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 calc ination 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 r esults provide no evidence for Co(III) and are consistent with all the framework species existing as Co(II) in both calcine and as-synthesiz ed samples. Although ESR measurements of calcined samples show a signi ficantly 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 a nd 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 resu lts are independently supported by titration adsorption measurements w ith reactive probe molecules, including isopropylamine, ethylamine, an d 2-propanol, which provide evidence for framework Co(II) but not for redox processes. Each of the probe molecules changed the calcined samp les from yellow-green to blue following adsorption at 295 K; however, there was no evidence for oxidation products in amounts even approachi ng the Co content, either on desorption or remaining in the zeolite. S imultaneous temperature-programmed desorption and thermogravimetric an alysis measurements of isopropylamine and ethylamine on calcined sampl es gave evidence for Bronsted acid sites in a concentration equal to t he Co content to near 1 mol % Co. Thus, the color changes observed dur ing calcination of CoAPO-5 do not indicate oxidation of Co(II).