Chemical and Biomolecular Engineering - Theses
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ItemElectric field-controlled admission of guest molecules in microporous materials( 2022)Microporous materials (MPMs) are sponge-like solids with pore diameters less than 20 angstrom. They have been used in various industries and technologies, such as clean energy, healthcare, and environmental protection. Gas adsorption is one of the most important applications of MPMs and is the basis of gas separation, storage, and detection. Active regulation of pore accessibility in MPMs by external stimuli has aroused great attention in recent years. Numerous MPMs undergo structural changes in response to physical or chemical stimulation, such as guest accommodation, temperature change, light absorption, etc., which enables selective molecular adsorption. Compared to other stimuli, the electric field (E-field) can be a faster and more energy-efficient alternative. However, to date, the feasibility of using an E-field to regulate molecule adsorption in MPMs has been mainly limited to computational studies, while the experimental demonstration is still very rare. Here, for the first time, we demonstrate that the adsorption of small gas molecules in dielectric MPMs can be regulated by an external E-field. The adsorption capacity of CO2 in the metal-organic framework (MOF) MIL-53 was significantly reduced by applying an external direct current E-field gradient at adsorption. Gas separation selectivities of zeolite molecular sieves were also improved by pre-activating the zeolites with an external E-field. Our findings demonstrate the feasibility of regulating molecular adsorption in MPMs using E-fields. It proves that E-fields can be exploited to sharpen the molecular sieving capability and opens up new avenues for regulating pore accessibility in porous materials. This thesis comprehensively describes the E-field-controlled gas adsorptions in various MPMs: Chapter 1 is an extensive literature review on the gating effect of gas adsorption, which reflects existing approaches to regulate guest admission in MPMs. Chapter 2 describes the E-field-controlled CO2 adsorption in the flexible MOF MIL-53 (Al). The adsorption capacity of CO2 in MIL-53 (Al) was significantly reduced while that of NH2-MIL-53 (Al) changed insignificantly under a direct current E-field at the intensity of 286 V/mm. The Ab initio computational calculations revealed that the E-field decreased the charge transfer between the CO2 molecule and the adsorption site in the MIL-53 framework, which resulted in reduced binding energy and consequently lowered CO2 adsorption capacity. This effect was only observed in the narrow pore state MIL-53 (Al) but not in its large pore configuration. This work demonstrated the feasibility of regulating the adsorption of molecules in microporous materials using moderate E-fields. Chapter 3 demonstrates pre-activating zeolites in an external E-field can change the gas adsorption capacities and improve the gas separation selectivities. After the E-field activation, the potassium chabazite showed a higher adsorption capacity for CO2, but a lower one for CH4 and N2. The separation selectivities of CO2/CH4 and CO2/N2 were greatly improved by at least 25% after the E-field activation. Ab initio computational studies revealed the possible cation relocation in chabazites caused by the E-field, which led to the expansion of zeolite frameworks and created more favorable adsorption sites for CO2 molecules. The change of gas adsorption capacity after E-field activations was also demonstrated in zeolites TMA-Y and ZSM-25. These findings prove E-field can be exploited to sharpen the molecular sieving capability. Chapter 4 studies the feasibility to use E-field activation to improve the N2/CH4 selectivity in ZSM-25 zeolites. The E-field pre-activation of ZSM-25-K led to a gate-opening effect which significantly increased the CH4 adsorption capacity at low temperatures. It was attributed to the relocation of trapdoor potassium cations induced by the E-field. In contrast, the CH4 adsorption capacity of ZSM-25-Na was decreased after the E-field activation due to the framework expansion and the decreased heat of adsorption. The N2 adsorptions in both sodium and potassium types of ZSM-25 were remarkably improved, which partially resulted from the increase of N2 adsorption sites at the eight-membered rings. Consequently, the changes in the adsorption capacities after the E-field activation led to a higher CH4/N2 selectivity in ZSM-25-Na at the temperature range of 252-294 K and in ZSM-25-K at 294 K.