Thermally stable trivalent iron-substituted hexagonal mesoporous aluminophosphate (FeHMA) molecular sieves: Synthesis, characterization, and catalytic properties

Thermally stable trivalent iron-substituted hexagonal mesoporous aluminophosphate (FeHMA) molecular sieves were synthesized and systematically characterized using analytical and spectroscopic techniques, including XRD, TEM, ICP–AES, TGA–DTA, N2 sorption, DRUV–vis, EPR, and Mössbauer spectroscopy. The characterization studies showed that framework substitution of trivalent iron in tetrahedral framework position of the mesoporous aluminophosphate has taken place. Mössbauer revealed the presence of a small amount of divalent iron. This material has high thermal stability with regard to the dislodgement of trivalent iron from the framework structure on calcination and showed excellent activity in the liquid-phase oxidation of cycloalkanes such as cyclohexane, cyclooctane, and cyclododecane in presence of molecular oxygen or air under mild reaction conditions. FeHMA behave as a heterogeneous catalyst; the catalytic activity was practically unaffected by recycling experiments. Furthermore, the oxidation was successful for the bulkier cycloalkane molecules. A comparative study with selected iron-containing molecular sieve-based catalysts indicated that FeHMA is a promising heterogeneous catalyst with respect to its microporous analogue (FeAPO-5) as well as the silicate analogues, microporous FeS-1 and mesoporous FeMCM-41.