Effects of hydrogen on heptane isomerization over zirconium oxide modified with tungsten oxide and platinum

摘要

Heptane isomerization was carried out over Pt/WO3-ZrO2 under a hydrogen pressure range 0-2 MPa to elucidate the effects of hydrogen on the activity and selectivity of the catalyst. Optimum catalyst was prepared by calcination of WO3-ZrO2 at 1053 K and loading of 0.75 wt% of Pt with a constant WO3 content of 11 wt%. The rate of isomerization was slightly negative in hydrogen pressure in a high pressure range (>0.3 MPa) and positive in a low pressure range (<0.3 MPa). Cracking was markedly suppressed in the reaction under a high hydrogen pressure, which is caused by a high concentration of hydride ion species on the surface. Hydride ions originating from hydrogen molecules play a key role in selective isomerization of heptane. Hydride transfer step was suggested to be slow as compared to skeletal rearrangement among mono- and di-branched carbenium ion intermediates. By controlling the reaction conditions, a high isomerization selectivity of 94% at a conversion of 80% was obtained at 553 K under a hydrogen pressure of 1.55 MPa.