Synthesis of MOFs
The study of MOFs developed from the study of zeolites, except for the use of preformed ligands. MOFs and zeolites are produced almost exclusively by hydrothermal or solvothermal techniques, where crystals are slowly grown from a hot solution. In contrast with zeolites, MOFs are constructed from bridging organic ligands that remain intact throughout the synthesis. Zeolite synthesis often makes use of a variety of templates, or structure-directing compounds, and a few examples of templating, particularly by organic anions. These templates are removed (of by oxidation) in the case of the zeolites, whereas in MOFs, the framework is templated by the SBU and the organic ligands. A templating approach that is useful for MOFs intended for gas storage is the use of metal-binding solvents such as N,N-diethylformamide and water. In these cases, metal sites are exposed when the solvent is evacuated, allowing hydrogen to bind at these sites.
Post-synthetic modification of MOFs opens up another dimension of structural possibilities that might not be achieved by conventional synthesis. A great deal of recent work explores covalent modification of the bridging ligands. Of particular interest to MOFs for hydrogen storage are modifications which expose metal sites. This has been demonstrated with post-synthetic coordination of additional metal ions to sites on the bridging ligands, and addition and removal of metal atoms to the metal site.
Since ligands in MOFs typically bind reversibly, the slow growth of crystals allows defects to be redissolved, resulting in a material with millimeter-scale crystals and a near-equilibrium defect density. Solvothermal synthesis is useful for growing crystals suitable to structure determination, because crystals grow over the course of hours to days. However, the use of MOFs as storage materials for consumer products demands an immense scale-up of their synthesis. Scale-up of MOFs has not been widely studied, though several groups have demonstrated that microwaves can be used to nucleate MOF crystals rapidly from solution. This technique, termed “microwave-assisted solvothermal synthesis”, is widely used in the zeolite literature, and produces micron-scale crystals in a matter of seconds to minutes, in yields similar to the slow growth methods.
A solvent-free synthesis of a range of crystalline MOFs has been described . Usually the metal acetate is and the organic ligand are ground and mixed with a ball bearing. Cu3(BTC)2 can be quickly synthesised in this way in quantitative yield. In the case of Cu3(BTC)2 the morphology of the solvent free synthesised product was the same as the industrially made Basolite C300. It is thought that localised melting when the components may assist the reaction. The formation of acetic acid as a by-product in the reactions in the ball mill may also help in the reaction having a solvent effect in the ball mill.
Read more about this topic: Metal-organic Framework
Famous quotes containing the words synthesis of and/or synthesis:
“It is in this impossibility of attaining to a synthesis of the inner life and the outward that the inferiority of the biographer to the novelist lies. The biographer quite clearly sees Peel, say, seated on his bench while his opponents overwhelm him with perhaps undeserved censure. He sees him motionless, miserable, his head bent on his breast. He asks himself: “What is he thinking?” and he knows nothing.”
—Andre Maurois (1885–1967)
“The spider-mind acquires a faculty of memory, and, with it, a singular skill of analysis and synthesis, taking apart and putting together in different relations the meshes of its trap. Man had in the beginning no power of analysis or synthesis approaching that of the spider, or even of the honey-bee; but he had acute sensibility to the higher forces.”
—Henry Brooks Adams (1838–1918)