X-ray Absorption Spectroscopic Study of Brønsted, Lewis, and Redox Centers in Cobalt-Substituted Aluminum Phosphate Catalysts

The structure of acid sites in cobalt-substituted aluminophosphates (AlPOs) catalysts has been investigated, with EXAFS spectroscopy. Data obtained, using in situ methods, for the Co K-edge spectra of cobalt-substituted AlPO-5, AlPO-18, AlPO-36, and APSO-44 in as-prepared, calcined, and reduced states yield the local structure of the Co ions. Whereas the as-prepared materials clearly contain Co(II) ions in regular four-coordinated sites, complex behavior is exhibited by their calcined analogues, with essentially complete oxidation of Co(II) to Co(III) in CoAlPO-18, with the local coordination of the high-spin Co(III) being distorted. By contrast incomplete oxidation of the Co(II) is observed for CoAlPO-5 and CoAlPO-36. A combination of EXAFS data analysis with the results of computer modeling studies suggests that this behavior is interpretable in terms of the formation of oxygen vacancies by dehydroxylation which lead to undercoordinated Lewis acid, Co(II) sites. IR data of the reduced samples reveal the presence of Brønsted acid sites which, from the EXAFS analysis, are seen to correspond to Co(II) species in a distorted environment owing to the presence of a neighboring protonated oxygen ion. The implications of the results for the catalytic properties of the systems are considered.