Metal-organic Frameworks for Sensing and Sorptive Treatment of Diverse Organic Compounds

Abstract

MOF-199 (71.7) > Carboxen-1000 (68.4) > Eu-MOF (27.9) > Carbopack X (24.3) > MOF-5 (12.7) >Tenax TA (10.6). In chapter 3, the sorptive removal mechanism of six MOFs (UiO-66, UiO-66(NH2), ZIF-67, MOF-199, MOF-5, and MIL-101(Fe)) was investigated against toluene under ambient conditions. Their interactions were assessed by the Henry’s law constant (KH) and the heat of adsorption (∆Hads¬). The equilibrated adsorption capacities of all MOFs were measured from 159 (MOF-199) to 252 mg g-1 (UiO-66(NH2). However, those values were reduced considerably with increases in humidity and temperature. Among them, the sorption pattern of UiO-66(NH2) was the most reproducible when tested over a three cycle (147 (1st) and 133 mg g-1 (3rd cycle)). The behavior of -NH terminated MOFs (UiO-66(NH2) and ZIF-67) was distinguished with those of –COOH as explained by a scheme of hypothetical potential energy profiles using mass transfer resistance and surface barrier phenomena. In chapter 4, the amino derivative of the UiO-66 MOF was investigated for its potential as a sensing probe for formaldehyde (FA) in an aqueous medium. The sensitivity and specificity of this MOF probe were virtually unaffected by the presence of other tested reference aldehydes, e.g., propionaldehyde (PA), butyraldehyde (BA), and valeraldehyde (VA). The maximum incubation time for the interaction of MOF probe against 100 ppm of FA standard was estimated as 2 min. The linear detection of FA molecules using this proposed probe was made in the range of 10 to 100 ppm with the limit of detection (LOD) at 4 ppm. The stability of the MOF for the detection of FA was estimated through a series of adsorption-desorption cycles. DFT calculations showed that the selective sensing/capture of FA corresponded to the formation of the stable non-covalent bond between FA and C6H3NH2 unit of the MOF. In addition, this interaction decreased the HOMO/LUMO gap of FA by about 0.8 eV, while such trend was not evident for all other aldehydes. Furthermore, the results of FTIR, TGA, and XPS analysis confirmed a small but visible change in chemical environment of the N-H and C-H bond of the UiO-66-NH2 upon their interactions with FA molecule. Therefore, the proposed MOF probe appeared suitable for the sensing of FA in an aqueous medium. In chapter 5, the feasibility of a highly water stable Zr-based metal-organic frameworks as a sensing probe material was explored for the detection of electron deficient nitrobenzene (NB) molecule in aqueous phase. The probe system was highly sensitive toward the NB molecule in the range of 10-100 ppm. Its quenching efficiency was measured as 95.4% at 100 ppm (linear range: 5-30 ppm). The limit of detection (LOD) of the proposed probe was estimated as 0.9 ppm. In contrast, it was not sensitive enough for other aromatic compounds (such as benzene (B), toluene (T), and chlorobenzene (CB)) with very low quenching efficiencies (<40%). For the proposed probe, the specificity toward NB molecules was highly appreciable in the presence of such co-existing components (B, T, and CB). Its sensing mechanism was ascribable to the electron transfer from the excited missed-linker induced sites of the Zr-OH to the NB molecule. This interaction was confirmed by the FTIR analysis of the UiO-66-NH2-NB material. In this thesis, I have studied the sorption and sensing properties of the MOF towards important odorous and hazardous gaseous pollutants. The novelty of this work can be explained as MOF applications have not sufficiently been made against target materials selected in this thesis (such as volatile fatty acids, phenols, and indoles). Furthermore, based on this study, the important property of the highly water-stable MOFs toward the capture of small and hazardous molecules in aqueous medium has been thoroughly evaluated for the first time. Therefore, the overall results of this study will be interpreted meaningfully to construct a roadmap for the applications of the MOF techniques in the field of sorptive treatment and sensing of volatile organic compounds.; In this thesis, the sorption and sensing properties of metal-organic frameworks (MOFs) were investigated to help develop management techniques for a list of volatile organic compounds (VOCs). The emphasis of this study was directed for the developing the treatment methods against: 1) benzene derivatives, 2) C2-C5 volatile fatty acids (VFAs), 3) phenolic, and 4) indolic compounds. The sensing property of the fluorescent MOFs was then evaluated against low molecular weight VOCs such as carbonyls (e.g., formaldehyde and propionaldehyde). In chapter 2, the sorptive behavior of three metal-organic frameworks (MOFs), i.e., MOF-5, Eu-MOF, and MOF-199 was investigated against a mixture of 14 volatile and semi-volatile organic compounds (four aromatic hydrocarbons (benzene, toluene, p-xylene, and styrene), six C2-C5 volatile fatty acids (VFAs), two phenols, and two indoles) at 5 to 10 mPa VOC partial pressures (25 °C). The selected MOFs generally exhibited the strongest affinity against semi-volatile (polar) VOC molecules (skatole), whereas the weakest affinity toward volatile (non-polar) VOC molecules (i.e., benzene). The experimental results were also supported through simulation analysis in which polar molecules were bound most strongly to MOF-199, reflecting the presence of strong interactions of Cu2+ with polar VOCs. In addition, the performance of selected MOFs was compared to three well-known commercial sorbents (Tenax TA, Carbopack X, and Carboxen 1000) under the same conditions. The estimated equilibrium adsorption capacity (mg.g-1) for all target VOCs was in the order of