Synergy between structure direction and alkalinity toward fast crystallization, controlled morphology and high phase purity of ZSM-12 zeolite

Abstract

Aiming for a control of the morphology and phase purity of the final product, two commercially available organic templates, supplying tetraethylammonium (TEA+) and methyltriethylammonium (MTEA+) cations in the synthesis mixture, were used for the hydrothermal synthesis of ZSM-12 zeolite. As key factors, alkalinity and water content of the synthesis mixture and silicon and aluminum source types were comprehensively studied using these two organic templates. All the products obtained were characterized by powder XRD, SEM, N2 physisorption, 29Si and 27Al-MAS solid state NMR and TGA techniques. Using TEAOH organic template and organic Si and Al sources along with the proper adjustment of water and sodium hydroxide contents in synthesis mixture, mono-sized polycrystalline particles of ZSM-12 with high crystallinity and purity were synthesized. Alleviating TEAOH drawbacks, MTEA+ organocation in the hydroxide form was used which led to the formation of mono-sized ZSM-12 single crystals. When using MTEAOH organic template, hydroxide ion and Na+ contents and the type of Al source were found to be playing major roles toward nucleation and growth of ZSM-12 crystals. Owing to increased alkalinity of the synthesis mixture, MTEAOH also promoted faster crystallization over a wide-range of Si/Al ratio (30 to ∼infinity). The discrete mono-dispersed synthesized crystals possessed a well-defined cubic morphology with no impurities.