Synthesis and Functionalization of Three-Dimensional Covalent Organic Frameworks

Abstract

Covalent organic frameworks (COFs) are an emerging class of cross-linked porous crystalline polymers constructed exclusively from rigid building blocks linked together by covalent interactions. Compared to conventional nanoporous materials, COFs possess a unique and highly desirable combination of attributes that are conducive to a variety of applications. Three-dimensional (3D) COFs are, in general, characterized by superior internal surface areas and pore sizes than their two-dimensional (2D) counterparts, and these two attributes are vital for many applications such as gas separation, gas storage, and catalysis. Unfortunately, 3D COFs are also significantly more difficult to construct, and the synthesis, application, and fundamental understanding of 3D COFs lag those of 2D COFs as a result. Thus, improving synthetic accessibility, practical utility, and basic understanding of 3D COFs is vital for realizing their full potential as controllable crystalline nanoporous materials.

This dissertation details the design and execution of an improved synthetic method to afford the archetypal imine-linked 3D COF-300, catalyzed by Lewis acidic scandium triflate (Sc(OTf)3), that allows the reduction of reaction temperature from 120°C to room temperature. A systematic investigation of temperature, catalyst loading, and water content as reaction variables elucidates the contribution of each factor towards the reaction equilibrium and facilitates identification of reaction conditions that result in the most crystalline framework formation, which is determined by powder X-ray diffraction (PXRD). Fourier transform infrared spectroscopy (FTIR) and Brunauer–Emmett–Teller (BET) surface area measurements confirm that COFs obtained via Sc(OTf)3 possess comparable properties as those produced by the conventional solvothermal method.

In addition, the synthesis of several functionalized aldehyde monomers is conducted utilizing a range of chemistries, and they were subsequently employed for functionalized COF-300 synthesis to embed reactive sites into the COF backbone for post-synthetic modification (PSM). The functionalized COFs are consistently amorphous despite systematic variations in reaction conditions under both solvothermal and Sc(OTf)3 catalyzed regimes, and decreasing ratios of functionalized to unfunctionalized aldehydes corresponded to increasingly crystalline frameworks. Steric hindrance is likely the culprit preventing rearrangement into crystalline structures. Lastly, the reduction of imine bonds in COF-300 to amides, multiphase synthesis of COF-300, and high-pressure transformation of COF-300 are explored as alternative methods of synthesis and functionalization.

The combination of these findings provides valuable insight into the imine formation and exchange process and tuning the equilibrium through reaction parameter adjustments, unlocking new synthetic possibilities in the field of 3D COFs via the utilization of a much more effective catalyst. In addition, the limitation of functionalized 3D COF syntheses due to steric hindrance is established and will inform future design efforts.