Preparation of a new metallomicelle catalyst for the hydrolysis of bis(4-nitrophenyl) phosphate

Article abstract of DOI:10.2478/s11696-012-0281-9

A new metallomicellar system containing cerium(III), a macrocylic polyamine ligand, and the nonionic surfactant Brij35(polyoxyethylene(23) lauryl ether) was prepared and used as a catalyst in the hydrolysis of bis(4-nitrophenyl) phosphate (BNPP). Catalyti

Full text of DOI:10.2478/s11696-012-0281-9

Chemical Papers 67 (4) 365–371 (2013)
DOI: 10.2478/s11696-012-0281-9
ORIGINAL PAPER
Preparation of a new metallomicelle catalyst for the hydrolysis
of bis(4-nitrophenyl) phosphate
a
b
a
a
Jia-Qing Xie, Ci Li, Min Wang, Bing-Ying Jiang*
aCollege of Chemistry and Chemical Engineering, bAcademic Affairs Division, Chongqing University of Technology,
Chongqing 400054, China
Received 3 June 2012; Revised 31 August 2012; Accepted 18 September 2012
A new metallomicellar system containing cerium(III), a macrocylic polyamine ligand, and the non-
ionic surfactant Brij35(polyoxyethylene(23) lauryl ether) was prepared and used as a catalyst in the
hydrolysis of bis(4-nitrophenyl) phosphate (BNPP). Catalytic rate of the BNPP hydrolysis was
measured kinetically using the UV-VIS spectrophotometric method. The results indicate that the
metallomicellar system has relatively high stability and excellent catalytic function in the BNPP
hydrolysis; also, the reaction rate of the BNPP catalytic hydrolysis increased by a factor of ca.
10
1
× 10 compared to the BNPP spontaneous hydrolysis due to the catalytic effect of the active
species and the local concentration effect of the micelles in the metallomicellar system. Experimental
results also showed that the mono-hydroxy complex containing the macrocyclic polyamine ligand
and cerium(III) is the real active species in the BNPP catalytic hydrolysis, and that the micelles
provide a useful catalytic environment for the reaction. On basis of the research results, the reaction
mechanism of BNPP catalytic hydrolysis has been proposed.
ꢀ
c 2012 Institute of Chemistry, Slovak Academy of Sciences
Keywords: metallomicelles, construction, catalysis, hydrolysis, mechanism
Introduction
hydrolysis of phosphoric acid esters and as potential
catalysts for the detoxification of anticholinesterase
agents in chemical warfare. Among these model com-
pounds, the lanthanide ion and its complexes have at-
tracted much attention as catalysts for the hydroly-
sis of the phosphoric acid ester (Katada et al., 2008;
Kuchma et al., 2010) because of their extremely strong
Lewis acidity, higher oxidation state and charge den-
sity, coordination number, and high ligand exchange
rates (Maldonado & Yatsimirsky, 2005; Jurek et al.,
2000). These characteristics make the lanthanide ions
well-suited to be catalytic centers in the develop-
ment of artificial enzymes (Franklin, 2001). Cerium is
unique among the lanthanides and it has been found
to be particularly effective in promoting phosphodi-
ester hydrolysis and DNA hydrolysis (Rammo et al.,
1996; Komiyama et al., 1999; Tjioet et al., 2012).
In a many biomimetic models of hydrolytic met-
alloenzymes, macrocyclic polyamine metal complexes
Organophosphorus compounds are ubiquitous in
nature. To fully understand their behavior it is very
important to establish mechanisms of their transfor-
mation in biological systems. Many researchers have
worked hard to develop biomimetic models for the
metalloenzyme with high efficiency and selectivity in
order to achieve friendly environment and economi-
cally favorable processes (Rossi et al., 2002; Iranzo et
al., 2003; Tonde et al., 2006; Ferreira et al., 2008).
In this research field, hydrolysis of phosphodiester is
of paramount importance in biological and industrial
processes (Westheimer, 1987). In recent decades, some
metal complexes, such as Mn(III), Cu(II), Zn(II),
Ni(II), and lanthanide (Ln(III)) complexes (Luedtke
&
Schepartz, 2005; Lombardo et al., 2010; Rossi et
al., 2002; Patel et al., 2011; Gunnlaugsson et al., 2002;
Anbu et al., 2010), have been used as catalysts for the
*Corresponding author, e-mail: xjq8686@163.com

3
66
J. Q. Xie et al./Chemical Papers 67 (4) 365–371 (2013)
employed as catalysts in the hydrolysis of phospho-
ric acid esters have attracted considerable attention
NaBH4 (1.9 g, 63 mmol) and NaOH (1.7 g, 42 mmol)
were added alternately while stirring at 0 C. After the
◦
(
Mancin & Tecilla, 2007; Franklin, 2001) due to the
addition was completed, the solution was stirred me-
chanically for 2 h, and then heated to reflux for 8 min.
After the solution was cooled to room temperature,
pure water (100 mL) and NaOH (5.0 g, 125 mmol)
were added and the solution was stirred vigorously
until precipitation occurred. The solution was then fil-
tered and the residue was washed with cold water and
dried. The product (5.2 g), 5,5,7,12,12,14-hexamethyl-
1,4,8,11-tetraazacyclotetradecane dihydrate, was ob-
unique properties of macrocyclic polyamines; e.g. sta-
bilizing metal ions with appropriate radii, introduc-
ing side-arms with different functional groups exhibit-
ing different catalytic activities. Moving along these
lines, in order to better mimic both the active sites
and the hydrophobic micro-environment of natural en-
zymes, to elucidate the role of lanthanide ions and the
mechanism of phosphate esters hydrolysis catalyzed
by the polyamine macrocyclic complex, a new metal-
lomicellar system formed by the cerium(III) ion and
the macrocylic polyamine ligand in the presence of
surfactant Brij35 micelles.
◦
tained as white crystals: yield 88.0 %, m.p. 148–149 C.
−
1
IR (KBr), ν˜ /cm : 3325 υ(NH), 3416 υ(OH). MS:
+
m/z = 284 [M ]. Composition found (calc.) (wi/%):
C 59.95 (60.00), H 12.46 (12.50), N 17.63 (17.50).
As it is well known, natural phosphodiesters inside
the organism are very complex and stable, so it is very
difficult to explore or elucidate the mechanism of natu-
ral phosphodiesters cleavage. To understand the cleav-
age mechanism of the P—O bond of natural phos-
phodiesters, bis(4-nitrophenyl) phosphate (BNPP), a
much more reactive analogue of natural phosphodi-
ester substrates, was used as the reactive substrate in
this work.
Methodology
−
2
Concentrated solution of Ce(III) nitrate, 5 × 10
−
3
mol dm , was prepared in diluted HNO solution of
3
the desired pH, and concentrated solution of the lig-
−
2
−3
and, 5 × 10 mol dm , was freshly prepared in pure
ethanol. The metallomicellar system was prepared by
adding concentrated Ce(III) nitrate and the ligand so-
lution to the surfactant Brij35 solution of the desired
pH under stirring. Stock solutions of BNPP were pre-
Experiment
−
3
pared in water in the concentration of 5.0 × 10
−
3
Instrumentation and materials
mol dm
.
Kinetic studies were carried out on a GBC 916 UV-
VIS spectrophotometer equipped with a thermostated
cell holder. The mixture solution (3 cm ) of Ce(III) ni-
Melting points were determined on a Yanaco
MP-500 micro-melting point apparatus (Yanaco-Mat,
USA) and are uncorrected. Infrared spectra were
recorded on a Nicolet-1705X IR spectrometer (Nico-
let, USA). Mass spectra were obtained on a Finni-
gan MAT 4510 and a Finnigan LCQ-DECA spectrom-
eters (Bruker, Germany). Elementary analysis was
performed on a Carlo Erba 1106 elemental analyzer
3
trate and the ligand of the desired concentration were
3
added in a cuvette (4 cm ) with a temperature con-
trol system and the reactions were initiated by inject-
3
−2
−3
ing 30 mm of a 5.0 × 10 mol dm stock solution
of BNPP into the cuvette. The characteristic spec-
trum of p-nitrophenol (product of BNPP hydrolysis)
was detected at the wavelength of 404 nm. Therefore,
the kinetics of BNPP cleavage was monitored spec-
trophotometrically by the absorbance change of the
p-nitrophenol at 404 nm. To obtain reliable trends of
kobsd, measurements were performed using the same
cells and stock solutions in one day. BNPP catalytic
hydrolysis is considered to be a first-order reaction as
the concentration of the catalyst is at least ten times
higher than that of BNPP and the observed first-order
rate constants were calculated by the integral method
from the kinetic curves followed by up 95 % or higher
conversion of the substrate.
(Carlo Erba, Italy). pH of the solution was deter-
mined by a Radiometer PHM 26 pH meter fitted with
G202C glass and K4122 calomel electrodes (Shanghai
photics apparatus, China). Spectral analysis and ki-
netic studies were carried out on a GBC 916 UV-VIS
spectrophotometer equipped with a thermostatic cell
holder (GBC, Sweden).
Polyoxyethylene(23) lauryl ether (Brij35), 2-ethyl-
hexyl sulfosuccinate sodium (AOT), hexadecyltrime-
thyl ammonium bromide (CTAB), and bis(p-nitrophe-
nyl) phosphate (BNPP) were purchased from Sigma
Chemicals, USA. All reagents, unless otherwise indi-
cated, were of analytical grade. Water used for kinetic
experiments was doubly distilled.
Results and discussion
Synthesis of the ligand (L, C16H40N O )
Activity and preparation of the metallomicellar
system
4
2
5
,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclo-
tetradeca-4,11-diene perchlorate (10.0 g, 21 mmol),
synthesized according to literature (Douglas, 1983),
was dissolved in pure methanol (50 mL) and then,
In preliminary experiments on the BNPP cleavage,
either the stock solution of Ce(NO3)3 or the nitrogen
heterocyclic ligand was employed as the catalyst in the

J. Q. Xie et al./Chemical Papers 67 (4) 365–371 (2013)
367
pH range of 5.7–8.0. Experimental results show that
the nitrogen heterocyclic ligand has no activity and
the Ce(NO3)3 solution shows low stability and activ-
ity. Therefore, the metallomicellar systems used as the
catalyst consisting of the cerium(III) ion, the ligand,
and the surfactant (including anionic AOT, cationic
CTAB, non-ionic Brij35) were tested in our work. The
results indicate that the anionic and cationic met-
allomicellar systems display low stability and activ-
ity. Surprisingly, the non-ionic Brij35 metallomicellar
system showed reproducible and remarkable catalytic
activity and stability in the phosphodiester cleavage.
The observed first-order rate constant (kobsd) of the
−
1
−1
BNPP catalytic cleavage was 4.18 × 10
s and it
Fig. 1. Characteristic spectra of ligand (A), ligand + Ce(III) in
10
is ca. 1 × 10 times the rate of the BNPP sponta-
aqueous solution (B), and ligand + Ce(III) in Brij35 so-
−
5
−3
◦
−11 −1
(Jones et
lution (C): [Ce(III)] = [ligand] = 1.5 × 10
mol dm
,
neous hydrolysis at 25 C (kobsd = 10
s
−
3
−3
◦
[
Brij35] = 4.0 × 10
mol dm , pH = 7.0, t = 25 C.
al., 1983)).
To get more information on the reaction mecha-
nism and the final products, hydrolytic rate of the
p-nitrophenyl phosphate (NPP) catalyzed by the met-
allomicellar system was also examined in this work.
The experimental results show that the hydrolytic rate
of the p-nitrophenyl phosphate (NPP) is ten times
higher than that of BNPP under the same conditions.
This indicates that one mole of BNPP liberates two
moles of p-nitrophenol in the BNPP catalytic hydrol-
ysis.
Analysis of the active species as catalyst in the
aqueous solution reaction system
Spectral analysis is an effective method for the
analysis of the species from the given reactive pro-
cess, and it was used in the analysis of the reac-
tion intermediate (Li et al., 2005). According to the
literature method, the characteristic spectra of lig-
and C H N O (the reference solution is the sol-
Fig. 2. Fluorescence spectra of three catalytic systems Ce(III)
A), Ce(III) + L (B), and Ce(III) + L + Brij35 (C):
(
−3 −3
mol dm , [Ce(III)] = [ligand]
[
Brij35] = 4.0 × 10
−6 −3 ◦
mol dm , pH = 7.5, t = 25 C.
=
1.6 × 10
16
40
4
2
vent of the ligand) and those of the mixture solu-
tion of Ce(NO3)3 + ligand (the reference solution is
Ce(NO3)3 solution with the same concentration) were
recorded on a UV-VIS absorption spectrophotometer
in the range of 190∼400 nm. The results are shown in
Fig. 1.
Comparing the characteristic spectra of the ligand
solution and the mixture solution in Fig. 1, it can be
seen that the characteristic peak of the ligand at λ
Fluorescence spectroscopy can also be used to
study the interaction between the metal ion and the
ligand. When the binding interaction between the
metal ion and the ligand occurs, the fluorescence in-
tensity of the system is enhanced or weakened due to
the energy transfer between the metal ion and the lig-
and, which decreases or increases complexes’ fluores-
cence (Wang et al., 2008). In order to further demon-
strate the interaction between metal ions and ligands,
the fluorescence intensity of three systems was ana-
lyzed and the results are shown in Fig. 2. As it can
be seen from the graph, the metal cerium ion fluores-
cence intensity strongly decreases with the addition of
the ligand. The fluorescence intensity decrease shows
that the metal ion and the ligand promote the binding
interaction. The results also show that, in the surfac-
tant as well as in the aqueous solution, the complex
fluorescence is the same, indicating that the complex
structure in the micellar solution did not change.
Kinetic version of the Job-plot is a convenient
method for the determination of the chelating stoi-
=
220 nm disappears and a new characteristic peak
appears at λ = 238nm in the mixture solution. The
signal of the ligand undergoes a very strong downfield
shift to λ = 238nm due to the complexation with the
cerium(III) ion. Apparently, this result indicates tight
binding and the formation of a new complex, CemLn,
between the cerium ion and the ligand. From curves B
and C it can be seen that the characteristic spectra of
complex CemLn do not change in the aqueous solution
and in surfactant Brij35. This shows that the structure
of complex CemLn is the same in water and also in
surfactant Brij35.

3
68
J. Q. Xie et al./Chemical Papers 67 (4) 365–371 (2013)
Fig. 4. Dependence of kobsd on the concentration of the CeL
Fig. 3. Job-plot for BNPP hydrolysis catalyzed by the system
−
2
−3
complex ( ) and surfactant Brij35 (ꢀ): [BNPP] =
Ce(III) + L: [Ce(III)] + [L] = 1 × 10
mol dm
mol dm , [BNPP] = 5.0 × 10
,
•
−
5
−3
◦
−
3
−3
−5
5 × 10
mol dm , pH = 7.5, t = 25 C: [CeL] =
[
Brij35] = 4.0 × 10
−
3
−3
−3
−3
−
3
◦
5 × 10 mol dm (ꢀ), [Brij35] = 4 × 10 mol dm
mol dm , pH = 7.5, t = 25 C.
( ).
•
−
5
chiometry of metal complexes; here, the observed rate
constants (kobsd) are plotted as a function of the mole
fraction (r) of the ligand or metal ion to achieve con-
stant total concentrations (Jiang et al., 2004). Thus,
to confirm the stoichiometry of the active species in
the hydrolysis of BNPP, the kinetic version of a Job
plot was drawn and it is shown in Fig. 3. This fig-
ure indicates that the r value is 0.5 for the maximum
rate constant obtained, which corresponds to the sto-
ichiometry of the metal ion to ligand of 1 : 1 for the
kinetically active species. This result also verifies the
presence of the complex in the catalytic hydrolysis of
BNPP. Due to the extremely strong Lewis acidity of
the cerium(III) ion, complex CeL can coordinate the
water molecule to form hydrated complex CeL(H2O)2
in the mixture solution of the cerium(III) ion and
the ligand. Thus, complex CeL or hydrated complexes
(critical micellar concentration (cmc) of 5.22 × 10
−3
mol dm (Xiang et al., 2002)) and L + Brij35 have no
catalytic activity, and the catalytic system Ce(III) +
Brij35 has low activity, indicating that the non-ionic
Brij35 micelles have no catalytic activity. From Ta-
ble 1 it can be also found that the rate of catalytic
hydrolysis of BNPP is more than ten times higher in
the system Ce(III) + L + Brij35 compared to the
system Ce(III) + L. The Job-plot in Fig. 3 shows
that the complex CeL can be considered as the ac-
tive species in the mixture solution. One of the most
important processes leading to micellar effects on the
reactions is the solubilization of solutes in the micelle
phase (Ta ¸s cio ˘g lu, 1996). In the present work, water-
solubility of both substrate (BNPP) and active species
(CeL) are low, so that an addition of surfactants leads
to better solubilization of BNPP and the catalyst in
the Stern layer of micellar aggregates by the hydropho-
bic interaction increasing thus the local concentration
and the collision frequency of the reagents and facil-
itating the BNPP catalytic hydrolysis. So, the cat-
alytic active species is still the complex in the non-
ionic Brij35 micelle, and the catalytic acceleration of
BNPP hydrolysis is ascribed to the solubilization ef-
fect of the Brij35 micelle.
−
CeL(H2O)OH and CeL(H2O)2 can be applied as the
active species, catalysts, in the mixture solution.
Effects of the non-ionic Brij35 micelles in
BNPP catalytic hydrolysis
Micelles play a very important role in the enzyme
catalytic process owing to the hydrophobic micro-
environment (Ta ¸s cio ˘g lu, 1996). The effects of non-
ionic Brij35 micelles on the BNPP reaction rate con-
Dependence of kobsd on the concentration of
the CeL complex and the surfactant in the met-
allomicellar system
−
1
stant, kobsd/s , of the BNPP hydrolysis in differ-
ent catalytic systems were investigated and they are
shown in Table 1.
Experimental results presented in Table 1 show
that the catalytic systems of Brij35 micelles solution
Effects of different concentrations of the CeL com-
plex and those of the surfactant on kobsd are shown
Table 1. BNPP hydrolysis reaction rate constant, kobsd, using different catalytic systems
System
Brij35
L + Brij35
inactive
Ce(III) + Brij35
Ce(III) + L
Ce(III) + L + Brij35
−
1
4.45 × 10−7
2.64 × 10−2
4.18 × 10−1
kobsd/s
inactive
Ce(III)] = [L] = 5 × 10− mol dm−3, [BNPP] = 5 × 10−5 mol dm−3, [Brij35] = 4 × 10−3 mol dm , pH = 7.5, t = 25 C.
3
−3
◦
[

J. Q. Xie et al./Chemical Papers 67 (4) 365–371 (2013)
369
Acid effect and catalytic mechanism of BNPP
hydrolysis in metallomicelles
The observed first-order rate constants of the hy-
drolytic reaction were obtained from experiments at
various pH values, and the results are shown in the
pH-rate profile in Fig. 5. From Fig. 5, the system’s op-
timal catalytic efficiency in the reactive medium can
be obtained at pH of ca. 7.5. This indicates that kobsd
of the catalytic hydrolysis of BNPP is correlated to the
acidity of the reaction system, implying that the active
species is neither the hydrated complex CeL(H2O)2
nor the CeL in the metallomicelles.
From the complex structure profile results that two
H O molecules of the hydrated complex CeL(H O)
Fig. 5. pH–reaction rate constant profile for BNPP catalytic
2
2
2
−
5
−3
mol dm , [CeL] =
hydrolysis: [BNPP] = 5 × 10
can dissociate in the metallomicelle solution, Fig. 6.
Therefore, the complex exists in three different pro-
tonization states with the first and the second order
acidic dissociation of H2O molecules coordinated to
the cerium(III) ion in the metallomicelle solution. The
−
3
−3
−3
−3
mol dm ,
5
× 10
mol dm , [Brij35] = 4 × 10
◦
t = 25 C.
−
in Fig. 4; catalytic reaction rate of BNPP increased
abruptly with the increasing Brij35 concentration be-
low the critical micellar concentration of Brij35 and
then stabilized at or above the cmc value of Brij35.
The results can be ascribed to the following rea-
sons: when the concentration of Brij35 increases above
its cmc value, the solubilization does not change sig-
nificantly so that the catalytic hydrolytic reaction rate
of BNPP remains almost constant.
2-hydroxy complex CeL(OH )2 is easily formed in the
exorbitant alkali solution, and the hydrated complex
CeL(H2O)2 is easily formed in the acidic solution;
however, neither environment is favorable for the for-
mation and stabilization of the mono-hydroxy com-
−
plex CeL(H2O)OH . Therefore, it is apparent that
the mono-hydroxy complex is most likely the real ac-
tive species (Jiang et al., 2008) promoting the BNPP
hydrolysis since the kobsd maximum of the bell-shaped
curve in Fig. 5 is achieved at ca. pH 7.5.
Experimental results indicate that the reaction
pathway of the BNPP catalytic hydrolysis is differ-
ent from that of its spontaneous hydrolysis because
the oxygen atom of the BNPP molecule has strong
affinity to the metallic ions. Phosphoryl oxygen of
the BNPP molecule can bind to the metal ion of the
complex when the BNPP is injected into the metal-
lomicelle solution, which promotes the formation of a
From Fig. 4 it can also be found that the ob-
served rate constants of the catalytic hydrolysis of
BNPP increase with the increasing complex concen-
−
3
−3
tration below 5 × 10 mol dm , stabilizing with the
−
3
increasing complex concentration between 5 × 10
−
3
−2
−3
mol dm and 1 × 10 mol dm . This implies that
the maximum catalytic efficiency of the catalytic sys-
tem is achieved at complex concentrations higher than
−
3
−3
.
5
× 10 mol dm
Fig. 6. Mechanism of BNPP hydrolysis catalyzed by metallomicelles.

3
70
J. Q. Xie et al./Chemical Papers 67 (4) 365–371 (2013)
Table 2. Changes of the BNPP catalytic hydrolysis rate with temperature
T/K
288.15
0.299
293.15
0.365
298.15
0.418
303.15
0.519
308.15
0.603
313.15
318.15
0.169
323.15
0.129
−
1
kobsd/s
0.389
CeL] = 5 × 10− mol dm−3, [BNPP] = 5 × 10−5 mol dm−3, [Brij35] = 4 × 10−3 mol dm , pH = 7.5.
3
−3
[
reaction intermediate containing the BNPP molecule
and the mono-hydroxy complex. In this process, there
are two modes of activation provided by the Ce(III)
ion accelerating the rate of the BNPP hydrolysis: the
Lewis acid activation, where the negative charge of
the substrate molecule is dispersed and the interme-
diate is stabilized by coordination of the phosphoryl
oxygen to the Ce(III) ion; the Ce(III) hydroxide acti-
vation, where the hydroxyl coordinated to the Ce(III)
ion is activated by the Ce(III) ion, promoting thus the
intramolecular nucleophilic reaction. The rate of the
BNPP hydrolysis is strongly accelerated by the syner-
gistic effects of the Lewis acid and the metal hydroxide
activation.
on the hydrolysis of BNPP was investigated kineti-
cally. Some interesting results were obtained:
i) the metallomicellar system presented in this
work is quite stable in the BNPP hydrolysis at neu-
tral pH when compared with other catalytic systems
containing lanthanide ions in the absence of micelle;
ii) the metallomicellar system containing the
cerium(III) ion and the macrocyclic ligand shows re-
markable catalytic activity in the BNPP hydrolysis
◦
under very mild conditions at pH 7.5 and 25 C, while
10
the hydrolytic rate of BNPP was by almost 10 fold
higher than that of its spontaneous hydrolysis in wa-
ter;
iii) catalytic efficiency of the metallomicellar sys-
tem correlates with pH values and temperature, and
the optimal catalytic efficiency is achieved in the re-
active medium at pH of ca. 7.5 and temperature of ca.
Temperature effect on the BNPP hydrolysis
◦
The activity of enzymes is closely related to the
temperature. Generally speaking, natural enzymes
have each their own optimal temperature at which
they exhibit excellent catalytic activity (Daniel, 1996).
In order to investigate the effect of temperature on
the catalytic activity of the present metallomicellar
system, and kinetic measurements of the BNPP hy-
drolysis were performed in the temperature range
from 288 K to 323 K; the results are shown in Ta-
ble 2. The data in Table 2 indicate that the reac-
tion rate of BNPP catalyzed by the metallomicellar
system increases with the increasing temperature to
the maximum rate achieved at 308 K and then de-
creases quickly when the temperature continues in-
creasing. This is probably caused by the higher tem-
perature of the reaction system compensating for the
reaction energy barrier and providing more activated
molecules increasing the collision frequency between
the molecules, which is apparently favorable for the
catalytic hydrolysis of BNPP. Meanwhile, a reaction
rate decrease was observed at T > 308 K caused by the
deactivation of the catalysts because of the complex
structure changes at such relatively high temperature.
On basis of the linearized form of the Arrhenius
35 C.
Acknowledgements. The authors gratefully acknowledge fi-
nancial support from the Chinese National Natural Science
Foundation (21173274), Natural Science Foundation Project
of CQ CSTC (2009BB5052), Scientific Research Foundation
for Returned Scholars, Ministry of Education of China, and
the Start-up Fund of Chongqing University of Technology.
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