Structure of the Catalytic Active Sites in Vanadium-Doped Aluminophosphate Microporous Materials. New Evidence from Spin Density Studies

Electron spin resonance and hyperfine sublevel correlation (HYSCORE) spectroscopy at X- and Q-band frequencies have been employed, in conjunction with DFT modeling, to determine the location of V­(IV) ions in AlPO-5 zeotype materials. Two EPR-active species are detected, whose spin Hamiltonian parameters are in accord with vanadyl ions (VO²⁺) experiencing slightly different local environments. Interactions of the unpaired electrons of the paramagnetic VO²⁺ species with all relevant nuclei (¹H, ³¹P, ²⁷Al, and ⁵¹V) could be resolved, allowing for the first detailed structural analysis of the VO²⁺ paramagnetic ions in AlPO materials. Dehydration treatments indicate that the observed ¹H hyperfine couplings stem from structural protons in the first coordination sphere of the VO²⁺ species, strongly pointing to charge compensating mechanisms associated with isomorphous framework substitution at Al³⁺ sites, in good agreement with the large ³¹P hyperfine couplings. Detection of fairly large ²⁷Al couplings point to the presence of VO²⁺–O–Al linkages associated with a different structural arrangement, in agreement with the presence of two EPR-active species. The interpretation of the experimental results is corroborated by DFT modeling, which affords a microscopic description of the system investigated. The two EPR-active species are found to be consistent with isolated VO²⁺ species isomorphously substituted in the AlPO framework at Al³⁺ sites and extraframework VO²⁺ species docked in the center of the 6-membered rings that line up the main channel of the AFI structure