Photoacoustic spectroscopy study of metal substituted molecular sieve catalysts

Abstract

Photoacoustic spectrometer system including photoacoustic cell capable of high temperature and vacuum treatment was constructed and photoacoustic spectroscopy(PAS) technique was applied to study the local environments of metal ions such as Fe(III) and Co(II) incorporated in molecular sieves as well as the effect of calcination on the state of these metal ions. During the calcination of these metal substituted molecular sieves to remove the organic templates carbogeneous residue showed strong absorption. These molecular sieves were photoacoustically unsaturated in the course of calcination although optically saturated. Thus absorption spectra of metal ions could be obtained by PAS technique during calcination of samples. It was suggested that several iron sites including coordinatively unsaturated site may exist in the matrix of Fe-silicalite. Co(II) ion seems to be located on the surface of the crystallite in as-synthesized as well as calcined coZSM-5. The local environment of the Co(II) ion was not affected by calcination. The PA spectrum of as-synthesized CoAPO-5 showed the three almost symmetrically spaced peaks and was assigned to Co(II) ions which are isomorphously substituted to the framework of ALPO4-5 structure. By calcination of as-synthesized CoAPO-5 some of Co(II) ions was oxidized to Co(III). There seems to be an interaction between framework Co (II) ion and organic template, and the removal of template by the calcination caused the removal of large portion of Co(II) ions in as-synthesized CoAPO-5 from the framework position. Some of Co(II) ions, however, remained in the framework position after the calcination.