TiO2 nanomaterials: Highly active catalysts for the oxidation of hydrocarbons

Article abstract of DOI:10.1016/j.molcata.2013.12.012

TiO₂ nanomaterials (nanoparticles, nanotubes and nanofibers) are active and selective heterogeneous catalysts for the oxidation of hydrocarbons under mild, solvent-free conditions. In addition, the reported results suggest that activities are not only related to the specific surface area of the catalysts, but that they can also be explained by the type of crystalline structure and surface morphology. Indeed, higher conversions and selectivity (up to 86% and 90%, respectively) were obtained in the oxidation of cyclohexene using TiO₂ rutile. Interestingly, these efficient catalytic systems do not require the presence of noble metals to achieve significant activities.

Full text of DOI:10.1016/j.molcata.2013.12.012

Journal of Molecular Catalysis A: Chemical 383–384 (2014) 225–230
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
TiO2 nanomaterials: Highly active catalysts for the oxidation of
hydrocarbons
Muhammad I. Qadir, Jackson D. Scholten, Jairton Dupont∗
Laboratory of Molecular Catalysis, Institute of Chemistry, Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonc¸ alves, 9500, P.O. Box 15003,
91501-970 Porto Alegre, RS, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
TiO2 nanomaterials (nanoparticles, nanotubes and nanofibers) are active and selective heterogeneous
catalysts for the oxidation of hydrocarbons under mild, solvent-free conditions. In addition, the reported
results suggest that activities are not only related to the specific surface area of the catalysts, but that
they can also be explained by the type of crystalline structure and surface morphology. Indeed, higher
conversions and selectivity (up to 86% and 90%, respectively) were obtained in the oxidation of cyclohex-
ene using TiO2 rutile. Interestingly, these efficient catalytic systems do not require the presence of noble
metals to achieve significant activities.
Received 19 September 2013
Received in revised form
20 December 2013
Accepted 21 December 2013
Available online 31 December 2013
Keywords:
© 2013 Elsevier B.V. All rights reserved.
Titanium dioxide
Nanomaterials
Heterogeneous catalysis
Oxidation
1. Introduction
catalyst, giving cyclohexanone and cyclohexanol with selectiv-
ity (80%), but a low conversion (1–10%) [3]. Nanomaterials such
Selective oxidation of hydrocarbons by air/molecular oxygen is a
significant process in the chemical industry and provides a greener
route for the synthesis of some fine chemicals and intermediates.
The aerobic oxidation of hydrocarbons is an important commer-
cial process for oxygen-containing compounds, such as alcohols,
aldehydes, ketones, carboxylic acids and epoxides [1]. However,
the reaction conditions are often harsh, the reagent mixtures are
corrosive (including Br− or Cl−) and the reactions are frequently
unselective. They also generate environmentally hazardous waste
and byproducts. Generally, low selectivities are observed in oxida-
tion reactions, which can be related to the formation of undesired
products due to easy over-oxidation of carbonyls and alcohols. In
avoiding such over-oxidation, conversions are often low, which is a
serious disadvantage. Thus, the selective transformation of hydro-
carbons – especially saturated hydrocarbons, such as alkanes –
to valuable oxygenated compounds is an important area in con-
temporary industrial chemistry. As an example, the oxidation of
cyclohexane to cyclohexanone and cyclohexanol is widely used in
the production of adipic acid and caprolactam, which are interme-
diates of nylon-6, fibers, food additives, plasticisers and nylon-66
polymers [2]. Industrially, cyclohexane is oxidised in air (15 atm)
at 160 ◦C in the presence of cobalt naphtenate as a homogeneous
as Fe2O3 [4], Au nanoparticles supported on porous solids like
Au/MCM-41 [5] have also been studied for cyclohexane oxida-
tion. Recently, the aerobic oxidation of cyclohexane was performed
using a supported cobalt tetra(4-carboxyl)phenylporphyrin cata-
lyst under 0.9 MPa and 150 ◦C [6]. On the other hand, the catalytic
aerobic transformation of alkenes into value-added oxygenated
derivatives is still a challenge in modern chemical processes. Allylic
oxidation of alkenes is an attractive process for the production of
␣,␤-unsaturated alcohols and ketones, which are intermediates for
the synthesis of medicines and pesticides. Ketone-derived prod-
ucts in particular are much desired due to the presence of the
carbonyl group, which is attractive for addition reactions [7]. The
solvent-free oxidation of cyclohexene over gold-supported cata-
lysts Au/OMS-2 and Au/La-OMS-2 with molecular oxygen has been
reported with less than 50% total conversion [8]. Ti-containing
mesoporous silica has also been examined for the oxidation of
alkenes [9]. Recently, a palladium oxide-based catalyst was applied
to the allylic oxidation of cyclohexene with t-BuOOH in acetoni-
trile achieving high conversions and selectivity to cyclohexenone
[10]. Toluene oxidation was also demonstrated, catalysed by Au-Pd
alloy nanoparticles supported on carbon or TiO2 under solvent-
free conditions [11]. In most cases, titanium dioxide has been
used only as a support for noble active metals or oxides in oxi-
dation reactions, but not as a catalyst [12–17]. Moreover, TiO2
was largely employed in photocatalytic oxidations of several com-
pounds [18–28]. For a more eco-friendly system it is desirable to
∗
Corresponding author. Tel.: +55 51 3308 6321; fax: +55 51 3308 7304.
E-mail address: jairton.dupont@ufrgs.br (J. Dupont).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.12.012

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M.I. Qadir et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 225–230
develop a simple, highly active and reusable catalyst with the ability
to oxidise hydrocarbons under mild conditions. We demonstrate
here that TiO2 nanomaterials, such as TiO2 nanoparticles (NPs),
nanofibers (NFs) and nanotubes (NTs), are effective catalysts for the
selective allylic oxidation of different cycloalkenes and for the oxi-
dation of cyclohexane. These catalytic systems displayed very high
activity and selectivity in the solvent-free oxidation of cyclohex-
ene, 1,5-cyclooctadiene, indane and cyclohexane using molecular
oxygen.
reaction mixture was stirred at 80 ◦C. After 12 h, 20 mL of water
was added to decrease the viscosity of ionic liquid. The produced
TiO2 NPs were collected by centrifugation, washed with acetonitrile
(8 × 25 mL) and CH2Cl2 (5 × 25 mL). Then, the product was dried at
100 ◦C for 24 h under vacuum.
2.3. Synthesis of TiO2 NTs
A well-known hydrothermal method was used for the prepa-
ration of TiO2 nanotubes [31]. In a typical procedure, commercial
TiO2 P25 (1.0 g) was placed into a teflon-lined autoclave, and filled
with 100 mL of 10 M NaOH solution. Then the autoclave was kept at
110 ◦C under autogenous pressure for 24 h under vigorous stirring.
The resulting white product was cooled, filtered and washed with
deionized water till neutral. Then the product was washed in a pH
1.6 solution adjust by HCl, and then washed again with deionized
water to pH 7. Finally, the white product was dried at 100 ◦C under
vacuum.
2. Experimental
2.1. General
The substrates were degassed and stored under argon prior
to use. The ionic liquid 1-n-butyl-3-methylimidazolium tetra-
fluoroborate (BMI.BF4) used for the synthesis of TiO2 NPs was
prepared from a reported procedure [29]. All the other chemi-
cals were purchased from commercial sources. Mass spectra were
obtained using a GC–MS Schimadzu QP-5050 (EI, 70 eV) and gas
chromatography analyses were performed with an Agilent Tech-
nologies 6820 gas chromatograph with a flame ionisation detector
and 30 m capillary column with a (5%-phenyl)-methylpolysiloxane
stationary phase. Transmission electron microscopy (TEM) anal-
ysis was measured in a JEOL JEM 1200 ExII 120 kV operating at
80 kV. SEM measurements were recorded in a JEOL JSM 5800 at
20 kV equipped with an EDS detector. X-ray powder diffraction
(XRD) experiments were conducted on a D/max-3B diffractome-
ter with Cu K␣ radiation. The scans were made in the 2ꢀ range
0–6◦ with a scan rate of 0.5◦/min (low angle diffraction), and in
the 2ꢀ range 20–70◦ with a scan rate of 10◦/min (wide angle
diffraction). The surface area of the supported metal catalyst was
measured using the Brunauer–Emmett–Teller (BET) method (N2
adsorption) with a Gemini apparatus (Micromeritics 2010 Instru-
ment Corporation). The infrared spectra were recorded at room
temperature on a Galactic ABB FTIR spectrometer equipped with an
MCT cryogenic detector, with the sample compartment modified
to accommodate the cryogenic head; 128 acquisitions (recorded
at 4 cm−1 resolution) were typically averaged for each spectrum.
Raman measurements were performed using home-made equip-
ment consisting of a iHR 320 Symphony-Jobin Yvon spectrometer
containing a CCD detector (cooled by liquid N2) and a laser beam
HeNe (632.8 nm, 10 mW, 2–3 ␮m of diameter, back reflection
geometry); a Super Notch filter has been used in order to elimi-
nate the Rayleigh scattering. Chemisorption analysis was recorded
using Micromeritics ASAP 2020 equipment. The catalytic oxida-
tion reactions were carried out in a modified Fischer-Porter steel
bottle immersed in a silicone oil bath. The substrates were added
with a gastight precision Hamilton syringe. For inductively cou-
pled plasma mass spectrometry (ICP-MS) analysis, the commercial
TiO2 P25 (Aeroxide® Degussa) sample was initially digested using
standard procedures with the aid of a microwave oven (MARS
6, 1800 W, CEM), diluted and the final solution was analysed in
Element 2 Thermo Fischer Scientific instrument. ICP-MS analysis
showed the presence of metals, such as iron (113.9 6.9 ␮g/g) and
nickel (0.68 0.02 ␮g/g) that may influence the catalytic properties
of the material.
2.4. Synthesis of TiO2 NFs
TiO2 nanofibers were also prepared using teflon-lined autoclave
by hydrothermal method [32]. In a teflon-lined autoclave, commer-
cial TiO2 P25 (1.0 g) was placed and filled with 100 mL of 10 M NaOH
solution. The autoclave was maintained at 180 ◦C under autoge-
nous pressure for 24 h under vigorous stirring. The resulting white
product was neutralised, washed and dried as described for TiO2
NTs.
2.5. Sample preparation for TEM
In order to perform the TEM analysis, a very small amount of
TiO2 nanomaterial was dispersed in acetone and put under sonica-
tion for 10 min. A small amount of this dispersion was placed in a
carbon-coated copper grid. Then this grid with nanomaterial was
placed under vacuum for 2–4 h. The nanoparticles’ diameter was
estimated from ensembles of particles chosen in arbitrary areas
of the enlarged micrograph. The diameter of the nanoparticles,
nanotubes and nanofibers in micrographs were measured using the
software Sigma Scan pro 5.
2.6. FTIR-carbon monoxide adsorption measurements
A KBr disc was prepared using 2–3 mg of TiO2. The sample was
placed in a tube-type cell. The system was evacuated and then 1 bar
of CO gas was dosed into the cell.
2.7. Chemisorption analysis
The desired amount of sample (113–157 mg) was placed in a
U-type glass tube. The chemisorption procedure begins with a pre-
treatment based on the following steps: (i) evacuation (110 ◦C,
60 min, fill with He); (ii) evacuation (200 ◦C, 120 min); (iii) evac-
uation (35 ◦C, 10 min); (iv) leak test (35 ◦C, 1 min); (v) evacuation
(25 ◦C, 20 min). Then, the analysis starts at 25 ◦C, 250 min in the
presence of CO.
2.8. Allylic oxidation of cyclohexene
2.2. Synthesis of TiO2 NPs
The catalytic reactions were carried out in a modified Fischer-
Porter gas reactor pressurised at the desired O2 pressure. For
each reaction the gas reactor was loaded with 25 mg catalyst
(0.31 mmol) and 12.30 mmol of alkene. The temperature was main-
tained at 75 ◦C. The reactor was filled with O2 with 4 bar pressure
and the reaction was conducted under magnetic stirring (700 rpm).
TiO2 NPs were prepared according to a reported method [30].
In a Schlenck tube, 0.5 mL of TiCl4 was mixed with 5 mL of 1-n-
butyl-3-methylimidazolium tetrafluoroborate ionic liquid under
argon atmosphere. The mixture was stirred hard for 10 min at room
temperature and 1 mL of deionized water was added slowly. The

M.I. Qadir et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 225–230
227
Table 1
Physicochemical properties of TiO2 nanomaterials.
Materials
Anatase:rutile (%)a
Specific surface area (m2/g)b
Pore size (nm)c
Pore size (nm)d
Pore volume (cm3/g)c
Pore volume (cm3/g)d
TiO2 NPs
TiO2 NTs
TiO2 NFs
TiO2 P25 [33]
73:27
52:48
47:53
81:19
218.4
205.7
14.4
56
6.95
14.8
11.3
17.5
6.67
14.4
36.7
17.5
0.32
0.66
0.05
0.25
0.38
0.70
0.04
0.25
a
Anatase:rutile content was determined by XRD using a previously reported method [34] (the content of amorphous TiO2 was not considered).
Calculated from nitrogen sorption using the BET method.
Calculated from the adsorption branch of the nitrogen sorption isotherm using the BJH method.
Calculated from the desorption branch of the nitrogen sorption isotherm using the BJH method.
b
c
d
After 16 h, the reactor was cooled at room temperature and depres-
monomodal size distribution for TiO2 NPs with a mean diameter
of 1.6 0.4 nm. In the case of NTs, a mean diameter was estimated
at 60.5 7.0 nm, whereas for the NFs, it was 138.3 1.2 nm (Fig.
S1). SEM-EDS measurements confirmed the chemical composition
of the TiO2 nanomaterials, consisting primarily of Ti and O (Fig. S2).
Analysing the crystallinity of TiO2 nanomaterials with X-ray diffrac-
tion (Fig. S3), the coexistence of anatase (JCPDS number 84-1286)
and rutile (JCPDS number 01-1292) phases was seen. Raman mea-
surements of the TiO2 catalysts also revealed the expected signals
from anatase and rutile crystalline phases (Fig. S5).
surised it. Conversion and selectivity were calculated based on peak
area of GC calibrated with an internal standard (n-undecane). The
validity of the identification and quantification of the GC peak were
verified by comparison the mass spectra and gas chromatographic
retention time with those of the authentic compounds.
2.9. Allylic oxidation of 1,5-cyclooctadiene
In
a Fischer-Porter gas steel reactor, 25 mg of catalyst
(0.31 mmol) was added and filled with 6.94 mmol of substrate. The
mixture was stirred for 10 min at room temperature and the reactor
was loaded with O2 (5 bar). The reactor was dipped in a silicon oil
bath that was maintained at 100 ◦C and stirred the reaction mixture
at 700 rpm. After 19 h the reactor was cooled at room temperature.
The products were quantitatively analysed and confirmed using GC
and GC–MS.
The surface properties of nanomaterials were also evaluated
by FTIR spectroscopy and chemisorption analyses in the presence
of CO gas (Figs. S6–S13). FTIR spectra of adsorbed CO provided
important information about the surface Lewis acidity of the nano-
materials. There is a general acceptance that the Lewis acidic sites
on anatase phase detected by the adsorption of CO are indicative for
the existence of Ti4+ cations. Moreover, it has been shown that the
␣-sites, the highest electrophilic sites, have to be assigned to highly
unsaturated titanium cations. Reported studies showed that the
peak at 2171 cm−1 can be ascribed to carbonyl adsorbed on 5-fold
coordinated Ti4+ sites (␣-sites) in the absence of adsorbed water
[35,36]. In addition, the calculated ꢁ(CO) frequencies for (1 0 1),
(1 0 0) and (1 1 2) of TiO2 surfaces are all grouped in a narrow inter-
val of 2185–2175 cm−1 [37–39]. The present results obtained in the
CO adsorption measurements revealed that CO coordinates on Ti4+
sites present on the different faces (2173–2167 cm−1). Therefore,
it is possible to assume that CO adsorbs at the TiO2 ␣-sites and
related surfaces. In addition, the presence of characteristic peaks
(2117–2115 cm−1), in all materials, was assigned to the adsorption
at the Ti3+ ions [40]. The concentration of ␣-sites was estimated
as 2.33, 2.23, 1.87 and 1.82 sites per nm2 for TiO2 P25, NPs, NTs
and NFs, respectively, after 2.4 h under 1 bar CO (Table S3). The
presence of hydroxyl group at the surface of the nanomaterials
was also detected. Indeed, the common broad absorption extending
from 3600 to 2500 cm−1 as well as the signal centred at 1620 cm−1
are due to the ꢁ(OH) modes perturbed by hydrogen bonding and
to the ı mode of H2O adsorbed on Ti4+ sites of faces and terra-
ces [35,36,39]. Interestingly, a control experiment showed that
the bands from adsorbed CO disappeared completely after sam-
ple evacuation. This may indicate that the CO molecules are only
physisorbed on the TiO2 surface. However, chemisorption analy-
sis revealed that a considerable amount of CO was chemisorbed on
the TiO2 nanomaterials (see Figs. S10–S13). Thus, the absence of CO
peaks after vacuum observed by IR technique does not mean that
there is no CO chemisorbed, but it can be a limitation due to the
small amount of sample used in the IR experiments (2–3 mg).
The oxidation of cyclohexene in a Fischer-Porter reactor with
O2 at 75 ◦C in the absence of a catalyst showed no oxidation reac-
tion. However, it was found that TiO2 NPs, NFs and NTs promoted
cyclohexene oxidation to yield cyclohexen-1-one, cyclohexen-1-ol
and cyclohexene oxide. With all these catalysts, no cyclohexene
hydroperoxide was observed. Except in the case of TiO2 NPs, the
major selectivity was for the cyclohexen-1-one product (2), prob-
ably due to rapid overoxidation of the allylic alcohol. Of note, the
2.10. Allylic oxidation of indane
In a modified Fischer-Porter gas steel reactor, 25 mg of catalyst
(0.31 mmol) was added and filled with 8.47 mmol of substrate. The
mixture was stirred for 10 min at room temperature and reactor
was loaded with O2 (5 bar). The reactor was dipped in a silicon oil
bath that was maintained at 100 ◦C and stirred the reaction mixture
at 700 rpm. After 17 h the reactor was cooled at room temperature.
The products were quantitatively analysed and confirmed using GC
and GC–MS.
2.11. Oxidation of cyclohexane
The oxidation of cyclohexane was also carried out in a modified
Fischer-Porter gas reactor, filled with 6 mg catalyst (0.075 mmol),
7.15 mmol cyclohexane, stirred at room temperature for 20 min.
Then, 0.67 mmol TBHP was added and the reactor was filled with
5 bar O2. The system was heated to 100 ◦C with a stirring rate of
700 rpm. After 4 h, the reactor was cooled to 25 ◦C. The products
were quantitatively analysed and confirmed by authentic com-
pounds using GC and GC–MS.
2.12. Recycling studies
Catalyst recycling experiments were carried out with repeated
uses of TiO2 NFs at 75 ◦C for 17 h in the oxidation of cyclohexene. In
order to regenerate the catalyst after reaction, it was separated by
centrifugation, washed with dichloromethane several times, dried
at 85 ◦C for 30 min under vacuum and then used with a fresh reac-
tion mixture.
3. Results and discussion
TiO2 nanomaterials were prepared according to known pro-
cedures [30–32] and their major physicochemical and textural
properties are summarised in Table 1. TEM analysis showed a

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M.I. Qadir et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 225–230
Table 2
Table 4
Allylic oxidation of cyclohexene.a
Allylic oxidation of indane.a
.
Selectivity (%)b
10
.
Entry
Catalyst
Conv. (%)b
Entry
Catalyst
Conv. (%)b
Selectivity (%)b
11
2
3
4
1
2
3
4
TiO2 P25
TiO2 NPs
TiO2 NTs
TiO2 NFs
38
65
52
71
69
25
53
82
31
75
47
18
1
2
3
4
5
TiO2 P25
TiO2 NPs
TiO2 NTs
TiO2 NFs
None
58
58
37
66
0
62
27
40
49
0
30
52
29
31
0
8
21
31
20
0
a
Reactions were carried out with TiO2 (0.31 mmol) in 9 (8.47 mmol) under O2
atmosphere (5 bar) at 100 ◦C for 17 h.
a
Reactions were carried out with TiO2 (0.31 mmol) in 1 (12.30 mmol) under O2
b
Determined by GC with n-undecane as an internal standard.
atmosphere (4 bar) at 75 ◦C for 16 h.
b
Determined by gas chromatography (GC) with n-undecane as an internal
standard.
indane is very unstable and decomposes into products 10 and 11
even at 40 ◦C [42].
The TiO2 nanocatalysts did not show significant activity for the
allylic oxidation of toluene using molecular oxygen, consistent with
previous results [11]. However, in the presence of TBHP (30 mol%)
as an initiator and using TiO2 NPs as the catalyst, 34% conversion
was obtained with total selectivity for benzaldehyde. In particular,
this difference between the allylic oxidation of indane and toluene
may be related to the presence of a secondary benzylic carbon
found in indane, which oxidises easily in radical chain reactions
as compare to the primary benzylic carbon of toluene.
It has been reported that the use of TiO2 for the oxidation of
cyclohexane was reported with low conversion (<15%) using TBHP.
The conversion was increased slightly, to almost 20% in the pres-
ence of Pt [43]. Here, we show that TiO2 NPs, NFs and NTs can
give significantly improved oxidation of cyclohexane under mild
and solvent-free conditions. The results of cyclohexane oxidation
catalysed by TiO2 nanomaterials are presented in Table 5.
Blank experiments in which one of the components, TiO2 nano-
material, TBHP or oxygen, was missing resulted in no oxidation.
Addition of TBHP was essential to initiate the reaction, suggesting
that this oxidation process proceeds via a radical chain mechanism
[2]. Indeed, it is reported that aerobic oxidation of cyclohexane
involves radical intermediates, and the product cyclohexanone
could also catalyse the initiation of the autoxidation process [44].
In all cases, cyclohexanol and cyclohexanone were obtained as the
primary products with preferable selectivity to the carbonyl prod-
uct; adipic acid was not formed. High conversions were obtained
catalytic activity of TiO2 NFs was slightly superior to the other cata-
lysts (entries 1–4, Table 2). In the case of cyclohexene oxidation, the
chemoselectivity of the products can be correlated with the surface
chemistry of the nanomaterials. It has been reported that the pres-
ence of surface Lewis acidic sites is responsible for the absence of
cyclohexene oxide because these sites induce the ring opening of
epoxide to allylic products [41]. This effect was strongly observed in
the case of TiO2 P25 catalyst, where due to the presence of a higher
amount of acidic sites (2.33 sites per nm2) compared to other mate-
rials, a small formation of product 4 was observed. These active
sites are responsible for the comparatively higher allylic products
as shown in Table 2.
The allylic oxidation of 1,5-cyclooctadiene with molecular oxy-
gen gave products 6, 7 and 8 (Table 3). For all catalysts tested,
product 8 was formed preferentially. Moreover, the NPs and NFs
showed the best catalytic activities. In fact, high conversion (89%)
was observed using TiO2 NFs, with selectivities of 65%, 32% and
3% for 8, 6 and 7, respectively (entry 4, Table 3). No hydroperoxide
products were observed under our experimental conditions. In this
case, there is no evidence for the correlation between the number
of acidic sites of the catalyst and the product’s selectivity, probably
due to the conformational nature of the substrate.
The oxidation of indane at 100 ◦C and 5 bar oxygen pressure also
gave promising results (Table 4). In this regime, allylic oxidation of
indane was achieved using TiO2 NFs (71%) and TiO2 NPs (65%) as the
best promoters for this reaction. The selectivity of 2,3-dihydro-1H-
inden-1-one (10) and 2,3-dihydro-1H-inden-1-ol (11) depended
on the type of catalyst used. It has been reported that indanyl 1-
hydroperoxide formed as a major product during the oxidation of
Table 5
Oxidation of cyclohexane.a
Table 3
Allylic oxidation of 1,5-cyclooctadiene.a
.
.
Entry
Catalyst
Initiator
Conv. (%)b
Selectivity (%)b
Entry
Catalyst
Conv. (%)b
Selectivity (%)b
13
14
6
7
8
1
2
3
4
5
TiO2 P25
TiO2 NPs
TiO2 NPs
TiO2 NTs
TiO2 NFs
TBHP
–
TBHP
TBHP
TBHP
16
–
33
68
63
78
–
75
68
64
22
–
25
32
36
1
2
3
4
TiO2 P25
TiO2 NPs
TiO2 NTs
TiO2 NFs
77
86
62
89
34
20
18
32
3
2
2
3
63
78
80
65
a
a
Reactions were carried out with TiO2 (0.31 mmol) in 5 (6.94 mmol) under O2
Reactions were carried out with TiO2 (0.075 mmol) in 12 (7.15 mmol), TBHP
(0.67 mmol) under O2 atmosphere (5 bar) at 100 ◦C for 4 h.
atmosphere (5 bar) at 100 ◦C for 19 h.
b
b
Determined by GC with n-undecane as an internal standard.
Determined by GC with n-undecane as an internal standard.

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229
using TiO2 NTs (68%) and NFs (63%), indicating that these catalysts
provide better conditions for oxygen adsorption and activation
under the conditions employed. Fortunately, this system was highly
selective for 13 and 14 using small amounts of radical initiator and
a low oxygen pressure, in contrast to previous reports where large
amounts of initiator and oxygen have been used [45,46]. Moreover,
the presence of cyclohexane hydroperoxide as a side product was
not detected probably due to of its higher instability under our
reaction condition at the end of 4 h reaction. It is noticed that the
ratios of cyclohexanone to cyclohexanol with TiO2 catalyst system
were higher than 2:1, suggesting a possible catalytic mechanism.
With the increase of reaction time (5 h) using TiO2 NFs, it was
observed 62% conversion with the selectivity of 64% of 13, 29% of
14 and byproducts (7%) including adipic acid, 2-hexenal and other
unknown.
Generally, TiO2 NFs provided slightly superior results as promo-
ters for oxidation reactions compared to the other catalysts tested.
Hence, it appears that activities are not only related to the specific
surface area of the catalysts. In fact, the observed catalytic activities
can also be explained by the type of crystalline structure and sur-
face morphology found in each catalyst. It is known that the rutile
is the most stable form of TiO2 at ambient conditions. Previously,
atomistic simulations of the surface structures of TiO2 polymorphs
revealed that the rutile phase contains exposed faces with high
energy compared to the anatase [47]. In addition, the exposed rutile
faces (0 1 1) and (1 1 1) were recognised as highly active surface
faces for oxidation process [48,49]. In our work, due to the dom-
inancy of rutile phase in TiO2 NFs (53%, see Table 1), this catalyst
has shown higher activity for the oxidation reactions compared to
the other nanomaterials. Indeed, the cyclohexene oxidation reac-
tion with P25 containing pure rutile (confirmed by XRD, Fig. S14),
gave 86% conversion with 90% selectivity of 2, 4% of 3 and 6% of 4.
The oxidation reactions are believed to proceed primarily via
radical mechanisms, involving both carbon- and oxygen-centred
radicals [50]. To provide support for this type of mechanism for
the allylic oxidation of cyclohexene, CCl4 and hydroquinone were
used as a radical inhibitor [51], which stopped the conversion
to oxidised products. Therefore it can be assumed that the free-
radical active oxygen is formed due to the activation of O2 by the
defect site in TiO2 that can easily accept electrons. The surface
Ti3+ cations coordinate to active oxygen species to form Ti4+OO•
(I) superoxo intermediate, which further attack the allylic position
of cyclohexene to form intermediate (V), undergo to the prod-
uct 3. The in situ produced intermediate Ti4+O• (VI) can further
oxidise alcohol through intermediate (VII) to product 2 with the
removal of water. Moreover, Ti4+OO• superoxo intermediate (I) can
go into oxidative addition to C-C double bond of cyclohexene to give
intermediate (II) that undergoes migratory insertion to yield cyclic
radical (IV) through intermediate (III). Cyclic radical (IV) can fur-
ther react with another molecule of cyclohexene to give epoxide 4
(Fig. S15). A similar mechanism has been proposed earlier [52,53].
In the case of 1,5-cyclooctadiene higher amount of product 8 was
observed that can be explained by the less probable allylic attack
of Ti4+OO• superoxo intermediate (I) to 1,5-cyclooctadiene due to
the presence of sterically hindered allylic hydrogens. Cyclohex-
ene possess chair conformation, which has double bonded carbons
as well as the allylic carbon is in the same plane. However, 1,5-
cyclooctadiene is in boat conformation and has the C C double
bonds with the allylic carbons in different planes [54,55]. In this
view, 1,5-cyclooctadiene is likely to have more steric hindrance
towards allylic attack by Ti4+OO• (I) resulting into small amount of
products 6 and 7.
Fig. 1. Catalyst recycling tested in the oxidation of cyclohexene catalysed by TiO2
NFs at 75 ◦C.
cyclohexyl radicals with the generation of water and t-BuOH, which
can be so-called initiation reaction [57]. Cyclohexyl-radical is active
specie that can coordinate to the OH group located at the surface
of TiO2, which with the loss of water further undergo cyclohex-
anone as reported in literature [58]. The deactivated TiO2 becomes
activated by the presence of O2 and water in our system. On the
other hand, cyclohexyl radical reacts with oxygen to produce cyclo-
hexyl hydroperoxide with the abstraction of proton from another
cyclohexane molecule, which can easily decompose into cyclohex-
anol and cyclohexanone (Fig. S16). The presence of such species
was confirmed in the middle of experiment with FTIR (Fig. S17).
The infrared spectrum of reaction mixture revealed three peaks at
1315, 1349 and 1364 cm−1 respectively, that can be assigned to
cyclohexyl hydroperoxide [59].
Finally, in order to give assessment for the catalyst recyclability,
the oxidation of cyclohexene was performed in the presence of TiO2
NFs. It is possible to observe from Fig. 1 that the activity of the
catalyst showed no decline after four recycles. In fact, the catalytic
system maintained its activity during at least five runs.
4. Conclusions
In summary, we demonstrate that TiO2 nanomaterials show
high efficiency for the allylic oxidation of cyclohexene, 1,5-
cyclooctadiene and indane with highly selectivity using molecular
oxygen in a solvent-free system under mild conditions. Addition-
ally, these catalysts showed significant activity in cyclohexane
oxidation. The presence of an initiator (TBHP) was found to be
essential for cyclohexane oxidation. It was shown that the nature
of the catalyst has a profound influence on catalytic activity and
selectivity. In fact, our results suggest that activities are not only
related to the specific surface area of the catalysts, but they can
also be explained by the type of crystalline structure and surface
morphology. Thus, it seems that the percentage of rutile – the most
active phase of TiO2 for oxidation reactions – also has an important
role in the catalytic activity of TiO2 nanomaterials. In addition, our
system does not need the presence of noble metals to achieve effi-
cient catalytic activities in the oxidation of hydrocarbons. Because
of their simplicity, stability and ease of synthesis, TiO2 nanoma-
terial catalysts may be useful in the selective oxidation of certain
important compounds in industrial processes.
In the case of cyclohexane oxidation, it has been assumed that
t-BuOOH undergo homolytic dissociation of hydroxide molecule
due to the O O weakest bond in the system, but the reaction is
slow [56]. TBHP reacts with cyclohexane molecules to produce

230
M.I. Qadir et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 225–230
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