The Effect of Acid Sites in Skeletal Isomerization of N-Butenes over Ferrierites and Coalpo-11

Abstract

Acid sites of H-, NaH-ferrierites and those partly or fully dehydroxylated, and CoAlPO-11 were characterized by TPD of ammonia, UV-VIS spectroscopy, IR spectroscopy of D₃-acetonitrile and by transformation of n-butenes. It has been shown that the high density of strong bridging OH groups or their simultaneous presence with strong Lewis sites support oligomerization reactions to higher olefins, leading via cracking to C₃-C₅ olefins, C₄ paraffins and some coking. Thus, a low density of strong OH groups and absence of Lewis sites led to the selective performance of ferrierite. Under the same reaction conditions CoAlPO-11 with very low number of acid sites exhibited the highest and moreover stable activity to isobutene (yield 45 % at 620 K) for at least 48 hrs. This is a result, besides very narrow channels, of cooperative effect of weakly acidic both P-OH groups and Co²⁺  Lewis sites. When P-OH groups were completely removed by calcination, skeletal isomerization selectively proceeds only on Co²⁺ sites.