Functionalized SBA-15 supported nickel (II)-oxime-imine catalysts for liquid phase oxidation of olefins under solvent-free conditions

Article abstract of DOI:10.1016/j.jssc.2016.01.018

A new oxime-imine functionalized highly ordered mesoporous SBA-15 (SBA-15-NH₂-DAMO) has been synthesized via post-synthesis functionalization of SBA-15 with 3-aminopropyl-triethoxysilane followed by the Schiff base condensation with diacetylmonooxime, which was further reacted with Ni(ClO₄)₂ to yield the functionalized nickel catalyst SBA-15-NH₂-DAMO-Ni. All the synthesized materials were thoroughly characterized using different characterization techniques. It was found that SBA-15-NH₂-DAMO-Ni catalyzes the one-pot oxidation of olefins like styrene, cyclohexene, cyclooctene, 1-hexene and 1-octene to the corresponding benzaldehyde, cyclohexene-1-ol and cyclooctene-oxide, respectively under solvent-free conditions by using tert-butylhydroperoxide as oxidant.

Full text of DOI:10.1016/j.jssc.2016.01.018

Journal of Solid State Chemistry 237 (2016) 105–112
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Functionalized SBA-15 supported nickel (II)–oxime–imine catalysts for
liquid phase oxidation of olefins under solvent-free conditions
a,
Luna Paul a, Biplab Banerjee b, Asim Bhaumik b, , Mahammad Ali
n
n
a Department of Chemistry, Jadavpur University, Kolkata 700 032, India
b Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new oxime–imine functionalized highly ordered mesoporous SBA-15 (SBA-15-NH2-DAMO) has been
synthesized via post-synthesis functionalization of SBA-15 with 3-aminopropyl-triethoxysilane followed
by the Schiff base condensation with diacetylmonooxime, which was further reacted with Ni(ClO4)2 to
yield the functionalized nickel catalyst SBA-15-NH2-DAMO-Ni. All the synthesized materials were thor-
oughly characterized using different characterization techniques. It was found that SBA-15-NH2-DAMO-
Ni catalyzes the one-pot oxidation of olefins like styrene, cyclohexene, cyclooctene, 1-hexene and
1-octene to the corresponding benzaldehyde, cyclohexene-1-ol and cyclooctene-oxide, respectively un-
der solvent-free conditions by using tert-butylhydroperoxide as oxidant.
Received 1 September 2015
Received in revised form
5 January 2016
Accepted 22 January 2016
Available online 25 January 2016
Keywords:
SBA-15
Grafting of Ni(II)
Functionalized mesopore silica
Liquid phase partial oxidation
Solvent free reaction
& 2016 Elsevier Inc. All rights reserved.
1. Introduction
olefin substrates and oxygen atom sources a good overall epox-
idation was obtained with Schiff-base complexes of manganese(III)
The oxime [1–4] and oxime–imine ligands [5,6] HL1–HL3 are
the most widely acclaimed chelates for stabilizing nickel(III/IV)
oxidation states through strong L-M s donation. Previous studies
have established that HL2 (15-amino-3-methyl-4,7,10,13-tetraaza-
pentadec-3-en-2-one oxime, Scheme 1) can stabilize nickel(III)
whereas both HL1 (6-amino-3-methyl-4- azahex-3-en-2-one
oxime) and H2L3 (3,14-dimethyl-4,7,10,13-tetraazahexadeca-3, 13-
diene-2,15-dione dioxime) can stabilize the nickel(IV) [6] oxida-
tion state. It is also significant in this context that (i) the nickel (IV)
complexes derived from H2L3 and HL1 contain two oximato groups
per nickel atom while HL2, with one oxime function, can stabilize
only Ni(III) oxidation state. Previously, we explored the electron
transfer kinetics of Ni(III)/(IV) complexes of these oxime–imine
ligands under homogeneous conditions [7–10]. Now we are in-
terested to explore the catalytic efficacy of these Ni-complexes
covalently tethered over high surface area matrices of SBA-15 to-
wards a wide range of organic transformations.
and nickel(II) as catalysts [11,12,16] leading to a significant en-
antiomeric excess, that was less pronounced for the analogous
manganese(II) [16] chromium(III) [17] ruthenium(II/III) complexes
[18]. In general, the oxygen-transfer processes mediated by the
above quoted complexes, and others such as iron(III), manganese
(III) and chromium(III) porphyrins [13], occur through the forma-
tion of high-valent metal-oxido intermediates [19].
Homogeneous catalysts provide much faster reaction rates than
heterogeneous catalysts, but the separation of catalyst from the
reaction mixture is tedious and considerably costly [20]. On the
other hand, heterogeneous catalysts can be recovered and recycled
rather easily. Heterogenization of catalysts can be achieved by
several approaches like encapsulation in zeolites [21], grafting on
polymers [22] and immobilization in polysiloxane membranes
[23]. In recent years, ordered mesoporous silicas like MCM-41,
SBA-15 etc. with high surface area, large pore volume and well-
defined hexagonal arrays of channels and cavities [24] have been
employed as solid support with great success [25,26]. Again, many
mononuclear and dinuclear nickel(II) complexes have been em-
ployed as oxidation catalysts [27] in a wide variety of reactions
under homogeneous conditions. However, the use of nickel-based
heterogeneous catalysts in epoxidation reaction are rare and they
often exhibit relatively poor catalytic activity [28].
In this context it is pertinent to mention that epoxidation of
olefins catalyzed by metal complexes has become an important
research area in both organic synthesis and bio-inorganic model-
ing of oxygen-transfer metalloenzymes [11–15]. For a variety of
n
Corresponding authors.
Here, we, for the first time, report a novel oxime–imine based
nickel(II) catalyst heterogenized on highly ordered mesoporous
E-mail addresses: msab@iacs.res.in (A. Bhaumik),
m_ali2062@yahoo.com (M. Ali).
http://dx.doi.org/10.1016/j.jssc.2016.01.018
0022-4596/& 2016 Elsevier Inc. All rights reserved.

106
L. Paul et al. / Journal of Solid State Chemistry 237 (2016) 105–112
Scheme 1. Structure of different oxime–imine ligands and their Ni(IV) complexes.
silica material SBA-15 and the resulting material efficiently cata-
lyzes the epoxidation of olefins under solvent free mild conditions.
temperature under N2 atmosphere for about 15 h. The resulting
white solid was filtered, washed with chloroform and di-
chloromethane and finally dried in air.
2. Experimental section
2.5. DAMO immobilized SBA15-NH2 (SBA-15-NH2-DAMO)
2.1. Materials
A solution of diacetylmonooxime (0.4 g) in diisopropyl ether
(5 ml) was added dropwise into the 3-APTES functionalized SBA-
15 material (0.50 g) dispersed into diisopropyl ether (20 ml) under
continuous stirring. After the complete addition, the reaction
mixture was refluxed for about 1 h at 333 K. The color of the re-
action mixture changed to yellow and no further color change was
observed on further reflux. The reaction mixture was cooled to
room temperature and yellow product was collected by filtration,
washed repeatedly with diisopropyl ether to remove unreacted
DAMO which was dried in air and was designated as SBA-15-NH2-
DAMO.
Tetraethylorthosilicate(TEOS), poly (ethylene oxide), poly
(propylene oxide), poly (ethylene oxide), pluronic P123 [EO20–
PO70–EO20] and 3-aminopropyl triethoxysilane were purchased
from Sigma-Aldrich. Diacetylmonooxime (DAMO) and diisopropyl
ether were obtained from Spectrochem and were used as received
without further purification.
2.2. Characterization techniques
FT-IR spectra were obtained on a Nicolet MAGNA-IR 750
spectrometer for the samples prepared in KBr pellets. Nitrogen
adsorption–desorption isotherms were obtained using a Quanta-
chrome Autosorb 1 C surface area analyzer at 77 K. Prior to gas
adsorption, all the samples were degassed for 4 h at 453 K.
Transmission electron microscopic images and selected area
electron diffraction (SAED) patterns were recorded on a JEOL 2010
TEM operated at 200 kV. Atomic absorption spectroscopy (AAS)
analysis was carried out using nickel standards on a Shimadzu AA-
6300 atomic absorption spectrophotometer. Thermogravimetric
analysis (TGA) and differential thermal analysis (DTA) of samples
were carried out in a TGA instruments thermal analyzer TASDTQ-
600 under N2 flow.The progress of the reactions was monitored by
an Agilent 7890D gas chromatograph (FID detector) fitted with a
capillary column. The products were identified comparing with
known standards.
2.6. Synthesis of nickel anchored mesoporous SBA-15 catalyst
(SBA-15-NH2-DAMO-Ni (sample a)
0.2 g dried DAMO anchored mesoporous SBA-15 (SBA-15-NH2-
DAMO) was suspended in absolute ethanol (20 ml). Then a solu-
tion of nickel(II) perchlorate (0.7 g) was added dropwise to the
suspension under refluxing condition for about 6 h at 363 K. The
color of the mesoporous material was slowly changed from light
yellow to light brown. The reaction mixture was cooled at room
temperature and the resulting mesoporous material was filtered
through suction and washed thoroughly with ethanol and then
dried under vacuum. The light brown colored mesoporous mate-
rial was designated as SBA-15-NH2-DAMO-Ni (sample a). The
detail of synthesis is highlighted in Scheme 2.
Maintaining the same reaction conditions and same solvent
system we have synthesized the following catalysts where only
the amount of reactant materials were varied. These are discussed
as below.
2.3. Synthesis of SBA-15
SBA-15 material was synthesized by using pluronic-123 tri-
block copolymer (EO20–PO70–EO20; Aldrich) as template. In this
typical preparation, 4.0 g of pluronic P123 was dissolved in 30 g of
water and 120 g (3.28 mol) of 2 M HCl solution with stirring at
35 °C. Then 8.50 g (40.8 mmol) of tetraethylorthosilicate (TEOS;
Aldrich) was added to the solution and stirred at 35 °C for 20 h.
The mixture was aged at 80 °C for 24 h under static condition. The
solid product was recovered by filtration, washed with distilled
water repeatedly to free from acid by centrifugation and air-dried
at room temperature overnight, followed by calcinations at 550 °C
for 6 h with air to remove the residual organic template materials,
yielding the final mesoporous SBA-15 material.
2.7. Synthesis of Nickel (Ni) anchored mesoporous SBA-15 catalyst
(SBA-15-NH2-DAMO-Ni, (sample b)
0.2 g of calcined SBA-15 was grafted with 0.19 g 3-APTES in
CHCl3 at room temperature to prepare SBA-15-NH2. Then 0.25 g of
SBA-15-NH2 was suspended in diisopropyl ether to which 0.101 g
of DAMO in diisopropyl ether was added and refluxed for 1 h to
get SBA-15-NH2-DAMO. To a 0.25 g of SBA-15-NH2-DAMO in
ethanol (15 ml) a solution of nickel(II) perchlorate (0.03 g) in
ethanol (20 ml) was added drop wise and refluxed for about 6 h at
363 K. It was then filtered to get the solid which was washed and
dried under vacuum to get light brown mesoporous material de-
signated as (SBA-15-NH2-DAMO-Ni), (sample b)
2.4. Synthesis of 3-aminopropyl functionalized SBA-15 (SBA-15-NH2)
The calcined SBA-15 was activated under vacuum at 150 °C for
3 h to remove the moisture absorbed on its surface. Then about
0.38 g of 3-aminopropyltriethoxysilane (3-APTES) was added to
0.2 g of calcined SBA-15 in CHCl3 under stirring condition at room
2.8. Synthesis of Nickel anchored mesoporous SBA-15 catalyst (SBA-
15-NH2-DAMO-Ni,(sample c)
0.20 g of calcined SBA-15 was grafted with 0.38 g 3-APTES in

L. Paul et al. / Journal of Solid State Chemistry 237 (2016) 105–112
107
Scheme 2. Route to synthesize functionalized SBA-15 supported nickel(II)-oxime–imine catalysts.
CHCl3 at room temperature to prepare SBA-15-NH2.Then 0.20 g of
Ni) , (sample e).
SBA-15-NH2 was suspended in diisopropyl ether to which 0.101 g
of DAMO was added and refluxed for 1 h to get SBA-15-NH2-
DAMO. To a 0.20 g of SBA-15-NH2-DAMO in ethanol (15 ml) a
solution of nickel(II) perchlorate (0.03 g) in ethanol (20 ml) was
added dropwise and refluxed for about 6 h at 363 K. It was then
filtered to get the solid which was washed and dried under va-
cuum to get light brown mesoporous material designated as (SBA-
15-NH2-DAMO-Ni), (sample c).
2.11. Synthesis Ni(II) Schiff base complex, [Ni(L)]
We have also prepared a complex derived from a Schiff base
ligand, obtained by condensation of diacetylmonooxime (5 mmol,
0.50 g) with n-propylamine (5 mmol, 0.29 g) under reflux condi-
tion for 1 h (Scheme 3) in diisopropyl ether. On keeping the re-
sultant solution in diisopropyl ether at 0 °C in refrigerator over-
night white crystalline solid was obtained which was filtered and
washed with diisopropyl ether to get the pure product (HL). Now,
Ni(ClO4)2 ꢀ 6H2O (2 mmol, 0.73 g) was refluxed with 2 equivalent of
the Schiff base ligand (0.57 g) in 20 ml ethanol solution and refluxed
for 4 hours. A red solution was obtained and after evaporation of the
solvent red crude product is obtained, which was recrystallized from
ethanol to get pure product. [Ni(HL)](ClO4)2:CHN analysis: Calc:C,
31.02; H, 5.2; N,10.34; Found: C, 31.12; H, 5.25; N,10.30.
2.9. Synthesis of Nickel anchored mesoporous SBA-15 catalyst (SBA-
15-NH2-DAMO-Ni) (sample d)
0.20 g of calcined SBA-15 was grafted with 0.38 g 3-APTES in
CHCl3 at room temperature to prepare SBA-15-NH2.Then 0.20 g of
SBA-15-NH2 was suspended in diisopropyl ether to which 0.40 g of
DAMO was added and refluxed for 1 h to get SBA-15-NH2-DAMO.
To a 0.20 g of SBA-15-NH2-DAMO in ethanol (15 ml) a solution of
nickel(II) perchlorate (0.35 g) in ethanol (20 ml) was added drop
wise and refluxed for about 6 h at 363 K. It was then filtered to get
the solid which was washed and dried under vacuum to get light
brown mesoporous material designated as (SBA-15-NH2-DAMO-
Ni), (sample d).
The details of syntheses of different materials are summarized
in Table 1.
2.12. Catalytic studies
Catalytic reactions were carried out over Ni-grafted materials
using different substrates in a similar manner. The heterogeneous
oxidation reactions were performed in a magnetically stirred two
necked round-bottomed flask fitted with a condenser placed in a
temperature controlled oil bath. All the reactions were carried out
under solvent-free condition. Typically, 0.50 g of the substrate
(olefins) was added to the flask followed by the addition of 50 mg
of the catalyst and the mixture was then preheated to 333 K. The
reaction was initiated by the addition of equimolar amount of tert-
butyl hydroperoxide (70% in H2O). Aliquots from reaction mixtures
were collected after 24 h. After cooling the product(s) of the re-
action were analyzed by Varian CP3800 gas chromatograph
equipped with an FID detector and a CP-Sil 8 CB capillary column
gas chromatography.
2.10. Synthesis of Nickel anchored mesoporous SBA-15 catalyst (SBA-
15-NH2-DAMO-Ni) (sample e)
0.20 g of calcined SBA-15 was grafted with 0.38 g 3-APTES in
CHCl3 at room temperature to prepare SBA-15-NH2.Then 0.20 g of
SBA-15-NH2 was suspended in diisopropyl ether to which 0.40 g of
DAMO was added and refluxed for 1 h to get SBA-15-NH2-DAMO.
To a 0.20 g of SBA-15-NH2-DAMO in ethanol (15 ml) a solution of
nickel (II) perchlorate (1.40 g) in ethanol (20 ml) was added drop
wise and refluxed for about 6 h at 363 K. It was then filtered to get
the solid which was washed and dried under vacuum to get light
brown mesoporous material designated as (SBA-15-NH2-DAMO-

108
L. Paul et al. / Journal of Solid State Chemistry 237 (2016) 105–112
Scheme 3. Preparation of 3-propylimino-2-one-oxime.
Table 1
Compositions of different SBA-15-NH2-DAMO-Ni samples.
Sample
name
Amount of Amount of Amount of Amount of Ni(ClO4)2
SBA-15
3-APTES
DAMO ta-
ken (g)
taken (g)
taken (g)
taken (g)
Sample a 0.2
Sample b 0.2
Sample c 0.2
Sample d 0.2
Sample e 0.2
0.38
0.19
0.38
0.38
0.38
0.40
0.101
0.101
0.40
0.40
0.70
0.03
0.03
0.35
1.40
3. Results and discussion
This heterogeneous catalyst SBA15-NH2-DAMO-Ni was devel-
oped via surface functionalization of SBA-15 with 3-aminopropyl-
triethoxysilane, followed by Schiff-base condensation of the sur-
face –NH2 groups with diacetylmonooxime and then complexation
with Ni(ClO4)2 in absolute ethanol (Scheme 2). The square planar
geometry around the Ni(II) center was confirmed by EPR spectrum
which was found to be EPR silent in the solid state (Fig. S1).
Small angle powder X-ray diffraction pattern of pure SBA-15,
SBA-15-NH2, SBA-15-NH2-DAMO, (a) SBA-15-NH2-DAMO-Ni
(sample a), (b) SBA-15-NH2-DAMO-Ni (sample b), (c) SBA-15-NH2-
DAMO-Ni (sample c), (d) SBA-15-NH2-DAMO-Ni (sample d) and
(e) SBA-15-NH2-DAMO-Ni (sample e) are shown in Fig. 1. The
sample preparations in different loading conditions are listed in
Fig. 1. Small angle powder X-ray diffraction pattern of pure SBA-15; SBA-15-NH2;
SBA-15-NH2-DAMO; SBA-15-NH2-DAMO-Ni (sample a); SBA-15-NH2-DAMO-Ni
(sample b); SBA-15-NH2-DAMO-Ni (sample c); SBA-15-NH2-DAMO-Ni (sample d);
and SBA-15-NH2-DAMO-Ni (sample e)(vide infra).
unfucntionalized SBA-15 material is 9.2 nm, which agrees very
well with literature [29c]. Whereas, for Ni-grafted material SBA-
15-NH2-DAMO-Ni estimated average pore dimension is ca. 6.0 nm.
Considerable decrease in the pore diameter than the traditional
SBA-15 material is a good agreement of the homogeneous func-
tionalization of the pore surface by the Ni-grafted Schiff-base
imine-oxime moiety in the parent SBA-15 material. We have also
shown the HR-TEM image of reused catalyst in Fig. 2d. This image
further suggests that the 2D hexagonal feature is intact after the
catalytic reactions. All the material also showed a bright diffrac-
tion ring in the fast Fourier transform (FFT) pattern (inset of
Fig. 2a, b, d).
Table 1. Three characteristic diffraction patterns in the 2θ region of
0.9 to 2.5 can be observed for both SBA-15 and SBA-15-NH2, which
can be attributed to the 100 (strong), 110 (weak) and 200 (weak)
reflections respectively, corresponding to the 2D-hexagonal me-
sostructure of SBA-15 and SBA-15-NH2 materials. When the amine
functionalized mesoporous silica was further subjected to the
Schiff-base condensation, followed by Ni-loading a considerable
decrease in the intensity and a shift of the peaks are observed. The
small shift in the XRD pattern as well as the decrease in intensity
can be attributed to the functionalization and loading of meso-
porous silica. After functionalization the mesoporous structure do
not damage at all in the final SBA-15-NH2-DAMO-Ni material, but
due to the incorporation of nickel in the porous framework by co-
ordination with Schiff base ligand the 2D ordering of the material
is disturbed so the higher order peaks are disappeared in the final
materials. In other words this decrease in the peak intensities is
attributed to the lowering of local order, viz. variations in the wall
thickness or reduction of scattering contrast between the channel
wall of the silicate framework or due to the presence of some
adsorbed Ni species within the channel structure [29].
Some representative HR-TEM images of the different functio-
nalized SBA-15 materials during the course of functionalization are
shown in Fig. 2(a–d). The uniform distribution of low electron
density spots (pores) in a hexagonally ordered honeycomb like
pores throughout the specimen for pure silica SBA-15 and SBA-15-
NH2-DAMO-Ni suggests that all the materials are composed of
hexagonally ordered pores. The average pore diameter of
The architectural porosity of the material SBA15-NH2-DAMO-
Ni is estimated from the N2 adsorption/desorption isotherms at
liquid N2 temperature (77 K). The isotherm is shown in Figs. S2
and S3. The isotherms for SBA-15-NH2-DAMO-Ni can be classified
as type IV with H1 type hysteresis loop with large mesopore. The
BET surface area of the material is 37 m2 gꢁ1 with a pore volume
of 0.0498 cm3 gꢁ1
.
The pore size distribution (PSD) estimated by using nonlocal
density functional theory (NLDFT) employing N2 adsorption on
silica with cylindrical pore as reference model is shown in Figs. S4
and S5. PSD plot suggested bimodal distribution of pores with
peak pore widths of 10.1 and 5.6 nm. The uniform decrease in the
pore diameter (5.6 nm; very close to the pore diameter obtained
from TEM analysis) and surface area compared to pure SBA-15
material are the indirect evidences of consecutive functionaliza-
tion of the mesopore surface [30]. On the other hand presence of
pores of dimension 10.1 nm suggested the existence of some un-
functionalized pores in SBA15-NH2-DAMO-Ni material also.

L. Paul et al. / Journal of Solid State Chemistry 237 (2016) 105–112
109
Fig. 2. HR-TEM images of SBA-15-NH2-DAMO-Ni showing 2D-hexagonal porous structure; in Inset FFT pattern of the material. (a) SBA-15; (b) SBA-15-NH2-DAMO-Ni
(sample a); (c) SBA-15-NH2-DAMO-Ni (sample b) (vide infra); (d) image of reused catalyst.
Table 2
BET surface area and total pore volume of SBA15-NH2-DAMO-Ni.
Sample name
Nickel load-
ing (wt%)
BET surface area Total pore vo-
(m2 gꢁ1
)
lume (cc gꢁ1
)
Sample (a)
Sample (d)
Sample (e)
Sample (a) after 5 re-
cycling experiments
1.29
0.63
1.95
1.27
37
45
32
36.77
0.0498
0.0543
0.0412
0.0478
All the datas are given in tabulated form in Table 2
FT-IR spectra of both the ligands and complexes showed strong
bands at 1636 and 1629 cmꢁ1 respectively corresponding to C¼N
bonds, which clearly demonstrate the retention of the C¼N bonds
in the complex (Fig. S6). A slight shift in stretching frequency to-
wards lower wave number may arise due to participation of C¼N
nitrogen atom towards bonding with Ni2þ ion. The stretching
frequency corresponding to oxime O–H bond occurs at 3440 cmꢁ1
for both the free ligand and complex and it is reasonable as oxime
–OH groups do not take part in bonding.
Fig. 3. TG-DTA profile of SBA15-NH2-DAMO-Ni samples.(a) TG and (a1) DTA for
sample b; and (b) TG and (b1) DTA for sample c respectively.
TG-DTA analysis was performed to know the thermal stabi-
lity of the prepared SBA-15-NH2-DAMO-Ni materials. The TG-
DTA profile (Fig. 3) shows four distinct weight losses. The first
stage ranging from 300–400 K may be attributed to the removal
of physically adsorbed water and solvent in the porous
materials. The next one, ranged between 500–600 K could be
ascribed to the desorption of the chemisorbed water on the
silica surface. The last two stage in the weight loss, ranging
from 600–700 K and 700–800 K, being exothermic based on
DTA analysis, is due to the decomposition of the pendant ligand
groups [29d].

110
L. Paul et al. / Journal of Solid State Chemistry 237 (2016) 105–112
Table 3
out under solvent-free condition. In Table 3 we have summarized
our results on the liquid phase partial oxidation of olefins over Ni-
catalyst. As seen from the Table 3 that although for styrene and
cyclohexene, benzaldehyde and 2-cyclohexen-1-ol were obtained
as major products, for cyclooctene corresponding epoxide is ob-
tained predominantly. Further, it was observed that although
SBA15-NH2-DAMO-Ni showed very high catalytic activity for the
oxidation of 1-hexene with almost equal yields of corresponding
epoxide and aldehyde, it showed very poor activity for 1-octene
(Table 3).
One blank reaction was carried out without adding any catalyst
(Table 3, blank experiment) keeping other conditions unchanged.
Very poor oxidation of styrene (ꢂ1.0%) suggested the catalytic
nature of partial oxidation reactions carried out over SBA15-NH2-
DAMO-Ni. Comparison of catalytic efficiency of SBA-15-NH2-
DAMO-Ni with other reported nickel based catalyst for the oxi-
dation of cyclohexene is listed in Table 4. A quick inspection of
Table 4 reveals that the conversion of cyclohexene is quite im-
pressive (73% conversion, 13% epoxide selectivity under solvent
free mild conditions) in comparison to [Ni(SAH)(H2O)]-Al2O3b (40%
conversion) -a heterogeneous catalyst without any epoxide for-
mation. However, under homogeneous conditions the other
homogeneous catalysts seem to be better than our catalyst with
respect to conversion and epoxide selectivity.
Liquid phase partial oxidation of olefins catalyzed by SBA15-NH2-DAMO-Ni (sample
a) under solvent-free conditionsa.
Substrates
Conversion (%) % of selectivity
Epoxide Others
TON
371
85.0b,c,d
10.58
17.80
98.08
53.06
89.41e
82.19f
1.92
73.0b
399
108
530
26.0b
98.0b
46.94
3.0b
1.0g
63.33
36.67
12.2
–
–
–
a
We have also carried out a catalytic run with a complex derived
from a Schiff base ligand, obtained by condensation of diace-
tylmonooxime with n-propylamine (Scheme 3), and Ni(ClO4)2
under homogeneous conditions using styrene and cyclohexene as
substrates where respective total conversion were found to be
only 35% and 20%, respectively. The catalytic oxidation of styrene
and cyclohexene using H2O2 under identical reaction conditions as
mentioned above showed very poor (10% and 4% respectively)
conversion, may be due to decomposition of the metal ion re-
ceptor part of the catalyst. It was observed that catalyst decom-
poses in presence of H2O2 within 3 h of mixing and we did not see
any major oxidation of cyclohexene. H2O2 being stronger oxidant
than tert-butylhydroperoxide, probably the Schiff base part of the
catalyst undergoes decomposition, as evidenced from color change
of the catalyst from reddish brown to white.
Turn over number¼moles of substrate converted/moles of active site. Cata-
lyst:
b
SBA-15-NH2-DAMO-Ni.
Ni-DAMO as catalyst yields 30% total conversion and 18% benzaldehyde se-
c
lectivity.
d
Ni-Schiff complex yields 35% total conversion and 12% benzaldehyde se-
lectivity.
e
Benzaldehyde.
2-cyclohexen-1-ol.
Blank.
f
g
3.1. Catalytic epoxidation
We have also prepared other two SBA15-NH2-DAMO-Ni sam-
ples by reacting SBA-15-NH2-DAMO with half and double the
amount of Ni(ClO4)2 as compared to sample (a) and designated as
sample (d) and sample (e), respectively and characterized by PXRD
studies which showed the similar pattern as that of sample (a)
(Fig. 1). AAS studies reveal that these two materials have Ni
Catalytic reactions were carried out over Ni-grafted materials
using different substrates in a similar manner. The heterogeneous
oxidation reactions were performed in a magnetically stirred two
necked round-bottomed flask fitted with a condenser and placed
in a temperature controlled oil bath. All the reactions were carried
Table 4
Comparison of catalytic efficiency of SBA-15-NH2-DAMO with other reported nickel based catalyst for the oxidation of cyclohexene.
Catalyst
Reaction temperature
Solvent, Oxidant
Conversion (%)
% yield of products
Reference
a
Epoxide
Other
b
[Ni(SAH)(H2O)]-Al2O3
Reflux condition
333
CH2Cl2, TBHP (80%)
MeCN, TBHP
(80%),
40.2
82
89
81
89
25.6
69
87
–
29.5
[32a]
[27c]
1c
82
72
87
84
–
2d
3e
–
4f
[Ni(H4C6N6S2)]g
[Ni II(Cyclam)]2þ
[Ni II(salen)]
Reflux condition
Ambient Temperature
Room temperature
333 K
TBHP
42.2
49
–
[32b]
Dicloromethane, PhIO
Dicloromethane, NaOCl
Solvent free, TBHP (70% in water )
36
23
13
[16(b)]
[16(a)]
this work
SBA-15-NH2-DAMO-Ni
73
60
a
2-Cyclohexene-1-ol.
b
SAH¼ bis(salicylaldiminato) hydrazone.
[Ni4L41(μ-tp-κ4-O)(H2O)2(μ-tp-κ2-O)] ꢀ 2C2H5OH ꢀ CH3OH ꢀ 3H2O(L1 ¼N-(3-aminopropyl)-5-bromosalicylaldimine )(1).
[Ni4L42(μ-tp-κ4-O)(H2O)2 (μ-tp-κ2-O)] ꢀ 3H2O(L2¼N-(3-aminopropyl)salicylaldimine] (2).
[Ni(L1)(H2O)-{Ag(CN)2}]α(3).
c
d
e
f
[Ni(L3)(MeOH){Ag(CN)2}]α(4) [L3¼N-(3-amino-2,2-dimethylpropyl)-5-bromosalicylaldimine].
H4C6N6S2¼1,2,5,6,8,11-hexaazacyclododeca-7,12-dithione-2,4,8,10-tetraene.
g

L. Paul et al. / Journal of Solid State Chemistry 237 (2016) 105–112
111
Table 5
Comparison of catalytic efficiency of SBA-15-NH2-DAMO under three different Ni-loading conditions. Other conditions of catalytic reactions remain the same as mentioned in
Table 3.
Catalyst
Ni-loading (From AAS)
1.29 wt%
Substrate
Conversion (%)
73.0
% of selectivity
epoxide
TON
399
Cyclohexene-1-ol
82.19
Sample (a)
17.80
Sample (d)
Sample (e)
0.63 wt%
1.95 wt%
32.53
78.28
20.72
8.57
79.28
91.42
363
283
catalyst in the oxidation of olefins [31a].The stability of the catalyst
was checked by BET and TEM analysis after five consecutive runs.
The reused catalyst showed type-IV isotherm (Fig. S3) and its pore
size distribution (Fig. S5) remained same as that of parent catalyst
(Figs. S2 and S4). Further, the hexagonal ordering of the pores is
similar to that of the initial TEM image (Fig. 2d) with Ni loading of
1.27 wt% vis-à-vis 1.29 wt% in the fresh catalyst. It is pertinent to
mention that no styrene oligomers were detected at the end of
reaction though styrene is known to be prone to polymerization
on heating, especially in the presence of tert-butylhydroperoxide, a
radical initiator [31b]. This result suggested that SBA15-NH2-
DAMO-Ni is a very selective catalyst for the epoxidation reactions
in the presence of TBHP as oxidant. The oxidation reactions are
typically believed to operate either through a radical or concerted
pathways involving metal-peroxo species. In order to ascertain
which one is operative here we decided to carry out the oxidations
in the presence of radical scavengers like 2,4-ditertiar-
ybutylphenol. It was interesting to observe that the reaction pro-
ceeds smoothly through concerted pathway (Scheme 4) as was
observed previously [31c].
4. Conclusions
A noveloxime–imine functionalized highly ordered mesopor-
ous SBA-15-NH2-DAMO-Ni has been synthesized and character-
ized by various physico-chemical techniques. The synthesized
heterogeneous catalyst was found to catalyze the one-pot oxida-
tion of olefins like styrene,cyclohexene, cyclooctene, 1-hexene and
1-octene to the corresponding benzaldehyde, cyclohexene-1-ol
and cyclooctene-oxide, respectively under solvent-free conditions.
The catalyst was found to be reusable, at least for 5 consecutive
runs, without much degradation as delineated by AAS studies for
Ni content. Radical trap experiments reveal that the catalytic
oxidation of olefins occurs through concerted mechanism.
Scheme 4. Catalytic oxidation of olefin following a concerted mechanism.
loading 0.63 and 1.95 wt%, respectively. The catalytic studies under
the similar reaction conditions showed 32% and 78% of total
conversion, the later being comparable to the original sample
(a) while the former showed almost half conversion. The TON
values were calculated to be 363 and 283 respectively. At this
point, we are getting higher conversion for higher Ni loading
sample yet the TON value is less compared to lower Ni loaded
samples. This may be attributed to the decrease of surface area and
ordering of 2D hexagonal mesophase due to higher Ni loading. All
these data are listed in Table 5.
The recycling test of the Ni-catalyst for five repetitive cycles for
the oxidation of styrene reveals that after the fifth run the con-
version decreased from 85.0% to 83.4% and content of Ni in the
catalyst after the fifth cycle estimated by AAS convincingly showed
there is no visible leaching of metal ion from the catalyst. The hot
filtration test also established the active participation of the
Acknowledgments
Financial supports from CSIR (Ref. 02(2490)/11/EMR-II) and UGC
(Ref. UGC [39-735/2010(SR)], New Delhi and DST, West Bengal are
gratefully acknowledged. L.P. thanks UGC (NET), New Delhi for SRF
fellowship.

112
L. Paul et al. / Journal of Solid State Chemistry 237 (2016) 105–112
Appendix A. Supplementary material
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