First principles study of enhanced CO2 adsorption on MOF-253 by salt-insertion

Abstract

We investigated the basic electron variation in salt-chelated MOF-253 and revealed increased CO2 adsorption by chelating PdCl2 and Cu(BF4)2 through the DFT-D2 method. The two N atoms preferentially orient to opposite side of the 2,2′-bipyridine (bpy) in bare MOF-253 to yield a CO2 adsorption energy of −0.15eV, which mainly contributes to the experimentally determined isotopic heat energy of −0.24eV. Following salt insertion, the two pyridine rings rotate to the same side to hold the salt that binds on NN atoms. While PdCl2 insertion does not significantly enhance CO2 adsorption, dipole polarization creates multiple adsorption sites by adding a PdCl bond and co-linker site, thereby increasing the adsorption capacity of the resultant material. Cu(BF4)2 chelation significantly increases the CO2 adsorption energy to −0.49eV, which is ascribed to the large positive charge of the Cu ion inducing significant electrostatic interactions with the O atom of CO2. Our calculation results demonstrate the effect of inherent electron distribution during salt insertion on CO2 adsorption and clearly explain the observed 7kJ/mol enhancement in CO2 absorption upon loading of Cu(BF4)2 on MOF-253. This study provides new insights into the selection of an ideal salt for chelation with the open bpy linker to improve gas uptake and illustrates the potential application of catalysis by MOF-253 to gas conversion.