Selective Production of Diethyl Maleate via Oxidative Cleavage of Lignin Aromatic Unit

Full text of DOI:10.1016/j.chempr.2019.05.021

Article
Selective Production of Diethyl Maleate via
Oxidative 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 gÀ1 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, 1–13
September 12, 2019 ª 2019 Elsevier Inc.
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Please cite this article in press as: Cai et al., Selective Production of Diethyl Maleate via Oxidative Cleavage of Lignin Aromatic Unit, Chem (2019),
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Article
Selective Production of Diethyl Maleate
via Oxidative Cleavage
of Lignin Aromatic Unit
Zhenping Cai,1,5 Jinxing Long,1,5 Yingwen Li,1 Lin Ye,2 Biaolin Yin,1 Liam John France,1 Juncai Dong,3
1,6,
Lirong Zheng,3 Hongyan He,4 Sijie Liu,1 Shik Chi Edman Tsang,2, and Xuehui Li
*
*
SUMMARY
The Bigger Picture
As a major component of
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 gꢀ1 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.
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
the advantages of ionic liquid and
polyoxometalate with the
INTRODUCTION
introduction of acidity, reduction-
oxidation, and miscibility
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, and most 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.
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.
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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 gꢀ1 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.
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),
1School of Chemistry and Chemical Engineering,
State Key Laboratory of Pulp & Paper
Engineering, South China University of
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 gꢀ1 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
Technology, Guangzhou 510640, China
2Wolfson Catalysis, Inorganic Chemistry
Laboratory, University of Oxford, Oxford OX1
3QR, UK
3Beijing Synchrotron Radiation Facility, Institute
of High Energy Physics, Chinese Academy of
Sciences, Beijing 100049, China
4Beijing Key Laboratory of Ionic Liquids Clean
Process, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing 100190,
China
5These authors contributed equally
6Lead Contact
and S9). POM-ILs are composed of [BSmim]+ cations and [MPW12O40 ꢀ polyoxome-
]
*Correspondence:
edman.tsang@chem.ox.ac.uk (S.C.E.T.),
cexhli@scut.edu.cn (X.L.)
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 gꢀ1, which
https://doi.org/10.1016/j.chempr.2019.05.021
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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 gꢀ1 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
3
3
˚
˚
volumes were 4,856.1(1) A for Cu-POM1 and 3,068.1(1) A 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
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Table 1. Selectively Catalytic Oxidation of Bagasse Lignin to DEM
Entry
Catalysts
Solventa
Conversion (%)
Yield of Volatile Product (mg gꢀ1
)
Selectivity (%)d
DEM
–
C4 Esterb
–
Othersc
14.7
32.2
9.3
Total
14.7
DEM
0
C4 Ester
e
1
–
80:20
80:20
80:20
80:20
80:20
80:20
80:20
80:20
80:20
80:20
48.2
82.4
81.3
80.9
80.6
84.3
90.7
80.8
86.8
90.0
0
2
[BSmim]HSO4
6.5
15.9
48.1
13.5
77.6
67.3
63.7
60.4
59.3
23.8
22.1
62.1
33.1
86.5
82.6
72.1
69.8
68.0
36.1
27.9
68.4
3
[BSmim]Na2PW12O40
[BSmim]MnPW12O40
[BSmim]CoPW12O40
[BSmim]NiPW12O40
[BSmim]CuPW12O40
[Bmim]CuPW12O40
H2SO4/CuSO4
53.6
87.7
93.3
100.4
153.6
11.0
16.5
100.9
59.8
69.1
4
107.6
105.6
116.0
176.2
16.7
22.7
40.9
50.3
82.9
29.5
53.7
51.4
130.3
146.5
166.3
259.1
46.2
5
6
7
8f
9g
10h
20.8
74.5
[BSmim]Na2PW12O40
CuSO4
/
111.2
162.6
11i
12
[BSmim]CuPW12O40
[BSmim]CuPW12O40
[BSmim]CuPW12O40
[BSmim]CuPW12O40
[BSmim]CuPW12O40
80:20
100:0
40:60
80:20
100:0
31.3
–
–
78.6
78.6
0
0
92.9
274.7
152.9
416.3
1,111.5
310.9
495.7
488.9
1,111.5
87.5
398.4
495.7
488.9
1,111.5
69.0
30.8
85.1
100.0
78.0
100.0
100.0
100.0
13j
14j
15j
100.0
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)
3ꢀ, 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
4
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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
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
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
.
.
aN, coordination number; R, distance between absorber and backscatter atoms;
G
b
˚
˚
O3 represent the first, second, and third nearest neighbor coordination O atoms. Fitting range: 2.5 % k (/A) % 12.5 and 1.6 % R (A) % 2.8.
c
d
˚
˚
˚
˚
Fitting range: 2.5 % k (/A) % 12.5 and 1.0 % R (A) % 2.0. Cu 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 have many 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.29 We 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.
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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 gꢀ1), 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-CuSO4 mixture (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 gꢀ1 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
6
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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 gꢀ1) 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 348.6 mg gꢀ1 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 system was also checked with 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 gꢀ1) with a high DEM
yield (404.8 mg gꢀ1) 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, 1–13, September 12, 2019
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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 in
the 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 cmꢀ1
1,129 cm
ꢀ1 35 in regenerated lignin than for those in original lignin (Figure 3C; Table
) and typical S unit (1,327 and
)
S18). However, two strong modes ca. 1,087 and 988 cmꢀ1 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
8
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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 gꢀ1 of volatile products containing 176.7 mg gꢀ1 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 gꢀ1, respectively). This effect can be ascribed to the favorability
of catechol- and hydroquinol-forming benzoquinone, an intermediate of phenol
oxidation and a key starting material 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
ꢀ1
example, the yields of total volatile products and DEM increased from 251.8
and 176.7 mg gꢀ1 to 321.6 ꢀ1 and 205.2 mg gꢀ1, 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 gꢀ1 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 gꢀ1) 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
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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
10
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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 gꢀ1 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.
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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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