Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over

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Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over Carbon Nanotube Supported Molybdenum Oxide Ling-Ping Xiao, Shuizhong Wang, Helong Li, Zhaowei Li, Zheng-Jun Shi, Liang Xiao, Run-Cang Sun, Yunming Fang, and Guoyong Song ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02563 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Catalysis

Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over Carbon Nanotube Supported Molybdenum Oxide Ling-Ping Xiao†, Shuizhong Wang†, Helong Li†, Zhaowei Li†, Zheng-Jun Shi‡, Liang Xiao§, Run-Cang Sun†, Yunming Fang*# and Guoyong Song*†⊥

†

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing

100083, China ‡

University Key Laboratory of Biomass Chemical Refinery & Synthesis, Southwest Forestry

University, Kunming 650224, China §

Hunan Engineering Laboratory for Ecological Applications of Miscanthus Resource, Hunan

Agricultural University, Changsha 410128, China #

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

Beijing 100029, China ⊥

College of Chemical Engineering, Northeast Electric Power University, Jilin 132000, China

KEYWORDS: lignin, molybdenum, unsaturated phenol, hydrogenolysis, heterogeneous catalysis

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ABSTRACT Lignin represents the most abundant source of renewable aromatic resources and the depolymerization of lignin has been identified as a prominent challenge to produce low-molecular-mass aromatic chemicals. Herein, we reported a nanostructured MoOx/CNT, which can serve as an efficient catalyst in hydrogenolysis of enzymatic mild acidolysis lignins (EMALs) derived from various lignocellulosic biomass, thus giving monomeric phenols in high yields (up to 47 wt%). This catalyst showed high selectivity towards phenolic compounds having an unsaturated substitute, because the cleavage of C−O bonds in β-O-4 units is prior to reduction of double bonds by MoOx/CNT under H2 atmosphere, which was confirmed by examination of lignin model compounds reactions. The effect of some key parameters such as the influence of solvent, temperature, reaction time and catalyst recyclability were also examined in view of monomer yields and average molecular weight. This method constitutes an economically responsible pathway for lignin valorization, which is comparable to those precious metal catalytic systems in term of activity, reusability and biomass feedstock compatibility.

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Introduction Lignin is an abundant aromatic biopolymer accounting for 15 to 30 wt% of the lignocellulosic biomass, which consists primarily syringyl (S), guaiacyl (G) and p-hydroxyphenyl (H)-containing units.1 As one of the few renewable sources of aromatic chemicals, depolymerization of lignin is increasingly recognized as an important starting point for lignin valorization, because it could provide a source of low-molecular-mass aromatic chemical feedstocks suitable for downstream processing.2 Currently, the bulk produced lignins from biorefinery industry is underutilized, where it is burned as fuel.1b,3 The natural complexity and high stability of lignin bonds makes it the most recalcitrant of the three components of biomass (cellulose, hemicellulose and lignin), and breaking down lignin into monomeric compounds in high yields is extremely challenging.4 Hydrogenolysis of lignin (reductive method) is one of the most prevalent and efficient strategies to produce aromatic compounds from lignin, taking advantage of increasing the overall energy density and value of the resulting products in a narrow distribution form.2,5 Such processes usually use a heterogeneous catalyst, based on ether precious or non-precious metals.1,2 Precious catalysts typically include Ru,6 Pd,6a,7 Rh,6a,8 Pt,6a,8 Ir8 and Re.9 B. F. Sels reported Ru/C-catalysed selectively hydrogenolysis of native lignin in birch into 4-propyl-substituted phenols with 52% yields, with remaining of unreacted carbohydrate fraction quantitatively.6b Similar results were also reported by Agrawal and Abu-Omar by using a bimetallic Zn/Pd/C catalyst for depolymerization native lignins in poplar, birch, eucalyptus and pine.7a Very recently, Luterbacher and co-workers reported that lignins having acetal species, generating from the reaction of formic aldehyde with 1,3-diols unit on lignin

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side-chains during biomass pretreatment, could be converted to phenolic monomers at near theoretical yields (21-78%), in which Ru/C was used as a catalyst.6d From a techno-economic perspective, it is desirable to use catalysts based on cheap and abundant sources to replace precious metal.2c Nonprecious metal catalysts are mainly based on Ni,10 W10a,11 and Cu12. Previous works by Zhang,10a Wang and Xu10b and Abu-Omar10c independently used Ni/C as a catalyst for depolymerization of native lignin in birch wood, affording monophenols in yields from 17.5% to 54%, and varies in monomers yield may be due to natural chemical variation in lignins varied across regions and growing seasons.10c Li and Zhang reported W2C/C-catalyzed hydrogenolysis of lignins from corn stalk, poplar, basswood, pine and spruce, leading to alkyl monomers up to 12.7 wt% yield.11 Westwood demonstrated the reduction of Cα-oxidized birch lignin in the presence of stoichiometric zinc, where a phenol product was isolated in 5 wt% yield.13 In addition, an organocatalyst, B(C6F5)3 was also employed for reductive depolymerization of various lignins using hydrosilanes as reductants, leading to formation silyl-substituted phenol derivatives.14 β-O-4 linkages are the most abundant connections in native lignin, which have been a principal target for depolymerization of lignin despite these linkages will be degraded totally or partially during biomass pretreatment.15,16 Late transition metal-catalyzed hydrogenolysis of lignin often proceeds through C−O bonds cleavage of β-O-4 units, dehydration to form C=C double bonds and hydrogenation of C=C double bonds,10b thus giving 4-propyl substituted phenols preferentially because competing reduction of C=C double bonds occurs faster than C−O bonds hydrogenolysis.7b Recent results indicated that the selectivity of phenolic products having different substitutes can be tuned by choice of catalysts and/or

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additives. Sels and coworkers reported that the native lignin in birch wood could be converted to 4-propanol phenols selectively with Pd/C catalyst,7e in contrast with Ru/C-catalyzed reactions, which always gave 4-propyl phenols as predominant products.6b 4-Ethyl substituted phenolic compounds were also observed in lignin depolymerization with the use of combination of Pd/C and NaOH in MeOH by Sels and coworkers,7f and Pd/C in dioxane by Hartwig and coworkers,7b possible proceeding via loss of a formaldehyde molecule (C−C cleavage), hydrogenation and C−O cleavage. It has been reported that catalytic hydrogenation of unsaturated side chain may act as a stabilization mechanism against fast recondensation in depolymerization of lignin,10k therefore, efficient hydrogenolysis of lignins leading to unsaturated phenols is still rare. In 2014, Samec group described Pd-catalyzed transfer hydrogenolysis of lignin from pine and birch wood using an endogenous hydrogen source, affording 4-propenyl phenols in a selective version.7g Given that the versatile applications of monomeric phenols bearing an unsaturated substitute in chemicals and materials, the search of cheap and recyclable catalysts, which can cleave aryl C−O bonds rather than hydrogenation of

double

bonds,

is

great

of

interest

and

importance

in

lignin

hydrogenolysis/depolymerization. Molybdenum is a cheap and earth-abundant metal,17 and pioneering works have employed Mo-based bimetallic catalysts (such as NiMo18 and CoMo19) in lignin hydrogenolysis. In view of their activity to cleave C−O bond20 and less activity on hydrogenation of molybdenum centers, we envisioned that monometallic Mo catalysts might serve as unique catalysts for the hydrogenolysis of lignin. Herein, we report that multi-walled carbon nanotube supported molybdenum oxide nanoparticles (MoOx/CNT) can be served as an efficient catalyst for

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hydrogenolysis of various enzymatic mild acidolysis lignins (EMALs) derived from different lignocellulosic biomass, including willow (Tamarix ramosissima), poplar (Triploid of Populus tomentosa Carr.), Eucalyptus grandis, birch (Betula alnoides), pine (Pinus yunnanensis) and herbaceous crop (Miscanthus lutarioriparius). This protocol can afford a series of monomeric phenols having unsaturated substitutes in high yields. The activity of lignin model compounds with MoOx/CNT was also presented in this paper.

Results and discussion

Preparation and characterizations of the catalysts. We prepared the MoOx/CNT catalyst using (H3O)2(Mo6Cl8)Cl6·6H2O and multi-walled carbon nanotubes as the precursors through a facile solution-based method combined with a subsequent calcination process at 400 oC under N2 flow condition. The weight percentage of molybdenum element was estimated about 4.8 wt% according to ICP analysis (Table S1). EDS spectra show the presence of Mo, C and O elements in MoOx/CNT (Figure 1e). It should be noted that Cl has been disappeared by comparison with EDS spectra before and after calcination (Figure S2). Powder X-ray diffraction (XRD) patterns of MoOx/CNT display typical peaks at 2θ = 26.1°, 37.1°, 53.7° and 60.4°, which are readily indexed to (011), (020), (022) and (031) planes of a pure monoclinic MoO2 phase (JCPDS No. 32-0671) (Figure 1a). No characteristic peaks were observed for other molybdenum species such as Mo2C and MoO3. Therefore, the formation of MoO2 specie was supposed as in situ oxidation as followed: (H3O)2(Mo6Cl8)Cl6·6H2O → MoO2 + H2 + HCl.21 In X-ray photoelectron spectroscopy (XPS) spectra (Figure 1b), the characteristic peaks at 239.2 eV and 229.7 eV are attributed to the 3d3/2 and 3d5/2 of Mo4+ in MoO2,22 and 6


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the oxidation state of Mo6+ in MoO3 specie appears as the peaks ranged at 236.1 eV (3d5/2) and 231.5 eV (3d3/2), which was possibly resulted from the surface oxidation of MoO2 in air. The HRTEM images in Figure 1c and 1d show the MoO2 nanocrystallites were distributed on CNT, whereas a lattice spacing was determined as 0.34 nm, consistent with the d spacing of the (011) planes of MoO2.22 The brightest regions in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image indicate that Mo nanoparticles are uniformly distributed on the CNT support (Figure 1e). The MoOx/CNT catalyst shows a special surface area of 178.3 m2 g-1 and pore volume of 0.84 m3 g-1, which are slighter higher than those of neat CNT (107.1 m2 g-1 and 0.69 m3 g-1, respectively) (Table S2). (011)

(a)

(b)

MoOx/CNT

4+

Mo 3d3/2 Mo 3d3/2

Intensity (a.y.)

(031)

(020)

(022)

6+

Intensity (a.u.)

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JCPDS card no. 32-0671 (MoO2)

6+

Mo 3d5/2 4+

Mo 3d5/2

JCPDS card no. 05-0508 (MoO3)

10

20

30

40

50

60

70

80

240 238 236 234 232 230 228 226

2θ θ (degree)

Bingding energy (eV)

(d)

(c)

0.342 nm MoO2 (011)

100 nm

5 nm

(e)

HAADF

50 nm

C

O

Mo

C-O-Mo

Figure 1. Morphology and structure characterization of the MoOx/CNT: (a) XRD patterns; (b) XPS spectrum of Mo 3d; (c) and (d) high-resolution TEM images; (e) HAADF-STEM image and corresponding EDS elemental mapping of C, O and Mo. Scale bars in panel c, d, e were redrawn according to the magnification.

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Catalytic hydrogenolysis of willow T. ramosissima EMAL. To probe the potential of the MoOx/CNT as a catalyst for depolymerization of lignin, initially we chose a lignin sample isolated from willow (T. ramosissima) through the combination treatment of enzymatic hydrolysis and mild acidolysis extraction.23,24 Biomass compositional analysis indicated that total lignin content (acid soluble and insoluble lignins) is about 81% (Table. S3). Both S- and G-subunits were found in this EMAL with a ratio of 5.7:1 on the basis of 2D HSQC NMR spectra analysis (Figs. S6). The β-O-4 (A, labeled in blue), β-5 (B, labeled in green) and β-β (C, labeled in purple) linkages were also detected in this lignin sample (Figure 2a). Treatment of willow EMAL with 10 wt% of MoOx/CNT at 240 °C and 3 MPa H2 in MeOH for 4 h in an autoclave, a brown soluble oily product, corresponding to 63 wt% of the original lignin, was obtained after extraction with CH2Cl2 (Table 1, entry 1). Acetylation of the oily product and analysis by gel permeation chromatography (GPC) shows a significant decrease of molecular weight (MW = 740 g mol-1) relative to the initial EMAL (MW = 8120 g mol-1) (Figure 3, Table S4). A 2D HSQC NMR spectrum of the lignin oil was collected and it is showed as Figure 2b. The signals of Aα (δC/δH: 71.4/4.87 ppm), Aβ (δC/δH for G-units: 84.1/4.32 ppm, for S-units: 87.1/4.11 ppm) corresponding to benzylic alcohols and secondary alkyl protons of β-O-4 linkages respectively, almost completely disappeared, evidenced by comparing HSQC NMR spectra of the oily product with initial lignin sample (Figure 2). Additionally, the signals of Bβ (δC/δH: 53.8/3.47 ppm) and Bγ (δC/δH: 62.1/3.76 ppm) from phenylcoumaran linkages, as well as Cα (δC/δH: 85/4.7 ppm), Cβ (δC/δH: 55.3/2.81 ppm) and Cγ (δC/δH: 71.1/3.81, 4.20 ppm) from resinol linkages are no longer observed after catalytic

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reaction. These results suggested that almost all linkages in the willow EMAL have been dissociated under current reaction conditions through C−O bonds cleavage.

(a)

Methoxyl

A: 64% B: 8% C: 11%

(b)

B

B Methoxyl

C

C

A

A

B

A

B

A

C

C

A (G)

A (G) C

A (S)

A (S)

C

Figure 2. HSQC NMR spectra of (a) EMAL isolated from willow (T. ramosissima) and (b) lignin oily product after catalytic reaction (reaction conditions from Table 1, entry 1). The relative volume integrals of characteristic peaks are given in Figure S6.

2.0

260 oC (blue)

1.5

W (log M)

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240 oC (purple)

1.0 Willow EMAL (red)

0.5

0.0 5e

3

1e

4

5e

4

1e

5

Molar mass [D]

Figure 3. Molecular weight distribution of willow EMAL (red) and lignin oil after catalytic hydrogenolysis reaction at 240 °C (purple) and 260 °C (blue). Reaction conditions: willow EMAL (50 mg), MoOx/CNT (5 mg), MeOH (10 mL), H2 (3 MPa), 4 h.

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Figure 4. Gas chromatogram and peak identification of the lignin monomers from catalytic hydrogenolysis of willow EMAL over MoOx/CNT (reaction conditions from Table 1, entry 1). The identification and quantification of lignin monomers in the oily product were assessed on GC-MS, by comparison with authentic samples acquired from commercial purchase or independent synthesis (for details, see Supporting Information). The hydrogenolysis of willow EMAL with MoOx/CNT resulted a series of monomers, with a combined yield of 33 wt% based on the total lignin content (Table 1, entry 1). The detailed distribution of phenolic monomers is depicted in Figure 4 and Table S5. Both syringyl- and guaiacyl-derived phenols are detected with an S/G ratio of 4.5:1, slight lower than that of S/G monomer composition in the original lignin (5.7:1). High ratios of phenolic compounds having unsaturated substitutes (47.2% selectivity, Table S5), such as vinyl (G2, S2), propenyl (G3, S3) and allyl (S4), are a unique feature by MoOx/CNT catalyst, in contrast with late transition metal-catalysed reactions, in which saturated alkyl substituted phenols were formed as major products via hydrogenation.6b,7a,10b In the case of S derivatives, 4-propenylsyringol (S3), 4-allylsyringol (S4) and 4-propylsyringol (S5) were all observed as depolymerized products, while in the case of G-derived product, isoeugenol (G3) is the major monomer with C3 chains. Treatment 4-allyl-2-methoxyphenol with MoOx/CNT (5 wt%) at 240 °C and 3 MPa H2 in MeOH generated

a

mixture

of

4-allyl-2-methoxyphenol,

10


isoeugenol

(G3)

and

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2-methoxy-4-propylphenol with a ratio value of 88: 6.5: 5.5, suggesting that MoOx/CNT is selective to cleave C−O bonds in linkages rather than reduction double bonds of monomer phenols in current hydrogenolysis reaction. A range of solvents were screened in the reductive depolymerization of willow EMAL in the presence of MoOx/CNT under H2 atmosphere and the results are summarized in Table 1. Analogous to MeOH, the hydrogenolysis reactions in EtOH and iPrOH proceeded to give phenolic monomers in 21 wt% and 30 wt% yields, respectively (Table 1, entries 2 and 3). In the case of ethylene glycol, syringyl-derived phenolic products were formed in a monomer yield of 14 wt%, with no observation of G-type derivatives (entry 4). When H2O was used instead of MeOH, the yields of monomers were decreased to 7 wt% (entry 5), probably because of the lower solubility of lignin and H2 in water than that in alcohols.10a Hydrogenolysis in the mixture solvents, such as MeOH/H2O, EtOH/H2O and MeOH/EtOH, were also tested with the MoOx/CNT; however, no further improvement in the products yields were observed (entries 6-8). A control experiment involving heating of the lignin under 3 MPa H2 without catalyst was conducted, from which a dark lignin oily product was obtained in a low yield (21 wt%) with only 3 wt% phenolic monomers (Table 1, entry 9, see also Figure S3). These results suggested that MoOx/CNT is essential in current hydrogenolysis reaction. When the catalytic reactions were carried out under ambient pressure of N2 or low pressure of H2 (1 MPa), the yields of both lignin oily product and monomers were significantly dropped (Table 1, entries 10-11, see also Figure S3). These results suggested that the external H2 is important for current transformation, probably because H2 may assist MoOx/CNT in reducing the extent of lignin

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recondensation reactions.10k The commercial available catalysts, such as Ru/C, Pd/C, and Raney Ni, were also tested for hydrogenolysis of willow EMAL, which afforded 4-propanol or 4-propyl-substituted phenolic monomers in 28-39 wt% yields. In these cases, poor selectivities to unsaturated substituted products were observed (entries 12-14, see also Figure S4 and Table S5). Table 1. MoOx/CNT catalysed hydrogenolysis of EMAL isolated from willow T. ramosissimaa

Lignin oil Monomers Unsaturated S/Gb (wt%) (wt%) selectivity (%)c 1 MoOx/CNT MeOH 63 4.5 33 47.2 2 MoOx/CNT EtOH 67 4.9 21 67.8 i 3 MoOx/CNT PrOH 56 8.2 30 74.6 4 MoOx/CNT ethylene glycol 29 14 24.3 5 MoOx/CNT H2O 11 6.8 7 6 MoOx/CNT MeOH/H2O 59 4.5 27 30.2 7 MoOx/CNT EtOH/H2O 74 3.8 27 51.6 8 MoOx/CNT MeOH/EtOH 62 4.6 30 62.4 9 MeOH 21 7.8 3 23.2 10d MoOx/CNT MeOH 38 7 16.7 e 11 MoOx/CNT MeOH 56 5.8 19 53.9 f 12 Ru/C MeOH 73 6.7 39 0 13g Pd/C MeOH 76 6.1 34 0 h 14 Raney Ni MeOH 73 7.5 28 2.8 a Reaction conditions: Willow EMAL (50 mg), MoOx/CNT (5 mg), solvent (10 mL), H2 (3 MPa), 240 °C, 4 h. bThe mole ratios were determined by comparison lignin degradation products with authentic samples on GC-MS. cThe mole fractions of phenols containing alkene specie. dReaction performed under N2 (0.1 MPa). eReaction performed under H2 (1 MPa). f,g,h5 mg 5% Ru/C, 2.5 mg 10% Pd/C and 10 mg 90% Raney Ni were used for hydrogenolysis of willow EMAL (50 mg), respectively. Entry

Catalyst

Solvent

Effect of reaction temperature and time. The influence of reaction temperature of MoOx/CNT-catalysed depolymerization of lignin was investigated (Figure 5a, Table S6). Lowing the reaction temperature to 200 oC led to a lignin oil with Mw = 1120 g mol-1 (Figure S8, Table S4), from which almost only syringyl phenolic monomers were detected in a low

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yield (10 wt%), maybe due to the lack cleavage of phenolic ether linkages in lignin G-units. This is consisted with previous reports by using late transition metal catalysts, in which S-unit always showed high reactivity in lignin hydrogenolysis process.6b,d The highest yields (38.7 wt%) of monomeric phenols were obtained at 260 oC, with high selectivity to alkenyl substituted phenols (56.4%, Table S6). Under such temperature, lignin oil with a small molecular weight (660 g mol-1) was also generated. Increasing the reaction temperature to 280 o

C caused the monomer yields decreasing (27.8 wt%) and Mw increasing (760 g mmol-1)

simultaneously, probably because of severe recondensation of resulted monomers under high reaction temperature. The selectivity to unsaturated phenols was also decreased to 26.3% at 280 oC. A similar parabolic trend was also observed under variant reaction times (Figure 5b, Table S7). In the case of shorter reaction time, the hydrogenolysis gave lower yields of monomers (2 h, 25.9 wt%; 3 h, 30.1 wt%) and higher Mw (2 h, 740 g mol-1; 3 h, 700 g mol-1), compared to those from 4 h reaction time (38.7 wt%, 660 g mol-1) (Figure S8, Table S4). Prolonging reaction time to 5 h resulted a decreased monomers yield (21.1 wt%). The observation of increasing of Mw (780 g mol-1) and S/G ratio value (16:1) indicated a severe recondensation of G-derived compounds. Recyclability of the catalyst. The recycling stability of the metal catalysts is important for their potential applications in industry. In our experiment, the spent catalyst was subjected to simple washing and was used directly in the following cycle. As shown in Figure 5c, the yields of monomers were still maintained at 26.2 wt% after four runs; an ICP analysis of the used catalyst showed insignificant loss of molybdenum content (4.03 wt%; Mo content for

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original catalyst: 4.79 wt%) (Table S1), demonstrating the high stability of MoOx/CNT

40

(c) 40

30

30

Monomer yield (wt %)

(a)


catalyst.

20

10

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o

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30

20

10

0 2

3

4

5

Time (h)

Figure 5. Influences of (a) temperature; (b) reaction time and (c) recyclability for the hydrogenolysis of willow EMAL with MoOx/CNT. Reaction conditions for (a): lignin (50 mg), MoOx/CNT (5 mg), MeOH (10 mL), H2 (3 MPa), 4 h. For (b): lignin (50 mg), MoOx/CNT (5 mg), MeOH (10 mL), H2 (3 MPa), 260 °C. For (c): lignin (50 mg), MoOx/CNT (5 mg), MeOH (10 mL), H2 (3 MPa), 260 °C, 4 h. Degradation of various EMAL. Having these results in hand, we investigated the degradation of various EMAL samples isolated from hardwood (birch, eucalyptus and poplar), softwood (pinus) and herbaceous crop (miscanthus) with MoOx/CNT under optimized conditions (260 oC, 3 MPa H2 and 4 h). The results are summarized in Table 2 and the distribution of monomers are showed in Figure S23 and Table S9. Similar to willow, the 14


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EMAL from birch24a underwent complete cleavage of β-O-4, β-5 and β-β linkages as determined by 2D HSQC NMR analysis (Figures S9 and S10). GPC analysis showed a significant decrease in the average molecular weight from 13570 g mol-1 to 590 g mol-1 (Table 2, entry 1, Figure S11). The hydrogenolysis of birch lignin with MoOx/CNT produced 47 wt% combined yields of monomers, which are comparable with the yields obtained by Ru(52%)6b and Pd-catalyzed (52%)7a direct hydrogenolysis of the wood resources and with the yields through established lignin monomer analysis methods.25 The occupancy of alkenyl substituted phenols reached 62.5% in total monomers (Table S9). Taking into account the ether linkages density in the birch lignin, it is disclosed that current 47 wt% yield is close to the expected theoretical maximum monomer yield of birch lignin.2c,6b EMAL sample isolated from Eucalyptus grandis24b was also compatible with current catalytic system, affording lignin oily product with a small Mw (470 g mol-1) and high yields of monomeric phenols (40 wt%) (Table 2, entry 2, Figure S13, Table S9). In the case of Poplar (Triploid of Populus tomentosa Carr.) EMAL,24c a slight drop in monomer yields was observed (28 wt%), while the average molecular weight remained low (440 g mol-1) (Table 2, entry 3, Figure S15, Table S9).

Further

analysis

indicated

that

4-methylhydroxybenzoate

derived

from

p-hydroxybenzoate unit in poplar24c was generated as a typical product in 2 wt% yield. When EMAL derived from softwood (Pinus yunnanensis)24c,d was treated with MoOx/CNT, an obvious decrease in Mw (410 g mol-1) was determined by GPC (Table 2, entry 4, Figure S18). Although ether linkages in this lignin has been cleaved by examination of HSQC data after catalytic reaction (Figs. S16 and S17), only 13 wt% of the monomers were detected, in line with previous reports by using late transition metal catalysts.6b,d,7b The relative low monomer

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yields may be attributed to the higher ratio of interunit C−C linkages in softwood than in hardwood.6d Herbaceous plant is one important fast-growing energy crops, which is highly relevant in context of biomass application. An EMAL containing S-, G- and H-(p-hydroxyphenyl) units with Mw = 8860 g mol-1 was isolated from Miscanthus.24d When this EMAL was treated with MoOx/CNT, an oily product with Mw = 700 g mol-1 was obtained (Table 2, entry 5, Figure S21). This reaction produced 25 wt% yields of phenolic monomers with 41.3% selectivity to unsaturated phenols, which is comparable to the results by using late transition metal catalysts in term of activity.6d,7c

Table 2. Catalytic hydrogenolysis of various EMAL from different feedstocks over MoOx/CNTa

Entry Lignin

Mw (g mol-1)

S/G

β-O-4 (%)b

before

after

b

c

before

d

after

Lignin oil (wt%)

Monomers (wt%)c

Unsaturated selectivity (%)e

d

1

Birch

61.9

1.7

3.0

13570

590

75

47

62.5

2

Eucalyptus

55.3

2.2

2.0

8100

470

69

40

41.5

3

Poplar

58.7

3.0

2.9

13420

440

60

28

19.3

4

Pinus

41.6

-

-

8600

410

33

13

27.5

5

Miscanthus

49.6

1.2

2.2

8860

700

64

25

41.3

a

Reaction conditions: EMAL (50 mg), MoOx/CNT (5 mg), MeOH (10 mL), H2 (3 MPa), 260 °C, 4 h. Determined by 2D HSQC NMR. cDetermined by comparison lignin degradation products with authentic samples on GC-MS. dDetermined by GPC after acetylation. eThe mole fractions of phenols containing alkene specie. b

Reactivity of β-O-4 model compounds. To gain further insight on reaction pathway of depolymerization of lignin with MoOx/CNT, we performed the reactions of β-O-4 model compounds under analogous conditions. Treatment of dimeric lignin model compound 8 with

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MoOx/CNT at 200 oC under H2 afforded guaiacol (90%) and alkenyl-substituted guaiacol 9 (90%) through aryl C−O bond cleavage (eq 1); no hydrogenation of alkene was observed. In the case of a more complex trimeric model compound 11, unsaturated-substituted phenols 12 and 13 were obtained as the predominant products (98% selectivity, see Fig S24) (eq 2). These results confirm again that C−O bond hydrogenolysis is prior to C=C bond hydrogenation with MoOx/CNT.

Conclusions In summary, we have developed a new heterogeneous MoOx/CNT catalyst, which can serve as an excellent catalyst in hydrogenolysis of enzymatic mild acidolysis lignins derived from various lignocellulosic biomass, furnishing unsaturated monomeric phenols in high yields. The present low-cost MoOx/CNT catalyst is comparable to precious metal-based catalysts in the terms of activity, reusability and biomass feedstock compatibility. The solvent, reaction temperature and time showed significant influences on both of the activity and selectivity of lignin hydrogenolysis. Reactions of lignin model compounds revealed that the cleavage of C−O bonds in β-O-4 units is prior to reduction of double bonds by 17


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MoOx/CNT catalyst under H2 atmosphere. This work provides new options for catalyst design and potential practical applications in lignin valorisation. Further studies on non-precious metal-catalysed depolymerization of lignin are in progress in our lab.

ASSOCIATED CONTENT

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

AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] (G. Song); [email protected] (Y. Fang) Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This work was supported by Fundamental Research Funds for the Central Universities (No. BLYJ2014-38), National Program for Thousand Young Talents of China, the National Natural Science Foundation of China (No. 21506013), the Open Funding Project of Jiangsu Key Lab of Biomass-based Green Fuels and Chemicals (No. JSBGFC14009), the Open Funding State Key Laboratory of Pulp and Paper Engineering (No. 201518), China

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Postdoctoral Science Foundation (No. 2015M570040), and International Science & Technology Program of China (No. 2015DFG31860).

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Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over

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