(388h) Synthesis of MOF-74(Ni) Using Segmented-Flow and Microwave-Assisted Methods with Chemical Modulation

The control of nanoparticle uniformity is one of the main challenges in nanoparticle synthetic methods. Particle uniformity often dictates both the physical and chemical properties of the final products. Batch synthesis methods, such as hot injection and extended LaMerâ??s method, lead to formation of products with very similar reaction history and consequent narrow particle size distributions.^1,2 Larger quantities of materials are required for industrial and precise control over size, and materials properties, becomes much more difficult. New scalable technologies are required for the precise control of nanomaterials during synthesis, and in this presentation we present a novel approach where we physically and temporally separate the nucleation and growth processes. For nucleation we use microwave radiation to minimize thermal gradients in the precursor solution while promoting high utilization of reagents with short reaction times.^3 A separate growth bath provides uniform heating of the nuclei during the growth stage, while segmented flow provides homogeneous mixing and constant reaction times throughout the process. A detailed understanding of each reaction step during colloidal nanoparticle synthesis is critical to obtain materials with the desired properties. There are several consecutive stages during nanoparticle synthesis, including: activation of precursors, formation of nuclei, growth of nanoparticles from the nuclei, and isolation of nanoparticles with the desired properties. The separation of the nucleation event from the growth stage is very important to the formation of nanoparticles with homogeneous size distributions. Using microwave radiation for fast and uniform heating for the nucleation zone, and using an oil bath at lower temperatures for the growth zone permits the distinct separation of nucleation processes from growth events. We will review the use of a segmented-flow, microwave-assisted reactor for the synthesis of a range of nanoparticles, including semiconductor nanoparticles for solar cells, quantum dots for lighting and display applications, as well as metal organic frameworks.^3â??6 A common characteristic in all the nanoparticle systems studied, was narrower nanoparticle size distributions, substantially shorter reaction times, and high utilization of reagents.

 The segmented-flow, microwave-assisted approach can be used to potentially support production of nanomaterials which still face commercialization difficulties due to the lack of scalable synthetic approaches, including metal-organic frameworks (MOFs).^7 MOFs are of considerable scientific and technological interest due to the flexibility in modifying their properties by changing either metal coordination species and/or organic linkers. MOFs can have three dimensional long-range order, high-surface area, large internal free volume space, and tunable adsorption properties. MOFs may be used in a wide range of applications including: gas storage, separation, catalysis, and more recently pharmaceuticals. However, the lack of scalable synthetic approaches still limits MOF production at industrial scales. In this study, we demonstrate the synthesis of MOF-74(Ni) using a segmented-flow, microwave-assisted reactor. The reaction chemistries were optimized so that the synthesis can be performed under relatively mild conditions. High yields (> 95 %) were obtained for short reaction times (~15 minutes), as opposed to days for typical batch reaction conditions. Optimization of reaction parameters for the microwave reactor has led to improvements in MOF crystallinity, conversion efficiencies, and production rates (> 4 g/h). We have found that chemical modulators significantly reduce the particle size distribution, increase the effective particle size, and enhance the adsorption properties of the MOFs. Characterization of the MOFs was performed using powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, BET isotherms, and UV-Vis spectroscopy to evaluate effects of microwave temperature and chemical modulator concentrations on MOF-74(Ni) properties.

References

1. Vreeland, E. C., et al., "Enhanced Nanoparticle Size Control by Extending LaMerâ??s Mechanism," Chem. Mater., 27,pp. 6059â??6066 (2015).

2. Thanh, N. T. K., et al., "Mechanisms of Nucleation and Growth of Nanoparticles in Solution," Chem. Rev., 114,pp. 7610â??7630 (2014).

3. Kim, K.-J., et al., "Continuous Microwave-Assisted Gasâ??Liquid Segmented Flow Reactor for Controlled Nucleation and Growth of Nanocrystals," Cryst. Growth Des., 14,pp. 5349â??5355 (2014).

4. Hostetler, E. B., et al., "Synthesis of colloidal PbSe nanoparticles using a microwave-assisted segmented flow reactor," Mater. Lett., 128,pp. 54â??59 (2014).

5. Albuquerque, G. H., et al., "Gasâ??liquid segmented flow microwave-assisted synthesis of MOF-74(Ni) under moderate pressures," CrystEngComm, 17,pp. 5502â??5510 (2015).

6. Fitzmorris, R. C., et al., "Structural and optical characterization of CuInS₂ quantum dots synthesized by microwave-assisted continuous flow methods," J. Nanoparticle Res., 17,(2015).

7. Chung, Y. G., et al., "Computation-Ready, Experimental Metalâ??Organic Frameworks: A Tool To Enable High-Throughput Screening of Nanoporous Crystals," Chem. Mater., 26, pp. 6185â??6192 (2014).