Visible light-driven oxidation of vanillyl alcohol in air with Au-Pd bimetallic nanoparticles on phosphorylated hydrotalcite

Article abstract of DOI:10.1039/C8NJ05477K

The selective photocatalytic oxidation of vanilla alcohol with air was studied preliminarily using supported Au-Pd bimetallic nanocatalysts. The support, phosphate-modified Mg-Al hydrotalcite (HT-PO₄^3?), was produced first with a sol-gel and ion-exchange method, and then Au-Pd alloy nanocatalysts supported on activated carbon (C) and HT-PO₄^3? were prepared, respectively, by an impregnated-reduction approach. Subsequently, the morphology and property of these two catalysts were characterized by XPS, SEM-EDX, ICP-OES, XRD, HR-TEM and DR-UV-vis. In terms of the photocatalytic reaction, Au-Pd@C and Au-Pd@HT-PO₄^3? exhibited photocatalytic activity and the latter displayed higher selectivity. Under optimized experimental conditions, the conversion rate and selectivity using Au-Pd@HT-PO₄^3? as catalyst were 52.8% and 49.6% respectively, due to the synergistically double dehydrogenative oxidation derived from alloy nanoparticles and basic support. Moreover, the catalysts had good recyclability without significant loss of activity after several recycles. Therefore, this catalyst could recurrently realize production of vanillin from vanilla alcohol under mild reaction conditions, including in the atmosphere, at ambient temperature and with no addition of alkali, which is very important for a potential “green” strategy towards clean and cost-efficient production of vanillin.

Full text of DOI:10.1039/C8NJ05477K

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Visible light-driven oxidation of vanillyl alcohol
in air with Au–Pd bimetallic nanoparticles on
phosphorylated hydrotalcite†
ab
Cite this:DOI: 10.1039/c8nj05477k
Miao Wu,
Jin-Hui Pang,c Ping-Ping Song,a Jian-Jun Peng,b Feng Xu,a
Qiang Li *d and Xue-Ming Zhang
*
a
The selective photocatalytic oxidation of vanilla alcohol with air was studied preliminarily using supported
Au–Pd bimetallic nanocatalysts. The support, phosphate-modified Mg–Al hydrotalcite (HT-PO43À), was
produced first with a sol–gel and ion-exchange method, and then Au–Pd alloy nanocatalysts supported
3À
on activated carbon (C) and HT-PO4
were prepared, respectively, by an impregnated-reduction
approach. Subsequently, the morphology and property of these two catalysts were characterized by XPS,
SEM-EDX, ICP-OES, XRD, HR-TEM and DR-UV-vis. In terms of the photocatalytic reaction, Au–Pd@C and
3À
Au–Pd@HT-PO4
exhibited photocatalytic activity and the latter displayed higher selectivity. Under
3À
optimized experimental conditions, the conversion rate and selectivity using Au–Pd@HT-PO4
as
catalyst were 52.8% and 49.6% respectively, due to the synergistically double dehydrogenative oxidation
derived from alloy nanoparticles and basic support. Moreover, the catalysts had good recyclability without
significant loss of activity after several recycles. Therefore, this catalyst could recurrently realize
production of vanillin from vanilla alcohol under mild reaction conditions, including in the atmosphere, at
ambient temperature and with no addition of alkali, which is very important for a potential ‘‘green’’
strategy towards clean and cost-efficient production of vanillin.
Received 29th October 2018,
Accepted 14th December 2018
DOI: 10.1039/c8nj05477k
rsc.li/njc
relative to the complicated lignin models.3 Traditionally, vanillin
is synthesized from guaiacol through glyoxylic and nitrose
Introduction
The selective oxidation of alcohols to their corresponding methods. Although these methods produce vanillin with higher
carbonyls (especially aldehydes) is one of the most widespread selectivity, two main serious drawbacks exist: formation of
and fundamental reactions in organic chemistry.1 In particular, undesirable byproducts such as nitrile and p-aminodimethyl
the conversion of vanillyl alcohol (4-hydroxy-3-methoxybenzylalcohol) aniline in the nitrose method resulting in the lower product
to produce vanillin (4-hydroxy-3-methoxybenzaldehyde) has attracted yield; utilization of toxic oxidants such as CuO, MnO2, and PbO2
immense research interest due to the extensive application of in the glyoxylic method.4 Also, some oxidation reactions must
vanillin as a flavouring agent in food, cosmetic, pharmaceutical, be undertaken under severe conditions, for instance, high
nutraceutical and fine-chemical industries, as well as in the temperature and high oxygen pressure.5 Therefore, it is of great
medicinal field.2 Vanillyl alcohol, as an aromatic alcohol, has practical importance to seek alternative, environmentally benign
also been used frequently as a model compound for the study of approaches to produce vanillin by ‘green’ chemical oxidation of
lignin conversion because it contains linkages that resemble vanillyl alcohol with some highly active, selective and reusable
those found in lignin and possesses inferior analytical challenges catalysts under mild conditions.
Accordingly, heterogeneous photocatalysis is being considered
as an advanced and promising technology for a broad range of
selective chemical transformations because it is clean, effective,
a Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University,
Beijing, 100083, P. R. China. E-mail: xm_zhang@bjfu.edu.cn
b National Engineering Laboratory for Pulp and Paper,
energy-saving, inexpensive, and combines the efficiency of catalysis
with the potential use of light. The common use of the generally
accepted term ‘photocatalysis’ and notable developments in this
field started mainly in the 1970s after the discovery of water
photolysis on a TiO2 electrode by Fujishima and Honda.6 To make
efficient use of light energy, the light used for irradiation in selective
photocatalytic reactions must be transferred from ultraviolet (UV)
China National Pulp and Paper Research Institute, Beijing, 100102, P. R. China
c College of Marine Science and Biological Engineering,
Qingdao University of Science and Technology, Qingdao, 266042, P. R. China
d School of Science, Beijing Forestry University, Beijing, 100083, P. R. China.
E-mail: liqiang@bjfu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
Fig. S1–S3. See DOI: 10.1039/c8nj05477k
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light to visible light. Hence, some researchers have employed a support, by combining the advantages of basic sites of the support
visible light-driven photocatalysis system through utilization of material and the photocatalytic properties of the metal nano-
metallic nanoparticles (including monometallic and bimetallic particles, show great potential in the oxidation of aromatic alcohols
compounds). Herein, bimetallic nanoparticles have drawn at moderate temperatures without the addition of alkali.
increasing attention owing to their enhanced activity, selectivity
In the present study, the green chemical oxidation of vanillyl
and stability in comparison with respective monometallic alcohol driven by visible light with Au–Pd bimetallic nano-
components. This difference can be attributed to not only a particles on a phosphorylated hydrotalcite support using air
3À
combination of the properties of two individual metals but also as the oxidant was studied. First, HT-PO4 was obtained by a
new properties and capabilities because of a synergistic effect sol–gel and impregnation process as a support, and then Au–Pd
3À
between two metals.7 Verma et al. produced a photoactive alloy nanoparticles were loaded onto the HT-PO4 support
bimetallic catalyst, AgPd@g-C3N4, which was efficient for various material. Meanwhile, activated carbon-supported Au–Pd was also
reactions under visible light: conversion of 4-hydroxy-3-methoxy- prepared for comparison through the same method. Subsequently,
benzaldehyde (vanillin) to 2-methoxy-4-methylphenol; generation both catalysts were characterized by X-ray photoelectron spectro-
of g-valerolactone from the biomass-derived levulinic acid; scopy (XPS), scanning electron microscopy with energy dispersive
concerted reductive formulation of aromatic nitro compounds.8–11 X-ray spectroscopy (SEM-EDX), inductively coupled plasma optical
Moreover, other bimetallic catalysts, such as CuAg@g-C3N4, have emission spectroscopy (ICP-OES), X-ray diffraction (XRD) and high-
been shown to be effective for the hydroxylation of benzene via C–H resolution transmission electron microscopy (HR-TEM). Finally,
activation.12 Recently, Au–Pd bimetallic nanoparticles have been the the catalytic performance was evaluated by selective oxidation of
subject of intense research on account of their superior catalytic vanillyl alcohol to vanillin in an atmosphere under neutral and
performance in various reactions, such as hydrogenation, oxidation benign reaction conditions using visible-light irradiation
and activation of the C–Cl bond.13–15 In particular, it has been (l, 420–780 nm). These results revealed a potential route toward
shown by several research groups that this catalyst exhibits a green commercial process for clean and efficient production
remarkably enhanced catalytic activity and product selectivity of vanillin.
in alcohol oxidation reactions under harsh reaction conditions.
For example, Villa et al. discovered that, under biphasic conditions,
benzyl alcohol is oxidized selectively to benzaldehyde with O2 as an
oxidant in the presence of NaOH using activated carbon-supported
Results and discussion
Au–Pd as the catalyst.16 It was reported by Enache and co-workers The phosphate-modified HT support (HT-PO43À) was prepared
that TiO2-supported Au–Pd alloy nanocrystals displayed significantly through the ‘‘memory effect’’ of HT, in which the layered double-
enhanced activity for alcohol oxidation at 373 K using a green hydroxide structure of HT was restored, whereas the phosphate
chemistry approach with O2 as an oxidant under solvent-free anion was between the layers, as reported previously.18 The
conditions.17
modified HT (HT-PO43À) and activated carbon (C) were used
Supported metal nanoparticles are attracting considerable separately as a support for the Au–Pd alloy catalyst.
attention because the catalyst support influences the catalysis
First, XPS detection was undertaken on Au–Pd@C and
3À
effect greatly. To improve the catalysis efficiency of noble metal Au–Pd@HT-PO4 catalysts to characterize the valence states
nanoparticles, many materials have been utilized as catalyst of Au and Pd. As shown in Fig. 1a, the XPS spectra of Au in
supports, among which activated carbon material is a typical Au–Pd@C showed that the two typical peaks at B84.0 and
and extensively studied support. However, in a recent study,18
a
B87.9 eV were attributed to Au 4f7/2 and Au 4f5/2, respectively,
unique acid-buffering support material, phosphorylated hydrotalcite indicating that the Au element was the one with metallic zero
(HT-PO43À), was prepared and shown to be effective in direct self- valence.19 Similarly, these two signals of zero-valence Au were
esterification of alcohols under mild and base-free conditions. also present in the signals of Au–Pd@HT-PO43À. In addition,
3À
Therefore, the Au–Pd alloy nanoparticles loaded onto the HT-PO4
the binding energies of Pd 3d5/2 and Pd 3d3/2 peaks were
3À
3À
Fig. 1 (a) XPS spectra of Au 4f in Au–Pd@C and Au–Pd@HT-PO4 catalysts. (b) XPS spectra of Pd 3d in Au–Pd@C and Au–Pd@HT-PO4 catalysts.
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Table 1 Specific percentages of elements in the catalysts detected by
ICP-OES
of these two supported Au–Pd catalysts were undertaken. As
shown in Fig. S1 (ESI†), the elements of Au and Pd were both
detected in the Au–Pd@C and Au–Pd@HT-PO43À catalysts and,
Catalyst (wt%)
Mg
Al
P
Au
Pd
3À
meanwhile, Mg, Al and P in Au–Pd@HT-PO4
were also
Au–Pd@C
Au–Pd@HT-PO4
NDa
22.52
ND
8.33
ND
ND
1.03
1.06
1.17
1.00
3À
observed, as expected. In particular, a clear signal of P in the
mapping image demonstrated that the doped phosphate anion
was distributed homogeneously in the HT sample. In addition,
the weight percentages of all elements in the catalysts were
a
ND, not detected.
detected, respectively, in the spectra of Au–Pd@C (341.0 and obtained as listed in Table S1 (ESI†). Simultaneously, the specific
3À
335.7 eV) and Au–Pd@HT-PO4 (340.5 and 335.1 eV) (Fig. 1b), contents of elements were further detected by ICP-OES (Table 1).
which also suggested the zero-valence state of Pd.20 Hence, Au and Both data indicated that the ratios of Au/Pd were approximately
Pd were both reduced in catalysts through the preparation process 1/1 in these two catalysts, and an Mg/Al ratio of 3/1 in Au–Pd@
3À
in this work. Certainly, ions of Au and Pd were also observed in HT-PO4 was also measured, results that were consistent with
XPS spectra, especially in that of Au–Pd@HT-PO43À; a high-energy the initial experimental design.
band at 89.0 eV in the XPS spectra of Au and an obvious peak at
The structural characteristics of the prepared catalysts was
3À
337.7 eV in the XPS spectra of Pd were assigned separately to the identified by XRD patterns. In the spectra of Au–Pd@HT-PO4
remaining metal ions.21 Moreover, by comparison with data (Fig. 2a), all diffraction peaks could be ascribed to the layered
reported previously,22 the data of Au 4f7/2 and 4f5/2 peaks of these HT structure, which remained unchanged after introducing
two Au–Pd bimetallic catalysts shifted apparently to the higher side phosphate anions and loading metal nanoparticles, as reported
in binding energy. Accordingly, the Pd 3d5/2 and 3d3/2 peaks moved previously.18 However, obvious signals that could be assignable
towards the lower energy compared with the corresponding to Au and Pd were not detected in XRD patterns, probably due
monometallic catalysts. These data revealed that electronics to the overlap of metal and HT peaks or the poor crystallinity of
were transferred from Au to Pd atoms by alloying interactions. the metal nanoparticles on the surface of HT.18 Simultaneously,
To determine the elemental composition in the prepared the reflection peak of activated carbon was observed in the spectra
photocatalysts, SEM combined with EDX elemental mappings of Au–Pd@C. Moreover, the signal attributed to the Au–Pd alloy
Fig. 2 (a) XRD patterns of catalysts. (b) HR-TEM images of Au–Pd@C. (c) HR-TEM images of Au–Pd@HT-PO43À. (d) DR-UV/vis extinction spectra of
catalysts.
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structure was discovered, and was located between that of pure condition (Fig. S3, ESI†). The quantitative results under this
Au (JCPDS-04-0784) and Pd (JCPDS-46-1043), strongly indicating light-radiation condition are shown in Fig. 3a. The conversion
that Au and Pd were synthesized in the form of an alloy in these of vanilla alcohol was 14.2% and the selectivity of vanillin
catalysts prepared by this method.23
catalyzed by Au–Pd@C was 47.0%, whereas the values for those
The specific morphologic properties of alloy nanoparticles catalyzed by Au–Pd@HT-PO43À were 12.8% and 57.8%, respectively.
in catalysts were characterized further using HR-TEM. Au–Pd Although the conversion rates of these two catalysts were
3À
alloy nanoparticles were distributed evenly on the support C comparative, the selectivity of Au–Pd@HT-PO4
was better
and HT surface (Fig. 2b and c), and the average diameter of the than that of Au–Pd@C. The difference was ascribed to the Lewis
nanoparticles in these two catalysts were both approximately basic sites in the HT-PO43À support, which could facilitate O–H
3–5 nm. Furthermore, the atom lattices of the Au–Pd alloy were cleavage in the reactant.18,25 However, compared with strong
revealed in HR-TEM images. Lattice fringe spacings of 0.21 and alkali, the relatively mild basic sites would not arouse the
0.19 nm in Au–Pd@C, respectively, corresponded to an inter- further reaction, thereby realizing preferable selective oxidation.
planar distance of (111) and (200) facets planes in the Au–Pd
The optimum reaction conditions, such as the catalyst dose,
alloy lattice. Similarly, (111) and (200) lattice planes were intensity of light radiation, and reactant concentration could be
3À
present in the HR-TEM image of Au–Pd@HT-PO4
.
acquired by monitoring the reaction system with high-performance
To understand preliminarily the photocatalytic properties liquid chromatography (HPLC). By comparing the quantitative
of catalysts, diffuse reflectance ultraviolet visible (DR-UV-vis) results of the product under different conditions, a favorable routine
absorption of samples was detected first. It has been reported was obtained (Fig. 3b–d). The influence of the catalyst dose on
3À
that HT-PO4 shows very weak absorption in the visible-light conversion and selectivity is shown in Fig. 3b. The conversion
region of 4400 nm, which means that this support itself has no rate increased gradually with an increase in catalyst dose because
photocatalytic activity.18 Furthermore, the absorption spectra more active sites were offered for catalysis, whereas the selectivity
of metal alloys and single metals differ on account of distinct remained almost unchanged.
dielectric constants.18 The local surface plasmon resonance
Fig. 3c shows the effects of light intensity on conversion and
absorption peak of pure Au nanoparticles was at 520 nm, and selectivity. With an increase in radiation intensity, the conversion
the absorption peak of pure Pd nanoparticles was mainly at rate increased dramatically because more energetic electrons
300 nm (UV range), which could have occurred due to light triggered by light were generated under stronger radiation, leading
absorption at solar wavelengths through light excitation of to the formation of a stronger electromagnetic field around the
electrons to higher energy levels.24 Therefore, the light absorption nanoparticles.18 However, the selectivity decreased gradually with
of each of the Au–Pd alloy catalysts in UV and visible-light regions increasing light intensity, indicating that more byproducts were
were similar (Fig. 2d). Hence, this alloy catalyst was considered to generated under stronger radiation. When the light intensity was
have a better ability to improve the catalytic performance under 0.55 W cmÀ2, the conversion reached a maximum rate, whereas
the whole region of visible-light irradiation than that of pure the selectivity levelled off.
metal particles. There was an abrupt drop from UV to visible
The relationships between the concentration of reactant and
region in the absorption spectra of Au–Pd@C, indicating that the conversion and selectivity are depicted in Fig. 3d. The reactant
catalytic effect of this catalyst under visible light was slightly concentration was adjusted by changing the solvent volume.
weaker than that under UV light, whereas this phenomenon did With a decrease in the reactant concentration, the conversion
not appear in Au–Pd@HT-PO43À. The whole absorption strength rate lowered gradually, which was possibly due to the deactivation
of Au–Pd@C was higher than that of Au–Pd@HT-PO43À, which of the catalyst. Nevertheless, selectivity increased rapidly at first,
was probably due to the dark absorption of active carbon.
and reached 75.6% when the solvent volume was 5.0 mL, but
Based on the results mentioned above, Au–Pd@C was used, began to decline subsequently with increasing volume of solvent
respectively, in the reaction under dark and visible radiation to from 5.0 mL to 6.0 mL. As reported previously,16 the reactant
evaluate its photocatalytic activity. The reaction container was concentration had a significant impact on selectivity. In the
wrapped with aluminum foil to ensure light avoidance in the present study, with water generation during the reaction, the
dark reaction, whereas the light intensity was 0.3 W cmÀ2 in the accessibility of vanillyl alcohol on the active sites of the catalyst
visible-light reaction. The reaction mixtures under dark and would be limited because it exhibited poor solubility in water,
light conditions were monitored with gas chromatography-mass thereby resulting in inhibition of its further oxidation.
spectrometry (GC-MS) (Fig. S2, ESI†). Obviously, only two strong
Therefore, the preferable Au–Pd@HT-PO43À catalyst was applied
peaks towered, which indicated that the reactant (vanilla alcohol) in the reaction with the optimized conditions mentioned above, and
and product (vanillin) were present in small amounts with other the results of conversion and selectivity were 52.8% and 49.6%,
byproducts. Moreover, the peak height of vanillin obtained under respectively (Fig. 4a). This moderate conversion rate might have
the light condition was significantly higher than that under the been due to the relatively low percentage of oxygen in air.
dark condition, suggesting that this catalyst indeed exhibited Simultaneously, a control experiment in the dark was also
photocatalytic activity. Simultaneously, the same reaction was carried out, the corresponding data were only 8.7% and 2.6%,
carried out with Au–Pd@HT-PO43À, and similar results were respectively, demonstrating that the conversion and selectivity
acquired. That is, the main product was vanillin, and more were enhanced considerably under visible radiation. This huge
product was produced under radiation than under the dark difference could be attributed to the synergistic effect of alloy
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3À
Fig. 3 (a) Quantitative results showing catalysis by Au–Pd@C and Au–Pd@HT-PO4 under the same light condition. (b) Effect of catalyst dose on
conversion and selectivity using Au–Pd@C as catalyst. (c) Effect of light intensity on conversion and selectivity using Au–Pd@C as catalyst. (d) Effect of
reactant concentration on conversion and selectivity using Au–Pd@C as catalyst.
Fig. 4 (a) Quantitative results of catalysis by Au–Pd@HT-PO43À under dark and optimized light conditions (catalyst dose, 100 mg; light intensity, 0.55 W cmÀ2
;
3À
solvent volume, 5.0 mL). (b) Recycling performance of Au–Pd@HT-PO4 in the oxidation of vanilla alcohol.
nanoparticles and the support, in which the Au–Pd alloy and
HT-PO4 were catalyzed separately under visible light.
Based on the literature,26,27 the photocatalytic mechanism
of oxidation of vanilla alcohol to vanillin under visible irradiation
3À
The recycling performance (a key attraction of heterogeneous using Au–Pd@HT-PO43À as catalyst could be proposed (Fig. 5). It
catalysis) of Au–Pd@HT-PO43À in the selective oxidation reaction of has been reported that the electronic property on the surface of
vanilla alcohol was assessed. Briefly, after each reaction cycle, the bimetallic alloy nanoparticles is the synergistic effect of two
used catalyst was centrifuged, washed thoroughly with ethanol, and metals, unlike a single metal.27 The electronegativity of Pd was
dried for the subsequent reaction. Conversion and selectivity 2.20, lower than that of Au (2.54), so the charge in heterogeneity
displayed almost no reduction after the second cycle (Fig. 4b) appeared on the Au–Pd alloy surface, which could increase
and, in addition, after three cycles of regeneration, selectivity could the interaction between alloy nanoparticles and electricity or
reach 489%. These results indicated that Au–Pd@HT-PO43À catalyst nucleophilic reactants. Moreover, once the alloy had been
possessed good recyclability without significant loss of activity.
formed, electrons would flow among metals to equilibrate the
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3À
Fig. 5 Reaction mechanism of Au–Pd@HT-PO4 (schematic).
electrochemical potential, and energetic electrons could be high-purity molecular oxygen was not required. This catalytic
generated under light radiation, focused mainly on the Pd process could provide a promising method for the green and
atoms of the alloy. Therefore, Pd rich in energetic electrons cost-effective production of vanillin from vanillyl alcohol, even
would interact with the reactant, and then the target product from other more complex aromatic alcohols.
aldehyde was obtained via a step-by-step dehydrogenation from
3À
the CH2 group in the reactant in the presence of Au–Pd@HT-PO4
Experimental
Preparation of supports of phosphate-modified Mg–Al HT
(HT-PO43À) and activated carbon (C)
catalyst, as well as the formation of alloy-H, which would finally go
back into the original catalyst by dehydrogenation. In addition,
besides the catalyzed effect of active Pd, the base site on the
surface of the HT-PO43À support could also combine with hydrogen
in the reactant, thereby promoting the cleavage of O–H.18 There-
fore, the synergistically double dehydrogenative oxidation (SDDO)
derived from alloy particles and base support was achieved and
resulted in high efficiency oxidation of vanillyl alcohol.
Phosphate-modified Mg–Al HT was prepared according to a
sol–gel and ion-exchange process reported previously.18 Briefly,
as shown in Fig. S4 (ESI†), Mg(NO3)2Á6H2O and Al(NO3)3Á9H2O
with a Mg/Al ratio of 3 were first dissolved in deionized water to
form an acidic aqueous solution. Correspondingly, NaOH and
Na2CO3 were dissolved in deionized water to form another alkaline
solution. Both of these solutions were heated to 75 1C, and then
added dropwise into deionized water at 75 1C simultaneously to
Conclusions
In the present study, a reusable catalyst of Au–Pd alloy supported obtain a precipitation identified as ‘‘HT’’. The suspension was
3À
on HT-PO4 was found to be active for direct oxidation of aged for 3 h at 85 1C under vigorous stirring, after which it was
aromatic alcohol driven by visible-light irradiation using air as filtered while being cooled to room temperature, and treated
a benign and inexpensive oxidant. HT-PO43À was obtained by a further hydrothermally in an autoclave at 80 1C for 16 h. The
sol–gel and impregnation process as a support, and then Au–Pd acquired gel was washed with deionized water until the lotion
3À
alloy nanoparticles were loaded onto the HT-PO4 support became neutral and was dried in an oven overnight at 80 1C. The
material. The catalytic performance was evaluated by selective resultant white powder was calcinated at 450 1C for 8 h in a flow of
oxidation of vanillyl alcohol to vanillin. When using Au–Pd@ dry air to produce mixed oxides of magnesium and aluminum.
HT-PO43À as the catalyst, conversion of 52.8% and selectivity of The calcined HT was dispersed into an aqueous solution of
49.6% were achieved in the atmosphere at ambient temperature Na3PO4 and stirred at room temperature for 12 h. Then, the
and under a neutral reaction environment with visible-light irra- obtained solid was washed and dried at 110 1C for 10 h, which
diation. This catalytic process was caused by the interaction of was denoted as ‘‘HT-PO43À’’ and served as the support material.
light-excited electrons from the catalyst reacting with substrate Another support, activated carbon (C), was purchased directly from
molecules, and high temperatures and pressures, and even a chemical reagent company.
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Preparation of supported Au–Pd bimetallic catalysts
(Au–Pd@HT-PO4 and Au–Pd@C)
HPLC using a 1260 system (Agilent) equipped with a C18 column
and UV detector, of which the detection wavelength was 280 nm,
and the mobile phase was acetonitrile and ammonium formate.
Calibration curves were obtained by injecting standard solutions of
vanillyl alcohol and vanillin. The concentrations of alcohol and
aldehyde during the reaction process were determined from
the corresponding peak areas detected by HPLC according to
calculation of a standard equation. Ultimately, percentages of
conversion and selectivity were calculated using the following
formulae:
3À
A supported Au–Pd bimetallic catalyst was prepared by an
impregnation-reduction method (Fig. S5, ESI†). Briefly, 2.0 g of
HT-PO43À powder, HAuCl4 (15.2 mL, 0.01 M) and PdCl2 (28.3 mL,
0.01 M) aqueous solutions were added together under magnetic
stirring at room temperature. Subsequently, lysine (16.0 mL,
0.53 M) aqueous solution was also added, and the suspension
was stirred vigorously for 30 min, the pH value of which was
measured to be 8–9. Then, a freshly prepared NaBH4 (3.0 mL,
0.35 M) aqueous solution was added dropwise over 20 min. After
standing for 24 h, the solid was separated by centrifugation,
washed with deionized water and ethanol, and finally dried at
Conversion = (Cr0 À Cr)/Cr0 Â 100%
Selectivity = Cp/(Cr0 – Cr) Â 100%
(1)
(2)
60 1C in a vacuum oven for 24 h. The dried sample could serve
Cr0 = the initial concentration of the reactant (vanillyl alcohol),
Cr = the concentration of the reactant (vanillyl alcohol) after the
reaction, Cp = the concentration of the product (vanillin) after
the reaction.
3À
directly as the photocatalyst Au–Pd@HT-PO4
. Similarly,
Au–Pd@C catalyst was prepared by the same method using
activated carbon as the support.
Characterization
Conflicts of interest
The surface composition and valence state of the prepared
catalysts were detected by the XPS (Thermo Fisher, ESCALAB
250Xi) using a system equipped with a microfocusing Al
monochromatic X-ray. SEM, elemental mapping, and EDX were
carried out using a Hitachi SU8010 SEM system at an accelerating
voltage of 30 kV. The specific contents of elements were determined
through ICP-OES. XRD patterns were obtained with a Bruker D8
ADVANCE instrument using a Cu Ka source operating at 40 keV at a
scan rate of 0.0251 sÀ1. Diffraction data were collected from 51 to 751
at a resolution of 0.011 (2y). The particle size and lattice parameter
of the catalyst samples were characterized by HR-TEM using a JEOL
JEM 2100 system equipped with a LaB6 filament operating at
200 kV. The diffuse reflectance UV-visible absorption of samples
was done with a Hitachi U-3010 spectrometer.
There are no conflicts of interest to declare.
Acknowledgements
We are grateful for the financial support of this research from the
Natural Science Foundation of China (31470606 and 21671021).
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Photocatalytic reactions
The photocatalytic reaction was undertaken in a photochemical
reaction instrument with a 10.0 mL quartz glass tube (j, 10 mm)
as the reaction container. After adding the reactant, solvent and
catalyst, the tube was agitated with a magnetic stirrer at room
temperature at a stirring speed of 800 rpm and exposed
to visible-light irradiation emitted by a 300 W Xe arc lamp
(CEL-HXUV 300, Beijing CEAuLight Technology) with a UV-CUT
filter to cut-off light of wavelength o420 nm. The initial experi-
mental conditions were that 0.2 mmol vanilla alcohol was
dissolved in 3.0 mL dioxane solvent, then 50 mg Au–Pd@C
catalyst was added, and the reaction was maintained at room
temperature and in air for 24 h. The reaction condition in
9 R. B. N. Baig, S. Verma, M. N. Nadagouda and R. S. Varma,
Green Chem., 2016, 18(4), 1019–1022.
the dark was maintained the same as that under irradiation 10 K. Tadele, S. Verma, M. A. Gonzalez and R. S. Varma, Green
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exposure of the reaction to light, the tube was wrapped with 11 S. Verma, R. B. N. Baig, M. N. Nadagouda and R. S. Varma,
aluminum foil. After the reaction, 1 mL aliquots were collected ChemCatChem, 2016, 8(4), 690–693.
and passed through a Millipore filter of pore size 0.45 mm to 12 S. Verma, R. B. N. Baig, M. N. Nadagouda and R. S. Varma,
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tion of products was first done by GC-MS (Agilent 6890-5973) with 13 Y. Tuo, G. Liu, B. Dong, J. Zhou, A. Wang, W. Jing, R. Jin,
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