Theoretical And Experimental Study On The Formation Of Defects In Uio-66 Related Materials And Their Adsorption And Separation Properties For CO₂

As a new type of porous material,the metal-organic Frameworks(MOFs)have attracted great interests in the last two decades,and more than 20,000 kinds of MOFs materials have been synthesized.Taking UiO-66 as an example,the different functionalized UiO-66 can be synthesized by changing the functional groups in the organic linkers.Because of their high surface area,large porosity,and fine tuned pore surface properties,they were widely studied in the fields including catalysis,sensing,gas adsorption and separation.The UiO-66 and UiO-66-NH2 materials have been studied for their good CO2 adsorption capacity,however,the optimal adsorption sites for CO2 in UiO-66(-NH2)were rarely reported.Recently,several research groups reported that these materials usually contain defects in UiO-66(-NH2)and the ratio of defects can be tuned by changing the conditions of synthesis.The gas adsorption.separation and catalytic performance of these materials were significantly improved because of the existence of the defects.However,the formation mechanism of defects were rarely reported.In addition,with the rapid development of computational chemistry,quantum chemistry and molecular simulations have been widely used to study the structure and electronic properties of the materials,as well as the reactions mechanism.In this study,the UiO-66 was treated w:ith ammonia hydroxide forming a new material NH3@UiO-66,and the obtained material showed a significant improvement in the adsorption capacity of CO2 gas.This thesis will discuss the following contents:(1)With the dispersion corrected density functional theory based method,DFT-D2,the binding energies of CO2 adsorption at various sites in UiO-66(-NH2)were computed and analyzed.Many different adsorption sites were considered during our calculations,and the results show that the energetically most favorable adsorption site for CO2 in UiO-66(-NH2)crystal structure is the "bottom" of the hydroxyl site,where strong hydrogen bonding exist between the O atom from the CO2 molecule and the H atoms from hydroxyl.Another favorable adsorption site for CO2 is the oxygen sites at the bottom of the cup.Compared to UiO-66,the CO2 binding in amine functionalized UiO-66-NH2 is stronger,e.g.,the binding energy for CO2 adsorption at a hydroxyl site in UiO-66 is-31.0 kJ/mol,smaller than the binding energy of-32.9 kJ/mol in UiO-66-NH2,indicating that the amine group has contribution to the improvement of CO2 adsorption.It is well-known that the effect of H2O vapor to the adsorbents is significant,and thus it is important to understand the effect of the H2O on gas adsorption and separation.Our theoretical calculations suggest that the presence of a certain amount of water in the material is beneficial to the adsorption of CO2 gas,which is supported by the enhanced binding energy of the CO2 molecules in the presence of water molecule.(2)We also discussed the formation mechanism of the defects in UiO-66 and UiO-66-NH2.Due to the complexity of the formation process of the defects,the started our research from the ab initio molecular dynamics simulations(AIMD).With the informative clues from AIMD,we tried to search for the transition states for the reaction pathways using the DFT-D2 method and the NEB method.The influence of H2O and NH3 solvent molecules on the formation of defects was considered.With the comparison of energy barriers under the different conditions,we analyzed the effect of the solvent molecule on the reation pathways and the results show that the solvent indeed reduces the energy barriers.It is worthwhile to mention that the solvent molecules can be used as the compensation of the defect sites to regulate the charge balance of the framework.(3)The UiO-66 crystal materials were synthesized according to the reported method in the literature.A new amorphous porous material was prepared by using ammonia water for further treatment.The adsorption isotherm of CO2 gas in the two materials was measured at 310 K-The comparison shows that the adsorption capabity for NH3@UiO-66 was greatly improved over UiO-66.Furthermore,the separation properties of CO2 gas in flue gas were performed for the two materials using the breakthrough experiments.Compared to UiO-66,NH3@UiO-66 performs better in the separation of CO2/N2 mixture in both dry and humid environment.