Catal Lett (2014) 144:1894–1899 DOI 10.1007/s10562-014-1360-z

Cycloaddition Reaction of Propylene Oxide and Carbon Dioxide Over NaX Zeolite Supported Metalloporphyrin Catalysts Fengyong Zhang • Yujia Xie • Pingle Liu Fang Hao • Zhengjie Yao • He’an Luo

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Received: 20 June 2014 / Accepted: 3 September 2014 / Published online: 14 September 2014 Ó Springer Science+Business Media New York 2014

Abstract Metalloporphyrin catalysts has attracted extensive attention as the result of their outstanding biomimetic catalytic properties. In this paper, different kinds of unsupported (Co(TPP), MnIII(TPP)Cl and CoIII(TPP)Cl) and NaX zeolite supported metalloporphyrin catalysts (Co(TPP)/NaX, MnIII(TPP)Cl/NaX, CoIII(TPP)Cl/NaX) were prepared and characterized. It has been found that these metalloporphyrin catalysts show high catalytic activity in cycloaddition reaction of propylene oxide and carbon dioxide under mild reaction conditions. The NaX supported metalloporphyrin catalysts are more stable and present better catalytic performance than the unsupported metalloporphyrins. CoIII(TPP)Cl/NaX was better than MnIII(TPP)Cl/NaX and CoTPP/NaX, the propylene oxide conversion is 94.5 % and the selectivity to propylene carbonate is 95.6 %. Keywords Supported-metalloporphyrin
Cycloaddition reaction
Carbon dioxide
Propylene carbonate

1 Introduction Carbon dioxide generated from the burning of fossil fuels is recognized as one of the greenhouse gases and major contributor to the global warming problem [1]. Therefore, carbon dioxide capture and utilization has attracted more and more researchers’ attentions [2, 3]. One of the most promising branches in this field is the synthesis of cyclic carbonates [4] (Scheme 1). Propylene carbonate can be used as electrolyte, apolar aprotic solvent [5, 6] and useful polymer materials

for the motor, electronic and communication industry because of their stable physico-chemical properties [7–9]. In the past decades, many different catalytic systems such as alkali metal salts, ionic liquids, quaternary ammonium salts and Schiff bases have been developed for the coupling reaction [10–19]. But most of them suffer from harsh reaction conditions and catalyst recycle [20–22]. As one of the electron-deficient compounds, metalloporphyrin have been used as mild Lewis acids catalysts [23–26]. It has been reported by many scholars that metalloporphyrins in conjunction with an organic Lewis base such as 4-dimethylaminopyridine (DMAP) or phenyltrimethylammonium tribromide (PTAT) can reach a high yield of carbonate under mild reaction condition [27–31]. However, the catalyst is difficult to be recycled. Bai et al. choose magnetic ferriferrous oxide as support to load metalloporphyrins and the yield of propylene carbonate only decreased by 3 % after several times recycle [32]. However, it is quite complex to prepare the catalyst. In this paper, the catalyst was prepared by using NaX zeolite as support to load metalloporphyrin [33–35], and it was applied in the production of propylene carbonate by the cycloaddition reaction of carbon dioxide and propylene oxide. The supported metalloporphyrin catalyst not only shows better catalytic performance but also is easier to be recycled.

2 Experimental 2.1 Catalytic Preparation 2.1.1 Materials

F. Zhang
Y. Xie
P. Liu (&)
F. Hao
Z. Yao
H. Luo College of Chemical Engineering, Xiangtan University, Xiangtan 411105, China e-mail: [email protected]

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Propylene oxide was distilled from CaH2. Propylene epoxide and PTAT and DMAP were purchased from Aldrich and used to reaction without further purification.

Cycloaddition Reaction of Propylene Oxide and Carbon Dioxide O O

Cat +

CO 2

O

O

Scheme 1 Cycloaddition reaction of propylene oxide and carbon dioxide

2.1.2 5,10,15,20-meso-Tetrametalloporphyrin Complexes(M(X)TPP) H2TPP was prepared according to the following procedures [36, 37]. Co(TPP), MnIII(TPP)Cl and CoIII(TPP)Cl were synthesized and purified by referring to the following procedures [36]. H2TPP was dissolved in DMF and the solution was heated to reflux with stirring. Then, cobalt acetate or manganous acetate was added in three portions within 30 min. When the thin-layer chromatography (silica) indicated no free base porphyrin, the solution was cooled to 70 °C, and then 40 mL of 6 M HCl was added and stirred for 4 h. The solution was cooled and the appeared solid was filtrated and washed with 3 M HCl until the filtrate no longer appeared red. The obtained solid was vacuum-dried, and it gave 85–98 % yield of M(Cl)TPP (Co(Cl)TPP, Mn(Cl)TPP).

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The amount of M(Cl)PP/NaX (M = Co and Mn) per gram of the support was measured by Inductively Coupled Plasma Atomic Emission Spectometer (IRIS Intrepid II XSP ICP-AES) (Thermo Electron Co.) under a microwave pressure digestion (MDS 200; CEM) with hydrofluoric and aquaregia. 2.3 Procedure for the Catalytic test Cycloaddition reaction of propylene epoxide and carbon dioxide was carried out in a 50 ml stainless-steel autoclave reactor with a Teflon liner and a magnetic stirrer. Typically, the catalyst (0.11 mmol), epoxide (100 mmol), and co-catalyst (0.22 mmol) were added into the autoclave reactor. And the reactor was then pressurized with the appropriate amount of CO2 and heated to the setting temperature. After the reaction, it was cooled in an ice bath. The reaction mixture was separated by refrigerated centrifugation. The reaction product was taken out and analyzed with gas chromatograph. The catalysts at the bottom of centrifuge tube were washed with ethanol and dried for 12 h for recycle in the next reaction.

2.1.3 Preparation of M(Cl)TPP/NaX

3 Results and Discussions

The M(Cl)TPP/NaX was prepared by referring to the following procedures. A certain amount of M(Cl)TPP was dissolved in toluene. The NaX zeolite was activated at 280 °C and added into the solvent. The mixture was heated to reflux with stirring for 10 h. After evaporating a large amount of the solvent and the mixture was vacuum dried for 12 h. The unsupported metalloporphyrin was washed away by toluene. The loading amount of metalloporphyrin was determined by ICP element analysis.

3.1 Characterization of Catalysts UV–Vis was used to confirm the formation of cobalt and manganese porphyrin. The results of UV–Vis absorption of M(Cl)TPP and MTPP were shown in Fig. 1. There is an obvious peak at the wavelength around 400 nm and more than one peaks at the wavelength exceed 500 nm, which is in accordance with the theory that there is one Soret band and more than one Q band in the ultraviolet visible region

2.2 Catalysts Characterization UV–Vis spectra was obtained by UV-2550 spectrophotometry with a scan range of 300–800 nm for metalloporphyrins chloride complexes using 1 cm quartz cuvette. M(Cl)PP/NaX was characterized by using barium sulphate as reference, a small amount of BaSO4 was pressed into a thin pellets, and some samples were put on the pellets and pressed again, then it was used to assay the spectra. TG-DTG curves were carried out on a TGA Q50 using air as purge gas (80 mL min-1). The temperature is between 30 and 900°Cwith a heating rate of 10 °C/min. X-ray diffraction (XRD) patterns were collected on a Japan Rigaku D/Max 2550 VB ? 18 kW X-ray diffractometer under the conditions of 40 kV, 30 mA, Cu Ka radiation. It was performed in the range of 2h = 5–35° at a scanning rate of 2°/min.

Fig. 1 UV–Vis spectra of different kinds of metalloporphyrin

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Fig. 4 TG-DTG for Co(Cl)TPP/NaX powder Fig. 2 UV–Vis spectra of NaX and NaX supported metalloporphyrin

Fig. 5 TG-DTG for Mn(Cl)TPP/NaX powder

Fig. 3 XRD patterns of NaX and NaX supported metalloporphyrin

of porphyrin. With the addition of metal, the symmetry of the porphyrin molecular structure will increase. As a result, the Q band will reduce to one or two. UV–Vis spectra of the metalloporphyrins supported on NaX zeolite are shown in Fig. 2. All the supported metalloporphyrins have the Soret peak between 400 nm and 500 nm. Mn(Cl)TPP/NaX presents adsorption peak at 570 and 720 nm. As to NaX support, there are no obvious adsorption peaks between 400 and 600 nm. The UV–Vis spectra indicates that the metalloporphyrins have been successfully supported on NaX zeolite. The XRD patterns of NaX, Mn(Cl)TPP/NaX, Co(Cl)TPP/ NaX, CoTPP/NaX are shown in Fig. 3. All of them exhibit characteristic broad peaks of the NaX molecular zeolite, and it indicates that metalloporphyrins particles are well dispersed on NaX zeolite.

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Thermal analysis was conducted to confirm the stability of the supported catalysts during the reaction. The results has been shown in Figs. 4, 5 and 6. The metalloporphyrins have a rapid weight loss between 400 and 600 °C and the structure is destroyed. A rapid weight loss between 60 and 200 °C in the supported metalloporphyrins is related to the adsorbed water and some impurities, such as free ligand, pyrrol etc. [38]. The loading amount of metalloporphyrins on the support was determined by Inductively Coupled Plasma Atomic Emission Spectometer (IRIS Intrepid II XSP ICP-AES). The results show that the amount of Co(Cl)TPP, CoTPP and Mn(Cl)TPP was 80, 75 and 200 mg per gram of NaX zeolite respectively. 3.2 Catalytic Performance The results of catalytic performance of different kinds of unsupported metalloporphyrins are shown in Table 1.

Cycloaddition Reaction of Propylene Oxide and Carbon Dioxide

1897 Table 2 Results of cycloaddition of CO2 and PO catalyzed by supported metalloporphyrins Entry

Catalysts

Conversion

Selectivity

Yield

1

Co(Cl)TPP/NaX

94.5

95.6

90.4

2

Mn(Cl)TPP/NaX

87.3

96.8

84.6

3

CoTPP/NaX

89.1

89.3

79.7

Reaction conditions: propylene oxide (100 mmol), catalysts (1.0 g, active component 0.11 mmol), co-catalyst (0.22 mmol), 120 °C for 5 h, carbon dioxide pressure (3 MPa), pumped once

3.3 Possible Reaction Mechanism Fig. 6 TG-DTG for CoTPP/NaX powder

Table 1 Results of cycloaddition of CO2 and PO catalyzed by metalloporphyrin Entry

Catalysts

Conversion

Selectivity

Yield

1

Co(Cl)TPP

88.9

97.1

86.3

2

Mn(Cl)TPP

74.1

90.0

66.7

3

CoTPP

77.3

90.3

69.8

Reaction conditions: propylene oxide (100 mmol), catalysts (0.11 mmol), co-catalyst (0.22 mmol), 30 °C for 3 h, carbon dioxide pressure (3 MPa), pumped once

Compared with the Mn(Cl)TPP and CoTPP, the Co(Cl)TPP exhibits the best catalytic performance under the same reaction conditions. The conversion of PO is 88.9 % and the selectivity to PC is 97.1 %. Compared with the Co(II)porphyin and Mn(III)porphyin, the Co(III)porphyin is a more electrophilic complex [27, 39] and thus has the trendency to form electrom-donor center which will accelerate the nucleophilic ring open of propylene oxide [38]. As a co-catalyst, on one hand, PTAT is necessary for the formation of a more electron-rich Co(III) center which can help to activate the carbon dioxide in the Co(III)porphyin-PTAT system [38]. On the other hand, the easier leaving group of Br3 of PTAT will result in the separation of propylene carbonate from the catalyst which contributes to a better catalytic performance [29]. We can see from Table 2 that the catalytic performance of supported metalloporphyrins was better than the unsupported catalysts. The conversion of PO is 94.5 % and the selectivity to PC is 95.6 %. We attribute this phenomenon to the site isolation of immobilized complexes which was caused by the unsupported metalloporphyrins’ partly accumulation during the reaction [40]. Furthermore, the special structure of NaX and better adsorption capacity of carbon dioxide on NaX zeolite may contribute to the delightful result.

It has been proved by Lu et al. [7, 41], Paddock et al. [27, 42] and Jing et al. [10] that both Lewis acid center and Lewis base are necessary in the cycloaddition of propylene oxide and carbon dioxide. In this paper, a possible mechanism for the reaction was proposed as follow (Scheme 2). After loaded on the surface of NaX zeolite, the axial ligand of metalloporphyrin will be removed from previous location in order to provide a Lewis acid center (b and c) for the incoming NaX zeolite and propylene epoxide. At the same time, the hydroxy of NaX zeolite will help to activate the oxygen ion of propylene epoxide. The activated propylene epoxide will be transferred to metalloporphyrin. With the addition of PTAT, Br3 will attack propylene epoxide and form an intermediate product d. The insertion of carbon dioxide occurs after the ring opening of d and produces e. After that, a stable five-membered ring product propylene carbonate f has been obtained. The leaving of Br3 indicates the end of this circulation and the beginning of the next reaction. 3.4 The Optimization of Reaction Conditions The influence of temperature and reaction time were studied and the results are shown in Fig. 7 and Fig. 8. The yield of PC grows sharply from 11.2 to 90.4 % with the increasing of temperature. But this phenomenon was terminated when the temperature went up to 140 °C. As to the supported catalysts, metalloporphyrins are dispersed on the support uniformly, however it is more difficult to activate propylene epoxide and carbon dioxide. Thus it is necessary to raise the reaction temperature so as to provide enough energy to activate the substrate and ensure the smoothly running of the reaction. Since the cycloaddition reaction of PO and carbon dioxide is an exothermic process, extra heat will hinder the proceed of the reaction. It has been tested that the best reaction temperature is 120 °C in this Co(Cl)TPP/NaX catalyst system. As to the unsupported metalloporphyrins, the yield of PC reaches the maximum value when the reaction time is

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Scheme 2 Monometallic mechanism for the reaction of propylene oxide and carbon dioxide

Fig. 7 The influence of temperature on the reaction results

Fig. 8 The influence of reaction time on the results

about 3 h. Because the number of activated molecular is abundant, Co(Cl)TPP/NaX system costs 5 h to obtain the maximum yield 90.4 % of PC, but the performance decreased when we extend the reaction time to 6 h.

When we conducted our experiment without metalloporphyrins, that is the cycloaddition reaction of PO and carbon dioxide was catalyzed by NaX/PTAT, the yield of

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Cycloaddition Reaction of Propylene Oxide and Carbon Dioxide Table 3 Recycle of Co(Cl)TPP/NaX catalyst Catalysts

Entry

Conversion

Selectivity

Yield

1

94.5

95.6

90.4

Co(Cl)TPP/NaX

2

93.3

96.2

89.8

3

88.8

89.8

79.8

Average

92.2

93.9

86.7

Reaction conditions: propylene oxide (100 mmol), catalysts (1.0 g, active component 0.11 mmol), co-catalyst (0.22 mmol), 120 °C for 5 h,carbon dioxide pressure (3 MPa), pumped once

PC is 73.4 %. To some extent, it proves the importance of co-catalyst PTAT. 3.5 Recycling Test Finally, recycling tests about Co(Cl)TPP/NaX catalyst were carried out under the same reaction conditions. The catalysts were washed with ethanol, dried and used for the next reaction. The results are shown in Table 3, the average conversion of PO and selectivity to PC are 92.2 and 93.9 % after three consecutive reactions. While unsupported metalloporphyrin catalysts are difficult to recover and reused.

4 Conclusions Different kinds of unsupported and supported metalloporphyrins were prepared and characterized. NaX zeolite supported metalloporphyrin catalysts show better catalytic performance than unsupported catalysts. The yield of PC is improved more than 4 %. Co(Cl)TPP/NaX presents the best results, it gives the selectivity to PC of 95.6 % at the PO conversion of 94.5 %. This complex may be an active and reusable catalyst for industrial cycloaddition process of propylene oxide and carbon dioxide. Acknowledgments This work was supported by the NSFC(21276218), SRFDP(20124301110007), Scientific Research Fund of Hunan provincial Education Department (13K043, CX2012B271) and the Project of Hunan provincial Science & Technology Department (2012FJ1001).

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