Chemistry of Preparation of Alumina Aerogels in Presence of a Complexing Agent

Transition alumina with both high specific surface area (SSA) and quite important pore volumes deserve some interest in the field of heterogeneous catalysis as such alumina can be used as supports for different active phases, namely those used in hydroprocessing reactions. Alumina has been synthesised by a sol-gel method for which a key parameter, examined in this study, is the introduction of chelating agents such as butan-1,3-diol or acetylacetone (Hacac) in the aluminium-tri-sec butylate (ASB, the chosen Al precursor) solution. The study of the complexation chemistry for a complexing agent/Al alkoxide molar ratio (C) equal to 1, 2 and 3 was carried out. Some species in initial solutions were identified by IR and 27Al NMR spectroscopies, and the dried solids were characterised by the combined use of elemental analysis, XRD and IR spectroscopy. After hydrolysis of the precursor, the dried solids exhibited the boehmitestructure, more or less crystallised whatever the C ratio when butan-1,3-diol was used whereas a formulation Al(acac)(OH)2 was obtained when Hacac was introduced with a ratio C = 1. It has been shown that the increase in the C ratio up to 2 or 3 induces the precipitation of well-defined Al(acac)3 particles before hydrolysis. This compound was also present in small quantities in the dried gel for C = 1. After calcination at 500°C, more or less well crystallised γ-alumina was obtained. Compared to those of the reference alumina, the textural parameters are strongly modified by the use of a complexing agent. Moreover, Hacac has a stronger effect than butane-1,3-diol on the textural characteristics of the support.