Metallo-deuteroporphyrin as a biomimetic catalyst for the catalytic oxidation of lignin to aromatics

Article abstract of DOI:10.1002/cssc.201500048

A series of metallo-deuteroporphyrins derived from hemin were prepared as models of the cytochrome P450 enzyme. With the aid of the highly active Co^II deuteroporphyrin complex, the catalytic oxidation system was applied for the oxidation of several lignin model compounds, and high yields of monomeric products were obtained under mild reaction conditions. It was found that the modified cobalt deuteroporphyrin that has no substituents at the meso sites but does have the disulfide linkage in the propionate side chains at the β sites exhibited much higher activity and stability than the synthetic tetraphenylporphyrin. The changes in the propionate side chains can divert the reactivity of cobalt deuteroporphyrins from the typical C-C bond cleavage to C-O bond cleavage. Furthermore, this novel oxidative system can convert enzymolysis lignin into depolymerized products including a significant portion of well-defined aromatic monomers.

Full text of DOI:10.1002/cssc.201500048

DOI: 10.1002/cssc.201500048
Full Papers
Metallo-Deuteroporphyrin as a Biomimetic Catalyst for the
Catalytic Oxidation of Lignin to Aromatics
Chenjie Zhu,[a] Weiwei Ding,[a] Tao Shen,[a] Chenglun Tang,[a] Chenguo Sun,[b] Shichao Xu,[b]
Yong Chen,[a] Jinglan Wu,[a] and Hanjie Ying*[a]
A series of metallo-deuteroporphyrins derived from hemin
were prepared as models of the cytochrome P450 enzyme.
With the aid of the highly active CoII deuteroporphyrin com-
plex, the catalytic oxidation system was applied for the oxida-
tion of several lignin model compounds, and high yields of
monomeric products were obtained under mild reaction condi-
tions. It was found that the modified cobalt deuteroporphyrin
that has no substituents at the meso sites but does have the
disulfide linkage in the propionate side chains at the b sites ex-
hibited much higher activity and stability than the synthetic
tetraphenylporphyrin. The changes in the propionate side
chains can divert the reactivity of cobalt deuteroporphyrins
from the typical CÀC bond cleavage to CÀO bond cleavage.
Furthermore, this novel oxidative system can convert enzymol-
ysis lignin into depolymerized products including a significant
portion of well-defined aromatic monomers.
Introduction
As a result of the depleting fossil fuel reserves and increasing
greenhouse gas emissions, the exploration of feasible path-
ways for the conversion of abundant and renewable biomass
into clean fuels and high-value-added chemicals to supple-
ment or gradually replace the petroleum-based industry is
highly desirable.[1] In this context, lignocellulosic biomass is
a promising candidate to substitute fossil resources in some in-
dustrial processes.[2] The chemical conversion of cellulose and
hemicellulose has been extensively studied,[3] whereas that of
lignin remains uncommon despite the fact that it is the second
most abundant terrestrial polymer after cellulose on earth and
constitutes up to 15–30% of the weight and 40% of the
energy content of lignocellulosic biomass. At present, plenty of
lignin is discharged into paper-making black liquor or burned
as a low-calorific-value fuel, which results in enormous re-
source waste and serious environmental pollution. The other
uses of lignin remain limited mainly to low-value products,[4]
such as surfactants, dispersants, or emulsifiers,[5] phenolic
resins,[6] epoxy resins,[7] carbon fibers,[8] thermoplastic films,[9]
and polyurethane foams.[10]
a sustainable precursor for basic aromatic building blocks that
are currently obtained from fossil-based feedstocks,[12] because
it is the only large-volume renewable feedstock that is com-
posed of aromatics. Moreover, the utilization of lignin does not
compete with food production, an ethical problem often en-
countered with renewable resources.[13] However, lignin is re-
sistant to normal chemical or biochemical processes for break-
ing down linkages among the structural units, as a result of its
huge molecular weight and robust 3D amorphous network
structure.[14] Generally, there are five major strategies for lignin
depolymerization: pyrolysis,[15] hydrolysis,[16] enzymolysis,[17] hy-
drogenolysis,[18] and oxidation.[19] Among these, the oxidative
depolymerization of lignin is a very promising way to go, be-
cause it leads to highly functionalized monomeric or oligomer-
ic products, which can be used directly as fine chemicals or as
platform chemicals.[12,20] However, the low conversions and
broad product distributions (usually <20% yield of well-de-
fined aromatic monomers) are the main problems for the oxi-
dative depolymerization of lignin. The incorporation of catalyt-
ic technologies into lignin oxidative depolymerization is a possi-
ble option for valorizing the feedstock as a renewable raw ma-
terial for aromatic chemical production, and the key to achiev-
ing this goal is the development of efficient and robust
catalysts.
The extraction of more value from lignin is increasingly rec-
ognized as crucial to the economic viability of integrated bio-
refineries.[11] Recently, lignin has received great attention as
Inspired by the peroxidase degradation of lignin (for exam-
ple, lignin peroxidase, manganese peroxidase),[21] the applica-
tion of synthetic metalloporphyrins as biomimetic catalysts for
[a] Dr. C. Zhu, W. Ding, T. Shen, C. Tang, Y. Chen, J. Wu, Prof. H. Ying
College of Biotechnology and Pharmaceutical Engineering
Nanjing University of Technology
lignin oxidative depolymerization attracted
a significant
30 S Puzhu Rd, Nanjing, 211816 (PR China)
E-mail: yinghanjie@njtech.edu.cn
amount of interest in the past.[22] However, there has been
very little recent progress in this area. The structures and ab-
breviated names of representative synthetic metalloporphyrins
are given in Figure 1. As can be seen, nearly all of the metallo-
porphyrins were based on the structure of synthetic meso-tet-
raphenylporphyrin (TPP). These biomimetic catalysts can yield
[b] Dr. C. Sun, S. Xu
School of Chemical Engineering
Nanjing University of Science and Technology
200 Xiaolingwei Rd, Nanjing, 210094 (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500048.
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Results and Discussion
Evaluation of the metallo-deuteroporphyrin catalysts
It is well known that lignin is a highly branched macromole-
cule consisting of p-hydroxybenzene, guaiacyl, and syringyl
phenylpropane units cross-linked by CÀC bonds (b-1, b-5, b-b,
5-5’) and CÀO bonds (4-O-5, a-O-4, b-O-4), of which the b-O-4
bonds account for more than 50% of the linkages in most
plants.[27] Therefore, initial experiments were carried out with
2-(2-methoxyphenoxy)-1-phenylethanol (1) as a simple non-
phenolic b-O-4-type model substrate to check the efficiency of
the deuteroporphyrin catalysts. Metallo-deuteroporphyrins
M(DPDME), M(DPNH2), M(DPBr2), M(DPCl2) and disulphide-de-
rivatized deuteroporphyrins M(DPS2) and M(DPCys) (Figure 2),
Figure 1. Structures and abbreviated names of representative synthetic met-
alloporphyrins.
highly oxidized metal–oxo species upon reaction with the oxi-
dant species. However, there are still several drawbacks of
using some of these porphyrin complexes from a practical
point of view: 1) they are usually unstable under oxidation con-
ditions as a result of their self-destruction or the formation of
inactive m-oxo dimers;[23] 2) it is usually difficult to maintain the
aromaticity of the degradation product because of the aromat-
ic nuclei oxidation;[22] and 3) the catalytic potential of the por-
phyrin complexes still needs to be improved. In view of the
aforementioned inadequacies of existing metalloporphyrins,
recent significant advances in biomimetic catalysis prompted
us to reconsider the metalloporphyrin catalysts and provided
an opportunity of designing more reactive and stable catalysts
for lignin depolymerization.
Figure 2. The structures and abbreviated names of the metallo-deuteropor-
phyrins used in this study.
Recently, the cytochrome P450 enzyme has drawn consider-
able attention as a mild and highly efficient catalyst for various
organic transformations.[24] Heme is usually considered as the
prosthetic group of the cytochrome P450 enzyme.[25] Metallo-
deuteroporphyrin, which is derived from natural hemin, is an
ideal candidate to mimic the P450 enzyme as a result of its ex-
cellent stability, high accessibility, and close relationship to the
naturally occurring hemes. To the best of our knowledge, there
is no report for the catalytic oxidation of lignin with metallo-
deuteroporphyrins as catalysts. We envisaged that the use of
metallo-deuteroporphyrins would properly address the effi-
ciency issues in porphyrin catalysts by providing a stable cata-
lyst with reasonably high activity. In a continuation of our ef-
forts to develop new methods for catalytic biomass conversion
and synthetic chemistry,[26] we report herein a novel and effi-
cient biomimetic catalytic oxidation system for the oxidation
of lignin model compounds and real lignin.
in which M represents MnIIICl, CoII, or FeIIICl, were synthesized
from protohemin by using our previous methods (for details,
see the Supporting Information).[28] We first examined the
effect of the oxidant with Fe(DPDME) as the catalyst. The
active oxidant, an iron–oxo-complex radical cation structurally
similar to that involved in the enzymatic reactions, is usually
formed by reaction of the FeIII–porphyrin with various oxygen
donors, such as hydrogen peroxide, tert-butylhydroperoxide
(TBHP), m-chloroperoxybenzoic acid, sodium hypochlorite, io-
dosylbenzene, magnesium monoperoxyphthalate, and molecu-
lar oxygen plus a reducing agent. The choice of the oxygen
donor depends on the substrate structure, the catalyst, and
the reaction medium. Previous research on the oxidation of
lignin usually employed H2O2 or TBHP as the oxidant.[22] The re-
sults indicated that both are not suitable under the present
system (Table 1, entries 1 and 2). The use of PhIO as the oxi-
dant did not show any improvement despite the fact that it is
often used as an oxygen donor in the porphyrin catalysis
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markedly with the change of the central metal atom.
CoII(DPDME) exhibited the highest activity in the series of
M(DPDME). If the central metal atom was Fe or Mn, 1 was oxi-
dized with lower conversion, and the following order of reac-
Table 1. Optimization studies for the catalytic oxidation of lignin model
compound 1.[a]
Entry
Oxidant
Catalyst
Solvent
T [8C]
Conv.[b] [%]
tivity
was
observed:
Co(DPDME)>ClMn(DPDME)>
1
2
3
4
5
6
7
8
H2O2
TBHP
PhIO
Fe(DPDME)
Fe(DPDME)
Fe(DPDME)
Fe(DPDME)
Mn(DPDME)
Co(DPDME)
Co(DPNH2)
Co(DPBr2)
Co(DPCl2)
Co(DPS2)
Co(DPCys)
Co(DPCys)
Co(DPCys)
Co(TPP)
Co(DPCys)
Co(DPCys)
Co(DPCys)
Co(DPCys)
Co(DPCys)
Co(DPCys)
Co(DPCys)
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
MeOH
CH2Cl2
MeOH
MeCN
MeOH/H2O
MeOH/H2O
MeOH/H2O
60
60
60
60
60
60
60
60
60
60
60
60
RT
RT
RT
RT
RT
RT
RT
RT
RT
<10
<10
16
29
37
71
78
54
63
89
96
91
90
48
56
51
58
ClFe(DPDME). This phenomenon may be caused by the differ-
ent redox potentials of the deuteroporphyrin catalysts; the
half-wave potential (E1/2) values (versus the SCE in DMF) of
these catalysts are in the order: Co(DPDME) (À803.7 mV)>
ClMn(DPDME) (À902.8 mV)>ClFe(DPDME) (À915.0 mV).[28]
There is a linear relationship between the catalytic activity and
the half-wave potential of M(DPDME); this is agreement with
earlier observations that the catalytic activity of metallopor-
phyrins increases with an increase in the redox potential.[32]
Also, there is another possibility that ClMn(DPDME) and
ClFe(DPDME) both need to drop the axial-ligand Cl group
before combination with the oxygen donor to produce the
highly oxidized metal–oxo complexes. Seen in this way,
Co(DPDME), without the axial-ligand Cl group, is prone to be
activated. The split of the axial ligand in the initiation step of
the reaction has been proven for ClMn(TTP) porphyrin[32c] and
(HO)FeIII porphyrin.[33]
oxone
oxone
oxone
oxone
oxone
oxone
oxone
oxone
oxone
oxone
oxone
oxone
oxone
TBAOX
oxone
oxone
oxone
oxone
9
10
11
12[c]
13[d]
14
15
16
17
18
19
20[e]
21[f]
55
100
88
91
[a] Reactions were performed by using 1 (0.5 mmol), catalyst (0.5 mol%),
and oxidant (2.5 mmol) in H2O (10 mL) for 8 h unless otherwise noted.
[b] The conversion was determined by GC analysis. [c] Co(DPCys)
(0.1 mol%) was used. [d] This reaction was performed at room tempera-
ture for 15 h. [e] Pyridine (0.05 mmol) was added. [f] Imidazole
(0.05 mmol) was added.
Besides the central metal atom of the complexes, the elec-
tronic and geometric effects of the substituents are the other
important factors that affect the catalytic activity of deutero-
porphyrins. It is well known that the introduction of electron-
À
withdrawing substituents such as F, Cl, Br, or SO3 groups into
metalloporphyrins increases the redox potential of porphyrin
system and hence increases the catalytic activity. Depending
on the size of the substituent, it may also exert a steric bulk in-
fluence. On the basis of these considerations, a series of Co–
system (Table 1, entry 3).[29] Other oxidants, such as urea hydro-
gen peroxide, K2S2O8, peracetic acid, or m-chloroperoxybenzoic
acid were also tested for this reaction; however, all of them
were unsatisfactory (data not shown). Recently, oxone
(2KHSO5·KHSO4·K2SO4) has been used extensively for a variety
of chemical transformations and particularly as a reagent in nu-
merous oxidation reactions as a result of its nontoxic nature,
affordability, and safety profile.[30] We were very pleased to find
that noticeable conversion was achieved with oxone as the ox-
idant (Table 1, entry 4). In the control experiments, there was
no product formation in the absence of the deuteroporphyrin
catalyst or oxone. This high activity of oxone probably occurs
because the peroxidic OÀO bond is more unsymmetrical in
deuteroporphyrins
with
electron-withdrawing
groups
[Co(DPBr2), Co(DPCl2)], and electron-donating groups
[Co(DPDME), Co(DPNH2), Co(DPS2)] on the propionate side
chains were synthesized and tested for the reaction. The order
of catalytic activity of these deuteroporphyrins is as follows:
Co(DPS2)>Co(DPNH2)>Co(DPDME)>Co(DPCl2)>Co(DPBr2)
(Table 1, entries 6–10). To our surprise, the order of the catalytic
activity is nearly opposite to the earlier observations. The cata-
lytic activities of Co(DPBr2) and Co(DPCl2) were found to be
much lower than those of the deuteroporphyrins containing
electron-donating groups [Co(DPNH2), Co(DPDME)]. Apparent-
ly, the character of electronic effect does not play a pro-
nounced influence on the catalytic properties of the deutero-
porphyrins. Based on what has been mentioned above, the
catalytic activity of the metalloporphyrins increases with an in-
crease in the redox potential. However, the half-wave potential
2À
oxone than in H2O2 and because SO4 is a better leaving
group than HOÀ; thus, the heterolytic cleavage step leading to
a
high-valence metal–oxo species is highly favored for
oxone.[22k] A similar phenomenon has also been observed in
DNA breaks catalyzed by (bleomycin)FeIII and cationic metallo-
porphyrins.[31] Beyond that, the acidic character of oxone may
be another important reason for its high activity (see below),
because it will favor the cleavage of inactive m-oxo metallo-
deuteroporphyrin dimers, which are known to be formed in
the oxidation of porphyrin complexes and represent a dead
end in the catalytic cycle.
(E1/2
Co(DPNH2) (À711.3 mV)>Co(DPCl2) (À778.8 mV)>Co(DPBr2)
(À782.6 mV)>Co(DPS2) (À786.6 mV)>Co(DPDME)
)
values of these catalysts are in the order:
(À803.7 mV). No obvious correlation between the two orders
is found. If we compare only Co(DPBr2) and Co(DPCl2), they are
still consistent with the above rule. We speculate that this
simple linear relation just exists within the same character of
structure, such as Co(DPBr2) and Co(DPCl2). For other deutero-
porphyrins bearing various substituents, the catalytic efficiency
does not correlate with the reduction potential. Thus, we be-
With the aim of finding an appropriate catalyst for lignin cat-
alytic oxidative depolymerization, we then evaluated the cata-
lytic performance of other deuteroporphyrin catalysts (Table 1,
entries 4–6). As shown in Table 1, the conversion of 1 varies
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lieve that the groups on the propionate side chains play an im-
portant role in the reaction. Recently, the effect of the propio-
nate groups in tuning the heme electronic properties and
ligand binding in cytochrome P450 monooxygenase has been
reported.[34] In accordance with that report, the electron trans-
fer (spin delocalization effect) between the metal center, the
deuteroporphyrin system, and the propionate groups of the
present Co–deuteroporphyrin is suggested in Figure 3. This in-
Table 2. Degradation of Co–deuteroporphyrins in the presence of H2O2.[a]
Entry
Catalyst
Soret band [nm]
Degradation [%]
1
Co(DPDME)
Co(DPNH2)
Co(DPS2)
Co(TPP)
Co(DPCys)
390
415
391
410
397
66
56
31
82
19
2[b]
3
4
5
[a] Reactions were performed by using catalyst:H2O2 in a 1:100 molar
ratio in MeOH at room temperature for 5 h. [b] The Co(DPNH2) was dis-
solved in DMF.
a closed ring that contributes to the stability of the Co–deuter-
oporphyrin toward oxidative degradation.
Thus, we conclude that the substitution of the propionate
side chains acts in a double role, both to affect the electron
transfer and to improve the stability. These results inspired us
to develop a more efficient catalyst, which has both NH2 and
SÀS groups on the propionate side chains and forms the
closed ring to enhance the stability and catalytic activity of the
deuteroporphyrin. Based on the observations, Co(DPCys) was
synthesized and tested for the reaction. As expected, Co(DP-
Cys) exhibited a much higher stability among the Co–deutero-
porphyrins (Table 2, entry 5), and we were very pleased to find
that Co(DPCys) shows excellent catalytic activity; the conver-
sion of 1 was 96% (Table 1, entry 11). With regard to the
Co(DPCys) dosage, it was possible to decrease the amount of
Co(DPCys) to as low as 0.1 mol% without a significant loss in
catalytic efficiency (Table 1, entry 12). Notably, the reaction can
proceed smoothly even at room temperature as a result of the
excellent catalytic activity of Co(DPCys), although a longer re-
action time was required (Table 1, entry 13). Under the same
conditions, Co(TPP) was also applied for the purpose of com-
parison; unfortunately, the conversion was much lower
(Table 1, entry 14), which clearly demonstrates the superior cat-
alytic activity of Co(DPCys).
Figure 3. Intramolecular electron transfer from the propionate lone pairs
into the deuteroporphyrin system and from the deuteroporphyrin system
into the Co metal center.
tramolecular electron-transfer effect will induce delocalization
of the spin density in the active high-valence metal–oxo spe-
cies, which can be used to explain the high catalytic activity of
Co(DMDPE). For Co(DPNH2) and Co(DPS2), the greater catalytic
activity is probably because the SÀS and NH2 groups on the
propionate side chains both have unpaired electrons that are
prone to bind the cobalt in another deuteroporphyrin core as
axial ligands by forming an intermolecular interaction, and this
coordination mode would be responsible for the high catalytic
activity of Co(DPS2) and Co(DPNH2). Such similar coordination
modes have been recently demonstrated for MnIII–aminophe-
nylporphyrins and ZnII–aminophenylporphyrins.[35]
A significant disadvantage of porphyrin complexes is that
they are subject to oxidation degradation in strong oxidizing
media, which is the main obstacle to their practical application.
In contrast to the central metal atom and the electronic and
geometric effects, little attention has been paid to the stability
of porphyrins during the oxidation process. The stability of
Co(TPP), Co(DPS2), Co(DPDME), and Co(DPNH2) was studied in
the presence of H2O2 (1:100 molar ratio of catalyst to H2O2) in
MeOH solvent, and the extent of degradation of the catalyst
was measured through the change in absorbance at the Soret
band of the Co–deuteroporphyrin in the UV/Vis spectrum. As
shown in Table 2, the stability of the catalysts is in the order:
Co(DPS2)>Co(DPNH2)>Co(DPDME). For the purpose of com-
parison, the traditional Co(TPP) was also treated to the same
conditions. The results indicate that approximately 82% of
Co(TPP) was degraded 5 h after the addition of H2O2. Under
the same conditions, only 31% of Co(DPS2) had been degrad-
ed. Interestingly, although the electronegativity of the SÀS
group was lower than that of OCH3 and NH2 groups, Co(DPS2)
is more stable than Co(DPDME) and Co(DPNH2). Apparently,
the groups on the propionate side chains of Co(DPS2) form
Interestingly, the nature of the solvent plays an important
role in the present system. Co(DPCys) has good solubility in
MeOH or CH2Cl2; however, the use of MeOH or CH2Cl2 as sol-
vents led to poor conversions (Table 1, entries 15 and 16),
which is probably because of the poor solubility of oxone in
these solvents. To check this point, tetra-n-butylammonium
peroxymonosulfate (nBu4NHSO5, TBAOX, an organic salt of
oxone), which is completely soluble in MeOH and CH2Cl2,[36]
was synthesized and used in the reaction. However, the use of
TBAOX instead of oxone was also unsuccessful (Table 1,
entry 17), which clearly indicates that H2O is essential for this
reaction. The use of MeCN as the solvent also did not give
a satisfactory result (Table 1, entry 18), despite the fact that it is
usually an efficient solvent (or co-solvent) for metalloporphyr-
in-catalyzed oxidations, because it may act as an axial ligand
and limit the formation of inactive m-oxo dimers or influence
the electron transfer between the substrate and the cata-
lyst.[22h,j,23a,37] Further optimization revealed that a minimum of
20 vol% of H2O was needed to ensure completely conversion
of 1, and a 50 vol% MeOH/H2O mixture was the best solvent
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for the reaction (Table 1, entry 19). Surprisingly, the addition of
pyridine or imidazole to accelerate the reaction did not show
any improvement, although they are usually used as axial li-
gands in metalloporphyrin-catalyzed oxidations (Table 1, en-
tries 20 and 21).[38]
Interestingly, all of the aromatic compounds, which bear
only one C6 ring structure, were all derived from ring A
through CbÀO bond or CaÀCb bond cleavage. Guaiacol, which
is generated from ring B, was not detected after the oxidation.
At the same time, a range of aliphatic dimethyl dicarboxylates,
1g–1j, were detected after the oxidation. We speculate that
these aliphatic compounds were obtained by the further oxi-
dation of guaiacol. To verify this hypothesis, a confirmatory ex-
periment was carried out with guaiacol as the substrate under
the same reaction conditions. The result shows that most of
the guaiacol was consumed during the reaction, and the same
compounds were detected after the oxidation. A similar phe-
Oxidation of lignin model compounds
By maintaining all of the key parameters of the reaction, the
optimal reaction conditions were applied for the catalytic oxi-
dation of nonphenolic b-O-4-type lignin model compound 1.
Five aromatic compounds were identified after the reaction,
with 1-phenyl-1,2-ethanediol (1c) as the dominant product
(Scheme 1). Aromatic compounds 1a and 1b are monoaromat-
nomenon was previously observed for the nitrogen-containing
[41]
graphene[19c] or CuFeS2
catalyzed oxidation of lignin with
TBHP or H2O2 as the terminal ox-
idant. The current hypothesis
favors the formation of o-qui-
none in situ by oxidation of
guaiacol, which is further oxi-
dized to open the ring and form
the dicarboxylic acids. Under our
system, the dicarboxylic acids
were further transformed into
the dimethyl dicarboxylates as
a
result of the presence of
MeOH in the solvent mixture.
Generally, the metalloporphyr-
in catalysis is believed to involve
an electron-transfer mechanism,
and the reaction proceeds
through two competing path-
ways that lead to either side-
chain oxidation or aromatic
nuclei oxidation.[11] For b-O-4-
type lignin model compound 1,
the main products were usually
aromatic aldehydes or acids by
Scheme 1. Degradation of nonphenolic b-O-4-type lignin model compound 1.
ic compounds derived from oxidative CaÀCb bond breakage,
and compounds 1c–1e are formed by CbÀO bond breakage.
Notably, the fact that 1c was identified as the dominant prod-
uct derived from ring A through hydroxylation indicates that
the major reaction pathway involves CbÀO bond cleavage. Diol
formation has also been found to be the first step in the deg-
radation of lignin in nature by the basidiomycete fungus Pha-
nerochaete chrysosporium.[39] The formation of the diol was
very meaningful, because it has great application potential for
the synthesis of bio-polyols derived from lignin, which are suit-
able to replace or partly replace petroleum-based polyols in
polyurethane foam synthesis.[40] Surprisingly, the compound 2-
(2-methoxyphenoxy)-1-phenylethanone (1 f) was not identified
after the oxidation; this is quite different from previous reports,
in which keto ether 1 f was often obtained as the major prod-
uct generated from the oxidation of the benzylic CÀOH bond
without CÀO or CÀC bond cleavage;[19] this highlights the fact
that Co(DPCys) is an effective catalyst for oxidative decoupling
of the lignin model compound by selective cleavage of the
CÀO bond.
side-chain oxidation through CÀC bond cleavage or quinones
(or muconates) by aromatic nuclei oxidation. However, under
the present Co–deuteroporphyrin catalytic system, 1-phenyl-
1,2-ethanediol (1c) was produced as the dominant product
through CÀO bond cleavage. A plausible mechanism for the
present process is proposed in Scheme 2. Compound 1 is first
protonated at the a-hydroxy group under the acidic reaction
conditions; this is followed by dehydration to form a carbocat-
ion. Abstraction of a proton from the carbocation produces
enol ether intermediate I.[42] Indeed, a trace amount of I was
detected during the reaction by 1H NMR spectroscopy and
GC–MS analysis. Co(DPCys) is activated by oxone to form
a cobalt–superoxo species, which can further react with Co(DP-
Cys) to produce the m-peroxo-bridged cobalt–deuteroporphyr-
in dimers and then gives the high-valence cobalt–oxo species
[(Por)CoIV=O] by homolytic cleavage of the OÀO bond.[43] The
activated (Por)CoIV=O then abstracts a single electron from the
aryl group of I, to form the Cb-centered cation radical II. The
combination of II and the reduced (Por)CoIIIÀOH produces b-
hydroxyquinonemethide cation ion intermediate III through
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Scheme 2. Plausible mechanism for the oxidation of the simple nonphenolic b-O-4-type lignin model compound 1.
hydroxyl transfer, and intermediate III immediately undergoes
intramolecular nucleophilic attack of the resulting b-hydroxy
group on the Ca atom with concomitant release of an H+ ion
to produce epoxide IV through CÀO bond cleavage. Epoxide
IV then undergoes noncatalytic hydrolysis through nucleophilic
attack of a hydroxide anion to produce the final product 1-
phenyl-1,2-ethanediol (1c). Additionally, epoxide IV can also
undergo nucleophilic attack by a hydroperoxide anion (HOOÀ)
to give the a-hydroperoxyl-b-hydroxide anion intermediate V,
which immediately decomposes to give the product, benzalde-
hyde (1a). Another route (path b in Scheme 2) is also possible,
in which the Co–superoxo intermediate reversibly binds to the
double bond of I and attacks the reaction site to produce in-
termediate VI through CÀO bond cleavage,[44] which results in
the formation of 1-phenyl-1,2-ethanediol (1c) and benzalde-
hyde (1a).
matically increased the reactivity of the substrate, probably as
a result of the electron-donating property of the methoxy
group.[23a,37a] The reaction was finished in 20 min with 100%
conversion; however, this substrate was converted into some
unfavorable polymers. HPLC-MS shows that a series of high-
molecular-weight compounds (m/z>500) was formed during
the reaction. The traditional products, such as 3,4-dimethoxy-
benzaldehyde through CaÀCb bond cleavage and ketones
through benzylic CÀOH bond oxidation, were not detected
after the reaction.
After the study of the nonphenolic b-O-4-type lignin model
compound, we were then interested in investigating the reac-
tivity of the present catalytic system with phenolic b-O-4-type
lignin model compound 2 (Scheme 3). Phenolic groups are
abundant in lignin; for example, spruce lignin is estimated to
To check the substituent ef-
fects on the lignin model com-
pound conversion, 1-(3,4-dime-
thoxyphenyl)-2-(2-methoxyphe-
noxy)ethanol, which has an alkyl
aryl ether moiety in positions 3
and 4 of the aromatic ring A,
was then tested in the reaction.
The result shows that the pres-
ence of the methoxy group dra- Scheme 3. Degradation of phenolic b-O-4-type lignin model compound 2.
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have 15–30 free phenolic groups
per
100
phenylpropanoid
units.[45] It has been reported
that the phenolic group could
influence the course of the reac-
tion.[42a,46] As shown in Scheme 3,
Scheme 5. Degradation of a-O-4-type lignin model compound 3.
the presence of a phenolic hy-
droxy group dramatically in-
creased the conversion rate, the reaction was finished in
15 min with 100% conversion, and three compounds were
identified after the reaction. Compound 2a was generally con-
sidered as the product of acid-catalyzed b-aryl ether cleavage,
as illustrated in Scheme 4. However, in the control experiment,
the starting material 2 was recovered with a yield of 91% in
(benzyloxy)-2-methoxybenzene (3) underwent a smooth trans-
formation to afford benzyl alcohol (3a, 85.1%) as the major
product through CÀO bond cleavage. The control experiment
shows that no reaction occurred in the absence of the deuter-
oporphyrin catalyst (see Figure S6 in the Supporting Informa-
tion). The reaction pathway can be inferred to be analogous to
the above-proposed one. The ac-
tivated (Por)CoIV=O first abstracts
a single electron from the aryl
group of 3 to produce the radi-
cal cation 3+ . The latter under-
C
went proton loss to produce the
a-phenoxybenzyl radical 3d,
which further reacts with (Por)-
CoIIIÀOH through hydroxyl trans-
fer to form 3e. Compound 3e is
subsequently hydrolyzed to give
the main product benzyl alcohol
(3a) and the corresponding oxi-
dative products 3b and 3c
(Scheme 6). The formed guaiacol
was further oxidized into aliphat-
ic acids, which is in full agree-
ment with the observations from
the reaction of the b-O-4-type
Scheme 4. Proposed mechanisms for A) the formation of 2a and B) the formation of 2b and 2c.
the absence of Co(DPCys), which
clearly demonstrates the impor-
tant catalytic effect of Co(DPCys)
during the reaction. The forma-
tion of enone derivative 2b was
surprising. Cobalt complexes
such as a Co Schiff base[47] or
cobalt salt[48] have been reported
Scheme 6. Proposed mechanism for the formation of 3a.
to break oxidatively the CÀC
bonds in similar lignin model
compounds, but cobalt-mediated CÀO bond cleavage is very
rare. Recently, Toste and co-workers investigated the reactivity
of a series of vanadium complexes with a nonphenolic b-O-4-
type lignin model compound, and a similar enone product was
obtained through CÀO bond cleavage.[49] An analogous one-
electron transfer pathway was proposed for the formation of
2b: the activated (Por)CoIV=O first abstracts the benzylic hy-
drogen atom of 2, to form the cobalt-bound ketyl radical inter-
mediate 2d, then elimination of the OH group from the result-
ing enolate produces the product 2b (Scheme 4).
model compound. Notably, the 4-O-5-type lignin model com-
pound diphenyl ether 4, which often resists oxidation by most
metalloporphyrin catalytic oxidation systems, also furnished
guaiacol with 37% conversion, although a higher reaction tem-
perature (1508C) and increased catalyst dosage (1 mol%) were
required (Scheme 7). In the control experiment, the conversion
(9%) was much lower in the absence of the deuteroporphyrin
catalyst (see Figure S8 in the Supporting Information). Al-
though the mechanistic details of this transformation are not
clear at the moment, on the basis of the results obtained from
the control experiment, we believe that the reaction pathway
was more than acid-catalyzed aryl ether cleavage; a synergistic
catalysis effect might be occurring in this reaction.
We were excited by this new reactivity for CÀO bond cleav-
age, so the present catalytic system was then tested for anoth-
er lignin model compound that contains the CÀO bond. As
shown in Scheme 5, the a-O-4-type lignin model compound 1-
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and repolymerized lignin fragments from the reaction mixture.
With consideration of the complex structure and low reactivity
of real lignin, a higher temperature was employed. Upon an in-
crease in the reaction temperature to 1208C, the yield was sig-
nificantly improved (Table 3, entry 2), which indicated that the
lignin oxidation was promoted at a higher temperature to
overcome the activation energy for the reaction. In the control
experiments, the yield of oily liquid fraction was much lower in
the absence of the deuteroporphyrin catalyst (Table 3, entry 3),
which clearly demonstrates the major catalytic role of Co(DP-
Cys) during the reaction. After the reaction, the solvent was
evaporated, the residue was extracted with ethyl acetate, and
a variety of aromatic compounds were identified by GC–MS.
These compounds could be classified into three types: H-type
compounds including benzoic acid, p-hydroxybenzaldehyde, p-
hydroxyacetophenone, and p-hydroxybenzoic acid; G-type
compounds including guaiacol, vanillin, acetovanillone, and va-
nillic acid; and S-type compounds including syringaldehyde,
acetosyringone, and syringic acid. Among these three types of
aromatic compounds, the main products were vanillin and sy-
ringaldehyde. Hence, these two compounds were selected to
determine the effects of the reaction conditions.
Scheme 7. Degradation of 4-O-5-type lignin model compound 4.
Oxidation of enzymolysis lignin
Based on the promising results obtained from the lignin model
compounds, we next conducted reactions on real lignin.
A huge amount of lignin is expected to be available as a by-
product in the production of second-generation bioethanol
from crop waste.[50] Lignin obtained from the residue of enzy-
matic hydrolysis, referred as “enzymolysis lignin” (EL), is as-
sumed to be close to its original status and generally exhibits
much smaller structural changes than lignin obtained by
chemical-treatment methods, such as kraft lignin and lignosul-
fonate. Therefore, enzymolysis lignin, which is derived from en-
zymatic hydrolysis of corn stover in bioethanol production,
was selected in the present study and purified according to
the literature procedure[51] to remove the remaining cellulose,
hemicellulose, and sugars. The Co(DPCys) catalytic oxidation
system was applied for the oxidation of EL, and two fractions,
that is, an oily liquid fraction and a solid residue fraction, were
obtained after the reaction. The yield of the reaction was cal-
culated as a percentage of the weight of recovered oily liquid
products divided by the weight of dry lignin. The molecular
structure of lignin is very complicated, so it is difficult to calcu-
late the molar yield of the product. As can be seen in Table 3,
the result obtained from oxidation of EL at room temperature
is very poor (Table 3, entry 1). 95.7 wt% of solid residue was re-
covered after the reaction, which contained unconverted lignin
We then screened a variety of parameters of this reaction,
such as the reaction temperature, dosage of oxidant and cata-
lyst, the ratio of mixed solvent, and the reaction time (see
Table S1 in the Supporting Information). Under the optimal re-
action conditions, a soluble oily liquid fraction corresponding
to 31.2 wt% of the original lignin was obtained, together with
an insoluble solid fraction that accounted for 53.3 wt% of the
lignin (Table 3, entry 4). Notably, 31.2 wt% of the oily liquid
fraction is not the highest yield because our target products
are aromatics. A further increase in the dosage of oxone or in
the reaction temperature has a favorable effect on the yield of
the oily liquid fraction (Table 3, entries 5 and 6). However,
a subsequent decrease in the selectivity of the aromatics and
the formation of coking were observed, probably as a result of
the further oxidation of in situ generated aromatics into ali-
phatic compounds, such as maleic acid, muconic acid, and
even carbon dioxide. The soluble oily liquid fraction was ana-
lyzed by GC–MS to identify the products of the reaction
(Figure 4), and a full listing of characterized products is provid-
ed in the Supporting Information (Figure S9).
Table 3. Catalytic oxidative depolymerization of real lignin.[a]
Entry
Product distribution[b] [%]
aliphatics [wt%] [wt%]
Yield Solid residue Mass balance
H
G
S
[wt%]
1[c]
2[d]
3[e]
4[f]
5[g]
6[h]
7[i]
–
–
–
–
1.1
22.3
5.7
31.2
37.8
39.1
33.4
29.8
13.6
95.7
64.8
82.8
53.3
40.2
34.7
47.9
53.7
71.5
96.8
87.1
89.5
84.5
78.0
73.8
81.3
83.5
85.1
22.8 45.2 24.1 6.1
–
–
–
–
To elucidate whether the present method could be broadly
used in the conversion of other types of lignin, we applied this
procedure to several organosolv lignins, including ethanol
lignin derived from bagasse, dioxane lignin derived from bag-
asse, and ethanol lignin derived from pine sawdust. It was
found that the source of lignin has a significant effect on the
conversion efficiency and the distribution of products. For in-
stance, ethanol lignin derived from bagasse could afford an
oily liquid fraction yield of 33.4wt% (Table 3, entry 7), and all
three types of aromatics were identified after the reaction (Fig-
ure S10 in the Supporting Information). The different organic
solvents (ethanol and dioxane) used for the lignin extraction
seem to have had little influence on the reaction (Table 3, en-
tries 7 and 8; Figure S10 and S11 in the Supporting Informa-
tion). Significantly different from the conversion of grass bio-
18.8 48.9 22.7 8.4
13.5 41.2 26.8 15.5
8.7 37.6 24.4 22.3
7.5 32.1 46.1 11.7
9.0 38.5 39.3 9.4
8[j]
9[k]
–
85.2
–
11.3
[a] Reaction were performed by using lignin (0.2 g), Co(DPCys)
(0.01 mmol), and oxone (8 mmol) in H2O (20 mL) at 1508C for 10 h unless
otherwise noted. [b] Product distribution was calculated by GC–MS.
[c] The reaction was performed at room temperature. [d] The reaction
was performed at 1208C. [e] The reaction was performed in the absence
of Co(DPCys). [f] The yield is the averaged data from three replicated ex-
periments. [g] Oxone (12 mmol) was used in the reaction. [h] The reaction
was performed at 1808C. [i] Ethanol lignin derived from bagasse was
used in the reaction. [j] Dioxane lignin derived from bagasse was used in
the reaction. [k] Ethanol lignin derived from pine sawdust was used in
the reaction.
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Figure 4. GC–MS chromatography spectrum of the soluble oily liquid fraction. Inset graphics depict the structures and selectivities of the detected aromat-
ics (in minutes). The mass spectra and reference mass spectra are given in the Supporting Information (see Figure S9).
mass lignin, when ethanol lignin derived from pine, a woody-
based biomass, was used as the substrate for the reaction
under the same conditions, it only gave rise to a relatively
poor oily liquid fraction yield (Table 3, entry 9), and only the G-
type aromatics were detected after the reaction (Figure S12 in
the Supporting Information). Therefore, the different structures
of lignin from grass-based biomass (including corn stover and
bagasse) and wood biomass might be an important reason for
the different results. It has been reported that the composition
and the abundance of primary units in wood lignin and grass
lignin varies remarkably.[12a] Sinapyl, coniferyl, and p-coumaryl
alcohols constitute approximately 70%, 25%, and 5% of grass
lignin,[52] whereas roughly equal proportions of coniferyl and si-
napyl alcohols appear in hard-wood lignin.[12a]
of the reaction, the mixture was extracted with EtOAc (3”10 mL).
The organic phase was dried and analyzed by GC–MS (for details,
see the Supporting Information).
Typical procedure for the catalytic oxidation of enzymolysis
lignin
A 100 mL thick-walled glass tube with a stirrer bar was charged
with dry enzymolysis lignin (0.2 g), Co(DPCys) (0.01 mmol), oxone
(8 mmol), and H2O (20 mL). The tube was sealed with a polytetra-
fluoroethylene screw cap and heated for 10 h at T=1508C. After
completion of the reaction, the mixture was first concentrated
under vacuum, and the residue was extracted with EtOAc (3”
20 mL). The organic soluble fraction was concentrated under
vacuum to afford a viscous oil, which was characterized by GC–MS.
The organic insoluble material was washed with hexane and then
water to remove all salts and was dried under vacuum. The result-
ing solid was further treated with acetic anhydride and pyridine (3/
3 mL) at room temperature for 48 h to acetylate all of the alcohol
groups. After 48 h, the acetylated polymer was extracted with
EtOAc and subjected to gel permeation chromatography analysis.
Conclusions
In conclusion, a series of metallo-deuteroporphyrin biomimetic
catalysts, which are derived from natural hemin, have been de-
signed and synthesized to mimic the cytochrome P450
enzyme. Various lignin model compounds underwent smooth
transformation to afford the corresponding CÀO bond cleav-
age product in moderate to excellent yields under mild reac-
tion conditions. This novel catalytic oxidative system can fur-
ther convert real lignin into depolymerized products including
a significant portion of well-defined aromatic monomers. Some
of these products can be used directly as fine chemicals (such
as vanillin and syringaldehyde) or as feedstocks for further up-
grading. Further studies on immobilizing the metallo-deutero-
porphyrins on heterogeneous supports and enhancing the
yield of monomeric chemicals from real lignin are under way.
Acknowledgements
The authors thank the financial support by the National Science
Fund for Distinguished Youth Scholars (Grant No.: 21025625), the
Program for Changjiang Scholars and Innovative Research Team
in University (Grant No.: IRT1066), the Major Research Plan of the
National Natural Science Foundation of China (Grant
No.:21390204), the National High-Tech Research and Develop-
ment Plan of China (863 Program; Grant No.: 2012AA021201),
the National Natural Science Foundation of China (Grant No.:
21406110), the Jiangsu Province Natural Science Foundation for
Youths (Grant No.: BK20140938), the State Key Laboratory of
Motor Vehicle Biofuel Technology (Grant No.: KFKT2013001), and
the Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
Experimental Section
General procedure for the catalytic oxidation of lignin
model compounds
Oxone (2.5 mmol) was added to a stirred solution of lignin model
Keywords: cleavage reactions
lignin · porphyrinoids · oxidation
· homogeneous catalysis ·
compound
(0.5 mmol)
and
metallo-deuteroporphyrin
(0.0025 mmol) in MeOH/H2O (5/5 mL). The solution was stirred at
room temperature for several hours, and the reaction progress was
checked by GC–MS or thin-layer chromatography. After completion
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