Mn(III) salen complex immobilised into pillared clays by in situ and simultaneous pillaring/encapsulation procedures : Application in the heterogeneous epoxidation of styrene

Abstract

The complex [Mn(saldPh)Cl], chloride-[ N, N′-bis(salicylaldehyde)1,2-diphenylethylenediamine]manganese(III), was encapsulated into an aluminium pillared clay (denoted as Al-WYO) by three different methodologies. Method A—two step liquid phase methodology: (i) adsorption of manganese(II) chloride in methanolic solution within an aluminium pillared clay, followed by (ii) diffusion of the ligand N, N′-bis(salicylaldehyde)-1,2-diphenylethylenediamine in methanol. Method B—simultaneous/pillaring encapsulation of the manganese(III) salen complex, [Mn(saldPh)Cl], and pillaring of a montmorillonite clay with aluminium polyoxycations. In this case, the [Mn(saldPh)Cl] complex was dissolved in ethanol and added to the oligomer solution at a stage where the oligomeric species are already formed. Method C—the simultaneous encapsulation also occurred but the complex, also dissolved in ethanol, was added to the clay dispersion. All materials were characterised by XRD, XPS, atomic absorption spectroscopy, UV–Vis, and FTIR. In all cases the manganese(III) salen complex is mainly physically entrapped within the matrix, although some host–guest interactions with the matrix could be present. All new materials, [Mn(saldPh)Cl]@Al-WYO_A, [Mn(saldPh)Cl]@Al-WYO_B, and [Mn(saldPh)Cl]@Al-WYO_C show catalytic activity in the epoxidation of styrene at room temperature using PhIO as oxygen source in acetonitrile. The solids presented high styrene epoxide selectivity and were reused for four times, but a small decrease in the catalytic activity was observed. Nevertheless, the catalysts prepared by simultaneous pillaring/encapsulation, methods B and C, are more stable upon reuse than the catalyst prepared by method A. FTIR spectra of catalysts after the catalytic reactions suggest that during the reaction no structural changes of the PILC took place, but some active phase leaching and deactivation have occurred which are responsible for the decrease in the catalytic activity on reutilisation.