Mild Redox-Neutral Depolymerization of Lignin with a Binuclear Rh Complex in Water

Article abstract of DOI:10.1021/acscatal.9b00669

A mild redox-neutral lignin depolymerization system featuring a water-soluble binuclear Rh complex has been developed. The catalytic system could be successfully applied to the depolymerization of a lignin-like polymer, alkaline lignin, as well as raw lignocellulose samples to produce aromatic ketones, providing a homogeneous catalytic system for "lignin-first" biorefinery in water. Mechanistic studies on the model substrate suggest that the reaction proceeds via a metal-catalyzed dehydrogenation step to afford a carbonyl intermediate, followed by C-O bond cleavage to afford ketone and phenol products. Deuterium labeling study shows that the hydrogen used for cleavage of the C-O bond originates from the alcohol moiety in the substrate.

Full text of DOI:10.1021/acscatal.9b00669

Letter
Cite This: ACS Catal. 2019, 9, 4441−4447
pubs.acs.org/acscatalysis
Mild Redox-Neutral Depolymerization of Lignin with a Binuclear Rh
Complex in Water
†
,‡
,‡,∥
Wang Miao, Weijun Tang, Dong Xue, Chaoqun Li, Bo Zhang,‡
†
‡
†
†
†
Yuxuan Liu, Changzhi Li,*
Jianliang Xiao,†,§ Aiqin Wang,‡,∥ Tao Zhang, and Chao Wang*
,†
†Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an, 710062, China
‡
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
§
Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, U.K.
∥
Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian, 116023, China
*
S Supporting Information
ABSTRACT: A mild redox-neutral lignin depolymerization system featuring a water-soluble binuclear Rh complex has been
developed. The catalytic system could be successfully applied to the depolymerization of a lignin-like polymer, alkaline lignin, as
well as raw lignocellulose samples to produce aromatic ketones, providing a homogeneous catalytic system for “lignin-first”
biorefinery in water. Mechanistic studies on the model substrate suggest that the reaction proceeds via a metal-catalyzed
dehydrogenation step to afford a carbonyl intermediate, followed by C−O bond cleavage to afford ketone and phenol products.
Deuterium labeling study shows that the hydrogen used for cleavage of the C−O bond originates from the alcohol moiety in the
substrate.
KEYWORDS: lignin depolymerization, redox-neutral, rhodium complex, lignin-first biorefinery, aqueous reaction
he development of effective depolymerization methods is
pivotal to lignin valorization. The catalytic cleavage of β-
O-4 linkages has attracted a great deal of attention because of
mild reaction conditions and provide models for mechanistic
studies. In addition, homogeneous catalysts might be superior
to heterogeneous catalysts for the depolymerization of native
lignin in terms of mass transfer, noting that native lignins are
generally insoluble solids. The first homogeneous catalytic
system for redox-neutral cleavage of C−O bond of β-O-4
model compounds, as well as a synthetic lignin model polymer,
to afford ketone and phenol products, was reported by
1
T
their high abundance (45−60% of all linkages) in native lignin
2
3
4
structure. Both oxidative and reductive catalytic systems for
the cleavage of β-O-4 linkages have been extensively studied by
1b,4a
several groups including our own.
More recently, a two-
step strategy combining oxidation and hydrogenation reactions
has also been regarded as an efficient approach to lignin
7
Bergman and Ellman and their co-workers, which was further
3
k,5
valorization. From the atom-economic and green chemistry
viewpoint, the redox-neutral cleavage of β-O-4 linkages would
be more ideal, as it avoids the use of extra oxidants or
reductants, which are often hazardous and generate waste.
The typical structure of β-O-4 linkage usually contains
hydroxyl groups in its side-chain and has the potential to be
dehydrogenated to offer hydrogen for the cleavage of bonds in
lignin, thus providing a good platform for redox-neutral
cleavage. Several heterogeneous catalytic systems have been
reported for the redox-neutral cleavage of β-O-4 linkage in
8
9
studied by James and Li and their co-workers. Klankermayer
and co-workers reported a Ru catalyzed redox-neutral cleavage
1
0
of C−C bond in lignin model substrates. Lancefield,
Bruijnincx, and co-workers also reported the cleavage of C−
C bond under redox-neutral conditions, which they have
applied to the depolymerization of extracted lignin and
11
lignocellulose. Despite this progress, homogeneous catalysts
Received: February 15, 2019
Revised: April 9, 2019
6
lignin. However, structurally well-defined homogeneous
catalysts have rarely been reported, although they might offer
©
XXXX American Chemical Society
4441
DOI: 10.1021/acscatal.9b00669
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Letter
Figure 1. Rh-catalyzed redox-neutral cleavage of a lignin model substrate in water. Reaction conditions: 2a (0.2 mmol), Catalyst (0.002 mmol),
H O (1 mL), 110 °C, 12 h, under Ar. Yields were determined by GC-FID with diphenyl as internal standard.
2
1
2
for redox-neutral “lignin-first” depolymerization of lignocel-
lulose are still highly desirable.
5 (89% yield), along with a small amount of the
dehydrogenated intermediate 3, with 1 mol % of 1 as catalyst
in the presence of NaOH in water at 110 °C for 18 h under Ar
atmosphere (Figure 1, entry 1). Both the catalyst 1 and NaOH
are essential for the reaction (Figure 1, entries 2, 3). The
reaction could take place at 100 °C or with lower catalyst
loading, albeit with lower conversion and yields (Figure 1,
entries 4, 5). Other Rh complexes showed much lower activity
than 1 (Figure 1, entries 6, 7).
Herein, we report a redox-neutral lignin depolymerization
system based on a water-soluble binuclear Rh complex. The
catalytic system can cleave a series of lignin model compounds
into ketones and phenols under mild conditions in water. The
water-soluble Rh catalyst could be recycled at least 18 times
without a noticeable decrease in activity for the model
substrate. Most notably, the catalytic system could be
successfully applied to the depolymerization of a lignin-like
polymer, alkaline lignin, as well as native lignin samples,
providing excellent yield of depolymerized aromatic oil with
the monomers mainly consisting of aromatic ketone
compounds. In particular, almost complete deconstruction of
lignin components from a raw lignocellulose sample has been
achieved, providing an example of “lignin-first” catalytic system
with a homogeneous catalyst.
Various other lignin model substrates 2b−i were then tested
under the above optimized conditions, and all of them could
be fully consumed in 18 or 48 h with moderate to high yields
of ketones and phenols (Figure 2). Methoxy groups at ortho-,
meta-, and para-positions on both phenyl rings, and a hydroxyl
group on the α-phenyl ring (Figure 2, entries 1−6), which are
commonly found in natural lignin structure, are all tolerated.
Cleaving β-O-4 model compounds containing γ−OH
In our previous study, we discovered that the binuclear Rh
complex 1 (Figure 1) displayed unique activity in dehydrogen-
ation of alcohols, and the hydrogen removed from alcohols
functionality with a redox-neutral homogeneous catalytic
7−9
system is a challenge,
are reported.
and only a few successful examples
1
0,11
Delightfully, such kind of substrates can be
could be released as hydrogen gas, intercepted by O , or used
2
13
cleaved in our binuclear Rh catalytic system (Figure 2, entries
−9). Full conversions were obtained, and two major ketone
for the reduction of olefinic bonds. This special property of
the binuclear Rh catalyst prompted us to investigate its
potential in the dehydrogenative depolymerization of lignin
because the alkyl−aryl ether linkage between aromatic rings
features different alcohol groups. If the alcohol structure in
lignin can be dehydrogenated by the Rh catalyst to afford a
ketone intermediate and a Rh hydride complex, the ether C−O
bond (bond energy 55.9 kcal/mol) of the ketone intermediate
would be weakened compared with that in lignin (bond energy
7
products were detected for these substrates. We noticed that
the yields of the major ketone products and phenols were
lower than expected. A detailed analysis of the products/
intermediates/byproducts, as well as a plausible mechanism for
their formation is given in the Supporting Information (see
Section 5 in the SI). The ability to cleave substrates with γ-OH
functionality suggests that the catalytic system has the potential
for the depolymerization of real lignin (vide infra). One major
challenge of the homogeneous catalysts is the recyclability. The
binuclear Rh catalyst is water-soluble while the ketone product
has very poor solubility in water. Thus, after each reaction, the
ketone product was readily extracted with petroleum ether, and
the solution containing the catalyst could be reused in the next
run.
5
b
6
9.2 kcal/mol), and consequently, the ether C−O bond in
the ketone intermediate might be cleaved by the Rh hydride
complex to afford ketone and phenol products. A typical lignin
model compound 2a was chosen as the substrate to verify our
hypothesis. After a series of optimization (Figure 1 and Table
S1−S2 in SI), compound 2a was indeed found to be
completely cleaved into a ketone 4 (86% yield) and a phenol
4
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Figure 2. Rh-catalyzed cleavage of different lignin model substrates. See SI for experimental details.
7
−10
As shown in Scheme S2, the catalyst could be reused in the
next run. As shown in Scheme S2, the catalyst could be
recycled at least 18 times for the transformation of the model
substrate 2a (see Section 6 in SI for details). The recyclability
of the catalytic system is superior to most other homogeneous
systems
and even a certain number of heterogeneous
6a−f
catalytic systems
(the number of recycles is normally within
10 for reported heterogeneous catalysts). The versatility of the
catalytic system is further demonstrated by a gram-scale
reaction with 1.2 g of 2a as the substrate; full conversion with
4
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Figure 3. 2D-HSQC-NMR analysis of an alkaline lignin sample and the oil obtained after depolymerization of the alkaline lignin and a native
lignocellulose. (a,b) Alkaline lignin sample; (c,d) Oil obtained from the alkaline lignin sample; (e,f) Oil obtained from the native lignocellulose.
high yields of 4a and 5a (88% and 90%, respectively) were
mechanism for real lignin, 2D-HSQC-NMR spectra of the
alkaline lignin and the oil products were compared (Figure 3).
Figure 3a,c illustrate the side chain region of the lignin and its
oil product. It was found that cross signals of methoxy groups
and β-O-4 aryl ether were the dominant linkages in basswood
lignin. In addition, a small amount of β-5/α-O-4 linkages
(resinol structure B 22.8%) and phenylcoumaran (β−β,
Structure C 2.1%) were observed. After the reaction, most of
the A, B, C linkages of the lignin disappeared in the side-chain
region of the oil (Figure 3, panel a vs c), indicating that the
three major linkages of A, B, and C are effectively cleaved by
the catalyst.
obtained in 40 h (Scheme S3). Moreover, a lignin-like polymer
14
was also synthesized and tested in the catalytic system, which
gave complete conversion with a good monomer yield of 80%
(Scheme S4).
The results obtained with the model compounds encouraged
us to apply the methodology to the conversion of authentic
lignin feedstock. One typical alkaline lignin extracted from
basswood was chosen as the substrate to be depolymerized by
1. Pleasingly, the alkaline lignin was effectively converted into
liquid oil with a high yield of 88 wt %. MALDI-TOF
characterization of the oil (Scheme S7) showed that the
obtained aromatic products are exclusively in the m/z = 0−400
range, indicating monomers and dimers as the major products.
The quantified monomers in the oil were about 11 wt % as
measured by GC-FID (Table S5), with aromatic ketones as the
major products. To gain some insight into the cleavage
Typically, basswood lignin belongs to G-S lignin, which
comprises a mixture of S and G units along with a trace
15
amount of the H unit. Hence, the aromatic region of both
the lignin material and the resulting bio-oil showed similar
S:G:H molar ratios (78.5%, 18.8% and 2.7% vs 80.7%, 18.4%
4
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Figure 4. Mechanistic studies. (a,b) Verification of intermediate. (c) Deuterium labeling experiment.
and 0.9%) in 2D-HSQC-NMR spectra (Figure 3b,d).
However, it is interesting to note that the S/S′ and G/G′
molar ratio of the oil products (Figure 3d, 0.88, 0.83) was
significantly lower than that of lignin (Figure 3b, 6.38, 13.78).
The increased molar ratio of S′ in the oil products indicates the
selective even for real lignin material. Aromatic ketone
products are valuable fine chemicals.
The mechanism of the catalytic system was then investigated
with model substrates. As there is no external hydrogen source
added and the binuclear Rh complex has been shown to
13b
dehydrogenation of the C −OH group. Similarly, certain
catalyze the dehydrogenation of alcohols to ketones,
we
α
amounts of G ′, G ′, and G ′ appeared in the oil products
reasoned that the hydroxyl group of 2a would be first
dehydrogenated to form a ketone intermediate 3, whose ether
C−O bond would then be cleaved by a Rh hydride
intermediate. As shown in Figure 1, compound 3 was observed
in the reaction. 3 was then synthesized, and it was almost
inactive under the standard catalytic condition for 18 h in the
absence of a hydrogen source (Figure 4a). Interestingly, when
1 equiv of 1-phenylethan-1-ol was added, 74% of 3 was
converted to 65% of 4a and 71% of 5a under the standard
conditions for 18 h (Figure 4b); meanwhile, 1-phenylethan-1-
ol was completely consumed and 74% acetophenone was
detected, suggesting that the ether C−O bond of 3 could be
selectively cleaved to afford the ketone and phenol product
with an external hydrogen source. These results suggest that 3
2
5
6
(
Figure 3, panel b vs d), indicating the dehydrogenation of
α
C −OH group in the neighboring G-type units. These results
suggest that the C −OH groups in the lignin materials have
α
been transformed into ketone groups and accordingly, the
cleavage of C−O bonds in real lignin likely follows the same
reaction pathway as that proposed in Figure 5 (vide infra).
Further, raw basswood powder (40−60 mesh, 1000 mg) without
pretreatment was also submitted to deconstruction catalyzed by
1
. Impressively, 26.6 wt % aromatic oil was obtained in water
at 110 °C for 24 h. The total yield of the monomers is 2.3 wt %
based on the starting raw basswood powder. HPLC-MS
analysis showed that the crude product did not contain any
sugar derivatives, such as glucose, xylose, 5-hydroxylmethyl-
furfural, and polyols, and the distribution of aromatic
monomers in the oil was similar to that obtained from isolated
lignin. 2D-HSQC-NMR characterization showed that the
S:G:H molar ratio of the oil was in agreement with that of
the alkaline lignin, and the S/S′, G/G′ molar ratio was 0.77,
6,7
is an intermediate during the reaction, and the same process
may be happening for real lignin cleavage.
Deuterium labeling study with deuterium labeled 2a′ as
substrate and toluene as solvent further confirmed the
hydrogen transfer process for the redox-neutral cleavage
reaction (Figure 4c). It clearly showed that the deuterium
atom on the carbon adjacent to the hydroxyl group could be
transferred to the methyl group of the ketone product,
implying that the removed hydrogens from the alcohol group
participate in the subsequent cleavage of the ether C−O bond.
Further, KIE experiments indicated that the dehydrogenation
of 2a to form 3 is the rate-limiting step (Scheme S13).
On the basis of these mechanistic studies, a catalytic
mechanism for the lignin cleavage is tentatively proposed
(Figure 5). Under basic conditions, the chloride in the Rh
catalyst might first be replaced by a hydroxide to form complex
I, which might be the real catalyst. Complex I could then
deprotonate the lignin substrate II to form an intermediate III,
which would undergo β-hydride elimination to produce a
ketone intermediate IV and a Rh hydride V. IV could then be
0
.46, respectively (Figure 3e,f). Noting that the lignin content
in this basswood powder is determined to be 28 wt % in our
1
5
previous work, it is clear that most of the lignin has been
selectively deconstructed to the aromatic oil with the
molecular weight in the range of 0−500 (Scheme S9), leaving
the other two components (cellulose and hemicellulose)
almost intact as a solid residue (see Section 11 in SI for
details). Enzymatic hydrolysis experiment confirms that the
solid residue is mainly consist of cellulose and hemicellulose
because glucose and xylose are the main hydrolysis products.
The rate of enzymatic hydrolysis of the solid residue is much
faster than that of raw basswood powder (Scheme S11),
12
showcasing the advantage of this lignin first system.
GC-FID analysis of the monomers obtained from both the
extracted lignin and raw basswood powder also showed that
similar products were obtained, and aromatic ketones were the
dominant monomer products (see Schemes S8 and S10, SI).
These results suggest that the catalytic system operates via the
same mechanism as that of the model substrates and is highly
cleaved by V with the aid of H O to afford the cleaved
2
products and regenerate I (Figure 5). As 2D-HSQC-NMR has
revealed that the C −OH groups in the lignin materials were
α
transformed into ketone groups and the depolymerization of
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*
E-mail: c.wang@snnu.edu.cn (C.W.).
ORCID
Changzhi Li: 0000-0001-6748-2575
Dong Xue: 0000-0002-7269-6356
Aiqin Wang: 0000-0003-4552-0360
Tao Zhang: 0000-0001-9470-7215
Chao Wang: 0000-0003-4812-6000
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge Ms. Qitian Huang and Prof.
Zongbao Kent Zhao for performing the enzymatic hydrolysis
reaction. This research was supported by the 2017 Royal
Society International Collaboration Award (IC170044), the
National Natural Science Foundation of China (21773145,
2
1473109, 21690080, 21690083, 21878288), Science and
Technology Program of Shaanxi Province (2016KJXX-26),
Projects for the Academic Leaders and Academic Backbones,
Shaanxi Normal University (16QNGG008), the 111 project
Figure 5. Proposed mechanism for the cleavage of lignin.
(
B14041), the Fundamental Research Funds for the Central
extracted lignin and lignocellulose mainly produced aromatic
ketone products, we thus propose that the cleavage of aryl C−
O bonds in real lignin follows the same mechanism as that for
the model substrates (Figure 5). It is therefore very interesting
to note that hydrogenolysis of real lignin does not involve an
external hydrogen source over the binuclear Rh complex under
such mild conditions (110 °C, 1 atm Ar) in water, a green and
readily available solvent. Hence, our catalytic system holds
great promise for the redox-neutral production of aromatic
compounds from lignin, which has been identified as a key
challenge in the biorefinery area.
In conclusion, a redox-neutral lignin depolymerization
system featuring a water-soluble binuclear Rh complex is
reported. The catalytic system can cleave a series of lignin
model compounds, lignin-like polymer, alkaline lignin, as well
as raw lignocellulose samples, into ketones and phenols under
mild conditions in water. Mechanistic studies suggest that
benzyl-hydroxyl units in the side-chain between the aromatic
units are the hydrogen source for the cleavage of the ether C−
O bonds, and an intramolecular dehydrogenation-hydro-
genolysis cascade pathway is coupled to afford excellent yield
of aromatics. In particular, almost complete deconstruction of
lignin component from a raw lignocellulose sample was
Universities (GK201803079), and DNL cooperation fund CAS
(DNL180302).
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ACS Catal. 2019, 9, 4441−4447