Influence of acid-base properties of the support on copper-based catalysts for catalytic dehydrogenation of 2-butanol

Article abstract of DOI:10.1007/s10562-012-0926-x

Copper-based catalysts, supported on γ-Al₂O₃, La₂O₃, and γ-Al₂O₃-La ₂O₃, respectively, were prepared by co-precipitation and tested in the continuous dehydrogenation of 2-butanol to methyl ethyl ketone. The catalytic performance of the catalysts was found to be markedly dependent on acid-base properties of the support. Three copper species were found in the calcined samples: Cu²⁺, [Cu-O-Cu] _n cluster, and bulky CuO oxides. Cu⁰ was shown to be the active species of the reduced copper-based catalysts. The synergistic effect between γ-Al ₂O₃ and La₂O₃ when used as a support for well dispersed Cu⁰ gave the system with the best activity and stability; whereas, loss of the active Cu⁰ during the reaction was believed to be the main reason for Cu-La₂O₃ deactivation. Moreover, a possible reaction mechanism was proposed based on GC-MS results. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-012-0926-x

Catal Lett (2013) 143:101–107
DOI 10.1007/s10562-012-0926-x
Influence of Acid–Base Properties of the Support
on Copper-Based Catalysts for Catalytic Dehydrogenation
of 2-Butanol
Guoyi Bai
Guofeng Chen
•
Yalong Wang
•
•
Fei Li
Na Li Xue Han
•
Zhen Zhao
•
•
Received: 13 June 2012 / Accepted: 10 October 2012 / Published online: 23 October 2012
Ó Springer Science+Business Media New York 2012
Abstract Copper-based catalysts, supported on c-Al O ,
1 Introduction
2
3
La O , and c-Al O –La O , respectively, were prepared by
2
3
2
3
2 3
co-precipitation and tested in the continuous dehydroge-
nation of 2-butanol to methyl ethyl ketone. The catalytic
performance of the catalysts was found to be markedly
dependent on acid–base properties of the support. Three
Supported copper-based catalysts are widely used in
various industrial processes, such as dehydrogenation,
hydrogenation, and the water–gas shift reaction [1–3].
Particularly, dehydrogenation of alcohols to aldehydes or
ketones over copper-based catalysts attracts more and more
attention due to its environmentally friendly nature, high
selectivity, and low cost [4, 5]. Thus, a number of inves-
tigations has focused on the influence of preparation
methods, additives, and supports on the activity of copper-
based catalysts in the catalytic dehydrogenation of alcohols
[6, 7]. For example, Wang and colleagues have found that
the surface structure and catalytic behavior of Cu-SiO2
depended greatly on the preparation methods, and the
catalyst prepared by a sol–gel method showed the best
catalytic performance in 2-butanol dehydrogenation [6]. Tu
and Chen [7] have reported the effect of alkali metal oxide
additives on Cu–SiO2 in ethanol dehydrogenation and
2
?
copper species were found in the calcined samples: Cu
,
0
Cu–O–Cu]n cluster, and bulky CuO oxides. Cu was
[
shown to be the active species of the reduced copper-based
catalysts. The synergistic effect between c-Al O and
2
3
0
La O when used as a support for well dispersed Cu gave
2
3
the system with the best activity and stability; whereas, loss
0
of the active Cu during the reaction was believed to be the
main reason for Cu–La O deactivation. Moreover, a pos-
2
3
sible reaction mechanism was proposed based on GC–MS
results.
Keywords Copper-based catalyst Á Support Á
Acid–base properties Á Dehydrogenation Á 2-Butanol
found that K O was an excellent promoter for copper-based
2
catalysts in terms of activity and stability.
On the other hand, the characteristics of supports are
believed to be crucial to the catalytic performance of the
supported catalysts [8–12]. Detailed experiments have
been carried out on the selection of supports and inter-
action between active species with supports. For instance,
the influence of support type and copper loading on the
activity of copper-based catalysts in dehydrogenation of
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-012-0926-x) contains supplementary
material, which is available to authorized users.
G. Bai (&) Á Y. Wang Á F. Li Á Z. Zhao Á G. Chen Á N. Li
College of Chemistry and Environmental Science,
Hebei University, No. 180 Wusi East Road, Baoding 071002,
Hebei, People Republic of China
2-butanol to methyl ethyl ketone have been investigated
by Keuler and a 15 wt% Cu/SiO catalyst was found to be
2
most active in this reaction [8]. Spencer et al. have
studied the role of zinc oxide in Cu/ZnO catalysts for
methanol synthesis and the water–gas shift reaction [9].
Shen et al. [10] have reported the synergistic effect
e-mail: baiguoyi@hotmail.com
X. Han
Hebei Geological Laboratory, Baoding 071051,
Hebei, People Republic of China
0
between Cu and MgO in transfer dehydrogenation of
123

1
02
G. Bai et al.
primary aliphatic alcohols. However, studies into the
influence of support acid–base properties on catalyst
performance are scarce and mainly limited to the sup-
ported noble metal catalysts [11].
2.3 Catalytic Runs
2-Butanol dehydrogenation was carried out in a tubular,
fixed-bed reactor with 20 mm i.d., and 5.0 g catalyst was
loaded into this reactor and reduced with hydrogen at
300 °C for 3 h. 2-Butanol was dosed into the reactor at a
speed of 0.30 mL/min by a syringe pump and reacted
continuously at 360 °C. Reaction mixtures were analyzed
by a gas chromatograph using a 30 m PEG-20000 capillary
column and the product structures were confirmed by a gas
chromatograph-mass spectroscopy (GC–MS) on an Agilent
5975C spectrometer.
In continuation of our previous work [12], we herein
report our recent research into the effect of support acid–
base properties on copper-based catalysts in the catalytic
dehydrogenation of 2-butanol to methyl ethyl ketone,
which is a widely used industrial intermediate and solvent
[
8]. c-Al O and La O were selected as representative of
2
3
2 3
acidic and basic supports, and their combination (c-Al O -
2
3
La O ) was particularly examined. The catalytic activity
2
3
and stability of the supported copper-based catalysts were
investigated and found to correlate well with acid–base
properties of the support.
3 Results and Discussion
XRD patterns of Cu–Al O , Cu–La O , and Cu–Al O –
2
3
2
3
2 3
2
Experimental
La O are shown in Fig. 1. As can be seen, typical dif-
2 3
fraction peaks due to CuO were obvious for the three
calcined copper-based catalysts (Fig. 1a). However,
0
detection of new characteristic peaks of Cu in the reduced
2
.1 Catalyst Preparation
Unless otherwise stated, all chemicals were purchased
from Baoding Huaxin Reagent and Apparatus Co., Ltd.
and were used without further purification. Copper-based
catalysts were prepared by a co-precipitation method. A
typical procedure is as follows: 11.4 g Cu(NO ) Á3H O
samples (Fig. 1b), together with the disappearance of CuO
a
CuO
2
Al O3
3
2
2
La O3
was first dissolved in 100 mL distilled water, and then
2
7
.0 g of the requisite powdered support was dispersed into
this solution with stirring at 40 °C. Subsequently, an
a
aqueous solution of 10.6 g Na CO made up to 100 mL
2
3
was added dropwise to the above mixture to achieve a
final pH value of 10. Finally, the precipitated mass was
thoroughly washed, filtered, extruded to form rods, dried
at 120 °C for 4 h and calcined at 500 °C for 4 h. The
supported catalysts so obtained were Cu–Al O ,
b
c
2
3
2
0
30
40
50
60
70
Cu–La O , Cu–Al O –La O (50 % c-Al O /50 % La O ,
weight ratio), respectively.
2
3
2
3
2
3
2
3
2 3
2
Theta / deg.
2
.2 Catalyst Characterization
b
Cu
Al O3
2
X-ray diffraction (XRD) patterns were acquired on a
Bruker D8-ADVANCE X-ray diffractometer. The nitrogen
adsorption was measured using a Micromeritics Tristar II
La O
2
3
3
020 surface area and pore analyzer. Copper content was
a
identified by inductively coupled plasma analysis (ICP) on
a Thermo VISTA-MPX spectrometer. DRUV–Vis diffuse
reflectance spectra were recorded on a Shimadzu UV-3600
spectrophotometer. X-ray photoelectron spectroscopy
b
c
(
XPS) measurements were carried out with a PHI 1600
2
0
30
40
50
60
70
spectroscope. Temperature programmed reduction (H2-
TPR) and temperature programmed desorption of NH3
2Theta / deg.
(
NH -TPD) were performed on a TP-5000 instrument from
3
Fig. 1 XRD patterns of the calcined (a) and reduced (b) copper-
based catalysts (a Cu–Al , b Cu–Al –La , c Cu–La
Xianquan Ltd.
O
2 3
2
O
3
O
2 3
2 3
O )
1
23

Influence of Acid–Base Properties of the Support on Copper-Based Catalysts
103
Table 1 Selected
Sample
Cu content
a
Surface atomic
ratio of Cu (%)
Cu particle
c
size (nm)
Surface
area (m /g)
Pore
volume
(
Pore
diameter
(nm)
physicochemical properties of
the supports and copper-based
catalysts
b
2
(
wt%)
3
cm /g)
Al
Cu–Al
Al –La
Cu–Al
La
Cu–La
2
O
3
–
–
–
162
109
86
77
9
0.24
0.14
0.13
0.12
0.04
0.02
5.9
5.3
2
O
3
28.6
–
1.8
–
20.4
–
O
2 3
2
O
3
6.2
O
2 3
–La
O
2 3
29.4
–
5.3
–
20.0
–
5.5
a
Based on ICP results
2
O
3
15.7
11.2
b
Based on XPS results
2
O
3
28.0
2.7
42.5
7
c
Based on XRD results
peaks, indicated the full reduction of CuO during the
0
reduction process. Thus, Cu should be the active species
oxygen in [Cu–O–Cu] surface species; whereas the peak
n
at 600–800 nm can be ascribed to bulky CuO oxides [5,
?
2
in the reduced copper-based catalysts. On the other hand,
13]. Mononuclear Cu species and oligonuclear [Cu–O–
Cu] clusters appear to play dominant roles in Cu–Al O
there were typical and intense diffraction peaks of c-Al O3
2
n
2
3
and La O in Cu–Al O and Cu–La O , respectively;
2
and Cu–Al O –La O .
2 3
2
3
2
3
3
2
3
whereas the intensity of c-Al O and La O3 peaks
H -TPR curves of the three catalysts are presented in
2
2
3
2
decreased markedly in Cu–Al O –La O , implying strong
3
Fig. 3. For Cu–Al O –La O , a main reduction peak was
3
2
3
2
2
2 3
interaction between c-Al O and La O in this catalyst.
observed at about 230 °C with a small shoulder at 270 °C,
which can be ascribed to the reduction of well dispersed
copper oxide species [14]. A similar reduction peak was
also detected at about 240 °C in Cu–Al O ; whereas the
2
3
2 3
Table 1 shows selected physicochemical properties of
the supports and copper-based catalysts. As can be seen,
the actual Cu contents of the three catalysts were slightly
lower than the nominal contents (30 wt%), with Cu–
Al O –La O having the highest amount of Cu. The par-
2
3
reduction peak was observed at about 360 °C in Cu–La O ,
2
3
probably due to the reduction of bulky CuO. Thus, the
2
3
2 3
0
ticle sizes of Cu in Cu–Al O and Cu–Al O –La O ,
3
synergistic effect between c-Al O and La O is believed
2
3
2
3
2
2
3
2 3
calculated from the Scherrer equation, were nearly the
same and both much smaller than that of Cu–La O . It
to weaken the interaction of CuO and La O in Cu–Al O –
2 3 2 3
La O , leading to well dispersed and reducible copper
2 3
2
3
appears that the copper species supported on c-Al O and
oxide species in this catalyst.
2
3
the mixed support were present in a well dispersed state
compared to those on La O . We also found that Cu–Al O
2 3
Figure 4 shows XPS spectra of the three reduced sam-
ples. As can be seen, there are more copper species on the
surface of Cu–Al O –La O , compared to the other two
2
3
had the maximum surface area and pore volume, and Cu–
La O had the minimum values. In contrast, the test of pore
2
3
2 3
catalysts (Table 1). This can be attributed to the synergistic
2
3
diameter gave reverse results for the three catalysts. Fur-
thermore, the results showed that the structure of Cu–
Al O –La O was close to that of Cu–Al O , both having
effect between c-Al O and La O , making the copper
species well dispersed on the surface of this catalyst. Fur-
0
thermore, only Cu was detected on the surface of Cu–
2
3
2 3
2
3
2
3
2 3
large surface area and pore volume. Thus, these two cata-
lysts are suggested to have porous structures with some
copper particles occupying the pores of the supports;
whereas Cu–La O has a nearly planar structure with most
Al O –La O (Fig. 4d), as well as the other two catalysts,
2
3
2 3
0
by applying the curve fitting procedure [15]. Thus, Cu is
believed to be the active species in the reduced copper-
based catalysts, in accordance with the XRD results.
2
3
of the copper particles covering the surface of the support,
as indicated by SEM results.
NH -TPD was used to study the influence of support
3
acid–base properties on copper-based catalysts and the
DRUV–Vis tests were carried out to understand the
nature and the co-ordination of the copper oxide species in
the catalysts. Their spectra are shown in Fig. 2. The
absorption value of Cu–Al O –La O was found to be in
results are shown in Fig. 5. The NH -TPD curve of Cu–
3
La O was close to a line (not shown), which can be
2
3
ascribed to the basic nature of La O , causing no adsorp-
3
2
tion of NH during the test. The spectrum of Cu–Al O
2 3
2
3
2
3
3
the middle of the three samples, probably due to the
presented a composite signal due to at least two compo-
nents, with a main desorption peak at 200 °C and a broad
shoulder at about 350 °C together with a long tail
extending up to 550 °C. This is indicative of a wide acidity
distribution of the acidic sites in this catalyst. The
desorption peak at 200 °C was ascribed to the weakly
acidic sites, while the desorption peak at 350 and 550 °C
represented the moderately acidic sites and strongly acidic
interaction between c-Al O and La O . Furthermore, the
2
3
2 3
curves were deconvoluted into three peaks, which can be
?
2
assigned to different copper species: Cu , [Cu–O–Cu]n
cluster, and bulky CuO oxides. The peak at about
2
50–400 nm is related to charge-transfer between mono-
2
nuclear Cu ion and oxygen, and the peak at 400–600 nm
?
2?
is related to charge-transfer between Cu
cluster and
123

1
04
G. Bai et al.
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
a
c
b
a
200
300
400
500
600
700
800
900
Wavelength / nm
1
00
200
300
400
500
1.2
Temperature /
b
2 2 3
Fig. 3 H -TPR profiles of the copper-based catalysts (a Cu–Al O ,
b Cu–Al O –La O , c Cu–La O )
2 3 2 3 2 3
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
ketone was only 83.5 %, which can be attributed to the
0
high surface area, well dispersed Cu and acidic nature of
c-Al O [18, 19]. In comparison, the conversion of
2
3
2-butanol was only 84.1 % and the selectivity for methyl
ethyl ketone was 96.4 % over Cu–La O , which can be
2
3
attributed to the low surface area and basic nature of La O
2
3
200
300
400
500
600
700
800
900
[
20]. As expected, the conversion of 2-butanol was 92.2 %
Wavelength / nm
and the selectivity for methyl ethyl ketone was 95.8 % over
Cu–Al O –La O , thus its yield (88.3 %) was superior to
1.2
2
3
2 3
the other two catalysts. This can be ascribed to the syner-
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
c
gistic effect between c-Al O and La O , giving this cat-
2
3
2 3
0
alyst suitable acid–base properties and well dispersed Cu ,
as shown by NH -TPD and XPS results. In contrast, the
3
three unreduced copper-based catalysts showed poor
activities in the dehydrogenation of 2-butanol, especially
for Cu–La O with only 1.4 % conversion of 2-butanol.
2
3
0
Thus, Cu appears to be the active species of the reduced
copper-based catalysts [6, 12], in accordance with XRD
and XPS results, and accounts for the catalysts high
activities.
2
00
300
400
500
600
700
800
900
Wavelength / nm
A possible reaction mechanism of 2-butanol dehydro-
genation is then proposed based on GC–MS results
Fig. 2 DRUV–Vis spectra of the copper-based catalysts. (a Cu–Al
b Cu–Al –La , c Cu–La
2 3
O ,
2
O
3
2
O
3
2 3
O )
(Scheme 1). Methyl ethyl ketone (1) was the main product
of 2-butanol dehydrogenation over copper-based catalysts.
The oligomerization of methyl ethyl ketone and 2-butanol
will produce small amount of 5-methyl-3-heptanone (3).
Indeed, 5-methyl-3-heptene (2) was obtained as a major
by-product over these catalysts, especially the c-Al O
sites, respectively [16, 17]. In contrast, Cu–Al O –La O
2
3
2
3
only showed one broad desorption peak centered at about
80 °C, suggesting that weakly acidic sites play a dominant
role in Cu–Al O –La O . Moreover, it is obvious that Cu–
2
2
3
2
3
2 3
Al O –La O had a lower desorption area than Cu–Al O ,
2 3
supported one. We ascribe this result to the acidity of the
supports, which catalyze the cleavage of the C–O bond to
form isobutylene or butylene intermediates that then
dimerize at the reaction temperature to yield 5-methyl-3-
heptene (2) and 2-octene (4), respectively. Further hydro-
genation of 5-methyl-3-heptene (2) by the in situ generated
hydrogen will produce trace amounts of 3-methylheptane
(5). Thus, suitable acid–base properties of the catalysts
2
3
2
3
indicating that it has lower amount of acidic sites. The
synergistic effect between c-Al O and La O is believed
2
3
2 3
to tune the acid–base properties of Cu–Al O –La O .
2
3
2 3
Table 2 summarizes the reaction results of catalytic
dehydrogenation of 2-butanol over the three reduced cop-
per-based catalysts. For Cu-Al O , the conversion of
2
3
2
-butanol was 93.1 % but the selectivity for methyl ethyl
1
23

Influence of Acid–Base Properties of the Support on Copper-Based Catalysts
105
2 3 2 3 2 3 2 3 2 3 2 3
Fig. 4 XPS spectra of the reduced copper-based catalysts (a Cu–Al O , b Cu–Al O –La O , c Cu–La O , d Cu 2p in Cu–Al O –La O )
should be crucial to the selective dehydrogenation of
2-butanol to methyl ethyl ketone.
Finally, the stability of the three copper-based catalysts
was tested under the optimized reaction conditions (Fig. 6).
As can be seen, both Cu–Al O –La O and Cu–Al O were
2
3
2
3
2 3
stable within the test period; whereas the conversion of
-butanol decreased markedly over Cu–La O . Certain
2
2
3
characterizations were then carried out to find the reason
0
for the above results. The particle sizes of Cu in the used
a
b
Cu–La O were found to increase from 42.5 to 45.6 nm,
3
2
0
but the particle sizes of Cu in the other two used catalysts
also increased similarly. So we think sintering of the active
100
200
300
400
500
600
700
800
0
Cu may be one reason for Cu–La O deactivation [21],
2
3
Temperature /
but not the main reason. The copper contents in the reac-
tion solutions of the three copper-based catalysts were then
analyzed by ICP-MS according to our previous work [22].
Fig. 5 NH
b Cu–Al
3
-TPD profiles of the copper-based catalysts (a Cu–Al
2 3
O ,
2
O
3
–La
2 3
O )
123

1
06
G. Bai et al.
Table 2 Dehydrogenation of 2-butanol over the copper-based
catalysts
planar structure of this catalyst. Particularly, Cu–Al O –
2 3
La O showed the best performance among the catalysts
2
3
studied, with the conversion of 2-butanol and the selec-
tivity for methyl ethyl ketone both higher than 90.0 %
during the whole test.
Catalysts
Cu–Al
2
O
3
Cu–Al
2
O
3 2
–La O
3
Cu–La
84.1
2 3
O
Conversion(%)
93.1
92.2
Selectivities(%)
Methyl ethyl ketone 83.5
95.8
4.1
96.4
2.9
4
Conclusion
5
-Methyl-3-heptene
6.4
Others
10.1
0.1
0.7
The catalytic performance of copper-based catalysts was
found to be remarkably dependent on acid–base properties
of the support in 2-butanol dehydrogenation to methyl
ethyl ketone. Compared with Cu–Al O and Cu–La O ,
Reaction conditions reaction temperature: 360 °C; atmospheric
pressure; flow rate of 2-butanol: 0.30 mL/min. Others 5-Methyl-3-
heptanone, 2-octene, 3-methylheptane
2
3
2 3
Cu–Al O –La O exhibits the best activity and stability.
2
3
2 3
The synergistic effect between c-Al O and La O is sug-
2
3
2 3
gested to contribute to the suitable acid–base properties and
0
well dispersed Cu in this catalyst, accounting for its good
2
5
performance.
or
Acknowledgments The authors thank Professor L. M. Harwood for
his kind assistance. Financial support by the National Natural Science
Foundation of China (20806018) and Natural Science Foundation of
Hebei Province (B2011201017) are gratefully acknowledged.
-
H2O
OH
4
-
H2
References
O
O
1
3
1. Figueiredo RT, Mart ´ı nez-Arias A, Granados ML, Fierro JLG
(
1998) J Catal 178:146
2. Yahiro H, Murawaki K, Saiki K, Yamamoto T, Yamaura H
2007) Catal Today 126:436
Scheme 1 Proposed routes for 2-butanol dehydrogenation
(
3
4
. Fridman VZ, Davydov AA, Titievsky K (2004) J Catal 222:545
. Sheldon RA, Kochi JK (1981) Metal-catalyzed oxidations of
organic compounds. Academic Press, New York
100
5
. B a´ lsamo NF, Chanqu ´ı a CM, Herrero ER, Casuscelli SG, Crivello
ME, Eimer GA (2010) Ind Eng Chem Res 49:12365
9
8
7
6
5
4
0
0
0
0
0
0
6
. Wang ZL, Liu QS, Yu JF, Wu TH, Wang GJ (2003) Appl Catal A
2
39:87
7
8
9
. Tu YJ, Chen YW (2001) Ind Eng Chem Res 40:5889
. Keuler JN, Lorenzen L, Miachon S (2001) Appl Catal A 218:171
. Spencer MS (1999) Top Catal 8:259
a: Cu-Al O
2
3
10. Shi RJ, Wang F, Mu XL, Li Y, Huang XM, Shen WJ (2009) Catal
Commun 11:306
11. Hoxha F, Schimmoeller B, Cakl Z, Urakawa A, Mallat T, Prat-
sinis SE, Baiker A (2010) J Catal 271:115
2. Bai GY, Wang HL, Ning HS, He F, Chen GF (2008) React Kinet
Catal Lett 94:375
13. Chanqu ´ı a CM, Sapag K, Rodr ´ı guez-Castell o´ n E, Herrero ER,
Eimer GA (2010) J Phys Chem C 114:1481
b: Cu-Al O -La O
3
2
3
2
c: Cu-La O
2
3
1
5
0
100 150 200 250 300 50 100 150 200 250 300
Time on stream / min
Fig. 6 Stability tests of the copper-based catalysts (a Cu–Al
b Cu–Al –La , c Cu–La
2
O
3
,
14. Chary KVR, Sagar GV, Naresh D, Seela KK, Sridhar B (2005) J
Phys Chem B 109:9437
O
2 3
O
2 3
2 3
O )
1
5. Nagaraja BM, Padmasri AH, Raju BD, Rao KSR (2011) Int J
Hydrogen Energy 36:3417
The copper content in reaction solution of Cu–La O was
3
16. Turco M, Bagnasco G, Cammarano C, Senese P, Costantino U,
Sisani M (2007) Appl Catal B 77:46
17. Turco M, Bagnasco G, Costantino U, Marmottini F, Montanari T,
Ramis G, Busca G (2004) J Catal 228:43
2
found to be markedly higher than those of the other two
0
catalysts. So we confirm that loss of the active Cu during
the reaction should be the main reason for Cu–La O
2
3
18. Zaccheria F, Ravasio N, Psaro R, Fusi A (2006) Chem Eur J
12:6426
deactivation [22, 23], which can be ascribed to the nearly
1
23

Influence of Acid–Base Properties of the Support on Copper-Based Catalysts
107
1
2
9. Huang L, Zhu YL, Huo CF, Zheng HG, Feng G, Zhang CH, Li
YW (2008) J Mol Catal A 288:109
0. Shi RJ, Wang F, Ta N, Li Y, Huang XM, Shen WJ (2010) Green
Chem 12:108
21. Chang FW, Kuo WY, Lee KC (2003) Appl Catal A 246:253
22. Bai GY, Chen LG, Li Y, Yan XL, He F, Xing P, Zeng T (2004)
Appl Catal A 277:253
23. Jung KD, Joo OS, Han SH, Uhm SJ (1995) Catal Lett 35:303
123