Synthesis And Gas Adsorption Of UiO-66Type Metal-organic Frameworks

Metal-organic frameworks （MOFs） are a new class of porous materials that are comprisedof metal ions or metal ion clusters and organic ligands （e.g. polycarboxylic aromaticmolecules, bipyridines, and polyazaheterocycles）. Nevertheless, the relatively lowphysicochemical stabilities （e.g. moisture, acid-base, and thermal stability） as compared to theconventional materials such as zeolites have greatly hindered the industrial applications of theMOFs. Admittedly, it is still a synthetic challenge to obtain a MOF simultaneously with highphysicochemical stabilities and excellent function performances.In this thesis, a new dimethyl-functionalized UiO-66type framework, UiO-66-（CH₃）₂, hasbeen successfully constructed by solvothermal synthesis. UiO-66-（CH₃）₂exhibits extremelyhigh physicochemical stability, large CO₂adsorption capacity, and high selectivity towardCO₂/N2, representing substantial improvements over other UiO-66type compounds. Thecombination of excellent thermal, water, and acid-base stability, high CO₂capacity andselectivity, and fast regenerability enables UiO-66-（CH₃）₂a promising material in the fields ofcatalysis and CO₂capture from gas streams such as post-combustion flue gas.The followings are the main points of this work.（1） A new microporous dimethyl-functionalized UiO-66（Zr） type solid, denoted asUiO-66-（CH₃）₂, was synthesized by direct self-assembly. For comparison, we also reproducedthe reported UiO-66and its functionalized analogues UiO-66-NH2, UiO-66-NO₂as well asUiO-66-Br.（2） The PXRD pattern shows that UiO-66-（CH₃）₂is crystalline and topologically identicalto the non-functionalized UiO-66and other functionalized analogues UiO-66-NH2,UiO-66-NO₂as well as UiO-66-Br. IR spectroscopy analysis of the sample displays bands（2950and2920cm-1） corresponding to the asymmetric stretchings of the methyl moieties onthe linkers. The BET and Langmuir surface areas for UiO-66-（CH₃）₂measured by N₂adsorption at77K are868and968m2g-1, respectively. TG analysis shows a steep weightloss occurring at around500°C corresponding to the decomposition of the structure. ThePXRD patterns of the UiO-66-（CH₃）₂after exposed to air or immersed in water, strong base orstrong acid solutions at room temperature showed that the crystallinity was fully maintained.（3） The low-pressure isotherms of CO₂adsorption/desorption for the UiO-66（Zr） typeMOFs were collected. UiO-66-（CH₃）₂exhibits the highest uptakes among all the UiO-66typeMOFs investigated throughout the whole range of pressure. The adsorption enthalpies of CO₂for the UiO-66MOFs were calculated by using the Clausius-Clapeyron equation. The highest Qstfor UiO-66-（CH₃）₂implied the strong interaction between CO₂and UiO-66-（CH₃）₂framework. Minimum N₂adsorption was observed for UiO-66-（CH₃）₂with CO₂/N₂selectivity of ca.58.