Percolation-assisted coating of metal-organic frameworks on porous substrates

Abstract

In the past two decades, significant advancements have been made toward thin film fabrication of metal-organic frameworks (MOFs) on porous solid substrates for gas separation and catalysis applications. To enhance the physical and chemical stability of the films, various top-down and bottom-up approaches have been implemented. While both approaches have some advantages, they are mostly limited due to a lack of adhesion with the substrate, cracking of the films, and non-uniform coverages. Therefore, there is a need for improvement in the coating processes for the fabrication of robust, uniform, and scalable films. Here, we develop a novel percolation-assisted coating (PAC) process that combines top-down and bottom-up approaches in a continuous-flow microfluidic device to deposit HKUST-1 on a porous substrate, yielding controlled film thicknesses and mass loading. The PAC process is optimized using a Multiphysics modeling approach to design the microfluidic reactor, which can be readily scaled up and deployed to fabricate MOF films on various porous substrates. The desired thickness in the range of 10–200 μm of the MOF film can be achieved by controlling the residence time and temperature of the reaction mixture. The synthesized film is characterized for physical adhesion using sonication, film coverage using scanning electron microscopy, and porosity using a porosimeter. The performance of synthesized films is benchmarked for the effective separation of 50 vol% CH4–H2 gas mixture with the separation factor of 4. The microfluidic device is also applicable to synthesize films of various MOFs like MOF-5, MOF-505 (also reported here), UIO-66, etc., over a wide range of porous substrates. Lastly, we propose a multi-chamber design of the microfluidic device for high-throughput screening of thin-film growth using the PAC process.