Catalytic Formation of Ethylene Carbonate from Supercritical Carbon

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Applied Catalysis A: General 234 (2002) 25–33

Catalytic formation of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture with tetradentate Schiff-base complexes as catalyst Xiao-Bing Lu, Xiou-Juan Feng, Ren He∗ State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China Received 29 October 2001; received in revised form 4 February 2002; accepted 23 February 2002

Abstract Various tetradentate Schiff-base aluminum complexes (designated as SalenAlX) were prepared and used as catalysts for the synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture. Their catalytic activities can be markedly enhanced in the presence of a Lewis base or quaternary salt. With SalenAlCl/n-Bu4NBr as catalyst, the formation ◦ rate of ethylene carbonate is up to 2220 turnovers/h and about two times that under 4.0 MPa CO2 constant pressure at 110 C. The high rate of reaction may be attributed to rapid diffusion and high miscibility of ethylene oxide in supercritical carbon dioxide under employed conditions. Axial X– group and substitution on the aromatic rings of SalenAlX also affect catalytic properties of these aluminum complexes. Compared to SalenAlX, some other metal–Salen complexes, alone or combined with a Lewis base as co-catalyst, show very little catalytic activity towards the reaction under employed conditions. However, with a quaternary salt as co-catalyst, these metal–Salen complexes all exhibit catalytic activities, which are in the following order: SalenCrCl > SalenCo > SalenNi > SalenMg, SalenCu, SalenZn. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Supercritical carbon dioxide; Ethylene oxide; Ethylene carbonate; Salen–aluminum complexes; Synergistic effect

1. Introduction percritical carbon dioxide can be adjusted by changing the pressure and temperature [5]; therefore, selective The use of supercritical carbon dioxide (designated separation directly from the reaction mixture seems as sc-CO2) as a substitute solvent for chemical synthe- possible. Furthermore, sc-CO2 may be a particularly sis is a very attractive area in view of resource utiliza- advantageous reaction medium when CO2 serves as tion and environmental problems [1–4]. Since CO2, both reactant and solvent. The improved rates for cat- which has an easily accessible critical point with a alytic hydrogenation of CO to formic acid in super- ◦ 2 Tc of 31 C and a Pc of 7.3 MPa, is non-toxic, non- critical conditions provided support for this approach ﬂammable and inexpensive, it can replace hazardous [6]. The success of Noyori and co-workers suggests organic solvents and thereby provides a valuable pollu- that investigation of other CO2 reactions in sc-CO2 tion prevention tool. Also, the solvent properties of su- is worthwhile. However, only limited reactions have been explored in supercritical conditions based on ∗ Corresponding author. Tel.: +86-411-3631333x3243; sc-CO2 as both reactant and solvent [7–9]. fax: +86-411-3633080. For a long time, the search for environmentally E-mail address: [email protected] (R. He). benign processes has been the impetus for much of

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26 X.-B. Lu et al. / Applied Catalysis A: General 234 (2002) 25–33

the research involving epoxide and CO2 coupling. effective initiators for the living polymerization of The synthesis of cyclic carbonates (which may be epoxides [19,20,27]. The novel properties of these used as aprotic polar solvents and are also monomers aluminum complexes motivate us to investigate their for polymer synthesis) was performed, using catalysts new applications in catalysis and synthesis. such as alkali metal salts [10], quaternary salts [11], In the present paper, various aluminum–Salen com- Ph3SnI [12,13] or Ph3SbX2 [14] with a high temper- plexes (designated as SalenAlX) have been prepared ature or high catalyst concentration. It has been re- and used as catalysts for the synthesis of ethylene ported that in the presence of 1-methylimidazole, alu- carbonate from supercritical carbon dioxide/ethylene minum porphyrin [15,16] or phthalocyanine [17,18] oxide mixture (Scheme 1). Effects of co-catalyst, having an axial aluminum alkoxide or chloride could X– group, and substitution on the aromatic rings on effectively catalyze the reaction between CO2 and the catalytic activity of SalenAlX were studied in epoxides to form cyclic carbonates, involving the in- detail. In order to make a systematic comparison, the sertion of CO2 into the aluminum alkoxide bond. Le catalytic properties of other metal–Salen complexes, Borgne and co-workers [19–22] have developed a new such as Mg-, ClCr-, Co-, Ni-, Cu-, ZnSalen, towards class of initiators for oligomerization of heterocycles, the reaction were also investigated. leading to oligomers with controlled structure. These initiators are aluminum complexes derived from 2. Experimental tetradentate Schiff-bases (Salen), which are reminis- cent of tetraphenylporphyrin or phthalocyanine. The 2.1. Materials Salen ligands feature two covalent and two coordi- nate covalent sites situated in a planar array, but their The Schiff-base ligand (Salen: N,N -bis(salicyli- preparation is generally easier and more inexpensive dene)ethylene diamine) was synthesized from sal- than that of porphyrin derivatives. The high coordi- icylaldehyde and ethylenediamine in ethanol and native activity of the Salen ligands towards metallic re-crystallized from chloroform/ethanol. Triethy- ions has led to their extensive use in transition-metal laluminum (Et3Al) and diethylaluminum chloride chemistry, particularly in modeling enzymes and in (Et2AlCl) were purified by fractional distillation under catalysis [23,24]. However, only sporadic reports of reduced pressure in a nitrogen atmosphere. Ethylene main-group–Salen complexes have appeared [25,26]. oxide was distilled after refluxing over a mixture of Despite the similarity between the third main-group potassium hydroxide and calcium hydride. Methanol elements and the trivalent transition metals, very little and ethanol were distilled after refluxing over the work has been done in this area until recent times. corresponding magnesium alkoxides under nitrogen Aluminum–Salen complexes have been proved to be atmosphere. CO2 was purified by passing through a column packed with 4 Å molecular sieves before use.

2.2. Preparation of catalysts

A round-bottom ﬂask equipped with a three-way stopcock containing the Salen ligand of 1.340 g (5 mmol) was purged with dry nitrogen, and then anhydrous chloroform (40 ml) was added via syringe in a nitrogen atmosphere to dissolve the ligand. To this solution was slowly added of 0.602 g Et2AlCl (5 mmol) under constant stirring. The reaction was highly exothermic and resulted in a yellow solution and a pale yellow solid. The mixture was stirred for 4 h at ambient temperature, and then ﬁltered under a nitrogen atmosphere. The solid was washed with Scheme 1. The cycloaddition reaction of CO2 with ethylene oxide and structure of SalenAlX. anhydrous chloroform three times and then dried 中国科技论文在线 http://www.paper.edu.cn

X.-B. Lu et al. / Applied Catalysis A: General 234 (2002) 25–33 27

in vacuo, yielding 1.477 g of SalenAlCl. 1H-NMR the volatiles were removed under reduced pressure. 1 (CDCl3): δ 3.74 (m, 2H, NCH2), 4.22 (m, 2H, NCH2), (t-Bu)2SalenAlCl: H-NMR (CDCl3): δ 1.30 (s, 18H, 6.74–7.44 (m, 8H, PhH), 8.35 (s, 2H, PhCH). FT-IR C(CH3)3), 1.54 (s, 18H, C(CH3)3), 3.74 (m, 2H, −1 (KBr, cm ): 3050 m, 1641 vs, 1605 s, 1550 s, 1478 NCH2), 4.16 (m, 2H, NCH2), 7.06 (m, 2H, PhH), s, 1340 m, 909 m, 759 s. Anal. Calcd.: C, 58.46; H, 7.56 (m, 2H, PhH), 8.39 (s, 2H, PhCH). FT-IR (KBr, 4.29; N, 8.52. Found: C, 58.18; H, 4.42; N, 8.35. cm−1): 2954 s, 1624 s, 1542 m, 1472 m, 1445 m, The method used in the preparation of SalenAlEt 1419 m, 1310 w, 1185 m, 757 s. (t-Bu)SalenAlCl: 1 was as the same as that described earlier and was sim- H-NMR (CDCl3): δ 1.29 (s, 18H, C(CH3)3), 3.78 1 ilar to the literature method [28]. H-NMR (CDCl3): δ (m, 2H, NCH2), 4.18 (m, 2H, NCH2), 6.85–7.57 (m, −1 −0.36 (q, 2H, AlCH2CH3), 0.73 (t, 3H, AlCH2CH3), 6H, PhH), 8.37 (s, 2H, PhCH). FT-IR (KBr, cm ): 3.65 (m, 2H, NCH2), 4.01 (m, 2H, NCH2), 6.65–7.38 2957 s, 1630 s, 1588 m, 1496 s, 1361 w, 1288 m, (m, 8H, PhH), 8.24 (s, 2H, PhCH). 1175 m, 795 s. These aluminum complexes are all SalenAlOCH3 was obtained from the reac- sensitive to air or moisture and should be stored in a tion of SalenAlEt with methanol [22]. 1H-NMR nitrogen atmosphere. (CDCl3/TMS): δ 2.92 (s, 3H, AlOCH3), 3.71 (m, SalenCo(II), Ni(II), Mg(II), Zn(II), Cu(II) were pre- 2H, NCH2), 4.14 (m, 2H, NCH2), 6.72–7.41 (m, 8H, pared by the addition of a hot solution of 0.05 mol of PhH), 8.19 (s, 2H, PhCH). anhydrous CoCl2, NiCl2, MgCl2, Zn(CH3COO)2, and SalenAl-OCH2CH2(OCH2CH2)2Cl was prepared Cu(CH3COO)2 in ethanol or methanol, respectively, by the oligomerization of ethylene oxide (EO) with to a hot solution of 0.05 mol of SalenH2 in 400 ml SalenAlCl, with the molar ratio [EO]0/[SalenAlCl]0 of hot ethanol. The solution was reﬂuxed under ni- of 3 in chloroform for 7 days at ambient tempera- trogen atmosphere for about 10 h. After mixture was ture. Compared to SalenAlCl, the complexes having cooled, the product was ﬁltered, washed and dried at a long oxyethylene chain present higher solubilities 100 ◦C in a vacuum oven. SalenCrCl was synthesized 1 in chloroform or ethylene oxide. H-NMR (CDCl3): by the reaction of CrCl3 with SalenH2 in CH3CN δ 3.38–3.75 (t, 12H, Al-(OCH2CH2)3Cl), 3.63 (m, solution. 2H, NCH2), 4.23 (m, 2H, NCH2), 6.75–7.44 (m, 8H, PhH), 8.36 (s, 2H, PhCH). 2.3. Procedure (Cl)2SalenAlCl and (NO2)2SalenAlCl were prepared by a similar method to that for the preparation The cycloaddition reaction of CO2 with ethylene of SalenAlCl with THF as solvent. (Cl)2SalenAlCl: oxide was carried out in a 60 ml stainless autoclave 1 H-NMR (DMSO): δ 3.32 (m, 2H, NCH2), 3.91 (m, with a magnetic stirrer. The autoclave with weighed 2H, NCH2), 7.56 (m, 2H, PhH), 7.68 (m, 2H, PhH), catalyst inside was sealed and purged with CO2 for −1 8.61 (s, 2H, PhCH). FT-IR (KBr, cm ): 2927 m, three times. Then, ethylene oxide and liquid CO2 were 1647 s, 1535 s, 1453 s, 1387 w, 1302 m, 1217 m, charged into the autoclave. The autoclave was put into 1 1185 m, 779 s. (NO2)2SalenAlCl: H-NMR (DMSO): a constant temperature bath and rapidly heated to the δ 3.43 (m, 2H, NCH2), 3.94 (m, 2H, NCH2), 7.01 (m, desired temperature. After the expiration of the desired 2H, PhH), 8.22 (m, 2H, PhH), 8.79 (s, 2H, PhCH). reaction time, the autoclave was half-submerged in FT-IR (KBr, cm−1): 2921 s, 1647 s, 1610 s, 1562 s, a bath of ice/water mixture. Thus, the autoclave was 1500 m, 1395 w, 1321 s, 1109 m, 757 m. cooled to room temperature and the excess gases were (t-Bu)2SalenAlCl and (t-Bu)SalenAlCl were vented. The remainder of the mixture was degassed synthesized from Et2AlCl with corresponding and dissolved in methanol for the measurement of Schiff-bases. The typical procedure is as follows: ethylene carbonate by gas chromatography method, 5 mmol (t-Bu)2SalenH2 or (t-Bu)SalenH2 was dis- with butyl acetate as an internal standard. Pure ethy- solved in 40 ml of CHCl3 and the mixture was cooled lene carbonate could be obtained via distillation un- ◦ to −30 C, and then a cooled solution of 5 mmol der reduced pressure and re-crystallization in Et2O. Et2AlCl in 30 ml of CHCl3 was slowly added. The The spectral data are listed as follows. IR: νC=O 1775, −1 −1 1 solution was gradually warmed to ambient temper- 1805 cm ; νC–O 1163, 1072, 973 cm ; H-NMR ◦ ature and stirred for an additional 12 h, and then (CDCl3/TMS): δ 4.51 (s, 4H); Tm: 36.5 C. 中国科技论文在线 http://www.paper.edu.cn

28 X.-B. Lu et al. / Applied Catalysis A: General 234 (2002) 25–33

Safety warning: The high-pressure equipment such According to the classification of Van Konynenburg as that required for these experiments should be and Scott [29], the CO2/ethylene oxide system belongs equipped with a relief value and/or (preferably) a rup- to type I binary system, which has a continuous criti- ture disk for minimizing the risk of personal injury. cal locus between the critical points of the pure com- ponents. In the present case, cycloaddition reactions 2.4. Measurement under various conditions all were performed above the critical temperature of CO2/ethylene oxide (2/1 molar Infrared spectra were measured using a Nicolet ratio) mixture. In other words, at the start of reaction, 50X FT-IR spectrophotometer. 1H-NMR spectra were reactants are only present in one phase, namely the recorded by a Varian INOVA-400 type spectrometer supercritical phase. at 399.7 MHz. The chemical shifts were determined Although sc-CO2 is a good solvent for most in ppm using TMS as an internal standard. Elemen- non-polar and some polar organic compounds with tal analyses were obtained on a Perkin-Elmer 2400 low molecular weight, it dissolves these aluminum analyzer. complexes derived from Schiff-bases hardly at all. On the contrary, these complexes were found to dissolve in sc-CO2/ethylene oxide mixture, perhaps resulting 3. Results and discussion from their solubility in ethylene oxide. This result in- dicates that sc-CO2/ethylene oxide mixture should ex- 3.1. Phase behavior and properties of hibit not only fundamental properties of supercritical CO2/ethylene oxide binary system fluids, but also some part of the properties of sc-CO2 and ethylene oxide. However, it is very difficult to It is generally known that CO2 exhibits high solu- investigate their accurate solubility in sc-CO2/ethylene bility in various epoxides. On the other hand, epox- oxide mixture, because aluminum–Salen complexes ides, particularly, ethylene oxide, also easily dissolve can catalyze the reaction of CO2 with ethylene oxide to in sc-CO2 under certain conditions, as shown in Fig. 1. form ethylene carbonate under employed conditions.

Fig. 1. Plots of compositions (curves A and B) and densities (curves C and D) of the phases present for sc-CO2/ethylene oxide system vs. temperature. Curves A and C are for the gas phase; curves B and D are for the liquid phase. Equilibrium condition: ethylene 3 oxide/CO2 = 1/2 (mol/mol); equilibrium time, 36 h. Average density of phases present in the equilibrium cell is 0.600 g/cm . 中国科技论文在线 http://www.paper.edu.cn

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Table 1 Table 2 Effects of base co-catalysts on the catalytic activity of SalenAlCla Effect of quaternary ammonium or phosphonium salts co-catalysts on the catalytic activity of SalenAlCla Entry Catalyst TOFb (turnovers/h) Entry Catalyst Temperature TOF 1 SalenH2 0 (◦C) (turnovers/h) 2 SalenAlCl 174 3 Pyridine <5 10 SalenAlCl/n-Bu4NBr 110 2220 b 4 1-MeIm <5 11 SalenAlCl/n-Bu4NBr 110 1140 5 SalenAlCl/pyridine 690 12 SalenAlCl/n-Bu4NCl 110 2190 6 SalenAlCl/1-MeIm 526 13 SalenAlCl/n-Bu4NI 110 2360 14 SalenAlCl/n-BuPPh3Br 110 1990 7 SalenAlCl/n-Bu3N 490 15 n-Bu NBr 110 78 8 SalenAlCl/Et3N 358 4 9 SalenAlCl/quinoline 217 16 n-Bu4NCl 110 48 17 n-Bu4NI 110 126 a = / / Reaction condition: SalenAlCl/base ligand/EO 1 5 2500 18 SalenAlCl/n-Bu4NBr 100 1520 ◦ (molar ratio); time, 2 h; temperature, 110 C; pressure, 15–16 MPa. 19 SalenAlCl/n-Bu4NBr 120 3070 b TOF = mole of product (ethylene carbonate)/mol of catalyst a = / / per hour. Reaction condition: SalenAlCl/co-catalyst/EO 1 1 5000 (molar ratio); time, 1 h; pressure, 15–16 MPa. b Reaction was carried out under 4.0 MPa CO2 pressure. 3.2. Promoting effect of Lewis base co-catalyst 3.3. Promoting effect of quaternary ammonium The experimental results (Table 1) indicate that CO2 or phosphonium salts can react with ethylene oxide to produce ethylene carbonate only in the presence of SalenAlX, while Salen SalenAlCl, combined with a quaternary ammonium ligand (SalenH2) itself does not catalyze the reaction. or phosphonium salt such as n-Bu4NBr, exhibits very The reaction temperature has a great effect on the cat- high catalytic activity up to 2220 turnovers/h towards alytic activity of the binary catalyst. The formation the reaction of CO2 and ethylene oxide to synthe- rate of ethylene carbonate increases remarkably with size ethylene carbonate. The formation rate of ethy- the enhancement of reaction temperature. lene carbonate is dozens of times that of SalenAlCl Similar to the cases of CO2 and propylene oxide or n-Bu4NBr alone as catalyst at same condition. The with Al(L)TPP [15,16] or Al(L)Pc(t-Bu)4 [17,18] results are shown in Table 2. To the best of our knowl- systems, in the presence of a Lewis base such as edge, such catalytic activity is one of the highest for pyridine, trialkylamine, 1-methylimidazole, Sale- the cycloaddition of CO2 to epoxides amongst the re- nAlCl can efﬁciently promote the conversion of the ported catalysts. The Inoue groups [30] have reported epoxide to cyclic carbonate at the same tempera- the living and alternating co-polymerization of CO2 ture, although these bases themselves have no cat- and epoxides to produce linear polycarbonates of con- alytic activity under employed conditions. Among trolled molecular weight with a narrow distribution, them, the binary catalytic systems of SalenAlX/ catalyzed by the aluminum porphyrin-quaternary or- pyridine, SalenAlX/1-methylimidazole, or SalenAlX/ ganic salt system at ambient temperature. However, tributyl-amine were found to be more effective. The in the catalytic system of SalenAlCl coupling with promoting effect of base co-catalyst probably results a quaternary salt, reaction between CO2 and ethy- from the formation of hexacoordinated complexes lene oxide mainly produces ethylene carbonate rather between SalenAlX and base ligands, as with the than polycarbonates, even for temperature as low as Al(L)TPP/1-methylimidazole system [16]. Owing to 10 ◦C. electron transfer from base ligands to central metal It is interesting that the formation rate of ethylene of SalenAlX, the Al–X bond of SalenAlX is reduced. carbonate does not have any obvious relationship to This might be beneﬁcial for the insertion of epoxide the nucleophilic abilities of anions of quaternary salts. and CO2 into the Al–X bond of SalenAlX to form On the other hand, with quaternary salt alone as cata- linear carbonate, which is transformed into ethylene lyst, the reaction appears to be dependent on the nu- carbonate by intramolecular cyclic elimination. cleophilicity of anions under the same condition. 中国科技论文在线 http://www.paper.edu.cn

30 X.-B. Lu et al. / Applied Catalysis A: General 234 (2002) 25–33

Table 3 Table 4 Effect of axial X– group on the catalytic activity of SalenAlXa Effect of substitution on the aromatic rings on the catalytic activity of SalenAlXa Entry Catalyst TOF (turnovers/h) Entry (R1)(R2)SalenAlCl/n-Bu4NBr TOF (turnovers/h) 2 SalenAlCl 174 20 SalenAl-OCH3 133 10 1 2220 21 SalenAl-(OCH2CH2)3Cl 318 25 2 1264 10 SalenAlCl/n-Bu4NBr 2220 26 3 820 22 SalenAlEt/n-Bu4NBr 1980 27 4 1025 23 SalenAlOCH3/n-Bu4NBr 2060 28 5 1161 24 SalenAl-(OCH CH ) Cl/n-Bu NBr 2340 2 2 3 4 a Reaction conditions are the same as Table 2. a Reaction conditions are the same as Table 2. of SalenAlEt [28], while substitution on the aromatic For a comparison, the cycloaddition reaction of rings of SalenAlX probably causes geometrical distor- tion of square pyramidal structure, and thus, may affect CO2 and ethylene oxide was also made to proceed under non-supercritical condition with the use of its properties significantly. The amount of the distor- tion depends on the size of the substituted groups, SalenAlCl/n-Bu4NBr as catalyst. It was observed that the formation rate of ethylene carbonate under and can be measured using the geometrical calcula- tion: τ = (β − α)/60 [31]. The τ value ranges from 4.0 MPa CO2 pressure (entry 11) was only half of that found under supercritical condition at the same 0 (perfectly square pyramidal) to 1 (perfectly trigonal temperature. The high rate of reaction was attributed bipyramidal). Alpha and beta are the angles that are to rapid diffusion and high miscibility of ethylene opposite each other in the xy plane (with the Al–X– oxide in supercritical carbon dioxide under employed group oriented along the z-axis). For Salen(t-Bu)AlMe τ conditions. [32], the value is 0.47, clearly indicating a dis- torted square pyramidal or trigonal bipyramidal geometry. 3.4. Effect of axial X– group of SalenAlX The experimental results (Table 4) show that substitution on the aromatic rings of SalenAlX Axial X– group also affects catalytic proper- has a negative effect on its catalytic activity for ties of SalenAlX. The results are listed in Table 3. the cycloaddition reaction. Under employed condi- SalenAl-(OCH2CH2)3Cl, possessing a long oxyethy- tions, catalytic activities of the series of substituted lene chain, exhibits higher catalytic activity than aluminum–Salen complexes are in the following order: other aluminum complexes towards the cycloaddition SalenAlCl > Salen(Cl)AlCl > Salen(NO2)AlCl > reaction of CO2 and ethylene oxide. The reason may Salen(t-Bu)AlCl, which is reverse to the order of the be that a long axial group is beneficial for the forma- size of the substituted groups. It is obvious that the tion of monomeric rather than dipolymeric SalenAlX, steric effect of these bulk substituted groups plays a and thus, favors obtaining maximum catalytic activ- key role in the decrease of catalytic property of these ity. Existence of halides in Axial X– group seems aluminum complexes. to improve catalytic activity of SalenAlX. However, catalytic activities of binary catalysts, consisting of 3.6. Effect of central metal ion of metal–Salen SalenAlX and n-Bu NBr, were only slightly influ- 4 complexes enced by changing the axial group of SalenAlX. In order to make a systematic comparison, the cat- 3.5. Effect of substitution on the aromatic rings alytic properties of SalenMg, SalenZn, and transition of SalenAlX metal–Salen (such as SalenCrCl, SalenCo, SalenNi, SalenCu), or combined with a Lewis base or qua- Monomeric SalenAlX has a structure of five-coordi- ternary salt as co-catalyst, were also investigated nate square pyramidal Al3+ strongly bound to both (Table 5). However, compared to SalenAlX, these the oxygen and nitrogen ligand atoms, similar to that metal–Salen complexes, alone or combined with a 中国科技论文在线 http://www.paper.edu.cn

X.-B. Lu et al. / Applied Catalysis A: General 234 (2002) 25–33 31

Table 5 3.7. Cycloaddition mechanism The formation rate of ethylene carbonate from sc-CO2/ethylene oxide mixture with various metal–Salen complexes as catalysta The formation of ethylene carbonate from CO2 and Entry Catalyst TOF (turnovers/h) ethylene oxide can proceed via various reaction path- 29 SalenCrCl 0 ways: direct insertion of CO2 into the C–O bond of 30 SalenCo 0 ethylene oxide, cyclic elimination of linear carbonate 31 SalenCrCl/1-MeIm <5 formed, polymerization/de-polymerization process, 32 SalenCo/1-MeIm <5 etc. These pathways may not operate exclusively of 33 SalenCrCl/n-Bu4NBr 2140 each other, but one of them might be expected to 34 SalenCo/n-Bu NBr 1320 4 predominate for a given catalytic system, or under 35 SalenNi/n-Bu4NBr 350 36 SalenCu/n-Bu4NBr 190 certain experimental conditions [33]. 37 SalenZn/n-Bu4NBr 174 For SalenAlX alone as catalyst, ethylene oxide 38 SalenMg/n-Bu4NBr 225 is ring-opened according to base-catalyzed cleavage a Reaction conditions are the same as Table 2. and inserts into Al–X bond of SalenAlX to produce SalenAl-OCH2CH2X, due to interaction between SalenAlX and ethylene oxide. The insertion of CO2 Lewis base as co-catalyst, hardly show any catalytic to Al–O bond of SalenAl-OCH2CH2X results in the activity towards the reaction under employed condi- formation of linear carbonate, which is transformed tion. It is thus strange that, with a quaternary salt as into ethylene carbonate by intramolecular cyclic elim- co-catalyst, these metal–Salen complexes all exhibit ination. catalytic activities, which are in the following or- In the presence of a Lewis base, duo to its der: SalenCrCl > SalenCo > SalenNi > SalenMg, electron-donor ability, a base ligand easily coordi- SalenCu, SalenZn. nates with the central metal ion of SalenAlX to form

Scheme 2. The possible formation mechanism of ethylene carbonate from sc-CO2/ethylene oxide mixture using SalenAlX alone or coupling with a Lewis base as catalyst. 中国科技论文在线 http://www.paper.edu.cn

32 X.-B. Lu et al. / Applied Catalysis A: General 234 (2002) 25–33

Scheme 3. The possible formation mechanism of ethylene carbonate from sc-CO2/ethylene oxide mixture in the presence of SalenAlX/n-Bu4NBr binary catalyst.

a hexacoordinated complex, as shown in Scheme 2. combined with n-Bu4NBr as co-catalyst, the forma- Electronic transfer from base ligands to Al3+ of Salen- tion of ethylene carbonate probably only is involved AlX must reduce the Al–X bond of SalenAlX, and in one mechanism, namely the former path. thus, benefit the insertion of epoxide and CO2 into the Al–X bond of SalenAlX. Therefore, SalenAlX, combined with a Lewis base, exhibits higher catalytic 4. Conclusion activity than SalenAlX alone as catalyst. The reaction of CO2 and ethylene oxide with Ethylene oxide has high miscibility with CO2. SalenAlX/n-Bu4NBr as catalyst shows a much higher According to the classification of Van Konynenburg rate than with SalenAlX alone, or combined a Lewis and Scott, the binary system shows fluid phase behav- base, as catalyst. The plausible mechanisms are pro- iors of type I, which possesses a continuous critical posed as shown in Scheme 3. One reaction path is locus between the critical points of the pure compo- that ethylene oxide is ring-opened in view of the syn- nents. The critical temperature of the binary system ergistic effect of the binary catalyst, which probably depends on the molar ratio of CO2 and ethylene results from nucleophilicities of highly reactive anions oxide. sc-CO2/ethylene oxide mixture exhibits not of quaternary salts and the electrophilic interaction of only fundamental properties of supercritical fluids, SalenAlX with ethylene oxide. Then, CO2 inserts into but also some part of the properties of sc-CO2 and Al–O bond of Salen(X)Al-OCH2CH2BrN(n-Bu)4 ethylene oxide. formed to produce a linear carbonate, which is further The Salen–aluminum complexes are effective transformed into ethylene carbonate by intramolec- catalysts for the synthesis of ethylene carbonate ular cyclic elimination. Another path is the same as from sc-CO2/ethylene oxide mixture. Their catalytic that described with SalenAlX alone as catalyst. The activities can be markedly enhanced in the presence proposed reaction paths may not operate exclusively, of a Lewis base or quaternary salt. Among them, the but the former path should predominate for the binary binary catalyst consisted of SalenAl-(OCH2CH2)3Cl catalytic system. For other metal–Salen complexes, and n-Bu4NBr was found to be most effective and 中国科技论文在线 http://www.paper.edu.cn

X.-B. Lu et al. / Applied Catalysis A: General 234 (2002) 25–33 33

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