Structural flexibility of a copper-based metal–organic framework: sorption of C4-hydrocarbons and in situ XRD

Abstract

Pure component sorption isotherms of n-butane, isobutane, 1-butene and isobutene on the metal–organic framework (MOF) ³_∞[Cu₄(μ₄-O)(μ₂-OH)₂(Me₂trz-pba)₄] at various temperatures between 283 K and 343 K and pressures up to 300 kPa are presented. The isotherms show a stepwise pore filling which is typical for structurally flexible materials with broad adsorption–desorption hysteresis loops. Gate opening pressures in their endemic characteristic depend on the used hydrocarbon gases. From all investigated gases only the isotherms of 1-butene present a second step at a relative pressure above p/p₀ = 0.55. As a consequence, only 1-butene can fully open the framework resulting in a pore volume of 0.54 cm³ g⁻¹. This result is in good agreement with the value of 0.59 cm³ g⁻¹ calculated based on single crystal structure data. The isosteric heat of adsorption was calculated from the experimental isotherms for all C₄-isomers. At low loadings the isosteric heat is in a narrow region between 41 and 49 kJ mol⁻¹. Moreover, in situ XRD measurements at different relative hydrocarbon pressures were performed at 298 K for the C₄-isomers. The differences in the pressure-depending powder diffraction patterns indicate phase transitions as a result of adsorption. Similar diffraction patterns were observed for all C₄-hydrocarbons, except 1-butene, where the second step at higher relative pressure (p/p₀ > 0.55) is accompanied by an additional phase transition. This powder pattern resembles that of the as-synthesized MOF material containing solvent molecules in the pore system. The resulting structural changes of the material during guest and pressure induced external stimuli are evidenced by the new coupled XRD adsorption equipment.