Robustness and Thermophysical Properties of MOF-5: A Prototypical Hydrogen Storage Material.

摘要

Metal-organic frameworks (MOFs) are an emerging class of microporous, crystalline materials with potential applications in the capture, storage, and separation of gasses. Of the many known MOFs, the compound known as MOF-5 has attracted considerable attention due to its ability to store gaseous fuels at low pressure with high densities. However, low thermal conductivity and limited robustness upon exposure to water and other reactive species are two challenges which limit the application of MOF-5; similar issues plague several other MOFs. The focus of this dissertation is to understand and overcome these shortcomings through detailed experimental and computational characterization of the prototype compound, MOF-5. The insight provided by this study regarding the properties of MOFs will aid in the transition of these materials from lab bench to applications.;Improvements to the thermal conductivity of MOF-5 are demonstrated using densified pellets consisting of a physical mixture of MOF-5 and expanded natural graphite (ENG). The high-aspect ratio of ENG particles, combined with uni-axial compression, results in anisotropic microstructural and thermal transport properties in the pellets. Perpendicular to the pressing direction the thermal conductivity was observed to be two to four times higher than in the orthogonal direction. We further demonstrate that this anisotropy can be exploited to enhance conductivity along a preferred direction in the pellets by altering the pellet processing conditions. We conclude that the low thermal conductivity typical of MOFs can be improved using a judicious combination of second phase additions and processing techniques.;Regarding robustness, we first quantify experimentally the impact of humid air exposure on the properties of MOF-5 as a function of exposure time, humidity level, and morphology (i.e., powders vs. pellets). For humidity levels below 50% only minor degradation is observed for exposure times up to several hours. In contrast, irreversible degradation occurs in a matter of minutes for exposures above the 50% threshold. This transition in performance can be linked to the shape of the water isotherm, which shows a large increase in uptake at ~50% relative humidity. Densification into pellets can slow the degradation of MOF-5 significantly, and may present a pathway to enhance the stability of some MOFs.;We subsequently examined the thermodynamics and kinetics of water adsorption/insertion into MOF-5 using van der Waals-augmented Density Functional Theory calculations and transition state finding techniques. Adsorption and insertion energetics were evaluated as a function of water coverage while accounting for the full periodicity of the MOF-5 crystal structure, i.e., without resorting to cluster approximations or structure simplification. We find that incoming water molecules preferentially adsorb at adjacent sites on Zn-O clusters rather than filling widely separated low energy sites. Our calculations also suggest that the thermodynamics of MOF hydrolysis are coverage dependent: water insertion into the framework becomes exothermic (with a low, 0.17eV activation barrier) only after a sufficient number of H2O molecules are adsorbed on a Zn-O cluster. This observation is in good agreement with experimental measurements, which show that hydrolysis is slow at low water coverages and is preceded by an incubation period.;The third component in our study of MOF-5 robustness involved cyclic and static exposure to impure hydrogen gas. Five impurity gas mixtures were prepared by introducing low levels of selected contaminants (NH3, H2S, HCl, H2O, CO, CO2, CH4, O 2, N2, and He) to high-purity hydrogen gas. MOF-5 was exposed to these mixtures over hundreds of adsorption/desorption pressure cycles and for extended periods of static exposure lasting up to 1 week. Hydrogen chloride was the only impurity that yielded a measurable decrease in hydrogen storage capacity. Post-cycling and post-storage samples were analyzed using FTIR spectroscopy and x-ray diffraction. These analyses reveal slight changes in the spectra (compared to the pristine samples) only for those samples exposed to HCl and NH3 impurities.;In closing, we briefly examine hydrogen permeation into- and the internal structure of- MOF-5 pellets using neutron and x-ray imaging techniques (tomography and radiography). Neutron tomography reveals the 3-dimensional distribution of the ENG network inside MOF-5/ENG composite pellets. MicroCT allows for the characterization of density variations within MOF-5 pucks. In situ neutron radiography shows that hydrogen permeation is rapid in densified MOF-5 bodies.