Isomerization of n-butane over zeolites depending on catalyst structure

Abstract

The isomerization of n-butane in the gasphase, catalyzed by H-Y zeolite, RE-Y zeolite, H- and Na-offretite, was studied at 200 °C and atmospheric pressure. The rate of isobutane formation was found to be higher for the Y zeolites (H- and cation-Y) than for the corresponding offretites (H- and cation-offretite), indicating some sterical restrictions of the offretites due to their smaller pore sizes and possible stacking faults. The protonated zeolites showed higher rates of isobutane formation than the corresponding cation exchanged catalysts. The selectivities of the zeolites during the first 20 h of the reaction were found to exceed 75%, whereas the maximum yield of isobutane for the active catalysts was in the range of 33–43%. Several physical and spectroscopical characterization methods were applied. The appropriate crystal structures of the offretite samples were verified by X-ray diffraction measurements. Furthermore, tetrahedrally coordinated Si- and Al-sites were identified by solid-state 29Si- and 27Al-MAS NMR spectroscopy, in both the offretites and the Y zeolites. Scanning electron microscopy (SEM) and chemical analysis disclosed the formation of coke during isomerization. Finally, XPS-measurements indicate the coexistence of exchangeable and non-exchangeable Na-ions in Na-offretite, the latter being located in a more electron-rich environment in the zeolite lattice.