Thermodynamic and Kinetic Reaction Regimes in the Isomerization of 1-Pentene over ZSM-5 Catalysts

Abstract

Two interesting observations have been made during isomerization of 1-pentene over H-ZSM-5 catalyst. (a) It is possible to switch between two thermodynamically controlled regimes by changing the reaction temperature. At low temperature, only double bond isomerization is occurring whereas the total isomerization equilibrium involving both n- and isopentenes is attained at high temperature. The temperature range of the kinetically controlled regime between these two equilibrium states is narrow. The transition is accompanied by a sharp increase in the 1-pentene conversion from 90% to almost 100%. The skeletal isomerization reactions are sufficiently fast to maintain an equilibrium distribution although pentenes are continuously withdrawn for the formation of dimerization and cracking products. Changing the space velocity or the feed partial pressure did not result in a stretching or squeezing but rather in a temperature shift of the kinetic regime. (b) Increasing 1-pentene partial pressures favor skeletal isomerization over dimerization and cracking reactions. This result, unexpected at first sight, suggests that reactions yielding isopentenes exhibit a higher reaction order in n-pentenes than the competing reactions. We have also investigated the dehydroisomerization of n-pentane over a single fixed bed of bifunctional Pt/H-ZSM-5 and over stacked beds of Pt/γ-Al2O3 and H-ZSM-5. The results confirm that very low partial pressures of n-pentenes, here generated in situ, have a negative effect on the yields of isopentenes.