Excitation generated preferential binding sites for ethane on porous carbon-copper porphyrin sorbents: ethane/ethylene adsorptive separation improved by light

Abstract

Energy-efficient separation of C₂H₆/C₂H₄ is a great challenge, for which adsorptive separation is very promising. C₂H₆-selective adsorption has big implications, while the design of C₂H₆-sorbents with ideal adsorption capability, particularly with the C₂H₆/C₂H₄-selectivity exceeded 2.0, is still challenging. Instead of the current strategies such as chemical modification or pore space modulation, we propose a new methodology for the design of C₂H₆-sorbents. With a Cu-TCPP [TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin] framework dispersed onto a microporous carbon and a hierarchical-pore carbon, two composite sorbents are fabricated. The composite sorbents exhibit enhanced C₂H₆-selective adsorption capabilities with visible light, particularly the composite sorbent based on the hierarchical-pore carbon, whose C₂H₆ and C₂H₄ adsorption capacities (0 °C, 1 bar) are targetedly increased by 27% and only 1.8% with visible light, and therefore, an C₂H₆-selectivity (C₂H₆/C₂H₄ = 10/90, v/v) of 4.8 can be realized. With visible light, the adsorption force of the C₂H₆ molecule can be asymmetrically enhanced by the excitation enriched electron density over the adsorption sites formed via the close interaction between the Cu-TCPP and the carbon layer, whereas that of the C₂H₄ molecule is symmetrically altered and the forces cancelled each other out. This strategy may open up a new route for energy-efficient adsorptive separation of C₂H₆/C₂H₄ with light.

Fig. 1. The scheme for composite sorbent construction and the adsorptive separation of C₂H₆/C₂H₄ enhanced by visible radiation.

Fig. 2. Textural and optoelectronic properties. (A) The HRTEM image of the representative CuT/HC sample. (B) The XPS deconvolution for the Cu 2p_3/2 of CuT, CuT/MC, and CuT/HC. (C) The N₂ adsorption–desorption isotherms of the pristine and composite materials. (D) The pore diameter distributions of the pristine and composite materials.

Fig. 3. The static adsorption and dynamic breakthrough results of C₂H₆ and C₂H₄ tested with visible radiation and in the dark over the composite sorbents at 0 °C. (A) The static adsorption isotherms, in which the blue arrow indicates the variation trend of the C₂H₆ adsorption isotherm under visible radiation with respect to that in the dark, and the red arrow shows the variation trend of the C₂H₄ adsorption isotherm. (B) The rates of change for the light-responsive adsorption capacities of C₂H₆ and C₂H₄ at 1 bar. (C) The breakthrough profiles of C₂H₄ and C₂H₆ over CuT/MC and CuT/HC with visible radiation and in the dark, in which the arrows indicate the variation trends of the breakthrough profiles under visible radiation with respect to that in the dark. (D) The calculated IAST selectivity at 0 °C and 1 bar on the variable of the molar fraction of C₂H₆.

Fig. 4. The DCD images of the excited composite sites adsorbing C₂H₄ and C₂H₆ with respect to their ground states. DCD = CD (excited state) − CD (ground state); DCD: differential charge density; CD: charge density. (A) and (B) The DCD images of the excited CP-M site respectively adsorbing C₂H₄ and C₂H₆, in which the CP-M simulates the micropore structures in the composite sorbent, constructed via placing two CuT 2D-framework layers near the outside of the two carbon layers with a c-axial distance of 1 nm. (C) and (D) The DCD images of the excited CP-L site respectively adsorbing C₂H₄ and C₂H₆, in which the CP-L simulates the mesopore and large pore structures in the composite sorbent, constructed via locating a CuT 2D-framework layer near a carbon layer.