04/24/2009
A Structural Resolution Cryo-TEM Study of the Early Stages of MFI Growth
Recent research from the research group of Professor R
Lee Penn.
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 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.
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:
- 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.
- 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.
|
