An epoxide ring-opening reaction by using sol–gel-synthesized palladium supported on a strontium hydroxyl fluoride catalyst

Article abstract of DOI:10.1016/j.crci.2016.07.008

Palladium supported on a strontium hydroxyl fluoride catalyst was synthesized by a one-pot fluorolytic sol–gel method. The prepared catalyst was characterized by various physicochemical techniques. The sol–gel method has led to the formation of a high surface area (57?m²g^?1), mesoporous (pore diameter?=?13.0?nm) catalyst with uniform dispersion of Pd nanoparticles of size ～7?nm on the surface of strontium hydroxyl fluoride. The catalyst was used for epoxide alcoholysis, and 100% conversion was obtained with 96% selectivity for β-alkoxy alcohols under mild conditions. The catalyst could be recycled for up to three catalytic cycles without any appreciable decrease in conversion and selectivity, indicating the stability of the catalyst under the reaction conditions. Further, the mechanism of alcoholysis was proposed on the basis of the physicochemical characteristics of the catalyst and on the basis of the products formed during the catalytic reaction.

Full text of DOI:10.1016/j.crci.2016.07.008

C. R. Chimie xxx (2016) 1e10
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Full paper/M eꢀ moire
An epoxide ring-opening reaction by using solegel-
synthesized palladium supported on a strontium hydroxyl
fluoride catalyst
Vaibhav R. Acham , Mohan K. Dongare b, Erhard Kemnitz c
a
a
,
, **
,
a, *, 1
Shubhangi B. Umbarkar
Catalysis and Inorganic Chemistry Division, CSIR e National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, 411008, India
Malati Fine Chemicals Pvt. Ltd., Panchwati, Pashan Road, Pune, 411008, India
a
b
c
Humboldt University Berlin, Institute of Chemistry, Brook-Taylor-Straße 2, 12489, Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium supported on a strontium hydroxyl fluoride catalyst was synthesized by a one-
pot fluorolytic solegel method. The prepared catalyst was characterized by various
physicochemical techniques. The solegel method has led to the formation of a high surface
area (57 m g ), mesoporous (pore diameter ¼ 13.0 nm) catalyst with uniform dispersion
of Pd nanoparticles of size ~7 nm on the surface of strontium hydroxyl fluoride. The
catalyst was used for epoxide alcoholysis, and 100% conversion was obtained with 96%
Received 20 July 2015
Accepted 11 July 2016
Available online xxxx
2
ꢀ1
Keywords:
Heterogeneous catalysis
Strontium fluoride
Epoxides
selectivity for b-alkoxy alcohols under mild conditions. The catalyst could be recycled for
up to three catalytic cycles without any appreciable decrease in conversion and selectivity,
indicating the stability of the catalyst under the reaction conditions. Further, the mecha-
nism of alcoholysis was proposed on the basis of the physicochemical characteristics of the
catalyst and on the basis of the products formed during the catalytic reaction.
Alcoholysis
b
-Hydroxy alcohols
©
2016 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
synthesis of important classes of intermediates in organic
chemistry. The opening of epoxides with alcohols is one of
Epoxides are resourceful and vital intermediates in the
organic synthesis which, undergoes ring-opening reactions
to give -substituted compounds with a variety of nucleo-
philic species [1]. Epoxides undergo a variety of ring-
opening reactions to give -amino alcohols [2], 1,2-
diacetates [3], carbonyl compounds [4], diols [5], -alkoxy
alcohols [6], and -alkoxy sulfides [7]. This is the conve-
the important transformations for the synthesis of b-alkoxy
alcohols which are mainly used as valuable organic sol-
vents, versatile synthons, and intermediates [8].
b
A variety of organic reactions that are catalyzed by
b
Brønsted acids such as H
2
SO
4
, HCl, HNO
3
, CH
3
COOH, etc. or
b
Lewis acids like AlCl , TiCl
3
4
, FeCl , ZnCl
3
2
, etc. has been
b
gradually replaced by heterogeneous catalysts with more
efficiency [9]. The conventional mineral acids or bases have
been used for alcoholysis of epoxides, which resulted in the
formation of polymerized products with low regio-
selectivity [10]. The use of conventional mineral acids in
industrial processes is still widespread, leading to large
amounts of inorganic waste, often imposing drastic reac-
tion conditions.
nient, practical and widely employed strategy for the
This article is dedicated to Prof. Edmond Payen on his 60th birthday.
*
Corresponding author.
** Corresponding author.
E-mail addresses: vaibhav.acham@gmail.com (V.R. Acham), dongare.
mohan@gmail.com (M.K. Dongare), erhard.kemnitz@chemie.hu-berlin.
de (E. Kemnitz), sb.umbarkar@ncl.res.in (S.B. Umbarkar).
Various catalysts have been used for this transformation
including Lewis acids such as FeCl [11], Cu(BF ) $nH O
3 4 2 2
1
URL http://academic.ncl.res.in/ncl_1/sb.umbarkar.
http://dx.doi.org/10.1016/j.crci.2016.07.008
1631-0748/© 2016 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

2
V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
[
12], InCl
lysts like polymer supported FeCl
Recently, the use of triflates Yb(OTf)
Fe(ClO [18] has been reported along with other catalysts
such as Cp ZrCl [19], K [CoW12 40]$3H O [20], CBr [21],
3
[13], Mg(HSO
4
)
2
[14] and heterogeneous cata-
[15] and AlPW12 40 [16].
[17] and perchlorates
2. Strontium is highly reactive with methanol to generate
hydrogen hence the rate of reaction needs to be
controlled by keeping the reaction flask in an ice bath.
3
O
3
4 3
)
2
2
5
O
2
4
Catalyst preparation was carried out under an inert at-
mosphere of argon using a standard Schlenk technique. In a
250 mL round bottom flask, metallic strontium (2.103 g,
24 mmol) was treated with dry methanol (300 mL) at room
temperature for 16 h in a Schlenk flask to form strontium
methoxide solution. A stoichiometric amount (Sr/F: 1/2) of
71% aqueous solution of hydrofluoric acid (5.3 mL,
48 mmol) was added to the solution of strontium meth-
oxide followed by the addition of alcoholic solution of
palladium acetate (0.630 g, 1 wt % loading of Pd metal,
dissolved in 15 mL of methanol). This solution turned into a
gray colored gel which was kept for 16 h for aging. Then the
gray gel was dried under vacuum at room temperature and
tin(IV) porphyrinato trifluoromethanesulfonate [22], and
Amberlyst-15 [23] for the alcoholysis of epoxides. Although
currently there are a number of methods available for
epoxide ring opening, they have one or more disadvan-
tages, such as a long reaction time, high catalyst loading,
high reaction temperature, tedious method of catalyst
synthesis, and low selectivity. However, in spite of high
catalytic activity, perchlorates and triflates are less favored
because of their explosive nature and high cost.
The use of harsh reaction conditions is necessary owing
to poor nucleophilicity of alcohols, which led to the
decrease in regioselectivity of the product [24]. Further-
more, significant and important progress has been made in
the development of efficient catalytic methods which are
successful under mild conditions [12,25].
ꢁ
70 C to remove solvents (methanol and water). The solid
ꢁ
product was further calcined at 250 C for 5 h. The prepared
catalyst is referred hereafter as Pd-SrF -71 indicating 71%
2
The novel nanoscopic partially hydroxylated inorganic
fluoride materials with bi-acidic (Lewis/Brønsted) proper-
ties were developed for catalytic applications [26]. The
materials were synthesized using classical solegel route
from metal alkoxide via fluorination with aqueous/non-
aqueous HF which led to high surface area metal fluo-
rides [27]. These types of catalysts have been already
applied successfully for various catalytic applications viz.
aqueous concentration of HF used for synthesis.
Similarly, other fluoride based catalysts Pd-MF -71
2
(where, M ¼ Mg, Sr, Ba) were also prepared for comparison
of catalytic activity.
2.3. Catalyst characterizations
synthesis of (all-rac)-[
a]-tocopherol [28], FriedeleCrafts
2
The Pd-SrF -71 was characterized using various physi-
reaction [29], Suzuki coupling reaction [30], synthesis of
menthol [31], synthesis of vitamin K-1 and K-2 chromanol
cochemical techniques as mentioned below.
[32], oxidation of ethylbenzene [33], dehydrohalogenation
2.3.1. Powder X-ray diffraction (PXRD) analysis
of 3-chloro-1,1,1,3,-tetrafluorobutane [34], catalytic CeH
bond activation [35] and glycerol acetylation [36].
Crystallinity and phase purity of the samples were
determined using powder X-ray diffraction (PXRD) anal-
ysis. Powder patterns were recorded on an X'pert Pro
Recently the palladium supported catalyst was used for
phenolysis of epoxides at a high temperature in the pres-
ence of bases [37]. The Pd supported on alkaline earth
metal fluoride is known for its dual (acidic/basic) proper-
ties. These properties play an important role in determining
not only the activity but also the selectivity of the catalytic
reactions. Therefore the study of synthesis and character-
ization of palladium supported strontium fluoride and its
catalytic activity for alcoholysis of epoxides has been car-
ried out and the results are presented in this article.
PANalytical X-ray diffractometer with Ni-filtered Cu-K
a
ꢁ
radiation (40 kV, 30 mA) in the 2
q
range of 10e80 at a scan
ꢀ1
rate of 4 min on the glass substrate.
2.3.2. FTIR spectroscopy
A Nicolet Nexus 670 FTIR instrument with a DTGS de-
tector was used to record the IR spectrum of the catalyst in
ꢀ1
the range 4000e400 cm with a KBr pallet technique in
ꢀ
1
transmission mode. The data were collected at 4 cm
resolution averaged over 100 scans.
2
. Experimental
2
.3.3. BET surface area measurements
The specific surface area (BET) of the sample was
determined by acquiring adsorptionedesorption isotherm
BET method) at 77 K for nitrogen gas using a Autosorb
2
.1. General
(
All chemicals were procured from Aldrich Chemical Co.,
Quanta Chrome corporation instrument. The micropore
volume was estimated from the t-plot and the pore diam-
eter was estimated using the BarretteJoynereHalenda
USA and used as received. Hydrofluoric acid (71% aq. so-
lution) and solvents were procured from Merck Chemicals,
Germany and used as obtained.
(BJH) model.
2
.2. Catalyst synthesis
2.3.4. Ammonia-temperature programmed desorption (NH3-
TPD) analysis
Cautions:
NH -TPD measurements were performed on a Micro-
3
meritics AutoChem 2910 instrument. In a typical experi-
1. HF is a highly toxic and irritant compound causing se-
ment, 0.1 g of the catalyst was taken in a U-shaped, flow-
vere burns if it comes in contact with the skin.
thru, quartz sample tube. Prior to measurements, the
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
3
3
ꢀ1
ꢁ
with a DTGS detector) and 1HNMR (Bruker AC 200,
200 MHZ spectrometer) spectral analysis and matched
with the literature (only for representative reaction alco-
holysis of cyclohexene oxide, styrene oxide and epichlo-
rohydrin using ethanol).
catalyst was pre-treated in helium (30 cm min ) at 250 C
for 1 h. A mixture of NH in helium (10%) was passed
30 cm3 min ) at 25 C for 1 h. The sample was subse-
3
ꢀ1
ꢁ
(
3
ꢀ1
ꢁ
quently flushed with helium (30 cm min ) at 100 C for
h. The TPD measurements were carried out in the range
1
ꢁ
ꢁ
ꢀ1
1
00e250 C at a heating rate of 10 C min . The ammonia
concentration in the effluent was monitored with a gold-
plated, filament thermal conductivity detector (TCD). The
amount of desorbed ammonia was determined based on
the area under the peak.
2.5. Procedure for the catalytic recycle study
To study the recyclability of the catalyst, after comple-
tion of the first reaction, the catalyst was separated using
centrifugation followed by filtration through Whatman
filter paper no. 1, washed with ethanol (5 mL ꢂ 3 times),
2
.3.5. X-ray photoelectron spectroscopy (XPS)
ꢁ
XPS was recorded using a VG Micro Tech ESCA 3000
and finally dried at 80 C for 30 min. A fresh charge of re-
ꢀ
9
instrument at a pressure below 10
Torr. The samples
actants was taken with the dried catalyst and subjected to
similar reaction conditions. The same procedure was
repeated for successive three catalytic runs with the same
catalyst.
were mounted on sample tubes. The wide scan C-1s,
O-1s, F-1s, Pd-3d and Mg-2p core level spectra were
recorded with monochromatic Al Ka radiation (photon
energy ¼ 1486.6 eV) at the pass energy of 50 eV and elec-
ꢁ
tron take-off angle of 60 . The core-level binding energies
2.6. Procedure for the filtration experiment study and ICP
were aligned taking ‘adventitious’ carbon's binding energy
as 284.6 eV. All peaks were fitted with GausseLorentz
peaks using XPSPEAK41 software to obtain peak informa-
tion. A Shirley's baseline was used in the fitting process.
analysis
To verify the Pd leaching in the reaction mixture, the
filtration test was carried out at room temperature. A 0.05 g
of the catalyst was stirred with 0.5 g of cyclohexene oxide
in 6 mL of ethanol under typical reaction conditions at
room temperature. After 1 h, the catalyst was separated
from the liquid phase by centrifugation followed by filtra-
tion through Whatman filter paper No. 1. Further, the re-
action was continued without the catalyst at room
temperature. The separated catalyst was further tested for
ICP-AES analysis to determine the Pd content in the cata-
lyst. The catalyst sample was mixed with a solution of conc.
2
.3.6. Scanning electron microscopy (SEM)
The elemental composition on the catalyst surface and
catalyst morphology was determined by EDAX analysis
coupled with FEI Quanta, 200 3D SEM using an aluminum
sample holder on the carbon film.
2
.3.7. Transmission electron microscopy (TEM)
The palladium particle size was determined using
HRTEM measurements on a Tecnai F 30 instrument oper-
ated at an accelerating voltage of 200 kV. The TEM grids
were prepared by dispersing the powder in 2-propanol
using ultrasound. The suspension was dropped by the
micropipette on a conventional carbon film with a thick-
ness of about 20 nm supported by a copper grid. After
complete evaporation of the 2-propanol from the spec-
imen, the TEM imaging was performed.
3
HNO (20% v/v) in a 50 mL PFA beaker (ratio of acid solution
to sample ¼ ~50 by weight). The sample was heated in the
ꢁ
bomb at 100 C in the oven for 3 h. After cooling the sample,
the solution was taken out in a volumetric flask. The solu-
tion was then diluted to 100 mL using milli Q water by
using a calibrated volumetric flask, which was further
subjected to ICP analysis. Metal leaching was studied by
ICP-AES analysis of the catalyst before and after the three
reaction cycles.
2
.4. Typical procedure for the catalytic reaction
3. Results and discussion
In a sample tube equipped with a magnetic needle, an
epoxide (1.0 mmol), alcohol (2.0 mL) and catalyst (10 wt %
with respect to epoxide) were added. The reaction mixture
was stirred at room temperature for the given time. The
progress of the reaction was monitored by thin layer
chromatography (TLC) and gas chromatography (GC). After
the completion of the reaction, the catalyst was separated
by filtration and washed with alcohol and diethyl ether. The
combined organic fractions were dried over anhydrous
sodium sulfate and the solvent was removed under
reduced pressure. The reaction was monitored by gas
chromatographic analysis using an Agilent 6890 Gas
Chromatograph equipped with a HP-5 dimethyl poly-
siloxane capillary column (60 m length, 0.25 mm internal
3.1. Catalyst synthesis
2
The one pot synthesis of the Pd-SrF -71 catalyst by the
fluorolytic solegel route using aq. HF (71%), resulted in
simultaneous fluorolysis and hydrolysis of MeOR bond to
form MeF and MeOH bonds, respectively [38,39]. Though
the molar ratio of Sr:HF was adjusted to 1:2, the hydrolysis
of strontium methoxide becomes competitive with the
fluorination due to water content in the fluorinating agent.
This reaction generates the composition in which both
fluorine and hydroxide are attached to strontium in a solid
structure and this led to the formation of strontium hy-
droxyl fluorides [SrF(2ꢀX)(OH)
further addition of methanolic solution of palladium ace-
tate forms Pd-SrF2ꢀX(OH) (henceforth referred to as Pd-
SrF -71) catalyst as represented in Scheme 1. The addition
of an alcoholic solution of palladium acetate led to the
X
] material, which upon
diameter, 0.25 mm film thickness) with a flame ionization
detector. Products were confirmed by comparison with GC
spectra of the authentic samples. Further, the structure was
confirmed by FTIR (Nicolet Nexus spectrometer equipped
X
2
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

4
V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
hydroxyl fluorides. However, no diffraction for Pd111 was
detected because of a very lowcontent of Pd with high order
of dispersion of Pd particles over the surface of the catalyst.
3.3. FTIR
2
The FTIR spectrum of the Pd-SrF -71 catalyst showed
the bands at 3450, 3007, 1631 1402, 1083, 743, and
ꢀ
1
ꢀ1
4
84 cm (Fig. 2). The intense bands at 484 and 743 cm
can be related to (SreF) and (SreO), respectively. The
bands around 3450 and 1650 cm
presence of surface hydroxyl groups or moisture. The band
n
n
ꢀ1
correspond to the
ꢀ1
at 3007 and 1083 cm observed due to n(CeH) and n(CeO)
stretching vibrations could be due to the hydrocarbon
residue in prepared catalysts. The presence of SreO and
OeH band confirms the formation of the partially hydrox-
ylated strontium fluoride phase in the final catalyst [41].
2
Scheme 1. Schematic representation for solegel synthesis of the Pd-SrF -71
catalyst.
formation of light red colored gel initially which upon aging
II
0
turned light gray. Typically Pd gets partially reduced to Pd
due to dissolved hydrogen liberated during the formation
of strontium methoxide from metallic strontium to give
gray color to the gel. In contrast to the non-aqueous solegel
route which results in the formation of clear sols and
transparent gels, the aqueous route results in opaque gels
of gray color due to the presence of partially reduced
palladium nanoparticles [40]. The bulk and surface struc-
ture of this material was characterized using various
physicochemical techniques.
3
.4. BET surface area
Nitrogen sorption studies were performed to study the
surface area, pore diameter and pore volume (Fig. 3). The
isotherms showed type IV character typical for mesoporous
materials with a H1 type hysteresis loop and porous
texture. The BET surface area and total pore volume of the
2
ꢀ1
ꢀ1
catalyst was found to be 57 m g and 0.11 cc g , respec-
tively. Typically the surface area of Pd-SrF -71 was
observed to be intermediate between metal fluorides from
the same group in the periodic table like Pd-MgF -71
-71 (8 m g ) prepared under
2
3
.2. XRD
The PXRD pattern of the prepared sample is shown in
2
2
ꢀ1
2 ꢀ1
(140 m g ) and Pd-BaF
2
identical conditions. The average pore size was determined
using BarretteJoynereHalenda (BJH) analysis and found to
be 13.1 nm, which confirms the mesoporous nature of the
catalyst.
Fig. 1. The diffraction peaks can be indexed as the peak
corresponding to metal fluorides and metal hydroxides on
comparison with strontium fluoride (JCPDS 06-0262) and
strontium hydroxide (JCPDS 27-847). Furthermore, PXRD
showed broad peaks which may correspond to strontium
fluoride and sharp peaks to strontium hydroxide. Addi-
tionally, few less intense peaks were observed; these peaks
indicate the formation of mixed phases of strontium
3
3
.5. Acidity measurements
3
.5.1. Ammonia-temperature programmed desorption (NH -TPD)
The total acidity as well as the strength of acidic sites on
the surface of the catalysts was determined by ammonia-
20
40
θ / degree
60
80
4000 3500 3000 2500 2000 1500 1000 500
-1
2
Wavenumber (cm )
Fig. 1. PXRD pattern of the Pd-SrF
2
-71 catalyst.
2
Fig. 2. FTIR spectrum of the Pd-SrF -71 catalyst.
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
5
100
1452
80
60
40
20
0
1490
1
640
610
1
1542
0.0
0.2
0.4
0.6
0.8
1.0
1700
1650
1600
1550
1500
-1
1450
1400
Relative Pressure (P/P0)
Wavenumber (cm )
2
Fig. 3. BET surface area plot of the Pd-SrF -71 catalyst.
Fig. 5. FTIR spectrum of adsorbed pyridine on the Pd-SrF
2
-71 surface at
ꢁ
100 C.
temperature programmed desorption (NH
3
-TPD) technique
and Brønsted acidity on the surface of oxide materials in
adsorbed pyridine spectrum [42]. Similarly, the bands in
as shown in Fig. 4. The NH desorption studies indicated
3
the presence of weak, moderate and strong acidic sites with
peak maxima corresponding to 111, 276, and 386 C,
ꢁ
the adsorbed pyridine spectrum of Pd-SrF -71 are assign-
2
able to coordinately bound pyridine [42,43]. The pyr-
respectively. Also, the total acidity of the catalyst was found
þ
ꢀ
1
idinium ion (PyH ) produced by the reaction of pyridine
to be 0.151 mmol g . The total acidity of the catalyst was
dependant on the degree of fluorination as well as on the
nature of the metal alkoxide. Normally a decreasing trend
in the acidity of the metal alkoxide was observed down the
group in the periodic table.
with Brønsted acid sites showed bands around 1542 and
ꢀ
1
1
640 cm . The coordinately bound pyridine on Lewis acid
ꢀ1
sites shows bands around 1452 and 1610 cm . This DRIFT
indicated the presence of Lewis and Brønsted acidic sites on
2
the surface of the Pd-SrF -71 catalyst.
3
.5.2. FTIR of adsorbed pyridine
The type of acidity present on the catalyst surface was
3.6. XPS
studied by DRIFT spectroscopy of adsorbed pyridine
pK
¼ 5.25). The subtracted FTIR spectrum of absorbed
pyridine on the Pd-SrF -71 surface is shown in Fig. 5. The
(
a
XPS was used to derive surface compositional infor-
mation of the Pd-SrF -71 catalyst in Fig. 6. The montage of
the survey scan after analysis of the Pd-SrF -71 catalyst is
2
2
ꢀ
1
FTIR peak at 1445 and 1545 cm shows the presence of
Lewis and Brønsted acidic sites, respectively, while the
2
shown in Fig. 6-A. The peaks were annotated to the Sr, F, O
and Pd with their core levels. The peaks were standardized
with respect to the carbon peak at 284.6 eV on the surface.
Due to non-conductive nature of strontium fluoride, some
charging of the sample was observed. In order to determine
the effect of the support on the oxidation state of palladium
of the as-synthesized samples, the region of Pd was studied
further (Fig. 6-B). Palladium showed the presence of two
ꢀ
1
peak at 1490 cm represents the presence of both Lewis
111
0
2þ
oxidation states namely Pd and Pd . The two different
2
þ
2þ
values of Pd may be due to variable coordination of Pd
0
species. The peak at 335.1 corresponds to Pd and peaks at
2
þ
3
38.9 and 340.4 relate to Pd species from palladium ac-
386
etate and palladium oxide, respectively [44]. The ratio of
2þ
273
relative abundance (Pd:Pd ) was observed to be 1:2
0
approximately. The presence of Pd can be attributed to the
partial reduction of palladium acetate in the presence of
dissolved hydrogen which was produced during catalyst
synthesis.
3.7. SEM
50
100
150
200
250
300
350
400
Temperature (°C)
The morphology of the Pd-SrF
by SEM. The representative micrograph of Pd-SrF
2
-71 catalyst was studied
-71 is
Fig. 4. NH
3
-temperature program desorption plot of the Pd-SrF
2
-71 catalyst.
2
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

6
V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
F 1s
A
B
335.1
340.4
Sr 3p
338.9
Sr 3d
C 2s
Pd 3d
O 1s
F 2s
0
200
400
600
800
1000
332 334 336 338 340 342 344 346
Binding energy (eV)
Binding energy (eV)
2
Fig. 6. XPS of the Pd-SrF -71 catalysts; (A) survey scan and (B) scan for binding energy of palladium species.
shown in Fig. 7-a. It showed the particles of SrF
2
-71 to be
m in
are in the range of 7e10 nm where as many Pd particles are
of large sizes of around 20e30 nm. Some agglomerated Pd
was also observed in TEM. The palladium particles were
found to be in 111 planes as identified from d spacing
(d111 ¼ 0.223 nm).
oval-shaped of 1.5e2.0 m in length and 0.5e1.0
m
m
width. The Pd particles were spread throughout evenly on
the strontium hydroxyl fluoride surface. Further, the sur-
face composition of the catalyst was also determined by
EDAX spectroscopy. The results showed the surface palla-
dium composition to be approximately 1.1 wt %.
3.9. Catalytic activity for alcoholysis of epoxides
3
.8. TEM
The catalytic activity of Pd-MF
2
-71 (where, M ¼ Mg, Sr,
Ba) was studied for alcoholysis of epoxide at room tem-
perature initially with cyclohexene oxide as the model
substrate and ethanol as nucleophile (Scheme 2). The re-
sults obtained with various catalysts are given in Table 1. In
the absence of the catalyst, the reaction did not take place
even after 2 h time (Table 1, entry 1). Further a series of
palladium supported metal fluoride catalysts were evalu-
The particle size of palladium was determined by
HRTEM analysis. The representative micrograph of Pd-SrF
1 is shown in Fig. 7-b. The particle size distribution
studied by TEM clearly showed the majority of particles
with ~7 nm diameter though the particle size distribution
spreads over a wide range. There is clearly the bimodal
distribution of Pd particles observed in TEM. Many particles
2
-
7
2 2
ated for catalytic reactions. The Pd-MgF -71, Pd-SrF-71 ,
2
Fig. 7. Representative electron microscopic image of the Pd-SrF -71 catalyst (A) SEM image and (B) TEM image SAED pattern.
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
7
and Pd-BaF
sion, respectively (Table 1, entry 2e4). It is well known that
the Lewis acidity of the catalyst decreases from PdeAlF to
PdeBaF with the increase in basicity [45]. In the case of a
2
-71 catalysts showed 43, 100 and 20% conver-
Table 1
a
Results of alcoholysis of epoxide.
Entry Catalyst
Conv.b (%) Sel.b (%) Activityc
(mmol h
3
ꢀ
1
ꢀ1) ꢂ 104
g
2
highly acidic catalyst, higher conversion has been reported
but the higher order of acidity is responsible for the
decrease in selectivity and generation of byproduct like 1,6-
1
2
3
4
5
None
Pd-MgF
Pd-SrF
Pd-BaF
Pd-AlF
0
e
e
2
-71 43
93
96
96
80
3.18
7.00
1.27
2.86
2
-71
100
20
59
2
-71
-71
hexandiol. Moreover, PdeBaF
with 100% selectivity due to its basic nature. In the case of
PdeSrF , the acidic and basic sites were found to be
balanced to get optimum conversion and selectivity. The
catalytic activity of the PdeAlF was compared which
showed 59% conversion under identical reaction conditions
Table 1, entry 5).
2
showed less conversion
3
a
Reaction conditions: Cyclohexene oxide: 1 mmol; ethanol: 2 mL;
2
ꢁ
catalyst: 0.01 g (1.0 wt % of Pd loading); temperature: RT (27 C), time: 2 h.
b
Conv. and Sel. were determined based on GC results.
Rate constants were determined by assuming the reaction to follow
c
3
first-order kinetics for 30 min of time.
(
3
3
.10. Effects of various reaction parameters
100
.10.1. Catalyst loading effect
8
6
4
2
0
0
0
0
0
The effect of catalyst loading on alcoholysis of cyclo-
hexene oxide was studied at room temperature in Fig. 8.
The conversion increased with gradual increase in catalyst
loading. The 1 wt % catalyst loading gave 9% conversion
which increased to 70% for 15 wt % loading in 1 h at room
temperature. There was a marginal decrease in selectivity
at high catalyst loading.
Conversion
Selectivity
3
.10.2. Temperature effect
Epoxide ring opening of cyclohexene oxide was studied
at various temperatures ranging from room temperature to
ꢁ
7
0
C in ethanol with 10 wt % loading of the Pd-SrF
2
-71
catalyst as shown in Fig. 9. It was observed that with the
gradual increase in the reaction temperature from room
0
2
4
6
8
10
12
14
16
Catalyst Loading (%)
ꢁ
ꢁ
temperature (27 C) to 70 C, the time for 100% conversion
of cyclohexene oxide decreased from 120 to 30 min without
much decrease in selectivity. The increase in the rate of
reaction can be correlated to increase in the rate of inter-
molecular collision. Although the rate of reaction was
2
Fig. 8. Catalyst loading effect using the Pd-SrF -71 catalyst. Reaction con-
ditions: cyclohexene oxide: 5 mmol (0.54 g); ethanol: 2 mL; temperature: RT
ꢁ
(
27 C). Time: 1 h.
ꢁ
3.11. Substrate scope study
higher at 70 C, room temperature is always preferred.
Therefore room temperature was considered as the opti-
mum temperature for carrying out further reactions.
After optimizing the reaction conditions for ethanol as a
nucleophile with cyclohexene oxide as a model reaction,
3
.10.3. Time profile study
The conversion of the cyclohexene oxide increased with
1
20
00
Time
Conversion
Selectivity 120
increase in the reaction time at room temperature in
ethanol as shown (Fig. 10). A virtually linear time-
conversion profile has been observed for ethanolysis of
cyclohexene oxide. The selectivity of the 2-
ethoxycyclohexanol decreased to 96% after complete con-
version of cyclohexene oxide after 2 h which was found to
be constant up to 2.5 h at room temperature.
1
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
35
40
45
50
55
60
65
70
75
Temperature (°C)
2
Fig. 9. Temperature effect study using the Pd-SrF -71 catalyst. Reaction
conditions: cyclohexene oxide: 2 mmol (0.196 g); catalyst (Pd-SrF
ethanol: 2 mL.
2
): 0.02 g;
Scheme 2. Alcoholysis of epoxides.
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

8
V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
the applicability of the catalyst to other epoxides and al-
cohols was evaluated. The reaction of cyclohexene oxide
with methanol and ethanol gave 98 and 96% yields,
respectively (Table 2, entry 1e2). The reaction of 2-
propanol gave a lower yield (57%) as compared to ethanol
as a nucleophilic source (Table 2, entry 3). This may be due
to the steric effect of two methyl groups, which hinders the
reaction site resulting in the lowering of cyclohexene oxide
conversion into the desired product. A similar behavior was
observed when the reaction was carried out with t-butyl
alcohol, which showed a 25% yield (Table 2, entry 4). This
observation can be explained by the fact that tertiary butyl
alcohol is even more bulky, having three methyl groups
which increases the steric hindrance and lower the activity
in the nucleophilic substitution reaction. When benzyl
alcohol was used as the nucleophile, a 99% yield was
observed (Table 2, entry 5). Although the steric hindrance
in benzyl alcohols is higher than that in t-butyl alcohol, the
higher nucleophilicity may be responsible for increment in
the rate of the reaction. Further, styrene oxide was used as
the substrate to check the effect of the catalyst on regio-
selectivity (Table 2, entry 5e10).
Table 2
Substrate scope study using the Pd-SrF
2
-71 catalyst.a
Entry Epoxide
Alcohol Products ratio (2:3) Yieldsb (%)
1
2
3
4
5
6
7
8
9
MeOH
EtOH
Trans
Trans
98
96
57
25
99
92
73
49
18
72
89
78
53
36
65
i-PrOH Trans
t-BuOH Trans
BnOH
MeOH
EtOH
Trans
100:0
100:0
i-PrOH 100:0
t-BuOH 100:0
10
11
12
13
BnOH
MeOH
EtOH
100:0
0:100
0:100
i-PrOH 0:100
t-BuOH 0:100
14
15
BnOH
0:100
a
Reaction conditions: substrate: 0.2 g; catalyst (Pd-SrF -71): 0.02 g
2
ꢁ
(10 wt % w.r.t. substrate); alcohols: 2 mL; temperature: RT (27 C). Time:
2
h.
b
Yields were determined based on GC.
1
benzylic carbocation, the reaction follows the SN pathway
and formed 2 as the preferable regio-isomer. In epichlo-
rohydrin, the reaction follows the SN pathway due to the
attack of the nucleophile from the rear side of the ring
The rate of the reaction was found to be dependant on
nucleophilicity of the alcohol but also on the steric hin-
2
drance of epoxide. The yields of
b-alkoxy alcohols
opening, yielding 3 as the preferable regioisomer.
decreased from methoxy to t-butoxy (92-18%) due to
increasing bulkier nature of nucleophile (Table 2, entry
3.12. Catalyst recycle study
5
e9), which increased to 72% in the case of benzyl alcohol
(
Table 2, entry 10). The reaction of epichlorohydrin with
2
The recyclability of Pd-SrF -71 was studied for the ring
methanol and ethanol showed 89 and 78% yields, respec-
tively (Table 2, entry 11e12). When bulkier alcohols like 2-
propanol and 2-methyl-2-propanol were used as the
nucleophile, the yield decreased to 53 and 36%, respectively
opening of cyclohexene oxide in ethanol under optimized
reaction conditions. After completion of the reaction, the
catalyst was separated from the reaction mixture by
centrifugation followed by filtration using Whatman filter
(
Table 2, entry 13e14), which again increased to 65% with
paper no.1. The catalyst was washed with ethanol
benzyl alcohol as the nucleophile (Table 2, entry 15).
Remarkably, the formation of a single regioisomer was
observed in all the cases. This formation of a stable inter-
mediate in each case led to the formation of a single
regioisomer. In styrene, because of formation of stable
ꢁ
(
5 mL ꢂ 2) and acetone (5 mL) and allowed to dry at 80 C
and used it for subsequent catalytic runs. The same pro-
cedure was repeated for three times. However, even after
three runs, the catalyst exhibited excellent activity as
shown in Fig. 11. It implied that the catalytic system can be
100
100
8
0
80
60
40
20
0
60
40
Conversion (%)
Selectivity (%)
20
0
20
40
60
80 100 120 140 160
Time (min)
0
1
2
3
Recycle number
Fig. 10. Time profile study using the Pd-SrF
2
-71 catalyst. Reaction condi-
Fig. 11. Recycle study using the Pd-SrF
2
-71 catalyst. Reaction conditions:
tions: cyclohexene oxide: 2 mmol (0.196 g); catalyst (Pd-SrF
2
-71): 0.02 g
2
cyclohexene oxide: 5 mmol (0.54 g); Pd-SrF -71: 0.05 g (10 wt% w.r.t. sub-
ꢁ
ꢁ
strate); ethanol: 2 mL; temperature: RT (27 C). Time: 2 h.
(
10 wt% of cyclohexene oxide); ethanol: 2 mL; temperature: RT (27 C).
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
9
change in cyclohexene oxide conversion was observed even
after additional 3 h which confirms no Pd leaching which
was also confirmed by ICP-AES analysis of the catalyst. The
Pd concentration was found to be 0.986% before the reac-
tion and 0.978% after the reaction. Therefore the Pd-SrF
2
-71
catalyst acts as a true heterogeneous catalytic system.
3.14. Possible reaction mechanism
Based on the results obtained during catalytic reactions
and by comparison with the literature reports, the mech-
anism for the alcoholysis of epoxide using the Pd-SrF -71
2
catalyst has been proposed in Scheme 3. In the case of
styrene oxide, the nucleophilic alkoxy group attacks in such
a way to produce 2-phenyl-2-alkoxy ethanol. While in the
epichlorohydrin case, the alkoxy group attacks epoxide ring
to get 3-chloro-1-alkoxy-2-propanol as the only product.
Initially, the adsorption of epoxide on the surface of the
catalyst takes place which activates epoxide via partial
transfer of electrons of oxygen to the empty orbital of
oxidized palladium to get intermediate B (Step 1). Further
attack of nucleophilic oxygen led to opening of the epoxide
ring in two possible ways depending on the nature of the
alkyl group. In the case of an electron withdrawing alkyl
2
Fig. 12. TEM image of the Pd-SrF -71 catalyst after third catalytic recycle.
used even for three cycles without significant loss of
N
group, the reaction follows the S 2 pathway to generate
activity.
intermediate carbon which undergoes simultaneous CeO
bond cleavage and rear side attack of nucleophilic O-atom
of alcohol followed by proton transfer to produce product 3.
In the case of an electron donating alkyl group, the reaction
3
.13. TEM and ICP analysis of recycled catalyst
The TEM image of the recycled catalyst after third cycle
N
follows the S 1 pathway to generate intermediate carbo-
revealed no agglomeration of Pd on the catalyst surface as
shown in Fig. 12. The size of the Pd nanoparticle was found
to be in the same range of 6e8 nm when compared with
that of the fresh catalyst (refer Fig. 7). There was no
decrease in the catalytic activity in successive cycles for
alcoholysis of epoxides. The filtration test was used to
check the Pd leaching in the reaction mixture. It was
observed that in the absence of the catalyst, no further
cation C′ by cleavage of CeO bond. This C′ carbocation in-
termediate gets stabilized via resonance and inductive
effects, which undergoes nucleophilic attack by the nucle-
ophilic O-atom of alcohol followed by proton transfer to
generate product 2. The high regioselectivity is due to
stabilization of intermediate species on the catalyst surface
and results in two different nucleophilic substitution
reactions.
2
Scheme 3. Possible reaction mechanism for regioselective alcoholysis of epoxide using the Pd-SrF -71 catalyst.
Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008

1
0
V.R. Acham et al. / C. R. Chimie xxx (2016) 1e10
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4
. Conclusions
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(
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Acknowledgements
(
We thank Council of Scientific and Industrial Research
1
(
(
CSIR), India and Federal Ministry of Education and
Research (BMBF), Germany for financial support for the
collaborative project. VRA acknowledges University Grant
Commission (UGC) for the research fellowship.
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Please cite this article in press as: V.R. Acham, et al., An epoxide ring-opening reaction by using solegel-synthesized palladium
supported on a strontium hydroxyl fluoride catalyst, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.07.008