Skeletal Isomerization of Hexenes on Tungsten Oxide Supported on Porous α-Alumina

The isomerization of hexenes on tungsten oxide supported on a porous α-Al2O3 catalyst with a coverage approaching one monolayer has been studied. The support used is a porous monolith of α-alumina prepared by a novel method. It appears that this catalyst exhibits high and very stable activity and isomerization selectivity under a hydrogen stream at 350°C or at higher temperatures. No pretreatment is necessary to induce the activity. The skeletal isomerization proceeds only through an acidic mechanism including carbenium ion rearrangements. The catalyst was characterized by XPS, XANES, XRD, and TPR. The acidic character of the fresh calcined surface was attributed to the presence of amorphous tungsten oxide species in interaction with the support, as Lewis acidic sites, and to the presence of OH groups as Brønsted acidic sites. Such a catalyst has less resistance to poisoning by carbonaceous residues at low temperature (200°C), but is fully rejuvenated by a simple exposure to hydrogen at 350°C.