Ion—molecule reactions of tetramethylethylene radical cations in zeolites

Abstract

Radical cations were produced by γ-irradiation of solutes in zeolites at 77 K. The radical cation of tetramethylethylene (TME) was examined by EPR in a variety of zeolites at different temperatures and loading. The TME monomer radical cation, the TME dimer radical cation and the neutral radical formed by proton transfer in the dimer cation were observed. The radical cation dimermonomer aggregation is reversible at low temperatures; above ≈ 110 K the neutral radical was observed. The effect of host zeolite interactions on the reactivity of tetramethylethylene radical cations in mazzite (Na-Ω-5) and pentasil (MFI-Na-ZSM-5 and silicalite S115) are reported. γ-irradiation of TME in Na-ZSM-5 and silicalite S115 gives rise to a dimer radical cation below 50 K when the concentration of TME is ⪢ 1 wt%. EPR spectra from 50 to 110 K correspond to the monomer radical cation and the neutral radical was observed above 110 K. EPR spectra observed in Na-Ω-5 show the co-existence of all three radical species in varying proportions at different temperatures. The differences in the zeolite interior surface topology (e.g., cage size and type of channel) are invoked to explain the observed effects on the radical cation chemistry.