Study of n-butene isomerization and polymerization on CaNaY-type zeolites by means of 13C-N.M.R. spectroscopy

With highly resolved carbon-13 n.m.r. spectroscopy, the isomerization and polymerization of but-1-ene sorbed in CaNaY-type zeolites have been investigated. The application of molecules partially enriched with carbon-13 nuclei allowed a detailed study of the transformtion of labelled groups.At lower temperatures (290 – 350 K) only the double bond isomerization of butenes proceeds in CaNaY-type zeolites with 65 – 70% of their sodium ions replaced by calcium. Within the limits of experimental error an enrichment of ¹³C nuclei was found in the CH₃ and CH groups of but-2-ene molecules if the but-1-enes were enriched in the positions CH₂ and CH, respectively. Relative equilibrium concentrations as determined from n.m.r. line intensities were in fair agreement with known data for the thermal equilibrium of the n-butene isomers in the gas phase. Thus, possible differences between Overhauser factors of the different carbon-13 resonance lines were so small that they did not give rise to significant errors in determining concentrations via the enchanced resonance line intensities.Polymerization reactions occurred at higher temperatures. They gave rise to mutually overlapping lines of saturated carbons which were difficult to detect unless carbon-13 enriched butene molecules were applied. Using pore filling factors of ≈ ≈ 0.6, partly branched octanes were formed. This is at variance with investigations at low pore filling factors of ≈ ≈ 0.1 where predominantly octenes were found. These experimental results are explained by a different availability of hydrogen as an important controlling factor in these reactions.