Role of Ester Intermediates in Isobutane Alkylation and Its Consequence for the Choice of Catalyst System

Abstract

Alkylation of isobutane by olefins catalyzed by Bronsted acids involves initial reaction of the olefin with the acid to form an ester intermediate. The esters formed by the reaction of olefins with triflic acid are very short-lived under typical alkylation reaction conditions but have been made by reaction of the olefin with frozen triflic acid and characterized by NMR spectroscopy. The ester intermediates are formed in an olefin-rich environment and reacts predominantly in an environment where the acid activity is high and the olefin concentration is low. This protects against oligomerization reactions, and the formation and stability of these ester intermediates in the absence of free acid, therefore, play an important role in the selectivity to the desired high-octane components typical of alkylate gasoline. The formation of relatively stable ester type intermediates represents a route to passivation of solid acid catalysts and thus favors a liquid acid catalyst for isobutane alkylation. A new fixed-bed alkylation technology using an SLP type catalyst has been developed for isobutane alkylation. It is a compromise between the wish for improved control of the acid catalyst and the chemistry of isobutane alkylation as it provides an effective control of an otherwise liquid acid catalyst without compromising the liquid phase chemistry. Results from bench-scale tests show product qualities comparable to the qualities obtained using the established technologies.