Capturing the H 2 -Metal Interaction in Mg-MOF-74 Using Classical Polarization Tony Pham, Katherine A. Forrest, Keith McLaughlin, Juergen Eckert, and Brian Space* Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, Florida 33620-5250, United States * S Supporting Information ABSTRACT: Grand canonical Monte Carlo (GCMC) simulations of H 2 sorption were performed in Mg-MOF-74, a metal- organic framework (MOF) that displays very high H 2 sorption affinity. Experimental H 2 sorption isotherms and isosteric heats of adsorption (Q st ) values were reproduced using a general purpose materials sorption potential that includes many-body polarization interactions. In contrast, using two models that include only charge-quadrupole interactions failed to reproduce such experimental measurements even though they are the type normally employed in such classical force field calculations. Utilizing the present explicit polarizable model in GCMC simulation resulted in a Mg 2+ -H 2 distance of 2.60 Å, which is close to a previously reported value that was obtained using electronic structure methods and comparable to similar experimental measurements. The induced dipole distribution obtained from simulation assisted in the characterization of two previously identified sorption sites in the MOF: the Mg 2+ ions and the oxido group of the linkers. The calculated two-dimensional quantum rotational levels for a H 2 molecule sorbed onto the Mg 2+ ion were in good agreement with experimental inelastic neutron scattering (INS) data. Although the H 2 -metal interaction in MOFs may be thought of as a quantum mechanical effect, this study demonstrates how the interaction between the sorbate molecules and the open-metal sites in a particular highly sorbing MOF can be captured using classical simulation techniques that involve a polarizable potential. I. INTRODUCTION Metal-organic frameworks (MOFs) are nanoporous crystalline materials that are constructed with metal ions and organic ligands. Most MOFs of interest contain a three-dimensional framework that incorporate a network of pores and channels that can be used to sorb guest molecules, such as molecular hydrogen (H 2 ). The sorption process in these materials is reversible as MOFs can release the gas molecules from a mild change in conditions. It is for these reasons that MOFs offer promising potential in applications in H 2 storage. Indeed, MOFs have been considered one of the most promising candidates to approach the U.S. Department of Energy (DOE) targets for hydrogen storage. A number of different MOFs have been evaluated for their hydrogen uptake capacity through experimental measure- ments. 1 Out of all the MOFs that have been synthesized thus far, members of the M-MOF-74 series 2-12 were shown to display the highest uptakes for hydrogen at very low loading and with high initial isosteric heats of adsorption (Q st ). These MOFs are constructed by combining metal ions in the 2+ oxdiation state with 2,5-dioxido-1,4-benzenedicarboxylate (dobdc) ligands. These MOFs exhibit a honeycomb-like structure with a pore diameter of approximately 12 Å. Further, these MOFs are based upon helical chains of an octahedra (consisting of M 2+ -O coordination) that are located at the intersections of the honeycomb. Each M 2+ ion in the evacuated structure of these materials bears an open-metal site, which is a highly favorable sorption site for various guest molecules. Different analogues of this series of MOFs have been synthesized by simply substituting the metal. The Mg variant of the MOF, known as Mg-MOF-74 (or CPO-27-Mg or Mg/DOBDC), has been studied extensively for its gas sorption properties (Figure 1). This MOF currently displays one of the best sorption performances for many Received: August 14, 2014 Revised: August 28, 2014 Published: September 8, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 22683 dx.doi.org/10.1021/jp508249c | J. Phys. Chem. C 2014, 118, 22683-22690