Study on the Ti and Al coincorporation into the MFI zeolitic structure
The coincorporation of Ti and Al into the MFI zeolite framework has been investigated using a method based on the wetness impregnation of amorphous Al2O3–TiO2–SiO2 solids with TPAOH solutions and subsequent crystallization under autogenous pressure. The influence of the precursor addition sequence and the conditions during the amorphous cogel formation on the properties of the zeolites obtained have been studied. The Al content in the synthesized zeolite is similar to that of the raw cogel, whereas the Ti content is lower and depends on the degree of Al incorporation in the starting material. The distribution of Al and Ti in the Al-TS-1 samples depends not only on the initial Al content but also on the procedure to prepare the raw cogel. The activity for oxidation process of the Al-TS-1 samples prepared has been tested in n-hexane oxidation. The progressive incorporation of Al into the framework increases the oxidation capacity of Ti atoms compared to TS-1 until a maximum is reached (Al/Ti molar ratio: 0.70) whereas further Al incorporation decreases the catalytic activity to even lower values than in TS-1. This effect has been assigned to changes in the hydrophilic‘vs70,21s\‘vs21,20s\/v\hydrophobic character of the catalyst caused by the interaction between Al and Ti centers, which strongly affects the affinity towards the different compounds in the reaction mixture.