Nondestructive sol-gel immobilization of metal(salen) catalysts in silica aerogels and xerogels

Nondestructive immobilization of cobalt and copper Schiff base complexes in silica aero- and xerogels was achieved via the sol-gel method using a prec ursor N,N'-ethylenebis(salicylidenaminato) (salen) ligand and modified with pendant silyl ethoxy groups. Aerogels were obtained by semicontinuous extr action of the wet gels with supercritical CO2 and xerogels by conventional drying. Cobalt. and copper(salen) containing silica gels were characterized by FTIR, UV-tis, and XPS spectroscopy, laser ablation-ICP-MS, and EPR stud ies. Aero- and xerogel incorporated salen compounds exhibited similar spect roscopic properties to cobalt/copper(salen) precursors and known metal(sale n) compounds. BET measurements confirmed the importance of supercritical CO 2 drying in maintaining the mesoporous structure of the aerogel. Laser abla tion-ICP-MS and EPR studies of the aerogels showed that a uniform distribut ion of the isolated metal(salen) complex was achieved via molecular mixing using the sol-gel method. Stability of these materials was demonstrated by thermogravimetric analyses in air and leaching studies conducted under typi cal liquid-phase oxidation conditions. XPS analyses showed surface relative atomic concentrations in the modified gels to be similar before and follow ing leaching studies.