Design And Syntheses Of Cationic Metal-Organic Frameworks For Catalytic CO₂ Conversions

The chemical fixation of carbon dioxide(CO2)into industrial feedstock like cyclic carbonates represents an important approach to achieve utilization of CO2 resource.This reaction converts CO2 into valuable chemicals on one hand,and provide many possibilities for green chemistry.Metal-Organic Frameworks(MOFs),as a newly emerging class of porous crystalline material,have been widely investigated in the cyclic addition reaction of CO2 and epoxide.However,most of MOFs catalyst systems need the use of co-catalyst such as organic base or high pressure conditions to get optimal catalytic results,which are uneconomic and risky.A high mount of co-catalyst may lead to deactivation of MOFs by pore plugging.In this dissertation,we designed and synthesized a series of functional cationic MOF catalysts,which showed efficient catalytic performance in this reaction without the use of co-catalyst and solvate under 1 bar CO2 conditions.We also discussed the catalytic roles of active sites of these MOFs for their properties.(1)Under the theory of 'acid/base synergetic catalysis',we synthesized the first microporous cationic imidazolium functionalized zirconium MOF,(I-)Me-UiO-66(2),via reticular chemistry and post-synthetic modifications.(I-)Me-UiO-66(2),containing Br(?)nsted acid sites and nucleophilic iodine ions,could effectively convert various epoxides into cyclic carbonates without adding any co-catalyst or solvate under 1 bar CO2 condition.Chloropropene carbonate could be obtained with a yield of over 93%and a selectivity over 93%.(I-)Me-UiO-66(2)could be recycled a few times without significant loss of the activity.The in situ FT-IR measurements monitoring the reaction process in the presence of(I-)Meim-UiO-66(2)revealed that chloropropene carbonate could be formed rapidly(in a few minutes)after heating.Additionally,the crystal size of(I-)Me-UiO-66(2)has a significant effect on its activity.The modifiable imidazole groups and nanocrystal size of Im-UiO-66(1)may provide opportunities for various MOF supported metal catalysts and others.(2)For the first time,we synthesized a microporous ionic MOF(iMTV-MOF),through mixed-ligand strategy and tandem post-synthetic modification technology,which contained both the zinc porphyrin and imidazolium bromide.A Zr-MOF containing porphyrin and free imidazole groups was first synthesized,then imidazolium bromide 'and zinc porphyrin were introduced via sequentially post-synthetic modifications to obtain ZnTCPP(?)(Br-)Etim-UiO-66(5).The structures and components of these iMTV-MOFs were fully characterized.It was found that the structural features and affinity toward CO2 of these MTV-MOFs could be tuned by introducing imidazolium groups or zinc sites.ZnTCPP(?)(Br-)Etim-UiO-66(5)showed more superior performance than TCPPc(Br-)Etim-UiO-66(4)and TCPPcIm-UiO-66(3)herein in the conversion of allyl glycidyl ether(AGE)into corresponding cyclic carbonates under 1 bar CO2 conditions.This was attributed to the cooperative effect of Zn2+ sites and Br-ions in this microporous ionic MTV-MOF.ZnTCPP(?)(Br-)Etim-UiO-66(5)can be recycled easily and used at least three times,but decomposed gradually during the catalytic process.This work has provided a feasible approach for the preparation of ionic metalloporphyrin based MOF materials.(3)Currently,most of reported microporous MOF catalysts suffers some problems such as low activities and slow mass transport under 1 bar CO2 conditions.For the first time,we constructed a mesoporous cationic Cr-MOF denoted compound 6,containing imidazolium moieties,Lewis acidic Cr3+ sites and free halogens,by a topology-guided hydrothermal synthesis.Compared with neutral mesoporous MIL-101,compound 6 exhibited enhanced CO2 adsorption ability(20.2 wt%,273 K)at 1 bar.On the other hand,compound 6 could convert various epoxides into cyclic carbonates without the use of co-catalyst under atmospheric pressure.Compound 6 achieved a TON of 494 in the catalytic synthesis of chloropropene carbonate under 1 bar conditions and could be recycled for a few times without showing significant decrease in activity.Based on the unique features of 6 and the catalytic results,a possible cooperative acid/base catalytic mechanism was proposed.