Web Date: November 5, 2014
Two-Dimensional MOFs Boost Gas Separation
Metal-organic framework (MOF) compounds ordinarily appear as three-dimensional crystals. An international team has now devised a method for preparing these materials in 2-D form—as flat platelets—and for embedding them in thin polymeric films (Nat. Mater. 2014, DOI: 10.1038/nmat4113). The reduction from 3-D to 2-D opens new areas of application for MOFs. The work also points to a way to customize the morphology of other crystalline substances.
MOFs are porous crystalline materials composed of metal ions or metal clusters bridged by organic linking groups. They have generated intense scientific interest over the past dozen years because of their outstanding adsorption properties and numerous potential applications, especially in gas storage and separation.
For gas applications, MOFs are typically used as powders. For example, the materials can be loaded in bulk form into tanks to increase the tanks’ gas storage capacity. Or MOFs can be used to separate gases by flowing mixtures though MOF-filled tubes or pipes.
These applications and others would be easier to implement, and engineers would have greater design flexibility, if MOFs could be prepared as thin, robust, and easily processed membranes. Efforts thus far to prepare MOF membranes and MOF-polymer hybrid membranes have seen limited success, often as a result of the membranes’ poor mechanical properties.
Now, a team of researchers led by Francesc X. Llabrés i Xamena of Polytechnic University of Valencia, in Spain, and Jorge Gascon of Delft University of Technology, in the Netherlands, has devised a method for preparing MOFs as nanometer-thick sheets or platelets with micrometer-scale lateral dimensions. The team dispersed the MOF sheets in a polymer matrix and demonstrated that the resulting membranes can separate gases.
To demonstrate the method, the team synthesized a copper benzenedicarboxylate MOF known as CuBDC. Rather than mixing precursors in a single solution, the team formed a three-tiered liquid synthesis medium in which solutions of the dicarboxylic acid and copper precursors resided at the bottom and top of the reaction vessel, respectively, and were separated according to their densities by a buffer solution. Diffusion of the reactants to the middle layer resulted in controlled growth of micrometer-sized MOF sheets that were less than 25 nm thick. Standard synthesis methods yield thick cubic CuBDC crystals.
Then the team prepared polyimide-MOF membranes with a range of MOF-sheet concentrations and evaluated their usefulness in separating CO2 from CO2-methane mixtures. In addition to being a greenhouse gas, CO2 is a common impurity that reduces the energy value of natural gas. The team found that polymer membranes hybridized with MOF sheets provided 80% higher separation efficiency than pure polymer membranes and up to eight times better separation performance than membranes loaded with standard CuBDC crystals.
“This work is a real breakthrough in gas-selective membrane technology,” says Jürgen Caro, a catalysis and materials specialist at the University of Hannover, in Germany. He adds that the surprisingly good separation results of the hybrid membranes are not completely understood.
Sankar Nair of Georgia Institute of Technology is similarly impressed. “This work demonstrates a clever and potentially generalizable synthesis strategy to shape MOF crystals into sheetlike morphologies,” he says. Such morphologies are difficult to obtain with conventional synthesis procedures, he adds.
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