Activating molecular oxygen with Au/CeO2 for the conversion of lignin model compounds and organosolv lignin

Article abstract of DOI:10.1039/c9ra04838c

Au/CeO₂ was demonstrated to be a high efficiency catalyst for the conversion of 2-phenoxyacetophenol (PP-ol) employing O₂ as an oxidant and methyl alcohol as the solvent without using an erosive strong base or acid. Mechanistic investigations, including emission quenching experiments, electron spin-resonance (ESR) and intermediate verification experiments, were carried out. The results verified that the superoxide anion activated by Au/CeO₂ from molecular oxygen plays a vital role in the oxidation of lignin model compounds, and the cleavage of both the β-O-4 and C_α-C_β linkages was involved. Au/CeO₂ also performed well in the oxidative conversion of organosolv lignin under mild conditions (453 K), producing vanillin (10.5 wt%), methyl vanillate (6.8 wt%), methylene syringate (3.4 wt%) and a ring-opened product. Based on the detailed characterization data and mechanistic results, Au/CeO₂ was confirmed to be a promising catalytic system.

Full text of DOI:10.1039/c9ra04838c

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Activating molecular oxygen with Au/CeO2 for the
conversion of lignin model compounds and
organosolv lignin†
Cite this: RSC Adv., 2019, 9, 31070
a
a
Wu-Lin Song,
Qingmeng Dong,a Liang Hong,ab Zhou-Qi Tian,
Li-Na Tang,a
Wenli Haoa and Hongxi Zhang
a
*
Au/CeO2 was demonstrated to be a high efficiency catalyst for the conversion of 2-phenoxyacetophenol
(PP-ol) employing O2 as an oxidant and methyl alcohol as the solvent without using an erosive strong
base or acid. Mechanistic investigations, including emission quenching experiments, electron spin-
resonance (ESR) and intermediate verification experiments, were carried out. The results verified that the
superoxide anion activated by Au/CeO2 from molecular oxygen plays a vital role in the oxidation of lignin
model compounds, and the cleavage of both the b-O-4 and Ca–Cb linkages was involved. Au/CeO2 also
performed well in the oxidative conversion of organosolv lignin under mild conditions (453 K), producing
vanillin (10.5 wt%), methyl vanillate (6.8 wt%), methylene syringate (3.4 wt%) and a ring-opened product.
Based on the detailed characterization data and mechanistic results, Au/CeO2 was confirmed to be
a promising catalytic system.
Received 26th June 2019
Accepted 25th September 2019
DOI: 10.1039/c9ra04838c
rsc.li/rsc-advances
feedstock to produce organic compounds, especially high-
value aromatics.4,5 Efficiently depolymerizing lignin into
Introduction
well-dened aromatics represents a key challenge that
limits the valorization of lignin.6 In the past decade,
increasing attention has been paid to the catalytic depoly-
merization of lignin in academia and industry.7–9 Many
efforts have focused on utilizing and manipulating the
inherent structure and functionality of lignin,10,11 espe-
cially the most abundant type, b-O-4 linkages, because they
make up approximately 50% of all linkages, making them
key to any depolymerization strategy.4
The best strategy for the depolymerization of lignin is still
under debate. Traditional strategies have been based on cata-
lytic cracking, hydrolysis, hydrogenolysis, reduction and
oxidation.12–16 Of these strategies, catalytic oxidization has
received substantial attention due to its potential for yielding
more highly functionalized monomers or oligomers, which are
widely applicable in the chemical industry. Several attempts
Despite their excellent performance and broad applica-
bility, fossil fuel reserves are dwindling and their use cau-
ses environmental pollution. Green and renewable
resources are considered key to the continuous develop-
ment of society. Lignocellulosic biomass can continuously
and steadily provide chemicals and fuel with zero net
carbon emissions. In recent years, there has been
increasing interest in the industrial production of organic
compounds and fuels from lignocellulosic biomass.1 As
a primary constituent of lignocellulosic biomass (15–30%
by weight and 40% by energy),2 lignin consists mainly of
three monolignols: coniferyl alcohol, sinapyl alcohol and p-
coumaryl alcohol (seen Fig. 1). Based on its structure,
lignin can be considered a source of value-added chem-
icals, such as aromatic aldehydes, ketones, acids and
esters. The annual output of lignin in nature is 150 billion
tons. More than 70 million tons of lignin are annually
produced by the paper industry, and historically, most of
this material is burned to provide energy.3 As a nonedible
and renewable biomass, lignin is a potentially valuable
aDepartment of Chemistry and Applied Chemistry, Changji University, Changji
831100, Xinjiang, China. E-mail: cjxyzhx@sina.cn
bProduct Quality Inspection Institute of Changji Hui Autonomous Prefecture, Changji
831100, Xinjiang, China
† Electronic supplementary information (ESI) available: TEM, XRD and XPS;
lignin model compounds synthesis and cycling tests on Au/CeO2. See DOI:
10.1039/c9ra04838c
Fig. 1 The basic structural units and connections in lignin.
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have been made toward an economically practical catalytic
oxidative decomposition of lignin to generate acids or alde-
hydes using O2 as the oxidant with a heterogeneous catalyst.17,18
In recent years, Au nanoparticles (NPs) supported on metal
oxides (Au/MnO2, Au/TiO2, Au/CeO2, etc.) have been investi-
gated to selectively catalyze the aerobic oxidation of alcohols to
the corresponding carbonyl compounds.19–21 Very recently, Yang
Song, etc. discussed a Au/hydroxide catalyst in which an alcohol
was converted to the corresponding carbonyl by a two-electron
process.22 To date, however, there has been little discussion of
the performance of Au/CeO2 in the catalytic conversion of
lignin.
In this paper, we introduce a heterogeneous catalyst, Au/
CeO2, for the depolymerization of lignin under molecular
oxygen. The high catalytic activity of Au/CeO2 with the lignin
model compounds, the study of the reaction mechanism and
the application of this catalyst to the conversion of lignin to
aromatics are highlighted in this manuscript.
Catalytic test
All catalytic reactions were carried out in an 80 mL autoclave
reactor. First, 0.47 mmol or 0.1 g of substrate, 20 mg of catalyst
and 25 mL of methanol were added to a stainless steel auto-
clave. Then, the reactor was pressurized with 1 MPa O2 and
heated to the target temperature under mechanical stirring. The
products were analyzed via HPLC and GC-MS (see the ESI†). The
conversion and the selectivity of conversion of the substrate
were calculated according to the following equations:
Conversion ¼ The mole of converted substrate/The mole of total
substrate ꢀ 100 (%)
Yield ¼ The mole of the product/The total mole of the substrate ꢀ
100 (%)
Results and discussion
Characterizations of the materials
Experimental
The phase purity and crystal structure of the catalyst were
determined by XRD, and the results are displayed in Fig. S1.†
Diffraction peaks from the (111), (200), (220), (311), (222), (400),
(331) and (420) planes of CeO2 are consistent with a cubic
uorite-type structure (JCPDS Card No. 65-2975). Obviously, the
strong diffraction peaks conrm the high crystallinity of the
sample. At the same time, no additional peaks were observed,
which means that the prepared CeO2 has better phase purity.
Aer loading the Au NPs on the CeO2 support, no Au peaks were
observed in the spectrum of 0.88 wt% Au/CeO2, suggesting an
ultralow loading of Au, a particle size less than 5 nm, and a good
dispersion of the Au NPs on the support. (The actual loading
weight content was 0.88 wt%, as determined by ICP-AES
analysis).
The transmission electron micrographs (Fig. S2†) show the
presence of Au NPs with an average particle size of 5 nm on the
as-prepared Au/CeO2. The observed lattice spacing of 0.23 nm
and 0.31 nm lattice fringes, which correspond to the Au (111)
and CeO2 (111) atomic planes, are in good agreement with the
XRD results (Fig. S1†).25
Materials and methods
Preparation of CeO2 nanorods.23 First, 25.2 g of NaOH and
1.73 g of Ce (NO3)3$6H2O were dissolved in 70 and 10 mL of
deionized water, respectively, in beakers. These two suspen-
sions were then mixed together in a Teon bottle and stirred for
1 h at room temperature. Then, the Teon bottle was placed in
an autoclave and heated in an oven at 373 K for 72 h. The solid
products were recovered by centrifugation, washed, and dried at
343 K for 12 h. Finally, the solid sample was calcined at 873 K in
air for 6 h.
Preparation of Au/CeO2.24 First, 0.5 g of CeO2 was added to
50 mL of an aqueous solution of 2.2 ꢀ 10ꢁ3 M HAuCl4 with
0.22 M urea. The suspension was vigorously stirred for 4 h at
353 K and then centrifuged. The solid was washed with deion-
ized water, dried at 343 K overnight and calcined at 573 K in air
for 4 h, other metal oxides are treated in the same way.
Catalyst characterization
A 50 mg sample of the catalyst was transferred into a U-shaped
The H2-TPR data from the CeO2 nanorods and Au/CeO2
quartz reactor and purged with He at 403 K for 3 h. Then, aer catalysts are shown in Fig. S3.† The pure CeO2 nanorods do not
cooling to ambient temperature, the owing gas was switched exhibit a reduction peak at 293-773 K.26 For the Au/CeO2 cata-
to 5% H2/Ar, and the sample was heated from room tempera- lyst, signicant changes due to the reaction/absorption of
ture to 973 K at a rate of 10 K minꢁ1. The X-ray diffraction (XRD) hydrogen on the solid were observed. Four reduction peaks, at
patterns of the catalyst were recorded using a Rigaku MiniFlex 398 K, 523 K, 661 K and 869 K, were present. The Au species in
600 diffractometer system. The morphology of the prepared Au/ the Au/CeO2 catalyst were reduced at 373–473 K. The peak at 398
CeO2 was observed by TEM on a Jeol 2100F electron microscope. K is attributed to Au reduction, while the peaks at 523 K and 661
X-ray photoelectron spectroscopy (XPS) measurements of the K are attributed to the reduction of surface oxygen by CeO2, and
Au/CeO2 were recorded on a Thermo SCIENTIFIC ESCALAB the reduction peak at 869 K is attributed to the lattice oxygen.27
250Xi. The binding energies were referenced to the C 1s level of This indicates that the deposition of the Au NPs on ceria
adventitious carbon at 284.6 eV. The electron spin-resonance remarkably alters the catalytic activity and reducibility of the
(ESR) signals were recorded on a Bruker EMX-10/12 spectrom- catalyst.
eter. The settings for the ESR spectrometer were as follows:
To further clarify the chemical states of the Au NPs, XPS
center eld, 3500.00 G; sweep width, 150.00 G; microwave analysis was conducted, as shown in Fig. S4.† The XPS spectrum
frequency, 9.85 G; power, 3.99 mW; conversion time, 40.0 ms.
of Au 4f displays typical doublet peaks with binding energies of
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83.9 eV (Au 4f7/2) and 87.6 eV (Au 4f5/2), indicating that Au is behavior under catalytic conditions. The results are shown in
present in its elementary metallic form.28
Table 2.
PP-one is transformed into the major products i.e., Phenol,
BA and MB, with a conversion of 21% (Table 2) without a cata-
lyst. Compared with PP-ol, it reacted more quickly under the
same operating conditions because PP-one also contains a b-O-4
bond, but the Ca–OH moiety is converted to a Ca]O group in
PP-ol. This reactivity is consistent with the idea that the
breakage of the b-O-4 linkage is easier when a Ca]O is present
instead of a Ca–OH moiety.29
Then, we investigated the oxidative activity of Au/CeO2
catalysts with different Au loadings (the actual loadings (weight
content) were determined by ICP-AES analysis). As shown in
Fig. 2, when the Au loading was increased from 0.08 wt% to
0.88 wt%, the conversion of PP-ol signicantly increased from
25% to 71%. These results suggest that the Au NPs play a key
role in the catalytic conversion of the substrate. Au NPs may
enrich and activate molecular oxygen, greatly reducing the
absorption energy of the reactants, as Wang reported.30 Because
Au NPs provide active sites for PP-ol oxidation in the Au/CeO2
systems, increasing the number of Au active sites enhances the
catalysis. However, it should be noted that the increase in the
PP-ol conversion and the yield of Phenol and MB decreased as
the Au loading exceeded 0.88 wt%. Moreover, the yield of DT
increased signicantly with increasing Au loading, and DT may
be an intermediate in repolymerization, making it undesirable
in the reaction system.
Metal oxides catalyze the oxidative of lignin model compound
The diffusion limit was eliminated by optimizing the agitation
speed before the catalytic experiments (Fig. S5†). We found that
when the rotation speed reached 400 rpm, the internal and
external diffusion phenomena were controlled.
Based on this result, we tested the catalytic activities of
several typical metal oxides and metal oxide-supported Au in the
oxidative conversion of 2-phenoxy-1-phenylethanol (PP-ol). The
results are shown in Table 1. When several of the typical metal
oxides or no catalyst were used, the substrate displayed a very
low conversion of 0.1–2.7% (Entries 1–5). When the Au NPs were
loaded into metal oxides, the PP-ol was effectively converted to
benzoic acid (BA), methyl benzoate (MB), phenol, dimethox-
ytoluene (DT), methyl phenylglyoxylate (MP) and uncleaved
product 2-phenoxy-1- phenylethanone (PP-one) (Entries 6–9).
Compared to several typical metal oxides (Entries 1–5), Au
loaded on a metal oxide signicantly accelerates the conversion
of PP-ol. Au/CeO2 showed the highest PP-ol conversion (71.5%).
Phenol, MB, PP-one and DT were the major products with yields
of 45.3%, 31.4%, 20.8% and 6%, respectively (Entry 9). The
appearance of MB also demonstrates that the cleavage of the
Ca–Cb bond is accompanied by the breakage of the b-O-4
linkage. Stahl et al. recently developed a novel methodology
for the conversion of lignin into aromatic compounds under
mild conditions.9,29 This method involves the catalytic conver-
sion of a Ca–hydroxyl group into the ketonic group and the
subsequent cleavage of the b-O-4 bond. This strategy was
developed by Deng and coworkers.4 Because PP-one was also
detected as a product (Table 1, Entry 6–9), we investigated its
Both b-O-4 bonds and methoxy groups at various substitu-
tion positions are abundant in the aromatic units of lignin, and
those methoxy groups may inuence the activation of the b-O-4
linkages. Therefore, we investigated the oxidative performance
of Au/CeO2 for the catalytic transformation of substituted PP-ol
at 453 K. As displayed in Table 3, the substituted compound is
more reactive than PP-ol. When a CH3O moiety was present,
more than 90% the model compound was converted to the
corresponding esters and phenols. This result indicates that Au/
Table 1 Catalytic conversion of PP-ol over several metal oxidesa
Table 2 Catalytic conversion of PP-one over several metal oxidesa
Yield (%)
Cons.
Entry Cat.
(%)
Phenol DT
BA
MB PP-one MP
1
2
3
4
5
6
7
8
9
Blank
SiO2
Al2O3
Pr6O11
CeO2
Au/SiO2
Au/Al2O3
Au/Pr6O11 44.8
Au/CeO2 71.5
<0.1
<0.1
2.9
1.7
2.7
0
0
0
0
0
0
0
1.9
0
0
0.24
0.1
1.7
0
0
2
0
0
0
0.9
0
1.7
0
0
0
0
0
0
7.1
14.6
6.7
Yield (%)
0.5
0.7
0
9.5
45.1
25.1
45.3
Entry Catalysis Cons. (%) Phenol DT BA
MB
MP AP
1.3
1
2
3
4
5
Blank
Al2O3
SiO2
CeO2
Au/CeO2 >99
21
23
26
76
15
20
19
58
0
0
0
0
9.2
8.2
13
3.8
4.2
4.6
56
0
0
0
0
0
0
0
4.7
0
54.8
62.7
21.2 24.6 18
14.6 15.4 4.3
6.7
6
14.9
17.4
26.2 4.3
4
2.8
31.4 20.8
71.1
1.5 12.3 58.6 6.8
a
a
Reaction conditions: substrate, 0.1 g (0.47 mmol); catalyst, 20 mg;
Reaction conditions: PP-one, 0.1 g (0.47 mmol); catalyst, 20 mg;
CH3OH, 25 mL; O2, 1 MPa; 453 K; 4 h.
CH3OH, 25 mL; O2, 1 MPa; 443 K; 2 h.
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Fig. 2 Effect of Au loadings on the catalytic behaviors of Au/CeO2 for
the oxidation of PP-ol. Reaction conditions: PP-ol, 0.1 g (0.47 mmol);
catalyst, 20 mg; CH3OH, 25 mL; O2, 1 MPa; 453 K; 4 h.
Fig. 3 Time course for the oxidation of PP-ol catalyzed by Au/CeO2.
Reaction conditions: 0.1 g (0.47 mmol); catalyst, 20 mg; CH3OH, 25
mL; O2, 1 MPa; 453 K.
Table 3 Catalytic performances of Au/CeO2 for the conversion of
substituted PP-ol model compoundsa
oxidation progressed, the selectivity for PP-one rapidly
decreased, and the selectivities for Phenol, DT and MB
increased. Aer 4 h, the difference in selectivity became less
signicant. Subsequently, we calculated the concentration of
the products, and depicted the changing trend of products
concentration vs. time, as Fig. S6† shows. These ndings
support the hypothesis that the reaction proceeded through PP-
one as an intermediate, and PP-one was further oxidized over
the Au/CeO2 catalyst. These results further indicate that the Au
NPs play a key role in the oxidation of PP-ol by catalyzing the
conversion of Ca-OH to Ca ¼ ¼O. A carbonyl compound was
formed, and cleavage of the b-O-4 and CaꢁCb bonds subse-
quently occurred. Such a preoxidation process may reduce the
energy barrier for the breakage of the b-O-4 linkage. Gregg T.
Beckham et al. previously reported that by converting the Ca-
Yield (%)
Cons.
Substrate
(%)
MB
BA
Phenol
DB
DT
Ketone
>99
82.5
73.7
1.7
5.6
85.3
72
0
0
1.9
3.7
6.4
98.2
10.2
hydroxyl group to a Ca-carbonyl, the b-O-4 bond dissociation
94.7
60.7
4.3
1.7
70.2
6.7
15.4
enthalpy was reduced from 69.5 to 60.6 kcal molꢁ1
.
31
To further investigate the role of O2, we probed the effect of
O2 pressure in the catalytic system. We observed a 29% trans-
formation of PP-ol with 0.1 MPa O2 (Table 4, entry 1). PP-one,
a
Reaction conditions: substrate, 0.47 mmol; Au/CeO2, 20 mg; CH3OH,
25 mL; O2, 1 MPa; 453 K; 4 h.
CeO2 performs well in the catalytic oxidative cleavage of b-O-4 in
the lignin model compounds.
Table 4 Catalytic conversion of PP-ol under different O2 pressuresa
The stability of the Au/CeO2 catalyst for the oxidative
conversion of PP-ol was investigated (Fig. S7†). The catalyst
could be recycled four times without a signicant decline in its
oxidative activity, and its product distribution remained stable.
The reduction in the conversion may be attributed to an
increase in the size of the Au NPs or a change in the catalyst
surface properties, such as the presence of adsorbed products
on the Au/CeO2 or the oxidization of the catalyst surface.
Yield (%)
Cons.
Catalysis
(%)
Phenol
DT
BA
MB
MP
PP-one
Reaction mechanism for the oxidation of PP-ol
O2 (0.1 Mpa)
O2 (1 Mpa)
N2 (1 Mpa)
29
71.5
0.7
15.1
45.3
0
4.90
2
0
5.1
4
0
5.5
31.4
0
0
6.7
0
8.3
20.8
0
To elucidate the reaction pathway, we examined the oxidation of
PP-ol over time on the Au/CeO2 catalyst, and the results are
shown in Fig. 3. As Fig. 3 shows, PP-one was the main product in
the initial stage of the reaction with ꢂ80% selectivity. As the
a
Reaction conditions: PP-ol, 0.1 g (0.47 mmol); catalyst, 20 mg; CH3OH,
25 mL; 453 K; 4 h.
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Table 6 Radical tripping experimentsa
Radical scavenger
Phenol and MB were the major products. An increase in O2
pressure efficiently promoted the conversion, suggesting that
O2 facilitates the breakage of both the Ca–Cb and b-O-4 bonds
(Table 4, Entry 2).
Cons.
(%)
Entry
Substrate
equivalent
When N2 was employed, only a 0.7% conversion of PP-ol
(Table 4, Entry 3) was observed. This nding suggests that Au/
CeO2 cannot catalyze this reaction in the presence of N2, but the
cleavage of the b-O-4 linkage could occur in the absence of O2.
It was also conrmed that PP-one could more readily than
PP-ol undergo this transformation because PP-one contains a b-
O-4 bond adjacent to a Ca]O moiety, while PP-ol has a Ca–OH
group. However, we observed 17% conversion of PP-one in the
presence of N2 (Table 5, Entry 3), which is obviously lower than
the conversions (56%, >99%) under O2 (Table 5, Entry 1 and 2).
Interestingly, phenol and acetophenone were the major prod-
ucts, and they were obtained in yields of 15.2 and 14%,
respectively. The appearance of MB conrms that the b-O-4
linkage was cleaved.
1
2
3
PP-ol
PP-ol
PP-ol
PP-ol
PP-one
PP-one
—
0.1
0.5
1
—
0.1
71.5
10.7
7
<0.1
>99
42
4
5b
6b
a
Reaction conditions: PP-one, 0.1 g (0.47 mmol); catalyst, 20 mg;
b
CH3OH, 25 mL; 453 K; O2, 1 MPa; 2 h. 443 K; 2 h.
characteristic of the superoxide radical anion was observed in
the ESR data, suggesting that oxygen is activated by Au/CeO2 to
form superoxide anion free radicals.28,33 Therefore, these
experiments conrm that the Au/CeO2 catalyst can strongly
activate oxygen.
Employing molecular oxygen as the oxidant is green,
economical, and attractive from an environmental viewpoint.34
Efficiently catalyzing the reactions between inert, ground
triplet-state O2 (3SgO2) and organic molecules (mainly in
singlet-state) is key to this process. Substantial effort has been
devoted to activating O2 into active oxygen species.35 Au NPs
show a unique ability to generate superoxide radicals (O2cꢁ)
through electron transfer processes.36
Tsukuda et al. reported that molecular oxygen is activated
through negatively charged Au NPs to form a peroxo species or
superoxide, which can then activate C–H bonds.37 As Woodham
et al.38 demonstrated, electrons can be added to the O2 p*
orbital of O2 via electron donation from loaded Au NPs, gener-
ating elongated O–O bonds that closely resemble those of the
superoxo-like (O2ꢁ) adsorbate state. The oxidation process is
subsequently accomplished through a series of deprotonation
and elimination steps, i.e., adsorbed molecular O2 obtains an
electron from Au/CeO2 to generate Au/CeO2–O2cꢁ, which then
reacts with PP-ol to form PP-one. Based on the results presented
and discussed above, a possible mechanism for the oxidation of
PP-ol over the Au/CeO2 catalyst is proposed in Scheme 1.
Radical trapping experiments
p-Benzoquinone is a scavenger of superoxide radicals,32 and it
was added to the reactor along with the substrate, and the
results are presented in Table 6.
As displayed in Table 6, the conversion of PP-ol reached
71.5% (entry 1) in the absence of p-benzoquinone. However,
when benzoquinone was added to the system, the conversion of
PP-ol dropped dramatically (Entry 2–4) from 10.7% to 0.1% with
increasing loading of the radical scavenger. When PP-one was
used as the substrate, similar results were observed. These
results demonstrated that a radical species played a vital role in
the catalytic oxidation of the lignin model compounds; thus, we
speculate that the oxidation occurred via a free radical process.
To verify this hypothesis, more mechanistic information was
obtained by identifying the surface intermediates using in situ
liquid-phase ESR spin-trapping experiments, as Fig. 4 shows.
Data were acquired from a suspension of 0.1 g Au/CeO2 sample
in methanol with DMPO and oxygen. An obvious 6-fold signal
Table 5 Catalytic conversion of PP-one under different O2 pressuresa
Yield (%)
Catalysis
Cons. (%)
Phenol
DT
BA
MB
MP
AP
O2 (0.1 Mpa)
O2 (1 Mpa)
N2 (1 Mpa)
56
>99
17
39
71.1
15.2
0.7
1.5
0
16.9
12.3
0
24
58.6
0
5
6.8
0
0
0
14
Fig. 4 ESR spectrum of DMPO-O2cꢁ recorded in a system containing
a
Reaction conditions: PP-one, 0.1 g (0.47 mmol); catalyst, 20 mg;
CH3OH, 25 mL; 443 K; 2 h.
methanol, the catalyst and O2 at 453 k.
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To investigate the breakage of the b-O-4 linkage, the oxida-
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tion of several potential intermediates by Au/CeO2 in an O2
atmosphere at 453 K was explored (Fig. 5). First, phenylglyoxal
can be fully converted to the corresponding breakage products,
i.e., Phenol, DT, BA and MB (eqn (1) in Fig. 5). These products
are in good agreement with the products of the Au/CeO2-cata-
lyzed oxidation of PP-ol (Table 1). Because phenylglyoxal was
also detected as a minor product by GC-MS, phenylglyoxal may
be the intermediate in the oxidation, and it would presumably
be produced from the breakage of b-O-4 linkage. Second,
benzaldehyde showed a low conversion (10.2%) to BA, MB and
DT (eqn (2)). Benzaldehyde is stable under the reaction condi-
tions, suggesting that little of its Ca–Cb bond was broken. BA
also showed a low transformation (24.4%) to MB (eqn (4)),
indicating that MB was mainly formed from phenylglyoxal (eqn
(1)). Third, methyl phenylglyoxylate showed a conversion of
32.9% under these conditions, and the major products were
MB, MA, benzaldehyde and DT (eqn (3)). CO2 was also released
by this reaction (Fig. S8†). These results indicate that Cb is
transformed via two pathways, including Ca–Cb bond and Cb–O
bond breakage to CO2 and methyl formate. Methyl phenyl-
glyoxylate is stable in the reaction, meaning that it is not
a possible intermediate.
Based on these results, a possible cleavage process for the
oxidation was proposed (Scheme 2). In short, the Ca–hydroxyl
group of PP-ol is primarily oxidized a Ca–carbonyl via the
catalytic action of Au NPs. The subsequent oxidation of the b-
O-4 linkage of PP-one over the Au/CeO2 catalyst via O2cꢁ could
afford phenylglyoxal, methyl phenylglyoxylate and phenol.
The catalytic breakage of the b-O-4 linkage over CeO2 affords
an oxidized intermediate (e.g., phenylglyoxal) and phenol. A
small amount of methyl phenylglyoxylate could also be
produced in this process. The catalytically generated inter-
mediate may undergo rapid oxidative cleavage of the C–C
bond, generating BA and then MB, benzaldehyde and ulti-
mately DT.
Fig. 5 Catalytic conversions of the possible intermediates under an O2
atmosphere over Au/CeO2.
Au/CeO2-catalyzed conversion of real lignin
The oxidative conversion of organosolv lignin by the Au/CeO2
catalyst system was subsequently investigated. Lignin was
extracted via the ethyl alcohol-based organosolv process from
cotton stalk according to a previous report. Then, the organo-
solv lignin was subjected to catalytic oxidation in methanol
under O2 with the Au/CeO2 catalyst at 453 K for 4 h.
The results are displayed in Fig. 6. Several monomeric
aromatic compounds, including vanillin (10.5 wt%), methyl
vanillate (6.8 wt%), 2,6-dimethoxy-1,4-benzoquinone, methy-
lene syringate (3.4 wt%) and a ring-opened compound, were
detected and quantied by GC-MS. According to earlier reports,
LaMnO3 and LaCoO3 are highly active for the catalytic oxidation
of lignin to aromatic aldehydes, and they offer high yields of
vanillin (ꢂ5%) and syringaldehyde (ꢂ10%).39,40 Perovskite-type
LaFe0.8Cu0.2O3 materials have also been studied for the wet
catalysis of lignin, and they provided maximum yields of
vanillin and 4-hydroxylbenzaldehyde of 4.56 wt% and 2.49 wt%,
respectively. However, an erosive strong base was required in
these methods. Recently, a Pd/Al2O3 catalyst was reported for
the oxidative conversion of alkaline lignin, affording only
vanillin in a yield of 1.6 wt%.41 Yang et al. investigated Au/Li–Al
LDH materials, which showed excellent activity in the oxidation
Our observations and mechanistic studies demonstrate the
vital role of the preoxidation of the Ca–hydroxyl group. In our
earlier research,4 we loaded Au NPs onto commercial CeO2 from
Alfa Aesar. This material could also catalyze the transformation
of PP-ol into monomeric aromatic compounds but a lower
conversion than that achieved over Pd/CeO2. However, when we
changed the support to CeO2 nanorods via a hydrothermal
method and loaded Au NPs (<5 nm) on these rods, the prepared
catalyst displayed excellent performance.
Scheme 1 Proposed mechanism for the aerobic oxidation of PP-ol Scheme 2 Possible reaction mechanism for the oxidative conversion
over Au/CeO2.
of PP-ol.
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Au/CeO2 was found to be a competent and environmentally
friendly heterogeneous catalyst for the oxidation of model
compounds and organosolv lignin using O2 as the oxidant and
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a superoxide anion plays a vital role in the catalytic system. The
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RSC Adv., 2019, 9, 31070–31077 | 31077