Coordination Environment and Redox Property of Co(II) in the Framework of CoAPO-36 Molecular Sieve

Abstract

Abstract The interaction of water, ammonia, methanol, and tetrahydrofuran molecules with Co(II) ions in CoAPO-36 was examined by means of diffuse reflectance UV-vis spectroscopy. The Co(II) ions in CoAPO-36 are in a tetrahedral environment. Upon adsorption of water or ammonia molecules, the coordination environment of a part of the Co(II) ions changes into octahedral, indicating the direct coordination of water or ammonia molecules to the Co(II) ions. The remainder of the Co(II) ions are slightly modified in their electronic state by the adsorption of these molecules in a close proximity. Adsorption of methanol and tetrahydrofuran molecules also causes a modification of the coordination environment, though they do not directly coordinate to the Co(II) ions. The redox properties of Co(II) ions in CoAPO-36 were also studied. Heating CoAPO-36 in oxygen at 673 K brings about oxidation of a part of the Co(II) ions into Co(III) ions, which are reduced to Co(II) by a hydrogen-treatment at 573 K. In addition, a reduction of the Co(III) ions occurs upon adsorption of water or ammonia on the oxidized CoAPO-36, even at room temperature. The Co(III) ions react with toluene at room temperature and with methane at 673 K. The oxidation-reduction cycle of the Co ions in CoAPO-36 is also brought about by the adsorption-desorption cycle of NO.