| CO2 is an abundant, inexpensive, notoxic and renewable C1 resource. In the view of the concepts of "atom economy", "sustainable development", and "green chemistry", the chemical fixation of CO2 into valuable compounds under mild conditions is attracting much attention. However, there have been a few commercial processes that use CO2 as a feedstock so far. The synthesis of cyclic carbonates via the cycloaddition of CO2 to epoxides is one of the few processes that have been commercialized. Many catalysts have been reported to show catalytic activity towards this reaction, while most of them suffered from some shortcomings, such as low catalytic activity, and/or difficult separation of catalysts from products. In this study, several high-efficiency homogeneous and heterogeneous catalysts were designed and fabricated for the synthesis of cyclic carbonates from CO2 and epoxides.A series of novel bismuth compounds bearing a nitrogen-bridged bis(phenolato) ligand, LnBiX (X=Cl, Br, I; n=1～5) and LnBi(OCH3) (n=1～5) were firstly synthesized and examined for the cycloaddition reaction. In the presence of NaI as co-catalyst, the bismuth biphenolate complexes show high catalytic activity and selectivity (>99.5%) for the cycloaddition reaction of CO2 to propylene oxide (PO). LnBiI (n=1～3) is much superior to other bismuth complexes in catalytic efficiency, showing above 95% propylene carbonate (PC) yield at 120℃and 2.5 MPa CO2 initial pressure within 1 h. However, the stability of the bismuth compounds is not satisfactory, which will limit their applications.Subsquently, the two-component catalyst systems composed of zinc salts and different phosphonium salts were examined for the cycloaddition reaction. It was found that the ZnBr2-Ph4PI shows the best catalytic activity (e.g., giving TOF 6272 h-1 at 120℃and 2.5 MPa CO2 initial pressure when propylene oxide is used as substrate). Furthermore, the catalytic activity of ZnBr2-Ph4PI didn't change after 10 recycles, showing good stability and reusability. A systematic investigation was also conducted on the effects of different zinc salts, ZnBr2/Ph4PI molar ratio, reaction parameters, and addition of water on the catalytic activity of ZnBr2-Ph4PI. It is noted that the presence of water even in trace amount has a negative effect on PO conversion.Heterogeneous catalysts of Zn-Al-O and alkaline-earth metal modified Zn-Al-O composite oxides (denoted as Zn-M-Al-O, M=Mg, Ca, Sr, Ba) were fabricated via calcination of the corresponding hydrotalcites. Among the synthesized composite oxides, Zn-Mg-Al-O (Zn/Mg=4.0, pH=10, without hydrothermal treatment) is the best in performance, showing PC yield of 88.8% and selectivity of 99.2% at 140℃and 2.5 MPa CO2 initial pressure within 12 h. Furthermore, the Zn-Mg-Al-O catalyst has good stability and reusability. The results of NH3- and CO2-TPD investigation indicate that the Zn-Mg-Al-O surface has both acid and basic sites. The basicity of the catalysts was measured by Hammett indicator method, and the results indicate that the sites of moderate basicity (6.1≤H0<8.9) are crucial to the reaction.In order to obtain high-efficiency heterognenous catalysts, SBA-15 immobilized hydroxyl ionic liquid (3-(2-hydroxyl-ethyl)-1-propyl-imidazolium bromide, HEPIMBr) catalysts were fabricated by grafting method. The catalyst (SBA-15-HEPIMBr) prepared by grafting method was found to show high catalytic efficiency. It gave 98.9% of PC yield at 140℃and initial CO2 pressure 2.0 MPa when propylene oxide was used as a substrate. Compared with that over ZnBr2-Ph4PI, the presence of a trace amount of water has positive effect on the cycloaddition reaction over SBA-15-HEPIMBr. The excellent performance of SBA-15-HEPIMBr is possibly related to the existence of OH groups. Furthermore, the SBA-15-HEPIMBr shows better catalytic performance than the pure hydroxyl ionic liquid (3-(2-hydroxyl-ethyl)-1-methyl-imidazolium bromide), possibly as a result of the synergistic effect between support and HEPIMBr. Although there was a distinct decrease of PC yield within the first three runs, the activity of SBA-15-HEPIMBr almost didn't change after 4 runs. In order to improve the stability and reusability of the SBA-15 immobilized HEPIMBr catalyst, hybrid SBA-15 immobilized HEPIMBr catalyst (H-SBA-15-HEPIMBr) was also fabricated by one-pot synthesis method. Although the catalytic activity of H-SBA-15-HEPIMBr is lower than that of SBA-15-HEPIMBr, the former shows better stability and reusability.In addition,3-(2-hydroxyl-ethyl)-1-vinylimidazolium bromide was copolymerized with the cross-linked divinylbenzene (DVB) to prepare a polymer immobilized hydroxyl ionic liquid (P-DVB-HEVIMBr). The P-DVB-HEVIMBr shows good catalytic activity and stability for the cycloaddition reaction of CO2 to epoxides. After 6 runs, there was only a little decrease in the catalytic activity of P-DVB-HEVIMBr. Moreover, the hydroxyl ionic liquid-grafting polydivinylbenzene catalyst (PDVB-HEIMBr) was fabricated through the reaction of 1-(3-amino-propyl) imidazole-functionlized polydivinyl-benzene with 2-bromoethanol. It was found that the PDVB-HEIMBr exhibits better catalytic performance than P-DVB-HEVIMBr. Furthermore, there was no loss of catalytic activity of PDVB-HEIMBr even after 8 recycles. Therefore, it can be deduced that the PDVB-HEIMBr will find potential industrial applications in cyclic carbonates synthesis. Moreover, a possible reaction mechanism was proposed on the basis of the role of hydroxyl groups of the PDVB-HEIMBr catalyst.
|
PDF Full Text Request