Surface and catalytic properties of Al2O3-ZrO2 solid solutions prepared by sol-gel methods

A series of ZrO2-Al2O3 solid solutions was prepared by co-hydrolysis of Al-tri-sec-butoxide and Zr-propoxide, having molar ratios X=Zr/(Zr+Al)=0.0.29, 0.53, 0.63, 0.71, 0.81, 1.0. A systematic study was performed by X-ray diffraction, BET-N-2 adsorption, electron microscopy, potentiometry (butylamine) and NH3-TPD. The as synthesized ZrO2 included monoclinic, tetragonal and cubic phases, i.e. XRD peaks at (2 Theta)=28.2 degrees and 31.5 degrees for the m-phase, (2 Theta)=30.2 degrees and 35.0 degrees for the t-phase and (2 Theta)=50.5 degrees and 59.5 degrees for the c-phase. The progressive formation of a solid solution occurs, between Al2O3 and ZrO2 along the composition axis, with poorly crystallized mixed phases. The BET-N-2 isotherms from X=0.2-1.0 showed a Type IV (BDDT) profile, typical of mesopores in the interval between 20<D-p<500 Angstrom, the S-BET varied from 200 to 250 m(2) g(-1) with (S-BET)(MAX)=246 m(2) g(-1) for X=0.2 The latter composition had a mean pore diameter (D-p) equal to 250 Angstrom but for X=0.5 the value increased to D-p=700 Angstrom. For X=0.5 and 0.6 there is a bimodal distribution centered at 25 and between 200 and 700 Angstrom, respectively. For X=0.8 and 1.0 the interval moves from 200 to about 1000 Angstrom. The acid sites of the series Al2O3-ZrO2 were titrated with n-butylamine and NH3, showing that higher acid sites densities (DAS) are obtained for X=0.20 and 0.50, i.e. 10.8 mu mol Bu m(-2) but ZrO2-rich solids with X=0.6 and 0.8 showed DAS values minor than 10%, while single ZrO2 phase was 50% more acidic, i.e. 15 mu mol Bu m(-2). For NH3 the highest DAS values were for ZrO2 and Al2O3, then for X=0.2 and 0.8 the total amount of NH3 adsorbed was 1.78 and 1.8 mu mol (NH3) g(-1), respectively. The reaction of 2-propanol at 453 K on the Al2O3-ZrO2 series showed for X=0.20, i.e. a conversion of 4 mu mol h(-1) m(-2), against 2.32 and 1.16 for the single Al2O3 and ZrO2 phases, respectively. The selectivity towards C-3(=) at 453 K (S-p) Was superior for the extremes having X=0.20 and 0.80, i.e. 87 and 100%, respectively, while for acetone (S-A) there is a maximum at X=0.6 and for di-isopropyl-ether (S-E) the alumina (X=0) showed the highest selectivity, i.e. 18%. At higher temperatures (i.e. 513 K) the selectivity towards C-3(=) was predominant, i.e. 91-98%. (C) 2000 Elsevier Science B.V. All rights reserved.