Covalent organic framework
Encyclopedia
The design and synthesis of crystalline extended organic structures in which the building blocks are linked by strong covalent bonds are core concepts of covalent organic frameworks (COFs). COFs are porous, and crystalline, and made entirely from light elements (H, B, C, N, and O) that are known to form strong covalent bonds in well-established and useful materials such as diamond
, graphite
, and boron nitride
.
The successful realization of COF materials through molecular building blocks would provide covalent frameworks that could be functionalized into lightweight materials optimized for gas storage, photonic, and catalytic applications.
An almost infinte 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 framework
s (MOFs), and Covalent Organic Frameworks (COFs).
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 (to temperatures up to 600°C), and low densities. They exhibit permanent porosity with specific surface areas surpassing those of well-known zeolites and porous silicates.
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.
In essence, reticular synthesis can be described as the process of assembling judiciously designed rigid secondary building units into predetermined ordered structures (networks), 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.
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 (GCMC) 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.
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.
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.
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 {C6H4[B(OH)2]2} and hexahydroxytriphenylene [C18H6(OH)6]. Powder X-ray diffraction studies of the highly crystalline products (C3H2BO)6&(C9H12)1 (COF-1) and C9H4BO2 (COF-5) revealed 2-dimensional expanded porous graphitic layers that are either staggered
(COF-1, P63/mmc) or eclipsed
(COF-5, P6/mmm). 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 (to temperatures up to 500 to 600 C), permanent porosity, and high surface areas (711 and 1590 square meters per gram, respectively).
The synthesis of 3D COFs has been hindered by longstanding practical and conceptual challenges. Unlike 0D and 1D system, the insolubility of 2D and 3D structures precludes the use of stepwise synthesis, making their isolation in crystalline form very difficult. The first challenge, however, was overcome by judiciously choosing building blocks and using reversible condensation reactions to crystallize COFs. Examples of 3D COFs are COF-102, 103, 105, 108, 202, and 300. Most of 3D COF show high surface area, which surpass those of 2D (3472, 4210, 3214, square meters per gram for COF-102, 103, and 202 respectively). COF-105 and 108 calaulated theoretically to perform exceptional hydrogen storage function which is the highest value reported for associative H2 storage of any material.
reaction which is a molecular dehydration reaction between boronic acids. In case of COF-1, three boronic acid molecules converge to form a planar six-membered B3O3 (boroxine
) ring with the elimination of three water molecules.
materials which can be achieved by dynamic trimerization reaction of simple, cheap, and abundant aromatic nitriles in ionothermal conditions (molten zinc chloride at high temperature (400 °C)). CTF-1 is a good example of this chemistry.
condensation
of aniline
with benzaldehyde
that results in imine bond formation with elimination of water. COF-300 is a good example of this chemistry.
is understood by scanning electron microscopy (SEM). Finally, porosity, in most cases surface area, is measured by a N2 sorption isotherm.
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
, graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
, and boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
.
The successful realization of COF materials through molecular building blocks would provide covalent frameworks that could be functionalized into lightweight materials optimized for gas storage, photonic, and catalytic applications.
Structure
Porous crystalline solids consists of secondary building units (SBUs) which assemble to form a periodic and porous framework.An almost infinte 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
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
, Metal-organic framework
Metal-organic framework
Metal-Organic Frameworks are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable to elimination of the guest molecules and can be used for...
s (MOFs), and Covalent Organic Frameworks (COFs).
Zeolites are microporous, aluminosilicate
Aluminosilicate
Aluminosilicate minerals are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals....
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 (to temperatures up to 600°C), and low densities. They exhibit permanent porosity with specific surface areas surpassing those of well-known zeolites and porous silicates.
Secondary building units
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’ and ‘heat and beat’. It was caused by that the starting entities do not maintain their structure during the reaction, leading to poor correlation between reactants and products. However, the design of an extended network that will maintain their 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 (networks), 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.
Hydrogen storage
Omar M. YaghiOmar M. Yaghi
Dr. Omar M. Yaghi is an American chemist, currently the Jean Stone Professor of Chemistry at University of California, Los Angeles. He and his research laboratories design and produce classes of compounds now known as metal-organic frameworks , zeolitic imidazolate frameworks , and covalent...
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
Surface area
Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
and free volume, by grand canonical Monte Carlo (GCMC) 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.
Optical properties
The ultimate highly ordered π-conjugation TP-COF, consisting of pyrenePyrene
Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This colourless solid is the smallest peri-fused PAH...
and triphenylene
Triphenylene
In chemistry, the organic compound triphenylene is a flat polycyclic aromatic hydrocarbon consisting of four fused benzene rings. Triphenylene can be isolated from coal tar, but it is also made synthetically using benzyn chemistry. One molecule of triphenylene has delocalized 18-π-electron systems...
functionalities alternately linked in a mesoporous hexagonal skeleton, is highly luminescent, harvests a wide wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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 areaSurface area
Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
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
Gas separation
Gas mixtures can be effectively separated by synthetic membranes. For other methods see adsorption, absorption, cryogenic distillation.Membranes are employed in:* separation of nitrogen or oxygen out of air...
etc.
History
Omar M. Yaghi, a professor at the University of California, Los AngelesUniversity of California, Los Angeles
The University of California, Los Angeles is a public research university located in the Westwood neighborhood of Los Angeles, California, USA. It was founded in 1919 as the "Southern Branch" of the University of California and is the second oldest of the ten campuses...
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 {C6H4[B(OH)2]2} and hexahydroxytriphenylene [C18H6(OH)6]. Powder X-ray diffraction studies of the highly crystalline products (C3H2BO)6&(C9H12)1 (COF-1) and C9H4BO2 (COF-5) revealed 2-dimensional expanded porous graphitic layers that are either staggered
Staggered
In organic chemistry, a staggered conformation is a chemical conformation of an ethane-like moiety abcX-Ydef in which the substituents a,b,and c are at the maximum distance from d,e,and f...
(COF-1, P63/mmc) or eclipsed
Eclipsed
In chemistry an eclipsed conformation is a conformation in which two substituents X and Y on adjacent atoms A, B are in closest proximity, implying that the torsion angle X-A-B-Y is 0°. Such a conformation exists in any open chain single chemical bond connecting two sp3 hybridised atoms, and is...
(COF-5, P6/mmm). 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 (to temperatures up to 500 to 600 C), permanent porosity, and high surface areas (711 and 1590 square meters per gram, respectively).
The synthesis of 3D COFs has been hindered by longstanding practical and conceptual challenges. Unlike 0D and 1D system, the insolubility of 2D and 3D structures precludes the use of stepwise synthesis, making their isolation in crystalline form very difficult. The first challenge, however, was overcome by judiciously choosing building blocks and using reversible condensation reactions to crystallize COFs. Examples of 3D COFs are COF-102, 103, 105, 108, 202, and 300. Most of 3D COF show high surface area, which surpass those of 2D (3472, 4210, 3214, square meters per gram for COF-102, 103, and 202 respectively). COF-105 and 108 calaulated theoretically to perform exceptional hydrogen storage function which is the highest value reported for associative H2 storage of any material.
Boron condensation
The most popular COF synthesis route is a boron condensationCondensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
reaction which is a molecular dehydration reaction between boronic acids. In case of COF-1, three boronic acid molecules converge to form a planar six-membered B3O3 (boroxine
Boroxine
Boroxine is a 6-membered, heterocyclic compound composed of alternating oxygen and singly-hydrated boron atoms. Boroxine derivatives such as trimethylboroxine and triphenylboroxine also make up a broader class of compounds called boroxines. These compounds are solids that are usually in...
) ring with the elimination of three water molecules.
Triazine based trimerization
Another class of high performance polymer frameworks with regular porosity and high surface area is based on triazineTriazine
A triazine is one of three organic chemicals, isomeric with each other, whose molecular formula is 333 and whose empirical formula is CHN.- Structure :...
materials which can be achieved by dynamic trimerization reaction of simple, cheap, and abundant aromatic nitriles in ionothermal conditions (molten zinc chloride at high temperature (400 °C)). CTF-1 is a good example of this chemistry.
Imine condensation
A new class of COFs can be obtained by imineImine
An imine is a functional group or chemical compound containing a carbon–nitrogen double bond, with the nitrogen attached to a hydrogen atom or an organic group. If this group is not a hydrogen atom, then the compound is known as a Schiff base...
condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
of aniline
Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...
with benzaldehyde
Benzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...
that results in imine bond formation with elimination of water. COF-300 is a good example of this chemistry.
Characterization
Even though COFs are usually harder to characterize properties than MOFs because COFs have no single crystal structure, COFs can be characterized by some following methods. Powder X-ray diffraction (PXDR) is used to determine structure. MorphologyMorphology
Morphology may mean:*Morphology , the study of the structure and content of word forms*Morphology , the study of the form or shape of an organism or part thereof...
is understood by scanning electron microscopy (SEM). Finally, porosity, in most cases surface area, is measured by a N2 sorption isotherm.
See also
- Coordination polymer
- Conjugated microporous polymerConjugated microporous polymerConjugated microporous polymers are a sub-class of porous materials, related to structures such as zeolites, metal-organic frameworks, and covalent organic frameworks, but amorphous in nature, rather than crystalline...
- Omar M. YaghiOmar M. YaghiDr. Omar M. Yaghi is an American chemist, currently the Jean Stone Professor of Chemistry at University of California, Los Angeles. He and his research laboratories design and produce classes of compounds now known as metal-organic frameworks , zeolitic imidazolate frameworks , and covalent...
- Metal-organic frameworkMetal-organic frameworkMetal-Organic Frameworks are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable to elimination of the guest molecules and can be used for...
- ZeoliteZeoliteZeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...