Aerobic oxidative esterification of benzyl alcohol and acetaldehyde over gold supported on nanostructured ceria-alumina mixed oxides

Article abstract of DOI:10.1002/cctc.201402923

Au nanoparticles supported on nanostructured ceria-alumina mixed oxides (10 and 30 wt% ceria) were prepared by a deposition-precipitation method. Their properties were studied by N₂ adsorption, XRD, TEM, X-ray photoelectron spectroscopy, and UV/Vis spectroscopy under temperature-programmed reduction or oxidation. The materials catalyzed the liquid-phase aerobic oxidative esterification of benzyl alcohol and benzaldehyde effectively and showed a much better performance than Au supported on the individual oxides. The reactions occurred with high turnover numbers (up to 19000) in methanol solutions in the absence of any auxiliary base and gave mainly methyl benzoate. The strong synergetic effect of ceria and alumina can be explained by the enhanced oxygen storage capacity of the materials prepared from mixed oxides compared to that of pure alumina and ceria. The order of the catalytic activity (Au/Al₂O₃2

Full text of DOI:10.1002/cctc.201402923

DOI: 10.1002/cctc.201402923
Full Papers
Aerobic Oxidative Esterification of Benzyl Alcohol and
Acetaldehyde over Gold Supported on Nanostructured
Ceria–Alumina Mixed Oxides
[
a]
[b]
[b]
[c, d]
Elena Smolentseva, Vinꢀcius V. Costa, Rafaela F. Cotta, Olga Simakova,
[e]
[b]
[a]
Sergey Beloshapkin, Elena V. Gusevskaya,* and Andrey Simakov*
Au nanoparticles supported on nanostructured ceria-alumina
mixed oxides (10 and 30 wt% ceria) were prepared by a deposi-
tion–precipitation method. Their properties were studied by N2
adsorption, XRD, TEM, X-ray photoelectron spectroscopy, and
UV/Vis spectroscopy under temperature-programmed reduc-
tion or oxidation. The materials catalyzed the liquid-phase
aerobic oxidative esterification of benzyl alcohol and benzalde-
hyde effectively and showed a much better performance than
Au supported on the individual oxides. The reactions occurred
with high turnover numbers (up to 19000) in methanol solu-
tions in the absence of any auxiliary base and gave mainly
methyl benzoate. The strong synergetic effect of ceria and alu-
mina can be explained by the enhanced oxygen storage ca-
pacity of the materials prepared from mixed oxides compared
to that of pure alumina and ceria. The order of the catalytic ac-
tivity (Au/Al O <Au/CeO <Au/Ce(10)-Al<Au/Ce(30)-Al) corre-
2
3
2
lated well with the oxygen activation capacity of the materials.
Introduction
Supported gold nanoparticles (AuNPs) have shown a remark-
able catalytic activity in a large number of chemical transfor-
mations, in particular, in the oxidation of CO, alkenes, and alco-
ported that supported AuNPs were effective catalysts for the
[9,10]
oxidation of polyols by O2.
Since then, a great number of
studies on the aerobic oxidation of alcohols over AuNPs has
[
1–8]
[11–23]
hols.
The oxidation of alcohols is one of the most important
been published.
If these reactions are performed in metha-
reactions in synthetic organic chemistry as the resulting car-
bonyl compounds are employed widely in various fields of the
chemical industry. Nearly 15 years ago, Prati and co-workers re-
nol solutions, which is rather inert to aerobic oxidation, methyl
esters can be obtained as final products from the oxidative
[23–33]
esterification of alcohols.
Methyl esters have found practical use as solvents, diluents,
extractants, and flavoring agents and are also useful intermedi-
ates for further transformations. Their syntheses usually involve
two-step procedures that consist of the production of carbox-
ylic acids or their derivatives followed by the esterification
step. Only a few catalysts able to promote the direct conver-
[
a] Dr. E. Smolentseva, Prof. A. Simakov
Centro de Nanociencias y Nanotecnologꢀa
Universidad Nacional Autꢁnoma de Mꢂxico
Km. 107 Carretera Tijuana a Ensenada, C.P. 22860, Ensenada
Baja California (Mꢂxico)
Fax: (+52)6461744603
E-mail: andrey@cnyn.unam.mx
sion of alcohols into methyl esters using O as the final oxidant
2
have been reported, all of which are based on AuNPs, as far as
[
b] MSc. V. V. Costa, R. F. Cotta, Prof. E. V. Gusevskaya
[23–33]
Departamento de Quꢀmica
we know.
However, most of the Au systems reported for
Universidade Federal de Minas Gerais
the oxidative esterification of alcohols require the presence of
base. Among the few examples of the efficient synthesis of
carboxylic esters from alcohols under neutral conditions, are
3
1270-901, Belo Horizonte, MG (Brazil)
Fax: (+55)31-34095700
E-mail: elena@ufmg.br
[26]
the processes that use AuNPs supported on b-Ga O
and
In both cases, it has been suggested that the nature
[
c] Dr. O. Simakova
2
3
[30,32]
Laboratory of Industrial Chemistry and
Reaction Engineering Process Chemistry Centre
ꢃbo Akademi University
MgO.
of the support determines the catalytic efficiency of the mate-
rial.
Biskopsgatan 8, FI-20500 ꢃbo/Turku (Finland)
It is well known that the catalytic activity of Au catalysts de-
pends on the nature of the support, the method of Au deposi-
tion, and the size and morphology of the AuNPs and their in-
teraction with the support. Of the various metal oxides used to
[
d] Dr. O. Simakova
Graduate School of Chemical Engineering
ꢃbo Akademi University
Biskopsgatan 8, FI-20500, ꢃbo/Turku (Finland)
[
e] Dr. S. Beloshapkin
Materials & Surface Science Institute
University of Limerick
stabilize AuNPs, ceria (CeO ) is considered to be one of the
2
most efficient and collaborative supports in oxidation reactions
because it is reducible and has an oxygen storage and release
Limerick (Ireland)
[1,4,34]
capacity.
Ceria-supported Au catalysts have been used
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402923.
successfully in the oxidation of alcohols, and a variety of so-
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phisticated approaches have
been applied to the synthesis
of the support to improve the
catalytic performance of the
Table 1. Characteristic data for the supports.
Sample
BET surface Average pore
Particle
size [nm]
Band-gap
energy [eV]
Concentration of surface Lewis acid sites
2
À1
À1
area [m g
]
diameter [nm]
[mmolg
medium
]
[
11–17,34,35]
alumina ceria
weak
strong
Au.
Along with conven-
CeO
Al
Ce(10)-Al 340
Ce(30)-Al 272
2
23
207
4 and 8
–
41
–
3.2 3.51
9.1 3.52
3.55
–
0
0
10
10
10
0
10
10
0
20
0
tional
ceria,
nanocrystalline
[
11,34]
2
O
3
9
12
7
6.4
5.5
5.1
ceria,
ceria synthesized
[
35]
under supercritical conditions,
0
[
15]
ceria foam,
and mesoporous
[
16]
Mn-doped ceria
have been
used to prepare Au catalysts for the aerobic oxidation of alco-
hols.
sample preparation and the size of ceria crystals because it is
related to the structural and electronic defects in the ceria
structure. However, the prepared Ce(10)-Al and Ce(30)-Al
oxides showed similar Egap values (3.51 and 3.52 eV, respective-
ly), which were surprisingly close to the value of bulk ceria
(3.55 eV) in spite of the great difference in the size of ceria
crystals in these samples (Figure S4). A plausible explanation is
that the direct contact between alumina and ceria could affect
the Egap value because ceria in mixed oxides is mostly embed-
ded into alumina. The incorporation of ceria into alumina did
not affect the chemical state of Ce according to X-ray photo-
Alumina is usually considered to be inert and inappropriate
[
20]
as a support for the preparation of active AuNPs. However,
several examples of efficient alumina-supported Au catalysts
for the oxidation of alcohols have been reported recent-
[
19,20,23,36]
ly.
In this work, the performance of AuNPs supported on binary
nanostructured Ce–Al oxides in the liquid-phase aerobic oxida-
tion of benzyl alcohol and benzaldehyde in methanol solutions
under neutral conditions has been studied, for the first time as
far as we know, in comparison with Au catalysts prepared on
only CeO or Al O . The effect of the catalyst pretreatment (re-
[37]
electron (XPS) spectroscopy for the Ce3d region (Figure S5).
The acidity of the supports was studied by IR spectroscopy
using pyridine as a probe molecule (described in detail in
Ref. [36]). All characterized materials contained surface Lewis
acid sites (LAS) only (Table 1). Bulk ceria showed only small
amounts of LAS of a medium strength, whereas strong LAS
were detected on the surface of nanostructured alumina in
double the concentration of that of ceria. The addition of ceria
to alumina decreased the strength of the surface Lewis acidity
but not the concentration of the surface LAS, which is inde-
pendent of the ceria content.
2
2
3
duction or oxidation) was also evaluated. It has been reported
[
37]
previously
that AuNPs supported on Ce–Al mixed oxides
show better activity in the oxidation of CO than those support-
ed on only cerium or aluminum oxides.
Results and Discussion
Support characterization
The characteristics of the supports are summarized in Table 1.
The introduction of ceria into alumina resulted in a significant
increase in the BET surface area compared to that of pure alu-
mina (N2 adsorption–desorption isotherms are presented in
Figure S1). This could be the modification of the morphology
of alumina as the incorporation of ceria affected the pore
structure of alumina dramatically (Figure S2). The mixed Ce–
Al–O oxides presented pores with a smaller average diameter
than pure alumina, which correlates with the ceria content.
The ceria sample had a bimodal pore size distribution of ap-
proximately 4 and 8 nm. The size of ceria and alumina crystals
was estimated from XRD data (Figure S3; details presented in
Ref. [37]). The incorporation of
Catalyst characterization
The Au particle size and total Au content determined by TEM
and inductively coupled atomic emission spectroscopy (ICP-
AES), respectively, are presented in Table 2 for the preoxidized
and prereduced samples. Typical TEM images are presented in
Figure S6. The average AuNPs diameter depended on both the
nature of the support and the catalyst pretreatment and varied
within the range of 2.0–5.8 nm. The AuNPs supported on Ce–
Al mixed oxides were characterized by a less uniform size dis-
tribution than those supported on the pure oxides because of
ceria into alumina did not affect
[
a]
the size of the alumina particles
significantly. However, the ceria
particles incorporated to alumina
were much smaller than that in
pure ceria. As the content of
ceria in the mixed Ce–Al oxide
increased, the size of the ceria
particles increased.
Table 2. Au content, Au particle size (dAu), and XPS data of the supported catalysts.
Sample
Au [wt%]
Sample pretreatment
O
2
H
2
XPS data
dAu [nm]
XPS data
dAu [nm]
0
0
Au/CeO
2
3.9
Au (83.8; 0.7; 68)
2.9
Au (83.7; 0.9, 70)
2.7
+
+
Au (84.1; 2.04; 32)
Au (84.1; 2.04, 30)
0
0
Au/Al
2
O
3
3.4
3.0
1.9
Au (83.8; 1.5; 100)
3.9
5.8
4.6
Au (83.7; 1.2; 100)
2.0
3.8
3.7
0
0
Au/Ce(10)-Al
Au/Ce(30)-Al
Au (83.6; 1.1; 100)
Au (83.7; 1.0; 100)
0
0
Au (83.6; 0.9; 100)
Au (83.6; 1.1; 100)
The value of the band-gap
energy (Egap) for ceria depends
usually on the method of
[
a] In parentheses, the peak position of Au4f7/2 [eV] with reference to the C1s peak (BE=284.8 eV), the full
width at half maximum (FWHM), and the relative content of Au species are shown.
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the difference in the interaction of Au species with alumina
and ceria in the course of the AuNPs formation (more details
can be found in Ref. [37]). The size of the AuNPs after thermal
lic AuNPs may be stabilized in the mixed oxides on both alumi-
na and ceria surface sites.
treatment under O was larger than that of the samples re-
2
Catalytic studies
duced in H . However, this difference in size decreased as the
2
ceria content in the sample increased. On the nonreducible
alumina support, the size of the AuNPs varied after the differ-
ent treatments, whereas the size of the AuNPs supported on
reducible ceria was nearly the same in both the preoxidized
and prereduced samples.
The catalytic activity of Au/Al O , Au/CeO Au/Ce(10)-Al, and
2
3
2
Au/Ce(30)-Al after two different types of pretreatment (reduc-
tion with H or oxidation with O ) was tested first in the oxida-
2
2
tion of benzyl alcohol (1) with O in a methanol solution. The
2
results are presented in Table 3. All samples demonstrated
a high efficiency in the oxidation and, particularly, in the oxida-
tive esterification of benzyl alcohol in the absence of any auxil-
iary base, although they showed a different performance. The
reaction resulted in two major products: benzaldehyde (2) and
methyl benzoate (4; Scheme 1). The combined selectivity for
these two products was 80–85% in all runs, and only small
amounts of dimethyl acetal (3), benzyl benzoate (5), and ben-
zoic acid (6) were detected. The relative amounts of the main
products 2 and 4 depended on the reaction time, and 4 was
the predominant product at the end of most runs. This kinetics
is consistent with a two-step process that involves the oxida-
tion of alcohol to aldehyde and the further oxidation of alde-
hyde to benzoic acid derivatives (Scheme 1).
The electronic state of Au was evaluated by XPS measure-
[
37]
ments. The results described in detail previously are sum-
marized in Table 2 and presented in Figure S7. The XPS spectra
of the Au4f region for Au/CeO samples treated either in O or
2
2
H exhibited two doublets separated by a binding energy (BE)
2
of 3.67 eV (Au4f7/2 and Au4f5/2). The doublets were attributed
to the metallic and cationic Au species, and the BE values for
[
38]
Au4f7/2 were 83.8 and 84.5 eV, respectively. Thus, even after
reduction at 3508C, some fraction of Au existed on the ceria
surface in a cationic form probably because of the stabilization
offered by this reducible support. However, only metallic Au
species were found on Au/Al O , Au/Ce(10)-Al, and Au/Ce(30)-
2
3
Al after both types of pretreatment.
The process of the AuNPs formation on the surface of ceria,
alumina, and Ce–Al mixed oxides under temperature-pro-
grammed reduction and oxidation was studied by UV/Vis spec-
Compounds 4 and 5 are formally the products of the
esterification of benzoic acid with methanol and benzyl alco-
hol, respectively. However, within the mechanism accepted
currently, their formation occurs through the dehydrogenation
of the corresponding hemiacetals that are considered as key
[37]
troscopy and reported in detail in our previous work (see
Supporting Information and Figure S8). Analysis of the UV/Vis
spectra of Au/Ce(10)-Al and Au/Ce(30)-Al indicated that metal-
[11,25,26,39]
intermediates.
The hemiacetals are suggested to be
[
a]
Table 3. Oxidation of 1 in methanol solutions catalyzed by Au catalysts.
[
b]
[c]
[d]
Run
Catalyst
Time
Conversion
[%]
Selectivity
S
[%]
mono/di
TOF
[s
TON
À1
[h]
[%]
]
2
3
4
5
6
Catalysts pretreated in H
2
1
2
3
4
Au/Al
2
O
3
1
13
67
25
92
39
93
72
100
48
43
45
31
51
24
62
20
6
38
36
27
53
35
60
15
66
8
11
6
9
7
12
–
4
–
–
–
–
–
–
–
54/46
53/47
67/33
38/62
58/42
28/72
85/15
23/76
0.91
–
2.07
–
6.77
–
17.86
–
–
10
1
10
1
10
1
10
12
22
7
7
4
2487
–
4436
–
9998
–
15625
Au/CeO
2
Au/Ce(10)-Al
Au/Ce(30)-Al
[
e]
23
3
6
Catalysts pretreated in O
2
5
6
7
8
Au/Al
2
O
3
1
0
1
0
1
0
1
0
30
74
24
96
45
99
78
100
49
38
48
25
50
26
58
22
0
3
14
4
10
5
21
2
36
45
30
63
35
52
15
63
7
14
8
8
5
12
2
3
8
0
–
–
–
5
4
10
49/51
41/59
62/38
29/71
60/40
31/69
79/21
24/76
4.08
–
2.10
–
12.02
–
23.40
–
–
5768
–
5194
–
16086
–
19130
1
1
1
1
Au/CeO
2
Au/Ce(10)-Al
Au/Ce(30)-Al
2
[a] Conditions: catalyst (10 mg); substrate (2.5 mmol); methanol (3 mL); 1108C; 10 atm (O ). Conversion and selectivity were determined by GC; [b] Selectiv-
ity for the products of mono- and dioxidation: (2+3)/(4+5+6); [c] Initial rate of the substrate conversion (initial TOF) per mol of the surface Au atoms. The
relative amount of surface Au atoms (Au dispersion) was calculated as 1.15 divided by the average diameter of Au particles; [d] TON calculated as a ratio
between the amounts of products formed and the amounts of surface Au. It was considered that products 4, 5, and 6 were formed via products 2 and 3
and both steps were catalyzed by Au: TON=[n(2+3)+2n(4+5+6)]/n (Au), where n represents the amount of the indicated compound or Au in moles;
[e] The catalyst was reused after this run without a significant loss of activity and selectivity.
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Figure 2. Oxidation of 1 in methanol solutions catalyzed by Au catalysts pre-
treated in O
2
at 3508C: effect of the support (runs 5–8, Table 3). Conditions:
).
Scheme 1. Oxidation of 1 into 2, 3, 4, 5, and 6.
catalyst (10 mg); substrate (2.5 mmol); methanol (3 mL); 1108C; 10 atm (O
2
which the reactions over Au/Ce(10)-Al and Au/Ce(30)-Al were
even faster than those over Au supported on pure ceria
(Table 3, runs 3 and 4 vs. run 2; runs 7 and 8 vs. run 6).
formed by the condensation of benzaldehyde formed in the
first reaction step with methanol or benzyl alcohol. Small
amounts of benzoic acid detected in the reaction solutions
may be the result of the dehydrogenation of a hydrate formed
by the interaction of the aldehyde with trace water instead of
alcohols.
The efficiency of the second reaction step, the aldehyde oxi-
dation into benzoic acid derivatives, was much higher over Au
supported on ceria or mixed oxides compared to that over
pure alumina. For example, in 10 h reactions over Au/Ce(30)-Al,
both prereduced and preoxidized, the products of dioxidation
4, 5, and 6 accounted for 76% of the mass balance at a com-
plete substrate conversion, and 4 was a predominant product
formed with a selectivity of approximately 65% (Table 3, runs 4
and 8). In these runs, the increase in ester selectivity with the
reaction time can be seen clearly.
The kinetic curves for the reactions over the catalysts pre-
treated in H or O are shown in Figures 1 and 2, respectively.
2
2
In all cases, the treatment in O resulted in more active cata-
2
lysts; however, the product distributions were similar for two
sets of catalysts. The remarkable effect of the composition of
the support on the catalytic performance of Au was observed,
the general tendencies of which are similar for the prereduced
and preoxidized materials.
The reactions occurred with very low catalyst loadings
(0.6 wt%) so that turnover numbers (TONs) reached high
values of up to 19000, which reflects the high catalyst stability.
The TON was calculated with respect to the amount of surface
Au; that is, the fraction of Au atoms located on the surface of
Au particles that are, therefore, accessible to the substrate.
The catalysts, which are solid materials insoluble in the reac-
tion mixture, can be separated from the products by simple
centrifugation or filtration. These features represent the impor-
tant technological advantages of the process. The catalyst after
the experiment described in run 4 in Table 3 was reused to
convert another portion of the substrate without a significant
loss of activity and selectivity. The inductively coupled plasma
analysis showed no Ce in the solution separated from the cata-
lyst after the reaction and very small concentrations of Al and
Au (0.05 and 0.12% loss of Al and Au, respectively). To further
verify the possible catalyst loss, fresh benzyl alcohol was
added to the recovered supernatant and the solution was
placed in the autoclave under oxidation conditions (1108C,
The oxidation of benzyl alcohol in the presence of Au/Al O3
2
occurred at 1108C at a moderate rate that resulted in an ap-
proximately 70% conversion in 10 h to give the products of
mono-oxidation (aldehyde 2 and its acetal 3) and dioxidation
(esters 4 and 5) in comparable amounts (Table 3, runs 1 and 5).
The introduction of ceria into alumina resulted in a significant
and gradual increase in the substrate conversion rate, for
1
0 atm of O ). No further conversion was observed, which indi-
2
cated the lack of significant Au leaching to the liquid phase
during the process and showed that the reaction was, there-
fore, truly heterogeneous.
The conversion of benzyl alcohol over the catalysts based
on mixed oxides occurred much faster than over those pre-
pared from the single oxides (Figures 1 and 2). Furthermore, if
the data are expressed in terms of TONs and turnover frequen-
Figure 1. Oxidation of 1 in methanol solutions catalyzed by Au catalysts pre-
treated in H
2
at 3508C: effect of the support (runs 1–4, Table 3). Conditions:
).
catalyst (10 mg); substrate (2.5 mmol); methanol (3 mL); 1108C; 10 atm (O
2
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For all samples, thermal pretreatment in O result-
[
a]
2
Table 4. Oxidation of 2 in methanol solution catalyzed by Au catalysts.
ed in a significant increase in the catalytic activity
compared to the reduced counterparts, similar to
that found in the oxidation of benzyl alcohol. The re-
markable effect of the support composition on the
catalytic performance of Au in the aldehyde oxida-
tion was also observed, the general tendencies of
which were similar for the prereduced and preoxi-
[
b]
[c]
Run
Catalyst
Conversion
%]
Selectivity
TOF
[s
TON
À1
[
[%]
]
4
6
Catalysts pretreated in H
2
1
2
3
4
Au/Al
2
O
3
25
67
33
65
83
94
92
93
17
0.35
0.99
1.27
2.48
631
1994
2062
5804
Au/CeO
2
6
8
7
Au/Ce(10)-Al
Au/Ce(30)-Al
dized samples. Au/CeO showed a much higher activ-
2
Catalysts pretreated in O
2
ity in this reaction as compared to Au/Al O , which
2
3
5
6
7
8
Au/Al
Au/CeO
Au/Ce(10)-Al
Au/Ce(30)-Al
2
O
3
31
67
50
84
81
91
85
85
19
9
15
15
1.02
1.49
2.00
4.83
1520
2120
4808
9130
converted only 25 and 31% of benzaldehyde in 10 h
Table 4, runs 1 and 5 vs. runs 2 and 6). The introduc-
2
(
tion of ceria into alumina enhanced the efficiency of
the material gradually in the oxidation of aldehyde.
The substrate consumption rate over Au/Ce(30)-Al
[
(
a] Conditions: catalyst (10 mg); substrate (2.5 mmol); methanol (3 mL); 1108C; 10 atm
); reaction time 10 h. Conversion and selectivity were determined by GC. At the
O
2
calculation of conversion, acetal 3 formed in small amounts as a result of the acetali-
zation of benzaldehyde was considered as the unconverted substrate; [b] Initial rate
of the substrate conversion (initial TOF) per mol of the surface Au. The relative
amount of surface Au atoms (Au dispersion) was calculated as 1.15 divided by the
average diameter of Au particles; [c] TON calculated as a ratio between the amount of
products formed and the amount of surface Au.
was as high as that over Au/CeO or even higher (Fig-
2
ures 3 and 4). The presentation of the results in
terms of TONs and initial TOFs illustrates the remark-
able synergetic effect between alumina and ceria on
the performance of AuNPs in the oxidative esterifica-
tion of benzaldehyde. The Au/Ce(30)-Al catalyst
showed 3–9 times higher TONs and 3–8 times higher
cies (TOFs) by considering only surface Au atoms, the synerget-
ic effect between alumina and ceria in Au/Ce(10)-Al and Au/
Ce(30)-Al becomes remarkable (Table 3). The Au/Ce(30)-Al cata-
lyst was efficient in both steps of the tandem catalytic process
and showed 3–6 times higher TONs and 6–20 times higher ini-
tial TOFs than AuNPs supported on pure alumina or ceria.
The kinetics of the product accumulation in the reaction sol-
utions implies that the slowest step of the process is the for-
mation of esters from the aldehyde. To clarify the effect of the
support on this particular step, we have studied the oxidation
of benzaldehyde with O over Au/Al O , Au/CeO , Au/Ce(10)-Al,
initial TOFs than AuNPs supported on pure alumina or ceria.
The materials were tested in the oxidation of a secondary
benzylic alcohol, 1-phenyl-1-propanol, to give ethyl phenyl
ketone. The reactions were performed under conditions similar
to those used for the oxidation of benzyl alcohol but at
a lower temperature because of the higher reactivity of the
secondary alcohol (substrate, 2.5 mmol; catalyst, 10 mg; meth-
anol, 3 mL; reaction time 2 h; 1008C; 10 atm of O ). The order
2
of catalytic activities in the oxidation of 1-phenyl-1-propanol:
Au/Al O3 (758/2550)<Au/CeO2 (1756/2152)<Au/Ce(10)-Al
2
(2500/6154)<Au/Ce(30)-Al (6339/8261), showed the same ten-
dencies, which confirms the strong synergetic effect of ceria
and alumina and the beneficial influence of the catalyst pre-
2
2
3
2
and Au/Ce(30)-Al (Table 4). The kinetic curves of these reac-
tions are shown in Figures 3 and 4 for the prereduced and pre-
oxidized samples, respectively. Indeed, all catalysts were less
active in the oxidation of benzaldehyde than in the oxidation
of benzyl alcohol under the same conditions. The main reac-
tion product was methyl benzoate along with a small amount
of benzoic acid.
treatment in O . TONs achieved in 2 h reactions calculated
2
with respect to the amount of surface Au for the catalysts pre-
treated in H and O , respectively, are given in parenthesis.
2
2
The order of the catalytic activity: Au/Al O <Au/CeO <Au/
2
3
2
Ce(10)-Al<Au/Ce(30)-Al, correlates well with the oxygen acti-
Figure 3. Oxidation of 2 in methanol solutions catalyzed by Au catalysts pre-
Figure 4. Oxidation of 2 in methanol solutions catalyzed by Au catalysts pre-
treated in H
2
at 3508C: effect of the support (runs 1–4, Table 4). Conditions:
).
treated in O
2
at 3508C: effect of the support (runs 5–8, Table 4). Conditions:
).
catalyst (10 mg); substrate (2.5 mmol); methanol (3 mL); 1108C; 10 atm (O
2
catalyst (10 mg); substrate (2.5 mmol); methanol (3 mL); 1108C; 10 atm (O
2
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vation capacity of the materials determined in our previous
mixture was heated to reflux with moderate agitation for 3 h. Hy-
drolysis was performed by the addition of the deionized water. The
gel obtained was aged for 10 h. The samples were dried under
[
37]
À1
work:
zero for Au/Al O , 0.18 mol mol
for Au/CeO ,
2
3
O
Au
2
À1
À1
0.48 mol mol
for Au/Ce(10)-Al, and 0.68 mol mol
Au
for
O
Au
O
À3
vacuum (ꢀ10 torr) at 1008C for 12 h then heated to 4508C
Au/Ce(30)-Al (details are presented in the Supporting Informa-
tion and Figure S9). The amount of adsorbed oxygen may be
considered as the measure of the relative content of (Au–ceria
vacancy) active sites on the catalyst surface.
under N and kept at this temperature for 12 h. Finally, the samples
2
were treated in O at 6508C for 4 h for the decomposition of or-
2
ganic residuals. Nanostructured alumina was prepared by the same
sol–gel method described above without the addition of the
cerium compound. Commercial CeO2 (Alfa-Aesar) was used as
a support as well. The mixed oxide supports were encoded as
Ce(X)-Al and Au/Ce(X)-Al, respectively, in which X (10 or 30) indi-
cates the ceria content (10 or 30 wt%).
Within the mechanism accepted currently for the oxidative
esterification of primary alcohols, the first reaction step in-
volves the dehydrogenation of the alcohol into the aldehyde
followed by the formation of a hemiacetal from the aldehyde
[
11,25,26,39]
and alcohol.
Finally, the dehydrogenation of the hemi-
Au (calculated as 3 wt%) was supported by deposition–precipita-
tion using urea as a precipitation agent according to the proce-
dure developed in Ref. [37]. The support (4.0 g) was added to an
acetal gives the ester. Thus, the introduction of ceria into alu-
mina facilitates the oxygen activation to improve the per-
formance of AuNPs in these oxidation processes because of
the better accessibility of active oxygen species in the Au–ceria
interfacial area. A specific Au–ceria interaction results in the in-
À3
aqueous solution (400 mL) of HAuCl4 (1.6ꢂ10 m) and urea
(
0.42m). The initial pH of the solution was ꢀ2. The suspension was
stirred intensively at 808C for 4 h and then filtered. The solid mate-
rial was washed with ammonium hydroxide (25.0m) for 30 min.
Then samples were washed with water until pH 7, filtered, and
dried at RT for 24 h. Finally, samples were treated in H or O with
[
40]
crease in oxygen mobility, which can also contribute to the
enhanced activity of materials that contain Au supported on
ceria compared to that on alumina. The negative effect of the
2
2
a continuous temperature increase from 50–3508C with a ramp
À1
catalyst pretreatment in H can be explained, at least for the
rate of 208Cmin . The resulting materials were used in the catalyt-
2
ceria-containing materials, by the partial reduction of ceria,
which could affect the capacity of the material to oxygen acti-
vation by increasing the OÀsupport bonding energy.
ic tests.
Catalyst characterization
Conclusions
The chemical composition of the Au samples was determined by
ICP-AES by using a Varian Liberty 110 ICP Emission Spectrometer.
In a typical procedure, the sample (30 mg) was dissolved in a mix-
ture of concentrated H SO , HCl, and HNO (20 mL, H SO /HCl/
Au nanoparticles supported on the Ce–Al mixed oxides pre-
pared by a sol–gel technique are more active in the liquid-
phase aerobic oxidative esterification of benzyl alcohol and
benzaldehyde than Au supported on single cerium or alumi-
num oxides. The reactions occur in methanol solutions in the
absence of any auxiliary base and give methyl benzoate as the
main product. High turnover numbers of up to 19000 reflect
the high stability of the catalysts, which can be reused. The
2
4
3
2
4
HNO =6:6:3, v/v) and heated to 1508C.
3
Specific surface areas were determined using the BET method by
N thermal adsorption measurements by using a Gemini 2600 Mi-
2
cromeritics instrument. Before the analysis, samples were heated in
an Ar flow at 3508C for 1 h. The pore distribution was evaluated
using N adsorption by using a TriStar II Micromeritics unit. Before
2
thermal pretreatment in O resulted in more active catalysts
2
the analysis, samples were heated under vacuum at 3508C for 12 h
to eliminate support impurities and traces of adsorbed water.
than pretreatment in H ; however, the product distributions
2
were similar for the two sets of catalysts. The remarkable syn-
ergetic effect between ceria and alumina may be explained by
the enhanced oxygen storage capacity of the materials pre-
pared from mixed oxides compared to neat alumina and ceria.
The order of the catalytic activity (Au/Al O <Au/CeO <Au/
The qualitative and quantitative analysis of the acid sites in the
supports was performed by IR spectroscopy using pyridine as
[36,42]
a probe molecule as described in detail previously.
The adsorp-
tion of pyridine and the measurement of FTIR spectra were per-
formed at 1008C. The pyridine desorbed at 250, 350, and 4508C
corresponds to weak, medium, and strong acid sites, respective-
2
3
2
Ce(10)-Al<Au/Ce(30)-Al) correlates well with the results of the
oxygen activation capacity of the materials. Thus, the introduc-
tion of ceria into alumina facilitates oxygen activation by Au
nanoparticles and improves their performance in the oxidation
processes strongly.
[42]
À1
ly. Spectral bands at n˜ =1545 and 1450 cm , which correspond
to Brønsted and Lewis acid sites, respectively, were analyzed.
TEM was performed by using a JEOL 2010 microscope. The sample
was dispersed in isopropanol and dropped onto a copper grid
coated with a carbon film. To determine the mean diameter of Au
particles, more than 150 particles were chosen. UV/Visible diffuse
reflectance spectra (UV/Vis DRS) were recorded at RT by using
a Varian Cary 300 scan spectrophotometer equipped with a stan-
Experimental Section
Catalyst preparation
[37]
dard diffuse reflectance unit as described previously. XPS meas-
urements were performed by using a Kratos AXIS 165 spectrome-
ter using monochromatic AlKa radiation (hv=1486.58 eV) and
a fixed analyzer pass energy of 20 eV. All measured BEs refer to the
C1s line of adventitious carbon at BE=284.8 eV. Spectrum decon-
volution was performed with background estimation using the
Nanostructured Ce–Al mixed oxides with different contents of ceria
(
10 and 30 wt%) were prepared by a sol–gel method using
[41]
organometallic precursors as described previously. A solution of
cerium(III) 2,4-pentanedionate hydrate (Alfa-Aesar) in ethanol with
moderate agitation was added to a mixture of aluminum sec-but-
oxide (Alfa-Aesar) in 2-methyl-2,4-pentanediol (Alfa-Aesar), and the
[43]
Shirley algorithm. XRD analysis was performed by using a Philips
&
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Full Papers
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and DGAPA-PAPIIT (UNAM, Mexico) through the grants 179619
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Received: November 18, 2014
Published online on && &&, 0000
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FULL PAPERS
E. Smolentseva, V. V. Costa, R. F. Cotta,
O. Simakova, S. Beloshapkin,
Mix it up: Au nanoparticles supported
on Ce–Al mixed oxides are more active
in the oxidation of benzyl alcohol and
acetaldehyde than catalysts prepared
from pure ceria or alumina, and the syn-
ergetic effect of the oxides on the per-
formance of the Au correlates well with
the oxygen activation capacity of the
materials.
E. V. Gusevskaya,* A. Simakov*
&& – &&
Aerobic Oxidative Esterification of
Benzyl Alcohol and Acetaldehyde over
Gold Supported on Nanostructured
Ceria–Alumina Mixed Oxides
&
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