Sol-gel synthesis, characterization and catalytic activity of silica aerogels functionalized with copper(II) complexes of cyclen and cyclam

Abstract

Mesoporous silica aerogels covalently functionalized with Cu(II) complexes of cyclen (1,4,7,10-tetraazacyclododecane) and cyclam (1,4,8,11-tetraazacyclotetradecane), denoted as CuAM12 and CuAM14, respectively, have been first prepared. Monolithic materials were synthesized from alkoxysilane-anchored macrocyclic complexes and TMOS through the base catalyzed sol-gel method followed by a supercritical CO2 drying. Presence of organic moieties in the silica structure was confirmed by FT-IR measurements, the amount of incorporated copper was determined by ICP-OES technique, resulting in 1.310 ± 0.028 and 2.060 ± 0.025 wt% copper for CuAM12 and CuAM14, respectively). Surface morphology was characterized by nitrogen adsorption/desorption porosimetry and scanning electron microscopy. Catalytic activities of the aerogels CuAM12 and CuAM14 were evaluated by oxidation of phenol with eco-friendly reagent hydrogen peroxide, without buffering or using organic co-solvents. Control experiments using unbound macrocyclic complexes in the homogenous phase were also conducted. Surprisingly, results revealed that the functionalized aerogels' turnover frequencies (TOF) were approximately 7–15 times higher than that of the free complexes in the homogeneous phase. The increase was attributed to the confinement effect. Reusability studies showed that the catalysts remained active even after four cycles completed. Certain reduction of catalytic activity was observed in the repeated cycles, which was the combined effect of poor mechanical properties of the aerogel, splitting-off and leaching of the less-bound surface macrocycles, and extensive adsorption of the oxidation products.