Hydrogen Adsorption in Ultramicroporous Metal-Organic Frameworks Featuring Silent Open Metal Sites

In this study, we utilized an ultramicroporous metal-organic framework (MOF) named [Ni₃(pzdc)₂(ade)₂(H₂O)₄]·2.18H₂O (where H₃pzdc represents pyrazole-3,5-dicarboxylic acid and ade represents adenine) for hydrogen (H₂) adsorption. Upon activation, [Ni₃(pzdc)₂(ade)₂] was obtained, and in situ carbon monoxide loading by transmission infrared spectroscopy revealed the generation of open Ni(II) sites. The MOF displayed a Brunauer-Emmett-Teller (BET) surface area of 160 m²/g and a pore size of 0.67 nm. Hydrogen adsorption measurements conducted on this MOF at 77 K showed a steep increase in uptake (up to 1.93 mmol/g at 0.04 bar) at low pressure, reaching a H₂ uptake saturation at 2.11 mmol/g at ∼0.15 bar. The affinity of this MOF for H₂ was determined to be 9.7 ± 1.0 kJ/mol. In situ H₂ loading experiments supported by molecular simulations confirmed that H₂ does not bind to the open Ni(II) sites of [Ni₃(pzdc)₂(ade)₂], and the high affinity of the MOF for H₂ is attributed to the interplay of pore size, shape, and functionality.