Phase selection of FAU and LTA zeolites by controlling synthesis parameters

The formation ratio of zeolite faujasite (FAU)/Linde type A (LTA) can be controlled in the presence of an organic structure-directing agent (SDA), tetramethylammonium cation (TMA+), by tuning synthesis parameters. The effects of aging process and the content of an inorganic cation on the phase selection of FAU/LTA are especially addressed. The initial composition used in the study is 10SiO2:2.53Al2O3:5.5(TMA)2O:xNa2O:570H2O. When x is higher than 0.3, pure LTA is synthesized, while FAU without any impure phase is obtained when x is lower than 0.08. No zeolitic phase is synthesized in the absence of Na+. The reason may be that the frameworks of FAU and LTA cannot occlude enough TMA+ to compensate for the negative charges of the aluminosilicate frameworks. Therefore, Na+, whose molecular size is much smaller than that of TMA+, has to be used as a countercation in the syntheses of FAU and LTA. The local environments and amounts of TMA+ in FAU and LTA are determined by thermogravimetric analysis equipped with mass spectroscopy (TGA/MS), differential thermal analysis (DTA), inductively coupled plasma mass spectrometry (ICP-MS) and 13C magic-angle spinning nuclear magnetic resonance (13C MAS NMR). It is revealed that each sodalite cage of FAU and LTA is occupied statistically by one TMA+, and TMA+ occluded in the supercage of FAU is more than that in the α cage of LTA.