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

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).