Dealumination of ferrierite by ammonium hexafluorosilicate treatment: characterization and testing in the skeletal isomerization of n-butene Original Research Article

Ferrierite zeolite has been treated with ammonium hexafluorosilicate (AHFS). The prepared catalysts were characterized by XRD, atomic absorption spectroscopy, ammonia TPD, FT-IR, SEM, BET/pore size distribution and pyridine chemisorption. Their catalytic activity has been evaluated in the skeletal isomerization of n-butene. Ammonium hexafluorosilicate treatment of ferrierite zeolite leads to a little but progressive framework dealumination, although bulk Si/Al ratio determined by chemical analysis only increases slightly, indicating that most of the extracted aluminum atoms remain in the pores. Higher fluorosilicate amounts decrease crystallinity, micropore area, and volume. Ammonium hexafluorosilicate treatment remove mainly ferrierite external acid sites with a marked improvement in the number of medium strength acid sites. The higher dealumination degree the lower isobutene yield, however, the presence of extraframework aluminum species, medium strength acid sites and lower acid site density reduces the conversion of n-butene through consecutive reactions like n-butene dimerization followed by cracking and hydrogen transfer. These reactions are responsible for the formation of by-products and hence dealuminated catalysts show isobutene selectivity values as high as ferrierite zeolite.