CO₂ Conversion Catalyzed By New Bifunctional Imidazolium-based Ionic Liquid Catalyst Or DMAB

Carbon dioxide（CO₂）is the main greenhouse gas that causes global warming,and the massive accumulation of carbon dioxide in the atmosphere has posed a serious threat to the environment and human health.In addition,carbon dioxide is regarded as inexhaustible,low-cost,non-toxic green C1 resources,which can be converted into high value-added products with certain economic benefits.In this paper,the green and efficient catalysts were developed to construct C-O bond and C-N bond with carbon dioxide as raw material to realize the chemical conversion and utilization of carbon dioxide,which is in line with the purpose of "green chemistry".The specific research work is as follows:A series of bifunctional imidazolium-based ionic liquids（ILs1-6）were designed and synthesized as environmentally friendly catalysts for the chemical conversion of carbon dioxide.Their structures were characterized by modern analytical techniques such as NMR,FTIR and elemental analysis.The results show that the ionic liquid catalyst can produce propylene carbonate（PC）from epoxy propane（PO）and CO₂ with high yield and high selectivity（> 99%）without any solvent or cocatalyst.Through the study of catalyst structure,it is found that the alkyl chain length on haloalkanes has little effect on the catalytic activity,and different halogen ions have an important influence on the catalytic activity.The sequence of catalytic activity is I^-> Br ^-> Cl^-.The reaction time,reaction temperature,CO₂pressure and the amount of ionic liquid were systematically investigated and the optimal conditions were obtained as（120℃,1.5 MPa,3 h,ILs loading 0.25 mol%）.Under these conditions,all the catalysts（ILs1-4）containing I^-can efficiently catalyze the cycloaddition reaction with the conversion rate of more than 90% and the selectivity of nearly 100%.In addition,under the optimum conditions,ILs2 shows a good substrate applicability,which can catalyze the reaction of CO₂ with a variety of epoxides.Moreover,ILs2 can also catalyze the cycloaddition reaction of CO₂ with various glycidyl ether under atmospheric pressure with high efficiency.Kinetic studies were carried out preliminarily for these catalysts and the formation activation energies（Ea）of cyclic carbonate were obtained（47.77 kJ/mol for ILs1,48.85 kJ/mol for ILs2,56.28 kJ/mol for ILs3,59.83 kJ/mol for ILs4,71.87 kJ/mol for ILs5,and 85.93 kJ/mol for ILs6）that it agrees well with the catalytic activity.Finally,an inferred mechanism was presented according to the kinetic studies and experimental results.Further research shows that the ionic liquid catalyst ILs2 can be used not only to construct C-O bonds for cyclic carbonates but also for C-N bonds to form oxazolidone in the CO₂chemical conversion.The catalyst ILs2 can catalyze the cycloaddition of CO₂ and aziridine to oxazolidinone without any organic solvent or additive,which can produce 5-substituted oxazolidinone with high yield,and the regioselectivity of 5-substituted product is much higher than that of 4-substituted product.The effects of various reaction parameters on the catalytic performance were studied in detail.The results showed that oxazolidinone could be high efficiency produced by using 10 mol% catalyst loading at room temperature and atmospheric pressure.In addition,the catalyst can produce oxazolidinone with good yield and excellent regioselectivity.Finally,a possible reaction mechanism was proposed based on the experimental results and literature.The use of CO₂ and reducing agents for N-formylation and N-methylation reduction of amines is an attractive method for introducing CO₂ into valuable organic compounds.Dimethylaminoborane（DMAB）is an environmentally friendly reducing agent,which has the advantages of easy availability,non-toxicity,non-flammability,high stability,low price,good water solubility,and high volume-to-mass density in comparison with hydrogen.Therefore,DMAB is used as a green hydrogen source for the synthesis of formamide and methylamine.In this paper,DMAB is used as a reducing agent to reduce cyclic amines and CO₂ directly to N-formylation reaction without using any catalyst and solvent.The effects of various reaction parameters on the reactivity are systematically studied to obtain the optimal reaction conditions: 100℃,1 MPa,6 h,and the molar ratio of substrate to DMAB is 2: 1.Under these conditions,the yield of formamide from morpholine and CO₂ reduced by DMAB is up to 95%.Moreover,the reactivity of DMAB to different cyclic amines is studied.The results show that most of them have excellent yields.In subsequent studies,DMF is used as the solvent,and DMAB could directly reduce the aromatic amine and CO₂ for N-methylation reaction.The optimal reaction conditions were obtained as follows: 100 ℃,1 MPa,24 h,molar ratio of substrate to DMAB.It is 1: 1 and can reduce most aromatic amines to methylamine.