Effectiveness of the tetramethylammonium size-modifier in the synthesis of faujasite nanocrystals

This paper reports the effects of alkalinity, Si/Al ratio and amount of tetramethylammonium (TMA) on the formation of nanocrystals of faujasite. The presence of TMA allows to form faujasite crystals smaller than 0.1 μm. In a wide range of composition, there is no proportionality between the concentration of TMA in the synthesis system and the size of the zeolite crystals formed. TMA concentrations as low as 0.05 mmol L−1 are enough to induce the formation of crystals of 30 nm. The role of TMA in the formation of nanocrystals corresponds to a shift of the balance between nucleation and growth kinetics due to the adsorption of organics at the surface of the nuclei of zeolite. The field of composition of the faujasite crystals shifts from zeolite Y to zeolite X according to the alkalinity of the synthesis system. As expected, the effectiveness of silicon incorporation decreases with the alkalinity of the synthesis system. However, the effectiveness of silicon incorporation significantly increases with the content of TMA, suggesting that the large TMA cations contrast the depolymeryzing effect of alkalinity on the silicate species implied in the formation of the zeolite. This effect is modulated by the concentration of silicate in the synthesis system.