Coordination and properties of cobalt in the molecular sieves CoAPO-5 and-11

Diffuse reflectance electronic spectra have been utilized to investigate the local geometry of cobalt centers substituted in the three-dimensional framework of CoAPO-5 and -11 molecular sieves. Structural changes induced by calcination in an oxidative atmosphere, evacuation and hydrogen treatment, as well as by adsorption of amines and water are discussed in terms of the steric conditions of cobalt incorporation in the aluminophosphate matrix. We suggest a template-framework interaction between the framework cobalt centers and neutral amine-type templates as an alternative stabilizing effect in the as-synthesized samples. Spectral changes observed after calcination in oxygen are interpreted as distortion-induced charge transfer effects without oxidation of the divalent cobalt ions. Formation of oxygen vacancies in the close vicinity of the cobalt sites is proposed to explain the adsorption properties of the framework cobalt centers. (C) 2000 Elsevier Science B.V. All rights reserved.