The metal organic framework, MOF-74IJNi), was synthesized in a continuous flow microwave-assisted reactor obtaining a high space-time yield (~90 g h −1 L −1 ) and 96.5% conversion of reagents. Separation of the nucleation and growth steps was performed by using uniform and rapid microwave heating to induce nucleation, which allowed a substantial increase in conversion for shorter reaction times under mild pressure. High yields were achieved in minutes, as opposed to days for typical batch syntheses, with excellent control over the material's properties due to more uniform nucleation, and the separation of the nucleation and growth steps. Optimization of the microwave reactor parameters led to improvements in crystallinity, reagent conversion, and production rates. Differences in MOF-74(Ni) crystallinity were observed as smaller grains were formed when higher microwave zone temperatures were used. Crystallinity differences led to different final adsorption properties and surface areas. Herein we show that a continuous high space-time yield synthesis of MOF-74(Ni) allows control over nucleation using microwave heating.
In
this study, microwaves and chemical modulators were used in
combination to improve the uniformity of synthesized MOF-74(Ni) materials.
A segmented flow, microwave-assisted reactor was used where rapid
nucleation in the microwave zone was separated from growth processes
in an oil bath. High temperatures in the microwave zone induce fast
nucleation, which can result in the formation of polydisperse products
through agglomeration that reduces the available surface area. Benzoic
acid was used as a chemical modulator where higher benzoic acid to
Ni2+ concentration ratios resulted in a substantial increase
in apparent reaction activation
energies for short reaction times (∼1 s). Optimization of the
synthetic process resulted in MOF-74(Ni) particles with high crystallinity,
high surface area, and narrow particle size distributions. These synthesized
MOF-74(Ni) particles were used to prepare metal–organic framework-Nafion
matrix based membranes to investigate the removal of metal ions from
aqueous solutions with high efficiencies.
The precise control of both the size and shape of Ag nanoparticles strongly influences their optical properties. Although the synthesis of Ag nanocubes with sharp corners and edges has been demonstrated, the ability to scale these approaches with high selectivity remains elusive. In this study, a continuous flow microwave-assisted reactor was used to separate nucleation from growth events, which provides a method to synthesize very uniform single crystalline Ag nanocubes. Nucleation in the microwave zone was enhanced through seed-mediated processes by sulfide formation, and a chemical regulator was used in the growth zone to further improve the sharpness of the edges of the nanocubes. Transmission electron microscopy and optical properties were used to optimize the reaction conditions and Ag nanocubes with edge lengths of 28 and 45 nm were readily synthesized with narrow particle size distributions and high selectivities (>70%). Nanocubes with 28 nm edge lengths were used to prepare films to demonstrate the detection of Rhodamine 6G with concentrations down to 10 nM using surface enhanced Raman spectroscopy. These results indicate that continuous flow approaches have the potential to produce large quantities of uniform Ag nanocubes that can be used for sensing or other applications.
Recent studies have indicated that nickel gallium alloys can be effective catalysts for the hydrogenation of CO2 to methanol. To simplify the characterization of NiGa catalysts, the authors are developing model systems using sputter deposited NiGa thin films. The NiGa thin films used in this study were deposited using an equimolar alloy target and annealed in ultrahigh vacuum. Atomic force microscopy (AFM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) were used to characterize the NiGa films before and after annealing. AFM results showed that film roughness and grain size significantly increased as the film was annealed above 700 °C. XRD patterns indicated that NiGa thin films were nanocrystalline as deposited and then transitioned to the Ni13Ga9 phase after annealing above 500 °C. XPS results indicated that gallium and oxygen segregated to the surface after annealing up to 600 °C, and formed a surface Ga2O3 layer. For anneals above 600 °C, the Ga2O3 XPS signal was reduced in intensity due to desorption/decomposition of Ga2O3 from the NiGa surface.
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