An Infrared Study of Acid Sites on Silica-Alumina Catalysts Poisoned with Sodium Hydroxide

Abstract

Abstract We investigated the effect of NaOH impregnation on the acidic properties (the type, strength, and amount of acid sites) of silica-alumina by means of IR spectroscopy of chemisorbed pyridine. Most acid sites on the unpoisoned catalyst were of the Lewis type. Some Lewis-bound pyridine was retained on the surface even at 400 °C, but the pyridinium ion was eliminated at 300 °C, indicating that the strong acid sites were also of the Lewis type. Impregnated NaOH affected both Lewis and Brønsted sites. An increase in the amount of NaOH resulted in the elimination of the Brønsted acid sites of medium strength, and also in a significant reduction in the number of Lewis acid sites over the whole range of acid strength. Weak Brønsted acid sites showed no marked change in number. When the NaOH-impregnated catalysts were exposed to pyridine, a new IR band appeared at about 1445 cm−1; this band is attributable to pyridine associated with the Na ions on the surface.