Decomposition of phenethyl phenyl ether to aromatics over Cs xH3.0 - XPW12O40 (X = 2.0-3.0) heteropolyacid catalysts

Article abstract of DOI:10.1016/j.catcom.2010.08.002

Cesium-exchanged Cs_xH_{3.0 - x}PW₁₂O ₄₀ heteropolyacids were prepared with a variation of cesium content (X = 2.0-3.0) and were applied to the decomposition of phenethyl phenyl ether to aromatics. Phenethyl phenyl ether was used as a lignin model compound for representing β-O-4 linkage of lignin. Phenol, benzene, toluene, and ethylbenzene were mainly produced by the decomposition of phenethyl phenyl ether. Conversion of phenethyl phenyl ether and total yield for main products increased with increasing surface acidity of the catalyst. Cs _2.5H_0.5PW₁₂O₄₀ with the largest surface acidity showed the highest conversion of phenethyl phenyl ether and the highest total yield for main products.

Full text of DOI:10.1016/j.catcom.2010.08.002

Catalysis Communications 12 (2010) 1–4
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Decomposition of phenethyl phenyl ether to aromatics over Cs H
PW O
x
3.0 − x
12 40
(X=2.0–3.0) heteropolyacid catalysts
Hai Woong Park, Sunyoung Park, Dong Ryul Park, Jung Ho Choi, In Kyu Song ⁎
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 May 2010
Received in revised form 23 July 2010
Accepted 4 August 2010
Available online 12 August 2010
x
Cesium-exchanged Cs H3.0 −xPW12O40 heteropolyacids were prepared with a variation of cesium content
(X=2.0–3.0) and were applied to the decomposition of phenethyl phenyl ether to aromatics. Phenethyl
phenyl ether was used as a lignin model compound for representing β-O-4 linkage of lignin. Phenol, benzene,
toluene, and ethylbenzene were mainly produced by the decomposition of phenethyl phenyl ether.
Conversion of phenethyl phenyl ether and total yield for main products increased with increasing surface
acidity of the catalyst. Cs2.5
phenethyl phenyl ether and the highest total yield for main products.
© 2010 Elsevier B.V. All rights reserved.
H0.5PW12O40 with the largest surface acidity showed the highest conversion of
Keywords:
Lignin
Phenethyl phenyl ether
Heteropolyacid
Aromatics
1
. Introduction
forming a tertiary structure [17,19]. It is known that surface acidity of
cation-exchanged HPAs is varied depending on cation content. Surface
Lignin has been generally produced by the delignification process
x
acidity of Cs H3.0 −xPW12O40 (X=2.0–3.0) shows a volcano-shaped
in the pulp and paper processes and burned for power generation
1,2]. Biorefinery process has provided resource of lignin because
curve with respect to cesium content within the range of X=2.0–3.0,
and shows maximum value when X is 2.5 [18–22].
[
lignin is simultaneously produced as a by-product of biorefinery
process [3]. Lignin has attracted much attention as an aromatic
resource because lignin is an amorphous polymer linked by aromatic
compounds [4–6]. Recently, researches on the decomposition of lignin
for production of aromatic compounds have been widely conducted
due to the depletion of fossil fuels.
Dimeric lignin model compounds have been generally used as a
lignin feedstock for research on lignin decomposition due to the
complex and various structure of lignin [23,24]. Among various model
compounds, phenethyl phenyl ether has been widely used as a lignin
model compound for representing β-O-4 bond that is dominant
linkage in the lignin [25–29].
Catalytic decomposition of lignin has attracted much attention as a
key technology for production of aromatic compounds because
polymeric lignin is thermally stable [7,8]. H-ZSM-5 has been used for
cleavage of β-O-4 bond and relatively weak C–C bond in the lignin inner
unit. The catalytic performance of H-ZSM-5 in the decomposition of
lignin strongly depends on the structure and acid property of the
catalyst [9]. Other catalysts examined for lignin decomposition include
x
In this work, cesium-exchanged Cs H3.0 −xPW12O40 heteropolyacid
catalysts were prepared with a variation of cesium content (X=2.0, 2.3,
2.5, 2.8, and 3.0), and were applied to the decomposition of phenethyl
phenyl ether (a lignin model compound) to aromatics. Correlations
between catalytic performance and surface acidity of Cs
were then established and discussed. This is the first example reporting
the catalytic performance of Cs 40 catalyst for the
x 40
H3.0−xPW12O
x
H3.0 − xPW12O
sulfided Co–Mo/Al
Fe –S/Al –S, NiO–MoO
and Raney Ni, Pd/C, Rh/C, Rh/Al
2
O
3
, Ni–W/Al
2
O
3
, and Ni–Mo/Al catalysts [10–13],
, and MoO /TiO –S catalysts [14],
2 3
, Ru/C, and Ru/Al O catalysts [15].
2
O
3
decomposition of lignin model compound to aromatics.
O
2 3
O
2 3
3
/Al
2
O
3
3
2
2
O
3
2. Experimental
Hydrocracking of lignin by these catalysts requires relatively high
temperature (over 400 °C) and high pressure (50–100 atm).
Heteropolyacids (HPAs) are inorganic acids. Acid strength of HPAs is
2.1. Catalyst preparation
+
stronger thanthat of conventionalsolid acids[16–19]. HPA salts with K ,
Cs
x
H
3.0 − xPW12
O
40 catalysts were prepared according to the
PW12 40 (Sigma-
+
+
Cs , and NH
4
cations have high surface area and porous structure by
reported method [19,22]. Commercially available H
3
O
Aldrich) was thermally treated at 300 °C for 2 h for precise quantifica-
tion, prior to the preparation of cesium-exchanged heteropolyacid
catalysts. A series of cesium-exchanged heteropolyacid catalysts
⁎
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
x
(Cs H3.0−xPW12O40) were prepared by an ion exchange method with
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.08.002

2
H.W. Park et al. / Catalysis Communications 12 (2010) 1–4
Primary reaction
a variation of cesium content (X=2.0, 2.3, 2.5, 2.8, and 3.0). A known
amount of cesium nitrate (CsNO3, Sigma-Aldrich) was dissolved in
distillated water. The solution was added dropwise into an aqueous
OH
H+
3
solution containing H PW12O40 with constant stirring. The resulting
solution was then slowly heated at 60 °C for 12 h to obtain a solid. The
solid product was dried overnight at 70 °C, and then it was calcined at
O
3
00 °C for 2 h to yield Cs
catalysts. Cesium content in Cs
was confirmed by ICP-AES analysis (Shimadz, ICP-1000 IV).
x
H
3.0−xPW12
O
40 (X=2.0, 2.3, 2.5, 2.8, and 3.0)
x
H3.0 − xPW12
O40 (X=2.0−3.0) catalysts
Secondary reaction
2
x 40
.2. Decomposition of phenethyl phenyl ether over Cs H3.0 − xPW12O
Decomposition of phenethyl phenyl ether over cesium-exchanged
x
H3.0 − xPW12O40 heteropolyacid catalysts was carried out in an
Cs
autoclave reactor under nitrogen atmosphere. 1,2,3,4-Tetrahydro-
naphthalene was used as a solvent for phenethyl phenyl ether and as a
hydrogen donor in the decomposition of phenethyl phenyl ether.
2
4.5 ml of 1,2,3,4-tetrahydronaphthalene (Sigma-Aldrich) and 0.5 ml
of phenethyl phenyl ether (Frinton Laboratory) were charged into the
reactor. 0.025 g of Cs 40 (X=2.0, 2.3, 2.5, 2.8, and 3.0) or
x
H3.0 − xPW12O
Fig. 1. Scheme for the decomposition of phenethyl phenyl ether over Cs
x 40
H3.0−xPW12O .
Raney Nickel catalyst (Sigma-Aldrich) was then added into the
reactor. The catalytic reaction was performed at 200 °C and 10 atm for
1
h with agitation speed of 100 rpm. Reaction products were analyzed
with a gas chromatograph (Younglin, ACME 6100) equipped with DB-
column and flame ionization detector. Conversion of phenethyl
Catalytic performance of Cs
and 3.0) in the decomposition of phenethyl phenyl ether performed at
00 °C and 10 atm for 1 h is listed in Table 1. Conversion of phenethyl
phenyl ether over Cs 40 was in the range of 31.2–68.1%.
Selectivity for phenol (40.7–51.3%) was much higher than that for
benzene, toluene, and ethylbenzene, because benzene, toluene, and
ethylbenzene were produced by secondary reaction of styrene.
Among the products produced by secondary reaction, selectivity for
ethylbenzene (17.6–19.2%) was a little higher than that for benzene
x
H3.0 − xPW12O40 (X=2.0, 2.3, 2.5, 2.8,
5
2
phenyl ether and selectivity for product (phenol, benzene, toluene, or
ethylbenzene) were calculated on the basis of carbon balance. Yield
for product (phenol, benzene, toluene, or ethylbenzene) was
calculated by multiplying conversion of phenethyl phenyl ether and
corresponding product selectivity.
x
H3.0 − xPW12O
Conversion of phenethyl phenyl ether
(
(
9.3–16.7%). Selectivity for by-products such as light hydrocarbons
–C ) and alcohols was in the range of 16.7–26.0%.
Because selectivity for phenol was higher than that for benzene,
moles of phenethyl phenyl ether reacted
moles of phenethyl phenyl ether supplied
C
2
7
=
ð1Þ
toluene, and ethylbenzene, yield for phenol (16.0–30.2%) was much
higher than that for benzene, toluene, and ethylbenzene. Yield for
ethylbenzene (6.0–12.3%) was a little higher than that for benzene
Selectivity for product (phenol, benzene, toluene, or ethylben-
zene)
(
2.9–11.4%). Total selectivity for main products (phenol, benzene,
moles of phenol; benzene; toluene; or ethylbenzene formed
toluene, and ethylbenzene) over Cs H3.0 − xPW12O40 was in the range
of 74.0–83.3%. Total yield for main products (phenol, benzene,
x
=
ð2Þ
moles of phenethyl phenyl ether reacted
toluene, and ethylbenzene) over Cs
x
H3.0 − xPW12O40 was in the
Total selectivity for main products
range of 25.4–54.9%. Among the catalysts tested, Cs2.5
H
0.5PW12O
40
catalyst showed the best catalytic performance in terms of conversion
of phenethyl phenyl ether and total yield for main products (Table 1).
Total moles of phenol; benzene; toluene; and ethylbenzene formed
=
moles of phenethyl phenyl ether reacted
ð3Þ
Table 1
Total yield for main products
Catalytic performance of Cs
ether.
x
H3.0 − xPW12O40 in the decomposition of phenethyl phenyl
=
ðConversion of phenethyl phenyl etherÞ
ðTotal selectivity for main productsÞ
ð4Þ
X in Cs
x
H3.0 − xPW12O40 (X=2.0–3.0)
×
X=1.5 c X=2.0 X=2.3 X=2.5 X=2.8 X=3.0
Conversion (%)
51.1
56.9
74.0
59.8
77.9
68.1
80.4
61.8
83.3
31.2
81.3
Total selectivity for main 70.8
a
3
. Results and discussion
products (%)
Phenol (%)
Benzene (%)
Toluene (%)
39.7
13.1
1.6
16.4
36.2
40.7
14.0
1.7
17.6
42.2
43.6
14.8
1.6
17.9
46.7
44.3
16.7
1.4
18.0
54.9
47.3
15.7
1.6
18.7
51.4
51.3
9.3
1.5
19.2
25.4
3
.1. Catalytic performance in the decomposition of phenethyl phenyl
ether
Ethylbenzene (%)
Total yield for main
b
products (%)
Fig. 1 shows that phenol and styrene are produced by the
primary cleavage of β-O-4 linkage in the phenethyl phenyl ether
over Cs 40 catalysts. Styrene is then further converted
Phenol (%)
Benzene (%)
Toluene (%)
20.3
6.7
0.8
23.2
8.0
1.0
26.1
8.9
1.0
30.2
11.4
1.0
29.2
9.7
1.0
16.0
2.9
0.5
x
H3.0 − xPW12O
to benzene, toluene, and ethylbenzene through the secondary
reaction over the catalyst. As shown in Fig. 1, aromatic compounds
such as phenol, benzene, toluene, and ethylbenzene are mainly
produced in the decomposition of phenethyl phenyl ether.
Ethylbenzene (%)
8.4
10.0
10.7
12.3
11.5
6.0
a
Calculated by Eq. (3).
Calculated by Eq. (4).
Conducted for comparison.
b
c

H.W. Park et al. / Catalysis Communications 12 (2010) 1–4
3
8
7
6
5
4
3
2
0
0
0
0
0
0
0
(
a)
Conversion of phenethyl phenyl ether
Total yield for main products
Cs H PW O
12 40
70
2.5 0.5
Cs H PW O
2
.0 1.0
12 40
Cs H PW O
12 40
6
0
2.8 0.2
Cs H PW O
2
.3 0.7
12 40
50
40
Cs PW O
12 40
3
0
0
3.0
2
2
.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
X in Cs H PW O
0
10
20
30
40
50
60
x
3.0-x
12 40
Surface acidity (µmmol-NH /g)
3
Fig. 2. Catalytic performance of Cs
phenyl ether plotted as a function of cesium content in Cs
condition: Temperature=200 °C, Pressure=10 bar (N ), Time=1 h.
x
H3.0 − xPW12
O
40 in the decomposition of phenethyl
(
b)6
x
H3.0 − xPW12O40. Reaction
0
2
Cs H PW O
2
.5 0.5
12 40
Cs H PW O
12 40
2
.8 0.2
3
.2. Effect of cesium content on the catalytic performance
50
Cs H PW O
2
.3 0.7
12 40
Fig. 2 shows the catalytic performance of Cs
decomposition of phenethyl phenyl ether plotted as a function of
cesium content in Cs 40. Conversion of phenethyl
phenyl ether showed a volcano-shaped curve with respect to cesium
content. Total yield for main products also showed a volcano-shaped
curve with respect to cesium content. Among the catalysts tested,
x
H3.0 − xPW12O40 in the
4
3
2
0
0
0
Cs H PW O
2.0 1.0 12 40
x
H3.0 − xPW12O
Cs2.5H
0.5PW12O40 showed the best catalytic performance in terms of
Cs3.0PW12
10
O
40
conversion of phenethyl phenyl ether and total yield for main
products.
Fig. 3 shows the yields for phenol, benzene, and ethylbenzene
x
plotted as a function of cesium content in Cs H3.0 − xPW12O40. Yields
0
20
30
40
50
60
Surface acidity (µmol-NH /g)
3
for phenol, benzene, and ethylbenzene showed volcano-shaped
curves with respect to cesium content. Among the catalysts tested,
Cs2.5H0.5PW12O40 catalyst showed the best catalytic performance in
Fig. 4. Correlations (a) between conversion of phenethyl phenyl ether over
Cs 40 and surface acidity of Cs 40, and (b) between total yield
for main products (phenol, benzene, toluene, and ethylbenzene) over Cs H3.0−xPW12O40
x
H
3.0 − xPW12
O
x
H3.0−xPW12O
x
terms of yields for phenol, benzene, and ethylbenzene. As mentioned
earlier, yield for phenol was much higher than that for ethylbenzene
or benzene.
and surface acidity of Cs
x
H
3.0−xPW12
O
40. Surface acidity data were taken from a literature
[
19]. Reaction condition: Temperature=200 °C, Pressure=10 bar (N
2
), Time=1 h.
Surface acidity data were taken from a literature [19]. The correlation
clearly shows that conversion of phenethyl phenyl ether is closely related
to the surface acidity of the catalyst. Conversion of phenethyl phenyl
ether increased with increasing surface acidity of the catalyst. Among the
3
.3. Effect of surface acidity on the catalytic performance
Fig. 4a shows the correlation between conversion of phenethyl phenyl
ether over Cs
x
H
3.0 −xPW12
O
40 and surface acidity of Cs
x
H
40
3.0 −xPW12O .
tested catalysts, Cs2.5H0.5PW12O40 catalyst with the largest surface acidity
showed the highest conversion of phenethyl phenyl ether.
Fig. 4b shows the correlation between total yield for main products
3
3
2
2
1
1
5
0
5
0
5
0
5
0
(
x 40
phenol, benzene, toluene, and ethylbenzene) over Cs H3.0 − xPW12O
Benzene
Ethylbenzene
Phenol
and surface acidity of Cs 40. The correlation shows that
x
H3.0 − xPW12O
total yield for main products is also closely related to the surface acidity
of the catalyst. Total yield for main products increased with increasing
surface acidity of the catalyst. It is concluded that cesium-exchanged
Cs
catalyst in the decomposition of phenethyl phenyl ether to aromatics.
Surface acidity of Cs 40 played an important role in
x
H3.0−xPW12O40 heteropolyacid catalysts served as an efficient acid
x
H3.0−xPW12O
determining the catalytic performance in the decomposition of
phenethyl phenyl ether.
Catalytic performance of Cs1.5H1.5PW12O40 in the decomposition of
phenethyl phenyl ether is also listed in Table 1 for comparison purpose.
Conversion of phenethyl phenyl ether (51.1%) and total yield for main
products (36.2%) over Cs1.5
Cs3.0PW12 40 (31.2% and 25.4%, respectively) but were lower than those
over Cs2.0 40 (56.9% and 42.2%, respectively). Judging from the
fact that surface acidity of Cs1.5 40 (2 μmol-NH /g-cat) was
higher than that of Cs3.0PW12 40 (1 μmol-NH /g-cat) but was lower
than that of Cs2.0 40 (5 μmol-NH /g-cat) [19], it can also be
H1.5PW12O40 were higher than those over
2
.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
X in Cs H PW O
O
x
3.0-x
12 40
H1.0PW12O
H
1.5PW12
O
3
Fig. 3. Yields for phenol, benzene, and ethylbenzene plotted as a function of cesium
content in Cs 40. Reaction condition: Temperature=200 °C, Pressure=10 -
bar (N ), Time=1 h.
O
3
x
H3.0 −xPW12O
H
1.0PW12
O
3
2

4
H.W. Park et al. / Catalysis Communications 12 (2010) 1–4
1
00
6
8.1% and 25.4–54.9%, respectively. Conversion of phenethyl phenyl
Conversion of phenethyl phenyl ether
Selectivity for main products
Total yield for main products
ether and total yield for main products were closely related to the
surface acidity of Cs
tested, Cs2.5
highest conversion of phenethyl phenyl ether and total yield for main
x 40
products. It is concluded that surface acidity of Cs H3.0 − xPW12O
played an important role in determining the catalytic performance in
the decomposition of phenethyl phenyl ether.
x
H3.0 − xPW12O40 catalysts. Among the catalysts
80
60
40
20
0
H0.5PW12O40 with the largest surface acidity showed the
Acknowledgement
This work was supported by the National Research Foundation of
Korea Grant funded by the Korean Government (MEST) (NRF-2009-
C1AAA001-0093292).
Raney Ni
Cs2.5H0.5PW12O40
Fig. 5. Catalytic performance in the decomposition of phenethyl phenyl ether over
Cs2.5 40 catalyst and Raney Nickel catalyst. Reaction condition:
Temperature=200 °C, Pressure=10 bar (N ), Time=1 h.
H
0.5PW12O
References
2
[
[
1] C. Amen, H. Pakdel, C. Roy, Bioresour. Technol. 79 (2001) 277.
2] V.O. Sippola, A.O.I. Krause, Catal. Today 100 (2005) 237.
concluded that surface acidity of Cs
important role in determining the catalytic performance in the target
reaction.
x
H
3.0 − xPW12
O
40 played an
[3] J.J. Bozell, Clean 36 (2008) 641.
[
[
[
[
[
[
4] H.H. Nimz, R. Casten, Holz Roh-Werkstoff 44 (1986) 207.
5] M. Kleinert, T. Barth, Chem. Eng. Technol. 31 (2008) 736.
6] H.L. Jairo, G.G. Wolfgang, J. Polym. Environ. 10 (2002) 39.
7] R.W. Thring, J. Breau, Fuels 75 (1996) 795.
8] R.K. Sharma, N.N. Bakhshi, Enegy Fuels 7 (1993) 306.
9] J.D. Adjaye, N.N. Bakhshi, Fuel Process. Technol. 45 (1995) 161.
3
.4. Comparison of catalytic performance between Cs2.5H0.5PW12O
40
catalyst and Raney Nickel catalyst in the decomposition of phenethyl
phenyl ether
[
10] Y.H.E. Sheu, R.G. Anthony, E.J. Soltes, Fuel Process. Technol. 19 (1988) 31.
[11] F.P. Petrocelli, M.T. Klein, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 635.
12] R.K.M.R. Kallury, W.M. Restivo, T.T. Tidwell, D.G.B. Boocock, A. Crimi, J. Douglas,
J. Catal. 96 (1985) 535.
13] E. Laurent, B. Delmon, Appl. Catal. A 109 (1994) 77.
[
[
Raney Nickel catalyst is a well known catalyst for lignin
decomposition [15]. Fig. 5 compares the catalytic performance
[14] M. Koyama, Bioresour. Technol. 44 (1993) 209.
[
[
15] J.M. Pepper, Y.W. Lee, Can. J. Chem. 47 (1969) 723.
16] Q. Yang, M.P. Kapoor, S. Inagaki, N. Shirokura, J.N. Kondo, K. Domen, J. Mol. Catal.
A: Chem. 230 (2005) 85.
[17] Q. Yang, J. Liu, J. Yang, M.P. Kapoor, S. Inagaki, C. Li, J. Catal. 228 (2004) 265.
18] S. Park, S.H. Lee, S.H. Song, D.R. Park, S.-H. Baeck, T.J. Kim, Y.M. Chung, S.H. Oh, I.K.
Song, Catal. Commun. 10 (2009) 391.
19] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113.
[20] J.A. Dias, E. Caliman, S.C.L. Dias, Microporous Mesoporous Mater 76 (2004) 221.
[21] A. Corma, A. Martinez, C. Martinez, J. Catal. 164 (1996) 422.
22] H. Lee, J.C. Jung, H. Kim, Y.M. Chung, T.J. Kim, S.J. Lee, S.H. Oh, S.Y. Kim, I.K. Song,
Korean J. Chem. Eng. 26 (2009) 994.
between Cs2.5
decomposition of phenethyl phenyl ether. Conversion of phenethyl
phenyl ether over Cs2.5 40 (68.1%) was much higher than
that over Raney Nickel catalyst (11.4%). Total yield for main products
produced by the acid catalysis of Cs2.5 40 (54.9%) was much
higher than that produced by Raney Nickel catalyst (6.7%). It is
concluded that Cs2.5 40 was more efficient than Raney
Nickel catalyst in the decomposition of phenethyl phenyl ether.
H0.5PW12O40 catalyst and Raney Nickel catalyst in the
H
0.5PW12
O
[
H0.5PW12O
[
H
0.5PW12O
[
[
23] Z. Joseph, B.C.A. Pieter, L.J. Anna, M.W. Bert, Chem. Rev. 110 (2010) 3552.
4
. Conclusions
[24] A.-C. Carlos, H. Pakdel, C. Roy, Bioresour. Technol. 79 (2001) 277.
[
[
[
25] M.T. Klein, P.S. Virk, Ind. Eng. Chem. Fundam. 22 (1983) 35.
26] P.F. Britt, M.K. Kidder, A.C. Buchanan, Energy Fuels 21 (2007) 3102.
27] P.F. Britt, A.C. Buchanan, M.J. Cooney, D.R. Martineau, J. Org. Chem. 65 (2000)
1314.
x
Catalytic performance of Cs H3.0−xPW12O40 (X=2.0, 2.3, 2.5, 2.8,
and 3.0) in the decomposition of phenethyl phenyl ether to aromatics
was studied in this work. Conversion of phenethyl phenyl ether and
total yield for main products (phenol, benzene, toluene, and ethylben-
[
[
28] J.B. Binder, M.J. Gray, J.F. White, Z.C. Zhang, J.E. Holladay, Biomass Bioenergy 33
(2009) 1122.
29] D.V. Evtuguin, A.I.D. Daniel, A.J.D. Silvestre, F.M.L. Amad, C.P. Neto, J. Mol. Catal. A:
Chem. 154 (2000) 217.
x
zene) over Cs H3.0−xPW12O40 (X=2.0–3.0) was in the range of 31.2–