Selective oxidation of benzyl alcohol to benzaldehyde over Co-metalloporphyrin supported on silica nanoparticles

Article abstract of DOI:10.1016/j.apcata.2012.08.029

Heterogenation of the metalloporhyrin ligand, [tetrakis(o-chlorophenyl) porphyrinato]Co(II) was effected by immobilizing the complex onto inorganic silica support to form spherical nanoparticles of RHAC-CoPor. This insoluble mesoporous hybrid showed a hig

Full text of DOI:10.1016/j.apcata.2012.08.029

Applied Catalysis A: General 445–446 (2012) 252–260
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
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Selective oxidation of benzyl alcohol to benzaldehyde over Co-metalloporphyrin
supported on silica nanoparticles
∗
Farook Adam , Wan-Ting Ooi
School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 June 2012
Received in revised form 11 August 2012
Accepted 21 August 2012
Available online 31 August 2012
Heterogenation of the metalloporhyrin ligand, [tetrakis(o-chlorophenyl)porphyrinato]Co(II) was effected
by immobilizing the complex onto inorganic silica support to form spherical nanoparticles of RHAC-
2
−1
CoPor. This insoluble mesoporous hybrid showed a high specific surface area of 114 m g and a pore
3
−1
volume of 0.177 cm g . The prepared catalyst possessed a narrow pore size distribution centered at
1
3
around 4.17 nm. The C MAS NMR showed that RHAC-CoPor had three chemical shifts at 12.60, 26.60
and 45.18 ppm consistent with the three carbon atoms of the propyl group and a series of chemical shifts
in the range of ı = 108–167 ppm consistent with the presence of the metalloporphyrin complex. Elemental
analysis showed the successful immobilization of metalloporphyrin complex onto functionalized RHA
silica support. RHAC-CoPor appeared to be an active catalyst in the oxidation of benzyl alcohol, producing
Keywords:
Rice husk ash
Metalloporphyrin
Sol–gel technique
Nano particle
Oxidation
◦
97.1% conversion and 97.7% selectivity for benzaldehyde under an ambient temperature of 70 C. RHAC-
◦
CoPor could be regenerated and reused several times by washing with water, followed by drying at 100 C.
Benzyl alcohol
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
enhance the stability of the metalloporphyrin catalyst and to uti-
lize some of the remarkable properties of porphyrin, it is important
to immobilize them on suitable insoluble solid supports, which
include alumina, silica, zeolite, clays and resins.
Rice husk (RH) is a major agricultural biomass residue in nearly
all developing countries in the world including Malaysia [1]. The
rice milling industry is always facing problems of RH disposal.
In order to solve this problem, RH can be transformed into ash.
Rice husk on burning gives rice husk ash (RHA) containing high
silica content [2]. Many researchers have utilized this as a renew-
able silica source. This insoluble RHA silica can be used as a
support to functionalize various organic groups and by different
immobilization techniques in order to generate novel materi-
als with interesting new properties [3]. For instance, RHA silica
functionalized with organohalotrialkoxysilane can facilitate the
immobilization of a wide variety of organic ligands. The use of this
inorganic silica support plays a very important role in the prepara-
tion of hybrid materials [4–6].
Over the past few decades, metalloporphyrin have become well
known for their catalytic activity. It has been used in a range of
reactions including oxidation, epoxidation as well as hydroxyl-
ation [7–12]. Metalloporphyrins used as a catalyst in homogeneous
catalytic systems have a number of disadvantages such as decom-
position of the catalyst during reaction and difficulty of recovery
at the end of the reaction for reuse [13–15]. Hence, in order to
Oxidation of benzyl alcohol to benzaldehyde is of an important
chemical reaction as benzaldehyde is one of the most industrially
useful compounds. For instance, it is widely used as a precur-
sor for certain dyes and other organic compounds, ranging from
pharmaceuticals to plastic additives [16–18]. Besides, benzalde-
hyde is also used in perfumery, cosmetic as well as food industries
as it is commonly used to confer flavor and odor [19,20]. Litera-
ture review shows that gold and palladium supported materials
have been used for the oxidation of benzyl alcohol but with a
relatively low selectivity toward benzaldehyde [17,21–26]. Only
a few cobalt or cobalt supported catalyst have been reported for
the partial oxidation of benzyl alcohol. Xavier et al. [27] have
reported the liquid phase partial oxidation of benzyl alcohol by
zeolite-encapsulated cobalt(II) complex, resulting in a very low
percentage of conversion and extremely low selectivity toward
benzaldehyde. Similar results were obtained when using various
alkali metals added to impregnated cobalt with inorganic oxides
including NaY, SiO and NaUSY (ultra-stable Y) zeolites in gas phase
2
oxidation [28,29].
Herein we present the synthesis of organic–inorganic hybrid
material as an effective catalyst by immobilizing metalloporphyrin
complex onto the silica support in a simple co-condensation sys-
tem. Functionalization of RHA silica with 3-aminopropyltriethoxy-
silane was carried out by a fast sol–gel technique at room
∗ Corresponding author at: School of Chemical Sciences, Universiti Sains Malaysia,
Minden Heights, 11800 Penang, Malaysia. Tel.: +60 46533567; fax: +60 46574854.
E-mail addresses: farook@usm.my, farook dr@yahoo.com (F. Adam).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.08.029

F. Adam, W.-T. Ooi / Applied Catalysis A: General 445–446 (2012) 252–260
253
temperature and pressure. While the heterogenation of metallo-
porphyrin complex, [tetrakis(o-chlorophenyl)porphyrinato]Co(II)
onto RHA–APTES was carried out under reflux condition in toluene.
To the best of our knowledge, the oxidation of benzyl alcohol has
not been carried out using catalyst synthesized from immobilized
of tetrakis(o-chlorophenyl)porphyrin. The synthesized tetrakis(o-
chlorophenyl)porphyrin was refluxed with cobalt(II) chloride in
dimethylformamide for 1 h and stirred in an ice bath before pour-
ing cold distilled water into the mixture. An immediate precipitate
of the complex [tetrakis(o-chlorophenyl)porphyrinato]Co(II) was
formed and it was recrystallized in a 4:1 chloroform–methanol
mixture. The product yield was 75.8%.
[
tetrakis(o-chlorophenyl)porphyrinato]Co(II). The search for green
chemistry prefers the use of H O2 compared to other oxidants as it
2
is inexpensive and it will not produce any hazardous waste since
water is its only by-product formed during the reaction [20,30,31].
Therefore, in this work we report the catalytic activity of RHAC-
CoPor in liquid phase oxidation of benzyl alcohol using hydrogen
peroxide as a green oxidant to form benzaldehyde. All the prepara-
tion methods used are simple, cost-effective, less time-consuming
and eco-friendly.
2.5. Preparation of RHAC-CoPor catalyst
Preparation of the catalyst was carried out by a modified
technique reported elsewhere [35]. The ligand, [tetrakis(o-
chlorophenyl)porphyrinato]Co(II) (12.0 mmol) was added to a
suspension of RHAC-NH2 (3.0 g) in dry toluene (50 mL) and triethyl-
amine (1.67 mL, 12.0 mmol). The reaction mixture was allowed to
◦
reflux at 110 C in an oil bath for 24 h. Then, the solid phase was fil-
2
. Experimental procedures
tered and washed with toluene, DCM, and distilled water. The solid
sample was then dried in an oven. Finally, it was ground to a fine
powder and the sample was labeled as RHAC-CoPor.
2.1. Materials
The RH was obtained from a local rice mill in Penang. Nitric
2
.6. Characterization of RHAC-CoPor
acid (65%) and sodium hydroxide (99%) were obtained from QReC.
Propionic acid was purchased from HmbG Chemicals. Pyrrole and
dichloromethane (DCM) (99%) were purchased from Merck. 3-
Aminopropyltriethoxy silane (APTES) (98%) was purchased from
Sigma–Aldrich. Toluene and acetonitrile were purchased from
J.T. Baker (99.8%). Benzyl alcohol was obtained from Unilab. All
chemicals are of AR grade and used as obtained without further
purification.
RHAC-CoPor was characterized by several spectroscopic and
physical methods. These includes FTIR spectroscopy (Perkin Elmer
System 2000), N2 adsorption–desorption analysis (Micromeritics
Instrument Corporation model ASAP 2000, Norcross), solid state
29
13
Si and C nuclear magnetic resonance, elemental analysis (CHN,
EDX) transmission electron microscopy (TEM, Phillips CM12), scan-
ning electron microscopy (SEM) and image analyzer.
2.2. Sources of silica
2.7. Catalytic activity
Rice husk ash (RHA) was chosen as the source of amorphous
silica from which the silica was extracted according to a previously
reported method [32].
The catalytic liquid phase oxidation of benzyl alcohol was car-
ried out in a 50 mL round-bottom flask that was equipped with a
reflux condenser. In a typical run, benzyl alcohol (1.08 g, 10 mmol)
and an amount of catalyst (0.08 g) was placed into the flask con-
2
.3. Functionalization of RHA with APTES
taining 10 mL of acetonitrile and the reaction mixture was heated
◦
in an oil bath with continuous stirring at 70 C. H O (15 mmol,
2
2
RHA silica was functionalized with APTES via a sol–gel reaction
3
0%) was then added drop wise slowly into the reaction flask
according to the method reported elsewhere [33] with some mod-
ification. About 3.0 g of RHA was stirred in 300 mL of 1.0 M NaOH
at room temperature overnight. The sodium silicate formed was
filtered to remove undissolved particles. APTES (6.0 mL, 0.026 mol)
was then added to this sodium silicate solution and the solution
was titrated slowly with 3.0 M nitric acid until pH 3 with constant
stirring. A white gel started to form when the pH decreased to
less than 10. The gel formed was aged for 24 h at room temper-
ature. The gel was separated by centrifuge at 4000 rpm for 15 min
within 5 min and the reaction was allowed to continue for 5 h.
During the reaction, 0.5 mL of the sample was withdrawn periodi-
cally from the mixture and filtered to remove the catalyst so as to
determine the percentage conversion by analyzing it with gas chro-
matography (Perkin Elmer Clarus 500) equipped with Elite Wax
(
30 m × 0.2 mm ID) using 20 ␮L of cyclohexanone as an internal
standard. The products formed were further confirmed by GC–MS
Perkin Elmer, Clarus 600) analysis. After the reaction, the catalyst
(
was filtered and washed thoroughly with water in order to sepa-
rate it from the reaction mixture. The separated catalyst was dried,
and then reused again for three times to test the reusability of the
catalyst.
(
Rotina 38, Hittich Zentrifugn) and washing 5 times with distilled
water. Final washing was done with hot water. The sample was
then dried in an oven overnight. Finally, it was ground to pro-
duce a fine powder. A 3.45 g of RHAC-NH2 was collected from
this method and it was then used as the support to anchor the
3. Results and discussion
[
tetrakis(o-chlorophenyl)porphyrinato] cobalt(II) complex.
3.1. Characterization of porphyrin and metalloporphyrin complex
2
.4. Synthesis of [tetrakis(o-chlorophenyl)porphyrinato]
cobalt(II)
The 1H NMR spectrum of the ligand, tetrakis(o-
chlorophenyl)porphyrin (Fig. 1a) displayed
a singlet highly
Tetrakis(o-chlorophenyl)porphyrin was synthesized first by
following the reported Alder–Longo method [34] with some mod-
ification. 2-Chlorobenzaldehyde (20.3 mL, 0.18 mol) and pyrrole
12.5 mL, 0.18 mol) were added simultaneously to the reflux-
ing propionic acid (300 mL). The mixture was then refluxed for
0 min and was allowed to cool to room temperature. The pre-
cipitate formed was filtered and washed with an appropriate
amount of cold methanol and hot water to give a violet solid
shielded peak at around −2.60 ppm. This was attributed to the
rapidly exchanging N H proton at the center of the porphyrin
ligand in the core of the macrocycle. Whereas, the aromatic
protons in the porphyrin ring resonated at 8.22 ppm, 7.88 ppm,
7.77 ppm and 7.68 ppm. Multiplet at ı = 8.22 ppm and 7.68 ppm
were assigned to the proton at the meso-position of the phenyl
rings. The proton of the m-phenyl which is located just beside
the substituted electronegative atom, chlorine was deshielded
(
3

2
54
F. Adam, W.-T. Ooi / Applied Catalysis A: General 445–446 (2012) 252–260
Fig. 1. The 1H NMR spectra of (a) tetrakis(o-chlorophenyl)porphyrin and (b) [tetrakis(o-chlorophenyl)porphyrinato]Co(II).
downfield compared to the one located further away from the
substituted chloro group. The multiplicity of the peak is due to the
proton coupling and long range coupling between neighboring pro-
tons. While a triplet at ı = 7.77 ppm and a doublet at ı = 7.88 ppm
was assigned to protons of the phenyl rings at the para- and
ortho-position, respectively. The ˇ-protons on the periphery of
the macrocycle were strongly deshielded by the diamagnetic ring
current. Hence the singlet peak resonated at ı = 8.75 ppm was
attributed to the eight proton of ˇ-pyrrole in the basic skeleton of
porphyrins. The disappearance of the inner NH peak at high field
for [tetrakis(o-chlorophenyl)porphyrinato]Co(II) can be observed
in Fig. 1b. This is good evidence that the metallation of porphyrin
ligand was successful as the cobalt had been incorporated in the
core of the macrocycle. The metallation caused shielding of the
ˇ-pyrrole which appeared at ı = 8.21 ppm and the deshielding of
all protons in the substituted phenyl ring shifted them to a more
downfield resonance.
Fig. 2 shows the IR spectra of tetrakis(o-chlorophenyl)porphyrin
and [tetrakis(o-chlorophenyl)porphyrinato]Co(II). From the spec-
tra, tetrakis(o-chlorophenyl)porphyrin showed the presence of
−
1
N
H stretching vibration of pyrrole at 3404 cm as well as ␯N
H
(in planarity) and ␯N H (out of planarity) transmission bands of
−
1
−1
porphyrin at 968 cm and 800 cm , respectively. The N H peak
disappeared after the metallation of porphyrin. This is because the
secondary amines (
cobalt ion as shown in the respective structures in Fig. 1. The N
stretching vibration is thus not observed in the FTIR spectrum of the
N H) lost the H atom when coordinated to the
H
−
1
cobalt coordinated complex. The transmission bands at 3325 cm
,
−
1
2
3061 cm
are due to the C H (sp ) stretching of macrocyclic
and the C H (sp ) stretching of the phenyl groups, respectively.
Another two bands at 1636 cm and 1469 cm were due to the
C stretching of the phenyl groups which showed the presence of
an aromatic ring. For [tetrakis(o-chlorophenyl)porphyrinato]Co(II),
2
−
1
−1
C
−
1
−1
these two bands appeared at 1628 cm and 1443 cm . The C
N

F. Adam, W.-T. Ooi / Applied Catalysis A: General 445–446 (2012) 252–260
255
peak at 1098 cm 1 was assigned to the asymmetric vibration of
the siloxane bond, Si Si associated with the condensed silica
network in the silica. C H stretching vibration can be observed
−
O
−
1
−1
in both RHAC-NH2 and RHAC-CoPor at 2981 cm and 2962 cm ,
−
1
respectively. Another strong band at 1378 cm which can be only
observed in both RHAC-NH2 and RHAC-CoPor was assigned to
the C N stretching of the primary amides. However, the intensity
of this band decreased once the metalloporphyrin complex was
immobilized onto the silica support. Apart from that, RHAC-CoPor
−
1
−1
−1
showed additional peaks at 1638 cm , 1486 cm and 1352 cm
which were ascribed to the C C and C N bonds of benzene and
−
1
−1
porphyrin ring as well as peaks at 793 cm and 702 cm which
corresponds to the skeletal vibration of porphyrin ring. These differ-
ences proved that the metalloporphyrin complex was successfully
incorporated onto the silica support, RHAC-NH . Since there is no
2
difference in FT-IR spectra of RHAC-CoPor before and after the reac-
tion, hence it was possible to conclude that the structure of catalyst
did not collapse after the catalytic reaction.
Fig. 2. The FT-IR spectra of (a) tetrakis(o-chlorophenyl)porphyrin and (b)
tetrakis(o-chlorophenyl)porphyrinato]Co(II).
[
Stronger evidence for the successful incorporation of tetrakis(o-
29
chlorophenyl)porphyrinato Cobalt(II) on silica comes from Si
−
1
13
29
13
stretching vibration of the porphyrin ring is seen at 1205 cm
(
and
C
MAS NMR studies. Both
Si and
C MAS NMR
−1
Fig. 2a) and 1243 cm (Fig. 2b).
.2. Characterization of catalyst
The IR spectra of RHA, RHAC-NH , and RHAC-CoPor (before and
spectra of RHAC-CoPor are shown in Fig. 4. The solid state
29
Si NMR spectrum of RHAC-NH2 (Fig. 4a) shows the pres-
ence of Q4 (Si(OSi)4) and Q3 (Si(OSi)3OH) silicon centers at
ı = −110.15 ppm and −101.28 ppm, respectively. These values
had shifted to ı = −99.23 ppm and −88.60 ppm in RHAC-CoPor
(Fig. 4b). A chemical shift at ı = −66.93 ppm for RHAC-NH2 indi-
3
2
after reaction) are shown in Fig. 3. Typically, a broad band with
high intensity in the range of 3500–3400 cm–1 corresponded to the
cates the formation of Si
i.e. (SiO2)(
O
Si linkages via three siloxane bonds,
3
O
H vibration of SiO H groups and HO H of adsorbed water [36].
O
)3Si CH2CH2CH2-Por (T ) and a chemical shift at
The bending vibration of the trapped water molecules in the silica
matrix was detected as an intense peak at 1648 cm . The vibration
ı = −59.04 ppm was attributed to the formation of two siloxane
−
1
2
linkages, i.e. (SiO2)( O )2Si(OH)CH2CH2CH2-Por (T ), to the silica.
2
3
While for RHAC-CoPor, T and T peaks appeared at ı = −48.71 ppm
29
and −57.18 ppm, respectively. The slight shift in the Si NMR
spectrum of RHAC-CoPor with respect to RHAC-NH2 proves the
formation of the new C N bond that immobilized the [tetrakis(o-
chlorophenyl)porphyrinato]Co(II) onto the silica.
While in the analysis of the 13C MAS NMR spectrum, the incorpo-
rated propyl group (C , C2 and C ) in RHAC-NH2 (Fig. 5a) appeared
1
3
at ı = 12.57 ppm, 24.17 ppm and 44.71 ppm, respectively. These
values were shifted to ı = 12.60, 26.60 and 45.18 ppm. Similar
shifts were observed by other researchers [2,37]. Several unre-
solved peaks at chemical shifts in the range of ı = 100–168 ppm
were ascribed to the carbons of the metalloporphyrin complex
as shown in Fig. 5b. Hence, it can be concluded that [tetrakis(o-
chlorophenyl)porphyrinato]Co(II) was successfully immobilized
onto the silica via the formation of new C N bond. To the best of our
knowledge, the 13C MAS NMR study has not been reported in the
literature before with respect to the supported metalloporphyrin
complexes.
The N2 adsorption–desorption isotherm of RHAC-NH2 and
RHAC-CoPor are shown in Fig. 6. Both RHAC-NH2 and RHAC-
CoPor showed type IV isotherm and H2 hysteresis loop according
to IUPAC classification, indicating the existence of mesopores.
The H2 hysteresis loop was observed at P/P = 0.3–0.9 and it is
0
a characteristic of mesoporous material consisting of agglomer-
ates or particles having a narrow neck and wide body pore such
as an “ink bottle” [38,39]. The BET analysis (Table 1) showed
that the specific surface area of RHAC-NH and RHAC-CoPor was
2
2
−1 −1
2
2
53 m g and 114 m g , respectively. The decrease in the sur-
face area of RHAC-CoPor could be due to the immobilization of
the porphyrin ligand network of the metalloporphyrin complex,
[
tetrakis(o-chlorophenyl)porphyrinato]Co(II) which resulted in the
reduction of surface area. RHAC-CoPor showed slight reduction in
pore size compared to RHAC-NH2 which may also be due to the
‘blockage’ by the metalloporphyrin complex moiety.
Fig. 3. The FT-IR spectra of (1) RHA, (2) RHAC-NH2, (3) RHAC-CoPor before the
reaction and (4) RHAC-CoPor after the catalytic reaction.

2
56
F. Adam, W.-T. Ooi / Applied Catalysis A: General 445–446 (2012) 252–260
Fig. 4. 29Si MAS NMR of (a) RHAC-NH2 and (b) RHAC-CoPor.
Table 1
The result of BET analysis for RHACNH2 and RHAC-CoPor.
the percentage of 5.61% and 6.60%, respectively. RHAC-CoPor also
showed the presence of cobalt with 0.52% in EDX analysis while
AAS analysis showed a cobalt content of 0.79%. Besides, there was
no chlorine atom present in RHAC-CoPor which showed that all
the chlorine had been eliminated after the immobilization. Once
again, this shows that the metalloporphyrin complex was success-
fully incorporated on the RHA silica support. The percentage of C
and H in RHAC-CoPor was significantly higher than that in RHA and
RHAC-NH2 as expected.
Sample
Specific surface
Average pore
volume (cm3
Average pore
diameter (nm)
area (m2
g
−1
)
g
−1
)
RHAC-NH2
RHAC-CoPor
253
114
0.343
0.177
5.92
4.17
The TEM micrographs of RHAC-NH and RHAC-CoPor are shown
2
in Fig. 7. From the micrographs, it can be seen that RHAC-NH2
appear as spherical nano particles with a diameter range of about
X-ray diffraction measurement was performed to investigate
the influence of the metalloporphyrin complex on the structure of
2
0–40 nm. Immobilization of the metalloporphyrin complex onto
RHAC-NH . Fig. 8 shows the XRD patterns of RHAC-NH and RHAC-
2
2
RHA silica caused the agglomeration of the particles and expan-
sion of the particle size to 35–75 nm. SEM analysis of RHAC-CoPor
showed the existence of porous surface which had resulted in a
high surface area. The elemental composition of RHAC-CoPor was
determined by CHN and EDX analysis as represented in Table 2,
which further confirmed the presence of carbon, oxygen, silicon,
nitrogen and cobalt. The EDX analysis showed that nitrogen was
only present in RHAC-NH2 and RHAC-CoPor as expected with
CoPor. It can be seen that RHAC-NH2 showed a broad diffraction
◦
peak at 2ꢀ angle 22.5 . While a slight shift of the peak to 2ꢀ angle
◦
2
3.3 can be seen in the X-ray diffraction pattern of RHAC-CoPor.
The broad diffraction bands show the amorphous nature of the sam-
ples. The immobilization of tetrakis(o-chlorophenyl)porphyrinato
cobalt(II) did not change the basic amorphous structure of silica as
in RHA silica [40].
Table 2
Percentages of C, H and N obtained by elemental analysis, EDX and AAS analysis for RHA, RHAC-NH2 and RHAC-CoPor.
Sample
Elemental analysis (%)
C
H
N
Si
Co
Cl
RHA
RHAC-NH2
RHAC-CoPor
(0.27)a
(1.10)a
–
36.66
24.66
30.06
–
–
–
–
–
a
a
a
a
13.15 (6.17)
(2.44)
5.61 (2.36)
6.60 (5.07)
17.88 (17.34)a
(2.68)a
0.52 [0.79] [0.67]c
b
a
Values in bracket were obtained by CHN analysis.
Cobalt content obtained by AAS analysis for fresh catalyst.
Cobalt content obtained by AAS analysis after 3rd reused analysis.
b
c

F. Adam, W.-T. Ooi / Applied Catalysis A: General 445–446 (2012) 252–260
257
Fig. 5. 13C MAS NMR of (a) RHAC-NH2 and (b) RHAC-CoPor.
3
.3. The oxidation benzyl alcohol
of products were studied (Tables 3–7). The major prod-
uct detected from GC and GC–MS was benzaldehyde while
benzoic acid and benzyl benzoate were detected as by-
products.
The effectiveness of the prepared organic-inorganic hybrid
material (RHAC-CoPor) as a catalyst was tested in the liquid
phase oxidation of benzyl alcohol. The influence of vari-
ous parameters on the percentage conversion and selectivity
3.3.1. Influence of reaction temperature
The effect of reaction temperature on the conversion of
benzyl alcohol and the selectivity of the products was demon-
strated by considering 6 different temperatures in the range
of 303–373 K. Conversion of benzyl alcohol increased from
4
4.9% to 91.9% (343 K) as the temperature increased as shown
in Table 3. There was no remarkable difference in the
percentage conversion when the temperature was increased
up to 373 K. Therefore, 343 K was taken to be an opti-
mum temperature in this study to achieve highest conver-
sion.
Selectivity toward benzaldehyde decreased slightly as the tem-
perature increased. This observation showed that the temperature
has only a very small impact on the selectivity of the product
but it has a strong influence on the progress of benzyl alcohol
oxidation. In previous studies, lower conversion was obtained
when the reaction was carried out in the temperature range
3
43–363 K. In order to get a high conversion (>95%), high tem-
perature (>500 K) was used [18–20,41]. However, in this study,
98% conversion was achieved at 343 K over a short contact time of
5 h.
∼
Fig. 6. The N2 adsorption–desorption isotherms and narrow pore size distribution
graphs (inset) of (a) RHAC-NH2 and (b) RHAC-CoPor.

2
58
F. Adam, W.-T. Ooi / Applied Catalysis A: General 445–446 (2012) 252–260
Fig. 7. TEM images of (a) RHAC-NH2 and (b) RHAC-CoPor catalyst showing the distribution of particle size.
Table 4
Influence of amount of catalyst used on the oxidation of benzyl alcohol.
Catalyst mass
Benzyl alcohol
conversion (%)
Selectivity (%)
TOFa (h−1)
(
mg)
Benzaldehyde Benzoic acid
3
0
84.6
91.9
97.1
96.2
94.1
96.5
96.3
97.7
96.4
93.1
3.5
3.7
2.2
3.6
6.9
406.0
264.1
176.9
138.4
87.1
50
80
1
1
00
50
Reaction condition: 343 K, molar ratio of substrate:H2O2 = 1:1.5, 10 mL of CH3CN
solvent, 5 h reaction time.
a
Turn over frequency (TOF): moles of substrate converted per mol of Co in the
catalyst/h.
ca. 97% compared to the works reported in literature. For instance,
Cristina et al. [43] reported the usage of large amount of bimetallic
gold–copper catalysts (about 0.2 g) in order to get similarly high
conversion (98%) and selectivity (>99%) toward benzaldehyde.
Fig. 8. The X-ray diffraction pattern for (a) RHAC-NH2 and (b) RHAC-CoPor.
3.4. Effect of mass of catalyst
3
.4.1. The effect of mole ratio of reactant (benzyl alcohol:H O )
2 2
Concentration of H O used in the reaction is another vital factor
influencing the progress of benzyl alcohol oxidation. Thus, several
2
2
Table 4 shows the changes in the conversion of benzyl alco-
hol by varying the mass of catalyst from 30 mg to 150 mg at 343 K.
The results indicate that the conversion of benzyl alcohol increased
with an increasing amount of catalyst until the maximum conver-
sion of 97.1% was achieved when 80 mg of catalyst was used. Lower
conversion of benzyl alcohol with lower mass of catalyst was due to
the fewer catalytic sites. Whereas, increasing catalyst mass beyond
benzyl alcohol:H O2 molar ratios were investigated in this study
2
and the results are shown in Table 5. With 1:0.5 molar ratio of ben-
zyl alcohol to H O , low conversion of benzyl alcohol was observed
2
2
(
85.6%) as compared to the percentage conversion obtained at 1:1.5.
The conversion was found to increase to 97.1% with a TOF value of
−
1
1
76.9 h when the molar ratio was increased to 1:1.5. Since oxy-
8
0 mg, decreased the selectivity of benzaldehyde. This is due to
gen produced on decomposition of H O2 oxidizes benzyl alcohol,
2
the excess catalyst resulted in further oxidation of benzaldehyde
to benzoic acid [42]. This study showed that only 80 mg of the cata-
lyst was needed for the reaction to reach an optimum conversion of
hence the conversion of benzyl alcohol was increased progressively
when the molar ratio of benzyl alcohol to H O was increased due to
2
2
increased amount of available oxygen [41]. The selectivity of prod-
ucts did not show significant variation as the molar ratio of benzyl
Table 3
alcohol to H O2 was varied. Therefore, 1:1.5 molar ratio of benzyl
2
Effect of temperature on the oxidation of benzyl alcohol using RHAC-CoPor as the
catalyst.
Table 5
Temperature (K) Benzyl alcohol
conversion (%)
Selectivity (%)
TOFa (h−1)
The effect of different benzyl alcohol:H2O2 molar ratio on the oxidation of benzyl
alcohol using RHAC-CoPor as the catalyst.
Benzaldehyde Benzoic acid
TOFa (h−1)
Benzyl alcohol:H2O2 Benzyl alcohol Selectivity (%)
molar ratio
conversion (%)
3
3
3
3
3
3
03
23
33
43
53
73
44.9
70.8
82.8
91.9
91.3
90.8
98.8
98.1
97.1
96.3
96.1
94.2
1.2
1.9
2.9
3.7
3.9
5.8
132.4
207.3
239.9
264.1
261.8
255.2
Benzaldehyde Benzoic acid
1:0.5
1:1
1:1.5
85.6
92.2
97.1
95.8
96.5
97.1
97.7
94.4
3.5
2.9
2.3
5.6
154.1
167.0
176.9
168.7
1:2
Reaction condition: 50 mg RHAC-CoPor, molar ratio of substrate:H2O2 = 1:1.5, 10 mL
CH3CN solvent, 5 h.
Reaction condition: 343 K, 80 mg RHAC-CoPor, 10 mL CH3CN solvent, 5 h.
a
a
Turn over frequency (TOF): moles of substrate converted per mol of Co in the
Turn over frequency (TOF): moles of substrate converted per mol of Co in the
−
1
−1
catalyst h
.
catalyst h
.

F. Adam, W.-T. Ooi / Applied Catalysis A: General 445–446 (2012) 252–260
259
Table 6
Influence of the types of solvents used on the oxidation of benzyl alcohol using RHAC-CoPor as the catalyst.
Types of solvent used
Benzyl alcohol conversion (%)
Selectivity (%)
Benzaldehyde
TOFa (h−1)
Benzoic acid
Benzyl benzoate
Solventless
92.7
17.2
49.2
97.1
38.9
99
99
34.7
1
1
18.1
67.3
31.8
90.8
Cyclohexane
Dichloromethane
Acetonitrile
–
–
–
97.7
2.3
176.9
Reaction condition: 343 K, 80 mg RHAC-CoPor, molar ratio of substrate:H2O2 = 1:1.5, 5 h.
a
−1
.
Turn over frequency (TOF): moles of substrate converted per mol of Co in the catalyst h
Table 7
catalyst, from 48.8% (1 h) to 49.9% (3 h). However, there was no fur-
ther increase in conversion at 5 h. Since significant increase in the
conversion of benzyl alcohol was not observed, hence it can be said
that leaching of cobalt metal during the reaction can be considered
as negligible. RHAC-CoPor was subjected for AAS analysis after the
reaction to further confirm the heterogeneity of the catalyst. It was
found that the cobalt content after the 3rd cycle of oxidation reac-
tion was 0.67% compared to the fresh catalyst which is 0.79% as
shown in Table 2. It can thus be concluded that the catalytic oxida-
tion of benzyl alcohol by RHAC-CoPor proceeds via a heterogeneous
process.
The result of leaching test on RHAC-CoPor during its use as the catalyst in the
oxidation of benzyl alcohol.
Time (h)
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzoic acid
1
48.8
49.9
50.1
99.9
99.2
98.9
0.1
0.8
1.1
a
3
a
5
Reaction condition: 343 K, 80 mg RHAC-CoPor, molar ratio of substrate:H2O2 = 1:1.5,
acetonitrile (10 mL), 5 h reaction time.
a
After removing the catalyst.
In order to further evaluate the performance of the catalyst,
RHAC-CoPor was used repeatedly for 3 cycles under the same con-
ditions to test its catalytic reusability as illustrated in Table 8.
The used catalyst was filtered and regenerated by washing it with
alcohol to H O was the optimum in terms of percentage conver-
2
2
sion. In the past, Yu and co-workers [16] were able to obtain a very
high conversion (100%) and high selectivity (100%) toward benz-
aldehyde. However, the reaction was carried out at higher ratio of
◦
distilled water and drying at 110 C for 24 h before using it in
the subsequent runs. From Table 8, it can be seen that only 3.8%
decrease in conversion was observed after three cycles. This shows
that the activity of catalyst was not affected even after three cycles.
Therefore, it can be concluded that the prepared catalyst can be eas-
ily recovered and recycled which is potentially useful for industrial
application.
H O to alcohol (4.5:1) with the help of a large amount of catalyst
2
2
(
0.2 g) over a longer reaction time (6 h) compared with the optimum
conditions in this study.
3.4.2. Effect of solvents on oxidation of benzyl alcohol
In order to determine the effects of solvents on the oxidation
of benzyl alcohol, the reaction was carried out separately by using
three different types of solvents (neat – no solvent, cyclohexane,
dichloromethane and acetonitrile) keeping all other parameters
4
. Conclusions
All the spectroscopic evidence obtained in this study showed
◦
fixed: namely mass of catalyst (80 mg), temperature (70 C), reac-
that an organic-inorganic hybrid material labeled as RHAC-CoPor
was successfully synthesized by immobilizing the metallopor-
phyrin complex onto the silica support prepared from RHA.
The procedure used in this study was simple, time-saving and
environmental friendly. RHAC-CoPor showed excellent catalytic
performance in the oxidation of benzyl alcohol with good conver-
sion of almost 97% and very high benzaldehyde selectivity by using
H O as an oxidant. In addition, the catalyst can be reused several
tion time (5 h) and benzyl alcohol to H O2 molar ratio (1:1.5). The
2
results are summarized in Table 6. It shows that a significantly
lower conversion and TOF values were obtained in non-polar sol-
vent, i.e. cyclohexane and a maximum conversion was obtained in
−1
acetonitrile (TOF = 176.9 h ). In addition, the selectivity of benz-
aldehyde decreased with increasing polarity of the solvent which
indicates that the polar solvent may favor the formation of acid
since benzoic acid is a more polar molecule compared with benz-
aldehyde [44]. Whereas the selectivity of benzaldehyde was quite
low and an additional by-product of benzyl benzoate was formed
when the reaction was run without using any solvent.
2
2
times without significant loss in the conversion and selectivity.
Acknowledgements
The authors would like to express their thanks to the Ministry
of Education, Malaysia for the RU grant (1001/PKIMIA/811092) and
USM for a fellowship to WT Ooi.
3
.4.3. Leaching and reusability of RHAC-CoPor
Table7 presentsthe data obtainedfor the leachingtest for RHAC-
CoPor. During the catalytic activity, the catalyst was removed after
h and the reaction was allowed to continue. A slight increase
1
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