Nucleation and crystallization of the MWW-type lamellar zeolitic precursor

Abstract

Abstract The nucleation and crystallization of the layered zeolitic precursor (P)MCM-22 with Si/Al gel ratio = 25 was studied by several characterization techniques such as X-ray Diffraction (XRD), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Infrared Spectroscopy (IR), Thermogravimetric Analysis (TGA) and Small-Angle X-ray Scattering (SAXS). The crystallization kinetics of the samples was firstly monitored by XRD identifying reflections from the sample obtained at 48 h. The ICP-OES analysis concluded that the Si/Al ratio was 23, which is very similar to the initial gel one. SEM analyses corroborated the XRD results observing the formation of smooth particles from 48 h of crystallization and, after 168 h, the formation of thin platelet-like particles. The IR results showed an increase of external silanol groups when the crystallization time increases, due to terminating silanol groups between zeolitic crystals, whereas the bands attributed to the structure directing agent- hexamethyleneimine (HMI) − increase up to 168 h. Finally, the SAXS results indicated the existence of primary nanoparticles population in the first stages of the reaction, allowing to follow their growth and transformation from spherical morphology to two-dimensional objects forming aggregates.