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Chemistry Letters Vol.34, No.5 (2005)
Active and Reusable Catalyst in the Friedel–Crafts Alkylation Derived from a Heteropoly Acid
Kazu Okumura,ꢀ Katsuhiko Yamashita, Miho Hirano, and Miki Niwa
Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680-8552
(Received March 1, 2005; CL-050269)
The H3PO4–WO3–Nb2O5 derived catalyst exhibited excel-
lent activity and reusability in the benzylation of anisole at low
temperature. On the basis of the XRD and IR data, the oxide de-
rived from heteropoly acid was ascribed to the active and insolu-
ble species in the reaction. It was found that the calcination tem-
perature was the key factor in the generation of active species.
tive in the reaction at 353 K. On the other hand, 35% of conver-
sion of benzyl alcohol was obtained on WO3–Nb2O5. The
H3PO4–WO3–Nb2O5 exhibited the highest activity among these
oxides, where 98% conversion of benzyl alcohol was reached at
343 K. The ortho- and para-substituted benzylanisole were ob-
tained as products, where the distribution of p-benzylanisole
was 54% at 353 K. The conversion of H3PO4–WO3–Nb2O5
was 4 times higher than that of K-10 treated with sulfuric acid
when comparison was made at 343 K, which was reported to be
active for this class of reaction.3 The activity of H3PO4–WO3–
Nb2O5 was higher than H-ꢀ zeolite (Si/Al2 ¼ 25, PQ co.) as
well. The recycle use of H3PO4–WO3–Nb2O5 was attempted
simply by separating the catalyst by filtration and washing with
anisole. The deactivation was not observed after the recycle use
at least three times at 353 K as given in Table 1. By contrast,
no further reaction took place over the filtrated solution under
the presence of remained benzyl alcohol, indicating the catalytic
reaction truly took place over the solid catalyst. The H3PO4–
WO3–Nb2O5 catalysts having different amount of H3PO4 were
subjected to the reaction at 333 K in order to optimize the compo-
sition of the catalyst. The highest activity was reached when the
concentration of H3PO4 was 8.6 wt % (54.8 wt %, WO3; 36.6 wt
%, Nb2O5) as analyzed by ICP method. As a comparable experi-
ment, HPW (60 wt %) were impregnated on Nb2O5, followed by
calcination at 773 K in air. Although the catalyst exhibited high
conversion (42%) of benzyl alcohol in the first run at 333 K,
the conversion decreased to 8.9% in the second run, meaning
the recycle use was not possible over the impregnated catalyst.
Figure 1 shows the relationship between calcination temper-
ature and the catalytic performance of H3PO4–WO3–Nb2O5. The
reaction was performed at 333 K. The conversion and yield of
products increased with increasing the calcination temperature,
and the optimum activity was attained when the catalyst was cal-
In organic synthesis using solid acid catalysts, not only high
activity but the reusability of the active species is desired in view
of the separation of the products and the recycle use of the cata-
lysts. Heteropoly acids (HPA) have been employed as catalysts
for the various kinds of organic reactions using their strong
Brꢀnsted acidity.1 However, the high solubility of HPA in the sol-
vent hampered the application of HPA to the liquid phase reac-
tion. The difficulty was overcome by inclusion of HPA in the sili-
ca matrix.1,2 Herein, we report that intermediate in the decompo-
sition of heteropoly acid derived from the mixed oxides (H3PO4–
WO3–Nb2O5) exhibited excellent catalytic performance in that it
showed facile reusability as well as high activity in the Friedel–
Crafts alkylation. Recently, much attention has been paid to the
Friedel–Crafts alkylation in which alkyl group could be readily
introduced on an aromatic ring. The scheme of the reaction con-
ducted here was given in eq 1. A side reaction occurs to form di-
benzylether.
MeO
MeO
OH
+
+
H2O
(1)
Niobium oxalate was prepared by the dissolution of 1.27 g of
niobic acid (supplied by CBMM Co.) in a oxalic acid (4.77 g) so-
.
lution in 100 mL on a hot plate. (NH4)10W12O41 5H2O (1.69 g)
was dissolved in deionized water (100 mL). The solutions of
niobium oxalate and (NH4)10W12O41 were mixed, followed by
addition of 85% phosphoric acid (0.22 g). The admixture solution
was evaporated on a hot plate with continuous stirring. The ob-
tained solid was calcined in air at 573 K to 773 K for 3 h. The
K-10 modified with sulfuric acid was prepared according to the
lieterature.3 A 0.1 g of catalyst was used for benzylation of ani-
sole. The pretreatment was carried out in an N2 flow at 673 K
for 1 h. For the sample calcined at 573 K in the synthesis, the pre-
treatment was conducted at 573 K (Figure 1). The reaction was
performed using 10 g of anisole and 0.675 g (6.25 mmol) of ben-
zyl alcohol in an oil bath at 333–353 K under an N2 atmosphere
for 3 h. In the recycled catalytic reaction, the catalyst was separat-
ed with filtration and washed with anisole. Then the catalyst was
repeatedly used for further reaction without pretreatment. The
products were analyzed by GC equipped with capillary column.
In the analysis, tridecane was used as an internal standard.
The data of catalytic performance over H3PO4–WO3–Nb2O5
as well as several combinations of oxides were listed in Table 1.
Nb2O5, WO3, H3PO4–Nb2O5, and H3PO4–WO3 prepared in a
similar manner as H3PO4–WO3–Nb2O5 were substantially inac-
Table 1. Data of the reaction between benzyl alcohol and anisole
over various catalysts
Benzyl
alcohol anisole
Benzyl Dibenzyl
Material
balance
/%
ether
yield
/%
Catalyst
conv.
/%
yield
/%
a
Nb2O5
a
0
1.2
0
0.2
0.3
0.1
0.1
1.5
0.1
3.5
0.3
0.3
11.5
3.5
6.6
102
99
WO3
H3PO4–Nb2O5
a
3.4
0.5
105
105
99
a
H3PO4–WO3
WO3–Nb2O5
0
a
35.0
100
100
98.0
25.4
40.6
26.8
93.4
94.0
93.5
19.5
23.2
a
b
c
H3PO4–WO3–Nb2O5
H3PO4–WO3–Nb2O5
H3PO4–WO3–Nb2O5
93
94
102
96
c
modified K-10
H-ꢀ (Si/Al2 ¼ 25)
a
96
a353 K, b3rd run in the repeated reaction at 353 K, c343 K, the oxides
were calcined at 773 K in the preparation.
Copyright Ó 2005 The Chemical Society of Japan
Okumura, Kazu
