Abstract
X-/Q-band electron paramagnetic resonance (EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopies have been employed, in conjunction with density functional theory (DFT) modeling, to determine the location of Cr5+ions in SAPO-5 zeotype materials. The interaction of the unpaired electron of the paramagnetic Cr5+ species with 27Al could be resolved, allowing for the first detailed structural analysis of Cr5+ paramagnetic ions in SAPO materials. The interpretation of the experimental results is corroborated by DFT modeling, which affords a microscopic description of the system investigated. The EPR-active species is found to be consistent with isolated Cr5+ species isomorphously substituted in the framework at P5+ sites.
Highlights
Aluminophosphate molecular sieves (AlPOs)[1] characterized by neutral lattices of alternating TO4 (T = Al or P) tetrahedra form a class of microporous crystalline materials comparable to zeolites, featuring characteristic properties linked to their unique composition
The CW-electron paramagnetic resonance (EPR) spectrum of the assynthesized CrSAPO-5 measured at the X-band (Figure 3a) showed a broad absorption between 120 and 200 mT, characteristic of Cr3+ species in CrAPO-511 and CrAPSO-1137 and associated with octahedrally distorted coordinations.[8,12,37,38]
(a) X-band Continuous wave EPR (CW-EPR) spectra of as-synthesized CrSAPO-5 show the characteristic spectrum of Cr3+ species
Summary
One of the peculiar features of AlPOs is the electroneutrality of the framework, which limits their use as acid catalysts. Redox functionalities can be tuned through the insertion of specific transition-metal ions (TMI), and a range of TMI-substituted SAPOs have been synthesized, displaying characteristic properties.[3] The combination of these two strategies, leading to the simultaneous presence of Brønsted and redox sites, is a viable path to synthesize selective catalysts with a peculiar bifunctional character, where the reactivity of redox and acidic functionalities is combined with the high surface area and the unique spatial constraints imposed by the molecular dimensions of the porous network.[4]
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