Preparation and application of surface activated Si-MCM-41 and SBA-16 as reusable supports for reduction of cyclic ketones with preferential stereoselectivity

Article abstract of DOI:10.1039/c6ra21480k

An efficient and benign procedure for the reduction of a few cyclic ketones adsorbed on the activated surface of calcined Si-MCM-41 and calcined SBA-16 using NaBH₄ as the reducing agent with ethanol as the medium resulted in the formation of two epimeric alcohols. After synthesis, these calcined materials were treated with concentrated HCl to activate surface silanol groups. Various instrumental techniques like FTIR, XRD, N₂ sorption isotherms, FESEM, TEM and XPS were carried out to examine the pre and post activated surface of the used supports. Five cyclic ketones were reduced. Reduction of 4-tert-butylcyclohexanone yielded only trans-4-tert-butylcyclohexanol. Moreover, exclusive formation of cis-3-methylcyclohexanol (equatorial -OH) was also observed. This work offers several advantages such as a simple operational procedure, short reaction time, and high yield of the product, along with maintaining the materials' diversity. This is due to the presence of the activated silanol groups of these materials, which cause nucleophilic activation of the carbonyl group of ketones leading to faster reaction rates. Beside this, these supports can be regenerated well from the reaction mixture using a calcination treatment followed by concentrated HCl, and reused several times without causing any serious malformation in the activated surfaces. Finally, this work opens up a new direction of research for the fabrication of solid reusable supports in the reduction of cyclic ketones.

Full text of DOI:10.1039/c6ra21480k

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Preparation and application of surface activated Si-MCM-41 and SBA-16
DOI: 10.1039/C6RA21480K
as reusable supports for reduction of cyclic ketones with preferential
stereoselectivity
Haribandhu Chaudhuri, Subhajit Dash, and Ashis Sarkar*
Organic Materials Research Laboratory, Department of Applied Chemistry, Indian School of
Mines, Dhanbad, Jharkhand-826004, India.
*Corresponding author: Ashis Sarkar, E-mail: a_sarkar_99@yahoo.com, Tel. +91
9430335255, Fax: +91 326-2307772.
Abstract
An efficient and benign procedure for the reduction of a few cyclic ketones adsorbed on the
activated surface of calcined Si-MCM-41 and calcined SBA-16 using NaBH4 as the reducing
agent with ethanol as the medium resulted in the formation of two epimeric alcohols. After
syntheses, these calcined materials were treated with concentrated HCl to activate surface
silanol groups. Various instrumental techniques like FTIR, XRD, N2 sorption isotherms,
FESEM, TEM and XPS were carried out to examine the pre and post activated surface of
used supports. Five cyclic ketones were reduced. Reduction of 4-tert-butylcyclohexanone
yielded only trans-4-tert-butylcyclohexanol. Moreover, exclusive formation of cis-3-
methylcyclohexanol (equatorial -OH) was also observed. This work offers several advantages
such as simple operational procedure, short reaction time, high yielded of the product along
with retainment of the materials diversity. This is due to the presence of activated silanol
groups of these materials which cause nucleophilic activation of the carbonyl group of
ketones leading to faster reaction rate. Beside this, these supports can be well regenerated
from the reaction mixture using calcination treatment followed by concentrated HCl and
reused several times without causing any serious malformation in the activated surfaces.
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Finally, this work opens up a new direction of research for the fabrication of soDliOdI: 1r0e.1u0s39a/bCl6eRA21480K
supports in the reduction of cyclic ketones.
Keywords
Activated mesoporous silicas; reduction of cyclic ketones; 100 % trans-4-tert-
butylcyclohexanol; 100 % cis-3-methylcyclohexanol; reusable supports.
1. Introduction
To be applicable as an excellent catalyst support, one material should have a number of
minimal requirements, such as a remarkable stability in working conditions, a fairly low-cost,
simple, friendly synthetic method and unambiguous characteristics to evaluate the quality of
the final synthesised material [1-6]. Moreover, other requirements include suitability of the
pores for the reagents along with controllable micro- and mesoporosity. The micro- and
mesoporosity can greatly elevate the activity and selectivity of the catalyst. In this context,
mesoporous silica materials have received a huge recognition due to their excellent surface
area and pore volume as well as nano sized pores (mesopores). Specially, both Si-MCM-41
and SBA-16 have hexagonally ordered pores throughout the surface of these materials. Those
surface properties make them useful as catalyst support [7-18]. The amorphous nature of the
silica walls most often results in poorer catalytic performance. So, it is necessary to prepare
uniformly distributed and preferably isolated active sites on the mesoporous silica surface,
which is crucial for high catalytic activity and selectivity [19]. The catalytic application of Si-
MCM-41 has received much attention for the past few years and the counterpart, SBA-16,
has not yet been studied vigorously due to difficulties encountered during the synthesis
procedure. But, the problem is now overcome and we have carried out a series of
stereochemical investigations on both Si-MCM-41 and SBA-16 to make a new vista of
research in the direction of stereochemistry and to check whether these materials are suitable
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as solid supports for stereochemical investigations [20, 21]. Previous workers have reported
DOI: 10.1039/C6RA21480K
stereoselective reduction of some cyclic and bicyclic ketones with lithium trialkylaluminium
hydride and lithium diisobutyl-tert-butylaluminium hydride [22, 23]. But, from the view point
of stable isomer formation and % of isolated yield, these results were not very much exciting.
So, from the understanding of that kind of problems, we introduce activated Si-MCM-41 and
SBA-16 as supports for reduction of some cyclic ketones with preferential stereoselectivity
using NaBH4 as reducing agent. We explored the possibility of using active mesoporous
silicas as adsorbent for ketones towards effective nucleophilic activation in the reduction with
sodium borohydride (NaBH4) [24, 25]. The purpose was twofold (a) increase the rate of
reduction (b) improvement of stereoselectivity in the reduction of some cyclic ketones. As a
consequence, some used ketones were fully converted to stabler alcohols with greater
chemical yields in short time, compared to that reported earlier [23]. Our initial effort to
reduce cyclic ketones with low carbon fly ash as adsorbent and NaBH4 as reducing agent was
not very encouraging [26, 27]. Reduction of the cyclic ketones namely 4-tert-
butylcyclohexanone,
3-methylcyclohexanone,
2-methylcyclohexanone,
3,3,5-
trimethylcyclohexanone, and 4-methylcyclohexanone were accompanied by similar
stereochemical results as obtained in the reduction of those ketones under homogeneous
condition (NaBH4 in ethanol). There was neither improvement in the rates of the reductions
nor in chemical yields. This problem has now been overcome by activation of surface silanol
groups of these materials using concentrated HCl, a report is being made herewith.
2. Experimental Section
2.1. Materials and reagents
Tetraethoxysilane (TEOS, ≥ 99%), CTAB ((C16H33)N(CH3)3Br, MW = 364.45), Pluronic F127
(EO106PO70EO106, MW = 12600), Chloroform-d (CDCl3), and tetramethylsilane (TMS) were
purchased from Sigma Aldrich. All used ketones were obtained from Alfa Aesar.
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Triethanolamine (TEA, N(CH2CH2OH)3, ≥ 98%), hydrochloric acid (HCl, ~37D%O)I:,10s.1o0d39i/uCm6RA21480K
borohydride (NaBH4), sodium sulphate (Na2SO4), potassium bromide (KBr), diethyl ether
and ethanol (EtOH, ≥ 99%) were procured from Merck India. The pre-coated aluminium
plates (Merck) were used for thin layer chromatography (TLC) and spots were taken under
UV light. In column chromatography, we used silica gel (60-120 mesh). Deionised water was
used without further purification.
2.2. Syntheses
Si-MCM-41 material was synthesised by a method where TEOS was added into aqueous
medium containing CTAB, ethanol, and TEA. The following process was reported
previously: [28] 2.56 mL of water (0.36 mol), 4.56 ml of ethanol (0.015 mol), 4.16 g of a 25
wt% CTAB solution (0.786 mmol), and 0.07 ml of TEA (0.19 mmol) were combined and
stirred in an oil bath at 60°C for 45 min. Then, 2.92 mL of TEOS (3.25 mmol) was added
dropwise into the mixture and stirred for 2 h. Then, the white solution was cooled to room
temperature and the solid product (yield ~98%) was filtered, washed with deionised water
and distilled ethanol. After that, the product was dried under vacuum at 70°C for 12 h.
Finally, the material was calcined at 550°C for 6 h at a heating rate at 15°C/min.
SBA-16 was prepared using a method comparable to that reported earlier [1]. Pluronic F127
(4.0 g) was dissolved in 81 mL of 2 M HCl and 30 mL of deionised water with stirring for 2 h
at room temperature. Then, 9.04 ml of TEOS was added to that homogeneous solution and
stirred for 20 h at room temperature. The mixture was then aged at 80°C for 24 h under
invariable conditions. Thereafter, the solid product (yield ~98%) was filtered, washed with
deionised water, distilled ethanol and air-dried overnight. Calcination was carried out by
slowly increasing the temperature to 500°C at a heating rate of 20°C/min. and heating further
at a constant temperature for 6 h.
After that, the virgin materials were refluxed in concentrated HCl for 12 h to activate the
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surface silanol groups. Thereafter, the materials were made acid free by washing with
DOI: 10.1039/C6RA21480K
distilled water and dried by heating at 110°C for 24 h.
2.3. General procedure for reduction of ketones
In a typical reduction procedure, different amount of ketones like 2 mmol, 5 mmol, and 10
mmol (respectively) were added to 1 g of acid treated calcined solid in presence of ether in
different round bottom flask (RB) and stirred for 12 h each. After ensuring that the ketones
have been thoroughly adsorbed on the surface of the activated solid, NaBH4 (2 mmol, 5
mmol, and 10 mmol, respectively) in 5 ml ethanol was added to the mixture and refluxed
with stirring for certain time (Table 2-Table 3). Those reactions were monitored using TLC
1
and H nuclear magnetic resonance (NMR) spectroscopy. After the reaction was complete,
the ethanolic layer was separated and the residue extracted with 10 ml of ethanol. The
combined ethanolic extract after drying over anhydrous Na2SO4 (2g) was evaporated to
obtain the corresponding alcohols. After reaction, the synthesised catalysts were collected in
distilled water and separated using calcination at 500°C followed by treatment with
concentrated HCl under reflux condition and reused for the next reaction. All reduced
products were characterized by TLC and 1H NMR.
2.4. Characterisations
Fourier transformed infrared (FTIR) spectroscopic analyses were recorded using FTIR
spectrometer (Model IR-Perkin Elmer, Spectrum 2000) using KBr pellet method. X-ray
diffractograms (XRD) patterns were collected using Thermal ARL X-ray diffractometer
associated with CuKα radiation source along with a graphite monochromator. N2 sorption
measurements were done at 77 K with Nova 3200e (Quantachrome, USA). The morphologies
of the acid treated calcined Si-MCM-41 and SBA-16 were determined using field emission
scanning electron microscopy (FESEM, Supra 55, Zeiss, Germany) and transmission electron
microscopy (TEM, JEM-2100, JEOL, Japan). Surface chemical state with elemental
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composition analysis was carried out using X-ray photoelectron spectroscopic (XPS)
DOI: 10.1039/C6RA21480K
instrument (PHI 1600 ESCA system, Perkin Elmer). 1H NMR spectra were taken in Bruker
advanced 300 MHz and 400 MHz spectrometers fitted with 5 mm broadband inverse probe
using CDCl3 as solvent and TMS as internal standard. δ values are reported in parts per
million (ppm) and coupling constants (J) in Hertz (Hz).
3. Results and discussion
3.1. Characterisations of activated materials
FTIR spectra of calcined materials and concentrated HCl treated materials are presented in
Fig. 1a. The main characteristic peaks of calcined materials are: broad bands near 3462 cm-1
and 3468 cm-1 which are due to silanol -OH stretching vibrational mode. The corresponding
bands due to bending vibrational mode are observed at 1639 cm-1 and 1637 cm-1 [29, 30]
respectively. Asymmetric stretching vibrational band near 1087 cm-1 and 1085 cm-1 are due to
the presence of Si-O-Si linkage in those calcined materials. The bands at 983 cm-1 and 986
cm-1 are characteristic one for the mesoporous materials [31] and may be linked to Si-OH
stretching vibration. Symmetric stretching bands of Si-O-Si near 803 cm-1 and 809 cm-1 are
also observed. After activation of surface silanol groups using concentrated HCl, the main
characteristic peaks that appear are: broad band near 3459 cm-1 and 3466 cm-1 which are due
to silanol -OH stretching vibrational mode. The corresponding band due to bending
vibrational modes are observed at 1638 cm-1 and 1641 cm-1 respectively [29]. Asymmetric
stretching vibrational bands near 1086 cm-1 and 1089 cm-1 are due to the presence of Si-O-Si
linkage on those materials. The bands at 981 cm-1 and 982 cm-1 are important peaks for the
mesoporous materials [29] and may be linked to Si-OH stretching vibrations. Symmetric
stretching band of Si-O-Si near 806 cm-1 and 807 cm-1 are also observed. So, there are no
significant changes observed for concentrated HCl treated calcined materials. Only the peak
intensities of -OH groups appeared much sharper due to the existence of activated silanol
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DOI: 10.1039/C6RA21480K
groups on the surface of those materials.
XRD patterns of calcined materials and concentrated HCl treated materials are summarised in
Fig. 1b. The obtained calcined Si-MCM-41 material exhibits some prominent peaks which
are assigned to (100), (110), (200), and (210). These peaks are indicative of typical ordered
mesoporous structures with hexagonal arrays [32]. There are no any significant changes
observed for concentrated HCl treated calcined Si-MCM-41 material. Only the assigned peak
intensities decreased a little. On the otherhand, the synthesized calcined SBA-16 exhibits
typical low-angle diffraction patterns of mesoporous structures. These prominent peaks are
assigned to (100), (110), and (200) which are typical of ordered mesoporous structures with
hexagonal arrays. There are no significant changes observed for concentrated HCl treated
calcined SBA-16 material. Only the (100) peak intensity decreased a little.
N2 sorption isotherms for the calcined materials and concentrated HCl treated calcined
materials were carried out (Fig. 1c). Specific surface area, average pore diameter, and total
pore volume data are given in Table 1. Brunauer–Emmett–Teller (BET) surface area for
SBA-16 is 652 m2/g having an average pore diameter of 5.71 nm with a total pore volume of
0.91 cc/g. The increase of nitrogen uptake at P/P0 > 0.95 is a characteristic feature of this type
of ordered mesoporous silicas and it is attributed to the presence of high textural porosity
[33]. As a result, calcined SBA-16 material possesses very high framework with average pore
diameter and total pore volume. Concentrated HCl treated calcined SBA-16 possesses greater
surface area (734 m2/g), average pore diameter (5.94 nm), and total pore volume (0.93 cm3/g)
due to the presence of activated surface silanol groups on the material compared to the
calcined material. Whereas, concentrated HCl treated calcined Si-MCM-41 possesses greater
surface area (1265 m2/g), average pore diameter (3.67 nm), and total pore volume (1.05
cm3/g) compared to the calcined Si-MCM-41 material having BET surface area of 1202 m2/g
having an average pore diameter of 3.61 nm with a total pore volume of 1.02 cc/g. This is due
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to the fact that after treatment of concentrated HCl, specific surface area, average pore
DOI: 10.1039/C6RA21480K
diameter along with total pore volume increases thus retaining the surface phenomenon of
these materials same as before.
Both FESEM and TEM are important tools for characterising the surface morphology of the
material. It is useful for determining the particle shape, porosity and approximate size
distribution of the material. Both FESEM and TEM were performed for concentrated HCl
treated materials (Fig. 2). FESEM images reveal that fine particles have been formed (Fig. 2a,
b). The hexagonal symmetries of the concentrated HCl treated materials were confirmed by
TEM images (Fig. 2c, d) and the observation of particle shape (Fig. 2c) and ordered lattice
array (Fig. 2d) suggests that the pore structure and size are well formed for these materials.
To study the chemical state, both calcined materials and acid treated calcined materials were
analysed by the XPS technique (Fig. 3). According to the survey scan, only silicon and
oxygen exist on the surface of pre and post acid treated materials. In order to examine the
composition of these used materials, the core level Si(2p) peaks of XPS spectra have been
studied. A perusal of Fig. 3 (c, d) reveals that each Si(2p) peak consists of two Si(2p3/2) and
Si(2p1/2) peaks due to the spin orbital coupling. The binding energy of Si(2p3/2) peaks in Fig.
3 (c, d) was determined to be about 103.1 eV (Fig. 3c) and 102.9 eV (Fig. 3d) for pre and
post acid treated materials, which are consistent with the binding energy for formation of
siloxane moieties [34]. Fig. 3 (e, f) shows the deconvoluted O(1s) of pre and post acid treated
mesoporous silicas. These peaks have been deconvoluted into two parts, in which binding
+
energy of the first part is assigned to OH- and OH2 groups, respectively and the second part
corresponds to the oxygen bound in the H2O molecule. Furthermore, a perusal of Fig. 3e, f
reveals that the presence of H2O molecules and OH-/+OH2 molar ratio is greater in acid
treated calcined materials compared to the calcined materials [35]. Therefore, based on the
XPS analysis, it can be assumed that activation of mesoporous silica surfaces have been
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DOI: 10.1039/C6RA21480K
successfully done using acid treatment.
In this context, refluxing the calcined Si-MCM-41 and SBA-16 materials with conc. HCl
yields activated surface silanol groups onto these materials in which –OH groups are
+
protonated to produce OH2, which is bigger in size compared to –OH group. Beside this,
being a neutral molecule, it is a good leaving group. Moreover, because of the presence of –
+OH2 groups, the Si atoms to which it is attached becomes more electron deficient. Thus,
some small changes have been observed for acid treated calcined solids and the above
discussions depict that the surfaces of these calcined materials have been activated
successfully.
3.2. Stereochemical studies
Both acid treated calcined Si-MCM-41 and SBA-16 materials were used as adsorbents for
five
ketones,
namely
4-tert-butylcyclohexanone,
2-methylcyclohexanone,
3-
methylcyclohexanone, 4-methylcyclohexanone, 3,3,5-trimethylcyclohexanone and reduced
with NaBH4. Those five ketones were chosen as the intention was to check whether there was
any change in stereochemical outcome of the reduction using present systems. Moreover,
their rates of reduction were points of interest.
The obtained product, which should be a mixture of cis and trans-cyclohexanol, should give a
complicated 1H NMR spectrum. However, we can easily use the spectrum to analyze the ratio
of cis to trans isomers in our product because the methine hydrogen, the hydrogen on the
carbon bearing the hydroxyl group, is shifted downfield of the other signals. Considering a
mixture of cis and trans-4-tert-butylcyclohexanol, there is a signal at about = 3.54 ppm for
the methine hydrogen of the trans isomer (ESI) and a signal at about = 4.02 ppm for the
methine hydrogen of the cis isomer (ESI). The integration values for these two signals
correspond to the relative amounts of the two isomers. The methine hydrogen of the trans
isomer is axially placed whereas the methine hydrogen of the cis isomer is equatorially
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placed. Studies of many different substituted cyclohexanones have shown that axiDaOl Ih: 1y0d.10r3o9g/Ce6nRA21480K
appear at about 0.5 ppm higher field than the corresponding equatorial hydrogen when cis
and trans isomers are compared. Using this information we can analyse that the 4-tert-
butylcyclohexanol formed consists only of trans-isomer and not the cis-isomer and reduction
product of 3-methylcyclohexanone gives only of cis-isomer and not the trans-isomer. In this
1
context, HNMR (CDCl3, 300 and 400 MHz, ppm) of 4-tert-butylcyclohexanol (δ 3.54, C-H
axial proton), 2-methylcyclohexanol (δ 3.11, C-H axial proton and δ 3.76, C-H equatorial
proton), 3-methylcyclohexanol (δ 3.58, C-H axial proton), 4-methylcyclohexanol (δ 3.54, C-
H axial proton and δ 3.92, C-H equatorial proton), and 3,3,5-trimethylcyclohexanol (δ 3.76,
C-H axial proton and δ 4.16, C-H equatorial proton) have been determined [26, 27]. All the
1H NMR spectra have been given in the electronic supplementary information (ESI).
Table 2 and Table 3 represent that the chemical yields (%) and time of completion (in h) for
the reduction of ketones using acid treated calcined Si-MCM-41 and SBA-16 materials as
supports and NaBH4 as reducing agent. A perusal of Table 2 and Table 3 reveal that the
overall chemical yields of various alcohols are very high and the time required for complete
reductions are short. For complete reduction of 3,3,5-trimethylcyclohexanone, the reduction
time was much longer as compared to other used ketones. 3,3,5-trimethylcyclohexanone is a
hindered ketone and the approach by the reducing species namely H- from either side is rather
difficult. A perusal of Table 4 and Table 5 reveals that the stereochemical outcomes of the
reductions in case of 4-tert-butylcyclohexanone and 3-methylcyclohexanone are
commendable. In both the cases, exclusive formation of thermodynamically stabler epimer
takes place (in which the -OH groups are equatorially disposed). In case of other ketones like
2-methylcyclohexanone, the formation of stabler alcohols (-OH equatorial) are not exclusive
(Table 4-Table 5). However, there is a greater preponderance of the stabler alcohol (-OH
equatorial). In case of 3,3,5-trimethylcyclohexanone, it cannot be expected that the formation
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of cis alcohol should be exclusive because of the fact that the approach by the reducing
DOI: 10.1039/C6RA21480K
species namely H- from the axial side is greatly hindered by two axial methyl groups (Table
4-Table 5). Reduction of 4-tert-butylcyclohexanone with NaBH4 under homogeneous
condition yields ~85% of the trans alcohol [36]. The exclusive formation of trans-4-tert-
butylcyclohexanonol in the present case can be rationalised only on the basis of nucleophilic
activation by the silanol groups present on the surface of both acid treated calcined Si-MCM-
41 and SBA-16 materials. It is expected that the silanol groups form hydrogen bonds with the
carbonyl groups (Scheme 1a). Under such condition, the attack by H- from the equatorial side
is blocked while H- adds up exclusively from the axial side. Since, 4-tert-butylcyclohexanone
is a conformationally locked ketone, the molecule cannot flip and behaves like a rigid system.
Under such circumstances, the H- has no other alternative but to attack from axial side
resulting in the formation of 100 % equatorial alcohol [37-39]. Here, for hydride reductions
of cyclic ketones, a four-centre transition state has been postulated [39]. Thus, structures
(Scheme 1b, I) and (Scheme 1b, II) represent at least qualitatively the transition states
respectively for equatorial and axial attack of sodium borohydride on 4-tert-
butylcyclohexanone. These transition states are compatible with the inverse isotope effect
observed in boro-deuteride reduction and would also account for the well-known role which
I-strain plays in hydride reduction. It has been reported that there is a considerable change in
the degree of hybridisation of the C-O bond from sp2 to sp3 contrary to what one can infer
from the Hammond postulate [40]. It does not necessarily imply that the geometry of the
transition state is tetrahedral (the strain in four-membered ring will be too much for that)
which fact would explain the substantial absence of product stability control in the
reductions. In case of 3-methylcyclohexanone, the reduction lead to the exclusive formation
of cis-3-methylcyclohexanol in which the -OH group is equatorially disposed (Table 4-Table
5). 3-methylcyclohexanone is also conformationally locked like 4-tert-butylcyclohexanone.
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The equatorial approach is severely blocked because of hydrogen bonding with the silanol
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groups from the active surface of Si-MCM-41 and SBA-16 materials. 3,3,5-
trimethylcyclohexanone is a sterically hindered ketone. The axial C3-methyl groups cause
hindrance to any approaching nucleophile. Hence, the attack to the carbonyl group from the
equatorial side is preferred. However, if there is substantial blockage towards approach by H-
from the equatorial side, preferably the attack will take place from the axial side. In the
present case, it appears that the complexation of the surface silanol groups with the carbonyl
group is not that strong owing to the presence of an equatorially placed methyl group. Hence,
exclusive formation of cis-3,3,5-trimethylcyclohexanonol does not take place (Table 4-Table
5).
FTIR spectroscopic analysis (Fig. 4) was performed for the activated material-ketone
complex (Considering 4-tert-butylcyclohexanone). A perusal of Fig. 4 represents that the
peak due to -OH (symmetrical stretching vibration, 3449 cm-1) is intense enough. Such
increase in intensities points towards increase in surface activated silanol groups. A probable
mechanism for opening up of siloxane (Si-O-Si) groups of the activated materials is that due
to nucleophilic attack of Si atoms attached to ‘O’ atoms by OH- ions is suggested for
increased surface active groups in case of acid treated activated solids (Fig. 4).
Determining the cause of the high activity for these materials requires understanding of the
importance of their BET surface areas and the bulk morphology [41]. It is well known that
reducing activity is increased by larger surface area and larger pore volume along with
ordered pore diameter [42]. It has been noted that the enhancement of reduction for acid
treated Si-MCM-41 is the highest, whereas acid treated SBA-16 is the lowest, compared to
that previous one. The experimental data (Table 2-Table 5) that are consistent with the active
materials being those with the largest surface area, larger pore diameter, and ordered
mesostructure (Table 1). The reducing activity of acid treated calcined SBA-16 is lower
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mainly due to its low surface area (Table 1). Therefore, the material with the higher surface
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area, ordered pore diameter along with larger pore volume is favorable for better accessibility
of the active sites, which helps to reduce those used ketone.
3.3. Reusability and recyclability of solid supports
We have attempted to investigate the recyclability of these materials. Therefore, we carried
out calcination followed by treatment with concentrated HCl of these used solids and utilised
for next reaction, which resulted in excellent yields along with reactivity of used ketones until
the 5th time reuse (Fig. 5). XRD patterns and nitrogen sorption isotherms of spent materials
(Fig. 6 and Table 6) have been carried out to check the effectiveness after five successive
cycles. It has been observed from XRD patterns (Fig. 6a) that peak intensities remain almost
the same as before. Only (210) peak has been diminished. Moreover, nitrogen adsorption-
desorption isotherms (Fig. 6b) almost retain type IV isotherm as acid treated solids. From
Table 6, it is clear that surface area (1187 m2/g for acid treated calcined Si-MCM-41 and 663
m2/g for acid treated calcined SBA-16), mean pore diameter (3.56 nm and 5.62 nm for acid
treated calcined Si-MCM-41 and SBA-16, respectively), and overall pore volume (1.01 cm3/g
for acid treated calcined Si-MCM-41 and 0.89 cm3/g for acid treated calcined SBA-16) have
decreased a little after five consecutive cycles. So, it may be concluded that these active
mesoporous silicas can act as highly active and reusable supports for reduction of cyclic
ketones with preferential stereoselectivity.
4. Conclusion
In the present work, surface activated Si-MCM-41 and SBA-16 act as facile activators in the
reduction of ketones. In some substituted cyclohexanones, the reductions are accompanied
with good stereochemical outcome. Specially, in case of 4-tert-butylcyclohexanone and 3-
methylcyclohexanone reduction using activated materials lead to exclusive formation of the
epimer with equatorially disposed -OH group. Apparently, activated surface silanol groups
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activate the carbonyl group towards nucleophilic activation for reduction ofDOthI:e10.1g0r3o9/uCp6R.A21480K
Furthermore, in the FTIR spectrum, C=O stretching vibrational band of those used ketones
generally appear near 1680 cm-1-1720 cm-1. But, a perusal of Fig. 4 reveals that the
corresponding ketone band moves to 1663 cm-1 due to the formation of the activated
material-ketone complex. Such an observation clearly supports the hypothesis that the
carbonyl groups are activated towards nucleophilic addition of H- from sodium borohydride
during ketone reduction (Scheme 1). Acid treated calcined Si-MCM-41 material shows
slightly higher reducing activity compared to the acid treated calcined SBA-16 material.
Finally, it has been observed that these supports could exhibit excellent activity after five
consecutive cycles of conversion and reuse.
Acknowledgement:
HC would like to thank Sophisticated Analytical Instrumentation Facility (SAIF), Punjab
University, India for NMR facility.
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DOI: 10.1039/C6RA21480K
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Table 1: Surface areas, average pore diameter, and total pore volume of calcined and
concentrated HCl treated materials.
Materials
BET Surface
Average pore
Total pore
area (m2/g)
1256
diameter (nm)
3.67
volume (cc/g)
1.05
Cal. Si-MCM-41/
Conc. HCl
Cal. Si-MCM-41
Cal. SBA-16
Cal. SBA-16/
Conc. HCl
1202
652
3.61
5.71
5.94
1.02
0.91
0.93
734
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DOI: 10.1039/C6RA21480K
Table 2. Chemical yields (%) and time of completion (in h) for the reduction of ketones
using acid treated calcined Si-MCM-41 as support and NaBH4 as reducing agent (Figures in
parentheses were results obtained using NaBH4 in ethanol, without support). For all cases,
Ethanol was used as solvent.
Ketones (reduced)
Amounts used Chemical yields (%) of
Time of
(mmol)
(i) 2
corresponding alcohols completion (h)
4-t-butylcyclohexanone
>99 (97)
>99 (96)
>99 (94)
97 (86)
96 (85)
95 (83)
>99 (92)
>99 (90)
>99 (89)
98 (94)
95 (90)
91 (86)
97 (85)
94 (84)
90 (82)
3 (7)
3 (7)
(ii) 5
(iii) 10
(i) 2
3 (7.5)
3 (8)
2-methylcyclohexanone
3-methylcyclohexanone
4-methylcyclohexanone
3,3,5-trimethylcyclohexanone
(ii) 5
3 (8.5)
3.5 (9)
2 (6)
(iii) 10
(i) 2
(ii) 5
2 (6.5)
2 (7)
(iii) 10
(i) 2
3 (7)
(ii) 5
3.5 (7)
5 (8)
(iii) 10
(i) 2
6 (9)
(ii) 5
6.5 (9)
7.5 (10)
(iii) 10
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Table 3. Chemical yields (%), amount of ketones and time of completion (in h) for the
DOI: 10.1039/C6RA21480K
reduction of ketones using acid treated calcined SBA-16 as support and NaBH4 as reducing
agent (Figures in parentheses are results obtained using NaBH4 in ethanol, without support).
For all cases, Ethanol was used as solvent.
Ketones (reduced)
Amounts used Chemical yields (%) of
Time of
(mmol)
(i) 2
corresponding alcohols completion (h)
4-t-butylcyclohexanone
>99 (97)
>99 (96)
>99 (94)
96 (86)
95 (85)
93 (83)
>99 (92)
>99 (90)
98 (89)
97 (94)
93 (90)
89 (86)
96 (85)
92 (84)
90 (82)
3 (7)
3 (7)
(ii) 5
(iii) 10
(i) 2
3 (7.5)
3.5 (8)
3.5 (8.5)
4 (9)
2-methylcyclohexanone
3-methylcyclohexanone
4-methylcyclohexanone
3,3,5-trimethylcyclohexanone
(ii) 5
(iii) 10
(i) 2
2 (6)
(ii) 5
2 (6.5)
2 (7)
(iii) 10
(i) 2
3 (7)
(ii) 5
4 (7)
(iii) 10
(i) 2
5.5 (8)
6.5 (9)
7 (9)
(ii) 5
(iii) 10
8 (10)
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Table 4: Percentage of stable alcohol after reduction of ketones using acid treated calcined Si-MCM-41 as support and their NMR peaks.
Ketones (reduced)
Amounts
used (mmol)
Stable
% of stable
alcohol formed
C-H axial
proton (ppm)
C-H equatorial
proton (ppm)
alcohol
H
OH
H
4-t-butylcyclohexanone
(i) 2
(ii) 5
100
100
100
71
3.54
3.54
3.54
3.11
3.11
3.11
-
-
OH
H
OH
H
(iii) 10
(i) 2
-
OH
H
2-methylcyclohexanone
3-methylcyclohexanone
3.76
3.76
3.76
OH
H
(ii) 5
67
OH
H
(iii) 10
63
OH
H
(i) 2
100
100
3.58
3.58
-
OH
H
(ii) 5
-
OH
(iii) 10
(i) 2
100
82
3.58
3.54
3.54
3.54
-
H
4-methylcyclohexanone
3.92
3.92
3.92
OH
H
OH
H
(ii) 5
81
OH
H
(iii) 10
76
OH
H
3,3,5-trimethylcyclohexanone
(i) 2
(ii) 5
26
29
34
3.76
3.76
3.76
4.16
4.16
4.16
OH
H
OH
(iii) 10
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Table 5: Percentage of stable alcohol after reduction of ketones using acid treated calcined SBA-16 as support and their NMR peaks.
Ketones (reduced)
Amounts
used (mmol)
Stable
% of stable
alcohol formed
C-H axial
proton (ppm)
C-H equatorial
proton (ppm)
alcohol
H
OH
4-t-butylcyclohexanone
(i) 2
(ii) 5
100
100
100
70
3.54
3.54
3.54
3.11
3.11
3.11
-
-
H
H
OH
OH
(iii) 10
(i) 2
-
H
H
H
OH
2-methylcyclohexanone
3-methylcyclohexanone
3.76
3.76
3.76
OH
OH
(ii) 5
66
(iii) 10
63
H
H
H
OH
OH
OH
(i) 2
100
100
3.58
3.58
-
(ii) 5
-
(iii) 10
(i) 2
100
82
3.58
3.54
3.54
3.54
-
H
OH
OH
OH
4-methylcyclohexanone
3.92
3.92
3.92
H
H
(ii) 5
79
(iii) 10
75
H
OH
3,3,5-trimethylcyclohexanone
(i) 2
(ii) 5
25
29
33
3.76
3.76
3.76
4.16
4.16
4.16
H
H
OH
OH
(iii) 10
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Table 6: BET surface area, average pore diameter, and total pore volume of used materials
DOI: 10.1039/C6RA21480K
after 5th cycle.
Materials
BET Surface
Average pore
Total pore
area (m2/g)
1187
diameter (nm)
3.56
volume (cc/g)
1.01
Cal. Si-MCM-41/
Conc. HCl
Cal. SBA-16/
Conc. HCl
663
5.62
0.89
Fig. 1: (a) FTIR spectra of calcined and concentrated HCl treated materials: From top to
bottom is calcined Si-MCM-41 (1), concentrated HCl treated calcined Si-MCM-41 (2),
calcined SBA-16 (3), and concentrated HCl treated calcined SBA-16 (4). (b) XRD spectra of
calcined and concentrated HCl treated materials. (c) N2 adsorption-desorption isotherms of
calcined and concentrated HCl treated materials.
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DOI: 10.1039/C6RA21480K
Fig. 2: (a) FESEM and (c) TEM spectra of concentrated HCl treated Si-MCM-41,
respectively. (b) FESEM and (d) TEM spectra of concentrated HCl treated SBA-16,
respectively.
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DOI: 10.1039/C6RA21480K
Fig. 3: Wide-scan XPS spectra of calcined materials (a) and acid treated calcined materials
(b). Deconvolution of Si(2p) XPS spectra of calcined materials (c) and acid treated calcined
materials (d). Deconvolution of O(1s) XPS spectra of calcined materials (e) and acid treated
calcined materials (f).
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Scheme 1: (a) Proposed nucleophilic activation of ketone by the silanol groups present in
acid treated synthesised solids and (b) possible sodium borohydride reduction mechanism
(considering 4-tert-butylcyclohexanone).
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Fig. 4: FTIR of the activated material-ketone complex (Considering 4-tert-
butylcyclohexanone).
Fig. 5: Conversion of different cyclic ketones in five successive cycles and reused with both
acid treated Si-MCM-41 and SBA-16 materials.
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Fig. 6: XRD patterns (a) and nitrogen adsorption-desorption isotherms (b) of used materials
after 5th cycle.
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Table of contents entry
Surface activated calcined Si-MCM-41 and SBA-16 materials as activators in the reduction
of cyclic ketones.