Evaluating and understanding the hydrothermal stability of alumina aerogel doped with yttrium oxide and used as a catalyst support for the thermo-catalytic cracking (TCC) process

Abstract

The current work presents results concerning the hydrothermal stability of yttria-doped alumina aerogel, which was prepared by the sol–gel method using a supercritical drying technique. The influence of atmospheric steam at 750 and 1000 °C on the rate of sintering of yttria-doped alumina aerogel with various yttria contents was investigated. BET N2 adsorption measurements, X-ray diffraction, thermogravimetric analysis, FTIR of adsorbed pyridine, and 1H and 27Al MAS NMR were used for characterization of solid samples. It was found that the hydrothermal stability of yttria-doped alumina aerogel was greatly improved compared with alumina aerogel and conventional alumina. This was evident from the decrease in the loss of surface area and pore volume, and the minimization of structural transformations. These significant improvements were attributed to the combined effect of sol–gel method and the presence of yttrium ions. Yttria-doped alumina aerogel with various yttria loadings, when compared with undoped alumina aerogel, exhibited lower concentrations of surface –OH groups, higher amounts of bridged –OH groups, and lower degrees of crystallinity with smaller particle sizes. It was also observed that the thermal and hydrothermal stabilities of both yttria-impregnated alumina aerogel and conventional alumina were improved but to a much lower extent. These improvements were attributed to the presence of yttria, and subsequently the formation of surface yttrium aluminate. This structure was found to be more thermally stable than bare alumina owing to the presence of less surface defects and –OH groups. Therefore, it was concluded that to observe the most effective role of yttria, it is quite essential to incorporate it into the alumina structure via the sol–gel technique.Key words: thermo-catalytic cracking (TCC), yttria-doped alumina aerogel, yttria-impregnated alumina, hydrothermal stability, surface –OH groups, Lewis sites.