An unusual H2 sorption mechanism in PCN-14: insights from molecular simulation

Abstract

Simulations of H₂ sorption were performed in PCN-14, a metal–organic framework (MOF) that consists of Cu²⁺ ions coordinated to 5,5′-(9,10-anthracenediyl)diisophthalate (adip) linkers. This MOF displays an excess H₂ uptake of 2.70 wt% at 77 K and 1.0 atm and an initial H₂Q_st value of 8.6 kJ mol⁻¹ according to previous experimental measurements. The experimental H₂ sorption isotherms and Q_st values in PCN-14 were reproduced in simulations using well-known H₂ potentials that have been widely used for MOF–H₂ theoretical studies. H₂ sorption in PCN-14 was dominated by repulsion/dispersion energetics; this allowed the experimental observables to be reproduced by a model that includes only Lennard-Jones parameters. The most energetically favorable H₂ sorption site in PCN-14 corresponds to sorption within a small cage that is enclosed by three [Cu₂(O₂CR)₄] units and three adip linkers. The anthracenyl rings of the adip linkers represent the secondary sorption sites within the MOF. In contrast to expectations, sorption of H₂ onto the Cu²⁺ ions of the copper paddlewheels was not observed within the simulations at low loading. The simulations revealed that the open-metal sites in PCN-14 were occupied at high loading. Control simulations of H₂ sorption in PCN-14 for different cases in which the partial positive charge of one of the paddlewheel Cu²⁺ ions was increased relative to the other revealed that sorption onto the open-metal sites can be captured if there is a very high positive charge on the metal. Otherwise, the calculated partial charge for the Cu²⁺ ions in PCN-14 in this work was not high enough in magnitude to facilitate strong H₂–metal interactions in simulation. This study shows the power of using molecular simulations to elucidate an unusual H₂ sorption behavior in a MOF.