Desilication of a TON zeolite with NaOH: Influence on porosity, acidity and catalytic properties

Abstract

A TON zeolite (Si/Al = 31) was treated at 353 K with NaOH solutions of different concentrations C (0.05–0.4 M) and for different times t (30–300 min). The resulting samples were characterized by various techniques: SEM, XRD, nitrogen adsorption–desorption, pyridine chemisorption followed by FTIR and 1-butene isomerisation at 333 K. The filtrate analysis shows a simultaneous dissolution of Si and Al with a large preference for Si (Si/Al from 35 to 150), especially at high t and low C values. The increase with t of the filtrate Si/Al ratio was related to the redeposition of Al on the zeolite under the form of extraframework species with Lewis acidity. The alkali treatment caused the creation of mesopores with ∼20 nm diameter, a limited decrease in the Brønsted acidity and a significant increase in the Lewis acidity. With all samples, 1-butene double bond shift selectively occurred but with a fast deactivation due to carbonaceous deposits which can be eliminated from the zeolite through nitrogen sweeping. Desilication had no effect on the catalyst stability but increased the initial activity. The activity per protonic site (TOF) was shown to be correlated with the volume of created mesopores, which shows that these mesopores facilitate the access of 1-butene molecules to the active sites.