COBALT-SUBSTITUTED ALUMINOPHOSPHATE MOLECULAR-SIEVES - X-RAY ABSORPTION, INFRARED SPECTROSCOPIC, AND CATALYTIC STUDIES

Cobalt was incorporated by direct synthesis into three distinct kinds of microcrystalline microporous aluminophosphates (ALPOs): the large-pore ALPO-5 (free diameter ca. 7 angstrom), medium-pore ALPO-11 (free diameter ca. 6 angstrom), and small-pore ALPO-21 (free diameter < 4 angstrom) molecular sieves. All three possess a unidimensional pore system. X-ray absorption spectroscopy (XAS) indicates that, in the CoAPO-6 and 11 precursors, the Co(II) cations are essentially four-coordinated but in CoAPO-21, which transforms into CoAPO-25 after calcination to remove the occluded template, there are some five-coordinated as well as four-coordinated framework Co(II) cations. After calcination at 550-degrees-C in an 02 atmosphere, the framework Co(III) cations are partly oxidized to Co(III), and once exposed to methanol they may be reduced to Co(II) cations to an extent that depends on the framework structure of the solid. The decreasing order of ease of reducibility is CoAPO-5 > CoAPO-11 greater-than-or-equal-to CoAPO-25. Some of the tetrahedrally coordinated Co(II) species are driven out of the precursor framework and become six-coordinated during the calcination. Hetero-atom-free ALPO-5, -11, and -25 contain various amounts of framework hydroxyls which function as Bronsted acid sites for the catalytic conversion of methanol to dimethyl ether. With incorporation of cobalt into its framework, ALPO-5 is able to bear hydroxyl groups with strong acidity, and as a result it catalyzes methanol conversion to hydrocarbons especially to propene. Incorporation of cobalt into ALPO-11 and ALPO-25, however, does not enhance their acidity to a comparable extent as is borne out by their catalytic performance for methanol conversion.