INFLUENCE OF THE CALCINATION TEMPERATURE ON THE ACIDIC AND CATALYTIC PROPERTIES OF SULFATED ZIRCONIA

The effect of the calcination temperature (T-c from 500 to 700 degrees C) of sulphated zirconia on acidic and catalytic properties was inves tigated. The sulphur content decreases from 2.5 to 1 wt% when T-c incr eases from 500 to 700 degrees C; the BET surface area is between 90 an d 100 m(2) g(-1) for all the samples with the exception of that calcin ed at 700 degrees C (80 m(2) g(-1)). IR spectroscopy has been used to characterize the hydroxyl groups and to determine the concentration of Bronsted- and Lewis-acid sites from pyridine adsorption. The activiti es, stabilities and selectivities of the sulphated zirconia samples we re compared for both n-butane and propane transformations at 250 degre es C. For T(c)less than or equal to 600 degrees C, the concentrations of Lewis- and Bronsted-acid sites are practically identical. For highe r calcination temperatures, the Lewis acidity increases at the expense of the Bronsted acidity. The increase with T-c in the wave number of the hydroxyl band, the decrease of the ratio between the number of pro tonic sites retaining pyridine adsorbed at 450 and 150 degrees C as we ll as the increase in the ratio of n-butane and propane reactivities s uggest that the acid strength decreases when T-c increases. In agreeme nt with this proposal and with the decrease in the number of protonic sites, there is a decrease in activity when T-c increases from 600 to 700 degrees C. However, the reverse is found for lower calcination tem peratures. The unexpected low activity of the samples calcined at lowe r temperatures may be due to their very fast initial deactivation (bef ore the first activity measurement). Deactivation is mainly due to poi soning of the acid sites by carbonaceous deposits. However, sulphur el imination with protonic sites consumption also plays a limited role. ( C) 1998 Elsevier Science B.V. All rights reserved.