Nanoscale Reactor Engineering: Hydrothermal Synthesis of Uniform Zeolite Particles in Massively Parallel Reaction Chambers

This communication highlights the importance of controlling surface interactions and processing conditions when zeolites are synthesized hydrothermally within the confinement of a nanoporous reactor (here, three-dimensionally ordered macroporous carbon or 3DOM C).  Nanocasting processes are becoming very important for preparations of nanostructures (including zeolites) with controlled architecture.  Typically products are assumed to take on the shape of the mold used for nanocasting.  We show in our paper that the template is not so inert and that product morphologies can depend on gel composition and several other parameters (especially surface charges) that can be engineered into a hydrothermal reaction system under confinement.  We observed several interesting product morphologies for zeolite that was hydrothermally grown within macropores of inverse opal carbon, including solid spheres with corrugated surfaces, with smooth surfaces, hollow geode-like structures and needles.  These can be grown in high yield (i.e., all macropores filled) depending on specific conditions.  Concentration gradients across a monolithic porous reactor also produced morphology gradients (a kind of combinatorial synthesis).  Finally, recovery of either colloidal crystals of the zeolite spheres (materials with hierarchical porosity) or discrete, monodisperse zeolite particles was possible by calcination of the carbon reactor.  This nanoreactor engineering approach promises to bring improved control over the morphology of materials prepared by confined syntheses, and the design principles should also be applicable to hydrothermal syntheses of other materials.

Figure: SEM images illustrating the different morphologies of silicalite products obtained after hydrothermal syntheses in 3DOM nanoreactors for varying processing conditions.  The effect of repeated infiltration/hydrothermal reaction (IHT) cycles is shown in the sequence (a, b, c) for 1, 2 and 4 cycles, respectively.  The images show core cross-sections of 3DOM C monoliths with an outermost anionic polyelectrolyte layer for reactions with a high silica concentration.  To illustrate the difference in product morphology between core and edge regions of the 3DOM C monolith, corresponding edge cross-sections are shown for 1 (d) and 2 (e) IHT cycles.  The effect of the charge of the outermost polyelectrolyte layer on particle growth is shown in the sequence (c, f).  Dense spheres were produced in 3DOM C reactors with an anionic outermost polyelectrolyte (c).  Hollow geode structures of silicalite particles formed after multiple IHTs in the confinement of 3DOM C with a cationic outermost polyelectrolyte (f).  The effect of reducing the silica precursor concentration ("low concentration") is shown in the SEM images (g) after 5 IHT cycles, (h) after 7 cycles and (i) after 12 cycles for silicalite prepared in 3DOM C with an anionic outermost polyelectrolyte layer.  Scale bars: 500 nm.

Acknowledgements:
Funding was provided by the NSF (mainly by CMMI-0707610 and in parts by DMR-0704312, DMR-0212302 and CBET-0522518) and the Petroleum Research Foundation, administered by the American Chemical Society (ACS-PRF #42751-AC10).  Parts of this work were carried out in the Institute of Technology Characterization Facility, University of Minnesota, which receives partial support from NSF through the NNIN program.

CharFac Offers New Instrument

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