Regulation on C2H2/CO2 adsorption and separation by molecular rotors in metal–organic frameworks

Abstract

The introduction of molecular rotors with distinct kinetic characteristics into metal–organic frameworks (MOFs) imparts these materials with heightened responsiveness, facilitating precise control over gas molecule recognition, adsorption as well as separation. This enhancement translates into improved performance in separating light hydrocarbons. In this investigation, three MOF materials (In-NDC, SNNU-118, and SNNU-128) were synthesized with two molecular rotor-containing ligands, 1,4-naphthalenedicarboxylic acid (1,4-NDC) and 9,10-anthracenedicarboxylic acid (9,10-ADC). Among these materials, In-NDC and SNNU-118 exist as supramolecular isomers. Through meticulous adjustment of synthesis conditions, successful structural transformations were achieved. Unlike In-NDC, the rotational flexibility of the molecular rotor 1,4-NDC endows SNNU-118 with distinctive adsorption behavior, manifesting as a gate-opening effect induced by the rotation of the molecular rotor. This phenomenon is facilitated by electrostatic repulsion between 1,4-NDC and CO₂ during the adsorption process. Additionally, the electrostatic attraction between the framework and C₂H₂ molecules significantly enhances C₂H₂ adsorption at low pressure, thereby improving selectivity for C₂H₂. Compared to In-NDC, the IAST selectivity values increased from 4.55 to 17.05. Dynamic breakthrough experiments demonstrate the effective separation capabilities of both In-NDC and SNNU-118 in separating C₂H₂/CO₂ mixed gases.