Nondestructive Sol−Gel Immobilization of Metal(salen) Catalysts in Silica Aerogels and Xerogels

Abstract

Nondestructive immobilization of cobalt and copper Schiff base complexes in silica aero- and xerogels was achieved via the sol−gel method using a precursor N,N‘-ethylenebis(salicylidenaminato) (salen) ligand modified with pendant silyl ethoxy groups. Aerogels were obtained by semicontinuous extraction of the wet gels with supercritical CO2 and xerogels by conventional drying. Cobalt and copper(salen) containing silica gels were characterized by FTIR, UV−vis, and XPS spectroscopy, laser ablation-ICP-MS, and EPR studies. Aero- and xerogel incorporated salen compounds exhibited similar spectroscopic properties to cobalt/copper(salen) precursors and known metal(salen) compounds. BET measurements confirmed the importance of supercritical CO2 drying in maintaining the mesoporous structure of the aerogel. Laser ablation-ICP-MS and EPR studies of the aerogels showed that a uniform distribution 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 typical liquid-phase oxidation conditions. XPS analyses showed surface relative atomic concentrations in the modified gels to be similar before and following leaching studies.

Nanoparticle-Supported Molecular Polymerization Catalysts

“…The high reactivity of A towards water (which is liberated in the condensation reaction) resulted in some hydrolysis and condensation of the silylmethoxy groups to form a small portion of insoluble materials. 19 However, after workup 2 could be isolated in 48% yield as a brownish oil.…”

Epoxidation of Alkenes Catalyzed by Phenyl Group‐Modified, Periodic Mesoporous Organosilica‐Entrapped, Dimeric Manganese–Salen Complexes

“…The catalytic activity of the heterogeneous Mn(salen) complexes, however, was higher than that of their corresponding homogeneous counterparts (Table 3). For example, at a similar Mn loading level, the heterogeneous dimeric Mn(salen) complexes with different structures followed the order of activity of 3 a/PMOPh(2.0) > 3 b/PMO-Ph(3.3) > 3 c/PMO-Ph(2.1) and the yields and TOFs reached 62.9 %, 16 (Table 3, Entries 4, 9, and 14). The a series of heterogeneous Mn(salen) complexes followed the order of activity of 1 a/PMO-Ph(1.2) < 2 a/PMO-Ph(2.4) < 3 a/PMO-Ph(2.0), and the yields and TOF reached 31.8 %, 6.4 h À1 ; 50.2 %, 10.0 h À1 ; and 62.9 %, 16.0 h À1 , respectively, after the reaction proceeded for 24 h (Table 3, Entries 2-4).…”

Waste-Reducing Catalytic Oxidation of m-Xylene to m-Toluic Acid

“…When using cobalt(II) naphthenate (0.19 mmol per mol xylene), a 10 % yield to TPA could be obtained in 135 min [19]. A few heterogeneous catalysts for alkylaromatic oxidation have also been developed [12,13,20,24,25]. The PX conversion could approach 100 with 99 % selectivity to TPA as performed in HAc/water solvent using CoMn 2 (O)-Y ([CoMn 2 (l 3 -O)(MeCO 2 ) 6 (py) 3 ] n inside the cage of zeolite HY, n = 0 or ?1) as the catalyst [20].…”