Intensified synthesis and post-synthetic modification of covalent organic frameworks using a continuous flow of microdroplets technique

Abstract

Covalent organic frameworks (COFs) are two-dimensional/three-dimensional crystalline polymeric materials with diverse molecular backbones and topologies and are strong candidates for numerous applications. However, the ready availability of these materials is a challenge. The key issues are slow production rates, harsher and longer reaction conditions, and a need for post-synthetic modification to obtain desired molecular functionalities. Only a few studies, including those using microfluidic techniques, have focused on refining these factors, but these reports lack synthetic continuity and scalability. Herein, we present a fast, intensified and continuous synthesis and post-synthetic modification of β-ketoenamine-linked COFs by confining organic building units into moving microdroplets in a transparent capillary. This study introduces a one-step, facile approach to serially modify NO2- to NH2-substituted COFs in a fraction of the time, effort and cost of traditional methods. These results may stimulate the development of novel COFs with unique chemistries and functions for various applications. Confining organic building blocks inside a moving system of microdroplets enables high-speed production of catalytic crystals. Covalent organic frameworks are promising for applications such as carbon capture, but it is challenging to rapidly synthesize high-quality, tailored structures. Dong-Pyo Kim from POSTECH, South Korea, and co-workers report that microfluidic devices can produce covalent organic frameworks at a rate of 40 milligrams per hour. This approach injects an aqueous reagent and a carrier oil separately into a capillary and they meet at a T-junction, creating tiny droplets that pass through mixing and heating stages. Energy released by microdroplet flow facilitated β-ketoenamine generation by locking its organic framework in place. The microfluidic setup easily handled post-synthetic modification; the team added palladium to the framework and modified its functional groups to create a catalyst for synthetic coupling reactions. β-ketoenamine-linked covalent organic frameworks and their post-synthetic modification is performed in a continuous-flow of microdroplets in a serial manner.