Encapsulation of SALEN- and SALHD-Mn(III) complexes in an Al-pillared clay for bicarbonate-assisted catalytic epoxidation of cyclohexene

Abstract

[Mn(3,5-dtSALEN)Cl] (I) and [Mn(3,5-dtSALHD)Cl] (II) complexes (3,5-dtSALEN = N,N′-bis(3,5-di-tert-butylsalicylaldehyde)ethylenediamine; 3,5-dtSALHD = N′N-bis-(3,5-di-tert-butylsalicylaldehyde)-1,2-cyclohexanediamine) were successfully encapsulated within a natural bentonite by using three preparative approaches: (A) direct adsorption of every metal complex on the previously Al-pillared bentonite, Al-PILC; (B) two-step liquid phase methodology: (i) cationic adsorption of Mn2+ in Al-PILC by substituting its residual cationic exchange capacity (CEC), followed by (ii) diffusion of either 3,5-dtSALEN or 3,5-dtSALHD ligands, for in-situ generation of the corresponding interlayered metal complexes; and (C) simultaneous pillaring/encapsulation of the complexes on the raw starting clay. The materials were characterized by cationic exchange capacity, X-ray diffraction, atomic absorption, FT-Infrared and UV–vis spectroscopies, and N2 adsorption at 77 K. The physical encapsulation of the complexes into final materials was proven by spectroscopic analyses. Method C yielded both highest metal incorporation and enhanced basal space on the modified clay. All materials showed to be active catalysts in cyclohexene epoxidation with hydrogen peroxide using acetonitrile as solvent (0.79 atm, 293 K). Addition of sodium bicarbonate as co-catalyst led to enhanced conversion (100%) and selectivity (70%) towards the epoxide in the presence of such a kind of heterogeneized metal-complex catalysts. The catalysts were stable and reusable along at least two catalytic cycles