Solvent-free synthesis of zeolites: new insights into the mechanism and non-mechanochemical route

The solvent-free (SF) synthesis is a green alternative to the conventional method of synthesis for zeolites. In this study, we show for the first time the feasibility of carrying out the SF synthesis of silicalite-1 (F- route) without any grinding. We also monitor the changes during the synthesis thanks to multiple characterization techniques: XRD, TGA, SEM, EDX, and multinuclear NMR (13C, 19F, 29Si). We show how the duration of the thermal treatment impacts the zeolite crystal size and morphology, as well as the local degree of order probed by solid-state NMR. The obtained results provide new insights on the SF formation mechanism based on solid phase transformations. Our findings also show that once the organic structure directing agent (OSDA), here tetrapropylammonium, is incorporated, the crystals are formed and no other intermediate phases incorporating the OSDA exist. Further, we study the role of water in the synthesis using 17O NMR. For this purpose, we develop a simple and efficient 17O enrichment method of zeolite frameworks. The efficiency of this enrichment demonstrates the role of water in the hydrolysis and condensation reactions and its need for a successful synthesis.