Supported ionic liquid phase catalysis for aerobic oxidation of primary alcohols

Article abstract of DOI:10.1016/j.apcata.2010.09.022

The copper-TEMPO-catalysed aerobic oxidation of primary alcohols based on the new supported ionic liquid phase (SILP) catalysts is presented. CuCl ₂ acts as a homogenous catalyst dissolved in the small quantity of ionic liquid dispersed in the form of film on the solid support. A bi-modal pore structure silica with a highly developed surface and an ionogel were used as solid supports. 1-Butyl-3-methylimidazolium octylsulfate as the most active catalyst-philic phase was found. The application of SILP catalysts for the oxidation of alcohols allows the high yields of corresponding aldehydes (92-95%). The catalysts were used in seven cycles without significant loss of activity. High recoveries of the catalysts were observed.

Full text of DOI:10.1016/j.apcata.2010.09.022

Applied Catalysis A: General 389 (2010) 179–185
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Supported ionic liquid phase catalysis for aerobic oxidation of primary alcohols
Anna Chroboka,∗, Stefan Baja, Wojciech Pudłob, Andrzej Jarze˛bskib
a Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland
b Department of Chemical and Process Engineering, Silesian University of Technology, Strzody 7, 44-100 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 May 2010
Received in revised form 29 August 2010
Accepted 21 September 2010
Available online 29 September 2010
The copper-TEMPO-catalysed aerobic oxidation of primary alcohols based on the new supported ionic
liquid phase (SILP) catalysts is presented. CuCl2 acts as a homogenous catalyst dissolved in the small quan-
tity of ionic liquid dispersed in the form of film on the solid support. A bi-modal pore structure silica with
a highly developed surface and an ionogel were used as solid supports. 1-Butyl-3-methylimidazolium
octylsulfate as the most active “catalyst-philic” phase was found. The application of SILP catalysts for the
oxidation of alcohols allows the high yields of corresponding aldehydes (92–95%). The catalysts were
used in seven cycles without significant loss of activity. High recoveries of the catalysts were observed.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Supported ionic liquid phase catalysis
Ionogel
Alcohol oxidation
Aldehydes
1. Introduction
hydroamination with a palladium catalyst under continuous flow
conditions [11–13].
The concept of supported liquid phase (SLP) catalysis involves a
thin film of a non-volatile liquid phase adsorbed on the solid sur-
face. SLP catalysis is a method developing a lot of interest for many
chemical reactions due to its many advantages over conventional
biphasic or homogenous systems, such as an easier separation of
the products, improved rates and selectivities, improved catalyst
stability, better catalytic efficiency and improved environmental
performance [1,2].
The supported ionic liquid phase (SILP) catalysis is a new gener-
ation of SLP catalysis [3–5]. In ionic liquid phase, which is adsorbed
on the solid surface, the catalyst is dissolved (e.g., metal complexes).
This technique has already been implemented in hydroformylation
and hydrogenation reactions. The activities of these systems are the
same, or slightly higher, as conventional biphasic catalysis, with the
catalysts showing good stability with no leaching of active species
or the ionic liquid [6–10]. Only limited information about SILP catal-
ysis can be found in the literature. One of the examples uses an
ionic liquid phase made from 1-butyl-3-methylimidazolium hex-
afluorophosphate supported on silica, modified with a monolayer
of covalently attached ionic liquid fragments, to dissolve the cat-
alyst for hydroformylation process [6]. Covalently attached ionic
liquid, functionalised with heteropoly acids, was also used for selec-
tive oxidation of alcohols [10]. Recently, SILP catalysis has been
demonstrated for hydroformylation with a rhodium catalyst or
The oxidation of alcohols to the corresponding aldehydes and
ketones is one of the processes in which SILP catalysis could
be implemented. This is one of the most important functional
group transformations in organic chemistry [14]. Among the sto-
ichiometric oxidants, molecular oxygen is a preferred terminal
oxidant for this process. Recently, oxygen with nitroxy radical
TEMPO (2,2,6,6-tetramethyl-piperidyl-1-oxy) and metal catalyst
has become a catalyst system of choice for the oxidation of alco-
hols to corresponding aldehydes and ketones [15,16]. This is a
very selective catalyst system preferred from both economic and
environmental perspectives [17]. This method can replace older
stoichiometric oxidants with either highly toxic chromium (VI)
and manganese (II) oxides or milder organic reagents such as
Dess–Martin periodinane. TEMPO is still an expensive chemical for
industry (80–100 $/kg) and therefore industry generally employs
nitroxyl derivatives functionalised in the 4 position such as 4-
hydroxy-TEMPO or 4-acetamido-TEMPO obtained from the cheap
(3 $/kg) precursor triacetoneamine. Example of industrial oxidative
processes based on the TEMPO catalyst include, e.g., Novartis pro-
cess with Cu+/O2 developed by Semmelack in the early 1980s to
synthesise retinal from retinol.
The copper-TEMPO-catalysed aerobic oxidation of alcohols in
ionic liquids as alternative solvents has already been performed.
High yields of aldehydes and ketones have been obtained and effec-
tive recycling of ionic liquid or ionic liquid with the whole catalyst
system has been demonstrated [18–20]. Although the ability of
homogenous catalysis in ionic liquids as a reaction medium has
been successfully performed, it would be interesting to use SILP
∗
Corresponding author. Fax: +48 32 237 10 32.
E-mail address: Anna.Chrobok@polsl.pl (A. Chrobok).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.09.022

180
A. Chrobok et al. / Applied Catalysis A: General 389 (2010) 179–185
catalysis for this process. The simplicity of catalyst separation and
the possibility of using a fixed-bed reactor make this idea attractive.
Herein, we report a concept for the copper-TEMPO-catalysed
aerobic oxidation of alcohols based on the new supported ionic
liquid phase catalysts.
2. Experimental
2.1. Materials and methods
[bmim]OSO3Oc (1-butyl-3-methyl-imidazolium octylsulfate,
ECOENG418, Merck), CuCl2·2H2O, alcohols, HNO3, TEMPO (Acros
Organics), tetraethoxysilane (Sigma–Aldrich) and oxygen (BOC,
purity 99.99%) were commercial materials, while [bmim]PF6
(1-butyl-3-methylimidazolium hexafluorophosphate) [21] and
[bmim]BF4 (1-butyl-3-methylimidazolium tetrafluoroborate) [22]
were prepared according to the known procedures.
All products were characterised by comparing their NMR spec-
tra with those of authentic samples. 1H NMR and 13C NMR spectra
were recorded at 300 MHz in CDCl3 (Varian Unity Inova plus, inter-
nal TMS). 13C NMR experiments in solid state were performed using
a Bruker 500 MHz NMR spectrometer. GC analysis was performed
using a PerkinElmer chromatograph with decane as the internal
standard and a PerkinElmer Column, ELITE-624 (30 m, 0.53 mm
ID, 3 ␮m df); elemental analysis using a Vario EL III (Elementar)
apparatus, Scanning Electron Microscopy micrograph (SEM) using
a Philips XL30 apparatus.
Fig. 1. 13C NMR of the solid-state samples of catalysts: A CuCl2/[bmim]OSO3Ocsup
and B CuCl2/[bmim]OSO3Ocsup
.
This powder contained 2.32 wt% of CuCl2 and was dried in a vac-
uum for 1 day (70 ◦C). The material was characterised by 13C NMR
(Fig. 1). Anal. Found: C, 27.62; H, 4.51; N, 4.22; S, 4.92.
B CuCl2/[bmim]BF4sup was prepared by mixing of 2.0 g of
[bmim]BF4 with CuCl2·2H2O (1.25 wt%, 0.025 g) and 2.0 g of a bi-
modal pore structure silica support in 50 ml of methanol at room
temperature. After stirring for 2 h, the methanol was evaporated
and the newly created catalyst was recovered as a solid powder
3.85 g (95% yield). This powder contained 0.50 wt% of CuCl2 and
was dried in a vacuum for 1 day (70 ◦C).
2.2. Catalysts preparation
13C NMR (solid state): ı = 17 (N-CH2-CH2-CH2-CH3), 34 (N-CH3;
N-CH2-CH2-CH2-CH3), 49 (N-CH2-C3H7), 122–124 (N-CH-CH-N),
136 (N-CH-N). Anal. Found: C, 21.29; H, 3.32; N, 6.21.
2.2.1. Supported ionic liquid phase catalysts – type A
A CuCl2/[bmim]OSO3Ocsup was prepared by dissolving 10 ml of
1 M HNO3 in 9 ml of tetraethoxysilane (TEOS). After creation of the
clear solution, 2 g of [bmim]OSO3Oc with dissolved CuCl2·2H2O
(6 wt%, 0.12 g) was added and mixed for 5 min. Next, the solu-
tion was left to gel at 40 ◦C, aged for 10 days at 40 ◦C and
dried for 4 days at 60 ◦C to evaporate the solvent (water and
ethanol), affording 4 g of ionogel. This material consisted of light
green particles (Aꢀꢀ CuCl2/[bmim]OSO3Ocsup, containing 2.37 wt%
of CuCl2), which were ground into a powder. The additional layer
of IL was then immobilised on the surface of the created ionogel
Aꢀꢀ CuCl2/[bmim]OSO3Ocsup by suspending 4 g of obtained ionogel
in 100 ml of methanol with 2.66 g of ionic liquid and CuCl2·2H2O
(6 wt%, 0.16 g). After stirring for 2 h, the methanol was evaporated
and the newly created catalyst A CuCl2/[bmim]OSO3Ocsup (6.5 g,
91% yield), containing 3.26 wt% of CuCl2 was dried in a vacuum for
1 day (70 ◦C). The material was characterised by 13C NMR (Fig. 1).
Anal. Found: C, 38.50; H, 6.55; N, 5.70; S, 6.89.
2.2.3. Preparation of B CuCl2 catalyst
It was prepared by mixing of 0.058 g of CuCl2·2H2O and 1.95 g
of a bi-modal pore structure silica support in 50 ml of methanol at
room temperature. After stirring for 2 h, the methanol was evap-
orated and the newly created catalyst was recovered as a solid
powder 1.88 g (94% yield). This powder contained 2.29 wt% of CuCl2
and was dried in a vacuum for 1 day (70 ◦C).
The values for CuCl2 and IL contents presented above are theo-
retical. Since no washing procedures were applied in the catalysts
preparation procedures it was assumed that all CuCl2 and IL remain
in the catalyst.
2.3. Typical oxidation procedures
2.3.1. Typical procedure for TEMPO-catalysed alcohol oxidation
in ionic liquid as the solvent
The same procedure was used for the preparation of
A CuCl2/[bmim]PF6sup, containing 3.26 wt% of CuCl2 which was
obtained with 90% yield. 13C NMR (solid state): ı = 18 (N-CH2-CH2-
CH2-CH3), 34 (N-CH3; N-CH2-CH2-CH2-CH3), 50 (N-CH2-C3H7),
122–124 (N-CH-CH-N), 136 (N-CH-N). Anal. Found: C, 23.64; H,
3.68; N, 6.80.
First, 0.2 mmol of TEMPO (0.028 g) and 0.1 mmol of CuCl2·2H2O
(0.017 g) were added to a solution of 2 mmol of benzyl alcohol
(0.22 g) in 2 ml of ionic liquid and stirred at 65 ◦C under a blan-
ket of oxygen at atmospheric pressure for 10–14 h (depending
on the reaction rate). The progress of the reaction was moni-
tored by GC analysis. The post-reaction mixture was extracted
with diethyl ether (6× 5 ml), and the organic extracts were fil-
tered over a short silica gel column (to isolate TEMPO) and
concentrated to yield 65–75% of benzaldehyde determined by
GC.
2.2.2. Supported ionic liquid phase catalyst – type B
B CuCl2/[bmim]OSO3Ocsup was prepared by mixing of 2.0 g of
[bmim]OSO3Oc with CuCl2·2H2O (6 wt%, 0.12 g) and 2.0 g of a bi-
modal pore structure silica support (macroporous, interconnected,
open network with bimodal system of macropores of microme-
ter size; apart from the macropores, the material also exhibited
textural mesopores, prepared according to the earlier described
procedure [23]) in 50 ml of methanol at room temperature. After
stirring for 2 h, the methanol was evaporated and the newly cre-
ated catalyst was recovered as a solid powder 3.95 g (96% yield).
2.3.2. Typical procedure for TEMPO-catalysed alcohol oxidation
in the presence of SILP catalyst
First, 0.1 mmol of TEMPO (0.014 g) and catalyst (containing
0.05 mmol of CuCl2) were added to a solution of 1 mmol of alco-
hol in 1 ml of solvent (dibutyl ether) and stirred at 65 ◦C under a

A. Chrobok et al. / Applied Catalysis A: General 389 (2010) 179–185
181
Table 1
TEMPO, CuCl2, O2
RCH2OH
RCHO
IL, 65oC
The copper-TEMPO-catalysed aerobic oxidation of benzyl alcohol in ionic liquids as solvents.a
.
Entry
Ionic liquid
[bmim]PF6
TEMPO [%]
Reaction time [h]
Conversionb [%]
Yieldb [%]
5
10
5
10
10
14
10
14
10
14
99
99
99
99
85
75
73
71
73
65
1
2
[bmim]OSO3Oc
[bmim]BF4
3
a
Reaction conditions: benzyl alcohol (1 mmol), CuCl2·2H2O (0.05 mmol), oxygen at atmospheric pressure, ionic liquid 1 ml, 65 ◦C.
b
Determined by GC.
blanket of oxygen at atmospheric pressure for 7–14 h (depending
Anal. Found for A CuCl2/[bmim]OSO3Ocsup after seven cycles: C,
35.50; H, 6.01; N, 5.42; S, 6.05.
Anal. Found for B CuCl2/[bmim]OSO3Ocsup after seven cycles: C,
23.20; H, 4.08; N, 3.55; S, 4.11.
on the reaction rate). The progress of the reaction was monitored
by GC analysis. After the reaction was complete the post-reaction
mixture was dissolved in diethyl ether and filtered. Next, the fil-
trate was passed over a short silica gel column (to isolate TEMPO),
concentrated and weighted. The desired aldehydes were obtained
with 92–93% of yield.
3. Results and discussion
When benzyl alcohol was used, the reaction was also car-
ried out as described above, but with no solvent. 0.1 mmol of
TEMPO (0.014 g) and 0.07 g of A CuCl2/[bmim]OSO3Ocsup (contain-
ing 0.015 mmol of CuCl2) were added to 1 mmol of benzyl alcohol
and stirred at 65 ◦C under a blanket of oxygen at atmospheric pres-
sure for 7 h.
3.1. Ionic liquid selection for SILP catalysis
In our preliminary study, the catalyst-philic phase for SILP
catalysis process of aerobic copper-TEMPO-catalysed oxidation
of alcohols was selected. Inspired by the work of Ansari and
Gree [18], we chose 1-butyl-3-methylimidazolium hexafluo-
rophosphate ([bmim]PF6) as the model ionic liquid. Additionally,
1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and
1-butyl-3-methylimidazolium octylsulfate ([bmim]OSO3Oc) were
tested. The ionic liquid with octylsulfate anion was especially
interesting. Recently, a large interest in halogen-free ionic liquids
containing alkyl sulphate anions has been observed due to their
biodegradability and that their toxicology is well documented [24].
The results of the aerobic copper-TEMPO-catalysed oxidation
of benzyl alcohol, as a model substrate, in homogenous conditions
with selected ionic liquids as solvents are presented in Table 1. We
applied the conditions from Ansari and Gree [18], conducting this
process at 65 ◦C. High conversions of benzyl alcohol and yields of
2.3.3. Recycling of catalysts
After isolation of the products from the reaction mixtures, the
catalysts were washed with diethyl ether and were dried under
vacuum (60 ◦C, 5 h).
For the experiments where A CuCl2/[bmim]OSO3Ocsup cata-
lyst was recycled 5 mmol of benzyl alcohol, 0.35 g of catalyst and
0.5 mmol of fresh portion of TEMPO in each cycle was used. For the
experiments where B CuCl2/[bmim]OSO3Ocsup catalyst was recy-
cled 3 mmol of benzyl alcohol, 0.87 g of catalyst and 0.3 mmol of
fresh portion of TEMPO in each cycle was used.
Scheme 1. The structure of SILP catalysts.

182
A. Chrobok et al. / Applied Catalysis A: General 389 (2010) 179–185
Table 2
SILP catalysts used in this study.
Entry
Catalyst
Content of CuCl2 [wt%]a
Ionic liquid loading [wt%]a
1
2
3
4
5
6
7
A CuCl2/[bmim]OSO3Ocsup
Aꢀ/[bmim]OSO3OcSup
Aꢀꢀ CuCl2/[bmim]OSO3OcSup
A CuCl2/[bmim]PF6sup
B CuCl2/[bmim]OSO3Ocsup
B CuCl2/[bmim]BF4sup
B CuCl2
3.26
–
65
65
50
65
50
50
–
2.37b
3.26
2.32
0.50
2.29
a
Recalculated values.
b
The content of CuCl2 in catalyst was recalculated assuming that all Si was converted into SiO2.
benzaldehyde were observed when [bmim]PF6 and [bmim]OSO3Oc
were used as solvents. Slightly lower conversion and yield were
obtained in the presence of [bmim]BF4 as the solvent.
3.2. Catalysts characterisation
Two types of catalysts (A, B) which were obtained by a post-
synthesis modification of selected supports were used. The applied
catalysts differ in the structure of the solid support (Scheme 1). The
solid support for catalysts of A-type was prepared by the sol–gel
method, which is the reaction of HNO3 with tetraethoxysilane, ionic
liquid and CuCl2 to create ionogel. The basic idea of this method
is to introduce the ionic liquid with CuCl2 within an oxide matrix
through a one step process [25]. In the case of B-type catalysts, a bi-
modal pore structure silica support was used. The preparation and
characterisation of that support was described in our earlier report
[23]. The main advantage of supports with a bi-modal hierarchical
pore structure, containing micro-, meso- and macropores, is the
high surface area.
We applied the following ionic liquids: [bmim]PF6, [bmim]BF4
and [bmim]OSO3Oc, as a catalyst-philic phases, based on the results
obtained from experiments demonstrated above (Table 1). The
ionic liquid phase was dispersed in the form of film on the solid
surface of the supports A and B in all cases. The catalysts were
prepared by stirring the suspension of support and CuCl2 in the
solution of ionic liquid in methanol. Solid catalysts were obtained
after the evaporation of methanol. The expected amounts of ionic
liquids and CuCl2 in the catalysts were in the range from 50% up to
65% and from 0.50% up to 3.26% respectively (Table 2). The lowest
Fig. 2. (a) Scanning Electron Microscopy micrograph of the ionogel with supported
ionic liquid phase (A CuCl2/[bmim]OSO3Ocsup) before and (b) isolated after the oxi-
dation reaction (bar – main picture: 20 ␮m, insertion: 5 ␮m).
amount of CuCl2 in catalyst B CuCl2/[bmim]BF4sup is a result of low
solubility of CuCl2 in [bmim]BF4.
The example of solid state 13C NMR spectrum of selected cat-
alysts A CuCl2/[bmim]OSO3Ocsup and B CuCl2/[bmim]OSO3Ocsup
are presented in Fig. 1. The significant line broadening in the NMR
spectrum arises from the anisotropy of the solid-state samples. The
solid-state NMR spectrum revealed that the chemical shifts for the
carbon atoms in the 1-butyl-3-methylimidazolium octylsulfate-
modified silica samples are adequate to the signals obtained
from the 13C NMR spectrum of the liquid sample [bmim]OSO3Oc
[26]. The structure of the catalysts A CuCl2/[bmim]OSO3Ocsup and
Table 3
The copper-TEMPO-catalysed aerobic oxidation of benzyl alcohola in the presence of A-type SILP catalysts.
Entry
Catalyst
Catalyst [g]/content of
CuCl2 [mmol]
Reaction time
[h]/TEMPO [mol%]
Conversionb of
benzyl alcohol [%]
Yieldc of
benzaldehyde [%]
TONd
Catalyst
recovered [%]
1
2
3
4
5
6
7
8
9
0.11/0.025
0.22/0.05
0.07/0.015
–
0.22/0.05
0.22/–
0.28/0.05
0.22/0.05
0.88/0.2
7/10
7/5
99
99
99
10
8
10
20
20
50
98 (92)
97 (92)
98 (93)
3920
1940
6533
–
94
93
93
–
95
93
95
91
94
A CuCl2/[bmim]OSO3Ocsup
7/10e
14/10
14/–
–
7
5
6
15
18
45
A CuCl2/[bmim]OSO3OcSup
Aꢀ/[bmim]OSO3OcSup
Aꢀꢀ CuCl2/[bmim]OSO3OcSup
–
–
–
–
14/10
14/10
14/10
14/10
A CuCl2/[bmim]PF6sup
225
a
Reaction conditions: benzyl alcohol (1 mmol), dibutyl ether as solvent (1 ml), oxygen at atmospheric pressure, 65 ◦C.
Determined by GC.
Determined by GC, in parenthesis isolated yields by filtration and concentration.
The number of moles of substrate that a mole of catalyst can convert to the product.
Without dibutyl ether as solvent.
b
c
d
e

A. Chrobok et al. / Applied Catalysis A: General 389 (2010) 179–185
183
that ionic liquid and CuCl2 was not soluble in the organic sol-
vent.
The results of the aerobic oxidation of benzyl alcohol using SILP
catalysis are presented in Table 3. A CuCl2/[bmim]OSO3Ocsup was
the most active catalyst in the oxidation reaction. High yields and
selectivities of benzaldehyde were obtained in 7 h even in case
of using very low amounts of catalyst (0.11 g of catalyst contain-
ing 0.025 mmol of CuCl2 for 1 mmol of alcohol, Table 3, entry 1).
The amount of CuCl2 used in this case is even lower than that
applied in the process with ionic liquids which are not immo-
bilised on a solid support (Table 1, entry 2). High concentration
of CuCl2 in the supported ionic liquid catalysts can result in high
oxidation rates. The study showed that the amount of TEMPO
used in the oxidation process can be lowered by larger amounts
of the catalyst A CuCl2/[bmim]OSO3Ocsup (Table 3, entry 2). We
also demonstrated that in the case of liquid substrate, e.g., benzyl
alcohol the reaction can be performed without additional solvent
(Table 3, entry 3).
In the case of using TEMPO without the catalyst very poor yield
of benzaldehyde was obtained. Similar results were observed for
the oxidation in the presence of catalyst without TEMPO (Table 3,
entries 4, 5). We also synthesised catalyst Aꢀ/[bmim]OSO3OcSup
which does not contain CuCl2 in the ionogel and in the supported
ionic liquid layer. The reaction in the presence of this catalyst
yielded benzadehyde only with 6% (Table 3, entry 6). This fact can
confirm that the ionic liquid does not catalyse the oxidation reac-
tion. We can conclude that in this particularly reaction system, it
is necessary to use both TEMPO and catalyst with CuCl2 to carry
out the oxidation reaction. Such catalytic system is very active as
indicate Turnover Numbers (Table 3).
We also checked the activity of support in the model oxi-
dation process. The catalyst Aꢀꢀ CuCl2/[bmim]OSO3Ocsup which
does not possess an extra liquid layer on the surface (the sup-
port for A-type catalyst) was not active in the model process
(Table 3, entry 7). Probably, CuCl2 in this ionogel is inaccessi-
ble for reagents. In order to check if the oxidation takes place
in the film of ionic liquid the additional test was performed. The
catalyst Aꢀꢀ CuCl2/[bmim]OSO3Ocsup and the amounts of CuCl2
and [bmim]OSO3Oc which are equivalent to those expected for
A CuCl2/[bmim]OSO3Ocsup were introduced to the reactor. Then,
the reactants were added and the experiment using the same reac-
tion conditions as for entry 3, Table 3 was performed. As a result we
obtained only 20% yield of benzadehyde. Additionally, on the NMR
spectrum of the post-reaction mixture after filtration, signals from
the ionic liquid were presented. The weight of the recover catalyst
was adequate to the weight of Aꢀꢀ CuCl2/[bmim]OSO3Ocsup which
was introduced to the reactor. Low yield of benzaldehyde and the
presence of the ionic liquid in the filtrate may indicate that under
Fig. 3. (a) Scanning Electron Microscopy micrograph of the bi-modal pore struc-
ture of silica modified with ionic liquid (B CuCl2/[bmim]OSO3Ocsup) before and (b)
isolated after the model oxidation reaction (bar – main picture: 20 ␮m, insertion:
5 ␮m).
B CuCl2/[bmim]OSO3Ocsup analysed by SEM method is depicted in
Figs. 2 and 3. The bi-modal pore structure of catalyst B support was
not destroyed neither by the immobilisation of the ionic liquid on
the solid surface nor by using them for oxidation processes.
3.3. SILP catalysis
The reaction system applied in our study is presented in
Scheme 2. The ionic liquid phase together with CuCl2 was
immobilised on the solid support. The studied alcohols and
TEMPO were dissolved in dibutyl ether as the organic sol-
vent and the gas phase consisted of oxygen. It was important
Scheme 2. The reaction system for the copper-TEMPO catalysed aerobic oxidation of alcohols in the presence of SILP catalysts.

184
A. Chrobok et al. / Applied Catalysis A: General 389 (2010) 179–185
Table 4
The copper-TEMPO-catalysed aerobic oxidation of benzyl alcohola in the presence of B-type SILP catalysts.
Entry
Catalyst
Catalyst [g]/content of
CuCl2 [mmol]
Reaction time
[h]/TEMPO [mol%]
Conversionb [%]
Yieldc [%]
TON
Catalyst
recovered [%]
1
2
3
4
B CuCl2/[bmim]OSO3Ocsup
B CuCl2/[bmim]OSO3Ocsup
B CuCl2/[bmim]BF4sup
B CuCl2
0.29/0.05
0.29/0.05
1.7/0.05
14/10
7/5
14/10
14/10
99
52
50
52
98 (93)
51
1960
1020
800
91
92
40
41
50 (leaching of IL)
70
0.29/0.05
820
a
Reaction conditions: benzyl alcohol (1 mmol), dibutyl ether as solvent (1 ml), oxygen at atmospheric pressure, 65 ◦C.
Conversion of benzyl alcohol determined by GC.
Yield of benzaldehyde determined by GC, in parenthesis isolated yields by filtration and concentration.
b
c
100
90
80
70
60
50
40
30
20
10
0
the reaction conditions it is not possible to create the film of ionic
liquid on the surface of ionogel. These results can confirm that the
reaction occurs inside the ionic liquid layer.
All post-reaction mixtures were analysed by NMR after filtration
of the catalyst, and no trace of over-oxidation to the carboxylic acids
was detected (Table 3). High recovery (about 91–95%) of catalyst
was possible.
2
1
Unfortunately,
the
second
catalyst,
A-type
3
A CuCl2/[bmim]PF6sup
, was not so efficient under the same
reaction conditions. The conversion was only 50% even in the
presence of larger amount of the catalyst (Table 3, entries 8, 9).
This result was surprising when compared to the results obtained
with [bmim]PF6 as non-supported solvent (Table 1, entry 1). It is
possible that the presence of long chain alkyl substituent in the
A CuCl2/[bmim]OSO3Ocsup catalyst can result in higher solubility
of oxygen and higher oxidation rate. This phenomenon can be
caused by creation of hydrogen bonds between hydrogen atoms of
octyl chain and oxygen molecule [27].
B-type catalysts were prepared by the very simple method. It
was simple impregnation of ionic liquid on the surface of bi-modal
pore structure silica. B CuCl2/[bmim]OSO3Ocsup was active in the
model oxidation of benzyl alcohol (Table 4, entry 1) and demon-
strated high recovery of catalyst. In order to compare A and B-type
catalysts the same reaction conditions with 5% of TEMPO were
applied (Table 4, entry 2). After 7 h the Turnover Number for cat-
alyst B was almost twice lower comparing to catalyst A (Table 3,
entry 2).
0
2
4
6
8
10
12
14
Reaction time [h]
Fig. 4. Variation of yield of benzaldehyde versus the time on stream during the oxi-
dation of benzyl alcohol in the presence of: A CuCl2/[bmim]OSO3Ocsup catalyst and
TEMPO (5 mol%) – curve 1; B CuCl2/[bmim]OSO3Ocsup catalyst and TEMPO (10 mol%)
– curve 2; B CuCl2 catalyst and TEMPO (10 mol%) – curve 3. Reaction conditions:
benzyl alcohol (1 mmol), catalyst containing 0.05 mmol of CuCl2, dibutyl ether as
solvent (1 ml), oxygen at atmospheric pressure, 65 ◦C, yields determined by GC.
We also tried to immobilise another ionic liquid, [bmim]BF4,
on a bi-modal pore structure of silica. We were unable to obtain a
stable system, and the ionic liquid easily leached from the support
during the reaction (Table 4, entry 3).
The variation of catalytic activity of selected catalysts ver-
sus the time of oxidation of benzyl alcohol under the same
conditions was compared (Fig. 4). Only in case of the catalyst
A CuCl2/[bmim]OSO3Ocsup lower amount of TEMPO was used (5%).
The highest reaction rate (curve 1) can be observed in the presence
of the A CuCl2/[bmim]OSO3Ocsup catalyst.
To prove the activity of SILP catalysts, B CuCl2 catalyst was pre-
pared. It consisted only from a bi-modal pore structure silica and
CuCl2. Low activity and low recovery of this catalyst was observed
in model oxidation (Table 4, entry 4). This fact can indicate that the
reaction takes place inside the ionic liquid layer in case of other
applied catalysts.
High yields of other aldehydes in relatively short times
were obtained in the presence of the most active catalyst
Table 5
SILP catalysis for the copper-TEMPO-catalysed aerobic oxidation of alcoholsa with A CuCl2/[bmim]OSO3Ocsup
.
Alcohol
Aldehyde
Time [h]
Conversionb [%]
Time [h]
Conversionb [%]
Yieldc [%]
Catalyst recovered [%]
CHO
OH
2
2
50
50
7
7
99
99
93
95
93
92
CHO
CHO
OH
O2N
O2N
MeO
a
OH
OH
MeO
3.5
5.5
50
50
10
15
99
99
94
92
91
90
CHO
Reaction conditions: alcohol (1 mmol), TEMPO (0.1 mmol), 0.025 mmol of CuCl2 included in A CuCl2/[bmim]OSO3Ocsup, dibutyl ether as solvent, oxygen at atmospheric
pressure, 65 ◦C.
b
Conversion of alcohols determined by GC.
c
Isolated yields by filtration and concentration.

A. Chrobok et al. / Applied Catalysis A: General 389 (2010) 179–185
185
demonstrated that the application of SILP catalysis for selective oxi-
dation can provide valuable alternatives to purely homogeneous or
heterogeneous processes. Furthermore, the use of a homogenous
reaction system requires a large amount of ionic liquids, which is
not necessary in the case of SILP catalysis.
Two types of catalysts differing in the structure of the solid sup-
port were employed for this process. CuCl2 acts as a homogenous
catalyst dissolved in the small quantity of the ionic liquid dispersed
in the form of film on the solid support. The catalysts based on the
ionogel or the bi-modal pore structure silica as solid supports with
[bmim]OSO3Oc as the liquid phase was highly advantageous for
the production of aldehydes. High yields of aldehydes and no over-
oxidation to carboxylic acids were observed using these catalysts.
The activity of these systems was the same or slightly higher as
conventional homogenous one, but after the reaction the catalyst
can be easily isolated and recycled. High activity and high recov-
ery of catalysts were observed in each cycle of the reaction. We
believe that application of these catalysts can be further expanded
into other catalysis areas.
In perspective, we will continue our work using SILP cataly-
sis, e.g., with catalysts based on the bi-modal pore structure silica
support for the continuous fixed-bed processing.
Fig. 5. Reusability of the A CuCl2/[bmim]OSO3Ocsup [reaction conditions: benzyl
alcohol (5 mmol), TEMPO (0.5 mmol), 0.35 g of catalyst containing 0.075 mmol of
CuCl2, oxygen at atmospheric pressure, 65 ◦C; isolated yields after 7 h with 98%
conversion of benzyl alcohol] and B CuCl2/[bmim]OSO3Ocsup [reaction conditions:
benzyl alcohol (3 mmol), TEMPO (0.3 mmol), 0.87 g of catalyst containing 0.15 mmol
of CuCl2, dibutyl ether as solvent (3 ml), oxygen at atmospheric pressure, 65 ◦C;
isolated yields after 14 h with 98% conversion of benzyl alcohol] catalysts on the
example of benzyl alcohol oxidation.
Acknowledgments
This work was cofinanced by the Ministry of Science and Higher
Education (Grant No. N N209 149236). We thank Dr. M. Pawlyta
from Institute of Engineering Materials and Biomaterials of Silesian
University of Technology for SEM analysis.
A CuCl2/[bmim]OSO3Ocsup (Table 5). In order to compare the
course of oxidation of different substrates the time needed to
achieve 50% conversion is also indicated. The most reactive are
benzyl alcohol and 4-nitrobenzyl alcohol.
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oxidation of the primary alcohols as the model reactions. It was