Crystallization mechanism of Al-TS-1 synthesised from amorphous wetness-impregnated Al2O3–TiO2–SiO2 xerogels: role of aluminium species

Abstract

The crystallization mechanism of Al-TS-1 zeolite has been investigated using amorphous wetness-impregnated Al2O3–TiO2–SiO2 xerogels as raw materials. Samples with different degrees of crystallinity have been prepared and characterised by means of conventional techniques. The catalytic activity of the Ti and Al centres along with the crystallization have been tested by oxyfunctionalization of n-hexane with H2O2 and alcohol etherification reactions, respectively. The crystallization mechanism involves a partial dissolution of the initial xerogel, which is more pronounced for Si and Ti species, whereas Al species remain in a high percentage within the solid. Simultaneously, the starting polymeric cogel is converted into a particulate amorphous material formed by a tight packing of primary units, which are aggregated leading to secondary particles that become independent of the cogel as they reach a critical size of 0.5 μm. During this aggregation process, the crystallization is governed by solid–solid transformations without any incorporation of species from the solution. The last crystallization stage is characterised by a slow incorporation of different species from the solution accompanied with an increase in the solid yield and a complete incorporation of Al species, which results in a densification of the secondary particles and their transformation into zeolite crystals. In addition, the environment of Ti and Al atoms is modified by the crystallization affecting significantly their catalytic activity. The aluminium content in the starting xerogel has an important influence on the crystallization process, with enhancement of the Ti and Si species dissolution as the Al content increases in the raw material. Moreover, a higher participation of the solution-mediated process in the global zeolite crystallization and a decrease of the Ti content in the final crystalline materials are evidenced with increasing Al content in the initial xerogel.