Gold nanoparticles supported on Ce-Zr oxides for the oxidative esterification of aldehydes to esters

Article abstract of DOI:10.1039/c5cy00241a

Au nanoparticles supported on Ce-Zr oxides were prepared and characterized in order to study the role of the support in the oxidative esterification of aldehydes in the presence of molecular oxygen. Ce-Zr solid solutions were synthesized by using (NH₄)₂Ce(NO₃)₆ as a precursor, while the mixed oxides were obtained by using a Ce(NO₃)₃ precursor. The solid solutions exhibited a smaller crystallite size, higher BET surface area, larger amount of H₂ consumption, and higher acidity and basicity than the mixed oxides at the same Ce/Zr molar ratio due to the incorporation of Zr⁴⁺ into the ceria lattice. The effect of the support was investigated because all the samples presented similar Au particle sizes, as confirmed by the HAADF-STEM study. Supports with higher reducibility showed better performance by activating methanol to methoxy and facilitating the β-H elimination of hemiacetal. We also found that the formation of hemiacetal was enhanced by the acidic sites and basic sites of Au catalysts supported on solid solutions possessing similar reducibility. A plausible reaction mechanism for the oxidative esterification of aldehydes on Ce-Zr solid solution-supported Au nanoparticles was proposed. The screened catalyst was also applicable to the oxidative esterification of different benzylic aldehydes, producing high yields. This catalyst could be reused after a simple separation eight times, keeping a high selectivity of above 99%.

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PAPER
Gold nanoparticles supported on Ce-Zr oxides for the oxidative
esterification of aldehydes to esters
Yuchao Li,a,b Lei Wang,a Ruiyi Yan,a Junxing Hana and Suojiang Zhang*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Au nanoparticles supported on CeꢀZr oxides catalysts were prepared and characterized in order to study
the role of support for the oxidative esterification of aldehydes in the presence of molecular oxygen. Ceꢀ
Zr solid solutions were synthesized by using (NH4)2Ce(NO3)6 as precursor, while the mixed oxides were
obtained by Ce(NO3)3 precursor. The solid solutions exhibited smaller crystallite size, higher BET surface
10 area, and larger amount of H2 consumption, acidity and basicity than the mixed oxides at the same Ce/Zr
mole ratio due to the incorporation of Zr4+ into ceria lattice. The effect of the support was investigated
owing to all samples presenting the similar Au particle size confirmed by HAADFꢀSTEM study. Supports
with higher reducibility showed better performances by activation of methanol to methoxy and facilitating
the βꢀH elimination of hemiacetal. We also found that the formation of hemiacetal was enhanced by
15 acidic sites and basic sites of Au catalysts supported on solid solutions possessing similar reducibility.
Plausible reaction mechanism for oxidative esterification of aldehydes on CeꢀZr solid solution supported
Au nanoparticles was proposed. The screening catalyst was also applicable to the oxidative esterification
of different benzyl aldehydes with high yields. The catalyst could be reused after a simple separation for
eight times keeping high selectivity above 99%.
45 particles and activation of reactants, 17ꢀ21 such as O2.
In the recent years, CeꢀZr mixed oxides have been widely used
as supports for Au in many oxidation reactions.22ꢀ24 CeꢀZr mixed
oxides were selected because of their unique redox properties and
high capacity of lattice oxygen, and these properties strongly
50 depend on the Ce/Zr molar ratio. In addition to these properties,
CeꢀZr materials present remarkable acid–base properties, which
have not been extensively investigated. The acidꢀbase nature of
the supports has a great impact on the performance of the
20 1. Introduction
Esterification, one of the most fundamental reactions in organic
synthesis, is extensively used in laboratories and industries.1, 2
Contrary to the traditional twoꢀstep esterification procedure that
includes the synthesis of carboxylic acids or activated carboxylic
25 acid derivatives, such as acid anhydrides or acid chlorides,3, 4 the
direct oxidative esterification of aldehydes with alcohols has
great economic and environmental benefits in the synthesis of
esters. Although several facile and selective esterification
methods have been developed, the direct oxidative esterification
30 of aldehydes with alcohols using the ultimate oxidation agent,
namely, molecular oxygen, by heterogeneous catalysts under
mild conditions is an attractive topic for both organic synthesis
and green chemistry.5,6
oxidation esterification of aldehydes, according to the literature.8,
11
55
To date, Au supported on CeꢀZr mixed oxides for oxidative
esterification of aldehydes have not been reported.
This paper reports the relation between the physicꢀchemical
properties of the CeꢀZr mixed oxides and the catalytic
performance of Au/Ce1ꢀxZrxO2 in the oxidative esterification of
60 aldehydes with methanol. Au/Ce1ꢀxZrxO2 (x = 0, 0.2, 0.4, 0.6, 0.8,
1) oxides and Au/Ce1ꢀxZrxO2 catalysts were prepared,
characterized, and tested in aerobic oxidative esterification of
aldehydes. The effect of the physicꢀchemical of the support has
been examined according to their influence on the oxidation
65 esterification reaction. The substrate scope and the cycle times of
the screening catalyst have also been investigated.
7
Since Haruta et al. reported the aerobic oxidation of carbon
35 monoxide using highly dispersed Au nanoparticles supported on
reducible and active supports have drawn much attention because
of their high activity in aerobic oxidation reactions. In particular,
several solid catalysts based on transition metal oxideꢀsupported
Au nanoparticles for oxidative esterification of aldehydes have
40 recently been reported.6,
The performance of the Au
8ꢀ11
nanoparticleꢀsupported metal oxides depends on various factors
covering the size and shape of the Au nanoparticles, the
electronic state and preparation procedures.12ꢀ16 In addition, the
support also plays an important role in the dispersion of the Au
2. Experimental
2.1. Catalyst preparation
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A series of CeꢀZr mixed oxides, CeO2 and ZrO2 were prepared
for 2 h. Then the sample was cooled to 50 °C, and subsequently
via the hydrothermal method. All supports were designed to 60 the test was performed with a 10% H2/Ar gas mixture (20
obtain Ce1ꢀxZrxO2 solid solution, where x was the atomic ratio of
Zr/(Ce+Zr) and varied at 0, 0.2, 0.4, 0.8, and 1.0. Two Ce
precursors, Ce(NO3)3 and (NH4)2Ce(NO3)6 , were used to
synthesize the CeꢀZr mixed oxides according to previously
mL/min) from room temperature to 900 °C (20 °C /min). The
temperatureꢀprogrammed desorption of ammonia and carbon
dioxide (NH3ꢀ and CO2ꢀTPD) experiments were carried out on
Autochem II 2920 (Micromeritics, USA). The samples were
5
reported methods25,26 with slight modification. The oxides are 65 pretreated under He flow (20 mL/min) at 200 °C for 2 h. The
referred to as AꢀCe1ꢀxZrxO2 (Ce(NO3)3 as precursor) and BꢀCe1ꢀ
xZrxO2 ((NH4)2Ce(NO3)6 as precursor) respectively.
The typical preparation procedure was described below. Ce
precursor (>99.0%, AR, Sinopharm Chemical Reagent Co. Ltd),
sample was cooled to 50 °C, and then exposed to 10% NH3/He
mixture or 10% CO2/He mixture flow (20 mL/min) for 1 h. Then,
the sample was flushed under He flow at 50 °C to remove
physically adsorbed CO2 or NH3. A CO2ꢀTPD (or NH3ꢀTPD)
10
ZrOCl2 (AR, Beijing Chem. Corp.) and urea (>99.0%, AR, 70 profile of the sample was recorded by increasing the temperature
Beijing Chem. Corp.) in the molar ratio of Cem+: ZrO2+: urea =
(1–x): x : 10 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were dissolved in
from 50 °C to 800 °C at a heating rate of 10 °C/min under 20
mL/min of He flow.
15 80 mL of deionized water in a Teflonꢀlined stainless steel
autoclave (inner volume: 100 mL). The total cation concentration
at 0.1 mol·L−1 was maintained. The autoclave was tightly sealed
and placed in an oven for hydrothermal treatment at 413 K for 24
h. The white and yellow precipitate was obtained respectively,
20 and then it was centrifugally separated, washed several times with
deionized water, and subsequently dried at 353 K in air overnight.
The asꢀprepared Ce1ꢀxZrxO2 oxides were calcined in still air at
773 K for 4 h and yellow powders were obtained.
The Au nanoparticles were deposited on metal oxides by
25 deposition–precipitation method. The pH level of an aqueous
solution of HAuCl4 (10ꢀ4 mol•Lꢀ1) was adjusted to pH 10 by
adding saturated Na2CO3 solution. Then, the solution was mixed
with corresponding support to fix the Auꢀsupported 1.5% and the
pH value was maintained at 10 by saturated Na2CO3 solution.
30 After stirring at 313 K for 3 h, the suspension was filtered and
washed with an excess amount of deionized water until no more
chlorine remained. The supported Au catalyst was dried in a
vacuum oven at 313 K overnight and then calcined at 250 °C for
4 h in air.
2.3. Catalytic reaction
The oxidative esterification of isobutyraldehyde and other
75 aldehydes with methanol to methyl esters was performed in a 50
mL steel autoclave. The substrate (50 mmol) and methanol (40
mL) were added into the reactor preꢀcharged with catalyst
(typically, 0.50 g) and K2CO3 (typically, 0.10 g). After O2 charge
with 0.3 MPa pressure, the mixture was heated to 343 K reaction
80 temperature, and then the catalytic reaction was started by
vigorous stirring. After a desired time (typically 2 h), stirring and
introducing oxygen were immediately stopped. The reactor was
quickly cooled down to room temperature. Next, the excess
oxygen was depressurized slowly. The products were analyzed
85 using a gas chromatograph equipped with an FID detector and a
capillary column (DBꢀ624, 30 m × 0.32 mm × 0.25 ꢁm) using
ethanol as an internal standard for quantification. Benzyl
aldehydes and products were quantified using heptane as an
internal standard.
90 3. Results and discussion
35 2.2. Catalyst characterization
3.1. Textural characterization of Ce1-xZrxO2 supports and
Au/Ce1-xZrxO2 catalysts
Transmission electron microscopy (TEM) and scanning
transmission electron microscopy (STEM) measurements were
performed using JEOL TEM 2011 operated at 200 kV. Samples
were deposited on the TEM grids after ultrasonic dispersion in
40 ethanol. Scanning electron microscopy (SEM) measurements
were performed by Sꢀ4700 microscope (Hitachi, Japan) using a
20 kV acceleration voltage.
Table 1 shows the textural properties of the two series of catalysts
prepared by AꢀCe1ꢀxZrxO2 and BꢀCe1ꢀxZrxO2. BET data indicates
95 that the catalysts supported on BꢀCe1ꢀxZrxO2 possess bigger
surface area in the range of 77.9ꢀ182.2 m2 gꢀ1 than those
supported on AꢀCe1ꢀxZrxO2 (50.1ꢀ105.4 m2 gꢀ1) at the same Ce/Zr
molar ratio. The specific surface areas of the two series of
catalysts presented almost the same change law, among which
100 Au/AꢀCe0.4Zr0.6O2 and Au/BꢀCe0.6Zr0.4O2 possessed the highest
surface area and the pure oxide ZrO2 had the lowest surface area.
The crystallite size of supports was calculated using Scherrer
equation, and the results are shown in Table 1. The mixed oxides
AꢀCe1ꢀxZrxO2 have larger crystallite sizes than BꢀCe1ꢀxZrxO2 at
105 the same Ce/Zr molar ratio. This characteristic resulted in the
larger surface area of series B than that of series of A. For
supports B, the crystallite size were small when the Zr content
was near to 50 mol %, and the sample BꢀCe0.6Zr0.4O2 presented
the highest BET surface and smallest crystallite size of 3.5 nm.
110 The lack of a clear trend for the crystallite size for supports A
with Zr content could be attributed to the fact that each
composition presents a mixture of crystalline structures, which
are indicated by the XRD patterns.
Xꢀray diffraction (XRD) measurements of the catalysts were
performed on
a Rigaku RINT 2500 diffractometer using
45 monochromated Cu Kα radiation with a scan speed of 15° min−1
and a scan range of 20–80° at 30 kV and 15 mA.
Nitrogen adsorption/desorption isotherms at 77
K were
measured using Sorptomatic 1900 apparatus (CarloꢀErba). Prior
to the measurement, all samples were degassed for 4 h at 573 K.
50 The specific surface area, SBET was calculated using BET
equation.
The Au lading was determined by inductive couple plasma
optical emission spectrometer (ICPꢀOES) by IRIS Intrepid II XSP
(Thermofisher, USA).
55
Hydrogen temperature programmed reduction (H2ꢀTPR) was
conducted on Autochem II 2920 (Micromeritics, USA). A total of
50 mg catalyst was filled in a quartz reactor. The catalyst
powders were pretreated under Ar flow (20 mL/min) at 200 ℃
The XRD patterns of Ce1ꢀxZrxO2 oxides are shown in Figure 1.
2
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The XRD patterns of AꢀCe1ꢀxZrxO2 correspond to two phases, the
cubic phase of CeO2 and tetragonal phase of ZrO2, indicating that
the mixed oxides have their corresponding independent structure.
Compared with pure oxides, the intense lines observed for the
mixed oxides of B became broader, which can be contributed to
the lattice distortion of cubic phase for CeO2 caused by the
The peak for (220) reflection of CeO2 decreases and the peak for 15 incorporation of ZrO2. The main diffraction peaks significantly
5
(220) reflection of ZrO2 increases with the increase in Zr content.
At the same time, the characteristic diffraction peaks shifted
slightly toward high 2θ values, reflecting that the mixed oxides
presented little solid solution structure with some Zr doped into
shift to higher diffraction angles with an increasing amount of Zr
content, which could be attributed to the shrinkage of lattice
parameter caused by the smaller Zr4+ (0.084 nm) cation
incorporated into the Ce4+ (0.097 nm) lattice.22 Therefore, the
the lattice of CeO2. All the BꢀCe1ꢀxZrxO2 formed a solid solution, 20 solid solutions B possessed smaller crystallite sizes than supports
10 because their XRD patterns are in accordance with the single
cubic or tetragonal phase without Zr as a segregated phase.
A and pure oxides as calculated by Scherrer equation (Table 1),
which agreed well with the results from HRTEM determination.
Table 1 Characterization of Au/Ce1ꢀxZrxO2 samples
a
Sample
Au content
(wt%) a
X value of Ce1ꢀxZrxO2
Lattice
Cubic
Crystallite size of support b Surface area c (m2 gꢀ1)
(nm)
A
B
A
B
A
B
A
B
A
B
Au/CeO2
Au/Ce0.8Zr0.2O2
Au/Ce0.6Zr0.4O2
Au/Ce0.4Zr0.6O2
Au/Ce0.2Zr0.8O2
ZrO2
1.42
1.40
0.0
12.6
97
0.95
0.99
1.01
0.94
0.18
0.41
0.60
0.81
1.0
0.17
0.35
0.48
0.75
ꢀd
ꢀ
ꢀ
Cubic
Cubic
13.8
8.4
11.1
6.7
7.8
3.5
3.9
5.3
80
85
105
50
131
182
125
78
1.19
1.09
0.75
Tetragonal
Tetragonal
ꢀ
0.90
Monoclinic
7.8
50
a Calculated from ICPꢀOES data. b Calculated from value of support plane (111) with Scherrer equation. c Determined by N2ꢀBET. d Supports A constituted
25 by cubic CeO2 and tetragonal ZrO2.
Fig. 1 XRD patterns of the supports: (a) AꢀCe1ꢀxZrxO2 (x=0ꢀ1); (b) BꢀCe1ꢀxZrxO2 (x=0ꢀ1).
XRD patterns of all catalysts shown in Fig. 2 were almost
similar to the patterns for the corresponding support with the
30 same Ce/Zr mole ratio. No obvious Au reflections could be
observed for all studied catalysts, indicating the low Au content
or small size of the Au particles, which could not be determined
by XRD, and the phases of supports were not transformed after
the loading of Au particles.
35
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Fig. 2 XRD patterns of the catalysts: (a) Au/AꢀCe1ꢀxZrxO2 (x=0ꢀ1); (b) Au/BꢀCe1ꢀxZrxO2 (x=0ꢀ1).No Au reflections could be observed
and the crystalline phases of Ce1−xZrxO2 supports were not altered after loading Au for all catalysts.
Fig. 3 SEM and TEM images of the supports and the catalysts: (a) AꢀCe0.6Zr0.4O2; (b) Au/ AꢀCe0.6Zr0.4O2; (c) AꢀCe0.6Zr0.4O2; (d) Au/ Aꢀ
Ce0.6Zr0.4O2; (e) BꢀCe0.6Zr0.4O2; (f) Au/ BꢀCe0.6Zr0.4O2; (g) BꢀCe0.6Zr0.4O2; (h) Au/ BꢀCe0.6Zr0.4O2. The representative Au particle was
indicated by the white circle.
5
respectively almost without solid solution. However, the mixed
oxides of B were composed of uniform small grain particles, and
more pores in the submicrometer range could be viewed, which
are in consistent with BET results for surface area. Comparison
25 with the SEM micrographs of supports, both catalysts A and B
present similar morphologies with their supports, indicating that
the Au nanoparticles did not change the phase of the supports.
The HRTEM pictures show highly dispersed nanoparticles of
mixed oxides and Au, and representative Au nanoparticles were
30 indicated by the white circle. All the catalysts possessed similar
mean sizes of Au particles of around 3 nm with narrow
distributions, and the particle sizes of mixed oxides were around
5ꢀ10 nm, which is in agreement with the XRD data.
HAADFꢀSTEM technique (Fig. 4) was used to determine the
35 average Au particle size more accurately, cosidering that some
overlapping particles of support could be easily regarded as Au
particles. A clear high dispersion of Au and support nanopartilces
were observed in the STEM pictures for the sample Au/Bꢀ
Ce0.6Zr0.4O2, and the Au particle size distributions were consistant
40 with the HRTEM results. The other catalysts also presented
similar Au particle size seen in Fig. S1.
3. 2. Reducibility characterization
Fig. 4 HAADFꢀSTEM images of Au/BꢀCe0.6Zr0.4O2 catalyst. The
representative Au particle was indicated by the black circle.
H2ꢀTPR is often used as an efficient tool to study the redox
property of materials. The supports of series A and B were
10
The morphology of supports and catalysts was viewed by SEM
and HRTEM and some representative micrographs are illustrated 45 analyzed by H2ꢀTPR, and the profiles of samples are presented in
in Fig. 3, which shows the similarities of the micrographs
obtained for two respective series of studied samples. From the
SEM micrographs of supports in Fig. 3(a) and (e), the supports of
Figure 5. Pure oxide CeO2 was reduced in two stages, that is,
with the profile peaks at 470 ℃ which can contribute to the
reduction of easily reducible surface capping oxygen, and peak at
750℃, which could be ascribed to the bulk reduction of CeO2
15 A and B series showed totally different morphologies. The mixed
oxides of A were characteristic structure of angular and faced 50 forming low valence cerium oxides.27ꢀ31 The pure oxide ZrO2 is
large particles, which is a typical structure for ZrO2, and some
smaller spherical grain particles aggregated on the large particles
were observed, which is typical structure for CeO2. These finding
20 could be ascribed to the formation of two phases by Ce and Zr
often considered to be nonꢀreducible at low temperature,32 and
only a small peak was observed around 700 ℃ from the TPR
profile of ZrO2. AꢀCe1ꢀxZrxO2 (x=0ꢀ0.8) showed the similar shape
profiles with CeO2 possessing two stages of reduction, while Aꢀ
4
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Ce0.2Zr0.8O2 showed no high temperature reduction peak ascribed 20 CeO2. This behavior could be attributed to the phase distortion of
to the reduction of bulk oxygen. An obvious decrease in the H2
consumption for supports A with increasing Zr content could be
observed from the profiles and the data are shown in Table 2.
Those phenomena may have been caused by the fact that the
mixed oxides, which would alternatively shorten or lengthen the
metalꢀoxygen bonds. Furthermore, the lengthening of the metalꢀ
oxygen bond would result in a lower energy barrier for the
32
5
oxygen migration in the bulk.31,
Consequently, reduction
mixed oxides of AꢀCe1ꢀxZrxO2 formed little structure of solid 25 process was no longer confined to the surface but extended
solution, and thus they mainly presented pure oxides properties as
Ce/Zr molar ratio. Even for the Zrꢀrich sample AꢀCe0.2Zr0.8O2,
only a small peak around 500 ℃ assigned to the reduction of
10 capping oxygen can be observed because of the thoroughly
dispersed CeO2 on the surface resulting in a total reduction.
Supports B showed totally different change law with supports
A. For the Ceꢀrich solid solutions, all exhibited broader capping
oxygen reduction peak than pure CeO2, which indicated that these
15 solid solutions showed a great promotion of reduction in the bulk
deeply into the bulk at lower temperature, resulting in the
disappearance of the peak of bulk reduction for Ceꢀrich solid
solutions. Interestingly, the additional shoulder weak peak at 330
℃ for BꢀCe0.8Zr0.2O2 and BꢀCe0.6Zr0.4O2 could be ascribed to the
30 reduction of adsorbed surface oxygen species on the free Ce
particles,33, 34 and we postulated that it may be because those two
samples possessed bigger surface area. The reduction of Bꢀ
Ce0.2Zr0.8O2 was similar to that of the two stages reduction of
pure CeO2, which may have resulted from the inhibition of the
oxygen of solid solution. In addition, for all BꢀCe1ꢀxZrxO2, a shift 35 migration of bulk oxygen to the surface and probable reduction of
of maximum of the capping oxygen reduction peak towards
higher temperature from 470℃ to 490 ℃ was observed and the
starting temperature for H2 consumption was lower than pure
bulk at high temperature because of the excessive content of ZrO2
confining the migrating oxygen.
Fig. 5 H2ꢀTPR profiles of supports: (a) AꢀCe1ꢀxZrxO2 (x=0ꢀ1); (b) BꢀCe1ꢀxZrxO2 (x=0ꢀ1).
40
The TPR profiles of catalysts are shown in Figure 6 and a
significant improvement in Au particles on the reducibility of the
oxide supports was observed because of the evident decrease in
corresponding support. A characteristic decrease in the reduction
temperature of catalysts was observed compared with oxide
supports. This phenomenon indicates that wellꢀdispersed Au
temperature for reduction. The reduction of all the catalysts 55 nanoparticles promotes the activation of hydrogen molecule on
occurred in two stage with a higher peak at approximately 100 ℃
metallic Au particles and results in the spillꢀover of dissociated
hydrogen from Au nanoparticles to support.31 Meanwhile, Au
could increase the amount of surface oxygen by partial lattice
filling of vacant Ce sites with Au.35 In this case, the presence of
45 . This condition could be ascribed to the reduction in the oxygen
species coordinated around finely dispersed Au particles and the
Ce surface layers at 400ꢀ600 ℃, reflecting the bulk reduction of
mixed oxides. Compared with the Au/AꢀCe1ꢀxZrxO2 with tiny 60 Au decreases the strength of the surface CeꢀO bonds adjacent to
peak of bulk reduction, the Au/BꢀCe1ꢀxZrxO2 catalysts possessed a
50 higher peak of reduction at 500 °C, which could be assigned to
the reduction of capping oxygen, which was similar to the
the Au atoms, thus leading to higher surface lattice oxygen
mobility.
Table 2 H2 consumption and surface acidityꢀbasicity for CexZr1−xO2 supports
Total acidity
Total basicity
Consumption of H2
(mmol NH •gꢀ1)
(mmol CO •gꢀ1)
(mmol•g−1)
Sample
3
2
A
B
A
B
A
B
CeO2
1.52
0.086
0.65
Ce0.8Zr0.2O2
Ce0.6Zr0.4O2
Ce0.4Zr0.6O2
Ce0.2Zr0.8O2
ZrO2
1.41
1.2
1.01
0.48
1.81
1.85
1.84
1.59
0.12
0.12
0.12
0.11
0.08
0.13
0.17
0.11
0.68
0.88
0.75
0.81
0.79
0.97
0.75
0.53
0.09
0.087
0.87
65 ZrO2 dopants in CeO2 can significantly change the acidꢀbase
3. 3. Acid-base property characterization of the support
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33
properties of the mixed metal oxides.32,
The loading of Au
amounts of weak acidic sites for BꢀCe1ꢀxZrxO2 ranged from 0.075
mmol to 0.090 mmol NH3 per gram catalysts, which is higher
than the AꢀCe1ꢀxZrxO2. The support BꢀCe0.4Zr0.6O2 with a broad
peak around 450 °C possessed the highest amount of strong
particles changes the acidꢀbase properties of support slightly.
Hence, we just use the acidꢀbase properties of support instead of
catalyst to research the effects of support in the following
5
discussion. The acidꢀbase properties of all oxides were revealed 20 acidic sites, which was twice that of other mixed oxides. The
by NH3ꢀTPD and CO2ꢀTPD analysis. The results are shown in
Figs. 7 and 8. Data are summarized in Table 2.
sample BꢀCe0.6Zr0.4O2 presented weakest acidic sites with a broad
peak around 200 °C. The phenomenon may be attributed to the
weak acidic sites connection of the surface area, with Bꢀ
Ce0.6Zr0.4O2 as having the largest surface area in all solid
The mixed oxides prepared by method A have similar profiles
and presented almost similar total acidity amount around 0.12
10 mmol NH3 per gram catalysts. Meanwhile, the mixed oxides of 25 solutions. It was reported that37 the incorporation of increasing
supports A had more strong acidic sites, which were assigned to
the shoulder peak around 550 °C, with increasing of Zr content.
However, the solid solutions presented different acidic site
concentrations and acidity distributions, which had two big peaks
quantities of Zr in the Ce lattice leads to a marked increase in the
total and strong acidity, while our catalyst BꢀCe0.4Zr0.6O2
possessed highest amount of acidic sites and strong acidity. The
present results may be not comparable between our results and
15 presenting the weak and strong acidic sites respectively. The 30 literature with using different synthesizing methods.
Fig. 6 H2ꢀTPR profiles of catalysts: (a) Au/AꢀCe1ꢀxZrxO2 (x=0ꢀ1); (b) Au/BꢀCe1ꢀxZrxO2 (x=0ꢀ1).
The base properties for all oxides are shown in Fig. 8. The 40 were presented as preparing by mechanical mixing according to
surface base concentrations of AꢀCe1ꢀxZrxO2 ranged from 0.68
35 mmol to 0.88 mmol CO2 per gram support. The surface base
concentrations of BꢀCe1ꢀxZrxO2 ranged from 0.53 mmol to 0.97
mmol CO2 per gram support. Similar to the change law of acidity
fixed Ce/Zr content. The solid solutions of BꢀCe1ꢀxZrxO2 had
more basic sites and higher concentrations than pure oxides. The
solid solutions BꢀCe1ꢀxZrxO2 had more kinds of basic sites, all of
which had a broad strong base peak with BꢀCe0.6Zr0.4O2 having
properties for AꢀCe1ꢀxZrxO2 and BꢀCe1ꢀxZrxO2, the base 45 the highest amount of basic sites.
concentrations and distribution of the basic sites for AꢀCe1ꢀxZrxO2
Fig. 7 NH3ꢀTPD profiles of CexZr1−xO2 supports: (a) AꢀCe1ꢀxZrxO2 (x=0ꢀ1); (b) BꢀCe1ꢀxZrxO2 (x=0ꢀ1).
Comparative results of the NH3ꢀTPD and CO2ꢀTPD data 50 a more pronounced basic character in terms of concentration of
showed that all supports possess both acidic and basic sites, with
basic sites and strong adsorption sites. Meanwhile, most solid
6
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solutions present more acidic and basic sites in terms of
surface oxygen anions as basic sites in the catalyst Thus, the
concentration and kinds of different sites than the series of 10 substitution of Ce4+ cation with the smaller Zr4+ cation distorts
support A, which possesses more acidic and basic sites than pure
oxides caused by the formed partial solid solution structure. A
thorough review of the literature suggests that oxygen mobility is
favored in CeꢀZr solid solutions because of the substitution of
the cubic structure of CeO2, and results in the solid solutions
supports B to exhibit stronger acidity/basicity as well as higher
amount of oxygen available. At the same time, the size of the Au
particles in all the catalysts differed slightly, which facilitated the
5
Ce4+ by the smaller Zr4+ cation. This phenomenon causes 15 understanding of the intrinsic support effects on these Au
shrinkage of the CeO2 fluoriteꢀtype lattice, which generates
catalysts in aldehyde oxidative esterification.
Fig. 8 CO2ꢀTPD profiles of CexZr1−xO2 supports: (a) AꢀCe1ꢀxZrxO2 (x=0ꢀ1); (b) BꢀCe1ꢀxZrxO2 (x=0ꢀ1).
3.4. Catalytic performance on oxidative esterification of
of a number of aldehydes. First, the catalyst screening was
obtained by oxidative esterification of isobutyraldehyde.
20 aldehyde
Catalytic activities were evaluated for the oxidative esterification
3.4.1. Catalyst screening and the effects of support
25 Table 3 Catalytic performance for oxidative esterification of isobutyraldehyde with methanol to form methyl isobutyratea
Selectivity/%b
Sample
Conversion/%b
Methyl isobutyrate
Dimers of ISB
Othersc
1.52
BꢀCe0.6Zr0.4O2
Au/CeO2
3.42
96.9
0
96.7
2.2
81.0
13.7
A
B
A
B
A
B
A
B
61.0
96.4
69.9
98.0
32.0
97.7
13.5
66.5
15.8
87.5
80.9
72.6
83.8
43.1
82.2
82.6
68.4
94.5
2.1
3.8
10.1
14.1
18.2
10.2
19.1
13.6
13.2
27.1
1.6
Au/Ce0.8Zr0.2O2
10.3
4.1
Au/Ce0.6Zr0.4O2
Au/Ce0.4Zr0.6O2
Au/Ce0.2Zr0.8O2
35.2
3.8
2.1
2.3
Au/ZrO2
2.2
a Reaction conditions: catalyst :0.50g; K2CO3:0.10 g; ISB=0.05 mol; methanol=40 mL; O2=15 ml/min; P=0.3 MPa；at 80 °C for 2 h. b Determined by GC
analysis. c others include dimers of the isobutyraldehyde, acetal, CO2, and some unknown products.
Two series of Au supported on different Ce/Zr oxides and pure
the increase in Zr content of the mixed oxides of A, the
oxides were tested for the oxidative esterification of 40 conversion first slightly increased a little from 61.0% to 69.9%,
30 isobutyraldehyde (ISB), and the catalytic results are shown in
Table 3. The support CeꢀZr oxides or pure oxides without loading
of Au nanoparticles presented no activity for oxidative
esterification. The reaction data is shown in table 3 using
then decreased sharply to 13.5%; the selectivity dropped first
from 87.5% to 43.1%, then increased to 82.2%. The main byꢀ
products during the direct oxidative esterification of the ISB are
dimers of itself, acetal, isobutyric acid and CO2, which mainly
representative BꢀCe0.4Zr0.6O2 as catalyst. Only little of ISB was 45 produced by the over oxidation of isobutyraldehyde. The series of
35 mainly transformed into dimers of itself and no methyl
isobutyrate was detected. Compared with Au/BꢀCe1ꢀxZrxO2
(x=0.2ꢀ0.8) catalysts, the series of Au/AꢀCe1ꢀxZrxO2 (x=0.2ꢀ0.8)
catalysts preformed badly with lower conversion by 30%. With
Au supported on mixed oxides of B all showed good performance
on the esterification of ISB with the yield of methyl isobutyrate
above 78%, except of Au/BꢀCe0.2Zr0.8O2 with low yield. The
highest yield of 82.1% for methyl isobutyrate in ISB oxidative
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esterification was obtained using the Au/BꢀCe0.6Zr0.4O2 catalyst.
To determine the essential reason for the different activities of all
mixed oxides, the relation of the yield of methyl isobutyrate and
higher yield of ester for each series studied catalysts, while the
catalysts prepared by supports with high content of Zr, particular
for pure oxide ZrO2, presented lower activity due to their weaker
physicalꢀchemical properties of supports were investigated in 15 reducibility. These findings indicate that the reducibility of the
5
detail. The effect of reducibility, acidity and basicity of mixed
oxides for the catalytic activity is shown in Figure 9. For both A
and B series, the sequence of increasing the H2 consumption
clearly follows the sequence of increasing activity of the catalyst
support plays an important role in the performance of studied
catalysts in oxidative esterification of ISB. However, high
activity with high consumption of H2 is not inevitable,
considering that catalysts B with almost the same reducibility
for corresponding support on the whole. Thus, the observed 20 possessed. Therefore, determining other factors of the support
10 correlations indicate that the higher H2 consumption of Au/Aꢀ
Ce0.8Zr0.2O2 and Au/BꢀCe0.6Zr0.4O2 might be responsible for the
that impact the activity is necessary.
Fig. 9 Effects of reducibility, acidity and basicity of supports on the catalytic activity. (a) AꢀCe1ꢀxZrxO2 (x=0ꢀ1); (b) BꢀCe1ꢀxZrxO2 (x=0ꢀ1)
The relation between yield and acidꢀbase properties of support 30 quite low activity. The sample Au/ZrO2 possessing high acidic
25 is also shown in Fig. 9 (b) for series B. Evident correlation was
found between the acidꢀbase properties of the support and yield
of ester. The catalyst Au/BꢀCe0.4Zr0.6O2 with most acidic and
basic sites illustrated the highest activity, while the catalyst Au/Bꢀ
Ce0.6Zr0.4O2 with small amount of acidic and basic sites showed
sites presented worst activity assigned to the unreducible of ZrO2.
These results allowed us to conclude that the acidity and basicity
of support also play an important role on oxidative esterification
of aldehydes in the proposed catalytic system.
35
Fig. 10 Plausible reaction mechanism for oxidative esterification of aldehydes on CeꢀZr solid solution supported Au nanoparticles
The reaction mechanism for the oxidative esterification of
aldehydes with alcohols over the Pdꢀ and Auꢀbased catalysts was
recently investigated in detail8, 10, 36ꢀ38. Generally, the reaction for
condensation reaction between aldehyde and alcohol. The
formation of hemiacetal is easily effected by the acidity/basicity
of support. Zheng et al.8 and Wang et al.11 found that stronger
40 the direct oxidation oxidative esterification of aldehydes to esters 45 acidity/basicity of support, respectively, was in favor of forming
involves two steps. Initially, hemiacetal is formed by
hemiacetal, which could lead to higher selectivity of esters. Then,
8
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the ester is produced by the abstraction of H from hemiacetal.
support presented no activity for the formation of ester and the
Thus, considering the correlation of reducibility and activity, we 25 samples with larger amounts of H2 consumption presented higher
speculate the reaction mechanism for oxidative esterification of
isobutyraldehyde on solid solution supported Au nanoparticles,
which is shown in Fig. 10. First, the formation of hemiacetal
could be enhanced through two routes by acidic and basic sites
activity. For the catalysts A supported on mixed oxides, the effect
of acidꢀbase properties did not show prominent role due to the
low consumption of H2 considering the step C as the rate
determining step. Therefore, for all the catalysts prepared by
5
respectively. The acidic sites could facilitate the aldehyde into 30 mixed oxides and pure oxides, higher reducibility and basicity of
oxonium ion, while the basic sites can activate methanol into
methoxy and sequentially, both the oxonium and methoxy have a
the support are in favor of ester formation.
3.4.2. Optimization of substrates scope and recycling uses of
Au/B-Ce0.6Zr0.4O2 catalyst
10 beneficial effect to form the intermediate hemiacetal by ease of
nucleophilic reaction as shown in step A and step B. It is
noteworthy that the hemiacetal might mainly formed by route B
due to all the catalysts possessing more basic sites than acidic
sites according the results of NH3ꢀTPD and CO2ꢀTPD. Then, the
15 ester is formed by the abstraction of βꢀH from hemiacetal. The
atomic oxygen plays a very important role during the whole
pathway, which is beneficial to the abstraction of H from
methanol to methoxy and the βꢀH elimination of hemiacetal to
ester. The active atomic oxygen can be produced by the direct
20 activation of molecular oxygen on the supported small enough
Au nanoparticles and be enhanced by the spillover from the
lattice oxygen of the oxides supports due to the interaction
between Au nanoparticles and the support39. That is why the bare
To examine the effect of the substrate structure on reactivity, the
35 reactions of a range of benzylic aldehyde were examined using
the Au catalyst supported on solid solution BꢀCe0.6Zr0.4O2. It is
because that BꢀCe0.6Zr0.4O2 has been found to be the most active
at the same preparation condition. The results are summarized in
Table 4. To obtain a high yield of the product, the reaction time is
40 not the major factor for consideration. Benzylic aldehyde with
electronꢀdonating and electronꢀwithdrawing groups gave
excellent yields of products (Table 4, entries 1–7). Similar to
paraꢀand metaꢀsubstituted substrates, the sterically more hindered
orthoꢀsubstituted aldehyde also provided excellent results under
45 the same reaction conditions.
Table 4 Auꢀcatalyzed oxidative esterification of benzylic aldehyde with methanola
Entry
1
Substrate
Product
Reaction time/h
2
Base/mol%
5
Conversion/%
98.0
Selectivity/%
89.5
O
O
OCH3
Cl
Cl
Cl
O
O
2
3
Cl
OCH3
12
12
3
3
91.2
94.3
98.5
95.6
O
O
Cl
Cl
OCH3
O
O
4
OCH3
12
1
63.7
94.2
H3CO
H3CO
O
O
5
6
OCH3
12
12
3
3
82.9
99.5
93.6
99.1
H3CO
H3CO
H3CO
H3CO
O
O
OCH3
a Footnote text.
Stability is a key requirement of an effective catalyst for its
practical application, besides the selectivity of methyl benzoate.
50 Thus, the recycling use of the Au/BꢀCe0.6Zr0.4O2 catalyst for the
decreased only slightly in the initial 7 recycles, and then kept
almost unchanged in the 8th recycle. Meanwhile, methyl
benzoate was a constant selectivity of as high as 99%. After 8
oxidative esterification of benzaldehyde with methanol was 60 recycles, the benzaldehyde conversion and the methyl benzoate
conducted. The catalyst was recovered by removing the
supernatant after the reaction mixture was left undisturbed for a
time, and washed with methanol, followed by supernatant
55 removal. The procedure was repeated twice, and then the catalyst
was reused. Fig. 10 shows that the benzaldehyde conversion
yield could still be sustained at >95%. This phenomenon
indicates that the Au/BꢀCe0.6Zr0.4O2 catalyst is stable during the
recycling uses for the conversion of benzaldehyde with methanol
in the presence of O2.
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40 Acknowledgements
This work was supported by the financial support of "Strategic
Priority Research Program" of the Chinese Academy of Sciences
(XDA07070600) and the National Natural Science Foundation of
China (21306203).
45 Notes and references
a Beijing Key Laboratory of Ionic Liquids Clean Process, State Key
Laboratory of Multiphase Complex System, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, China
Fax: XX XXXX XXXX; Tel: 86-10-82544875; E-mail: sjzhang@ipe.ac.cn
50 b College of Chemical and Engineering, University of Chinese Academy
of Sciences, Beijing 100049, China
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
Fig. 11 Catalytic performance of the recycled Au/Ce0.6Zr0.4O2 for
the oxidative esterfication of benzaldehyde. Reaction conditions:
Catalyst 0.5g, benzaldehyde 0.04mol, K2CO3 0.15 g, methanol 40
mL, P(O2)=0.3 MPa, T=353 K, t=2 h.
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Au nanoparticles supported on Ce0.6Zr0.4O2 solid solution with high reducibility and basicity is efficient
catalyst for oxidative esterification.
39x19mm (300 x 300 DPI)