Selectively Upgrading Lignin Derivatives to Carboxylates through Electrochemical Oxidative C(OH)?C Bond Cleavage by a Mn-Doped Cobalt Oxyhydroxide Catalyst

Article abstract of DOI:10.1002/anie.202015431

Oxidative cleavage of C(OH)?C bonds to afford carboxylates is of significant importance for the petrochemical industry and biomass valorization. Here we report an efficient electrochemical strategy for the selective upgrading of lignin derivatives to carboxylates by a manganese-doped cobalt oxyhydroxide (MnCoOOH) catalyst. A wide range of lignin-derived substrates with C(OH)-C or C(O)-C units undergo efficient cleavage to corresponding carboxylates in excellent yields (80–99 %) and operational stability (200 h). Detailed investigations reveal a tandem oxidation mechanism that base from the electrolyte converts secondary alcohols and their derived ketones to reactive nucleophiles, which are oxidized by electrophilic oxygen species on MnCoOOH from water. As proof of concept, this approach was applied to upgrade lignin derivatives with C(OH)-C or C(O)-C motifs, achieving convergent transformation of lignin-derived mixtures to benzoate and KA oil to adipate with 91.5 % and 64.2 % yields, respectively.

Full text of DOI:10.1002/anie.202015431

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 8976–8982
International Edition:
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doi.org/10.1002/anie.202015431
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Lignin Upgrading
Hot Paper
Selectively Upgrading Lignin Derivatives to Carboxylates through
À
Electrochemical Oxidative C(OH) C Bond Cleavage by a Mn-Doped
Cobalt Oxyhydroxide Catalyst
Hua Zhou+, Zhenhua Li+, Si-Min Xu, Lilin Lu, Ming Xu, Kaiyue Ji, Ruixiang Ge, Yifan Yan,
Lina Ma, Xianggui Kong, Lirong Zheng, and Haohong Duan*
À
Abstract: Oxidative cleavage of C(OH) C bonds to afford
carboxylates is of significant importance for the petrochemical
industry and biomass valorization. Here we report an efficient
electrochemical strategy for the selective upgrading of lignin
derivatives to carboxylates by a manganese-doped cobalt
oxyhydroxide (MnCoOOH) catalyst. A wide range of lignin-
derived substrates with C(OH)-C or C(O)-C units undergo
efficient cleavage to corresponding carboxylates in excellent
yields (80–99%) and operational stability (200 h). Detailed
investigations reveal a tandem oxidation mechanism that base
from the electrolyte converts secondary alcohols and their
derived ketones to reactive nucleophiles, which are oxidized by
electrophilic oxygen species on MnCoOOH from water. As
proof of concept, this approach was applied to upgrade lignin
derivatives with C(OH)-C or C(O)-C motifs, achieving con-
vergent transformation of lignin-derived mixtures to benzoate
and KA oil to adipate with 91.5% and 64.2% yields,
respectively.
Introduction
Carboxylic acids represent an important class of com-
pounds in the fine and bulk chemical industry with wideap-
plications as food preservatives, pharmaceuticals, and build-
Scheme 1. a) Industrial processes for the production of carboxylic
acids. b) Oxidative C(OH)-C cleavage of lignin derivatives. c) Proposed
electrochemical strategy for upgrading lignin-derived platform mole-
ing blocks of polymers.[1] Most of these commodities are
currently produced from fossil-derived hydrocarbons via
tandem oxidation (Scheme 1a). Alternatively, lignin, a main
constituent of biomass, offers a sustainable source to alleviate
cules coupled with HER.
the petroleum depleting and associated environmental is-
sues.[2] Notably, carbohydrates and lignin features C(OH)-C
motifs,[3] therefore providing a feasible route to carboxylic
[*] Dr. H. Zhou,[+] K. Ji, Dr. R. Ge, Prof. H. Duan
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
À
acids via selective C C bonds cleavage (Scheme 1b). In this
respect, several thermocatalytic methods have been recently
E-mail: hhduan@mail.tsinghua.edu.cn
Prof. Z. Li,[+] Dr. S.-M. Xu, Prof. M. Xu, Y. Yan, Dr. L. Ma, Dr. X. Kong
State Key Laboratory of Chemical Resource Engineering
College of Chemistry, Beijing University of Chemical Technology
Beijing 100029 (China)
À
developed for aerobic oxidative cleavage of C(OH) C bonds,
achieving efficient transformation of lignin models to corre-
sponding carboxylic acids.[2b–d] Nevertheless, these processes
often require homogeneous catalysts (e.g., copper or vana-
dium) or toxic nitrogen oxides, and are operated at elevated
temperature (Table S1).[2c,d,4] Therefore, the development of
more sustainable procedures for oxidative transformation of
Dr. L. Lu
School of Chemistry and Chemical Engineering
Wuhan University of Science and Technology
Wuhan 430081 (China)
Dr. L. Zheng
À
C(OH) C bonds in lignin and its derivatives to produce
Institute of High Energy Physics, Chinese Academy of Sciences
Beijing 100049 (China)
carboxylic acids continues to attract significant interest.
Electrochemical oxidation provides a promising alterna-
tive to synthesize value-added organic oxygenates instead of
oxygen evolution reaction (OER). Advantageously, the
thermodynamically favorable anodic organic oxidation facil-
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202015431.
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itates the improvement of energy efficiency of the overall
electrolysis paired with cathodic half reactions, for example,
hydrogen evolution reaction (HER) and CO2 reduction.[5] To
that end, much research interests have focused on exploring
oxidative transformation of organic molecules.[5b, 6] Among
them, biomass platform chemicals have received special
concern for carboxylic acids production from primary alco-
hols.[5b, 7] Despite these efforts, the electrochemical oxidation
À
of lignin-derived products via oxidative C(OH) C bonds
cleavage is underexplored possibly due to its high binding
dissociation energy (260–300 kJmolÀ1).[8]
Herein we report an electrochemical oxidation strategy
for selective upgrading of lignin-derived secondary alcohols
or ketones into carboxylates over a MnCoOOH catalyst
integrated with water splitting (Scheme 1c). The MnCoOOH
catalyst shows good generality to various secondary alcohols
and ketones to corresponding carboxylates in satisfactory
yield (64–99%) and excellent operational stability (200 h) at
room temperature without external oxidant. Comprehensive
studies indicate that the process involves tandem nucleophilic
oxidation reactions (NOR). The base from alkaline electro-
lyte drives the deprotonation of alcohols and ketones to
reactive nucleophiles (i.e., alkoxide and carbanion, respec-
tively), which are subsequently oxidized by electrophilic
oxygen species over MnCoOOH prior to OER. As proof of
concept, this strategy is efficient for electrorefining diverse
lignin aromatics and KA oil featuring C(OH)-C and C(O)-C
units to benzoate and adipate in high yields, respectively
(91.5% and 64.2%).
Figure 1. a) Schematic illustration of the synthesis of MnCoOOH/NF.
b) SEM image, c) EDS mapping, and d) HR-TEM of MnCoOOH.
e) Normalized XANES adsorption profiles and f) EXAFS Fourier trans-
form (FT) spectra of Co K-edge for samples. g) EXAFS spectra of Co K-
edge for CoOOH and Mn K-edge for Mn-based samples.
X-ray diffraction (XRD) patterns indicate that g-phase
MnCoOOH/NF and CoOOH/NF were successfully prepared
by CV activation of a-phase MnCo(OH)2/NF and Co(OH)2/
NF, respectively through anodic deprotonation process (Fig-
ure S1).[9c,10c] Scanning electron microscope (SEM) analysis
shows the honeycomb-like array structure composed of
MnCoOOH nanosheets (Figure 1b), which display thinner
average thickness (9.3 nm) than that of CoOOH (31.2 nm),
suggesting Mn doping have effect on limiting the growth of
Co(OH)2 at c orientation (Figures S2,3). Energy dispersive
spectrometry (EDS) mapping shows that Mn is uniformly
distributed on MnCoOOH nanosheets (Figure 1c). High-
resolution transmission electron microscope (HR-TEM)
image of MnCoOOH (Figure 1d) displays an interplanar
spacing of 0.246 nm, which corresponds to the (110) plane of
hexagonal g-CoOOH.[9c] We further investigated the elec-
tronic structure of MnCo(OH)2, MnCoOOH, and the coun-
terparts using X-ray absorption near-edge structure
(XANES), extended X-ray absorption fine structure spec-
troscopy (EXAFS), and X-ray photoelectron spectroscopy
(XPS). As shown in Figure 1e, the Co K-edge adsorption
edges of both MnCo(OH)2 and Co(OH)2 are coincident with
that of CoO reference, reflecting the prominence of Co2+
species in the two materials. Upon the CV activation applied,
the adsorption edges of MnCoOOH and CoOOH shift
positively and are next to that of LiCoO3, indicating the
formation of Co3+ species as corroborated by Co2p XPS
results (Figure S4a). The oxidation of metal cations in basal
Results and Discussion
Cobalt-oxyhydroxide (CoOOH) is a promising anodic
material for electrochemical oxidation,[9] and recognized to be
the catalytically active phase of Co-based catalysts during
anodic reactions such as OER[10] and organic transforma-
tions.[11] In addition, high-valence metallic heteroatoms dop-
ing, such as Mn[12] and Fe,[10c] has shown to be an efficient
strategy to modulate the electronic structure of CoOOH and
thereby to enhance the intrinsic activity. Considering these
facts, we prepared Mn-doped CoOOH supported on nickel
foam (MnCoOOH/NF) as anode to investigate electrochem-
À
ical oxidative cleavage of C(OH) C bonds. As illustrated in
Figure 1a, the MnCoOOH/NF was synthesized firstly using
an electrodeposition method to obtain Mn-doped Co(OH)2
on NF (MnCo(OH)2/NF), followed by cyclic voltammetry
(CV) activation. Specifically, the generated local OHÀ ions on
the surface of NF electrode under negative potential (À1 V vs.
SCE) mainly reacted with Co cations in aqueous metal
solution (200 mM Co2+ and 100 M Mn2+) and led to MnCo-
(OH)2/NF. The low content (atomic Mn/Co ratio ꢀ 1/20) of
Mn in MnCo(OH)2/NF is attributed to that Mn2+ ions prone
to be oxidized and deposited on the counter electrode
(Table S2). Then, the as-obtained MnCo(OH)2/NF material
was activated in 1 M KOH electrolyte using CV method (1.0– plane contributes to deprotonation of the hydroxyls of the a-
1.8 V vs. RHE, 20 cycles) to give the desired MnCoOOH/NF.
The comparative CoOOH/NF sample was also synthesized
using the similar method except the absence of Mn.
phase to form O-bridged metal-oxygen-metal (M-O-M)
moieties (Figure S4b) and contraction of the local Co-O and
Co-Co distances.[9b, 10c] EXAFS analysis confirm this trend
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charge transfer resistance for MnCoOOH catalyst (Fig-
ure S7).
To have an insight into the reaction process, the distribu-
tion of 1 and its derivatives upon the course of reaction time
were analyzed by high-performance liquid chromatography
(HPLC). As shown in Figure S8, the HPLC chromatograms
demonstrate that acetophenone (2) is the only observable
intermediate of 1 before the formation of final product-
benzoate. Kinetic curves show that 1 undergoes rapid
oxidation (88.5%) in 30 minutes at 1.5 V vs. RHE, giving
79.7% of 2 and 7.8% of benzoate (Figure 2c) with a total
faradaic efficiency of 85.3% (Figure S9). Subsequently, 2 is
gradually converted into benzoate and formate, achieving
maximum benzoate yield (94.2%) after 10 hours reaction
(Figure 2c and Figure S9). These results reveal the reaction
pathways of anodic 1 oxidation over MnCoOOH/NF involves
two key steps: i. dehydrogenation of secondary alcohol (1) to
À
ketone (2), and ii. ketone mediated oxidative C C bond
cleavage to carboxylates. Notably, no benzoate is detected
when using ethylbenzene as substrate under the same reaction
conditions (Figure S10), highlighting the key role of C(OH)-C
Figure 2. Electrochemical evaluation. a) Cyclic voltammetric (CV)
curves of MnCoOOH/NF in 1 M KOH with or without 1 at a scan rate
of 10 mVsÀ1. b) Cathodic H2 evolution at 1.5 V vs. RHE in 1 M KOH
with or without 1. c) Kinetic curves for 1 transformation as a function
of reaction time. Reaction conditions: 0.5 mmol 1, 50 mL 1 M KOH,
MnCoOOH/NF(+)//Pt foil(À), 1.5 V vs. RHE. d) Comparison of the
catalytic performances of other common catalysts and MnCoOOH/NF.
Reaction conditions: 0.5 mmol 1, 50 mL 1 M KOH, anodic catalyst-
(+)//Pt foil(À), 1.5 V vs. RHE, 8 h.
À
and C(O)-C for scissoring C C bonds in this process. To the
best of our knowledge, the transformation of 1 to benzoate
using an electrochemical strategy has not been reported
previously, thus offer a sustainable alternative to conventional
thermo-driven processes performed at elevated temperature
using external oxidants (Table S1).
To confirm the unique activity of MnCoOOH/NF, we
prepared and evaluated the catalytic performances of other
common catalysts (Figures S11–15 and Figure 2d), including
non-precious metal oxy(hydroxides) on NF (MnOx/NF,
NiFeOOH/NF) and noble-metal nanocrystals on carbon cloth
(Pt/CC and Pd/CC). As shown in Figure 2d, NF and MnOx/
NF give much lower conversion of 1 (16.8% and 45.4%,
respectively) than that of CoOOH/NF (> 99%) and
MnCoOOH/NF (> 99%), indicating the high intrinsic activity
of Co-based oxyhydroxide. As for the well-known OER
catalyst in alkaline media, NiFeOOH/NF, the conversion of
1 and the selectivity of benzoate are much inferior to that of
the MnCoOOH/NF, suggesting the different reaction mech-
anism of 1 oxidation and OER processes. Moreover, both Pt/
CC and Pd/CC catalysts give poor conversion (< 7%), and
benzoate is the only detected product. The low activity of Pt/
CC and Pd/CC can be explained by the inefficient binding
between the relatively hydrophobic metal surface and the
substrate that contains numerous alkoxide groups in alkaline
media.[13] The electrode material dependent activity for
1 oxidation reflects the nature of an inner-sphere reaction
of this reaction, which require strong interaction between
substrate and electrode for electron transfer.[14]
toward shorter distance of first shell Co-O and second shell
Co-Co (Figure 1 f). Moreover, the Mn K-edge EXFAS
spectra show that the length of Mn-O and Mn-Mn in
MnCoOOH are equal to that of Co-O and Co-Co, respec-
tively (Figure 1g). Therefore, it can be deduced that Mn
atoms replaced Co atoms to form octahedral structure in the
MnCoOOH.[12] Overall, these characterization techniques
comprehensively identify the successful synthesis of metal
oxyhydroxide structure of MnCoOOH and CoOOH.
The as-prepared samples were initially evaluated for
electrochemical oxidation of 1-phenylethanol (denoted as 1),
a typical lignin model compound with C(OH)-C structure,[2b–d]
in undivided cell with 1 M KOH electrolyte. As shown in
Figure 2a, the CV curve of MnCoOOH/NF shows the good
OER activity in 1.0 M KOH electrolyte with an onset
overpotential of 310 mV. To our delight, introducing 1 into
the electrolyte lead to significant reduction of onset potential
to 130 mV, indicating the lower energy barrier of 1 oxidation
than that of OER. As a result, the cathodic H2 production rate
increased from 0.1 mmolcmÀ2 hÀ1 of traditional water splitting
to 1.41 mmolcmÀ2 hÀ1 at 1.5 V vs. RHE (Figure 2b), leading to
the enhancement of the energy efficiency of the whole
process.[5c] Notably, the MnCoOOH catalyst exhibits approx-
imately twofold improvement in current density for 1 oxida-
tion, compared with CoOOH under constant potential (1.5 V
vs. RHE) (Figure S5). This can be partially explained by the
increased electrochemical active surface area of MnCoOOH
(3.3 mFcmÀ2) compared to that of CoOOH (1.6 mFcmÀ2)
(Figure S6). Moreover, electrochemical impedance spectros-
copy reveal that Mn doping induces significant decrease of
To have an insight into the mechanism of this reaction on
MnCoOOH/NF electrode, we carried out isotope labeling
experiments to investigate inappreciable intermediates dur-
ing electrochemical 1 oxidation using alkaline H2O, D2O, and
H218O electrolyte (1 M KOH), respectively. The isotopomer
distribution in the intermediate (2) and benzoate were
analyzed by mass spectrometry (MS) equipped on gas
chromatography (GC). First, we notice that the H atom on
OH group of 1 can spontaneously exchange with water under
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Figure 3. Mechanism study. a,b) Mass spectra of a) acetophenone (m/z=120) and b) benzoic acid (m/z=122) derived from 1 in isotope-labeled
electrolyte. c) The effect of electrolyte pH on electrocatalytic oxidation of 1. Reaction conditions: 0.5 mmol 1, 50 mL 1 M KOH, MnCoOOH/
NF(+)//Pt foil(À), 1.5 V vs. RHE, 10 h. d) Illustration of the formation of electrophiles for OER (red oxygen species represent electrophiles)[16] and
NOR. e) Free energy diagram for electrocatalytic oxidation of 1 over MnCoOOH catalyst.
reaction conditions, suggesting the formation of alkoxide via
acid-base equilibrium (Figure S16 a–c). Moreover, the MS
spectra of 2 show that one 18O atom from H218O and three D
atoms from D2O are incorporated into as-formed 2 from
1 oxidation (Figure 3a and Figure S16 a). The plausible
reasons for the results are base promoted reversible ketone
hydration and keto-enol tautomerism, respectively (Fig-
ure S16d, e, detailed analysis shown in SI). Notably, these
results reveal the formation of nucleophilic intermediates
(alkoxide and carbanion) from 1 and 2 by deprotonation in
the presence of base (Figure S16d, e, and S17). Simultane-
ously, the MS spectra of benzoic acid reveal that two 18O
atoms in benzoate are originated from 18O labeled water
(Figure 3b), suggesting that water derived oxygen species
involved in this reaction.
We recognize that these results are different from a recent
publication of Lei and co-workers,[6e] who reported a neutral
electrocatalytic process for selective 1 oxidation to 2 without
À
C C bond cleavage. Thus, we investigated the influence of pH
on the reaction rates and product distribution of electro-
chemical 1 oxidation over MnCoOOH/NF and CoOOH/NF
under our reaction conditions (Figure 3c and Figure S18).
The results show that in near neutral electrolyte (pH 8.3), 1 is
quantitatively converted into 2 with moderate reaction rate
À
without C C bond cleavage, which is in agreement with Leiꢀs
results. However, both the conversion of 1 and the selectivity
to benzoate are dramatically increased with higher pH
(pH 13.9). These results suggest that high concentration of
hydroxide ions promotes both the dehydrogenation of 1 and
subsequent C(O)-C cleavage process. The pH-dependent
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reactivity could be explained by the high pKa of 1 (14.4) and 2
(18.4), which requires high pH to drive their deprotonation to
nucleophilic alkoxide and carbanion, respectively.[15]
Nucleophilic oxidation reaction (NOR) mechanism is
wildly adopted in inner-sphere OER or organic reaction over
anode, where molecules react with interfacial oxidizing
species.[14,17] Significantly, electrophilic oxygen species (i.e.
OH*, O*, OOH*) have been identified on the anodic
catalysts surface during alkaline OER process by experiments
and/or theoretical calculations.[9c,12,16,18] It is well demonstrat-
ed that the rate-determining-step over Co-based hydroxides
or oxyhydroxides is the deprotonation of adsorbed OH* to
O* (step ii),[19] being consistent with our density functional
theory (DFT) calculations over MnCoOOH catalyst (Fig-
ure 3d, left circle and Figures S19,20). In our work, the
applied working potential can only overcome the energy
barrier of generating OH* (1.29 eV), rather than that of
deprotonation of OH* (1.66 eV). Therefore, we infer that
OH* is the main electrophile on the surface of MnCoOOH/
NF during 1 transformation because of the relatively low
working potential (1.5 V vs. RHE). While, compound 1 and its
derived intermediates nucleophilic attack on OH* over
MnCoOOH/NF (Figure 3d, right cycle), along with proton-
electron transfers. To validate this assumption, we performed
DFT calculations for 1 oxidation towards benzoate. As shown
in Figure 3e, 1 can smoothly transforms to a carbanion, which
further reacts with OH* on MnCoOOH/NF to give 2-
hydroxyacetophenone (3). In addition, compound 3 is grad-
ually transformed into final benzoate, via 2-oxo-2-phenyl-
acetaldehyde (4) and benzoylformic acid (5) intermediates,
which are verified by experiments (Figure S21). Notably, the
calculated RDS of the tandem nucleophilic oxidation pro-
cesses is the conversion of 2-derived carbanion to 3 (1.07 eV),
which is in agreement with experimentally kinetic curves
(Figure 2c).
On the basis of the above results and previous literatures,
we propose a tandem NOR pathways for electrochemical
transformation of 1 to benzoate in alkaline electrolyte. As
shown in Figure 4, the secondary alcohol initially undergoes
Ha deprotonation into more active alkoxide via base catalysis,
which is then oxidized by the electrophilic OH* on the surface
of MnCoOOH for b-H elimination to corresponding ketone
(2).[15a,16] Then the hydroxide ions induce keto-enol tautome-
rization of 2 to enol via carbanion intermediate. The
nucleophilic attack of carbanion intermediate to electrophilic
OH* on MnCoOOH generates a-ketol (3). Subsequently, 3 is
converted to 4 via electrocatalytic dehydrogenation. Then,
the base from electrolyte promotes the reversible trans-
formation of 4 to reactive geminal diol for electrocatalytic
Figure 4. A plausible tandem nucleophilic oxidation reaction (NOR)
mechanism for electrochemical oxidation of 1.
and + 4 in MnCoOOH (Figure S22a). Consistent with the
previous study, high-valence Mn-doping results in the in-
crease of white line peak intensity of MnCoOOH compared
to CoOOH (Figure S22b), suggesting a relatively stronger
adsorption of the OHÀ ions for the formation of electrophilic
OH* species, which accounts for the enhanced activity of
MnCoOOH, together with the above-mentioned higher
electrochemical active surface area (Figure S5) and lower
charge transfer resistance (Figure S6).
To demonstrate the generality of this electrochemical
process, a wide range of lignin model compounds with
C(OH)-C or C(O)-C motifs were examined as substrates
using MnCoOOH/NF catalyst under the established reaction
conditions. As shown in Figure 5, excellent yields (84.1–
99.2%) of benzoate are obtained from benzylic compounds
(2, 3–7, 13, 14). In addition, the electrochemical oxidation of
substrates with substituent groups on phenyl, such as methyl
(11), methoxy (8–10, 15), carbonyl (12), also afford satisfac-
tory yield (80.1–96.7%) to substituted benzoates. Moreover,
this approach also exhibits excellent catalytic results for
oxidizing primary alcohols (16 and 18) and aldehydes (17 and
18) to corresponding carboxylic acids with near theoretical
À
dehydrogenation to 5, which is followed by an oxidative C C
cleavage process to generate benzoate and formate as the
final products.[6c,20]
Recent publications have shown that doping of high-
valence Mn4+ with an unpaired 3d3 configuration into Co is an
effective method that is capable of extracting local electrons
from Co sites, then reducing the adsorption free energy of
CoOOH for OHÀ, therefore leading to enhanced OER
performance.[12] In our work, Mn K-edge XANES reveal that
the valence of Mn dopants mainly exist in + 3 in MnCo(OH)2
Figure 5. Substrate scope investigation.
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yield. For example, 99.3% yield of 2,5-furandicarboxylic acid,
ical strategy provides an efficient approach to convergently
an emerging renewable substitute of terephthalic acid,[21] can
be obtained from biomass-derived 5-hydroxymethylfurfural
(18) via this electrocatalytic strategy in 2 h.
transform these diverse lignin-derived products into relatively
single commodity, acting an “electrochemical funnel”.
On the other hand, KA oil is an important platform
chemical that can be produced from lignin-derived phenols or
aromatic ethers via reductive routes (Figure 6a).[23] Consid-
Moreover, we evaluated the long-term stability of
MnCoOOH/NF electrode for anodic oxidation of 1 in batch
reactions. As shown in Figure S23, the conversion of 1 and
yield of benzoate remain stable after 20 batches (200 h),
which is possibly attributed to the self-supporting structure of
MnCoOOH/NF. In addition, the SEM images and XPS
spectra of the fresh and used MnCoOOH/NF anode demon-
strate the morphological and chemical structure stability
under reaction conditions (Figures S24,25).
A significant issue for lignin valorization is downstream
processing of lignin-derived products to commercial com-
modities, with the advancements in lignin fractionation.[22] For
example, selective oxidation has been demonstrated to be an
effective route for lignin fractionation (Figure 6a), leading to
heterogenous portfolio of molecules include aromatic ke-
tones, aldehydes, and acids featuring Ca(O)-R (R = H, OH,
C(O)Me, CH3, C2H5).[3] However, the purification of these
À
ering the C(OH)-C and C(O) C bonds exist in KA oil, we
expect our electrochemical strategy could also exhibit cata-
lytic activity in their oxidative transformation. The catalytic
results show that the starting cyclohexanol was tandemly
converted to adipate via cyclohexanone intermediate, as
demonstrated by 1H NMR spectra (Figure 6c). NMR and
HPLC analysis show that 100% conversion of cyclohexanol
and 64.2% yield of adipate is achieved. This work provides
a sustainable way to produce adipate, a monomer of nylon 66
which has wide applications. Although the selectivity to
adipate in this preliminary study requires further optimiza-
tion, this electrochemical strategy offers a promising route to
produce monomer of polymers from renewable biomass.
chemicals with similar property remains a huge challenge. In Conclusion
this context, we demonstrated the convergent transformation
of a mixture composed of 11 kinds of aromatic substrates (1–
7, 13, 14, 16, 17, as shown in Figure 5) with benzylic Ca(OH)-R
or Ca(O)-R motifs using our electrochemical strategy. As
shown in Figure 6b, these lignin models with different
functional groups are convergently transformed into benzoate
with 91.5% yield. Notably, benzoate is an important com-
modity chemical that is conventionally produced by oxidation
of petroleum-based toluene at elevated temperature and
pressure (150–1708C, 1 MPa).[7a] Therefore, our electrochem-
In summary, we synthesized a Mn-doped CoOOH elec-
trocatalyst, which can selectively upgrade lignin derivatives
À
into carboxylates through electrochemical oxidative C(OH)
C bonds cleavage. Detailed mechanistic studies indicate that
base catalyzed the deprotonation of secondary alcohols and
its derived ketones to reactive nucleophiles, which subse-
quently oxidized by electrophilic OH adsorbed on the surface
of MnCoOOH. This methodology can be generalized to
various lignin derivatives with C(OH)-C or C(O)-C motifs for
producing carboxylic acids in high yield. As proof of concept,
this process demonstrated its applications in electro-funneling
lignin-derived mixture products into single benzoate and
transforming lignin-derived KA oil to adipate with high
efficiency. We believe this work provides a promising avenue
À
toward carboxylates via C(OH)-C and C(O) C bonds
cleavage under mild and environmentally-benign conditions.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21978147, 21935001,
22090031). The authors thank the BL1W1B in the Beijing
Synchrotron Radiation Facility (BSRF).
Conflict of interest
The authors declare no conflict of interest.
Figure 6. a) Schematic illustration of lignin fractionation to platform
molecules. b) Total ion chromatograms of various lignin model com-
pounds (top) and derived benzoic acid (bottom) after electro-funnel-
ing. Inset pictures are the electrolyte before and after electro-funneling.
Reaction conditions: 0.1 mmol of each substrate, 50 mL 1 M KOH,
MnCoOOH/NF(+)//Pt foil(À), 1.5 V vs. RHE, 12 h. c) 1H NMR
spectra of cyclohexanol and its derived products during electrolysis.
Reaction conditions: 1 mmol cyclohexanol, 50 mL 1 M KOH,
MnCoOOH/NF(+)//Pt foil(À), 1.45 V vs. RHE, 17 h.
À
Keywords: carboxylic acids ·C C bond cleavage ·
electrochemical oxidation ·hydrogen evolution ·
lignin upgrading
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Manuscript received: November 18, 2020
Revised manuscript received: January 18, 2021
Accepted manuscript online: February 8, 2021
S. D. Karlen, S. S. Stahl, J. Am. Chem. Soc. 2019, 141, 15266 – Version of record online: March 10, 2021
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Angew. Chem. Int. Ed. 2021, 60, 8976 –8982