Synthesis and Characterization of Chromium Silicalite

Abstract

Infrared spectroscopy (mid-IR), 29Sia nd 27Alm agic angle spinning nuclear magnetic resonance (MAS-NMR), powder X-ray diffraction (PXRD), electron paramagnetic resonance (EPR), and photoacoustic spectroscopy (PAS), in combination, have produced useful information about a series of crystalline MFI-type chromium silicalite, synthesized in a fluoride medium. Aluminum is an impurity and occupies framework sites, as evaluated by the 27Al MAS-NMR. The mid-IR spectra are very similar to the zeolite ZSM-5 with the addition of a weak band at ~68c0m -l, which we tentatively assign to symmetric stretching of (Cr-0-Si), groups. The samples are crystalline materials and have expanded unit cells, as PXRD measurements pointed out. The as-synthesized sample shows a distribution of Cr sites, detected by EPR, comprising structure and channel occlusion sites; the nonstructural sites being extensively oxidized to Cr(V1) species after calcination in the presence of dioxygen. The changes in the photoacoustically measured physical properties, such as nonradiative relaxation time, the thermal diffusivity and optical absorption coefficient of both calcined and noncalcined chromium silicalite samples are also reported.