中国科技论文在线 http://www.paper.edu.cn Applied Catalysis A: General 275 (2004) 73–78 www.elsevier.com/locate/apcata Chemical fixation of CO2 to ethylene carbonate under supercritical conditions: continuous and selective Xiao-Bing Lu*, Jing-Hai Xiu, Ren He, Kun Jin, Li-Mei Luo, Xiu-Juan Feng State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, PR China Received 17 January 2004; received in revised form 5 July 2004; accepted 21 July 2004 Available online 1 September 2004 Abstract Chemical fixation of CO2 to ethylene carbonate proceeds effectively under supercritical conditions by using as catalyst the system of an immobilized cobalt complex in conjunction with a quaternary ammonium salt in a flow apparatus, based on supercritical fluid used as both a solvent and a reactant for continuous synthesis of organic compound. A conversion of up to 85.6% for ethylene oxide was achieved at 110 8C and 12.5 MPa, which value is about 4.5 times that under subcritical condition (4.0 MPa). No byproduct such as polycarbonates or polyester was observed in the obtained products. The immobilized cobalt complex exhibited good stability and was subjected to utilization for 24 h with no loss of activity. The powder X-ray diffraction patterns indicate no change in the structure of the supported mesoporous silica during the reaction. # 2004 Elsevier B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Ethylene oxide; Ethylene carbonate; Cycloaddition; Salen–cobalt complex; Quaternary ammonium salt; MCM-41; Flow reactor. 1. Introduction mediates, and in many biomedical applications [4–6].In recent decades, numerous catalyst systems have been From the standpoint of environmental protection and developed for this transformation [7–22]. resource utilization, the development of a truly environmen- On the other hand, supercritical CO2 (SC-CO2) is an attrac- tally benign process utilizing CO2, which is the largest single tive substitute solvent, because it combines an environmen- source of greenhouse gas, has drawn current interest in tally benign character with favorable physico-chemical industrial chemistry and biotechnology. Chemical fixation properties for chemical synthesis [23–25]. Also, CO2 may of CO2 is one of the most attractive methods since there are be a particularly advantageous reaction medium when CO2 many possibilities for CO2 to be used as a safe and cheap C1 serves as both a reactant and a solvent. The improved rates for building block in organic synthesis [1,2]. One of the most catalytic hydrogenation of CO2 to formic acid and its promising methodologies in this area is the synthesis of five- derivatives in supercritical conditions provided support for membered cyclic carbonates via the cycloaddition reaction this approach [26–28]. The success of Noyori and co-workers of CO2 and epoxides [3]. These carbonates are valuable as suggests that investigation of other CO2 reactions in SC-CO2 precursors for polymeric materials such as polycarbonates, is worthwhile. In fact, the cycloaddition reaction of CO2 to aprotic polar solvents, pharmaceutical/fine chemical inter- epoxides is very suited for proceeding under supercritical conditions, because SC-CO2 possesses high solvating power towards various epoxides [29–31]. In previous paper [30], * Corresponding author. Tel.: +86 411 88993864; fax: +86 411 83633080. we reported that ethylene carbonate could be rapidly E-mail address: [email protected] (X.-B. Lu). synthesized from supercritical CO2/ethylene oxide mixture 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.022 转载 中国科技论文在线 http://www.paper.edu.cn 74 X.-B. Lu et al. / Applied Catalysis A: General 275 (2004) 73–78 Scheme 1. Continuous cycloadditon of CO2 and ethylene oxide with an immobilized cobalt complex (the metal complex covalently bonded on both the inside and the outside of the MCM-41 pores) in conjunction with a quaternary ammonium salt. in the presence of homogeneously bifunctional catalyst. 2.2. Preparation and characterization of the anchored However, irreversible phase separation, resulting from the cobalt complexes formation of product during the reaction, was observed and thus reduces the rate significantly. A similar phase separation Ordered MCM-41 silica, which was prepared according was also reported for propylene carbonate synthesis under to a previously described method [36], was chosen as supercritical CO2 conditions [31]. Very recently, Kawanami support material for immobilizing Schiff-base cobalt and coworkers found that in the presence of 1-octyl-3- complexes. MCM-41 silica, discovered by Mobil scientists methylimidazolium tetrafluoroborate cyclic carbonates in 1992 [37,38], possesses regularly hexagonal arrays of could be rapidly synthesized in nearly 100% selectivity cylindrical mesopores and changeable pore diameter from corresponding epoxides and CO2 at 14 MPa and between 1.5 and 20 nm. These properties make MCM-41 100 8C [32]. materials to be the best candidates of catalyst or catalyst Indeed, the low viscosity and the high diffusivity inherent support, and the hosts for many guest materials [39,40].The to supercritical fluids are ideally suited for continuous flow cobalt complexes were grafted onto inorganic supports in reactors rather than batch reactors [33,34]. Furthermore, the three steps, as outlined in Scheme 2. use of an immobilized homogeneous catalyst in the flow In a typical synthesis process, activated MCM-41 silica 1 reactor not only provides a direct and quantitative separation (5 g, 300 8C, under vacuum) was first treated with a reflux- of the products from the catalyst and avoids any solubility ing anhydrous toluene solution (100 ml) of 3-chloropropyl- limitation of homogeneous catalysts [35], but also easily trimethoxysilane (3 ml) under N2 atmosphere followed by overcomes the influence of phase change during some washing with diethyl ether-dichloromethane in a Soxhlet reactions. Herein, we report the use of a flow reactor system apparatus yielding covalently anchored 3-chloropropylsi- À1 for continuous cycloaddition reaction from SC-CO2/ lane moieties 2 (Cl, 1.02 mmol g ). The later (3 g) was then ethylene oxide mixture fluid by using as catalyst the system treated with a refluxing toluene solution (60 ml) of 3-[N,N’- of an immobilized cobalt complex in conjunction with a bis-(2-salicylidenamino)ethyl]amine (SalenH2) or 3-[N,N’- quaternary ammonium salt (Scheme 1), based on super- bis-2-(5-tert-butylsalicylidenamino)ethyl]amine (t-BuSa- critical fluid used as both a solvent and a reactant for lenH2) (15 mmol) followed by washing with dichloro- continuous synthesis of organic compounds. methane in a Soxhlet apparatus leading to the formation of SalenH2-MCM-41 3a or t-BuSalenH2-MCM-41 3b. The anchored ligand 3a or 3b (2 g) was further reacted with 2. Experimental anhydrous CoCl2 (0.4 g) or Co(OAc)2Á4H2Oinrefluxing ethanol (50 ml) under N2 for 24 h followed by washing in a 2.1. Materials Soxhlet apparatus with ethanol in a N2 atmosphere and then drying at 100 8C in vacuum to yield SalenCo-MCM-41 4a The Schiff-base ligands 3-[N,N’-bis-(2-salicylidenami- (Co, 0.39 mmol gÀ1)ort-BuSalenCo-MCM-41 4b (Co, À1 no)ethyl]amine (SalenH2) and 3-[N,N’-bis-2-(5-tert-butylsa- 0.18 mmol g ). À1 licylidenamino)ethyl]amine (t-BuSalenH2) were synthesized SalenH2: FTIR (Neat, cm ): 2845 m, 1632 versus, from diethyltriamine and corresponding salicylaldehyde in 1581(s), 1497(s), 1461(s), 1279 versus, 757 versus; 1H- ethanol. Ethylene oxide was distilled after refluxing over NMR (CDCl3/TMS): d 2.94 (s, 4H, NCH2), 3.64 (s, 4H, a mixture of potassium hydroxide and calcium hydride. NCH2), 6.81 (t, 2H, PhH), 6.90 (m, 2H, PhH), 7.17 (m, 2H, Methanol and ethanol were distilled after refluxing over the PhH), 7.23 (t, 2H, PhH), 8.28 (s, 2H, PhCH); 13C-NMR corresponding magnesium alkoxides under nitrogen atmo- (CDCl3): d49.8 (NHCH2), 59.5 (CH2N=C), 116.9, 118.5, sphere. CO2 was purified by passing through a column 131.3, 132.1 (aromatic CH), 118.7, 161.1 (aromatic C), À1 packed with 4A molecular sieves before use. 165.9 (C=N). (t-Bu)SalenH2: FTIR (Neat, cm ): 2961 中国科技论文在线 http://www.paper.edu.cn X.-B. Lu et al. / Applied Catalysis A: General 275 (2004) 73–78 75 Scheme 2. Synthesis of SalenCo(II) or (t-Bu)SalenCo(II) complexes covalently anchored to MCM-41 silica. versus 2866(s), 1635 versus 1590(s), 1493 versus 1463(s), MCM-41: FTIR (KBr, cmÀ1): 2900–3100, 1631, 1571, 1 1267 versus 829(s); H-NMR (CDCl3): d1.25 (s, 18H, 1458, 814. 2 À1 C(CH3)3), 2.99 (s, 4H, NCH2), 3.69 (s, 4H, NCH2), 6.73 (m, N2 sorption: 1; surface area (S) = 868 m g , mesoporous 3 À1 2H, PhH), 6.88 (t, 2H, PhH), 7.22 (t, 2H, PhH), 7.33 (m, 2H, volume (Vm) = 0.881 cm g , BJH pore diameter (DBJH)= 13 2 À1 3 PhH), 8.36 (s, 2H, PhCH); C-NMR (CDCl3): d31.7 (CH3), 2.81 nm. For 4a; S = 655 m g , Vm = 0.512 cm , DBJH = 34.2 (C(CH3)3), 50.1 (NHCH2), 59.7 (CH2N=C), 115.2, 2.33 nm. XRD (d100 value): for 1, 3.97 nm; 2, 3.97 nm; 3, 127.8, 129.7 (aromatic CH), 116.9, 141.4, 158.9 (aromatic 3.98 nm; 4, 4.01 nm. 13 C), 166.5 (C=N). C MAS NMR of SalenH2-MCM-41: d10.5 (CH2Si), 26.4, 30.4 (CH2CH2N), 46.0, 46.8 2.3. Measurement (NCH2CH2N=), 114.3, 118.1, 126.2, 127.1, 132.4, 151.2 (aromatic C), 156.2 (C=N). SalenCo-MCM-41: FTIR (KBr, The MCM-41 samples were characterized by powder cmÀ1): 2900-2960, 1629, 1560, 1457, 801.