A team of researchers based in the Netherlands has made the most porous crystalline framework known so far, a metal-organic framework with 90% of its structure as empty space.

Metal-organic frameworks, commonly called MOFs, are porous structures made from organic ligands connecting metal clusters. Researchers have been pushing the porosity limits of MOF crystalline frameworks in recent years, in part to understand the fundamental principles behind the stability of these structures. Larger pores could be beneficial for frameworks, for example those used as air filters, energy storage materials, and containers for toxic gas. But as pores get larger, the frameworks tend to collapse during the final stages of synthesis.

To make this new material, a team of researchers led by Irena Senkovska and Stefan Kaskel, both at Technische Universität Dresden, wanted to increase the pore size in one of their previously synthesized MOFs called DUT-6. This MOF contains zinc clusters, each connected to five or six other clusters with carboxylate-tipped organic linkers. The high degree of connectivity strengthens the framework, Kaskel says.

To increase the MOF pore size, the researchers wanted to lengthen the organic linkers between the zinc clusters. Other MOF makers, including the research group that made the previous most porous MOF called NU-110, have used ligand lengthening as a design strategy to increase porosity. Senkovska and Kaskel’s group modeled the framework of DUT-6 and identified the structure of lengthened ligands that maintained the same cluster connectivity. Next, the research team calculated the bulk modulus of the optimized enlarged framework to determine whether the material would resist collapse during the last stages of synthesis. The predicted average bulk modulus of 4.97 GPa was strong enough to encourage the researchers to make the material.

However, in their early attempts, the ingredients tended to assemble into MOFs with different structures. The researchers optimized the synthesis over more than five years to eventually produce the enlarged MOF, called DUT-60, with high purity and yield. They confirmed its structure using powder diffraction x-ray crystallography. The crystallographic density of the material was 0.187 g cm^-3, near the record low density of 0.126 g cm^-3 in a MOF called MOF-399. The MOF-399 framework, however, collapsed after synthesis, so its accessible pore volume was never reported.

But since DUT-60 was strong enough to hold its shape after removing solvent trapped inside, the researchers were able to determine its pore volume by measuring the amount of nitrogen gas that the framework adsorbed. The framework’s measured accessible pore volume of 5.02 cm³ g^-1 is the highest of any crystalline framework known thus far. Kaskel says his team has not yet produced enough of DUT-60 to test its ability to store gases for practical applications, but they plan to do so in the future.

Most MOFs store low-volume gases like hydrogen and methane in nanosized pores, a property that has led to collaborations between materials scientists synthesizing these materials and chemical engineers developing energy storage systems, says Banglin Chen, at the University of Texas at San Antonio. The large pores in this new MOF could lead to collaborations with other fields, such as biochemistry, he says, because the large pores might be useful for storing biomolecules.

Read the abstract in Angewandte Chemie International Edition.