Ionic Liquids Modifying Porous Materials For Synergistic Synthesis Of Diols

Diols,including ethylene glycol(EG)and propylene glycol(PG),are important raw materials and intermediates in chemical industry.As the most concerned and simplest diol,EG becomes the second major derivative of ethylene.China is the largest consumer of EG.The traditional technology has some disadvantages,such as large water ratio(H2O/EO),high energy consumption and low EG yields.Under the pressure of energy crisis and high demand,technological innovation has become an inevitable trend.There are two most promising alternative processes,including CO2 based process via ethylene carbonate(EC)and catalytic hydration process of ethylene oxide(EO).All the two new technologies become international research frontiers,of which the most important is catalyst development.To overcome the disadvantages of current catalysts,including uncombined activity and stability of carbonylation catalysts in EC process,as well as scarce activity and selectivity of EO hydration catalysts,new catalysts would be designed by combining the feature of ionic liquids and porous materials.With the good activity and clustering effect,ionic liquids could be used to modify and assemble with stable porous materials to function synergistically in efficient catalytic synthesis of EG.In this work,investigations were conducted around three aspects,including ionic liquids adjusting methods on formation and assembly of porous materials,the structure-activity relationship between materials and catalytic performance,and synergistic catalytic mechanisms by ionic liquids and materials.The study will open a new approach for development of highly efficient catalysts in EG synthesis process and provide the basis for catalyst development of other diols.The innovative achievements of the thesis are as follows:(1)Ionic liquids were used as both templates and dopants in the preparation of carbon nitrides for epoxides carbonylation.Based on characterization and catalytic evaluation,the structure-activity relationship with varying ionic liquids concentration was obtained and the optimal addition of ionic liquids(BmimBF4/Melamine=0.01)and boron doping amount(<1.59 atm%)was achieved.With the best catalyst,the propylene carbonate(PC)yield could reach 95.7%under 130 ?.By DFT calculation,the regulation law for more exposed edge defects is obtained and the corresponding boron enhanced catalytic mechanism is proposed.(2)Ionic liquids adjusting law for one-step assembly of mesoporous silica(mSiO2)and confinement of ionic liquids is revealed.The materials owning high dispersity and stability were obtained under low ionic liquids concentration.Under the optimal confined content of EmimBr(6.9 wt.%),a higher TOF(112.6 h-1)up to nearly 1.7 times of bulk phases,is achieved.The high dispersity of active sites(Si/Br=25)and the strong confining effect(Si-OH/Br=8)of mSiO2 skeleton are the main factors affecting the catalytic performance,and the catalytic mechanism based on synergistic function of Si-OH and Br anion is proposed.(3)The influence law of ionic liquid cations on mSiO2 skeletons and confining effect is obtained.It was found that ionic liquids with long carbon chain(carbon number>8)were not suitable for confined catalysis of epoxides conversion,and the symmetric ionic liquids with four carbon chains functioned best both in stability(Si-OH/23)and recyclability with PO conversion up to 87.0%after three runs.Analyzed by XPS,the reason leading to the higher stability of four carbon chains is explained,which is mainly due to a propping effect in four directions,both to ensure stability and create more space for mobility of active anions.(4)CO2 functionalized carboxylic ionic liquids were designed to catalyze the EO hydration.By adjusting the chain lengths of ionic liquids,the strongest restraining effect from cation is observed with the shortest chain length.The ionic liquids with symmetric shortest chain length(DMIC)could catalyze EO hydration with 99.9%EO conversion and EG selectivity under low water ratio(H2O/EO=5:1)and short reaction time(0.5 h).Through characterization and catalytic evaluation,the possible catalytic mechanism,based on CO2 leaving,anion transformation and base catalysis,was proposed finally.(5)With ionic liquids as the template and directing agents,ionic liquids were confined during the formation of silicagels via sol-gel way.By tailoring the concentration and structures of ionic liquids,the pore sizes of silicagels were selectively modulated for efficient catalytic conversion of EO and selectivity of EG.With low ionic liquids confined(5.5 wt.%),both 99.9%of EO conversion and EG selectivity were achieved under low water ratio(H2O/EO=5:1)and short reaction time(0.5 h).Compared with bulk phases,only 10%of ionic liquids were used.The structure-activity relationship and catalytic performance of confined ionic liquids was preliminarily studied.Ionic liquids with four symmetric carbon chains(N(Et)4Ac)were found to be most suitable for confined catalysis of EO hydration due to the cooperative effect of the suitable micropore sizes for high EG selectivity and large quantities of mesopores for high EO conversion.This study laid the foundation for the subsequent development of similar catalyst systems.