Support effects on Brønsted acid site densities and alcohol dehydration turnover rates on tungsten oxide domains
The effects of support identity on catalytic 2-butanol dehydration rates, Brønsted acid site density, and reducibility are examined for WO x domains supported on ZrO 2, Al 2O 3, SiO 2 (MCM41), and SnO 2. On WO x –Al 2O 3, 2-butanol dehydration rates (per W atom) increased with increasing WO x surfac...
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Veröffentlicht in: | Journal of catalysis 2004-10, Vol.227 (2), p.479-491 |
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Sprache: | eng |
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Zusammenfassung: | The effects of support identity on catalytic 2-butanol dehydration rates, Brønsted acid site density, and reducibility are examined for WO
x
domains supported on ZrO
2, Al
2O
3, SiO
2 (MCM41), and SnO
2. On WO
x
–Al
2O
3, 2-butanol dehydration rates (per W atom) increased with increasing WO
x
surface density and reached maximum values at WO
x
surface densities (9–10 W nm
−2) similar to those required for two-dimensional polytungstates, as also found on WO
x
–ZrO
2. UV–visible edge energies showed that WO
x
domains become larger as WO
x
surface density increases. Selective titration of Brønsted acid sites by sterically hindered 2,6-di-
tert-butylpyridine during 2-butanol dehydration reaction showed that this reaction occurs predominately on Brønsted acid sites for WO
x
domains on ZrO
2, Al
2O
3, SiO
2, and SnO
2 supports. Pre-edge features appear in the UV–visible spectra of WO
x
–Al
2O
3 samples during 2-butanol dehydration and their intensity increases with WO
x
surface density in parallel with measured Brønsted acid site densities and dehydration rates (per W atom). These d–d electronic transitions reflect the formation of reduced centers, consisting of acidic
H
δ
+
(
WO
3
)
n
δ
−
species, using 2-butanol as a stoichiometric reductant. These processes resemble those on WO
x
–ZrO
2, indicating that temporary acid sites generally form from neutral WO
x
precursors on all supports. Dehydration turnover rates (per Brønsted acid site) were unaffected by the identity of the support, but for a given WO
x
surface density, the number of reduced centers and the density of Brønsted acid sites, but not their intrinsic reactivity, depend on the identity of the support; both reduced centers and Brønsted acid sites are more abundant on ZrO
2-supported than on Al
2O
3-supported samples, as a result of electronic isolation of WO
x
domains on the more insulating and unreducible Al
2O
3 supports. The dehydration regioselectivity on Brønsted acid sites is strongly influenced by support, with more electropositive support cations leading to stronger interactions between
α-hydrogens in reactants and lattice oxygens, favoring sterically hindered transition states required for the formation of
cis-2-butene. |
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ISSN: | 0021-9517 1090-2694 |
DOI: | 10.1016/j.jcat.2004.08.014 |