Mechanism on solvent-free crystallization of NaA zeolite

Abstract

It is of great importance to deeply understand the crystallization mechanism of zeolite in both fundamental research and industrial applications. Here we have systemically investigated the crystallization of zeolite NaA using hydrated silica (SiO2·3H2O) and sodium aluminate solids in the absence of water solvent, which is well characterized by X-ray diffraction (XRD), scanning electron micrography (SEM), UV-Raman, and 27Al MQMAS and 29Si MAS NMR techniques. The XRD patterns show that the complete crystallization requires for 5 h at 80 °C. The SEM images suggest that the nucleation mainly occurs on the surface of solid particles. The UV-Raman spectra exhibit that a large amount of the zeolite building units of four-membered rings (4R) formed in the starting solid particles, which even occurs before the crystallization, while crystal growth is always along with the formation of zeolite building units of six-membered rings (6R). The 27Al 2DMQMAS NMR spectra show that the D4R species are dominant in the precursors for crystallization of NaA. According to these results, we believe that the rearrangement of zeolite building units such as aluminosilicate D4R plays a critical role for the crystallization of zeolite NaA in the absence of water solvent.