Revisiting alkaline aerobic lignin oxidation

Article abstract of DOI:10.1039/c8gc00502h

Lignin conversion to renewable chemicals is a promising means to improve the economic viability of lignocellulosic biorefineries. Alkaline aerobic oxidation of lignin has long been employed for production of aromatic compounds such as vanillin and syringaldehyde, but this approach primarily focuses on condensed substrates such as Kraft lignin and lignosulfonates. Conversely, emerging lignocellulosic biorefinery schemes enable the production of more native-like, reactive lignin. Here, we revisit alkaline aerobic oxidation of highly reactive lignin substrates to understand the impact of reaction conditions and catalyst choice on product yield and distribution. The oxidation of native poplar lignin was studied as a function of temperature, NaOH loading, reaction time, and oxygen partial pressure. Besides vanillin and syringaldehyde, other oxidation products include acetosyringone and vanillic, syringic, and p-hydroxybenzoic acids. Reactions with vanillin and syringaldehyde indicated that these compounds are further oxidized to non-aromatic carboxylic acids during alkaline aerobic oxidation, with syringaldehyde being substantially more reactive than vanillin. The production of phenolic compounds from lignin is favored by high NaOH loadings and temperatures, but short reaction times, as the products degrade rapidly, which is further exacerbated by the presence of oxygen. Under optimal conditions, a phenolic monomer yield of 30 wt% was obtained from poplar lignin. Testing a range of catalysts showed that Cu-containing catalysts, such as CuSO₄ and LaMn_0.8Cu_0.2O₃, accelerate product formation; specifically, the catalyst does not increase the maximum yield, but expands the operating window in which high product yields are obtainable. We also demonstrate that other native and isolated lignin substrates that are significantly chemically modified are effectively converted to phenolic compounds. Finally, alkaline aerobic oxidation of native lignins was compared to nitrobenzene oxidation and reductive catalytic fractionation, as these methods constitute suitable benchmarks for lignin depolymerization. While nitrobenzene oxidation achieved a somewhat higher yield, similar monomer yields were obtained through RCF and alkaline aerobic oxidation, especially for lignins with a high guaiacyl- and/or p-hydroxyphenyl-content, as syringyl units are more unstable during oxidation. Overall, this study highlights the potential for aerobic lignin oxidation revisited on native-like lignin substrates.

Full text of DOI:10.1039/c8gc00502h

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Roman-Leshkov and G. T. Beckham, Green Chem., 2018, DOI: 10.1039/C8GC00502H.
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Heterogeneous catalytic oxidation for lignin valorization into valuable
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†
Revisiting alkaline aerobic lignin oxidation
a,b
a
a
a
a
Wouter Schutyser , Jacob S. Kruger , Allison M. Robinson , Rui Katahira , David G. Brandner ,
Nicholas S. Cleveland , Ashutosh Mittal , Darren J. Peterson , Richard Meilan , Yuriy Románꢀ
a
a
a
c
d
a
Leshkov , and Gregg T. Beckham
Abstract: Lignin conversion to renewable chemicals is a promising means to improve the economic viability
of lignocellulosic biorefineries. Alkaline aerobic oxidation of lignin has long been employed for production of
aromatic compounds such as vanillin and syringaldehyde, but this approach primarily focuses on
condensed substrates such as Kraft lignin and lignosulfonates. Conversely, emerging lignocellulosic
biorefinery schemes enable the production of more nativeꢀlike, reactive lignin. Here, we revisit alkaline
aerobic oxidation of highly reactive lignin substrates to understand the impact of reaction conditions and
catalyst choice on product yield and distribution. The oxidation of native poplar lignin was studied as a
function of temperature, NaOH loading, reaction time, and oxygen partial pressure. Besides vanillin and
syringaldehyde, other oxidation products include acetosyringone and vanillic, syringic, and pꢀ
hydroxybenzoic acids. Reactions with vanillin and syringaldehyde indicated that these compounds are
further oxidized to nonꢀaromatic carboxylic acids during alkaline aerobic oxidation, with syringaldehyde
being substantially more reactive than vanillin. The production of phenolic compounds from lignin is favored
by high NaOH loadings and temperatures, but short reaction times, as the products degrade rapidly, which is
further exacerbated by the presence of oxygen. Under optimal conditions, a phenolic monomer yield of 30
wt% was obtained from poplar lignin. Testing a range of catalysts showed that Cuꢀcontaining catalysts, such
as CuSO
4
and LaMn0.8Cu0.2
O , accelerate product formation; specifically, the catalyst does not increase the
3
maximum yield, but expands the operating window in which high product yields are obtainable. We also
demonstrate that other native and isolated lignin substrates that are significantly chemically modified are
effectively converted to phenolic compounds. Finally, alkaline aerobic oxidation of native lignins was
compared to nitrobenzene oxidation and reductive catalytic fractionation, as these methods constitute
suitable benchmarks for lignin depolymerization. While nitrobenzene oxidation achieved a somewhat higher
yield, similar monomer yields were obtained through RCF and alkaline aerobic oxidation, especially for lignins
with a high guaiacylꢀ and/or pꢀhydroxyphenylꢀcontent, as syringyl units are more unstable during oxidation.
Overall, this study highlights the potential for aerobic lignin oxidation revisited on nativeꢀlike lignin substrates.
1
ꢀ5
a. National Bioenergy Center, National Renewable Energy
Laboratory, Golden CO 80401, USA
with βꢀether bonds being the most prominent. Other
important lignin building blocks are pꢀhydroxycinnamic
acids (pꢀcoumaric and ferulic acid), found in herbaceous
b. Center for Surface Chemistry and Catalysis, KU Leuven,
Celestijnenlaan 200F, 3001 Heverlee, Belgium
c. Department of Forestry and Natural Resources and Purdue
Center for Plant Biology, Purdue University, West Lafayette,
IN 47907, USA
6, 7
crops, and pꢀhydroxybenzoic acid, found in feedstocks
like poplar and palm. These acids are mainly connected
to lignin through ester linkages and can account for a
significant fraction of the lignin mass.
8, 9
d. Department of Chemical Engineering, Massachusetts Institute
of Technology, Cambridge MA 02139, USA
Due to its aromatic structure, lignin is regarded as a
1
ꢀ4, 10ꢀ15
Lignin valorization | Oxidation | Vanillin | Oxidative catalysis |
Lignocellulosic Biomass
Email: gregg.beckham@nrel.gov; yroman@mit.edu
promising source for renewable chemicals.
A
problem, however, is that traditional lignocellulosic
biorefinery processes primarily focus on the valorization of
the carbohydrates, while the lignin is only utilized for lowꢀ
value applications (heat and power) without considering
the fate of the lignin stream. For instance, pulping
processes such as Kraft, sulphite, and organosolv, aim to
remove lignin from wood to produce a pure carbohydrate
†
Electronic supplementary information (ESI) available.
Introduction
Lignin, a heterogeneous aromatic biopolymer, is one of the
main constituents of lignocellulosic biomass, comprising
1
ꢀ4
16, 17
1
5 to 30% of its weight.
It is an integral part of
pulp.
In the production of cellulosic biofuels such as
terrestrialꢀplant cell walls, where it is responsible for
mechanical strength, defence against microbial and insect
attack, and preventing the collapse of waterꢀconducting
elements. Lignin is mainly assembled from three phenylꢀ
propanoid units (pꢀcoumaryl, coniferyl, and sinapyl
alcohols), which are linked through CꢀO and CꢀC bonds,
bioethanol, a pretreatment step is applied to disrupt the
lignocellulosic cellꢀwall structure and remove or relocate
the lignin, with the primary aim of making the
1ꢀ5
18,
carbohydrates more amenable for biological conversion.
19
In these processes, the lignin either ends up in a
pretreatment liquor (from which it can be precipitated), or
1

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in
a
residue after enzymatic hydrolysis of the
vanillin and syringaldehyde from lignin, it is performed
commercially to produce vanillin from lignosulfonates.
The production volume has, however, dropped
considerably since 1980.
10
43, 44
carbohydrates. In most biorefinery processes, the lignin
is heavily degraded, which involves cleavage of labile
ether bonds (mainly βꢀOꢀ4 bonds) and condensation of
3, 4, 11,
reactive intermediates through CꢀC bond formation.
2
0ꢀ22
Alkaline lignin oxidation has been extensively studied in
the last decade, mainly on condensed lignin substrates such
The resulting degraded lignin streams are resistant to
depolymerisation and are, therefore, usually incinerated for
51, 59, 68ꢀ70
49ꢀ53, 71
10, 23
as lignosulfonates
and Kraft lignins.
For
energy recovery.
and sustainability of lignocellulosic biorefineries, a higher
value utilization of the lignin fraction is critical.
However, to improve the economics
instance, the influence of process parameters including
temperature, oxygen pressure, and NaOH concentration on
oxidation of Kraft lignin and lignosulfonates was studied
1
0, 24
49, 50, 52
54, 56, 70
In this context, strategies to convert lignin into chemicals
are receiving increasing attention. Research efforts are
directed both toward the development of biorefinery
processes in which lignin degradation is avoided,
and the development of effective and selective methods to
by Rodrigues,
Tarabanko,
and Evtuguin and
59
coworkers. New biorefinery processes, however, enable
the generation of more reactive lignins, and, therefore, we
decided to examine the alkaline aerobic oxidation of a
highly reactive, nonꢀdegraded, unmodified substrate,
namely native poplar lignin. Oxidation of the entire poplar
substrate is likely not relevant from a process perspective
(although the carbohydrates are also converted to valuable
compounds, vide infra), but is performed here to assess the
oxidation behaviour of unmodified lignin. We thoroughly
investigated the influence of the process parameters and
evaluated a series of catalysts on the reaction, in order to
gain more insight into the alkaline oxidation process and to
clarify some apparent contradictions between previous
reports, for instance on the role of the catalysts.
Furthermore, we assessed the conversion of other
biorefinery lignins and also compared the performance of
alkaline aerobic oxidation to RCF for lignin valorisation in
a biorefining context.
3, 4, 14, 25
1ꢀ4, 11ꢀ13
depolymerize the biorefinery lignin streams.
Degradation of the lignin structure (CꢀC bond formation)
can, for instance, be prevented through chemical
stabilization of the lignin structure during pulping, as in
formaldehydeꢀassisted pulping or reductive catalytic
fractionation (RCF).
2
6
2
7ꢀ32
In the latter case, pulping is
combined with lignin depolymerization and reductive
stabilisation of the lignin products. Another option to
prevent degradation is to utilize mild conditions, as in the
isolation of nativeꢀlike lignin substrates such as milled
wood lignin (MWL), cellulolytic enzyme lignin (CEL), or
34
33,
enzymatic mild acidolysis lignin (EMAL).
Pretreatment of biomass is often performed through
thermochemical methods such as diluteꢀacid pretreatment
(
DAP) or hydrothermal pretreatment, which are known to
1
3, 19, 21
induce significant lignin degradation.
Pretreatment
Results
methods that do not use acids or high temperatures, such as
Oxidation of poplar lignin: parameter investigation
To examine the oxidation of nativeꢀlike lignin, we
performed reactions with poplar sawdust, with a total
lignin content (Klason lignin and acidꢀsoluble lignin) of
29.4 wt%. Reactions were performed in 2 M NaOH
aqueous solutions under an oxygen partial pressure of 5
bar and a total pressure of 20 bar (He as makeꢀup gas;
pressurized at room temperature) for temperatures ranging
from 125 to 200°C and reaction times of 0 to 60 min (the
heating time of 30 min is not included in the reaction
time). After reaction, the reaction mixture was acidified
and extracted with ethyl acetate. The ethyl acetateꢀ
extracted phase (EA oil) was derivatized through silylation
and subjected to gas chromatography (GC) analysis to
quantify the phenolic monomers. The results are shown in
Figure 1. Oxidation of poplar lignin mainly yielded the
aromatic aldehydes vanillin and syringaldehyde, along
with other compounds such as vanillic and syringic acid,
35
deacetylation and mechanical refining (DMR)
or
3
6, 37
anhydrous ammonia pretreatment (AAP),
likely cause
less degradation, and are, thus, expected to generate more
reactive lignin streams.
Depolymerization of the resulting lignin streams has been
explored via reductive, oxidative, acidꢀcatalyzed, baseꢀ
catalyzed, and thermal methods, and many promising
1
ꢀ4, 11ꢀ13
results have been reported.
For instance, high yields
of phenolic monomers can be obtained through reductive
depolymerization, especially from native lignins (as in
2
7ꢀ32
26, 38
RCF)
and other reactive lignins,
but also from
39, 40
recalcitrant substrates such as Kraft and soda lignin.
An effective depolymerization method that was recently
introduced is acidꢀcatalyzed depolymerization combined
with in situ stabilization of the products through acetal
41, 42
formation.
Herein, ethylene glycol forms acetal
structures with the reactive carbonyl functionalities in the
sideꢀchains of the phenolic products. Through oxidation,
lignin can be selectively converted to valuable chemicals
such as aromatic aldehydes, aromatic acids, and aliphatic
and acetosyringone. These compounds were also obtained
53, 58
in the oxidation of isolated hardwood lignins.
In
addition, pꢀhydroxybenzoic acid was obtained. This
compound is not an oxidation product, but is part of the
43ꢀ48
carboxylic acids.
Lignin oxidation is mostly performed
structure of poplar lignin (it is esterified to the γꢀOH
in alkaline reaction media with oxygen as oxidant (i.e.,
8, 9
43, 49ꢀ61
groups of syringyl structures).
During the reaction, it
aerobically),
but has also been successfully
was released through hydrolysis of the ester bond. Under
these oxidative conditions, the carbohydrates were also
transformed. Only under mild oxidation conditions (e.g.,
low temperature and short reaction time) was some
performed in acidic solutions, such as diluted inorganic
6
2, 63
64
65
acids
or concentrated acetic acid, organic solvents,
6
6, 67
and ionic liquids.
As alkaline aerobic oxidation
enables the selective production of the aromatic aldehydes
2

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carbohydrateꢀrich residue preserved, but increasing
reactor with a much faster heating rate would likely further
enhance the product yield at temperatures above 175°C.
temperature and reaction time induced almost complete
conversion of the substrate. Figure 2 shows a gas
chromatogram of the EA oil after poplar conversion at
1
75°C for 10 min. The main carboxylic acids were formic
acid and acetic acid (not shown in the chromatogram),
followed by lactic acid (A); glycolic acid (B); and smaller
amounts of other compounds, such as oxalic acid (D), 3ꢀ
hydroxypropionic acid (E), fumaric acid (H), and malic
acid (I). Most of these acids originated from the cellulose
and hemicellulose in poplar, although some of these also
derived from consecutive oxidation of the phenolic
compounds (vide infra). Thus, the carbohydrate fractions
were also converted to valuable compounds during
alkaline aerobic oxidation.
Figure
2 Gas chromatogram of the silylated EA oil from poplar
2
oxidation at 175°C for 10 min (2 M NaOH, 5 bar O + 15 bar He at RT).
The external standard (ES) is 4ꢀnꢀpropylguaiacol.
To examine the oxidative degradation of the phenolic
monomers, we performed reactions with vanillin and
syringaldehyde at 150°C for 10 min (Table 1).
Syringaldehyde was much more reactive than vanillin,
with a conversion of 88%, compared to 31% for vanillin.
The higher reactivity of syringyl (S)ꢀtype compounds
compared to guaiacyl (G)ꢀtype compounds observed here
5
3, 59, 60, 73
Figure 1 Phenolic monomer yields (on a lignin basis) from alkaline
aerobic oxidation of poplar at various temperatures and reaction
times. Reaction conditions: 0.5 g poplar (150 mg lignin), 30 mL of 2 M
is in accordance to previous studies
and is
obvious from the timeꢀcourse plots in Figure 1, which
shows a faster degradation of syringaldehyde and syringic
acid compared to their guaiacyl analogues.
2
NaOH aqueous solution, 5 bar O + 15 bar He (at RT).
Both vanillin and syringaldehyde were mainly converted to
small carboxylic acids, with formic acid as the main
product from vanillin. Syringaldehyde yielded both formic
and malonic acid as main products. Malic acid was
obtained from syringaldehyde, while this was only
scarcely obtained from vanillin. Other products from both
aldehydes are acetic acid, glycolic acid, lactic acid, and
oxalic acid, although the latter could not be quantified.
Vanillin oxidation yielded a small amount of vanillic acid,
while reaction with syringaldehyde generated only trace
amounts of syringic acid. So although these acids are
usually proposed to be derived from vanillin and
Figure 1 shows that, especially in oxidation at higher
temperatures, the phenolic monomers were rapidly
produced, but also quickly consumed through oxidative
degradation, resulting in sharp maxima in monomer yield
as function of reaction time. The maximum product yield
increased with temperature, from 7 wt% at 125°C up to
about 30 wt% at 175°C, but did not increase further by
raising the temperature to 200°C. The 30 wt% monomer
yield obtained after 10ꢀmin reaction at 175°C comprises 24
wt% oxidation products (including 13 wt% syringaldehyde
and 7 wt% vanillin) and 6 wt% pꢀhydroxybenzoic acid. As
was also observed in the oxidation of condensed lignin
68
5
0, 52, 59, 72
syringaldehyde, the majority of vanillic and syringic acid
obtained during lignin oxidation is likely produced via a
substrates such as Kraft lignin,
these results show
that increasing the reaction temperature enhances the
maximum product yield, but also shortens the reaction
time needed to reach this maximum, because both the
formation and degradation of phenolic monomers are
accelerated. Thus, the reaction necessitates careful control
of the reaction time. For a high product yield, a very short
reaction at high temperature is ideal. However, this is
difficult to accomplish with a batch Parr reactor setup,
because it requires a long heating time (~ 30 min), during
which considerable lignin conversion takes place. Using a
74
different route, such as that proposed by Gierer et al.
3

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acid, which was expected from baseꢀcatalyzed hydrolysis
of ester linkages). Other studies have found that
conversion of lignosulfonates in the absence of oxygen
yields a considerable amount of depolymerization products
Table 1 Conversion and product yields from alkaline aerobic oxidation of vanillin
a
and syringaldehyde.
Substrate
Vanillin
10
Syringaldehyde
(
vanillin, syringaldehyde, vanillic acid, and syringic
time (min)
10
59, 69
acid),
but the different functional structure of
b
conversion (wt%)
yield (wt%)
31
88 (4)
lignosulfonates compared to native lignin likely facilitates
the different chemistry. In contrast, high monomer yields
were obtained from reaction under 5 bar oxygen, but
increasing the pressure up to 10 bar resulted in a drop in
yields. Increasing the oxygen pressure is known to
vanillic /syringic acid
formic acid
acetic acid
2
12
3
0
28
5
50, 72
accelerate both product formation and degradation,
and, therefore, shortens the time needed to reach the
maximum product yield. The fact that oxygen facilitates
product degradation is also illustrated in Table 1 by the
high conversion of syringaldehyde under oxygen (88
wt%), compared to only slight conversion under an inert
atmosphere (4 wt%, in brackets). Due to the long heating
time, the maximum yield under 10 bar oxygen was likely
obtained during the heating period. This again indicates
that while oxygen is needed to generate oxidation products
from native lignin, rapid heating of the reaction mixture is
also critical to avoid subsequent oxidation of the desired
products.
glycolic acid
lactic acid
1
3
3
5
malonic acid
malic acid
5
31
5
0
sum
23
76
a
Reaction conditions: 100 mg substrate, 30 mL of aqueous 2 M NaOH
solution, 150°C, 5 bar O + 15 bar He (at RT). Substrate conversion and
vanillic and syringic acid yield were determined via GCꢀFID. The yield of the
other acids was determined via HPLC. Oxalic acid was observed in both the
GC and HPLC analyses, but could not be quantified. Syringaldehyde
conversion under inert atmosphere (20 bar He) is indicated in brackets.
2
b
In Figure 3b, the GPC profiles of the EA oils for reaction
under inert atmosphere and 5 bar oxygen are depicted. The
M , M , and PD values are provided in Table S1. The
n
w
peak at 200 Da corresponds to the aromatic aldehydes and
acetosyringone, the peak at 300 Da to the aromatic acids
(
vanillic, syringic, and pꢀhydroxybenzoic) and the broad
signal at higher molecular weights (> 350 Da) to the
oligomeric products. Reaction under inert atmosphere
mainly yielded oligomeric products, while reaction under
oxygen primarily generated monomeric products, which
underpins the need for oxygen to depolymerize lignin. To
further assess the chemical transformations occurring
under oxygen and inert atmosphere, poplar and the
reaction products (EA oils) were analyzed through HSQC
NMR (Figure 4). Poplar wood showed large peaks of aryl
ether (βꢀOꢀ4), phenylcoumaran (βꢀ5), and resinol units (βꢀ
β), while there were almost no aldehyde peaks (Figure
4
a). In the aromatic region, the poplar spectrum mainly
displayed signals of guaiacyl (G), syringyl (S), and pꢀ
hydroxybenzoate (PB) units. The correlation peaks of pꢀ
hydroxyphenyl units (H2/6) at δC/δH 128.0/7.2 ppm were
extremely small, with the H/G ratio being only 0.0004.
This indicates that Hꢀunits (other than PB) are only present
in minor amounts in poplar. Based on the peak integral, an
S/G molar ratio of 1.18 was estimated, which was
calculated using both normal and oxidized S components.
Besides the Sꢀ, Gꢀ and Hꢀsignals, a clear signal of PB2/6 is
present at 131/7.71 ppm. The EA oil generated under inert
atmosphere contained less resinol and phenylcoumaran
units than poplar, and had no aryl ether and cinnamyl
alcohol units (Figure 4b), suggesting, as expected, that βꢀ
Oꢀ4 linkages were also cleaved under nonꢀoxidative
conditions, likely through baseꢀcatalyzed transformations.
Phenylcoumaran and resinol structures appeared to
partially withstand these conditions. In the aromatic
Figure
monomer yields (a) and molecular weight distribution of the EA oils
b) obtained from alkaline aerobic oxidation of poplar. The EA oils for
GPC analysis were obtained after 10 min reaction and were
acetylated prior to analysis. Reaction conditions: 0.5 g poplar, 30 mL
of 2 M NaOH aqueous solution, 175°C, 20 bar total pressure (at RT; He
as makeꢀup gas).
3 Impact of oxygen partial pressure (at RT) on phenolic
(
Subsequently, we assessed the influence of the oxygen
pressure on poplar oxidation. Figure 3a shows the product
yields for poplar oxidation in 2 M NaOH at 175°C, under
oxygen partial pressures of 0, 5, and 10 bar (20 bar total
pressure). Under inert atmosphere (0 bar oxygen), no
oxidation products were obtained (only pꢀhydroxybenzoic
4

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region, the PB signal decreased because the ester bonds in
aromatic region, the S2/6 and G2/5/6 peaks drastically
reduced and the S’2/6 and G’2 signals increased,
supporting that 30% of the poplar lignin was degraded into
PB units were readily saponified to generate pꢀ
hydroxybenzoic acid, which is evidenced by the phenolic
monomer yield distribution (Figure 3a). Also, oxidized
guaiacyl unit (G’2) signals at around 110ꢀ114/7.4ꢀ7.7 ppm
and G’6 at 124.5/7.57 ppm clearly appear, suggesting that
αꢀketone structures were generated from βꢀOꢀ4 units under
alkaline conditions. The S/G ratio in the EA oil generated
under inert atmosphere was 1.73, higher than that in
poplar, likely due to baseꢀcatalyzed repolymerization of Gꢀ
αꢀketone monomers by 5 bar O treatment. (Figure 5). The
2
decrease of aromatic peaks also suggests that some
74
aromatic ring cleavage occurred. The interunit linkages
were thus effectively disrupted under the oxidative
conditions. The S/G ratio in the EA oil generated under
oxygen decreased to 1.14, likely due to a faster oxidative
degradation of Sꢀtype units compared to Gꢀtype units. In
addition, the aldehyde peak was much more prominent in
the EA oil generated under oxygen than in the oil obtained
without oxygen, due to generation of vanillin and
syringaldehyde.
3
units. The spectrum of the EA oil obtained under oxygen
exhibited far fewer signals than the spectra of the poplar
and the EA oil obtained in absence of oxygen (Figure 4c).
Specifically, there were no βꢀOꢀ4, phenylcoumaran,
resinol, and much fewer carbohydrate peaks. In the
Figure 4. HSQC NMR spectra of (a) Poplar wood, (b) EA oils for poplar conversion under inert atmosphere and (c) 5 bar oxygen. Reaction conditions for
b) and (c): 0.5 g poplar, 30 mL of 2 M NaOH aqueous solution, 175°C, 10 min, 20 bar He at RT (b) and 5 bar O + 15 bar He at RT (c). G: Guaiacyl, G’:
(
2
Guaiacyl with Cα=O, S: Syringyl, S’: Syringyl with Cα=O, PB: pꢀHydroxybenzoate, AL: Benzaldehydes, CA: Cinnamyl alcohol, A: βꢀAryl ether (βꢀOꢀ4), B:
Phenylcoumaran (βꢀ5), C: Resinol (βꢀβ)
Figure 5 Impact of NaOH concentration on the phenolic monomer
yields from alkaline aerobic oxidation of poplar. Reaction conditions:
0.5 g poplar, 30 mL of 1 or 2 M NaOH aqueous solution, 175°C, 5 bar
O
2
+ 15 bar He (at RT).
Finally, we assessed the impact of the NaOH concentration
on poplar oxidation. Figure 5 shows the monomer yields
for oxidation at 175°C under 5 bar oxygen partial pressure
in 1 and 2 M NaOH solutions. In line with previous studies
43, 50, 54, 71
on Kraft lignin and lignosulfonates,
the highest
monomer yields were obtained in 2 M NaOH. A high
NaOH concentration is suspected to be necessary to
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facilitate lignin oxidation, viz. for ionization of the
In Figure 6b, the catalytic results for a range of oxideꢀ
based catalysts are depicted. range of oxides,
perovskites, and noble metal catalysts were tested. Co O
4
3, 44
phenolic hydroxyl groups
and likely also for the
A
4
3, 54ꢀ
conversion of reactive intermediates (see Discussion),
3
4
5
7
2
43,
81, 82
and to retard degradation of the phenolic compounds.
For instance, Rodrigues et al. showed that raising the pH
is a commonly used oxidation catalyst,
and CuO has
7
been used frequently in lignin oxidation, either in the
8
3, 84
of the reaction solution over 12 significantly slows down
absence (itself being the oxidant)
oxygen.
or presence of
7
2
60, 75
the oxidative degradation of vanillin. On the other hand,
utilizing an NaOH concentration that is too high (e.g., 4
M) causes operational problems such as salt deposition and
fouling in the reactor, and, therefore, a 2 M concentration
The use of perovskites is based on the work of
Lin and coꢀworkers, who found that Coꢀ, Mnꢀ, and Feꢀ
based perovskites significantly enhanced the product
yields in alkaline lignin oxidation, and that their
43
77ꢀ80
is usually advised.
performance can be further improved by Cu doping.
Sales et al. showed that Pd/Al O ꢀcatalyzed lignin
2
3
Catalytic oxidation of poplar: catalyst screening and
examination of the catalyst effect
Alkaline oxidation of lignin is often performed in the
presence of a catalyst. The catalyst is suggested to
facilitate lignin oxidation through electron abstraction
oxidation enabled much higher product yields than
76
uncatalyzed oxidation, while Villar et al. observed that
the use of Pt/Al O had an adverse effect on the product
2
3
71
yield. In poplar oxidation, the highest yield was achieved
with CuO (17 wt%), similar to the yield obtained with
CuSO . Following oxidation with Co O , Pd/Al O , and
(
oxidation) of the phenolate groups to phenoxy radicals,
4
3
4
2
3
after which the catalyst is reoxidized by oxygen (thus
4
3, 59, 75
Pt/Al
2
O
3
, no increase in product yield was observed
completing the redox cycles).
The catalyst is usually
compared to the uncatalyzed reaction. To test the catalytic
activity of perovskites, we prepared Feꢀ, Coꢀ, and Mnꢀ
based perovskites through solꢀgel synthesis and confirmed
their perovskite crystal structure confirmed via XRD
(Figure S1). The highest yield was obtained when poplar
CuSO or another metal salt, but also solid catalysts such
4
as metal oxides and noble metal catalysts have been
studied. Numerous studies have shown that the catalyst
accelerates product formation and also raises the maximum
58ꢀ60, 76ꢀ80
product yield,
observe an increase in maximum product yield.
although a few studies did not
50, 71
was oxidized with LaMnO , while the use of LaFeO or
3
3
To
LaCoO resulted in no or a very minor yield increase,
3
verify the catalyst effect in alkaline lignin oxidation, we
first compared the catalytic activity of a range of reported
catalysts in the alkaline oxidation of poplar. The oxidation
was performed in 2 M NaOH at 150°C under 5 bar oxygen
for 0 min. Figure 6a shows the catalytic results for
reactions with metal salts. The uncatalyzed reaction
reached a total monomer yield of 14 wt%, including 8 wt%
oxidation products (without pꢀhydroxybenzoic acid). The
latter yield is used as benchmark and is indicated with a
compared to the uncatalyzed reaction. In line with the
results of Lin et al., the catalyst performance could be
enhanced by doping the perovskites with Cu (see results
for LaCo Cu O and LaMn Cu O ). The highest yield
0.8
0.2
3
0.8
0.2
3
of oxidation products was achieved with LaMn Cu O
0.8
0.2
3
(
13 wt%).
dashed line. CuSO reached the highest yield of oxidation
4
products (17 wt%), followed by CoCl and MnCl (11ꢀ12
2
2
wt%). Reactions with FeCl3 and Cr (SO ) achieved
2
4 3
approximately the same yields as the uncatalyzed run. The
superior catalytic performance of CuSO is likely due to
4
2
+
the intermediate reduction potential of Cu (ꢀ0.16 V for
the CuO/Cu O redox pair at pH 14), as was postulated by
2
Tarabanko et al. to be sufficient for oxidation of lignin to
aldehydes 5 with limited subsequent oxidation of
7
aldehydes. The promising performance of Cu relative to
other metals explains why it is a commonly used catalyst
5
0, 58, 59, 68, 70
in alkaline lignin oxidation.
Regarding the Co,
Mn, and Fe salts, Lin et al. also found that Co and Mn salts
7
8, 79
improve lignin oxidation,
while FeCl had no effect on
3
80
3+
the reaction. This can be explained by the fact that Fe
cannot abstract an electron from the phenolate groups and,
thus, cannot follow the oxidationꢀreduction cycles.
75
However, some studies reported that FeCl3 facilitates
lignin oxidation, which was attributed to the formation of
O ꢀFeꢀlignin complexes that act as oxygen carriers that can
2
58, 60, 75
attack the lignin.
These studies also showed that
CuSO4 and FeCl3 exert a synergistic effect on the
oxidation. However, in this work, no further increase in
product yield was observed with the addition of FeCl to
3
CuSO (Figure 6a).
4
6

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LaCoO , its reduction started at lower temperature (the
3
maximum of the first TPR signal of LaMnO and LaCoO3
3
was at about 320°C and 380°C, respectively), which might
also have contributed to its higher activity in lignin
oxidation. Incorporation of Cu in LaMnO and LaCoO did
3
3
not affect the specific surface area of the catalysts (Figure
a), but clearly lowered the temperature at which reduction
takes place (Figure 7b). The first TPR signal (at about
7
2+
4+
3+
88
2
00ꢀ350°C) is due to both Cu and Mn /Co reduction.
The higher activity of the Cuꢀdoped catalysts is, thus,
attributed to their easier reducibility, which can also
89
explain the high activity of CuO.
Figure
6 Phenolic monomer yields from oxidation of poplar with
various metal salts (a) and oxideꢀbased catalysts (b). Reaction
conditions: 0.5 g poplar, 30 mL of 2 M NaOH aqueous solution, 40
Figure 7 Brunauer–Emmett–Teller specific surface areas (SBET) obtained
from adsorption isotherms (a) and ꢀTPR profiles (b) of the
perovskite catalysts.
N
2
H
2
ꢁ
mol metal salt or 10 mg solid catalyst, 150°C, 0 min (the reactor was
quenched when reaching the reaction temperature, 30 min heating
time), 5 bar O + 15 bar He (at RT). The dashed line indicates the yield
2
Due to their high catalytic activity, CuSO4 and
LaMn Cu O were further used to examine the influence
of oxidation products obtained in the uncatalyzed reaction (8 wt%).
0.8
0.2
3
of the catalyst in alkaline aerobic oxidation. First, the
To elucidate the catalytic behaviour of the perovskite
catalysts, some physicochemical properties were
determined. Figure 7a shows the specific surface area,
determined by N2 physisorption, and Figure 7b the
temperature programmed reduction (TPR) profiles, which
provide valuable information about the reducibility of
conversion of poplar was studied with CuSO at 150°C and
4
1
75°C at times ranging from 0 to 60 min. The monomer
yields for catalyzed and uncatalyzed oxidation are depicted
in Figure 8. At both temperatures, CuSO accelerated
4
product formation and retarded product degradation,
compared to uncatalyzed oxidation, but did not increase
the maximum monomer yield. As shown in Figure 8c-d,
LaMn Cu O exerted the same effect on the product
8
5, 86
oxidation catalysts.
A high specific surface area is
advantageous because more active surface sites are
available, while a high catalyst reducibility facilitates the
oxidation (electron abstraction) of the lignin. Regarding
0.8
0.2
3
yields. So, although addition of a catalyst does not enhance
the product yield, it accelerates the reaction and widens the
time window in which a high product yield is obtained,
which makes the process less time sensitive.
LaMn Cu O was further employed in oxidation of other
the Cuꢀfree perovskites, LaMnO exhibited a considerably
3
higher specific surface area than LaFeO and LaCoO (22
3
3
2
vs. 6ꢀ9 m /g), which can explain its higher catalytic
0.8
0.2
3
activity. The TPR profiles indicated a negligible hydrogen
substrates, as discussed in the next section.
uptake for LaFeO , in line with the very low reducibility of
3
3
+ 80,
87
Fe .
LaMnO3 showed
a
higher hydrogen
LaMn Cu O -catalyzed oxidation of other substrates
0.8
0.2
3
4+
consumption, which can be attributed to reduction of Mn
to Mn at lower temperature and Mn to Mn at higher
temperature. The highest hydrogen consumption
was observed for LaCoO , due to reduction of Co to Co
In addition to poplar, we subjected other lignocellulosic
feedstocks, viz. highꢀS poplar, pine, and corn stover to
alkaline aerobic oxidation with LaMn Cu O . Figure 9
3
+
3+
2+
79, 88, 89
0.8
0.2
3
3
+
2+
3
shows the monomer yields for oxidation at 175°C for 0
min under 5 bar oxygen partial pressure. From wildꢀtype
(WT) poplar, a 31 wt% monomer yield was obtained,
2
+
0
at lower temperature (300ꢀ450°C) and Co to Co at
8
8, 90
higher temperature (500ꢀ650°C).
Although LaMnO3
showed a much lower hydrogen consumption than
7

Green Chemistry
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comprising 25 wt% oxidation products. Oxidation of
transgenic poplar with a high Sꢀlignin content (highꢀS
9
1, 92
poplar; line F5Hꢀ64ꢀE)
towards Sꢀtype
resulted in a higher selectivity
compounds (syringaldehyde,
acetosyringone, and syringic acid) than WT poplar, but the
yield of oxidation products was only a little higher (29 vs.
2
5 wt%) and the total monomer yield was the same (31
wt%). This is quite unexpected because a higher S content
9
3, 94
in lignin results in a higher βꢀether content,
and,
therefore, should enable higher monomer yield.
a
However, the maximum attainable monomer yield was
likely limited due to the fast oxidative degradation of Sꢀ
type compounds. HighꢀS poplar oxidation also generated
pꢀhydroxybenzoic acid, but with a considerably lower
yield than WT poplar (2 vs. 6 wt%).
The oxidation of pine yielded only Gꢀtype compounds
(
vanillin, acetovanillone, and vanillic acid), in line with the
lignin composition of softwoods, with high selectivity
towards vanillin. While the maximum product yield was
obtained at a reaction time of 0 min for all other substrates
(
Table S2 for the monomer yields after 0 and 10 min), the
maximum yield from pine (22 wt%) was reached after 10
min, which indicates that oxidation of Gꢀrich lignin occurs
more slowly than oxidation of Sꢀrich lignin. In oxidation
of corn stover, pꢀcoumaric acid, ferulic acid, and pꢀ
hydroxybenzaldehyde were obtained, along with the
previously described
products. Similar
to
pꢀ
hydroxybenzoic acid in poplar, the hydroxycinnamic acids
pꢀcoumaric and ferulic acid were not produced through
lignin oxidation, but through alkaline hydrolysis of ester
bonds. These compounds were obtained in 9 and 2 wt%
yield, respectively, with the total monomer yield being 27
wt%.
Figure 8 Phenolic monomer yields from uncatalyzed and catalyzed
oxidation of poplar wood at 150°C (a and b) and 175°C (c and d).
Panels a and c depict the total monomer yield, while the yields of the
individual compounds are indicated in panels b and d. Reaction
conditions: 0.5 g poplar, 30 mL of 2 M NaOH aqueous solution, 40
4 2
ꢁmol CuSO or 10 mg LaMn0.8Cu0.2O3, 5 bar O + 15 bar He (at RT).
8

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In addition to the native lignins, we examined the
oxidation of a range of isolated lignins, comprising one
softwood lignin (spruce Kraft lignin) and three herbaceous
lignins (corn stover DMRꢀEH, DAPꢀEH, and AAPꢀAE
lignin) (Figure 9). The results not only indicated the
potential of oxidation to yield phenolic products from
these substrates, but also made it possible to evaluate the
impact of the isolation method on the structural integrity of
the lignin, by comparing the results of the native and
isolated lignins for the same (or similar) feedstocks.
Oxidation of spruce Kraft lignin, an isolated softwood
lignin, yielded 12 wt% monomers, with a very similar
monomer distribution as from pine oxidation. The
monomer yield, however, was much lower, which is due to
the degraded state of the Kraft lignin structure. Next, three
isolated corn stover lignins were subjected to oxidation,
which enables a direct evaluation of the isolation method
on the reactivity of lignin toward depolymerization.
Oxidation of DMR enzymatic hydrolysis residue (DMRꢀ
EH lignin) from corn stover rendered a similar monomer
yield (27 wt%) and monomer distribution as corn stover
oxidation, thus, indicating that the original lignin reactivity
is well preserved during the mechanical pretreatment and
enzymatic hydrolysis steps. On the other hand, oxidation
of dilute acidꢀpretreated enzymatic hydrolysis residue
Figure
3
9 Monomer yields (on lignin basis) from LaMn0.8Cu0.2O ꢀ
(
DAPꢀEH lignin) from corn stover generated a much lower
catalyzed oxidation of various lignocellulosic feedstocks and isolated
lignins. Reaction conditions: 0.5 g lignocellulose (95ꢀ150 mg lignin),
monomer yield (13 wt%), indicating that the lignin
structure is significantly degraded during DAP. The
individual yield of most monomers was lower than in
oxidation of corn stover or DMRꢀEH lignin, but the main
difference is observed for the hydroxycinnamic acids. The
pꢀcoumaric acid yield was only 3 wt% and almost no
ferulic acid was obtained, compared to a pꢀcoumaric and
ferulic acid yield of 10 and 1 wt%, respectively, for corn
stover DMRꢀEH lignin. These acids were likely removed
during the DAP step, as aqueous acid can also hydrolyse
ester bonds. Lastly, we tested alkalineꢀextracted lignin
from anhydrous ammoniaꢀpretreated corn stover (AAPꢀAE
lignin) in the reaction. The ammonia pretreated corn stover
was treated with a 0.1 M NaOH aqueous solution, which
extracted 70 wt% of the lignin. A portion of this alkaline
extraction liquor was subsequently combined with a
concentrated NaOH solution to obtain a 2 M NaOH
aqueous solution, and subjected to oxidation. The
monomer distribution was quite similar to that from corn
stover and DMRꢀEH lignin oxidation, while the monomer
yield was even higher (31 wt% on extracted lignin basis).
However, in oxidation of corn stover and DMRꢀEH lignin,
the yields were based on the entire corn stover lignin
fraction (given that no lignin is lost during DMRꢀEH),
while in oxidation of AAPꢀAE lignin, only the extracted
lignin was considered (i.e., 70 wt% of the total lignin).
Taking into account the extracted lignin yield, 22 wt% of
the original corn stover lignin was converted to phenolic
monomers through AAPꢀAE and oxidation. Although this
yield is somewhat lower than the yields from raw corn
stover and corn stover DMRꢀEH lignin (27 wt%), it can be
concluded that AAPꢀAE is also an effective method to
isolate a highly reactive lignin substrate from corn stover.
1
50ꢀ300 mg isolated lignin (140ꢀ160 mg lignin) or 15 g corn stover AAPꢀ
AE liquor (150 mg lignin), 30 mL of 2 M NaOH aqueous solution (for
reaction with the corn stover AAPꢀAE lignin, 15 mL of the liquor (0.1 M
NaOH) was combined with 15 mL of a 3.9 M NaOH aqueous solution
to obtain 30 mL of a 2 M NaOH solution), 10 mg LaMn0.8Cu0.2
0
O3, 175°C,
min (10 min for pine), 5 bar O + 15 bar He (at RT).
2
The molecular weight distribution of the oxidation
products from the lignocellulosic feedstocks is depicted in
Figure 10 (see Table S1 for M , M and PD values). For
the woody substrates, the peaks at about 200 and 300 Da
correspond to the monomeric compounds. The signal of
the oligomeric compounds (> 350 Da) decreased in the
following order: pine > WT poplar > highꢀS poplar. The
GPC profile of the corn stover oxidation product showed a
large peak at 350ꢀ400 Da, corresponding to the
hydroxycinnamic acids.
n
w
Figure 10 GPC profiles of the lignocellulose oxidation products. The
reaction conditions are indicated in the caption of Figure 9.
9

Green Chemistry
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carboxylic acid stream that is obtained in addition to the
lignin products could, for instance, be separated into
individual acids analogously to recovery of acids from
95
fermentation broth,
or they could be upgraded
96
biologically to other valueꢀadded products. In Table 2,
the yields of phenolic compounds and aliphatic carboxylic
acids (on total substrate basis) are indicated. Spruce Kraft
lignin has a very high lignin content (93 wt%) and, thus,
almost all carboxylic acids derived from the lignin
fraction. On the other hand, the isolated corn stover lignins
have a much lower lignin content (30ꢀ63 wt%) and contain
a significant fraction of carbohydrates (15ꢀ33 wt%). Thus,
for these substrates, the carboxylic acids originated from
both the lignin and carbohydrates. Besides the phenolic
monomer compounds, spruce Kraft lignin oxidation
mainly yielded formic acid and smaller amounts of acetic,
glycolic, lactic, malonic, and oxalic acid (the lattermost
was not quantified). Only traces of succinic acid and no
malic acid were obtained. The carboxylic acid distribution
was, thus, rather similar to that from vanillin oxidation
(
Table 1). A total product yield of 39 wt% was obtained.
In oxidation of the corn stover lignins, formic acid was
also the main product, followed by acetic, glycolic, and
lactic acid. Malonic and malic acid were minor products
Figure 11 GPC profiles of the isolated lignins (a) and oxidation
products of the isolated lignins (b). The reaction conditions used to
obtain the oxidation products are indicated in the caption of Figure 9.
(
2ꢀ3 wt% yield), while succinic acid was only present in
trace amounts. Also some oxalic acid was obtained from
these lignins, although not quantified. Remarkably, AAPꢀ
AE lignin oxidation generated similar amounts of formic
and acetic acid, while the acetic acid yield was much lower
than the formic acid yield for the other corn stover lignins.
The total quantified product yields from the corn stover
lignins amounted to 46ꢀ49 wt%. Thus, a targeted
valorization strategy can be rationally designed for up to
half of the substrate mass.
In Figure 11, the molecular weight distribution of the
isolated lignins (a) and their oxidation products (b) is
displayed (see Table S1 for M , M and PD values). The
GPC profiles of the isolated lignins and the oxidation
products are plotted together in Figure S2 for each lignin
substrate. Spruce Kraft lignin, corn stover DMRꢀEH, and
DAPꢀEH lignin exhibited a high molecular weight (Figure
n
w
1
1a) and were clearly converted to lowꢀmolecularꢀweight
products (200ꢀ4,000 Da) by oxidation (Figure 11b).
Similar to Figure 10, the main peaks were present at 200
Da, corresponding to the aromatic aldehydes and
acetovanillone and ꢀsyringone (especially for spruce Kraft
lignin), and 350ꢀ400 Da, corresponding to the
hydroxycinnamic acids (for the corn stover lignins). In
contrast to the other isolated lignins, corn stover AAPꢀAE
lignin exhibited a low molecular weight (Figure 11a). The
GPC profile showed a large signal at 350ꢀ400 Da,
indicating that the lignin in the AAPꢀAE liquor was mainly
composed of ‘free’ (hydrolyzed) hydroxycinnamic acids.
Oxidation of the AAPꢀAE lignin enabled the conversion of
other lignin structures in the liquor to small monomeric
compounds, as indicated by the signal strength at 200 Da
Comparison of lignocellulose alkaline aerobic oxidation
with nitrobenzene oxidation and RCF
Through alkaline aerobic oxidation, the native lignin in
lignocellulosic feedstocks can be effectively converted to
phenolic monomers. To evaluate the effectiveness of this
depolymerization method, we compared alkaline aerobic
oxidation to nitrobenzene oxidation and reductive
depolymerization. Nitrobenzene oxidation is commonly
used for lignin characterization, as it enables an effective
51, 53, 83
depolymerization of lignin into phenolic monomers.
However, as the oxidant (nitrobenzene) and its products
are harmful, nitrobenzene oxidation is only relevant from a
characterization perspective. Reductive depolymerization
of native lignin, which is usually termed RCF when
applied to whole biomass, enables very high (closeꢀtoꢀ
theoretical) monomer yields from native lignin (based on
(
Figure 11b and Figure S2).
Besides phenolic compounds, alkaline aerobic oxidation of
the lignocellulosic feedstocks and isolated lignins also
generated a large amount of aliphatic carboxylic acids,
which derived from both the lignin and the carbohydrate
fractions. While this study only focuses on the lignin
fraction in the reactions with the lignocellulosic feedstocks
3, 11, 13, 14
ester and βꢀOꢀ4 ether cleavage).
Both of these
methods constitute suitable benchmarks for alkaline
aerobic oxidation. It should be noted that while RCF is an
appropriate method to valorize the entire lignocellulose
substrate (i.e., both the lignin and carbohydrates; the latter
are largely retained in a pulp that can be used for further
processing), alkaline aerobic oxidation is likely not, as the
carbohydrates are converted into a complex mixture of
aliphatic carboxylic acids. Therefore, alkaline aerobic
(
and, therefore, only looks at the phenolic compounds), the
fate of the entire substrate is of interest in the reactions
with the isolated lignins, because these represent highly
relevant feedstocks for chemical production. The mixed
10

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Green Chemistry
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oxidation and RCF are solely compared at the lignin level
here.
Table 2 Product yield from alkaline aerobic oxidation of isolated lignin on total
a
substrate basis.
Figure 12 depicts the monomer yields obtained through
Spruce Corn stover Corn stover Corn stover
alkaline aerobic oxidation (Ox; same results as in Figure
Substrate
Kraft
lignin
DMR-EH
lignin
DAP-EH
lignin
AAP-AE
NB
b
9
), nitrobenzene oxidation (Ox ) and RCF (Red). Only
lignin
poplar was subjected to nitrobenzene oxidation. The same
products and a similar product distribution were obtained
as in alkaline aerobic oxidation of poplar, but a rather
higher total monomer yield was achieved (35 vs. 31 wt%).
RCF was applied to all lignocellulose substrates in Figure
m substrate
(mg)
1
50
300
250
520
lignin content
9
3
53
63
30
(wt%)
m lignin (mg)
yield (wt%)
140
160
158
154
9
, and was performed with Pd/C in methanol at 225°C
under hydrogen pressure. The main products were
propanolguaiacol and ꢀsyringol, with minor products being
phenolic
12
14
8
9
compounds
guaiacol and syringol with
a
propyl, propenyl,
formic acid
acetic acid
glycolic acid
lactic acid
malonic acid
succinic acid
malic acid
sum
15
4
13
4
16
5
11
11
6
methoxypropyl, or ethyl sideꢀchain (these compounds are
combined under ‘other guaiacols’ and ‘other syringols’).
The pꢀhydroxybenzoic acid in poplar and highꢀS poplar
was converted to methyl pꢀhydroxybenzoate (through
3
7
6
97
esterification) and phenol (through decarboxylation) and
the pꢀcoumaric and ferulic acid in corn stover were
converted to methyl dihydrocoumarate and dihydroferulate
3
7
5
7
2
2
3
2
(
through sideꢀchain hydrogenation and esterification,
< 1
0
< 1
2
< 1
2
< 1
2
28
respectively).
Because the monomeric compounds
produced through oxidation and reduction are different
and, thus, have different molecular weights, the product
yields cannot be compared on weight basis. Therefore, the
yields presented in Figure 12 are indicated on a molar
basis (mmol/g). The total monomer yields on weight basis
are also shown.
39
49
46
49
a
Reaction conditions: 30 mL of 2 M NaOH aqueous solution, 10 mg
, 175°C, 0 min, 5 bar O + 15 bar He (at RT). The yield of
LaMn0.8Cu0.2
O
3
2
phenolic compounds and other acids was determined via GCꢀFID and HPLC,
respectively. Oxalic acid was observed in both the GC and HPLC analyses,
but could not be quantified. 15 mL of the AAPꢀAE liquor (0.1 M NaOH,
b
containing 520 mg solids and 154 mg lignin) was combined with 15 mL of a
3
.9 M NaOH aqueous solution to obtain 30 mL of a 2 M NaOH solution.
For poplar, pine, and corn stover, alkaline aerobic
oxidation and reduction achieved comparable monomer
yields. Also, starting from poplar and corn stover, a similar
distribution of Sꢀ, Gꢀ, and pꢀhydroxyphenyl (H)ꢀtype
compounds (indicated in red, blue, and green, respectively,
in Figure 12) was obtained through both methods. Pine
conversion yielded only Gꢀtype products. For corn stover,
reduction generated a significantly higher yield of pꢀ
coumaric acid (as methyl dihydrocoumarate) and ferulic
acid (as methyl dihydroferulate) than oxidation, while the
total yield of Hꢀ and Gꢀtype products was fairly similar.
This can be explained by the fact that these acids were
further transformed to other Hꢀ and Gꢀtype products
61
during oxidation, while they were stabilized during
reduction (as methyl dihydrocoumarate and
dihydroferulate). Oxidation achieved a considerably lower
monomer yield than reduction only for highꢀS poplar. The
main difference was in the Sꢀtype products. This yield
difference is likely due to the low stability of Sꢀtype
compounds during oxidation, while they are stable under
reductive conditions. These results show that an effective
oxidative depolymerization is especially difficult for Sꢀrich
lignins.
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the esterꢀlinked pꢀhydroxybenzoic acid, are produced
while heating without oxygen, as indicated in Figure 3,
but significant monomer production is evident by the time
the reactors reach reaction temperature when they are
heated under 5 bar O . These results are consistent with
2
Rodrigues et al., who showed that pine Kraft lignins
produce minimal vanillin under nonꢀoxidizing alkaline
hydrolysis conditions, but that vanillin is both rapidly
50
produced and consumed when O is present. On the other
2
hand, Pacek et al. found significant vanillin production
69
without O from Naꢀlignosulfonates. In future work, it
2
would be useful to assess the influence of introducing the
oxidizing atmosphere before and after heating the reactor,
although it is expected that heating in absence of oxygen
will induce lignin degradation through baseꢀcatalyzed
transformations, as illustrated by the loss of βꢀether
structures in the reaction under inert atmosphere (see
Figure 4) and, therefore, lower the product yields. While
not typically reported, the stirring rate, which affects the
gasꢀliquid mass transfer of O into solution, may also
2
influence reported yields, though Pacek et al. found
vanillin yields in lignosulfonate oxidation were relatively
Figure 12 Monomer yields (on lignin basis) from alkaline aerobic
oxidation (Ox), nitrobenzene oxidation (OxNB) and reductive catalytic
fractionation (Red) of various lignocellulosic substrates. Syringyl (S)ꢀ,
guaiacyl (G)ꢀ and p-hydroxyphenyl (H)ꢀtype products are indicated in
red, blue and green, respectively. Reaction conditions for alkaline
aerobic oxidation: 0.5 g lignocellulose (95ꢀ150 mg lignin), 30 mL of 2 M
insensitive to agitation, at least for stirring speeds above
69
5
00 rpm at pH > 12.
Second, the observation in the literature that a catalyst
sometimes increases lignin monomer yields and other
times does not may be due to both heating rate
3
NaOH aqueous solution, 10 mg LaMn0.8Cu0.2O , 175°C, 0 min (10 min
for pine), 5 bar O + 15 bar He (at RT). Reaction conditions for
2
(
with/without O ) and lignin chemistry. Lignin in the
2
nitrobenzene oxidation: 0.5 g poplar, 27 mL of 2 M NaOH aqueous
solution, 3 mL nitrobenzene, 170°C, 120 min. Reaction conditions for
reduction: 1 g lignocellulose, 30 mL methanol, 0.1 g Pd/C, 225°C, 6 h,
native biomass substrates explored here has a high
reactivity even without catalyst, and while some catalysts
appear to have a substantial effect at lower temperatures
and short reaction times (150°C, 0 min in Figure 6 and
Figure 8a), the apparent benefit is not as prominent at
higher temperatures or intermediate reaction times. At
longer reaction times, monomer yields are again higher
with catalyst than without, though overall yields are again
lower. There are at least three mechanisms by which the
catalyst could lead to these trends, which would be
sensitive to the chemistry of the substrate. First, the
catalyst may be breaking lignin linkages that are not
broken (or not broken as quickly) without a catalyst,
leading to higher aldehyde yields initially. Second, as the
aldehydeꢀproducing linkages in the lignin are consumed,
the catalyst may preserve the aldehyde products, inhibiting
their degradation. Third, the catalyst may preferentially
shuttle active oxygen species to other reactions (e.g.,
2
30 bar H (at RT).
Discussion and Conclusions
Consistent with previous work, we have shown that
alkaline oxidation is an effective lignin depolymerization
strategy for both native and technical lignins, provided that
the latter are not overly degraded in upstream operations.
Up to about 30 wt% aromatic monomers can be achieved,
depending on the substrate and conditions, with short
reaction times, high temperatures, and high NaOH
concentrations being favourable. Addition of Cuꢀbased
catalysts does not appear to improve phenolic monomer
yields, but makes the process less sensitive to reaction
time.
These results provide some insight into the puzzling and
sometimes contradictory results of previous work. First,
because short reaction times are required and reactors are
converting carboxylic acids to CO ), reducing the O2
2
pressure in the reactor and leading to slower aldehyde
degradation. The first mechanism is difficult to prove
because the HSQC NMR spectra are somewhat
ambiguous. The second mechanism is unlikely, as
preliminary results suggest that Cu catalysts increase the
rate of degradation of vanillin and syringaldehyde. In the
third mechanism, the reactor pressure should decrease
typically pressurized with O or air at room temperature,
2
the heating rate of the reactors is a critical parameter that
could significantly affect results across experimental
setups. For example, Wu et al. employed a system that
could be heated from room temperature to 170°C in 3ꢀ5
60, 75
min.
In contrast, other configurations, including the
one used here, were limited by a 30ꢀmin heating time,
which we show allows the reaction to proceed to a
significant extent. The reaction during heating is likely due
to both hydrolysis of ether and ester bonds in the alkaline
solution and oxidation reactions. Few monomers, such as
more during the reaction with catalyst as more O is
2
consumed. However, in our reactions, the reactor pressures
with and without catalyst were similar, rendering the third
mechanism unlikely, as well. Therefore, the catalytic
12

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DOI: 10.1039/C8GC00502H
trends cannot yet be explained, and further research efforts
are needed to understand the role of the catalyst.
have to rely on differences in molecular weight and/or
boiling point between monomers, dimers, and oligomers.
Alkaline oxidation, on the other hand, yields a highly
alkaline reaction solution containing a mixture of
Third, further study is needed regarding the congruence of
monomeric product yields in oxidative and reductive lignin
depolymerization approaches. RCF has been proposed to
cleave mainly interꢀunit ether and ester linkages,
generating stabilized products. On the other hand, aromatic
monomers are rapidly degraded under alkaline oxidation
conditions, with loss of aromaticity as a primary
contributor. This, combined with the much shorter reaction
times in alkaline oxidation, indicates that the reported
maximum aromatic monomer yields are more snapshots of
a much more dynamic reaction system than an indication
that both depolymerization strategies attack the same
functional linkages (although there is clear NMR evidence
that ester and βꢀOꢀ4 linkages do not survive either
phenolics
(aromatic
aldehydes,
aromatic
acids,
acetophenoneꢀlike compounds, and oligomer fragments)
and aliphatic carboxylic acids. Isolation of the phenolics
from the reaction is typically performed through
43, 44, 99
acidification and extraction with organic solvents.
This procedure, however, requires large amounts of acid
and organic solvent, and generates a significant amount of
highly saline wastewater. To avoid extraction of the
acidified solution with organic solvents, alternatives such
as extraction with supercritical CO or adsorption of the
2
phenolics on zeolites or macroporous resins have been
43, 44, 99
examined.
To circumvent the acidification of the
28ꢀ30
entire reaction mixture, strategies to directly isolate the
phenolates from the mixture, such as extraction with
specific solvents (e.g., butanol), adsorption on cationꢀ
process).
Fourth, and similarly, further study is needed to
conclusively elucidate the mechanism of alkaline oxidation
with and without catalysts. Several mechanisms have been
proposed in literature, comprising both radical and ionic
+
+
exchange resins (in the Na or H form), or ultrafiltration
to remove highꢀmolecularꢀweight lignin fragments, have
43, 44, 99
been investigated.
Despite advances being made,
48, 54ꢀ57, 74, 75
pathways.
The requirement of a highly alkaline
further research is necessary to enable a more sustainable
isolation of the phenolic products from the reaction
solution, and, in addition, separation of individual products
from the phenolics mixture. For instance, separating
vanillin in high purity requires an elaborate purification
procedure, involving crystallization, extraction, and
distillation steps. However, an advantage of alkaline
oxidation compared to reductive procedures is that the
market for certain oxidation products, such as vanillin, is
well established, while the market for reductively
produced monomers is smaller and/or lessꢀdefined.
solution can in part be rationalized on the basis of
deprotonating phenolic functional groups and maintaining
alkalinity in the presence of produced acids. However,
even in presence of the produced acids, which constitute
much less than 0.5 M of neutralizing equivalents,
deprotonation of the phenolic groups should still be
possible at much lower NaOH concentrations than 2 M, as
43
the pKa of phenolic groups is around 10ꢀ11.5 at 25°C.
Thus, the need for high alkalinity likely indicates a critical
step in the depolymerization that depends on deprotonation
of different functional groups with higher pKa. According
to the mechanism proposed by Tarabanko et al. (Figure
S3), a high alkalinity is necessary for deprotonation of
certain reaction intermediates and retroꢀaldol cleavage of a
cinnamaldehydeꢀlike intermediate to obtain the final
Materials and methods
Chemicals and materials
N,OꢀBis(trimethylsilyl)trifluoroacetamide
(BSFTA),
pyridine (anhydrous, +99%), dichloromethane (+99%),
nitrobenzene (+99%), 2ꢀisopropylphenol (+98%), HCl
(37%), 4ꢀpropylguaiacol (+99%), citric acid (+99%),
spruce Kraft lignin (alkali lignin; 93.2 wt% lignin content),
54ꢀ57
aromatic aldehydes and acetophenoneꢀlike products.
The mechanism involving retroꢀaldol cleavage is
supported by the fact that the kinetics of vanillin formation
from lignin are similarly affected by the pH of the reaction
medium as oxidation of vanillideneacetone, which is a
representative compound for the cinnamaldehydeꢀlike
intermediate. Furthermore, this mechanism can also
explain the formation of the acetophenoneꢀderived
products, and is, therefore, considered to be a plausible
mechanism for alkaline lignin oxidation. However, a more
conclusive elucidation of the reaction mechanism likely
requires a combined experimental (model compound) and
computational study, as was previously performed to
verify the mechanism of acidꢀ and baseꢀcatalyzed lignin
Na
CuO
Co(NO
(+99%), MnCl
· xH
SO
(+99%), Pt/Al
nanopowder,
O
(5 wt%), Pd/Al
La(NO ·6H
·5H
· 6H O (+97%), Cr (SO )
2 4
O
(5 wt%),
(+99%),
2
4
2
3
2
3
)
O
2
3
2
)
·6H
O (+99%), CuSO
(+99%), CoCl
O (+99%), FeCl
3
2
2
4
2
3
3
2
2
2
O (+99%), were all purchased from SigmaꢀAldrich.
2
NaOH (99%), acetone (+99%), tetrahydrofuran (THF,
Baker, HPLC grade), methanol (+99%) and ethyl acetate
(+99%) were purchased from Fischer Scientific.
Fe(NO
Cu(NO
)
·9H
·3H
O
2
(98%), Mn(NO
O (99%), Co O nanopowder (99%) were
3 4
)
·4H
O
(99%),
3
3
2
3
2
2
)
3
2
purchased from Alfa Aesar. H
Ricca Chemical Company.
SO was purchased from
2
4
22, 98
conversion.
Finally, alkaline oxidation requires a different workup
procedure than reductive approaches, such as RCF.
Reductive methods are usually performed in lowꢀboiling
reaction solvents, which can be easily removed and
recycled by distillation. The products exhibit similar
functionality and polarity, implying that separations will
The poplar wood used for this project was obtained from a
farm in Alexandria, Minnesota, provided through the
USDA Northern Research Station in Rhinelander,
Wisconsin. The poplar sawdust was extracted with water
and ethanol for 48 h, respectively. The total lignin content
(Klason lignin and acidꢀsoluble lignin) of the (extractivesꢀ
13

Green Chemistry
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DOI: 10.1039/C8GC00502H
free) poplar sawdust is 29.1 wt%. Pine sawdust was
provided by Idaho National Laboratory (total lignin
content of 29 wt%). Corn stover was provided by Idaho
National Laboratory (total lignin content of 15.1 wt%).
The genetically engineered highꢀS poplar line (F5Hꢀ64ꢀE)
concentrated HCl to pH < 2 and extracted with ethyl
acetate (4 x 5 mL). The nitrobenzene oxidation reaction
mixtures were first extracted with dichloromethane (3 x 5
mL) before acidification. The ethyl acetate was evaporated
with a rotavap, resulting in a brown lignin oil (EA oil). 25
mg of 4ꢀnꢀpropylguaiacol was added as a standard and 7
mL acetone was added to solubilize the EA oil and
standard. 200 µL of this solution was added to a GC vial
and subsequently dried under vacuum. Next, 1 mL of
pyridine and 100 µL of BSTFA were added, the vial was
sealed, thoroughly shaken and left for 24 h before GC
analysis.
91
was originally produced as described by Franke et al. and
was provided by Carl Huetteman (Managing Director,
C3Bio, Purdue University) (total lignin content of 22.9
92
wt%). The preparation of corn stover DMRꢀEH and
DAPꢀEH lignin is described elsewhere (total lignin content
1
00
of 52.8 and 67.3 wt%, respectively). Corn stover AAPꢀ
3
6
AE lignin was prepared as described elsewhere. Briefly,
0 g milled corn stover was treated with anhydrous
5
For GPC and NMR, the remainder of the reaction solution
ammonia (3 g/g corn stover) in a stainless steel Parr batch
reactor at 130°C for 60 min. After evaporation of the
ammonia, alkali extraction was performed by treating the
pretreated corn stover in an Erlenmeyer flask with 500 mL
of a 0.1 M NaOH solution at 25°C for 2 h. The insoluble
material was subsequently separated by centrifugation. The
separated solid was mixed with 30 mL of deionised water
and the obtained slurry was centrifuged again. This step
was repeated twice, and the two wash fractions were added
to the supernatant from the first centrifugation, making up
the corn stover AAPꢀAE liquor. A portion of this liquor
was neutralized with H SO and freezeꢀdried to determine
(
~ 20 mL) was acidified and extracted with ethyl acetate in
a separatory funnel (4 x 25 mL). The ethyl acetate phase
was dried with NaSO and the ethyl acetate evaporated
using a rotavap, yielding the EA oil.
4
Gas chromatography (GC) analysis
For quantitative analysis of the volatile fraction, the
duplicate samples were analyzed on an Agilent GC (6890
series) equipped with a HP5ꢀcolumn and a flame
ionization detector (FID). The following operating
conditions were used: injection temperature of 260°C;
column temperature program: 50°C, 2ꢀmin hold, 10°C
min to 280°C, 5ꢀmin hold; detection temperature of
300°C. Sensitivity factors of the reagents and products
were obtained by calibration with commercial standards.
When using lignocellulosic feedstocks and isolated lignins,
the product yields were calculated on a weight basis using
the following equation:
2
4
ꢀ1
the total solids concentration (34.4 g/L, corrected for
Na SO weight) and lignin concentration (10.3 g/L).
2
4
Oxidation experiments
Alkaline aerobic oxidation reactions were performed in a
Parr multiꢀbatch reactor system. 30 mL of a NaOH
aqueous solution (deionized water; typically
2 M
ꢅꢆꢇꢇꢈꢉꢊꢋꢌꢍꢎ
concentration), substrate and catalyst were added to 75ꢀmL
reactor cups (type 316 stainless steel). The reactors were
closed and stirring was performed using magnetic stir bars
at 800 rpm. Next, the reactors were flushed with highꢀ
pressure He (three cycles), pressurized with He, and
ꢀ
ꢁꢂꢃꢄ =
× 100ꢖꢟꢙ%
ꢅꢆꢇꢇꢏꢌꢐꢏꢎꢉꢑꢎꢒ,ꢓꢔꢓꢎꢓꢑꢕ ꢖ× ꢖꢇꢗꢘꢇꢙꢚꢆꢙꢂꢖꢃꢁꢛꢜꢁꢜꢖꢝꢞꢜꢙꢂꢜꢙꢖ
In the experiments with vanillin and syringaldehyde
Table 1), the conversion and yield were calculated as
follows:
(
further pressurized with O to achieve a total pressure of
2
ꢅꢆꢇꢇꢏꢌꢐꢏꢎꢉꢑꢎꢒ,ꢓꢔꢓꢎꢑꢕ − ꢅꢆꢇꢇꢏꢌꢐꢏꢎꢉꢑꢎꢒ
2
0 bar. The applied He pressure depended on the targeted
O2 partial pressure and was typically 15 bar
corresponding with an O partial pressure of 5 bar). The
ꢠꢞꢜꢡꢂꢚꢇꢁꢞꢜ =
× 100ꢖꢟꢙ%
ꢅꢆꢇꢇꢏꢌꢐꢏꢎꢉꢑꢎꢒ,ꢓꢔꢓꢎꢑꢕ
ꢖ
ꢅꢆꢇꢇꢈꢉꢊꢋꢌꢍꢎ
(
ꢀꢁꢂꢃꢄ =
× 100ꢖꢟꢙ%
2
ꢅꢆꢇꢇꢏꢌꢐꢏꢎꢉꢑꢎꢒ,ꢓꢔꢓꢎꢑꢕ
ꢖ
reactors were then heated to the desired temperature
heating time of 30 min), at which point the reaction was
(
The yields and conversions indicated in the manuscript
constitute the average of the yields from the duplicate
samples. The standard deviation was always less than 1%.
Selected reactions were performed in triplicate, with the
standard deviation being less than 1%. Qualitative analysis
of the product solution was performed by analyzing the
samples on an Agilent GC (6890 series) equipped with a
HP5ꢀcolumn and
Technologies). The following operating conditions were
used: injection temperature of 260°C; column temperature
started (i.e., reaction time was set to 0 min). After reaction,
the reactors were quenched in water and depressurized at
room temperature.
For the nitrobenzene oxidation reactions of poplar, 27 mL
of a 2 M NaOH aqueous solution, 3 mL nitrobenzene and
0
.5 poplar were added to the reactor cups. After flushing
a
5973N MS detector (Agilent
with He, the reactors were heated to 170°C and left at this
temperature for 120 min.
ꢀ
1
CAUTIONARY REMARK: Using highly alkaline
solutions at high temperature can cause stress corrosion
cracking in stainless pressure vessels. Therefore, Monel
program: 50°C, 2ꢀmin hold, 10°C min to 280°C, 5ꢀmin
hold; MS transfer line temperature: 280°C.
HPLC analysis
4
00 pressure vessels were purchased for future research.
Highꢀperformance liquid chromatography (HPLC) analysis
was performed to quantify the aliphatic carboxylic acids in
the reaction mixture. Before HPLC analysis, a 1 mL
reaction sample was acidified with concentrated HCl to pH
< 2 and filtered with a 0.45 µm nylon membrane syringe
Work-up procedure
For GC analysis, the following procedure was performed
in duplicate for each reaction: 5 mL of the reaction
solution was added to a 15ꢀmL glass vial, acidified with
14

Page 15 of 18
Green Chemistry
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DOI: 10.1039/C8GC00502H
filter. The samples were analyzed on an Agilent HPLC
1100 series, Santa Clara, CA, USA) outfitted with a
refractive index detector and using an Aminex HPXꢀ87H
For the synthesis of the LaCoO , LaFeO , and LaMnO
3 3 3
(
perovskite catalysts, 15 mmol of La(NO ) ·6H O (6.495
3 2 2
g), 15 mmol of either Co(NO ) ·6H O (4.3655 g),
3
2
2
+
(
300 × 7.8 mm) organic acid column and Cation H guard
Fe(NO ) ·9H O (6.06 g) or Mn(NO ) ·4H O (3.765 g), and
3 3 2 3 2 2
cartridge (Biorad Laboratories, Hercules, CA, USA). The
column was maintained at 65°C with mobile phase
consisting of 0.05 N sulfuric acid with a flow rate of 0.6
mL min . The mobile phase was prepared from 10 N
sulfuric acid (Ricca Chemical Company, Arlington, TX,
USA). Analytes were identified by comparing retention
times and spectral profiles with pure standards.
45 mmol citric acid (8.645 g) were added to a 200ꢀmL
glass beaker together with 120 mL deionized water. For
the LaCo Cu O and LaMn Cu O catalysts, 12 mmol
0.8
0.2
3
0.8
0.2
3
ꢀ1
of either Co(NO ) ·6H O (3.492 g) or Mn(NO ) ·4H O (g)
3 2 2 3 2 2
and 3 mmol Cu(NO ) ·3H O (0.7248 g) were combined
3
2
2
with 15 mmol La(NO ) ·6H O, 45 mmol citric acid, and
3
2
2
120 mL deionised water. The mixture was stirred with a
magnetic stirring bar, heated to 90°C and left until most of
the water was evaporated and a gel remained. The sample
Gel permeation chromatography (GPC) analysis
GPC analysis was performed to assess the molecular
weight distribution of the aromatic lignin oxidation
products. The EA oils and the lignin substrates (20 mg)
were acetylated in a mixture of pyridine (0.5 mL) and
acetic anhydride (0.5 mL) at 40°C for 24 h with stirring.
The reaction was terminated by addition of methanol (0.5
mL). The acetylation solvents were then evaporated from
was subsequently dried at 100°C overnight and calcined at
ꢀ1
6
50°C for 6 h (5°C min heating rate). XRD was
performed on a Rigaku Ultima IV Xꢀray diffraction system
using Cu Kα radiation, with operating voltage and current
ꢀ1
of 40 kV and 44 mA, respectively, scan speed of 5° min ,
and point spacing of 0.02°. N physisorption was carried
2
out at 77K with a Quantachrome Quadrasorb SI
instrument, with outgassing under vacuum at 350°C for a
minimum of 12 h before analysis, and equilibration time of
the samples at 40°C under a stream of N gas. Additional
2
methanol was added in 1ꢀmL increments during N drying
2
to assist in removing the remaining acetylation solvents by
converting any remaining acetic anhydride to methyl
acetate. The samples were further dried overnight in a
vacuum oven at 40°C. The dried, acetylated samples were
dissolved in THF. The dissolved samples were filtered
3
0 s for both adsorption and desorption. Temperature
programmed reduction (TPR) was performed on an AMIꢀ
90 (Altamira Instruments) system equipped with a
3
thermal conductivity detector (TCD) to measure hydrogen
consumption during a temperature ramp. A molecular
sieve was placed before the TCD to trap water coming
from the catalyst bed. Approximately 100 mg of catalyst
was loaded between quartz wool plugs into a quartz Uꢀtube
reactor. A thermocouple located in the furnace wall was
used to control the experiment temperature and another
thermocouple located on top quartz wool plug monitored
(
0.2ꢀꢁm nylon membrane syringe filters) before GPC
analysis. GPC analysis was performed using an Agilent
HPLC with 3 GPC columns (Polymer Laboratories, 300 ×
7
.5 mm) packed with polystyreneꢀdivinylbenzene
copolymer gel (10ꢀꢁm beads) having nominal pore
4
3
diameters of 10 , 10 , and 50 Å. The eluent was THF and
ꢀ1
the flow rate 1 mL min . An injection volume of 25 ꢁL
was used. The HPLC was attached to a diode array
detector measuring absorbance at 260 nm (bandwidth 80
nm). Retention time was converted into molecular weight
the sample temperature. A pretreatment step consisting of
ꢀ1
heating in 10% O /He at 10°C min up to 500°C, followed
2
by holding for 30 min, was performed in situ. TPR
experiments were then carried out in 35 sccm of 5% H /Ar
2
(
MW) by applying a calibration curve established using6
ꢀ1
with a ramp rate of 10°C min from 100 to 800°C.
polystyrene standards of known molecular weight (1 × 10
to 580 Da) plus toluene (92 Da). The detection method is
mainly sensitive to aromatics, while other (sugar and
ligninꢀderived) products only contribute little to the signal.
RCF experiments
RCF experiments were performed in a Parr multiꢀbatch
reactor system. 30 mL of methanol, 1 g of substrate and
1
00 mg of Pd/C catalyst were added to the 75ꢀmL reactor
cups. The reactors were closed and stirring was performed
800 rpm). The reactors were flushed with highꢀpressure
He (three cycles), pressurized with 30 bar H , heated to
NMR analysis
Heteronuclear Single Quantum Coherence (HSQC) nuclear
magnetic resonance (NMR) spectra were acquired for the
EA oils and poplar wood. The EA oil (80 mg) was
dissolved in 0.5 mL of deuterated acetone. The MWL (100
mg) was dissolved into 0.5 mL of d ꢀDMSO/d ꢀpyridine
(
2
2
25°C (heating time of 30 min) and left at this temperature
for 6 h. After reaction, the reactors were quenched in water
and depressurized at room temperature. The reaction
solutions were subsequently filtered to separate the pulp
and catalyst from the liquor. The liquors were dried in a
rotavap and the resulting brown oils were subjected to a
threeꢀfold liquidꢀliquid extraction with dichloromethane
and water. The dichloromethane phase, containing the
lignin products, was dried, 40 mg 2ꢀisopropylphenol was
added as a standard and 7 mL acetone was added to
solubilize the standard and lignin products. The resulting
mixtures were quantitatively and qualitatively analysed by
GCꢀFID and GCꢀMS, respectively. The operating
conditions of the GCꢀFID and GCꢀMS, and the yield
6
5
(
4:1, v/v). Spectra were acquired on a Bruker 400 MHz
spectrometer using a BBO probe with Z gradient, with
1
1
,024 points and a sweep width of 15 ppm in the F2 ( H)
dimension and 512 points and 220 ppm of sweep width in
13
the F1 ( C) dimension. The delay time was 1.5 s. TMS
was used as an internal reference for the oil samples and
DMSO peak (δ 2.5, δ 39.51 ppm) for the MWL. Peak
H
C
1
01ꢀ103
assignment was performed according to the literature.
Synthesis and characterization of the perovskite
catalysts
15

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DOI: 10.1039/C8GC00502H
calculations were the same as described for the oxidation
reactions.
J. N. Saddler, T. J. Tschaplinski, G. A. Tuskan and
C. E. Wyman, Science, 2014, 344, 709.
1
1. R. Rinaldi, R. Jastrzebski, M. T. Clough, J. Ralph,
M. Kennema, P. C. A. Bruijnincx and B. M.
Weckhuysen, Angewandte Chemie International
Edition, 2016, 55, 8164ꢀ8215.
Conflicts of Interest
There are no conflicts of interest to declare.
Acknowledgements
12. S. Gillet, M. Aguedo, L. Petitjean, A. R. C. Morais,
A. M. da Costa Lopes, R. M. Lukasik and P. T.
Anastas, Green Chemistry, 2017, 19, 4200ꢀ4233.
This work was authored in part by Alliance for Sustainable
Energy, LLC, the manager and operator of the National
Renewable Energy Laboratory for the U.S. Department of
Energy (DOE) under Contract No. DEꢀAC36ꢀ08GO28308.
Funding provided by the U.S. Department of Energy
Office of Energy Efficiency and Renewable Energy
BioEnergy Technologies Office. W.S. acknowledges the
Research Foundation Flanders (FWO Vlaanderen) and the
internal funds of KU Leuven for funding. R.M. was
supported by the Center for Direct Catalytic Conversion of
Biomass to Biofuels (C3Bio), an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office
of Science, Office of Basic Energy Sciences, Award DEꢀ
SC0000997. The views expressed in the article do not
necessarily represent the views of the DOE or the U.S.
Government. The U.S. Government retains and the
publisher, by accepting the article for publication,
1
3. W. Schutyser, T. Renders, G. Van den Bossche, S.
Van den Bosch, S.ꢀF. Koelewijn, T. Ennaert and B.
F. Sels, in Nanotechnology in Catalysis, WileyꢀVCH
Verlag GmbH
&
Co. KGaA, 2017, DOI:
1
0.1002/9783527699827.ch23, pp. 537ꢀ584.
1
4. T. Renders, S. Van den Bosch, S. F. Koelewijn, W.
Schutyser and B. F. Sels, Energy & Environmental
Science, 2017, 10, 1551ꢀ1557.
1
1
5. L. Shuai and B. Saha, Green Chemistry, 2017, 19,
3
752–3758.
6. M. Ragnar, G. Henriksson, M. E. Lindström, M.
Wimby, J. Blechschmidt and S. Heinemann, in
Ullmann's Encyclopedia of Industrial Chemistry,
WileyꢀVCH Verlag GmbH & Co. KGaA, 2000, DOI:
1
0.1002/14356007.a18_545.pub4.
1
7. P. Sannigrahi and A. J. Ragauskas, in Aqueous
Pretreatment of Plant Biomass for Biological and
Chemical Conversion to Fuels and Chemicals, John
acknowledges that the U.S. Government retains
a
nonexclusive, paidꢀup, irrevocable, worldwide license to
publish or reproduce the published form of this work, or
allow others to do so, for U.S. Government purposes.
Wiley
&
Sons,
Ltd,
2013,
DOI:
1
0.1002/9780470975831.ch10, pp. 201ꢀ222.
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8. S. P. S. Chundawat, G. T. Beckham, M. E. Himmel
and B. E. Dale, Annual Review of Chemical and
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