Covalent Organic Frameworks (COFs) are an exciting new class of microporous polymers with unprecedented properties in organic material chemistry. They are generally built from rigid, geometrically defined organic building blocks resulting in robust, covalently bonded crystalline networks that extend in two or three dimensions. By strategically combining monomers with specific structures and properties, synthesized COF materials can be fine-tuned and controlled at the atomic level, with unparalleled precision on intrapore chemical environment; moreover, the unusually high pore accessibility allows for easy post-synthetic pore wall modification after the COF is synthesized. Overall, COFs combine high, permanent porosity and surface area with high thermal and chemical stability, crystallinity and customizability, making them ideal candidates for a myriad of promising new solutions in a vast number of scientific fields, with widely varying applications such as gas adsorption and storage, pollutant removal, degradation and separation, advanced filtration, heterogeneous catalysis, chemical sensing, biomedical applications, energy storage and production and a vast array of optoelectronic solutions. This review attempts to give a brief insight on COF history, the overall strategies and techniques for rational COF synthesis and post-synthetic functionalization, as well as a glance at the exponentially growing field of COF research, summarizing their main properties and introducing the numerous technological and industrial state of the art applications, with noteworthy examples found in the literature.

Water pollution due to global economic activity is one of the greatest environmental concerns, and many efforts are currently being made toward developing materials capable of selectively and efficiently removing pollutants and contaminants. A series of β-ketoenamine covalent organic frameworks (COFs) have been synthesized, by reacting 1,3,5-triformylphloroglucinol (TFP) with different C2-functionalized and nonfunctionalized diamines, in order to evaluate the influence of wall functionalization and pore size on the adsorption capacity toward dye and heavy metal pollutants. The obtained COFs were characterized by different techniques. The adsorption of methylene blue (MB), which was used as a model for the adsorption of pharmaceuticals and dyes, was initially evaluated. Adsorption studies showed that –NO₂ and –SO₃H functional groups were favorable for MB adsorption, with TpBd(SO₃H)₂-COF [100%], prepared between TFP and 4,4′-diamine- [1,1′-biphenyl]-2,2′-disulfonic acid, achieving the highest adsorption capacity (166 ± 13 mg g⁻¹). The adsorption of anionic pollutants was less effective and decreased, in general, with the increase in –SO₃H and –NO₂ group content. The effect of ionic interactions on the COF performance was further assessed by carrying out adsorption experiments involving metal ions. Isotherms showed that nonfunctionalized and functionalized COFs were better described by the Langmuir and Freundlich sorption models, respectively, confirming the influence of functionalization on surface heterogeneity. Sorption kinetics experiments were better adjusted according to a second-order rate equation, confirming the existence of surface chemical interactions in the adsorption process. These results confirm the influence of selective COF functionalization on adsorption processes and the role of functional groups on the adsorption selectivity, thus clearly demonstrating the potential of this new class of materials in the efficient and selective capture and removal of pollutants in aqueous solutions.