New Hybrid Iron Phosphonate Material as an Efficient Catalyst for the Synthesis of Adipic Acid in Air and Water

Abstract

A new organic–inorganic hybrid iron phosphonate material (FePO-1-2) has been synthesized hydrothermally using etidronic acid (1-hydroxyethylidene-1,1-diphosphonic acid) as an organophosphorus precursor. Under optimized reaction conditions the synthesis has been carried out hydrothermally for 3 days at 180 °C temperature at near neutral pH. The material has been characterized thoroughly by various techniques and its crystal structure has been indexed to a new orthorhombic phase with unit cell parameters of a = 10.995 Å, b = 10.395 Å, c = 11.793 Å, and α = β = γ = 90°. The considerably good Brunauer–Emmett–Teller (BET) surface area of 236 m2 g–1, pore volume of 0.229 cm3 g–1, and robust nature of FePO-1-2 have motivated us to explore its catalytic activity in liquid phase partial oxidation reactions under green conditions. FePO-1-2 exhibits excellent catalytic activity (96% product selectivity, 72% conversion) for selective liquid phase oxidation of cyclohexanone to adipic acid in the presence of molecular O2 under atmospheric pressure and in aqueous medium. This selective liquid (aqueous) phase oxidation pathway is highly green and sustainable as it does not involving any need of nitric acid, initiator, peroxides, and other organic solvents.