Kai-Shiun Chang

Residual Solvent Effects on Free Volume and Performance of Fluorinated Polyimide Membranes: A Molecular Simulation Study

In this study, molecular simulation techniques were used to investigate the residual solvent effects on the free volume and performance of 6FDA-mPDA polyimide (PI) membranes. A molecular dynamics (MD) simulation was used to analyze how the residual solvent in the 6FDA-mPDA PI membrane affects the fluctuation and flexibility of the polymer segment and the free volume. The gas sorption in the membrane was analyzed by a Monte Carlo (MC) technique. The energy analysis of the MD simulation indicates that the presence of solvent molecules tends to favor the fluctuation, and flexibility of polymer segments tended to be encouraged due to the presence of solvent molecules through the energy analysis by MD simulation. The free volume analysis revealed that the free space in the membrane would be enlarged by extracting the solvent from the membrane. The gas sorption behavior, analyzed by the MC technique, showed that gas solubility increased in the lower residual solvent membranes. This was caused by the free volume released by extracting the solvent, thus providing more suitable sites for gas sorption. The sorption ability was also affected by the intermolecular attractive energy, while the free volumes inside the membranes were similar. In addition, the residual solvent effect on gas sorption would be eliminated at higher pressure. From the thermal motion analysis of gas molecules, it was found that the effective thermal motion of gaseous molecules improved as residual solvent molecules remained in the membrane matrix, but this did not have an effective influence on gas permeation.

Mesoporous Fluorocarbon-Modified Silica Aerogel Membranes Enabling Long-Term Continuous CO2 Capture with Large Absorption Flux Enhancements

The use of a membrane contactor combined with a hydrophobic porous membrane and an amine absorbent has attracted considerable attention for the capture of CO2 because of its extensive use, low operational costs, and low energy consumption. The hydrophobic porous membrane interface prevents the passage of the amine absorbent but allows the penetration of CO2 molecules that are captured by the amine absorbent. Herein, highly porous SiO2 aerogels modified with hydrophobic fluorocarbon functional groups (CF3 ) were successfully coated onto a macroporous Al2 O3 membrane; their performance in a membrane contactor for CO2 absorption is discussed. The SiO2 aerogel membrane modified with CF3 functional groups exhibits the highest CO2 absorption flux and can be continuously operated for CO2 absorption for extended periods of time. This study suggests that a SiO2 aerogel membrane modified with CF3 functional groups could potentially be used in a membrane contactor for CO2 absorption. Also, the resulting hydrophobic SiO2 aerogel membrane contactor is a promising technology for large-scale CO2 absorption during the post-combustion process in power plants.

Reusable methyltrimethoxysilane-based mesoporous water-repellent silica aerogel membranes for CO2 capture

For the first time, highly mesoporous and water-repellent SiO2 aerogels were successfully coated onto a macroporous Al2O3 membrane using methyltrimethoxysilane (MTMS) precursors. The MTMS-derived hydrophobic SiO2 aerogel membranes exhibited at least 500% higher CO2 absorption flux than the uncoated MTMS-based aerogel membranes and could be reused and continuously operated for CO2 absorption for extended periods of time. As a result, MTMS-derived water-repellent silica aerogel membrane contactors are a potential candidate for large-scale CO2 absorption in industrial power plants.

Growth of zirconia and yttria-stabilized zirconia nanorod arrays assisted by phase transition

Well-aligned, densely distributed ZrO2 nanorod arrays were fabricated using a non-catalytic, template-free metal–organic chemical vapour deposition process at a reaction temperature of 1000 °C. The reaction temperature was found to play a key role in product morphology, with particle thin films obtained at 550 °C and nanorod arrays produced at 1000 °C. Increasing the reaction temperature led to the emergence of the medium-temperature tetragonal phase from the dominant low-temperature monoclinic phase, which is advantageous for anisotropic growth necessary for the nanorod formation. With the same deposition process, yttria-stabilized zirconia nanorod arrays of polycrystalline cubic phase were fabricated by co-feeding the dopant precursor, YCl3, with the zirconia precursor, Zr(C5H7O2)4. The present work demonstrated the first example of monoclinic to tetragonal phase-transition assisted one-dimensional (1D) growth, and the concept can be extended to the formation of 1D nanostructures of materials possessing the monoclinic-tetragonal polymorphism.

Polyvinylidene Fluoride/Siloxane Nanofibrous Membranes for Long‐Term Continuous CO2‐Capture with Large Absorption‐Flux Enhancement

In a CO2 membrane contactor system, CO2 passes through a hydrophobic porous membrane in the gas phase to contact the amine absorbent in the liquid phase. Consequently, additional CO2 gas is absorbed by amine absorbents. This study examines highly porous polyvinylidene fluoride (PVDF)/siloxane nanofibrous layers that are modified with hydrophobic fluoroalkylsilane (FAS) functional groups and successfully coated onto a macroporous Al2 O3 membrane. The performance of these materials in a membrane contactor system for CO2 absorption is also investigated. Compared with pristine PVDF nanofibrous membranes, the PVDF/siloxane nanofibrous membranes exhibit greater solvent resistance and mechanical strength, making them more suitable for use in CO2 capture by the membrane contactor. The PVDF/siloxane nanofibrous layer in highly porous FAS-modified membranes can prevent the wetting of the membrane by the amine absorbent; this extends the periods of continuous CO2 absorption and results in a high CO2 absorption flux with a minimum of 500 % enhancement over that of the uncoated membranes. This study suggests the potential use of an FAS-modified PVDF/siloxane nanofibrous membrane in a membrane contactor system for CO2 absorption. The resulting hydrophobic membrane contactor also demonstrates the potential for large-scale CO2 absorption during post-combustion processes in power plants.

Molecular modelling of polyimides with intrinsic microporosity: from structural characteristics to transport behaviour

Molecular dynamics (MD) simulations and the Monte Carlo (MC) method were adopted to investigate the characteristics of polyimide (PI)-derived membranes with intrinsic microporosity. Two PI-derived membrane models, PIM-PI-1 and PIM-PI-8, were constructed to analyse the effect of the spiro-centres on the membrane structure and gas transport behaviour. The torsional angle, wide angle X-ray diffraction, free volume and cavity size of these membranes were analysed to characterize the membrane structure. The sorption and diffusion behaviours of various gases in the membranes were also investigated to evaluate the transport properties of the membranes. The spiro-centres stiffened the PI-derived membrane, resulting in the formation of a larger and more effective free volume in the PIM-PI-1 and PIM-PI-8 membranes. The free volume did not have a significant influence on gas sorption, however the nature of the gas species was the predominant factor affecting gas solubility in the membrane matrix. The diffusion analysis revealed that the increased effective free volume of the PIM-PI-1 and PIM-PI-8 membranes provided more pathways for gas molecules to transfer from one cavity to another. The increased diffusivity in these PI-derived membranes results in an increase in gas permeability. In addition, the solubility of gases, including CO2 and CH4, in the membrane increased, further improving gas permeability. The quantitative results from the molecular simulation technique agreed well with the experimental data. The combined MD and MC techniques proved to be a promising tool for the characterization of the membrane structure and gas separation processes of PI-derived membranes with spiro-centres.

Insight into the roles of ethylenediamine and hydrazine for the synthesis of ZnO micro/nanostructures using solvothermal process

In this study, we investigated the roles of two commonly used amine solvents, ethylenediamine and hydrazine, in the solvothermal process used for the formation of ZnO mirowires (MWs) and nanowires (NWs), respectively. The strong coordination between ethylenediamine and the Zn atoms of the (0001) plane of hexagonal ZnO crystalloid restricts the anisotropic [0001] growth direction of hexagonal crystalloids, resulting in the formation of ZnO MWs. Also, ethylenediamine was able to coordinate with Te to form [Te–EN]2− complexes, decreasing the possibility for coordination between ethylenediamine and the Zn atoms of the (0001) plane of the hexagonal ZnO crystalloid. As a result, when Te was added in the solvothermal process using ethylenediamine, ZnO NWs were prepared instead of ZnO MWs.