The Isomerization of n-Butenes over Acidic Ion-exchange Resin

The isomerization of n-butenes and the polymerization of propylene were studied in a gas-phase reaction on a sulfonic acid-type resin catalyst. These reactions were considered to proceed via the carbonium ion formation on the functional groups, and a strong correlation was found between the activities in these two reactions. The acidity and the acid strength of the resin catalyst were measured by means of amine titration in a nonpolar solvent using Hammett indicators. The value of the acidity obtained at pKa=6.8 seemed to correspond to the number of the ion-exchangeable hydrogen on the functional groups. The effects of poisoning by ion exchange with Na+, Mg2+ or by adsorption of NH3 were also studied. The decrease in the activity for the isomerization was not proportional to the amount of cations exchanged. The catalytic activities of the protonic hydrogen seemed not to be equal as was suggested by the result of resin catalysts in polar solvents. The possible heterogeneity of the active sites on the resin catalyst was discussed. The six relative rate constants were calculated on the basis of the mutual interconversion among the three isomers of n-butene. The apparent energy of activation for the double-bond migration of 1-butene was found to be 17 kcal/mol on the H-form resin, but it was changed by ion exchange with Na+. The selectivities were characteristic of solid acid catalysts of the protonic type.