The development of new materials with high adsorption capacity and selectivity is becoming attractive for the applications of clean energy and environment pollution control. Metal-organic frameworks (MOFs) are promising adsorbents for gas storage and separation, such as H2 and CH4 storage, and CO2 capture, due to their extraordinarily high porosity, adjustable pore sizes, controllable surface functionality and potential scalability for industrial applications.
This thesis focuses on developing novel MOFs for selective gas adsorption with large adsorption capacities and high selectivity, as well as good thermal and chemical stability. The studies include optimizing the activation conditions for uniform and empty pores with MOFs Cu-BTC taken as a case study, designing novel MOFs structure with desired functional groups for high selectivity of CO2 over other gases, fabricating MOFs contained composites with desired electrostatic force with ZIF-8/CNTs as an example, and evaluating the selective gas adsorption performance of all the prepared materials. This thesis aims to establish the relationship between the structure features of MOFs (pore size, metal centres, surface functional groups and electrostatic force) and gas adsorption performance of MOFs (including adsorption capacity and selectivity).
The first part of experimental chapters focuses on the preparation and activation of copper-based MOFs Cu-BTC. The materials were synthesized by solvothermal method and activated by six different solvents (chloroform, dichloromethane, acetone, ethanol, methanol and water) in the activation process. The effects of different activation solvents on the thermal stability, porous structure and CO2 adsorption of Cu-BTC were investigated. The more DMF molecules were evacuated from the pores of Cu-BTC, the better adsorption performance was reflected in the material. The high crystalline and nearly solvent-free frameworks with highest BET surface area (2042 m2/g), largest pore volume (0.823 cm3/g) as well as highest CO2 loading (11.60 mmol/g at 0 °C and 132 kPa) can be achieved while using methanol as activation solvent. Then the selective adsorption of CO2/N2 and CO2/CH4 on Cu-BTC were examined through the experimental measurement of equilibrium adsorption capacities from pure fluids (CO2, CH4 and N2) and mixtures of CO2/N2 and CO2/CH4. Grand Canonical Monte Carlo (GCMC) model was performed to predict the adsorption capacities from pure fluids and binary mixtures. The GCMC model gives reasonable predictions of the measured adsorption capacities for pure gases at low pressures (<5 bar), but significantly over predicts that at pressures greater than 5 bar. The GCMC model fails to provide a satisfactory fit of the binary adsorption measurements across the entire pressure range studied. The Ideal Adsorbed Solution Theory (IAST) model using best-fit parameters for Langmuir isotherms of each pure fluid provides more satisfactory predictions of CO2/N2 and CO2/CH4 than the GCMC model. This combined experimental and modelling approach can provide criteria to screen MOFs for the separation of gas mixtures at industrially relevant compositions, temperatures and pressures. The second part of experimental chapters mainly focuses on synthesising MOFs with enhanced affinity for CO2 and selectivity of CO2 over other gases. Three novel amino-functionalized MOFs with both open metal sites (OMSs) and Levis basic sites (LBSs) were synthesized by solvothermal reactions. Single crystal structure analysis showed that Mg-ABDC and Co-ABDC were isostructural comprising two-dimensional layer structures, while Sr-ABDC contained a three-dimensional motif. These amino-functionalized MOFs were further characterized by powder X-ray diffraction, thermal gravimetric analysis and N2 ads-desorption. Adsorption isotherms of CO2 and N2 were obtained at various temperatures (0, 25 and 35 °C) and then the adsorption capacity and CO2/N2 selectivity for these MOFs were compared. Based on results, both Mg-ABDC and Co-ABDC decorated by the -NH2 groups and the open metal sites exhibit high heat of CO2 adsorption (> 30 kJ/mol) and excellent adsorption selectivity of CO2 over N2 (>375). In contrast, Sr-ABDC displays poor adsorption properties due to small pore size, low surface area and small pore volume.
Introducing desired electrostatic force into MOF structures by the incorporation of carbon nanotubes (CNTs) into MOFs can obtain better crystals and enhance the properties of composite. A series of ZIF-8/CNTs composites were successfully synthesized by solvothermal method. The contents of ZIF-8 and CNTs in the composites were calculated from Thermogravimetric Analysis data. CO2 and N2 adsorption at 273 K on the composites were also investigated and compared. Results show that there are interactions (synergetic effect) between ZIF-8 crystals and CNTs in the composites, reflected in the change of crystallinity, morphology, thermal stability, and adsorption properties. The surface area and adsorption capacities of ZIF-8/CNTs composites can be controlled by adjusting the CNTs content in the composites. In optimal CNTs loading ratio, the ZIF-8/CNTs composite showed improved adsorption capacities and selectivity of CO2/N2, illustrating that the incorporation of CNTs into MOFs synthesis is a promising approach to enhance the adsorption performance of MOFs.