Synthesis of zeolitic imidazolate frameworks in droplet microfluidic system

Abstract

The zeolitic imidazolate frameworks (ZIFs) have been extensively investigated in adsorption, separation and catalysis for their unique properties, e.g . , high surface area, ordered pore structures, tailorable frameworks and high chemical and thermal stability. Zn 2+ and Co 2+ are mostly used metal cations in ZIFs synthesis, because they prefer to form tetrahedron structures with imidazolates and their derived organic ligands. However the conventional preparation methods, which are generally performed at solvothermal condition in the sealed vessel, are time consuming and inhomogeneous process at micro-scale. As a result, the products are always different from batch to batch, which limits the scale-up of ZIFs by conventional synthesis methods. Therefore, facile ways to avoid these problems occurred in ZIFs synthesis are still needed to develop. The microfluidic system is of great advantages such as high surface to volume ratio, high mass and heat transfer efficiency and continuous flow processing. The continuous droplet microfluidic system is effective in mixing for the dynamic micro-environment inside droplets, and it is also effective in reducing the clogging problems for the confined synthesis of nanoparticles inside the microdroplets. Hence the droplet microfluidic system has been widely applied in nanomaterials synthesis. In this work, we design and fabricate a dual T type droplet microreactor to overcome the disadvantages of batch synthesis. The PDMS microchip was fabricated by a scaffold template assisting thermal curing process. Various ZIFs, including monoclinic C2 / c ZIF-7, ZIF-8 and Co doped ZIF-8, were first synthesized in this microfluidic system to demonstrate a universal applicable synthesis method. The concentration of starting material and reaction time were studied in the synthesis of all ZIFs in this work. The results indicated that the increasing of concentration and retention time led to the increasing particles size in all ZIFs, as well as the increasing of product yield. However, the space-time yield was dramatically decreased while the retention time increased from 1 to 5 min, while the yield was just slightly increased. It was because that the micro-synthesis process had been significantly intensified in 1 min for the high mixing and diffusion efficiency in the microfluidic system. Moreover, the mixture solvent of methanol and ammonia water strongly facilitated the formation of ZIFs as well. The high throughput microfluidic system showed an estimated space-time yield of monoclinic C2 / c ZIF-7 (461 kg/(cm 3 d)), ZIF-8 (267 kg/(cm 3 d)) and Co-ZIF-8 (160 kg/(cm 3 d)) in a single microreactor, respectively. It paves the way to massively produce ZIFs without scale-up effect. All the ZIFs obtained from microreactions had high crystallinity except ZIF-67. ZIF-7 synthesized in these conditions showed a typical monoclinic C2 / c ZIF-7 phase respect to the existence of water in the mixture solvents, which drove the structure from porous phase into the dense phase of monoclinic C2 / c ZIF-7. ZIF-8 and Co-ZIF-8 both exhibited sodalite structure respect to the tetra coordination properties of Zn 2+ and Co 2+ . The small polyhedron ZIF-8 particles had grown into large cubic particles with reaction time increased to 5 min. The Co doping exhibited strong effect on the particles morphology. The Co-ZIF-8 prepared from low concentration reactants showed spherical octadecahedron morphology, where {110} surfaces were the main surface. On the contrary, the high concentrated reactants resulted in cubic octadecahedron, where the {100} were mainly occupied the surface. The reason could be that the pure Co was difficult to form ZIF-67 in this condition; therefore the Co changes the formation of ZIF-8. Accordingly, this work provides a facile approach to massively produce diverse ZIFs with controllable morphology in microfluidic system.