Effects of acid gas interactions on the adsorption and separation properties of zeolitic imidazolate framework (ZIF) materials
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Separation technology is central to industrial chemical processes, and accounts for ~15% of the world’s energy consumption. Large energy reduction opportunities exist if traditional separation technologies such as distillation can be replaced or augmented by materials-based, energy efficient processes such as adsorption or membrane separation. The efficacy of such technologies depends on the performance of the materials used as separation agents, which interact at a molecular level with the targeted components of the feed stream in order to accomplish their separation. Among various classes of materials, nanoporous Zeolitic Imidazolate Frameworks (ZIFs), a subclass of Metal Organic Frameworks (MOFs), have been shown to have tremendous potential for applications in chemical separations. This can be attributed to their diverse capabilities for selective separation of molecules, relative thermal and chemical stability among MOFs, and the possibility of fine control over their pore dimensions and functionality through judicious selection of linkers and synthesis conditions. However, the successful translation of these materials in industrial separations not only depends on their selectivity and throughput in a particular separation process, but also on their stability under realistic process conditions. A number of practical applications involve complex mixtures of molecules including acid gas species (e.g., CO2, SOx, NOx, H2S), whose presence can lead to irreversible structural changes in ZIFs and have a detrimental effect on the separation performance. The overall goal of this thesis is to systematically investigate the effects of various acid gases (CO2, SO2, and NO2) on the stability of ZIF materials under different conditions of interest, leading to a more quantitative and generalized mechanistic understanding of ZIF-acid gas interactions. Three objectives are defined to achieve this goal: i) establish a general framework for systematic investigation of acid gas stability of ZIFs, with particular focus on the underlying mechanistic routes of ZIF degradation, specific to each acid gas, ii) create an extensive information database on the stability of different ZIF materials to acid gas species for their immediate selection and application in separation processes, and iii) develop a quantitative approach to the ZIF degradation process from fundamental knowledge, allowing predictions of material stability in acid gas environments. This first objective led to the establishment of a general set of characterization tools, to probe the bulk stability of all ZIF materials to different acid gases and the associated reaction mechanisms. This toolkit was first applied to study the effects of the acid gas CO2 on ZIF stability in Chapter 2, which motivated further investigations on interactions of ZIFs with stronger acid gases SO2 and NO2 in Chapters 3-6. The general investigative framework was applied successfully to 16 ZIF materials and all three acid gases studied in the thesis, revealing striking differences in reaction pathways specific to the properties of individual ZIFs and the acid gas. The second objective led to a large expansion of the (previously very limited) knowledge base on the acid gas stability of ZIF materials, through documentation of individual observations for a library of more than 15 different ZIFs with varying functionalities and crystal structures, towards humid air, water, dry and humid CO2 and SO2 gases. The stability chart in Chapter 4 summarizes these observations, allowing rapid selection of stable ZIF materials towards practical separation applications. Three characteristic ZIFs were then selected to study ZIF-NO2 stability in Chapter 5 providing valuable insights on the action of this acid gas which is different from CO2 or SO2. The third objective led to the emergence of a quantitative approach to the stability of ZIF materials on acid gas exposure. The rate of humid SO2 induced degradation reactions were measured for different ZIFs and statistically correlated with multiple characteristic material properties. The surprising results of this approach provided new insights on material stability in acid gases and are presented in Chapter 4. The relation between the degradation rate and relative humidity is explored in Chapter 6, leading to the development of the first predictive model of the durability of a ZIF material towards an acid gas over long time scales.