Imperial College London

Researchers tune the chiral chemistry of MOFs for high value molecules

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Schematic of the metal-organic framework structure, below which is a 'defect mixer' that is used to illustrate the tunability of this structure by incorporating defects.

Researchers at the Barrer Centre in Imperial College have tuned a chiral metal-organic framework (MOF) to improve its enantiomeric separation ability.

Chiral molecules are those that can exist as enantiomers, which are non-superposable mirror images of the same molecule. Racemic chemical mixtures are those that consist of equal proportions of each enantiomer of a given chiral compound, and are commonplace in the pharmaceutical industry. In some cases, each enantiomer can exhibit strikingly contrasting pharmacological and toxicological properties. As such, the separation of such chemicals plays a significant role in pharmaceutical development. However, efficient separation of chiral compounds remains a very challenging task due to each enantiomer displaying identical physical and chemical properties. Further, enantiomers only behave differently from each other in the presence of other chiral species.

Conventional techniques for effecting chiral separation, such as chiral chromatography, have displayed several shortcomings, namely a high price and low separation efficiency. In this regard, homochiral metal-organic frameworks (MOFs), a class of crystalline porous materials, have received considerable attention as a promising candidate for enantiomeric separations. In addition, the highly porous nature and tunable structure of these materials enables them to function over a wide range of applications; these include catalysis, drug delivery, and gas separation.

One of the key aspects of tuning the structure of these materials is incorporating structural defects. The location and quantity of structural defects play an important role in the properties of MOFs and can significantly alter the pore network architecture. However, careful control and tuning of defect engineering in MOFs continues to remain a challenging prospect in the field of enantiomeric separation.

Researchers from the Barrer Centre, at the Department of Chemical Engineering, in Imperial College London have now developed a new method to produce homochiral MOFs exhibiting enhanced enantiomeric separation of racemic mixtures through a tunable defective framework. Ben Slater, a member of the research team, explained in more detail: “The tunable defective framework was achieved through introducing ‘missing linker defects’ into the chiral MOF through swapping the existing bi-coordinating ligands with monocoordinating ligands. This in turn increased the chiral separation capacity by enhancing the host-guest interactions between the framework and the chiral molecule to be separated.” An increase in enantiomeric separation was observed with increasing defect concentration for the 1-phenylethanol chiral species.  

The results, published in JACS, are highly relevant to industry as they show an effective, energy-efficient alternative to traditional chiral separation techniques, while enabling tunable porosity and pore architecture. Importantly, in developing this new technique, the researchers have also provided a thorough characterization of the tuned MOF structures, which will assist in applications such as pharmaceutical development and drug delivery.

Reference

Benjamin Slater, Zeru Wang, Shanxue Jiang, Matthew R. Hill, and Bradley P. Ladewig. Missing Linker Defects in a Homochiral Metal Organic Framework: Tuning the Chiral Separation Capacity. Journal of the American Chemical Society, 2017. DOI: 10.1021/jacs.7b10112 

[Article written by Ravi Shankar, a PhD student in the Department of Chemical Engineering and Creative Content Developer for the Barrer Centre.]

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Ravi Shankar

Ravi Shankar
Department of Chemical Engineering

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Email: ravi.shankar12@imperial.ac.uk

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