Experimental and Theoretical Investigations of CO 2 Sorption by a 3D In‐MOF with Multiple 1D Channels

Abstract

The CO2 sorption capacity of the 3D In-MOF (Et2NH2)[In(2,6-NDC)2]·2H2O·DEF (I) (where 2,6-NDC is 2,6-naphthalenedicarboxylate and DEF is N,N-diethylformamide) was investigated. The solvent-free I contains three distinct types of 1D micropores with different shapes and pore dimensions. The evacuated I sorbed 297.2 cm3 g–1 (13.3 mmol g–1) of CO2 at 196 K, 72.2 cm3 g–1 (3.22 mmol g–1) at 273 K, and 39.8 cm3 g–1 (1.78 mmol g–1) at 298 K. The difference between the uptake at 196 K and those at 273 and 298 K is relatively large. The shapes of the sorption isotherms are also dramatically different. At 196 K, the adsorption–desorption isotherms are S-shaped without significant hysteretic behavior between the adsorption and desorption branches. On the contrary, the adsorption isotherms measured at 273 and 298 K fit well with the Langmuir–Freundlich equation. The low-surface-coverage isosteric heat (Qst) of CO2 adsorption by I is 19.6 kJ mol–1. Detailed estimations of the adsorption sites for CO2 were then performed by DFT calculations. The calculated binding energies were typically dependent on the dimension of the micropores when nonbonding (van der Waals) interactions were considered; the larger the micropore dimensions, the smaller the calculated binding energy. In addition, not only the framework CH···OCO contacts for Ch3 but also the counter-cation CH···OCO contacts for Ch1 and Ch2 were found to be important.