Breakdown of lignins, lignin model compounds, and hydroxy-aromatics, to C1 and C2 chemicals via metal-free oxidation with peroxide or persulfate under mild conditions

Article abstract of DOI:10.1039/c4ra00742e

The aromatic components of lignin model compounds and lignins are degraded in basic, aqueous solutions using H₂O₂ or K ₂S₂O₈, even at ambient temperatures, to mainly MeOH, formate, carbonate, and oxal

Full text of DOI:10.1039/c4ra00742e

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Breakdown of lignins, lignin model compounds,
and hydroxy-aromatics, to C1 and C2 chemicals via
metal-free oxidation with peroxide or persulfate
under mild conditions†
Cite this: RSC Adv., 2014, 4, 17931
Received 24th January 2014
Accepted 7th April 2014
*
Adam Wu, Jean Michel Lauzon, Indah Andriani and Brian R. James
DOI: 10.1039/c4ra00742e
www.rsc.org/advances
The aromatic components of lignin model compounds and lignins are imply effectiveness for lignins!9 The rst report on the use of a
degraded in basic, aqueous solutions using H2O2 or K2S2O8, even at Ru-catalyst did not appear until 2010,10 which is surprising
ambient temperatures, to mainly MeOH, formate, carbonate, and considering that Ru species appear to be the most widely used
oxalate. The MeOH derives from aromatic OMe substituents, and the in catalytic oxidations; indeed, every type of catalyzed organic
other products are formed from other substituents and/or C–H bonds oxidation process (with or without bond-cleavage) can be
in the aromatic rings.
exemplied by a Ru system, particularly using ruthenium-oxo
species.11 Griffith's group had reported in 1993 an oxidation of
3,4-dimethoxybenzyl alcohol (veratryl alcohol) to the acid (6)
using RuCl3$3H2O in the presence of KBrO3 (bromate) or
K2S2O8 (persulfate) as co-oxidants in basic aqueous solution at
room temperature (r.t., ꢀ20 ꢁC), these generating, respectively,
Lignocellulosic biomass is an attractive, potential renewable
source of biofuels and other useful aromatic compounds, and
research interest this area has intensied enormously in the last
decade;1–3 it is evident that progress in the lignin area2 is slower
than in the cellulose area,3 presumably because of the more
complex nature of the lignin structure. The interest in lignin is
particularly signicant within the pulp and paper industry,
which generates large amounts of this currently largely ‘waste’
material.4 Lignin is a cross-linked, racemic and phenolic
macromolecule comprised of three basic phenyl propane
monomers (p-hydroxycinnamyl, coniferyl and sinapyl alcohols)
connected by a network of C–C and C–O bonds.2d,5 Phenol,
guaiacol (1), and syringyl derivatives (2–4), may be considered
basic structural units of lignin and, along with vanillic (5) and
veratric (6) acids (Chart 1) are loosely termed lignin model
compounds (LMCs) in this current paper; the Chart also shows
examples of so-called ‘dimeric’ LMCs (i.e. containing two
aromatic rings) that contain, like lignins, a benzylic OH group
(7 and 8).
2À
perruthenate (RuO4À) or ruthenate (RuO4 );12 other benzyl
alcohols and aldehydes, and cinnamyl alcohol and aldehyde,
were all similarly oxidized to the corresponding carboxylic acid.
The ndings were presented as new, efficient methods for
selective oxidation of organic alcohols and aldehydes, and no
mention was made of LMCs. The bromate systems were cata-
lytic in RuO4À, whereas the persulfate systems were stoichio-
2À
12
metric and not catalytic in RuO4
. Based on this report, we
initiated a project in 2010 using such RuO4À or RuO42À systems
for oxidation of LMCs and lignins.
Catalytic oxidative or hydrogenolysis breakdown of LMCs of
the type shown in Chart 1 has been reported for decades, the
most recent studies using V-, Cu-, Ni-, and Ru-based systems;2c,6
however, reports on potentially useful breakdown of lignins (as
opposed to fuel value by non-selective, complete combustion to
CO2)7 are, as expected, more limited,1b,2b,8 although discussion
on catalytic model systems has sometimes been embellished to
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver,
BC, V6T 1Z1, Canada. E-mail: brj@chem.ubc.ca
† Electronic supplementary information (ESI) available: Tabulated experimental
results and associated NMR spectra. See DOI: 10.1039/c4ra00742e
Chart 1 Lignin model compounds (LMCs).
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1
Table 1 Reactions of LMCs and lignins in 1.0 M KOH (from H NMR
data)
This current communication describes the surprising and
remarkable discovery that, even in the absence of a Ru-oxo
species, the aromatic components of LMCs and lignins can be
Consumptn
(%)
MeOH
(mmol)
HCO2
(mmol)
À
2À
Substratea
Oxidantb
oxidized by S2O8 and by H2O2 with up to complete loss of
aromaticity in basic aqueous solutions even at ambient
temperatures; control experiments gave #15% consumptions,
showing that it is the oxidants that effect the degradation to
products, mainly MeOH, formate, carbonate, and oxalate. Non-
catalysed, complete combustion of aromatics typically requires
temperatures of 500 ꢁC.7
Each of the commercially available model substrates (1–6),
the synthesized dimers 7 and 8,6a and several lignin samples13
were reacted in an NMR tube under the general conditions
shown in Scheme 1. The use of H2O2 (inexpensive and green)14
seemed particularly attractive, and was the primary oxidant
used for this study.
Guaiacol (1)
Syringic acid (2)
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
61
94
89
100
100
100
100
100
40
96
4
51
n/a
n/a
0.025
0.028
0.073
0.069
0.096
0.086
0.100
0.096
0.004
0.020
0.000
0.025
0.022
0.019
0.021
0.008
0.022
0.002
0.085
0.016
0.093
0.013
0.003
0.002
0.000
0.000
0.039
0.002
Syringyl aldehyde
(3)
Syringyl alcohol (4)
Veratric acid (5)
Vanillic acid (6)
LMC dimer (7)c
The initial 1H NMR spectra were recorded at r.t. prior to
addition of the oxidant, and the tube was then heated to 60 ꢁC
with substrate consumption monitored over time by the
decreasing intensities of aromatic 1H NMR signals in the dH
7.5–6.5 region. A 3 h reaction time was generally used to dene a
standard set of conditions because 100% consumption of the
monomer LMCs were oen seen at this time; remaining
aromatic content in the lignins ranged from 1 to 22%. The
reaction mixture was allowed to cool to room temperature
before pivalic acid was added as a standard to determine the
yields of MeOH and formate (Table 1), which both possess an
1H NMR signal. The other major products formed were
carbonate and oxalate, which we tried to quantify for substrates
Ligninsd
9
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
H2O2
K2S2O8
80
84
97
95
99
89
82
82
91
78
0.040
0.018
0.022
0.033
0.093
0.090
0.047
0.019
0.055
0.037
0.046
0.006
0.031
0.007
0.050
0.004
0.054
0.003
0.033
0.003
10
11
12
13
a
0.050 mmol in 1 mL D2O with 1.0 M KOH, except for 7 (0.025 mmol).
0.50 mmol H2O2, 0.25 mmol K2S2O8. c Not fully dissolved; dimer 8 was
b
insoluble. d 15 mg of lignin sample used.
2, 3, and the highest yielding lignin (11) using quantitative 13
C
{1H} NMR spectroscopy (Table 2).15
Each experiment was carried out at least twice, with data
showing variation of up to Æ15% in the substrate consumptions
and yields;16 the average values are presented in Table 1. Slow
degradation of the LMCs is observed at room temperature; for
example, consumption of syringyl alcohol (4) using H2O2 is
ꢀ20% is aer 2 h, ꢀ35% aer 4 h, and 100% aer 48 h and NMR
data conrm production of the same four major products.
Preliminary room temperature studies with lignin 11 are
promising with >90% consumption observed aer 24 h; signals
for MeOH, formate, carbonate, and oxalate are all present in the
13C{1H} NMR spectrum.
Table 2 Reactions of LMCs and lignins with H2O2 in 1.0 M KOH (from
13C{1H} NMR data)
À
À
2À
MeOH
HCO2
CO3
C2O4
Substratea
(mmol)
(mmol)
(mmol)
(mmol)
Syringic acid (2)
Syringyl aldehyde (3)
Lignin 11
0.072
0.080
0.081
0.029
0.080
0.052
0.116
0.079
0.082
0.037
0.046
0.027
a
0.050 mmol of 2 and 3, and 15 mg of 11, in 1 mL D2O with 1.0 M KOH
and 0.50 mmol H2O2.
The quantitative data for MeOH and formate derived from
1H NMR data for substrates 2, 3, and 11 with H2O2 are consid-
ered in reasonable agreement with those determined by 13C{1H}
NMR data (Table 2). Interestingly, the 13C{1H} NMR spectra
reveal other C-containing products beside carbonate and
oxalate.17 For example, for 3, the total carbon detected by 13C
{1H} NMR is in the 87–97% range of the theoretical value
of 0.450 mmol [0.050 mmol substrate  9 (the number of
C-atoms)];18 the amounts of MeOH, HCO2À, CO32À, C2O42À, and
unidentied C-atoms (integration of seven smaller resonances),
are 0.080, 0.079, 0.080, 0.046, and 0.11 mmol, respectively
(Table S5†). Similar data for 2 (Table S4†) show 74–86% of the
theoretical value; corresponding data for the lignin (Table S6†)
show the same four major products and only 14% unidentied
C-atoms. Of note, the same unidentied 13C{1H} NMR reso-
nances are seen for the lignin and the syringyl substrates.
The highest substrate consumptions (>90%) are realized
2À
with 1–5, particularly using S2O8 as the oxidant, with the
syringyl substrates 2–4 giving the most MeOH. The MeOH is not
formed via hydrolysis in the basic conditions, consistent with a
study showing that OMe substituents within aromatic
Scheme 1 Generalized reaction scheme for oxidation of LMCs and
lignins.
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compounds are stable toward hydrolysis in 20% NaOH at (2) system under Ar; similarly, exclusion of ambient light (by
100 C.19 Nevertheless the MeOH likely derives from the OMe wrapping the NMR tube in aluminium foil) had no effect on the
ꢁ
groups since the data for 2–4, which contain two OMe substit- consumption and product formation. Photochemical induced
uents, compared with those for 1 and 5 with one OMe, suggest a degradation of LMCs and lignins in alkaline H2O2 solutions has
correlation between the number of OMe groups and MeOH been widely studied,25 but there are no reports on formation of
production (and substrate consumption). Support for this is the C1 and C2 compounds found herein. Further, Stahl's group
found in corresponding data for o- and p-ethoxyphenols which, has reportedꢁthe use of conditions similar to ours (H2O2 in 2 M
with the H2O2/KOH treatment, generate EtOH and formate, NaOH at 50 C for 10 h) in a 1 : 1 : 1 H2O–THF–MeOH solvent
analogous to the reactivity of 1. The data for 6, which contains mixture for C–C bond cleavage of a dimer LMC to give mainly 1
two OMe groups, does not t the correlation, and implies that and 6 with no evidence for breakdown of the aromatic rings.2b
the presence of an OH group (as in 5) in some way ‘activates’ the Solvent effects clearly play a dominant role in peroxide oxida-
system.
tion processes, and further work is necessary to gain insight
NMR studies show that MeOH, formate, and oxalate are not into the mechanism.
oxidised under the standard conditions using H2O2, and simi-
Irrespective of the mechanism, formation of MeOH from the
larly the syringyl reactants 2–4 are not oxidised stepwise (i.e. methoxy group of 1 might generate catechol, and so this (and
alcohol to aldehyde to acid) prior to de-aromatization. That the phenol) were also subjected to the oxidation conditions using
formate is not formed from the MeOH is supported further by H2O2 or K2S2O8 (at 1 M KOH). Complete consumption of the
data for 3 and 4, where the amount of MeOH produced corre- substrates was observed, though, consistent with the absence of
sponds to (or approaches) that expected for formation from the OMe groups, no MeOH is formed. Notably, the major products
OMe groups. The dimer model 7 (with two OMe groups) did not present were formate, carbonate, and oxalate. In contrast,
completely dissolve under the reaction conditions even when oxidation of catechol and phenol in aqueous solution by O2
0.025 mmol of substrate was used, but both MeOH and formate (which requires high temperature and pressure) is complex and
are formed. Qualitatively the data suggest that the MeOH likely can plausibly generate many products, including formate.26
comes from just one OMe, again likely the one in the phenoxide
De-polymerization of lignin to, for example, phenolic-type
ring. Dimer 8 was essentially insoluble, and thus gave no MeOH chemicals in the hope of establishing its use as a feedstock for
or formate. Of interest, the data for lignin 11 correspond closely the sustainable production of bulk chemicals remains, of
to complete conversion of the OMe substituents to MeOH course, a key goal1,2 and hopefully, by ne-tuning our demon-
1
(ꢀ17%), since an independent H NMR method gives a more strated mild oxidation conditions especially using a variety of
precise value of 18.7%. Such a breakdown of lignins to MeOH solvent mixtures (with or without the involvement of transition
could provide a simple method for determining their OMe metal catalysts), such a useful type of degradation can be
content, typically in the 11.4–22.9% range (by weight),20 as accomplished.
determined traditionally via conversion to MeI by treatment
with HI.21
We thank NSERC Lignoworks for nancial support, and Dr.
Andrew Lewis at Simon Fraser University for assistance with,
A lower pH solution was also tested for all the substrates and use of, the Bruker AVANCE 600 MHz NMR spectrometer.
using K2CO3 because carbonate had been used in the studies of
Griffith et al. for in situ formation of RuO4À (KOH had been used
Notes and references
2À
for generating RuO4 ).11,12 However, lower consumptions
(#20%) were generally seen for the LMCs, and none of the
lignins was fully soluble in the K2CO3 solution. Substrate 2 on
treatment with RuO4 (formed by addition of RuCl3$3H2O to
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and cSO4À 23 respectively), and a radical process at the aromatic
,
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