Chromium(III) supported on aluminum-nitride-surfaced alumina: characteristics and dehydrogenation activity

摘要

Chromium catalysts were prepared by the saturating chemisorption of chromium(III) acetylacetonate (Cr(acac)_3) at 200 deg C on alumina and on alumina that had been surfaced with aluminum nitride. According to elemental analysis, inrfared spectroscopy, and electron spin resonance spectroscopy, chemisorption occurred through liand exchange reaction of Cr(acac)_3 with surface Oh and NH_x groups and dissociation of Cr(acac)_3 or hacac on coordinatively unsaturated sites on the surface. Steric hindrance imposed by the acac ligands defined saturation of the surface with adsorbed species; the same chromium loading (0.64 +- 0.06 atoms per nm~2) was obtained on all supports. After removal of the ligands with oxygen, water, or ammonia, the samples were active in isobutane dehydrogenation. For chromium sites with high activity, nearby oxygen ions were needed. The activity of a Cr~(3+) ion was mainly determined by its environment and not by whether it had been formed through reduction.