Anomalous aggregation state of deuterium molecules in the nanoscale pores of a metal organic framework

Abstract

The behavior of hydrogen molecules adsorbed onto the nanospace of porous materials is of great interest but is poorly understood. Here we show direct visualization of deuterium molecules adsorbed on isoreticular metal organic frameworks ͑IRMOF-1͒ at 77 K using reverse Monte Carlo simulation based on first principle molecular dynamics simulation and neutron diffraction. Results show that the two types of adsorption sites around a ZnO 4 cluster are almost fully occupied by the deuterium molecules but that other sites are not fully occupied. Moreover an interesting information about the aggregation state of deuterium molecules was directly obtained from the deuteriumdeuterium partial pair distribution function. Namely, the average distance of deuterium molecules adsorbed onto IRMOF-1 is slightly longer than that in a solid state but much shorter than that in the corresponding gas state.

Structural Analysis of Molecular Materials Using the Pair Distribution Function

“…The loading of molecular gas and liquid species into microporous hosts has also been studied, for example, the substructures of H 2 and N 2 loaded into Prussian Blue analogues. , H 2 showed a disordered, nondirectional orientation distribution within the pore, without any direct binding to Mn 2+ sites, whereas different adsorption sites with varying occupation were observed for deuterium loading into IRMOF-1 . The confinement driven structuring has also been studied for various species within nanoconfined graphitic pores. − I 2 incorporation into ZIF-8 for radioiodine capture and storage has also been investigated, showing that I 2 loading is consistent between crystalline ZIF-8 and after amorphization, which helps to inhibit subsequent release of the guest molecules .…”

“…Besides the structural states of the frameworks and their components, real-space studies have flourished in helping to verify and elucidate the mechanisms of functionalization processes. PDFs are uniquely sensitive for identifying the presence, site-isolation, and structural states of species added into the pores and/or bound to coordinatively unsaturated sites on the clusters (see section for more details), for instance by gas and liquid adsorption, − atomic layer deposition, ,− ligand exchange, nanocasting, and the incorporation or nucleation of larger complexes and nanoparticles. ,− These processes will be especially important for enabling applications in gas loading/storage, remediation, catalysis, and more. Further effects on the structural state of the framework have been investigated due to electrochemical cycling for battery applications and degradation, , and due to in situ loading and detoxification of chemical warfare agent simulants …”

Design of Improved Metal-Organic Framework (MOF) H2 Adsorbents

“…Neutron diffraction measurements were performed to obtain adsorption sites; the result showed that the main sites were around the metallic cores [16]. Taking advantage of its design flexibility, the MOF structure shown in [ Figure 1(a)], Cu 2 (3,5-pyridinedicarboxylate) 2 (=Cu 2 PDC 2 ), was designed specifically for target of supporting high volumetric and gravimetric hydrogen uptake.…”

“…[ The following are the characteristics of Cu 2 PDC 2 that we designed to provide improved hydrogen storage properties: (1) The metallic cores, which are major adsorption sites [16], were placed with a high volumetric density. The use of long linker ligands in conventional MOFs to increase specific surface area [15] lead a decrease of volumetric hydrogen uptake; (2) Linkers containing N were used to localize electrons to increase the adsorption energy; (3) A paddle wheel-shaped structure that exposes much of the metallic core was selected; (4) ABAB stacking was chosen to stabilize the exposed metallic core sites as open sites.…”