Hydroisomerization of n-Butane over Platinum-Promoted Cesium Hydrogen Salt of 12-Tungstophosphoric Acid

The hydroisomerization of n-butane was carried out in a fixed-bed gas-flow reactor over Pt-promoted Cs_2.5H_0.5PW₁₂O₄₀ (denoted as Cs2.5). Two kinds of catalysts, a direct impregnation of Pt on Cs2.5 (denoted as Pt/Cs2.5), as well as a mechanical mixture of Pt/Al₂O₃ and Cs2.5 (denoted as Pt/Al₂O₃+Cs2.5), were used for the hydroisomerization. Pt/Al₂O₃+Cs2.5 showed a higher stationary activity than Pt/Cs2.5 because the Pt particles supported on Al₂O₃ were much smaller than those supported on Cs2.5. The initial activity decreased with increasing H₂ pressure over Pt/Al₂O₃+Cs2.5. This indicates that the hydroisomerization of n-butane over Pt/Al₂O₃+Cs2.5 proceeded through a bifunctional mechanism, in which n-butane was hydrogenated/dehydrogenated on Pt sites and was isomerized on acid sites of Cs2.5. For the hydroisomerization of n-butane over Pt/Al₂O₃+Cs2.5 the hydrogenation/dehydrogenation on Pt sites is a limiting step at a low Pt loading and the isomerization on solid acid sites is a limiting step at a high Pt loading. During the reaction, hydrogen molecules were dissociated to active hydrogen atoms on Pt sites, and then the formed active hydrogen atoms moved to the solid acid sites of Cs2.5 (spillover effect) to eliminate the carbonaceous deposits and suppress the catalyst deactivation. Because Cs2.5 has suitably strong and uniformly-distributed solid acid sites, Pt/Al₂O₃+Cs2.5 showed a higher stationary activity than Pt/Al₂O₃+H-ZSM-5 and Pt/Al₂O₃+SO₄/ZrO₂ for the hydroisomerization of n-butane at a low H₂ pressure.