Energetics of van der Waals Adsorption on the Metal-Organic Framework NU-1000 with Zr₆-oxo, Hydroxo, and Aqua Nodes

We report measurements of adsorption isotherms and the determination of the isosteric heats of adsorption of several small gases (H₂, D₂, Ne, N₂, CO, CH₄, C₂H₆, Ar, Kr, and Xe) on the metal-organic framework (MOF) NU-1000, which is one of the most thermally stable MOFs. It has transition-metal nodes of formula Zr₆(μ₃-OH)₄(μ₃-O)₄(OH)₄(OH₂)₄ that resemble hydrated ZrO₂ clusters and can serve as catalysts or catalyst supports. The linkers in this MOF are pyrenes linked to the nodes via the carboxylate groups of benzoates. The broad range of adsorbates studied here allows us to compare trends both with adsorption on other surfaces and with density functional calculations also presented here. The experimental isotherms indicate similar filling of the MOF surface by the different gases, starting with strong adsorption sites near the Zr atoms, a result corroborated by the density functional calculations. This adsorption is followed by the filling of other adsorption sites on the nodes and organic framework. Capillary condensation occurs in wide pores after completion of a monolayer. The total amount adsorbed for all the gases is the equivalent of two complete monolayers. The experimental isosteric heats of adsorption are nearly proportional to the atom-atom (or molecule-molecule) Lennard-Jones well-depth parameters of the adsorbates but ∼13-fold larger. The density functional calculations show a similar trend but with much more scatter and heats that are usually greater (by 30%, on average).