SI-29 MAGIC-ANGLE-SPINNING NMR-STUDY OF PHILLIPS CATALYSTS - MULTIEXPONENTIAL SPIN-LATTICE RELAXATION RESULTING FROM ADSORPTION OF PARAMAGNETIC CHROMIUM SPECIES ONTO A SILICA SURFACE

Si-29 NMR longitudinal relaxation times (T-1) of a series of Phillips (chromia/silica) catalysts are reported for different stages in the pr eparation of the active catalysts, from precursor through to post-poly merisation. The relaxation behaviour of a catalyst precursor was found to be highly dependent on the nature of the chromium(III) salt adsorb ed on the surface of the silica support, whereas in their post-calcina tion stages all of the catalysts behaved in an identical fashion. Rela xation behaviour was found to be also extremely sensitive to the stage of catalyst preparation and at each stage non-exponential relaxation was observed. The time dependence of the magnetisation recovery was an alysed both by the stretched exponential expression and by deconvoluti on into individual T-1 components. The latter are considered in relati on both to the oxidation states of the paramagnetic chromium centres a nd to the distances of the Si-29 nuclei from those centres.