Abstract
Omega zeolite was synthesized by a hydrothermal method and then modified by ionic exchange using Na, Ca, K, Mg, Co and Cu cations. Analyses through CO2- and H2O-TPD revealed an improvement of the Lewis basicity as compared to the parent zeolite. CO2 adsorption was found to take place on two types of sites: i. strong sites on lattice oxygen located in 12- and 8-ring channels; ii. medium and weak sites located in 12-ring channels involving mainly silanol groups and adsorbed water. Weak Bronsted acidity and high porosity of alkali-exchanged Omega turned out to be essential requirements for high catalytic activity. Such catalysts exhibit heterogeneous charge dispersion but with beneficial effect for catalysis. The lowest conversion yields were registered with reagents bearing high electron density chlorine and/or nitro-groups, which may hinder adsorption at room temperature. MCR processes on Omega turned out to be governed by acid-base interactions between the reactants and zeolite surface.
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