Date of Award

2008

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Keywords

Applied sciences, Pure sciences, Heterogeneous acid catalysts, Niobium oxides, Tantalum oxides

Supervisor

David M. Antonelli

Rights

info:eu-repo/semantics/openAccess

Abstract

In order to probe into the structural and coordination geometry of mesoporous Nb oxide and in efforts to make meaningful comparisons of mesoporous niobia prepared by the amine-templating method with the corresponding bulk sol-gel prepared Nb2O5 phase, 17O magic-angle-spinning solid-state NMR studies were conducted. The results showed a very high local order in the mesoporous sample. The oxygen atoms are coordinated only as ONb 2 in contrast with bulk phases in which the oxygen atoms are always present in a mixture of ONb2 and ONb3 coordination environments.

To enhance their surface acidities and thus improve their performance as solid acid catalysts in the acid-catalyzed reactions mentioned above, pure mesoporous Nb and Ta oxides were further treated with 1M sulfuric acid or phosphoric acid. Their surface acidities before and after acid treatment were measured by Fourier transform infraRed (FT IR), amine titration and temperature programmed desorption of ammonia (NH3-TPD).

Results obtained in this study showed that sulfated mesoporous Nb and Ta oxides materials possess relative high surface areas (up to 612 m 2/g) and amorphous wormhole structure. These mesoporous structures are thus quite stable to acid treatment. It was also found that Brønsted (1540 cm-1) and Lewis (1450 cm-1) acid sites coexist in a roughly 50:50 mixture on the surfaces of the parent pristine materials. However, the acid-treated Nb and Ta materials both exhibited a strong dominance of Brønsted acid sites, which is believed to be responsible for their higher catalytic activities with respect to the untreated materials.

Although Ta samples (C6, C12, C13) have relatively lower surface areas than the Nb samples, they always show much higher catalytic activity as well as selectivity. For instance, in the isomerization of 1-hexene and alkylation of benzene with bulky olefins, the best results were achieved using C12 H2SO4 mesoporous Ta oxide, which has a BET surface area of 292.19 m2/g, far less than its sulfated C12 Nb counterpart (413.97 m2/g). This demonstrates that the catalytic performance of these materials does not only depend on their surface areas, but is also influenced by the pore size and surface acidity. To investigate this unusual phenomenon, ammonia TPD was used to precisely measure the surface acidities of the Nb and Ta samples and show that the Ta materials have a much greater concentration of Brønsted acid sites than the Nb materials. This explains the observed higher activities.

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