Catalysis and transition metal ions in zeolites

Abstract

Physical and chemical properties of copper (I and II) X and Y zeolites were examined in relation to their method of preparation. NaX and NaY zeolites were ion-exchanged with Cu 2+ ions in acidic and alkaline solutions. X and Y zeolites exchanged in acidic solutions were stoicheiometric. X zeolites exchanged at pH 8.5 in the presence of NaOH had excess copper due to ion exchange of products of hydrolysis of the copper ions (non-stoicheiometric zeolites). Exchange in ammoniacal solution of CuCl^ (ρΗ=9·65) resulted in a zeolite without excess copper; some NH^+ ions were also exchanged. In addition, protons were exchanged during the preparation of low exchangedzeolites. The zeolites prepared were X-ray crystalline but some loss of crystallinity occurred during heat treatment. The unit cell of Cu2+-exchanged X zeolites was smaller than that of the parent NaX zeolite and v/as dependent on the Cu2+ content of the unit cell. The water content of NaCuX and NaCuY zeolites showed that there was no loss of micropo rosity in the stoicheiometric zeolites but some blocking of the micropores occurred in the non-stoicheiometric zeolites. The specific surface area measured, after dehydration at 673 K, from the nitrogen adsorption isotherm decreased with increasing copper content in the unit cell. Surface studies on a variety of copper exchanged zeolites using secondary ion mass spectrometry (SIMS) showed that the outermost layer of zeolites exchanged in acidic conditions was slightly dealuminated 2+ and that migration of the Cu ions to the internal regions of the crystallites occured during dehydration. Oxygen -bridged copper specieswere identified by SIMS in dehydrated non-stoicheiometri c zeolites. In stoicheiometric NaCuX zeolites "loose" water filled the pores. Water loss from hydrolyzed copper species appeared with anendotherm in the DIA profiles and a step in the TG curves in the region around 600 K. The temperature of collapse of the zeolite structure ?/as clearly dependent on the copper content for all the NaCuX zeolites. The collapse of the lattice did not always result in an amorphous phase as it did for the stoicheiometrie NaCuX zeolites. At higher temperatures recrystallization to CuAl^O^ and other phases occurred. Normal probability plots of the water loss on heating could distinguish two main stages in dehydration. -Finally, models and kinetic equations applicable to gas-solid reactions were tested for the reduction of oxidized Cu( I )-exchanged 2+ e X zeolites. The reduction of Cu ions i n thse zeolites could be described by an equation (Jander*s) for reaction at an interfaceformed between the products of raction and the unreacted solid; the rates were controlled by the diffusion of reactants through the product layer. Two stages could be distinguished in the reduction of Cu^+ ions. The first one corresponded to the reduction of Cu ^ 1" to Cu+ ions (Ea ~ 75. kJ mol 1) and the second one to the reduction of Cu+ ions to metallic copper (Ea ~ 30 kJ mol-1).