Covalent organic framework
Covalent organic frameworks are two-dimensional and three-dimensional organic solids with extended structures in which building blocks are linked by strong covalent bonds. COFs are porous and crystalline and are made entirely from light elements that are known to form strong covalent bonds in well-established and useful materials such as diamond, graphite, and boron nitride. Preparation of COF materials from molecular building blocks would provide covalent frameworks that could be functionalized into lightweight materials for diverse applications.
Structure
solids consists of secondary building units which assemble to form a periodic and porous framework. An almost infinite numbers of frameworks can be formed through various SBU combinations leading to unique material properties for applications in separations, storage, and heterogeneous catalysis.Porous crystalline solids can be used to describe materials such as Zeolite, Metal-organic frameworks, and Covalent Organic Frameworks. Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. MOFs are a class of porous polymeric material, consisting of metal ions linked together by organic bridging ligands and are a new development on the interface between molecular coordination chemistry and materials science.
COFs are another class of porous polymeric materials, consisting of porous, crystalline, covalent bonds that usually have rigid structures, exceptional thermal stabilities, are stable in water and low densities. They exhibit permanent porosity with specific surface areas surpassing those of well-known zeolites and porous silicates.
Secondary building units
The term ‘secondary building unit’ has been used for some time to describe conceptual fragments which can be compared as bricks used to build a house of zeolites; in the context of this page it refers to the geometry of the units defined by the points of extension.Reticular synthesis
Although the synthesis of new materials has long been recognized as the most essential element in advancing technology, it generally remains more of an art than a science—in that the discovery of new compounds has mostly been serendipitous, using methods referred to by critics as 'shake and bake', ‘mix and wait', 'mash and smash' and 'heat and beat'. The reason is that the starting entities do maintain their structure during the reaction, leading to poor correlation between reactants and products. However, the design of an extended network that will maintain its structural integrity throughout the construction process can be realized by starting with well-defined and rigid molecular building blocks.In essence, reticular synthesis can be described as the process of assembling judiciously designed rigid secondary building units into predetermined ordered structures, which are held together by strong bonding. It is different from retrosynthesis of organic compounds, because the structural integrity and rigidity of the building blocks in reticular synthesis remain unaltered throughout the construction process—an important aspect that could help to fully realize the benefits of design in crystalline solid-state frameworks. Similarly, reticular synthesis should be distinguished from supramolecular assembly, because in the former, building blocks are linked by strong bonds throughout the crystal.
Applications
Hydrogen storage
and William A. Goddard III reported COFs as exceptional hydrogen storage materials. They predicted the highest excess H2 uptakes at 77 K are 10.0 wt % at 80 bar for COF-105, and 10.0 wt % at 100 bar for COF-108, which have higher surface area and free volume, by grand canonical Monte Carlo simulations as a function of temperature and pressure. This is the highest value reported for associative H2 storage of any material. Thus 3-D COFs are most promising new candidates in the quest for practical H2 storage materials. In 2012, the lab of William A. Goddard III reported the uptake for COF102, COF103, and COF202 at 298 K and they also proposed new strategies to obtain higher interaction with H2. Such strategy consist on metalating the COF with alkaline metals such as Li. Uptake in Li-, Na-, K-Metalated Covalent Organic Frameworks and Metal Organic Frameworks at 298 K. J. Phys. Chem. A. 2012, 116, pp 1621–1631. These complexes composed of Li, Na and K with benzene ligands have been synthesized by Krieck et al. and Goddard showed that the THF is important of their stability. If the metalation with alkaline is performed in the COFs, Goddard et al. calculated that some COFs can reach 2010 DOE gravimetric target in delivery units at 298 K of 4.5 wt %: COF102-Li, COF103-Li, COF102-Na and COF103-Na. COFs also perform better in delivery units than MOFs because the best volumetric performance is for COF102-Na, COF102-Li, COF103-Na, and COF103-Li, all using delivery g H2/L units for 1–100 bar. These are the highest gravimetric molecular hydrogen uptakes for a porous material under these thermodynamic conditions.Methane storage
and William A. Goddard III also reported COFs as exceptional methane storage materials. The best COF in terms of total volume of CH4 per unit volume COF absorbent is COF-1, which can store 195 v/v at 298 K and 30 bar, exceeding the U.S. Department of Energy target for CH4 storage of 180 v/v at 298 K and 35 bar. The best COFs on a delivery amount basis are COF-102 and COF-103 with values of 230 and 234 v/v, respectively, making these promising materials for practical methane storage. More recently, new COFs with better delivery amount have been designed in the lab of William A. Goddard III, and they have been shown to be stable and overcome the DOE target in delivery basis. COF-103-Eth-trans and COF-102-Ant, are found to exceed the DOE target of 180 v/v at 35 bar for methane storage. They reported that using thin vinyl bridging groups aid performance by minimizing the interaction methane-COF at low pressure.Optical properties
A highly ordered π-conjugation TP-COF, consisting of pyrene and triphenylene functionalities alternately linked in a mesoporous hexagonal skeleton, is highly luminescent, harvests a wide wavelength range of photons, and allows energy transfer and migration. Furthermore, TP-COF is electrically conductive and capable of repetitive on–off current switching at room temperature.Porosity/surface-area effects
Most studies to date have focused on the development of synthetic methodologies with the aim of maximizing pore size and surface area for gas storage. That means the functions of COFs have not yet been well explored, but COFs can be used as catalyst, or gas separation etc.Carbon capture
In 2015 the use of highly porous, catalyst-decorated COFs for converting carbon dioxide into carbon monoxide.Electrocatalysis
COFs have been studied as non-metallic electrocatalyst for energy-related catalysis, including carbon dioxide electro-reduction and water splitting reaction. However, such researches are still in the very earlier stage. Most of the efforts have been focusing on solving the key issues, such as conductivity, stability in electrochemical processes.History
While at UMich, Omar M. Yaghi and Adrien P Cote published the first paper of COF. They reported the design and successful synthesis of COFs by condensation reactions of phenyl diboronic acid and hexahydroxytriphenylene. Powder X-ray diffraction studies of the highly crystalline products having empirical formulas 6·1 and C9H4BO2 revealed 2-dimensional expanded porous graphitic layers that have either staggered conformation or eclipsed conformation. Their crystal structures are entirely held by strong bonds between B, C, and O atoms to form rigid porous architectures with pore sizes ranging from 7 to 27 Angstroms. COF-1 and COF-5 exhibit high thermal stability, permanent porosity, and high surface areas.The synthesis of 3D COFs has been hindered by longstanding practical and conceptual challenges. Unlike 0D and 1D systems, which are soluble, the insolubility of 2D and 3D structures precludes the use of stepwise synthesis, making their isolation in crystalline form very difficult. This first challenge, however, was overcome by judiciously choosing building blocks and using reversible condensation reactions to crystallize COFs.