Effect of alumina polymorph on the dehydrogenation activity of supported chromia/alumina catalysts

Abstract

In this work, the effect of surface properties of different alumina supports for CrOx/Al2O3-catalysts on Cr-speciation and dehydrogenation activity was studied. For this purpose, a series of CrOx/Al2O3-catalysts with a monolayer coverage of CrOx (ca. 4.4 at. Cr/nm2) was prepared with γ-, δ-, (δ + θ)-, η-, θ-Al2O3 as supports. Systematic investigation of fresh catalysts with a set of techniques (chemical analysis, N2 adsorption, XRD, Raman and ESR spectroscopy) showed that (i) chemical state distribution of Cr (Cr6+, Cr5+, Cr3+) was the same for all of the catalysts; (ii) CrOx-species were in X-ray amorphous state; (iii) surface chromate species were virtually of the same oligomerization degree. However, aggregation of Cr3+ ions both in calcined and reduced catalysts is support-dependent. ESR data and infrared spectroscopic results of adsorbed CO showed that increase of Lewis acid sites (LAS) surface density on bare aluminas promotes the growth of the relative amount of Cr3+ ions in a relatively large Cr2O3-like clusters in the fresh catalysts. Catalytic testing in a cycling dehydrogenation-regeneration mode leads to catalyst deactivation. Investigation of spent catalysts revealed that deactivation is accompanied with sintering of Cr3+Ox-species with partial migration of Cr3+ ions inside alumina. It was observed that increase of LAS surface density on aluminas promotes the growth of both the initial dehydrogenation activity and stability upon cycling. On the basis of experimental results, it was concluded that (i) Cr3+ ions on the surface of relatively large Cr2O3-like clusters are more active than isolated and poorly agglomerated Cr3+ ions; (ii) increase in LAS concentration on alumina induces growth of surface coverage by Cr2O3-like clusters with concomitant modification of intrinsic activity of active sites through their interaction with support.