Gold nanoparticles supported on the periodic mesoporous organosilicas as efficient and reusable catalyst for room temperature aerobic oxidation of alcohols

Article abstract of DOI:10.1002/adsc.201100802

Gold nanoparticles supported on the channels of a bifunctional periodic mesoporous organosilica, were found to be a highly efficient catalyst system for the aerobic oxidation of various types of alcohols into their corresponding aldehydes and ketones at room temperature. The catalyst showed no significant loss of efficiency for the aerobic oxidation of benzyl alcohol to give benzaldehyde after 7 reaction cycles. Copyright

Full text of DOI:10.1002/adsc.201100802

FULL PAPERS
DOI: 10.1002/adsc.201100802
Gold Nanoparticles Supported on the Periodic Mesoporous
Organosilicas as Efficient and Reusable Catalyst for Room
Temperature Aerobic Oxidation of Alcohols
Babak Karimia,* and Farhad Kabiri Esfahania
a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box 45137-6731, Gava Zang,
Zanjan, Iran
Fax : (+98)-241-4214949; e-mail: karimi@iasbs.ac.ir
Received: October 17, 2011; Revised: January 6, 2012; Published online: April 19, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201100802.
Abstract: Gold nanoparticles supported on the chan- of benzyl alcohol to give benzaldehyde after 7 reac-
nels of a bifunctional periodic mesoporous organosil- tion cycles.
ica, were found to be a highly efficient catalyst
system for the aerobic oxidation of various types of
alcohols into their corresponding aldehydes and ke- Keywords: aerobic oxidation; alcohols; gold nano-
tones at room temperature. The catalyst showed no particles; heterogeneous catalysts; periodic mesopo-
significant loss of efficiency for the aerobic oxidation rous organosilica (PMO)
Introduction
hols using air or molecular oxygen at atmospheric
pressure and low temperature.
In the current cutting edge catalysis research, gold ap-
Although a significant number of metal-catalyzed
pears to be one of the most important catalysts for aerobic oxidation processes based on gold,[7] gold/
a variety of organic transformations.[1] Although gold platinum,[8] palladium,[9] gold/palladium,[10] Ru,[11] and
has been traditionally considered as an inert and Cu[12] have given remarkable results, in many cases,
highly stable metal without catalytic activity, nowa- they require high reaction temperatures which are not
days, both homogeneous and heterogeneous gold spe- suitable for synthesis of complex and thermolabile
cies show excellent catalytic activities in a wide range compounds. In this regard, satisfactory results for the
of chemical processes.[2,3]
room-temperature aerobic oxidation of alcohols were
In particular, the small size of gold nanoparticles attained in only few cases.[7f–k,8,13] Therefore, the devel-
has been considered as one of the crucial criteria for opment of new reusable catalysts for the aerobic oxi-
their catalytic activities.[4] However, the great difficul- dation of alcohols under ambient conditions should
ty when using small particles is that they are thermo- attract a great deal of attention in both green chemis-
dynamically susceptible to agglomeration and the for- try and organic synthesis. Recently, several research
mation of larger inactive particles. A possible solution groups have demonstrated that the excellent catalytic
is based on the development of solid supports in performance of gold is not only influenced by the size
order to increase the catalystꢀs stability and to facili- and shape of the particles but also depends on the in-
tate the catalyst recovery.[5]
teraction between gold nanoparticles and support in-
Among the different reactions, the selective oxida- terface.[14] In particular, there are considerable rele-
tion of alcohols is one of the most important and fun- vant reports verifying that even a weak interaction of
damental transformations in organic synthesis, be- aromatic groups such as the benzene ring through p-
cause the corresponding carbonyl compounds are ver- electrons with gold nanoclusters appears to be suffi-
satile intermediates of valuable compounds such as cient to increase the catalystꢀs stability and to improve
pharmaceuticals, agricultural chemicals, and fine its performance.[7f,15] In recent years, the periodic mes-
chemicals.[6] An alternative hot topic in this area is oporous organosilicas (PMOs) with a large surface
the development of reusable heterogeneous catalysts area, tunable pore structure, and tailored composi-
that are able to perform the aerobic oxidation of alco- tions have received increasing interest in broad appli-
cations ranging from material chemistry to catalysis.[16]
Adv. Synth. Catal. 2012, 354, 1319 – 1326
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Babak Karimi and Farhad Kabiri Esfahani
FULL PAPERS
These materials can be built up through the surfactant nanoparticles would seem to be a versatile catalyst in
templated self-assembly of bridged silsesquioxane various types of transformations.
À À
precursor of the type [(R’O)3Si R SiACTHUNTRGNEU(GN OR’)3] similar
to that used in the preparation of ordered mesopo-
rous silicas. Unlike functionalized mesoporous sili- Results and Discussion
cates in which the organic groups are anchored in or
onto the pore walls, in PMOs the group R is an inte- To prove this hypothesis, we have chosen a known
gral part of the mesoporous channel walls, so they are benzene-ethane PMO which can be easily prepared in
homogeneously distributed throughout the framework the presence of Pluronic P123 as a template under
of materials. Moreover, the bridging organic moieties acidic conditions according to a reported simple pro-
in the framework of PMO not only improve hydro- cedure with slight modification.[19] The resulting bi-
thermal and mechanical stabilities, but also higher or- functional PMO was then used to support gold nano-
ganic loading and greater avoidance of channel block- particles. To do this, briefly, a dispersion of a surfac-
age can be achieved in comparison with mesoporous tant-free PMO sample in water was first sonicated to
materials functionalized with terminal groups. On the obtain a homogeneous distribution of PMO particles
other hand, the bridging organic groups inside the in- in water. The resulting dispersion was then subjected
organic matrix maintain characteristics similar to to mixing under vigorous stirring with an appropriate
those of free organic molecules and at the same time, concentration of aqueous NaAuCl4 after which all of
depending on the nature of these groups, the degree the NaAuCl4 was essentially adsorbed onto PMO ma-
of hydrophobicity of the silica framework can be terials to give a yellow slurry.[20] Next, the obtained
tuned toward specific applications. Along the line of slurry were allowed to react with a freshly prepared
these unique characteristics, a wide range of bridging aqueous solution of a stoichiometric amount of
organic units, especially those containing aromatic NaBH4 (0.1M) at room temperature while continuing
sections, has been integrated into inorganic matrixes, the vigorous stirring to obtain the corresponding Au
resulting in the preparation of different types of nanoparticles supported PMO (Scheme 1, Au@PMO
PMOs with novel chemical and physical properties.[17] 1).
While there are many reports on the synthesis of
The color of the matrix immediately turned from
PMOs, the number of applications involving PMO- yellow to wine red. TEM analysis of 1 illustrates a uni-
supported noble metal nanoparticles are quite limit- form tube-like structure with two-dimensional hexag-
ed.[18]
onal symmetry in which Au nanoparticles are well dis-
In particular, to the best our knowledge there are tributed with an average size of 3–4 nm in the meso-
currently, no reports of employing PMOs as support porous channels. The surface plasmon band at
for metal nanoparticles in the aerobic oxidation of al- ~518 nm appears in the diffuse-reflectance UV-vis
cohols. Considering the fact that even an aromatic (DR-UV-Vis) spectra, a value which is in good agree-
ring system could eventually improve the catalytic ment with the TEM images (Figure 1).[21] The XPS
performance of Au nanoclusters, we reasoned that spectrum indicates gold nanoparticles have to a great
PMOs having an aromatic bridge in their framework extent been settled on the PMO framework since
might be an excellent alternative matrix in order to there is a weak characteristic peak around the 80–
stabilize gold nanoclusters. If this hypothesis could be 90 eV.[20,21a]
realized in practice, the resulting supported gold
Scheme 1. A typical schematic pathway for the preparation of Au@PMO 1.
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Gold Nanoparticles Supported on the Periodic Mesoporous Organosilicas
Table 1. Effects of various solvents and bases on the aerobic
oxidation of benzyl alcohol with Au@PMO 1 catalyst.[a]
Entry
n
1
Solvent
toluene
Base
Time [h] Yield [%][b]
1
2
3
4
5
6
7
8
K2CO3
Cs2CO3
Cs2CO3
Et3N
24
0.75
1
10
99
99
14
9
0.4 toluene
0.4 toluene
0.4 toluene
0.4 toluene
0.4 toluene
0.4 toluene
0.4 TFT
0.4 CH2Cl2
0.4 H2O
0.4 TFT:H2O Cs2CO3
24
Figure 1. TEM images of Au@PMO 1.
AcOÀNa+ 24
K3PO4
–
Cs2CO3
Cs2CO3
Cs2CO3
24
24
24
24
24
24
11
<5
78
65
12
25
The activity of 1 was tested as a solid catalyst for
the selective aerobic oxidation of various types of al-
cohols under a normal pressure of molecular oxygen
and at room temperature.[22] Very recently, we showed
that NaAuCl4 in combination with Cs2CO3 is an effi-
cient catalyst system for the selective aerobic oxida-
tion of alcohols into their corresponding carbonyl
compounds in the absence of any polymeric and/or
oxidic support. However, despite the user-friendly
feature of this catalyst system, it suffers from the dis-
9
10
11
[a]
Reaction conditions: benzyl alcohol (1 mmol), Au@PMO
1 (0.4 mol%), toluene, Cs2CO3 (3 mmol), O2 (1 atm),
room temperature.
[b]
GC yield.
advantage of significant reduction of its reactivity 40% and 15% yields of oxidized products were ob-
after two further reaction cycles owing to extensive tained, respectively. The results clearly confirm the
agglomeration of the Au nanoparticles. Moreover, it beneficial role of the PMO phase in obtaining accept-
was also noted that due to the hygroscopic nature of able catalytic activities (Table 2). The much higher ac-
Cs2CO3 it is actually impossible to maintain the cata- tivity of 1, as compared to 2, 3 and 4 may be attribut-
lytic activity of stored samples.
ed to the high concentration of benzene units inside
First, we focused our attention on the aerobic oxi- the channel walls of PMO, which would favor p-inter-
dation of benzyl alcohol (1 mmol) using molecular actions with surface of Au nanoclusters and thus en-
oxygen (1 atm) in the presence of 1 mol% of 1, hance the catalytic activity of 1 (Table 2).
K2CO3 (3 mmol) in toluene at room temperature.
However, the actual activity of supported metal
Under these conditions, only 10% of the correspond- nanoparticles is not only influenced by the nature of
ing aldehyde was obtained even after 24 h (Table 1, the functionalized groups on a support but it can also
entry 1). Interestingly, the same reaction rapidly went be affected by other factors including size of particles,
to completion within 45 min when Cs2CO3 was used surface area and pore size distribution of the support.
as base instead of K2CO3, giving an excellent yield of
99% of benzaldehyde as sole product (Table 1,
Table 2. Effects of various Au-based catalysts on the aerobic
entry 2). This reaction also proceeded quickly in the
oxidation of benzyl alcohol.[a]
presence of 0.4 mol% of 1 (Table 1, entry 3). To deter-
mine the optimal experimental conditions, the impact
of various bases such as Et3N, Na+ AcOÀ, K3PO4 was
also studied, but all of them resulted in poor yields of
the corresponding aldehyde (Table 1, entries 4–6). A
variety of solvents such as TFT (1,1,1-trifluoroto-
luene), toluene/water, water, and CH2Cl2 were also in-
vestigated and it turned out that toluene was better
suited as solvent for this system (Table 1, entries 8–
11).
For the purpose of comparison, the aerobic oxida-
tion of benzyl alcohol was also tested using a number
of selected Au-supported catalysts such as Au@SBA-
15 2, Au@SBA-15-Pr-NR3+ XÀ 3, Au@SBA-15-Ph 4,
Au@TiO2 5 under the same reaction conditions and
with the same loading of Au, but only <5%, 28%,
Entry Catalyst
Time [h] Yield [%][b]
1
2
3
4
5
6
Au@SBA-15 2
24
<5
28
42
99
15
97
Au@SBA-15-Pr-NR3 XÀ 3 24
+
Au@SBA-15-Ph 4
Au@PMO 1
Au@TiO2 5
24
1
24
NaAuCl4
1.75
[a]
Reaction conditions: benzyl alcohol (1 mmol), catalyst
(0.4 mol%), toluene, Cs2CO3 (3 mmol), O2 (1 atm), room
temperature.
GC yield.
[b]
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Babak Karimi and Farhad Kabiri Esfahani
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To address this issue, we have also performed a careful primary benzylic alcohols to the corresponding car-
study on these catalysts using transmission electron bonyl compounds.
microscopy (TEM) and N2 adsorption-desoption anal-
Substituted benzyl alcohols bearing either electron-
ysis. TEM as well as high resolution TEM (HR-TEM) releasing or electron-withdrawing groups were also
of all AuNP supported catalyst systems are shown in oxidized smoothly and affording the corresponding
the Supporting Information (Figures S12–S15). Ac- carbonyl compounds in excellent yields in a relatively
cording to TEM images of the catalysts, if we are only short reaction time under the optimized reaction con-
taking the AuNPs size into account for the observed ditions (Table 3, entries 3–7). Despite the low reactivi-
catalysis, we may expect that Au@SBA-15-Pr- ty of secondary aliphatic alcohols, high catalytic activ-
NR3+ XÀ 3 should be even more reactive than our ities were also observed with both open-chain and
Au@PMO 1 and Au@SBA-15-Ph 4 in the aerobic oxi- cyclic aliphatic substrates, affording the corresponding
dation of alcohols. However, this is in sharp contrast ketones in excellent yields under the same reaction
to our observations. As can be interestingly seen, for conditions (Table 3, entries 16–19).
example, the size distribution of AuNPs supported on
It should be noted that the same transformations
Au@SBA-15-Pr-NR3+ XÀ 3 is smaller than that ob- for open-chain secondary alcohols using our recently
tained in our Au@PMO 1 catalyst system. Therefore, developed catalyst system (Au/Cs2CO3) were very
we believe the fact that Au@PMO consistently shows sluggish giving the corresponding ketones in poor
higher activity than those observed for example yields (Table 3, entries 20 and 21).[7g]
Au@SBA-15-Pr-NR3+ XÀ 3 (and of course other stud-
It is worth noting that 1 also exhibits a high catalyt-
ied AuNP supported systems) most likely highlight ic activity for the room temperature oxidation of vari-
the notion that the large differences between the vari- ous types of allylic alcohols affording the correspond-
ous gold-based catalysts are caused mainly by the sur- ing carbonyl compounds in good to excellent yields
face properties of PMO, and most probably originate and selectivities (Table 3, entries 22–24). Even dicy-
from the interaction of phenylene groups embedded clopropyl carbinol, which is remarkably sensitive,
in the silica frameworks with AuNPs. Moreover, we could also be oxidized to dicyclopropyl ketone in
have also determined both size and surface area infor- good yield in 10 h (Table 3, entry 17). On the other
mation regarding all catalysts based on mesoporous hand, while the catalytic activity of 1 for the oxida-
SBA-15 (Figures S6–S11 in the Supporting Informa- tion of primary aliphatic alcohols was found to be sig-
tion). As can be clearly seen, BET (Brunauer– nificantly high and gave conversions of 98–99% of the
Emmett–Teller) surface area and BJH (Barrett– corresponding carbonyl compounds without any de-
Joyner–Halenda) pore size distributions for tectable over oxidation to carboxylic acids, a feature
Au@SBA-15 2, Au@SBA-15-Pr-NR3+ XÀ 3, and not observed for almost all of the previous heteroge-
Au@SBA-15-Ph 4 are very close to each other. There- neous gold catalysts at room temperature (Table 3,
fore, for the surface analysis data it would be reasona- entries 25 and 26). For example Au/Cs2CO3 is com-
ble to speculate that the difference in catalytic activity pletely inactive in the oxidation of the same primary
of these three catalysts is most likely (and mainly) aliphatic substrate under identical reaction conditions
due to the difference in their functionalized organic (Table 3, entries 27 and 28).
groups. This observation again highlights the impor-
The recyclability of 1 was also examined by isolat-
tant role of phenyl groups in the observed catalytic ing it from the reaction mixture of aerobic oxidation
performances (Table 1, entries 1–3). It is worth men- of benzyl alcohol through successive microfiltration,
tioning that the positive effect of weak interactions of washing with toluene and drying. In each run the
aromatic ring systems and AuNPs has been previously leaching of Au species into solution was negligible as
well-documented in improving catalyst performance determined by atomic absorption spectroscopy (AAS)
in the aerobic oxidation of alcohols.[7f]
(less than the detection limit of 2 ppm). Furthermore,
Having the optimized reaction conditions, we next no appreciable catalytic activity was observed from
managed to examine further the scope and limita- the residue of toluene extraction of 1 even after pro-
tiond of this Au-catalyzed aerobic oxidation reaction, longed reaction times. The recycled catalyst was then
As shown in Table 3, a wide variety of benzylic alco- successfully used in at least 7 reaction cycles and dis-
hols, including either primary or secondary ones, were played consistent catalytic activities and selectivities.
converted to the corresponding aldehydes and ke- Interestingly, we found that even after the 7th reac-
tones in almost quantitative yields (Table 3, entries 1– tion cycle, the recovered catalyst still exhibited high
15).
activity in transforming benzyl alcohol to benzalde-
Noteworthy is that in the case of primary alcohols, hyde in excellent yield, albeit it was necessary to in-
we did not observe any over oxidation to the corre- crease the reaction time (from 1.75 h to 2.20 h for the
sponding carboxylic acids even after prolonged reac- 8th run) to ensure quantitative conversion. The exact
tion times (Table 3, entries 1–9), further confirming role of p-interactions on the catalystꢀs performance is
that this method is highly selective for oxidation of unclear for us at this stage. One plausible explanation
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Adv. Synth. Catal. 2012, 354, 1319 – 1326

Gold Nanoparticles Supported on the Periodic Mesoporous Organosilicas
Table 3. Aerobic oxidation of alcohols using Au@PMO 1 catalyst system at room temperature.[a]
Entry
1
Substrate
Time [h]
1
Yield [%][b]
97
Entry
15
Substrate
Time [h]
1.5
Yield [%][b]
100
2
3
1.75
0.75
88[c]
100
16
17
9
95
88
10
4
1.25
100
18
5
95
5
6
15
99
19
20
24
24
95
8.5
100
15[d]
7
8
2.75
24
100
–
21
22
24
8
10[d]
52
9
6
75
23
4
80
10
11
12
2.5
2.5
2.5
100
100
72
24
25
26
30
14
15
98
1-octanol
1-octanol
98[e]
99[e]
[d]
13
8.5
9
95
90
27
28
24
24
–
[d]
14
–
[a]
Reaction conditions: alcohols (1 mmol), Au@PMO (0.4 mol%), toluene, Cs2CO3 (3 mmol), O2 (1 atm), room temperature.
GC yield.
Reaction conditions: benzyl alcohol (1 mmol), Au@PMO (0.4 mol%), toluene, Cs2CO3 (3 mmol), air (1 atm), room tem-
perature.
[b]
[c]
[d]
[e]
Reaction conditions: alcohols (1 mmol), NaAuCl4 (0.4 mol%), toluene, Cs2CO3 (3 mmol), O2 (1 atm), room tempera-
ACHTUNGTRENNUNG
ture.[7g]
Reaction conditions: alcohols (1 mmol), Au@PMO (2 mol%), toluene, Cs2CO3 (3 mmol), O2 (1 atm), room temperature.
for the improved activity of 1 might be due to the hy- up the adsorption of organic substrates over the sur-
drophobic environment provided by phenyl groups face of catalyst, where AuNPs are located. It is also
inside the mesoporous channels, which likely speeds not surprising to think that the weak interactions of
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Babak Karimi and Farhad Kabiri Esfahani
1 (0.05 g, 0.4 mol%) and Cs2CO3 (0.978 g,
FULL PAPERS
Au@PMO
the benzene ring through their p-electrons with gold
nanoparticles may in turn prevent (or slow down) the
agglomeration of nanoparticles and led to a more
stable and durable catalyst system. However, further
in-depth investigations would be necessary to clarify
the actual synergistic effect between the bridging ben-
zene rings in PMO and supported Au nanoparticles in
achieving such a high activity. This study is currently
ongoing in our laboratories.
3 mmol) in toluene (8 mL). The flask was then evacuated
and refilled with pure oxygen or air for three times (bal-
loon-filled). The resulting mixture was stirred at room tem-
perature under an oxygen or air atmosphere. The progress
of the reaction was monitored by GC. After completion of
the reaction, the reaction mixture was filtered off and the
catalyst rinsed three times with Et2O (3ꢂ25 mL), the excess
of solvent was then removed under reduced pressure to give
the corresponding carbonyl compounds. In most cases, the
purity of products was analyzed by GC to be more than
99% without any chromatographic purification.
Conclusions
In conclusion, we have shown that bifunctional
phenyl-ethane PMO having aromatic bridges in their
framework can be used as an excellent alternative
matrix in order to stabilize gold nanoclusters. This
new material was found to be an efficient catalyst
system for the aerobic oxidation of a variety of struc-
turally diverse alcohols. The present catalyst system
showed superior catalytic activity than those of previ-
ously reported Au-based catalysts, especially in the
case of aerial oxidation of the most challenging pri-
mary aliphatic alcohols to the corresponding alde-
hydes at room temperature. The catalyst also showed
no significant loss of efficiency for the aerobic oxida-
tion of benzyl alcohol to give benzaldehyde within 7
reaction cycles.
Acknowledgements
The authors acknowledge IASBS and INSF Research Coun-
cils for support of this work. We also acknowledge Dr. Hoja-
tollah Vali from McGill University for TEM images.
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conditions and 300 g of acetone at 568C under stirring for
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tails.
Adv. Synth. Catal. 2012, 354, 1319 – 1326
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1325

Babak Karimi and Farhad Kabiri Esfahani
FULL PAPERS
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1326
asc.wiley-vch.de
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 1319 – 1326