Heats of Adsorption of N2, CO, Ar, and CH4 versus Coverage on the Zr-Based MOF NU-1000: Measurements and DFT Calculations

Abstract

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.