FT-IR study of the isomerization of n-butene over different zeolites

Adsorption and reaction of 1-butene, cis- and trans-2-butene, and isobutene on silicalite H-ZSM-5 and H-FER were studied by FT-IR spectroscopy. On sil icalite the different butenes were only physisorbed and no interaction took place. Over H-ZSM-5, hydrogen-bonded cis- and trans-2-butenes were initial ly formed followed by dimerization. Over H-FER the adsorbed 1-butene was co nverted into cis- and trans-a-butenes which were hydrogen-bonded to the aci dic OH-groups of the zeolite and no further dimerization was found. Since t he Bronsted acidity of H-FER and H-ZSM-8 is comparable it can be concluded that the pore size and pore structure and not the acidity plays the importa nt role for the observed behavior. From this study and literature results, a monomolecular mechanism is proposed for the isomerization of n-butenes to isobutene over both types of zeolites (H-ZSM-5 and H-FER).