Effect of Broensted and Lewis sites in ferrierites on skeletal isomerization of n-butenes

Skeletal isomerization of n-butenes to isobutene at 620 K was followed over NaH-ferrierites (FER) with different concentration of Broensted sites, prepared by NH₄⁺ vs. Na⁺ ion exchange, and over H-FER containing both, the Broensted and Lewis sites formed by zeolite dehydroxylation at temperatures ranging from 670 to 990 K. Quantitative analysis of IR spectra of OH groups and adsorbed d₃-acetonitrile on ferrierite enabled determination of the concentration of Broensted and Lewis sites, and detected also those present in 8-ring ferrierite channels. With H-FER, the yield of isobutene is linearly proportional to the concentration of acidic OH groups. Formation of C₃ and C₅ olefins, C₂-C₅ paraffins and higher molecular weight products, resulting from di-, oligomerization of butenes followed by cracking/hydrogen transfer reactions, is enhanced by the presence of Lewis sites, if simultaneously present with the Broensted sites, that is in a substantial change of the selectivity of the reaction. On the other hand, both the conversion and yield of isobutene are nearly constant at decreasing concentrations of OH groups in NaH-FER, and only at low-concentration range of acidic hydroxyls a decrease in conversion was found. This indicates that the exchanged Na⁺ ions are placed preferably in small ferrierite channels, as also evidenced by ²³Na MAS NMR, which size is not large enough to accommodate isobutene molecules. Thus, besides the well-known positive effect of the inner geometry of ferrierite channels on isobutene selectivity, it has been shown that the reaction of n-butenes to isobutene is proportional to the number of hydroxyls located in large channels of ferrierite, and simultaneous presence of Lewis sites enhances di-,oligomerization/cracking reactions leading to higher yields of by-products.