Inelastic Neutron Scattering and Theoretical Studies of H₂ Sorption in a Dy(III)-Based Phosphine Coordination Material

Abstract

A combined inelastic neutron scattering (INS) and theoretical study of H2 sorption was performed in PCM-16, a phosphine coordination material (PCM) with the empirical formula [(CH3)2NH2]­[Dy2(tctpo)2(O2CH)] (tctpo = tris­(p-carboxylato)­triphenylphosphine oxide). INS measurements at different loadings of H2 revealed a peak occurring at low rotational tunnelling energies (ca. 5–8 meV), which corresponds to a high barrier to rotation and, therefore, a strong interaction with the host. Molecular simulations of H2 sorption in PCM-16 revealed that the H2 molecules sorbed at two main sites in the material: (1) the (CH3)2NH2 + counterions and (2) within the small pores of the framework. Two-dimensional quantum rotation calculations revealed that the peak occurring from approximately 5–8 meV in the INS spectra for PCM-16 is associated with sorption onto the (CH3)2NH2 + ions. These counterions provide for the strongest H2 sorption sites in the material, which corresponds to an isosteric heat of adsorption (Q st) value of close to 8 kJ mol–1. The calculated rotational barrier for the (CH3)2NH2 +–H2 interaction in PCM-16 (45.60 meV) is higher than those for a number of extant metal–organic frameworks (MOFs), especially those that contain open-metal sites. This study provides insights into the H2 sorption mechanism in a PCM for the first time and shows how the inclusion of counterions in porous materials is a promising method to increase the H2 sorption energetics in such materials.

High H2 Sorption Energetics in Zeolitic Imidazolate Frameworks

“…While broad INS spectra have been observed for H 2 sorbed in certain materials, [64][65][66][67][68][69] such materials are known to contain extra-framework counterions. Since the framework of ZIF-68 and ZIF-69 is not charged, the broad features of the INS spectrum for these ZIFs represent an unexpected finding.…”

“…This type of calculation was previously used to predict the rotational transitions for H 2 sorbed in various MOFs. 52,53,69,[71][72][73][74][75][76][77][78] Here, the results are discussed for calculations using the polarizable H 2 potential. For H 2 sorbed at site 1 in ZIF-68, a rotational energy level of 6.95 meV was calculated for the transition between j = 0 and the lowest j = 1 sublevel (Table S8).…”

MPMC and MCMD: Free High‐Performance Simulation Software for Atomistic Systems

“…Generally, atomistic insight is either difficult or expensive to obtain. Neutron powder diffraction, for example, has been performed on deuterium residing in the pores of MOF crystallites to obtain precise, atomistic spatial information on the location of the gas in the MOF . In general, however, it is easier to probe microscopic properties of these materials on a computer, which allows exploration of any set of thermodynamic conditions and offers exceptional numerical precision.…”

“…Our aim with MPMC and MCMD is to leverage the best hardware available to date, increase the physical accuracy of traditional force field modeling through the inclusion of explicit, long‐range many‐body polarization in both the energetics and forces. We have used both codes in many published theoretical studies but have not formally introduced them until now.…”

Chemistry of Metal-organic Frameworks Monitored by Advanced X-ray Diffraction and Scattering Techniques

“…Evaluation of framework-tohydrogen interaction during H 2 or D 2 adsorption by different neutron diffraction or scattering techniques was performed on numerous MOF systems. [28][29][30][31] Similarly, adsorption processes of other hydrogenous hosting molecules within MOFs were studied as well. Additionally, neutrons scatters on magnetic moments of nucleus, thus ND (particularly small angle neutron scattering or SANS) methods can be used for probing magnetic structure features of MOFs.…”