Electron Paramagnetic Resonance Spectroscopy of Chromium in CrAPO-5 Molecular Sieves

A new synthesis strategy with the use of a co-template allowed for the framework substitution of chromium and the preparation of CrAPO-5 with high sorption capacities for nitrogen and benzene (HS samples). For these materials as well as for those synthesized with classical methods and exhibiting low sorption capacities (LS samples), X-band electron paramagnetic resonance (EPR) spectra were measured at room and liquid nitrogen temperatures. The spectra of the as-synthesized HS and LS samples consist of a broad intense line ( g eff = 1.971 ± 0.001, Δ H pp ≅ 500 G) with a positive lobe at g eff = 5.15 ± 0.01, assigned to the Cr 3+ (3d 3, 4 F 3/2 ) ions with severely (rhombic) distorted octahedral coordination. The calcined samples show an additional narrow EPR signal of axial symmetry with g ∥ = 1.971 ± 0.001 and g ⊥ = 1.959 ± 0.001 that is assigned to the Cr 5+ (3d 1, 2 D 3/2 ) ions in octahedral coordination as well. The HS samples heated under a vacuum exhibit a decrease in intensity of the Cr 3+ lines as well as several new signals. Two of these signals are characterized by an axially symmetric g factor and belong to the Cr 5+ centers in square pyramidal and tetrahedral coordinations. The third signal may be assigned to the Cr 1+ (3d 5, 6 S 5/2 ) ions. The spectra of the LS samples heated under a vacuum show a strong decrease in intensity of the Cr 3+ lines and only the two signals of the Cr 5+ centers in the same coordinations. Their g factors are similar to those in the HS samples. The thermal treatments under oxidative (O 2 ) and reductive (H 2 ) atmospheres reveal reproducible redox properties of the Cr 5+ ions and a high stability of the Cr 3+ ones, especially in the HS samples. The EPR results give strong support for the framework incorporation of the Cr 3+ ions in the HS materials.