13C NMR investigation of adsorbed hydrocarbons

Abstract

By means of Fourier transform techniques 13C nmr spectra and longitudinal 13C magnetic relaxation times of various butene isomers sorbed on NaY and NaX zeolites have been studied. In contrast to 1H resonance where only two broad lines can be observed, the 13C spectra show separated sharp lines. The shift Δσ = σ ads- σ free of the carbon-13 lines of adsorbed molecules with respect to the resonance positions of free molecules and the longitudinal relaxation times indicate a peculiar behaviour of the groups  CH and = C ▪ of the adsorbed n-butene and isobutene molecules, respectively. The resonance deviations Δσ to lower fields in these groups decrease in the order isobutene ( δσ = − 10.3 ppm), but-1-ene (δσ ≈ −5.7 ppm) and but-2-enes (Δσ ≈ − 2.2–2.4 ppm), whereby the spectra of cis-but-2-ene and trans-but-2-ene show similar differences as found in the liquid state. When adsorbed on NaY and NaX types (silica/alumina ratios 5 and 3.5) the 13C nmr line positions of but-1-ene molecules are the same within the limits of experimental error, which demonstrates that electrostatic fields have no noticeable influence on the molecular parameters in these systems. Therefore one may conclude that chemical bonds to Na + cations have the dominant effect on the line shift Δσ. CNDO-MO calculations based on such a model have shown that a charge transfer occurs from the π-electrons and the electrons of the methyl group hydrogen atoms to the cation.