Chemistry of CoAPO-11 and VAPO-5: ESR studies of molecular oxygen adducts.

CoAPO-11 and VAPO-5 were synthesized in the presence of diisopropylamine. XRD indicates high crystalline materials with the isotopic structure of respectively ALPO-11 and ALPO-5. CoAPO-11 UV-spectra and the existence of acidic OH revealed by infrared, indicate isomorphous substitution of divalent cobalt for aluminium Removal of the template by O-2- treatment at 773 K produced a diamagnetic ore-type cobalt Co = O. This species activates molecular oxygen only after H-2-treatment at high temperature. The esr spectra of the oxygen adduct formed upon O-2 adsorption on H-2-reduced CoAPO-11 correspond to the paramagnetic species Co(2+)-O-O-.. The amount of this peroxo cobalt adduct increases with the severity of the H-2-treatment. One concludes that Co = O is converted into an active Co(2+) species by H-2-treatment. Co(2+)-O-O-. is very reactive toward H-2, hydrocarbons, and is regenerated by cyclic O-2 adsorption. At high temperature about 573-773 K Co(2+)-O-O-. is converted into Co = O oxo species non active to activate molecular oxygen. VAPO-5 was studied by esr. The esr spectra revealed the presence of V4+ both in the sample as synthesized and in the 773 K O-2-treated sample to remove the template. Tetravalent V is rather stable in O-2 at high temperature which indicates that V4+ ion substitutional framework position is resistant towards oxidation. Like CoAPO-11, oxygen adducts V-O-O-. are formed, as revealed by esr, only after H-2-treatment at high temperature. The esr results are interpreted similarly to those on CoAPO-11.