CO2 Separation by Imide/Imine Organic Cages

Abstract

Two novel imide/imine-based organic cages have been prepared and studied as materials for the selective separation of CO₂ from N₂ and CH₄ under vacuum swing adsorption conditions. Gas adsorption on the new compounds showed selectivity for CO₂ over N₂ and CH₄ . The cages were also tested as fillers in mixed-matrix membranes for gas separation. Dense and robust membranes were obtained by loading the cages in either Matrimid® or PEEK-WC polymers. Improved gas-transport properties and selectivity for CO₂ were achieved compared to the neat polymer membranes.

Keywords: carbon capture; gas separation; mixed-matrix membranes; organic cages; porous materials.

Scheme 1

Sketches of C1 and C2 (R=ethyl).

Figure 1

a) Front and b) top views of the crystal structure of C1 in C1‐α crystals; MeCN and water molecules are omitted for clarity. c) Focus on the central portion of the cage cavity (top view), dashed cyan lines show minimum distances between HC=CH hydrogens.

Figure 2

A view of the packing along the b crystallographic axis, for the C1‐α phase (MeCN and crystallization water molecules have been omitted for clarity). The free volume, as determined by the solvent‐accessible surface for a molecular probe of 1.72 Å radius, is reported in yellow. H‐bonding interactions are highlighted as dashed lines. H atoms are drawn as small sticks, except for the ones involved in C−H⋅⋅⋅O hydrogen bonds.

Figure 3

a) Front and b) top views of the crystal structure of C2 in C2‐β; DMSO and water molecules are omitted for clarity; c) Focus on the central portion of the cage cavity (top view), dashed cyan lines show minimum distances between HC=CH hydrogens.

Figure 4

A view of the packing along the b‐axis for the C2‐β phase, obtained by omitting DMSO and water crystallization molecules. The free volume is reported in yellow, as calculated by the solvent‐accessible surface for a molecular probe of 1.72 Å radius; H‐bonding interactions are highlighted by dashed lines. H‐atoms are drawn as small sticks, except for the ones involved in C−H⋅⋅⋅O hydrogen bonds.

Figure 5

a) Front and b) top views of the two cage molecules per asymmetric unit (highlighted in blue and yellow) of the C2‐δ phase. Hydrogens and solvent molecules are omitted for clarity.

Figure 6

Overlay of CO₂, CH₄, and N₂ adsorption isotherms, measured at 298 K. C1 (blue) and C2 (green). p ₀=1 bar, which is the maximum pressure reached by the instrument.

Scheme 2

Chemical structures of Matrimid®9725 and PEEK‐WC.

Figure 7

Robeson plots for the a) CO₂/N₂, b) CO₂/CH₄, gas pairs. Blue and red lines correspond to 1991 and 2008 upper bounds, respectively.[ ⁶³ , ⁶⁴ ] The purple line corresponds to the 2019 upper bound. The data are reported as: •: Matrimid® 9725, ▪: Matrimid® 9725/C2, : Matrimid® 9725/C1, •: PEEK‐WC, ▪: PEEK‐WC/C2, and : PEEK‐WC/C1.

Figure 8

Correlation of the effective diffusion coefficient as a function of the molecular diameter of four light gases in a) PEEK‐WC and b) Matrimid® 9725. The various symbols are referred to: •: Matrimid® 9725, ▪: Matrimid® 9725/C2, : Matrimid® 9725/C1, •: PEEK‐WC, ▪: PEEK‐WC/C2 and : PEEK‐WC/C1.