Efficient pathway for CO2 transformation to cyclic carbonates by heterogeneous Cu and Zn salen complexes

Journal of Organometallic Chemistry 713 (2012) 104e111

Contents lists available at SciVerse ScienceDirect

Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

Efficient pathway for CO2 transformation to cyclic carbonates by heterogeneous Cu and Zn salen complexes Zeynep Tas¸cı a, b, *, Mahmut Ulusoy c, ** a

Department of Chemistry, Ege University, 35100 Bornova, Izmir, Turkey Department of Chemistry, Mugla University, 48000 Mugla, Turkey c Department of Chemistry, University of Harran, 63190 Sanliurfa, Turkey b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 November 2011 Received in revised form 20 April 2012 Accepted 23 April 2012

A series of silica supported Salen type Zn(II) and Cu(II) complexes were prepared and characterized. The obtained materials were characterized by X-ray diffraction (XRD), sorption measurement (BET), Fourier transform infrared spectroscopy (FT-IR), atomic absorption spectroscopy (AAS), atomic force microscopy (AFM), scanning electron micrograph (SEM), thermal and elemental analysis. The chemical fixation of CO2 to form cyclic carbonates in the presence of a Lewis base was efficiently performed by this heterogeneous catalyst system. Very high yield and conversions were obtained at 1.0 MPa CO2 atmosphere without an addition of any solvent. The active heterogeneous Cu-DTBSA was efficiently transformed CO2 into cyclic carbonates while the reusability of the Cu-DTBSA is eight times with minimal decrease in yield. Ó 2012 Elsevier B.V. All rights reserved.

Dedicated to Prof. Dr. Bekir Çetinkaya in honor of his retirement. Keywords: Heterogeneous Cu (II) and Zn (II) complexes Catalysts Carbon dioxide Salicylaldimine ligand Cyclic carbonate

1. Introduction Carbon dioxide is the industrial waste product and the main components of the greenhouse gases. Research studies to convert CO2 into many useful products to reduce the carbon dioxide emission are increased over the past decades. Since CO2 is a nontoxic, abundant, natural and cheap C resource, it has seena large interest in converting it to organic products (e.g. cyclic carbonates, polycarbonates). The synthesis of cyclic carbonates via the chemical fixation of CO2 and epoxides has received an increasing attention due to the wide application areas such as electrochemical applications (e.g. as polar solvents, curing agents, cosmetics, resins, etc.) and their usage as intermediates for polymers and fine chemicals [1]. Up to now, various catalytic systems have been developed for the synthesis of cyclic carbonates from epoxides and CO2, including the transition-metal complexes [2], ionic liquids [3], supercritical CO2 (scCO2) [4], organic bases [5], metal oxides [6], etc.

* Corresponding author. Department of Chemistry, Ege University, 35100 Bornova, Izmir, Turkey. Tel.: þ90 252 2115098; fax: þ90 252 2111472. ** Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Tas¸cı), [email protected] (M. Ulusoy). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2012.04.019

The usage of homogeneous catalyst in industry is unfavorable due to the difficulties of catalyst separation (e.g. decomposition of catalyst, energy consumption, etc.) [7]. Although there are several heterogeneous catalysts that have been reported for the formation of cyclic carbonates in the literature, there are still many problems of using them as catalyst systems such as their low catalyst activity and/or selectivity, necessity of low catalyst amount, required high CO2 pressure, long reaction time and requirement of an additional co-solvent for the process. Metal complexes of Schiff base ligands have found wide application area in catalyst chemistry such as oxidation, ring opening reactions and many notable organic transformations [8]. The chemical fixation of CO2 to form polycarbonate and/or cyclic carbonates have been reported by using salen complexes of Al, Zn, Cr, Co, Mn, Sn and Ru [9,10]. However, some metals are highly toxic and/or expensive and/or unstable to moisture and oxygen. Therefore the immobilization of the catalyst system is crucial. He and co-workers have demonstrated a bifunctional Co-salen complex that has a metal center and phosphonium salt parts which was effective (71e94% propylene carbonate (PC) yield) for the cycloaddition of epoxides (4.0 MPa, 100
C, 4 h, 0.5% mol cat.) [10e]. Similarly, Lee and co-workers reported a bifunctional Co-salen complex that has an ammonium salt part which was effective for the

Z. Tas¸cı, M. Ulusoy / Journal of Organometallic Chemistry 713 (2012) 104e111

105

copolymerization of CO2/epoxides [9e]. Garcia immobilized a Crsalen complex to silica and used it for the formation of cyclic carbonates under ScCO2 (100 bar, 80
C, 6 h, 0.2% mol cat.) with styrene oxide [10h]. They obtained 74% conversion in CH2Cl2 [10h]. Baiker et al have studied immobilized Mn-salen complex as the catalyst with styrene oxide (SO), where the yield was 95% (TOF ¼ 196) under ScCO2 condition [10e]. Kleij has reported that Znsalphen compounds can be used as an effective and green catalysts (10 bar, 45
C, 18 h, 2.5 mol % cat.) using epoxy hexane [10g].

a Perkin Elmer Spectrum 100 series. AFM measurements were carried out at room temperature and ambient conditions using Ambios Q-Scope 250 instrument. SEM observations were performed using Phillips XL-30S FEG microscope. Elemental analyses of samples were done using a LECO CHNS model 932 elemental analyzer. AAS determinations of metals were done using Thermo Elemental Solaar AA Spectrometer. The samples (20 mg) were digested in a mixture of concentrated HNO3 (65%, 3 mL), HF (40%, 2 mL) and HClO4 (65%, 1 mL) using Cem Mars 5 Microwave Oven.

As an alternative to salen complexes, Kaneda’s group [11] synthesized ZnHAP as a catalyst. Immobilized ionic liquids to poly-styrene (PSIL) [12] and silica [13] were also reported in the literature for the chemical fixation of CO2. In this paper, the preparation and characterization of the immobilized Cu(II) and Zn(II) salen complexes and their usage as a catalyst for the chemical fixation of CO2 and epoxides are reported. Recently cyclohexene oxidation using molecular oxygen and hydrogen peroxides was reported using immobilized Cu(II) salicylaldimine complex on amorphous and mesoporous silica support [14]. In this study, the use of similar heterogeneous Cu complexes for another catalytic process is proposed. These complexes are found to be efficient catalysts for the formation of cyclic carbonates without an additional co-solvent. They exhibit high TOF value, and can be reused up to eight times with minimal decrease on conversion.

Catalytic tests were performed in a 50 mL PARR 4843 stainless steel pressure reactor.

2. Experimental 2.1. Materials Silica gel (Davisil grade, surface area: 480 m2/g; pore size: 0.75 cm3/g; particle size: 35e60 mesh) was commercially supplied by SigmaeAldrich Company. 3-aminopropyltrimethoxysilane (97%, Alfa Aesar), 4-diethylaminosalicylaldehyde (99%, Alfa Aesar), Zn(OAc)2$2H2O (99%, Fluka) and Cu(OAc)2$H2O (99%, Fluka) were used in the study. 3,5-ditert-butylsalisilaldehyde was synthesized according to the literature [15]. All epoxides were supplied from Alfa Aesar Company, and carbon dioxide with the purity of 99.9% was used. All solvents were distilled prior to use under the inert atmosphere (Et2O, ethanol and toluene over Na and dichloromethane over P2O5).

2.3. Catalyst preparation Surface modification of silica was done as reported earlier [16]. Briefly, amorphous silica (3 g) was treated with 50 mL 0.01 M acetic acid solution in an ultrasonic bath for 1 h at ambient temperature. Then, pretreated silica was washed with ultra pure water until a pH of 7 was obtained. It is dried at 200
C for 2 days under vacuum. Amorphous silica (3 g) was suspended in dry toluene. 4.5 mmol of 3-aminopropyl-trimethoxysilane was added to this suspension, and the mixture was stirred under reflux for 24 h. Modified silica was recovered by filtration. Ina Soxhlet extractor, it is extracted by refluxing dichloromethane over a period of 4 h and then dried at 80
C under vacuum. 3-Aminopropylated silica bearing 1.5 mmol of organic amine (1.1 g SiO2eNH2, according to the elemental analysis) was suspended into 30 mL ethanol. 3,5-Ditert-butylsalicylaldehyde (DTBSA) (3 mmol) or 4-diethylaminosalicylaldehyde (DEASA) (3 mmol) was added, and the mixture was stirred under reflux for 2 days under inert atmosphere. The yellow colored functionalized silica was filtered, washed with ethanol and then extracted under refluxing dichloromethane in a Soxhlet extractor over a period of 8 h and dried at 60
C under vacuum overnight. 0.6 mmol Cu(OAc)2 or Zn(OAc)2 was dissolved in acetone, and the functionalized silica which contains 1.2 mmol immobilized ligand according to elemental analysis test [SiO2eNEt2 (1.15 g) or Table 1 Elemental analysis results of prepared compounds. Comp.

%C

%N

%H

(C/N) measured

(C/N) calca

Metal cont. (mmol/g)b

SiO2eNH2 SiO2eNEt2 SiO2-DTBSA ZneNEt2 CueNEt2 Zn-DTBSA Cu-DTBSA

5.69 17.81 20.06 12.32 13.10 23.05 21.79

1.92 2.91 1.56 2.13 2.23 1.56 1.68

1.61 2.57 2.95 1.90 1.97 3.32 2.37

2.96 6.12 12.86 5.78 5.78 14.78 12.97

2.57 6.01 15.43 6.01 6.01 15.44 15.44

0.135 0.095 0.162 0.094

2.2. Measurements The synthesized heterogeneous compounds were characterized by XRD, BET, FT-IR spectroscopy, AAS, AFM, SEM, thermal and elemental analysis. X-ray diffractogram were recorded on a Phillips X’Pert Pro diffractometer. Nitrogen adsorptionedesorption isotherms were obtained at 77 K using Micromeritics ASAP 2010 apparatus. TG measurements were performed on a Perkin Elmer Diamond TG/DTA Instrument. FT-IR Spectra were recorded on

a b

Calculated suggesting all ethoxy sides attached to silica. Measured from AAS.

106

Z. Tas¸cı, M. Ulusoy / Journal of Organometallic Chemistry 713 (2012) 104e111

t Bu

N Cu

N

O O

t Bu t Bu

t Bu

Cu-salen Scheme 1. Synthesis route of immobilized complexes and the structure of homogeneous Cu-salen complex.

SiO2-DTBSA (1.08 g)] was added to the solution. The suspension was stirred at 60
C for 24 h. Physisorbed metal species were removed by extraction with refluxing methanol in a Soxhlet extractor over a period of 24 h and dried at 100
C under vacuum. In Table 1, elemental analysis and AAS results of the prepared compounds are given.

with dichloromethane, and used for recycling test after drying. Conversion of the crude reaction mixture was determined by 1H NMR spectra. For recycle experiments, the catalyst was easily recovered by simple filtration after the catalytic reaction, washed with CH2Cl2, dried under vacuum and finally used for the subsequent reaction.

2.4. Catalytic reactions

3. Result and discussion

The coupling reaction of carbon dioxide and epoxide was carried out in a 50 mL stainless steel Parr autoclave. In a typical reaction process, the supported metal complex (0.01 mmol), DMAP (0.067 mmol) and epoxide (10 mmol) were charged into the reactor without using any solvent. The reactor was placed under a constant pressure of carbon dioxide and heated up to the required temperature. After the reaction was completed, the reactor was cooled to 5e10
C in an ice bath and excess gases were vented out slowly. Immobilized metal complex was separated by filtration, washed

The immobilization of complexes on amorphous silica is illustrated in Scheme 1. 3.1. Catalyst characterization The immobilization of metal complexes on silica was performed with the condensation of 3-aminopropyltrimethoxysilane as mentioned in Section 2.3.

Fig. 1. TG/DTG profile of Cu-DTBSA.

Z. Tas¸cı, M. Ulusoy / Journal of Organometallic Chemistry 713 (2012) 104e111

107

Fig. 2. FT-IR spectra of silica-DTBSA (a) and Cu-DTBSA (b).

Elemental analysis and TG/DTA were used to determine the amount of supported organic ligands. The synthesized Cu-DTBSA’s TG/DTA profile is shown in Fig. 1 as an example. From the TG/DTA profile, it is apparent that immobilized complex decomposes in two different temperature regions. The first decomposition is remarked between 30 and 200
C due to the loss of loosely bound water (1.5%). The second decomposition is observed between 200 and 700
C in two stages which possibly corresponding to covalently bonded organic groups. It can be seen from the TG/DTA curves of the ligands, in the immobilized complex structure, that the aldehyde group is decomposed first and then 3aminopropylsilyl group is broken up. Similar results are obtained for the other immobilized complexes. The results from elemental analysis and TG data are consistent with the calculated amount of the supported ligand. The IR spectra of the metal complexes were compared with those of the free ligands in order to determine the coordination sites that may be involved in chelation. The position and/or the intensities of these peaks were expected to be changed upon chelation. The bands located at around 1652 cm1 are assigned to

the y(C]N) stretching vibrations of the azomethine of the ligands (Fig. 2). These bands are shifted 44 cm1 to a lower wavenumber which support the participation of the azomethine group of these ligands in binding to the copper ion [17e20]. XRD measurements of powdered silica and Cu-DTBSA functionalized silica are shown in Fig. 3. X-ray diffraction pattern for silica shows characteristic low angle peaks which attributed to nonhomogeneous structure. Immobilized Cu-DTBSA complex has also low angle peaks. Thus, immobilization of Cu complexes on silica did not significantly affect the structure of silica. The physicochemical properties of the Cu-DTBSA catalyst and silica are given in Table 2. The nitrogen isotherms of silica and Cu-DTBSA catalyst are of the type II that indicates the formation of multimolecular adsorption (Fig. 4). The decrease in the surface area and the increase in the pore volume are comparable with amorphous silica, which was due to the surface modification. Surface morphology and particle size distribution of silica and Cu-DTBSA functionalized silica were analyzed by AFM and SEM. According to SEM images of silica (Fig. 5a) and Cu-DTBSA functionalized silica (Fig. 5b), the particles have non-uniform size distribution within the range of 0.1e10 mm and 5e25 mm for CuDTBSA and silica, respectively. Consistent with the SEM images, AFM images of Cu-DTBSA shows that the grain size is nonhomogeneous. AFM sample of amorphous silica could not be prepared due to its big particle size. AFM and SEM images were consistent with the sorption measurements, and it is concluded that particle size of silica decreased during modification with organic ligands and metal particles. Table 2 Physicochemical properties of catalyst and silica. Cat.

A)c Ligand (mmol g1)d Surf. area (m2 g1)a n (cm3 g1)b Pore size (

Silica 209 Cu-DTBSA 166 a b c

Fig. 3. XRD pattern of Cu-DTBSA.

d

0.324 0.276

Calculated from BET isotherm. Calculated from BET isotherm (4 V/A). BET analysis. Calculated from elemental analysis.

0.630 1.055

e 0.76

108

Z. Tas¸cı, M. Ulusoy / Journal of Organometallic Chemistry 713 (2012) 104e111

Fig. 4. Nitrogen isotherms of Cu-DTBSA.

3.2. Catalytic activity The cycloaddition reaction is catalyzed by a Lewis acid and a Lewis base. According to the proposed reaction mechanism [11], Lewis acidic center (i.e. metal center) activates epoxide via bonding to O atom and then Lewis basic species (co-catalyst) attacks to the sterically less hindered carbon atom of epoxide. Afterwards, insertion of carbon dioxide to opened epoxide occurs, cyclic carbonate forms following ring-closing step. Under solvent less conditions, the activities of copper and zinc/ salen complexes immobilized on silica are tested for the chemical fixation of carbon dioxide into cyclic carbonate with different epoxides. In this process, 4-dimethylaminopyridine (DMAP) is used as a Lewis base (Scheme 2).

The comparison of the metal complexes as the catalyst was done using styrene oxide (SO) as the substrate. The results are summarized in Table 2. All catalysts were effective for the conversion of SO to styrene carbonate (SC). From the results, the Cu-DTBSA having the DTBSA as the ligand was determined to be the best catalyst (Table 3, entry 1). It is also found that Cu complexes were slightly more efficient than Zn complexes when they were tested with the same ligand (DTBSA). When Cu complexes used in the test, DTBSA was more efficient than DEASA as the ligand. As a result, we concluded that both steric effect and electron donor capability are decisive for the catalyst activation. The classical homogeneous salen complex of Cu(II) (Cu-salen) was synthesized according to literature [21] to compare it with CuDTBSA (Table 3, entry 5). It is shown that there is no significant

Fig. 5. SEM images of a) silica, b) Cu-DTBSA and c) AFM image of Cu-DTBSA.

Z. Tas¸cı, M. Ulusoy / Journal of Organometallic Chemistry 713 (2012) 104e111

109

O

O

CO2 R

Cat., base temp., press.

O

O R

Scheme 2. Synthesis of cyclic carbonates from epoxides and CO2.

difference in catalytic activity between the homogeneous (83%) and heterogeneous (70%) complexes at the same catalytic conditions. In the absence of a catalyst, the reaction did not work at all (Table 3, entry 6). Moreover, silica gel alone was inactive for SC synthesis (Table 3, entry 7). We have also evaluated the effect of co-catalyst on the catalytic performance (Table 3, entries 8e11). For testing this, instead of DMAP, other bases (KOH, Na2CO3 and NEt3) and ionic salts (NBu4Br and [bmim]I) were used as co-catalyst. The catalytic efficiencies of these co-catalysts were found to be in the following order: KOH < Na2CO3 < NEt3 < DMAP < NBu4Br < [bmim]I. Organic cocatalysts were found to be more effective than inorganic bases, which is probably due to the solubilities of bases in the epoxides. 3.2.1. Influence of temperature The effect of temperature on the conversion of SO is shown in Fig. 6a. As can be seen from the Figure, the conversion of SO increases sharply with increasing temperature (0
Ce100
C), and it stays almost constant between 100
C and 125
C. Interestingly, as temperature further increases and reaches the range of 125e150
C, it is observed that the selectivity of SC decreases, even though SO conversion remains the same. Probably, this is due to the result of side reactions such as diol formation and SO isomerization [22]. Therefore, the optimum temperature range for this reaction is chosen as 100e125
C. 3.2.2. Influence of pressure The influence of CO2 pressure on the conversion of cyclic carbonates was investigated at 100
C for 1 h with Cu-DTBSA as the catalyst (Fig. 6b). In the low pressure region (0.5e1.5 MPa), increase in conversion efficiency and selectivity of SO are in proportion with increase in pressure, on the other hand, in the Table 3 Synthesis of styrene carbonate from styrene oxide and CO2 catalyzed by immobilized complexes.

O

O

CO2 Ph

0.1% cat., 0.6% DMAP

O

1 h, 100 oC, 1.0 Mpa

Entry

Catalyst

Base

Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13

Cu-DTBSA Cu-DEASA Zn-DTBSA Zn-DEASA Cu-salen e Silica e Cu-DTBSA Cu-DTBSA Cu-DTBSA Cu-DTBSA Cu-DTBSA

DMAP DMAP DMAP DMAP DMAP e

72 65 70 60 83 Trace 5 30 14 4 32 75 82

DMAP Na2CO3 KOH NEt3 NBu4Br [bmim]I

O Ph

a

TOFb 720 650 700 600 830 50 300 140 40 320 750 820

a Conversions were determined by comparing the ratio of product to substrate in the 1H NMR. b TOF (mol of product (mol of catalyst h)1) ¼ (TON/h).

Fig. 6. Conversion of styrene oxide as a function of temperature (a), pressure (b) and time (c) with complex Cu-DTBSA.

high pressure region (2.0e5.0 MPa) there is an inverse proportion between SO conversion and CO2 pressure. Similar effect of CO2 pressure on catalytic activity was observed in other catalytic systems in the literature [2,4,10d]. According to these reports, at very high CO2 pressure medium, carbon dioxide may hinder the interaction between the epoxide and the catalyst, causing a low concentration of epoxides in the surrounding of the catalyst and thus resulting in a low conversion of SC. [2,4,10d]. In our studies, the maximum conversion of SC is observed at around 1.0e1.5 MPa pressure region. 3.2.3. Influence of time SO conversion efficiency as a function of reaction time is shown in Fig. 6c. The results show that the reaction progressed within the first 1 h with a 72% SO conversion (100
C, 1.0 MPa CO2). At the end of 2 h, the conversion of SO has only reached to 74% with 93% selectivity. Under the same conditions (100
C, 1.0 MPa CO2), further increase in reaction time didn’t change the conversion

110

Z. Tas¸cı, M. Ulusoy / Journal of Organometallic Chemistry 713 (2012) 104e111

Table 4 Coupling of CO2 and various epoxides catalyzed by immobilized complexes.a Entry

Carbonate

Conversionb

Selectivity

TON

TOF (h1)

O 1 2

O

O c

Ph O

3

O 4

72

93

670

670

99

98

970

323

53

99

525

525

96

99

950

317

99

98

970

323

50

97

485

485

96

97

931

310

O

c

O 5

O

O

Cl O 6

O 7c

O

a Reaction conditions: Cu-DTBSA (0.1 mmol), DMAP (0.067 mmol), epoxide (10 mmol), CO2 (1.0 MPa), 100
C, 1 h. b Determined by 1H NMR. c 120
C, 1.5 MPa, 3 h.

efficiency. As future studies, the temperature and pressure might be varied to get higher conversion values. Our results shows that CuDTBSA can be used as an effective catalyst for cycloaddition of SO with CO2 under solvent-free conditions. 3.2.4. Applicability of different epoxides In order to appraise the applicability of various epoxides, the cycloaddition reaction of other epoxides possessing aromatic, aliphatic and both electron donating and with-drawing substituents were also examined. The data is presented in Table 4. As can be seen from the data (Table 4), all epoxides were converted into corresponding cyclic carbonates in acceptable conversions. Among all the epoxides surveyed, the epichlorohydrin is shown to be the most reactive epoxide with a 99% conversion in 1 h. 3.2.5. Recycle experiments Recycle experiments were performed with epichlorohydrin since it is the most effective epoxide among the examined ones (PO, EB, SO and ECH). At the end of eight-cycle, the conversion is decreased slowly from 99% to 92% within 1 h (at 100
C, 1.0 MPa CO2 pressure). After each cycle, Cu-DTBSA catalyst was easily

Table 5 The comparison of metal catalysts for the formation of epichlorohydrin to corresponding cyclic carbonate at assorted catalytic conditions. Entry Catalyst

Solvent Temp. (K) Press. t (h) (MPa)

TOF (h) Ref.

1

Cu-DTBSA/DMAP

No

373

1.6

9555

2 3 4 5 6 7 8 9 10 11

iminRu/DMAP Et3N/salenZn ZnHAP/DMAP PSIL [C4mim][BF4]/SiO2 [bmim]Br/ZnCl2 ZnCl2/PPh3C6H13Br PPh3/SiO2 Ni/Zn/TBAB HEMIMB

No CH2Cl2 No No CO2 No No No No No

373 373 373 383 433 373 393 363 393 398

1.6 3.45 1.0 6.0 8.0 1.5 1.5 1.0 2.5 2.0

3 1 2 24 3 4 1 1 20 1 0.33

4050 100 1250 47 11 4887 3386 43 3234 189

*this work [2b] [10b] [11] [12] [13] [23] [24] [25] [26] [27]

recovered by simple filtration, washed with CH2Cl2, dried under vacuum, and then reused for the subsequent reaction. The results are illustrated in Fig. 7. To test the recyclability of Cu-DTBSA/DMAP catalytic system where PO was used as a substrate, experiments were done under the optimized reaction conditions (120
C, 1.5 MPa CO2 pressure, 3 h). As it can be seen from Fig. 7 that during the catalytic reuse of Cu-DTBSA/DMAP, there is a small reduction in the formation of propylene carbonate. At the end of the eighth cycle, the obtained conversion efficiency is14% lower than the initial value (Fig. 7). Similarly, we found that the catalyst can be reused at least eight times with only a slight decrease in conversion efficiency while using the epoxides such as PO and ECH as the substrates. In Table 5, the comparison of the Cu-DTBSA/DMAP and the conventional catalyst systems for the formation of 4-chloromethyl1,3-dioxolan-2-one are shown. The obtained TOF value of CuDTBSA was found higher from the reported values. A high conversion of cyclic carbonate is acquired with ZnHAP [11], ZnCl2 in ionic liquid [23] and phosphonium salt [24] catalysts at 100
C and low CO2 pressure. However, these catalyst systems have much lower TOF value compared with the proposed catalyst system. A silica supported imidazolium salt [13], PPh3 [25] and polymer supported ionic liquid [12] can also catalyze this reaction. Nevertheless, higher CO2 pressure and reaction temperature were required when compared with the present catalyst system. Homogeneous Zn-salen complex [10b] requires an organic solvent and high CO2 pressure. Ni/Zn/TBAB catalyst system [26] requires a higher temperature and CO2 pressure. HEMIMB compound [27] catalyzes this reaction effectively, but the high catalyst loading is a disadvantage. Thus, the present catalyst system is an effective system and further studies are in progress to obtain an effective catalyst system working at low temperature and low catalyst loading. 4. Conclusions

Fig. 7. Catalyst recycle experiments in coupling CO2 and propylene oxidea or epichlorohydrinb. a120
C, 3 h, 1.5 MPa CO2, b100 C, 2 h, 1.5 MPa CO2.

The silica supported heterogeneous Salen type Zn(II) and Cu(II) complexes were prepared and efficiently used for the chemical fixation of CO2 to organic cyclic carbonates. When the effectual heterogeneous Cu-DTBSA was compared with the other heterogeneous and homogeneous catalyst systems in the literature, the obtained TOF value of Cu-DTBSA catalyst was found to be higher than those in the literature. The reported catalyst systems have much lower TOF value compared with the present catalyst system. It is also found that this effective catalyst (CuDTBSA) transforms CO2 to cyclic carbonates efficiently while it can be reused at least eight times with minimal decrease on conversion.

Z. Tas¸cı, M. Ulusoy / Journal of Organometallic Chemistry 713 (2012) 104e111

Acknowledgements _ This work was supported by the TÜBITAK (The Scientific and Technological Research Council of Turkey-110T655) and ÖYP (State Plannig Organisation-2005DPT003/7). We also thank to Dr. Zeynep Özkan Aracı at Arizona University for the proof reading. References

[11]

[1] A.-A.G. Shaikh, S. Sivaram, Chem. Rev. 153 (1996) 155e174. [2] a) R.L. Addock, S.T. Nguyen, J. Am. Chem. Soc. 123 (2001) 11498e11499; b) M. Ulusoy, E. Cetinkaya, B. Cetinkaya, Appl. Organomet. Chem. 23 (2009) 68e74; c) A. Kilic, M. Ulusoy, M. Durgun, Z. Tasci, I. Yilmaz, B. Cetinkaya, Appl. Organomet. Chem. 24 (2010) 446e453. [3] J.J. Peng, Y. Deng, New J. Chem. 25 (2001) 639e641. [4] H. Kawanami, A. Sasaki, K. Matsui, Y. Ikushima, Chem. Commun. (2003) 896e897. [5] H. Kawanami, Y. Ikushima, Chem. Commun. (2000) 2089e2090. [6] K. Yamaguchi, K. Ebitani, T. Yoshida, H. Yoshida, K. Kaneda, J. Am. Chem. Soc. 121 (1999) 4526e4527. [7] Y. Zhou, S. Hu, X. Ma, S. Liang, T. Jiang, B. Han, J. Mol. Catal. A 284 (2008) 52e57. [8] a) C. Baleizao, H. Garcia, Chem. Rev. 106 (2006) 3987e4043; b) N.E. Leadbeater, M. Marco, Chem. Rev. 102 (2002) 3217e3274; c) E.M. McGarrigle, D.G. Gilheany, Chem. Rev. 105 (2005) 1563e1602. [9] a) M.H. Chisholm, Z.-P. Zhou, J. Am. Chem. Soc. 126 (2004) 11030e11039; b) G.W. Coates, D.R. Moore, Angew. Chem. 116 (2004) 6784e6806; Angew. Chem. Int. Ed. 43 (2004) 6618e6639; c) D.J. Darensbourg, R.M. Mackiewiwicz, J. Am. Chem. Soc. 127 (2005) 14026e14038; d) K. Nakano, K. Nozaki, T. Hiyama, J. Am. Chem. Soc. 125 (2003) 5501e5510; e) E.K. Noh, S.J. Na, S. S, S.-W. Kim, B.Y. Lee, J. Am. Chem. Soc. 129 (2007) 8082e8083. [10] a) A. Berkessel, M. Brandenburg, Org. Lett. 8 (2006) 4401e4404; b) Y.-M. Shen, W.-L. Duan, M. Shi, J. Org. Chem. 68 (2003) 1559e1562; c) R.L. Paddock, Y. Hiyama, J.M. Mekay, S.T. Nguyen, Tetrahedron Lett. 45 (2004) 2023e2026;

[12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

[22] [23] [24] [25] [26] [27]

111

d) C.-X. Miao, J.-Q. Wang, Y. Wu, Y. Du, L.N. He, ChemSusChem 1 (2008) 236e241; e) F. Jutz, J.-D. Grunwaldt, A. Baiker, J. Mol. Catal. A 297 (2009) 63e72; f) M. Alvaro, C. Baleizao, D. Das, E. Carbonell, H. Garcia, J. Catal. 228 (2004) 254e258; g) A. Decortes, M.M. Belmonte, J.B. Buchholz, A.W. Kleij, Chem. Commun. 46 (2010) 4580e4582; h) A. Decortes, A.M. Castilla, A.W. Kleij, Angew. Chem. Int. Ed. 49 (2010) 9822e9837; i) A. Decortes, A.W. Kleij, ChemCatChem (2011) 831e834. K. Mori, Y. Mitani, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, Chem. Commun. (2005) 3331e3333. Y. Xie, Z. Zhang, T. Jiang, J. He, B. Han, T. Wu, K. Ding, Angew. Chem. Int. Ed. 46 (2007) 7255e7258. J.-Q. Wang, X.-D. Yue, F. Cai, L.-N. He, Catal. Commun. 8 (2007) 167e172. N. Malumbazo, S.F. Mapolie, J. Mol. Catal. A 312 (2009) 70e77. J.F. Larrow, E.N. Jacobsen, Y. Gao, Y. Hong, X. Nie, C.M. Zepp, J. Org. Chem. 59 (1994) 1939. doi:10.1021/jo00086a062. J. Horniakova, T. Raja, Y. Kubota, Y. Sugi, J. Mol. Catal. A 217 (2004) 73e80. R. Kılınçarslan, H. Karabıyık, M. Ulusoy, M. Aygün, B. Çetinkaya, O. Büyükgüngör, J. Coord. Chem. 59 (2006) 1649. A. Kilic, E. Tas, B. Gumgum, I. Yilmaz, Heteroatom Chem. 18 (2007) 657. Y. Li, Z.Y. Yang, Inorg. Chim. Acta 13 (2009) 4823. _ Yılmaz, Transit. Met. Chem. 29 (2004) 180e184. E. Tas¸, M. Ulusoy, M. Güler, I. a) S. Bunce, R.J. Cross, L.J. Farrugia, S. Kunchandy, L.L. Meason, K.W. Muir, M. O’Donnell, R.D. .Peacock, D. Stirling, J.S. Teat, Polyhedron 17 (23e24) (1998) 4179e4187; b) F. Thomas, O. Jarjayes, C. Duboc, C. Philouze, E. Saint-Aman, J.L. Pierre, Dalton Trans. 17 (2004) 2662e2669; c) V.T. Kasumov, F. Koksal, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 61A (1e2) (2004) 225e231. K.M.K. Yu, I. Curcic, J. Gabriel, H. Morganstewart, S.C. Tsang, J. Phys. Chem. A 114 (2010) 3863e3872. S. Zhang, Y. Chen, F. Li, X. Lu, W. Dai, R. Mori, Catal. Today 115 (2006) 61e69. J. Sun, L. Wang, S. Zhang, Z. Li, X. Zhang, W. Dai, R. Mori, J. Mol. Catal. A 256 (2006) 295e300. T. Sakai, Y. Tsutsumi, T. Ema, Green Chem. 10 (2008) 337e341. F. Li, C. Xia, L. Xu, W. Sun, G. Chen, Chem. Commun. 16 (2003) 2042e2043. J. Sun, S. Zhang, W. Cheng, J. Ren, Tetrahedron Lett. 49 (2008) 3588e3591.