Materials chemistry

Liquefied molecular holes

Porous solids have many uses in the chemical industry, which has stimulated the development of several generations of such materials. A new generation has now arrived, with the report of permanently porous liquids. See Letter p.216

Although many kinds of porous solid are known, permanently porous liquids are rare — most liquids contain only transiently formed cavities between their molecules. But on page 216 of this issue, Giri et al.1 report a trick that enables permanently porous liquids to be formed from organic cage-like molecules. Their compounds represent a new class of material that might one day be used in applications such as trapping carbon dioxide emitted from industrial sites, for which a liquid capture system has practical advantages.

Porosity is a common natural phenomenon often found in living systems, including tree bark and marine sponges. Pores have also been found in certain inorganic minerals such as stilbite2. Heating these minerals causes steam to be released, because the pores contain large amounts of water. This class of mineral was therefore named zeolites — a word derived from the greek zeo and lithos, meaning boiling stone.

Zeolites are chemically and thermally robust, and have found several applications, especially in the chemical industry. They are particularly useful as catalysts because their surface areas are larger than those of similar non-porous compounds, making them more catalytically active. But only a limited number of zeolites are available.

This problem was solved by the advent of porous solids called metal–organic frameworks (MOFs)3. In these materials, metal ions or charged metal–oxygen clusters act as the nodes of 2D or 3D molecular frameworks, with organic linker molecules as the connecting struts. This construction principle enables porous networks to be designed, and has been hugely influential — a vast number of porous solids has become available in the past few decades4 using this or related synthetic strategies. Some of these materials have large surface areas and distinct properties, such as a high adsorption capacity for CO2.

In 2009, another branch in the evolutionary tree of porous materials appeared: shape-persistent molecular cages5. In these systems, organic cage-like molecules form crystalline materials that retain the void space of the cages. An advantage of molecular cages is that they can be dissolved without any of the chemical bonds disconnecting. Such cages can therefore be thought of as soluble porous units, which means that they can be processed for incorporation into other materials or to make thin-film devices6,7.

At ambient temperature, liquids are usually composed of molecules. Organic cage molecules could therefore in principle be used to make porous organic liquids8. Liquids are desirable, because they are easier to transport than solids (because they can be pumped through tubes) and can be easier to process — for instance, they can be 'painted' onto surfaces to make thin films. So what are the molecular requirements for a compound to be liquid at ambient conditions?

The main requirement is to minimize the weak intermolecular forces that pack molecules together. Organic chemists usually do this by attaching long, sometimes branched, hydrocarbon (alkyl) chains to molecules that normally pack together. Researchers from Giri and colleagues' laboratory previously reported9 that the melting points of organic cages are indeed substantially lowered by the attachment of long alkyl chains (Fig. 1). Unfortunately, it turned out that these liquids are not porous, possibly because the alkyl chains penetrate the cage cavities and block the pores.

Figure 1: How to make permanently porous liquids.
figure1

a, Organic cage molecules pack together to form crystalline porous solids. b, When long alkyl chains are attached to the cages, the number of packing interactions decreases, and a solid cannot form at 50 °C. However, the resulting liquid is not porous because the chains penetrate the cages' cavities. c, Giri et al.1 report that cages with attached cyclic oligoether units form a permanently porous liquid when mixed with a solvent (15-crown-5), because neither the solvent nor the oligoethers can enter the cage cavities.

Giri et al. now report the successful production of permanently porous liquids using a simple trick — their cage molecules still have flexible chains attached, but interpenetration is avoided by attaching looped chains instead of linear chains. This was achieved using oligoether units (small hydrocarbon chains that are connected by oxygen atoms). The resulting compound has a melting point higher than 180 °C, and so is not a porous liquid by itself, but the authors created a porous liquid by combining the cages with a solvent called 15-crown-5 (the ratio of cage to solvent was 1:12, and probably corresponds to the highest concentration of cages that could be achieved in 15-crown-5). The molecules of this solvent are too big to enter the cages' cavities.

The 15-crown-5 solvent has a major role in maintaining the cage as part of a flowing liquid at ambient conditions. Other solvents that are too bulky to enter the cages could also be used, and the oligoether groups can be replaced by smaller groups that cannot penetrate the cavities. However, the solubilities of cage compounds that have small peripheral groups are, in general, relatively low in common organic solvents, and so the number of pores that they would create in the liquid is also relatively low.

Giri et al. demonstrate that cages with small peripheral groups can also be used to create porous liquids by making scrambled cages — a blend of cage compounds in which the six oligoether groups are replaced by a statistical mixture of pairs of methyl groups and six-membered hydrocarbon rings (see Fig. 4a of the paper1). The solubility of the scrambled cages in hexachloropropene (another solvent that is too large to enter the cavities) is greater than those of cages that contain only methyl pairs or six-membered rings as peripheral groups. The resulting liquid has a similar porosity to that of the material generated from oligoether cages and 15-crown-5, but is much less viscous. For both porous liquids, obtaining the correct combination of molecular structure and solvent is essential for success. Because the scrambled cages are easier to prepare than the oligoether cages, the porous liquid made from the scrambled cages might have more promise for future applications.

The researchers proved that their liquids are porous using a sophisticated spectroscopic technique, but also used the naked eye. When gas was adsorbed in a porous liquid and a solvent small enough to enter the cages' cavities was added, displacement of the gas by solvent was immediately detectable by the evolution of bubbles in the liquid.

The surface area per unit volume and the overall uptake of gases in the porous liquids are much lower than those of solid porous materials, so the liquids cannot compete for applications immediately — they should instead be seen as a prototype of a new class of material. But if the amount of gas that can be adsorbed within these liquids can be increased, then this new generation of porous materials will undoubtedly find technological applications, such as in liquid porous beds for efficient gas separations or gas chromatography10.Footnote 1

Notes

  1. 1.

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Mastalerz, M. Liquefied molecular holes. Nature 527, 174–175 (2015). https://doi.org/10.1038/527174a

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