Dimerization of Linear Butenes and Pentenes in an Acidic Zeolite (H-MFI)

Abstract

Dimerization of linear butenes and pentenes at zeolitic Brønsted acid sites yields, rather than branched alkenes, alkanes featuring a cyclohexane ring. The absence of any C=C double bond in the latter explains not only the observed change of the IR spectra, but also the observation that oligomerization stops at the dimer. Quantum chemical evidence is produced to show that dimerization of linear butenes and pentenes at zeolitic Brønsted sites in H-MFI yields alkanes featuring cyclohexane rings rather than branched alkenes. The absence of any C=C double bond in the formed cyclic alkane explains the observations that oligomerization stops at the dimer. The calculated reaction enthalpies for the dimerization of 2-pentene in the gas phase are −84 kJ mol−1 for branched alkenes, but −153 and −154 kJ mol−1 for alkyl-cyclopentane and -hexane, respectively. Together with calculated adsorption enthalpies of the dimers, −111 and −127 kJ mol−1, respectively, this implies surface dimer formation enthalpies of −264 and −281 kJ mol−1, respectively, in close agreement with the experimental value of −285 kJ mol−1. In contrast, the predicted enthalpy for formation of branched alkoxides, −198 kJ mol−1, deviates by 87 kJ mol−1 from experiment. Calculated IR spectra for the Brønsted OH group show the observed conversion of the band at approximately 3000 cm−1 (hydrogen bond with alkene) to a less intense band at approximately 3450–3500 cm−1 ( interaction with alkane ).