Activation of sulfated zirconia catalysts Effect of water content on their activity in n-butane isomerization

Abstract

The effect of water content in sulfated zirconia catalysts on their activities for n-butane isomerization was investigated using catalytic testing, Fourier transform infrared (FTIR) spectroscopy and thermogravimetry analysis. It was found that minor amounts of water promote the catalytic activity while excess water diminishes it. The FTIR spectrum of pyridine adsorbed on the catalysts showed that a decrease in water content resulted in a decrease in Bronsted acidity with a concurrent increase in Lewis acidity. In the most active catalyst, approximately two-thirds of the acid sites are Bronsted sites, and the rest are Lewis sites; an appropriate amount of water is needed to produce this ratio. The Bronsted acid sites presumably contribute to the catalysis by protonating butene formed at redox sites on the catalyst to create carbocations. The high catalytic capability of the Bronsted acid sites requires the presence of adjacent Lewis acid sites, which withdraw electrons from bisulfate SO–H bonds through an induction effect, giving rise to more acidic protons.