Stepwise Gel Preparation for High-Quality CHA Zeolite Synthesis: A Common Tool for Synthesis Diversification

Aluminosilicate molecular-sieve zeolites are widely used in industrial processes, mostly as catalysts or adsorbents, and advances in the synthesis are still required in order to progress further applications. To further expand the zeolite synthesis system, herein we report the stepwise preparations of synthesis gels through divided compositional control; this strategy is applied to the effective synthesis of a CHA zeolite in the presence of an inexpensive organic structure directing agent (OSDA), benzyltrimethylammonium. Highly crystalline nanosized CHA zeolites were prepared at higher crystallization rates compared to those prepared using the conventional one-step gel-preparation method. The stepwise method also provided CHA zeolite in a wide range of starting gel compositions, and aluminum content within the formed CHA could be tuned. The aluminosilicate-formation process, investigated by the combination of analytical methods including X-ray diffractometry, NMR and Raman spectroscopy, and electrospray-ionization mass spectrometry (ESI-MS) and several synthetic experiments, revealed that the aluminosilicate cluster that forms in the highly alkaline aluminum-rich intermediate gel during the stepwise method helps to effectively form the CHA zeolite in the final synthesis step. We also demonstrate that this strategy is applicable to other synthesis systems with different OSDAs and target zeolites. Furthermore, the stepwise method provides efficient zeolite catalysts with high activities and durabilities for emission-gas purification applications. This sharable concept is expected to become a common tool that brings additional synthetic diversity to a variety of zeolite-synthesis systems.