Polymers for Advanced Technologies Volume 7, pp . 701-703

Synthesis of Propylene Carbonate and Some Dialkyl Carbonates in the Presence of Bifunctional Catalyst Compositions Hong Zhu,’ Li-Ban Chen’* and Ying-Yan Jiang2 Guangzhou Institute of Chemistry, Academia Sinica, Guangzhou, 520650, China Institute of Chemistry, Academia Sinica, Beijing 200080, China

ABSTRACT

Poly(ethy1ene glycolJ-potassium iodide complexes were found to have high catalytic activities in the esterification of propylene oxide and carbon dioxide under smooth experimental conditions. Furthermore, the complexes were combined with sodium methoxide to form bifunctional catalyst compositions sequentially for the above esterifica- tion, as well as the transesterification of the propylene carbonate formed in situ with some aliphatic alcohols, such as methanol, ethanol, propanol and butanol. It was found that the compositions were very effective, giving corresponding dialkyl carbonate, propylene carbonate and propylene glycol. It was noticed that the resulting dialkyl carbonate from n-butanol was diisobutyl carbonate, instead of di-n-butyl carbonate.

KEYWORDS: bifunctional catalyst composition; propylene carbonate; dialkyl carbonate; carbon dioxide; poly(ethy1ene glycol); alcohols

INTRODUCTION The chemical fixation of carbon dioxide has received much attention from the viewpoint of preservation of the environment. The reaction of carbon dioxide with propylene oxide (PO) to produce propylene carbon- ate (PC) is an effective method of fixing and utilizing carbon dioxide (eq. (1)).

* To whom correspondence should be addressed.

The synthesis of dialkyl carbonates is very interesting because of their extensive applications 111. They can be produced through the transester- ification of PC and an alcohol (eq. (2)).

The above reactions have been investigated individually. In this paper, attempts have been made by using bifunctional catalyst compositions so as to produce dialkyl carbonate, PC and propylene glycol (PG) directly from carbon dioxide, aliphatic alcohol and PO (eq. (3)).

The complexes of alkali metal halides and crown ethers have been reported to be effective for the esterification [2, 31. It is expected that cheap and nontoxic poly(ethy1ene glycol) (PEG) can be used instead of crown ethers for the same purpose. On the other hand, various soluble bases such as alkali metal alkoxides, sodium and potassium hydroxide are useful catalysts for the transesterification 14,51. Herein sodium methoxide is combined with PEG- potassium iodide complexes to catalyze the two-step reactions sequentially.

EXPERIMENTAL

Materials Reagents and solvents were obtained commercially and, if necessary, dried with 3 %, molecular sieves.

CCC 1042-71 47/ 96 / 070701 -03 0 1996 by John Wiley & Sons, Ltd.

Received 2 November 1995 Revised 30 Janua y 1996

702 / Zhu et a/.

TABLE 1. of PEG, KI or KBr

Synthesis of PC from C02 and PO in the Presence

M,of Time Temp. No. Catalyst PEG [hr) ("C)

1 KBr 400 5 150 KI 400 2.5 150

3 KI - 5 150 2

4 KI 400 5 150 5 KI 400 5 100 6 KI 600 5 150 7 KI 1000 5 150

Experimental conditions: CO,/PO= 1.2, PO/PEG= KI=3 (molar ratio), 2 MPa of pressure.

Yield 1%) 13 91 8

100 75 93 86

00, PEG/

Instruments Gas liquid chromatography (GLC) analyses were performed on a SC-7 gas chromatography with SE-54 column (@ 0.25 mm x 30 m) within 80-220°C at a rate of 16"C/min. Mass spectra were recorded on a JEOL JMS-D300 spectrometer with SE-54 column (@ 0.25 mm x25 m) within 50-250°C at a rate of 5"C/min.

Procedures for the Esterification Into a 110ml autoclave, the potassium halide was dissolved in a small amount of distilled water. PEG (M,, = 400) and propylene oxide were added accord- ing to the required molar ratio. Carbon dioxide was then introduced from the containing cylinders and its amount was determined by weighing the auto- clave before and after the addition. The reaction mixture was stirred magnetically at desired tem- peratures for a certain period. The final liquid product was determined by GLC.

Sequential Esterification and Transesterification in One Pot Into a 110ml autoclave, potassium iodide and sodium methoxide were mixed with PEG (Mn=400) according to the required molar ratio. Then propyl- ene oxide and an alcohol were added, and certain amount of carbon dioxide was introduced from the containing cylinders. The reaction solution was stirred magnetically at the desired temperatures for a certain period. The resulting liquid mixture was submitted for GLC and mass spectrum analyses.

RESULTS AND DISCUSSION The complexes composed of PEG and KI or KBr were used to catalyze the reaction of carbon dioxide with PO (eq. (l)), and the results are shown in Table 1. It can be seen that the yields of PC exceeded 90% in the presence of PEG-KI complexes under the pressure of 2 MPa at 150°C in 2.5 hr and reached 100% in 5 hr. In contrast, the activity of either PEG-KBr complexes or KI in the absence of PEG was very low at the same conditions. The nuclephilicity of Br- anions was weak in PEG-KBr complexes, KI without PEG did not produce "naked I- anions", thus they possessed poor activities. On the other hand, the reaction temperatures greatly affected the reaction. The yields for 5 hr reaction were 75% at 100°C and 100% at 150°C respectively. Besides, the catalytic activities were also related to the average molecular weight of PEG. For the catalytic system, PEG (400) (Mn=400) was better than PEG (600) and PEG (1000).

The reaction of carbon dioxide, PO and some aliphatic alcohols was investigated in the presence of the catalysts composed of PEG, KI and NaOCH,, as shown in Table 2. Indeed, the desired products including dialkyl carbonate, PC and PG were pro-

TABLE 2. Co eneration of DAC, PG and PC from C02, PO and ROH in t/e Presence of PEG, KI and NaOCH,"

Conversion Product composition (Wt%) (%I

No. R P@ PO

1 Me 39.5 100 2 Me 55.7 100 3 Et 51.0 100 4 n-Pr 62.7 100 5 n-Bu 41.4 100 6 n-Bu 50.5 100

ROH DAC PG PC

30.5 14.7 15.2 36.6 59.3 12.1 10.5 16.3 58.5 10.4 9.3 20.1 47.1 19.8 8.9 13.2 41.7 21.9' 4.0 23.1 53.8 14.3' 12.4 15.2

Experimental conditions: ROH (ali hatic alcohol), PO (propylene oxide), PC (prop lene carLnate), PG (propy-

Feed compositions: (1 ) MeOH/PO= 1.8 (molar ratio, the same below), (2): MeOH/PO=5.2; (3) EtOH/PO=3.0, (4): PrOH/PO=3.0, (5): BuOH/PO=2.0, (6): BuOH/PO=3.0; COJPO= 1.2, PO/PEG= 100, PEG/KI=2.5, KI/

lene glycol), DAC (dialkyl car L onate).

NaOCH,= 1 .O.

the in situ formed PC.

At 150°C for 6 hr under the pressure of 2 MPa. Determined by the molar ratio of PC converted into DAC to

Diisobutyl carbonate.

Synthesis of Propylene Carbonate / 703

2

S I , I

50 100 MIE

FIGURE 1. Mass spectra of the products from C02, PO and methanol: 1 ) methanol; (2) dimethyl carbonate;

experimental conditions were the same as no. 1 in Table 2. (3) propylene g I ycol; (4) propylene carbonate. The

duced in the presence of the above catalysts. It was found that the conversions of PO reached loo%, while the conversions of PC derived from the esterification of PO and carbon dioxide were related to the sorts of alcohol and their molar ratio to PO in feed compositions under the experimental condi- tions.

l 4 50 100 150 MIE

FIGURE 2. Mass spectra of the products from C02, PO and n-butanol: ( I ) n-butanol; 2 ) prop lene glycol;

experimental conditions were the same as no. 5 of Table 2. (3) propylene carbonate; (4) 4.. risobuiy r carbonate. The

The product from MeOH/PO in the molar ratio of ca. 1.8 was observed to consist of dimethyl carbonate (DMC) (14.7%), PC (36.6%) and PG (15.2%); the conversion of PC was 39.5%. When the MeOH/PO ratio increased to ca. 5.2, the composi- tions changed to 12.1%, 16.3% and 10.5%, and the conversion increased to 55.7%. The mass spectra of the products from carbon dioxide, PO and methanol are shown in Fig. 1. It is seen by gas chromatography that few by-products appeared in addition to the desired products, indicating a high selectivity. When either ethanol or n-propanol was used, similar results were observed, and the expected products were obtained as diethyl carbonate or di-n-propyl carbon- ate, PC and PG.

Interestingly, when the alcohol applied was n- butanol, the observed mass spectra (Fig. 2) revealed that the resulting carbonate was diisobutyl carbonate instead of di-n-butyl carbonate. The phenomenon was likely to be related to the isomerization during the reaction. Table 2 indicated that 41.4% of PC was converted and 21.9% of diisobutyl carbonate in the product mixture was observed in a BuOH/PO feed molar ratio of ca. 2.0. As the ratio changed into ca. 3.0, the conversion of PC and the content of diisobutyl carbonate turned out to be 50.5% and 14.3%, respec- tively. When isobutanol was used, diisobutyl carbonate was obtained and no other isomerized products were found.

For the catalytic system, it was suggested that the K' or Na' cations were captured by complexation with the PEG chains. AT the same time, the I- and OCH; anions with enhanced nucleophilicity were released; this greatly accelerated the catalytic reac- tions.

ACKNOWLEDGMENT We are grateful to the Natural Science Foundation of China for supporting the work.

REFERENCES 1. U. Romano, D. Nicola and R. Franco, I.E.C. Prod. Res.

2. G. Rokicki and W. Kuran, Bull. Chem. SOC. Jpn., 57, 1662

3. G. Rokicki and W. Kuran, Monatshe. Chem., 115, 205

4. H. J. Buysch, H. Krimm and H. Rudolph, US Patent

5. D. J. Brunelle and W. E. Smith, US Patent 4,349,486

Dev., 19(3), 393 (1980).

(1984).

(1984).

4,181,676 (1980).

(1982).