Preparation And Catalytic Properties Of Triazolium-functionalized MIL-101 Metal-organic Frameworks

Carbon dioxide?CO₂?is the major greenhouse gas causing global warming but also deemed as an abundant,cheap,non-toxic and renewable C1 resource.The conversion of CO₂ has become a hot topic in sustainable chemistry and green carbon science for it can not only help to mitigate the greenhouse effect,but also provides useful chemical intermediates and products.The cycloaddition of CO₂ to epoxides?CCE?into cyclocarbonate is one of the most promising strategies to fix CO₂,owing to the high atomic economy of the reaction and the wide utilizability of the products.Metal-organic frameworks?MOFs?,which feature ordered hybrid structures,high surface area,tunable pore size and tailorable functionalities,especially could introduce functional site by pre-functionalization and post-synthetic modification,are promising porous heterogeneous catalysts.MOFs can activate the epoxide ring by metal center in CCE reactions.But most of the MOFs catalytic systems require the co-catalytic such as tetrabutylammonium bromide?TBAB?to open the epoxide ring.The homogeneous co-catalysts are toxic and irritant and complicate separation,recycling,and continuous fixed-bed applications.To avoid this problem,it is critical to optimize materials and realize a fully heterogeneous system without additional co-catalyst.A feasible strategy is to install organic cations onto the frameworks so that“free”nucleophilic counteranions are encapsulated and thus immobilized within the pores and construct an ionic metal-organic framework?ionic MOFs,IMOFs?containing both metal center and the nucleophilic anion.MIL-101?Cr?and its derivatives are well known for their excellent thermal and chemical stability and tolerance,so we chose them as an excellent platform for post-synthetic modification.Two triazolium-functionalized IMOFs have synthesized by multi-step tandem PSM for the first time.Cycloaddition of CO₂ to epoxides without co-catalyst was chosen as a model reaction to study the catalytic properties of the IMOFs.We studied the catalytic sites,optimum conditions,heterogeneity and substrate universality.This thesis is mainly divided into two parts:1.Synthesis and characterization of triazolium-functionalized IMOFsIn this part,we synthesized IMOFs containing both metal center and nucleophilic anion by multi-step tandem PSM and ion exchange reaction.Azide-tagged MIL-101-N₃ was prepared first.1,2,3-triazonium cation and counter anion were introduced by azide–alkyne cycloaddition and N-alkylation reactions for achieving IMOFs.Finally,the nucleophilic anion Br^-was introduced by ion exchange reaction and the IMOFs catalysts MIL-101-tzm-Br and MIL-101-tzmOH-Br were obtained.The latter cotains hydroxyl group,which can act as potential hydrogen bond donors.The composition and structure of the modified products were characterized by PXRD,¹H NMR,IR,elemental analysis,ICP and ion chromatography.The results showed that the framework retained the structural integrity throughout the PSM steps and N-methylation ratio was about 50%.N₂ and CO₂ adsorption studies showed higher surface area,total pore volumes and high CO₂ adsorption capacity of these IMOFs.2.Catalyticpropertiesoftriazolium-functionalized IMOFs for cycloaddition of CO₂ to epoxidesIn this part,we studied the catalytic properties of MIL-101-tzm-Br and MIL-101-tzmOH-Br for cycloaddition of CO₂ to epoxides.We found the optimum temperature and CO₂ pressure for CCE are 80°C and 1 MPa,respectively.The catalytic activity of MIL-101-tzmOH-Br was higher than those for the previous IMOF catalysts under similar conditions.The turnover frequency of MIL-101-tzmOH-Br with respect to Br^-reached to 200 h^-1 under the optimum condition.Various control experiments using different MOFs indicated that the intrinsic metal center at the node,the hydroxyl group grafted to the cationic linker and the bromide anion encapsulated within the pore all play important roles on high catalytic activity.The substrate scope of MIL-101-tzmOH-Br as catalyst for cycloaddition was examined using different epoxides and the results proved that the calatyst was suitable for a wide range of substrates and was a highly efficient CCE catalyst.Heterogeneity and recycling catalytic test indicated there is no loss of active sites in the reaction process.The activity,structure and specific surface areas of the catalyst were basically consistent with the fresh one after four catalytic cycles.