Investigation of Silicalite-1 Crystallization Using Attenuated Total Reflection/Fourier Transform Infrared Spectroscopy

The crystallization of silicalite-1 in aqueous sols has been investigated using ATR/FTIR spectroscopy as a function of sol composition. The existence of oligomeric siliceous species has been verified for all compositions after the end of tetraethoxysilane (TEOS) hydrolysis. On the other hand, more condensed and highly cross-linked siliceous structures have been observed only at the sols of high SiO 2 concentrations. These structures have been considered to be the “primary” nanoparticles reported by other researchers, and their IR spectra revealed that their structures are different from that of the silicalite-1 crystals. The evolution of the nanoparticle structure as a function of crystallization time has been monitored. It has been found that, upon heating of the synthesis sols, the “primary” nanoparticles transform to a new form that has an increased number of SiOSi bonds. These “intermediate” particles do not have the structure of silicalite-1, and they are present for most of the crystallization. Finally, once the silicalite-1 nuclei are formed, they grow in a suspension of these “intermediate” particles.