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A Structural Resolution Cryo-TEM Study of the Early Stages of MFI Growth

Sandeep Kumar, Zhuopeng Wang, R. Lee Penn and Michael Tsapatsis
Journal of the American Chemical Society, 130 (51), 17284, 2 Dec 2008

Understanding the nucleation and growth phenomena in solution is of fundamental and practical significance for the synthesis of various materials including zeolites.  In this context, the synthesis of all-silica zeolite with MFI-type framework (referred to as MFI) has been studied extensively as a model system.  Of fundamental importance is the role of nanoparticles (ca. 5 nm) present in the precursor sol in nucleation and crystallization Recently we proposed a mechanism in which the nanoparticles evolve towards the zeolite crystal structure, which can then contribute to crystal growth by aggregation.1, 2  However, information regarding the structure of the evolving nanoparticles, nuclei and early aggregates is limited since they are vaninshingly small components of the sols at the early stages of growth and due to the challenges of sample preparation and characterization.

high-resolution cryo-TEM image
Figure: A high-resolution cryo-TEM image of a representative crystal in synthesis sol aged for 220 days at room-temperature.  Fast Fourier transform (FFT), shown in the inset, indicates the crystal to be oriented either along [100]- or [010]-axis.  The bright and dark dots within the high-lighted perimeter represent the straight or sinusoidal pores of the MFI framework structure.  Image obtained using the FEI Tecnai G2 F30 Cryo-TEM.
In this communication, the authors present a cryogenic transmission electron microscopy (cryo-TEM) study on precursor sols prior to and during the early stages of MFI formation.  Cryo-TEM images with structural resolution were obtained and yielded new insights in MFI growth.  The importance of this study is two-fold.  First, the authors provide evidence supporting the recently proposed mechanism of evolution of nanoparticles followed by aggregative crystal growth while adding a new element.  The new element is the formation of predominantly amorphous aggregates before MFI crystallization and points to the importance of intra-aggregate rearrangements in nucleation and growth.  Second, the authors demonstrate that electron-beam sensitive materials such as zeolites can be imaged by cryo-TEM with structural resolution in their parent sols.  Similar studies for other zeolites under different conditions may reveal useful structural information for the understanding of hydrothermal nucleation and growth.

References:

1. Davis, T. M.; Drews, T. O.; Ramanan, H.; He, C.; Dong, J.; Schnablegger, H.; Katsoulakis, M. A.; Kokkoli, E.; McCormick, A. V.; Penn, R. L.; Tsapatsis, M., Nat. Mater. 2006, 5, 400-408.
2. Kumar, S.; Davis, T. M.; Ramanan, H.; Penn, R. L.; Tsapatsis, M., J. Phys. Chem. B 2007, 111, 3398-3403.

Acknowledgements:

Financial support was provided by the NSF (NIRT-CMMI-0707610, -CBET-0522518 and the MRSEC Program: DMR-0212302).  Characterization was carried out at the Characterization Facility, University of Minnesota, which receives support from NSF through the NNIN.  ZW received support from China Scholorship Council (2007) for his stay at the University of Minnesota.