Zn-Ni double metal cyanide complex: A novel effective catalyst for copolymerization of propylene oxide and carbon dioxide

CHEN Shang(陈  上)¹, MA Ming-you(麻明友)¹, XIAO Zhuo-bing(肖卓炳)¹,

LIU Jian-ben(刘建本)¹, ZHANG Xing-hong(张兴宏)², QI Guo-rong (戚国荣)²

1. College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China;

2. Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

Received 20 April 2006; accepted 30 June 2006

Abstract: A novel double metal cyanide complex (DMC) based on Zn[Ni(CN)₄] was prepared using K₂[Ni(CN)₄] and ZnCl₂, and employed as catalyst for the copolymerization of carbon dioxide and propylene oxide (PO). The resulting copolymers were characterized by IR, ¹HNMR. The results show that this catalyst exhibits catalytic efficiency at approximate 500 g copolymer per gram of Zn[Ni(CN)₄] under proper conditions. The mole fraction of CO₂ for copolymer can reach about 0.3. Propylene carbonate is also produced as by-product, but its content is as low as 4%-6% of overall products under suitable conditions.

Key words: carbon dioxide; propylene oxide; catalyst; nickel

1 Introduction

Carbon dioxide, a kind of inexhaustive carbon resource, is suitable to be chosen as potential feed in chemical industry due to low cost and high security. Using it as chemical feed is also beneficial to the balance of carbon resource and environmental protection. Since INOUE et al [1] discovered the production of aliphatic polycarbonate via the reaction of carbon dioxide and epoxide (the copolymerization reaction of PO with CO₂ as shown in reaction (1)), the copolymerization attracted great interest. In recent years a great deal of catalytic systems were developed and remarkable progress has been made.

ZnEt₂-alcohol, zinc carboxylate as well as metal porphyrin have been employed as catalyst and fully studied in earlier years [2, 3]. However, these catalysts demonstrated relatively lower catalytic efficiency, which was in the range of 10-80 g copolymer per gram catalyst. In recent years, zinc bis(2,6-diphenylphenoxide) derivatives [4-7] and zinc β-diiminate[8-11] were developed as catalysts, and they exhibited improved catalytic efficiency. Except for zinc carboxylate, most catalysts mentioned above are sensitive to moisture and substances containing active group, which can make them lose partial or even entire catalytic activity, thus strict conditions should be ensured during preparation and use. Double metal cyanide complexes (DMC), a category of catalysts for ring-opening polymerization (ROP) of epoxide, have common formula as M^II_a[M(CN)_b]_c?xM^IIX_d ? yL? zH₂O (M_II is Zn²⁺ etc, M is transition metal, X is halogen, L is water-soluble organic solvent, namely complexing agent). They are easy to be prepared and not sensitive to moisture, among them DMC catalyst based on Zn₃[Fe(CN)₆]₂ (Zn-Fe DMC) has been used for copolymerization of carbon dioxide and propylene oxide and exhibited catalytic efficiency at approximately 50 g copolymer per gram catalyst [12]. We found another effective DMC catalyst by modification of central metal, in which nickel was used as center mental, and it exhibited balanced performance including much lower content of byproduct, cyclic carbonate as well as remarkably enhanced efficiency compared with other DMC complex. Herein, we report its preparation and features towards copolymerization.

2 Experimental

K₂[Ni(CN)₄] was synthesized by KCN and NiSO₄, and the crude product was recrystallized twice before use. ZnCl₂ and tert-butyl alcohol (TBA) were analytical grade and used without further purification. PO was distilled over calcium hydride. CO₂ (＞99.9%) was used as received.

The composition of catalyst was analyzed by Hitachi 180-50 atomic absorption spectrophotometer and Flash EA1112 elemental analyzer. FT-IR spectra were recorded on Brucker Vector 22, ¹HNMR spectra were recorded on AVANCE DMX 500 superconducting high-resolution spectrometer (500 MHz) with CDCl₃ as solvent, the molar fraction of CO₂ was calculated by integrating area of peaks (A). The relative molecular mass and distribution of copolymers were determined by gel permeation chromatography (GPC) using 2×PL gel columns at 35 ℃, running in THF at 0.8 mL/min, 6 monodisperse polystyrene standards were used to generate a calibration curve (580-3×10⁴ u).

Under vigorous stirring, 10 mL of potassium tetracyanonickelate solution (0.2 mol/L) was added dropwise into ZnCl₂ solution (8 g ZnCl₂ in mixture of 40 mL water and 20 mL TBA) at 35°C, the resulting white suspension was filtered to isolate precipitate of DMC catalyst, and resuspended in a solution composed of TBA and water with volume ratio of 1∶1 under vigorous stirring, then precipitate was filtered again. The precipitate was washed three times with gradually increasing portion of TBA against water. Finally the solid was resuspended in TBA to exclude water, filtered and dried at 50 ℃ over 8 h, then was pulverized for use.

2.3 Copolymerization of CO₂ and PO

Copolymerization of PO and CO₂ was performed in a 50 mL autoclave equipped with magnetic stirring and pressure indicator. Prior to the experiment, the autoclave was heated to 80 ℃ for 1 h, desired amount of catalyst and 20 mL PO were added, then autoclave was heated to reaction temperature rapidly and charged with CO₂ to proper pressure. After desired time, it was cooled down and pressure was slowly released. Catalyst precipitate should be removed by centrifugation. In some cases, extra PO could be added before centrifugation to facilitate separation by reducing viscosity of mixture. Finally the excess PO was evacuated by heating product at 80 ℃, thus product free of catalyst was obtained.

3 Results and discussion

In general, DMC complex refers to complex composed of zinc cyanometalate and zinc chloride,  as well as complexing agent in most case. Although other metals could be used to substitute for zinc, they behave obviously inferior to zinc analog for copolymerization. It was developed for the production of high relative molecular mass and low unsaturation polyether by ring opening polymerization of PO since 1960’s. KRUPER et al[12] as well as KUYPER et al[13] employed DMC complex based on Zn₃[Fe(CN)₆]₂  as catalyst for copolymerization of PO and CO₂. However, this kind of catalyst received less attention in previous decades because it demonstrated relatively low catalytic activity. Recently, we investigated the DMC catalyst based on Zn₃[Co(CN)₆]₂ (Zn-Co DMC), which showed obvious increase on catalytic efficiency [14, 15]. It prompted us to take extensive exploration in DMC with various center metals for copolymerization. Although accurate stru- ctures of these complexes have not been well defined, the typical structures of their main composition, Zn_u[M(CN)_n]_v, has been well characterized in Ref.[16]. They construct three-dimensional network though bridged bond as Zn—N ≡≡ C—M. It is generally acknowledged that zinc is coordination site in catalytic reaction and involves coordination with epoxide in the ROP of epoxide. In copolymerization, it supposedly includes coordination of CO₂ as well. Thus coordinative status of zinc will play significant impact on catalytic behavior and resulting composition of product. Since zinc and central metal (Mⁿ⁺) are bridged by cyanide ion, the influence of central metal on the coordination status of zinc can be imposed by changing electron density of nitrogen atom. Our previous works have confirmed that the efficiency of DMC catalyst is sensitive to central metal of coordination compound. Based on above facts, a series of transition metals were tested as central metal. It was found that Ni²⁺ was a good candidate besides Fe³⁺ and Co³⁺，and showed difference on catalytic chara- cteristics for copolymerization of PO and CO₂.

Like other DMC catalyst, the DMC catalyst based on Zn[Ni(CN)₄] was prepared by precipitating  reaction of K₂Ni[CN]₄ and ZnCl₂ solution. Herein, the excess amount of ZnCl₂ relative to K₂Ni[CN]₄ was used, which could ensure better catalytic efficiency of final catalyst as most literatures disclosed. In addition, TBA, a most common used complexing agent was adopted in preparation. The lone pair electrons of its oxygen atom provided the capacity to coordinate with ZnCl₂ in solution, thus influenced precipitating reaction to achieve different morphologies of precipitates and higher catalytic efficiency. Perhaps, TBA may not be the most suitable complexing agent for promotion of catalytic efficiency of Zn-Ni DMC complex, thus it is necessary to examine other complexing agents extensively in further study.

The resulting Zn-Ni DMC catalyst was washed several times to exclude potassium ion, which was detrimental to the catalytic efficiency, then dried for further use. It is noted that this DMC catalyst was a complex substance of Zn[Ni(CN)₄], ZnCl₂, TBA and H₂O, and the contents of the later three compositions were variable under different preparative conditions, thereby the proper evaluation of catalytic efficiency of DMC catalyst should be based on the amount of included Zn[Ni(CN)₄] in DMC complex. It was also ensured that the comparisons of catalytic efficiency among different kinds of DMC complex were reasonable. The elemental analysis indicated that the empirical formula of catalyst we used in following experiments was Zn[Ni(CN)₄]? 0.7ZnCl₂?1.3TBA?2.0H₂O, and the mass of Zn[Ni(CN)₄] occupied 48.1% of overall mass of DMC catalyst.

Fig.1 shows a typical FT-IR spectrum of product. It is seen that the peaks at 1 745 cm^-1 and 1 264 cm^-1 exist in product, which are ascribed to strength vibration of    C == O and C—O in oxycarbonyl group, respectively. It indicates the success of incorporation of CO₂ into polymer chain. It is also seen that there is a peak at 1 799 cm^-1, which is ascribed to the strength vibration of C == O in propylene carbonate (PC). The intensities of the peaks vary among different products, which indicates that coupling reaction of PO and CO₂ is presented more or less in catalytic system, as most investigated catalytic systems do. ¹HNMR spectrum also confirms the existence of both polycarbonate and PC, as shown in Fig.2. The signals at 5.0 and 4.0-4.3 are ascribed to hydrogen of CH and CH₂ group in carbonate segments of copolymer, respectively. The signals at 3.2-3.8 are ascribed to hydrogen of both CH and CH₂ group in ether segments, and the signals indicate that copolymer is not alternative copolymer. At the same time, there are small peaks at 4.89, 4.58, 4.04, 1.50. The positions of these peaks fully accord with standard spectrum of PC, which also confirms the formation of cyclic carbonate. According to integrating area of these signals, the accurate mole fraction of CO₂ in copolymer (x(CO₂)) and mass fraction of PC (w(PC)) can be calculated.

Table 1 lists catalytic efficiency and composition of catalytic product at different temperatures. It is seen that the catalytic efficiencies are at the range of 18-582 g product per gram of Zn[Ni(CN)₄] when temperature varies from 80 ℃ to 150 ℃, and the catalytic efficiency sharply ascends from 119.3 to 482.6 g product per gram Zn[Ni(CN)₄] when temperature increases from

Fig.1 FT-IR spectrum of catalytic product

Fig.2 ¹HNMR spectrum of catalytic product

120 ℃ to 130 ℃. It reveals that this DMC catalyst can only be activated effectively under such high temperature as 130 ℃, which is apparently higher than that of Zn-Fe DMC or Zn-Co DMC (about 50 ℃ and 90 ℃ respectively). With respect to catalytic efficiency for copolymer, it could reach 438.7 g copolymer per gram Zn[Ni(CN)₄] at 130 ℃, approximately ten times against Zn-Fe DMC (40-50 g polymer per gram of catalyst) and six times against zinc glutarate (60-70 g polymer per gram of the catalyst) [17], but lower than that of Zn-Co DMC. The x(CO₂) of copolymer can reach as high as 0.35 at 80 ℃, but corresponding catalytic efficiency is only 17.5 g copolymer per gram Zn[Ni(CN)₄]. On the contrary, the higher temperature above 130 ℃ could result in abrupt decrease of x(CO₂). Thus the suitable reaction temperature should be 110-130 ℃, under which can achieve moderate catalytic efficiency along with moderate x(CO₂) ( 0.24-0.30). However, the x(CO₂) of copolymer is slightly lower than that of copolymer derived from Zn-Fe DMC (approximately 0.36). With respect to content of PC, it slightly increases with temperature ascending, and the content of PC is merely between 6.6% and 9.1% when temperature is at the range

Table 1 Effect of temperature on catalytic efficiency of catalyst and compositions of products

of 110-130 ℃. It apparently contrasts with performance of Zn-Fe and Zn-Co DMC, by which the content of PC is between 20% and 28% (mass fraction) under the suitable conditions, e.g. for Zn-Co DMC, the content of PC is 28.1% under 110 ℃ and 5.5 MPa [14]. Herein in order to denote the relative tendency of CO₂ transformation into copolymer against product of coupling reaction, propylene carbonate, R value is used, which is define as the ratio of the mole of CO₂ incorporated into copolymer to the mole of CO₂ incorporated into PC. From Table 1, R values for Zn-Ni DMC are at the range of 4.3-8.0 at 110-130 ℃, generally larger than that for Zn-Co DMC, whose R values are 1.2-1.5 under optimum temperature (110 ℃). It means that for Zn-Ni DMC over 85% of CO₂ was transformed into copolymer instead of cyclic carbonate under optimum conditions, and these values presented a contrast with that for Zn-Co DMC (approximately 58%). From view point of R, it demonstrates an advantage over other kinds of DMC catalysts.

The number average relative molecular masses of copolymers are below 10 000 and steadily increase with increasing temperature. It should attribute to increase of percent conversion with increasing temperature. From this point Zn-Ni DMC has the same feature as Zn-Co DMC. Because the resulting copolymers of entry 1 and 2 have so low relative molecular masses that it is beyond lower limit of GPC column, their relative molecular masses are not shown in Table 1.

Table 2 lists the influence of pressure on results of copolymerization. Unexpectedly, unlike other kinds of DMC catalysts(Zn-Co DMC or Zn-Fe DMC), the content of PC decreases with increasing CO₂ pressure at temperature of 110 ℃ or 130 ℃ catalyzed by Zn-Ni DMC. For instance, the content of PC decreases from 12.7% to 6.7% when CO₂ pressure increases from 3.0 MPa to 6.5 MPa at 130 ℃. At the same time, it is accompanied with increased catalytic efficiency (479.8 g/g for copolymer) and higher x(CO₂) (x(CO₂)=0.29). It means that higher concentration of CO₂ in reaction system is more favorable for copolymerization than coupling reaction, thus it is possible that the content of PC could be suppressed to lower level and x(CO₂) could be higher under even higher CO₂ pressure above 6.5 MPa. With regard to the reason for difference between them, it relates to mechanism of catalysis, which is still obscure until now due to difficulty in direct characterization of active site.

It is known that DMC catalyst can exhibit different catalytic efficiencies towards ring-opening polymeri- zation of PO when subtle change of preparative conditions occurs. The most important factor is kind of complexing agent employed, which influences catalytic characteristics of DMC catalyst via change of condensed state stemming from variation of precipitating surrounding. Except for TBA, several other water-soluble organic solvents were also employed in preparation of Zn-Ni DMC (Table 3), and both catalytic efficiency and composition of copolymer were investigated. It is seen that catalysts with glycol ether as complexing agent exhibited nearly the same or slightly higher catalytic efficiency, and the catalyst without any complexing agent (catalyst 6) exhibits only one third of the highest catalyst efficiency. Except for catalyst 6, x(CO₂) of copolymer and the content of PC change in

Table 2 Effect of pressure on catalytic efficiency of catalyst and compositions of products

Table 3 Effects of complexing agent on results of copolymerizations

very narrow range, indicating that the different complexing agents bear no influence on composition of copolymer. For reasons of its advantages, it is worthy to investigate more kinds of complexing agent in further research, then seek more effective complexing agent.

4 Conclusions

DMC complex based on Zn[Ni(CN)₄] was prepared by reaction of  K₂[Ni(CN)₄] and ZnCl₂ in the presence of complexing agent, TBA and its highest catalytic efficiency towards copolymerization could reach approximately 500 g copolymer per gram Zn[Ni(CN)₄], which is several fold over that of Zn-Fe DMC and zinc carboxylate and so forth, but lower than that of Zn-Co DMC. Moreover, an advantage of this novel DMC catalyst is that the mass of PC only occupies about 4%-6% in overall mass of product under optimum conditions. It is in contrast with other DMC complexes such as Zn-Co DMC etc, by which over 20% mass of final product is PC. The mole fraction of CO₂ in copolymer can reach approximately 0.3, close to that derived from other DMC catalysts. Considering the features of this novel catalyst for copolymerization of PO and CO₂, it could be regarded as an effective catalyst.

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(Edited by YANG You-ping)

Foundation item: Project(50273031) supported by the National Natural Science Foundation of China

Corresponding author: CHEN Shang; Tel: +86-743-8563911; E-mail: shangchen@jsu.edu.cn