Design And Synthesis Of Heterogeneous Catalysts For The Cycloaddition Of Carbon Dioxide And Epoxides

CO2 as a cheap and abundant C1 resource can be used to synthesize five member cyclic carbonates with epoxides,which has become an attractive craft because of its sustainability and environmental friendliness.Until now,quaternary ammonium salts and metal halides are most common catalysts for COZ cycloaddition in industry,but these catalytic systems have some problems,such as low activity,harsh reaction conditions and difficult recovery of catalysts.The development of novel,efficient,stable and reproducible heterogeneous catalysts is the key issue in CO2 cycloaddition.Exploring the relationship between the composition and catalytic performance of catalysts and explaining possible mechanism in COZ cycloaddition reaction is the research focus.In this paper,several novel and effi cient heterogeneous catalysts were synthesized based on the conception of multi-functional active sites(acidic and basic sites)synergistically catalyzing cycloaddition of CO2 with epoxides.The main research contents are as follows:1.Because most metal-organic frameworks(MOFs)contain only one kind of active site,they can not efficiently catalyze CO2 cycloaddition without co-catalysts.In order to solve this problem,a kind of multi-functional sites MOF composite(MIL-101-IMBr)was successfully prepared by post-synthesis modification of MIL-101 with imidazolium-based ionic liquids.The results showed that MIL-101-IMBr-6 containing imidazole group,halogen anion and tetra-coordination Cr presented the high catalytic activity under co-catalyst free conditions for cycloaddition reaction of CO2 with propylene oxide(PO).The conversion of PO and the selectivity of propylene carbonate reached 95.8%and 97.6%,respectively.2.In order to make full use of templates in zeolite,mesoporous titanium silicate(MTS-1)containing organic templates polyquaternary ammonium salt-6(PDDA)and tetrapropyl ammonium hydroxide(TPAOH)were synthesized.Not required to be removed by calcination,the organic templates endow as-synthesized mesoporous TS-1 zeolite basic sites,which cooperate synergistically with framework Ti species,leading to an efficient bi-functional catalyst for cycloaddition reaction of CO2 with epichlorohydrin(ECH).Infrared characterization of CO2 adsorption showed that MTS-1 could activate CO2.The reaction results showed that the increased content of accessible basic sites in the catalysts was favor for the conversion of ECH,and the catalysts containing two kind of active centers,titanium species and PDDA,have excellent catalytic activity.3.In order to avoid the waste of TPAOH and reduce the cost of catalyst,the organic-inorganic hybrid mesoporous titanium-silicon catalyst(Ti-mSiO2)with a single basic center PDDA was further prepared.PDDA endow Ti-mSiO2 basic sites,which cooperate synergistically with Ti species,leading to an efficient bi-functional catalyst for cycloaddition reaction of CO2 with ECH.The reaction results showed that the catalysts containing both titanium species and PDDA have higher activity than those containing only one kind of active site,and the catalytic activity enhances with the increased content of accessible basic sites.4.In order to improve the mass transfer in catalyst,multifunctional iron-based mesoporous catalysts were synthesized by using PDDA and conventional surfactants as double templates.The results of N2 physical adsorption and TEM characterization showed that the catalyst has hexagonal ordered and wormlike mesoporous channels with pore size of 2-3 nm.The reaction results showed that the Fe-HMS catalyst containing both iron species and PDDA possesses the good catalytic performance in the cycloaddition reaction of CO2 with ECH.