Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopic Studies of Chromium Ion Location and Adsorbate Interactions in Calcined CrAPSO-11

Abstract

The local environment of the chromium ion in calcined CrAPSO-11, which may involve framework sites, is compared with the Cr ion environment in solid-state ion-exchanged (S)Cr−SAPO-11 which involves ion exchange into nonframework sites at high temperature. Powder X-ray diffraction confirms that CrAPSO-11 has the SAPO-11 framework and is highly crystalline. The 27Al and 29Si magic-angle-spinning nuclear magnetic resonance spectra are similar to those of corresponding SAPO-11 showing only one type of tetrahedral atom. As-synthesized CrAPSO-11 shows an ESR spectrum assignable to Cr(III), but on calcination this converts to Cr(V). (S)Cr−SAPO-11 forms Cr(V) directly since solid-state ion exchange occurs at high temperature. The UV−vis spectrum of calcined, hydrated CrAPSO-11 is assigned to Cr(V), and after dehydration the coordination for Cr(V) seems consistent with tetrahedral as expected for a framework site. Electron spin resonance (ESR) of calcined, hydrated CrAPSO-11 shows Cr(V) which is consistent with sq...