The catalytic isomerization of 1-hexene on H-ZSM-5 zeolite: The effects of a shape-selective catalyst

Abstract

The isomerization of 1-hexene on 60 80 -mesh ZSM-5 zeolite was studied in the temperature range 200–280 °C and the results were compared with those previously obtained for HY at 200 °C. The observed products were formed through a variety of processes including double bond shift, cis-trans isomerization, skeletal rearrangement, cracking, hydrogen transfer, polymerization and coke formation. By applying the time-on-stream theory, the products were classified as primary, secondary, or both according to their optimum performance envelope curves on product selectivity plots. At all levels of conversion, cis- and trans-2-hexene were found to be the principal products. The ratio of the initial selectivities of cis- to trans-2-hexene at 200 °C was 0.54, significantly closer to the equilibrium value than previously found for HY zeolite. A possible explanation is given, relating to the difference in pore structure of these zeolites. The ratio of the initial rate of deprotonation to that of hydrogen shift in the hexyl carbenium ion was found to decrease with temperature. At 200 °C this ratio was greater than that observed on HY. Skeletal rearrangement, polymerization, cracking and hydrogen transfer reactions were all found to increase with temperature. At 200 °C the total contribution from these processes accounts for significantly less product than on HY. All products of skeletal rearrangement were observed to be secondary. The formation of 2,3-dimethyl-1-butene appears to be restricted on ZSM-5 due to the size of this molecule. Skeletal rearrangement of 1-hexene gives cis-3-methyl-2-pentene and trans-3-methyl-2-pentene as secondary products. These isomers are also formed as initial products by double bond shift of the principal impurity present in our feed, 2-ethyl-1-butene. Coke formation decreases with increasing temperature. The composition of the coke indicated that it initially consists mainly of adsorbed olefins. No aromatic products could be detected and polymerization appeared to be restricted to the formation of dimers. The small amount of paraffinic products found and the lack of cyclization and dehydrogenation to aromatic structures appear to be related to the pore size of ZSM-5.