Surface characteristics and catalytic activity of Al-pillared(AZA) and Fe-Al-pillared (FAZA) clays for isopropanol decomposition

Fractionated Al-pillared (code name AZA) and Fe-Al-pillared (code name FAZA) clays were examined for the dependence of their specific surface area on the particle size. The results indicate a very weak dependence and a fractal dimension of D approximate to 3 for both materials, characteristic of highly porous solids. The a(s) plots of the fractionated particles show that the part of the surface area due to the mesoporosity plus the external surface, decreases as the particle size increases and possess a fractal dimension equal almost to 2.8. For both kinds of solids, AZA and FAZA as well as zeolite-Y, the acidity estimated by NH3 adsorption was found equal to 1.4 x 10(18), 2.2 x 10(18) and 2.1 x 10(18) sites per m(2) respectively. The experimental results of catalytic activity of AZA, FAZA and zeolite-Y, using the decomposition of isopropanol as a probe reaction in the range of 60 to 130 degrees C, suggest that AZA is catalytically more active compared to FAZA for the total conversion of isopropanol. The reaction rate calculated per g is one order of magnitude higher on the zeolite-Y, as compared to AZA and/or FAZA, due to its higher surface area but it is almost equal if the comparison is made per unit surface area. The products obtained were mainly propene and diisopropyl ether with a selectivity for propene laying between 0.8-0.9 irrespective of particle size of the solids or the temperature. On the contrary, zeolite-Y shows a high selectivity for propene (approximate to 0.9) only at low conversions while at higher conversion selectivity tends to 0.5. The kinetic investigation of the reaction showed single Arrhenius plots for AZA and zeolite-Y with apparent activation energies (E(app)) around 80-90 kJ/mol for AZA and 70 kJ/mol for zeolite. On the contrary, FAZA showed two distinct regions in the Arrhenius plots. One at low temperatures (T < 100 degrees C) with E(app) 25-35 kJ/mol and another at high temperatures (T > 100 degrees C) with E(app) approximate to 70-80 kJ/mol, depending on the fraction of the solid. This alternation of catalytic behaviour in FAZA was attributed to the extensive adsorption of water at higher conversions that leads to kinetic expression inhibited by the adsorbed water, while at lower conversions first order kinetic is proposed. AZA on the other hand as well as zeolite-Y shows kinetic behaviour inhibited by water adsorption even at very low conversions.