Catalytic performance of ceria nanorods in liquid-phase oxidations of hydrocarbons with tert-butyl hydroperoxide

Article abstract of DOI:10.3390/molecules15020747

The CeO₂ nanorods (CeNR) promote the oxidation of ethylbenzene (PhEt) and cyclohexene with t-BuOOH, at temperatures as low as 55 °C. For both substrates the saturated C-H bonds are preferentially activated over the unsaturated ones. The catalyst seems fairly stable towards leaching phenomena. The liquid-phase oxidation catalysis may be associated with the Ce ³⁺/Ce⁴⁺ inter-conversion in the one-electron redox processes mediating the formation of tert-butyl-(per)oxy radicals. CeNR is very effective in H₂O² disproportionation. Pre-treatment of CeNR with H₂O₂ or t-BuOOH prior to the catalytic reaction enhances the reaction rate of PhEt with t-BuOOH in comparison to CeNR. Textural characterization and spectroscopic studies suggest that catalytic activation is associated to defect sites.

Full text of DOI:10.3390/molecules15020747

Molecules 2010, 15, 747-765; doi:10.3390/molecules15020747
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Catalytic Performance of Ceria Nanorods in Liquid-Phase
Oxidations of Hydrocarbons with tert-Butyl Hydroperoxide
1
,2
2
2
1
Andreia G. Macedo , Sílvia E. M. Fernandes , Anabela A. Valente , Rute. A. S. Ferreira ,
1
2,
Luís D. Carlos , and João Rocha *
1
Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal;
E-Mail: agmacedo@ua.pt (A.G.M.)
2
Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal;
E-Mail: atav@ua.pt (A.A.V.)
*
Author to whom correspondence should be addressed; E-Mail: rocha@ua.pt;
Tel.: +35-223-437-0730; Fax: +35-123-437-0084.
Received: 9 December 2009; in revised form: 28 January 2010 / Accepted: 29 January 2010 /
Published: 2 February 2010
2
Abstract: The CeO nanorods (CeNR) promote the oxidation of ethylbenzene (PhEt) and
cyclohexene with t-BuOOH, at temperatures as low as 55 ºC. For both substrates the
saturated C-H bonds are preferentially activated over the unsaturated ones. The catalyst
seems fairly stable towards leaching phenomena. The liquid-phase oxidation catalysis may
3
+
4+
be associated with the Ce /Ce inter-conversion in the one-electron redox processes
mediating the formation of tert-butyl-(per)oxy radicals. CeNR is very effective in H O2
2
disproportionation. Pre-treatment of CeNR with H O or t-BuOOH prior to the catalytic
2
2
reaction enhances the reaction rate of PhEt with t-BuOOH in comparison to CeNR.
Textural characterization and spectroscopic studies suggest that catalytic activation is
associated to defect sites.
Keywords: cerium oxide; nanorods; catalysis; oxidation; liquid phase; free radical
mechanism

Molecules 2010, 15
. Introduction
Cerium oxide has attracted much attention for several catalytic and environmental applications,
748
1
such as depollution of noxious compounds from gaseous streams originating from industrial
productions and from automobiles, the (industrial) gas-phase catalytic dehydrogenation of
ethylbenzene to styrene, and the degradation of organic pollutants in wastewater through catalytic wet
3
+
4+
oxidation [1]. The interest arises from its remarkable Ce /Ce redox and oxygen storage capacity
properties. According to the literature the reduction of the ceria is controlled by the nature of oxygen
vacancies, albeit the mechanism of creation of these favourable defects and their roles in the activation
mechanisms, reducibility and activity of nanosized ceria at the atomic level is still lacking [2]. Cerium
oxide has also been used in reactions for therapeutic applications to avoid reactive oxygen
intermediates (ROI, like hydrogen peroxide) that increases retinal neurons degeneration (cerium
nanoparticles were effectively used to scavenge these ROI within to retinal cells and inhibit the
progressive degeneration) [3].The preparation route determines structural and surface properties of
cerium oxide and there is a growing interest in controlling the morphology to obtain cerium oxide
nanotubes. Very recently, cerium oxide nanotubes have been produced by hydrothermal alkali
treatment [4], precipitation method [5], surfactant-assisted method [6], simple solid-liquid interface
reaction route [7], by etching Ce(OH) nanotubes/nanorods with H O [8] or using carbon nanotubes as
3
2
2
templates by a liquid phase deposition method [9–11].
Studies of the influence of ceria materials in oxidation reactions are of interest since ceria is used as
catalyst support in several oxidation applications. Here cerium oxide nanorods were tested as catalysts
2 2
in the liquid phase oxidation of ethylbenzene using tert-butyl hydroperoxide (t-BuOOH) or H O as
oxidising agent: a possible reaction product is acetophenone which is used as ingredient in perfumes
and as a chemical intermediate in the production of pharmaceuticals, resins and flavouring agents.
3
+
4+
Additionally, the catalytic oxidation of cyclohexene is investigated. The role of the Ce /Ce redox
and defect sites on the catalytic reaction is discussed. Additionally, the UV-vis absorption and
3
+
luminescence techniques were used to study the presence and environment of Ce in the samples
3
+
before and after exposure to t-BuOOH or H
2
O
2
. The fluorescence of Ce activated compounds is
3
+
widely studied [12]. Usually, the Ce emission consists of the allowed electric dipole 5d→4f
transitions, since the Ce has a 4f configuration; the ground states have a doublet with an energy
3
+
-1
2
2
separation around 2000 cm ( F5/2 and F7/2). The lower excited states are the crystal field components
of the 5d configuration. Also a broad band in a lower energy position has been attributed to the
Ce4f→O2p charge transfer [13].
2
. Results and Discussion
The powder XRD patterns of CeNR, CeNR-BuOOH, CeNR-H
2
O
2
and CeNR recovered after
(JCPDS34-394)
Figure 1). The TEM shows that CeNR consists essentially of nanorods with a few tens of diameter
catalysis (CeNR-PhEt-BuOOH-run2) are indexed to cubic fluorite-type CeO
2
(
and lengths of up to several hundreds of nanometers. Some fragmentation of the nanorods and
formation of small 6–8 nm spherical nanoparticles occurred for CeNR-PhEt-BuOOH and CeNR-
2 2 2 2
BuOOH and is much more significant for CeNR-H O and CeNR-PhEt-H O (Figure 2); this is

Molecules 2010, 15
749
consistent with ref. [5]. The formation of small nanoparticles is consistent with the broadening of the
powder XRD reflections, which is clear in the case of CeNR-H (Figure 1).
2
O
2
The nitrogen adsorption-desorption isotherm (-196 ºC) of CeNR shows a substantial increase in the
uptake of nitrogen at relative pressures greater than ca. 0.8, which is accompanied by hysteresis,
possibly due to condensation/evaporation of nitrogen in/from the mesoporous network (Figure 3). The
2
BET specific surface area is 78 m /g (Table 1), and the PSD curve is rather broad with maximum pore
widths of 16–20 nm (Figure 3): this may correspond to inter-particle voids and/or to open pores of a
possible minor fraction of nanotubes. It is worth mentioning that the PSD curve calculated from the
adsorption branch is not well-defined. The specific surface area of the pre-treated samples, CeNR-
BuOOH and CeNR-H O are comparable to that of CeNR.
2
2
Figure 1. Powder XRD patterns of (a) CeNR, (b) CeNR-PhEt-BuOOH, (c) CeNR-BuOOH
and (d) CeNR-H
2
2
O .
(
111)
(
220)
(
200)
(
311)
222)
(
(
400) (d)
(c)
(b)
(a)
1
0
20
30
40
50
60
70
80
2
.1. Catalytic Oxidation of Ethylbenzene
2
.1.1. General
CeNR promotes the liquid-phase oxidation of ethylbenzene (PhEt), using tert-butyl hydroperoxide
t-BuOOH) as the oxidant and acetonitrile (CH CN) as the solvent, at 55–105 ºC. At 55 ºC, 1-phenyl-
ethyl-tert-butyl-peroxide (PhEtOOT) is the only reaction product until 51% conversion (reached after
20 h) (Figure 4): no reaction takes place without a catalyst. To the best of our knowledge, the
(
3
1
formation of PhEtOOT in the catalytic oxidation of PhEt with t-BuOOH has not yet been reported.
Syntheses procedures for PhEtOOT are based on the reaction of α-phenyl-ethyl bromide with
potassium salt of t-butyl hydroperoxide [14]. In the range 70–105 ºC, PhEtOOT selectivity decreases
with PhEt conversion, while that to acetophenone increases (Figure 4B). These results suggest that
acetophenone is formed from PhEtOOT. On the other hand, the PhEtOOT selectivity tends to decrease
with the reaction temperature whereas that of acetophenone increases reaching 70% at 89%
conversion, at 105 ºC (Figure 4B, Table 2). Aromatic ring oxidation products were never detected
revealing excellent regioselectivity of this catalytic system towards the side-aliphatic chain oxidation.

Molecules 2010, 15
750
The absence of carboxylic acids (which could be formed in consecutive oxidations) was checked by
analysing derivatized samples by GC-MS.
2 2
Figure 2. TEM images of (a) CeNR, (b) CeNR-BuOOH, (c) CeNR-H O , (d) CeNR-PhEt-
BuOOH and (e) CeNR-PhEt-H O .
2
2
Table 1. BET specific surface area.
2
Sample
S
BET (m /g)
CeNR
CeNR-BuOOH
CeNR- H O
2 2
78
55
90

Molecules 2010, 15
Figure 3. CeNR nitrogen adsorption-desorption isotherm (77 K) and (inset) pore-size
751
distribution curve calculated for the desorption branch, using the Barrett-Joiner- Halenda
method.
300
250
200
150
100
0
25 50 75 100
d (nm)
5
0
0
0
.0
0.2
0.4
0.6
0.8
1.0
p/p0
Increasing the temperature from 55 ºC to 90 ºC enhances the reaction rate: conversion at 6 h/120 h
increases from 6%/51%, at 55 ºC, to 31%/96%, at 90 ºC (Figure 4A, Table 2). Increasing the
temperature above 90 ºC does not further increase the PhEt conversion. For the reaction carried out at
1
1
05 ºC, the conversion of PhEt at 6 h/120 h is 22/88% and after 120 h remains nearly constant (89% at
75 h). Probably, the thermal decomposition (disproportionation) of t-BuOOH becomes a major
competitive reaction for PhEt oxidation at high temperatures (see section 2.1.3) [15].
When a radical scavenger (2,6-di-tert-butyl-4-methyl-phenol) is added initially to the reaction of
PhEt with t-BuOOH in the presence of CeNT, at 70 ºC, no reaction takes place, in contrast to that
observed without the radical scavenger. These results indicate that a radical-chain process is involved.
The initial induction periods of at least 120 min and 10 min observed for the reactions carried out at
5
5 ºC and 70 ºC, respectively (at higher temperatures no induction periods were detected), may be due
to mechanistic factors. Possibly, the CeNR material acts as an initiator in the homolysis of t-BuOOH
•
•
into free alkoxy (t-BuO ) and alkylperoxy radicals (t-BuO
2
) via the net redox reaction represented in
4
+
3+
Equation 1: Ce is a relatively strong one-electron oxidant, and the resulting Ce species may be re-
•
oxidized by t-BuOOH [16]. The t-BuO radical can abstract a proton from the substrate to give the
•
•
corresponding radical (PhEt ), Equation 2 [17], which reacts with t-BuO
2
radicals to give PhEtOOT
(Equation 3). It has been reported for the liquid-phase oxidation of arylbenzene compounds in
3
homogeneous phase, in the presence of cerium ammonium nitrate/KBrO that radicals of the substrates
4
+
3+
may reduce Ce into Ce species, forming intermediate carbocations, which are subsequently
oxidized [18]. Accordingly, cerium ions may promote the introduction of the alkylperoxy group of t-
•
BuOOH into PhEt via the net redox reaction represented in Equation 4. The reactions represented by
Equations 5–6 are typically involved in radical chain processes of oxidation of hydrocarbons, using t-
BuOOH as oxidant oxidant [16]. A control experiment showed that the oxidation of PhEt in the

Molecules 2010, 15
752
presence of CeNT and in the absence of t-BuOOH does not take place. Hence, the autoxidation of the
substrate with molecular oxygen may be neglected (ethyl benzene hydroperoxide was not detected in
the reaction mixtures by GC-MS): the observed induction periods may be due to intermediate rate
limiting elementary steps.
The PhEtOOT product may be transformed into acetophenone (PhEt=O) plus t-BuOH via radical
intermediates [14,17] (Equation 7). This hypothesis is supported by the fact that when CeNR is filtered
from the reaction mixture after 24 h, at 70 ºC (at this point PhEtOOT is the only product), and the
obtained solution is left to react in the absence of CeNR until 48 h, at the same temperature, the
formation of acetophenone is observed. On the other hand, the addition of a radical scavenger to the
reaction of PhEt with t-BuOOH in the presence of CeNR, after 24 h, at 70 ºC, not only inhibits the
reaction of PhEt, but also no acetophenone is formed after 48 h, in contrast to that observed without a
radical scavenger.
3
+
4+
[
Ce / Ce ]
•
•
2
t-BuOOH
→
t-BuO + t-BuO + H O
(1)
(2)
(3)
(4)
(5)
(6)
(7)
2
2
•
•
t-BuO + PhEt
→
t-BuOH + PhEt
•
•
PhEt + t-BuO
2
→
[
PhEtOOT
4
+
3+
Ce / Ce ]
•
+
PhEt + t-BuOOH
→
PhEtOOT + H
•
•
t-BuO + t-BuOOH →
t-BuO
2
+ t-BuOH
•
•
2
t-BuO
2
→
2 t-BuO + O
2
PhEtOOT
PhEt=O + t-BuOH
When 1,2-dichloroethane is used as solvent instead of acetonitrile, the reaction at 70 ºC, gives 45%
conversion at 48 h and PhEtOOT is the only product. These results are comparable to those observed
for CeNR suggesting that a similar reaction mechanism is involved for both solvents. Further studies
were carried out using acetonitrile a solvent.
2
.1.2. Stability of CeNR in the Liquid-Phase Oxidation Reaction
In order to get some insight into the stability of the CeNR under the applied oxidizing conditions,
the solid was recovered from the reaction of PhEt with t-BuOOH, using acetonitrile as solvent, at
0 ºC and used in a second batch run. The solid was separated from the reaction medium by
7
centrifugation, washed consecutively with n-hexane, dichloromethane and acetone, and finally dried at
room temperature prior to reuse (giving CeNR-PhEt-BuOOH). The kinetic curves for runs 1 and 2 are
roughly coincident as well as the dependence of products selectivity on conversion (Figure 4, Table 2).
The powder XRD patterns of CeNR and CeNR-PhEt-BuOOH are similar (Figure 1). The TGA
analyses of the fresh and used solids gave similar weight losses between 30 ºC and 600 ºC
(
ca. 8 wt. %), under air, and therefore coking is negligible. These results suggest that CeNR is fairly
stable under the applied oxidizing conditions.

Molecules 2010, 15
753
Carrying out the reaction of PhEt in homogeneous phase in the presence of the precursor used for
preparing CeNR, e.g., cerium sulphate (using the same mass as CeNR), at 70 ºC, gives 15%
conversion after 120 h, and PhEtOOT and acetophenone are formed in 34% and 66% selectivity,
respectively (Table 2). The reaction is much faster in the presence of CeNT than that of
Ce (SO ) .9H O, under similar reaction conditions.
2
4 3
2
Table 2. Catalytic oxidation of PhEt with t-BuOOH, in the presence of the cerium-based catalysts.
Catalyst
Temperature Conversion
PhEtOOT
Selectivity
PhEt=O
Selectivity
6/120 h
0/0
(
ºC)
6/120 h
6
/120 h
CeNR
55
70
70
70
70
90
105
70
6/51
15/64
11/64
16/97
10/78
31/96
22/88
0/15
100/100
a
a
a
CeNR (run 1)
CeNR (run 2)
CeNR-BuOOH
100/82
0/18
100/82
100/34
0/18
0/64
a
a
a
CeNR-H
CeNR
2
O
2
100/71
100/43
100/35
-/66
0/29
0/57
0/65
-/34
CeNR
Ce(SO ) .9H O
4 2 2
a
Reaction time:150 h.
Commercially available CeO (powder, < 5 μm particle size, cubic fluorite-type, irregular
2
morphology and apparently wide particle size distribution, Aldrich) gives 29% conversion after 150 h,
at 70 ºC, and PhEtOOT and acetophenone are formed in 63% and 37% selectivity, respectively; under
similar conditions, the reaction is faster in the presence of CeNR (64% conversion), and PhEtOOT and
acetophenone are formed in 18% and 82% selectivity, respectively.
In order to assess the homo/heterogeneous nature of the catalytic reaction in the presence of CeNR,
the solid was separated by filtration (Whatman 0.2 μm PVDF w/GMF membrane) after 6 h reaction, at
7
0 ºC, and the obtained solution was left to react at this temperature until 48 h. Conversion between 6
h and 48 h was approximately a third of that observed in the presence of CeNR (11% vs. 32%):
possibly, the reaction proceeds in homogeneous phase due to (i) active CeNR, which was not separated
by filtration and/or (ii) free radicals formed prior to catalyst filtration. Hypothesis (i) is ruled out by
two experiments. The solid was separated from the reaction medium after 120 h by filtration at 70 ºC,
and the obtained solution was analysed by ICP-AES for cerium, which was not detected (detection
limit ca. 5 ppb). Nevertheless, the obtained solution was evaporated to dryness and to the same vessel
PhEt, t-BuOOH and CH
stirring this mixture during 150 h, at 70 ºC, no PhEt reaction was observed, indicating that the
residue’ obtained after evaporation did not contain active cerium compounds. Hypothesis (ii) is
3
CN were added in the same amounts as those used for the first batch run: after
‘
supported by the fact that hypothesis (i) is ruled out and the reaction stops when a radical scavenger is
added, as mentioned in section 2.1.1. Based on these results, it seems clear that CeNR performs as a
heterogeneous redox catalyst for generating free radicals, accounting for a radical chain process in the
homogeneous phase.
When pristine CeNR was heated up to 600 ºC in air no structural modifications were observed for
the resulting solid (denoted CeNR600), as ascertained by powder XRD, SEM and FTIR (not shown).

Molecules 2010, 15
754
However, CeNR600 is inactive in the reaction of PhEt with t-BuOOH, at 70 ºC. The calcination
process induces marked differences in the colour (CeNR is yellow and CeNT600 is light maroon) and
3
+
4+
in the emission spectra of these materials, leading changes in the distribution of Ce /Ce ions (as
discussed ahead in section 2.3). The solid recovered (washed and dried) from the reaction of PhEt with
t-BuOOH in the presence of CeNR, during 150 h, at 70 ºC, was heated at 280 ºC, during 3 h, under
static air, giving a solid denoted CeNT280. The reaction of PhEt with t-BuOOH in the presence of
CeNR280, at 70 ºC, is slower than that observed for CeNR and CeNR-PhEt-BuOOH, and PhEtOOT is
the only product until 54% conversion, reached after 120 h. Thermal treatments seem to gradually
destroy the active sites, which may be defect sites in the original material. It has been reported in the
3
+
4+
literature that defect sites of the type Ce -O-Ce are responsible for activating PhEt using N O as
2
2
oxidant, at 325 ºC, and increasing temperature of H treatment from 300 ºC to 350 ºC led to a drastic
decrease in concentration of these defect sites [19]. UV-vis and photoluminescence studies revealed
3
+
4+
the presence of both Ce and Ce in CeNR before and after it exposure to t-BuOOH, as
discussed ahead.
Figure 4. (A) - Ethylbenzene oxidation with t-BuOOH, in the presence of CeNR, at 55 ºC
(
‘), 70 ºC (run 1 - (O); run 2 - (z)), 90 ºC (U), 105 ºC (ꢀ), using acetonitrile as solvent.
(B) - Dependence of selectivity to PhEtOOT (solid lines) and acetophenone (dashed lines)
on conversion for the reactions conditions indicated in (A) - the same symbols were used
for each reaction.
100
(
A)
100
(
B)
80
60
40
20
0
80
60
40
20
0
0
30
60
Time (h)
90
120
150
0
20
40
60
80
100
Conversion (%)
2
.1.3. Oxidant Effect
2 2
When H O (30% aq.) is used as oxidant instead of t-BuOOH, no PhEt reaction is observed in the
presence of CeNR during 150 h, at 70 ºC. It was previously reported for a ceria possessing a cubic
2
-1
crystal structure of the fluorite type (67 m g , 12 nm crystallite size), used as catalyst in the oxidation
of PhEt with H and acetonitrile as solvent, that the reaction is sluggish giving ca. 4% conversion
2
O
2
after 6 h, at 60 ºC, and benzaldehyde and acetophenone were the only products [20]. The
decomposition of H O has been used as a test reaction for investigating the redox properties of ceria
2
2
2 2
materials [1]. In the case of the CeNR/H O system, the poor catalytic results may be due to the

Molecules 2010, 15
755
relatively fast ‘non-productive’ decomposition of H
2
O
2
by CeNR. In order to investigate this
hypothesis, iodometric titrations were performed for the reactions of CeNR with H
2 2
O or t-BuOOH
(
without PhEt) after 4 h, at 70 ºC. These results revealed that H was totally consumed, whereas
2 2
O
t-BuOOH conversion after 4 h was negligible, supporting the above hypothesis. It is worth mentioning
that acetamide, which may be formed from the reaction of CH CN with H O via the intermediate
3
2
2
formation of the correspondent peroxyimidic acid was not detected in the reaction mixtures [21].
According to the literature, CeNR may undergo morphological changes in the presence of H O [8]. A
2
2
similar destructive effect of H
2
O
2
has also been reported for cerium-containing SBA-15, in that cerium
crystallites on the surface [22].
was removed from the silica framework, forming fine CeO
2
To the best of our knowledge, the influence of t-BuOOH on the physico-chemical properties of
CeNR has not been reported. In order to get some insight into the effect of the oxidant on the catalytic
activity, the solids used in the reactions of PhEt with t-BuOOH or H O , at 70 ºC, were recovered after
2
2
1
20 h in a similar fashion to that described above for the catalyst recycling experiments, and
subsequently characterized: the resulting solids are denoted CeNR-PhEt-BuOOH and CeNR-PhEt-
H O , respectively. On the other hand, for comparative studies, CeNR was treated with H O or
2
2
2
2
t-BuOOH, in the absence of PhEt, in an ultrasonic bath, as described in the experimental part, giving
CeNR-H and CeNR-BuOOH, respectively. The TEM images of CeNR-BuOOH and CeNR-PhEt-
BuOOH show some fragmentation of the nanorods and the formation of 6–8 nm spherical particles,
and this effect is major for CeNR-H and CeNR-PhEt-H (Figure 2). The ATR-FTIR spectra of
CeNR-BuOOH and CeNR-H are similar to that of CeNR in the region 1300–1600 cm , with the
main difference being that the band at ca. 1640 cm assigned to H-O-H bending vibration is very weak
in the case of CeNR-BuOOH (not shown).The powder XRD patterns of CeNR-H and CeNR-
BuOOH are similar to that of CeNR (see introduction of the section 2), albeit the peaks are slightly
broader, especially in the case of CeNR-H (Figure 1).
In order to assess the effect of the oxidizing pre-treatment on the reaction of PhEt, the CeNR-H
2
O
2
2
O
2
2 2
O
-1
2
O
2
-1
2
O
2
2
O
2
2
O
2
and CeNR-BuOOH materials were tested as catalysts in the oxidation of PhEt with t-BuOOH, at 70 ºC.
No major differences in products selectivity were observed between the pre-treated solids and CeNR,
at least until ca. 70% conversion, suggesting that a similar reaction mechanism is involved (Figure 5).
However, the reaction rate (based on conversion at 24 h/120 h) is higher for the pre-treated materials
2 2
than for CeNR, increasing in the order CeNR (35%/~60%) < CeNR-H O (42%/78%) < CeNR-
BuOOH (50%/~90%), Table 2. Altogether, the results support the hypothesis that CeNR is not
effective in the reaction of PhEt with H O because of its effectiveness in H O disproportionation.
2
2
2
2
The reaction of t-BuOOH on the surface of CeNR may primarily involve the coordination of the
oxidant to defect sites (possibly co-ordinately unsaturated cerium), followed by redox reactions to
form radical species, according to the above mechanistic considerations.
2
.2. Oxidation of Different Substrates
CeNR was further tested as catalyst in the oxidation of cyclohexene and cyclohexanol, at 55 ºC,
during 24 h (Table 3). For both substrates, when H
negligible, similar to that observed for PhEt. These results contrast with those reported for mesoporous
Ce-SBA-15 in the oxidation of cyclohexene and cyclohexanol with H , under similar conditions to
2 2
O was used as oxidant, conversions were
2
O
2

Molecules 2010, 15
756
those used in the present work: 40% cyclohexene conversion was reached after 10 h, at 50 ºC, forming
the epoxide as the main product (other products included the 1,2-diol, -en-1-ol and -en-1-one) and, on
2 2
the other hand, the reaction of cyclohexanol with H O gave a maximum of ca. 15% conversion after
7
h, at 60 ºC [22]. The competitive ‘non-productive’ decomposition of H O is probably less important
2
2
for the referred supported ceria catalyst than for CeNR. Differences in catalyst surface polarity may
lead to different competitive adsorption effects, which may partly explain the different catalytic
performances of CeNR and Ce-SBA-15 using H O as oxidant.
2
2
Figure 5. (A) - Ethylbenzene oxidation with t-BuOOH, at 70 ºC, using acetonitrile as
solvent, in the presence of CeNR (O), CeNR calcined at 280 ºC (¯),CeNR pre-treated with
H O (ꢀ) or t-BuOOH (U). (B) - Dependence of selectivity to PhEtOOT (solid lines) and
2
2
acetophenone (dashed lines) on conversion for the reactions conditions indicated in (A) -
the same symbols were used for each reaction.
1
00
100
80
(
A)
(B)
8
6
4
2
0
0
0
0
0
60
40
2
0
0
0
30
60
90
120 150 180
0
20
40
60
80
100
Time (h)
Conversion (%)
The reaction of cyclohexene with t-BuOOH in the presence of CeNR gives 22% conversion after
2
4 h and the main product is 1-(tert-butyl-peroxy)-2-cyclohexene (86% selectivity): by-products
include 2-cyclohexen-1-one and minor amounts of 2-cyclohexen-1-ol (when no catalyst is added, no
reaction is observed). The products selectivity for cyclohexene somewhat parallels that obtained for
PhEt, in that for both substrates the major product is a tert-butyl-peroxy (alkyl or aryl) compound, and
saturated C-H bonds are preferentially activated over the unsaturated ones. The 1-(tert-butyl-peroxy)-
2
-cyclohexene product has been identified in the oxidation of cyclohexene with t-BuOOH via radical
reactions in the presence of free and alumina-supported transition metal complexes bearing Schiff base
ligands [23–25].
The reaction of cyclohexanol with t-BuOOH gives cyclohexanone as the only product in 33% yield,
after 24 h, at 55 ºC (when no catalyst is added, cyclohexanone yield is 11%), Table 3. Based on these
results the choice of a solvent for the catalytic oxidation reactions should preferably not include
alcohols. For all substrates, CeNR600 gave negligible conversions, similar to that observed for PhEt.

Molecules 2010, 15
Table 3. Oxidation of different substrates with t-BuOOH, in the presence of CeNR, using
757
acetonitrile as solvent, during 24 h, at 55 ºC.
Conversion
Substrate
Selectivity
(%)
Product
(
%)
1
-phenyl-ethyl-tert-butyl-peroxide
Ethylbenzene
Cyclohexene
2
5
O
O
100
1
-(tert-butyl-peroxy)-2-cyclohexene
a
2
2
3
86
O O
Cyclohexanol
Cyclohexanone
OH
O
3
100
a
By-products are mainly 2-cyclohexen-1-one and minor amounts of 2-cyclohexen-1-ol.
2
.3. UV-Vis and Photoluminescence Spectroscopy
Figure 6A displays the UV/Vis absorbance spectra of the as-prepared CeNR, CeNR-BuOOH, and
3
+
2 2
CeNR-H O powders. The peaks attributed to the 4f→5d transitions of Ce (220–320 nm), are
3
+
observed in all spectra. The excitation spectra (Figure 7) also exhibit Ce 4f→5d transitions. A broad
band (cut-off wavelength 480–550 nm) is also observed due to charge transfer transitions from O 2p to
Ce 4f [27–30]: a broadening of this band is observed especially in the case of CeNR-H
CeNR possibly due to the increasing number of surface defects [13]. The optical energy gap, ε
the indirect band gap, ε , were calculated for a suspension of CeNR in ethanol (Figures 6B and 6C).
The values obtained (ε = 3.1 eV and ε = 2.5 eV) are in accord with the band gaps of ceria bulk
2
O
2
relative to
d
, and
i
d
i
[
2 2
31,32]. The band gap values were not estimated for Ce-BuOOH and CeNR-H O due their low
dispersion and stability in suspension. However, the similarity of the absorption spectra measured for
the CeNR and CeNR-BuOOH powders (Figure 6A) suggest that the energy gaps of both materials may
2 2
be comparable, albeit may differ somewhat from that of CeNR-H O .

Molecules 2010, 15
Figure 6. UV-Vis absorbance spectra of (A) CeNR (black), CeNR-BuOOH (blue) and
758
2
1/2
2 2
CeNR-H O (red) powders. Plots of (B) (αhν) versus (hν) and (C) α versus (hν) for the
CeNR ethanol suspension.
(
A)
0
0
0
0
0
0
.5
.4
.3
.2
.1
.0
O 2p → Ce 4f
4
f→ 5d
2
50 300 350 400 450 500 550 600
(
B)
(C)
9
9
9
9
120
4
3
2
1
x10
x10
x10
x10
1
00
8
0
60
4
2
0
0
0
0
2
3
4
5
6
2
3
4
5
6
Figure 7. −11 K excitation spectra of CeNT monitoring at 370.1 nm (black), 390.2 nm
red) and 450 nm (blue).
(
3
-1
E nergy (10 cm
)
3
8
37
36
35
34
33
32
31
30
2
70
280
290
300
310
320
330
340
W avelength (nm )
The emission spectra at -262 ºC of CeNR, CeNR-H
32 nm) display two overlapping broad bands at ca. 375 and 410 nm, exemplified for CeNR-H
spectral deconvolution (Figure 8A). In contrast to that observed for CeNT-H , in the case of CeNR
2
O
2
and CeNR-PhEt-BuOOH (excitation at
3
2 2
O
by
2
O
2

Molecules 2010, 15
759
and CeNR-PhEt-BuOOH, a series of sharp peaks at ca. 370.1, 375.8, 380.6, 386.3 and 390.5 nm were
discerned superimposed to the broad band envelope. Whereas the two broad bands are unequivocally
-1
3+
2
(
because their energy separation is 2270 cm ) attributed to Ce 5d→ F5/2,7/2 transitions [33,34], the
nature of the sharp peaks is still open to discussion: some authors suggest that the peaks are attributed
3
+
to the hopping from localized Ce states or different defect levels to the O 2p band [35].
Figure 8. (A) 11 K emission spectra of CeNR (red), CeNR-PhEt-BuOOH (green) and
CeNR-H O (magenta) excited at 316 nm: the dotted lines represent spectral deconvolution
2
2
2
using two Gaussian functions attributed to the 5d→ F5/2,7/2 transitions. The inset shows the
emission spectra of CeNR (black) and CeNR600 (cyan) excited at 332 nm. (B) Emission
intensity as a function of temperature of CeNR (red squares) and the Mott-Seitz fitting
(solid line) using Equation. (8).
(A)
(
B)
3
75
450
525
Wavelength (nm)
3
40 360 380 400 420 440 460 480 500 520
30 60 90 120 150 180 210 240 270 300
Temperature (K)
Wavelength (nm)
The emission spectra of CeNR and CeNR600 excited at 332 nm are displayed in Figure 8 (inset)
3
+
showing the same sharp peaks (360–400 nm) associated with the hopping from localized Ce states or
2
different defect levels to the O 2p band superimposed to the 5d→ F5/2,7/2 broad transitions. Although
3
+
the emission spectra of all the different materials clearly show the presence of Ce ions, the
2
differences between the relative intensity of the sharp peaks and the 5d→ F5
and 4f→O 2p broad
/2,7/2
transitions when comparing CeNR600 to the non-calcined samples suggest that the calcination process
3
+
modifies the distribution of Ce ions present in bulk regions and/or in surface defects [19]. These
3+
4+
results together with the catalytic ones, suggest that the presence of Ce /Ce in the material is not
sufficient for the existence of catalytic activity and most likely catalytic activation is associated to a
certain population of defect sites.
Emission spectra were acquired as a function of temperature, wavelength excitation of 282 nm, and
the evolution of the integrated emission is depicted in Figure 8B. At room temperature, CeNR material
does not emit. Several reasons have been put forward to explain the quenching of luminescence
including the thermally-excited interconfigurational system crossing, energy transfer from 4f level to
nearby centres, electron transfer or photoionization [36]. Between -262 and -173 ºC the CeNR
emission intensity remains constant and then it decreases markedly, almost disappearing at ca. -53 ºC.

Molecules 2010, 15
760
This could be the result of defects (including oxygen vacancies site in the crystal) with electronic
energy levels between the O2p and 4f bands. The number of these defects is temperature dependent
[
37–39]. This behaviour can be described by a thermally-activated non-radioactive mechanism
modelled by the Mott Seitz Equation [40,41]:
I0
I =
+ aexp(
−ΔE kBT
)
(8)
1
where a is a constant and ΔE is the activation energy that corresponds to the energy gap between the 4f
level and that of the de-excitation states. A fit of the experimental values by Equation (8) (Figure 8B)
-1
gives an activation energy of ca. 651 cm (81 meV). This value is consistent with a previous report
where the defect levels are located in the range of 1 eV below the Ce 4f band [38,39].
3. Experimental Section
3
.1. Samples
Cerium oxide nanorods (CeNR) were obtained by hydrothermal synthesis using a published
procedure [8]: some nanotubes were formed (ascertained by TEM analysis as discussed above), albeit
apparently they represent a minor fraction of this sample. In a typical reaction, 0.6 g of
3
Ce
2
(SO
4
)
3
.9H
2
O (Aldrich) was dispersed in 35 cm of NaOH (12 M). This mixture was stirred during 1
®
3
h, then transferred to a Teflon autoclave (inner volume 42 cm ) and placed in a pre-heated furnace at
35 ºC for 15 h. The yellowish Ce(OH) precipitates were then filtered, washed and dried at 60 ºC
1
3
under nitrogen atmosphere. The materials were left under air for a week giving CeNR.
The CeNR material was partially oxidised in air, at room temperature, for 24 h and then dispersed
in distilled water and treated ultrasonically for 2 h with 70% aqueous tert-butyl hydroperoxide
(
t-BuOOH) or 15% aqueous hydrogen peroxide to give CeNR-BuOOH and CeNR-H
2 2
O , respectively.
3.2. Structure and Texture Characterisation
TEM measurements were done on H-9NA microscope over the ceria samples on amorphous
®
carbon-Fomvar grid support. These samples were previously dispersed in distilled water (0.5 mg/mL)
with ultrasonic stirring during 10 min; the TEM grid was deep coated in this suspension and dried
in air.
The XRD patterns were recorded using a Philips X’Pert MPD powder X-ray diffractometer. The
samples were exposed to the CuK radiation (1.54 Å) in a 2θ range between 1 and 80º with a step of
α
0
.04 and acquisition time of 40 s per step.
The nitrogen adsorption-desorption isotherms were measured at -196 ºC, using a Micromeritics
®
Gemini equipment. Prior to analysis the powdered samples were pre-treated at 150 ºC. The BET
specific surface areas were calculated for p/p
PSD) curves were calculated from the desorption branches of the isotherms using the Barrett-Joiner-
Halenda method.
0
in the range 0.015–0.15. The pore size distribution
(

Molecules 2010, 15
.3. Catalysis
The liquid-phase oxidation of ethylbenzene was carried out under air and autogeneous pressure, in
761
3
the temperature range 55–105 ºC, using a micro reactor equipped with a valve for sampling, a
magnetic stirrer and immersed in a thermostated oil bath. The micro vessel was loaded with 30 mg of
catalyst, 1.5 mmol of substrate, 3 mmol of oxidant (70% aq. tert-butyl hydroperoxide or 30% aq.
3
H
2
O
2
) and 1.5 cm of acetonitrile (is of modest toxicity and seems fairly stable under the applied
reaction conditions) as solvent. The reaction mixture was cooled in an ice bath during 30 seconds prior
to opening the valve for sampling, which was performed under stirring. Cyclohexanol and cyclohexene
were also tested as substrates. Samples were analysed using a Varian 3800 gas chromatograph
equipped with a DB-5 30 m × 0.25 mm capillary column and a flame ionisation detector, using decane
as internal standard added to the withdrawn samples. The reaction products were identified by GC-MS
(
HP 5890 Series II GC; HP 5970 Series Mass Selective Detector) using He as carrier gas.
3
.4. UV/Vis Spectroscopy
UV/Vis absorbance spectra were acquired from CeNR, CeNR-BuOOH, CeNR-H O and CeNR-
2
2
PhEt-BuOOH powders and from CeNR dispersion on a Jasco V 560 UV/Vis spectrometer. The
absorbance spectrum from CeNR dispersion was recorded in a quartz cell (1 cm path length), and
hexane or ethanol were used as blank. The optical absorption coefficient α was calculated according to
-3
2
α = (Aρ)/lc 13], where A is the absorbance of the sample, ρ the density of CeO (7.28 g cm ), l the
-3
path length (1 cm), and c the concentration of the ceria suspension (0.00026 g cm ).
3
.5. Photoluminescence Spectroscopy
Emission and excitation spectra were recorded between -262 ºC and room temperature on a
®
Fluorolog -3 Model FL3–2T with double excitation spectrometer (Triax 320), fitted with a
-1
1
200 grooves mm grating blazed at 330 nm, and a single emission spectrometer (Triax 320), fitted
with a 1200 grooves/mm grating blazed at 500 nm, coupled to R928 photomultiplier. Excitation
spectra were corrected from 240 to 580 nm for the spectral distribution of the lamp intensity using a
photodiode reference detector. Emission spectra were also corrected for the spectral response of the
monochromators and the detector using typical correction spectra provided by the manufacturer.
4
. Conclusions
The CeO nanorods (CeNR) promote the oxidation of ethylbenzene (PhEt) and cyclohexene with
2
t-BuOOH, at temperatures as low as 55 ºC. The reaction of PhEt gives 1-phenyl-ethyl-tert-butyl-
peroxide (39% yield after 48 h, 55 ºC), which may be converted into acetophenone (70% selectivity at
8
9% conversion, 105 ºC). The reaction of cyclohexene with t-BuOOH, at 55 ºC, gives mainly 1-(tert-
butyl-peroxy)-2-cyclohexene (86% selectivity at 22% conversion). For both substrates the saturated
C-H bonds are preferentially activated over the unsaturated ones.
3
+
4+
The liquid-phase oxidation catalysis with CeNR/t-BuOOH may be associated with the Ce /Ce
inter-conversion in the one-electron redox processes mediating the formation of tert-butyl-(per)oxy
radicals. The tert-butyl-peroxy products of the reactions of cyclohexene and ethylbenzene are most

Molecules 2010, 15
762
likely formed via the homolytic addition of tert-butyl-peroxy radicals (these results contribute to the
list of the main applications of t-BuOOH as a source for tert-butyl derivatives, including alkyl
peroxides). CeNR is fairly stable towards leaching of cerium and the catalytic results obtained for the
used CeNR are similar to those observed in the first batch run.
Pre-treatment of CeNR with H O or t-BuOOH prior to reaction of PhEt with t-BuOOH leads to
2
2
faster reaction than CeNR. While ATR-FTIR does not show major differences in the bulk features of
CeNR (before and after exposure to the oxidants), UV-vis and photoluminescence spectroscopies
shows noticeable differences in the case of CeNR-H O , which are probably associated to the defect
2
2
sites present in the materials (supported by the powder XRD data and TEM images): UV-vis and
3
+
4+
photoluminescence spectroscopy confirm the presence of both Ce and Ce in all investigated
materials. Thermal treatment at 600 ºC of CeNR has a detrimental effect on catalytic activity. While
the powder XRD and TEM data are similar for CeNR and CeNR600, the emission spectra of these
3
+
4+
materials shows changes in the distribution of Ce /Ce ions probably in defect sites. Although
exposure to H seems to give a more defective material (CeNR is very effective in the
disproportionation of H O ) than that with t-BuOOH, the PhEt reaction is faster for CeNR treated with
2
O
2
2
2
t-BuOOH. The enhanced catalytic activity associated with the generation of defect sites may be
levelled off by the concomitant morphological changes (extensive fragmentation) leading to defect
sites with different structures and electronic properties (and expectedly different intrinsic catalytic
activity). A detailed study using XPS spectroscopy could give further insights on structure-activity
relationships.
Acknowledgements
We acknowledge the financial support of PTDC, FEDER, FSE and FCT (SFRH/BD/28433/2006).
The authors wish to thank Patrícia Neves and Salete Balula for their assistance in performing some
preliminary catalytic experiments.
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Sample Availability: Samples of the compounds CeO
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from MDPI).
©
2010 by the authors; licensee Molecular Diversity Preservation International; Basel; Switzerland.
This article is an open-access article distributed under the terms and conditions of the Creative
Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).