Chlorinated alumina as an alkylation catalyst: influence of acidity moderators
Résumé
Chlorinated alumina catalysts obtained by reacting a gamma-alumina with hydrogen chloride show high initial activity in isobutane-butene alkylation. After a few hours on stream however, the conversion decays due to self poisoning. This work attempts at improving the catalyst lifetime by modifying the Al2O3-Cl catalyst. Best results were obtained by adding small amounts of alkaline cations to the alumina before chlorination. For a particular cation, lithium or sodium, the improvement in catalyst lifetime depends on the chlorination procedure (pre-treatment with CCl4, final temperature). Addition of cations also enhances the selectivity for octanes, mainly by decreasing the amount of cracked products. Degradation of 2,2,4-trimethylpentane was found to occur over the Al2O3-Cl catalysts at 273 K. The influence of cation addition on this process was examined. Thus, the occurrence of dimethylhexanes and cracked or heavier alkanes among alkylation products may be explained by the degradation of 2,2,4-TMP.