Selective oxidation of veratryl alcohol with composites of Au nanoparticles and graphene quantum dots as catalysts

Article abstract of DOI:10.1039/c5cc00061k

Veratryl alcohol can be oxidized to veratryl aldehyde or veratric acid with excellent selectivity and efficient conversion under acidic and alkaline conditions using Au nanoparticles and graphene quantum dot composites (Au/GQDs) as catalysts. This journal is

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Selective Oxidation of Veratryl Alcohol with
Composites of Au Nanoparticles and Graphene
Quantum Dots as Catalysts
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
Received 00th January 2012,
Accepted 00th January 2012
Xiaochen Wu , Shouwu Guo * and Jingyan Zhang *
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract Veratryl alcohol can be oxidized into veratryl aldehyde or lateral sizes of GQDs range in 25ꢀ35 nm, and the average height of
GQDs
veratric acid with excellent selectivity and efficient conversion under
acidic and alkali conditions using Au nanoparticles and graphene
quantum dots composites (Au/GQDs) as catalysts.
Veratryl alcohol (VA) has been often utilized as a model
compound to study the reaction pathways of decomposition or
chemical transformation of natural abounded lignin for the
1
ꢀ3
production of biofuels and aromatic chemicals. VA could be
selectively oxidized to veratryl aldehyde in the presence of catalysts
4
5, 6
including enzymes, metal porphyrin complexes,
complexes,
cobalt(salen)
3, 7, 8
9
or zeolitic imidazolate frameworks etc. However,
the high cost and poor stability of the enzyme and small molecular
catalysts severely impede their practical application. Having higher
10
selectivity and less possibility to be deactivated by oxygen, Au
nanoparticles (NPs) supported on various matrices also have been
1
1
employed to catalyze alcohol oxidation. Recently, it has been
reported that the Au NPs supported on carbonaceous materials
exhibited higher catalytic activity, selectivity, and better
environmental suitability than those attached onto other substrates,
1
2ꢀ14
especially the commonly used metal oxides.
However, Au NPs
centered catalysts are almost inactive in the absence of base or basic
1
5ꢀ17
supports in many oxidation reactions.
We and others recently
discovered that graphene quantum dots (GQDs) alone and the
nanoarchitectures assembled with GQDs could dramatically improve
the activity and stability of the catalysts, due to the synergetic effect
raised by GQDs that possess excellent electron conductivity and Figure 1. Structure of Au/GQDs composites. TEM images of the
1
8, 19
good chemical/physical stabilities.
On the basis of these Au/GQDs composites: (a) overall images, (b) 5ꢀfold magnification,
findings, in this work we explored the catalytic properties of the and (c) 20ꢀfold magnification. (d) Schematic representation of their
composites of Au NPs and GQDs (abbreviated as Au/GQDs) in VA structures.
oxidation. As will be shown, with the Au/GQDs as catalysts, VA can
is ~ 1 nm, revealing the single atomic layered feature (Figure S1).
be oxidized selectively into veratryl aldehyde or veratric acid using
The Au/GQDs composites were prepared through a facile oneꢀpot
hydrogen peroxide with excellent yield and selectivity. Due to the
reaction using GQDs, HAuCl and trisodium citrate as raw materials
4
good stability and high affinity to aromatic alcohols of the GQDs,
22
(
detailed in the experimental part). In contrast to the bare Au NPs
prepared under the same condition (Figure S2), the TEM
transmission electron microscopy) images (Figure 1a, b) show that
and the strong interaction between Au NPs and GQDs, the Au/GQDs
composites show a synergistically enhanced catalytic activity in VA
oxidation, which is much higher that of Au NPs or GQDs.
(
the Au NPs are coated by GQDs forming a coreꢀshell structure in the
Au/GQDs composites, which was further confirmed by the HRTEM
The GQDs used in this work were fabricated through photoꢀ
20, 21
Fenton reaction of graphene oxide as previously reported.
The
(high resolution TEM) image as shown in Figure 1c (Figure S3). On
the basis of TEM images, the structure of Au/GQDs is schematically
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Jou5Crnal Na1Kme
depicted in Figure 1d. To elaborate the chemical composition of converted to veratryl aldehyde, with above 99% selectivity. The
Au/GQDs, Raman, FTꢀIR (Fourier transform infrared) and XPS (Xꢀ detailed optimizations of the Au/GQDs and H O in VA oxidation
2
2
ray photoelectron spectroscopy) of Au/GQDs were acquired and were summarized in Figure S5a and b. For comparison, the oxidation
compared with those of GQDs. As illustrated in Figure S4a, the ratio of VA with H O2 in the presence of bare Au NPs, GQDs, and
2
of D to G bands intensities in the Raman spectra of Au/GQDs (~ physical mixture of GQDs with Au NPs (GQDs + Au) were also
0
.855) is higher than that of bare GQDs (~ 0.825), indicating the carried out under the same conditions, respectively. Figure 2b shows
partial reduction of GQDs. Additionally, by comparing the FTꢀIR that within 2 h of the reaction, only a small peak of veratryl aldehyde
spectra of Au/GQDs and GQDs, it can be noted that within the was observed using the mixture of GQDs and Au NPs as a catalyst
composites there are certain oxygen containing groups from GQDs, (green line in Figure 2b). The Au/GQDs composites also exhibit
including epoxy, alkoxy, and carbonyl groups, but these groups are great stability, they still maintain more than 80% activity after 6
2
3
much less than that in the bare GQDs (Figure S4b). It is possible cycles (Figure S5c).
that the oxygen containing groups on GQDs were reduced during the
The catalytic activity of Au NPs related catalysts depends strongly
Au NPs formation due to the presence of trisodium citrate in the
preparation. XPS data further confirmed this assumption. As
depicted in Figure S3c and d, all of the C1s XPS peak intensities at
25ꢀ27
on temperature and the pH of the reaction system,
hence, these
two factors of the reaction system with Au/GQDs as catalysts were
also investigated. As illustrated in Figure 2c, the conversion of VA
to veratryl aldehyde at 4 °C is less than 10%, but it can reach up to
2
86.4, 287.8 and 288.5 eV, corresponding to CꢀO (epoxy and
alkoxy), C=O (carbonyl) and COOH (carboxylic) groups in
Au/GQDs composites, respectively, were decreased dramatically in
comparing with those in bare GQDs. Owing to these oxygen
containing groups can effectively prevent the Au NPs from
aggregation, the asꢀobtained Au/GQDs composites exhibit much
better dispensability and stability in aqueous solution than bare Au
9
0% at 70 °C (reaction time 1 h, pH 8.5). Apparently, high
temperature is beneficial to the catalytic activity of the Au/GQDs.
To examine the effect of pH on the catalytic activity of Au/GQDs,
the reactions were performed under acidic and basic conditions, and
the results were analyzed with GCꢀMS. As illustrated in Figure 2d,
the Au/GQDs composites possess the catalytic ability even in pH 3.3
solution, and the conversion of VA to veratryl aldehyde was 17%
within 2 h; when the reaction mixture is at pH 5, the conversion can
reach up to 45% with extraordinary selectivity to veratryl aldehyde,
and only about 30% of acid was formed after 21 h of reaction
2
0
NPs.
(Figure S6, Table S2). This result is different from many published
works that the Au catalytic systems are inactive in the absence of
1
5ꢀ17
base or basic supports.
In other words, the Au/GQDs can
catalyze the oxidation of VA under acidic condition. This is
important in lignin depolymerization, because the first step in the
most currently used methods of depolymerization is acid treatment,
which thus requires the subsequent used catalysts including enzymes
have very high stability under acidic conditions. The stability and
catalytic activity of the Au/GQDs under acidic condition may
contributed by its out inert layer of GQDs on Au NPs (Figure 1).
When the pH value was further increased to above 7, the
conversion of VA to veratryl aldehyde raised up quickly to more
than 90% in 2 h of the reaction. Besides the veratryl aldehyde,
veratric acid was also produced. The conversion of VA to veratric
acid was further improved to more than 99%, when the pH of the
reaction reaches to 13. Apparently, under the basic condition, the
catalytic activity of the Au/GQDs was enhanced. In fact, VA was
first converted to veratryl aldehyde, then to veratric acid under either
basic or acidic conditions (Figure S6 and Table S2). These results
indicate that with Au/GQDs catalysts, VA can be oxidized into
veratraldehyde or veratric acid selectively by tuning the reaction
time or pH (Table 1). For instance, under basic condition, only
aldehyde was formed in a short time, while in a longer reaction time,
such as 6.5 h (Table S2), only acid was obtained.
Figure 2. Oxidation of VA catalyzed by the Au/GQDs composites.
a) UVꢀvis spectra of the oxidation products of veratryl alcohol
versus reaction time at pH 8.5. The experiments were carried out
with 15 ꢁg of Au/GQDs, 3 mM of H O and 0.3 mM of VA at 50
(
2
2
°C; (b) The oxidation reaction of VA was carried out with different
catalysts under the same condition for 2 h; (c) Conversion rates of
VA catalyzed by the Au/GQDs at pH 8.5 with different reaction
temperature for 1 h; (d) Conversion rates of VA catalyzed by the
Au/GQDs at 50 °C for 2 h with varying pH.
To explore the catalytic activity of the asꢀobtained Au/GQDs
The high activity and selectivity of Au/GQDs may be contributed
composites, oxidation of VA using H O as an oxidizing agent was by GQDs, Au NPs, and probably their synergistic effect too. It was
2
2
employed as a model system. In a typical experiment, VA (0.3 generally accepted that the residual oxygenꢀcontaining functional
mol), H O (3 ꢁmol) was mixed with 15 ꢁg of Au/GQDs groups on the surface of supported Au catalysts play a key role in
ꢁ
2
2
composites in 1 mL of aqueous solution at 50 °C. The reaction was their catalytic performances, especially for the dehydrogenation and
28
monitored with UVꢀvis spectroscopy. As shown in Figure 2a, after oxidization reactions. The GQDs with plenty of the periphery
5 min of the reaction, an absorption peak centered at 310 nm, carboxylic groups show high peroxidaseꢀlike activity under acidic
1
2
4
29
corresponding to veratryl aldehyde, could be detected clearly, and environment.
That’s possibly the reason why Au/GQDs
the peak intensity increased rapidly with the reaction time increased, composites exhibit catalytic activity under acidic condition, since Au
indicating that the Au/GQDs and H O could convert VA to veratryl NPs alone are inactive. In fact, graphene oxide (GO) was reported to
2
2
aldehyde. The conversion percentage and selectivity of VA to enhance the chemiluminescence of luminolꢀH
O
2
system in a weakly
veratryl aldehyde were analyzed using GCꢀMS quantitatively. As alkaline medium. In luminolꢀH O system, the GO functioned
2 2
2
30
1
summarized in Table S1, after 2 h of the reaction, 97% of VA was through the generation singlet oxygen O intermediate. We thus
2
1
infer that under acidic condition, O is possibly involved in the
2
2
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DOI: 10.1039/C5CC000 K
reaction catalyzed by the Au/GQDs. Reactions with the inhibitor of species. The assumption, however, need to be further proved. During
1
O supported this assumption. As shown in Figure S7a, NaN could the preparation of this manuscript, a paper showed that graphene
2
3
effectively inhibit the reaction at pH 5. In addition, the reaction rate covered on the Pt surface could promote Ptꢀcatalyzed carbon
was enhanced when the reaction was carried out in D O instead of monoxide oxidation was published online. The authors believed that
2
1
H O (Figure S7b), because the life time of O is longer in D O than graphene weakens the
in H O. Apparently, under alkali condition, O was not, or only
2
2
2
3
1
1
2
2
involved partially since the reaction was barely inhibited at pH 8.5 or
1 by NaN . To further confirm this conclusion, the reaction was
1
3
1
monitored with O trapping agents. As shown in Figure 3a, when
2
1
O trapping agent 2,2,6,6ꢀtetramethylpiperidine (TEMP) was added
2
to the mixture of Au/GQDs and H O the signal of TEMP oxide
2
2,
(TEMPO) was detected, and the signal intensity was increased with
the amount of Au/GQDs used. While in all control samples, only a
very weak signal was observed possibly from impurity of the trace
oxidized TEMP. Interestingly, the signal intensities are similar in the
1
samples of different pH, suggesting that O was present in all pH
2
conditions. It is likely that singlet oxygen was converted from other
3
2
active species under basic condition.
Figure 3. Catalytic active species generated by the Au/GQDs
Table 1. The results of VA oxidation at different pH using the
Au/GQDs composites as catalysts for 24 h. Reaction conditions:
composites with H O . Left: EPR spectra of H O +Au/GQDs (5
2
2
2
2
ꢁg/mL) with singlet oxygen trapping agent TEMP (20 mM) at pH 5,
1
5 ꢁg of Au/GQDs, 3 mM of H O , 0.3 mM of VA, 50 °C.
2
2
8
.5 and 11. Controls are H O (5 mM), Au/GQDs (2.5 ꢁg/mL), and
2
2
Product selectivity (%)
Conversion
Au/GQDs (2.5 ꢁg/mL) with H
Right: EPR spectra of Au/GQDs (10 ꢁg/mL)+H O
2 2
2
O
2
in the presence of TEMP at pH 5.
pH
.5
(
%)
with superoxide
Veratraldehyde Veratric acid
anion trapping agent Tiron (20 mM) at pH 11, 8.5, and 5. Controls
are H O (5 mM), Au/GQDs (5 ꢁg/mL), and Au/GQDs (5 ꢁg/mL)
2
34
42
59
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
66
57
51
17
2
ꢀ
ꢀ
ꢀ
2
2
3
with H O in the presence of Tiron at pH 11.
2
2
4
5
6
7
8
9
strong interaction between carbon monoxide and Pt, consequently
facilitates the CO oxidation with lower apparent activation energy
34
43
49
83
98
>99
>99
34
based on density function calculation. Importantly, we found that
Au/GQDs composites are almost inactive to the oxidization of
primary aliphatic alcohols with H O . As shown in Table S3, under
2
2
the same reaction condition, only trace aldehydes were detected with
aliphatic alcohols, including cyclohexane methanol, 4ꢀ
methylcyclohexane methanol, 1ꢀhexanol and 1ꢀpentanol. Differently,
1
0
ꢀ
ꢀ
1
1
Since under basic condition, singlet oxygen inhibitor cannot the aromatic alcohols, such as benzyl alcohol, 4ꢀmethylbenzyl
inhibit the oxidation reaction catalyzed by Au/GQDs, a superoxide alcohol, 4ꢀmethoxybenzyl alcohol, and cinnamyl alcohol were
anion trapping agent, disodium
4,5ꢀdihydroxyꢀ1,3ꢀ oxidized to the corresponding aldehydes or acids with pronounced
benzenedisulfonate (Tiron), was employed to examine the existence conversion rates. Since the molecules with aromatic structural unit
35
of superoxide anion. Strong signal of Tiron oxidation product was interact easily with GQDs through πꢀπ stacking, the large aromatic
detected at pH above 7, but not below 7 (Figure 3b), confirming that basal plane of GQDs in Au/GQDs thus absorbs the aromatic
superoxide anion was the active species under alkali condition. reactant, which then is easily oxidized by the active oxygen species
Interestingly, a weak signal of hydroxyl radical in alkali condition generated in situ by GQDs with H O . The confinement of the
2 2
was also detected (Figure S8). However, the reaction cannot be aromatic reactants by GQDs accelerates their oxidation, in which Au
inhibited by any hydroxyl radical quenchers, such as isopropanol and NPs apparently are involved. This result may be similar to the
36
tertꢀbutanol (Figure S9), suggesting that trace hydroxyl radical we palladium on graphene catalytic system. While for aliphatic
ꢀ
.
observed was possibly converted from O . Especially under basic alcohols, their hydrophobic tail is unfavorable to the interaction with
2
condition, H O decomposes into O very easily, which then reacts GQDs, thus cannot be efficiently oxidized by the active species
2
2
2
ꢀ
.
with GQDs forming O that is more stable in high pH solution, generated by Au/GQDs with H O . The mechanism of the oxidation
2 2
2
3
0
similar to luminolꢀH O with graphene oxide system. The oxygen reaction catalyzed by Au/GQDs was thus summarized in Scheme 1.
2
2
containing radical species involved in the catalytic oxidation reaction Through πꢀπ stacking, VA was absorbed on the Au/GQDs, the active
was also supported by the reaction with oxygen instead of H O , but species singlet oxygen primarily generated by GQDs with H
O
2
2
2
2
the reaction with oxygen was less efficient (Figure S10).
under acidic condition oxidize reactant; the superoxide anion and
trace singlet oxygen generated by both Au
The role of Au NPs apparently is not only a carrier of GQDs,
because the activity of Au/GQDs is much higher than that of GQDs
(
Figure 2 and Table S1). We found that under different ratio of
GQDs to Au NPs, the activities of the composites are different
Figure S11) suggesting that both of them contributed to the activity.
(
Our previous work showed that GQDs could enhance nuclease
activity of the copper complexes through efficient electronꢀtransfer
3
3
from the electronꢀrich GQDs to the copper complexes. There may
also exist interaction between Au NPs and GQDs, which enhances
the activity of GQDs with H O , or increases the lifetime of active
Scheme 1. Schematic illustration of possible mechanism for VA
oxidation with Au/GQDs as catalysts at different pH.
2
2
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2
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2
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