Salicylaldiminato chromium complex supported on chemically modified silica as highly active catalysts for the oxidation of cyclohexene

Article abstract of DOI:10.1016/j.cattod.2012.08.040

Immobilization of chromium complexes on modified silica support was achieved via two different synthetic routes using 3-aminopropyl triethoxysilane (APTS) as a linking group. The FTIR spectra, elemental and solid-state NMR analyses demonstrated incorporation of amine functional groups on the surface of the APTS-functionalized silica support, which was further confirmed by ²⁹Si solid-state CP MAS NMR spectroscopy. Oxidation of cyclohexene was carried out over the chromium complex supported on organically modified silica using H₂O₂ as an oxidant under various conditions and different atmospheres viz. oxygen, nitrogen and air. The supported catalysts exhibited excellent activity (>94%) after 6 h reaction time under O ₂ atmosphere using THF as solvent. The catalysts showed high chemoselectivity towards formation 2-cyclohexen-1-ol and 2-cyclohexen-1-one. Activity of the immobilized catalysts remains nearly the same after three consecutive cycles, suggesting the true heterogeneous nature of the catalyst.

Full text of DOI:10.1016/j.cattod.2012.08.040

Catalysis Today 204 (2013) 114–124
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Salicylaldiminato chromium complex supported on chemically modified silica as
highly active catalysts for the oxidation of cyclohexene
Salam J.J. Titinchi∗, Hanna S. Abbo
Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, Cape Town, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 May 2012
Received in revised form 16 August 2012
Accepted 20 August 2012
Available online 7 October 2012
Immobilization of chromium complexes on modified silica support was achieved via two different syn-
thetic routes using 3-aminopropyl triethoxysilane (APTS) as a linking group. The FTIR spectra, elemental
and solid-state NMR analyses demonstrated incorporation of amine functional groups on the surface of
the APTS-functionalized silica support, which was further confirmed by 29Si solid-state CP MAS NMR
spectroscopy.
Oxidation of cyclohexene was carried out over the chromium complex supported on organically mod-
ified silica using H2O2 as an oxidant under various conditions and different atmospheres viz. oxygen,
nitrogen and air. The supported catalysts exhibited excellent activity (>94%) after 6 h reaction time under
O2 atmosphere using THF as solvent. The catalysts showed high chemoselectivity towards formation 2-
cyclohexen-1-ol and 2-cyclohexen-1-one. Activity of the immobilized catalysts remains nearly the same
after three consecutive cycles, suggesting the true heterogeneous nature of the catalyst.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Salicylaldiminato chromium-complex
Immobilization
Cyclohexene oxidation
3-Aminopropyltriethoxysilane
Davisil silica
1. Introduction
been developed to covalently attach transition-metal complexes
to mesoporous inorganic materials. These methods overcome the
In recent years, heterogenization of organocatalytic systems has
been achieved by immobilization of homogeneous catalysts on var-
ious types of organic and inorganic supports. These heterogeneous
systems have received serious attention as alternatives to conven-
tional homogeneous catalysts. Several immobilization strategies
have been widely applied for creating active sites on the support
surface [1–3]. Surface modification using organic functional groups
has been found to be useful for their immobilization on the silica
surface, which represents a significant feature of the catalyst het-
erogenization [4]. Use of inorganic supports with chemically bound
active centres as heterogenized catalysts provides the homoge-
neous systems with attractive features and additional advantages
such as catalyst stability, easy separation, handling, and recovery
to reduce environmental problems [5].
3-Aminopropyltrialkoxysilane is one of the most commonly
used precursors for the modification of silica surfaces [6,7]. Fur-
thermore, amino-functionalized silica plays a vital role in the
development of materials for several applications, such as adsorp-
tion of metal ions [8], enzyme immobilization [9], drug delivery
[10] and catalysis [11].
problems of metal leaching in comparison with the ion exchange or
adsorption methods used for immobilization on porous supports.
The silica mesoporous material with pore size >20 A is well-known
˚
to be suitable for liquid-phase reactions that allow easy diffusion of
reactants to the active sites [4]. Thus, the silica material utilized in
˚
this work with pore size of 88 A was used as a support for chromium
complexes in liquid phase oxidation of cyclohexene.
Catalytic oxidation of cyclohexene by peroxides is of both
chemical and biochemical interest due to the formation of differ-
ent oxygenated functionalized products. These products are very
important as starting materials in pharmaceutical industries and
in synthesizing fine chemicals [12].
There are several reports on homogenous chromium–salen
complexes as catalysts for alkene oxidation [13]. Also a variety of
chromium-based reagents have been used as catalysts for selective
oxidation of allylic compounds [14]. Recently, Kawanami et al. [15],
Shiraishi et al. [16] and Mohapatra et al. [17] described their studies
on the oxidation of olefins with molecular O2 using Cr-containing
mesoporous molecular sieves in which Cr metal was directly incor-
porated within the silica matrix. Jacobs and Dioos have prepared
dimeric Cr(III)salen impregnated or supported on silica for epoxide
ring opening reactions [18].
Functionalization of mesoporous silica surfaces by the immo-
bilization of active sites via tethering or grafting methods have
To date, several reports on chromium complexes immobilized
on functionalized silica have been reported for various oxidation
reactions particularly epoxidation [19,20] and epoxide ring open-
ing reaction [21]. On the other hand, Koner et al. described Cu(II)
∗
Corresponding author. Tel.: +27 21 959 2217; fax: +27 21 959 3055.
E-mail address: stitinchi@uwc.ac.za (S.J.J. Titinchi).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.08.040

S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
115
complex supported catalysts containing a ligand derived from salic-
2.3. Preparations
ylaldehyde and APTS anchored on MCM-41 [22]. The resulting
catalyst was shown to be highly efficient for epoxidation of olefins
using tert-BuOOH (TBHP) as an oxidant.
Catalysts employed in this paper are based on the salicylaldim-
inato chromium(III) complex bound to a silica gel surface via
the hydrophobic propylsilane linking group between organic moi-
eties and inorganic support. The modified silica gel supported
chromium(III) catalysts were synthesized via two synthetic routes,
characterized and screened for the liquid phase catalytic oxidation
of cyclohexene with hydrogen peroxide.
All reactions were performed under a dry nitrogen atmosphere
using standard Schlenk techniques. Freshly dried ethanol (dried
by standard method; distillation over magnesium turnings and
iodine). Solvent was degassed with nitrogen for 15 min prior to the
catalytic tests.
2.3.1. Synthesis and immobilization of the ligands and their
Cr-complex
Preparation of Cr(III) salicylidene propylene silane supported on
silica gel.
Two different synthetic routes were followed to prepare this
catalyst (Scheme 1).
2. Experimental
2.3.1.1. Route 1 (Pre-grafting): Ligand – Complexation – Immobiliza-
tion (L.C.I.).
2.1. Materials
2.3.1.1.1. Preparation
of
2[(E)-{(3-triethoxysilyl)propyl}-
Salicylaldehyde, (3-aminopropyl)triethoxysilane, silica gel
(Davisil, grade 710, 4–20 ␮, 60◦, 99%), chromium acetate hydrox-
ide [Cr3(CH3COO)7(OH)2], chromium trichloride anhydrous, 30%
aqueous H2O2, cyclohexene redistilled), cyclohexene oxide, 2-
cyclohexen-1-ol, 2-cyclohexen-1-one, 1,2-cyclohexandiol were
purchased from Aldrich, tetrahydrofuran from Sarachem, acetoni-
trile from Riedel-de Haen. 1,2-Dichloroethane, benzene, ethanol
and acetone from Sigma-Aldrich.
imino]phenol (APTSsal) (L1). Equal moles of (3-aminopropyl)-
triethoxysilane (APTS) (2.77 g, 12.5 mmol) and salicylaldehyde
(Sal) (1.53 g, 12.5 mmole) were dissolved in freshly dried ethanol
(50 mL) under nitrogen atmosphere at room temperature. The
reaction mixture immediately changed to yellow and was left to
reflux for 3 h. A yellow semi-viscous liquid was obtained after
evaporation of ethanol using a rotary evaporator (yield 3.59 g, 88%)
which was characterized directly.
IR, ꢀ = 1632 cm−1 (C N); 1H NMR (CDCl3, ppm): 13.56 (s, 1H,
phenolic OH), 8.34 (s, 1H, CH N), 7.34–6.82 (m, 2H, phenyl
2-H and 3-H), 6.97–6.82 (m, 2H, phenyl 4-H and 5-H), 3.88–3.77
(q, 6H, CH2O), 3.63–3.56 (t, 2H, CH2N), 1.87–1.75 (quintet, 2H,
␤-propyl CH2 ), 1.26–1.19 (t, 9H, CH3 of ethoxy group), 0.73–0.64
(t, 2H, CH2 Si ); 13C NMR (CDCl3, ppm): 7.63, 17.97, 24.09, 58.03,
61.60, 116.62, 118.00, 118.50, 130.84, 131.67, 161.12, 164.52. Anal.
Calcd. for C16H27NO4Si: C 59.1, H 8.31, N 4.31. Found: C 57.84, H
8.03, N 4.51.
2.2. Physical methods and analysis
1H and 13-C NMR spectra were recorded in CDCl3 using a Var-
ian XR200 spectrometer (1H at 200 MHz, 13C at 50.3 MHz). Sample
signals are relative to the resonance of residual protons on carbons
in the solvent. The solid-state NMR was carried out on a Varian
VNMRS 500 MHz two channel spectrometer using 6 mm zirconia
rotors and a 6 mm Chemagnetics TM T3 HX probe. The direct polar-
ization (DP/SPE) spectrum was recorded at ambient temperature
with proton decoupling, a 3.75 ␮s 90◦ pulse, and a recycle delay
of 30 s. All cross-polarization (CP) spectra were recorded at ambi-
ent temperature with proton decoupling, a 3.75 ␮s 90◦ pulse, and a
recycle delay of 5 s. The power parameters were optimised for the
Hartmann-Hahn match. The contact times for cross-polarization
was 1 ms. The free induction decay for both was 640 points, Fourier
transformed with 1280 points and 50 Hz line broadening. Magic-
angle-spinning (MAS) was performed at 5 kHz and adamantane was
used as an external chemical shift standard.
The elemental analysis was performed on a Carlo Erba ana-
lyzer, thermal analysis using Mettler Toledo TGA/SDTA and the
BET surface area using a Tristar 3000 micromeritics Surface Area
and Porosity Analyzer at the University of Cape Town, South Africa.
The percentage metal content was determined using Pye Unicam
9100 atomic absorption spectrophotometer and ICP. The ATR-IR
measurements were carried out on a Perkin-Elmer Spectrum 100
FTIR spectrometer and Paragon 1000 PC FT-IR spectrometer. Elec-
tronic spectra were recorded on a GBC UV/VIS 920 UV-Visible
spectrophotometer in absolute ethanol or in Nujol (by layering the
mull of the sample to the inside of one of the cuvette while keep-
ing another one layered with Nujol as reference). In some cases
a diffuse reflectance technique was used (recorded under ambi-
ent conditions equipped with a quartz cell). All catalyzed reaction
products were analyzed using Varian CP3800 gas chromatograph
fitted with flame ionization detector. A HP-PONA capillary column
(50 m × 0.35 mm (id) × 0.5 ␮m film thickness, Agilent technologies,
J&W Scientific) and Star workstation computer software were used.
The retention time of all the peaks were compared with authentic
samples and from GC-MS data.
2.3.1.1.2. Complexation (C1):[L4Cr2(ꢁ-OH)(ꢁ-O2CCH3)]. Two
different chromium metal sources were used to prepare the
unsupported catalyst:
(i) Chromium acetate hydroxide: Cr3(Ac)7(OH)2 (1.04 g, 1.7 mmol)
was dissolved in dry ethanol (20 mL) and added to L1 in
1:6 (Cr-acetate: APTSsal) molar ratio. The reaction mixture
was refluxed with stirring overnight. A green semi-solid was
obtained (5.4 g; yield 60%). IR, ꢀ = 1614 cm−1 (C N); 1712 cm−1
(C O). Anal. Calcd. for C66H108N4O19Si4Cr2: C 53.66, H 7.32, N
3.79. Found: C 51.93, H 7.32, N 3.79. Anal. by ICP (Atomic %): Cr,
6.91. M/N = 1.82; C/N = 13.70 (Calcd. 14.14).
(ii) Chromium chloride anhydrous: On the other hand using CrCl3
anhydrous as a metal source, no complexation took place.
2.3.1.1.3. Immobilization. Silica gel (Davisil, grade 710, 4–20 ␮)
(5 g) was added directly to C1 in dry ethanol (50 mL). The mixture
was left to stir overnight under nitrogen atmosphere. A green solid
(6.93 g) was obtained. The solid was filtered and washed thoroughly
several times with boiling ethanol to get rid of any free metal salt
immobilized on the silica surface. The resulting solid was oven dried
at 80 ◦C for 3 h. IR, ꢀ = 1614 cm−1 (C N); 1712 cm−1 (C O). Anal.
Found: C, 4.03; N, 0.29; Anal. atomic % (ICP): Cr, 0.52. % M/N = 1.79;
C/N = 13.90 (Calcd. 14.14).
2.3.1.2. Route 2 (post-grafting): Functionalization – Condensation –
Complexation (F.C.C.).
2.3.1.2.1. Functionalization (SG-APTS). The inorganic support
materials, was functionalized with amino groups. (3-
Aminopropyl)-triethylsilane (2.77 g, 12.5 mmol) in dry ethanol
(10 mL) was added directly to a suspension of silica gel (5.0 g) in

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S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
Scheme 1. Synthetic routes of Cr-supported catalyst.
dry ethanol (50 mL). The mixture was left to stir overnight under
nitrogen atmosphere. A colourless solid was obtained, filtered and
washed thoroughly several times with boiling dry ethanol to get
rid of any unimmobilized silane. The resulting solid was oven dried
at 80 ◦C for 3 h. (7.2 g), IR, ꢀ = 1550 cm−1 (symmetric NH2 bending
vibration). Anal. Found: C, 4.21; N, 1.64; C/N = 2.57 (Calcd. 2.57).
2.3.1.2.2. Preparation of supported salicylidene propylene tri-
ethoxy silane (SG-APTSsal) (L2). An excess amount of salic-
ylaldehyde (Sal) (1.53 g, 12.5 mmol) in dry ethanol (10 mL)
was added to an equivalent mole of the assumed moles of
(3-aminopropyl)triethoxysilane attached to silica gel surface
(APTS-SG) (2.7 g) in freshly dried ethanol (50 mL) in a Schlenk
tube under nitrogen atmosphere. The reaction mixture immedi-
ately changed to yellow and was left to reflux for 3 h. A pale yellow
powder was obtained (3.2 g). Anal. Found: C, 5.86; N, 0.44; IR,
ꢀ = 1632 cm−1 (C N).
to the supported ligand (L2) suspended in ethanol (20 mL) in 1:6
(Cr-acetate:APTSsal)molar ratio. The reaction mixture was refluxed
with stirring overnight. A pale green powder was obtained (3.8 g).
IR, ␯ = 1614 cm−1 (C N); 1712 cm−1 (C O). Anal. Found: C, 4.32;
N, 0.31; Anal. atomic % (ICP): Cr, 0.57. % M/N = 1.84; C/N = 13.94
(Calcd. 14.14).
2.4. Catalytic evaluation
Oxidation of cyclohexene using the prepared catalysts was
carried out in a 12 place parallel reactor using 50 mL glass reac-
tor vessels (a Radley’s Discovery Technologies 12 place Heated
Carousel Reaction Station fitted with a reflux unit as well as a
gas distribution system). In a typical reaction, cyclohexene (2.05 g,
0.025 mol) and 30% solution of H2O2 (3.58 g, 0.025 mol) were mixed
in 5 mL of the desired solvent unless otherwise stated, and the
reaction mixture was heated at 80 ◦C with continuous stirring.
Toluene was added as an internal standard. The reactions were
2.3.1.2.3. Complexation (C2). Chromium acetate hydroxide
(1.04 g, 1.7 mmol) was dissolved in dry ethanol (20 mL) and added

S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
117
Fig. 1. 13C DP-MAS NMR spectra of immobilized Cr-catalyst (LCI).
carried out under an oxygen atmosphere. Requisite amount of cat-
alyst (0.010 g) was added to the reaction mixture, and this was
taken as the starting point of the reaction. The resulting mixture
was stirred at 80 ◦C for 24 h under constant flow of the appropri-
ate atmosphere. The reaction products were analyzed using a gas
chromatograph and monitored at set time intervals by withdrawing
small aliquots. Samples were filtered before analysis. The oxidation
products were identified by comparison with authentic samples.
ethoxy), 86.21 (NCH2, propyl), 116.62 (Ar), 118.00 (Ar), 118.54 (Ar),
130.85 (Ar), 131.64(Ar), 161.16 (COH) and 164.32 (CN) ppm.
The supported ligand on silica was characterized by C-13 solid-
state NMR spectroscopy to confirm the incorporation of silane
Schiff base ligand on the support. The 13C CP-MAS experiment of
the supported complex was not successful due to the presence of
paramagnetic chromium.
Fig. 1 shows the 13C DP/SPE-NMR spectrum of the immobilized
catalyst (LCI). The spectrum shows peaks at 16.8, 28.3 and 49.8 ppm
which were attributed to the different methylene carbons of the
propyl group in the linker showing the incorporation of amine func-
tional groups. This is in agreement with the reported results on
similar silane modified silica [23]. The peak at 24 ppm is assigned to
the CH3 [24] whereas, azomethine carbon could be hardly observed.
The peak ∼178 ppm is assigned to carbonyl carbon of the bridging
acetate group.
3. Results and discussion
The synthesis of the organic–inorganic hybrid materials immo-
bilized chromium catalysts are illustrated in Scheme 1.
Two different synthetic routes were followed to immobilize
the chromium(III) complex on the surface of modified silica gel
depending on the sequence of the reaction steps. Route 1, the pre-
grafting (LCI), consists of Schiff base condensation between APTS
and salicylaldehyde and subsequent complexation with chromium
metal ion followed by immobilization to generate the silica sup-
ported chromium (III) catalyst. Route 2, the post grafting (FCC): the
inorganic support material was functionalized with amino groups
(APTS) followed by Schiff base condensation. The heterogenized
ligand was then complexed with chromium acetate hydroxide.
The catalysts employed herein were based on salicylaldiminato
chromium complex bound to silica gel surfaces via propyl silane
spacer.
The presence of peak in the high field region (114 ppm) is typical
of the carbon resonances for an aromatic system.
The 29Si CPMAS NMR spectrum of the silica support shows three
resonance peaks at chemical shifts ı = −111 (Q4), −101 (Q3), and
−92 (Q2) ppm which are assigned to the silica framework [25].
Immobilization of the ligand onto the surface makes the Q2 peak
featureless, whereas the intensities of Q3 peak decreases and con-
comitantly increases the Q4 peak as shown in Fig. 2. This is due to
the consumption of isolated Si-OH groups and geminal silandiols
during the reaction.
The 29Si NMR proofs that the APTS was successfully grafted on
the surface in agreement with C/N analysis. This is concluded by
the absence of the ungrafted APTS resonance signal which would
appear at −45 ppm and the appearance of an additional broad reso-
nance peak at −64 ppm created by overlapping two peaks at distinct
chemical shifts corresponding T2 and T3 [26].
The synthesized ligand and supported complexes were charac-
terized by a variety of spectroscopic techniques to confirm their
structures. These include solid-state NMR, FTIR, UV–vis, elemental
analyses and metal/N ratio, all of which support the structure of the
proposed complex. The M/N ratio suggested a 2:1 ligand to metal
stoichiometry.
Structural characterization of the synthesized catalysts via
the two routes is comparable albeit with a slight difference in
metal loading. The loading of the amino groups was 0.10 and
0.11 mmol g−1 silica for 0.52 and 0.57% Cr, respectively.
3.2. UV-visible spectra:
The electronic spectra of the unsupported complex and immo-
bilized catalyst were obtained as DR-UV or as nujol mulls (as
described in the experimental section) due to their insolubility. The
UV-visible spectra of these two complexes show four absorption
bands at 222, 261(sh), 391 and 556 nm (Fig. 3). The first two bands
are assigned to ꢂ → ꢂ* and ꢃ → ꢃ* (which belong to an intra-ligand
transition at 213 and 256 nm, respectively). The other two bands are
assigned to symmetry forbidden charge transfer transitions from
3.1. NMR spectra
The H-NMR spectral data (ı in ppm) of silane Schiff base
ligand (APTSsal) exhibits singlets at 8.34 and 13.56 ppm, which are
attributed to azomethine and phenolic OH protons, respectively.
A multiplet peak in the region 6.82–7.34 ppm is attributed to the
aromatic protons. 2-Proton triplet peaks between 0.64–0.73 and
3.56–3.63 ppm are assigned to (Si CH2 ) and (N CH2 ) protons,
respectively. A quintet in the region 1.75–1.87 ppm is assigned for
the metal to the anti-bonding orbital of the ligand 4A2g(F) → T1g(F),
4
which appeared as a broad band at 391 nm similar to that reported
for analogue systems [27]. A very weak and broad absorption band
appearing in the complex at 556 nm (highly concentrated sample
4
(
CH2 ) of the propyl group. Protons of ethoxy groups exhibit a
was used), due to d–d transition, 4A2g(F) → T2g(F) (ꢀ) [28], merged
triplet at 1.19–1.26 ppm and quartet at 3.77–3.88 ppm.
Carbon-13 NMR of the ligand shows peaks at 9.47 (SiCH2,
propyl), 11.32 (CH3, ethoxy), 23.74 (CH2, propyl), 60.80 (CH2,
with the n → ꢃ* spin allowed internal ligand transition band [10].
A weak shoulder appearing in the ligand spectrum at 299 nm is
assigned as intramolecular hydrogen bonding, which disappeared

118
S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
Fig. 2. 29Si CP-MAS NMR spectra of immobilized Cr-catalyst (LCI).
in the spectra of the complexes. This indicates the coordination of
phenolic oxygen to the metal ion. The electronic spectra of the sup-
ported catalysts exhibit similar bands to that reported for similar
systems [29], which indicate the existence of the same coordination
environment around chromium(III).
vibration, ı (O–H phenolic)in plane overlapped with the Si–O asym-
metric stretching vibration band of silica.
Further absorptions in the range of 3000, 2880–2980 and
1455 cm−1 are assigned to aromatic C H stretching, aliphatic
C
H stretching and deformation vibrations, respectively [33]. The
Si
O Si stretching and bending vibrations in the silanol Schiff base
ligand which appeared at ∼1040 and ∼805 cm−1 were not affected
[31,34].
3.3. Infrared spectra
The supported Cr-catalyst shows a broad band at ∼3480 cm−1
which is assigned to O H stretching vibrations of silanol groups
overlapped with the symmetric stretching vibrations of H2O [35].
The supported catalyst shows a strong absorption at 1712 cm−1
due to the C O stretching vibration of the acetate group. Addi-
tionally, the catalyst shows stretching vibration peaks at 1543 and
1410 cm−1, which are attributed to the asymmetric and symmet-
ric stretching modes of the CO2 group in the bridging acetate. The
difference between the two band positions (ꢄ = 133 cm−1) is con-
FT-IR spectroscopy was used to confirm the functionlization of
the support (Fig. 4). The intensities of the bands of the supported
metal complex were weak due to low concentration of the metal
complex.
The supported and unsupported ligand (APTSsal) exhibit a band
at 1632 cm−1 assigned to ꢀ (C N) of the azomethine stretching
vibration band which is shifted to a lower frequency (1614 cm−1
)
on complexation due to coordination of the azomethine nitrogen
atom to the metal ion. The appearance of two to three bands in the
spectra of the complex in the low frequency region (between 410
and 527 cm−1) is attributed to ꢀ (M N and M O) stretching, which
indicates the coordination of the phenolic oxygen as well as the
azomethine nitrogen to the metal ion [14b],[30].
siderably less than that found for ionic acetate (164–171 cm−1
)
and can be attributed to the presence of the bridging carboxylate
The ligand (APTSsal) exhibits a band at 3058 cm−1, that is
assignable to ꢀ (OH phenolic) which disappeared on complexa-
tion [31,32]. The band at 1280 cm−1 is assigned to the deformation
Fig. 3. Electronic spectra of the unsupported and supported ligand, unsupported
Fig. 4. FT-IR spectral pattern of the support and the supported and unsupported
and supported complex (a) diluted and (b) concentrated.
catalysts.

S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
119
Fig. 5. TGA-DTG thermograms for supported Cr-catalysts.
group [36]. A similar bridging dinuclear structure was confirmed
by X-ray crystal structure analysis for copper complex related sys-
tems, which further supports the structure of our complex [37]. The
proposed structure of the bridging unit is in agreement with the
elemental analysis and supported by a number of previous exam-
ples of binuclear Cr(III) complexes [38] in which the Cr(III) centres
are linked by one acetate and one hydroxo bridge. From the above
results it may be concluded that the donor ligands which act as NO
bidentate, and the bridging groups, fill the octahedral coordina-
tion about the chromium ion. Interestingly, the bridging binuclear
complex was obtained on using chromium acetate hydroxide as a
metal source, while mononuclear ◦Ctahedral complex was obtained
using chromium acetate [7]. These results confirm introduction of
the bridged chromium complex onto the silica support.
steps between 180 and 450 ◦C with an overall weight loss of 17.4%
for FCC and 16.6% for LCI.
3.5. Surface area analysis
Nitrogen physiosorption was used to determine the changes in
the BET surface area, average pore diameter, and pore volume of
the silica resulting from immobilization of the Cr-complex. The sur-
face area and total pore volume of silica gel and the immobilized
catalysts prepared via routes 1 and 2 are shown in Table 1. This
information is quite important to compare the adsorption proper-
ties of silica gel and immobilized catalysts.
Nitrogen adsorption–desorption isotherms of the catalysts
revealed a reversible type IV behaviour [40]. The supported Cr-
catalysts synthesized via routes 1 and 2 show a decrease in the
value of the surface area indicating the influence of immobilized
complex on the silica gel surface. The surface areas, pore volumes
and pore sizes of the catalysts decrease as a function of increasing
the loading; however the increase in loading results in increased
crowding of the functionalized groups in the pores. Thus, the pore
size of LCI catalyst is less affected by the crowding as a result of less
loading. Although the two materials are synthesized under similar
conditions with equal amounts of precursor solution, metal loading
in catalyst FCC is ∼10% higher than LCI.
3.4. Thermal analysis
TGA-DTG thermograms of supported Cr-catalysts show that the
amount of the metal loading in both routes is equivalent to the
% metal obtained from atomic absorption and ICP (Fig. 5) i.e. the
content of combustible organic ligand and bridging groups is in
stoichiometric proportion to the metal amount found by ICP. The
supported complexes synthesized via route 1 and 2 (Scheme 1)
exhibit similar TGA-DTG traces.
The immobilized complex shows a three-stage loss. The weight
loss peaks below 120 ◦C with a mass loss of 4.1–4.3% is due to
physically adsorbed water. The following decomposition steps con-
sist of: (i) elimination of bridging acetato and hydroxyl moieties
at about 160–180 ◦C, which is expected to fall in the range of
120–300 ◦C [39]; (ii) decomposition of the ligand starts in the range
of 300–320 ◦C. This step starts immediately after the previous step
and ends at ca. 450 ◦C. Unfortunately, it is difficult to estimate loss
percent in both stages separately due to overlapping of the two
3.6. Catalytic activity study
To evaluate the catalytic behaviour of the Cr-catalysts prepared,
oxidation of cyclohexene was carried out in different solvents and
under various reaction conditions. Cyclohexene is a suitable test
reagent for the oxidation of alkenes due to the low stability of
the resultant epoxide as well as the range of products formed
(Equation 1).

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S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
Table 1
Textural properties of the support and the catalysts.
Support/catalyst
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size* (Å)
%wt M content
Silica gel (Davisil 710)
Cr-catalyst LCI (Route 1)
Cr-catalyst FCC (Route 2)
347.5
235.0
221.6
0.77
0.52
0.49
88.30
87.79
83.13
–
0.52
0.57
*
Pore size distributions were calculated from the desorption branch by using BJH method.
CHO
100
CH-OL
CH-ONE
CH-diOL
80
60
40
20
0
.
3.7. Oxidation under aerobic conditions
The catalytic activity of the silica supported Cr-complex (LCI;
route 1) as a representative cyclohexene catalyst was initially stud-
ied in an aerobic atmosphere using a wide range of solvents and
H2O2 as an oxidant (Table 2). It was found that the % conversion
was dependent on the polarity of the solvent. Using polar solvents
such as THF, MeCN, EtOH and acetone, the % cyclohexene conver-
sion varied between 8–72% over 6 h reaction period. Extending the
reaction time to 24 h increased the conversion to 38-80% depend-
ing on the solvent used. In non-polar solvents such as benzene
and 1,2-dichloroethane, cyclohexene was oxidized to a consider-
ably lower extent. This may be due to a non-homogeneous reaction
mixture (two-phase solvent system). The same effect was found
when the oxidation reaction was carried out in bulk (solventless).
The reaction time also has an impact on the product selectivity.
In general, under these conditions cyclohexene oxidation pro-
ceeds with higher selectivity for 2-cyclohexen-1-ol (21–37%) and
2-cyclohexen-1-one (14–46%).
Neat
EtOH
CH3CN
Acetone
THF
Fig. 6. Selectivity (%) to various oxidation products under O2 atmosphere using
Cr-supported catalyst (LCI) at 6 h reaction time under the optimized conditions:
cyclohexene (0.025 mol), catalyst (10 mg), cyclohexene:oxidant (1:1 molar ratio),
solvent (THF; 2 mL) and temperature (80 ◦C).
after 24 h (3–44%). From the results above it can be concluded that
oxygen plays a major role in the oxidation process as a co-oxidant.
3.9. Oxidation in oxygen atmosphere
To further confirm the effect of oxygen and to prove its role as a
co-oxidant, the reaction was investigated under an oxygen atmo-
sphere using four selected solvents (Table 2). A significant increase
in the cyclohexene conversion was observed reaching 94% in THF
after 6 h and quantitative after 24 h. Selectivity in the formation
of various cyclohexene oxidation products under an oxygen atmo-
sphere in different solvents is shown in Fig. 6. Cyclohexene oxide
was only detected when the reaction was conducted in THF sol-
vent along with other expected products. In neat (solvent-free)
solution and ethanol, only three products viz. 2-cyclohexene-
1-ol, cyclohexene-1-one and 1,2-cyclohexandiol were detected.
Using acetonitrile, 100% selectivity towards 2-cyclohexen-1-one
was obtained. It is noteworthy to mention that the product selec-
tivity was dependent upon the type of solvent used [41–43].
Furthermore, when these catalysts were tested under an oxygen
atmosphere in the absence of H2O2, no activity at all was observed.
3.8. Oxidation in inert atmosphere
To ascertain the extent to which molecular oxygen in air may
play a role in the oxidation, similar oxidation reactions were car-
ried out under anaerobic conditions (under N2), to gain further
insight into the role of oxygen on the % conversion and selectivity
of cyclohexene oxidation.
Table 2 shows that the catalyst was most inactive or much less
active (∼1–37% conversion, 6 h). On the other hand, the effect of
solvent under an anaerobic atmosphere has a similar pattern com-
pared to that under an aerobic atmosphere with almost similar
products selectivity. There is no significant increase in conversion
Table 2
Cyclohexene conversion (%) under various atmospheres at two different time intervals in different solvents using Cr-supported catalyst (LCI).
Atmosphere
Time (h)
Cyclohexene conversion (%)/Solvent
Neat
EtOH
MeCN
Benzene
DCE
Acetone
THF
72.1
Air
6
0
29.1
8.0
38.4
40.2
55.5
1.2
5.4
0.6
21.5
62.7
71.2
24
80.1
Nitrogen
Oxygen
6
24
0
5.8
14.0
15.1
0
31.3
0
2.4
3.6
7.7
36.7
43.8
19.5
23.4
6
24
52.2
81.5
69.0
85.2
61.6
89.4
6.3
29.2
16.7
48.9
66.8
69.3
93.6
>99.0
Reaction conditions: cyclohexene (0.025 mol); H2O2 (0.025 mol); catalyst (10 mg); temp. 80 ◦C; solvent (2 mL).

S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
121
Table 3
Effect of H2O2:cyclohexene molar ratio on cyclohexene conversion (%) and %H2O2 efficiency.
Reaction time(h)
H2O2:cyclohexene molar ratio
0.5:1
1:1
2:1
% Conversion
% H2O2 efficiency
% Conversion
% H2O2 efficiency
% Conversion
% H2O2 efficiency
1
2
6
28.4
33.7
76.8
44.6
52.9
100.0
40.7
48.2
93.6
32.7
37.5
72.4
62.1
81.1
100.0
24.1
31.2
40.4
Reaction conditions: cyclohexene (0.025 mol); catalyst (10 mg); temperature 80o C, solvent THF (2 mL). %H2O2 efficiency = (moles of H2O2 utilized in products formation/moles
of H2O2 added) × 100.
These results verified that the oxygen plays an important role in
the oxidation process as a co-oxidant.
3.13. Effect of cyclohexene: oxidant molar ratio
The influence of H2O2 to cyclohexene molar ratio on the oxi-
dation of cyclohexene under an oxygen atmosphere as a function
of time is presented in Table 3. Three different H2O2:cyclohexene
molar ratios (0.5:1, 1:1 and 2:1) were used in this study while keep-
ing all the other parameters fixed. It is clearly shown that 2:1 molar
ratio gave the maximum percentage conversion (100%) with 40%
H2O2 efficiency after 6 h. On the other hand, the 1:1 molar ratio
of the oxidant:cyclohexene gave 94% overall cyclohexene conver-
sion with 72% H2O2 efficiency. Reducing the molar ratio to 0.5:1,
i.e. H2O2 in half molar amount of cyclohexene, the overall percent-
age conversion was lowered to ca. 77% with 100% H2O2 efficiency
(Table 3).
These results suggest that 1:1 molar ratio is the minimum
requirement for the effective oxidation of cyclohexene along with
better percentage H2O2 efficiency. This clearly suggests that a large
concentration of oxidant is not an essential condition to maximize
cyclohexene transformation. Obviously, an increase in H2O2 con-
centration enhanced the decomposition of H2O2 to form oxygen
and hence the side reactions, including allylic oxidation [43,44].
After 8 h of using 1:1 molar ratio, a maximum conversion was
achieved ∼100%, whereas on using 0.5:1 molar ratio the maximum
conversion of 91% was levelled off after 12 h reaction time.
3.10. Optimization of other reaction conditions
To optimize the reaction conditions under an oxygen atmo-
sphere for cyclohexene oxidation, other parameters such as
temperature, time, substrate:oxidant molar ratio, volume of sol-
vent, catalyst weight, additive, and oxidant were studied in detail.
3.11. Effect of amount of catalyst
Different amounts of catalyst were employed to investigate the
corresponding variation on the oxidation results as shown in Fig. 7.
The % cyclohexene conversion increased from 80 to 94% on
increasing the catalyst weight from 5 to 10 mg. Even 5 mg cata-
lyst was efficient to catalyze the oxidation reaction. These results
confirm that Cr-supported catalyst is highly active for oxidation of
cyclohexene. As the catalyst amount increased the % H2O2 decom-
position will increase which in turn decrease the oxidation process
i.e. lower conversion.
3.12. Effect of volume of solvent
3.14. Effect of other oxidant and additive
The volume of solvent has a significant effect on the % conversion
when using THF. Increasing the volume of solvent from 2 to 5 mL
causes the % conversion to decrease from 94 to 42% at 6 h and to 61%
after 24 h. Further increases in the volume of solvent results in dra-
matic decreases in conversion as shown in Fig. 8. The reduced rate
of reaction observed upon increasing the volume of solvent may
be due, in part, to the decrease in the bulk density of the catalyst
particles in the reaction media. The effect of the solvent’s volume
in agreement with the effect of amount of catalyst (vide infra).
The oxidation of cyclohexene was carried out using an oxidant
other than H2O2, namely 70% aqueous tert-butylhydroperoxide.
This oxidant was unable to oxidize cyclohexene under the opti-
mized reaction conditions. The overall conversion obtained was
less than 4%. It seems that this oxidant failed to generate active
100
80
60
40
20
0
1h
6h
24h
100
80
60
40
20
0
2ml
5ml
20ml
5
10
20
30
Volume of solvent
Catalyst wt. (mg)
Fig. 7. Effect of amount of catalyst on cyclohexene conversion under O2 atmosphere
using Cr-supported catalyst (LCI) at 6 h reaction time under the optimized condi-
tions: cyclohexene (0.025 mol), cyclohexene:oxidant (1:1 molar ratio), solvent (THF;
2 mL) and temperature (80 ◦C).
Fig. 8. Effect of volume of solvent on % cyclohexene conversion under O2 atmo-
sphere using Cr-supported catalyst (LCI) at 6 h reaction time under optimized
conditions: cyclohexene (0.025 mol), catalyst (10 mg), cyclohexene:oxidant (1:1
molar ratio), solvent (THF) and temperature (80 ◦C).

122
S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
Table 4
Comparison study between the as-prepared catalysts and their metal source.
Reaction time (h)
(L.C.I.) route 1
Conv. (%)
(F.C.C.) route 2
Conv. (%)
Cr-acetate hydroxide
Conv. (%)
TOF (h−1
)
TOF (h−1
)
TOF (h−1
)
1
2
6
40.7
48.2
93.6
10174
6025
3900
63.0
71.3
>99.0
14367
8130
3763
82.2
87.4
>99.0
413
209
83
Reaction conditions: cyclohexene (0.025 mol); H2O2 (0.025 mol); catalyst (10 mg); temperature 80 ◦C, THF (2 mL). Turn over frequency (TOF).
oxidant species and also had solubility problems associated with
the reactant vis-a-vis the oxidant. Hence, hydrogen peroxide is a
preferred oxidant as it is environmentally friendly and produces
only water as by-product and shows good selectivity.
Studies have shown that ascorbic acid as an additive may play
a role in the epoxiadtion of alkenes using H2O2 as oxidant [45]. In
this study, ascorbic acid in the presence of H2O2 was also used. No
marked effect was observed using ascorbic acid on either activity
or selectivity.
time to 24 h, the conversion was increased slightly with a slight
change in the selectivity of the products.
The insignificant difference in activity between the two catalysts
(FCC and LCI) may be due to the minor difference in % metal content.
Both catalysts catalyze olefin oxidation efficiently with very high
selectivity for partially oxidized products (>90%).
3.16. Reusability
In order to examine the reusability of the catalyst, it was sep-
arated from the reaction mixture by filtration, thoroughly washed
with acetonitrile and finally dried at 120 ◦C and reused three times
under similar conditions. The recycled catalyst was found to exhibit
almost the same conversion, 94.2, 93.7 and 92.4% after the first,
second and third recycling, respectively.
To test for the absence of metal leaching, the catalyst was
separated from the reaction mixture by filtration at the reaction
temperature after 2 h and the reaction mixture was allowed to react
further. No additional conversion of cyclohexene was observed
after removal of the catalyst. The ICP analysis of the reaction
mixture confirms that no chromium metal was detected. This posi-
tively suggests that metal leaching does not ◦Ccur and is indicative
of chemical bonding between the metal and the support of the
catalyst. However, the selectivity towards the products after 6 h
remains unaffected. Comparable IR spectra between fresh and used
catalysts indicates no change in the functionalities and that cat-
alytic behaviour should still be possible.
3.15. Comparison study for catalysts’ performance
After optimizing the reaction conditions (cyclohexene
(0.025 mol), catalyst (10 mg), 1:1 cyclohexene:oxidant molar
ratio, THF (2 mL) and temperature (80 ◦C) under an oxygen atmo-
sphere), a comparison study between Cr-based catalysts (LCI and
FCC) and chromium acetate hydroxide was performed for the
oxidation of cyclohexene. From Table 4, it is evident that the
Cr-based catalyst (LCI) shows lower performance (41% conversion)
in comparison with FCC catalyst 63% after 1 h reaction time.
However the LCI catalyst performs much better after 6 h and
catalyzes the oxidation of cyclohexene with a maximum of 94%
conversion comparable to FCC and the homogenous catalysts.
For a meaningful comparison of the relative performances of the
catalysts, turn over frequency values (TOF) should be used. TOF val-
ues have been calculated on the basis of the total chromium ions
present in the solid catalyst. The immobilized complexes exhibit
much higher TOF values by several orders of magnitude than the
metal source used in this work (>45 times). In fact, significant
enhancement in the intrinsic activity compared to metal source
is a strong indication that it is the catalytic behaviour of the immo-
bilized metal complexes which is responsible for the observed
enhancement. As no leaching was detected the catalytic activity
may be described to the immobilized complexes.
On comparing the catalytic data obtained herein for the oxi-
dation of cyclohexene using H2O2 with the reported data (%
conversion, catalyst wt., reaction time, oxidant); Cr-MCM-41 (52%,
20 mg, 24 h, O2) [15], Cr-MCM-48 (67%, 50 mg, 12 h, TBHP) [42]
as well as other chromium-based catalyst (∼70%, 600 mg, 6 h, O2)
[46], our catalysts show excellent conversion at a shorter reaction
time and required less amount of catalyst. While our catalysts show
very low activity under dioxygen atmosphere or using TBHP as an
oxidant.
3.17. 3D modelling optimized geometry
In order to estimate the geometry of the complex 3-
dimensional molecular modelling for (␮-acetato)(␮-hydroxo)-
bridged dichromium(III) complex using HyperChem Version 6.01
100
80
60
Conv
CHO
CHOL
CHONE
CHdiOL
40
20
0
In terms of product selectivities, 2-cyclohexen-1-ol was the only
product detected after the first hour of reaction and then decreased
with time to level off after 3 h. Meanwhile, selectivity towards
2-cyclohexen-1-one increased markedly to a maximum after 3 h
(∼80%). Cyclohexene oxide starts to form after 1 h and reaches the
maximum of ∼32% at 2 h and then demolish within the next hour.
1,2-Cyclohexandiol starts to form after ∼2 h and its concentration
increased with increasing reaction time. After 6 h the selectivity for
the products levels off. Cyclohexene oxide was obtained only in THF
at moderate-low percentage, and decreased with time, possibly due
to instability of the epoxide, which undergoes ring-opening reac-
tion (Fig. 9). Under similar conditions and extending the reaction
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (h)
Fig. 9. Effect of reaction time on % cyclohexene conversion and selectivity under O2
atmosphere using Cr-supported catalyst (LCI) under optimized conditions: cyclo-
hexene (0.025 mol), catalyst (10 mg), cyclohexene:oxidant (1:1 molar ratio), solvent
(THF; 2 mL) and temperature (80 ◦C).

S.J.J. Titinchi, H.S. Abbo / Catalysis Today 204 (2013) 114–124
123
Factors that influence the oxidation were also rigorously inves-
tigated and the optimum reaction conditions were: cyclohexene
(0.025 mol), catalyst (10 mg), cyclohexene:oxidant (1:1 molar
ratio), solvent (THF; 2 mL) and temperature (80 ◦C) under an oxygen
atmosphere.
Cyclohexene oxide was obtained only in THF (32%) after 2 h and
decreased with time due to epoxide ring-opening.
Furthermore, the silica-supported chromium could be recov-
ered and recycled by a simple filtration of the reaction mixture
and used for 3 consecutive trials without loss of activity. Recycled
catalysts exhibited similar activity which suggests the true hetero-
geneous nature of the catalyst.
Acknowledgments
The authors are grateful to the University of the Western Cape
for their financial support.
Fig. 10. Structural model of chromium complex immobilized on silica gel surface;
hydrogen atoms have been omitted for clarity. The black curve represents the silica
surface.
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