Volume 92 Issue 45 | p. 8 | News of The Week
Issue Date: November 10, 2014 | Web Date: November 6, 2014

Two-Dimensional MOFs Boost Gas Separation

Materials: Thin sheets of metal-organic framework compounds improve membrane properties
Department: Science & Technology
News Channels: Materials SCENE, Analytical SCENE
Keywords: MOF, membrane, gas separation
Credit: Nat. Mater.
This SEM image shows sheetlike crystals of a Cu-based MOF.
Credit: Nat. Mater.
Credit: Nat. Mater.
The photo of colored solutions (left) and schematic depict a synthesis strategy for making sheet-like MOF crystals. The method relies on forming a 3-tiered liquid in which reactants diffuse from the top and bottom to react in the center.
Credit: Nat. Mater.

A three-phase synthesis medium controls diffusion of metal (blue) and organic (yellow) MOF precursors and leads to micrometer-sized sheetlike crystals (micrograph) instead of standard cube-shaped crystals.

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. When used to store or separate gases, these sorbents 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 would be easier to implement 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 showed 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 and evaluated their usefulness in separating CO2 from CO2-methane mixtures. 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. The surprising separation results of the hybrid membranes are not completely understood, he adds.

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.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
A.Chandrasekaran (Thu Nov 27 12:02:13 EST 2014)
This layering technique is a common single crystal growing technique. Often two layers will be used, though sometimes a separator will also be used, such as glass frit or silica gel. It usually leads to bigger crystals than changing the crystal type or shape (crystal habit), as long as same medium is used.
Based on that, here also it should be the size of the crystals than the shape that changed with this technique. To put in another way, the long range order has increased more than in the conventional material.
If both the medium (solvents) and method has changed, difficult to isolate the effects.
The effect is really interesting indeed.
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