Characteristics of the active sites on sulfated zirconia for n-butane isomerization

Abstract

n-Butane isomerization on sulfated zirconia (SZ) has been studied with the goal of increasing our understanding of the nature of the active sites and the role of the acid (Brønsted and Lewis) sites. Isotopic transient kinetic analysis (ITKA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), butene addition, CO addition, and pretreatment of the catalyst at different temperatures have been used to investigate the nature of the active sites involved in the formation of reaction intermediates and of isobutane. It was found that CO, added from the beginning of the reaction, inhibited the formation of isobutane but it did not prevent formation of carbon from n-butane, resulting in a deactivation of the catalyst proportional with the exposure time to CO. Butene added for 2 min at the beginning of the reaction increased the reaction rate and was involved in the formation of isobutane during multiple turnovers as a result of creation of olefinic modified sites. Addition of CO (for different period of times) along with butene (for 2 min at the beginning of the reaction) did not prevent the formation of the olefinic modified sites when CO was added for a short time. However, such sites participated in the formation of isobutane only after the CO feed was stopped and it desorbed from the surface of the catalyst. These results indicate that Lewis acid sites are not involved in the formation of either deactivating carbon or the olefinic modified sites. Pretreatment of SZ at 500 °C improved the activity of SZ due to an increase in the concentration of surface intermediates, N iso-C4, compared to when it was pretreated at 200 and 315 °C and the Brønsted/Lewis acid sites ratio was greater. The results of this study support the hypothesis that the “active centers” for reaction are probably a combination of Brønsted and Lewis acid sites.