Porous Liquids

Abstract

The aim of this article is to put forward the novel concept of porous liquids, or, more precisely, liquids with permanent microporosity. In contrast to the small, transient cavities that exist between the molecules of any liquid (here called "extrinsic" porosity), we suggest that a truly microporous liquid could exist if it had empty pores within the molecules of the liquid ("intrinsic" porosity). By using rigid host molecules with restricted access windows, any unwanted occupation of the pores could be prevented (i.e., the pores could be kept empty and available so that the liquid would be genuinely microporous). The liquid could have permanent, well-defined, empty pores capable of molecular recognition when exposed to other species (e.g., gases etc.). We stress that these phases are not the same as simple solutions of host species, in which any pores would normally be occupied by solvent molecules. In microporous liquids, any solvent molecules, if present, would be deliberately sterically excluded from the host cavities, to leave them readily accessible. Microporous liquids would be of considerable fundamental interest. They could combine properties of microporous solids, such as size- and shape-selective sorption and so forth, with the rapid mass transfer, fluidity and fast kinetics of liquids. Some synthetic approaches to these materials are discussed in this article. Also, whilst the overall concept of microporous liquids is new, literature is described which suggests that some examples have arguably already been reported, even if they have not previously been recognised and characterised in such terms.

Understanding gas capacity, guest selectivity, and diffusion in porous liquids

“…For Type 2 systems, 1 which comprise a cage or macrocycle dissolved in a bulky solvent, this means that high solubilities of the cavity-containing molecule are required. This is a difficult challenge for porous organic cages, 10,11 which often have modest solubilities ( e.g.…”

“…1 Porous liquids should exhibit properties that are familiar for porous solids, such as enhanced gas uptakes and the potential for molecular selectivity, albeit perhaps with lower pore volumes. Such materials might have unique applications in the future: for example, liquids can be pumped around in a continuous system, which could facilitate guest loading and unloading steps.…”

Porous Molecular Solids and Liquids

“…The concept and technical challenges were first discussed by James in 2007. 118 It proved challenging, however, to translate these ideas into reality. The “dam” 119 for porous liquids has now burst; since 2012, six papers have been published that describe these materials and their properties, 47−49,120−122 and five of these studies were based on shape-persistent [4 + 6] porous organic imine cages.…”

Application of computational methods to the design and characterisation of porous molecular materials

“…Porous liquids were initially a hypothetical concept. 65 However, recently, experimental evidence of these novel materials has been demonstrated. 18,66,67 This now burgeoning area of materials science 68 will undoubtedly benefit from the application of computer simulation.…”