Ultra-High Purity and Productivity Separation of CO2 and C2H2 from CH4 in Rigid Layered Ultramicroporous Material

Abstract

Efficiently obtaining both high-purity gas-phase and adsorbed-phase products in a single physisorption process presents the challenge of simultaneously achieving high selectivity and uptake and rapid diffusion in adsorbents. With a focus on natural gas purification and high-purity acetylene production, we report for the first time that the synergistic ligand/anion binding mode and multiple diffusion pathways in a robust 2D layered ultramicroporous framework (ZUL-100) enable unprecedented carbon dioxide/methane and acetylene/methane separation performance. Taking advantage of its rich anion, functional ligand ,and rigid 3D interpenetrated ultramicroporous channels, ZUL-100 achieved record IAST selectivities for equimolar carbon dioxide/methane (3.2 × 10⁵) and acetylene/methane (1.7 × 10¹⁰) mixtures, accompanied by record dynamic uptakes of carbon dioxide (3.10 mmol/g) and acetylene (4.79 mmol/g), respectively. The strong affinity and fast mass transfer of carbon dioxide and acetylene on ZUL-100 were systematically elucidated by a combination of in situ FTIR, single-crystal XRD, kinetic tests, and DFT-D adsorption/diffusion modeling. In particular, high-purity (≥99.999%) methane and carbon dioxide (acetylene) can both be obtained on ZUL-100 through a single adsorption-desorption cycle, with exceptional productivity (2.81-4.22 mmol/g of methane, 2.96 mmol/g of carbon dioxide, and 4.31 mmol/g of acetylene) and high yield (95.5% for carbon dioxide and 90.0% for acetylene).

Figure 1

(a) Relationship between pore size and selectivity of CO₂/CH₄ and C₂H₂/CH₄. (b) Topology types and pore channels in typical HUM and LHUM. (c) Three profiles of CO₂ breakthrough curves in typical HUMs and LHUM.

Figure 2

(a) Structure changes of ZUL-100 after activation. (b) Energy difference of frameworks (ΔE_framework) upon activation of ZUL-100 and representative materials with the same sql topology. (c) Local surface electrostatic potential (ESP) of activated ZUL-100. (d) ESP of guest molecules. Color code: F, cyan; C, light gray in ZUL-100 and yellow in molecules; H, white; N, blue; Cu, green; O, pink in ZUL-100 and red in CO₂; S, bright yellow; Ti, purple. Scale of the molecule mapped spans: −0.03 (red) through 0 (white) to 0.1 (blue) au of ZUL-100, −0.03 (red) through 0 (white) to 0.03 (blue) au of molecules.

Figure 3

(a) CO₂ and CH₄ adsorption isotherms on ZUL-100 at 273, 298, and 313 K. (b) C₂H₂ and CH₄ adsorption isotherms on ZUL-100 at 273, 298, and 313 K. (c) Q_st of C₂H₂, CO₂, and CH₄ of ZUL-100. (d,e) Comparison of the IAST selectivities and gas adsorption uptakes at 0.5 bar and 298 K of ZUL-100 with representative porous materials. (f) Comparison of the IAST simulated C₂H₂ purity in the adsorbed phase of ZUL-100 and representative porous materials for adsorption of an equimolar C₂H₂/CH₄ mixture.

Figure 4

(a–e) Different binding sites in the single-crystal structures of CO₂/C₂H₂-loaded ZUL-100: (a) CO₂ binding site I, (b) CO₂ binding site II, (c) C₂H₂ binding site I, (d) C₂H₂ binding site II, and (e) C₂H₂ binding site III. (f) In situ FTIR of activated ZUL-100 under a CO₂ atmosphere. Color code: F, cyan; C, light gray in ZUL-100 and orange in CO₂/C₂H₂; H, white; N, blue; Cu, green; O, red; S, bright yellow; Ti, purple.

Figure 5

Diffusion barriers of CO₂ and C₂H₂ by DFT-D calculation based on the activated ZUL-100 crystal structure. (a) CO₂ diffusion through the interlayer channel. (b) CO₂ diffusion through the intralayer channel. (c) C₂H₂ diffusion through the interlayer channel. (d) C₂H₂ diffusion through the intralayer channel.

Figure 6

(a) Breakthrough curves of equimolar CO₂/CH₄ mixture on ZUL-100 (red), SIFSIX-3-Cu (yellow), and SIFSIX-14-Cu-i (blue) at 298 K. (b) Breakthrough curves of equimolar C₂H₂/CH₄ mixture on ZUL-100 under different flow rates. (c,d) Comparison of the breakthrough uptake of ZUL-100 with those of representative MOFs for equimolar CO₂/CH₄ and C₂H₂/CH₄ mixtures at 298 K.

Figure 7

(a) Desorption curves for saturated ZUL-100 and SIFSIX-3-Cu columns of equimolar CO₂/CH₄ mixture with He purge at 313 K. (b) Desorption curves for saturated ZUL-100 of an equimolar C₂H₂/CH₄ mixture with He purge at 353 K. (c) Breakthrough time and high-purity CH₄ productivity for five consecutive cycles for the equimolar CO₂/CH₄ mixture and three cycles for the equimolar C₂H₂/CH₄ mixture at 298 K.