Surface Modification Of Several MOFs And Their Adsorption Of CO₂/N₂/H₂O

Based on studying new metal organic framework materials (MOFs) for capture and separation of CO2as the application background of potential, this dissertation mainly focused on study of the surface modification of several MOFs and their adsorption toward of CO2/N2/H2O. With the help of molecular simulation, theoretical analysis and experimental study, technically, some MOFs were modified by postsynthetic method to obtain novel modified MOFs with high adsorption capacity toward CO2and higher selectivity for CO2/N2/H2O. Theoretically, the adsorption mechanism and the adsorption selectivity of CO2/N2/H2O on the modified MOFs were investigated. In addition, the effects of water vapor on adsorption of these MOFs were also studied. This study belongs to the fields of chemical engineering, material engineering and surface science, and has great scientific research value and practical significance.Adsorption equilibrium and kinetics of CO2on homemade metal-organic framework, MIL-101, were systematically investigated. The CO2adsorption isotherms and kinetic curves were determined by gravimetric method. Then the diffusion coefficients, the isosteric heat of adsorption and the adsorption activation energies of CO2on the MIL-101crystals were estimated. Results showed that the CO2uptake of the MIL-101was up to22.9mmol/g at30bar and298K, and was3.62mmol/g at1bar, being much higher than those of conventional AC and zeolite13X. The isosteric adsorption heat of CO2on the MIL-101crystals was in the range of4.0-28.6kJ/mol, and it was decreased with the increase of CO2uptake. And the diffusion coefficients of CO2were in the range of4.11×10-11to2.54×10-10cm2/s. Furthermore, the three consecutive CO2adsorpiton-desorption runs showed the desorption efficiency of CO2on the MIL-101was up to98%, suggesting the reproducibility and stability of this material.The imidazolate framework ZIF-8samples were modified separately by using ammonia impregnation and thermal treatment in atmosphere of N2or H2for improving their adsorption properties. Results showed that the sample modified by ammonia impregnation (A-ZIF-8) exhibited the highest uptake of CO2, having an increase of45%in comparison with the original ZIF-8sample. The adsorption selectivity of the A-ZIF-8sample for CO2/N2was increased by56.18%as compared to the original ZIF-8samples. Furthermore, the interaction between water molecule and the modified ZIF-8sample decreased, indicating that the hydropobilities of these samples were greatly improved. The application of ethylenediamine (ED) to modify the ZIF-8samples were proposed by using using postsynthetic method. Results showed that the BET surface area of the modified ZIF-8(ED-ZIF-8) was increased by39%. More importantly, its adsorption capacity of CO2per surface area was almost two times of that on the ZIF-8at298K and25bar. The IAST model was applied to estimate the CO2/N2selectivity in the mixed gas of the ED-ZIF-8. For the equimolar gas mixtures of CO2and N2, the selectivities of the ED-ZIF-8sample for CO2/N2were almost twice of those of the ZIF-8at0.1bar and0.5bar. The uptake of H2O on the ED-ZIF-8became obviously lower compared to the ZIF-8, indicating the hydrophobility of ED-ZIF-8was further enhanced.The GCMC molecular simulation and experimental methods were combined together to study the isotherms and adsorption selectivities of CO2/N2on "linker-pillar" MOFs and their modified samples. Results showed that the adsorption isotherms calculated from GCMC simulation were identical with those determined by experiments. The isotherms and adsorption selectivies of binary mixtures calculated by GCMC molecular simulation and predicted by IAST model were almost the same. The noninterpenetrated sample, NI3#, produced by introduction of-NH2group into NI1#, had not only higher CO2uptakes, but also higher CO2/N2selectivity, showing an increase of134%. For the interpenetrated sample I2#, prepared by introduction of methyl group into I1#, its adsorption selectivity of CO2/N2selectivity was increased to be as high as800.The adsorption of CO2, N2and their mitures on a new rht type of MOF, Cu-TDPAT, were investigated with the help of the GCMC molecular simulation. The effect of water on its adsorption performance was examined. Results showed that the CO2uptake on the Cu-TDPAT was far higher than those of other MOFs under low pressure due to the existence of open metal sites (OMSs) and Lewis bases sites (LBSs), and its CO2/N2selectivity was up to88(15%CO2,1atm bulk pressure). The presence of water would led to a decrease in CO2and N2uptake of the Cu-TDPAT due to strong competive adsorption of H2O with CO2and N2. However, due to much stronger competition with N2, the N2uptake decreased much seriously. Finally, in the presence of H2O, the amplitude of decrease in N2uptake uas much higher than that in CO2uptake due to much weaker competition of N2with H2O. As a result, the CO2/N2selectivity of the Cu-TDPAT was increased.The effects of electrostatic interaction, functional groups and existence of water vapor on the CO2adsorption performance of MOFs were investigated. The molecular simulation results showed that the contribution of electrostatic interactions between CO2and MOF for the CO2uptake of Cu-TDPAT was up to60%, indicating the atoms density of MOFs played a very important role in its adsorption performance. The open metal sites (OMSs), lewis basis sites (LBSs) and other functional sites (OFSs) on the MOFs usually had stronger interaction with quadrupole moment of CO2, which were helpful to enhance CO2adsorption on the MOFs. H2O molecules would have stronger competive adsorption with CO2on the OMSs active sites of MOF74-Mg and Cu-TDPAT, which could lead to a great decrease in the uptakes of CO2on these MOFs. Importantly, the uptake of N2on the Cu-TDPAT decreased more seriously, which made the CO2/N2selectivity of the Cu-TDPAT was obviously increased to be187. For other MOFs with saturated metal sites (SMSs), such as Cu(bpy-1)2(SiF6) and Cu(bpy-2)2(SiF6), they had stronger ability of withstanding water negative effects since the interaction between CO2and the SMSs was weaker. These findings have important instructional significance on preparing high performance MOFs toward CO2adsorption and separation.