In situ analysis of zirconia based catalysts during n-butane isomerization.

摘要

The objective of this project was to further our understanding of the phenomena occurring on the surface of sulfated zirconia (SZ) and tungstated zirconia (WZ), thus allowing us to clarify the nature of the active sites involved in acid catalyzed reactions. n-Butane isomerization was used as a probe reaction to clarify the complexity of those sites.; In the first part of the thesis, butene, an intermediate in the reaction, was added during reaction at 150°C and at 250°C to better understand the reaction pathway. Added olefin contributed to catalyst deactivation without eliminating the reaction induction period, both at 150°C and 250°C. However, at 150°C, it increased the activity of SZ before 30 min time-on-stream (TOS). Isotopic transient kinetic-analysis at 5 min TOS showed that added olefin increased the concentration of surface intermediates, N _iso-C4, both at 150°C and 250°C. However, it decreased the activity of the sites at 250°C. Butene added for 2 min at the beginning of reaction increased the activity of the catalyst during 250 min TOS. The added 1-butene contributed to the formation of isobutane for multiple times before its elimination, suggesting that it may have been involved in the formation of additional active sites.; In the second part of the thesis, the nature of the active acid sites was investigated. Pretreatment of SZ at 500°C improved the activity of SZ due to an increase in the concentration of surface intermediates, N_iso-C4. CO (known to adsorb on Lewis acid sites) suppressed isobutane formation.; Butene, added along with CO, for 2 min, had a promoting impact on the activity of SZ once CO and butene feed were terminated. The results support the hypothesis that the “active centers” for reaction are probably Brønsted acid sites neighboring Lewis acid sites.; The final part of the thesis focused on n-butane isomerization on WZ in the temperature range 280°C–360°C. Butene addition increased dramatically the activity of WZ and shortened its reaction induction period. H₂ pretreatment decreased the concentration of active sites thus, the activity of WZ. Our results indicate that formation of butene is a limiting step in the reaction on WZ.