one of the few renewable sources of aromatic chemicals, in principle, is recognized as the most straightforward and economical method for the production of monolignols and their corresponding derivatives and could


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reduce the cost of lignocellulose biorefinery.1,2 However, monolignols are not the direct products of common lignin depolymerization routes;12 they are prepared by conventional organic synthesis13 and are currently costly.14 Compared to carbohydrate components (cellulose and hemicellulose),15,16 the harnessing of lignin resources in a practical manner is still unexplored and is one of the foremost challenges12,17-23 due to the complexity and stability of the structures formed during biomass pretreatment.17,18 Recently, the reductive catalytic fractionation (RCF) of raw lignocellulose, enabling the fractionation of biomass components through the preferential fragmentation of the lignin component, has become a new biorefinery paradigm.17,18,24-33 Such processes usually utilize a heterogeneous metal catalyst, cleaving C−O bonds in β-O-4 units to produce monomeric phenols, where monolignols are initially formed.17,26 Generally, the loss of the chemical functionalities of monolignols

via hydrogenation and/or reductive deoxygenation of their end-chains is deemed to act as a stabilization mechanism against fast recondensation and may explain the high yields of phenolic monomers.17,25, 26,35,36 Late transition metal catalysts, such as Ru,24,32 Ni25-29 or the combination of Pd with a Lewis acid,30,31 produced 4-propyl substituted


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phenols due to their high activity for the reduction of C=C bonds and the deoxygenation of alcohol (Figure 1). When Pd/C was used as a catalyst, the chemoselectivity of the phenolic products could be tuned by hydrogen donor selection, enabling the formation of 4-propanol phenols via hydrogenation of monolignols under H232 or 4-propenyl phenols via alcohol hydrogenolysis under transfer hydrogenation conditions (Figure 1).33 Bimetallic catalysts, such as Ni-W2C/AC (AC: activated carbon), showed good activity towards both lignin and carbohydrate components, thus resulting in multiple products, including saturated phenols and a series of diols.34 An organocatalyst, B(C6F5)3, resulted in 4-propyl and 4-silyloxypropyl substituted aromatics when using hydrosilanes as reductants.35 To the best of our knowledge, monolignols were only observed in limited lignin depolymerization cases,26,29,37,38 and the production of monolignols in a controllable fashion has not yet been achieved.


5


MeO

lignin

primary monolignols

O

OH

O

p-coumaryl alcohol (H)

O

Ru/C Pd-Zn/C Ni/C

OMe

OH MeO

OMe

H2

O

MeO

HO



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-5 OH HO

MeO

MeO

OH

lignin peroxidase

HO coniferyl alcohol (G)

oxidative polymerization

HO

OMe

O

-O-4

HO

OMe

OH

HO

OMe

via "monolignols"

OH

HO

MeO

reductive depolymerization

O

MeO MeO

Pd/C, H2

OH OH

Pd/C EtOH/H2O

MeO

B(C6F5)3

[Si]O

OMe

MeO

MeO sinapyl alcohol (S)

O HO

OH

HO O O O

O[Si] O[Si]

OMe

[Si]

Figure 1. Overview of the reductive catalytic fractionation (RCF) of lignocellulose

The major challenge in the selective extraction of monolignols from raw lignocellulosic biomass is to find the proper balance between 1) lignin depolymerization and (hemi)cellulose preservation and 2) stopping at the monolignol (or derivative) stage and simultaneous stabilization. We have previously showed that supported molybdenum catalysts, such as MoOx/CNT (CNT: carbon nanotubes), can depolymerize enzymatic mild acidolysis lignins (EMALs) and biorefinery corncob lignin into monomeric phenols in high yields.39,40 In lignin model compound reactions, we observed the generation of coniferyl methyl ether (G2). These results encouraged us to examine whether the


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ability to easily cleave C−O bonds, together with the low hydrogenation activity of the molybdenum catalysts could promote the production of primary monolignols and their analogs from woody biomass. We report here our studies on the direct treatment of various hardwood and softwood sawdust biomass by the supported molybdenum catalysts, from which the lignin component is first removed from biomass, with well-preserved carbohydrate pulp. Moreover, the molybdenum catalysts enabled the formation of monolignols and their analogous allyl ethers in high yields with high selectivity, which is difficult to achieve with other catalysts. The reactivity of the lignin model compounds, including their deuterated analogous, was also examined, offering important insight into the mechanistic aspects of the catalytic process.

Results and Discussion

Monomeric phenols from eucalyptus sawdust. First, eucalyptus sawdust (particle size

ca. 0.5-1 mm) was treated in MeOH at 260 °C and 30 atm H2 for 4 h in the presence of a series of supported molybdenum catalysts, including the newly prepared MoOx/SBA-


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15 (for characterization, see Figure S1-S3 and Table S1, S2), MoOx/AC and MoOx/ASA (ASA: amorphous-silica-alumina). The lignin monomer yields, the selectivity, the degree of delignification and the retention of sugars in the carbohydrate pulp are summarized in Table 1 (see also Table S4 and Figure S5). Both the phenolic monomers yields and the selectivity were influenced by the catalyst supports. MoOx/CNT (Mo content: 4.8 wt %; 10 wt% dosage based on eucalyptus sawdust), which was previously reported to show high activity in the hydrogenolysis of isolated EMALs,39 was capable of fragmenting the lignin component of eucalyptus sawdust into monomers in 33.9 wt% yield. The carbohydrate pulp remained as an insoluble fraction with 96% sugar retention at a 91% degree of delignification (Table 1, entry 1). The identification and quantification of lignin monomers in the soluble fraction were assessed with GC-MS by comparison with authentic samples. The S/G ratio of the monomeric phenols was 2.0, which is similar to the syringyl/guaiacyl ratio of eucalyptus wood lignin.39,41 Primary monolignols, such as coniferyl alcohol (G1) and sinapyl alcohol (S1), as well as their methyl etherified analogs (G2 and S2), were identified as the main monomeric products with 91% selectivity based on a 33.9 wt% total monomers yield. Deoxygenation or hydrogenation at the


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side-chains of 1 and 2 was rare or even absent, which slightly differs from the MoOx/CNT-catalyzed hydrogenolysis of eucalyptus EMALs, where 4-propenyl and 4propyl substituted phenols were obtained via deoxygenation and/or hydrogenation.39

Table 1. MoOx/SBA-15-Catalyzed RCF of Eucalyptus Sawdusta

MeO Eucalyptus Sawdust

[Mo] (10 wt%)

(0.5-1.0 mm)

Entry

Catalyst

Solvent

MeO

G1, R = H G2, Me

OR

HO

+

HO

H2 (30 atm) 260 oC, 4 h

MeO

OR

soluble fraction

+ S1, R = H S2, Me

carbohydrate pulp

insoluble fraction

Phenolic monomers yield (wt%)b G1 G2 S1

Sugar retention Selectivity (wt%)e for 1 and Mw Delignificationc 2 (g/mol) (wt%) S2 S/G Total (mol%) C5 C6 total

1

MoOx/CNT

MeOH 1.7 8.4 4.3 17.3 2.0 33.9

91

820

91

90

98

96

2

MoOx/AC

MeOH 1.8 7.9 5.6 18.3 2.3 37.8

86

730

92

90

98

96

3

MoOx/ASA

MeOH 1.7 6.8 5.1 16.9 2.3 35.5

82

790

90

82

93

90

4

MoOx/SBA-15

MeOH 2.5 9.0 6.8 20.1 2.1 43.4

86

730

95(93d)

89

98

95

5f

MoOx/SBA-15

EtOH 1.4 2.7 14.9 7.6 3.5 34.5

70

800

86

86

95

92

6f

MoOx/SBA-15

iPrOH

60

920

72

90

98

96

7

MoOx/SBA-15

H2O

2.4

-

400

-

8

39

31

8

no

MeOH ND 0.4 0.3

1.7 9.5

4.2

72

1370

49

90

99

95

9g

no

MeOH 0.8 1.8 2.1

3.7 3.2 11.2

73

1330

90

83

96

91

MeOH 2.6 8.9 6.9 20.1 2.1 42.9

87

730

94

88

98

94

10h MoOx/SBA-15

1.0 2.0 5.6

1.9 3.6 16.8

ND ND ND ND 2.4

a

Reaction conditions: eucalyptus sawdust (500 mg), catalyst (50 mg, 10 wt%), methanol (15 mL), H2 (30 atm at RT), 260 °C, 4 h. bBased on the total lignin in eucalyptus. cBased on the weight of the dichloromethane extracted fraction (lignin oil) and the Klason lignin weight. dBased on total lignin by the chemical composition analysis of carbohydrate pulp according to the NREL method. eBased on the hemicellulose (C5) and cellulose (C6) components in eucalyptus by chemical composition analysis. fG2 and S2 refer to ethyl and isopropyl ethers. g Reaction time is 8 h. h A large scale reaction used 5 g of eucalyptus sawdust, 500 mg of MoOx/SBA-15 and 100 mL of methanol, and was performed at 260 °C under H2 (30 atm at RT) in a 160 mL batch reactor.

Further catalyst screening indicated that MoOx/SBA-15 (Mo content: 5.7 wt%; 10 wt% dosage based on eucalyptus sawdust) was superior to the other Mo catalysts, by which a


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combined yield of total phenolic monomers of 43.4 wt% of the original lignin was obtained with high selectivity (86%) for 1 and 2 (Table 1, entry 4, Figure 2d, see also Table S4). This monomers yield is close to the expected theoretical maximum monomer yield of native eucalyptus lignin (see also SI),35 and is comparable to the yields obtained by Ru24 and Pdcatalyzed30 RCF of wood resources (see also Table S5 and Figure S7), as well as the yields obtained by the catalytic hydrogenolysis of acetal-protected lignin by the reaction of aldehyde with the 1,3-diol unit on lignin side chains during biomass pretreatment.23,42 The RCF of eucalyptus sawdust was also carried out on large scale (5 g), which gave similar results in terms of monomer yields, selectivity, degree of delignification and sugar retention(Table 1, entry 10). Both treatments of coniferyl alcohol G1 with or without MoOx/SBA-15 in MeOH led to G2 in high yields (Figure S13), suggesting that etherification is a noncatalytic process and that the MoOx/SBA-15 catalyst has low activity towards deoxygenation or hydrogenation. Generally, phenolic compounds bearing an allylic alcohol specie are formed initially in lignin hydrogenolysis, which need to be stabilized through either hydrogenation of C=C double bonds,26,28 or dehydroxylation of γ-OH33 to prevent repolymerization. In the current Mo catalytic system, phenols having an unsaturated allyl ether substituent could be generated in an efficient and controllable fashion, suggesting that the etherification of monolignols can serve as an alternative stabilization mechanism, instead of hydrogenation and dehydroxylation. A 2D HSQC NMR spectrum of the lignin oil generated from the MoOx/SBA-15-catalyzed reaction is shown in Figure 2a. In the side-chain region of the spectrum, the Cα–Hα and Cβ–Hβ correlation signals of the substructure with ether bonds has disappeared, suggesting that most of the ether bonds in β-O-4 (A), phenylcoumarin (B) and resinol (C) have been dissociated. The correlation signals observed at δC/δH = 128.5/6.4, 127.9/6.2 and 62.2/4.1 ppm (labeled in pink)


10

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are ascribed to the Cα, Cβ and Cγ of compound 1, respectively. The Cα (δC/δH = 132.4/6.4 ppm), Cβ (δC/δH = 123.5/6.2 ppm), Cγ (δC/δH = 72.9/4.0 ppm) and OMe (δC/δH = 57.5/3.2 ppm) signals (labeled in red) from compound 2, were also observed. These results are in agreement with the GC/MS structural analysis. Gel permeation chromatography (GPC) showed a major signal ca. 220 g mol-1, suggesting a successful depolymerization mainly towards monomers (Figure 2e).

c

d MoOx/SBA-15 control reaction

S2 G2

1

S1 IS

2

2

G1

15

20

25

30

retention time (min)

S2,6

S`2,6

S2,6

S`2,6

G2

MoOx/SBA-15 control reaction

G2

G2

G 5/G6

220 g mol-1

e

S2,6

S`2,6

G 5/G 6

G5/G6

2-

2-

1 1 2-

2-

25

30

35

40

45

retention time (min) OH

OH HO HO



  O 4



HO  OMe

 O

5 

O

4

1

MeO

OMe

B Phenylcoumaran (-5)

OMe

 1

 O



 

A -Aryl ether -O-4)

MeO

C Resinol (-)

1

OH



 

OMe

2

Figure 2. Catalytic and control reactions for the conversion of eucalyptus lignocellulose. (a) HSQC NMR spectra of lignin oil from the MoOx/SBA-15-catalyzed RCF of eucalyptus (Table 1, entry 4). (b) HSQC NMR spectra of lignin oil from a 4 h control reaction using eucalyptus as a substrate (Table 1, entry 8). (c) HSQC NMR spectra of the oily product from the MoOx/SBA-15 catalyzed reaction using the 4 h control reaction lignin oil as a substrate (Scheme 1). (d) GC spectra of the lignin oils from the catalytic


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and 4 h control reactions. (e) GPC of lignin oils from the catalytic and 4 h control reactions. Parameter investigation. A range of solvents were screened with MoOx/SBA-15 (Table 1, entries 5-7, see also Figure S6). In alcoholic solvents, the degree of delignification and monomer yields are directly related to the solvent polarity.43 The reactions in EtOH (ET(30) = 51.9 kcal mol-1) and iPrOH (ET(30) = 48.4 kcal mol-1) resulted in phenolic monomers in 34.5 wt% and 16.8 wt% yields, wherein the selectivity for compounds 1 and 2 analogs decreased to 70% and 60%, respectively. Reduced degrees of delignification were also observed in EtOH (86%) and iPrOH (72%), while the sugar retentions remained high (92% and 96%, respectively). More polar alcoholic solvents, such as MeOH with a high ET(30) value (55.4 kcal mol-1), can promote the degree of delignification not only through penetrating the lignocellulosic matrix and extracting the lignin but also through the fragmentation of the lignin oligomers to produce monomeric phenols.44,45 When water, a more polar solvent with an ET(30) value of 63.1 kcal mol-1, was used as the solvent, a soluble fraction containing phenolic monomers in poor yield (2.4 wt%), and polyols from almost entirely carbohydrate components was generated. A possible explanation is that the high temperature allowed the autoionization of water into acidic hydronium ions (H3O+), which can catalyze the hydrolysis of (hemi)cellulose.46 Therefore, water is not a suitable solvent for lignin depolymerization and biomass component fractionation in the current case, which is in line with the results of using Ru and Pd catalysts.24,44


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The influences of catalyst amount, hydrogen pressure, reaction temperature and reaction time were investigated. A linear relationship between monomer yields and the Mo/sawdust ratio (wt/wt) was found during the first 4 h, where the selectivity for 1 and 2 remained constant, suggesting the crucial role of the Mo center in the current RCF process (Figure 3a, see also Table S6). In the H2 pressure profile, the formation of monomers is linear with the H2 pressure above 30 atm, and became independent of H2 pressure over 40 atm, probably because of the solubility of H2 in MeOH (Figure 3b, see also Table S7 and Figure S8).26 For reaction temperature and reaction time, parabolic trends were observed, and the highest yields of monomers were obtained at 260 °C and 4 h, respectively (Figure 3c, 3d; see also Tables S8, S9 and Figures S9, S10). Continued increase of the reaction temperature or prolonging the reaction time led to a drop in both the monomer yields and the selectivity, possibly due to severe condensation under harsh conditions.39

50

80

40

60 y = 0.7834x + 19.682 R2 = 0.9947

y = 0.5007x+17.628 R2= 0.9865

30

40

20

20

0

10 1.14

60

Selectivity to 1 and 2 (mol%)

100

(b)

(a)

2.28 3.42 4.56 Mo/sawdust (wt/wt) x1000

5.70

0

10

20

30

40

50

H2 pressure (atm)

(c)

100

(d)

50

80

40

60

30

40

20

20

10 240

260 280 Reaction temperature (oC)

300


Monomers yield (wt %)

60

Monomers yield (wt%)

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


0 2

4

6

8

Reaction time (h)

Figure 3. The influences of catalyst dosage (a), H2 pressure (b), reaction temperature (c) and time (d).


13


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Control reactions. Control experiments in the absence of catalyst were conducted for 4 h and 8 h. GPC data showed that the molecular weights of the lignin oils from the control reactions (Mn = 1370 g mol-1 for 4 h and 1330 g mol-1 for 8 h) were higher than that of the catalytic reaction (Mn = 730 g mol-1) (Figure 2e). In the 4 h control experiment, a poor monomer yield (4.2 wt%) and moderate delignification (49%) were obtained (Table 1, entry 8). By biomass compositional analysis, ca. 10 wt% hemicellulose was degraded and solubilized in MeOH, which is in line with that of the catalytic process (11 wt%), suggesting that MoOx/SBA-15 is a selective catalyst for the lignin component rather than the hemicellulose component. Analysis of the HSQC NMR spectrum of the lignin oil from the 4 h control reaction indicated that the lignin ether bond linkages, such as β-O-4, β-β and β-5, still remained (Figure 2b), suggesting that the solubilization of lignin fragments is faster than the full cleavage of ether linkages in the control reaction and that the Mo catalyst can accelerate the C–O bond cleavage. Since ether linkages were present in the soluble lignin oil, and the lignin component was retained in the insoluble fraction, we then treated the samples with MoOx/SBA-15 (Scheme 1). As a result, the ether bond linkages of both samples were completely cleaved (Figure 2c), thus affording monomeric phenols in a 15.2 wt% yield from the oil fraction and an 11 wt% yield from the solid residue, respectively. High selectivity (85%) for 1 and 2 was also observed (Figure S11). The 26.2 wt% combined yield is in between that of the 8 h control experiment (11.2 wt%) and the 4 h catalytic reaction (43.4 wt%), suggesting that lignin fragments with cleavable ether bonds are generated in the 4 h control experiment through a solvolytic process, accompanied by partial recondensation. In the 8 h control experiment, 90% of the lignin component was solubilized in methanol, with a 11.2 wt% monomer yield (Table 1, entries 9). Analysis of the oily product by HSQC NMR spectroscopy showed that the ether linkages completely disappeared (Figure S23). These results indicate that


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the lignin component can be decomposed and totally extracted by a single solvolysis process, which is in accordance with a previous report by Sels.26 After treatment of this oily product (containing 11.2 wt% monomers) with MoOx/SBA-15 at 260 ° C and under H2 (30 atm), no more phenolic monomers were generated (containing 11.1 wt% monomers), indicating that irreversible recondensation had occurred in the control reaction (Scheme 1). Based on the above results, it was deduced that the decomposition of the β-O-4 unit, recondensation and the etherification of monolignols are all involved in the Mo-catalyzed lignin hydrogenolysis. The Mo catalyst can promote the fragmentation of lignin into monomeric molecules, and what is more, it can suppress the recondensation reaction, thus leading to stable monomers. G1 + G2

Eucalyptus Sawdust

control reaction

+

S1 + S2

+

others

lignin oil

0.8%

4.4%

1.1%

6.4%

2.5%

solid residue

1.1%

1.2%

5.0%

2.3%

1.4%

4 h (4.2 wt%) lignin oil solid residue 8 h (11.2 wt%)

MoOx/SBA-15 MeOH H2 (30 atm) 260 oC, 4 h

combined monomers yields: 26.2 wt% n.d

2.4%

n.d

5.1%

3.0%

n.d

n.d

n.d

n.d

0.6%

combined monomers yields: 11.1 wt%

Scheme 1. MoOx/SBA-15-catalyzed hydrogenolysis of lignin oil and solid residue from the control experiments.

Lignin model compounds experiments. To gain further insight into the hydrogenolysis of lignin by MoOx/SBA-15 reaction pathway, we performed the reactions with β-O-4 model compounds (Scheme 2, eq 1). The reaction of the dimeric lignin model compound 3a with MoOx/SBA-15 at 200 °C afforded G2 (51%) and guaiacol (61%) in 3 h. No hydrogenation at the C=C double bond or deoxygenation at the γ-position were detected. This scenario is slightly different than the reaction of 3a with MoOx/CNT, in which 4-propenyl guaiacol was also


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generated via deoxygenation at the γ-position.39 In the case of the α-methoxylated model compound 3b, a proposed intermediate generated via nucleophilic attack of methanol at the benzylic position,26,47 Cβ–O bond cleavage gave G2 and guaiacol in 55% and 70% yields, respectively. These results are consistent with those of the hydrogenolysis of native lignin in eucalyptus wood. The treatment of the deuterated model compound 3a-D with MoOx/SBA-15 gave G2-D in 52% yield, in which 100% deuterium incorporation was observed at the α and β positions (Scheme 2, eq 2, see also Figure S16). This result suggests that the possible dimeric intermediates, such as enol ether generated from an α,β-dehydration reaction48,49 or Cα carbonyl compound generated via dehydrogenation,50,51 were not involved in the catalytic cycle (Scheme S1). The pathway involving the direct hydrogenolysis of 3a leading to 1,3-dihydroxypropylsubstituted guaiacol, followed by dehydration or demethanolization,25 can also be ruled out from the possible mechanisms (Scheme S1), since deuterium would be partially lost. In view of the formation of a C=C double bond, together with the preservation of the α and β deuterium atoms, we deduced that the Cα–O and Cβ–O bonds are cleaved simultaneously through a concerted process. OR MeO



 O



HO

OMe

MoOx/SBA-15 HO (10 wt%) MeOH, H2 (3 MPa) 200 oC, 3 h

OH

3a, R = H 3b, R = Me OH MeO

D

OMe O

D HO

OH 3a-D

MoOx/SBA-15 (10 wt%) MeOH, H2 (3 MPa) 200 oC, 3 h

OMe OH (1)

+ OMe G2 51% 55%

OMe 61% 70%

OMe HO

D

OH + OMe

D G2-D, 52%

(2) OMe 58%

Scheme 2. Fragmentation of lignin model compounds by MoOx/SBA-15


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Based on our results and previous reports, a proposed pathway for the MoOx/SBA-15catalyzed fragmentation of native lignin from lignocellulose was rationalized (Figure 4). The disentanglement of the lignin component through a noncatalytic solvolytic cleavage of lignincarbohydrates complexes (LCCs) releases soluble lignin fragments from the lignocellulosic matrix, during which nucleophilic substitution at the Cα position with methanol occurs to form an α-methoxylated β-O-4 intermediate.26 The subsequent β-O-4 bond cleavage gives the monolignols G1 and S1, which can be converted into their etherified analogs G2 and S2. The βO-4 bond cleavage is a redox process, and methanol or H2 may act as a reducing agent. In this procedure, it was supposed that Cα–OH (or Cα–OMe) and Cβ–OAr bonds are ruptured synchronously, probably through a concerted process, thus leading to the formation an allyl alcohol specie at the side-chain without the participation of protons at the Cα and Cβ positions. The Mo catalyst plays important roles not only in accelerating the cleavage of the β-O-4 structures but also in preventing the formation of active species that lead to recondensation. The poor hydrogenation ability of the Mo catalyst allows for the maintaining of C=C bonds in the allyl alcohol specie, and the following etherification stabilization mechanism may act as an alternative to reductive stabilization.26,28 HO MeOH

RO

Lignin O OMe

LCC

[Mo]

OH HO

OMe

Lignin Lignin hemicellulose

O

OMe HO

OMe

OMe

R = H, Me

soluble fragment

cellulose carbohydrate pulp (insoluble fraction)

Figure 4. Plausible pathway for the MoOx/SBA-15-catalyzed fragmentation of native lignin from lignocellulose.


17


Catalyst regeneration. MoOx/SBA-15 could be separated from the carbohydrate pulp by mesh screening or a simple gravity process, as described in the Supplemental Information. The direct use of spent catalyst (2nd) gave a decreased phenolic monomer yield (23.4 wt%), with 81% selectivity for 1 and 2 (Figure 5, see also Table S10). Physicochemical characterization of spent MoOx/SBA-15 showed an altered special surface area, suggesting that catalyst deactivation is the result of fouling of the Mo active sites (Table S2). Upon treatment of spent MoOx/SBA-15 at 400 °C under H2 (5%) flow conditions, the catalytic performance can be recovered, thus giving monophenols in 40.1 wt% yield in the 3rd run and 32 wt% yield in the 5th run (Figure 5 and Table S10). The XRD patterns showed no difference between the fresh and H2-treated spent catalysts (Figure S17). In the X-ray photoelectron spectroscopy (XPS) spectra, the characteristic peaks for the 3d5/2 of Mo4+ (229.4 ev) disappeared in the spent MoOx/SBA-15, but reappear after the H2thermal treatment (Figure S17). It should be noted that the Mo content was determined to be 5.2 wt% after the first H2-treatment (Table S1), and the relationship between the Mo/sawdust ratio and monomer yield (40.1%, 3rd) was fit to the linear equation in Figure 3a. 60

80

40 60 30 40 20 20

10 0


100

50

monomers yield (wt%)

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

Page 18 of 30

0 1

2

G1

G2

3

4

S1

S2

run times

5 others


18

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Figure 5. The results of recycling the MoOx/SBA-15 catalyst for eucalyptus sawdust. Reaction conditions: see Table 1, entry 4. The 2nd run used the recycled catalyst without regeneration. The 3rd, 4th and 5th runs used the H2-treated spent catalyst.

Utilization of carbohydrate pulp. A biomass compositional analysis indicated that almost quantitative cellulose and ca. 89% of the hemicellulose were retained in the carbohydrate pulp after MoOx/SBA-15 treatment, which could be readily separated with 93% and 76% retentions, respectively, through a basic treatment (see SI).52 The separated hemicellulose could be converted into xylose efficiently via an acid-catalyzed hydrolysis. This, together with the xylose solubilized during the Mo/SBA-15-catalyzed process, resulted in an 86% total yield of xylose based on the hemicellulose component of eucalyptus lignocellulose. Two parallel reactions, where the isolated hemicellulose was treated with or without MoOx/SBA-15, resulted in similar alterations in the polymeric structure of hemicellulose, confirming again that MoOx/SBA-15 has no effect on hemicellulose (Scheme S2 and Figure S21). The carbohydrate pulp and separated cellulose showed slight alterations in their crystallinity by XRD analyses (Figure S17), and both underwent typical enzymatic hydrolysis processes without any further treatment to give glucose in 88% and 92% yields, respectively (Figures S19 and S20). These results suggest that the Mocatalyzed fragmentation of lignin facilitated the valorization of the carbohydrate fractions. Overall, the treatment of eucalyptus wood with MoOx/SBA-15 leads to overall yields of 43.4% phenolic monomers from lignin, 88% glucose from cellulose and 86% xylose from hemicellulose.


19


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RCF of various woody biomass. Having these results in hand, we investigated the RCF of various fast-growing woody lignocelluloses by MoOx/SBA-15 (Figure 6, see also Table S12 and Figure S22). The hard woods, such as beech, birch, poplar and weeping willow, gave phenolic monomers in high yields (37-42 wt%), corresponding to high selectivities for 1 and 2 (81-88%). High degrees of delignification (89-91%) and carbohydrate retentions in the pulp (89-92%) were also realized. In the case of softwoods with low contents of cleavable linkages in the lignin,23,24 decreases in both the monomer yields and the delignification were observed, as seen in the case of cypress (14.5 wt%, 63%) and pine (12.7 wt%, 59%), while the carbohydrate retentions remained high (95% and 96%). Nevertheless, since only guaiacyl units exist in softwoods, the selectivities for the single compound G2 in cypress and pine reached 86% and 83%, respectively. phenolic monomers, delignification, sugar retention ch ee B

91%, 92%

h irc B

92%, 90%

ar pl Po

92%, 96%

ow

89%, 89%

ss re yp C

63%, 95%

ne Pi

59%, 96%

ng pi ee W

ill w

0

10

20

30

40

50

Monomers yield (wt%) G1

G2

S1

S2

others

Figure 6. RCF of various woody lignocelluloses by MoOx/SBA-15

Conclusions


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In summary, we showed that MoOx/SBA-15 can preferentially fragmentize the lignin component of woody lignocelluloses, producing monolignols and etherized derivatives in high yields with high selectivities for the first time. The carbohydrate pulp was compatible with the current Mo catalyst and can be quantitatively retained. The fragmentation of lignin facilitated the valorization of cellulose and hemicellulose, leading to high yields of glucose and xylose. The Mo-catalyzed RCF process is low-cost, highly efficient and selective; it preserves the carbohydrate pulp and is compatible with biomass feedstock, thus paving an economically profitable way for the fractionation and subsequent utilization of lignocellulosic biomass components.

ASSOCIATED CONTENT

Supporting Information. Detailed experimental procedures, chemical compositions of the woody lignocelluloses, additional GPC and GC chromatograms and NMR spectra, tabulated monomer yields and identification and quantitation of lignin monomers (PDF)

AUTHOR INFORMATION

Corresponding Author *EMAIL for G.S.: [email protected]


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Author Contributions G.S. conceived the idea and supervised the research. J.S. conducted the experiments and performed the characterizations. H.L. performed the model compounds reactions. L.P.X. and X.G. analyzed the 2D-HSQC NMR data. G.S. and J.S. wrote the manuscript. G.S., Y.F. and R.C.S. edited the manuscript. All of the authors commented on the manuscript during its preparation.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful for the National Natural Science Foundation of China (No. 21776020) and the Fundamental Research Funds for the Central Universities (No. 2018BLRD12) for their support of this research. G.S. thanks the National Program for Thousand Young Talents of China.

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Compounds

for

Understanding

Acid-Catalyzed

Lignin

Depolymerization:

Identification of Renewable Aromatics and a Lignin-Derived Solvent. J. Am. Chem. Soc. 2016, 138, 8900-8911. 50. Gao, F.; Webb, J. D.; Sorek, H.; Wemmer, D. E.; Hartwig, J. F., Fragmentation of Lignin Samples with Commercial Pd/C under Ambient Pressure of Hydrogen. ACS Catal. 2016, 6, 7385-7392. 51. Sun, Z.; Bottari, G.; Afanasenko, A.; Stuart, M. C. A.; Deuss, P. J.; Fridrich, B.; Barta, K., Complete lignocellulose conversion with integrated catalyst recycling yielding valuable aromatics and fuels. Nat. Cata. 2018, 1, 82-92. 52. Peng, F.; Ren, J.-L.; Xu, F.; Bian, J.; Peng, P.; Sun, R.-C. Comparative Study of Hemicelluloses Obtained by Graded Ethanol Precipitation from Sugarcane Bagasse. J. Agric. Food Chem. 2009, 57, 6305-6317.


29


Page 30 of 30

TOC

Woody Biomass

CO2

MoOx/SBA-15 High monophenols yield ( 43.4 wt%) High selectivity ( 86%) lignin W ell carbohydrate preservation ( 95%) f ragmentation Low-cost and recyable catalyst f iltration carbohydrate pulp

OH

OH OMe

O

HO HO

OH OH

glucose (88%)

OMe

+

OH

OH

MeO

OH

O OH OH

xylose (86%)

OR

R = H, Me

OR

monolignols and derivatives (43.4%)

The molybdenum-catalyzed RCF of woody lignocelluloses led to the selective formation of monolignols and their derivatives, along with well preserved (hemi)cellulose components that are amenable to valorization.


30

Fragmentation of Woody Lignocellulose into Primary Monolignols and

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