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 fram ework substitution of chromium and the preparation of CrAPO-5 with high sor ption 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 sp ectra of the as-synthesized HS and LS samples consist of a broad intense li ne (g(eff) = 1.971 +/- 0.001, DeltaH(pp) congruent to 500 G) with a positiv e lobe at g(eff) = 5.15 +/- 0.01, assigned to the Cr3+ (3d(3), F-4(3/2)) io ns with severely (rhombic) distorted octahedral coordination. The calcined samples show an additional narrow EPR signal of axial symmetry with g(paral lel to) = 1.971 +/- 0.001 and g(perpendicular to) = 1.959 +/- 0.001 that is assigned to the Cr5+ (3d(1), D-2(3/2)) ions in octahedral coordination as well. The HS samples heated under a vacuum exhibit a decrease in intensity of the Cr3+ lines as well as several new signals. Two of these signals are characterized by an axially symmetric g factor and belong to the Cr5+ cente rs in square pyramidal and tetrahedral coordinations. The third signal may be assigned to the Cr1+ (3d(5), S-6(5/2)) ions. The spectra of the LS sampl es heated under a vacuum show a strong decrease in intensity of the Cr3+ li nes and only the two signals of the Cr5+ centers in the same coordinations. Their g factors are similar to those in the HS samples. The thermal treatm ents under oxidative (O-2) and reductive (H-2) atmospheres reveal reproduci ble redox properties of the Cr5+ ions and a high stability of the Cr3+ ones , especially in the HS samples. The EPR results give strong support for the framework incorporation of the Cr3+ ions in the HS materials.