EPR AND XRD SPECTROSCOPIES OF THE CHROMIUM IONS IN THE ZSM-5 ZEOLITE

Abstract

The introduction of chromium into silica-high zeolite of ZSM-5 type was carried out by heating homogeneous mechanical mixtures of H-form zeolite with the CrO 3 compound. X-ray diffraction and paramagnetic resonance methods were used to follow the thermal stability of the zeolite, the framework alteration, as well as the nature of the chromium complexes formed during solid phase reactions. For low contents of oxide (x≤5% wt. CrO 3) in mixtures, the thermal stability, T=1070°C, of the zeolite phase is not affected by the presence of the reaction products. The treatment of mixtures in air and in vacuum leads to the location of several isolated Cr(III) and Cr(V) complexes in cation positions. The low-field resonances at g=3.72 and/or g=5–6 indicate the presence of isolated Cr(III) species in orthorhombically distorted octahedral environments. Several Cr(V) species were also detected, two of them exhibiting a superhyperfine structure (a Al =0.63–0.73 mT ) caused by the interaction of the Cr(V) ions with 27 Al (I=5/2) of the zeolite framework. Generally, the signals arising from the isolated ions are overlapped by broad resonances typical for chromium-rich phases. Drastic changes of the spectra, upon ingress of gas-phase molecules (air, H 2 O , O 2) over the calcined samples, certify the unsaturated character of the Cr complexes formed during the treatments. It is one characteristic feature of the H-ZSM-5 zeolites which results in the special catalytic activity of the chromium-containing zeolites.