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Immobilisation of Liquid Solvents Inside PDMS for Carbon Capture

In recent years, a number of novel liquid solvents have been proposed for carbon capture and storage operations, including ionic liquids [1], pipperazine [2], polarity swing solvents [3] and potassium carbonate solutions [4,5]. These solvents typically have some benefits over traditional solvents such as aqueous monoethanolamine; for example, ionic liquids may have extremely small vapour pressures [1], and many new solvents require (in principle) much less energy for regeneration [6]. However these `advanced’ solvents may also be viscous, volatile, corrosive, precipitating, or absorb carbon dioxide slowly, making them difficult to handle in practice. Microencapsulation of liquid solvents (MECS) has recently been proposed as a means of utilising such solvents in a practical way [7]. MECS are small (~200 micron) capsules with a polydimethylsiloxane (PMDS) shell and an interior containing the liquid solvent of interest. Due to their small size, MECS have 1-2 orders of magnitude greater surface area than a traditional packed column (substantially increasing mass transfer rates) while the isolation of the solvent from the environment potentially renders inconvenient physical properties of the solvent irrelevant. In principle, the MECS paradigm may reduce absorber volumes in a wide range of gas-separation operations. At present MECS are manufactured inside microfluidic devices at the rate of a few grams per hour, and the most significant challenge the technology faces is upscaling manufacture to the large (multi-tonne) scale required for industrial gas-separation. We propose an alternative material, with many of the desirable properties of MECS, but which can be manufactured via a scalable, one-pot method. Rather than creating capsules, we create a solventin-PDMS gel, which can be shaped into thin particles or sheets with very high surface areas. The gel is manufactured by first creating a concentrated solvent-in-PDMS emulsion, stabilised by suitable surfactants. In order to make particles, this emulsion is then dispersed as a double-emulsion in an aqueous solution of similar composition to the internal aqueous phase (to minimise loss of solvent), though small quantities of ethanol or Tween 20 may be added to the continuous phase to control the droplet size. Alternatively, the emulsion may spread as a thin film. Finally, the PDMS is cross-linked in situ with UV light, solidifying the gel. As PDMS has a very large CO2 permeability, gas flux into these materials is comparable with that into MECS particles, though they may prove simpler to manufacture at scale. Following Vericella et al. [7], gels materials were manufactured containing 50wt% of a potassium carbonate solution (3-30wt%) and 50wt% of a commercially available PDMS, Semicosil 949 UV. Mass transfer measurements were conducted, and were found to be in good agreement with a continuum model (Figure 1). The internal structure of the material was characterised via confocal microscopy (Figure 2). Finally, cyclic stability experiments were conducted, in which the material