Kinetics of oxidation of an organic amine with a Cr(V) salen complex in homogeneous aqueous solution and on the surface of mesoporous silica

Abstract

A comparative study of catalytic activity under homogeneous and heterogeneous conditions was carried out using the (salen)Cr^III-catalyzed oxidation of tetramethylbenzidine (TMB) with iodosobenzene as a model reaction. Amine-functionalized mesoporous silica nanoparticles (MSN) were synthesized in a co-condensation reaction and functionalized with salenvia a covalent Si–C bond. A Cr(III) complex of this supported ligand, MSN-(salen)Cr^III, was prepared and characterized. Data from powder XRD, BET isotherms and BJH pore size distribution all showed that MSN-(salen)Cr^III still had the typical MSN high surface area, narrow pore size distribution, and ordered hexagonal pore structure, which were further confirmed by transmission electron microscopy (TEM) images. ¹³C and ²⁹Si solid-state NMR data provided structural information about the catalyst and verified successful functionalization of the salen ligand and coordination to Cr(III). No unreacted salen or Cr(III) were observed. The loadings of salen and salen-Cr^III complex were determined viaTGA and EDX, respectively. Both measurements indicated that approximately 0.5 mmol/g of catalyst was loaded on the surface of MSN. The oxidation of TMB with iodosobenzene using MSN-(salen)Cr^III as a heterogeneous catalyst exhibited both similarities and differences with the analogous homogeneous reaction using (salen)Cr^III(H₂O)⁺ as a catalyst in aqueous acetonitrile. In the presence of 0.10 M HClO₄, the two catalytic reactions proceeded at similar rates and generated the doubly oxidized product TMB²⁺. In the absence of acid, the radical cation TMB˙⁺ was produced. The kinetics of the heterogeneous reaction in the absence of added acid responded to concentrations of all three reagents, i.e.(salen)Cr^III, TMB, and PhIO.