1965 Volume 38 Issue 11 Pages 1857-1865
The dispersed state of chromium and the effect of heat treatment have been compared in Part I in chromium dispersed on different supporting materials. The behavior of the EPR absorption of the same materials in the reaction of several chemical reagents have been observed in the present work. The catalytic activities of these samples in the polymerization of ethylene and in the decomposition of hydrogen peroxide have also been measured.
The γ-absorption sites of the samples had different stabilities. In the sample with a silica gel base, this absorption disappeared quite easily in a reductive atmosphere, but that of the sample with a silica : alumina 1 : 1 base was stable in the same atmosphere. The former showed the greatest initial reaction rate in the polymerization of ethylene at 1 atm., while the latter had no catalytic activity in the polymerization, even at 40 kg./cm2. The sample with a gamma-alumina base showed an intermediate character in many respects. The activity in the decomposition of hydrogen peroxide generally corresponded to the oxidation number of the samples, even after reduction with cyclo-hexane, but in the case of the silica gel base different behavior was observed.
Taking the present and the preceding results into account, the character of this type of catalyst has been discussed. Its nature must essentially be due to the cluster-form dispersion of chromia; the addition of alumina to the supporting material gives an effect of stabilization to the active site, which extends the life of the catalyst, though an excess in the addition results in an inactive material. The 3d3 state of the chromium ions is inactive in the polymerization of ethylene, probably because of the lack of a beneficial adsorptive property to the monomer. The ease in the transfer among d0, d1 and d2 states of chromium ions may decide the activity, while the transfer into the d3 state may decide tha life of the catalyst. Water accelerates the latter and deactivates the catalyst.
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