Preparation Of Ionic Bifunctional Solid Catalysts And Their Performance Study For CO₂ Cycloaddition Reaction

Carbon dioxide?CO₂?is not only a main greenhouse gas,but also abundant,non-toxic,inexpensive and renewable carbon source.So its chemical conversion has attracted more and more attention.The cycloaddition of epoxides and CO₂ is 100% atom-economical reaction,which has become one of the most promising strategies of chemical fixation of CO₂.Cyclic carbonates as products of CO₂ cycloaddition can be used as polar aprotic solvents,electrolytes for lithium-ion batteries,precursors for polycarbonates,and intermediates for the synthesis of pharmaceuticals and fine chemicals.At present,ionic liquids and Salen-metal compounds have good catalytic activity in the catalysis system for cycloaddition of CO₂.The nature of the organic ionic polymer can be modulated by cationic functionalization,and the cross-linked polymer network backbone structure can impart heterogeneous properties to the catalyst.However,most of them have different kinds of shortcomings,such as harsh reaction conditions,requirement of solvent/cocatalyst,difficult separation of catalyst and so on.Therefore,this paper is intended to prepare a class of highly reactive heterogeneous catalysts that catalyze the CO₂ cycloaddition under mild conditions without solvent and cocatalyst.In conclusion,based on the diversity and designability of ionic liquids,Salen-metal compounds and organic ionic polymers,this dissertation designs and prepares heterogeneous catalysts of high cross-linking ionic polymers from the molecular level,and studies their catalytic performance for the cycloaddition of CO₂ and epoxides.The specific research contents are as follows:?1?A new type of highly cross-linked cationic polymer microspheres with uniform distribution of Br anions was synthesized by a simple procedure.The influence of solvent used on the material structure and morphology was studied and the obtained cationic polymers were fully characterized by ¹H NMR,CHN elemental analysis,FT-IR,TG,SEM,and EDS.Its catalytic performance for the cycloaddition of CO₂ and epoxides under solvent-free and metal-free conditions was investigated,along with comparisons to various counterparts.The results demonstrated that the solvents used in the synthetic media are an important factor for the formation of uniform microspheres structure,and the spherical particles TBB-Bpy-a formed in THF solvent are the most stable.The obtained spherical particles are proved to be highly efficient heterogeneous catalyst for the transformation of CO₂ and epoxides into cyclic carbonates,showing the best catalytic activity?conversion and selectivity of 99% and 100%,respectively?.The excellent activity of the catalyst is due to the good dispersion and high surface area of TBB-Bpy-a that allow the Br anions on the catalyst surface giving full play as active centres and to contact the reaction substrate more easily.Recovery and reuse of the catalyst can be achieved by simple filtration.After repeated use of the catalyst for six times,no significant reduction in the efficiency of the catalyst was observed.Control experiments demonstrated that the featured cross-linked covalent cationic structure imparts good recyclability of the catalyst.?2?The highly cross-linked ionic polymer microspheres?TBB-Bpy?was prepared by reacting 1,2,4,5-tetrakis?bromomethyl?benzene with 4,4'-bipyridine at a molar ratio of 1:1 in a one-pot solvothermal method.Then,the pre-synthesized Salen-Co Schiff base was covalently attached onto the surface of TBB-Bpy by anucleophilic reaction to obtain an ionic bifunctional solid catalyst TBB-Bpy@Salen-Co with a Salen-Co shell.The catalyst was fully characterized by ¹H NMR,CHN elemental analysis,FT-IR,TG-DTG,SEM,EDS,and ICP-OES.Its catalytic performance for the cycloaddition of CO₂ and epoxides was investigated under mild,solvent-free,and cocatalyst-free conditions.Control experiments revealed that the high catalytic activity of the catalyst is attributed to the synergistic effect between the high density of metal active center Co and the Br anions with good leaving ability.The catalyst is superior to other mono-and bifunctional catalysts at 60 °C,even at low CO₂ pressure?0.2 MPa?.It was also found that Br anions with high leaving ability and density promote the nucleophilic attack and cycloaddition reaction,and because of the nonporous structure of the catalyst,many catalytic active sites?Co metal centers and Br anions?were distributed across the outside surface of catalyst,which facilitates the contact of different epoxides,even for bulkier epoxides.After the reaction,the catalyst can be simply separated and reused five times without significant loss of catalytic activity and active species Co.Moreover,based on previouswork and our experimental results,a synergistic mechanism of cycloaddition reaction was proposed.?3?On the basis of the above,a four-connected structural unit Salen ligand was synthesized and then a Salen-metal-bridged ionic polymer network?M-Salen-TBB?was prepared by one-pot method.Finally,M-Salen-TBB were further modified with different nitrogen-containing units to give the final ionic bifunctional solid catalyst?M-Salen-TBB-R?containing appended ionic liquid substituents.And the effects of different N-containing ionic liquid cations and metal active sites on the catalytic activity of CO₂ and epoxides cycloaddition reaction were studied.The catalysts were characterized by ¹H NMR,CHN elemental analysis,FT-IR,TG,SEM,EDS,and ICP-OES.Under the condition of mild,solvent-cocatalyst-free,the catalytic performance of the catalyst for the cycloaddition of CO₂ and epoxides was investigated.The results indicated that the nitrogen N+ play a essential role as an organo-catalyst in the synthesis of cyclic carbonates.It is because the nitrogen atoms resulting from the organic groups have different electron-donating ability and form different close degree of ion pairs with Br anions,which affects the electron density on Br-and the ability to leave of Br-.Moreover,leaving ability of Br-further affects Br-nucleophilic attack to the epoxide ring.The optimum catalyst Co-Salen-TBB-Py exhibited the highest conversion of 97.8% with 100% selectivity under the optimal reaction conditions?0.5 MPa,80 °C,8 h?,and the catalyst could be easily separated and steadily recycled five times without significant loss in its catalytic activity and leaching of active Co species.The excellent catalytic activity of the catalyst for the cycloaddition under milder reaction conditions is attributed to the synergistic effect between functionalized active sites including metal center,nucleophile Br-,and organic cations.Moreover,unique highly cross-linked polymer framework structure provides high stiffness and stability of the catalyst.