Dataset Information

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

ABSTRACT: 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).

SUBMITTER: Berger F

PROVIDER: S-EPMC7898720 | biostudies-literature | 2021 Feb

REPOSITORIES: biostudies-literature

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

Berger Fabian F Sauer Joachim J

Angewandte Chemie (International ed. in English) 20201214 7

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<sup>-1</sup> for branched alkenes, but -153 and -154 kJ mol<sup>- ...[more]

PMID: 33314606

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Project description:Because of the small particle size, orientation-dependent diffusion measurements in microporous materials remains a challenging task. We highlight here the potential of micro-imaging by interference microscopy in a case study with MFI-type crystals in which, although with different accuracies, transient concentration profiles in all three directions can be observed. The measurements, which were performed with "rounded-boat" shaped crystals, reproduce the evolution patterns of the guest profiles recorded in previous studies with the more common "coffin-shaped" MFI crystals. The uptake and release patterns through the four principal faces (which in the coffin-shaped crystals extend in the longitudinal direction) are essentially coincident and there is no perceptible mass transfer in the direction of the long axis. The surface resistances of the four crystal faces through which mass transfer occurs are relatively small and have only a minor effect on the mass transfer rate. As a result of the pore structure, diffusion in the crystallographic c direction (which corresponds to the direction of the long axis) is expected to be much slower than in the transverse directions. This could explain the very low rate of mass transfer observed in the direction of the long axis, but it is also possible that the small end faces of the crystal may have high surface resistance. It is not possible to distinguish unequivocally between these two possibilities. All guest molecules studied (methyl-butane, benzene and 4-methyl-2-pentyne) show the same orientation dependence of mass transfer. The long 4-methyl-2-pentyne molecules would be expected to propagate at very different rates through the straight and sinusoidal channels. The coinciding patterns for uptake through the mutually perpendicular crystal faces therefore provide clear evidence that both the coffin shaped crystals and the rounded-boat-shaped crystals considered in this study, must be intergrowths rather than pure single crystals.

Dataset Information

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

Publications

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

Similar Datasets

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