29Si magic-angle spinning NMR study of Phillips catalysts: multi-exponential spin–lattice relaxation resulting from adsorption of paramagnetic chromium species onto a silica surface
29 Si NMR longitudinal relaxation times (T1) of a series of Phillips (chromia/silica) catalysts are reported for different stages in the preparation of the active catalysts, from precursor through to post-polymerisation. The relaxation behaviour of a catalyst precursor was found to be highly dependent on the nature of the chromium(III) salt adsorbed on the surface of the silica support, whereas in their post-calcination stages all of the catalysts behaved in an identical fashion. Relaxation 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 analysed both by the stretched exponential expression and by deconvolution into individual T1 components. The latter are considered in relation both to the oxidation states of the paramagnetic chromium centres and to the distances of the 29Si nuclei from those centres.