Abstract EN:

Transition aluminas, and especially gamma-alumina, are widely used as catalyst supports. Their synthesis requires a thermal activation step. This transformation is a key industrial process since it will determine the structural, textural and surface properties of these materials. Thus the development of innovative catalyst supports implies that this thermal activation process should be understood and mastered. The goal of this study was to understand and control the physicochemical processes involved during the thermal activation of boehmite. First, we studied the impact of boehmite crystallite morphology on the dehydration temperature. We showed that the dehydration temperature was strongly dependent on the size and shape of the boehmite crystallites. A thermodynamic model, taking into account the surface energy contribution, allowed to fully accounting for these experimental findings. For instance, the transformation temperature increases from 260 to 360°C when diamond shaped (50/50 basal/lateral) crystallite grew up from 3 to 10 nm. Furthermore we studied the effect of steam on the kinetics of the transformation. The reaction order with respect to water was negative which was expected because steam is a product of the reaction, and it changed with the extent of reaction. Finally we applied these results to the optimization of the thermal activation step for hydrotreating catalyst supports, which enabled to lower the activation temperature from 540 to 450°C. These results were validated by catalytic tests showing that only catalysts supported on boehmite heated at 450°C had similar activities as the reference catalyst evincing the thermal activation control benefit.