Thermal stability of alumina aerogel doped with yttrium oxide, used as a catalyst support for the thermocatalytic cracking (TCC) process: An investigation of its textural and structural properties

Undoped and yttria doped alumina aerogel are prepared by the (solution) sol-gel method using a supercritical drying technique. The doping concentrations are in the range of 2.5-20.0 wt.% Y2O3, using yttrium(III) nitrate as a precursor. The effect of yttria doping on the structural and textural properties of alumina aerogel is investigated by BET surface area measurement, X-ray diffraction (XRD), TEM and Al-27 MAS NMR. This method of preparation of alumina acrogel provides the most important increases in surface area at 750 degrees C and up to 1000 degrees C, when compared to conventional alumina that is obtained via thermal dehydration of boehmite. These results show that the much smaller size of crystallites of alumina aerogel is quite efficient to retard the sintering occurring at these high temperatures. Further improvements in thermal stability are obtained by incorporation of yttrium species during the aerogel preparation. The stabilizing effect goes through a maximum with a yttrium content of ca. 10 wt.%. The presence of yttrium significantly delays the lattice rearrangement into new phases, as normally experienced by the conventional alumina when the temperature increases. These yttrium species induce the formation of distorted tetrahedral Al sites. Such binding effect of Y to Al is clearly detected by Al-27 MAS NMR already at 750 degrees C. However, at 1200 degrees C, even the doped acrogel cannot withstand the combined effect of sintering and a-phase transformation, so that extremely low surface areas corresponding to those of alumina a-phase and segregated Y-Al2O3 are obtained. (c) 2006 Elsevier B.V. All rights reserved.