Gas Adsorption Properties and Selectivity in CuII/Adeninato/Carboxylato Metal–Biomolecule Frameworks



Herein we report the analysis of the porosity of a family of MBioFs with formula [Cu23-ade)2(μ-OOCCH3)2]n (compound 1), [Cu23-ade)2(μ-OOCCH2CH3)2]n (compound 2) and [Cu23-ade)2(μ-OOC(CH2)2CH3)2]n (compound 3) [ade = adeninato], by means of the measurements of N2 (77 K), CO2 (273 K) and H2 (77 K) experimental adsorption isotherms. The experimental results for each adsorptive species show that the uptake capacity decreases on increasing the length of the aliphatic chain of the carboxylato ligand. Although, in general, all compounds show a moderate experimental uptake capacity, the amount of CO2 adsorbed in compound 1 exceeds the values reported for many other well-known MOFs, which is related to the presence of the Watson–Crick face in the pore wall. The experimental data of all compounds is discussed on the basis of simulated isotherms computed from grand canonical Monte Carlo (GCMC) calculations. To assess the suitability of pores decorated by the Watson–Crick faces for the selective capture of CO2 and CO, the selectivity towards CO2/H2 and CO/H2 gas mixtures was analysed by computing the Henry constants and the binary adsorption isotherms. The results show that the selectivity towards CO2 and CO can be tuned by the carboxylato ligand, increasing its value with increasing length of the aliphatic chain. These selectivity values are relatively high in all compounds, especially for compound 3 in which the pore narrowing promoted by the lengthening of the aliphatic chain favours a preferential adsorption of CO2 and CO, leading to one of the highest values computed for MOFs to date. Moreover, the high affinity towards CO2 and CO, and the marked lowering of the selectivity with increasing pressure observed in compound 3 is explained on the basis of the analysis of the potential energy maps obtained by the simulations, which reveals two different adsorption sites.