Title : Hydrothermal synthesis of (Al)ZSM 5 and (Ga)ZSM 5 type zeolites
Abstract:
Zeolites are crystalline aluminosilicates, composed of TO4 tetrahedra (T = Si, Al) with O atoms connecting neighboring tetrahedral, that contain pores and cavities of molecular dimensions. Many occur as natural minerals, but it is the synthetic varieties which are among the most widely used sorbents, catalysts and ion-exchange materials in the world. The isomorphous substitution of Si4+ or Al3+ by Ga3+ is interesting from practical point of view. Catalytic processes for fine chemical productions and oxidation reactions are amenable to the employment of tuned and novel zeolites. The phenomenon of isomorphous substitution is well-known in the field of mineralogy. By isomorphous substitution, framework atoms of crystalline compounds are replaced by atoms of other elements without changing the type of the crystal structure.
This study reported on the hydrothermal synthesis of AlZSM-5 and GaZSM-5 zeolite from systems containing tetrapropylammonium bromide as an organic structure determining agent. The crystallization conditions were optimized by varying the heating time for both systems. AlZSM-5 was obtained from a system 50SiO2 : Al2O3 : 5.0Na2O : 5.0TPABr : 2000H2O after 72 hours at a crystallization temperature 170oC. Pure GaZSM-5 is obtained from an initial gel 30SiO2 : Ga2O3 : 4.4Na2O : 18TPABr : 1000H2O after 114 hours and crystallization temperature 150oC. Post synthesis treatment is applied in order to increase the specific surface area and providing easy access to the active zeolite centers. Secondary pores in crystals were formed by etching at room temperature with solution of NH4F and 0,25? HF acid, varying the time of treatment. All samples have been characterized by X-ray diffraction analysis, scanning electron microscopy (SEM), infrared spectroscopy (IR), solid-state nuclear magnetic resonance (NMR) spectroscopy and physical absorption-desorption of nitrogen.
