(395g) Electrochemical Synthesis of Metal-Organic Framework Based Microseparators

Electrochemical synthesis of metal-organic framework based microseparators

Tom R.C. Van Assche^a, Nicolo Compagnol’^b, Jan Fransaer^b, Joeri F.M. Denayer^a

^a Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium
^bDepartment of Metallurgy and Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium

Microstructured devices offer the possibility of enhanced mass and heat transfer. Combining these devices with adsorbents allows for faster adsorption/desorption cycles using less material and enhanced chromatographic separations. Incorporating tailormade metal-organic frameworks (MOF) in such devices opens up new applications for specific separation problems, or by making use of their catalytic activity (i.e. unsaturated metal sites). In contrast to their inorganic counterparts (zeolites), MOF can be synthesized using a variety of synthesis strategies. From microdevice fabrication point of view, an interesting method is oxidative electrochemical synthesis of MOF.             
During oxidative electrochemical synthesis, part of a metal electrode is electrochemically oxidized to form cations [1]. These cations subsequently coordinate with the linker present in the synthesis mixture. Advantages are the low temperature, short synthesis times, good atom efficiency and high process controllability. Under suitable conditions, the MOF can be grown as a well adhered and patterned film on the electrode, rather than a powder in the synthesis mixture. In this work, electrochemical MOF synthesis is combined with a facile microdevice fabrication technique to create an adsorption based microseparator [2]. This device is subsequently shown to outperform conventional packed bed technology in terms of adsorption kinetics.          
A patterned MOF layer is achieved by either starting form a patterned conductive metal electrode, or more simply by using a patterned non-conductive mask on a metal electrode surface. By utilizing a self adhering patterned PEEK mask, microchannels can be produced, where the critical dimension is the PEEK layer thickness (200 or 500 µm). After 20-60 minutes of electrochemical synthesis, the masked electrode sheets can simply be clamped in a holder, utilizing the secondary function of the PEEK, as a gasket. The result is a rapid and facile method to produce a MOF-based microseparator, which was tested to 130°C and beyond.       
For copper- benzene-1,3,5-tricarboxylate (HKUST-1), synthesis times are short and the synthesis can be performed at temperatures as lows as 30°C. The morphology can be controlled to a high degree, by varying the applied electrical potential, synthesis solvent and water content of the synthesis mixture. This allows the synthesis of HKUST-1 crystals of >0.5 µm - 50 µm diameter. By varying the temperature, octahedral or cubic crystals can be obtained.            
For other MOF structures, well adhered layers are less evident to obtain by electrochemical synthesis. However, by combining the classical solvothermal synthesis methodology with an electrochemical cell, MIL-100 (Fe) coatings can be obtained on both electrochemically deposited (pure) iron, as well as steel [3]. Adsorption and separation properties of MIL-100 and HKUST-1 layers, were tested by fabrication of a microseparator device with a channel cross section of 2000 x 200 µm², and by performing breakthrough experiments.   
For HKUST-1, the methanol adsorption kinetics on the MOF microdevice were improved compared to a conventional packed bed column (pellet size >0.6 mm). Here, the advantages of the microseparator are the absence of any large macropore diffusion resistance, low pressure drop and good heat transfer due to the intimate contact between the MOF layer and the conductive support. This fabrication methodology provides a facile, rapid manner to create MOF based microseparators and microreactors.

Acknowledgements:
T.R.C. Van Assche is grateful to the Agency for Innovation by Science and Technology (IWT) Flanders for financial support.

[1] Ameloot, R., Stappers, L., Fransaer, J., Alaerts, L., Sels, B.F., De Vos, D.E., 2009. Patterned growth of metal-organic framework coatings by electrochemical synthesis. Chem. Mater. 21, 2580–2582.
[2] Van Assche, T.R.C, Denayer, J.F.M, 2013. Fabrication and separation performance evaluation of a metal–organic framework based microseparator device. Chem. Eng. Sci. 95, 65-72.
[3] Compagnol’, N., Van Assche, T., Boudewijns, T., Denayer, J., Binnemans, K., De Vos, D., Fransaer, J., 2013. High pressure, high temperature electrochemical synthesis of metal–organic frameworks: films of MIL-100 (Fe) and HKUST-1 in different morphologies. J. Mater. Chem. A 1, 5827-5830.