Studies On The Adsorption And Membrane Separation Performance Of Metal-Organic Frameworks

In recent years, porous materials such as zeolites and metal-organic frameworks (MOFs) have experienced rapid development in the fields of chemical engineering, materials science, etc. In particular, MOFs have unique properties including pore size adjustability, structural designability and chemical functionality, making them a hot research field in adsorption/membrane separation, catalysis and biomedicine. Additionally, as a useful alternative to experiment, molecular simulation has the advantage of understanding the microscopic mechanism and structure-property relationships of MOFs as well as designing high-performance materials.In the present work, we study the adsorption and separation behavior of aromatic molecules in MOFs from aqueous solution and gas separation performance of MOF membranes, by a combination of experimental measurements and molecular simulations. The main contents and findings are given as follows.1. Water-stable MIL-68(Al) was selected as representative MOF to conduct the experimental adsorption isotherms of aniline from aqueous solution and the computational model was validated in this work. Furthermore, the adsorption behavior of aniline in 32 MOFs from solution was investigated computationally, and the structure-property relationships that can correlate the recovery performance of the MOFs to their physicochemical features were analysized. The results show that the interplay of the isosteric heat of adsorption of aniline and the free volume of materials is crucial to the aniline recovery capability of MOFs at low concentrations. In contrast, the aniline uptake at high concentrations is dominated by the material’s free volume as usually expected. In addition, the requirements of the best MOF candidates for aniline recovery were suggested in this study. Based on the conclusions drawn from the screening route, several novel MOFs were further computationally designed which can possess high aniline uptakes within the whole range of concentrations examined.2. Due to the similarity of their boiling points and dimensions, it is a technical challenge to separate aniline and phenol efficiently. In this work, experimental measurements were combined with molecular simulations to investigate the adsorption and separation of aniline/phenol mixtures from aqueous solutions by the aluminium terephthalate MIL-53. The results show that the framework flexibility of this material plays the crucial role in the adsorption process and thus can greatly enhance the separation of the aniline/phenol mixture from their solutions. Compared with the conventional adsorbents, MIL-53(Al) shows the best performance for such systems, both from the viewpoints of the adsorption capacities and selectivities for aniline.3. Molecular simulations were performed to build the structure-property relationships for 112 MOFs with large diversity in chemistry and structure for natural gas purification under humid condition, by taking pressure swing adsorption as example. Three evaluation criteria, namely selectivity, working capacity of targeted component and the regenerability were adopted in the present work, and the requirements of the best MOF candidates for natural gas purification under humid condition were suggested. Based on the conclusions, promising MOFs were further screened for this target separation.4. A cheap and biocompatible MOF, namely MIL-88B(Fe) was synthesized and incorporated into Matrimid(?)5218 to fabricate mixed-matrix membranes (MMMs) for gas separation. The separation performances of the MIL-88B(Fe)/Matrimid MMMs were tested for the single gas permeation of H2 and CH4 as well as the mixture gas separation of equimolar binary H2/CH4 mixture. Due to the molecular sieving effect, the incorporation of MIL-88B(Fe) can enhance H2 permeability but hinder the transport of CH4 in MIL-88B(Fe)/Matrimid MMMs, resulting in enhanced separation selectivity of H2/CH4. At 298 K and △P= 3.0 bar, compared with those of pure polymeric membrane, the H2 permeability and H2/CH4 mixture separation factor of MMMs with a MIL-88B(Fe) loading of 10% increased by 16% and 66%, respectively. In addition, operation temperature has significantly positive effect on the separation of H2/CH4 mixture.5. Post-synthesis modification is an effective strategy to enhance the gas separation performance of MOF membranes. However, the previously reported methods require that MOF surface should have reactive functional groups, thus limiting their applications in some cases. In this work, a facile modification of MOF membrane by bio-inspired polydopamine (PDA) was proposed for efficient CO2 capture. Biomimic PDA can adhere to all kinds of organic and inorganic surfaces under mild condition. Moreover, PDA has a large amount of amino and hydroxy groups, which may strongly interact with CO2. As the proof-of-concept experiment, typical ZIF-8 membranes were synthesized and modified by PDA for gas separations. The result shows that at 298 K and 1 bar, the ideal selectivity of H2, CH4 and N2 over CO2 for the modified ZIF-8 membrane was improved to 19.0,4.56 and 5.91 respectively, which far exceeded the corresponding Knudsen coefficients (4.69,1.65 and 1.25). In addition, to illustrate the versatility of this general method, graphene oxide was modified by PDA to fabricate the CO2 selective graphene oxide membrane. The obtained information shows that PDA can serve as a facile and powerful platform for the post-synthesis modification of membranes.