Accelerated robotic discovery of type II porous liquids

Abstract

Porous liquids are an emerging class of materials and to date little is known about how to best design their properties. For example, bulky solvents are required that are size-excluded from the pores in the liquid, along with high concentrations of the porous component, but both of these factors may also contribute to higher viscosities, which are undesirable. Hence, the inherent multivariate nature of porous liquids makes them amenable to high-throughput optimisation strategies. Here we develop a high-throughput robotic workflow, encompassing the synthesis, characterisation and property testing of highly-soluble, vertex-disordered porous organic cages dissolved in a range of cavity-excluded solvents. As a result, we identified 29 cage-solvent combinations that combine both higher cage-cavity concentrations and more acceptable carrier solvents than the best previous examples. The most soluble materials gave three times the pore concentration of the best previously reported scrambled cage porous liquid, as demonstrated by increased gas uptake. We were also able to explore alternative methods for gas capture and release, including liberation of the gas by increasing the temperature. We also found that porous liquids can form gels at higher concentrations, trapping the gas in the pores, which could have potential applications in gas storage and transportation.

Fig. 1. (a) The three different classes of permanently porous liquids: Type I – a neat liquid containing molecules with rigid intrinsic cavities; Type II – a porous material dissolved in a cavity-excluded solvent; Type III – a porous material dispersed in a cavity-excluded solvent; (b) synthesis of a highly-soluble scrambled 3³:13³ cage mixture: relative positional isomers present in the scrambled mixture are shown, with cyclohexane vertices in red, dimethyl vertices in green, and hydrogens omitted for clarity.

Fig. 2. Graphical representation of the scrambled cage mixtures targeted in the high-throughput synthetic screen: the feed ratio of precursors added to each reaction vessel is defined by the diamine combination, Aⁿ:X^6–n (where A is 1,2-diamino-2-methyl-propane, X represents the partner diamine (B–K), and n is the number of equivalents). Unsuccessful reactions are marked with a grey circle.

Fig. 3. (a) The 14 bulky solvents screened for both cage solubility and size-exclusion from the cage cavity; (b) solubility screen of the scrambled 3³:13³ cage in these bulky solvents; the highly soluble combinations (∼300 mg mL^–1) are highlighted in green; any combinations which fell below a 50 mg mL^–1 solubility threshold (i.e., solvents 8 & 14) were not investigated further; (c) scheme illustrating the size-exclusion screen for potential porous liquid solvents – each candidate solvent was added to a known xenon-loaded porous liquid (20% w/v 3³:13³ in PCP); if little or no gas was evolved upon solvent addition, the solvent was deemed to be size-excluded; (d) the five highly solubilising candidate solvents all displaced small volumes of gas from the known porous liquid (blue bars), but not equating to the total volume in the system, as demonstrated by a subsequent addition of chloroform (grey bars). It could be concluded that the candidate solvents were size-excluded from the cage cavities.

Fig. 4. Graphical summary of the results from the high-throughput solubility screen – the solubility of the scrambled cage library was tested in six different solvents (5 new solvents, plus PCP). A ‘hit’ was determined to be a cage/solvent combination with a concentration ≥300 mg mL^–1 (green); cage–solvent combinations with a concentration between 150 and 300 mg mL^–1 in yellow; 150 to 100 mg mL^–1 in red; combinations below the 100 mg mL^–1 threshold are grey. All combinations involved diamine A. Each potential porous liquid was assigned a name based on the partner scrambling amine used in the HT synthetic screen (A–K, left), plus a number assigned to each solvent/scrambling ratio combination (1–36, bottom); for example, the 3 : 3 scrambling ratio of diamine A with diamine B in solvent 2 is B9; the 2 : 4 scrambling ratio of diamine A with diamine I in solvent 3 is I16, etc.

Fig. 5. (a) Plot summarising the results from the porosity screen using xenon evolution as a measure for porosity; (b) naming system for the porous liquid families arising from the tiered screens, for example, the scrambled cage 3³:13³ (A³:E³) in 2,4-dichlorobenzyl chloride (DCBC) is referred to as 3³:13³_DCBC. The colours show the parent cages and corresponding solvents.

Fig. 6. Effect on gas uptake of changing the solvent in Type II porous liquids. (a) Average xenon uptake measured by displacement from the neat solvents (1 mL, dashed lines) and from each porous liquid (200 mg 3³:13³ with 1 mL solvent, 20% w/v) using chloroform (1.0 molar equivalent relative to cage) – average of 3 measurements, with the standard deviation shown as error bars; (b) methane uptake in neat solvents and porous liquids (20% w/v) measured using ¹H NMR spectroscopy; (c) comparison of the methane upfield shifts in the neat solvents and the porous liquids (Sol – indicates CH₄ in neat solvent, PL – indicates CH₄ in porous liquid); (d) comparison of the difference in upfield shift of methane between the neat solvent and the porous liquid, plotted against the increase in methane uptake in each porous liquid with respect to the neat solvent.

Fig. 7. Effect of changing the scrambled cage components in Type II porous liquids on gas uptake. (a) Effect on methane (upper) and xenon (lower) uptake of changing the scrambled diamine feed ratio; (b) effect on methane (upper) and xenon (lower) uptake of changing the peripheral functionality on the scrambled cage.

Fig. 8. Changing the concentration of scrambled Type II porous liquids allows us to exceed the porosity levels in our previous scrambled porous liquids. (a) Xenon uptake in 3³:13³_HAP at different concentrations (200, 300, and 400 mg in 1 mL solvent, equating to 16, 22 and 27 wt% solutions) measured by displacement with chloroform (1.0 molar equiv. per cage), compared to the uptake in the neat solvent (HAP – dashed line); the maximum Xe uptake attained in our earlier 3³:13³_PCP porous liquid is also marked for reference; (b) xenon uptake in 3³:13³_TBA at different concentrations (200 and 400 mg in 1 mL solvent, equating to 14 and 24 wt% solutions) measured by displacement with chloroform (1.0 molar equiv. per cage), compared to the uptake in the neat solvent (TBA – dashed line); (c) comparison of the amount of xenon evolved from the two porous liquids at different concentrations compared to their viscosities; (d) methane uptake in 3³:13³_HAP at different concentrations measured by ¹H NMR spectroscopy; (e) comparison of the amount of xenon evolved from 3³:13³_HAP at different concentrations and using different release mechanisms (chemical displacement with chloroform – green; increased temperature – orange; formation of gel and release on heating – blue).