Cu-ATC vs. Cu-BTC: comparing the H2 adsorption mechanism through experiment, molecular simulation, and inelastic neutron scattering studies

Abstract

A combined experimental, inelastic neutron scattering (INS), and theoretical study of H₂ adsorption was carried out in Cu-ATC and Cu-BTC, two metal–organic frameworks (MOFs) that consist of Cu²⁺ ions coordinated to 1,3,5,7-adamantanetetracarboxylate (ATC) and 1,3,5-benzenetricarboxylate (BTC) linkers, respectively. Experimental measurements revealed that Cu-ATC exhibits higher H₂ uptake at low pressures than Cu-BTC, but saturates more quickly on account of its lower surface area. This results in a higher isosteric heat of adsorption (Q_st) value at zero-coverage for Cu-ATC (12.63 kJ mol⁻¹). Grand canonical Monte Carlo (GCMC) simulations of H₂ adsorption in both MOFs produced isotherms that are in outstanding agreement with the corresponding experimental measurements at 77 and 87 K and pressures up to 1 atm. The simulations revealed that the H₂ molecules initially bind onto the Cu²⁺ ions of the copper paddlewheel ([Cu₂(O₂CR)₄]) units in both MOFs. In Cu-ATC, however, a H₂ molecule can interact with two Cu²⁺ ions of adjacent paddlewheels simultaneously, which provides for a favorable, synergistic interactions. The INS spectra of H₂ adsorbed in Cu-ATC and Cu-BTC showed neutron energy transfer peaks occurring at approximately 7.5 and 8.9 meV, respectively; these peaks correspond to the binding of H₂ onto the open-metal sites in both MOFs. The lower energy peak for Cu-ATC indicates that the adsorbed H₂ molecules experience a higher barrier to rotation and a stronger interaction with the host relative to Cu-BTC. These results were supported by two-dimensional quantum rotation calculations. This study demonstrates how differences in the H₂ adsorption mechanism between two prototypal MOFs with copper paddlewheel units can be discerned through a combination of experimental measurements and theoretical calculations.