On the formation of the acid sites in lanthanum exchanged X zeolites used for isobutane/ cis-2-butene alkylation

Abstract

The acid sites generated at different steps during the preparation of La-H-X zeolites were characterized by physicochemical methods. The resulting materials were tested in isobutane/ cis-2-butene alkylation in a continuously operated stirred tank reactor, under industrially relevant conditions. The concentration and strength of acid sites depend subtly on the ion exchange procedure. Especially, the rehydration of materials calcined for the first time after ion exchange changes the distribution of hydroxyl groups, the Brønsted acidic bridging hydroxyl groups (3640 cm −1) being strongly affected. Rehydration leads to dealumination and, as consequence, the concentration of silanol groups (3740 cm −1) and of Lewis acid sites increases. This in turn results in an enhanced stability towards the subsequent thermal treatments and rehydration processes of the rare earth zeolite in the next steps of catalyst preparation. The strength of the Brønsted acid sites was shown to be a function of the hydrolysis of hydrated lanthanum cations and removal of sodium cations. The catalytic activity in isobutane/ cis-2-butene alkylation and the fraction of strong Brønsted to total Brønsted acid sites are directly related. Catalysts with similar concentration of strong Brønsted acid sites and higher concentration of weak Brønsted sites showed shorter lifetime.