SAXS and USAXS investigation on nanometer-scaled precursors in organic-mediated zeolite crystallization from gelating systems

The formation of precursor particles in the crystallization of zeolites from gelating systems has been studied using X-ray scattering at small angles. The crystallization of Si-MFI using trimethylene-bis(N-hexyl, N-methyl-piperidinium) as the structure-directing agent shows the formation of two categories of precursors: gel particles and nanometer-scaled primary units. The size of the primary units for the crystallization of Si-MFI is found to be 2.8 nm both for gelating and nongelating systems using different structure-directing agents. Zeolites Si-BEA and Si-MTW have been prepared using the same organic (trimethylene-bis(N-benzyl, N-methyl-piperidinium)) at different concentrations. Primary units with a size of 2.6 nm are found to be present in the synthesis of Si-BEA, while their size is 1,5 nm in the synthesis mixture directing to Si-MTW. Our data suggest that the nanometer-scaled primary units are specific for the zeolite topology formed.