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

Diffuse reflectance electronic spectra have been utilized to investigate th e local geometry of cobalt centers substituted in the three-dimensional fra mework of CoAPO-5 and -11 molecular sieves. Structural changes induced by c alcination in an oxidative atmosphere, evacuation and hydrogen treatment, a s well as by adsorption of amines and water are discussed in terms of the s teric conditions of cobalt incorporation in the aluminophosphate matrix. We suggest a template-framework interaction between the framework cobalt cent ers and neutral amine-type templates as an alternative stabilizing effect i n the as-synthesized samples. Spectral changes observed after calcination i n oxygen are interpreted as distortion-induced charge transfer effects with out oxidation of the divalent cobalt ions. Formation of oxygen vacancies in the close vicinity of the cobalt sites is proposed to explain the adsorpti on properties of the framework cobalt centers. (C) 2000 Elsevier Science B. V. All rights reserved.