Gas Adsorption Characteristics of Metal-Organic Frameworks via Quartz Crystal Microbalance Techniques

We report an experimental investigation of the adsorption properties of two important small-pore metal-organic framework (MOF) materials recently identified for gas separation applications, through the development and use of a high-pressure/high-temperature quartz crystal microbalance (QCM) device. In particular, we characterize in detail the CO2, CH4, and N-2 adsorption characteristics of the MOFs Cu(4,4'-(hexafluoroisopropylidene)bisbenzoate)(1.5) (referred to as Cu-hfipbb) and zeolitic imidazolate framework-90 (ZIF-90). We first describe the construction of a QCM-based adsorption measurement apparatus. Single-component adsorption isotherms of CO2, CH4, and N-2 in the two MOFs were then measured at temperatures ranging from 30 to 70 C and pressures ranging from 0.3 to 110 psi. In both materials, the order of adsorption strength is CO2 > CH4 > N-2. We find that adsorption in the 1-D channels of Cu-hfipbb can be well described by a single-site Langmuir model. On the other hand, adsorption in ZIF-90 follows a more complex behavior, commensurate with its pore structure consisting of large porous cages connected in three dimensions by small windows. The nongravimetric QCM-based measurement techniques are shown to be a valuable microanalytical tool for the study of molecular adsorption in MOFs.