Equilibrium adsorption isotherms at very low coverage (0 to ∼0.1 monolayer) have been measured for four simple gases, N2, CO, Ar, and CH4, at several temperatures above 98 K on the metal-organic framework (MOF) NU-1000 (which has Zr6(μ3-OH)4(μ3-O)4(OH)4(OH2)4 nodes linked by pyrenes with -COO- end groups). From these, the differential isosteric heats of adsorption (Qst) were determined versus coverage. These were compared to density functional calculations of the adsorption enthalpies on different sites, assuming that they are filled in a sequential order from the strongest to weakest binding. This comparison shows excellent quantitative agreement on the trends for the four gases, as well as reasonable agreement in the absolute magnitude of the adsorption energies. This agreement indicates that the sites predicted by the density functional calculations to be populated at different coverages for the different gases are correct, thus further increasing our understanding of adsorption on this prototype MOF.
Bibliographical noteFunding Information:
This work was supported by DOE-OBES Chemical Sciences Division under grant #DE-FG02-96ER14630 and 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 DESC0012702. Initial funding for O.E.V.’s contributions to this project was provided by NSF DMR 1206208. We gratefully thank J. T. Hupp and O. K. Farha of Northwestern University for kindly providing the NU-1000 samples and discussing the manuscript.