8
J.K. Kim et al. / Catalysis Communications 27 (2012) 5–8
100
80
60
40
20
80
C4H9-O-C4H9
(Di-n-butyl ether)
C4H9 –OH
(n-butanol)
H3PW12O40
C4H8
(Butenes)
60
H4SiW12O40
100
80
60
40
20
0
H5BW12O40
40
H6CoW12O40
20
450
500
550
600
Desorption peak temperature of γ-peak (ºC)
Fig. 5. Correlations of desorption peak temperature of γ-peak with conversion of n-
butanol and with yield for di-n-butyl ether at 200 °C after a 3 h-reaction.
determined by NH3-TPD measurements. Desorption peak temperature
of γ-peak (acid strength) increased in the order H6CoW12O40 (469 °C)b
H5BW12O40 (501 °C)bH4SiW12O40 (568 °C)bH3PW12O40 (593 °C). In
the etherification of n-butanol to di-n-butyl ether over HnXW12O40
Keggin HPA catalysts, yield for di-n-butyl ether increased with increasing
acid strength of the catalysts. It is concluded that acid strength of HPA cat-
alysts, which was easily controlled by changing heteroatom, played an
important role in the etherification of n-butanol to di-n-butyl ether
over HnXW12O40 (X_Co2+, B3+, Si4+, and P5+) Keggin HPA catalysts.
H CoW
O
H BW
O
H SiW
O
H PW O
3 12 40
6
12 40
5
12 40
4
12 40
Fig. 4. Catalytic performance of HnXW12O40 (X_Co2+, B3+, Si4+, and P5+) Keggin HPA
catalysts in the etherification of n-butanol to di-n-butyl ether at 200 °C after a 3 h-
reaction.
intermediates including oxonium or carbenium ions can be selectively
coordinated and stabilized on the surface of heteropolyanion via forma-
Acknowledgments
tion of intermediate-heteropolyanion complex due to the softness of het-
4−
eropolyanion [14]. It is known that softness of SiW
O
12 40
is greater than
This subject is supported by Korea Ministry of Environment as
“Converging Technology Project (202-101-009)”.
3−
that of PW O
12 40
[14]. In some particular aqueous reactions, softness
becomes a dominant factor due to complete dissociation of hetero-
polyacid into heteropolyanion. In this case, the catalytic performance of
H4SiW12O40 catalyst would be better than that of H3PW12O40 catalyst.
In the etherification of n-butanol to di-n-butyl ether, however, the cata-
lytic performance of HPA catalysts was not proportional to the softness
of heteropolyanions. Therefore, our next investigation was focused on
the acid properties of HPA catalysts. However, no correlation between
acidity and catalytic performance was found. Instead, experimental re-
sults revealed that the trend of yield for di-n-butyl ether was well
consistent with the trend of acid strength (desorption peak temperature
of γ-peak). Therefore, it is concluded that acid strength of HPA catalysts
dominantly affected the catalytic performance in the etherification of n-
butanol.
Fig. 5 shows the correlations of desorption peak temperature of γ-
peak with conversion of n-butanol and with yield for di-n-butyl ether
at 200 °C after a 3 h-reaction. Conversion of n-butanol and yield for
di-n-butyl ether increased with increasing desorption peak tempera-
ture of γ-peak (with increasing acid strength) of HPA catalysts. This re-
sult indicates that acid strength played a key role in the etherification of
n-butanol over HnXW12O40 Keggin HPA catalysts.
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4. Conclusion
Etherification of n-butanol to di-n-butyl ether was carried out over
heteroatom-substituted HnXW12O40 (X_Co2+, B3+, Si4+, and P5+
)
Keggin HPA catalysts in a liquid-phase batch reactor. HnXW12O40 Keggin
HPA catalysts were prepared and characterized by FT-IR and ICP-AES
analyses. Acid properties of HnXW12O40 Keggin HPA catalysts were