Structural Design And Separation Performance Of Advanced Nanoporous Materials

Advanced nanoporous materials, such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have unique properties including pore size adjustability, structural designability and chemical functionality, making them an intense subject of scientific research in adsorption/membrane separation. In this work, the applications of MOFs in liquid adsorption separation and COFs in gas adsorption/membrane separation were studied by using both computational and experimental methods. The separation performance of the designed MOFs and COFs were greatly enhanced by revealing the underlying mechanisms and the structure-property relationships. The main contents and findings are given as follows.1.46 COFs were collected as a database to study the structure-property relationship of COFs for postcombustion gas (CO2/N2) separation. It was revealed that the separation performance of 2D-layered COFs can be greatly enhanced by generating "splint effects" on CO2 molecules, which can be achieved through structural realignment to form slit-like pores (6-7 A, distance between vertical center of mass in adjacent layers) with suitable size in the structures. Electrostatic property study shows that a significant change with a much larger gradient between each layer in the pore channel of the realigned 2D-COF, which provides favorable micro-environment for CO2 adsorption and thus the selectivities of 2D-COFs were improved by the ratio of 5-12. On the basis of these observations, a new design strategy was proposed to strengthen the separation performance of COFs.2. The 46 COFs were further adopted to computationally predict their separation performance for other three industrial gas mixtures, CH4/H2, CO2/H2 and CO2/CH4, using pressure swing adsorption (PSA) process. The results show that the interpenetrated COFs with small pores (size 3-7 A) and structures with strong electrostatic functional groups show good capture performance for CO2. For the CH4/H2 system, COFs with small pores that can provide strong van der Waals interactions for CH4 have outstanding separation performance. Moreover, the design strategy proposed in chapter II was extended to the system of CH4/H2 and CO2/H2. It was found that the realigned 2D-COFs show high separation selectivity and working capacity, which are superior to many industrialized zeolites. This work further proved that the general applicability of the proposed "structural realigned" design strategy for 2D-COFs with enhanced separation performance.3. The "structural realigned" strategy was further applied on the design of membrane separation materials. With the aid of computational methods, a series of few-layered 2D-COF membranes were constructed based on CTF-1 to explore their capability for CO2/N2 separation. The results showed that various few-layered 2D-COF membranes can be fabricated to show very different separation performances, from nonselective to highly selective, and even to the molecular sieving level with the CO2 permeances on the order of magnitude of 105-106 GPU. Furthermore, tuning the stacking modes of the nanosheets to construct a favorable energetic microenvironment can be considered as an effective regulation and control strategy for achieving few-layered ultrathin membranes with both high flux and high selectivity. Such energetic microenvironments can be accomplished by introducing surface interacted van der Waals potential sites near the narrow interlayer passages. The designed membranes show superior separation performance than the traditional industrial membrane materials, and provide promising potential materials for the membrane separation field.4. Highly porous water-stable MIL-100(Fe, Cr), which have the same linker but different metal ions, were selected to adsorb anionic methyl orange (MO) and the cationic methylene blue (MB) in water. It is experimentally found that MIL-100(Fe, Cr) show similar adsorption performance for MB, while exhibit big difference on MO adsorption. Motivated by this interesting phenomenon MIL-100(Fe, Cr) were further used to separate a dye mixture with equal amounts of MO and MB. The results show that MIL-100(Fe) does well in the simultaneous adsorption of MO and MB, while MIL-100(Cr) selectively adsorbs MB over MO showing its advantage in MO/MB mixture separation. The DFT calculations suggest that the competitive adsorption of MO and H2O is more pronounced at the surface of the Cr solid. This work demonstrates that MOFs are promising adsorbents for dye capture and highlights that framework metal ion replacement is an efficient way to tailor MOFs for different applications in liquid separation.5. The adsorptive removal of organoarsenic compounds such as p-arsanilic acid (ASA) and roxarsone (ROX) using water stable MOFs has been investigated for the first time. MIL-100(Fe) exhibits a much higher adsorption capacity for ASA and ROX than activated carbon, zeolite (HY), goethite, and other MOFs. This selective and high adsorption over MIL-100(Fe), different from other analogous MIL-100 species, can be explained (through calculations) by the facile desorption of water from MIL-100(Fe) as well as the large replacement energyexhibited by MIL 100(Fe). Moreover, the used MIL-100(Fe) can be recycled by simply washing with acidic ethanol.