Isomerization of cis-2-butene and trans-2-butene catalyzed by acid- and ion-exchanged smectite-type clays

Two commercial acid-activated montmorillonites were exchanged with different cations (Al3+, Cr3+, Fe3+, and Ni2+). The catalytic activity of the acid- and the ion-exchanged clays was measured using the isomerization of cis- and trans-2-butene at 300 °C to yield the corresponding isomers. The order of activity for the acid- and the different cation-exchanged forms was: acid clays >Al3+≈Ni2+>Cr3+>Fe3+, which slightly correlated with the acidity values determined by cyclohexylamine desorption and the d001 spacing obtained from integral series of reflections. The maximum conversion for the isomerization of cis-2-butene was ≈75% for F24 and F124, while the maximum conversion for the isomerization of trans-2-butene was ≈53% for F24 and F124. Cracking produced propylene and methane and their amounts increased with the acidity.

Furthermore, no study was reported where the octahedral sheet of a clay mineral had been partially delaminated and then exchanged with cations of high water polarization power to control the acid nature of the surface. Therefore, the objective of this work is to prepare different ion-exchanged clays from two commercial acid montmorillonites, to study their textural characteristics, to evaluate the catalytic activity of the synthesized materials towards the isomerization of cis- and trans-2-butene, and to compare with the results previously reported using acid-activated organoclays.

Besides, positional double bond isomerization of 1-butene to trans-2-butene and cis-2-butene was the main side reaction observed at all experimental conditions, especially at low temperature. At these conditions, the equilibrium of isomerization reaction is shifted to the formation of 2-alkenes because the trans-2-butene is thermodynamically favored. With regards to other byproducts, individual identification of every obtained byproduct in each experiment was not always possible due to low side reactions extension in the explored conditions.
The synthesized ion-exchanged clays showed good catalytic activity for the isomerization of butenes, comparable to that obtained using acid-activated organoclays. The conversion was slightly correlated with the acidity and the d-spacing. It is believed that the cis-2-butene is more thermodynamically favored that the trans-2-butene and thus higher conversions are obtained in the former. The isomerization reaction takes place by protons created by the water polarization power of the cations.