Novel protonated Fe-containing mesoporous silica nanoparticle catalyst: Excellent performance cyclohexane oxidation

Article abstract of DOI:10.1039/c7ra02280h

A mesoporous silica structure (MSN) was synthesized using the sol-gel method, followed by iron incorporation and protonation which afforded Fe-MSN and H/Fe-MSN with Si/Fe ratios of 20. Nitrogen physisorption confirmed their mesoporous structures with pore

Full text of DOI:10.1039/c7ra02280h

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Novel protonated Fe-containing mesoporous silica
nanoparticle catalyst: excellent performance
cyclohexane oxidation†
Cite this: RSC Adv., 2017, 7, 27506
Mohammad Reza Sazegar, * Aysan Dadvand and Ali Mahmoudi
A mesoporous silica structure (MSN) was synthesized using the sol–gel method, followed by iron
incorporation and protonation which afforded Fe-MSN and H/Fe-MSN with Si/Fe ratios of 20. Nitrogen
physisorption confirmed their mesoporous structures with pore diameters of 3.9 nm. Iron incorporation
decreased the degree of catalyst crystallinity. Fe-MSN and H/Fe-MSN catalyzed the oxidation of
cyclohexane to cyclohexanone using hydrogen peroxide at 298 K in 1 h. The significant advantages of
these catalysts were high conversion, short reaction time, easy work-up, and compatibility with various
organic and aqueous solvents. The cyclohexanone product was obtained with excellent conversions of
Received 23rd February 2017
Accepted 7th May 2017
8
6 and 97% at 298 K using Fe-MSN and H/Fe-MSN, respectively. H/Fe-MSN catalyst gave a higher
conversion than Fe-MSN. A comparative study of cyclohexane oxidation in the presence of H over
H/Fe-MSN heterogeneous catalyst showed that H/Fe-MSN had excellent activity at low temperature.
DOI: 10.1039/c7ra02280h
2 2
O
rsc.li/rsc-advances
In this study, Fe-graed MSN (denoted Fe-MSN) and
protonated iron-containing MSN (denoted H/Fe-MSN) were
Introduction
Mesoporous silica nanoparticles (MSN) have been widely used synthesized using the sol–gel method, and their activities and
in various scientic elds, including medicinal chemistry, magnetic properties were investigated in cyclohexane oxida-
environmental science, and petrochemical industries, and in tion. Reaction conditions for cyclohexane oxidation were
chemical reactions for their special properties, such as large optimized for a high oxidation process. The physical proper-
surface area, and high and homogenous pore size and ties of the catalysts were conrmed by nitrogen physisorption,
1
–3
volume.
have limited catalytic activities, metal-loaded MSN have been conrmed by
Although, pure mesoporous silica nanomaterials XRD, FTIR, XRF, ICP, XPS, and VSM. The products were
1
13
H
and
C NMR, and melting point
4,5
2
reported as active catalysts. Recently, the incorporation of Al , measurements.
6
4
7
Ti , Ni , and Mn into MCM-41 was reported to catalyze various
cracking, methanation, and oxidation reactions. Mesoporous
silica materials with magnetic properties have been prepared
Experimental
8–10
Synthesis of Fe-MSN catalyst
using Fe, Co, Ba, and Ni
and applied as magnetic adsorbents
to remove organic and inorganic pollutants.
Fe-MSN and H/Fe-MSN were synthesized from MSN using post-
Modied mesoporous silica nanomaterials are capable of synthesis methods, followed by protonation for H/Fe-MSN.
catalyzing the oxidation of organic compounds. Fe-containing Cetyltrimethylammonium bromide (CTAB, 7.44 g) was dis-
MCM-41 (denoted as Fe-MCM-41) has shown great potential solved in a solution containing water (170 mL), ethanol (40 mL),
as a catalyst in many reactions and as a sensor for molecular and aqueous sodium hydroxide solution (20 mL, 10%). Aer
8
identication. Fe-MCM-41 has shown good catalytic activity in stirring vigorously for approximately 30 min at 298 K, tetraethyl
11,12
oxidation reactions.
Continuing efforts are being made to orthosilicate (TEOS, 17.68 mL) was added to the mixture. The
improve or synthesize new catalysts to oxidize cyclohexane resulting mixture was stirred for an additional 2 h at 298 K and
1
2,13
under mild conditions.
Ti-MCM-41 has been used to catalyze allowed to rest for 20 h at the same temperature. The sample
6
the epoxidation of cyclohexene with H O , while Mn-MCM-41 was collected by centrifugation at 20 000 rpm for 30 min and
2
2
has been prepared and shown to have high selectivity for stil- washed with deionized water and absolute ethanol three times.
7
bene epoxidation using tert-butyl peroxide or TBHP.
Surfactant was removed by washing the MSN (1 g) with a solu-
tion of NH NO (0.3 g) in ethanol (40 mL) at 298 K. The
surfactant-free product was collected by centrifugation, dried at
83 K for 20 h, and calcined in air at 823 K for 3 h. The sample
4
3
Dept. of Chemistry, Fac. of Science, North Tehran Branch, Islamic Azad University,
Tehran, Iran. E-mail: m_r_sazegar@yahoo.com; Tel: +98-2122262564
3
was prepared by graing iron onto the pure MSN, followed by
centrifugation, drying at 383 K for 20 h, and calcination in air at
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra02280h
1
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8
23 K for 3 h. Ferric nitrate (Fe(NO
precursor. Iron loading occurred via an impregnation method
Si/Fe molar ratio, 20 : 1). Fe-graed MSN was denoted as Fe-
3 3 2
) $9H O) was used as an iron Catalytic activity
The catalytic activity of the H/Fe-MSN catalyst was investigated
in the cyclohexane oxidation reaction. Details of this study are
provided in the following section.
(
MSN.
Synthesis of H/Fe-MSN catalyst
Typical experimental protocol
Protonated Fe-MSN (H/Fe-MSN) was prepared by shaking To a stirring mixture of cyclohexane (0.02 mol) was added
Fe-MSN (1 g) in an aqueous solution of NH Cl (2.5 g in 50 hydrogen peroxide (0.04 mol) and H/Fe-MSN (0.01 wt%) at room
4
mL water) at 298 K for 16 h, followed by removal of the solu- temperature. Stirring was continued for 30 min and the mixture
tion, drying at 383 K for 20 h, and calcination at 823 K for 3 h was heated at 323 K under an air atmosphere for 3 h. The
in air.
sample was taken at regular intervals and the concentrations of
cyclohexane and cyclohexanone were identied by gas
chromatography-mass spectrometry (GCMS, HP 5971 detector)
and analyzed using a chrompak CP 9000 GC equipped with an
Catalyst characterization
Catalyst crystallinity was measured using a Bruker Advance D8 RTX-50 capillary column and the same ionization detector.
˚
X-ray powder diffractometer with Cu Ka radiation (l ¼ 1.5418 A) Dodecane was chosen as the internal standard.
as the diffracted monochromatic beam, operating at 40 kV and
The conversion of cyclohexane to cyclohexanone
40 mA. The bulk Si/Fe ratios of the catalysts were determined (Xoxidation%), yield of cyclohexanone (Yoxidation%), and selectivity
using X-ray uorescence spectroscopy (XRF, Bruker S4 Explorer) for cyclohexanone (Soxidation%) were calculated according to eqn
using Rh as the anode target material and operated at 20 mA (1)–(3), respectively:
14
and 50 kV.
½
KETꢀ ꢁ ½KETꢀ
i
f
Nitrogen physisorption analysis was conducted on a Quan-
tachrome Autosorb-1 at 77 K. Before measurement, the sample
was evacuated at 573 K for 3 h. Scanning electron microscopy
with energy dispersion X-ray spectrometry (SEM-EDX) was
conducted on a JEOL JSM-6701 F to observe the morphology and
perform elemental analysis. Before observation by SEM, the
sample was coated with Pt using a sputtering instrument. The
morphology and average particle size of the catalyst were esti-
mated from transmission electron microscopy (TEM) images
captured using a JEOL JEM-2100 transmission electron micro-
scope. Prior to TEM measurements, the powder samples were
ground and subjected to ultrasonic treatment in hexane for
X
oxidation ð%Þ ¼
ꢂ 100
(1)
(2)
(3)
½
KETꢀ
f
ny
Y
oxidation ð%Þ ¼ nc ꢂ 100
n
y
S
oxidation ð%Þ ¼ n
ꢂ 100
f
i
ꢁ n
where [KET]
i
f
and [KET] are the initial and nal cyclohexane
concentrations, n and n are the yielded and calculated moles
of cyclohexanone, and n and n are the initial and nal moles of
y
c
i
f
cyclohexane, respectively.
1
0 min. A drop of the suspension was dried on a copper TEM
Reusability testing
3
sample grid.
A catalyst reusability test was carried out for H/Fe-MSN in
Fourier transform infrared (FTIR) measurements were
carried out using an Agilent Carry 640 FTIR Spectrometer. The
catalyst was prepared as a self-supported wafer and activated
under vacuum at 673 K for 3 h. UV–vis spectra were obtained
on a Thermo Spectronic spectrophotometer model Genesys
18
cyclohexanone production (%) ve times at 298 K. The reac-
tivity was determined by washing with hot dichloromethane (2
ꢂ 10 mL) and drying under vacuum at 343 K for 4 h. The catalyst
weight, catalytic activity, and amount of product were measured
aer each reaction.
15
10UV, from 300–700 nm using a 1 cm quartz cuvette.
The amount of iron in the samples was identied using
inductively coupled plasma with optical emission spectrometry
Results and discussion
Catalyst characterization
(ICP-OES, Varian 735). The samples were digested using
a traditional acid method (HCl, HF, HClO , and HNO ), diluted
4
3
adequately, and analyzed for Fe.
Fig. 1A shows the XRD patterns of MSN, Fe-MSN, and H/Fe-
The magnetic properties of the catalysts were measured MSN. All samples exhibited ordered mesoporous silica and
using a vibrating sample magnetometer (VSM, LakeShore 7037/ iron-containing structures with three diffraction peaks at 2q ¼
1
6
ꢃ ꢃ ꢃ
9
509-P). X-ray photoelectron spectroscopy (XPS) was per- 2.26 , 4.0 , and 4.54 , indexed as (100), (110), and (200) reec-
19
formed using an ESCALAB 250Xi spectrometer armed with an Al tions, respectively. These peaks conrmed the presence of
Ka X-ray source (Thermo Fisher Company). The scans were a two-dimensional hexagonal (p6mm) structure with a d100
17
-
conducted and data plotted with respect to the binding energy. spacing of approximately 3.9 nm. The presence of an intense
ꢃ
Melting points were taken on a Koer hot stage apparatus peak at 2q ¼ 2.26 demonstrated a highly ordered mesoporous
1
13
and are uncorrected. H and C NMR spectra were recorded on structure. Introduction of iron into the mesoporous silica
a Bruker FT-500 spectrometer, using TMS as the internal nanoparticles (Fe-MSN) led to a reduction in the diffraction
standard.
peaks due to the decreasing scatter contrast between the pore
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thickness from 1.08 to 0.94 nm for MSN and Fe-MSN, respec-
tively, which increased again to 1.14 nm in H/Fe-MSN due to
widening of the pore size of MSN. Plug removal by protonation
restored the pore size, volume, and wall thickness to almost
original values.
XRF analysis showed Si/Fe molar ratios of 19.2 and 19.7 with
ꢁ
1
iron amounts of 0.398 and 0.404 mmol g in Fe-MSN and H/Fe-
MSN, respectively (Table 2). Furthermore, the amount of iron in
these samples was identied using ICP-OES (Varian ICP-OES
735), which conrmed the XRF results (Appendix A). The
amount of Fe in these samples was close to the actual loading
amount. Most of the loaded iron existed in the catalyst frame-
works. Therefore, when protonating the iron-containing cata-
lyst, the increase in the amount of Fe might be due to partial
22
desilication of the Fe-MSN framework.
Fig. 2A and B show nitrogen adsorption and desorption
isotherms and the pore size distributions of MSN, Fe-MSN, and
Fe/H-MSN. All catalysts exhibited isotherms classied as Type
IV with a Type H4 hysteresis loop, which can be attributed to
their mesoporous structures (Fig. 2A).
Fig. 1 XRD patterns of the MSN, Fe-MSN, and H/Fe-MSN catalysts. (A)
Low region of 1–9 ; (B) long region of 10–70 .
ꢃ
ꢃ
The isotherms for these samples showed characteristic sharp
inections of capillary condensation within uniform pores at
2
0
walls and space. Therefore, the order of crystallinity in Fe-MSN
was lower. Protonation of Fe-graed MSN (H/Fe-MSN) resulted
in a decrease in the intensity of all peaks, similar to that of the
Fe-MSN catalyst, which indicated that protonation occurred in
a P/P
0
of around 0.2–0.4, which indicated the presence of small
pore diameters and volumes. At a higher relative pressure (P/P
0
of 0.8–1.0), no increase in adsorbed nitrogen was observed,
which was attributed to Fe loading into the MSN framework and
indicated to the success of the extra-framework structures. At
a lower relative pressure (P/P0 < 0.2), a rounded transition
conrmed the presence of a small number of micropores,
whereas the pore size distribution conrmed the presence of
a narrow peak at a pore diameter of around 1.21 nm (Fig. 2B).
Iron graing and protonation of Fe-MSN changed the MSN
inection isotherms at <0.4 and >0.8. The absence of hysteresis
on the MSN conrmed that nitrogen sorption occurred on
a nonporous surface material. These results veried that the
MSN pores might be partially blocked by Fe atoms. Protonation
of Fe-MSN recovered the MSN isotherm.
21
the MSN framework. High-angle XRD patterns of MSN, Fe-
MSN, and H/Fe-MSN are shown in Fig. 1B. No obvious diffrac-
tion peaks were observed in the high-angle diffraction range for
incorporated iron in both Fe-MSN and H/Fe-MSN, suggesting
the Fe species were well dispersed in the MSN framework.
Table 1 shows the physical properties of the MSN, Fe-MSN,
and H/Fe-MSN catalysts. The surface areas of MSN, Fe-MSN,
2
ꢁ1
and H/Fe-MSN were 876, 410, and 858 m
g , respectively.
Lower surface areas were caused by changes in the pore size
and/or the collapse of the 2D hexagonal structure of MSN.
Introduction of iron into the MSN framework decreased the
pore size from 3.15 to 2.60 nm and the pore volume from 0.35 to
3
ꢁ1
0
.10 cm g , which indicated the plugging of MSN pores with
iron species. Protonation of Fe-MSN partially removed iron Table 2 Fe content in MSN, Fe-MSN, and H/Fe-MSN catalysts
atoms inside the pores or walls, which reopened the pores of the
ꢁ
1
b
Sample
Fe (mmol g
)
XRF (Si/Fe)
structure. This was conrmed by the total pore volume, size,
and wall thickness of these catalysts, which were analyzed using
the BJH method (Table 1). Plug removal from Fe-MSN increased
MSN
—
0.398
0.404
—
19.2
19.7
a
Fe-MSN (Si/Fe ¼ 20)
3
ꢁ1
a
the total pore volume from 0.20 to 0.67 cm g and the pore H/Fe-MSN (Si/Fe ¼ 20)
size from 2.60 to 3.53 nm. Iron loading decreased the pore-wall
a
b
Theoretical Si/Fe. Analyzed by XRF and ICP.
a
Table 1 Physical properties of MSN, Fe-MSN, and H/Fe-MSN catalysts
S
2
ꢁ1
3
ꢁ1
Sample
d100 (nm)
a
0
(nm)
(m g
)
V
p
(cm g
)
W (nm)
t (nm)
MSN
Fe-MSN
Fe/H-MSN
3.93
3.80
3.91
4.20
3.54
4.64
876
410
858
0.35
0.20
0.67
3.15
2.60
3.53
1.08
0.94
1.14
a
2 ꢁ1
d
100, d-value 100 reections; a
0
, pore center distance (equal to d100 ꢂ 2/O3); S, BET surface area (m g ) obtained from N
2
p
adsorption; V , total pore
ꢁ1
volume (mL g ); W, pore size (nm) obtained from BJH method; t, pore-wall thickness (equal to a
0
ꢁ W).
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in the framework. Fig. 3B shows the FT-IR spectra of these
ꢁ
1
catalysts in the range 400–1600 cm . The vibration peak at
ꢁ
1
3
432 cm was attributed to hydroxyl groups in the catalyst
structures. This peak was broad IR for pure MSN, but changed
to sharper peaks in Fe-MSN and H/Fe-MSN, which indicated the
decrease in hydrogen bonding in the Fe-containing samples.
This change was due to the substitution of hydrogen in some
hydroxyl groups with iron atoms.
The morphologies of MSN, Fe-MSN, and H/Fe-MSN were
observed using FESEM and HRTEM images. The FESEM images
(Fig. 4A–C) showed uniform spherical particles with particle
sizes of 70–120 nm for MSN, Fe-MSN, and H/Fe-MSN. The TEM
images (Fig. 4D–F) and their diffraction patterns (inset gures)
clearly showed well-ordered pores with parallel and cylindrical
channels and honeycomb structures, which indicated 2D
3
hexagonal p6mm mesostructures in these catalysts. These
results were in accordance with the low-angle XRD patterns and
nitrogen sorption analysis. XED (Fig. 5), ICP (Appendix A), and
XRF (Appendix B) analyses conrmed the presence of iron in
these nanomaterials.
Fig. 2 (A) Nitrogen sorption isotherms and (B) pore size distributions
for MSN, Fe-MSN, and H/Fe-MSN catalysts.
Fig. 5 shows the EDX spectrum of the H/Fe-MSN catalyst. The
presence of iron, silicon, oxygen, and carbon elements clearly
conrmed the synthesis of H/Fe-MSN. According to this result,
an iron loading of 2.63 wt% was present in H/Fe-MSN, with ICP
and XRF results (Si/Fe ¼ 19.7) matching with the obtained
results of EDX pattern analysis. Moreover, ICP and XRF analysis
(Appendices A and B) and EDX results (not shown) indicated
a molar ratio of Si/Fe ¼ 19.2 for Fe-MSN.
FTIR spectroscopy was used to determine structural differ-
ences between the MSN and iron-modied catalysts. Fig. 3A
shows the FTIR spectra of the MSN samples in the range 3100–
ꢁ
1
3
400 cm . All MSN catalysts showed IR peaks attributed to Si–
ꢁ
1
O–Si bending (455 cm ), Si–O–Si symmetric stretching (794
cm ), Si–O–Si asymmetric stretching (1050 cm ), and water
molecules retained by silica materials (1627 cm ).
ꢁ
1
ꢁ1
ꢁ1
23,24
FTIR spectra showed no major differences among the MSN,
Fe-MSN, and H/Fe-MSN frameworks, which indicated that the
MSN catalysts retained their siliceous structure even with iron
incorporation. Notably, the bands relating to MSN at 455, 794,
ꢁ
1
1
050, and 1627 cm were decreased by iron graing, which
might be due to the formation of ferrosilicate groups (Si–O–Fe)
Fig. 3 FT-IR spectra of MSN, Fe-MSN, and H/Fe-MSN catalysts in the Fig. 4 FESEM images of (A) MSN, (B) Fe-MSN, and (C) H/Fe-MSN; and
ꢁ1
ꢁ1
region (A) 3700–3100 cm and (B) 1600–400 cm
.
HRTEM images of (D) MSN, (E) Fe-MSN, and (F) H/Fe-MSN.
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Fig. 5 EDX pattern of H/Fe-MSN.
np
Fig. 6 shows the magnetic properties of H/Fe -MSN catalyst
through magnetic measurements of this catalyst at 298 K and 5
np
np
Tesla. The Fe -MSN and H/Fe -MSN samples exhibited
ꢁ1
magnetic moments of 3.3 and 4.1 emu g , respectively, and
coercivities of about 13 kOe at room temperature. Notably, the
magnetic moment is calculated per total mass of the sample
Fig. 7 XPS spectra of Fe-MSN and H/Fe-MSN.
ꢁ
1
16
(
emu g ), including the total weight of iron oxide and silica.
nanomaterials under mild conditions has been of great interest
to researchers and industry. Much effort has been made to
The prepared iron-coated samples displayed this magnetic
property in their structures, which was increased in H/Fe-MSN,
showing an enhanced amount of Fe in the framework, perhaps
due to the desilication process.
XPS spectra of the Fe-MSN and H/Fe-MSN catalysts are
shown in Fig. 7. For both catalysts, two peaks were observed at
27
determine mild conditions to promote oxidation conversion
and the selectivity of cyclohexane using biomimetic systems to
catalyze the selective oxidation of hydrocarbons at room
temperature with different oxidants. Hydrogen peroxide is
a suitable oxidant which can produce water and singlet oxygen
25
7
2
12 eV and 725 eV, corresponding to the binding energies of Fe
3/2 and Fe 2p1/2. These two weak specic peaks showed vari-
28
during an environmentally friendly reaction. The results of
p
2 2
classical cyclohexane oxidation with H O shows low conversion
ation satellites in the Fe 2p peaks. The H/Fe-MSN peak shied
by 1 eV toward a higher binding energy, which indicated that
the iron atoms were incorporated into the framework and that
29–32
and selectivity,
industries.
which are major problems in the chemical
33
Recently, Shul' pin et al. reported cyclohexane oxidation
17,26
the Fe–O–Si bonds were stable.
using a binuclear manganese(IV) complex as catalyst in the
Cyclohexanone is an important compound used in the
petrochemical industries and medicinal chemistry. Therefore,
cyclohexanone can be considered a strategic material in
different sciences. Selective oxidation of cyclohexane over some
2 2
presence of acetic acid to avoid H O decomposition to water
and oxygen. The reaction afforded cyclohexanone and cyclo-
hexanol with 46% cyclohexane conversion at 293 K aer 2 h. A
recent study catalyzed cyclohexene oxidation over metal-
2 2
loporphyrins with MCM-41 in the presence of aqueous H O .
Metalloporphyrins containing Fe, Mn, and Co were encapsu-
lated into the MCM-41 mesopore structure and hydrogen
18
peroxide was used as a clean oxidant agent. The results
showed that the supported metalloporphyrin complexes were
the active sites for cyclohexene oxidation reactions, giving 26,
12, and 15% for Fe, Co, and Mn-immobilized metals,
respectively.
This study concerns cyclohexane oxidation using pure MSN
and iron-containing MSN catalysts under mild conditions.
Fig. 8A shows cyclohexanone formation during cyclohexane
oxidation over three catalysts: (a) pure MSN; (b) Fe-graed MSN
(Fe-MSN) with an initial Si/Fe molar ratio of 20; and (c)
protonated Fe-MSN (H/Fe-MSN). The results showed that the H/
Fe-MSN catalyst exhibited higher performance in cyclohexane
oxidation to form cyclohexanone than Fe-MSN and pure MSN.
Both H/Fe-MSN and Fe-MSN showed the same drastically
increasing trend in cyclohexanone formation. MSN has no
np
Fig. 6 Room temperature magnetization isotherms for Fe -MSN and
H/Fe -MSN.
np
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Scheme 1 Proposed mechanism of cyclohexanone synthesis from
cyclohexane in the presence of H O over H/Fe-MSN at 298 K.
2 2
Fig. 8 (A) Cyclohexanone production with reaction time. (B) Cyclo- Table 3 Product distribution for cyclohexane oxidation over H/Fe-
hexane consumption with reaction time.
MSN at 298 K in 1 h
Fe-MSN
86
H/Fe-MSN
97
2
catalytic activity due to the lack of acidic sites in its structure.
Conversion (%)
The results indicate the high catalytic activity of H/Fe-MSN,
which formed about 0.8 g L cyclohexanone aer 25 min, Selectivity (%)
while Fe-MSN formed about 0.7 g L in the same time.
Fig. 8B shows the consumption of cyclohexane during
oxidation over MSN and Fe-modied MSN catalysts. MSN
showed a constant amount of unreacted cyclohexane, while Fe-
ꢁ1
ꢁ
1
Cyclohexanone
Cyclohexanol
88
12
96
4
Yield (%)
Cyclohexanone
76.5
9.5
14
93.1
3.9
3
MSN and H/Fe-MSN showed a decreasing trend in cyclohexane Cyclohexanol
consumption during the oxidation reaction. The cyclohexane
Cyclohexane
consumption trend was almost converse to the cyclohexanone
formation trend, which indicated the conversion of cyclohexane
into cyclohexanone through oxidation.
presence of protonic sites. The selectivities for cyclohexanone
formation were 88 and 94%, and the cyclohexanone yields were
7
6.5 and 93.1%, for Fe-MSN and H/Fe-MSN, respectively.
The H and C NMR spectra of the puried product of
Proposed mechanism for cyclohexane oxidation reaction
1
13
Published studies have described both free-radical and non- cyclohexane oxidation over H/Fe-MSN (Appendix C) conrmed
1
radical mechanisms that can be applied to the oxidation of the formation of cyclohexanone in high yield ( H NMR: d C4,
1
3
olens, aromatic hydrocarbons, and alcohols over metal oxides 1.65 ppm, 2H, t; C3, 78 ppm, 4H, m; C2, 2.26 ppm, 4H, m;
C
34–36
using H
2
O
2
.
Based on these studies, we propose a reason- NMR: CH
2
, 25 ppm; CH
2
, 27 ppm; CH
2
, 42 ppm; CO, 212 ppm).
ꢃ
able reaction mechanism for the oxidation of cyclohexane using Moreover, the measured boiling point (154 C) and melting
ꢃ
hydrogen peroxide in the presence of H/Fe-MSN catalyst.
point of the 2,4-dinitrophenylhydrazone derivative (158–160 C)
Scheme 1 shows the mechanism of cyclohexane oxidation to of the product conrmed that it was cyclohexanone formed in
cyclohexanone over the H/Fe-MSN catalyst. Notably, the mech- high yield.
3
+
anism explained the active sites of tetrahedral Fe on the
The activities of Fe-MSN and H/Fe-MSN catalysts in the
catalyst surface. Hydrogen peroxide is activated on H/Fe-MSN by oxidation of cyclohexane was compared with recent studies
chemisorption on the iron surface, accompanied by the using Fe-coated mesoporous/microporous silica nanomaterials
formation of a bi-radical Fe–peroxo complex. These radicals as catalysts for the conversion of cyclohexane to cyclohexanone
coordinate to Fe in H/Fe-MSN to form an iron–peroxo complex. (Table 4). Although there are a few reports of cyclohexane
This formed radical takes a hydrogen atom from a cyclohexane oxidation over iron-containing mesoporous siliceous catalysts,
molecule to form a free biradical cyclohexanol form, followed by none reported a high conversion of cyclohexane to cyclohexa-
37
the attack of a carbon free radical to form cyclohexanone.
The detailed product distribution of cyclohexane oxidation is
none in a short time.
Fe-containing MCM-41 catalysts have exhibited 8.1 and
listed in Table 3. At a low temperature (298 K), cyclohexanone 19.5% yields for cyclohexane oxidation. Furthermore, a very low
was observed as the main product using both catalysts. The H/ number of active sites were observed in silica-supported iron
Fe-MSN showed a higher cyclohexane conversion and cyclo- oxide for the oxidation of cyclohexane. In this study, pure MSN
hexanone selectivity than Fe-MSN, which might be due to the showed a trace conversion of cyclohexane (2%), while high
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2 2
Table 4 Comparison of activities of various Fe-containing silica-based catalysts for cyclohexane oxidation using H O as oxidant
t
(h)
a
b
Catalyst
Conv . (%)
m (wt%)
T (K)
Reference
MSN
Fe-MSN
H/Fe-MSN
Fe-MCM-41(1.64)
2
86
97
8.1
19.5
1.6
66
0.01
0.01
0.01
0.1
0.1
0.1
298
298
298
373
373
348
356
1
1
1
12
12
24
96
This study
This study
This study
38
39
40
41
Fe-MCM-41-NH
Fe –SiO
2
2
O
3
2
6
ꢁ
ꢁ6
g-SiW10{Fe(OH
2
)}
2
O
38
8 ꢂ 10
a
b
Cyclohexane conversion.
mcatal.(wt%).
cyclohexane conversions of 86 and 97% were observed using the Catalyst reusability
Fe-modied MSN catalysts in comparison with other meso-
The reusability of the H/Fe-MSN catalyst was investigated. Aer
the oxidation reaction, the catalyst was recycled by washing with
hot dichloromethane (2 ꢂ 10 mL), drying under vacuum at 323
K for 3 h, and then heating at 623 K under oxygen stream for 1 h.
Using this method, the catalyst was reused ve times, and
a minor loss in catalyst weight was observed aer each recycle
structured samples. The signicant differences between Fe-
graed MSNs and other samples are the high conversion of
cyclohexane in a low temperature reaction within a short time.
All reactions in this study were carried out at room temper-
ature (298 K) with a short time reaction of 1 h, while the
oxidation of cyclohexane required higher temperatures and
long reaction times (more than 12 h) over other catalysts. The
high activity of H/Fe-MSN might be attributed to its high surface
area, large pore size and volume, and the presence of 3D
channels, which result in the high mass transfer of cyclohexane
on the H/Fe-MSN surface. In the cyclohexane oxidation reac-
tion, the turnover frequency (TOF) is the number of moles of
cyclohexanone formed by a mole of Fe-MSN or H/Fe-MSN per
unit time before becoming inactive. Here, the TOF was deter-
mined from the rate of cyclohexanone production relative to the
amount of catalyst. The TOFs of cyclohexanone formed over Fe-
MSN and H/Fe-MSN are shown in Fig. 9. Although, the TOF
result exhibited an increasing trends for these catalysts with
reaction time, the TOF using H/Fe-MSN was higher than that
using Fe-MSN during a 1 h reaction.
(total loss of 6%). No signicant decrease in the conversion of
cyclohexane to cyclohexanone was observed. The yield
Fig. 10 FESEM images of (A) Fe-MSN and (B) H/Fe-MSN after cyclo-
hexane oxidation reaction.
Fig. 9 Turnover frequency of cyclohexanone production during Fig. 11 FTIR spectra of H/Fe-MSN before (fresh) and after (used)
cyclohexane oxidation using H at 298 K. reaction: (A) C–H stretching and (B) C]O bond.
2 2
O
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decreased from 97 to 94%, which exhibited the good reusability
of this catalyst in the oxidation reaction.
2 M. R. Sazegar, A. A. Jalil, S. Triwahyono, R. R. Mukti, M. Aziz,
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Fig. 10 shows FESEM images of both Fe-MSN and H/Fe-MSN
catalysts aer the nal oxidation reaction. The spherical parti-
cles indicated the stable structures of these nanomaterials. XRD
patterns (not shown) indicated that there was no change aer
the oxidation reaction, but the presence of a carbon atom was
observed in the used H/Fe-MSN, as shown in Fig. 11. Two weak
ꢁ
1
bands observed at 2865 and 2986 cm indicated the presence
of a C–H stretching vibration, which was related to the aliphatic
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1
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porous structures, with pore sizes of 3.9 nm, and surface areas
of 876, 410, and 858 m g for MSN, Fe-MSN, and H/Fe-MSN,
respectively. FTIR and XRD results conrmed that iron gra-
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structure of MSN led to a decrease in the degree of catalyst
crystallinity. XRF and ICP analysis showed Si/Fe molar ratios of
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frameworks of these catalysts. Cyclohexanone, an important
material in the petrochemicals industry, was produced over the
Fe-MSN and H/Fe-MSN catalysts with high conversions of 86
and 97%, and selectivities of 88 and 96%, respectively, at 298 K.
The results showed that the conversion of cyclohexane using H/
Fe-MSN was higher than that using Fe-MSN perhaps due to the
presence of protonic sites. The advantages of the H/Fe-MSN
solid catalyst include excellent cyclohexane conversion, low
temperature, short reaction time, and fast work-up. This cata-
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The results of the oxidation reaction indicate that H/Fe-MSN
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low energy consumption. The stable structure of the H/Fe-MSN
catalyst allowed it to be recycled several times in oxidation
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