Influence of the calcination temperature on the acidic and catalytic properties of sulphated zirconia Original Research Article

The effect of the calcination temperature (Tc from 500 to 700°C) of sulphated zirconia on acidic and catalytic properties was investigated. The sulphur content decreases from 2.5 to 1 wt% when Tc increases from 500 to 700°C; the BET surface area is between 90 and 100 m2 g−1 for all the samples with the exception of that calcined at 700°C (80 m2 g−1). IR spectroscopy has been used to characterize the hydroxyl groups and to determine the concentration of Brönsted- and Lewis-acid sites from pyridine adsorption. The activities, stabilities and selectivities of the sulphated zirconia samples were compared for both n-butane and propane transformations at 250°C. For Tc≤600°C, the concentrations of Lewis- and Brönsted-acid sites are practically identical. For higher calcination temperatures, the Lewis acidity increases at the expense of the Brönsted acidity. The increase with Tc in the wave number of the hydroxyl band, the decrease of the ratio between the number of protonic sites retaining pyridine adsorbed at 450 and 150°C as well as the increase in the ratio of n-butane and propane reactivities suggest that the acid strength decreases when Tc increases. In agreement with this proposal and with the decrease in the number of protonic sites, there is a decrease in activity when Tc increases from 600 to 700°C. However, the reverse is found for lower calcination temperatures. The unexpected low activity of the samples calcined at lower temperatures may be due to their very fast initial deactivation (before the first activity measurement). Deactivation is mainly due to poisoning of the acid sites by carbonaceous deposits. However, sulphur elimination with protonic sites consumption also plays a limited role.