Molecular separation

Flexing with the flow

The tuning of metal–organic frameworks (MOFs) to control molecular adsorption has attracted considerable interest for many applications, including catalysis and molecular separation. Now, Pei-Qin Liao and colleagues report the performance of a hydrophilic MOF for the challenging separation of 1,3-butadiene (C4H6) from other C4 hydrocarbons (Science 356, 1193–1196; 2017).

C4H6 is the main ingredient for the production of synthetic rubber. It is obtained from the purification of a mixture of C4 hydrocarbons, with impurities composing 40–70% of the total mix. However, the distillation used to remove these impurities — 1-butene (n-C4H8), isobutene (i-C4H8) and butane (C4H10), which possess similar polarizabilities, boiling points and sizes — is environmentally inefficient, requiring high temperatures and pressures that may result in polymerization of C4H6, impacting rubber synthesis.

MOFs separate gases via two mechanisms: thermodynamically, by binding of target (or impurity) molecules on open metal sites; or by shape selectivity of pores that permit only target (or impurity) molecules to diffuse through. Liao and colleagues screened ten different MOFs for this purpose, and measured the breakthrough curves — the amount of gas mixture that is forced through the membrane before parity is reached between concentrations of the specific compound in the inlet and outlet — for the C4 hydrocarbons (pictured, left). Most MOFs did not permit C4H6 to breakthrough first, but one did, showing exceptional performance. This was Zn-BTM, where H2btm is bis(5-methyl-1H-1,2,4-triazol-3-yl)methane. In this MOF, different breakthrough times were recorded for each C4 hydrocarbon, enabling efficient separation. For realistic gas mixtures, the highest C4H6 purity was found to be 99.9%, above the 99.5% threshold required for rubber production.

Credit: AAAS

The mechanism underlying this separation behaviour relies on two key factors. The first is the ability of C4 hydrocarbons to rotate the central C–C bond, forming either a cis (short and thick) or a trans (long and thin) isomer (pictured, middle). These isomers are expected to show different adsorption energies and especially transport properties when passing through a MOF with narrow apertures. Second, the unique structure of Zn-BTM, containing a mix of discrete cavities interconnected by narrow apertures (pictured, left, inset), allows the preferential passage of C4 hydrocarbons in the trans isomer (pictured, right). With respect to the other molecules, C4H6 has a higher barrier to adopt the cis configuration; in fact, X-ray diffraction experiments showed that C4H6 existed solely in the trans isomer, which most easily traverses the small apertures connecting the cavities. Using periodic density functional theory, the researchers also explored the thermodynamics of binding for all host–guest structures. They found that the other molecules have higher probability to adopt the bulkier cis configuration and bind to the framework more strongly than C4H6, both factors hindering diffusion. This combination of different bonding and diffusion selectivity for the trans isomer of C4H6 resulted in substantially faster transport through the MOF, and a more efficient separation from the other C4 molecules.

It would be interesting to see if this approach of manipulating molecule isomer structure could be utilized for larger and more complicated molecules, but, as it is, this work demonstrates the potential for MOFs with well-controlled cavity networks to be used for advanced gas separation applications.

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Shevlin, S. Flexing with the flow. Nature Mater 16, 785 (2017). https://doi.org/10.1038/nmat4955

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