The Effect of Surface Composition on the Selective Capture of Atmospheric CO 2 by ZIF Nanoparticles: The Case of ZIF-8.

We performed theoretical studies of CO 2 capture in atmospheric conditions by the zeolitic imidazolate framework-8 (ZIF-8) via classical Monte Carlo (MC) simulations with Metropolis sampling and classical molecular dynamics (MD) simulations in the NVT and NPT ensembles and different thermodynamic conditions. The ZIF-8 framework was described by varying unit cell dimensions in the presence of pure gases of CO 2 , N 2 , O 2 , Ar, and H 2 O steam as well as binary mixtures of CO 2 :N 2 and CO 2 :H 2 O in s 1:1 concentration. Different chemical compositions of the framework surface was considered to provide an accurate treatment of charge and charge distribution in the nanoparticle. Hence, surface groups were represented as unsaturated zinc atom (Zn +2 ), 2-methylimidazole (mImH), and deprotonated 2-methylimidazole (mIm - ). Force field reparameterization of the surface sites was required to reproduce the interactions of the gas molecules with the ZIF-8 surface consistent with quantum mechanics (QM) calculations and Born-Oppenheimer molecular dynamics (BOMD). It was observed that ZIF-8 selectively captures CO 2 due to the negligible concentrations of N 2 , O 2 , Ar, and H 2 O. These molecules spontaneously migrate to the inner pores of the framework. At the surface, there is a competitive interaction between H 2 O, CO 2 , and N 2 , for the positively charged ZIF-8 nanoparticle with a large binding energy advantage for water molecules (on average -62, -15, and -8 kcal/mol respectively). For the neutral ZIF-8 nanoparticle, the water molecules dominate the interactions due to the occurrence of hydrogen bond with the imidazolate groups at the surface. Simulations of binary mixtures of CO 2 /water steam and CO 2 /N 2 were performed to investigate binding competition between these molecules for the framework positively charged and neutral surfaces. It was found that water molecules drastically block the interaction between CO 2 molecules and the framework surface, decreasing CO 2 capture in the central pore, and CO 2 molecules fully block the interaction between N 2 molecules and the framework. These findings show that CO 2 capture by ZIF-8 is possible in atmospheric environments only upon dehydration of the atmospheric gas. It further shows that ZIF-8 capture of CO 2 from the atmospheric environment is dependent on thermodynamic conditions and can be increased by decreasing temperature and/or increasing pressure.