Cyclopropane Adsorption on Zeolites Studied by IR Spectroscopy: II. Adsorption and Conversion of Cyclopropane on Na- and Ca-Exchanged Zeolite Y

The adsorption and isomerization of cyclopropane on the calcium- and sodium-exchanged zeolite Y are studied by IR spectroscopy. Cyclopropane is adsorbed on CaY in two different forms. Weaker adsorption is related to residual sodium ions, which were not completely removed from the zeolite by ion exchange. This form can be removed from the surface after evacuation of the samples at room temperature. Stronger adsorption is attributed to the Ca ions. It is stable up to 100°C. The corresponding diffuse-reflectance spectrum indicates the C _2v symmetry of the cyclopropane complex with Ca ions. At 200°C cyclopropane adsorbed on the calcium-exchanged zeolite converts to propylene. On the sodium-exchanged zeolite, this reaction only occurs at 400°C. The reaction coordinate of cyclopropane isomerization on CaY is related to the simultaneous cleavage of the C–C bond in the cyclopropane ring and hydrogen atom transfer from one of the CH₂ fragments to another. The reaction coordinate corresponds to a combination of the stretching vibration of the C–C bond with the fan vibration of the CH₂ group and the stretching vibration of the CH bond. These composite vibrations result in the strong polarization of the C–C and C–H bonds and, hence, exhibit anomalously high molar absorption coefficients in the IR spectrum.