Article

Selective Production of Diethyl Maleate viaOxidative Cleavage of Lignin Aromatic Unit

Zhenping Cai, Jinxing Long,

Yingwen Li, ..., Sijie Liu, Shik Chi

Edman Tsang, Xuehui Li

edman.tsang@chem.ox.ac.uk (S.C.E.T.)

cexhli@scut.edu.cn (X.L.)

HIGHLIGHTS

Selective cleavage of lignin

aromatic unit into diethyl maleate

was achieved

A polyoxometalate ionic liquid

system was provided for lignin-

selective oxidation

556.7 mg g�1 volatiles were

obtained with 72.7% DEM

selectivity at optimized conditions

A five-coordinated Cu+ species is

the active center for lignin

aromatic ring cleavage

A highly efficient process for producing bulk chemical diethyl maleate is achieved

with polyoxometalate ionic liquids from a cleavage lignin aromatic unit with high

yield and selectivity, which is ascribed to the intensive synergistic effect between

the acidic depolymerization, oxidative aromatic ring cleavage, and in situ

esterification. This work offers new insight into the versatile petroleum-based

chemical production from renewable resources.

Cai et al., Chem 5, 2365–2377

September 12, 2019 ª 2019 Elsevier Inc.

https://doi.org/10.1016/j.chempr.2019.05.021

Article

Selective Production of Diethyl Maleatevia Oxidative Cleavageof Lignin Aromatic UnitZhenping Cai,1,5 Jinxing Long,1,5 Yingwen Li,1 Lin Ye,2 Biaolin Yin,1 Liam John France,1 Juncai Dong,3

Lirong Zheng,3 Hongyan He,4 Sijie Liu,1 Shik Chi Edman Tsang,2,* and Xuehui Li1,6,*

The Bigger Picture

As a major component of

biomass, lignin is regarded as an

ideal renewable feedstock for the

production of versatile chemicals.

However, because of its wide

structural diversity, currently

reported chemical conversions of

lignin hardly lead to high yields of

specific products, which

significantly limits the economic

efficiency of biomass process.

Here, we have designed a series of

polyoxometalate ionic liquid

(POM-IL) catalysts that combine

SUMMARY

Green production of bulk chemicals traditionally obtained from fossil resources

is of great importance. One potential route toward realizing this goal is through

the utilization of renewable lignin; however, current techniques generally lead

to low product specificity because of the structural diversity of this recalcitrant

biopolymer. Herein, we devised a new catalytic system to promote selectively

oxidative lignin in air, and diethyl maleate was formed at impressively high yield

of 404.8 mg g�1 and selectivity of 72.7% over the polyoxometalate ionic

liquid of [BSmim]CuPW12O40. This high catalytic activity is ascribed to a five-co-

ordinated Cu+ species, which, through the formation of end-on dioxygen

species in vacant orbitals, facilitates the selective oxidation of basic lignin aro-

matic units (phenylpropane C9 units). Therefore, these results represent signif-

icant progress toward the realization of an industrially applicable and highly

selective lignin oxidation process for the generation of value-added and bulk

chemicals.

the advantages of ionic liquid and

polyoxometalate with the

introduction of acidity, reduction-

oxidation, and miscibility

properties for lignin-selective

depolymerization. The bulk

chemical diethyl maleate is

generated as a single product with

an impressively high yield and

selectivity through controlled

oxidative cleavage of lignin

aromatic ring, suggesting that

lignin-selective oxidation would

be a promising method for diethyl

maleate production via a

sustainable route.

INTRODUCTION

Lignin, as a major component of lignocellulosic biomass (15%–30% by weight), is an

ideal renewable feedstock for the production of platform chemicals;1–4 this natural

polymer is a kind of high-volume ‘‘waste’’ in the pulp and paper industry and in mod-

ern bio-refinery processes.5,6 If converted to useful chemicals traditionally obtained

from fossil resources, it can bring huge benefits to the chemical industry and environ-

ment. However, the efficient utilization of lignin is a major challenge and has long

been recognized as a bottleneck in biomass valorization, mainly because of its com-

plex molecular structure and highly recalcitrant chemical nature.7,8 Typically, less

than 2% of lignin is currently utilized, andmost of it is directly burned for energy gen-

eration.9 To date, bench-scale depolymerization technologies, such as hydrogenol-

ysis, alcoholysis, pyrolysis, liquefaction, and oxidation, have been shown to trans-

form lignin into fine chemicals or biofuel.9,10 Furthermore, carboxylic acids,

including formic acid, acetic acid, and unsaturated dicarboxylic acids, have been ob-

tained from the oxidation of lignin and its model compounds.11 On the other hand,

development of these catalytic methods is severely limited by poor miscibility to

lignin, uncontrolled oxidation leading to a range of products, and the formation of

undesirable interunit C–C bonds, generating more complex intermediates. Shuai

et al. employed formaldehyde as a blocking group in lignin hydrogenolysis, conse-

quently reducing C–C bond formation tendency and resulting in improved mono-

phenol yield.12 Nevertheless, poor catalytic efficiency and product specificity render

the conversion of lignin to useful chemicals very difficult for industrial practice.

Chem 5, 2365–2377, September 12, 2019 ª 2019 Elsevier Inc. 2365

Polyoxometalate catalysts, known for their high redox activity and satisfactory oper-

ational stability, have been employed previously for delignification and transforma-

tion of lignin by breaking up its corresponding linkage bonds.13,14 For example,

Zhao et al. recently reported a novel polyoxometalate-mediated biomass fuel cell,

which could directly convert lignin to electricity with high power output and faradic

efficiency.15 However, the development of effective catalysts is still important for the

generation of value-added chemicals from lignin. As a novel catalyst, ionic liquid

shows uniform catalytic center and good reusability, and it has also shown impres-

sive performance in lignin conversion.3 Therefore, considering the combined advan-

tages of ionic liquid and polyoxometalate with the introduction of desirable acidity,

reduction-oxidation, and miscibility properties, we designed a series of polyoxome-

talate-ionic liquid catalysts (POM-ILs; Scheme 1) for the selective oxidative cleavage

of lignin basic aromatic units (phenylpropane units [C9 units]; the contents of the

lignin structural units are shown in Table S1) to produce diethyl maleate (DEM) in

ethanol. A typical result over the [BSmim]CuPW12O40 catalyst shows that 94.5% of

wheat stalk lignin is partially converted to 556.7 mg g�1 volatile liquid products,

of which 72.7% is composed of DEM. It is well known that DEM is an industrially

important chemical and widely used in the production of polymers, spices, and

pesticides.16 More importantly, it is a feasible biomass-derived platform chemical

for the production of maleic acid and its derivatives,17 which are high-value com-

modities in the petrochemical industry. DEM has been previously produced via a

two-step process, where benzene (derived from petroleum) is oxidized to maleic

anhydride and maleic acid and then esterified with ethanol (Scheme 1).18 Because

of rigid oxidation conditions and serious pollution, a butane-derived route (from

petroleum refinery) is used as an alternative, accounting for approximately

1.8 million tons/p.a.19 However, the heavy dependence of fossil resources, high

reaction temperature (673–723 K), and low process efficiency (70%–85% conversion

and 65%–70% selectivity of maleic anhydride)20 make this novel renewable

route competitive. Thus, the present process has a great potential in lignin valoriza-

tion and DEM production for replacement of the current petroleum-based

technology.

1School of Chemistry and Chemical Engineering,State Key Laboratory of Pulp & PaperEngineering, South China University ofTechnology, Guangzhou 510640, China

2Wolfson Catalysis, Inorganic ChemistryLaboratory, University of Oxford, Oxford OX13QR, UK

3Beijing Synchrotron Radiation Facility, Instituteof High Energy Physics, Chinese Academy ofSciences, Beijing 100049, China

4Beijing Key Laboratory of Ionic Liquids CleanProcess, Institute of Process Engineering,Chinese Academy of Sciences, Beijing 100190,China

5These authors contributed equally

6Lead Contact

*Correspondence:edman.tsang@chem.ox.ac.uk (S.C.E.T.),cexhli@scut.edu.cn (X.L.)

https://doi.org/10.1016/j.chempr.2019.05.021

RESULTS AND DISCUSSION

Selective Oxidation Cleavage of Lignin Aromatic Unit

We began our study by using an organosolv bagasse lignin, a typical herbaceous

lignin containing all phenylpropane units (C9 unit) of p-coumaryl (H), coniferyl (G),

and sinapyl (S) alcohols. No products attributable to cleavage of the benzene ring

units were detected in the absence of catalyst (Table 1, entry 1). However, 48.2%

lignin was depolymerized, and products such as 4-hydroxybenzaldehyde and

vanillin were formed by aerobic oxidation (Table S9).21 A significant increase in lignin

depolymerization was found for the –SO3H-functionalized acidic ionic liquid,

[BSmim]HSO4, which as an acid is capable of catalyzing cleavage of ether bonds.22

However, only 6.5 and 15.9 mg g�1 DEM and C4 esters, respectively, were detected.

The C4 esters are consistent with cleavage of the benzene ring and were composed

largely of diethyl maleate, diethyl succinate, diethyl fumarate, and diethyl malate

(Table 1, entry 2; Figures S5 and S6; Tables S9 and S10). Lignin depolymerization

and volatile product yields were remarkably enhanced when POM-ILs ([BSmim]

MPW12O40, M=Na+, Mn2+, Co2+, Ni2+, and Cu2+) were used as catalysts (Tables 1

and S9). POM-ILs are composed of [BSmim]+ cations and [MPW12O40]� polyoxome-

talate anions, which form a reversible oxidant that facilitates the transfer of one

electron.23 For example, 81.3% bagasse lignin is depolymerized in the presence

of [BSmim]Na2PW12O40, producing a volatile liquid yield of 69.1 mg g�1, which

2366 Chem 5, 2365–2377, September 12, 2019

Scheme 1. The Novel Renewable Strategy from Lignin Oxidation (Green) versus the Petroleum-Based Process (Red) for DEM Production

POM-ILs: [BSmim]MPW12O40, where BS = –CH2CH2CH2CH2SO3H and M = Na, Mn, Co, Ni, and Cu (Figure S1; the catalyst structure and property

characterization results are shown in Figures S2–S4 and Tables S2–S8); the basic phenylpropane units (C9 units) of lignin are marked accordingly.

consists of 77.6% DEM (Table 1, entry 3; the conversion of lignin to model com-

pound, the product yield, and selectivity were calculated according to Supplemental

Equations 1–4 via gas chromatography excluding the oligomer fraction). It can be

seen that selective oxidative cleavage of aromatic units was significantly promoted

by replacement of Na+ with transition-metal ions such as Mn2+, Co2+, Ni2+, and Cu2+

(Table 1, entries 4–7). Overall, [BSmim]CuPW12O40 (Cu-POM-IL) showed the best

activity at 90.7% lignin depolymerization, 259.1 mg g�1 volatile product yield, and

59.3% DEM selectivity (Table 1, entry 7), suggesting that the transition-metal ion

plays a key role in this process. The above results clearly demonstrate the selective

nature of this novel process for DEM production, an observation that stands in

contrast to other lignin depolymerization technologies.10

To obtain the probable reason for this high activity of POM-IL, we conducted several

catalyst characterization techniques. As shown in Figures 1A and S3, X-ray diffraction

(XRD) clearly demonstrated the pattern with a space group of Pcca for the polyoxo-

metalate structure where the Mn+ can substitute into the octahedral W position. Our

synchrotron powder X-ray diffraction (PXRD) study also illustrated the high crystal-

linity and purity of all POM-IL catalysts prepared for this study. For the Cu-POM-IL

([BSmim]CuPW12O40) catalyst, the quality of the Le Bail refinement for synchrotron

PXRD data from Diamond Light Source (UK) was assured with a low goodness-of-

fit factor (gof), a low weighted profile factor, and a well-fitted pattern (Figure S3B).

The Le Bail refinement result clearly depicted that the common space group of

the POM had been altered from Pcca to Pncn (Cu-POM) as a result of the structural

distortion from the M substitution.24–26 The best fitting also suggested that there

were two groups of closely related unit cell parameters, presumably as a result of

a slight difference in the Cu-POM units packing (Tables S2 and S3). The unit cell

volumes were 4,856.1(1) A3 for Cu-POM1 and 3,068.1(1) A3 for Cu-POM2. The

distortion of the POM with the incorporation of Cu2+ ions can be appreciated via

the transformation matrix in WinGX with the input of the Le Bail profile refinement

data as the distorted Cu-POM1 unit (Figure 1A). The presentation of the full struc-

tural details in terms of atomic positions, connectivity, or coordination geometry is

also shown in Tables S2 and S3 by Rietveld refinement according to the initial model

Chem 5, 2365–2377, September 12, 2019 2367

Table 1. Selectively Catalytic Oxidation of Bagasse Lignin to DEM

Entry Catalysts Solventa Conversion (%) Yield of Volatile Product (mg g�1) Selectivity (%)d

DEM C4 Esterb Othersc Total DEM C4 Ester

1 –e 80:20 48.2 – – 14.7 14.7 0 0

2 [BSmim]HSO4 80:20 82.4 6.5 15.9 32.2 48.1 13.5 33.1

3 [BSmim]Na2PW12O40 80:20 81.3 53.6 59.8 9.3 69.1 77.6 86.5

4 [BSmim]MnPW12O40 80:20 80.9 87.7 107.6 22.7 130.3 67.3 82.6

5 [BSmim]CoPW12O40 80:20 80.6 93.3 105.6 40.9 146.5 63.7 72.1

6 [BSmim]NiPW12O40 80:20 84.3 100.4 116.0 50.3 166.3 60.4 69.8

7 [BSmim]CuPW12O40 80:20 90.7 153.6 176.2 82.9 259.1 59.3 68.0

8f [Bmim]CuPW12O40 80:20 80.8 11.0 16.7 29.5 46.2 23.8 36.1

9g H2SO4/CuSO4 80:20 86.8 16.5 20.8 53.7 74.5 22.1 27.9

10h [BSmim]Na2PW12O40/CuSO4

80:20 90.0 100.9 111.2 51.4 162.6 62.1 68.4

11i [BSmim]CuPW12O40 80:20 31.3 – – 78.6 78.6 0 0

12 [BSmim]CuPW12O40 100:0 92.9 274.7 310.9 87.5 398.4 69.0 78.0

13j [BSmim]CuPW12O40 40:60 100.0 152.9 495.7 – 495.7 30.8 100.0

14j [BSmim]CuPW12O40 80:20 100.0 416.3 488.9 – 488.9 85.1 100.0

15j [BSmim]CuPW12O40 100:0 100.0 1,111.5 1,111.5 – 1,111.5 100.0 100.0

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol catalyst, 20 mL ethanol-water mixture, 433 K, 5 h, 0.8 MPa O2.aVolume ratio of ethanol to water (v/v).bIncluding diethyl maleate, diethyl succinate, diethyl fumarate, and diethyl malate.cIn addition to C4 ester of volatile products.dThe selectivity of DEM is based on the volatile product determined by gas chromatography equipped with both a mass spectrometer and a flame ionization

detector.eNot added or detected.fThe yield of volatile product was obtained by esterification of lignin depolymerization product (after removal of original solvent) with ethanol at 373 K for 2.0 h by

adding 0.45 mmol H2SO4.g0.45 mmol H2SO4 with 0.9 mmol CuSO4.hAdding 0.9 mmol CuSO4.iUsing 0.8 MPa N2.j0.25 g maleic acid as substrate.

by Le Bail-WinGX. As seen from the refinement of Cu-POM-IL using the Rietveld

method, after putting the above two groups of the structure and adding some

free waters, we obtained the best fit with an acceptable R-weighted pattern (Rwp)

of 9.975, expected R (Rexp) of 6.605, and goodness of fit (gof) of 1.510. As a result,

the four original six-coordinated (6-CN) W/Cu as Keggin ion units were distorted to

form 5-CN W/Cu units in the general molecular formula (PW12O40)3�, the charge of

which was counter-balanced by [BSmim] cations. In addition, the Cu-POM anions

were inter-connected through the corner sharing of m-oxo ligands of the two 5-CN

W/Cu sites as a result of condensation. Similar m-oxo oligomers and polymers of

W- or V-based POM structures have been reported.27 It is important to establish

the superior activity and selectivity with the Cu incorporation into this new catalyst

system and to anticipate that the two remaining 5-CN Cu sites could be the active

sites that give vacant orbitals to take up the molecular oxygen as end-on species

on Cu sites to approach the distorted near 6-CN sites for this oxidation reaction.

Additionally, X-ray absorption spectroscopy (XAS) analysis (Figures 1B and 1C; Ta-

bles S4–S7) indeed showed that the oxidized Cu-POM gave a coordination number

(CN) of around 6. Figure 1D shows that the Cu2+ was in a distorted octahedral

environment with a CN of 5.6. This [BSmim]CuPW12O40 system displayed the

2368 Chem 5, 2365–2377, September 12, 2019

Figure 1. Structural Characterization of POM-IL

(A) The structure of Cu-POM was obtained by transforming the space group Pcca to Pncn with winGX software, which was based on the Le Bail profile

refinement data. The framework of Cu-POM uses the ball-stick mode (unit cell parameters are illustrated in Table S2).

(B) EXAFS and XANES analyses of POM-ILs.

(C) XANES analyses of [BSmim]CuPW12O40.

(D) Cu K-edge and W L3-edge XAS analysis of [BSmim]CuPW12O40.aN, coordination number; R, distance between absorber and backscatter atoms;

s2, Debye-Waller factor to account for both thermal and structural disorders; DE0, inner potential correction; R factor (%), goodness of fit. Error bounds

(accuracies) that characterize the structural parameters obtained by EXAFS spectroscopy were estimated as N G 20%, R G 1%, s2 G 20%, and DE0 G

20%. S02 were determined from CuO and WO3 standard fitting and fixed. Bold numbers indicate fixed N according to the crystal structure. O1, O2, and

O3 represent the first, second, and third nearest neighbor coordination O atoms. bFitting range: 2.5 % k (/A) % 12.5 and 1.6 % R (A) % 2.8.cFitting range: 2.5 % k (/A) % 12.5 and 1.0 % R (A) % 2.0. dCu K-edge fitting range: 2.9 % k (/A) % 12.5 and 1.0 % R (A) % 2.6; W L3-edge fitting

range: 2.5 % k (/A) % 14.5 and 1.0 % R (A) % 2.4.

largest degree of reduction among all POM-IL studied (Tables S4–S7) such that the

Cu CN dropped to 5 in H2 (reduced after H2 pre-treatment at 473 K to mimic the

Cu2+ reduction in organic media) and a characteristic Cu+ pre-edge peak arose

at � 8,984 eV and was attenuated in white line signal at � 8,996 eV in the X-ray

absorption near-edge structure (XANES) analysis (Figures 1A, 1C, and S4). The

reduction can create vacant orbitals to facilitate the formation of dioxygen activation

from oxygen on activated ‘‘Cu’’ in the reduced structure, as previously discussed.

Cu-substituted heteropolyoxometalates havemany similarities to active Cu enzymes

for O2 transport and oxidation developed in biological systems because they

possess coordination sites surrounding an isolated Cu. End-on Cu dioxygen adducts

as ‘‘superoxo’’ complexes have been proposed as reactive but selective intermedi-

ates in the catalytic cycle of mononuclear Cu enzymes, such as peptidylglycine

a-hydroxylating monooxygenase (PHM) or dopamine b-monooxygenase (DbH).28

Indeed, the existence of such a species could be demonstrated by X-ray crystallog-

raphy for a precatalytic PHM complex.29We believe that a similar superoxo is formed

over the reduced Cu+ in polyoxometalate in oxygen, which selectively oxidizes the

specific linkages of lignin to give DEM in the catalytic process.

Chem 5, 2365–2377, September 12, 2019 2369

As shown in Table 1, it is obvious that the lignin transformation remarkably declined

when the ionic liquid [Bmim]CuPW12O40 was used without the sulfonic group (Ta-

ble 1, entry 8), giving lower lignin conversion (80.8%), volatile product yield

(46.2 mg g�1), and DEM selectivity (23.8%) than Cu-POM-IL (Table 1, entry 7), which

demonstrated the importance of acidic functionality for the formation of DEM. Con-

trol experiments employing a H2SO4-CuSO4mixture (Table 1, entry 9) revealed a low

DEM selectivity (22.1%), indicating that polyoxometalate framework and ionic liquid

moiety also play important roles in the process. Interestingly, when CuSO4 and

[BSmim]Na2PW12O40 were combined, the yield of volatile products and DEM was

significantly lower than that of Cu-POM-IL (Table 1, entry 10 versus 7). However,

comparison of the former with [BSmim]Na2PW12O40 revealed an obvious enhance-

ment (Table 1, entry 10 versus 3). It is plausible that this effect may have arisen as

a result of a degree of in situ Cu-POM-IL formation, which is caused by the exchange

of Na+ with Cu2+. It should be noted that in the absence of oxygen, lignin depoly-

merization was notably low (Table 1, entry 11) and exhibited an aromatic product

distribution akin to the acid-catalyzed process (Table S9, entry 11). Moreover,

when POM-IL was added, the weight-average molecular weight (Mw) and number-

average molecular weight (Mn) of regenerated lignin (Table S9) were significantly

smaller than those of bagasse lignin, demonstrating that lignin is depolymerized

via an oxidative process. We can therefore conclude that promoted lignin depoly-

merization, coupled with selective formation of DEM, arises as a result of an efficient

synergistic effect between acidic functionality and transition-metal ions and

improved miscibility between polyoxometalate and lignin rather than any single

factor.

Detailed investigation about the dosage of Cu-POM-IL content, reaction tempera-

ture, and time for oxidative cleavage of lignin to DEM were also evaluated (Figures

S7–S9). The Cu-POM-IL catalyst showed characteristic temperature-controlled parti-

tion and recyclability (Figure S1) such that it was miscible with the reaction mixture at

elevated temperature and precipitated at room temperature,30 and no significant

activity loss was exhibited after six consecutive runs (Figure S10). Furthermore, the

effect of ethanol concentration—whereby the DEM yield and selectivity increased

as ethanol concentrations increased (Table 1, entry 12; Table S11, entries 1–6) until

they reached a maximum of 274.7 mg g�1 and 69.0% selectivity, respectively, in

100% ethanol—seemed to be critical. These findings support the idea that the pri-

mary product, maleic acid, undergoes rapid solvent-mediated reactions to produce

the desired product DEM. To convincingly demonstrate the validity of this effect, we

evaluated the reaction of maleic acid in ethanol-water mixtures. As shown in Tables 1

and S11, DEM selectivity reached 100% only in the presence of pure ethanol.

Lowering the ethanol concentration coincided with the formation of diethyl fumarate

and diethyl malate, which were obviously generated from the isomerization and

hydration of maleic acid, respectively (Table S11, entries 7–9). Therefore, this in

situ esterification process inhibits side reactions during the conversion of the inter-

mediate maleic acid, which could explain the high selectivity of DEM in the volatile

products.

In addition, we measured the carbon balance and the C9 unit utilization efficiency of

the optimized reaction, which are generally key factors for lignin depolymerization

according to elemental analysis (Table S12).9,10 As shown in Figure 2, under the opti-

mized conditions (433 K for 5 h), 18.1% of the lignin carbon, including 12.9% of DEM

(Figure 2B), was converted to volatile products (Figure 2A). However, 56.4 % of lignin

carbon was transformed into phenolic oligomers (Figure 2A), which is still recog-

nized as a big challenge during lignin valorization because of its robust interunit

2370 Chem 5, 2365–2377, September 12, 2019

Figure 2. Process Efficiency of Lignin Oxidation Catalyzed by [BSmim]CuPW12O40

(A) Carbon balance of process (a).

(B) Volatile products from processes (a) and (b). Reaction conditions of process (a): 0.25 g bagasse

lignin, 0.9 mmol [BSmim]CuPW12O40, 20 mL 100% ethanol, 433 K, 5 h, 0.8 MPa O2. Process (b) was

purged by an extra 0.8 MPa oxygen when reaction (a) was cooled to room temperature and then

heated to 433 K for another 2 h. Detailed calculation procedures can be found in Supplemental

Equations 5–10.

linkage.3,31 Interestingly, DEM production increased notably (from 274.7 to

522.3 mg g�1) when the reaction mixture was refilled with pure oxygen and reheated

for 2 h (Table S13, entries 1 and 4, respectively). Relatively, the carbon yield of DEM

from lignin substantially increased from 12.9% to 24.5% with extra oxygen presence.

The C9-unit-utilization efficiency further illustrates that the converted lignin basic

structure unit for DEM changed from 29.0% to 55.0% with a turnover number

(TON) of 0.84 at the optimized condition and with extra 0.8 MPa oxygen (Figure 2B).

It is noteworthy that both carbon-utilization efficiencies to a single chemical were

much higher than those of other lignin- conversion processes.3,10,31 Furthermore,

the adjustment of oxygen concentration (0.8 and 0.2 MPa for O2 and N2, respec-

tively) still converted 94.2% lignin and yielded 348.6mg g�1 DEM (71.7% selectivity),

showing minor variation from the established result (Table S13, entry 5 versus 1).

Moreover, the gaseous fraction exhibited a corresponding decrease in oxygen con-

tent after the oxidation reaction (Tables S14 and S15). Therefore, lignin carbon is well

utilized in this POM-IL catalytic system, implying that it may have significant poten-

tial in lignin valorization processes.

The adaptability of this POM-IL catalytic systemwas also checkedwith a series of typical

feedstocks, such as rice stalk lignin, pine lignin, corn stalk lignin, wheat stalk lignin, and

dealkaline lignin (Figure 3A; Tables S16 and S17). All types of lignin were efficiently con-

verted, producing both high yield and selectivity of DEM. In particular, the wheat stalk

lignin exhibited the highest yield of volatile products (556.7 mg g�1) with a high DEM

yield (404.8 mg g�1) and selectivity (72.7%), which clearly depicts an alternative to

the current butane-to-DEM technology.32 Moreover, the catalytic efficiency found

with the dealkaline lignin feedstock (Figure 3A) indicates that our technology is also

applicable to byproducts generated by the paper and pulp industry.

The Evolution of Lignin Structure during the Process

We examined the evolution of the lignin structure by comparing heteronuclear

single quantum correlation (HSQC) spectra of the original and regenerated lignin.

As shown in the aromatic regions (Figure 3B), typical basal structure units consisting

of methoxylated phenylpropanoid (syringyl [S] and guaiacyl [G]) subunits linked by

b–O–4 (b-aryl ether) bonds33 in the original lignin disappeared completely after

the reaction, implying that S and G units are more flexible than the H unit for this

lignin oxidative cleavage process catalyzed by POM-ILs. This result is inconsistent

Chem 5, 2365–2377, September 12, 2019 2371

Figure 3. Feedstock Adaptability and Lignin Structural Evolution

(A) Oxidative cleavage of lignin from various resources. Errors bars represent the mean G SEM.

(B) HSQC spectra of original and regenerated bagasse lignin.

(C) FT-IR spectra of original and regenerated bagasse lignin.

Reaction condition: 0.25 g lignin, 0.9 mmol [BSmim]CuPW12O40, 20 mL 100% ethanol, 433 K, 5 h, 0.8 MPa O2.

with previous studies showing that H is more active than G and S units in lignin mole-

cule cleavage,22,34 indicating a different pathway for lignin conversion with POM-ILs.

In addition, we also noticed that a small fraction of ketone S0 and G033 was present inthe regenerated lignin. This could also be observed in the aliphatic regions, where

the signals of branched chains disappeared as the ketone structure arose (Figure 3B).

Fourier transform infrared spectroscopy (FT-IR) demonstrated lower band intensity

for the condensed S-G unit (1,327 cm�1) and typical S unit (1,327 and

1,129 cm�1)35 in regenerated lignin than for those in original lignin (Figure 3C; Table

S18). However, two strong modes ca. 1,087 and 988 cm�1 appeared after the reac-

tion and are characteristic of C=O and C–O, respectively,36 verifying the formation

of ketonic G0 and S0 units. Additionally, 13C nuclear magnetic resonance (13C NMR)

analysis showed that the signals of 152.73, 130.74, 115.52, and 56.28 ppm, repre-

senting typical H, G, and S lignin units and –OCH3, respectively,37 were significantly

2372 Chem 5, 2365–2377, September 12, 2019

weakened in the regenerated lignin samples, clearly demonstrating the efficient

lignin depolymerization, and the appearance of peak at 191.64 ppm indicates the

formation of aromatic ketone (such as G0 and S0) during the process. In particular,

the significantly declined signal at 56.28 ppm confirms that regenerated lignin con-

tains fewer G and S units than original lignin, further verifying that the H unit exhibits

the lowest reactivity in this process (Figure S11).

Selective Oxidation of Model Compound from Lignin Depolymerization

To gain further insight into the selectively oxidative cleavage of the lignin aromatic

unit to DEM, we employed a series of phenol, guaiacol, syringol, and their deriva-

tives, the most representative chemicals from lignin depolymerization,3,38 as model

compounds. Interestingly, most model compounds could be efficiently converted

and exhibited relatively high yield and selectivity to DEM (Figure 4; Table S19).

However, the oxidative aromatic ring cleavage performances of these chemicals

were dependent remarkably upon their substituent groups. For instance, phenol

(1) produced 251.8 mg g�1 of volatile products containing 176.7 mg g�1 DEM (Fig-

ure 4); however, the addition of another hydroxyl group in either the ortho or para

position (model compounds 2 and 3) caused a significant increase in DEM yield

(442.4 or 446.5 mg g�1, respectively). This effect can be ascribed to the favorability

of catechol- and hydroquinol-forming benzoquinone, an intermediate of phenol

oxidation and a key startingmaterial for maleic acid generation.39 Figure 4 also dem-

onstrates that oxidative ring cleavage of the phenolic monomer was enhanced when

–OCH3 was substituted at the ortho position (guaiacols 4 and syringols 5). For

example, the yields of total volatile products and DEM increased from 251.8 �1

and 176.7 mg g�1 to 321.6 �1 and 205.2 mg g�1, respectively, when phenol was re-

placed by guaiacol. This improvement was more evident when syringol was used,

which gave yields of 388.1 and 257.3 mg g�1 for volatile products and DEM, respec-

tively. These findings indicate that the depolymerization products from the G and S

lignin units are more likely to produce larger quantities of DEM than H units.

In general, the charge density of the aromatic ring is enhanced by electron-

donating substituents (such as ethyl and methoxyl groups) and weakened by elec-

tron-withdrawing substituents (such as aldehyde and carboxyl groups).40 Hence,

the reactivity of the aromatic ring and the product regioselectivity are influenced

by the nature of the substituents in homogeneous reactions.33,40 Here, higher

charge density facilitated the oxidation of the aromatic ring,41 resulting in higher

DEM yield (6–8). For example, an electron-donating methoxyl group in the ortho

position of 4-ethyl phenol (6) elevated both total product yield (from 336.7 to

502.4 mg g�1) and DEM selectivity (from 67.8% to 73.3%; Table S19, entry 6

versus 7). On the contrary, phenolic monomers with electron-withdrawing substit-

uents weakened the electron density of the aromatic ring, which reduced oxida-

tive ring cleavage activity (9–13). Typically, aldehyde functionalities exhibit a

stronger electron-withdrawing effect than carboxyl groups.11 However, 4-hydrox-

ybenzaldehyde (14), vanillin (15), and syringe aldehyde (16) were more readily

activated than their corresponding acids (Figure 4, model compounds 9–11), es-

ters (Figure 4, model compounds 12 and 13), and monolignols (Figure 4, phenol

1, guaiacol 4, and syringol 5), yielding significantly increased DEM. In this reac-

tion, the aldehyde condensed quickly with ethanol, which was readily catalyzed

by the acidic functionality of the catalyst. Therefore, the electron-withdrawing

group, C=O, was transformed into an electron-donating group, –OC2H5 (this ace-

talization process could be verified in the absence of O2), which resulted in

enhanced C–C bond cleavage of the aromatic ring. These trends were further re-

flected by 4-hydroxylbenzaldehyde (14) and its corresponding acetal (8), such that

Chem 5, 2365–2377, September 12, 2019 2373

Figure 4. Catalytic Oxidation of Lignin Model Compounds into DEM

acetal compound 8 produced a larger DEM yield. Thus, the results about the se-

lective oxidative ring cleavage of model chemicals clearly illustrate that the lignin

structure with more electron-donating groups (such as –OCH3) is much more

2374 Chem 5, 2365–2377, September 12, 2019

easily converted. Namely, G and S, the most predominant structure units in

lignin,10 were more reactive than H, which accords well with the comparative

characterization results of the lignin structure evolution. This result was further

confirmed by the conversion of several H-type dimers with typical linkages,

such as 4–O–5, a–O–4, 5–5, and b–1. Table S19 shows that the cleavages of

C–O bonds, such as 4–O–5 and a–O–4, were effectively carried out but that

less DEM was generated. In comparison with the high reactivity of the C–O

bond, that of C–C bond remained insignificant because of its relatively higher

activation energy.3 On the basis of the discussion above, we proposed a plau-

sible pathway for lignin depolymerization and in situ oxidative ring cleavage (Fig-

ure S12), which we further verified by using controlled reactions with several qui-

nones as model compounds (Table S20).

In summary, we have achieved a highly efficient and selective process for the pro-

duction of DEM, an important, versatile, and widely used bulk chemical currently

derived from fossil fuel, by using POM-IL catalysts. Under optimized conditions,

94.5% wheat stalk lignin was oxidized, producing 556.7 mg g�1 of volatile products

and a DEM selectivity of 72.7% over [BSmim]CuPW12O40. The presence of a syner-

gistic effect attributable to acidic functionality and selective superoxo species (the

latter of which was formed by the Cu-containing polyoxometalate) could explain

the observed improvement. Furthermore, the solvent of ethanol also showed an

intensification effect on DEM production via the in situ esterification with the formed

maleic acid. Moreover, this process is highly feedstock adaptable for the depolymer-

ization of many typical lignin sources and has the substantial advantage of facile

catalyst separation and recycling. Therefore, we have demonstrated that the present

process has great potential in lignin valorization and sustainable DEM production

because of its high process efficiency and DEM selectivity.

EXPERIMENTAL PROCEDURES

Full experimental procedures are provided in the Supplemental Information.

SUPPLEMENTAL INFORMATION

Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.

2019.05.021.

ACKNOWLEDGMENTS

The financial support of the Natural Science Foundation of China (21736003,

21336002, 21690083, 21878111, 21676108, and 11605225), the Science and Tech-

nology Program of Guangzhou, China (201804020014), and the Fundamental

Research Funds for the Central Universities (SCUT) is gratefully acknowledged. Ac-

cess to beamline I11 of Diamond Light Source (UK) for the SXRD characterization

is also acknowledged. The authors would like to thank Wenxing Chen of Tsinghua

University (China) for helping with sample preparation.

AUTHOR CONTRIBUTIONS

C.Z.P., L.J.X., and L.X.H. designed the catalysts and experiments; C.Z.P., L.Y.W.,

L.J.F., and L.S.J. synthesized and characterized the catalysts and conducted the ex-

periments; S.C.E.T. performed the PXRD and XAS characterization and analysis with

help from Y.L, D.J.C., and Z.L.R.; C.Z.P., L.J.X., L.J.F., and L.X.H. analyzed the data

and wrote the manuscript with help from S.C.E.T.; Y.L., D.J.C., Z.L.R., Y.B.L., and

H.H.Y. contributed to catalyst characterization and product determination; and all

authors participated in data analyses and discussions.

Chem 5, 2365–2377, September 12, 2019 2375

DECLARATION OF INTERESTS

The authors declare no competing interests.

Received: November 20, 2018

Revised: February 19, 2019

Accepted: May 28, 2019

Published: June 17, 2019

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Chem 5, 2365–2377, September 12, 2019 2377

Chem, Volume 5

Supplemental Information

Selective Production of Diethyl Maleate

via Oxidative Cleavage

of Lignin Aromatic Unit

Zhenping Cai, Jinxing Long, Yingwen Li, Lin Ye, Biaolin Yin, Liam John France, JuncaiDong, Lirong Zheng, Hongyan He, Sijie Liu, Shik Chi Edman Tsang, and Xuehui Li

Materials and Methods

Materials. Tetrahydrofuran (THF, HPLC grade), N-methyl imidazole, 1, 4-butane sulfonate,

dealkaline lignin and the model compounds were purchased from J&K Chemical Co. Ltd.

CuSO4.5H2O, H3PW12O40

.5H2O, CuCO3.Cu(OH)2

.H2O, MnCO3, Na2CO3, NiCO3.2Ni(OH)2

.4H2O

and CoCO3 were purchased from Guangzhou Guanghua Chemical Reagents Factory Co. Ltd. All

materials and reagents were of analytical grade and used without further purification. Organosolv

lignins were extracted from raw biomass sources (10.0 g) using 150 mL 80% v/v aqueous ethanol

solution (containing 1.0 g 98% sulfuric acid) at 393 K for 4 h.1 The relative lignin concentrations

of H, G and S structural units were measured by quantitative 13C-nuclear magnetic resonance

(13C-NMR) spectroscopy.2,3

Table S1. The structural unit contents of different lignin.

Lignin Structural unit content (%)

H G S

Wheat stalk 29.9 32.8 37.3

Corn 13.5 47.2 39.3

Bagasse 14.8 45.6 39.6

Pine 0 81.9 18.1

Rice straw 22.2 37.8 40.0

Dealkaline 0 82.3 17.7

Preparation and characterization of polyoxometalate ionic liquids (POM-ILs).

Following reported procedures,4 N-methyl imidazole and equimolar 1, 4-butane sulfonate

were mixed and stirred magnetically at 313 K for 24 h forming a white precipitate (denoted as

[BSmim]). The as-obtained solid was washed with diethyl ether three times to remove residual

precursors and was then dried under vacuum.

Metal carbonate (consistent with 10 mmol transition metal ion) was dispersed in 20 mL

deionized water and added dropwise to a 20 mL aqueous solution of H3PW12O40.5H2O (10 mmol)

under agitation at ambient conditions. The resulting mixture was then stirred at room temperature

for 6.0 h. MHPW12O40 (M= Na, Mn, Co, Ni and Cu) was obtained after removing water and

vacuum drying at 353 K for 24 h.5 An aqueous MHPW12O40 solution was added dropwise to an

equivalent molar quantity of BSmim and stirred at room temperature for 24 h. The solvent was

then removed at 353 K for 24 h under vacuum,6 yielding [BSmim]MPW12O40 POM-IL.

1H-NMR and 13C-NMR spectra of POM-ILs were obtained on a Bruker AV-400 spectrometer

using D2O and DMSO-d6 solvents respectively. Fourier transform infrared spectroscopy (FT-IR)

was undertaken on a Bruker Tensor 27 FT-IR spectrophotometer (the range of 400-4000 cm-1,

KBr pelleting method). The thermal stability of ILs was analyzed thermogravimetrically (TG,

NETZSCH STA449C) from 313 to 923 K at 10 K min-1. The relative elemental contents of P, W,

Cu, Ni, Co, Mn and Na were determined on an Agilent 7700 Series ICP-MS apparatus. X-ray

diffraction (XRD) patterns were obtained using a Rigaku D/MAX-3A Auto X-ray diffractometer

with Cu Kα radiation (λ= 1.5418 Å). Intensity data were collected over a 2θ range of 5-80° with a

0.01° step. Hydrogen-temperature-programmed reduction (H2-TPR) was performed with a

Micromeritics Autochem II instrument (Micromeritics), equipped with a thermal conductivity

detector (TCD). Approximately 100 mg of catalyst was loaded into a U-shaped quartz reactor,

pre-treated at 373 K (20 K min-1) in Ar for 1 h, then cooled to 323 K, where it was maintained in

flowing 10% H2/Ar (30 mL min-1) for 2 h. The sample was heated from 323 K to 1073 K at 20 K

min-1 under the same reductive gas mixture and held at the final temperature for 10 min.

General procedure for catalytic oxidation of lignin and model compound.

In a typical process, 0.25 g lignin, 0.9 mmol POM-IL catalyst and 20 mL 100% ethanol were

charged into a 100 mL stainless autoclave (Andorra MED1220, Premex Co. Ltd.). After air

purging with pure oxygen five times and pressurizing to 0.8 MPa, the reactor was heated to the

designated temperature and maintained for the desired time. Once the latter elapsed, the autoclave

was cooled rapidly to room temperature in an ice water bath. The reaction mixture was removed

and the reactor was washed with anhydrous ethanol (3 5.0 mL). The IL catalyst was precipitated

at room temperature and used for the next run after drying (extra fresh catalyst was added to

offset transfer losses). The liquid mixture was then diluted by ethanol to 50 mL for qualitative and

quantitative analysis, while dimethyl phthalate was used as the internal standard. When aqueous

solutions of ethanol were used, the spent mixture was rotary evaporated under reduced pressure

for solvent recovery. The concentrated liquor was esterified with 10 mL anhydrous ethanol at 373

K for 2 h and then diluted to 50 mL with ethanol. Volatile products were qualitatively and

quantitatively analyzed via gas chromatography-mass spectrometry (GC-MS) and gas

chromatography-flame ionization detection (GC-FID). Residual lignin can be obtained through

simple precipitation processes. Organosolv lignin was recovered as follows: 60 mL deionized

water was added into 20 mL of the above reaction mixture causing precipitation. The mixture was

then separated using centrifugation and was dried until a constant weight was obtained. For the

recovery of dealkaline lignin the mixture obtained after reaction was acidified to pH=2 with 1.0

mol L-1 HCl solution and the same procedure described for organosolv lignin was conducted.

In the atmosphere investigation, a mixture of nitrogen and oxygen with various molar ratios

was used, while depolymerization of lignin was conducted at 433 K for 5.0 h in the single stage

experiments. For a typical two-stage process, the lignin was first depolymerized employing the

aforementioned conditions. When the mixture was cooled to room temperature, an extra 0.8 MPa

nitrogen or oxygen was purged into the reactor and the reaction was heated to 433 K for 1.0 or 2.0

h. The product separation and analysis procedure remained unchanged to that described

previously. In comparative and control experiments, a series of model compounds (monolignols

and potential intermediate products) were tested under the same procedures as that for lignin (i.e.,

0.25 g model compound, 0.9 mmol POM-IL catalyst and 20 mL 100% ethanol solvent). Triplicate

experiments were conducted and the data shown in this study is the average.

The conversion of lignin/model compound was calculated according to Eq.1, while the

product yield and selectivity were determined according to Eq. 2 to 4 respectively.

R

F

WConversion(%) (1- ) 100%

W Eq.1

P1

F

WYield(mg g )

W Eq.2

D

P

WSelectivity of DEM (%) 100%

W Eq.3

E

4

P

WSelectivity of C ester (%) 100%

W Eq.4

WR: the weight of regenerated lignin (g); WF: the weight of feed lignin/model compound (g); WP:

the weight of total volatile products determined by GC-FID (mg); WD: the weight of DEM (mg);

WE: the weight of C4 ester including DEM, diethyl succinate, diethyl fumarate, diethyl malate.

Product analysis. The reaction products were identified by GC-MS based on an Agilent library

(Agilent 7890B/5977A GC-MS apparatus). The capillary column of HP-5 MS (5% phenyl Methyl

silox, 30 m×250 μm×0.25 μm) was used for chemical separation. The oven temperature was

programmed from 323 (held for 3 min) to 553 K (held for another 5 min) with the rate of 8 K

min-1. The quantitative analysis of these products was conducted on an Agilent 7890B GC with a

flame ionization detector (GC-FID) using the same chromatography column and temperature

program as the GC-MS analysis. Dimethyl phthalate was selected as the internal standard

compound.

Gel permeation chromatography (GPC) analysis of the original and regenerated lignin was

carried out on an Agilent 1260 HPLC apparatus with a differential refraction detector (RID). THF

was used as eluent at a flow rate of 1 mL min-1 with the polystyrene standards for the molecular

weight calibration curve measurement. FT-IR spectra were obtained by Bruker Tensor 27 FT-IR

spectrophotometer (the range of 400-4000 cm-1, KBr pelleting method). 13C-NMR and

heteronuclear single quantum (HSQC) spectra were performed on a Bruker ABANCE III HD 600

spectrometer using DMSO-d6 as solvent. The elemental analysis was characterized by vario EL III

element analyzer and oxygen content were based on the assumption that the residue only contains

C, H, N, S, and O.

The carbon balance, C9 unit utilization efficiency and TON were calculated according to

Eq.5 to 10 respectively:

O O

F F

W CCarbon yield of oligomer (%)= 100%

W C

Eq.5

P P

F F

W CCarbon yield of volatile products (%) 100%

W C

Eq.6

9

F CC

C

W CN (mmol) 1000 mmol / mol

M 9

Eq.7

DEM FDEM

DEM

Y WN (mmol)

M

Eq.8

9

DEM9

C

N C unit utilization efficiency (%) 100%

N Eq.9

9C DEM

catalyst

N TON 100%

N

Eq.10

WO: the weight of oligomer (g); CO: the carbon content of oligomer determined by element

analysis (%); WF: the weight of feed lignin (g); CF: the carbon content of raw bagasse lignin; WP:

the weight of volatile products determined by GC-FID; CP: the carbon content of volatile products

= carbon obtained from lignin/molecular weight; NC9: the content of C9 unit in lignin (mmol); CC:

the content of Carbon based on the elemental analysis (%); MC: the mass number of carbon, 12 g

mol-1; YDEM: the yield of DEM determined by GC-FID (mg g-1); MDEM: the molecular mass of

DEM, 172 g mol-1; NC9-DEM: the C9 unit transformed into DEM (mmol); Ncatalyst: the catalyst used

in this process (mmol).

Characterization of polyoxometalate ionic liquid catalysts

13C-NMR, 1H-NMR, FT-IR and TG analysis of POM-ILs

[BSmim]CuPW12O40.a 1H-NMR (400 MHz, D2O): δ ppm 8.71 (s, 1H), 7.59-7.54 (m, 2H), 4.37

(m, 2H), 4.07 (s, 3H), 3.05 (m, 2H), 2.14 (m, 2H), 1.84 (m, 2H). 13C-NMR (400 MHz, DMSO-d6):

δ ppm 135.51, 123.25, 121.82, 49.82, 48.47, 35.35, 27.78, 20.70. FT-IR (KBr): ν (cm-1) 3562,

3490, 3153, 2938, 1971, 1606, 1448, 1169, 1076, 976, 890, 789, 596, 518. Onset decomposition

temperature: 572.4 K.

[BSmim]NiPW12O40. 1H-NMR (400 MHz, D2O): δ ppm 8.70 (s, 1H), 7.61-7.56 (m, 2H), 4.39 (m,

2H), 4.11 (s, 3H), 3.08 (m, 2H), 2.17 (m, 2H), 1.86 (m, 2H). 13C-NMR (400 MHz, DMSO-d6): δ

ppm 134.57, 122.13, 120.75, 48.83, 47.30, 34.29, 26.80, 19.78. FT-IR (KBr): ν (cm-1) 3554, 3440,

3203, 2945, 1971, 1621, 1448, 1162, 1076, 970, 904, 789, 603, 518. Onset decomposition

temperature: 592.5 K.

[BSmim]CoPW12O40. 1H-NMR (400 MHz, D2O): δ ppm 8.36 (s, 1H), 7.29 (s, 1H), 7.23 (s, 1H),

4.11 (m, 2H), 3.77 (s, 3H), 2.85 (m, 2H), 1.90 (m, 2H), 1.56 (m, 2H). 13C-NMR (400 MHz,

DMSO-d6): δ ppm 136.33, 123.81, 122.44, 51.16, 48.94, 35.98, 28.56, 21.58. FT-IR (KBr): ν

(cm-1) 3504, 3368, 3218, 2945, 1971, 1621, 1456, 1162, 1076, 983, 897, 789, 610, 518. Onset

decomposition temperature: 566.2 K.

[BSmim]MnPW12O40. 1H-NMR (400 MHz, D2O): δ ppm 8.62 (s, 1H), 7.39-7.33 (m, 2H), 4.14

(m, 2H), 3.78 (s, 3H), 2.84 (m, 2H), 1.91 (m, 2H), 1.63 (m, 2H). 13C-NMR (400 MHz, DMSO-d6):

δ ppm 135.73, 123.33, 121.95, 49.21, 48.49, 35.66, 28.08, 21.03. FT-IR (KBr): ν (cm-1) 3447,

3354, 3153, 2953, 1971, 1621, 1456, 1169, 1084, 983, 897, 783, 603, 510. Onset decomposition

temperature: 607.9 K.

[BSmim]Na2PW12O40. 1H NMR (400 MHz, D2O): δ ppm 8.71 (s, 1H), 7.59 (s, 1H), 7.54 (s, 1H),

4.37 (t, J = 7.0 Hz, 2H), 4.07 (s, 3H), 2.98 (t, J = 7.4 Hz, 2H), 2.17-2.09 (m, 2H), 1.85-1.77 (m,

2H). 13C-NMR (400 MHz, DMSO-d6): δ ppm 136.21, 123.83, 122.43, 50.39, 49.01, 35.98, 28.46,

21.41. FT-IR (KBr): ν (cm-1) 3525, 3419, 3160, 2945, 1978, 1628, 1442, 1162, 1076, 976, 897,

783, 603, 518. Onset decomposition temperature: 563.1 K.

Recovered [BSmim]CuPW12O40. 1H-NMR (400 MHz, D2O): δ ppm 8.72 (s, 1H), 7.55 (m, 2H),

4.37 (m, 2H), 4.06 (s, 3H), 3.05 (m, 2H), 2.14 (m, 2H), 1.83 (m, 2H). 13C-NMR (400 MHz,

DMSO-d6): δ ppm 136.23, 123.83, 122.44, 50.49, 49.00, 35.98, 28.47, 21.42. FT-IR (KBr): ν

(cm-1) 3568, 3483, 3160, 2960, 1978, 1606, 1448, 1162, 1076, 983, 890, 789, 603, 518. Onset

decomposition temperature: 569.0 K.

Notes: [BSmim] represents the Methyl-3-(butyl-4-sulfonate) imidazolium cation.

f g h i j Ba b c d e

A

Figure S1. The ILs [BSmim]MPW12O40 (from a to e, M= Cu, Ni, Co, Mn, and Na) (A) and the

solubility of CuHPW12O40 and [BSmim]CuPW12O40 (B). (f): 0.5 g CuHPW12O40 in 5 mL 80%

ethanol-water solvent at room temperature. (g): mixture of 0.5 g [BSmim]CuPW12O40 and 5 mL

80% ethanol-water solvent at room temperature. (h) g at 353 K; (i) added 5 mL of ethanol to f. (j):

added 5 mL of ethanol to h.

50 150 250 350 450 550 650 750

a

TC

D S

ign

al (

a.u

.)

Temperature (oC)

a--BSmimCuP12O40

c

c--BSmimCoP12O40

d

d--BSmimMnP12O40

b

b--BSmimNiP12O40

f

e

e--BSmimNa2P12O40

f--H3PW12O40

Figure S2. Temperature programmed reduction (TPR) data of the POM-ILs showing their

reduction from 160oC

Figure S3. XRD patterns of POM-IL catalysts and synchrotron PXRD refinement and the

structure of Cu-POM. (A) XRD patterns of IL catalysts. (B) Comparison of the experiment data

(black line) and Le Bail profile refinement (red circle). Red circle is the sum of the phase

1(Cu-POM1) blue line and the phase 2 (Cu-POM2) green line and the difference between them

(grey line) from 2.5 ~ 30°. (C) The structure of Cu-POM is obtained by transforming the space

group Pcca to Pncn using winGX software, which was based on the Le Bail profile refinement

data. The framework of Cu-POM is using the ball-stick mode, framework O: red, WVI: blue,

(W/Cu)V: green, P: pink. (Unit cell parameters illustrate in Supplementary Table S2)

High resolution Synchrotron PXRD data were collected on Beamline I11, Diamond Light Source,

UK. Detailed description of the beamline can be found elsewhere.7 The incident X-ray beam energy

was set at 15 keV. The wavelength and the 2θ zero-point correction were refined using a diffraction

pattern obtained from a high-quality silicon powder (SRM640c). Unit cell parameters for Cu-POM1

and Cu-POM2 were obtained from powder diffraction data which was based on a Le Bail profile

refinement using TOPAS and summarized in Supplementary Table S2. The Pncn structure of the

A B

C

Cu-POM was deduced from the Pcca space group transformed through the matrix

using WinGX software.

Table S2. Crystallographic data and details of the Cu-POM sample.

Samples Cu-POM1 Cu-POM2

Crystal system Orthorhombic Orthorhombic

Space group Pncn Pncn

Chemical formula [P(W/Cu)V4W

VI8O39 ]

n- n (O2/H2O) [P(W/Cu)V4W

VI8O39 ]

n-·n (O2/H2O)

2θ range refinement (o) 2.5 – 30

Detector Multi-analyser crystals

Refinement methods Rietveld refinement

a (Å) 20.48624(9) 17.61397(4)

b (Å) 14.40534(9) 11.46489(5)

c (Å) 16.48389(1) 15.19459(7)

V (Å3) 4856.1(1) 3068.1(1)

Rwp / Rp / Rexp (%) 9.975/7.861/6.605

Wavelength (Å) 0.825255 (2)

2θ Zero point (°) -0.0001 (2)

Gof χ2 1.510

Table S3. The detailed parameters Rietveld refinement of POM-IL.

Species Atom X Y Z SOF Beq(Å2)

Cu-POM1/2 site P1 0 0.4457 0.25 1 2

site W1 0.2467 0.4503 0.3217 1 2

site W2 0.1126 0.4413 0.0953 1 2

site W3 0.0729 0.5802 0.3617 1 2

site W4 -0.0606 0.3122 0.1352 1 2

site W5 0.1816 0.5794 0.2128 1 2

site W6 0.1764 0.3123 0.2041 1 2

site O1 0.0863 0.4926 0.2749 1 2

site O2 0.0392 0.3988 0.1955 1 2

site O3 0.1124 0.2879 0.2824 1 2

site O4 0.0526 0.6029 0.1784 1 2

site O7 0.0013 0.5058 0.0967 1 2

site O8 0.1606 0.3848 0.3646 1 2

site O9 0.1442 0.6214 0.2923 1 2

site O10 0.0655 0.2699 0.1595 1 2

site O11 0.2769 0.5237 0.2598 1 2

site O12 0.1992 0.3682 0.127 1 2

site O13 0.1955 0.5232 0.3757 1 2

site O14 0.0148 0.3686 0.0753 1 2

site O15 0.3633 0.4354 0.3536 1 2

site O16 0.1686 0.4567 0.0216 1 2

site O17 0.0856 0.6408 0.4208 1 2

site O18 -0.1119 0.2507 0.0859 1 2

site O19 0.258 0.641 0.1791 1 2

site O20 0.2729 0.2521 0.1968 1 2

site O5 0.1821 0.5061 0.1497 1 2

site O6 0.2422 0.3856 0.2522 1 2

O atoms of water O 0.32482 -0.15407 1.36937 5.78(6) 2

For Cu-POM-1 O 0.73191 -0.27770 -0.50310 1.96(6) 2

O 0.30770 1.18390 -1.05108 12.56(7) 2

O -0.72126 1.16347 0.48646 7.09(6) 2

Table S4. Mn K-edge and W L3-edge EXAFS curve Fitting parameters of [BSmim]MnPW12O40.a

sample Shell N R (Å) σ2 (Å2) ΔE0 (eV) R, %

MnO2b Mn−O 6 1.90 0.005 4.3 0.01

[BSmim]MnPW12O40c Mn−O 6.0 2.16 0.007 8.7 0.01

W−O1

W−O2

W−O3

1.0

3.9

1.0

1.70

1.90

2.41

0.001

0.004

0.011

1.4 0.03

[BSmim]MnPW12O40_H2c Mn−O 5.6 2.15 0.007 8.9 0.01

W−O1

W−O2

W−O3

0.9

4.1

1.0

1.71

1.91

2.43

0.001

0.004

0.011

2.5 0.05

a N, coordination number; R, distance between absorber and backscatter atoms; σ2, Debye–Waller factor to

account for both thermal and structural disorders; ΔE0, inner potential correction; R factor (%) indicates the

goodness of the fit. Error bounds (accuracies) that characterize the structural parameters obtained by EXAFS

spectroscopy were estimated as N ± 20%; R ± 1%; σ2 ± 20%; ΔE0 ± 20%. S02 were determined from MnO2 and

WO3 standard fitting and fixed. Bold numbers indicate fixed coordination number (N) according to the crystal

structure. O1, O2, and O3 represent the first, second, and third nearest neighbor coordination O atoms. b Fitting

range: 2.0 ≤ k (/Å) ≤ 12.6 and 1.0 ≤ R (Å) ≤ 2.0. cMn K-edge fitting range: 2.5 ≤ k (/Å) ≤ 12.0 and 1.0 ≤ R (Å) ≤

2.4; W L3-edge fitting range: 2.5 ≤ k (/Å) ≤ 14.5 and 1.0 ≤ R (Å) ≤ 2.4.

Table S5. Co K-edge and W L3-edge EXAFS curve fitting parameters of [BSmim]CoPW12O40.a

sample Shell N R (Å) σ2 (Å2) ΔE0 (eV) R, %

Co bulkb Co−Co 12 2.49 0.008 0.7 0.01

CoOc Co−O

Co−Co

6

12

2.12

3.01

0.009

0.009 -9.9 1.0

[BSmim]CoPW12O40d Co−O 6.0 2.08 0.006 -3.1 0.01

W−O1

W−O2

W−O3

1.0

3.9

1.0

1.70

1.91

2.42

0.001

0.004

0.011

3.0 0.09

[BSmim]CoPW12O40_H2d Co−O 5.8 2.08 0.006 -3.3 0.01

W−O1

W−O2

W−O3

0.8

3.8

1.0

1.71

1.91

2.44

0.001

0.004

0.011

4.7 0.05

a N, coordination number; R, distance between absorber and backscatter atoms; σ2, Debye–Waller factor to

account for both thermal and structural disorders; ΔE0, inner potential correction; R factor (%) indicates the

goodness of the fit. Error bounds (accuracies) that characterize the structural parameters obtained by EXAFS

spectroscopy were estimated as N ± 20%; R ± 1%; σ2 ± 20%; ΔE0 ± 20%. S02 were determined from CoO and

WO3 standard fitting and fixed. Bold numbers indicate fixed coordination number (N) according to the crystal

structure. O1, O2, and O3 represent the first, second, and third nearest neighbor coordination O atoms. b Fitting

range: 3.2 ≤ k (/Å) ≤ 14.0 and 1.4 ≤ R (Å) ≤ 2.7. cFitting range: 3.2 ≤ k (/Å) ≤ 14.5 and 1.0 ≤ R (Å) ≤ 3.2. dCo

K-edge fitting range: 3.2 ≤ k (/Å) ≤ 11.9 and 1.0 ≤ R (Å) ≤ 2.4; W L3-edge: fitting range: 2.5 ≤ k (/Å) ≤ 14.5 and

1.0 ≤ R (Å) ≤ 2.4.

Table S6. Ni K-edge and W L3-edge EXAFS curve fitting parameters of [BSmim]NiPW12O40.a

sample Shell N R (Å) σ2 (Å2) ΔE0 (eV) R, %

Ni bulkb Ni-Ni 12 2.48 0.008 0.7 0.01

NiOc Ni-O

Ni-Ni

6

12

2.08

2.95

0.005

0.006 1.8 0.01

[BSmim]NiPW12O40d Ni−O 5.9 2.04 0.007 -10.9 0.01

W−O1

W−O2

W−O3

1.0

3.9

1.0

1.70

1.91

2.49

0.001

0.004

0.011

3.2 0.07

[BSmim]NiPW12O40_H2d Ni−O 5.7 2.03 0.007 -11.9 0.2

W−O1

W−O2

W−O3

1.0

3.8

0.5

1.72

1.92

2.44

0.002

0.004

0.011

5.1 0.08

a N, coordination number; R, distance between absorber and backscatter atoms; σ2, Debye–Waller factor to

account for both thermal and structural disorders; ΔE0, inner potential correction; R factor (%) indicates the

goodness of the fit. Error bounds (accuracies) that characterize the structural parameters obtained by EXAFS

spectroscopy were estimated as N ± 20%; R ± 1%; σ2 ± 20%; ΔE0 ± 20%. S02 were determined from NiO and

WO3 standard fitting and fixed. Bold numbers indicate fixed coordination number (N) according to the crystal

structure. O1, O2, and O3 represent the first, second, and third nearest neighbor coordination O atoms. b Fitting

range: 3.9 ≤ k (/Å) ≤ 13.9 and 1.6 ≤ R (Å) ≤ 2.6. cFitting range: 3.7 ≤ k (/Å) ≤ 14.5 and 1.0 ≤ R (Å) ≤ 3.2. dNi

K-edge fitting range: 3.2 ≤ k (/Å) ≤ 11.9 and 1.0 ≤ R (Å) ≤ 2.4; W L3-edge fitting range: 2.5 ≤ k (/Å) ≤ 14.5 and

1.0 ≤ R (Å) ≤ 2.4.

Table S7. Cu K-edge and W L3-edge EXAFS curve fitting parameters of [BSmim]CuPW12O40.a

sample Shell Na R (Å) σ2 (Å2) ΔE0 (eV) R, %

Cu bulkb Cu−Cu 12 2.54 0.008 1.0 0.01

Cu2Oc Cu−O 2 1.86 0.003 4.0 0.04

CuOc Cu−O 4 1.96 0.005 2.1 0.01

[BSmim]CuPW12O40d Cu−O1

Cu−O2

4.6

1.0

1.96

2.26

0.005

0.010 3.6 0.01

W−O1

W−O2

W−O3

1.0

4.0

1.0

1.70

1.91

2.49

0.001

0.004

0.011

3.2 0.07

[BSmim]CuPW12O40_H2d Cu−O1

Cu−O2

4.0

1.0

1.96

2.21

0.005

0.010 2.5 0.05

W−O1

W−O2

W−O3

1.0

3.6

0.8

1.71

1.91

2.49

0.001

0.004

0.011

3.3 0.14

a N, coordination number; R, distance between absorber and backscatter atoms; σ2, Debye–Waller factor to

account for both thermal and structural disorders; ΔE0, inner potential correction; R factor (%) indicates the

goodness of the fit. Error bounds (accuracies) that characterize the structural parameters obtained by

EXAFS spectroscopy were estimated as N ± 20%; R ± 1%; σ2 ± 20%; ΔE0 ± 20%. S02 were determined

from CuO and WO3 standard fitting and fixed. Bold numbers indicate fixed coordination number (N)

according to the crystal structure. O1, O2, and O3 represent the first, second, and third nearest neighbor

coordination O atoms. b Fitting range: 2.5 ≤ k (/Å) ≤ 12.5 and 1.6 ≤ R (Å) ≤ 2.8. cFitting range: 2.5 ≤ k (/Å)

≤ 12.5 and 1.0 ≤ R (Å) ≤ 2.0. dCu K-edge fitting range: 2.9 ≤ k (/Å) ≤ 12.5 and 1.0 ≤ R (Å) ≤ 2.6; W

L3-edge fitting range: 2.5 ≤ k (/Å) ≤ 14.5 and 1.0 ≤ R (Å) ≤ 2.4.

To understand the coordination environment and chemical states of M/W in POM-ILs,

X-ray absorption fine structure spectra were collected at 1W1B beamline of Beijing

Synchrotron Radiation source in China in fluorescence mode using a Si (111) double crystal

monochromator under ambient condition. The incident and fluorescence X-ray intensities were

detected by standard ionization chambers and Lytle-type detectors, respectively. The EXAFS

raw data were then background-subtracted, normalized and Fourier transformed by the standard

procedures with the ATHENA program;8 least-squares curve fitting analysis of the EXAFS χ(k)

data was carried out using the ARTEMIS program.

Figure S4a. EXAFS and XANES analyses of POM-ILs

Figure S4b. XANES analyses of [BSmim]CuPW12O40

Table S8. ICP-MS analysis of POM-ILs.a

IL catalyst Element Concentration

(mg L-1) b

Analysis 1

(mg L-1)

Analysis 2

(mg L-1)

Average

(mg L-1)

Error

(%)

[BSmim]CuPW12O40

Cu 6.06 5.97 5.88 5.93 2.15

P 3.18 3.21 3.15 3.18 0.06

W 20.93 20.16 19.98 20.07 4.11

[BSmim]NiPW12O40

Ni 5.60 5.34 5.40 5.37 4.11

P 3.23 3.20 3.27 3.24 0.28

W 20.96 20.76 20.84 20.80 0.76

[BSmim]CoPW12O40

Co 5.69 5.35 5.47 5.41 4.92

P 3.20 3.21 3.23 3.22 0.77

W 20.95 20.79 20.81 20.80 0.72

[BSmim]MnPW12O40

Mn 5.22 5.11 5.09 5.10 2.30

P 3.17 3.20 3.19 3.20 0.69

W 20.98 20.14 20.28 20.21 3.67

[BSmim]Na2PW12O40

Na 4.39 4.15 4.21 4.18 4.78

P 3.20 3.26 3.19 3.23 0.80

W 21.05 19.98 19.87 19.93 5.32

a The concentration of IL catalyst is 300 mg.L-1 for Cu, Co, Mn, Ni, Na and P, and is 30 mg.L-1 for

W; b Theoretical value.

The selective oxidation of lignin and model compounds

Table S9. Selective oxidation of bagasse lignin over various catalysts.

Entry

Catalysts

Conv.

(%)

Yield (mg g-1)

DEM

selectivitya

(%)

Average molar mass of

regenerated lignin

1 2 3 4 5 6 7 8 9 10 total Mw Mn D

1 None 48.2 —b — — — — 1.1 2.0 3.4 5.7 2.5 14.7 0 1643 990 1.66

2 [BSmim]HSO4 82.4 6.5 4.4 4.1 0.9 — 10.0 1.8 17.9 2.5 — 48.1 13.5 688 375 1.83

3 [BSmim]Na2PW12O40 81.3 53.6 3.7 2.5 — — 1.6 7.7 — — — 69.1 77.6 999 688 1.45

4 [BSmim]MnPW12O40 80.9 87.7 12.5 4.0 3.4 7.1 — 15.6 — — — 130.3 67.3 949 663 1.43

5 [BSmim]CoPW12O40 80.6 93.3 6.8 4.1 1.4 7.3 7.6 26.0 — — — 146.5 63.7 933 652 1.43

6 [BSmim]NiPW12O40 84.3 100.4 5.1 7.3 3.2 8.1 11.2 27.1 — 3.9 — 166.3 60.4 920 647 1.42

7 [BSmim]CuPW12O40 90.7 153.6 4.8 12.0 5.8 16.9 25.2 31.7 — 9.1 — 259.1 59.3 786 613 1.28

8c [Bmim]CuPW12O40 80.8 11.0 1.7 2.1 1.9 1.4 13.7 1.8 9.7 2.9 — 46.2 23.8 1132 846 1.34

9d H2SO4/CuSO4 86.8 16.5 0.9 1.6 1.8 11.4 12.0 10.7 10.2 9.4 — 74.5 22.1 898 648 1.39

10e [BSmim]Na2PW12O40/CuSO4 90.0 100.9 3.1 5.9 1.3 15.2 17.9 16.7 — 1.6 — 162.6 62.1 886 682 1.30

11f [BSmim]CuPW12O40 31.3 — — — — — — — — — — 78.6 0 954 726 1.31

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol catalyst, 20 mL of 80% ethanol-water, 0.8 MPa O2, 433 K, 5 h. a the selectivity of DEM is based on the volatile product determined by GC-MS; b not

added or detected; c After removed the solvents, then 10 mL ethanol and 0.45 mmol H2SO4 were added at 373 K for 2 h; d 0.45 mmol H2SO4/0.9 mmol CuSO4; e 0.9 mmol [BSmim]Na2PW12O40/(0.9 mmol)

CuSO4. f 0.8 MPa N2, volatile products: guaiacol (1.6 mg g-1), phenol (2.7 mg g-1), 4-ethylguaiacol (5.3 mg g-1), 4-ethylphenol (54.1 mg g-1), 5-methyl-2-isopropylphenol (4.2 mg g-1), 2,6-dimethoxyphenol

(3.6 mg g-1), 2-hydroxy-1-(4-hydroxy-3-methoxyphenyl) ethanone (7.1 mg g-1). 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl fumarate; 4: Diethyl malate; 5: Ethyl ethoxyacetate; 6: Ethyl

4-hydroxybenzoate; 7: Ethyl vanillate; 8: 4-Hydroxybenzaldehyde; 9: Vanillin; 10: Syringaldehyde. Original bagasse lignin with the Mw, Mn and D values of 2698, 1156 and 2.33, respectively, see Table S16,

D=Mw/Mn.

4 8 12 16 20

b

10

987

6

5

4

3

2

Inte

nsity

Retention time (min)

1

a

Figure S5. GC-MS analysis of the volatile products with (a) and without (b) [BSmim]CuPW12O40.

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol [BSmim]CuPW12O40 catalyst, 20 mL 80%

ethanol-water, 433 K, 5 h, 0.8 MPa O2.

Table S10. The volatile products detected by GC-MS.

Entry Retention

time (min) Products Structure Formula

Molecular weight

(g mol-1)

1 5.94 Ethyl ethoxyacetate

C6H12O3 132

2 11.68 Diethyl maleate

C8H12O4 172

3 12.03 Diethyl succinate

C8H14O4 174

4 12.95 Diethyl fumarate

C8H12O4 172

5 13.58 Diethyl malate

C8H14O5 190

6 15.80 4-hydroxybenzaldehyde

C7H6O2 122

7 16.08 Vanillin

C8H8O3 152

8 18.23 Ethyl 4-hydeoxybenzoate

C9H10O3 166

9 19.15 Ethyl vanillate

C10H12O4 196

10 20.26 Syringaldehyde

C9H10O4 182

Diethyl maleate

(a)

(b)

Diethyl succinate

(a)

(b)

Figure S6a. The MS spectra of C4 esters produced in this process. a: the data from the reaction

mixture; b: the data from MS library.

(a)

(b)

Diethyl fumarate

Diethyl malate

(a)

(b)

Figure S6b. The MS spectra of C4 esters produced in this process. a: the data from the reaction

mixture; b: the data from MS library.

Figure S7. Effect of POM-IL catalysts dosage on the oxidation of lignin.

Reaction condition: 0.25 g bagasse lignin, [BSmim]CuPW12O40 as catalyst, 20 mL 80%

ethanol-water, 433 K, 3 h, 0.8 MPa O2. 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl

fumarate; 4: Diethyl malate; 5: Ethyl ethoxyacetate; 6: Ethyl 4-hydroxybenzoate; 7: Ethyl

vanillate; 8: 4-Hydroxybenzaldehyde; 9: Vanillin; 10: Syringaldehyde.

Figure S8. Effect of reaction temperature on the oxidation of lignin.

Reaction condition: 0.25 g bagasse lignin, 0.9 mmol [BSmim]CuPW12O40, 20 mL 80%

ethanol-water, 3 h, 0.8 MPa O2. 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl fumarate; 4:

Diethyl malate; 5: Ethyl ethoxyacetate; 6: Ethyl 4-hydroxybenzoate; 7: Ethyl vanillate; 8:

4-Hydroxybenzaldehyde; 9: Vanillin; 10: Syringaldehyde.

Figure S9. Effect of reaction time on the oxidation of lignin.

Reaction condition: 0.25 g bagasse lignin, 0.9 mmol [BSmim]CuPW12O40, 20 mL 80%

ethanol-water, 433 K, 0.8 MPa O2. 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl fumarate;

4: Diethyl malate; 5: Ethyl ethoxyacetate; 6: Ethyl 4-hydroxybenzoate; 7: Ethyl vanillate; 8:

4-Hydroxybenzaldehyde; 9: Vanillin; 10: Syringaldehyde.

Figure S10. Recycle performances of IL [BSmim]CuPW12O40 catalyst.

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol [BSmim]CuPW12O40, 20 mL 100% ethanol,

433 K, 5 h, 0.8 MPa O2. 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl fumarate; 4: Ethyl

ethoxyacetate; 5: Ethyl 3-ethoxypropionate; 6: Diethyl oxalate; 7: Diethyl malonate; 8: Ethyl

diethoxyacetate; 9: Ethyl 3,3-diethoxypropionate; 10: Ethyl 4-hydroxybenzoate; 11: Ethyl

vanillate.

Table S11. Selective oxidation of bagasse lignin in different aqueous ethanol.

Entry

Ethanol

concentration

(%)

Conversion

(%)

Yield (mg g-1) DEM

selectivity

(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 Total

1 0 29.3 1.7 2.7 2.0 1.9 —a — — — — — — — — — 8.3 20.5

2 20 46.8 16.6 2.6 7.3 9.5 — 4.2 1.1 8.4 4.2 — — — — — 53.9 30.8

3 40 84.4 61.0 8.0 16.6 25.6 8.3 — — — — — — — — — 119.5 51.0

4 60 87.3 83.1 12.5 16.3 23.1 0.9 16.5 5.1 19.5 11.3 0.3 — — — 188.6 44.1

5 80 90.7 153.6 4.8 12.0 5.8 16.9 25.2 31.7 — 9.1 — — — — — 259.1 59.3

6 100 92.9 274.7 13.8 22.4 — 3.4 8.3 12.1 — — — 3.9 13.8 44.4 1.6 398.4 69.0

7b 40 100 152.9 — 163.5 179.3 — — — — — — — — — — 495.7 30.8

8b 80 100 416.3 — 41.3 31.3 — — — — — — — — — — 488.9 85.1

9b 100 100 1111.5 — — — — — — — — — — — — — 1111.5 100.0

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol [BSmim]CuPW12O40 catalyst, 20 mL ethanol-water, 433 K, 5 h, 0.8 MPa O2. a not detected. b 0.25 g of maleic aicd as substrate. 1: Diethyl

maleate. 2: Diethyl succinate. 3: Diethyl fumarate. 4: Diethyl malate. 5: Ethyl ethoxyacetate. 6: Ethyl 4-hydroxybenzoate. 7: Ethyl vanillate. 8: 4-Hydroxybenzaldehyde. 9: Vanillin. 10:

Syringaldehyde. 11: Ethyl 3-ethoxypropionate. 12: Diehtyl malonate. 13: Ethyl diethoxyacetate. 14: Ethyl 3,3-diethoxypropionate.

Table S12. The elemental analysis of lignin before and after the reaction.

Entry Material Element content (%)

C H O[a] N S

1 Raw bagasse lignin 59.59 6.42 33.12 0.40 0.47

2 Oligomer 42.97 4.42 52.53 0.08 0.00

3 Regenerated lignin 43.11 4.27 52.31 0.31 0.00

Reaction condition: 0.25 g bagasse lignin, 0.9 mmol [BSmim]CuPW12O40, 20 mL 100% ethanol, 433 K, 5 h,

0.8 MPa O2. a Oxygen content (%) = (100－C－H－N－S)%; raw bagasse lignin :C9H11.65O3.75N0.05S0.02

(180.99 g/mol)

Table S13. The selective oxidation of bagasse lignin.

Entry

Atmosphere/time(h) Conv.

(%)

Yield (mg g-1)a DEM Sel.

(%) 1st run 2nd runb 1 2 3 4 5 6 7 8 9 10 11 Total

1 0.8 MPa O2/5.0 — 92.9 274.7 13.8 22.4 — 3.4 13.8 3.9 44.4 1.6 8.3 12.1 398.4 69.0

2 0.8 MPa O2/5.0 0.8 MPa N2/1.0 91.4 281.6 13.8 23.3 8.3 9.6 18.2 5.3 8.2 4.1 20.2 12.2 404.8 69.6

3 0.8 MPa O2/5.0 0.8 MPa O2/1.0 96.5 483.8 21.0 47.5 13.3 10.0 24.1 5.7 10.1 4.3 15.4 9.3 644.5 75.1

4 0.8 MPa O2/5.0 0.8 MPa O2/2.0 100 522.3 30.3 56.1 22.5 9.6 23.5 5.3 9.8 3.9 — — 683.3 76.4

5

0.2MPa N2-0.8 MPa

O2/5.0

— 94.2 348.6 17.3 32.7 12.0 10.1 20.1 4.2 9.3 4.6 16.2 11.2 486.3 71.7

6

0.4MPa N2-0.8 MPa

O2/5.0

— 94.2 350.1 18.7 31.2 13.2 11.3 20.2 4.7 10.1 5.2 18.7 12.4 495.8 70.6

7

0.2MPa N2-1.0 MPa

O2/5.0

— 94.4 357.2 18.1 33.6 14.1 10.9 19.8 5.3 9.7 5.8 19.3 13.1 506.9 70.5

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol BSmimCuPW12O40 catalyst, 20 mL 100% ethanol, 433 K. a 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl fumarate; 4: Diethyl malate;

5. Ethyl ethoxyacetate; 6: Diethyl malonate; 7: Ethyl 3-ethoxypropionate; 8: Ethyl diethoxyacetate; 9: Ethyl 3,3-diethoxypropionate; 10: Ethyl 4-hydroxybenzoate; 11: Ethyl vanillate. b change

in conversion and product distribution after re-testing the obtained product mixture from 1st run by injecting additional gas into the autoclave (2nd run) for the time specified.

Table S14. Gaseous composition before and after reaction.

Entry

Before/after reaction (vol. %)

O2 N2 CO2 CO

1a 94.1/79.5 5.9/16.1 -/4.4 -/-

2b 78.6/65.5 21.4/28.9 -/4.9 -/0.7

3c 65.9/51.3 34.1/44.2 -/3.7 -/0.8

4d 83.2/73.3 16.8/21.9 -/4.1 -/0.7

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol BSmimCuPW12O40 catalyst, 20 mL

ethanol, 433 K, 5h. a 0.8 MPa O2; b 0.2MPa N2 and 0.8 MPa O2;

c 0.4 MPa N2 and 0.8 MPa

O2; d 0.2 MPa N2 and 1.0 MPa O2.

Table S15. The molar content changes of gaseous fraction.

Entry

Before/after reaction(mmol)

O2 CO2 CO

1a 25.4/15.7 -/1.17 -/0.17

2b 25.5/15.3 -/1.11 -/0.24

3c 32.2/21.8 -/1.22 -/0.21

Reaction conditions: 0.25 g bagasse lignin, 0.9 mmol BSmimCuPW12O40 catalyst, 20 mL

ethanol, 433 K, 5h. a 0.2MPa N2 and 0.8 MPa O2; b 0.4 MPa N2 and 0.8 MPa O2;

c 0.2

MPa N2 and 1.0 MPa O2.

Table S16. Average molecular weight of raw lignin.

Sample Mw (g mol-1) Mn (g mol-1) Da

Rice stalk lignin 3319 2170 1.53

Wheat stalk lignin 3094 1693 1.83

Pine lignin 2984 1338 2.23

Bagasse lignin 2698 1156 2.33

Corn stalk lignin 2090 1170 1.79

Dealkaline ligninb 17029 15613 1.09

a D= Mw/Mn. b the acetylated dealkaline lignin.

Table S17. The oxidative ring cleavage of various lignin feedstocks.

Entry Lignin resource

Yield (mg g-1) Selectivity of

DEM (%) 1 2 3 4 5 6 7 8 9 10 11 Total

1 Wheat stalk 404.8 26.6 35.0 5.0 —a — 18.0 50.3 3.9 — 13.1 556.7 72.7

2 Corn stalk 308.7 15.9 22.9 3.5 — 13.4 12.2 46.1 1.9 7.9 8.7 441.2 70.0

3 Bagasse 274.7 13.8 22.4 3.4 3.9 — 13.8 44.4 1.6 8.3 12.1 398.4 69.0

4 Pine 255.2 19.4 32.0 2.4 — — 5.9 25.5 2.8 — 18.3 361.5 70.6

5 Rice straw 254.3 15.8 18.3 3.1 3.3 17.0 13.3 46.6 2.6 — 8.4 382.7 66.4

6 Dealkaline 210.6 13.7 24.6 — 3.3 — 3.8 20.6 2.4 — 17.9 296.9 70.9

Reaction conditions: 0.25 g lignin, 0.9 mmol [BSmim]CuPW12O40, 20 mL 100% ethanol, 433 K, 5 h, 0.8 MPa O2. a not detected. 1: Diethyl maleate; 2: Diethyl succinate; 3:

Diethyl fumarate; 4: Ethyl ethoxyacetate; 5: Ethyl 3-ethoxypropionate; 6: Diethyl oxalate; 7: Diethyl malonate; 8: Ethyl diethoxyacetate; 9: Ethyl 3,3-diethoxypropionate; 10:

Ethyl 4-hydroxybenzoate; 11: Ethyl vanillate.

Table S18. Assignment of FT-IR spectra.

Wave numbers (cm-1) The attributions of the main absorptions

3,443 OH stretching

2,941 C-H stretching in methyl and methylene groups

2,859 -CH2-symmetry stretching in methyl and methylene groups

1,702 C=O stretching in unconjugated ketones, carbonyls and in ester groups

1,608 Aromatic skeleton and C=O (S>G, G condensed>G etherified)

1,509 Aromatic skeleton (G>S)

1,467 C-H deformation (asymmetric CH3 and CH2)

1,426 Aromatic skeletal vibrations combined with C-H in plane deformations

1,368 C-H stretching of side groups (CH3)

1,327 S unit plus G unit condensed (G unit bound via position 5)

1,274 G ring and C=O stretching

1,233 C-C, C-O and C=O stretching

1,129 typical of S unit

1,087 secondary alcohol and aliphatic ether

1040 C-O of primary alcohol, Guaiacyl C-H

988 C-O valence vibration

894 C-H bending in aromatic skeleton

831 C-H out plane deformation

S: syringyl unit. G: guaiacyl unit

Figure S11. 13C-NMR spectra of original and regenerated bagasse lignin

Table S19. The oxidation of model compounds over [BSmim]CuPW12O40.

Entry Substrates Conversion

(%)

Yield (mg g-1)a DEM

selectivity

(%) 1 2 3 4 5 6 7 8 9 10 Total

1

100 176.7 16.9 20.6 3.5 — 7.7 23.9 2.5 — — 251.8 70.2

2

100 442.4 27.1 32.6 6.8 18.1 9.9 40.4 4.4 — — 581.7 76.1

3

100 446.5 28.0 36.2 6.9 17.5 11.5 38.7 2.9 — — 588.2 75.9

4

100 205.2 14.7 15.4 4.5 30.3 15.0 33.3 3.2 — — 321.6 63.8

5

100 257.3 12.9 21.7 4.9 25.8 21.3 41.8 2.4 — — 388.1 66.3

6

100 228.2 13.3 14.1 5.0 23.0 14.1 35.6 3.4 — — 336.7 67.8

7

100 368.4 19.7 24.1 5.0 14.6 14.1 51.7 4.8 — — 502.4 73.3

8

86.0 243.9 16.1 21.8 3.8 15.8 8.0 29.8 2.9 108.0 — 450.1 54.2

9

86.2 315.1 15.8 18.4 9.2 30.0 6.3 9.2 2.9 490.2b 64.3

10

100 357.8 14.6 16.2 3.9 21.5 13.7 37.4 3.2 — 48.6 516.9 69.2

11

100 360.5 13.7 21.8 4.6 36.5 24.1 48.1 2.3 — — 511.6 70.5

12

100 8.2 — — — — — — — 944.7 — 952.9 0.9

13

100 114.8 5.8 6.1 1.3 9.6 8.7 24.8 2.4 — 457.2 630.7 18.2

14

100 223.2 12.9 31.6 4.2 25.4 31.5 66.1 3.0 — — 397.9 56.1

15

3.5 5.1 — — — — — — — — — 5.1 100.0

16

63.0 102.5 3.0 4.6 2.0 11.8 0.6 21.1 2.0 — — 147.6 69.4

17

(4-O-5)

100 36.6 — — — — — — — — — 48.7c 75.2

18

(α-O-4)

91.7 27.5 — — 20.7 — — — — — — 902.8d 3.0

19

(5-5)

0.0 — — — — — — — — — — — —

20

(β-1)

100 30.4 0.8 — — — — — — — — 529.5e 5.7

Reaction conditions: 0.25 g model compound, 0.9 mmol [BSmim]CuPW12O40 catalyst, 20 mL 100% ethanol, 433 K, 5 h, 0.8 MPa O2. a not detected;

b including 83.3 mg g-1 of other volatile products; c propanedioic acid, diethyl ester: 12.1 mg g-1; d benzoic acid, ethyl ester: 839.9 mg g-1, benzoic

acid:14.7 mg g-1; e benzene, 1-methoxy-4-methyl-: 10.6 mg g-1; benzene, 1-ethyl-4-methoxy-: 6.6 mg g-1; benzenemethanol, 4-methoxy-, formate:

19.3 mg g-1; benzaldehyde, 4-methoxy-: 50.1 mg g-1; benzoic acid, 4-methoxy-, ethyl ester: 396.3 mg g-1; benzoic acid, 4-methoxy-,4-ethylphenyl

ester: 15.4 mg g-1. 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl fumarate; 4: Ethyl ethoxyacetate; 5: Diethyl oxalate; 6: Diethyl malonate; 7:

Ethyl diethoxyacetate; 8: Ethyl 3,3-diethoxypropionate; 9: Ethyl 4-hydroxybenzoate; 10: Ethyl vanillate.

Plausible pathway for lignin depolymerization and oxidative ring cleavage

Based on above mentioned process of lignin oxidative ring cleavage, product distribution and

typical lignin depolymerization product oxidation, a plausible pathway for this efficient

depolymerization of lignin and DEM formation from the renewable lignin is proposed in the context

of literature (Figure S12).9-12 For the DEM production, the lignin (1) is firstly dehydrated under

strong acidic condition to form vinyl ether (2)11 in adjacent to the β-aryl ether bond probably via

benzylic carbocation formation. Without catalyst, the reactive carbocation may attack electron-rich

lignin aromatic rings leading to C-C formation (Shuai et al. used formaldehyde to block this

reactive species during hydrogenolysis of lignin to produce improved yields of monomers).13-15

However, we show that in the intimate contact of Cu2+ polyoxometalate in air, this ether reacts with

superoxo species from the Cu active site (see Cu catalyst elucidation in next section) to produce a

dioxetane intermediate (3). Subsequently, the C−C and O−O bond cleavage of compound 4 occurs

to afford ester (5) and aldehyde (6). It should be emphasized here that the initial aromatic aldehydes

and glycolic acid which is produced by the above-mentioned intermediates can be directly detected

by gas chromatography mass spectrometry (GC-MS) in form of esters (Table S10).

Figure S12. The pathway for lignin depolymerization and in situ aromatic ring oxidation.

Then, extensively acetalization of aldehyde (6) in ethanol occurs with

4-(diethoxymethyl)-2-methoxyphenol as the product. This compound can be easily oxidized to

benzoquinones, the main intermediate for maleic acid production in the phenol oxidation

technology, after a series continuous process of oxidation and demethoxylation.10,16,17 Further, the

quinones are converted to maleic acid via ring cleavage.18 Finally, the product of stable DEM is

achieved by the rapid esterification of maleic acid in ethanol without much side reactions. On the

other hand, lignin (1) can also be initially oxidized to ketones (7), which undergoes Keto-enol

tautomerism step (this process can be confirmed by above HSQC, FT-IR, and 13C-NMR

characterization of the original and regenerated lignin as shown in Figure 3 and Figure S11) to form

the vinyl ether (8). The compound (8) is then transformed to ester (5) and carboxylic acid (10)

through intermediate 9 via a similar C-C bond and O-O bond cleavage as that of chemical (3).9,12,19

Simultaneously, the carboxylic acid (10) is esterified with ethanol. Both the carboxylic acid and

ester would be decarboxylated20 as anticipated over the POM-IL catalyst. The decarboxylation

product is then oxidized to benzoquinones, which finally gives the target product of DEM through

the same oxidation and esterification process as that of aldehyde (6). To support this cleavage

pathway, oxidation of quinones is investigated. As shown in Table S20, the intermediates could be

converted efficiently with 100% conversion in the ethanol, giving DEM selectivity of higher than

70% for each run.

Table S20. Selective oxidation of typical model compounds.

Entry Substrates

Conversion

(%)

Yield (mg g-1)a DEM

selectivity

(%)

1 2 3 4 5 6 7 8 total

1

100 447.2 26.4 32.4 6.4 18.3 10.8 37.9 2.7 582.1 76.8

2

100 377.3 19.7 23.2 7.5 25.2 12.7 36.3 — 501.9 75.2

3

100 585.4 29.6 43.0 7.3 — 14.0 40.3 2.4 722.0 81.1

4

100 412.5 24.3 31.2 6.3 25.6 15.6 44.1 2.8 562.4 73.3

5

100 389.5 20.2 27.2 5.7 14.5 9.7 31.7 2.4 500.9 77.8

6

100 356.2 21.6 34.8 5.8 22.1 13.1 43.1 2.2 498.9 71.4

7

100 451.5 21.2 37.9 5.3 10.2 11.9 44.3 2.5 584.8 77.2

Reaction conditions: 0.25 g substrate, 0.9 mmol [BSmim]CuPW12O40, 20 mL 100% ethanol, 433 K, 5 h, 0.8 MPa O2. a not

detected. 1: Diethyl maleate; 2: Diethyl succinate; 3: Diethyl fumarate 4: Ethyl ethoxyacetate; 5: Diethyl oxalate; 6:

Diethyl malonate; 7: Ethyl diethoxyacetate; 8: Ethyl 3,3-diethoxypropionate.

Potential Separation of DEM

DEM can be separated with chromatographic techniques using an eluent composed of

n-hexane and petroleum ether at the bench-scale. At larger scale it can be separated using

rectification on the basis of boiling point differences between the products. There are several filed

patents regarding the recovery and purification of maleic acid esters: process for the recovery of

dialkyl succinate or dialkyl maleate (WO2016174388A1), process for the production of dimethyl

maleate (EP1678117B1), process for the preparation of dimethyl maleate (US4827022A),

dimethyl maleate purification method (CN106187775A).

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