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

by Tony Pham, Katherine A. Forrest, Zheng Niu, Brant Tudor, Chloe B. Starkey, Yue Wang, Mohamed Eddaoudi, Nathaniel Rosi, Gisela Orcajo, Juergen Eckert, Shengqian Ma, Brian Space
Year: 2023 DOI: 10.1039/D3TA04748B


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 (Qst) 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.