Hydroisomerization of n-hexane and n-heptane over platinum-promoted Cs2.5H0.5PW12O40 (Cs2.5) studied in comparison with several other solid acids

Isomerization of n-hexane and n-heptane was carried out over Cs2.5H0.5PW12O 40 (denoted by Cs2.5) promoted by Pt which was introduced by either impregn ation of H2PtCl6 or mechanical mixing of Pt/Al2O3 and over non-promoted Cs2 .5H0.5PW12O40 in the presence of hydrogen at atmospheric pressure. The reac tion temperature studied was relatively low (typically 453 and 423 K for n- hexane and n-heptane, respectively) and the hydrogen pressure was also rath er low (standard conditions: feed = n-alkane 0.05 atm, H-2 0.20 atm, N-2 ba lance; W/F = 40 g h mol(-1)). Results were compared with those obtained und er the same conditions for other Pt-promoted solid acids, where particular attention was paid to the time courses of the reaction (initial vs. station ary performance). Both the activity and selectivity of Cs2.5 at the initial stage (after 5 min) increased by the addition of the Pt component. Pressur e dependencies of the rate at the initial stage were approximately first an d -0.5th orders in alkane and hydrogen, respectively. Most remarkable was t he suppression of the deactivation during the reaction in the presence of b oth Pt and hydrogen. For example, the mechanical mixture of Pt/Al2O3 and Cs 2.5 (abbreviated as Pt+Cs2.5) showed little deactivation and much improved selectivity; resulting in high stationary conversion and selectivity; e.g., 98.4 and 92.1% selectivities for n-hexane and n-heptane at the conversions of 58.6 and 39.4%, respectively. Most of the results were well explained b y a classical bifunctional mechanism, although other mechanisms are not all excluded. As for the other solid acids, the initial activity of Pt-promote d SO4/ZrO2 was high, but decreased rapidly. The deactivation was small with Pt-promoted H-ZSM-5, but the activity was low. The stationary yields of is omerized products were higher for Pt-promoted beta zeolite and Al-pillared saponite (tested only for n-heptane), although higher reaction temperatures were necessary.