Studies on kinetics and mechanism of iridium(III) catalysed cerium(IV) oxidation of D-mannitol and D-glucose in aqueous acid media

Abstract

The kinetics and mechanism of Ce(IV) oxidation of _D-mannitol and _D-glucose in aqueous H₂SO₄media in the presence and absence of Ir(III) have been studied and compared under the conditions, [S]_T˃˃ [Ce(IV)]_T ˃˃[Ir]_T , where [S]_T gives the total substrate (_D-mannitol or _D-glucose) concentration and [Ir]_T denotes the total concentration of Ir(III) catalyst added. Ir(III) has been found to catalyse the title reactions very efficiently even at trace concentration (<i>ca.</i>10^-6<i> </i>to 10^-7 mol dm^-3). Under the kinetic conditions, both the uncatalysed and catalysed reactions take place simultaneously. The overall process shows a first order dependence on [Ce(IV)] and [S]_T. The catalytic path is first order in [Ir]_T. The catalytic path is suggested to involve the Ir(III)/Ir(IV) catalytic cycle. The process is acid catalysed and inhibited by [H₂SO^4-]. From the H₂SO^4-

dependence, both Ce(SO₄)²⁺ and Ce(SO₄)₂ have been found kinetically active in _D-mannitol oxidation while in _D -glucose oxidation Ce(SO₄)₂ is the main kinetically active species under comparable conditions. The different kinetic parameters for both the Ir(III) catalysed and uncatalysed paths have been estimated.