ON THE ROLE OF THE CALCINATION STEP IN THE PREPARATION OF ACTIVE (SUPERACID) SULFATED ZIRCONIA CATALYSTS

In the preparation of active SO4-ZrO2 catalysts, several steps involvi ng various chemical and/or physical processes are necessary. In partic ular it has been reported that, after sulfation of amorphous Zr hydrat es, a calcination at T-calc > 773 K is needed to guarantee the crystal lization of ZrO2 in the tetragonal phase. By the use of a stabilized t etragonal ZrO2, it is here demonstrated that a calcination at T-calc > 773 K is indeed necessary for all SO4-ZrO2 systems, and that its actu al role is the selective elimination of sulfates from highly energetic crystallographic defects. The calcination step at T-calc > 773 K so c reates the conditions for the formation of strong Lewis acid centres, that are necessary in the catalytic process, and the presence of which is here monitored spectroscopically by the reversible adsorption of c arbonmonoxide.