Aerobic oxidation of alcohols catalyzed by in situ generated gold nanoparticles inside the channels of periodic mesoporous organosilica with ionic liquid framework

Article abstract of DOI:10.1021/acscombsci.9b00160

In situ generated gold nanoparticles inside the nanospaces of periodic mesoporous organosilica with an imidazolium framework (Au?PMO-IL) were found to be highly active, selective, and reusable catalysts for the aerobic oxidation of activated and nonactivated alcohols under mild reaction conditions. The catalyst was characterized by nitrogen adsorption-desorption measurement, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), elemental analysis (EA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The catalyst exhibited excellent catalytic activity in the presence of either Cs₂CO₃ (35 °C) or K₂CO₃ (60 °C) as reaction bases in toluene as a reaction solvent. Under both reaction conditions, various types of alcohols (up to 35 examples) including activated benzylic, primary and secondary aliphatic, heterocyclic, and challenging cyclic aliphatic alcohols converted to the expected carbonyl compounds in good to excellent yields and selectivity. The catalyst was also recovered and reused for at least seven reaction cycles. Data from three independent leaching tests indicated that amounts of leached gold particles were negligible (<0.2 ppm). It is believed that the combination of bridged imidazolium groups and confined nanospaces of PMO-IL might be a major reason explaining the remarkable stabilization and homogeneous distribution of in situ generated gold nanoparticles, thus resulting in the highly active and recyclable catalyst system.

Full text of DOI:10.1021/acscombsci.9b00160

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Article
Aerobic Oxidation of Alcohols Catalyzed by In Situ Generated
Gold Nanoparticles inside the Channels of Periodic
Mesoporous Organosilica with Ionic Liquid Framework
Babak Karimi, Akram Bigdeli, Ali Asghar Safari, Mojtaba
Khorasani, Hojatollah Vali, and Somaiyeh Khodadadi Karimvand
ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.9b00160 • Publication Date (Web): 16 Jan 2020
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Aerobic Oxidation of Alcohols Catalyzed by In Situ
Generated Gold Nanoparticles inside the Channels
of Periodic Mesoporous Organosilica with Ionic
Liquid Framework
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Babak Karimi*†‡, Akram Bigdeli†, Ali Asghar Safari†, Mojtaba Khorasani†‡, Hojatollah
Vali§ and Somaiyeh Khodadadi Karimvand†
† Institute for Advanced Studies in Basic Sciences, No. 444, Prof. Sobouti Boulevard, P.O.
Box 45195-1159, Zanjan 45137-66731, Iran.
‡
Research Center for Basic Sciences & Modern Technologies (RBST), Institute for
Advanced Studies in Basic Sciences (IASBS), Prof. Sobouti Boulevard, Zanjan 45137-
66731, Iran.
§ Department of Anatomy and Cell Biology and Facility for Electron Microscopy Research,
McGill University, Montreal, Quebec H3A 2A7, Canada.
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KEYWORDS: Periodic Mesoporous Organosilica (PMO), Au Nanoparticles, Alcohol
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Oxidation, Heterogeneous Catalyst, Ionic Liquid.
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Abstract
In situ generated gold nanoparticles inside the nanospaces of periodic mesoporous organosilica
with imidazolium framework (Au@PMO-IL) were found to be a highly active, selective and
reusable catalyst for the aerobic oxidation of activated and non-activated alcohols under mild
reaction conditions. The catalyst was characterized by nitrogen adsorption-desorption
measurement, thermogravimetric analysis (TGA), transmission electron microscopy (TEM),
elemental analysis (EA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), X-
ray photoelectron spectroscopy (XPS) and inductively coupled plasma atomic emission
spectroscopy (ICP-AES). The catalyst exhibited excellent catalytic activity in the presence of
either Cs2CO3 (35 °C) or K2CO3 (60 °C) as reaction bases in toluene as a reaction solvent. Under
both reaction conditions, various types of alcohols (up to 35 examples) including activated
benzylic, primary and secondary aliphatic, heterocyclic and challenging cyclic aliphatic alcohols
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converted to the expected carbonyl compounds in good to excellent yields and selectivity. The
catalyst also recovered and reused at least for seven reaction cycles. Data from three independent
leaching tests indicated that amounts of leached gold particles were negligible (< 0.2 ppm). It is
believed that the combination of bridged imidazolium groups and confined nanospaces of PMO-
IL might be major reason explaining remarkable stabilization and homogeneous distribution of in-
situ generated gold nanoparticles, thus resulting in the highly active and recyclable catalyst system.
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Introduction
Selective oxidation of alcohols to the related carbonyl compounds such as aldehydes, ketones
and carboxylic acids, considered as one of the most interesting research topics since desired
products are extensively used in the synthesis of pharmaceuticals, agricultural and fine chemicals.1
These transformations have been traditionally performed by using stoichiometric amounts of
harmful organic as well as inorganic oxidants. Therefore, during recent two decades, in viewpoints
of green chemistry and specially atom economy, a plethora of researches have been directed
toward the development of new and efficient catalytic systems on the basis of molecular oxygen
or hydrogen peroxide as green terminal oxidants.2 In this regard, many sophisticated protocols
based on transition-metal catalyst were developed in either homogeneous or heterogeneous form.
However, for many practical applications heterogeneous compared to homogeneous catalysis
stands with greater promise because of simple recovery of the catalyst as well as minimization of
metal contamination in the final products.3-7
Various supported metal catalysts on the basis of gold, palladium, platinum, and ruthenium and
etc. in the form of metal nanoparticles (NPs) has thus far been employed in the liquid phase aerobic
oxidation of alcohols.8-18 Not only these systems provide the advantages of recovering and reusing
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the catalysts, but it has been demonstrated that synergistic effect between support and supported
metal NPs may also improve the selectivity and life time of catalyst.19-42 Among several metal
catalysts reported so far, gold catalysts have received particular attentions due to their excellent
performance in the aerobic oxidation of alcohols under ambient conditions.
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Similar to other noble metals, nano and sub-nano gold particles have shown very interesting
activities, selectivities as well as turn over numbers in the variety of catalytic processes.43 In this
context, it has been well documented that the activity, selectivity and life time of the gold
nanoparticles depend not only on size and shape of the particles, but it can be significantly
influenced by the nature of support, location of gold on the support, and synergistic interactions
between gold and supporting materials.44 In order to prevent the agglomeration of Au nanoparticles
and retain high catalytic activity, several stabilizing agents such as poly-vinylpyrrolidone (PVP),
sugars, surfactants, and various types of solid supports have been used during the synthesis steps
and/or when they acted in desired reaction mediums.45-46 Although, considerable improvements
have been achieved in the catalytic activity, selectivity and substrate scope, there are still major
problems with Au nanoparticle based catalysts. Primarily, the control in size of Au nanoparticles
is a highly challenging because of very low melting point of gold.47-48 Recently, we reported the
preparation of Au nanoparticles on a bifunctional phenylene/ethylene periodic mesoporous
organosilica (Au@PMO).49 The catalyst was then successfully employed in the aerobic oxidation
of alcohols under ambient temperature. Meanwhile we discovered that incorporating an
appropriate concentration of bridged imidazolium group inside the framework of mesoporous
silica led to innovative functional materials (so called PMO-IL) which serve as superior supported
ionic liquid and/or effective support for immobilization and stabilization of metal nanoparticles in
several synthetically important transformations.50-57 It was demonstrated that in these materials the
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molecular diversity of IL-like nanostructures that act as effective solid ionic stabilizer for metal
nanoparticles is genuinely combined with the advantages of an integrated reaction and purification
strategy. In particular, by using XPS studies we showed that Au supported on PMO-IL
(Au@PMO-IL) in the form of Au(III) species is an efficient catalyst in the A3-coupling reaction
of aldehyde with amine and phenylacetylen to give the corresponding propargylamines.58
Considering the excellent capability of PMO-IL in immobilization and stabilization of metal
nanoclusters, in our present work, we attempt to explore the use of Au@PMO-IL in the aerobic
oxidation of alcohols under mild reaction conditions. With this goal in mind, the aerobic oxidation
of several alcohols was investigated for confirming the general applicability of Au@PMO-IL
catalyst in the presence of either Cs2CO3 (at room temperature) or K2CO3 (at 60-80 °C). The
advantage of our catalyst systems relies on the fact that in contrary to most of the reported gold
catalysts the pre-generation of gold nanoparticles by employing an exogenous reducing agent is
not required. More importantly, the open pore structure of Au@PMO-IL not only exposes a highly
catalytically active species, but it also combines the possibility of separating and recycling both
ionic liquid and supported Au NPs at the same times, the features that are all together of paramount
importance for heterogeneous catalysis.
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Result and Discussion
The synthesis of bridged ionic liquid 1,3-bis(3-trimethoxysilylpropyl)-imidazolium chloride
(BTMSPC) was accomplished according to our reported procedures with a few modification.50
The PMO-IL was then prepared using hydrolysis and co-polymerization of both BTMSPCl and
tetramethoxyorthosilicate (TMOS) in the presence of Pluronic P123 as effective structure directing
agent (SDA) under acidic conditions.50 Surfactant was then removed by acidic ethanol to provide
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PMO-IL. The resulting PMO-IL was then used to immobilize anionic gold species (AuCl4 ) via a
simple ion-exchange-impregnation technique according to our previous reported procedure with
slight changes.58 In this regard, PMO-IL was first sonicated in water to obtain a homogeneous
distribution of particles of materials and then a solution of NaAuCl4 was added to replace some of
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PMO-IL chloride counter ions with AuCl4 until the final catalyst which is denoted as Au@PMO-
IL was obtained.
The textural and structural feature of PMO scaffold and Au@PMO-IL were evaluated by
nitrogen sorption analysis. Figure 1 displays N2 adsorption–desorption isotherms and BJH pore
size distributions (inset) for all materials. N2 adsorption–desorption isotherms exhibit type IV
adsorption isotherms with H1 hysteresis loops which are characteristics of mesoporous materials
with narrow pore size distributions.59 As evidenced by these investigations, the shape of isotherm
in Au@PMO-IL is well-retained as compared with that of pristine PMO-IL, suggesting that the
structure ordering survived during the mere ion-exchange process. Moreover, narrow pore size
distributions for both PMO-IL and Au@PMO-IL indicate high-ordered structure in the mentioned
materials (Figure 1, inset). The textural parameters of PMO-IL, fresh Au@PMO-IL and recovered
catalyst are summarized in Table 1. These results show that after immobilization of the anionic
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AuCl4 species into the PMO-IL channels, the specific surface area, total pore volume, and average
pore diameter decreased from initial amounts of 530 m2 g-1, 0.99 cm3 g-1 and 10.6 nm to 432 m2
g-1, 0.77 cm3 g-1 and 9.2 nm, respectively. On the basis of these measurements, the incorporation
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of AuCl4 species shows minor effects on the pore size, pore volume and surface area. While the
decrease in nitrogen uptake under the relative pressure 0.6-0.8 demonstrates successful
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immobilization of AuCl4 ions inside the mesochannels of PMO-IL, the isotherms clearly reflect
the excellent pore opening in the materials after ion-exchange process.
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Figure 1. N2 adsorption-desorption isotherms and BJH Pore size distributions (inset) for PMO-IL,
Au@PMO-IL and recovered Au@PMO-IL.
Table1. Textural properties of the PMO-IL, Au@PMO-IL and recovered Au@PMO-IL.
[a)
[c]
SBET
Vt[b]
DBJH
(nm)
10.6
9.21
9.2
F.G.[d]
Material
(m2g-1)
530
(cm3g-1)
0.98
(mmol g-1)
IL(1.1)
PMO-IL
Au@PMO-IL
432
0.77
Au (0.090)
Au (0.085)
Recovered Au@PMO-IL
355
0.58
[a] SBET: specific surface area from linear part of adsorbed nitrogen in the range from 0.05 to
0.3. [b] Vt: total pore volume derived from adsorbed nitrogen from relative pressure ≈ 0.99. [c]
DBJH: average pore size diameter calculated from the adsorption branch using BJH method. [d]
Loading of functional groups estimated by TGA, elemental analysis or ICP-AES.
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TEM studies were performed on PMO-IL and Au@PMO-IL, in order to confirm uniform
mesoporous structure in the materials. TEM image of PMO-IL (Figure 2a) clearly demonstrates
formation of a long-range ordered mesoporous structure with estimated pore size of 10 nm. On the
other hand, TEM image of catalyst Au@PMO-IL also establishes that periodicity in the
mesoporous structure is retained during catalyst preparation (Figure 2b). From this image it is clear
that no formation of Au nanoparticles or aggregation was occurred either on the external surface
or inside the mesochannels of PMO-IL. Overall, these finding are in good agreement with results
obtained by N2 adsorption-desorption analysis (Table 1).
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Figure 2. TEM images of PMO-IL (a) and Au@PMO-IL (b and c)
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The integrity of organic groups inside the PMO-IL channels was also examined by diffuse
reflectance infrared Fourier transform spectroscopy (DRIFT). Both materials displayed similar
patterns and peak intensity (Figures S1-S2). The peaks observed at ca. 950 and 1100 cm-1, can be
assigned to asymmetric and symmetric stretching vibrations of Si-O-Si bonds.60 In addition the
vibration around 2950 cm-1 corresponds to the asymmetric stretching vibration of aliphatic or C-
H bonds in the bridged imidazolium units.61 Moreover, the presence of imidazolium moieties may
be further confirmed by the vibration at 1633 cm-1 which is related to the stretching vibration of
C=N bonds.62 Peaks at 800 and 3440 cm-1 can be attributed to bending vibrations of Si-C bonds
and stretching vibration of O-H in surface silanol groups, respectively.50 These results indicate that
PMO precursors well incorporated during the Sol-Gel process, and retained during the aging,
surfactant extraction stage and catalyst preparation through ion exchanging process.
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The thermal stability as well as organic content of catalyst was evaluated by thermogravimetric
analysis (TGA) in temperatures ranging from 20 to 800 °C under air flow (Figure 3). The first
stage (ca. 6 wt%) occurring below 150 °C may be due to desorption of solvent molecules that
remained from either solvent-extraction processes or during catalyst preparation stages. The
subsequent main mass-loss of ca. 21 wt% between 350 and 600 °C is most likely related to the
decomposition of the bridged imidazolium ionic liquids in the framework of catalyst. From the
last data, the amounts of ionic liquid were approximately determined to be 1.1 mmol.g-1. Finally,
the gold contents in the catalyst were found to be 0.09 mmol g-1 using inductively coupled plasma
atomic emission spectroscopy (ICP-AES) from its acid washed solution.
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Figure 3. TG pattern for Au@PMO-IL
Once the final catalyst was characterized, its activity was first evaluated as recoverable catalyst
for the aerobic oxidation of alcohols under a normal pressure of molecular oxygen and at the
ambient temperature. The aerobic oxidation of benzyl alcohol (0.5 mmol) was first tested in
various solvents, catalyst loadings, and in the presence of Cs2CO3 following to our previous
successful experience of using this base in the gold-catalyzed aerobic oxidation of alcohols to find
the best optimized reaction conditions (Table 2).63 Our studies revealed that Cs2CO3 with
combination of toluene gave the best result as compared with the other solvents like CH3CN:H2O,
CH3CN, H2O, CH2Cl2 and α,α,α-trifluorotoluene (TFT) (Table 2, entry 6 vs. entries 1-5). Here we
attribute the high catalytic activity of Au@PMO-IL in toluene to superior solubility of molecular
oxygen in this solvent, a feature which is in good agreement to our previous reports.64-66 In the
next stage, the activity of catalyst was also probed in the presence of various bases in the toluene
as solvent in the same reaction. In this regard, several bases such as Et3N, Na2CO3, K2CO3 and
CaCO3 were also examined, but all of them resulted in poor yields of benzaldehyde (Table 2,
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entries 7-9). Although, all carbonate bases have relatively the same basicity, either the higher
solubility of Cs2CO3 at lower temperature in a nonpolar solvent as toluene or a phenomena so-
called "Cesium effect" might be to some extent the reasons for superior activity observed in the
case of using Cs2CO3 as exogenous base.67,68 The initial experiments using K2CO3 gave 88%
conversion after 12 h, under otherwise the same reaction conditions, but it was impossible to obtain
complete conversion even after prolonged reaction times (48h) (Table 1, entry 10). While
decreasing the catalyst loading from 0.4 to either 0.2 or 0.1 mol% did not apparently affect the
overall efficiency of the process in the case of using benzyl alcohol (Table 1, entries 11 and 12),
these conditions were proved to be significantly less efficient for oxidation of other alcohols.
Therefore, the best conditions were found to be alcohol (0.5 mmol), Au@PMO-IL (0.4 mol%),
Cs2CO3 (1.5 mmol), O2 (1 atm.) in toluene (0.7 mL) at 35 °C during 3.5 h (Table 2, entry 6).
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Table 2. Effects of solvents, catalyst loadings and bases in the aerobic oxidation of benzyl alcohol
catalyzed by Au@PMO-IL.[a]
Au based catalyst (Y mol%)
OH
O
Base (1.5 mmol)
Solvent (0.7 ml)
35 °C
Entry
Catalyst (Y)
Solvent
CH3CN:H2O
CH3CN
H2O
Base
t (h)
Con.[b] (%) Sel. [c] (%)
TON[d]
12.5
20
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2
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4
5
Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Cs2CO3 24
Cs2CO3 24
Cs2CO3 24
Cs2CO3 12
5
>99
>99
93
8
11
83
90
27.5
207.5
225
CH2Cl2
TFT
>99
>99
Cs2CO3
8
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Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Au@PMO-IL (0.4)
Au@PMO-IL (0.2)
Au@PMO-IL (0.1)
Au-Catalyst-(1) (0.4)
Au-Catalyst-(1) (0.4)
Au-Catalyst-(2) (0.4)
Au@SBA-15 (0.4)
Toluene
Cs2CO3 3.5
Et3N 24
Na2CO3 24
>99
25
10
25
88
97
85
98
37
10
<5
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
250
62.5
25
7
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
8
9
CaCO3
K2CO3
Cs2CO3
24
12
6
62.5
220
242.5
212.5
245
92.5
25
10
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13
14
15
16[e]
Cs2CO3 12
Cs2CO3
K2CO3
6
24
Cs2CO3 24
Cs2CO3 24
12.5
[a] Reaction conditions: benzyl alcohol (0.5 mmol), catalyst (0.1-0.4 mol%, 5-20 mg), solvent (0.7 ml),
base (1.5 mmol), O2 ( 1atm.) at 35 °C. [b] GC yields using internal standard method. [c] Reaction selectivity
푦푖푒푙푑(%)
to benzaldehyde. [d] Turn over frequency, TON = 퐶푎푡. (푚표푙 %) . [e] Au@SBA-15 was prepared according to
our literature procedure.49 Although due to some limitations the reactivity differences have been compared
in all data at the high end conversions, a more realistic comparison could be achieved at lower initial
conversion.
In the next stage, the influence of the preparation method on the catalytic performance of our
catalyst was also studied in oxidation of benzyl alcohol under optimized conditions (Table 2, entry
13-16). To do this, Au-catalyst-(1) and Au-catalyst-(2) with the same metal loading were prepared
via impregnation followed by reduction of Au(III) species by employing NaBH4 (for catalyst 1)
and benzyl alcohol (for catalyst 2) as reducing agents, respectively (see Experimental Section for
details). Although, Au-catalyst-(1) showed also remarkable catalytic activity for oxidation of
benzyl alcohol (Table 2, entry 13), the activity was dramatically declined in the case of oxidation
of non-activated and hindered alcohols which often need much longer reaction time (Table S2).
This suggests that pre-generated Au nanoparticles could not provide durable catalytic activity most
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likely because of their agglomeration and subsequent deactivation. On the other hand, Au-catalyst-
(2) prepared via benzyl alcohol reduction protocol, showed poor activity and only generated
benzaldehyde in 10% yields after 24 h (Table 2, entry 15). Therefore, the much higher as well as
durable activity of our Au@PMO-IL as compared to Au-catalyst-(1) and -(2) may be also
attributed to the different Au composition on the surface of these materials. We will discuss this
issue later in the manuscript. It is also worth noting that a catalyst prepared by immobilization of
NaAuCl4 on SBA-15 showed very poor activity in the aerobic oxidation of benzyl alcohol (Table
2, entry 16). This finding clearly demonstrates that presence of imidazolium groups in the interior
of PMO channels can provide efficient environment for both excellent distributions of AuNPs and
their long term stabilization.58
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
With the optimal reaction conditions in hand, we then evaluated the generality of the catalytic
system in aerobic oxidation of various primary, secondary, and even sterically hindered alcohols
(Table 3). A wide range of primary or secondary benzylic alcohols comprising either electron
withdrawing, releasing group or sterically hindrances were converted to the corresponding
carbonyl compounds in excellent yields and superior selectivity (Table 3, entries 1-16). Selective
oxidation of linear primary aliphatic alcohols catalyzed by gold catalysts, regarded as a highly
challenging process at room temperature. Although, somewhat higher catalyst loading was needed,
3-phenyl propanol and n-heptanol were selectively oxidized to the related aldehyde within 24h at
room temperature in excellent yields and selectivity, (Table 3, entries 11-12).
The alcohols comprising heteroatoms, also considered as another highly challenging substrates
for aerobic oxidation in the most transition-metal catalyst systems because the strong coordination
of these alcohols to metal center deactivates the catalyst. However, we found that some related
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alcohols can convert to the expected aldehyde in moderate to good yield and excellent selectivity
(Table 3, entries 17-20).
7
8
9
While many reports are concerned to the gold-catalyzed oxidation of primary alcohols to the
corresponding aldehydes, the oxidation of secondary aliphatic alcohols to the corresponding
ketones has been less studied. Using the present procedure, Au@PMO-IL also showed excellent
activity for the selective oxidation of open chain secondary aliphatic alcohols at room temperature
(Table 3, entry 24). In particular, under the optimized reaction conditions, alcohols with sensitive
functional groups such as propargylic and c-propyl moieties afforded the corresponding ketones
in excellent yields and selectivity without any detectable side-products (Table 3, entries 25–26).
Interestingly, the oxidation of cyclic aliphatic alcohols under the same reaction conditions
furnished the expected ketones in good to excellent yields (Table 3, entries 27–32). However, the
oxidation of borneol and menthol only produced moderate yields of the corresponding ketones
(Table 3, entries 33 and 34). To the best of our knowledge, oxidation of such highly encumbered
alcohols in the presence of gold catalyst at room temperature has not yet been reported.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3. Aerobic oxidation of alcohol catalyzed by Au@PMO-IL in the presence of Cs2CO3.[a]
OH
R2
O
Au@PMO-IL (Y mol%)
R1
R1
R2
Cs2CO3 (1.5 mmol)
toluene (0.7 ml)
35°C
Entry R1
R2
Y
t (h) Con.[b] (%) Sel. [c] (%) TON[d]
1
2
3
C6H5
H
H
H
0.4 3.5 >99
>99
>99
>99
250
250
250
4-CH3-C6H4
0.4 6
0.4 3
>99
>99
4-CH3O-C6H4
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4
4-i-C3H7-C6H4
4-Cl-C6H4
4-NO2-C6H4
2-CH3-C6H4
2-NO2-C6H4
3-Cl-C6H4
2,4-di-Cl-C6H3
C6H5CH2CH2
n-C6H13
H
0.4 12
0.4 12
0.4 20
0.4 7
>99
>99
>99
>99
>99
>99
>99
>99
95
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
250
250
250
250
250
250
250
50
5
H
6
H
7
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8
H
0.4 16
0.4 8
9
H
10
11
12
13
14
15
16
17
18[e]
19[e]
20[e]
21
22
23
24
25
26
H
0.4 16
H
2
5
2
24
24
30
H
19
(CH3)2C=CH
C6H5
H
75
36
C6H5
CO C6H5
0.4 5
0.4 7
>99
>99
95
250
250
238
48
C6H5
4-Cl-C6H4
4-CH3S-C6H4
2-Furyl
CO C6H4-4-Cl 0.4 24
H
2
2
2
2
48
12
4
96
H
65
32
3-Pyridyl
3-Pyridyl
n-C5H11
H
45
22
CH3
CH3
CH3
CH3
C6H4
c-C3H5
C≡CH
24
50
24
0.4 7
97
242
238
238
250
200
41
C6H5CH2CH2
c-C6H13
0.4 12
0.4 12
0.4 12
0.4 12
95
95
c-C6H11
>99
80
c-C3H5
n-C5H11
2
2
24
24
80
27
28
95
95
>99
>99
OH
46
46
2
24
OH
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OH
4
5
6
7
8
9
29
30
31
0.4 8
>99
75
>99
>99
>99
250
37
2
8
OH
OH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.4 4
>99
250
OH
32
33
34
0.4 10
>99
45
>99
>99
>99
250
22
2
2
24
24
OH
OH
20
10
[a] Reaction conditions: alcohol (0.5 mmol), catalyst (0.4-5 mol%), toluene (0.7 ml), Cs2CO3
(1.5 mmol), O2 ( 1atm.) at 35 °C unless otherwise stated. [b] GC yields using internal standard
푦푖푒푙푑(%)
method. [c] Reaction selectivity to desired product. [d] TON = 퐶푎푡.( 푚표푙 %)) . [e] TFT was used as
solvent.
To gain a better insight into the nature of the Au species in the Au@PMO-IL and more
importantly the significant difference between various Au catalysts investigated in this study, a
thin film of the materials was prepared and analyzed using X-Ray photoelectron spectroscopy
(XPS). Figure 4 displays the XPS spectra of the catalysts prepared by the different methods in the
binding energy range for Au4f signals. The XPS spectra for Au-Catalyst-(1) and Au@PMO-IL
show only peaks located at 85-88 and 87-90 eV, respectively. These peaks assigned to the spin–
orbit split components of the Au(0)4f7/2/Au(0)4f5/2 and Au(3+) 4f7/2/Au(3+)4f5/2 levels in the materials,
respectively (Figure 4).58 The spectra of the fresh Au@PMO-IL mainly contain Au(3+) as indicated
by intense peaks at 86.5 and 90.5 eV. In contrary, the XPS spectrum of Au-catalyst-(1) only
contains peaks at 84.7 and 88.4 and a spin-orbit splitting of ~3.7 eV representing gold in metallic
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form Au(0). In this regard, apparently Au(3+) ions play a crucial role in enhancement of catalytic
activity. Therefore, the much higher activity of Au@PMO-IL as compared to Au-catalyst-(1)
might be attributed to higher Au(3+)/Au(0) ratios on the surface of the materials. It is speculated that
in the case of Au@PMO-IL, Au(3+) species may act as Lewis acid and coordinated with substrate,
a feature allowing starting alcohols to present in the nanospaces of the catalyst in high
concentration, where they can more efficiently oxidize at available Au active sites.
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17
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19
20
21
22
23
24
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34
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40
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43
44
45
46
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48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. XPS spectra for Au@PMO-IL (red line) Au-catalysts-(1) (blue line) and recovered
Au@PMO-IL (yellow line).
In the next stage, we were very interested to study the scope of present catalytic system by
employing K2CO3 as a cheaper base than Cs2CO3 (Table 4, entries 1–7). As it was already pointed
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out before in the manuscript, in the presence of K2CO3, toluene and 0.4 mol % of Au@OMO-IL,
benzyl alcohol was efficiently converted to benzaldehyde in good yields of 88% at 35 °C (Table
2, entry 10). However, the reaction was not completed even after prolonged (24 h) reaction times.
Our preliminary investigations also showed that this condition was not suitable for the efficient
oxidation of other alcohols. With the aim to optimum reaction conditions, several parameters were
checked and it was found that the use of K2CO3 (1.5 mmol), catalyst Au@PMO-IL (0.2 mol%),
toluene as solvent at 60 °C, are the best conditions for this catalytic system (Table 4, entry 5).
Table 4. Effects of solvents, catalyst loadings and temperature in the aerobic oxidation of
benzyl alcohol catalyzed by Au@PMO-IL in the presence of K2CO3. [a]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Au@PMO-IL (Y mol%)
OH
O
K2CO3 (1.5 mmol)
toluene (0.7 ml)
T (°C)
Entry
Y
T (°C)
35
Time(h) Con.[b] (%)
Sel. [c] (%)
>99
TON[d]
95
1
2
3
4
5
6
7
0.4
0.1
0.1
0.1
0.2
0.2
0.2
12
12
12
6
88
60
38
>99
378
540
990
500
500
500
70
54
>99
80
89
>99
60
6
>99
>99
>99
>99
70
6
>99
80
5
>99
[a] Reaction conditions: benzyl alcohol (0.25 mmol), catalyst (0.4 mol%), toluene (0.7
ml), K2CO3 (1.5 m0mol), O2 ( 1atm.) [b] GC yields using internal standard method. [c]
푦푖푒푙푑(%)
Reaction selectivity to benzaldehyde. [d] TON = 퐶푎푡. (푚표푙%)
Interestingly, under the same reaction conditions, various benzylic alcohols gave the respective
benzaldehyde in high yields and selectivities (Table 5, entries 1-12). Even in the case of more
challenging aliphatic alcohols, the corresponding aldehydes were obtained in excellent yields and
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selectivities (Table 5, entries 13 and 14). The oxidation of cinnamyl alcohol, gave cinnamaldehyde
in excellent yields and selectivity without the formation of any detectable side products (Table 5,
entry 15). However, the oxidation of 3-methyl-2-buten-1-ol only produced moderate yields of the
corresponding aldehyde even in the presence of relatively 1 mol% of Au@PMO-IL (Table 5, entry
16). The alcohols bearing heteroatoms were also oxidized to furnish the expected carbonyl
compounds in moderate to excellent yields (Table 5, entries 17-20). It is also worth mentioning
that under essentially identical reaction conditions, various linear secondary aliphatic alcohols
were easily converted to the corresponding ketones in good to excellent yields (Table 5, entries
21-29). However, in the case of dicyclopropylmethanol and 1-octyn-3-ol somewhat higher catalyst
loadings were needed to obtain acceptable yield of the corresponding ketones (Table 5, entries 30
and 31). Some activated and non-activated cyclic secondary alcohols could also be converted to
the related ketones in excellent yields (Table 5, entries 32-37). Unfortunately, menthol and borneol
were oxidized in the moderate yields even after prolonged reaction times in the presence of 5 mol%
of catalyst (Table 5, entries 38-39).
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13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 5. Aerobic oxidation of alcohol catalyzed by Au@PMO-IL in the presence of
K2CO3. [a]
OH
R2
O
Au@PMO-IL (Y mol%)
R1
R1
R2
K2CO3 (1.5 mmol)
toluene (0.7 ml)
60°C
Entry R1
R2
Y
t (h) Con.[b] %) Sel.[c] (%) TON[d]
1
2
3
4
C6H5
H
H
H
H
0.2 6
0.2 7
0.2 4
0.2 16
>99
>99
>99
>99
>99
>99
>99
>99
500
500
500
500
4-CH3-C6H4
4-CH3O-C6H4
4-i-C3H7-C6H4
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5
4-Cl-C6H4
4-Br-C6H4
4-NO2-C6H4
2-CH3-C6H4
2-NO2-C6H4
3-Cl-C6H4
3-Br-C6H4
2,4-di-Cl-C6H3
C6H5CH2CH2
n-C6H13
H
0.2 24
0.2 24
0.2 20
0.2 12
0.2 24
0.2 14
0.2 30
0.2 24
>99
95
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
500
475
500
500
500
500
500
500
99
6
H
7
H
>99
>99
>99
>99
>99
>99
>99
95
8
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
H
10
11
12
13
14[e]
15
16
17
18[f]
19[f]
20[f]
21
22
23
24
25
H
H
H
H
1
5
30
24
H
19
C6H5CH=CH
(CH3)2C=CH
4-CH3S-C6H4
2-Furyl
H
0.2 7
>99
10
500
10
H
1
2
2
2
2
24
H
24
24
30
30
75
36
H
65
31
3-Pyridyl
3-Pyridyl
n-C5H11
H
45
21
CH3
CH3
CH3
C2H5
C6H5
CO C6H5
50
24
0.2 30
0.2 12
0.2 14
0.2 4
97
483
500
500
500
500
C6H5
>99
>99
>99
>99
C6H5
C6H5
C6H5
0.2 8
CO C6H4-4-
Cl
26
4-Cl-C6H4
0.2 30
95
>99
474
475
500
500
80
27
28
29
30
C6H5CH2CH2
c-C6H13
CH3
0.2 24
0.2 24
0.2 14
95
>99
>99
>99
>99
CH3
>99
>99
80
c-C6H11
C6H5
c-C3H5
c-C3H5
1
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n-C5H11
C≡CH
1
48
45
95
>99
>99
43
OH
0.2 20
474
7
8
9
OH
OH
33
34
35
36
1
24
95
>99
>99
>99
>99
94
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.2 14
>99
75
500
74
OH
1
24
OH
0.2 4
>99
500
OH
37
0.2 12
>99
45
>99
>99
>99
500
7
38[e]
39[e]
5
5
24
24
OH
OH
30
5
[a] Reaction conditions: alcohol (0.25 mmol), catalyst (0.4-5 mol%), toluene (0.7 ml),
K2CO3 (1.5 mmol), O2 ( 1atm.) at 35 °C unless otherwise stated. [b] Conversion of
alcohols determined by GC using internal standard method. [c] Reaction selectivity to
푦푖푒푙푑(%)
benzaldehyde. [d] TON = 퐶푎푡. (푚표푙%)) . [e] At 80 °C. [f] TFT was used as solvent.
When using a heterogeneous catalyst, the questions regarding the reuse and the leaching of active
species in solution should be resolved. Therefore, it is very important to determine whether or not
support itself survives the reaction conditions and the actual catalysis was from leached or
supported metal. In this regard, the recycling of Au@PMO-IL was assessed in the aerobic
oxidation of benzyl alcohols under described reaction conditions in entry 6 of Table 2. After first
run, the catalyst was recovered by simple filtration, followed by washing with excess amount of
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water and acetone, and dried under vacuum. The recovered catalyst was then reused for the next
run, as shown in Figure S10, the catalyst still shows good activity and excellent selectivity over
seven consecutive reactions. A filtration test at 35 °C showed that no soluble Au was detected
using ICP-AES within the detection limit (0.2 ppm). Moreover, the ICP-AES analysis indicated
that the Au contents in the catalyst after 7th catalytic cycle remained unchanged (0.085 mmol g-1).
We also conducted a filtration test for oxidation of benzyl alcohol catalyzed by Au@PMO-IL in
the presence of cesium carbonate (Cs2CO3) as base. In this regard, the aerobic oxidation of benzyl
alcohol was initially carried out under optimized reaction (1.5h, 45%) [See the Experimental
Sections for details]. We found that the reaction did not carefully progress and benzaldehyde was
obtained in only 2%. These findings clearly demonstrate that alkyl imidazolium groups in the
interior of mesochannels of Au@PMO-IL not only serve as a handle for in situ generation and
stabilization of gold nanoparticles but they can also effectively prevent the leaching of Au species
into solution during the reaction process. In addition to investigating the loss of Au from the
catalyst upon treatment under reaction condition, the structural integrity of catalyst was assessed
in several ways. Nitrogen adsorption-desorption isotherm of the recovered Au@PMO-IL showed
a type IV isotherm with a narrow hysteresis loop which confirms structural order of Au@PMO-IL
largely retained during reaction process (Figure 1, S11). In comparison to fresh catalyst, the BET
surface area and mesoporous volume for recovered catalyst were decreased from 432 and 0.77 to
355 m2 g-1 and 0.58 cm3 g-1, respectively (Table 1). Interestingly, BJH calculations revealed a
relatively sharp pore size distribution centered around 9.2 nm similar to fresh catalyst (Figure 1,
S12). These findings clearly demonstrate that structural ordering of catalyst significantly remained
intact under basic reaction conditions. However, it should be noted a relatively two-stage
desorption in N2 adsorption-desorption isotherm of the recovered Au@PMO-IL observed which
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is very similar to those of mesoporous silica/organosilica with confinement channels so called
plugged hexagonal templated silica (PHTS) materials.69 Such adsorption-desorption isotherm is
very similar to those materials having both open and plugged (constricted) cylindrical mesopores.
As we have already explained and by providing several compelling evidences including TEM
miscroscopy of both fresh and recycled Au@PMO-IL (See Figure 5), Au nanoparticles are
produced both inside and outside the mesochannels of PMO-IL during the catalytic process.
Therefore, it is most likely that the generated Au nanoparticles inside the mesochannels could
behave as "plugs" in some mesopores. In this case, open pores are gradually desorbed at reduced
relative pressures in a normal way but blocked pores with AuNPs will remain filled until the vapor
pressure is lowered to P/P0 = 0.45 (lower limit for Hysteresis P/P0) when condensed N2 eventually
desorbs.70
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Transmission electron microscopy (TEM) image of recovered Au@PMO-IL after the seventh
reaction cycle not only proved the retention of ordered structure in the catalyst after reaction and
recovery, but also confirm that agglomeration of Au nanoparticles was negligible during reaction
processes and recycling (Figure 5). Finally, TG diagram of the recovered catalyst showed weight-
loss of ca. 25% between 350 and 600 °C (Figure S13). This observation also indicated that bridged
imidazolium ionic liquids in the catalyst framework remained intact under basic reaction
conditions. Interestingly, the XPS spectrum of the recovered catalyst Au@PMO-IL confirmed the
presence of both Au(3+) and Au(0) species in the reused catalyst, which highlights the notion that
the Au(III) species remain on the surface of Au in metallic form even in the recycled catalyst (Figure
4).
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Figure 5. TEM image of recovered Au@PMO-IL catalyst after 7th run in the oxidation of benzyl
alcohol
Conclusion
In conclusion, among three distinct Au supported catalysts onto periodic mesoporous
organosilica with imidazolium framework (PMO-IL), in situ generated gold nanoparticles inside
the nanospaces of PMO-IL (Au@PMO-IL) exhibits superior activity, selectivity and high stability
as well as good reusability in the aerobic oxidation of activated and non-activated alcohols in the
presence of either Cs2CO3 (35 °C) or K2CO3 (60 °C) as reaction bases under mild reaction
conditions. It was found that the activity is strongly influenced by the method of generating the
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Au nanoparticles inside the well-ordered mesopore networks of PMO-IL. Our studies imply that
the co-existence of Au(3+) ions and Au(0) on the surface of catalyst play a crucial role in
enhancement of catalytic activity. This is most likely explained by the fact that high local Au(3+)
concentration of the surface of catalyst act as Lewis acid and coordinated with substrate, a feature
allowing starting alcohols to present in the nanospaces of the catalyst in high concentration, where
they can more efficiently oxidize at available Au active sites before they can release back to the
solution. The amounts of leached catalyst were very negligible (<0.2 ppm) and catalyst was
recovered at least seven reaction cycles. High stability and reusability catalyst can be attributed to
the presence of ionic liquid in the catalyst, a finding that is in good agreement with reference
catalyst experiment and structural analysis of a sample of recovered catalyst.
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ASSOCIATED CONTENT
Supporting Information including reaction procedures, additional characterization and
spectroscopic data for products is available.
AUTHOR INFORMATION
Corresponding Author
* Babak Karimi, 0000-0002-0453-0047
† Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
No. 444, Prof. Yousef Sobouti Blvd., Zanjan 45137-66731, Iran.
Email: karimi@iasbs.ac.ir; Fax: +98-24-3341-4747; Tel: +98-24-3315-3225
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‡ Research Center for Basic Sciences & Modern Technologies (RBST), Institute for
Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
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ACKNOWLEDGMENT
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The authors acknowledge IASBS Research Councils (Grant no. G2019IASBS31100), and
Iranian National Science Foundation (INSF, Grant no. G020), NanoQuebec and Natural Sciences
and Engineering Research Council of Canada (H.V.) for support of this work.
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