Preparation of Sodium Cobalt Tetracarbonyl and Optimization of Process Conditions for Hydroesterification of Ethylene Oxide

Wen Liyuan; Zhang Zhanjun; Chen Xiaoping;Wang Shiqin; Yu Wenli

(1. Institute of Industrial Catalysis, Guangdong University of Petrochemical Technology, Maoming 525000;2. National Quality Supervision and Inspection Center of Dangerous Chemicals (Guangdong), Maoming 525000)

Abstract: In this paper, sodium cobalt tetracarbonyl (NaCo(CO)4) was synthesized by using sodium dithionite and zinc powder as the reduction system and cobalt hexahydrate acetate as the precursor in the presence of methanol solvent. Methyl 3-hydroxypropionate was synthesized via hydroesterification of ethylene oxide (EO) catalyzed by NaCo(CO)4. The influencing factors on the reaction results were discussed, including the different ligands, the molar ratio of solvent and ethylene oxide, the reaction temperature, the reaction time, and the reaction pressure. An optimal catalytic system was obtained by using 3-hydroxypyridine as the ligand under reaction conditions covering a reaction temperature 65 °C, a reaction time of 7 h, a reaction pressure of 6 MPa, and a methanol/EO molar ratio of 3:2. Under the optimal conditions, the conversion of ethylene oxide was equal to 97.86%, while the selectivity and yield of methyl 3-hydroxypropionate reached 88.19% and 86.30%, respectively. Finally, the reaction mechanism of hydroesterification of ethylene oxide catalyzed by NaCo(CO)4 was proposed.

Key words: sodium cobalt tetracarbonyl; optimization; hydroesterification; reaction mechanism

1 Introduction

At present, the situation of environmental protection has been becoming more and more serious. “Atomic Economic Response” has drawn much attention thanks to its very few by-products and its extremely high efficiency[1-2]. The carbonylation reaction over the transition metal-catalysts is an atomic economy reaction,and an important carbonyl functional group is directly introduced in the compound. The carbonylation of methanol to acetic acid, and hydroformylation of propene to butyraldehyde are typical representatives of industrial applications of carbonylation reaction[3].

Hydroesterification reaction is an important class of carbonylation, and cobalt carbonyl catalyst and rhodium catalyst are commonly used in carbonylation synthesis reaction[4-6]. Although the latter’s activity was better, it is more expensive and requires higher purity of syngas,which has limited its application. Cobalt carbonyl is a very important homogeneous catalyst and its synthesis and application has received widespread attention. Although many methods have been reported for the preparation of cobalt carbonyl and its salts, they are not ideal. The

preparation method carried out at high temperature and high pressure should require the high-quality equipment coupled with large investment cost. Li Guangxing[6-7]has for the first time reported the potassium cyanide conversion method. Toxic raw materials were used in the reaction such as potassium cyanide and sodium cyanide. The operation of iron-manganese alloy reduction method was complex and the product after the reaction was difficult to be separated[7]. The synthesis of cobalt tetracarbonyl was reported by Edgell, et al[8]. In recent years, Satyanarayana[9]has reported that NaCo(CO)4was prepared by the reduction of cobalt chloride with sodium borohydride, and mentioned that sodium borohydride could be used as a reductant to prepare various types of metal complex catalyst.

1,3-Propanediol (1,3-PDO) is an important organic chemical raw material, which is usually synthesized through methyl 3-hydroxypropionate (MHP) or 3-hydroxypropionaldehyde (3-HPA) serving as the intermediate in the chemical synthesis[10-11]. Since the intermediate 3-HPA is unstable and flammable substances are produced during the reaction, the hydrogenation of MHP for preparing 1,3-propanediol is favored by scholars[12-13]. Therefore, the research on preparation of the intermediate MHP also has been attracting the attention of the chemical community.

In this paper, cobalt sodium tetracarbonyl (NaCo(CO)4)was synthesized by using sodium dithionite and zinc powder as the reduction system. MHP was synthesized through hydroesterification of ethylene oxide catalyzed by NaCo(CO)4in the original reactor, in order to find a new way for preparing MHP.

2 Experimental

2.1 Materials

The raw materials included: EO (CP, produced by the Guoyao Group Chemical Reagent Co. Ltd.); methanol(AR, produced by the Tianjin Guangfu Science and Technology Development Co., Ltd.); Zinc powder(AR, produced by the Guangzhou Chemical Reagent Factory); sodium dithionite (AR), tetrabutylammonium bromide (AR, produced by the Tianjin Yongda Chemical Reagent Co., Ltd.); cobalt acetate, 3-hydroxypyridine,triphenylphosphine oxide and imidazole (which all were of the analytically pure grade and produced by the Tianjin Guangfu Fine Chemical Research Institute).

2.2 Experimental methods

Specified amounts of methanol, cobalt acetate, sodium dithionite, and zinc powder were weighed prior to being fed into a stainless steel reactor with a PTFE inner liner.The airtightness of the reactor was checked with CO,which was used to purge the reactor to displace the air for 3 times, and then the reactor was filled with CO to a desired pressure. The reactants were heated to the desired temperature for carrying out the synthesis reaction. After a few hours, the reaction was terminated and was cooled down rapidly. A certain amount of methanol solution with accelerator and ethylene oxide was added into the reactor,which was quickly closed with the temperature and pressure adjusted to the desired values for implementing the catalytic reaction. After several hours, the reactor was cooled with an ice-water bath, depressurized, and then was purged with nitrogen stream for several times.Finally, sampling and analytical operations were carried out as required.

2.3 Analytical methods

The structure of NaCo(CO)4was characterized by the IR spectrometry (the liquid membrane method). MHP was analyzed by GC and GC-MS for qualitative and quantitative analyses. Qualitative analysis was performed on a Shimadzu QP-2100Plus gas chromatograph-mass spectrometer, equipped with a HP-5 capillary column(φ0.5 μm×30 m) with a gasification temperature of 200 °C. Quantitative analysis was performed on an Agilent 7890B gas chromatograph, equipped with a hydrogen flame ionization detector, and an OV-17 capillary column(φ0.5 μm×30 m) using nitrogen as the carrier gas, with the gasification temperature, the oven temperature, and the detector temperature equating to 180 °C, 170 °C,and 200 °C, respectively. The products were determined according to the area normalization method.

3 Results and Discussion

3.1 Preparation and characterization of NaCo(CO)4

The optimal conditions for the synthesis of sodium cobalt tetracarbonyl was obtained as follows: Co(Ac)2·6H2O and CO used as raw materials, zinc powder used as the reductant, sodium dithionite used as the adjuvant, and methanol used as the solvent took part in the reaction at a temperature of 80 °C, a CO pressure of 3 MPa, and a reaction time of 5 hours. When preparation of the catalyst was completed, the reaction solution was taken out and characterized by infrared spectroscopy. The IR spectra of NaCo(CO)4are shown in Figure 1.

Figure 1 IR spectra of NaCo(CO)4

It can be seen from Figure 1 that the curve ranging from 2 800 cm-1to 3 550 cm-1was the absorption band formed from strong association between the carbonyl group and the hydroxyl group[7]. The absorption peaks at 2 045 cm-1and 2 525 cm-1were probably assigned to the adsorption of CO on Zn and Co. The peaks at 1 020—1030 cm-1and 1 400—1 500 cm-1were caused by C-O stretching vibration and C-H deformation vibration in CH3OH solution, respectively. The absorption peak at 665 cm-1might be the bending of Co-C-O vibration peak.The characteristic absorption peak of NaCo(CO)4was identified at 1905 cm-1[14]. Edgell[15]studied the effect of tetrahydrofuran on three different enclosures of Na+and[Co(CO)4]-, indicating that absorption peaks might be found at 1 887 cm-1, 1 899 cm-1and 1 906 cm-1. Generally,the free CO has a ν (CO) peak at 2 155 cm-1, while most carbonyl groups move toward the lower frequency direction due to the σ-π complexation between the CO molecule and the metal atom[16-17]. Miura, et al.[18]reported that the IR spectrum of NaCo(CO)4compound had a strong absorption peak at 1 913 cm-1with Na2S serving as the reducing agent and isopropanol-water serving as the solvent. The difference was caused by the solvent effect.Li Guangxing[5-6]showed that the strong absorption peak of NaCo(CO)4was observed at 1 903 cm-1which was synthesized with iron powder acting as the reductant and methanol serving as the solvent. It was very similar to our report.

3.2 Optimization of process conditions

3.2.1 Selection of ligand

Phosphine ligands and N-ligands have received widespread attention in ligand selection for hydroformylation and hydro-methyl esterification of epoxide derivatives conducted over the carbonyl cobalt catalysts[19-20]. Under the reaction conditions covering a temperature of 65 °C, a time of 7 h, a pressure of 6 MPa, and a methanol/EO molar ratio of 3:2, the effects of ligand-free condition, the phosphine ligand(triphenylphosphine oxide), the quaternary ammonium salt ligand (tetrabutylammonium bromide), and the N-containing ligand (imidazole, 3-hydroxypyridine) on hydroesterification of ethylene oxide were studied, with the results shown in Table 1.

Table 1 Effect of ligands on hydroesterification reaction

It can be seen from Table 1 that the ligand involved in the reaction was obviously superior to the ligandfree reaction. When the phosphine ligand was selected,the conversion of ethylene oxide and selectivity of MHP were increased. The possible reason is that the phosphine ligand itself is a good electron sigma donor,which can make the electrons of the complex catalyst metal center increase the density, and help to improve the stability of the catalyst. Upon using the quaternary ammonium salt ligands, the conversion of ethylene oxide was significantly increased to 89.83% probably because of the alkaline nature of tetrabutylammonium bromide, which was in good agreement with that reported by Yu[21], et al., who had confirmed that the hydroesterification reaction could be promoted by a system having an appropriate alkalinity. The selectivity of MHP was decreased, because the formation of the by-products methyl acrylate and 2-methoxyethanol was facilitated under alkaline conditions. The selectivity and yield of MHP were increased after using the azaheterocyclic ligands. Especially when the 3-hydroxypyridine ligand was used, the conversion of ethylene oxide reached 97.86%. This may be ascribed to the fact that the azacyclic ligand has a gentle alkaline nature and its stability is stronger than the phosphine ligands and the quaternary ammonium salt ligands.Experimental results have showed that 3-hydroxypyridine is a better ligand for hydroesterification of ethylene oxide.

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3.2.2 Selection of molar ratio of solvent to ethylene oxide

The molar ratio of solvent to ethylene oxide reflects the concentration of ethylene oxide in the reactants.Because ethylene oxide has a very low boiling point and is volatile at room temperature, it is important to control the concentration of ethylene oxide in the reaction.Upon using 3-hydroxypyridine as the ligand, the effects of methanol solvent and the molar ratio of methanol to ethylene oxide were investigated at 65 °C for 7 h with the results shown in Table 2.

Table 2 Effect of ethylene oxide concentration on hydroesterification

It can be seen from Table 2 that when the ratio of solvent to EO was increased, the concentration of ethylene oxide was decreased, and the conversion of ethylene oxide at first increased and then decreased, while the selectivity of MHP did not change significantly, because increasing the amount of ethylene oxide was favorable to the reaction.When the amount of ethylene oxide was increased to a certain extent, the reaction was fully carried out to reach an equilibrium state, and then an continued increase of ethylene oxide was unfavorable to the reaction. When the molar ratio of methanol to ethylene oxide was 1.5, the yield of MHP reached 86.31%.

3.2.3 Selection of reaction temperature

The effect of reaction temperature on hydroesterification of EO is shown in Figure 2. The reaction conditions covered a reaction time of 7 h, a reaction pressure of 6 MPa, and a methanol/ethylene oxide molar ratio of 3:2,with 3-hydroxypyridine serving as the ligand.

Figure 2 Effect of reaction temperature on hydroesterification

It can be seen from Figure 2 that the reaction temperature is an important factor that can affect the synthesis reaction.With the increase of temperature, the selectivity of MHP showed a declining trend. This might be ascribed to the fact that MHP possessed more active hydroxyl groups in the β position, and when the temperature was higher, the dehydration reaction could more readily happen, and the selectivity was decreased. The conversion of ethylene oxide increased with an increasing temperature. This might occur because the lower the temperature was, the slower the movement between molecules would be, and the activation energy required for the reaction could not be met, and a higher temperature was conducive to the ring-opening of ethylene oxide, which could lead to the formation of MHP. Upon taking into account the yield of MHP, the optimum hydrogenation reaction temperature was determined to be 65 °C.

3.2.4 Selection of reaction time

The effect of reaction time on hydroesterification of EO is shown in Figure 3. The reaction conditions covered a reaction temperature of 65 °C, a reaction pressure of 6 MPa, and a methanol/ethylene oxide molar ratio of 3:2 using 3-hydroxypyridine as the ligand.

Figure 3 Effect of time on hydroesterification reaction

It can be seen from Figure 3 that the conversion of ethylene oxide showed an increasing trend when the reaction time was between 5 h and 7 h, but the conversion rate remained basically unchanged when the reaction time covered between 8 h and 9 h. However, the selectivity of MHP first increased and then decreased, because the occurrence of side reactions was increased with the extension of reaction time. When the reaction time was equal to 7 h, the selectivity reached a maximum value of 88.19%, and the highest yield was 86.31%.

3.2.5 Selection of reaction pressure

The effect of reaction pressure on hydroesterification of EO is shown in Figure 4. Reaction conditions covered a reaction temperature of 65 °C, a reaction time of 7 h,and a methanol/ethylene oxide molar ratio of 3:2, using 3-hydroxypyridine as the ligand.

■—EO conversion; ●—MHP selectivity; ▲—MHP yield

The reaction was a gas-liquid two-phase reaction.Increase of the pressure can promote the dissolution of CO molecules and speed up the reaction. It can be seen from Figure 4 that the selectivity and yield of MHP increased with an increasing pressure which should be less than 5 MPa, and then decreased slowly with a further increase in pressure. At a pressure ranging between 3―4 MPa, the conversion of ethylene oxide basically remained at close to 74%, and then the conversion rate was significantly increased when the pressure increased to 5 MPa, reaching a maximum value of 97.86%. When the pressure continued to increase, the EO conversion rate was reduced. Because the reaction was a gas reduction reaction, carbon monoxide pressure was too low and the reaction was relatively slow.Increasing the pressure was conducive to promoting the insertion of carbon monoxide into the Co-alkyl bond,and the catalyst NaCo(CO)4could play its catalytic role more effectively[22], leading to the increase of the MHP yield. However, if the pressure was too high, it could easily promote the side reaction and the yield of MHP was reduced. So the optimal reaction pressure for synthesizing MHP was 5 MPa.

4 Discussion on Reaction Mechanism

Many studies on the catalytic mechanism of co-catalyzed hydroformylation of olefins were reported, but few studies on the Co-catalyzed hydroesterification reaction were reported[23-24].

The coordination mechanism of olefin in cobalt carbonyl hydride and the dissociation mechanism were two kinds of theory, but the association mechanism was recognized by most researchers[25-27]. Based on the above reaction mechanism, the possible cycle mechanism on hydroesterification of EO was discussed, as shown in Figure 5.

Firstly, NaCo(CO)4was converted to its active catalytic form HCo(CO)4(Ⅰ). Secondly, the carbon atom of EO was attacked by the nucleophile anionic [Co(CO)4]-groups to form alkyl cobalt carbonyls (Ⅱ). Ethylene oxide was activated to open the ring and the hydrogen proton was transferred to the oxygen atom of ethylene oxide. Thirdly, the CO molecules activated by the Co center were inserted into C-Co bond to form cobalt carbonyl (Ⅲ). Finally the desired product was obtained via the alcoholysis reaction of Ⅲ. The catalyst then was turned to HCo(CO)4and the catalytic cycle was completed.

Figure 5 Proposed catalytic mechanism of hydroesterification reaction

5 Conclusions

NaCo(CO)4was synthesized by using sodium dithionite and zinc powder as the reduction system and cobalt hexahydrate acetate as the precursor in the presence of methanol solvent. The process featured a short reaction time and a convenient preparation method. The obtained solution was observed at wavenumber of 1905 cm-1by infrared spectroscopy. The strong absorption peak was the characteristic absorption peak of [Co(CO)4]-.

Upon using NaCo(CO)4as catalyst, the influencing factors on the reaction results were discussed, including the different ligands, the molar ratio of solvent and ethylene oxide, the reaction temperature, the reaction time, and the reaction pressure. The optimal catalytic system was obtained by using 3-hydroxypyridine as ligand, with the reaction conditions covering a reaction temperature of 65 °C, a reaction time of 7 h, a reaction pressure of 6 MPa, and a methanol/EO molar ratio of 3:2. Under the optimal conditions, the conversion of ethylene oxide was equal to 97.86% and the selectivity of MHP reached 88.19%. Based on the literature descriptions, the catalytic mechanism of hydroesterification with NaCo(CO)4was proposed. Upon considering that the reaction was based on the association mechanism, it was suggested that the reaction had undergone four stages, viz.: the HCo(CO)4complexation, the CO insertion, the CO association, and the intermediate reaction.

Acknowledgements:The project is financially supported by the Guangdong Province Natural Science Foundation(No.10152500002000019) and the Maoming City Science and Technology Planning Project (No.2014069).