Preparation of sulfated zirconia catalysts with improved control of sulfur content .2. Effect of sulfur content on physical properties and catalytic activity

The sulfur content of sulfated zirconia can be varied by changing the quantity of sulfuric acid used for impregnation in the controlled impregnation method, in which a pre-set quantity of sulfuric acid is added and evaporated without filtration or decanting. The material is then dried and calcined (in this work, for 5 h at 610 degrees C). The sulfur content of the calcined material increases with the increase in the quantity of sulfuric acid, but the ratio of sulfate retained after calcination to sulfate impregnated decreases at first and then increases. The surface area gradually increases with the quantity of the acid up to 3 ml of acid per gram of zirconium hydroxide (2.5% S in SZ after calcination), then decreases abruptly. The crystal structure of the sulfated zirconia is also affected by the sulfur content. XRD data show that the sulfated zirconia with a low or medium sulfur content crystallizes only in the tetragonal form, whereas at higher sulfur content a minor fraction of monoclinic zirconia begins to be seen, in addition to the major tetragonal form. XPS data indicate that at the higher sulfate loading part of the sulfate is present in the bulk phase, rather than on the surface. Thus, the increase in sulfur uptake, decrease in surface area and change in crystallinity appear related to sulfate migration inside the particles. The catalytic activity for the isomerization of methylcyclopentane at 65 degrees C shows a maximum for a sulfur content around 3%. It appears that the optimum catalyst should contain the maximum amount of sulfate near the surface, but not necessarily on the surface, and crystallize in the tetragonal form.