Hydrothermal synthesis of alkali-free chabazite zeolites

Alkali-free siliceous CHA zeolites with a wide Si/Al ratio ranging from 11 to infinity have been synthesized in hydroxide and fluoride media using either N,N,N-Trimethyl-1-adamantammonium (TMAda), N,N,N-Dimethylethylcyclohexyl ammonium (DMECHA) cations, or a mixture thereof. TMAda, in combination with other low-cost organic templates, allowed the formation of pure silica CHA without any impurities in the synthesis gel with an H2O/SiO2 ratio of 3. Single-phase crystalline pure silica CHA was produced by DMECHA within a very narrow range of H2O/SiO2 ratio, and small deviations in the water content from this range led to the formation of a slightly less dense zeolite beta phase. Alkali-free siliceous CHA zeolites possessing a high crystallinity and a variety of particle sizes were successfully prepared for the first time using a low-cost DMECHA template, particularly by a direct interzeolite transformation of FAU zeolites having the same secondary building unit of double-6-rings (D6Rs). This makes DMECHA an attractive organic structure-directing agent (OSDA) for wide applications of CHA zeolites as catalysts and adsorbents.