Effect of alumina surface chemistry on chromia dispersion and dehydrogenation activity of CrOx/η-Al2O3 catalysts with high Cr content

In this work, effect of alumina surface chemistry on physicochemical properties and isobutane dehydrogenation activity of CrOx/eta-Al2O3 catalysts with ca. 13 wt. % Cr was investigated. For this purpose, three catalysts were synthesized from eta-Al2O3 samples which have identical textural properties but different density of Lewis acid sites (LAS) and OH groups as revealed by IR spectroscopy of adsorbed CO and pyridine. Systematic investigation of CrOx/eta-Al2O3 catalysts with a number of techniques (chemical analysis, N-2 adsorption, X-ray diffraction, Raman spectroscopy and transmission electron microscopy) showed that (a) Cr6+ and Cr3+ quantities as well as surface areas are the same; (b) quantity of low-active large alpha-Cr2O3 (> ca. 5 nm) and very small CrOx species (< ca. 1 nm) diminishes while (c) content of partially crystallized Cr2O3 particles from ca. 1 to about 5 nm size increases with the LAS density on parent eta-Al2O3 and its dehydroxylation. Correspondingly, isobutane dehydrogenation activity at low times on stream (up to 40 min at 570 degrees C) increases. This activity difference was observed in the course of 50 dehydrogenation-regeneration cycles. Therefore, alumina surface chemistry strongly affects the Cr2O3 dispersion and dehydrogenation performance of high-loaded chromia/alumina catalysts. On the basis of the results, a model of surface structure for CrOx/Al2O3 catalysts was proposed including surface migration of chromium being dependent upon LAS and OH groups of Al2O3.