16558 Phys. Chem. Chem. Phys., 2011, 13, 16558–16568 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 16558–16568 From microscopic insights of H 2 adsorption to uptake estimations in MOFsw Diego A. Gomez and German Sastre* Received 9th June 2011, Accepted 15th July 2011 DOI: 10.1039/c1cp21865d The adsorption of hydrogen in MOFs takes place mainly close to the inorganic secondary building unit (IBU). The adsorption capacities on MIL-88, UiO-66, MIL-47 and MFU-1 were investigated. Quantum chemical calculations at the ab initio HF/MP2 theoretical level were employed to estimate the maximum uptake of H 2 molecules per metallic centre. Extrapolating the results on small clusters to the unit cell of each particular MOF, the H 2 uptakes (gravimetric and volumetric) were estimated. The loading of hydrogen per metal atom (H 2 molecules/M-atom) and the density of metal atoms (M-atoms A ˚ 3 ) were defined as useful parameters to assess hydrogen storage properties and to estimate the optimum density that the material should have to be a good H 2 adsorbent. It was found that values above 3 H 2 molecules/M-atom and around 0.004 M-atoms A ˚ 3 for MOFs with densities around 0.7–1.0 g cm 3 are required to reach the 2015 storage targets. Introduction One of the most promising strategies for hydrogen storage is physisorption in porous solid materials. 1–3 Gas storage by adsorption favours fast charge and discharge kinetics and the reusability of the adsorbent. However, the main problem is the low uptake obtained at the conditions of pressure and temperature commercially required, owing to the weak physisorption energy of hydrogen. Metal–organic frameworks (MOFs) 4,5 comprise one of this kind of porous solid materials, composed of two types of secondary building units (SBUs). 6,7 An organic building unit, which is an organic skeleton with two or more donor groups (usually carboxylate or nitrogenated groups), linked to an inorganic part containing one or several transition metal atoms, the inorganic building unit (IBU), through covalent bonds. 8 MOFs have been broadly studied experimentally and theore- tically for hydrogen storage. 9–16 These studies indicate that an appropriate selection/design of those units is required 17 in order to find the proposed targets (4.5–5.5 wt% and 28–40 g L 1 for 2010–2015, respectively 18 ). The main limitations of MOFs as hydrogen storage materials can be rationalised from a macroscopic and microscopic view. Attending to macroscopic parameters, the limitations can be summarised in that, so far, there is not a MOF with a relation of density, void volume, surface area and isosteric heat of adsorption that, combined with appropriate kinetic and thermodynamic requirements for the compression/delivery process, leads to the development of a practical device for H 2 storage. Down to a microscopic level, the low isosteric heats of adsorption found can be rationalised through computational chemistry studies that show the strength, number, and location of H 2 molecules that can be adsorbed at the different active sites. Thus, it should be possible to relate the adsorption energy and the number of molecules adsorbed per centre with the chemical and topologic properties of the material. 19,20 Our goal in this study is to define the necessary conditions to find an optimum combination of chemistry and structure (topology) which, in addition to the appropriated MOF density and specific area, lead to a material with H 2 uptakes above the target proposed. To this aim we will employ first- principles calculations, and highlight the fact that most of the previous studies 21–28 have been based on the archetypal MOF-5. While this has the advantage that the computational results can be properly compared to experiments, this does not allow us to extract trends or to compare the chemical and structural traits that define high adsorption strength and large uptakes. This is why we have extended our previous study on MOF-5 and we have selected four other structures in order to achieve a more general insight. Although adsorption experiments allow us to obtain the gravimetric and volumetric uptakes as well as the isosteric Instituto de Tecnologı´a Quı´mica U.P.V.-C.S.I.C. Universidad Polite´cnica de Valencia, Avenida Los Naranjos s/n, 46022 Valencia, Spain. E-mail: gsastre@itq.upv.es w Electronic supplementary information (ESI) available: Input files of cluster structures and comparison with experimental data. Results of PM6 calculations. Adsorption of H 2 molecules obtained from HF/6-31G and MP2/6-31G calculations. Additional information about the calculation of gravimetric and volumetric uptakes. See DOI: 10.1039/c1cp21865d PCCP Dynamic Article Links www.rsc.org/pccp PAPER