Fourier-transform infrared, scanning electron microscopy and magic-angle-spinning nuclear magnetic resonance studies of silicalite synthesized in non-alkaline media

Abstract

The crystallization of silicalite prepared using the ‘fluoride method’ has been examined using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), ²⁹Si magic-angle-spinning (MAS) and ¹³C cross-polarization (CP/MAS) NMR. The aim was to obtain information about the composition and structure of the solid at different stages of crystallization. FTIR monitors the transformation of the amorphous gel into the crystalline zeolite phase. The spectrum of the final product is much better resolved than earlier spectra. ²⁹Si MAS NMR shows the details of structural ordering which takes place in the solid as the synthesis mixture is aged for progressively longer periods, and ¹³C CP/MAS NMR gives well resolved spectra of the occluded TPA⁺ cations. All carbon signals are split, an effect not found in silicalite prepared under alkaline conditions. The spectra do not change significantly with crystallization time, indicating that silicate microcrystallites are formed at an early stage of the synthesis even though X-ray diffraction detects no crystalline material. It appears that no structural reorganization of the solid phase takes place in the course of the synthesis.