New Facts about the Influence of the χ-Al ₂ O ₃ Content on Properties of the CrOx/η-Al ₂ O ₃ Catalyst Dehydrogenation of C4 Hydrocarbons

In this study, the effect of the gibbsite content in bayerite on the properties of the CrOx/Al2O3 catalyst based on a support prepared from this bayerite was investigated. Two bayerites with gibbsite impurities were used for the preparation of granulated supports and CrOx/Al2O3 catalysts. The amounts of Cr in the catalysts were 5.5 ± 0.5 and 13.1 ± 0.5 wt %, respectively. Pure gibbsite and pseudoboehmite (binding agent) were used to prepare the reference samples. The gibbsite content in the supports was 20 wt %, 30, 80, and 100 wt %. Thermal analysis, laser diffraction method, 1H MAS NMR, low-temperature nitrogen adsorption–desorption, TPD-NH3, XRD, potentiometric titration of Cr6+, ESR, Raman spectroscopy, and XPS were used to study the initial powders of alumina precursors, Al2O3 supports, and CrOx/Al2O3 catalysts. The catalysts were tested in the isobutane and butane dehydrogenation processes. The gibbsite content in bayerite powder correlates with the content of the χ-Al2O3 phase in the support. The presence of the χ-Al2O3 phase in the support promotes a higher proportion of Cr5+ and a lower proportion of Cr3+ in the Cr2O3 species of catalysts containing 5.5 ± 0.5 wt % Cr. It further leads to a decrease in the number of active sites and a decrease in the activity in isobutane conversion and isobutene yield while selectivity remains unchanged. As the content of the χ-Al2O3 phase in the support increased, the total Cr6+ content and the water-insoluble Cr6+ content also increased in catalysts containing 13.1 ± 0.5 wt % Cr. Increasing the χ-Al2O3 content in the support also led to decreased n-butane conversion and butadiene yield.