AN INVESTIGATION OF FRAMEWORK SUBSTITUTION OF COBALT INTO ALUMINOPHOSPHATE-5 USING ELECTRON-SPIN-RESONANCE AND TEMPERATURE-PROGRAMMED DESORPTION MEASUREMENTS

As-synthesized CoAPO-5 is blue, and it becomes yellow-green after calcination in oxygen, This was interpreted by several research groups as indicating a valence slate change from Co(II) to Co(III); however, our electron spin resonance (ESR) and temperature-programmed desorption results provide no evidence for Co(III) and are consistent with all the framework species existing as Co(II) in both calcine and as-synthesized samples. Although ESR measurements of calcined samples show a significantly lower signal intensity than reduced samples at 4 K, a result which has been used as proof of Co(III) formation, measurements at 20 K or higher give the same intensities for both yellow-green calcined and blue as-synthesized samples. The anomalous change in ESR intensity with temperature can be fully understood in terms of the energy level structure of Co(II) in a crystal field of nearly tetrahedral symmetry and, therefore, provides evidence for incorporation of Co(II) into the AlPO4-5 framework in tetrahedral sites and no evidence for oxidation of Co(II) to Co(III) during calcination. Conclusions from the ESR results are independently supported by titration adsorption measurements with reactive probe molecules, including isopropylamine, ethylamine, and 2-propanol, which provide evidence for framework Co(II) but not for redox processes. Each of the probe molecules changed the calcined samples from yellow-green to blue following adsorption at 295 K; however, there was no evidence for oxidation products in amounts even approaching the Co content, either on desorption or remaining in the zeolite. Simultaneous temperature-programmed desorption and thermogravimetric analysis measurements of isopropylamine and ethylamine on calcined samples gave evidence for Bronsted acid sites in a concentration equal to the Co content to near 1 mol % Co. Thus, the color changes observed during calcination of CoAPO-5 do not indicate oxidation of Co(II).