Thermal and hydrothermal stability of pure and silica-doped mesoporous aluminas

Thermal and hydrothermal stabilities of solvent deficient derived pure alumina (PA) as well as silica-doped aluminas (SDA) with three different dopant concentrations (5, 15, 27% by weight) have been investigated for changes in phase and pore structure after calcination at high temperatures under air and steam/air atmospheres. For all aluminas, heat treatment results in decreased surface area and increased pore diameter. Starting at 700 degrees C, metastable to stable phase transformation occurs, causing a substantial change in the pore structure of all aluminas. By 1100 (PA), 1200 (15 SDA) and 1200 degrees C (27 SDA), the transition to alpha-Al2O3 was complete, respectively. No alpha-Al2O3 is observed for 5 wt % silica doped alumina (5SDA) under thermal calcination. Calcination in steam and air, instead of in air only, results in enlarged pore size and depressed temperature of alpha phase transformation for all aluminas. Results show that the silica dopant modifies the pores of the alumina, stabilizes the oxygen lattice of gamma-Al2O3, and retards alpha-Al2O3 formation. This study of the thermal and hydrothermal stability not only suggests the applicability of 5SDA for catalytic reactions occurring at high temperature in the presence of steam, but also contributes to a better understanding of the structural features responsible for the improved thermal stability of these aluminas.