Synthesis and characterization of divalent cobalt-substituted mesoporous aluminophosphate molecular sieves and their application as novel heterogeneous catalysts for the oxidation of cycloalkanes

Abstract

Thermally stable divalent cobalt-substituted hexagonal mesoporous aluminophosphate, CoHMA molecular sieves were hydrothermally synthesized and systematically characterized by several analytical and spectroscopic techniques. XRD, TEM, ED, ICP-AES, TGA-DTA, N 2 sorption, DR-UV–vis, XANES, and NH 3-TPD studies showed isomorphous substitution of divalent cobalt in a tetrahedral framework of a mesoporous aluminophosphate matrix. Furthermore, the blue color of CoHMA, before and after thermal treatment, further confirmed the tetrahedral environment of divalent cobalt in the matrix. As a result, the CoHMA catalyst showed excellent activity for the oxidation of cycloalkanes, that is, cyclohexane, cyclooctane and cyclododecane, under mild reaction conditions. Unlike the many other cobalt-based heterogeneous catalysts reported so far, CoHMA does not show any dislodgement or segregation of cobalt with calcination or any other postsynthesis treatments. In this study, the performance of CoHMA was also compared with the cobalt-containing microporous aluminophosphate, that is, CoAPO-5 with AFI topology, as well as with both microporous and mesoporous analogues of cobalt silicate molecular sieves that have MFI or MCM-41 structures.