Large vertices dramatically increase pore volume of metal-organic frameworks
16 Jan 2012
By increasing the size of the vertices in metal-organic frameworks (MOFs), researchers at the University of Pittsburgh, Northwestern University, and Durham University (United Kingdom) have produced an MOF with the largest pore volume reported to date - 4.3cm3/g.
This is about 20-per cent higher than the previous record pore volume for an MOF, which constitutes ''a significant leap,'' according to Nathaniel L. Rosi of the department of chemistry at the University of Pittsburgh.
High pore volume is important for these structures because they can be used for gas storage, separations, catalysis, and drug delivery - applications where volume matters.
The researchers achieved this record by changing the strategy normally used to enhance pore volume in these materials. MOFs typically have metal ions or metal clusters as vertices, which are connected by organic linker molecules to form a three-dimensional framework.
Most work to date has involved increasing the length of the linkers to increase the pore volume enclosed by the lattice-like structure. But increasing the linker molecule length requires skillful, time-consuming chemical synthesis, and the resulting long molecules are often less soluble than shorter ones, requiring further fine tuning to add functional groups to restore solubility. So Rosi and his colleagues decided to focus on the vertices instead.
''What we've done in this work is shown that we can make what we call a 'metal-organic vertex,' a vertex that is composed of both organic linkers, in this case the biomolecule adenine, and zinc metal ions,'' Rosi says.
This metal-adeninate cluster vertex is much larger than the typical metal clusters that are used in MOF chemistry.
''The new strategy that we've presented by using this discrete metal-organic vertex leads to high porosity because we've made a framework now with this very large cluster and a relatively short linker [biphenyl dicarboxylic acid],'' Rosi explains.
The underlying network topology is another factor to be considered in addition to vertex size and linker length, but the system these researchers worked with has an intrinsically open topology. An added benefit of this technology is that all the components of this new MOF are available commercially, eliminating the need for a chemist to synthesize ever-longer linking molecules.
Future work will involve creating larger metal-organic vertices to try to further increase the pore volume. Besides the possible applications mentioned above, these highly porous MOFs might be useful for organizing large macromolecuels - biomolecules, enzymes, proteins, polymers, and nanoparticles.
''That's the direction we're moving in,''Rosi says, noting that this work has not yet begun. ''We're thinking of this material as a scaffold for organising large macromolecules that are otherwise difficult to organise over multiple length scales.''
