Gas exclusion zones in Type II porous liquids

TL;DR

Porous liquids can have regions around their pores where gas solubility is reduced, and a new design reduces this effect for better gas uptake.

Contribution

The first observation of gas-exclusion zones around pores in Type II porous liquids and the first Cryptophane-based PL to mitigate this effect.

Findings

NoriaOEt@15C5 has equal or lower CO2 solubility than neat 15C5 at pressures above 1 bar.

Molecular dynamics reveal an exclusion zone around pores where CO2 binding is disfavored.

Cryptophane-A@Cyrene shows improved CO2 uptake compared to its neat solvent, confirming the exclusion zone hypothesis.

Abstract

Porous liquids combine permanent porosity with fluidity and may ultimately find uses which are not possible for conventional liquids or porous solids. An important general characteristic of porous liquids studied to date is that they exhibit very high gas solubilities. Here, we examine this aspect in more detail than has been done previously, in particular with regard to CO2 and CH4 solubility in the Type II porous liquid NoriaOEt@15C5 (15C5 = 15-crown-5). Whilst this porous liquid exhibits increased CH4 solubility compared to neat 15-crown-5, counterintuitively it actually exhibits equal or lower CO2 solubility than the neat solvent 15C5 at pressures above 1 bar. Molecular dynamics modelling reveals that although the pore space does provide a good binding site for gas molecules, there is an ‘exclusion zone’ around the pore space within which binding of CO2 molecules is disfavoured compared to binding within the bulk solvent. The unfavourable binding in this region arises from a number of effects, including (i) steric exclusion from the bulky covalent framework of the NoriaOEt host, and (ii) ordering of 15C5 solvent molecules in the solvation shell around the NoriaOEt. The first porous liquid to be based on the host Cryptophane-A, Cryptophane-A@Cyrene, was prepared in the expectation that the smaller framework bulk of Cryptophane-A compared to that of NoriaOEt should result in a smaller exclusion zone. Correspondingly, this porous liquid did indeed exhibit improved CO2 uptake compared to its neat solvent, supporting the assertion that the exclusion zone is at least in part due to exclusion of gas from the framework of the host. Overall, the work provides a more sophisticated understanding of gas solubility in Type II PLs and suggests some additional design considerations for achieving high solubility for a given gas. It also shows that, as well as being able to increase the solubility of certain gases PLs can also conceivably be designed to suppress the solubility of gases under some conditions, which could be useful in tuning selective dissolution.

Although gas binding in the pores of porous liquids (PLs) is favourable, we show for the first time that there is an exclusion zone around the pore which decreases gas uptake. We introduce the first Cryptophane-based PL to reduce this effect.

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