Prolonging catalyst lifetime in supercritical isomerization of 1-hexene over a platinum/alumina catalyst

Abstract

A series of experiments were carried out for the isomerization of 1-hexene in the temperature range 200 – 300 ∘ C and pressure range 10–100 bar, representing operating points both above and below the critical point of 1-hexene. At constant temperature of 300 ∘ C , increasing the pressure from 10 to 100 bar led to a substantial conversion increase up to a maximum of 78%. At each pressure, loss of conversion of 1-hexene was observed over the course of 8 h reaction time, which was attributed to the formation of oligomers and eventually coke upon the catalyst surface. Loss of conversion occurred initially more rapidly at the intermediate pressures of 40 and 70 bar, compared with at 10 and 100 bar, where a sustained but gradual decrease of conversion occurred. With increasing temperature in the range 200 – 300 ∘ C , conversion was highest at the condition 235 ∘ C , 40 bar, which is closest to the critical point of 1-hexene. Higher concentrations of oligomers, which act as coke precursors, were detected with increasing temperature and pressure of the reaction. The mass fraction of coke deposited upon the catalysts was dependent upon operating conditions and was within the range 0.63–1.36%, whilst the metal dispersion reduced from 26.8% for the fresh catalyst to 2.82–4.61% for the range of coked samples. A detailed examination of the void space structural changes, occurring after coking under both sub-and super-critical conditions, has been made. In particular, the void space structures were characterised in terms of external accessibility using percolation theory. The catalysts which were operated at reaction conditions where rapid initial deactivation occurred displayed a lower apparent connectivity in comparison with catalysts operated at conditions favouring a gradual sustained deactivation. It has also been found that, under certain conditions, the apparent connectivity of the remaining pore network can, unexpectedly, appear to increase following coking. This has been attributed to the initial loss of the most inaccessible pores within the network.