Generation of New Paramagnetic Rhodium Species in NaX Zeolite and Coordination with Adsorbates

Abstract

Rh(II) species are formed in NaX by heating in flowing oxygen. The various species formed after activation, oxidation, reduction and adsorption of various molecules are characterized by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. Two dominant species, A and C, are generated after activation at 320-400° C and 450-500° C respectively. An activation temperature of 410-450° C generates diamag-netic Rh(II) dimers. Species A is suggested to be located in the β-cage and species C in the hexagonal prisms, based on interaction distances with adsorbates and on the effect of O2 on the ESR linewidth. Reduction of samples, activated at 240-400° C, with H2 generated a species designated as HI which has an isotropic g tensor at 77 K and shows a dynamic Jahn-Teller effect. HI is suggested to be complexed with hydrogen molecules and located in the β-cage. H2 is formed after reducing samples activated at 400-500° C and is probably not located in the α-cage. Oxidation with O2 generated additional species which did not interact with benzene or ethylene however, adsorption of water or methanol generated two major species, 01 and 02 which are probably located in the β-cage and are directly coordinated to water or hydroxyl groups.