ELECTRON-SPIN-RESONANCE AND ELECTRON-SPIN ECHO MODULATION SPECTROSCOPIC STUDIES OF CHROMIUM ION LOCATION AND ADSORBATE INTERACTIONS IN CALCINED CRAPSO-11

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 cryst alline. The Al-27 and Si-29 magic-angle-spinning nuclear magnetic reso nance spectra are similar to those of corresponding SAPO-11 showing on ly one type of tetrahedral atom. As-synthesized CrAPSO-11 shows an ESR spectrum assignable to Cr(III), but on calcination this converts to C r(V). (S)Cr-SAPO-11 forms Cr(V) directly since solid-state ion exchang e occurs at high temperature. The UV-vis spectrum of calcined, hydrate d CrAPSO-11 is assigned to Cr(V), and after dehydration the coordinati on for Cr(V) seems consistent with tetrahedral as expected for a frame work site. Electron spin resonance (ESR) of calcined, hydrated CrAPSO- 11 shows Cr(V) which is consistent with square-pyramidal coordination which gradually converts Cr(V) in distorted tetrahedral coordination u pon dehydration by heating in vacuum. The ESR spectrum of ion-exchange d (S)Cr-SAPO-11 shows Cr(V), but it does not convert to tetrahedral co ordination upon heating in vacuum. Hydrogen reduction shows that Cr(V) in (S)-Cr-SAPO-11 can be reduced to a several hundred gauss broad ESR signal assignable to Cr(III). Reduction by H-2 of calcined CrAPSO-11 does not produce Cr(III) observable by ESR. The interactions between C r(V) in calcined CrAPSO-11 and (S)Cr-SAPO-11 with D2O, CH3OD, CD3OH, N D3, and pyridine adsorbates are also compared. Differences in the kine tics of absorbate coordination between calcined CrAPSO-11 and (S)-Cr-A PSO-11 are observed. All these differences support different Cr(V) sit es in these two materials. P-31 electron spin echo modulation of CrAPS O-11 shows that Cr(V) is surrounded by about 11-12 phosphorus nuclei a t 0.58 nm, which is consistent with Cr(V) in CrAPSO-11 being in a fram ework phosphorus position.