Initial activity of reduced chromia/alumina catalyst in n-butane dehydrogenation monitored by on-line FT-IR gas analysis

The initial activity of chromia/alumina catalyst (13 wt% Cr) in n-butane dehydrogenation was studied in a flow reactor at 853 K. The initial activity was determined by on-line FT-IR gas analysis, which enabled sampling of the gaseous product mixture at a time resolution of seconds. The catalysts were processed in repeated cycles of oxidation, reduction, and dehydrogenation using n-butane, methane, hydrogen, or carbon monoxide as reducing agents. With n-butane, methane, and hydrogen the dehydrogenation activity was associated with Cr3+ species apparently formed in the reduction of high-valence Cr species. The catalyst reduced with carbon monoxide at 853 K showed poor initial selectivity for butenes and, relative to the other catalysts, stronger formation of coke in dehydrogenation. The initial selectivity for butenes increased with time on stream, however, and within 2 min reached the level obtained with the other catalysts. Simultaneous data relating the initial activity, coke content, and some of the physicochemical properties of the catalyst indicated that the surfaces of all catalysts were modified to some extent by the successive reaction cycles. (C) 1996 Academic Press, Inc.