Comparison of heterogeneous B(OiPr)3-MCM-41 and homogeneous B(OiPr)3, B(OEt)3 catalysts for chemoselective MPV reductions of unsaturated aldehydes and ketones

Article abstract of DOI:10.1016/j.apcata.2012.06.004

Boron tri-isopropoxide, B(OⁱPr)₃, was immobilized on mesoporous material, MCM-41, and denoted as "B(OⁱPr) ₃-MCM-41". The prepared new heterogeneous catalyst, B(O ⁱPr)₃-MCM-41, was characterized in details by using PXRD, FT-IR-, ¹¹B NMR-, ²⁹Si NMR-, ¹³C NMR-, TEM, EDX, N₂ adsorption and ICP-OES. The results demonstrated the successful homogenous distribution of the B(OⁱPr)₃ on the MCM-41 support. Heterogeneous B(OⁱPr)₃-MCM-41 catalyst in comparison with the homogeneous B(OⁱPr)₃ and B(OEt) ₃ catalysts, display similiar catalytic activity in the Meerwein-Ponndorf-Verley (MPV) reduction of unsaturated aldehydes and ketones with alcohols as reductants. Reduced reaction times, higher rate constants and very high selectivities for the unsaturated alcohols were obtained with the heterogenous catalyst than the homogeneous catalysts. In most cases, there were no side products other than the desired alcohol. The B(OⁱPr) ₃-MCM-41 catalyst was found to be encouraging as the catalyst is recyclable up to six cycles without any significant loss in its catalytic activity. This work enriches the family of heterogeneous MPV catalysts for chemoselective reductions of unsaturated aldehydes and ketones.

Full text of DOI:10.1016/j.apcata.2012.06.004

Applied Catalysis A: General 435–436 (2012) 204–216
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Comparison of heterogeneous B(OiPr)3-MCM-41 and homogeneous B(OiPr)3,
B(OEt)3 catalysts for chemoselective MPV reductions of unsaturated aldehydes
and ketones
Burcu Uysal, Birsen S. Oksal∗
Department of Chemistry, Akdeniz University, 07058 Antalya, Turkey
a r t i c l e i n f o
a b s t r a c t
Boron tri-isopropoxide, B(OiPr)3, was immobilized on mesoporous material, MCM-41, and denoted as
“B(OiPr)3-MCM-41”. The prepared new heterogeneous catalyst, B(OiPr)3-MCM-41, was characterized
in details by using PXRD, FT-IR-, 11B NMR-, 29Si NMR-, 13C NMR-, TEM, EDX, N2 adsorption and ICP-
OES. The results demonstrated the successful homogenous distribution of the B(OiPr)3 on the MCM-41
support. Heterogeneous B(OiPr)3-MCM-41 catalyst in comparison with the homogeneous B(OiPr)3 and
B(OEt)3 catalysts, display similiar catalytic activity in the Meerwein–Ponndorf–Verley (MPV) reduction
of unsaturated aldehydes and ketones with alcohols as reductants. Reduced reaction times, higher rate
constants and very high selectivities for the unsaturated alcohols were obtained with the heterogenous
catalyst than the homogeneous catalysts. In most cases, there were no side products other than the
desired alcohol. The B(OiPr)3-MCM-41 catalyst was found to be encouraging as the catalyst is recyclable
up to six cycles without any significant loss in its catalytic activity. This work enriches the family of
heterogeneous MPV catalysts for chemoselective reductions of unsaturated aldehydes and ketones.
© 2012 Elsevier B.V. All rights reserved.
Article history:
Received 1 February 2012
Received in revised form 31 May 2012
Accepted 3 June 2012
Available online 12 June 2012
Keywords:
Mesoporous MCM-41
Boron tri-isopropoxide
Chemoselectivity
Unsaturated aldehydes and ketones
MPV reduction
1. Introduction
hydrogenations, hydrogen transfer reactions of the Meerwein-
Ponndorf-Verley (MPV) type form an attractive approach [11–13].
Unsaturated alcohols are important and versatile intermedi-
ates in the production of pharmaceuticals, agrochemicals and
fragrances [1,2]. They may be produced by oxidation of olefins [3],
or by alkylation of unsaturated aldehydes [4]; but a more gen-
eral route is the selective reduction of unsaturated carbonyls to
unsaturated alcohols.
In homogeneous catalysis, a major breakthrough was the
discovery of the chemoselective reduction with H2 and a Ru-
phosphine-diamine catalyst [5,6]. However, this catalyst requires
extra base for its activation; it is not easily recuperated from the
reaction mixture and may in the recycling loose its activity. There-
fore, considerable effort has been done to develop heterogeneous
catalysts for this type of reduction [7]. Especially the combina-
tion of group VIII B metals (Pt, Rh, Ru) and some metal additives
(Sn, Fe) has been found to be suitable for the selective reduction
of unsaturated aldehydes to allylic alcohols [8,9]. However, when
applied to the selective reduction of ␣,␤-unsaturated ketones,
these catalysts often show high selectivity for the saturated
ketone or the alcohol [10]. As an alternative to metal-catalysed
The reduction of carbonyl compounds by hydrogen transfer
from an alcohol is known as the “Meerwein–Ponndorf–Verley reac-
tion” or “MPV reaction” in Organic Chemistry and can be performed
under mild conditions using Lewis acids as catalyst [14,15]. The
MPV reduction of carbonyl compounds using secondary alcohols
as hydrogen donor and the Oppenauer oxidation of alcohols with
oxidants are highly selective reactions since only the carbonyl
group coordinates with the Lewis acid centre, whilst the double
bond remains unreacted [16]. Therefore, the MPV reduction offers
a facile access to unsaturated alcohols, many of which are impor-
tant starting materials for the production of fine chemicals [17].
Traditionally, the MPV reaction has been conducted by using a
metal (Al, Zr, Ti) alkoxide as catalyst in a homogeneous process
[18,19].
A major advantage of heterogeneously over homogeneously
catalyzed MPV reactions is that the catalysts can easily be
separated from the liquid reaction mixture [20,21]. So far,
a
modest number of heterogeneously catalyzed MPV reac-
tions have been reported. Heterogeneous catalysts for the
Meerwein–Ponndorf–Verley–Oppenauer (MPVO) reactions
include zeolites [22–29], grafted alkoxides [30,31], (modified)
metal oxides which exhibit either Lewis acid or basic properties
[21,31–35] such as magnesium oxide, zirconia, silica, alumina, etc.
∗
Corresponding author. Tel.: +90 2423102316; fax: +90 2422278911.
E-mail address: bbirsen@akdeniz.edu.tr (B.S. Oksal).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.06.004

B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
205
The heterogenation of the MPV catalysts also offer the advantage
of reusability [36]. In order to heterogenise 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 metalorganic moieties
[37,38]. Since its discovery by Mobile in 1992 [39,40], MCM-41 and
related mesoporous molecular sieves have attracted great atten-
tion. As it is known, MCM-41 possesses unidirectional channel-like
pores of rather uniform size which are arranged in a regular hexag-
containing the grafted alkoxide contains 2.54 mmol B per g final
material and is denoted as B(OiPr)3-MCM-41.
2.3. Characterisation
The surface area, average pore diameter and pore size distri-
bution of MCM-41 support and prepared heterogeneous catalyst
were determined by nitrogen adsorption using a Micromeritics
Gemini III 2375 Surface Area Analyzer. Prior to measurements,
the samples were degassed at 300 ◦C and 0.15 mbar at least
for 6 h. The surface areas were calculated 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–Joyner–Halenda) method. The bulk crystalline phases of
the samples was determined by powder X-ray diffraction (XRD).
This was conducted using a X’Pert Pro MPD Diffractometer from
PANalytical, with a CuK␣1 radiation wavelength 0.154 nm.
Energy-dispersive X-ray spectrometry (EDX) analysis to deter-
mine the elemental distribution throughout the sample granules
were acquired using a Hitachi S-4000 microscope equipped with
an SAMX EDX detector located at the ZELMI, TU Berlin. Transmis-
sion electron microscopy (TEM) images were recorded on a tecnai
G2 20 S-TWIN with an energy dispersive X-ray spectrometer (EDAX,
r-TEM SUTW) located at the ZELMI, TU Berlin.
˚
onal 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 [41].
Modification of the MCM-41 may provide an alternative strategy
to obtain suitable supports to maintain high activity in the hetere-
geneous catalyst system for the reduction of carbonyl compounds.
Several MPV reduction catalysts such as Al-alkoxides, Zr-alkoxides
as well as Hf and Zr alkyl complexes have been anchored on MCM-
41 or on amorphous SiO2 in order to generate potential catalytic
activity [33,21,31,32]. Many of these catalysts have been applied in
reactions of saturated ketones (e.g., substituted cyclohexanones),
which are much easier to reduce than unsaturated ketones. Only a
few heterogeneous catalysts were applied in the MPV reduction of
␣,␤-unsaturated ketones [2,21,34].
In the present study, a new heterogeneous MCM-41-supported
boron tri-isopropoxide catalyst was prepared. Catalytic activity of
this heterogeneous catalyst, B(OiPr)3-MCM-41, was compared with
homogeneous B(OiPr)3 and B(OEt)3 catalysts. Up to the present
time, there has not been any literature reporting about the com-
parison of boron alkoxide-grafted MCM-41 heterogeneous catalyst
and homogeneous boron alkoxide catalysts for the reduction of
unsaturated ketones and aldehydes to unsaturated alcohols.
The boron content of the grafted catalyst was determined by
inductively coupled plasma-optical emission spectroscopy (ICP-
OES). The mesoporous material (B(OiPr)3-MCM-41) was dissolved
in a mixture solution of HCl and HF. An aqueous HCl and HF solu-
tion containing boron was used as standard reference. ICP-OES was
performed using PerkinElmer Optima 4300DV.
The anchoring of boron tri-isopropoxide species on MCM-41
was followed by FT-IR and solid state NMR spectroscopy.
2. Experimental
Solid state 13C MAS NMR, 29Si MAS NMR and 11B MAS NMR
spectra were recorded on a Bruker Advance 400 NMR spectrometer
with a resonance frequency of 100.6, 79.46 and 128.3 MHz for 13C,
29Si and 11B using a Bruker 4 mm double-resonance probe head
operating at a spinning rate of 10 kHz for 13C and 12 kHz for 11B
and 29Si, respectively. 13C spectra were collected with 70◦ rf pulses,
5 s delay whilst 29Si spectra were collected with 70◦ rf pulses, 30 s
delay and in both cases with ∼6000 scans. 11B MAS NMR spectra
were accumulated for 1024–4096 scans with ꢀ/4 pulse width of
1.1 ␮s and 2 s recycle delay.
2.1. Preparation of mesoporous support
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 solu-
tion was obtained. 19.27 g (128.3 mmol) of LUDOX AS-40 (Dupont)
was added to the solution with stirring. After 15 min an addi-
tional 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 1 h. The molar composition of the final gel mixture was
SiO2:0.3CTMACl:0.2TEAOH:46.3H2O.
The mixture was placed in a Teflon-lined stainless-steel auto-
clave (BERGHOF BR-200 pressure reactor). The rotating autoclave
was heated at 110 ◦C for 48 h. The resulting mixture obtained was
then filtered, washed with deionized water till filtrate showed
a neutral pH and was then dried at room temperature for 24 h.
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.
2.4. Catalytic MPV reduction reactions
2.4.1. Catalytic activity of B(OiPr)3-MCM-41
The B(OiPr)3-MCM-41 catalyst was tested on the MPV reduction
of a series of unsaturated aldehydes and ketones.
The reaction mixture containing 30 mmol of the aldehyde or
ketone, 200 mmol (12.02 g; 15.4 mL) 2-propanol 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 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
analysed by GC/MS. Firstly, suitable reduction time for each alde-
hyde and ketone was examined. Then, kinetic of reduction for each
aldehyde and ketone was studied.
2.2. Grafting of boron tri-isopropoxide, B(OiPr)3
Boron alkoxide-containing ordered mesoporous silica material
(B(OiPr)3-MCM-41) was prepared by the grafting method [42].
Prior to grafting, the support (MCM-41) (2.0 g) was dried at 250 ◦C
for 3 h. It was then added to a solution of (1.9 mL; 5.6 mmol) B(OiPr)3
(as a 70 wt% solution in 2-propanol) in dry hexane (24 mL). The sus-
pension was stirred (500–750 rpm) for 4 h at room temperature.
The product was filtered under N2, washed three times with 10 mL
of hexane and finally dried under an inert gas flow. The material
The products were identified on the basis of their retention
times by comparing with authentic samples and their mass spec-
tral fragmentation patterns with those stored in the data bank
(Wiley/NIST library). Analyses were performed on a Varian CP 3800

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B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
Table 1
gas-chromatograph equipped with a Varian Saturn 2200 MS detec-
tor (Walnut Creek, CA, USA) and a VF-5 ms capillary column (30 m
length and 0.25 mm I.D., 0.25 ␮m film thickness) (Palo Alto, CA,
USA).
Surface properties of MCM-41 and B(OiPr)3-MCM-41.
Sample
BET surface
area (m2/g)
Pore volume
(cm3/g)
Pore
diameter (A)
˚
The same procedure was used to reduce other unsaturated
aldehydes and ketones in the presence of B(OiPr)3-MCM-41. The
selectivity towards the unsaturated alcohol (SUOL) at variable con-
version levels was calculated from the following expression [15]:
MCM-41
1119
809
0.37
0.13
24.4
20.6
B(OiPr)3-MCM-41
ꢀ
ꢁ
method, BET), FT-IR and multi nuclear NMR-spectroscopies, PXRD,
TEM and EDX techniques.
mmol unsaturated alcohol
mmol ␣, ␤ − unsaturated carbonyl compound converted
SUOL
=
× 100.
Fig. 1 shows the nitrogen adsorption-desorption isotherms for
MCM-41 and B(OiPr)3-MCM-41. The BJH pore size distribution
curves for MCM-41 and B(OiPr)3-MCM-41 are shown in Fig. 2. It is
observed that the MCM-41 shows a type IV isotherm according to
the IUPAC nomenclature without any hysteresis, and capillary con-
densation of nitrogen occurs at a relative pressure of p/p0 = 0.2–0.4,
which is very characteristic for mesoporous materials with uniform
pores. The isotherm can be divided in three parts: the monolayer-
multiple adsorption of N2 on the wall of the mesopores, the
capillary condensation of the nitrogen within the mesopores and
then the saturation.
To test for any leaching, about 500 mg of the grafted catalyst
was refluxed in 200 mmol of 2-propanol at 80 ◦C for 6 h. The solu-
tion was filtered and the filtrate was tested for activity in the MPV
reduction of cinnamaldehyde. The reuse of catalyst was also tested
for the MPV reduction of cinnamaldehyde. The yield was practically
unaffected up to six cycles. The filtrate and quenched solutions of
the catalyst did not show any catalytic activity. This confirms that
the activity observed is due only to the solid catalyst and not partly
due to the leached active species. Characterization of reused cat-
alyst were determined by 11B NMR and powder X-ray diffraction
(XRD).
Compared to the untreated MCM-41 sample, the sharp capillary
condensation step in the case of incorporated MCM-41 samples
shifts towards lower p/p0 region. This suggests that the intro-
duction of B(OiPr)3 into the channel causes changes in the pore
structure of the support during the incorporation process. These
changes could be observed more clearly in Table 1. As expected after
introduction of boron complexes into the MCM-41 framework, the
BET surface area, pore volume and pore diameter decreased.
Also, N2 adsorption measurements were conducted to check the
mesoporous structure of the supports after grafting of the B(OiPr)3
compounds. As we can see on Table 1 before functionalisation with
B(OiPr)3, the calcined MCM-41 has a surface area of 1119 m2/g,
2.4.2. General procedure for catalytic MPV reduction of
unsaturated aldehydes and ketones with homogeneous boron
alkoxide catalysts: B(OiPr)3, B(OEt)3
The catalytic MPV reduction reaction was conducted in a 50 mL
two-mouthed flask with a side stopcock equipped with a 100 cm
condenser and it was furnished with a magnetic stirrer. 30 mmol
iPrOH/EtOH was placed in the flask, and the flask immersed in a
water bath. 10 mmol B(OiPr)3/B(OEt)3 were added drop-wise to
iPrOH/EtOH. The solution was stirred for 5 min, then 30 mmol sub-
strate (unsaturated aldehyde or ketone) were injected into the
solution during the reduction slow stream of dry nitrogen was
passed just over the surface of reaction mixture. In this way, aris-
ing acetone/acetaldehyde was removed by nitrogen flow. Thus the
equilibrium reaction was slipped to the right hand. The resulting
solution was stirred for determining suitable reaction time.
After suitable reduction time was determined for each unsatu-
rated aldehydes and ketones, same experimental procedure was
repeated for studying reduction kinetics of each aldehydes and
ketones. At regular times which last for 1 h, 1 ␮L of solution
was removed and injected into water–isopropanol/water–ethanol
(1 mL H2O/1 mL alcohol) solution. In this case, boron alkoxide was
hydrolysed to boric acid (H3BO3) and dissolved in water. To sep-
arate water and organic phase, water phase was saturated with
K2CO3. Thus, product alcohol and unreducted aldehyde/ketone
remain in organic phase. The rate of reaction was followed by
removing aliquots at regular intervals during the course of the reac-
tion and analysing by GC–MS. Analyses were performed on a Varian
CP 3800 gaschromatograph equipped with a Varian Saturn 2200MS
detector (Walnut Creek, CA, USA) and a VF-5 ms capillary column
(30 m length and 0.25 mm I.D., 0.25 ␮m film thickness) (Palo Alto,
CA, USA). The same procedure was used to reduce all unsaturated
aldehydes and the ketones in the presence of boron alkoxides. All
chemicals were purchased from Merck, Fluka or Aldrich, and used
as received. All reactions were carried out under a nitrogen atmo-
sphere using modified Schlenk techniques.
˚
with an average pore diameter of 24.4 A as deduced using the
Barrett–Joyner–Halenda method. After reaction with B(OiPr)3, the
MCM-41 displays a type IV adsorption isotherm according to the
IUPAC nomenclature without any hysteresis between the adsorp-
tion and the desorption curves (Fig. 1). This is an indication for
uniform pore sizes arranged in networks. The specific surface
2
˚
decreases to 809 m /g, with an average pore diameter of 20.6 A. This
implies that the surface functionalisation to some extent decreases
the available pore volume and diameter, but the material still has
all characteristics of an ordered, mesoporous support.
The mesostructural and ordered characteristics of the pure
MCM-41 and B(OiPr)3-MCM-41 samples were determined by
XRD measurements. The low-angle XRD patterns of MCM-41 and
B(OiPr)3-MCM-41 are shown in Fig. 3(a) and (b), respectively. The
XRD pattern of the untreated MCM-41 exhibits a high intensity
(1 0 0) and two low intensity reflections (1 1 0) and (2 0 0) which are
characteristic of the hexagonal mesoporous MCM-41 [43]. After the
incorporation of boron alkoxide complex into framework, the main
peak intensity decreases. This results indicates that the incorpora-
tion of boron alkoxide complexes in the channel of MCM-41 leads
to a substantial loss in the scattering contrast between the chan-
nel and the wall, and reduces the intensity of the scattered X-ray
in the powder diffraction experiment [44]. Furthermore, the inten-
sity of the two less intense reflections (1 1 0) and (2 0 0) decreases
much more. This observation is supported by a drop in the pore vol-
ume and the surface area (Table 1). In addition, the incorporation of
boron alkoxide complexes into the framework leads to a shift of the
main peak to lower 2ꢁ values. Similar observations were previously
reported with different metal complexes incorporated in MCM-41
[45].
3. Results and discussion
3.1. Characterisation of materials
The as-obtained heterogenous catalyst systems were inves-
tigated with various analytical methods including nitrogen
adsorption/desorption measurements (Brunauer–Emmett–Teller
Fig. 4 shows the FT-IR spectra of the MCM-41 and B(OiPr)3-
MCM-41 materials. The peak at 971 cm−1 was usually assigned to
the stretch vibration of Si
O
H [46], whilst the peak at 802 cm−1

B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
207
Fig. 1. N2 adsorption/desorption isotherms of (a) MCM-41 and (b) B(OiPr)3-MCM-41.
was attributed to the symmetric stretch vibration of Si
O
Si [45].
To confirm the presence of Boron inside MCM-41 host, EDX anal-
ysis was carried out. The results are shown in Fig. 6. In comparison
with EDX spectra of pure MCM-41, B signals clearly appeared in EDX
spectra of B(OiPr)3-MCM-41 heterogeneous catalyst. This directly
proves the presence of boron in the prepared heterogeneous cata-
lyst.
Also, in order to obtain evidence of the homogenous dispersion
of boron alkoxides in MCM-41, quantification of elemental compo-
sition using EDX spectroscopy was carried out on different catalyst
particles. The EDX spectra of B(OiPr)3-MCM-41 were shown in Fig. 6
confirm clearly the homogenous dispersion of boron in B(OiPr)3-
MCM-41 without any phase separation into local domains. The
quantitative analysis of B, C, O and Si showed no significant change
in relative intensities. This provides strong indication that most of
the B are uniformly distributed into or onto the support materials.
The material was free from impurities in the studied range.
The grafting of the B(OiPr)3 on the calcined MCM-41 material
was followed with 29Si solid state NMR. 29Si MAS NMR spectra
of calcined mesoporous MCM-41 and B(OiPr)3-MCM-41 samples
were shown in Fig. 7. The presence of broad resonance peaks from
The strongest band at 1064 cm−1 due to asymmetric stretch vibra-
tion of Si
O Si bridges [47]. Intensity of the strongest peak at
1061 cm−1 decreased for B(OiPr)3-MCM-41.
It was found that the 948 cm−1 peak of the B(OiPr)3-MCM-41
material shifts towards the lower wave number region compared
to the 971 cm−1 peak of untreated MCM-41 sample (Fig. 4). This
could be explained by the interaction between boron and silicon
atoms and formation of B
O Si bonds. In fact, the bond length of
B
O (128–143 pm) was higher than that of H O (96 pm), and thus
led to a decrease in the bond force constant (k). The atomic weight
of boron is higher than that of hydrogen, so the reduced mass (ꢂ)
would increase, hence the vibration frequencies decreased accord-
√
ing to the frequency formula v= 1/2ꢀc (k/ꢂ) (c is speed of the
light).
A small intensity band at 1385 cm−1 was due to the tricoordi-
nated boron present in the incorporated MCM-41 [48,49]. This bond
was completely absent in the spectrum of the pure-MCM-41.
ICP-OES analysis showed 2.75 wt% of boron content in the cata-
lyst. To determine boron content, B(OiPr)3-MCM-41 was dissolved
in a mixture solution of HCl and HF. An aqueous HCl and HF solu-
tion containing boron was used as standard reference. 2.54 mmol
B was found per g of B(OiPr)3-MCM-41.
−90 to −110 ppm, indicative for a range of Si
O Si bond angles and
the formation of more tetrahedral silicon environments. The peaks
observed at −110, −98, −92 ppm, are usually assigned to the Q4
(Si(OSi)4), Q3 (Si(OH)(OSi)3), and Q2 (Si(OH)2(OSi)2) sites, respec-
tively. Compared to MCM-41, B(OiPr)3-grafted mesoporous silica
material shows a marked decrease in the intensity of the Q3 and Q2
signals, showing that some OH groups belonging to Q2 or Q3 have
bound with the boron alkoxide-complex during the incorporation
process.
Fig.
and B(OiPr)3-MCM-41. The well ordered hexagonal arrays of
mesoporous channels confirm the existence of hexagonal
5 shows the representative TEM images of MCM-41
a
mesostructure for MCM-41 material. The TEM images also sug-
gested that the ordered mesoporous structure of the MCM-41
support is not really affected after modification with B(OiPr)3.

208
B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
Fig. 2. BJH pore size distribution curves of (a) MCM-41 and (b) B(OiPr)3-MCM-41.
Solid state 13C NMR spectra of MCM-41 and B(OiPr)3-MCM-41
are shown in Fig. 8. The spectrum of unmodified MCM-41 exhibit
no carbonaceous compound as impurities on the unmodified
MCM-41. The signals at 22.7 and 65.5 ppm in the solid state 13C
NMR spectrum of B(OiPr)3-MCM-41 are characteristic for the iso-
proxy group. 13C NMR (300 MHz, CDCl3, 25 ◦C): ı = 22.7 (CH3), 65.5
(
CH(CH3)2). In addition the 11B NMR of the composize material
reveals the presence of 3-fold coordinated boron with a very
characteristic peak around 22 ppm as shown in Fig. 9(a) indicating
the successful modification of MCM-41 with the desired molecule.
3.2. Characterisation of reused catalyst
The reused catalyst was characterized with 11B NMR-
spectroscopy and PXRD measurement.
The low-angle XRD pattern of reused B(OiPr)3-MCM-41 catalyst
is shown in Fig. 3(c). After using catalyst for six cycle the XRD pat-
tern of the reused B(OiPr)3-MCM-41 catalyst still exhibits a high
intensity (1 0 0) and two low intensity reflections (1 1 0) and (2 0 0)
which are characteristic of the hexagonal mesoporous MCM-41
material. In comparison with XRD pattern of unused B(OiPr)3-
MCM-41 catalyst, intensity of the reflections for six cycle reused
catalyst almost same. In addition, also the 11B NMR of the reused
catalyst still exhibits the presence of characteristic peak around
22 ppm (Fig. 9(b)) which is important to determine of 3-fold coor-
dinated boron in the material.
3.3. Activity of heterogenized B(OiPr)3-MCM-41 catalyst and
homogeneous boron alkoxide catalysts
Homogeneous boron alkoxide catalysts have previously been
proposed as efficient catalysts for the MPV reduction of various
aldehydes and ketones [50–52]. Beyond the preparation of a new
Fig. 3. Low-angle XRD patterns of (a) unmodified MCM-41, (b) B(OiPr)3-MCM-41
and (c) reused B(OiPr)3-MCM-41 catalyst.

B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
209
Fig. 4. FT-IR spectra of (a) MCM-41 and (b) B(OiPr)3-MCM-41.
composite material we aimed further the applicability of the new
material as heterogenous catalyst in the MPV reduction and study
the possible advantages respect to the unsupported analogues. For
the comparison with homogeneous B(OiPr)3 and B(OEt)3 catalysts,
B(OiPr)3-MCM-41 heterogeneous catalyst was tested for the reduc-
tion of unsaturated aldehydes and ketones. The rate constant of
each carbonyl compound was also determined in the presence of
heterogeneous and homogeneous catalysts.
In the presence of homogeneous and heterogenized boron
alkoxide catalysts all unsaturated aldehydes and ketones exhibited
a linear correlation between the natural logarithm of the concen-
tration of the carbonyl group and the reaction time. This suggests
that the reaction is first order in the aldehyde and ketone concen-
tration and allowed us to calculate the rate constant from the slope
of a plot of ln(c0/c) as a function of time:
almost same with homogeneous boron alkoxide catalysts. In addi-
tion the reduced reaction time with B(OiPr)3-MCM-41 catalyst is
quite remarkable.
Also, the studied carbonyl compounds, the yields of the
reduction products in the presence of heterogeneous and homo-
geneous catalysts, and the rate constants of various unsaturated
aldehydes and ketones were summarized in Table 2. As a heteroge-
neous catalyst, B(OiPr)3-MCM-41, have similiar catalytic activity
with homogeneous analogues. The rate constants of carbonyl
compounds and selectivities for the corresponding unsaturated
alcohols are higher in the presence of heterogeneous catalyst than
the homogeneous catalysts. The hexagonal pore structure of MCM-
41 probably favours easy access of the reactants to the B centres,
and thus increases the reaction rate. Because immobilized catalytic
active sites are more accessible to the reactant molecules than
homogeneous counterpart. This is consistent with the tendency of
B-alkoxides to form aggregates in homogeneous reaction medium.
When in contrast the boron alkoxide is dispersed over the siliceous
surface by reaction with the surface silanol groups, more B centres
are available for reaction [35].
ꢀ
ꢁ
c0
c
ln
= kt
where c0 is the initial aldehyde or ketone concentration, c that at a
given time t and k is the rate constant.
To compare catalytic activity of heterogenized B(OiPr)3-MCM-
41 and homogeneous boron alkoxide catalysts the variation of
conversion with time for cinnamaldehyde was given in Fig. 10.
As can be seen in Fig. 10 the activity of B(OiPr)3-MCM-41 het-
erogeneous catalyst for the MPV reduction of cinnamaldehyde is
As it can be seen in Table 2, some differences were observed
in reduction yields, rate constants and reduction times for the
selected unsaturated aldehydes and ketones. These differences
were similar for heterogeneous and homogeneous catalysts.
Within the group of cyclohexenones, it is clear that an increasing
Fig. 5. TEM images of: (a) MCM-41 and (b) B(OiPr)3-MCM-41.

210
B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
Fig. 6. EDX analysis for heterogenised B(OiPr)3-MCM-41catalyst.
number of alkyl substituents leads to a decreasing in reactivity,
partially due to electronic effects. In, e.g., 3-methyl-2-cyclohexen-
1-one, the presence of an alkyl group in the ␤-position to the
them are not selective to target unsaturated alcohols. Selective
hydrogenation of unsaturated aldehydes and ketones such as
cinnamaldehyde, 2-cyclohexen-1-one, ␤-ionone, ␣-ionone, ben-
zalacetone to their corresponding unsaturated alcohols is highly
desirable due to the corresponding unsaturated alcohols are useful
as intermediates, pharmaceuticals, and as flavour chemicals [53].
The bond energy of C C bond is smaller (615 kcal/mol) than that
of C O bond (715 kcal/mol), so which makes the hydrogenation of
C
O group decreases the susceptibility of the carbonyl group
to hydrogen transfer. In comparison with 2-cyclohexen-1-one
and 3-methly-2-cyclohexen-1-one, the smallest rate constant for
3,5-dimethyl-2-cylohexene-1-one can be attributed to its steric
hindrance due to the presence of two methyl groups.
Unmodified MCM-41 was not active in the MPV reduction of
aldehydes and ketones. However, once boron tri-isopropoxide was
grafted onto MCM-41, the heterogenised material showed good
activity and selectivity to the corresponding alcohols. The filtrate
obtained after refluxing the grafted catalyst in 2-propanol was inac-
tive for the MPV reduction of unsaturated aldehydes and ketones,
showing the absence of leaching into the reaction medium. The cat-
alyst recuperated from a first run conserves considerably its activity
until sixth cycle (Table 3).
C O bond difficult [53]. In this instance, the selective MPV reduc-
tion of C O bond using MPV catalysts will be promising process for
perfumery industry.
3-methyl-2-butenal, citral, 2-cyclohexene-1-one, 3-methyl-2-
cyclohexen-1-one, 3,5-dimethyl-2-cyclohexen-1-one, ␣-ionone,
␤-ionone, cinnamaldehyde, 1-cyclohexene-1-carboxaldehyde and
benzalacetone are important unsaturated aldehydes and ketones.
These important carbonyl compounds possess conjugated C
C
and C O bonds. The C C bonds are more readily hydrogenated
than the C O bonds in most usual techniques [53]. Although
reduction of the C O bonds without affecting the C C bonds is
highly difficult, C O bond of these compounds were reduced using
homogeneous B(OiPr)3, B(OEt)3 catalysts. Notably, not only the
3.4. Chemoselectivity of heterogenized B(OiPr)3-MCM-41 and
homogeneous boron alkoxide catalysts
In Table 2 are summarized the results of catalytic MPV reduction
of several unsaturated carbonyls in the presence of homogeneous
B(OiPr)3, B(OEt)3 and heterogeneous B(OiPr)3-MCM-41 catalysts.
As can be seen from Table 2, the catalysts exhibited excellent
chemoselectivity towards unsaturated alcohols.
Catalysis in the perfumery industry is one of the new research
fields using hydrogenation catalysts. There are numerous hydro-
genation processes both in gas and liquid phases, but most of
C C double bond in the enone group of ␣-ionone and ␤-ionone,
but also the endocyclic C C bond are left totally intact. Same
catalytic activity was observed with B(OiPr)3-MCM-41 catalyst.
The catalyst selectively reduces the carbonyl functions with-
out affecting the olefinic bonds. Conversion yield of unsaturated
aldehydes and ketones to corresponding unsaturated alcohols
range from 59.2 to 87.6% within 7–11 h of reaction time with
B(OiPr)3-MCM-41.

B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
211
Reduction product, geraniol, is a monoterpenoid and an alco-
hol. It is the primary part of rose oil, palmarosa oil, and citronella
oil (Java type). It appears as a clear to pale-yellow oil that is insol-
uble in water, but soluble in most common organic solvents. It
has a rose-like odour and is commonly used in perfumes. Geraniol
also has been suggested to help prevent cancer. Carnesecchi et al
demonstrated in his study “Geraniol inhibits growth and polyamine
biosynthesis in human colon cancer cells”, [55] that geraniol cause
a 50% decrease of ornithine decarboxylase activity, a key enzyme
of polyamine biosynthesis, which is enhanced in cancer growth.
3.5. MPV reaction mechanism
The reaction mechanism for the homogeneous MPV reaction
involves a cyclic six-membered transition state [56]. Scheme 1
shows the proposed mechanism for the carbonyl compounds in
the presence of B(OEt)3. First, the carbonyl compound is coordi-
nated to the boron of the boron alkoxide. The reaction proceeds by
hydride-transfer to the carbonyl compound from the sec-alcohol,
which is bound to the boron centre as an alkoxide. Since the reduc-
tion is reversible, the acetaldehyde was removed from the medium
by a slow stream of nitrogen. The removal of the acetaldehyde from
the reaction solution leads to the progress of reaction to the right
hand side. B(OiPr)3 also reduces carbonyl compounds with a simi-
lar mechanism (Scheme 2); the acetone produced was removed by
the same procedure. The removal of acetone (bp 81 ◦C) is more dif-
ficult than acetaldehyde (bp 56 ◦C). Therefore, as shown in Table 3,
the yields of the alcohol products from reduction are greater than
those using B(OiPr)3.
Recent studies by Creyghton et al. proposed that over heteroge-
neous 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 [35]. Scheme 3 shows
the proposed mechanism for the MPV reduction of carbonyl com-
pounds in the presence of B(OiPr)3-MCM-41. First, the carbonyl
compound is coordinated to the boron of the boron alkoxide. The
reaction proceeds by hydride-transfer to the carbonyl compound
from the alcohol, which is bound to the boron centre as an alkox-
ide. Since the reduction is reversible, the acetone was removed from
the medium by a slow stream of nitrogen. The removal of the ace-
tone from the reaction solution leads to the progress of the reaction
to the right hand side (see Scheme 3).
Fig. 7. 29Si solid-state MAS NMR spectra of (a) pure MCM-41 and (b) B(OiPr)3-MCM-
41.
Conversions of citral to nerol and geraniol above 82% within
11 h of reaction were achieved in the presence of B(OiPr)3-MCM-
41 whilst 20 h with homogeneous boron alkoxide catalysts. The
preferential formation of geraniol as compared to nerol can be
explained by steric hindrance of the bulky chain [54]. The carbonyl
group of the geranial is trans to the bulky chain of the molecule,
minimizing any steric hindrance for the binding of C O to the boron
centre. Hence, the rate of reaction for geranial is faster than for
neral. Geranial has a strong lemon odour.
Fig. 8. 13C MAS NMR spectra of (a) MCM-41 and (b) B(OiPr)3-MCM-41.

212
B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
Fig. 9. 11B NMR spectra of (a) B(OiPr)3-MCM-41 and (b) reused B(OiPr)3-MCM-41 catalysts.
Fig. 10. Variation of conversion with time for cinnamaldehyde. Conditions: ( ) 500 mg B(OiPr)3-MCM-41 catalyst, 200 mmol iPrOH, 30 mmol cinnamaldehyde; ( ) 10 mmol
B(OiPr)3, 30 mmol iPrOH, 30 mmol cinnamaldehyde and ( ) 10 mmol B(OEt)3, 30 mmol EtOH, 30 mmol cinnamaldehyde.
3.6. Catalytic MPV reduction reactions: comparison of
homogeneous and heterogeneous catalysts
to its potential for incorporating higher amounts of heteroatoms
in the frame wall positions as well as due to the presence of
higher amounts of silanol groups, for the stabilisation of boron
tri-isopropoxide.
The heterogeneous solid supported catalyst offers further
advantages like the easy catalyst and product recovery as well as
a possible reuse of the catalyst systems. Hence mesoporous mate-
rial, MCM-41, was selected for the grafting of boron alkoxides due
The synthesized boron tri-isopropoxide-containing B(OiPr)3-
MCM-41 was first applied as catalyst for the MPV reduction reaction
of unsaturated aldehydes and ketones. Results showed that the
Scheme 1. A proposed reaction mechanism for the reduction of unsaturated aldehydes and ketones by B(OEt)3.

B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
213
Scheme 2. A proposed reaction mechanism for the reduction of unsaturated aldehydes and ketones by B(OiPr)3.
catalyst showed good conversion and indicate that the Lewis acid-
ity is strong enough and the amount of framework boron is big
to proceed the reaction effectively. Hence, B(OiPr)3-grafted meso-
porous MCM-41 have potential for appling in the MPV reduction of
different unsaturated aldehydes and ketones.
The rate of reaction was found that higher with heterogeneous
catalyst than homogeneous catalysts. An interesting result was that
the selectivity towards the reduced product was ≥97% in the pres-
ence of heterogeneous catalyst, showing that the supported boron
alkoxide catalyst is highly selective. The resulted improved conver-
sion for B(OiPr)3-MCM-41 catalyst shows that B(OiPr)3 is grafted as
a monolayer over the support surface and the higher concentration
of boron helps to increase the number of Lewis acid sites for the
conversion of unsaturated aldehydes and ketones [57].
side reactions due to the presence of Lewis and Brönsted acid-
ity, generated after the calcination process [54,58]. However, the
absence of undesired side products even after 11 h, for the boron
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. Moreover, as pointed out by
Anwander et al., the surface confinements effects in mesoporous
hosts may prevents the boron alkoxide groups from self-association
[59].
In comparison with homogeneous boron alkoxide catalysts,
the structure as well as pore dimensions added to its particu-
lar Lewis acid properties, B(OPr)-MCM-41 shows a higher activity
and better selectivity. Also, the catalyst can be recycled several
times and can be used without important loss in the activities, this
being of clear interest for a potential industrial application of the
catalyst.
Comparing these results with the Al-beta, Zr-beta and Sn-beta
zeolites it was inferred that the zeolite catalysts enhance undesired
Scheme 3. Proposed mechanism of MPV reduction over B(OiPr)3-MCM-41.

Table 2
MPV reduction of various unsaturated aldehydes and ketones in the presence of heterogenised B(OiPr)3-MCM-41a and homogeneous B(OiPr)3, B(OEt)3 catalysts: selected substrats, reduction times, product alcohol yields
(selectivity) and rate constants.
Entry
Substrate
t (h)
Yield (%), (selectivity (%))
B(OiPr)3-MCM-41a
k × 10−3 (min−1
B(OiPr)3-MCM-41a
)
B(OiPr)3-MCM-41a
B(OiPr)3
15
B(OEt)3
15
B(OiPr)3
B(OEt)3
B(OiPr)3
2.478
B(OEt)3
2.54
1.
8
86.8 (100)
83.8 (97)
87.9 (98)
88.3 (98)
4.168
2.
11
20
20
84.3 (93)
86.6 (94)
3.489
2.052
2.470
3.
11
20
20
82.8 (97)
83.1 (93)
85.2(94)
3.328
1.948
2.280
4.
5.
8
9
13
14
13
14
84.53 (100)
82.66 (99)
86.8 (98)
86.5 (97)
87.9 (98)
87(98)
4.137
3.403
2.43
2.487
2.318
2.258
6.
7.
6
7
9
9
87.03 (100)
72.30 (100)
88.7 (99)
73.6 (98)
89.5 (99)
74.8 (99)
6.152
3.389
4.155
2.41
4.278
2.553
10
10
8.
7
10
10
59.2 (99)
62.6 (96)
64.8 (97)
2.492
1.775
1.883

B. Uysal, B.S. Oksal / Applied Catalysis A: General 435–436 (2012) 204–216
215
Table 3
Reduction of cinnamaldehyde to cinnamyl alcohol under different reaction
conditions.
Entry
System
Yield [%]
1
2
3
4
5
6
B(OiPr)3-MCM-41
B(OiPr)3-MCM-41
No catalyst
84.5a
78.7b
c
–
–
–
c
Filtrate
c
Quenched solution
B(OiPr)3 free MCM-41
–
c
Reaction condition: cinnamaldehyde (30 mmol), catalyst (500 mg) in 2-propanol
(200 mmol, 15.4 mL) refluxed at 353 K for 8 h.
a
Yield on single experiment.
Yield after sixth run.
No reaction.
b
c
4. Conclusion
Boron alkoxide catalysts are active in the chemoselective reduc-
tion of unsaturated aldehydes and ketones by hydrogen transfer
from an alcohol, which constitutes the Meerwein–Ponndorf–Verley
(MPV) reaction. However, activity and selectivity of the catalysts in
the process vary markedly depending on the selected boron alkox-
ide and also on any modifications used to heterogenise the catalyst.
The catalyst synthetic procedure and modifications used in this
work (grafting with boron alkoxide) were found to affect surface
catalytic activity as a result. B(OiPr)3-MCM-41 catalyst was pre-
pared by grafting of B(OiPr)3 on ordered siliceous matrice MCM-41.
In optimised reaction conditions, and with suitable alcohol as
reductant, this material catalyses the chemoselective reduction
of most unsaturated ketones and aldehydes to the correspond-
ing unsaturated alcohols. Since the pores in MCM-type materials
allow access of even large molecules, complex substrates such
as the 2-cyclohexen-1-one, 3-methyl-2-cyclohexen-1-one, 3,5-
dimethyl-2-cyclohexen-1-one, ␣-ionone, ␤-ionone, cinnamalde-
hyde, 1-cyclohexene-1-carboxaldehyde and benzalacetone could
be reduced with good yields. The mesoporous support favour a
high dispersion of B(OiPr)3 MPV catalyst, and hence have a strongly
enhancing effect on its catalytic activity. Modifying MCM-41 with
an boron alkoxide leading to a decreased reduction time, increased
selectivity and rate constant.
Eventually, we concluded that the B(OiPr)3-MCM-41 is indeed
an excellent heterogeneous catalyst for the chemoselective reduc-
tion of unsaturated aldehydes and ketones to unsaturated alcohols
by a MPV process. The yield and reaction time are highly encour-
aging. Heterogeneous B(OiPr)3-MCM-41 catalyst also offer the
advantage of ease of separation and reusability. The catalyst is recy-
clable up to six cycles without any significant loss in their catalytic
activity.
Acknowledgement
Financial support from TUBITAK (Turkish Scientific and Techni-
cal Research Institute) (grant no. 110T465) and Scientific Research
Projects Unit of Akdeniz University (grant no. 2009.03.0121.005) is
gratefully acknowledged.
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