Catalysts For Promoting CO₂ Cycloaddition:Synthesis And Their Catalytic Performance

As a ubiquitous greenhouse gas,carbon dioxide(CO₂)has been rapidly increasing in its content with fossil energy consumption,hence,exhibit a serious threat to the climate and environment.Therefore,the development of CO₂ utilization technology is of relatively significance,both for academic research and sustainable development.In addition,CO₂ as a cheap,abundant,and non-toxic C1 resource has been widely used as a starting material to synthesize fine chemicals,of which five-membered cyclic carbonates derived from CO₂ cycloaddition have been employed widely in industrial application prospects.Metal organic framework materials(MOFs)are formed via regular coordination between metal ions and organic ligands,of which metal nodes can be used as Lewis acid sites,while organic ligands can often possess Lewis basic sites.MOFs with dual functions have the ability to enhance CO₂ cycloaddition.In this paper,a series of metal-organic framework materials were designed and synthesized,which were further used as catalysts for CO₂ cycloaddition.Besides,a reasonable reaction pathway was proposed,and further confirmed by density functional theory(DFT)calculations.The research content and conclusions are summarized as follows:(1)Four kinds of metal organic framework materials structured with the same ligand(dimethylamine),[(CH₃)₂NH₂][M(HCOO)₃](DMA-MF,M=Mn,Co,Ni,Zn)were successfully synthesized under solvothermal conditions,and thereafter further used as highly efficient catalysts for enhancing CO₂ coupling with propylene oxide.DMA-MnF was found to possess the highest catalytic activity among the four samples,as this is associated with the longest Mn-O coordination bond,which made propylene oxide/propylene carbonate to connect/detach from metal nodes more easily.When the reaction system was maintained at 2.0MPa,2.0 wt.%catalyst and 120? for 6 hours,99.6%conversion of propylene oxide and 98.0%selectivity to propylene carbonate were obtained.In addition,various epoxides including 1,2-butylene oxide,allyl-glycidyl ether,epichlorohydrin and styrene oxide were respectively used as a substrate for CO₂ coupling reaction under the same conditions.The two possible paths were proposed,with further confirmation by DFT calculations,of which the intermediate closure occurred when the metal node participation underwent lower state energy,and is very close to the real reaction mechanism.(2)Three kinds of manganese-based metal organic framework structured with different ligands(methylamine,ethylamine,imidazole),[CH₃NH₃][Mn(HCOO)₃](MA-MnF),[CH₃CH₂NH₃][Mn(HCOO)₃](EA-MnF)and[C₃H₅N₂][Mn(HCOO)₃](Im-MnF)were synthesized under room temperature conditions.The catalytic activity of the synthesized samples was studied for catalyzing CO₂ insertion into allyl-glycidyl ether,of which the Im-MnF sample was observed with the highest catalytic activity.The results of various tests showed that the catalytic activity of the synthesized sample was not only related to the number of basic sites,but also on the base strength of the active sites.Under the optimal catalytic conditions(100?,1.5 MPa,6 h,1.0 wt.%catalyst),97.2%conversion of allyl-glycidyl ether and 97.6%selectivity to allyl-glycidyl carbonate were achieved.Additionally,CO₂ coupling with various epoxides(epichlorohydrin,styrene oxide,cyclohexane oxide,and propylene oxide)were respectively performed over Im-MnF under the optimal conditions.Moreover,a proposed reaction path is in a good agreement with the DFT calculations.(3)Three kinds of metal-organic framework materials constituted with trimesic acid and formic acid ligands,[(CH₃)₂NH₂][M(BTC)(HCOO)₄(H₂O)]·H₂O(M-BTC,M=Mn,Ni,Co)were synthesized under solvothermal conditions,after which the catalytic activity was evaluated with epichlorohydrin as the probe.The highest catalytic activity appeared in the presence of Mn-BTC,while 98.0%epichlorohydrin conversion and 96.0%selectivity to chloropropylene carbonate were observed under optimal conditions(105?,3.0 MPa,9 h,1.5wt.%catalyst)when Mn-BTC was employed.In addition,the XRD results of fourth recovered sample showed that the lattice structure of Mn-BTC remains unchanged during the liquidation process,suggesting that Mn-BTC has a higher thermal stability.In addition,the yield of CO₂coupling with various epoxides follows the order:epichlorohydrin>1,2-butylene oxide>propylene oxide>allyl-glycidyl ether.(4)2-methylimidazole(2MeIm)functionalized Co-BTC(2MeIm@Co-BTC-x)was synthesized under solvothermal conditions,and further applied as the efficient catalyst for accelerating CO₂ coupling with epichlorohydrin.The XRD results showed that apart from the crystal planes of ZIF-67(0 1 1)and Co-BTC(-1 0 2),a new crystal phase was also observed at12.22°for functionalized Co-BTC,which is related to the re-coordination between 2MeIm and the unsaturated Co ion within Co-BTC lattice.Besides,a hexagonal prism structure was formed on the surface of the functionalized Co-BTC.Also,the XPS analysis showed that 2MeIm mainly captures the cobalt ion coordinated with formic acid,forming ZIF-67 structure.Significantly,the catalytic activity of the functionalized sample was obviously enhanced compared with that of Co-BTC,indicating that 2MeIm played a key role in the coupling reaction.In addition,2MeIm@Co-BTC-1.0 can achieve 97.2%epichlorohydrin conversion and98.7%chloropropylene carbonate selectivity under the optimal conditions(3.0 MPa,90?,5h,0.75 wt.%catalyst),while a slight reduction in the catalytic activity was found after the best synthesized catalyst was reused three times.(5)Zn₂(C₉H₃O₆)(C₄H₅N₂)(C₄H₆N₂)₃(Zn-BTC-2MeIm)was hydrothermally synthesized with dual ligands(2-methylimidazole,trimellitic acid)and zinc ions,and further used to catalyze the coupling reaction of CO₂ and epichlorohydrin.It was found when the stirring speed was higher than 1000 rpm and the catalyst particles are less than 120 mesh,the influence of internal and external diffusion on the reaction process can almost be eliminated.The highest catalytic activity was observed with 98.9%epichlorohydrin conversion and 98.3%chloropropylene carbonate selectivity under the optimal conditions(100?,3.0 MPa,6 h,0.75wt.%catalyst).In addition,only a slight decrease in catalytic activity was found when Zn-BTC-2MeIm was recovered three times.Moreover,Zn-BTC-2MeIm was also found with the ability to effectively promote various epoxides conversion into corresponding cyclic carbonates.Furthermore,the kinetic study shows that CO₂ coupling with epichlorohydrin conforms to first-order kinetics,and the activation energy(E_a)of the reaction is 113.38k J/mol.(6)Three mixed bimetallic organic framework materials of[CoZn][(BDC)(DABCO)_0.5](CZ-BDO),[Co Ni][(BDC)(DABCO)_0.5](CN-BDO)and[Ni Zn][(BDC)(DABCO)_0.5](NZ-BDO)containing triethylenediamine and terephthalic acid dual ligands were synthesized under solvothermal conditions.The catalytic activity of the synthesized samples was evaluated with epichlorohydrin as substrate,of which CZ-BDO sample possesses the highest catalytic activity.This may be associated with the existence of solid solutions within the Co and Zn bimetallic samples,which can synergistically catalyze CO₂ cycloaddition.In addition,the NH₃-TPD profiles shows that the number of acidic sites within CZ-BDO sample is highest among the studied samples,which is attributed to the strong interaction between Co and Zn nodes to form a new LUMO orbital between the two metal nodes,and as a result,more ammonia molecule was adsorbed to the metal surface.When the reaction system was carried out at 0.5 wt.%catalyst,3.0 MPa,and 100? for 5 hours,99.3%conversion of epichlorohydrin and 97.0%yield of chloropropylene carbonate was obtained over CZ-BDO.In addition,bimetallic sample can also be employed as the highly active catalyst for catalytic conversion of various epoxides into corresponding cyclic carbonates.(7)Six kinds of imidazolium ionic liquids were applied to catalyze the CO₂ cycloaddition,among which 1-butyl-3-methylimidazole bromide was found to process the highest catalytic activity.Subsequently,two ionic liquids constructed with 1-methylimidazole were respectively employed to synthesize functionalized UiO-66-NH₂,and the functionalized samples were used to enhance CO₂ insertion into epichlorohydrin.It was found that when UiO-66-NH₂ and 1-(4-carboxybutyl)-3-methylimidazole bromide underwent amidation reaction through H₂N–/HOOC–,the surface of the octahedral structure of UiO-66-NH₂ becomes rough,while some small crystal grains around UiO-66-NH₂ species were formed after UiO-66-NH₂ interacted with1-(3-sulfopropyl)-3-methylimidazole bromide([SPMIM]Br)through H₂N–and HSO3–,making more ionic liquid to be grafted to the UiO-66-NH₂ lattice framework,hence show higher catalytic activity.Besides,98.1%epichlorohydrin conversion and 96.3%chloropropylene carbonate selectivity were obtained under the optimal reaction conditions(CO₂ initial pressure of 2.5 MPa,1.2wt.%ECH catalyst weight,reaction at 95? for 8 h).In addition,a slight decrease in conversion of epichlorohydrin was observed when the functionalized UiO-66-NH₂ was recovered thrice.This phenomenon is attributed to the gradual leaching of the ionic liquid from functionalized UiO-66-NH₂,rather than the total collapse of the lattice structure.