Hydrogen Adsorption in Ultramicroporous Metal-organic Frameworks Possessing Silent Open Metal Sites

Herein, we report the use of an ultramicroporous (pore size <0.7 nm) metal-organic framework (MOF), [Ni3(pzdc)2(ade)2(H2O)4]2.18H2O (H3pzdc: pyrazole-3,5-dicarboxylic acid, ade: adenine), for hydrogen (H2) adsorption. Upon activation, [Ni3(pzdc)2(ade)2] was generated, and in situ carbon monoxide loading transmission infrared spectroscopy revealed that open Ni(II) sites could be generated. The MOF displayed a Brunauer-Emmett-Teller (BET) surface area of 160 m2/g. Hydrogen adsorption collected on this MOF at 77 K revealed a steep uptake at low pressure, and H2 uptake saturation was achieved at 0.15 bar. The affinity of this MOF for H2 is 9.7 1.0 kJ/mol. An interplay of in situ H2 loading experiments and computations confirmed that H2 does not bind to the open Ni(II) sites of the MOF, and the observed high affinity of the MOF for H2 is mainly attributed to its narrow pore size. To shed light on the impact of ultramicropores on H2 uptake, we experimentally compared the H2 uptake per surface area unit as a function of the pore size of other ultramicroporous, microporous, and mesoporous MOFs. Our results showcase that ultramicropores contribute the most to H2 uptake, and the size, shape, and functionality of our MOF are ideal and can be used as guiding principles for the design and synthesis of novel adsorbents for efficient H2 storage and delivery.