Effect of rare earth doping on the catalytic activity of copper-containing hydrotalcites in phenol hydroxylation

Article abstract of DOI:10.1002/adsc.200505004

The copper-containing hydrotalcites (Cu-HTLcs) are synthesized under microwave irradiation, the effect of rare earth elements (RE) on the synthesis of Cu-HTLcs, wherein RE stands for rare earth elements, e.g., La, Y, Sm and Ce, is also investigated in the present work. The hydrotalcite structure of the synthesized samples is verified by XRD and FT-IR. The results of their catalytic performances in phenol hydroxylation show that the doped rare earth elements can promote the catalytic activity of Cu-HTLcs, exhibiting a good trend as follows: La > Y > Sm > Ce. XPS results show that the Cu⁺ species are produced after the interaction of La-Cu-HTLcs with H ₂O₂. Combining with the catalytic test results, we propose a new mechanism about the generation of HO^? radicals in phenol hydroxylation, it is assumed that HO-Cu⁺-OH species are first formed by the reduction of HO-Cu²⁺-OH located in hydrotalcites in the presence of H₂O₂, and then react with H₂O ₂ to give rise to HO^? radicals.

Full text of DOI:10.1002/adsc.200505004

FULL PAPERS
DOI: 10.1002/adsc.200505004
Effect of Rare Earth Doping on the Catalytic Activity of
Copper-Containing Hydrotalcites in Phenol Hydroxylation
Chunxia Chen,a, b Chenghua Xu,a,* Liangrong Feng,a Zijian Li,a Jishuan Suo,a
Fali Qiu,a,* Yingchun Yangc
a
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, SiChuan 610041, People)s Republic of
China
Fax: (þ86)-28-8522-3978, e-mail address: xuch@cioc.ac.cn
b
Graduate School of the Chinese Academy of Sciences, Beijing 100039, People)s Republic of China
c
Department of Earth and Environment Sciences, Chengdu University of Information Technology, Chengdu, SiChuan
610041, People)s Republic of China
Received: January 4, 2005; Revised: May 9, 2005; Accepted: June 9, 2005
Abstract: The copper-containing hydrotalcites (Cu- after the interaction of La-Cu-HTLcs with H2O2.
HTLcs) are synthesized under microwave irradiation, Combining with the catalytic test results, we propose
.
the effect of rare earth elements (RE) on the syn- a new mechanism about the generation of HO radi-
thesis of Cu-HTLcs, wherein RE stands for rare earth cals in phenol hydroxylation, it is assumed that HO-
elements, e.g., La, Y, Sm and Ce, is also investigated Cuþ-OH species are first formed by the reduction
in the present work. The hydrotalcite structure of of HO-Cu2þ-OH located in hydrotalcites in the pres-
the synthesized samples is verified by XRD and FT- ence of H2O2, and then react with H2O2 to give rise to
.
IR. The results of their catalytic performances in phe- HO radicals.
nol hydroxylation show that the doped rare earth ele-
ments can promote the catalytic activity of Cu-HTLcs, Keywords: copper-containing hydrotalcites; micro-
exhibiting a good trend as follows: La>Y>Sm>Ce. wave irradiation; phenol hydroxylation; rare earth el-
XPS results show that the Cuþ species are produced ements
Introduction
that copper-containing HTLcs (Cu-HTLcs) exhibit
good catalytic performances in the phenol hydroxyla-
Phenol hydroxylation is an industrially important reac- tion with hydrogen peroxide as hydroxylating agent.[5]
tion, its main products include catechol and hydroqui- As far as the synthesis of HTLcs is concerned, the clas-
none, which are involved in diverse applications.[1] The sical method is hydrothermal crystallization under the
processes using H2O2 as oxidant are considered to be conventional heating, however, it needs a long crystalli-
the best foreground technique for environmentally be- zation time of over 15 hours.[6,7] In recent developments,
nign chemistry due to their final by-product being water. microwave-assisted synthesis has become popular in the
Hydrotalcite-like compounds (HTLcs), especially cop- crystallization of the solid catalyst materials,[8,9] because
per-containing HTLcs as catalysts, have attracted in- it can give rise to high yields, represents a clean synthetic
creasing attention in recent years for this reaction owing method and has short crystallization times in compari-
to their peculiar configuration and properties.[2,3] The son with the conventional method. However, until
chemical structure of HTLcs is
now, there are only few reports about the application
of microwaves on the synthesis of HTLcs. Therefore,
in the present work, we have synthesized the copper-
containing hydrotalcite samples (Cu-HTLcs) and modi-
fied themthrough the doping of rare earth elements un-
wherein MII represents divalent metal ions and MIII tri- der microwave irradiation. At the same time, the thus
valent metal ions, XmÀ stands for the negative ions synthesized samples were applied in the catalytic hy-
such as SO24À and CO32À and so on, n is the amount of wa- droxylation of phenol with aqueous H2O2 solution as hy-
ter in the crystal, and x is the molar ratio of trivalent met- droxylating agent. The present work also investigated
al ions to all metal ions.[4] Recent researches have shown the effects of reaction medium and reaction time on phe-
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Rare Earth Doping and Catalytic Activity of Copper-Containing Hydrotalcites
FULL PAPERS
nol hydroxylation in detail. Based on the catalytic hy-
droxylation of phenol and the XPS characterization of
catalysts, a new mechanism for phenol hydroxylation
has been proposed.
Results and Discussion
Synthesis and Characterization
Copper as an active metal component was widely ap-
plied in selective oxidation reactions,[10–12] the previous
studies[1,5] showed that Cu-HTLcs could give rise to
good catalytic performances in phenol hydroxylation.
Therefore, in the present work we synthesized Cu-
HTLcs under microwave irradiation and, in order to in-
vestigate the effect of rare earth elements on the catalyt-
ic activities of Cu-HTLcs, we also synthesized La-Cu-
HTLcs, Sm-Cu-HTLcs, Y-Cu-HTLcs and Ce-Cu-HTLcs
under the same conditions through the doping of the cor-
responding rare earth elements. The RE-doped amount
was 4 mol % of the total metal elements in HTLcs. The
formulae of the various HTLcs is calculated to be
Cu7.68RE0.40Al1.92(OH)20(CO3)1.16 ·7.77 H2O, which is al-
most coincident with the results obtained by the ICP-
AES techniques.
Figure 1. X-ray diffraction pattern of the copper-containing
hydrotalcites.
The structures of Cu-HTLcs and RE-Cu-HTLcs were
characterized by XRD and IR techniques. The spectra
are shown in Figures 1 and 2, respectively. It can be ob-
served that the XRD patterns of all samples give some
typical reflections at 2q¼12.8, 25.7, 33.5, 60.98, which
are almost identical to the typical pattern characteristic
of the hydrotalcite-like phase as previously report-
ed.[4,13–15] These reflection peaks are ascribed to the dif-
fraction by basal planes (003), (006), (009), (110), re-
spectively. At the same time, the IR spectra of all sam-
ples give two main absorption bands centered at
3445 cmÀ1 and 1388 cmÀ1, respectively, and as well as
a group of other small bands below 1200 cmÀ1. These
further prove that all the synthesized copper-containing
samples exhibit the structure of hydrotalcite-like com-
pounds. The band centered at around 3445 cmÀ1 is at-
tributed to the nOH stretching mode of hydroxy groups
fromwater molecules in the layers and interlayer of hy-
Figure 2. FT-IR spectra of the copper-containing hydrotal-
cites.
In Figure 3 are illustrated the XPS spectra of all cop-
drotalcites[16–18] and the O H vibrations of M (OH). per-containing hydrotalcites samples. It can be observed
The sharp band at 1388 cmÀ1 is due to the antisymmetric that all samples give a peak near 934.5 eV, which is as-
mode of interlayer carbonate anions. The group of peaks signed to Cu 2P3/2 of Cu2þ ions according to the litera-
À
À
in the region below 1200 cmÀ1 are fromthe M OH ture.[19–21]. At the same time, the Cu 2P1/2 peak near
À
[16]
À À
stretching and M O M’ bending modes wherein M 954.5 eV can also be clearly seen in all samples. Howev-
and M’ represent the metal elements such as Cu, Al and er, the peaks for both Cu 2P3/2 and Cu 2P1/2 exhibit a pos-
RE so on. Fromthe XRD and IR spectra of samples, it
itive shift as the order of Cu-HTLcs<Ce-Cu-HTLcs<
can be concluded that microwave irradiation results in Y-Cu-HTLcs<Sm-Cu-HTLcs<La-Cu-HTLcs. This in-
the formation of the copper-containing hydrotalcite-type dicates that the Cu2þ species in the RE-doped copper-
materials (Cu-HTLcs) in a short crystallization time, containing hydrotalcites will have an increasing elec-
and the doping of rare earth elements has no obvious in- tron-withdrawing capacity with the order of Ce<Y<
fluence on the formation of the hydrotalcite structure.
Sm<La.
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products is almost similar for all catalysts in phenol hy-
droxylation.
In the case of RE-Cu-HTLcs samples as catalysts, phe-
nolconversionsarealwayshigherthanthose ofCu-HTLcs.
At the same time, from Table 1 it can be also clearly ob-
served that the La-HTLcs free fromcopper has no catalyt-
ic activity inphenol hydroxylation. This shows that the rare
earth elements in hydrotalcites cannot act as the catalyti-
cally active centers. However, the doping of rare earth el-
ements does lead to an improvement in the catalytic activ-
ity of the copper-containing hydrotalcites. The increase in
the catalytic activities of RE-Cu-HTLcs probably comes
fromthe synergistic effect of rare earth element on the
Cu2þ species in hydrotalcites, is also probablydue to an im-
provement of the H2O2 efficiency. Moreover, fromTa-
ble 1, the different rare earth elements are observed to ex-
ert different degrees of improvement in the catalytic activ-
ity of RE-Cu-HTLcs. The catalytic activity of RE-Cu-
HTLcs is increased in the order of Ce <Y<Sm<La,
which is probably due to the increase of electron-with-
drawing capacities of Cu2þ species in hydrotalcites. It
can be deduced that the catalytic active centers of Cu2þ
species probably absorb electrons to formthe transition
state Cuþ species in the phenol hydroxylation.
Figure 3. Cu 2p XPS region in the corresponding XPS spectra
of (1) La-Cu-HTLcs, (2) Sm-Cu-HTLcs, (3) Y-Cu-HTLcs, (4)
Ce-Cu-HTLcs and (5) Cu-HTLcs.
Phenol Hydroxylation
Effect of Catalyst Types
Additionally, fromthe above-obtained results in Ta-
ble 1, it can be observed that the La-doped Cu-HTLcs
Copper-containing hydrotalcites are efficient catalysts (La-Cu-HTLcs) exhibits the highest phenol conversion,
for phenol hydroxylation, as has been reported previ- indicating that La-Cu-HTLcs is the best catalyst among
ously.[1,5,17] However, there are still only few reports all the synthesized copper-containing hydrotalcites in
about phenol hydroxylation catalyzed by the RE-doped phenol hydroxylation. Therefore, the present work fur-
Cu-HTLcs, wherein RE stands for the rare earth ele- ther investigates the catalytic performance of La-Cu-
ments, e.g., La, Sm, Y and Ce. In the present work, the HTLcs in phenol hydroxylation with various reaction
catalytic performances of RE-Cu-HTLcs with the RE/ parameters in details.
(REþAlþCu) ratio of 4 mol % are investigated and
compared with those of Cu-HTLcs in the phenol hy-
droxylation, the results are summarized in Table 1. Effect of Reaction Medium
FromTable 1, it can be observed that the two main prod-
ucts are catechol and hydroquinone, and the main by- In order to investigate the effect of the reaction medium
product is benzoquinone, and that the distribution of on the phenol hydroxylation catalyzed by La-Cu-
Table 1. Catalytic properties of copper-containing HTLcs in phenol hydroxylation.*
Catalysts
Phenol conversion
[mol %]
Products distribution [mol %]
CAT/HQ
CAT[a]
HQ[b]
BQ[c]
La-Cu-HTLcs
Sm-Cu-HTLcs
Y-Cu-HTLcs
Ce-Cu-HTLcs
Cu-HTLcs
53.2
50.2
47.9
46.8
44.8
0.0
54.7
55.0
61.3
63.4
62.9
–
42.4
41.7
35.1
33.5
33.2
–
3.0
3.3
3.6
3.1
4.0
–
1.3
1.3
1.3
1.9
1.9
–
La-HTLcs[d]
* Reaction conditions: 0.5 g phenol, 0.025 g catalyst, 1 mL 30 wt % H2O2 solution, 10 mL distilled water, temperature 708C,
time 120 min.
[a]
CAT¼catechol.
[b]
HQ¼hydroquinone.
[c]
BQ¼benzoquinone.
[d]
La content being 4 mol %.
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Rare Earth Doping and Catalytic Activity of Copper-Containing Hydrotalcites
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Table 2. Effect of reaction medium on phenol hydroxylation catalyzed by La-Cu-HTLcs.*
Solvent
Phenol conversion
[mol %]
Product distribution [mol %]
CAT/HQ
CAT
HQ
BQ
Water
53.2
16.1
0.0
54.7
61.5
–
42.4
36.9
–
3.0
1.5
–
1.3
1.7
–
Waterþacetone (1: 1)
Acetone
Acetonitrile
0.0
–
–
–
–
* Reaction conditions: 0.5 g phenol, 0.025 g La-Cu-HTLcs, 1 mL 30 wt % H2O2 solution, temperature 708C, time 120 min.
Table 3. Effect of phenol/H2O2 molar ratio on the phenol hydroxylation.*
Phenol/H2O2
(mole ratio)
Phenol conversion
[mol %]
Product distribution [mol%]
CAT/HQ
H2O2 utilization
ratio [mol%]
CAT
HQ
BQ
3: 1
2: 1
1: 1
1: 2
1: 3
18.2
26.4
34.7
53.2
56.7
58.9
53.1
54.1
54.5
53.2
37.0
42.6
41.4
42.3
43.4
4.1
4.3
4.5
3.3
3.5
1.6
1.3
1.3
1.3
1.2
52.3
50.5
33.1
25.8
18.2
* Reaction conditions: 0.5 g phenol, 0.025 g La-Cu-HTLcs, 10 mL distilled water as solvent, temperature 708C, time 120 min.
HTLcs, the reaction was performed in various solvents ratio decreases with the decreasing molar ratio of phe-
such as water, acetone, mixtures thereof, and acetoni- nol to H2O2, this is because most of the H2O2 is probably
trile. The results are summarized in Table 2. From Ta- decomposed to O2 on the catalytically active sites.[5] That
ble 2, it can be clearly seen that the La-Cu-HTLcs-cata- is to say, the higher the added amount ofH2O2 is, the low-
lyzed phenol hydroxylation gives the highest phenol er is the H2O2 utilization ratio obtained. Therefore, for
conversion in water as solvent, however, and almost the purpose of keeping both a high phenol conversion
has no catalytic activity in acetone and acetonitrile. and a high H2O2 utilization ratio, it is very important
This is perhaps due to the following reasons: (1) acetone for the phenol hydroxylation to select a proper molar ra-
and acetonitrile simply trap the hydroxyl radicals in con- tio of phenol to H2O2.
trast to water, thereby preventing the hydroxylation of
phenol; (2) water is a kind of perfect solvent for the
easy and simultaneous dissolution for both phenol and Effect of Reaction Time
H2O2, leading to the rapid approach on the catalytically
active centers; (3) water is beneficial to the generation of The present work investigates the phenol conversion
.
HO radicals being the active species involved in phenol and the distribution of products in different hydroxyla-
.
hydroxylation with H2O2; and (4) the HO radicals are tion times using La-Cu-HTLcs as catalyst, the results
easier stabilized in water than in other organic sol- are shown in the Figure 4. FromFigure 4, it can be
vents.[17] On the other hand, water is also beneficial to seen that phenol conversion increases drastically with
both industrial production and the elimination of envi- an increasing reaction time at the beginning and then
ronmental pollution. Hence, water is indicated to be levels off at 53 mol % after 120 min, because of the reac-
the best choice as solvent for the phenol hydroxylation. tion fulfillment. However, the selectivities of the three
products, such as catechol, hydroquinone and even ben-
zoquinone, remain unchanged during the whole hydrox-
Effect of the Ratio of Phenol to H2O2
ylation process, indicating that the production of three
products may be carried out through three parallel reac-
The present work investigates the effect of the molar ra- tions.
tio of phenol to H2O2 on the phenol hydroxylation, the
results are listed in Table 3. The phenol conversion can
be observed to increase with the decreasing molar ratio Investigation of the Mechanism of Phenol Hydroxyla-
of phenol to H2O2, indicating that it is possible to im- tion
prove the per pass conversion of phenol and the produc-
tion of dihydroxylbenzenes through reducing the molar In the case of copper-containing hydrotalcites as cata-
ratio of phenol to H2O2. However, the H2O2 utilization lyst, the HO-Cu2þ-OH species, located in the layer of
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Figure 4. Effect of reaction time on phenol hydroxylation. Reaction conditions: 0.5 g phenol, 0.025 g La-Cu-HTLcs, 1 mL 30
wt % H2O2 solution, 10 mL distilled water, temperature 708C.
the hydrotalcites, have been considered as the catalyti- lysts, we surveyed the valence change of copper in the
cally active centers for phenol hydroxylation with HO-Cu2þ-OH species of La-Cu-HTLcs mixed with
H2O2 ashydroxylating agent, and it has also been consid- aqueous H2O2 solution by the XPS technique, the results
ered that the hydroxylation reaction is carried out are depicted in Figure 5. It can be clearly observed that
.
through the produced HO radicals on the HO-Cu2þ
-
the fresh La-Cu-HTLcs sample only exhibits four sig-
.
OH species.[5,18] Therefore, the generation of HO radi- nals, which are attributed to the Cu 2p electron binding
cals is an important factor for the phenol hydroxylation energies of Cu2þ ions. However, the La-Cu-HTLcs/H2
with H2O2 aqueous solution. Zhu et al.[5] assumed that O2 system, resulting from the solid-liquid interaction be-
.
the HO radicals were produced by the reduction of tween La-Cu-HTLcs and H2O2, gives two additional
H2O2 on HO-Cu2þ-OH species, leading to the fact that strong signals located at the binding energies of 932.1
HO-Cu2þ-OH species were oxidized to HO-Cu3þ-OH.
Fromthe results in the present work, RE-Cu-HTLcs
and 953.9 eV, attributed to Cu 2p3/2 and Cu 2p1/2 of
Cuþ, respectively.[22] The same results are also found in
samples exhibit higher catalytic activities than Cu- the Cu-HTLcs/H2O2 system. Associated with the effect
HTLcs in phenol hydroxylation. It has also been testi- of RE-doping on Cu 2p binding energies and catalytic
fied that the increase in the catalytic activity of RE- activities of RE-Cu-HTLcs, it can be concluded that
Cu-HTLcs is not from the simple summation between
the activities of Cu-HTLcs and RE-HTLcs. Moreover,
the different rare earth elements exert a different effect
on the catalytic activity of copper-containing hydrotal-
cites, the catalytic activity of the RE-doped Cu-HTLcs
decreases in the order of La-Cu-HTLcs>Sm-Cu-
HTLcs>Y-Cu-HTLcs>Ce-Cu-HTLcs>Cu-HTLcs. If
the HO-Cu2þ-OH species are oxidized to HO-Cu3þ-OH
.
during the generation of HO radicals as reported in pre-
vious references,[5,18] the HO-Cu2þ-OH species in the
layer of hydrotalcites should be a kind of electron-do-
nor. Thus, for RE-Cu-HTLcs samples, the order of elec-
tron-donating capability should also decreases in the or-
der of La-Cu-HTLcs>Sm-Cu-HTLcs>Y-Cu-HTLcs>
Ce-Cu-HTLcs>Cu-HTLcs, and their Cu 2p electron
binding energies should exhibit a reverse order. Howev-
er, this is contrary to the detected results about the Cu 2p
electron binding energy of the RE-doped Cu-HTLcs by
XPS described above.
For the purpose of the further investigation on the
Figure 5. Comparison of Cu 2p XPS spectra of La-Cu-HTLcs
generation mechanism of HO free radicals on the cata- before (a) and after (b) H2O2 treatment.
.
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Rare Earth Doping and Catalytic Activity of Copper-Containing Hydrotalcites
FULL PAPERS
radicals to formO 2, which is regarded as the chain-ter-
mination step. If a proper solvent, such as water, is
used in the reaction system, the mutual collisions of
.
.
HOO with HO radicals can be avoided, resulting in
the increase of the probability for phenol hydroxylation,
which has been proved by the effect of solvent types on
phenol hydroxylation in the present work. On the other
hand, Equations (2) and (6) are main processes for the
side reactions, namely decomposition of H2O2.
Conclusion
Under microwave irradiation, the copper-containing hy-
drotalcites with a good crystalline structure can be syn-
thesized in a short time of 10 min. Moreover, doping
with rare earth elements improves the catalytic activity
of Cu-HTLcs. And the catalytic activity of RE-Cu-
HTLcs increases in the order of Ce<Y<Sm<La. The
binding energy of Cu 2p in copper-containing hydrotal-
cites increases in the order of Cu-HTLcs<Ce-Cu-
HTLcs<Y-Cu-HTLcs <Sm-Cu-HTLcs<La-Cu-HTLcs,
which is identical to the increased order of their catalytic
activities in phenol hydroxylation. Combining with the
XPS results of the La-Cu-HTLcs/H2O2 system, we pro-
pose that HO-Cu2þ-OH species in hydrotalcites are first
reduced by H2O2 to HO-Cuþ-OH, and then HO-Cuþ-
.
OH species react with H2O2 to produce HO radicals.
Scheme 1. Reaction mechanism for phenol hydroxylation.
Experimental Section
the transition state HO-Cuþ-OH species will be pro-
duced in the phenol hydroxylation with H2O2 catalyzed
Synthesis and Doping of Copper-Containing HTLcs
by copper-containing hydrotalcites. Therefore, the gen-
eration of HO radicals is deduced to be carried out not in a three-necked, round-bottomflask. A mixed aqueous solu-
The copper-containing HTLcs (Cu-HTLcs) were synthesized
.
through the oxidation of HO-Cu2þ-OH to HO-Cu3þ
-
tion containing metal nitrates [such as Cu(NO3)2 and
Al(NO3)3, respectively, the ratio of Cu2þ to Al3þ being 4]
formed solution A, another mixed solution containing
0.6 mol/L NaOH and 0.45 mol/L Na2CO3 was solution B. The
solutions A and B were synchronously titrated into the three-
necked, round-bottomflask under vigorous stirring at 70 8C,
the titration rates of solutions A and B were controlled in order
to maintain the pH value of mixture solution at about 9. After
the titration, the stirring was continued for another one hour, a
heavy precursor gel was obtained and then crystallized at 708C
OH as previously reported,[5,18] but through the reduc-
tion of HO-Cu2þ-OH to HO-Cuþ-OH as shown in the
following reactions (see Scheme 1):
.
The HO radicals are generated through the redox re-
action of H2O2 with HO-Cu2þ-OH species. HO-Cu2þ-OH
species in hydrotalcites are firstly reduced by H2O2 to
HO-Cuþ-OH according to Equations (1) and (2), and
then HO-Cuþ-OH species react with H2O2 to produce
.
.
HO radicals. This path for the generation of HO radi- for 10 min under microwave irradiation in a domestic micro-
cals can also be supported by the previous reports.[23–
wave oven (LG, the maximum power of 700 Wand a frequency
27] According to the proposed mechanism, Equation (1)
of 2.45 GHz), the temperature was controlled by the tempera-
.
ture feedback monitoring system with dual IR sensors. Finally,
the precipitates were filtered, washed with double-distilled wa-
ter to pH 7. The synthesized samples were dried for 30 min un-
der microwave irradiation, thus the obtained copper-contain-
ing sample was denoted as Cu-HTLcs.
The doping of rare earth elements in Cu-HTLcs was carried
out in the preparation of solution A. Rare earth (RE) nitrate
with the amount of RE/(REþCuþAl) being 4 mol % was
added to solution A, the other processes were the same, the ob-
is a chain-initiation step, the produced HO radicals at-
tack phenol to formtwo kinds of transition complexes
[see Equations (4) and (4’)], and then the transition com-
plexes continue to react with H2O2 to yield catechol and
.
hydroquinone, respectively, and also give rise to the HO
radicals, simultaneously. Thus, Equations (2) to (5) can
be considered as the chain-propagation steps. Of course,
.
.
the produced HO radicals can also contact with HOO
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Chunxia Chen et al.
FULL PAPERS
tained samples were denoted as RE-Cu-HTLcs, whereby RE
stood for rare earth elements, e.g., La, Sm, Y, and Ce.
[5] K. Z. Zhu, C. B. Liu, X. K. Ye, Y. Wu, Appl. Catal. A
1998, 168, 365.
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Characterization Methods
X-ray powder diffraction (XRD) patterns of the samples were
obtained on a Philips analytical X-Ray diffractometer in the
range of 108ꢀ2qꢀ708, using CuKa radiation combined with
a nickel filter. Fourier transforminfrared spectra (FT-IR) of
samples were recorded on the Nicolet MX-1E FT-IR in the
range of 500–4000 cmÀ1 using the KBr pellet technique. The
X-ray photoelectron spectra (XPS) were collected using a Kra-
tos Model XSAM 800 surface science instrument equipped
with an Al-Ka anode (1486.6 eV) operating at 180 W, 12 kV
and 15 mA. All binding energy values [BE (eV)] were deter-
mined with respect to the scale calibrated versus the Au4f line
at 84.0 eVand Ag3d line at 386.3 eV.
ˇ
ˇ ´
[10] V. Caplar, Z. Raza, M. Roje, V. Tomisic, G. Horvat, J. Po-
ˇ
ˇ
´
zar, I. Piantanida, M. Zinic, Tetrahedron 2004, 60, 8079.
[11] S. Kameoka, T. Tanabe, A. P. Tsai, Catal. Today 2004,
93–95, 23.
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ution was added through dropping wisely to the reaction mix-
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