In situ Observation of Nucleation and Crystal Growth in Zeolite Synthesis. A Small-Angle X-ray Scattering Investigation on Si-TPA-MFI

The formation and consumption of nanometer scale precursor particles during the hydrothermal synthesis of Si-TPA-MFI from a clear solution has been studied in situ using a combination of X-ray scattering techniques and with electron microscpy. The combination of wide-, small-, and ultra-small-angle X-ray scattering allowed us to obtain information on a continuous range of length scales spanning over four orders of magnitude (0.17−6000 nm), covering all particle populations present during the complete course of the crystallization process. The use of high-brilliance synchrotron radiation allows us to perform time-resolved experiments. Two types of precursor particles were observed: 2.8 nm sized primary units and aggregates (≈10 nm). Variation of the alkalinity of the synthesis mixture revealed a strong correlation between the concentration of the aggregates and the rate of the crystal nucleation. The presence of the 2.8 nm sized primary units appears to be independent on the alkalinity. The addition of seed crystals to a synthesis mixture that does not show spontaneous nucleation (no aggregates observed) resulted in normal crystal growth. The size distribution of the growing crystals could be followed in situ by fitting calculated scattering patterns to experimental curves and showed good agreement with electron microscopy results. The apparent activation energy for crystal growth is determined to be 83 kJ/mol by following in situ the crystal growth process at various reaction temperatures. These data show that the formation of aggregates of primary units is an essential step in the nucleation process and suggest that the crystal growth step is the reaction-controlled inclusion of the 2.8 nm sized primary units at the crystal surface.