Electron paramagnetic resonance study of hydrocarbon accommodation in copper(II)-exchanged zeolite ZSM-5 and related materials

Abstract

The EPR spectra due to copper(II) exchanged into the zeolites H-[Al]-ZSM-5, H-[Al]-ZSM-11 and H-[Ga]-ZSM-5, from which spin Hamiltonian and linewidth parameters have been obtained by computer simulations, have been used to identify the exchange sites in the zeolites. In keeping with the framework structural features of H-[Al]-ZSM-11, the copper(II) ions are distributed about equally between two sites, whereas in H-[Al]-ZSM-5 the copper(II) resides largely on one type of site, with a minor portion of copper(II) on a site similar to that found in H-[Al]-ZSM-11 and thought to arise from a small amount of intergrowth of this zeolite within the H-[Al]-ZSM-5. The wider EPR spectral linewidths due to copper(II) exchange into H-[GA]-ZSM-5 are thought to arise from a larger distribution of site symmetries brought about by trivalent metal ion framework substitution. The ESR spectral features which result from the introduction of xenon into CuH-[Al]-ZSM-5 and CuH-[Al]-ZSM-11 arise from minor zeolite framework distortions which are about equal to those which result from framework substitution of aluminium(III) by gallium(III). The similarity of the EPR spectra due to CuH-[Al]-ZSM-5 exposed separately to cyclopentane and n-hexane or methylcyclopentane and 2-methylbutane suggests that the straight-chain alkanes adopt a coiled orientation resembling a ring structure when inside the zeolite. Comparison of the EPR spectral data from the zeolite treated with n-hexane and hex-1-ene or cyclohexane and cyclohexene indicates that the double bond reduces hydrocarbon flexibility, while large framework distortions result from the accommodation of benzene. The framework distortions already present in CuH-[Ga]-ZSM-5 framework result in a loss of sensitivity to further framework distortion owing to the introduction of hydrocarbons.