Journal of Physical Chemistry B, Vol.101, No.50, 10763-10768, 1997
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 crystalline. The Al-27 and Si-29 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 square-pyramidal coordination which gradually converts Cr(V) in distorted tetrahedral coordination upon dehydration by heating in vacuum. The ESR spectrum of ion-exchanged (S)Cr-SAPO-11 shows Cr(V), but it does not convert to tetrahedral coordination 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 Cr(V) in calcined CrAPSO-11 and (S)Cr-SAPO-11 with D2O, CH3OD, CD3OH, ND3, and pyridine adsorbates are also compared. Differences in the kinetics of absorbate coordination between calcined CrAPSO-11 and (S)-Cr-APSO-11 are observed. All these differences support different Cr(V) sites in these two materials. P-31 electron spin echo modulation of CrAPSO-11 shows that Cr(V) is surrounded by about 11-12 phosphorus nuclei at 0.58 nm, which is consistent with Cr(V) in CrAPSO-11 being in a framework phosphorus position.
Keywords:MOLECULAR-SIEVES;SOLID-STATE;SILICOALUMINOPHOSPHATE TYPE-5;FRAMEWORK INCORPORATION;NI(I);ENVIRONMENT;SAPO-11;EPR;CR