Adsorption behavior of volatile organic componds in metal-organic frameworks

The purpose of this dissertation is to explore the applicability of Metal-Organic Frameworks (MOFs) to the adsorption of trace level of volatile organic compounds (VOCs) from the gas phase. The adsorption of VOCs on high surface area materials is an important process for many applications including purification and gas sampling (preconcentration). These applications often involve the complete removal of often toxic compounds that can be present at a wide range of concentrations (ppt-ppm). An effective adsorbent in applications such as gas sampling must often have the following properties: 1) complete capture of the targeted analytes without ???breakthrough???, 2) chemically inert to prevent changes in the composition of adsorbed analytes, 3) unaffected by the presence of humidity and other common gas stream components, 4) moderate interactions to allow for easy desorption and/or reactivation, and 5) sufficient chemical and thermal stability. All of the traditional microporous adsorbents which could be effectively applied to gas sampling applications (e.g. zeolites, silica gels, activated carbons, porous polymers) have limitations in one or more of the above criteria and there is an important need for novel adsorbents with improved performance. MOFs are an exciting class of crystalline nanomaterials which have been frequently explored for roughly the past decade. There is a large library of MOF materials which have been synthesized representing a wide variety of geometrical, chemical, and mechanical properties. In addition, the synthesis of MOFs allows for the capability of a priori designability of new structures possessing desired properties. MOFs are an ideal material for many adsorbent applications due to their well-defined geometries and chemistries combined with their extremely high surface areas. In addition, many MOFs show extreme thermal and chemical stability and most MOFs typically have lower heats of adsorption than other adsorbents. Despite their suitability to gas sampling applications, very little is known about the ability and mechanisms for VOC adsorption in MOFs. First, initial experiments were done investigating the applicability of zero-length column (ZLC) equilibrium and temperature-programmed desorption (TPD) applications to the characterization of VOC adsorption in MOFs. It was determined that significant complications are present in the applicability of both of these established techniques for VOC adsorption in MOFs. The analysis of TPD data in porous materials is an impossibly complex combination of competing mass transfer and equilibrium processes, and therefore it was determined that TPD was not a suitable technique for rapid screening of adsorption properties. The suitability of ZLC equilibrium studies is more promising to VOC adsorption in MOFs, however, the technique has some complications and presents no advantage over traditional chromatographic analysis. In the second part of this work, an inverse-gas chromatography (IGC) was employed to study the adsorption of a wide range of organic vapors on multiple samples of IRMOF-1 (one of the most widely studied MOFs). A wide range of molecular properties was represented in the studied analytes and standard analytical techniques were employed to extract equilibrium and thermodynamic data. Dispersive surface energies were calculated for the MOF samples and adsorption data was correlated to vapor pressure, deformation polarizability, and the Abraham linear free energy relationship (LFER). Despite differences in the surface areas of the multiple samples, the calculate heats of adsorption were found to be very similar. It was determined that the adsorption on IRMOF-1 is more complicated than previously believed. Hydrogen-bonding interactions appear to dominate the interactions of IRMOF-1 with adsorbed compounds. The effect of polarizability (resulting from nonbonded and ??-electrons) was found to have a negative effect which was attributed to a steric effect which was unaccounted for in the Abraham LFER. The dispersive surface energy was found to be much lower than traditional microporous adsorbents and this was attributed to the large pore sizes and largely organic nature of the material surface. Next, this same IGC technique was expanded to novel MOFs which are structural analogues of IRMOF-1 with added trifluoromethoxy- and methyl-functionalities. The presence of these functional groups was found to significantly alter the adsorption properties of organic compounds in these MOFs. The equilibrium constants, heats of adsorption, and dispersive surface energies of these novel MOFs were all found to be greatly reduced from the values calculated for IRMOF-1. The observed differences in adsorption are attributed to the geometric and chemical effects of the added functional groups in the MOF structures. The dominating nature of the hydrogen-bonding interactions were found to be largely unaffected by the structural modification implying that this behavior is an effect of the inorganic component of the MOFs. In the final experimental section, the IGC methodology was applied to a commercially available MOF, ZIF-8, with greatly different geometric and chemical properties from the other MOFs. A molecular-sieving effect was observed preventing the adsorption of branched-alkanes, aromatics, and heavily halogenated compounds. Hydrogen-bonding interactions were found to be important for the adsorption of alcohols and amines. However, the adsorption of hydrogen-bond bases was found to be greatly lessened from that of the other MOFs. Thermodynamics of adsorption for the organic compounds studied were calculated to be significantly reduced from IRMOF-1 and the polarizability was used to calculate the specific component of free energy. The specific component of the free energy was found to correlate well to the dipole moment of the adsorbed species. In addition, significant enhancement was seen in the adsorption of ethylene and propylene over their respective alkanes and this was attributed to a strong interaction between the ??-electrons present in the alkene with the framework structure