Organic-Free, ZnO-Assisted Synthesis of Zeolite FAU with Tunable SiO 2 /Al 2 O 3 Molar Ratio.

Zeolite FAU with tunable SiO2 /Al2 O3 molar ratio has been successfully synthesized in the absence of organic structure-directing agents (OSDA). Specifically, the addition of zinc species contributes to the feasible and effective adjustment of the framework SiO2 /Al2 O3 molar ratio between about 4 and 6 depending on the amount of zinc species added in the batch composition. In contrast, a typical OSDA such as tetramethylammonium hydroxide (TMAOH) has a limited effect on the SiO2 /Al2 O3 molar ratio of the zeolite. The role of zinc species is essential for the crystallization of zeolite FAU with a higher SiO2 /Al2 O3 molar ratio under the particular synthesis conditions. It is speculated that zinc species may suppress the incorporation of aluminum into the aluminosilicate framework, which is due to the Coulombic repulsive interaction. A higher SiO2 /Al2 O3 molar ratio is also found to be accompanied by a lower CO2 adsorption heat for CO2 /CH4 separation.