Résumé
Tailoring of physicochemical properties of synthetic zeolites is of paramount importance when these materials are applied as heterogenous catalysts. In this work, the effect of the synthesis system and in particular of the organic structure directing agent (OSDA) on acidity, crystal morphology, and catalytic behavior of FER-type materials is studied. FER-type zeolites with similar Si/Al ratio and crystal size range 0.3-15 µm are synthesized by changing OSDA type or by adding surfactant. A crystallization kinetic analysis shows that the stability of the zeolitic phase strongly depends on the OSDA type. In particular, the crystallization time strongly affects the purity of the FER phase when tetrahydrofuran is used as a template. Furthermore, it is found that long amines as 1,8-diaminooctane can play a role of OSDA obtaining highly crystalline FER-type crystals and with a high Lewis acid sites concentration. FT-IR and 27 Al-NMR analyses show that all of the synthesized samples possess different Lewis acid sites concentration associated with extra-framework Al-OH species that are not detected by NMR analysis. Catalytic tests in methanol-to-dimethyl ether reaction reveal that synthesized samples exhibit different catalytic behavior. In particular, large (10-15 µm) FER-type catalyst synthesized with tetrahydrofuran suffer deactivation caused by a higher carbon deposit level. On the contrary, nano-sized FER-type crystals, prepared by using pyrrolidine as OSDA and a surfactant, exhibits high selectivity and considerable resistance to coke deposition.