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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).
Bibliographical noteFunding Information:
This work was supported as part of the Inorganometallic Catalysis Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0012702. Initial funding for OEV contributions to this project was provided by NSF DMR 1206208. We are very grateful to J. T. Hupp and O. K. Farha from Northwestern University for providing us with samples of the NU-1000 Zr-MOF. E. Lindhal from the UW Physics Glassblowing shop fabricated the various Pyrex glass vials used in the measurements. O.E.V. acknowledges very useful conversations with Milton W. Cole (Pennsylvania State U.) regarding van der Waals adsorption parameters and potentials. I.S.-L. was supported by a postdoctoral fellowship from the National Council for Science and Technology of Mexico (Consejo Nacional de Ciencia y Tecnologia, CON-ACyT). Y.M. acknowledges the financial support from the National Natural Science Foundation of China (Grant 21401148).
I.S.-L. was supported by a postdoctoral fellowship from the National Council for Science and Technology of Mexico (Consejo Nacional de Ciencia y Tecnologia, CONACyT). Y.M. acknowledges the financial support from the National Natural Science Foundation of China (Grant 21401148).
© 2017 American Chemical Society.