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.
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ItemA narrow pore zeolite: ZSM-25 for natural gas purification( 2019)Due to both increased greenhouse gas emissions and increased natural gas demand, the development of separating CO2 and N2 from methane-rich streams (e.g. natural gas, biogas and landfill gas) has arisen worldwide research interest. Greenhouse gas emissions can be mitigated by post-combustion technology and switching the energy structure to CH4-based energy sources. Natural gas is the most significant source of CH4, which typically contains around 80%-95% CH4, less than 10% CO2 and N2, and small amounts of hydrocarbons. Hence, removing the CO2 and N2 is critical for purifying natural gas, with the respect of both increasing the energy density and preventing the corrosion of the pipeline caused by acid CO2 for transporting CH4. Adsorption-based capture of CO2 and N2 from natural gas has attracted tremendous interest owing to its economic advantages. Porous materials play very important roles in the adsorption process where the material is exposed to the gas mixture at high pressure and then desorbs at low pressure or vacuum. The significant index of evaluating a porous material is the selectivity, capacity, adsorption kinetics and regenerability. Narrow-pore zeolite (8MR zeolite) has significant potential in natural gas purification via pressure-swing adsorption (PSA), which is attributed to its pore size fitting between CO2 and CH4, and close to N2. Hence, the selectivity is relatively much higher than other zeolites (e.g. FAU, ZSM-5). However, the slow adsorption kinetics are limiting its application in the natural gas industry, and no zeolites have been found with preferential adsorbing N2 from CH4 at equilibrium, unable to effectively separate N2 from natural gas. This thesis describes the development of small-pore ZSM-25 based zeolites, and their applications in membrane separations. The study provides a rational strategy of designing ZSM-25 zeolite for effective CO2/CH4 and N2/CH4 separation in the natural gas purification industry. In this thesis, an extensive literature on 8MR zeolite for natural gas purification and their modification approaches has been sourced and analyzed in Chapter 1. Chapter 2 a Li+/ZSM-25 zeolite (LZZ) was developed via partial ion exchange of the Na+ with Li+. This exchange enabled higher CO2 capacity and adsorption kinetics due to higher pore volume and stronger affinity of CO2 with Li+, and the ultra-high CO2/CH4 selectivity remained. The CO2 isotherms showed deviation from typical Type I isotherm and 'breathing' behavior. This observation was explained by synchrotron in situ X-ray powder diffraction, demonstrating a gradual structural expansion induced by CO2. This expansion resulted in the increased CH4 admission in binary gas adsorption. This work enables the possibility of applying small-pore zeolites in natural gas purification which are kinetically-limited. Chapter 3 The Li+/ZSM-25 zeolite (LZZ) was incorporated into a commercial polymer Matrimid 5218 yielding a mixed-matrix-membrane (MMM). Li+/ZSM-25 was chosen as filler because of its fitting pore diameter between CO2 and CH4, which merely adsorbed CH4 while allowing considerable CO2 transport. The CO2/CH4 separation performance of the optimal MMMs at 5 wt% filler loading, showed higher CO2/ CH4 selectivity than that of the pristine Matrimid in both single- and mixed-gas separation. The dominant molecular sieving effect contributed to the increasing selectivity with increased pressure, showing unusual plasticization-resistance behavior. The optimized membrane (M-5) achieved ideal CO2/CH4 selectivity of 169, which surpassed the latest CO2/CH4 upper bound. Chapter 4 A new 'trapdoor' material K-ZSM-25 was designed for N2/CH4 separation by incorporating K+ as a 'door-keeping' cation. The extent of the temperature-dependent oscillations of the K+ cation regulated the accessibility of the cage, controlling the adsorption capacity of the material. There were distinguishable gate-opening temperatures (Ts) between N2 and CH4 molecules. Within this temperature range, N2 molecules had full access to the pathway into the cage, while CH4 molecules were hindered due to the blockage of K+. Both the experimental results and simulations demonstrated that K-ZSM-25 can achieve effective N2/CH4 separation at around ambient temperature with outstanding selectivity of over 30 in single gas adsorption and 5.7 in dynamic breakthrough simulation. The large N2 capacity, outstanding N2/CH4 selectivity, fast kinetics of K-ZSM-25, and it is readily regenerated ataround room temperature, all of which makes this adsorbent ideally suited to PSA-based industrial separations.