Origins of Catalyst Inhibition in the Manganese-Catalysed Oxidation of Lignin Model Compounds with H2O2

Article abstract of DOI:10.1002/cssc.201900689

The upgrading of complex bio-renewable feedstock, such as lignocellulose, through depolymerisation benefits from the selective reactions at key functional groups. Applying homogeneous catalysts developed for selective organic oxidative transformations to complex feedstock such as lignin is challenged by the presence of interfering components. The selection of appropriate model compounds is essential in applying new catalytic systems and identifying such interferences. Here, it was shown by using as an example the oxidation of a model substrate containing a β-O-4 linkage with H₂O₂ and an in situ-prepared manganese-based catalyst, capable of efficient oxidation of benzylic alcohols, that interference from compounds liberated during the reaction can prevent its application to lignocellulose depolymerisation.

Full text of DOI:10.1002/cssc.201900689

Accepted Article
Title: Origins of Catalyst Inhibition in the Manganese Catalysed
Oxidation of Lignin Model Compounds with H2O2
Authors: Alessia Barbieri, Johann Bartold Kasper, Francesco Mecozzi,
Osvaldo Lanzalunga, and Wesley Richard Browne
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Origins of Catalyst Inhibition in the Manganese Catalysed
Oxidation of Lignin Model Compounds with H
2
O
2
[
a]
[b]
[b]
[a]
Alessia Barbieri, Johann B. Kasper, Francesco Mecozzi, Osvaldo Lanzalunga and Wesley R.
Browne*[b]
Abstract: The upgrading of complex bio-renewable feedstock, such
as lignocellulose, through depolymerisation benefits from the
selective reactions at key functional groups. Applying homogeneous
catalysts developed for selective organic oxidative transformations to
complex feedstock such as lignin is challenged by the presence of
interfering components. The selection of appropriate model
compounds is essential in applying new catalytic systems and
identifying such interferences. We show here using the oxidation of a
Figure 1. β-O-4 linkage (red) in lignin and a lignin model compound (1) bearing
the characteristic β-O-4 linkage and a guaiacol motif.
model substrate containing a β-O-4 linkage with H
O
2 2
and an in situ
[16]
prepared manganese based catalyst, capable of efficient oxidation of
benzylic alcohols, that interference from compounds liberated during
the reaction can prevent its application to lignocellulose
depolymerisation.
Selective oxidative depolymerisation of
lignin using
homogeneous catalysts is a promising approach in terms of
energy efficiency and offers opportunities to make use of a wide
range of ligands/complexes already available for small molecule
oxidation. Given the scale of the process, catalysts based on first-
row transition metals together with simple ligands are especially
relevant. A further consideration is to distinguish between hard
and soft wood pulp and especially the relative abundance of
chemical linkages, with softwood lignin containing primarily
coniferyl alcohol based components and hardwood a large
component form sinapyl alcohol.[17] Furthermore, atom-economic
Introduction
Lignin is one of the most abundant biopolymers on earth. It is a
heterogeneous tri-dimensional phenolic polymer built from phenyl
propane units linked by various groups.[1–3] In combination with
cellulose and hemicellulose it forms cellulosic fibre walls that
imparts rigidity to trees and protection from oxidative degradation
caused by microorganisms.[4] The structural complexity of lignin is
a key aspect of its functionality (protection for plants), but presents
a challenge to its use as a source of chemicals and complicates
processes such as cellulose-based ethanol production.[5–8] Its
separation from the carbohydrate components in pulp and paper
manufacturing is challenging and energy intensive.[9–11] Efficient,
economic and sustainable depolymerisation pathways that
enable liberation of cellulose from lignocellulosic materials has
been a major focus over the last decades.[2] The β-O-4 linkage
2 2 2
terminal oxidants O and H O are favoured due to their non-
persistent toxicity and environmental impact.
Biomimetic metalloporphyrin catalysts, functionalized with
[
18–21]
[12]
halogens and sulfonate groups,
as well as, Fe-porphyrins,
[
15,22–24]
[9–11,25–
Co(salen)
and polyoxometalate-based compounds,
have been applied in oxidation catalysis over the last decades,
2
8]
including in delignification. In contrast, non-porphyrin based metal
complexes have drawn only modest attention for example with the
ligands tetra-amido macrocycle (TAML), 1,4,7-trimethyl-1,4,7-
triazacyclononane (Me
3
TACN), and 1,2-bis-(4,7-dimethyl-1,4,7-
(
Figure 1) is the most (ca. 55%) abundant linkage in lignin
triazacyclonon-1-yl)-ethane (DTNE).[
29]
Nevertheless catalysts
(µ-O) ](ClO and
6 2
) have shown good performance
polymers.[
2,3]
Hence, the oxidation of the functional groups
such
(Me
as
[(Me
(µ-O)
4
DTNE)Mn(IV)
](PF
2
3
4 2
)
adjacent to this linkage and particularly at benzylic positions,
represents an attractive starting point for lignin
depolymerisation.
[
3
TACN)Mn(IV)
2
3
in the delignification of softwood (e.g., Kraft-AQ) pulps with
[
1,2,12–15]
[30–32]
H
2
O
2
.
It is notable that biphenyl (5-5) and stilbene structures
are degraded preferentially, with -O-4, -5, - linkages
undergoing degradation to a lesser extent and they are therefore
more efficient in the delignification of the soft rather than
hardwood pulp. Hence, there is a need for catalysts that target the
breakup of lignin through attack of, e.g., -O-4 linkages.
Recently, we reported a manganese(II) catalyst prepared in
situ with pyridine-2-carboxylic acid (PCA) and sub-stoichiometric
2 2
ketones for the oxidation with H O of a broad range of organic
[
[
a]
b]
Dr. A Barbieri, Prof Dr O. Lanzalunga
Dipartimento di Chimica, Sapienza Universita`di Roma “La
Sapienza”, P.le A. Moro 5,I-00185 Rome, Italy.
Mr. J. B. Kasper, Dr F. Mecozzi, Prof Dr W. R. Browne
Molecular Inorganic Chemistry, Stratingh Institute for Chemistry,
Faculty of Science and Engineering, University of Groningen
Nijenborgh 4, 9747AG, Groningen, The Netherlands
w.r.browne@rug.nl
compounds as alkanes, olefins, aliphatic and benzylic alcohols
under ambient conditions with high turnover numbers (up to 300
Supporting information for this article is given via a link at the end of
the document.
000 for the epoxidation of electron-rich alkenes) and low catalyst
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ChemSusChem
10.1002/cssc.201900689
FULL PAPER
loadings (Scheme 1).[33–39] The simplicity of the catalyst in
preparation and, importantly, its wide solvent scope make it a
potential candidate for large scale application. In particular, the
good conversion shown in the oxidation of secondary benzylic
secondary aliphatic and benzylic alcohols.[34] With the in situ
prepared manganese catalyst (Scheme 1), 1-phenyl-1,2-
ethanediol undergoes 60% conversion to the corresponding α-
hydroxyketone, as reported in the earlier.[45] Surprisingly, only
minor (<10%) conversion of 1 was achieved under these
conditions (Figure S1). The low conversion was not due to loss of
3
alcohols, such as p-X-1-phenylethanol (X=H, Br, OCH ) and
aromatic vicinal diols to ketones, even when the benzylic alcohol
positions are protected as ethers, encouraged us to consider the
potential of this Mn system in catalytic lignin degradation, in
particular attack at the β-O-4 linkage.[34,39]
2 2
H O since it remained in solution unreacted. Notably, the
conversion to ketone product, i.e. the turn over number was ca.
100 regardless of whether the initial concentration of 1 was 0.05
M or 0.5 M (figure S1), which is consistent with the zero order
dependence on substrate of the reaction observed earlier.[
46]
Scheme 1. (a) Typical conditions employed in the manganese catalysed
oxidation of 1-phenyl-1,2-ethanediol to the corresponding ketone product, see
main text for details and (b) potential inhibitors studied.
II
We show here that the Mn /PCA/butanedione catalytic system
when applied to lignin model compounds such as 2-(2-
Figure 2. Formation of 2-hydroxy-acetophenone upon oxidation of 1-pheny-1,2-
ethanediol in the absence (blue) and presence of 5, 20, 50, and 100 mol% of 1
followed through the intensity of the Raman band (λexc 785nm) of the carbonyl
stretch of the product at 1692 cm . With 0.01 mol% Mn(II). See Scheme 1 for
conditions.
methoxyphenoxy)-1-phenylethanol
(1)
(Figure
1),
an
unexpectedly low conversion was observed. Catalytic oxidation in
the presence of mono and di-oxygenated aromatic compounds
such as catechol, that are typical motifs present in lignin, are
-
1
responsible
for
(temporary)
deactivation
of
our
II
Mn /PCA/butanedione based catalyst through coordination to the
metal centre. The binding of catechol type compounds to
manganese is well documented,[40] as is the formation of high
valent manganese complexes from such ligands in natural
Inhibition of manganese catalyst by 1 and phenols
The origin of the lack of conversion was explored through the
effect of 1 on the conversion of 1-phenyl-1,2-ethanediol (Figure 2)
to the corresponding α-hydroxyketone achieved. The conversion
decreases from 60% to 0% as the concentration of 1 is increased,
with the most pronounced decrease between 5 and 20 mol% of 1
systems.[41]
A combination of spectroscopic studies and
competition experiments establish the propensity of catechol
motifs to interact with the catalyst. The coordination of phenolic
groups to manganese is explored, however, in contrast to most
nonheme catalysts, above all nonheme iron complexes, where
complexation leads to a loss of catalytic activity,[42–44] in the
(
with respect to 1-phenyl-1,2-ethanediol). Notably the duration
over which oxidation of 1-phenyl-1,2-ethanediol was observed
with < 50 mol% 1) was unaffected but instead the rate decreased.
Furthermore, the H was consumed to a concomitantly lesser
(
2
O
2
II
Mn /PCA/butanedione system, inhibition is shown to be
extent excluding that wasteful degradation of H
for the decrease in conversion (Figure S2).
2 2
O is responsible
dependent on inhibitor concentration as well as time. The
conclusions reached hold implications for the application of other
transition metal based complexes and especially in understanding
catalyst inhibition mechanisms.
The rate of oxidation of 1-phenyl-1,2-ethanediol is lower
when the diol is added 5 min after addition of H , compared to
that when it is present before addition of H , confirming the
2
O
2
2
O
2
effect on catalytic activity is due to the presence of 1. The
difference between 1-phenyl-1,2-ethanediol and 1, lies in the
guaiacol moiety of the latter and indeed addition of 5 mol% of
guaiacol is sufficient to see a completely suppression of the
catalytic oxidation of 1-phenyl-1,2-ethanediol (Figure 3). These
data indicate that guaiacol or simple phenolic compounds are
Results and Discussion
The oxidation of 1-phenyl-1,2-ethanediol and 1 with H
studied here using conditions optimized earlier for the oxidation of
2 2
O is
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(
pink). The formation of the ketone product, 2-hydroxyacetophenone, was
responsible for the loss in catalytic activity, as confirmed by the
inhibition observed with several mono and di-oxygenated
aromatic compounds. The inhibition by 1,2-dimethoxybenzene is
much less pronounced than the inhibition observed with phenol
and especially ortho-dihydroxylated catechol (which shows
complete inhibition), indicating that alcohols available for
coordination are important (Figure 3).
-
1
monitored by following the increase of the band at 1692 cm by in line Raman
spectroscopy (λexc 785nm). Solvent bands served as internal reference. See
Scheme 1 for conditions.
Reaction of Mn(II) with catechol in absence of H
The reaction of catechol with the manganese catalyst prior to the
addition of H was studied by (resonance) Raman, UV-Vis
absorption spectroscopy and cyclic voltammetry. Addition of
catechol (in a hundred-fold excess with respect to Mn(ClO ) to
a mixture of Mn(ClO , PCA and 1-phenyl-1,2-ethanediol in
2 2
O
2
O
2
4 2
)
4 2
)
acetonitrile results in the appearance of an absorption band at 600
nm, reaching a maximum absorbance within 2-3 min. An
additional absorption band at 410 nm is noticeable after 6 min, but
in contrast to the band at 600 nm, the 410 nm band continues to
increase in absorbance over one hour (Figure 5a). With a ten-fold
increase in [Mn(II)] (and hence 10:1 ratio of catechol to Mn(II)) the
maximum absorbance at 600 nm increases ten-fold also (Figure
5b), however although the band at 410 nm appears more rapidly
and with greater absorbance, it does not increase more than a
few fold over that with the lower concentration of Mn(II) (Figure
5b).
Figure 3. Formation of 2-hydroxy-acetophenone by oxidation of 1-phenyl-1,2-
ethanediol with 0.01 mol% Mn(II) in the presence of
dimethoxybenzene (light blue), phenol (orange), guaiacol (grey), catechol
green), and 5 mol% (25 mM) 1 (yellow) and guaiacol (dark blue); 1-phenyl-1,2-
ethanediol was added 5 min before addition of H . The formation of the
1 mol% (5 mM)
(
2 2
O
ketone product, 2-hydroxyacetophenone, was monitored by following the
increase of the band at 1692 cm-1 by in line Raman spectroscopy (λexc 785nm).
Solvent bands served as internal reference. See Scheme 1 for conditions.
Notable, however, a ten-fold increase in [Mn(II)] (to 0.5 mM)
results in inhibition only by catechol and even so after a short
delay the reaction proceeds at the same rate as without inhibitor
(
Figure 4 and vide infra).
Figure 5. UV/vis absorption spectrum (path length 2 mm) of 1-phenyl-1,2-
ethanediol (0.5 M), PCA (2.5 mM) and Mn(ClO 6H O ((a) 0.05, and (b) 0.5
)
4 2
2
mM) in acetonitrile before (blue) and following addition of catechol (5 mM).(left
Change in absorbance at 413 (blue) and 620 nm (orange) over time.
These bands are not observed in the absence of 1-phenyl-1,2-
ethanediol or of catechol (Figure S3 and S4). Surprisingly,
although it is present in a large excess, the relative amount of 1-
phenyl-1,2-ethanediol has
a significant effect on the two
Figure 4. Formation of 2-hydroxy-acetophenone by oxidation of 1-phenyl-1,2-
ethanediol with 0.1 mol% Mn(II) (black) in the presence of 1 mol% of
dimethoxybenzene (blue), phenol (red), guaiacol (green), catechol (orange), 1
absorption bands also (Figure S5). The maximum absorbance at
410 nm and 600 nm reached increases with the concentration of
-phenyl-1,2-ethanediol until 0.5 M and thereafter the changes
1
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are less pronounced. The large excess needed indicates that
complexation to manganese by the diol is not significant but
instead the relative basicity of the solution is important (vide infra).
The rate and extent of formation of the absorption band at
nm is typical of LMCT bands of MnIV catecholate species. The
absorbance reached (ca. 0.45, Figure 5b) is consistent with the
IV
2-
-1
-1 [47–52]
3
molar absorptivity of [Mn (catecholato) ] (4500 M cm ).
These data indicate that in the first 2-3 minutes of the reaction the
IV
]2- is formed.
6
00 nm is unaffected by the absence of O
absorption band at 410 nm, which increased more rapidly with O
purged solutions and hardly appeared in N purged solutions (i.e.
2
, in contrast to the
deep blue [Mn (catecholato)
3
2
2
only residual oxygen reacted, Figure 6). Notably the rate of
increase in absorption at 630 nm is similar regardless of the
concentration of O
equilibrated solutions is absent in O
2
, but the induction period observed in air
purged solutions indicating
2
that oxygen plays the role of initiator. The presence of both
catechol and 1-phenyl-1,2-ethanediol (in large excess) is
essential for the appearance of both bands. Aliphatic diols such
as glycerol do not show similar effects. The presence of PCA,
which is essential for catalysis with H
changes observed (Figure S6).
2 2
O , has no effect on the
Figure 6. Absorbance (2 mm path length) at 457 and 620 nm over time for
solutions of PCA (2.5 mM) and 1-phenyl-1,2-ethanediol (0.5 M) in CH CN after
the addition of Mn(ClO 6H O (0.5 mM) and catechol (5 mM) in air equilibrated
CH CN, and with prior O (blue) and N (grey) purge. The absorbance at 475
3
)
4 2
2
3
2
2
nm was used to track the increase in the band at 410 nm since the absorbance
at 410 nm reached > 2 within a few min.
Raman spectra recorded at λexc 632.8 nm (i.e. in resonance with
the 600 nm absorption band) show the appearance of bands at
24 cm , 1264 cm , 1324 cm , 1474 cm and 1569 cm-1
-
1
-1
-1
-1
6
concomitant with the changes in absorbance at 600 nm (Figure
). The bands at 1264 cm-1 and 1474 cm-1 are characteristic of
the catecholate C-O stretching modes and the absorbance at 600
7
Figure 7. Catechol (5 mM) was added to Mn(ClO
4 2 2
) 6H O (0.5 mM), PCA (2.5
mM) and 1-phenyl-1,2-ethanediol (0.5 M) in CH CN. (a) Raman spectra (exc
3
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632.8 nm) recorded before (red) addition of catechol and after the absorption
-1
-1
-1
-1
cm , 1235 cm , 1550 cm and 1665 cm , after a delay of ca. 6
min (Figure 8) that is consistent with the lag time observed by
UV/Vis spectroscopy (Figure 5a). Moreover the change in
intensity of the Raman bands at 1550 cm-1 and 1665 cm-1 over
time tracks the change in absorbance at 410 nm. The Raman
bands at 1550 and 1665 cm-1 are characteristic of quinone C-O
stretching modes and may indicate the formation of a quinone
dimer.[53–55] Addition of catechol (1 mol%) to 1-phenyl-1,2-
band at 600 nm had reached a maximum (black). The difference spectrum
showing resonantly enhanced bands only is in blue. (b) Spectra recorded over
time. (c) Intensity of bands at 1250 and 1324 cm-1 over time.
4 2
ethanediol in the absence of Mn(ClO ) results in the appearance
of an absorption band at 410 nm over 60 min together with an
unassigned band at 340 nm (Figure S7) that is absent in the
presence of Mn(ClO
4 2
) . The maximum absorbance reached,
however, is much less than in the presence of Mn(ClO ) .
4 2
Influence of reaction components on redox chemistry and
UV/Vis spectroscopy of catechol
The cyclic voltammetry of catechol in acetonitrile shows the
expected oxidation wave at ca. 1.0 V and reduction of dimer
formed by radical coupling at 0.5 V. Addition of 1-phenyl-1,2-
ethanediol, which is itself redox inactive, has a pronounced effect
on the cyclic voltammetry of catechol with an almost 1 V shift in
the redox potential of catechol towards negative potentials (Figure
9). The addition of Mn(II) salts and PCA at the same
concentrations as present under catalytic conditions does not
affect the voltammetry significantly. These changes indicate that
the diol is facilitating oxidation of the catechol by Mn(II) and O
simply as a proton acceptor (vide infra). Indeed the preparation of
2
IV
2-
[47–
3
the complex Mn (catecholato) ] requires addition of a base.
5
2]
4 6
Figure 9. Cyclic voltammetry in acetonitrile, 0.1 M Bu PF as electrolyte with
saturated Ag/AgCl as reference electrode and scan rate of 0.1 V/s: (a) of
catechol 5 mM; (b) of catechol 5 mM, PCA 2.5 mM,1-phenyl 1,2-ethanediol 0.5
4 2 2
M and Mn(ClO ) 6H O 0.5 mM
Figure 8. Raman spectra (λexc 355 nm) of a Mn(ClO
4
)
2
6H
2
O (0.05 mM), PCA
2.5 mM and 1-phenyl-1,2-ethanediol (0.5 M) in CH
3
CN, (a) before black and
after the addition of catechol (5 mM). (b) Difference spectrum (Raman spectrum
at 33.3 min (red) minus spectrum at 8 min (blue)). (c) Raman intensity at 1665
-
1
cm and (inset) absorbance at 410 nm over time.
Raman spectroscopy at exc 355 nm (i.e. resonant with the 410
nm absorption band), shows the appearance of the bands at 795
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addition of AcOH, which is formed in situ rapidly through oxidation
of butandione under reaction conditions, results in immediate
disappearance of the bands at 410 and 600 nm, with the former
band largely obscured by the butanedione present. These
changes are consistent with the positive shift in the oxidation
potential of the catechol (Figure S9) and inhibition of complexation
to manganese expected upon protonation of catechol.
Effect of catechol on induction period in the catalysed
oxidation of phenyl-1-2ethanediol
Although UV/Vis absorption spectroscopy indicates formation of
IV
catechol oxidation products and the Mn (catecholato)
3
species,
these species undergo reduction in the presence of butanedione
and AcOH. However the relation between inhibition period and
catechol concentration (Figure 11) indicates that the oxidation of
catechol proceeds under reaction conditions. When the
concentration of catechol is close (0.05 mol% and 0.2 mol% w.r.t.
diol/ 5 and 20 equiv. w.r.t. Mn(II)) to that of Mn(II) then the
oxidation of 1-phenyl-1,2-ethanediol with
H
2
O
2
proceeds
smoothly. When the catechol concentration exceeds that of the
Mn(II) (40-60 equiv) then inhibition is observed in regard to the
initial lag phase but once conversion commences it proceeds at
the same rate regardless of initial catechol concentration (Figure
1
2). Above 60-100 equivalents of catechol with respect to
manganese, however, conversion is not seen even over several
hours (Figure 12) and there is negligible consumption of H also.
2
O
2
Hence, although we cannot be certain that the inhibition periods
duration is related to the rate of catechol oxidation there is present
a clear correlation indicating this.
Figure 10. UV/vis absorption spectrum (2 mm path length) of Mn(ClO
)
4 2
6H
2
O
(
0.5 mM), PCA (2.5 mM) and 1-phenyl-1,2-ethanediol (0.5 M) in CH CN (black)
3
and (a) after addition of catechol (5 mM) (i), immediately after addition of 0.5
equiv. Butanedione at 3.2 min (ii), and subsequent changes (iii), and after
addition of acetic acid at 18 min (iv)and (b) with addition of butanedione and
AcOH 39 min and 46 min, respectively, after addition of catechol. The
absorbance at 600 nm and 410 nm over time are shown as insets, respectively.
The addition of butanedione, a major reaction component present
in 25 mol% typically, induces immediately a reduction of both the
IV
putative Mn (catecholato)
3
species (absorbing at 600 nm) and
the species responsible for the absorption band at 410 nm.
However, the absorbance at 600 nm reappears eventually after
addition of butanedione (Figure 10a, see abs at 600 nm vs time).
The open circuit potential in time (Figure S8) as well as the cyclic
voltammetry (Figure S9) indicates that the recovery of the Mn(IV)
complex is inhibited by the lower redox potential of the solution,
highlighted by a shift in the oxidation potential by 0.5 V. The
Figure 11. Oxidation of 1-phenyl-1,2-ethanediol (0.5 M) in the presence of
Mn(ClO ) 6H O 0.05 mM and 0.25 mM (black), 1 mM (red), 2.0 mM (blue), 2.5
4 2 2
mM (green), 3.0 mM (pink) and 5.0 mM catechol (orange); The formation of 2-
hydroxy-acetophenone is followed through the Raman band at 1692 cm-1 (λexc
2 2
785nm). See Scheme 1 for conditions and Figure S10 for changes at 864 (H O )
-
1
and 842 (substrate) cm .
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Addition of 0.5 mol% of catechol 10 min after addition of H
2
O
2
butanedione and/or acetic acid, the Mn(IV) complex rapidly
disappears, presumably due to reduction to the relatively
colorless Mn(II) or Mn(III) state. These changes are consistent
with the effect of alcohol, butanedione and acetic acid on the
cyclic voltammetry of catechol in acetonitrile, specifically the
potential for catechol oxidation. Hence, although the formation of
catechol manganese(IV) complexes can occur, the formation of
similar complexes under reaction conditions can only be
speculated upon. However, the inhibition of the catalyst under
reaction conditions by catechol depends on its concentration
relative to the concentration of manganese ions. When near
stoichiometric amounts are used the inhibition is apparent. After
a lag time that depends on initial catechol concentration (relative
to manganese) the oxidation of 1-phenyl-1,2-ethanediol proceeds
with the same rate as in the absence of catechol. Hence, phenolic
compounds act as sacrificial inhibitors rather than permanently
deactivating the catalyst.
results in a halt to the oxidation of 1-phenyl-1,2-ethanediol, with
the same induction period (50 min) as observed when the
catechol and 1-phenyl-1,2-ethanediol are present before addition
of H
2
O
2
(Figure 12). Similarly, when 1-phenyl-1,2-ethanediol is
(Figure 12), the induction
added 5 min after addition of H
2
O
2
period is also ca. 50 min, indicating that regardless of the
sequence addition of reagents, the same lag time is observed.
The inhibition of the catalyst in the oxidation of 1 is unlikely
to be due to oxidation products of 1 such as mandelic acid as the
concentrations of these compounds under reaction conditions are
insufficient to inhibit reactivity also (Figure S12) nor is it due to
guaiacol residue from the synthesis of 1 as it is not present in
1
sufficient amounts (by H NMR spectroscopy) to cause the
inhibition observed in the oxidation of 1-phenyl-1,2-ethanediol in
the presence of 1. The formation of guaiacol from 1, e.g. by one
electron oxidation,[16] is reasonable but due to its subsequent
oxidation under reaction conditions it is not possible to confirm its
presence under reaction conditions.
Conclusions
Figure 12. Oxidation of 1-phenyl-1,2-ethanediol (0.5 M) in the presence of 1
mM catechol: catechol and 1-phenyl-1,2-ethanediol present before addition of
H
H
H
2
2
2
O
O
O
2
2
2
(red), catechol added 10 min after addition of 1-phenyl-1,2-ethanediol and
(black), 1-phenyl-1,2-ethanediol added 5 min after addition of catechol and
(blue); The formation of the ketone product was determined from the
Spectroscopic analysis confirms the tendency of phenolic
compounds such as catechol, to chelate manganese and to
undergo oxidation in the presence of aromatic diols such as 1-
phenyl-1,2-ethanediol. Under reaction conditions, catechol can
inhibit the activity of the catalyst through chelation but oxidation
eventually ‘releases’ the manganese ions to reform the catalytic
system. The presence of such species in lignin therefore is likely
to impact the effectiveness of the catalytic system and increases
the concentration of manganese required for the oxidative
degradation of the polymer. However, the formation of other
potentially chelating compounds (such as mandelic acid
derivatives) during the oxidation is likely to have a greater impact
as these are less susceptible to removal by further oxidation.
These results emphasise the challenge in translating results from
reactions with model compounds to the complex mixtures of
compounds encountered with biorenewable resources. Indeed
-
1
intensity of the band at 1692 cm in the Raman (λexc 785nm) of the reaction
2
mixture. See Scheme 1 for conditions and Figure S11 for changes at 864 (H O )
and 842 (substrate) cm .
2
-
1
Mechanistic considerations
Although the oxidation of 1 to its corresponding ketone proceeds
with ca. TON of 100, it is clear that inhibition by a species
produced during oxidation occurs. The inhibition in the oxidation
of 1-phenyl-1,2-ethanediol in the presence of 1 confirms this
conclusion. In the first instance the release[56] of guaiacol from 1
is a likely candidate as inhibitor. Indeed, inhibition by catechol and
a variety of phenolic compounds is observed. This inhibition is of
relevance to the use of this catalytic system in the oxidation of
lignocellulose as such motifs are abundant. UV/Vis spectroscopic
and resonance Raman data show that catechol forms the known
the success of robust complexes such as [(Me
4
2
DTNE)Mn(IV) (µ-
O) ](ClO and [(Me TACN)Mn(IV) (µ-O) ](PF )
3
4
)
2
3
2
3
6 2
used in soft
blue complex [Mn(IV)(catecholato)
,2-ethanediol and under aerobic conditions the catechol
undergoes oxidation manifested in the absorption band at 410
3
]2- in the presence of 1-phenyl-
wood oxidation highlight the need to build similar robust catalysts
for oxidation of β-O-4 linkages in lignin. In the present study we
have explored the use of a manganese based catalyst that is
prepared in situ. Although simple and economic, the lability of the
ligand (PCA) as well as the low concentrations used mean that
even low concentrations of other potential ligands can bind the
manganese ions and inhibit oxidation. Hence the development of
1
[
57],[58]
nm.
The presence of 1-phenyl-1,2-ethanediol is essential to
the observation of this species due to the need for a proton
acceptor; it should be noted that the formation of the Mn(IV)
complex requires the addition of a base. In the presence of
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ChemSusChem
10.1002/cssc.201900689
FULL PAPER
polydentate ligand based catalysts is ultimately necessary in
applications involving complex mixtures such as lignin.
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