Unexpected selectivity of ferrierite for the conversion of isobutanol to linear butenes and water effects

Abstract : Ferrierite was shown to be highly efficient in the conversion of isobutanol to butenes with selectivity values higher than 98%. Furthermore, its isomerisation activity is remarkable since proportion of linear butenes higher than 80% was obtained in the present study confirming patents claims. This selectivity was shown to increase with temperature and contact time as well as with time on stream. Neither water added to the feed nor water generated by dehydration has an impact on the structure of ferrierite as shown by XRD and 27Al NMR. A slight enhancement of catalytic activity was observed below 250 °C and could be due to an increase in the number of BAS as suggested by in situ acidity measurements achieved at the reaction temperatures in presence of water vapor while competition of adsorption would inhibit the catalytic activity above 250 °C. Furthermore, water was shown to improve dramatically selectivity to linear butenes at low conversion. We propose that water inhibits the proton shift of isobutylcarbenium ions or deprotonation sites leading to isobutene but not acid sites able to isomerize isobutylcarbenium ions into linear carbocations leading to linear butenes. At high conversions, both coke formation and water generated by dehydration could improve selectivity to linear butenes by neutralization of unselective sites responsible for proton-shift reaction.