Boosting Adsorption and Selectivity of Acetylene by Nitro Functionalisation in Copper(II)-Based Metal-Organic Frameworks

Abstract

Purification and storage of acetylene (C₂H₂) are important to many industrial processes. The exploitation of metal-organic framework (MOF) materials to address the balance between selectivity for C₂H₂ vs carbon dioxide (CO₂) against maximising uptake of C₂H₂ has attracted much interest. Herein, we report that the synergy between unsaturated Cu(II) sites and functional groups, fluoro (-F), methyl (-CH₃), nitro (-NO₂) in a series of isostructural MOF materials MFM-190(R) that show exceptional adsorption and selectivity of C₂H₂. At 298 K, MFM-190(NO₂) exhibits an C₂H₂ uptake of 216 cm³ g^-1 (320 cm³ g^-1 at 273 K) at 1.0 bar and a high selectivity for C₂H₂/CO₂ (up to ~150 for v/v = 2/1) relevant to that in the industrial cracking stream. Dynamic breakthrough studies validate and confirm the excellent separation of C₂H₂/CO₂ by MFM-190(NO₂) under ambient conditions. In situ neutron powder diffraction reveals the cooperative binding, packing and selectivity of C₂H₂ by unsaturated Cu(II) sites and free -NO₂ groups.

Figure 1

Views of the crystal structures MFM‐101 and MFM‐190(R) derived from the NPD studies (Cu, cyan; C, black; O, red; N, blue; H, grey; R, green; R represents −F, −CH₃, and −NO₂). Two types of metal‐ligand cages are interconnected by sharing three [Cu₂(O₂CR)₄] paddlewheel units and three iso‐phthalate moieties.

Figure 2

(a) Adsorption isotherms of C₂H₂ in MFM‐190(NO₂), MFM‐190(CH₃) and MFM‐190(F) at 273–298 K. (b) Adsorption isotherms of C₂H₂ and CO₂ in MFM‐190(NO₂), MFM‐190(CH₃) and MFM‐190(F) at 298 K. Desorption isotherms are omitted for clarity and can be found in the Supporting Information . (c) IAST selectivities of C₂H₂/CO₂ (v/v = 1/1, 1/2) of MFM‐190(NO₂) at 298 K. (d) Comparison of state‐of‐art sorbents for C₂H₂ capacity and C₂H₂/CO₂ selectivity at 298 K and 1.0 bar.

Figure 3

(a, b) Breakthrough curves for mixtures of C₂H₂/CO₂ (a) v/v = 1/1 and (b) 2/1 diluted in He at a flow rate of 20 mL min⁻¹ at 298 K and 1.0 bar over a fixed‐bed of MFM‐190(NO₂) (sample weight: 0.55 g) and regeneration of sorbent. (c) The recyclability of MFM‐190(NO₂) for the separation of C₂H₂/CO₂ (v/v = 2/1) over three cycles (the saturated sorbent was regenerated by heating at 373 K under a flow of He for 30 mins between cycles).

Figure 4

Views of host–guest interactions in C₂D₂‐loaded MFM‐190(NO₂); all structural models were determined from Rietveld refinements of in situ NPD data collected at 10 K. The interatomic distances are quoted in angstroms (Å). The occupancy of each site has been converted into C₂D₂ per Cu for clarity. The radii of the coloured spheres are proportional to the corresponding crystallographic occupancies. Cu, cyan; C, black; O, red; N, blue; H, grey. (a) Views of the distribution of C₂D₂ in MFM‐190(NO₂)⋅(C₂D₂)_5.2. (b) Detailed views of host–guest interactions between MFM‐190(NO₂) and C₂D₂. (c) Views of the packing of adsorbed C₂D₂ molecules within cage B . (site I: yellow, site II: purple, site III: orange, site IV: green, site V: pink, site VI: violet, site VII: black)

Figure 5

Views of host–guest interactions in CO₂‐loaded MFM‐190(NO₂); all structural models were determined from Rietveld refinements of in situ NPD data collected at 10 K. The interatomic distances quoted in angstroms (Å). The occupancy of each site has been converted into CO₂ per Cu for clarity. The radii of the coloured spheres are proportional to the corresponding crystallographic occupancies. Cu, cyan; C, black; O, red; N, blue; H, grey. (a) Views of the distribution of CO₂ in MFM‐190(NO₂)⋅(CO₂)_3.1. (b) Detailed views of host–guest interactions between MFM‐190(NO₂) and CO₂ (site I: yellow, site II: purple, site III: orange, site IV: green).