Skeletal isomerisation of n-butenes present in C4 pyrolysis residue fraction Review Article

Butenes present in butadiene and isobutene free C4 fraction (a stream from the production of methyl-tert-butyl ether — so-called raffinate II) were isomerised over the hydrogen-chlorided and sulphated catalysts at 350–515°C. A natural clay (bentonite) and commercial γ-alumina were used as catalyst precursors. Hydrogen-chlorided catalysts were deactivated in about 6 h by 10–15% with respect to butene conversion, and a regeneration with air doped hydrogen chloride did not allow to renew the catalytic activity to full extent. This behaviour should be ascribed to high volatility of chlorides which, moreover, pollute the product as itself and in the form of chlorinated organic compounds. On the other hand, catalysts prepared by treatment of calcinated and evacuated γ-alumina with diluted sulphuric acid, proved to be sufficiently active in 2500 h lasting experiments, whereas regeneration with air was necessary every 4–6 h. This stability is probably caused by high melting point of aluminium sulphate which is formed in the course of the catalytic preparation and treatment. The selectivity, 50% of the isobutene formation, was achieved at approximately 50% conversion of butenes, 450°C, GHSV=0.6 g cm−3 h−1. Experiments at higher pressure allowed to increase the conversion, but larger amount of components with higher molar weight was formed. Methanol and methyl-tert-butyl ether did not effect the process of skeletal isomerisation significantly, but a presence of butadiene strongly enhanced deactivation of the catalyst.