Greenhouse scrubbers: Predicting adsorptive materials

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  • Published: Aug 15, 2013
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
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Simulate and synthesise

Computational simulation and nuclear magnetic resonance (NMR) spectroscopy have been combined by chemists in the USA to help them predict accurately the adsorptive properties of highly porous synthetic materials known as crystalline multivariate-metal organic framework (MTV-MOF). The work should allow researchers to design new materials with optimal functionality for adsorbing the greenhouse gas carbon dioxide, for instance.

MTV-MOFs were first described by Omar Yaghi, currently at the University of California, Berkeley, but designing and synthesizing the optimal example of the class for a particular application remains a significant chemical challenge. But, Yaghi's team has not been reticent in striving to overcome the problems. Working with colleagues at Lawrence Berkeley National Laboratory Jeffrey Reimer, Berend Smit, Xueqian Kong, Hexiang Deng, Fangyong Yan, Jihan Kim and Joseph Swisher, a way to accurately predict a given MTV-MOF's physical properties has now been developed so that systems synthesized with different combinations of functional chemical groups can be made knowing in advance that they will behave in a specific way with respect to adsorption of a given gas.

Selecting selectivity

"By combining solid-state NMR measurements with molecular-level computational simulations we've identified a strategy for determining the underlying structure of MTV-MOFs that can help optimize function," NMR expert Reimer explains. "Our method is a first step in resolving the more general problem of spatial disorder in other ordered materials, including mesoporous materials, functionalized polymers and defect distributions within crystalline solids."

MOFs are essentially crystalline molecular sponges and have been touted as high-capacity solid state storage media for various gases, including the fuel for a future so-called "hydrogen economy". They consist of a metal oxide centre surrounded by organic linker molecules that give rise to the highly porous three-dimensional framework potentially having a vast internal surface area to overall volume ratio. It is often likened by pundits to the idea of being able to fold up the surface of a football field into a sugar cube-sized lump of MOF. Such a vast surface area to volume ratio and pores wide enough to accommodate small gaseous molecules points to the possibility of sequestering carbon dioxide from power station exhaust gases from the burning of fossil fuels as well as safe hydrogen storage.

Yaghi and colleagues found that multiple organic linkers with different functional groups can be incorporated into a given MOF to generate a heterogeneous interior with greater selectivity for a given small molecule over others, adsorbing carbon dioxide preferentially rather than methane or hydrogen, for example. "MTV-MOFs with different functionalities showed an increase in their ability to selectively separate carbon dioxide by two orders of magnitude compared to corresponding single-component MOFs," Yaghi explains. The apportioning of the different functional groups within the MOF's crystal structure is key to specificity but this cannot be determined by diffraction techniques.

Linker distribution

"The problem of linker distribution with MTV-MOF’s turns out to be an example of a larger problem in materials science, which is how does one quantify disorder when it is distributed upon a system that is governed by crystalline order," Yaghi explains. "In the case of MTV-MOFs, were it not for the differing functional groups on the linkers, all would exhibit the same regular crystalline order, which means diffraction methods cannot discern the apportionment of the linkers."

In order to investigate spatial apportionment within MTV-MOF crystals, the researchers have augmented their synthetic studies with an NMR-computational approach to generate three-dimensional maps of the apportionment of functional groups within and between the pores of a model MTV-MOF system. They found that, depending on the composition of the functional groups and their ratios, MTV-MOFs exist in either clusters, random or alternating apportionments, and these apportionments govern the carbon dioxide adsorption selectivity that was observed in Yaghi’s earlier work.

The research suggests that it might now be possible to synthesize structures with specific patterns of linker functionality and apportionment so that complex functions might be carried out by a designer MOF - whether that's storage, sequestration, separation, sensing or catalysis, all of which have been promised by advances in MOF technology. "Our work also shows that a combined NMR-computational approach might be able to provide similar insights into the many other systems in nature where disorder precludes a molecular understanding of a material’s structure", Yaghi adds.

Related Links

Science 2013, online:"Mapping of Functional Groups in Metal-Organic Frameworks"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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