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.