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 square-pyramidal coordination which gradually converts to 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 H2 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. 31P 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.

Chromium Environment within Cr-Doped Silico-Aluminophosphate Molecular Sieves from Spin Density Studies

“…The CW-EPR spectrum of the as-synthesized CrSAPO-5 measured at the X-band ( Figure 3 a) showed a broad absorption between 120 and 200 mT ( g eff = 5.2–3.2), characteristic of Cr 3+ species in CrAPO-5 11 and CrAPSO-11 37 and associated with octahedrally distorted coordinations. 8 , 12 , 37 , 38 …”

EPR and electron spin echo modulation spectroscopy of CrIII and CrV in CrAPSO-5 molecular sieve: Evidence for framework substitution

“…2 Chen and co-workers studied the catalytic activity of CrAPO-5 and supported framework substitution of Cr.6 X-Ray photoelectron spectroscopy (XPS) and reductionÈ oxidation cycles were used to ascertain the incorporation of chromium into the SAPO-11 framework.11 By comparison of calcined, synthesized CrAPSO-11 with solid-state ionexchanged Cr-SAPO-11, we suggested that CrV is in a tetrahedral framework site in calcined CrAPSO-11. 12 A key point is whether tetrahedral CrIII is possible. Some authors have suggested that crystal Ðeld stabilization energies of CrIII in tetrahedral coordination (66.9 kJ mol~1) are so much less than for octahedral coordination (224.5 kJ mol~1) that CrIII cannot be incorporated into a tetrahedral framework.9,10 However, this interpretation may not be valid.…”

Novel mesoporous (Cr)MCM-48 molecular sieves: Promising heterogeneous catalysts for selective oxidation reactions

“…4d) show absorption bands centering at 620 and 440 corresponding to trivalent chromium in octahedral coordination. 27,31,[39][40][41][42] On the other hand, the calcined samples display a new prominent band (Fig. 4a) centering at 367 nm with a shoulder around 440 nm.…”