New In(OiPr)3-MCM-41 heterogeneous catalyst in MPV reductions of unsaturated carbonyl compounds: effect of mesoporous SBA-15 and MCM-41 as supporting surfaces on catalytic activity of In(OiPr)3

Article abstract of DOI:10.1007/s10847-016-0680-6

Indium tri-isopropoxide, In(OⁱPr)₃, was immobilized on mesoporous material, MCM-41, and denoted as “In(OⁱPr)₃-MCM-41”. This new heterogeneous catalyst was characterized by XRD, ²⁹Si NMR-, N₂ adsorption–desorption isotherms and ICP-OES techniques. The new heterogeneous catalyst, In(OⁱPr)₃-MCM-41, was tested for the capable of catalyzed Meerwein–Ponndorf–Verley (MPV) reduction of unsaturated aldehydes and ketones with low catalyst loadings under mild conditions and showed good to excellent catalytic activities. Also, effect of supporting surfaces, both of SBA-15 and MCM-41, on catalytic activity of In(OⁱPr)₃ were examined. In(OⁱPr)₃-SBA-15 heterogeneous catalyst in comparison with the In(OⁱPr)₃-MCM-41 catalyst, display comparatively higher catalytic activity in the MPV reduction of unsaturated aldehydes and ketones. Also, similiar reaction times and selectivities for the unsaturated alcohols were obtained with the In(OⁱPr)₃-SBA-15 catalyst compared with the In(OⁱPr)₃-MCM-41 catalyst. The reason for the lower activity observed for MCM-41 sample may be due to smaller pore size of the In(OⁱPr)₃-MCM-41 catalyst as compared with In(OⁱPr)₃-SBA-15 catalyst can creat restrict site accessibility for the carbonyl compounds. Eventually, effect of supporting surfaces, SBA-15 and MCM-41, on catalytic activity of In(OⁱPr)₃ insignificant for MPV reduction of unsaturated carbonyl compounds.

Full text of DOI:10.1007/s10847-016-0680-6

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-016-0680-6
ORIGINAL ARTICLE
New In(OiPr)3-MCM-41 heterogeneous catalyst in MPV
reductions of unsaturated carbonyl compounds: effect
of mesoporous SBA-15 and MCM-41 as supporting surfaces
on catalytic activity of In(OiPr)3
Burcu Uysal Karatas1 Birsen S. Oksal2 Erhan Karatas3
•
•
Received: 5 August 2016 / Accepted: 9 November 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Indium tri-isopropoxide, In(OiPr)3, was immo-
bilized on mesoporous material, MCM-41, and denoted as
‘‘In(OiPr)3-MCM-41’’. This new heterogeneous catalyst
was characterized by XRD, 29Si NMR-, N2 adsorption–
desorption isotherms and ICP-OES techniques. The new
heterogeneous catalyst, In(OiPr)3-MCM-41, was tested for
the capable of catalyzed Meerwein–Ponndorf–Verley
(MPV) reduction of unsaturated aldehydes and ketones
with low catalyst loadings under mild conditions and
showed good to excellent catalytic activities. Also, effect
of supporting surfaces, both of SBA-15 and MCM-41, on
catalytic activity of In(OiPr)3 were examined. In(OiPr)3-
SBA-15 heterogeneous catalyst in comparison with the
In(OiPr)3-MCM-41 catalyst, display comparatively higher
catalytic activity in the MPV reduction of unsaturated
aldehydes and ketones. Also, similiar reaction times and
selectivities for the unsaturated alcohols were obtained
with the In(OiPr)3-SBA-15 catalyst compared with the
In(OiPr)3-MCM-41 catalyst. The reason for the lower
activity observed for MCM-41 sample may be due to
smaller pore size of the In(OiPr)3-MCM-41 catalyst as
compared with In(OiPr)3-SBA-15 catalyst can creat restrict
site accessibility for the carbonyl compounds. Eventually,
effect of supporting surfaces, SBA-15 and MCM-41, on
catalytic activity of In(OiPr)3 insignificant for MPV
reduction of unsaturated carbonyl compounds.
Keywords Mesoporous MCM-41 Á Mesoporous SBA-15 Á
Indium tri-isopropoxide Á Unsaturated aldehydes and
ketones Á Chemoselectivity
Introduction
The reduction of carbonyl compounds by hydrogen transfer
from an alcohol is known as the ‘Meerwein–Ponndorf–
Verley reaction’ or ‘MPV reaction’ in organic chemistry
and can be performed under mild conditions using Lewis
acids as catalyst [1, 2]. The classical MPV reduction
reaction is carried out in the presence of aluminum
i
alkoxides like Al(OiPr)3 and PrOH hydride source which
offers many practical advantages like the mild reaction
conditions, safe and simple operations and chemo selective
nature-that unsaturated aldehydes and ketones are not
reduced to saturated alcohols [3]. However, the usage of
stoichiometric amount of aluminum alkoxides, need for
excess alcohols, low reaction rate due to the poor reactivity
of traditional aluminum catalyst, as well as the formation
of condensed products limits its wide applicability [4, 5]. In
addition to the classical (Al(OiPr)3) catalyst, catalytic
applications of other isopropoxides, such as zirconium(IV)
isopropoxide [6, 7], lanthanide isopropoxides [8], and
indium isopropoxide [9] in the MPV reaction have been
reported. The potential of boron reagents, in particular
B(OiPr)3, in the MPV reduction reactions is also well
documented [10–13].
& Burcu Uysal Karatas
burcuu@pau.edu.tr
1
Technical Sciences Vocational High School, Pamukkale
University, Denizli, Turkey
2
Department of Chemistry, Art and Science Faculty, Giresun
University, Giresun, Turkey
In recent years, MPV reduction of carbonyl compounds
to the corresponding alcohols has been one of the standard
procedures for the selective reduction of unsaturated
3
Department of Chemistry, Faculty of Science, Selcuk
University, Konya, Turkey
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J Incl Phenom Macrocycl Chem
carbonyls in laboratories and industries [14]. Extensive
studies have been carried out to expand the potential of the
classical MPV reduction.
reactions of unsaturated ketones [42]. Lately, Indium tri-
isopropoxide catalyst has been grafted on SBA-15 meso-
porous silica, and the catalyst denoted as In(OiPr)3-SBA-15
[43]. In(OiPr)3-SBA-15 has been found that active catalyst
in the MPV reactions of carbonyl compounds including
unsaturated aldehydes and ketones with excellent conver-
sion, selectivity and reusability [43].
Heterogenous catalysts for these reductions are superior
to homogenous ones from the viewpoint of cost and are
utilized for the industrial scale reduction of carbonyls in
many applications [15].
Heterogeneous catalysts for the MPV reactions include
Sn-, Ti-, Zr- and Al-beta-zeolites [16–19], grafted alkoxides
[20, 21] and metal oxides such as magnesium oxide, zirconia,
silica, alumina [22], etc. A major advantage of heteroge-
neously over homogeneously catalyzed MPV reactions is
that the catalysts can easily be separated from the liquid
reaction mixture [18, 23]. The heterogenation of the MPV
catalysts also offers the advantage of reusability [24].
In order to heterogenize the metal complexes used in
MPV reductions, grafting onto inorganic supports has been
carried out. Recently, the mesoporous silicate MCM-41
was explored as a versatile support material for metalor-
ganic moieties [25, 26]. Since its discovery by Mobile in
1992 [27, 28], MCM-41 and related mesoporous molecular
sieves such as SBA-15 have attracted great attention. As is
known, MCM-41 possesses unidirectional channel-like
pores of rather uniform size which are arranged in a regular
hexagonal pattern. The pores diameters are adjustable from
15 to 100 A° depending on the synthesis conditions, such
as temperature, type and size of the templating detergent
cations [29].
In the present study, we reported a new heterogeneous
catalyst In(OiPr)3-MCM-41, characterized its structure, and
showed the selective reduction of a variety of unsaturated
aldehydes and ketones using heterogenised Indium tri-
isopropoxide catalyst.
Heterogeneous MPV catalyst of similar type with dif-
ferent supporting surface has just previously applied to the
reduction of unsaturated aldehydes and ketones using
heterogenized In(OiPr)3-SBA-15, and the results have been
very encouraging [43]. Also, effect of supporting surfaces,
SBA-15, MCM-41, on catalytic activity of In(OiPr)3 was
examined. Up to the present time, there has not been any
literature reporting on comparison of the Indium tri-iso-
propoxide grafted MCM-41 and SBA-15 heterogeneous
catalysts for the MVP reduction of unsaturated ketones and
aldehydes to unsaturated alcohols.
Experimental
All experiments were carried out under purified dry nitro-
gen atmosphere by using standard Schlenk techniques. All
reagents were purchased from Merck or Sigma–Aldrich
and used as received.
Among different ordered mesoporous silicas, SBA-type
silicas are the most frequently studied [30–32]. SBA-15
silica (SBA = Santa Barbara amorphous) exhibits inter-
esting textural properties, such as large specific surface
areas (above 1000 m2 g-1), uniform-sized pores (in range
4–30 nm), thick framework walls, small crystallite size of
primary particles and complementary textural porosity. The
advantage of the use of SBA-15 material as support includes
also its high surface-to-volume ratio, variable framework
compositions and high thermal stability [30–34].
Preparation of mesoporous support
Pure siliceous MCM-41 was synthesized following a previ-
ously reported procedure [10]. Silica MCM-41 was prepared
by dissolving 19.43 g (26.4 mmol) of tetraethylammonium
hydroxide (TEAOH, 20%) and 16.16 g (12.6 mmol)
cetyltrimethylammonium chloride (CTMACl, 25%) in
20 mL of deionised water with stirring (1000 rpm) until a
clear solution was obtained. 19.27 g (128.3 mmol) of
LUDOX AS-40 (Dupont) was added to the solution with
stirring. After 15 min an additional amount of 32.33 g
(25.3 mmol) CTMACl and 20 mL H2O were added. The
resulting mixture was vigorously stirred (1000 rpm) for
another 30 min. The molar composition of the final gel
mixture was as follows:
Modification of the MCM-41 and SBA-15 may provide
an alternative strategy to obtain suitable supports to
maintain high activity in the heteregeneous catalyst system
for the reduction of carbonyl compounds. These materials
display an open porous structure with pore sizes in the
range of mesopores, which facilitates the diffusion of bulk
molecules inside the material, minimizing mass transfer
hindrances [35].
Several MPV reduction catalysts, such as Al-alkoxides,
Zr-alkoxides, Hf and Zr alkyl complexes, as well as Boron-
alkoxides have been anchored on MCM-41 or on amor-
phous SiO2 in order to generate potential catalytic activity
[10, 21, 23, 36–41].
SiO2:0.3CTMACl:0.2TEAOH:46.3H2
The mixture was transferred into a Teflon-lined stain-
less-steel autoclave (BERGHOF BR-200 pressure reactor)
and kept at 110 °C under static conditions for 48 h. The
obtained solid material was then filtered and washed well
Zirconium alkoxides anchored to silica surfaces, e.g. Zr-
SBA-15, have been used as effective catalysts for MPV
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J Incl Phenom Macrocycl Chem
with copious amounts of water till filtrate showed a neutral
pH and was then air-dried. The surfactant inside the pores
of the mesoporous material was removed by calcination at
550 °C for 6 h with a heating ramp of 1 °C/min.
Alto, CA, USA). The same procedure was used to reduce
other a,b-unsaturated aldehydes and ketones in the pres-
ence of In(OiPr)3-SBA-15. To present the results we have
used the conversion and selectivity values defined as Ref.
[10, 39].
Grafting of indium tri-isopropoxide (In(OiPr)3)
on MCM-41 support
Characterization
Measurement of the surface area, average pore diameter
and pore size distribution of MCM-41 support and the
prepared heterogeneous catalyst were determined by a
Micromeritics Gemini III 2375 Surface Area Analyzer,
using nitrogen adsorption at -196 °C. Before measure-
ment, the supported samples were degassed at 300 °C and
0.15 mbar at least for 6 h. The surface areas were calcu-
lated by the method of Brunauer, Emmett and Teller
(BET). The pore size distribution curves were obtained
from the analysis of desorption branch of the nitrogen
adsorption–desorption isotherm by the BJH (Barrett–Joy-
ner–Halenda) method.
The grafting of indium tri-isopropoxide, In(OiPr)3, over
ordered mesoporous silica support (MCM-41) was carried
out by known literature reports [39, 43]. The siliceous
MCM-41 supports were calcined and used immediately.
Indium tri-isopropoxide grafted MCM-41 materials were
prepared by stirring (750 rpm) 2.0 g of the support with a
solution of (3 mL; 0.514 mmol) In(OiPr)3 (as a 5 wt%
solution in iso-propanol) in 10 mL dry hexane for 4 h at
room temperature. The product was filtered under N2
atmosphere, washed three times with 10 mL of hexane and
finally dried under the same inert gas flow. The material
containing the grafted alkoxide contains 0.214 mmol In per
g final material and is denoted as In(OiPr)3-MCM-41.
In (OiPr)3-SBA-15 catalyst were prepared and charac-
terised in Ref. [43].
Powder X-ray diffraction (XRD) was performed to
determine the bulk crystalline phases of samples. This was
conducted using a X’Pert Pro MPD Diffractometer from
PANalytical, with
a
CuKa1 radiation wavelength
0.154 nm. The spectra were scanned at a rate of 2.4°/min in
the range 2h = 10–80o.
Typical procedure employed for MPV reactions
The indium content of the grafted catalyst was deter-
mined by inductively coupled plasma-optical emission
spectroscopy (ICP-OES). The mesoporous material In(Oi-
Pr)3–MCM-41 was dissolved in a mixture solution of HCl
and HF. An aqueous HCl and HF solution containing
indium was used as standard reference. ICP-OES was
performed using Perkin Elmer Optima 4300DV.
The new heterogeneous In(OiPr)3-MCM-41 catalyst was
tested on the MPV reduction of a series of unsaturated
aldehydes and ketones. The typical procedure was used as
Ref. [37, 39]. Catalytic MPV reduction reaction was car-
ried out as follows:
The reaction mixture containing 30 mmol of the alde-
hyde or ketone, 200 mmol (12.02 g; 15.4 mL) isopropanol
and 500 mg of dried catalyst were placed in a 50 mL two
mouthed flask with a side stopcock equipped with a
100 cm condenser. The flask was immersed in an ther-
mostated oil bath. The rigorously stirred reaction mixture
was gently heated to reflux at 80 °C. During the reduction
reaction time a slow stream of dry nitrogen was passed just
over the surface of reaction mixture. By this way, the
formed acetone was removed by nitrogen flow. Thus the
equilibrium reaction was slipped to the right hand. Aliquots
were removed at different reaction times and reduction
products were analysed by GC/MS.
The anchoring of indium tri-isopropoxide species on
SBA-15 was followed by 29Si MAS NMR spectroscopy.
Solid state 29Si MAS NMR spectra were recorded on a
Bruker Advance 400 NMR spectrometer with a resonance
frequency of 79.46 MHz for 29Si using a Bruker 4 mm
double resonance probe head operating at a spinning rate of
12 kHz for 29Si, respectively. 29Si spectra were collected
with 70° rf pulses, 30 s delay and *6000 scans.
Results and discussion
The reduction products were identified on the basis of
their retention times by comparing with authentic samples
and their mass spectral fragmentation patterns with those
stored in the data bank (Wiley/NIST library). Analyses
were performed on a Varian CP 3800 gas-chromatograph
equipped with a Varian Saturn 2200 MS detector (Walnut
Creek, CA, USA) and a VF-5 ms capillary column (30 m
length and 0.25 mm I.D., 0.25 lm film thickness) (Palo
Structural as well as textural features
XRD patterns of mesoporous MCM-41 silica and indium
alkoxide-grafted mesoporous MCM-41 samples are shown
in Fig. 1. X-ray diffraction is one of the most important
techniques for the characterization of ordered mesoporous
materials. MCM-41 and In(OiPr)3-MCM-41 samples
showed three (hkl) reflections of (100), (110) and (200) for
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J Incl Phenom Macrocycl Chem
Fig. 2 N2 adsorption/desorption isotherms of
b In(OiPr)3-MCM-41
a MCM-41 and
Fig. 1 Low-angle XRD patterns of a unmodified MCM-41, b In(Oi-
Pr)3-MCM-41
the pore structure of the support during the incorporation
process.
The observed changes of the specific surface area and
pore volume as well as pore size are shown in Table 1. The
BET surface area and pore volume of MCM-41 were 1079
and 0.45 cm3/g, respectively. These two values for In(Oi-
Pr)3-MCM-41 dropped to 846 and 0.19 cm3/g, due to the
introduction of In(OiPr)3.
a highly ordered hexagonal structure [3]. The highest peak
observed for two samples (corresponding to 100 plane)
indicates the order of the mesopores, as expected in MCM-
41 type ordered mesoporous structure [3]. The presence of
characteristic reflections even after grafting of In(OiPr)3
complexes suggest that the mesoporous materials are
ordered with uniform pore arrangements and the grafting
process had not damaged the pore structure of the meso-
porous materials [3]. After grafting indium tri-isopropox-
ide, the intensity of the (100) peak decreases which may
relate to incorporation of indium isopropoxide complexes
in the channel of MCM-41 support. This result indicates
that the substantial loss in the scattering contrast between
the channel and the wall, and reduces the intensity of the
scattered X-ray in the powder diffraction experiment [44].
In addition, a slight decrease in the peak intensity shows
the reduction in the pore size with the grafting of indium
tri-isopropoxide [43].
ICP-OES analysis showed 2.46 wt% of indium content
in the catalyst. To determine indium content, In(OiPr)3-
MCM-41 was dissolved in a mixture solution of HCl and
HF. An aqueous HCl and HF solution containing indium
was used as standard reference. 0.214 mmol In was found
per g of In(OiPr)3-MCM-41.
29Si MAS NMR spectra of mesoporous MCM-41 and
In(OiPr)3-MCM-41 samples were shown in Fig. 3. 29Si
MAS NMR of MCM-41 shows signals with chemical shifts
at -93, -100 and -110 ppm resulting from Q2, Q3 and Q4
silicon nuclei, respectively, The presence of broad reso-
nance peaks from -90 to -110 ppm, indicative for a range
of Si–O–Si bond angles and the formation of more tetra-
hedral silicon environments. Compared to MCM-41, the
29Si NMR spectrum of In(OiPr)3-MCM-41 shows a marked
decrease in the intensity of the Q3 and Q2 signals, due to
the transformation of Si–OH groups into Si–OC during the
incorporation process. The presence of resonance origi-
nating from Q3 Si(OSi)3(OH) and Q4 Si(OSi)4 in the cat-
alyst indicates that MCM-41 also retains its structure.
Figure 2 shows the nitrogen adsorption and desorption
isotherms at 77 K which provides the indications of the
presence of the pore size and structure. The nitrogen
adsorption–desorption isotherm of MCM-41 sample exhi-
bits type IV sorption isotherm according to IUPAC clas-
sification, which indicates the existence of mesopores in
the corresponding material. The N2 uptake in the range of
0.2–0.4 P/P0 indicates capillary condensation in the
mesoporous of MCM-41 sample. The sharpness of this step
reflects the uniform pore size [45]. Compared to the
untreated MCM-41 sample, the sharp capillary condensa-
tion step in the case of incorporated MCM-41 samples
shifts toward lower P/P0 region. This suggests that the
introduction of In(OiPr)3 into the channel causes changes in
Activity of heterogenized In(OiPr)3-MCM-41
catalyst
To evaluate the catalytic performance of In(OiPr)3-MCM-
41 catalyst, its activity for the MPV reduction of unsatu-
rated aldehydes and ketones was investigated. Unmodified
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J Incl Phenom Macrocycl Chem
Table 1 Surface properties of
MCM-41 and In(OiPr)3-MCM-4
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
MCM-41
In(OiPr)3-MCM-41
1079
846
0.45
0.19
2.5
2.1
yields using unsaturated aldehydes and ketones and iso-
propanol as the solvent and the reducing agent (Table 2).
As can be seen in Table 2, unsaturated aldehydes and
ketones were reduced to the corresponding unsaturated
alcohols. No other reduction products were detected,
showing the high selectivity of the reaction.
The reaction mechanism for the homogeneous MPV
reaction involves a cyclic six membered transition state
[12, 46]. Recent studies by Creyghton et al. [18] proposed
that, over heterogeneous catalysts like zeolite beta, the
MPV mechanism also involves a six-membered transition
state where both the alcohol and the ketone coordinate to
the same Lewis acid site. Also, in our previous studies, we
proposed that, over grafted alkoxides on SBA-15 and
MCM-41, the MPV reduction mechanism of unsaturated
carbonyls involves a cyclic six membered transition state
[39, 43] In this regard, the reaction mechanism for MPV
reduction of unstaurated aldehydes and ketones with
In(OiPr)3-MCM-41 catalyst can be presumed to be similar
to the classical MPV reduction of carbonyls reported for
aluminium isopropoxide [46]. Scheme 1 shows the pro-
posed mechanism for the MPV reduction of carbonyl
compounds in the presence of In(OiPr)3-MCM-41. First,
the carbonyl compound is coordinated to the indium of the
indium alkoxide. The reaction proceeds by hydride-transfer
to the carbonyl compound from the alcohol, which is bound
to the indium centre as an alkoxide. Since the reduction is
reversible, the acetone was removed from the medium by a
slow stream of nitrogen. The removal of the acetone from
the reaction solution leads to the progress of the reaction to
the right hand side (see Scheme 1).
Fig. 3 29Si solid-state MAS NMR spectra of (a) pure MCM-41 and
In(OiPr)3-MCM-41
Comparison of heterogenized In(OiPr)3-MCM-41
and In(OiPr)3-SBA-15 catalysts: effect
of mesoporous support surface
MCM-41 was not active in the MPV reduction of carbonyl
compounds [39]. However, once indium tri-isopropoxide
was grafted onto MCM-41, the heterogenised material
showed good activity (Table 2).
The surface properties and catalytic activity differences of
indium-containing mesoporous catalysts prepared by
grafting method were compared in the MPV reduction of
carbonyl compounds. In(OiPr)3-SBA-15 heterogeneous
catalyst was previously reported by us [43] while the
preparation and characterisation of In(OiPr)3-MCM-41 is
described in this work. We have previously reported that
In(OiPr)3-SBA-15 showed excellent catalytic activity and
chemoselectivity for MPV reduction [43].
In Table 2 are summarized the results of the catalytic
MPV reduction of several unsaturated carbonyls in the
presence of heterogeneous In(OiPr)3-MCM-41 catalyst.
Good catalytic activity and selectivity to the reduced
unsaturated alcohols, citronellol, cinnamalcohol, a-ionol,
3-methyl-2-butanol, geraniol, nerol were observed over
indium tri-isopropoxide supported on silica. Herein we
wish to report that the MVP reaction can be effectively
catalysed by In(OiPr)3-MCM-41 catalyst to very good
In(OiPr)3-MCM-41 heterogeneous catalyst was tested in
the MPV reduction of the same unsaturated aldehydes and
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J Incl Phenom Macrocycl Chem
In(OiPr)3-SBA-15 [42]
Table 2 MPV reduction of
various unsaturated aldehydes
and ketones in the presence of
heterogenized In(OiPr)3-MCM-
41 and In(OiPr)3-SBA-15
Substrate
In(OiPr)3-MCM-41
t (h)
Yield (%)
Chemo
selectivity
t (h)
Yield (%)
Chemo
selectivity
catalysts: selected substrats,
reduction times, product alcohol
yields and selectivity
8
76.4
100
100
8
80.2
100
100
O
Citronellal
9
81.5
9
83.9
O
H
Cinnamaldehyde
7
7
87.2
85.4
98a
98a
7
7
89.4
87.6
98a
98a
O
Citral A (geranial)
O
Citral B (neral)
O
7
8
88.7
83.4
99a
98a
7
8
89.6
85.8
98a
97a
3-methyl-crotonaldehyde
O
α–
ionone
a
Chemoselectivity to allylic. Alcohol reaction conditions 30 mmol of substrate, 200 mmol 2-propanol,
500 mg catalyst, under reflux and stirring at 80 °C
Scheme 1 In(OiPr)3-MCM-41 catalysed MPV reduction of carbonyl compounds
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J Incl Phenom Macrocycl Chem
ketones. For comparison, In(OiPr)3-MCM-41 was prepared
using same amount of indium tri-isopropoxide as used in
In(OiPr)3-SBA-15. The amount of indium-grafted over
different support samples was determined by the ICP-OES
analysis. The percentage loading of indium was found to be
almost similar over two periodic mesoporous silica mate-
rials, SBA-15 and MCM-41. ICP-OES analysis showed
2.46 wt% of indium content in the In(OiPr)3-MCM-41
catalyst. Between them, In(OiPr)3-SBA-15 shows the
highest amount of indium grafting revealing a greater
percentage of silanol groups for the stabilization of indium
species. 2.72 wt% of indium content was found in the
In(OiPr)3–SBA-15 catalyst [43]. This result shows that the
number of silanol groups available for grafting is different
for the silicas thus the unstabilized indium species in silica
samples get washed out during the filtration steps. Thus the
low percentage of indium observed for the In(OiPr)3-
MCM-41 sample relate to the limited accessibility of the
surface hydroxyl groups for the In(OiPr)3 species or due to
the different surface properties between the two classes of
mesoporous solid materials.
MPV reduction of the C=O bond using MPV catalysts will
be a promising process for produce unsaturated alcohols.
The reduction product, citronellol, is an acyclic
monoterpenoid and an alcohol. Citronellol is used in per-
fumes and as insect repellents [47]. Citronellol is a good
mosquito repellent at short distances, but protection greatly
lessens when the subject is slightly further from the source
[48]. When complexed with b-cyclodextrin, it has on
average a 1.5 h protection duration against mosquitoes
[49]. Citronellol is used as a raw material for the produc-
tion of rose oxide. Citronellol also has been suggested to
help cure cancer. Song et al. [50] demonstrated in his study
‘‘Citronellol terpenoid inhibits cancer cell proliferation and
induces apoptosis in Non-Small Cell Lung Carcinoma’’.
As can be seen from Table 2, two heterogeneous cata-
lyst selectively reduces the carbonyl functions without
affecting the olefinic bonds. Conversion yield of unsatu-
rated aldehydes and ketones to corresponding unsaturated
alcohols range from 76.4 to 88.7% within 7–9 h of reaction
time with In(OiPr)3-MCM-41. Also, conversion yield of
same unsaturated carbonyls to corresponding unsaturated
alcohols range from 80.2 to 89.6% within 7–9 h of reaction
time with In(OiPr)3-SBA-15 [43]. In comparison with the
In(OiPr)3-MCM-41 catalyst, the In(OiPr)3-SBA-15 hetero-
geneous catalyst shows slightly higher unsaturated alcohol
yield after the same time. The reason for the lower activity
observed for MCM-41 sample may be due to the presence
of smaller pore volume and pore diameter than SBA-15
sample (Table 3). The indium isopropoxide-grafted may
block these pores and had limited access for the substrate
molecules for the formation of transition state species.
As shown in Table 2, two heterogeneous catalysts
exhibited excellent chemoselectivity towards unsaturated
alcohols. It was found that, for MPV reduction of unsatu-
rated aldehydes and ketones, In(OiPr)3-MCM-41 has a
similiar activity with In(OiPr)3-SBA-15 catalyst. These
results show that the structural features of the support
materials, SBA-15, MCM-41, slightly affect the catalytic
The studied carbonyl compounds, the yields of the
reduction product, selectivity and reaction times in the
presence of heterogeneous catalysts are summarized in
Table 2. Here, in the presence of In(OiPr)3-MCM-41 cat-
alyst, the yield of citronellol was 76.4% after 8 h. The yield
of citronellol over the supported In(OiPr)3-SBA-15 catalyst
was slightly higher: the yield reached 80.2% in the same
time [43]. In the conversions of citronellal, the isolated
double bond was left intact and citronellol was obtained
(Scheme 2). The bond energy of C=C bond is smaller
(615 kcal/mol) than that of the C=O bond (715 kcal/mol),
which makes the hydrogenation of C=O bond difficult [3].
Selective hydrogenation of unsaturated aldehydes and
ketones to their corresponding unsaturated alcohols is
highly desirable due to the corresponding unsaturated
alcohols being useful as intermediates, pharmaceuticals,
and as flavor chemicals [3]. In this instance, the selective
Scheme 2 MPV reduction of citronellal to citronellol using In(OiPr)3-MCM-41 catalyst
123

J Incl Phenom Macrocycl Chem
Table 3 Comparison of surface
properties of MCM-41,
In(OiPr)3-MCM-41, SBA-15
and In(OiPr)3-SBA-15 [43]
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
MCM-41
1079
879
846
548
0.45
1.24
0.19
0.68
2.5
SBA-15
In(OiPr)3-MCM-41
In(OiPr)3-SBA-15
7.8 [43]
2.1
6.6 [43]
activity of the indium catalyst that the In(OiPr)3-SBA-15
shows slightly higher activity than In(OiPr)3-MCM-41.
High surface area mesoporous materials like MCM-41,
SBA-15 provide a useful support for the heterogenisation
of indium tri-isopropoxide. These support materials,
MCM-41 and SBA-15, with the hexagonal pore structure,
probably favor easy access of the reactants to the In cen-
ters, and thus facilitate the MPV reduction reaction.
Comparing these results with the Al-beta, Zr-beta and
Sn-beta zeolites it was inferred that the zeolite catalysts
enhance undesired side reactions due to the presence of
Lewis and Bronsted acidity, generated after the calcination
process [48, 51]. However, the absence of undesired side
products even after 9 h, for the indium alkoxide-grafted
mesoporous materials suggest that polarisation of carbonyl
groups gets enhanced over the Lewis acid sites or Lewis
acid sites be the active catalytic sites. Following this, it
appears that the activity and selectivity of MPV catalysts
can be improved by introducing the adequate Lewis acid
sites in the mesoporous surface that can produce the right
polarization of the carbonyl group [52]. Moreover, as
pointed out by Anwander et al., the surface confinements
effects in mesoporous hosts may prevents the indium
alkoxide groups from self-association [21].
accessibility for the carbonyl compounds. Both of the
heterogeneous indium alkoxide catalysts facilitate the
chemoselective reduction of a variety of unsaturated
ketones and aldehydes to unsaturated alcohols. Hence,
In(OiPr)3-MCM-41 and In(OiPr)3-SBA-15 have great
potential for applying in the MPV reduction of different
unsaturated aldehydes and ketones.
Acknowledgements The financial support of Scientific and Techni-
cal Research Council of Turkey (TUBITAK) under Grant No.
113Z389 is gratefully acknowledged.
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