ethylene oxide-mediated reverse water-gas shift reaction catalyzed by ruthenium complexes

Pergamon PII: S0360-5442(96)00098-9

Energy Vol, 22, No. 2•3, pp. 169-176, 1997 Copyright © 1996 Elsevier Science Ltd

Printed in Great Britain. All fights reserved 0360-5442/97 $17.00 + 0.00

ETHYLENE OXIDE-MEDIATED REVERSE WATER-GAS SHIFT REACTION CATALYZED BY RUTHENIUM COMPLEXES

KEN-ICHI TOMINAGA,* YOSHIYUKI SASAKI,** TAIKI WATANABE ~ and MASAHIRO SAITO*

*National Institute for Resources and Environment (NIRE) 16-30nogawa, Tsukuba, Ibaraki 305, Japan and ~Research Institute of Innovative Technology for the Earth (RITE) 9-2 Kizugawadai, Kizu-cho, Soraku-gun,

Kyoto 619-02, Japan

(Received 30 September 1995)

Abstract--The very efficient hydrogenation of CO2 in the presence of ethylene oxide has been catalyzed by soluble ruthenium complexes to obtain CO and ethylene glycol in good yields; over 70% of the CO2 was converted into these products at 140-180°C, indicating that the reaction does not suffer from thermodynamic or kinetic restrictions at these reaction temperatures. The reaction mechanism may include CO 2 insertion into ethylene oxide to form ethylene carbonate in the first step, followed by its hydrogenation. Therefore, ethylene carbonate can be regarded as an activated CO2 with respect to its hydrogenation. Copyright © 1996 Elsevier Science Ltd.

INTRODUCTION

Among the various catalytic reactions of CO2, its hydrogenation is considered of great importance because of the potential availability of a large amount of hydrogen through the photo-assisted decompo- sition of water in the future) The initial stage of CO2 hydrogenation is the reverse water-gas shift reaction (RWGSR), which will be a key step for CO2 utilisation because CO can be used as an important synthetic carbon source in industry. Hydrogenation of CO2 proceeds according to the reaction

H 2 + C O 2 --} C O + H 2 0 , A H ° = + 4 1 k J / m o l . (1)

During the course of our study on CO2 hydrogenation using transition metal complexes, 2~ we have reported that ruthenium carbonylate cluster anions catalyze the above reaction in the presence of halide salts. 4.5 However, this reaction is not favored at lower temperatures because of its endothermicity. According to the thermodynamic equilibrium curve for this reaction [Fig. 1, curve (a)], only a few percent of CO are expected to be formed below 200°C. In order to avoid this restriction, we have recently proposed 6 a novel technique for CO2 hydrogenation which is carded out in the presence of ethylene oxide as follows:

H 2 + CO 2 + ~ ' 7 O

. C O + HO OH , AH ° =-172kJ /mol . (2)

Since this reaction is exothermic, a high yield of CO can be expected even at low temperatures as is shown by curve (b) in Fig. 1. In fact, this reaction has been found to be efficiently catalyzed by ruthenium complexes in the presence of tertiary amines. We report here the effects of transition metal complexes on this reaction, the temperature dependence of the reaction with a ruthenium complex, and the direct hydrogenation of ethylene carbonate to CO and ethylene glycol.

EXPERIMENTAL STUDIES

All chemicals were of reagent grade. RuC12(PPh3) 3, RhCl(PPh3)3, PdCl2(PPh3)2, and NiCI2(PPh3) 2 were purchased from Kanto Chemicals and used as received. CoCI(PPh3) 3 was prepared according to

*Author for correspondence; Fax: (81) 298 58 8158; e-mail: [email protected] 169

170 K.-I. Tominaga et al

80"

0.

0-

20- (a)

(b)

0 1 ~ I I I I

0 1 ~ 2 ~ 3 ~ 400 5 ~ 600

Temperature, "C

Fig. 1. Thermodynamic equilibrium curves for (a) Hs + COs - . CO + H20 and (b) H2 + COs + ethylene oxide --. CO + ethylene glycol.

the procedure in the literature. 7 N-methyl-2-pyrrolidone (NMP), N-methyl-pyrrolidine (Tokyo Kasei), N-methyl-piperidine, triethylenediamine (Nacalai Tesque), and triethylamine (Wako Chemicals) were dried over MS-5A and used without further purification. Ethylene oxide (Kanto Chemicals) and ethylene carbonate (Tokyo Kasei) were used as received. Authentic samples of ethylene glycol were purchased from Wako Chemicals, and ethylene glycol mono-formate was synthesized according to the procedure in the literature and isolated using column chromatography. 8

The catalytic reactions were carded out in a 50 ml stainless steel autoclave (Nitto Koatsu). Typically, we introduced into a solution of RuCI2(PPh3)3 (0.1 mmol), N-methyl-pyrrolidine (1.0 mmol) and NMP (8.0 ml), ethylene oxide (30 mmol) the CO2 (30 mmol) at -30°C. After being pressurized with 80 atm of H2 at room temperature, the autoclave was heated at a nominal reaction temperature for a designated period. After the reaction was finished, the autoclave was cooled down to room temperature.

Gas products were recovered and introduced into a sampling bag through an integrating flowmeter and quantitatively analyzed using GC (Shimadzu GC-20B) with an MS-13X column for H2 and CO, and a Porapack Q column for CO2. Liquid products were also analyzed using GC (Shimadzu GC-8A) with a Porapack Q column.

RESULTS AND DISCUSSION

Most of the transition-metal hydride complexes are known to react with CO2 to produce formate complexes.~ Koinuma et al 9 applied this reaction to the catalytic synthesis of 1,2-propanediol formates from CO2, H2, and propylene oxide in the presence of a rhodium complex [see Eq. (3)], in which a 23% yield of the mono-formate esters was obtained together with 29% propylene carbonate. There is, however, no description of CO formation in their report. The catalytic synthesis proceeds as follows:

Ethylene oxide-mediated reverse water-gas shift reaction catalyzed by ruthenium complexes 171

O O

OH H2 CO2 ~ O RhCI2(PPh3)3 H ~ O ~ T / O H H A O ~ " / ~ "x/ + + • + + Oi i O

~, ~ O 36

23% 29%

(3)

The results of C O 2 hydrogenation in the presence of ethylene oxide are summarized in Table 1. In the absence of H2, the Ru complex selectively catalyzes a reaction of CO2 with ethylene oxide to give ethylene carbonate at a yield of 93% (entry 1). A small amount of acetaldehyde (4%) is also formed by isomerization of ethylene oxide. When H 2 is introduced (entry 2), CO and ethylene glycol are formed with yields of 23% and 29%, respectively, while a decrease occurs in ethylene carbonate to 56%. The addition of tertiary amines increases the yields of CO and ethylene glycol to about 70% and almost completely reduces the formation of ethylene carbonate, independently of the structure of amines

Table 1. Hydrogenation of CO2 in the presence of ethylene oxide/

Yield / %*

Enlry Complex Base OH OH [.-O~ c o

OH OCHO EtOH

1 | RuC12(PPh3)3 ncae 0 3 0 93 4 0

2 RuCI2(PPh3)3 none 23 29 2 56 7 2

3 RuCI2(PPh3)3 O N - CH 3 71 75 5 0 0 3

4 RuCi2(PPh3)3 0 - CH 3 69 76 7 0 1 2

5 RuCl2(PPh3)3 ~ l q 64 72 12 1 0 2

6 RuCI2(-PPh3)3 EtaN 70 56 5 0 0 7

7 RuCIy3H20 C N - CH 3 66 58 4 0 1 1

8 Ru3(CO)12 C N - CH 3 54 60 15 0 1 2

9 RhCI(PPha)3 C N - CH 3 9 12 0 24 8 2

10 PdCI2(PPh3)2 C N - CH 3 33 28 0 21 3 0

11 CoCI(PPh3)3 C N - CH 3 26 18 0 19 3 0

12 NiCI2(PPh3)2 C N . CH 3 29 33 4 10 1 0

tCcodiflom: ccmpk~ (0.1 retool), base (1.0 retool), NMP (8.0 nil), CO2 (30 retool), ethylene oxide (30 retool), H2 (80 atmat r.t.), 140"C, 15 h. SBued on ethylene oxide except for CO (baaed on CO2). tWithout H2.

100

80-

0 wp Wl~ I I • T ml~


40 60 80 100 120 140 160 180 200

Temperature, "C

Fig. 2. Effect of reaction temperature on the hydrogenation of CO2 in the presence of ethylene oxide. The reaction conditions were RuCi2(PPh3)3 (0.1 mmol), N-methyl-pyrrolidine (1.0 retool), NMP (8.0 ml), ethylene oxide (30 retool), CO2 (30 retool), H2 (80 atm at room temperature), 15 h, • =CO, • =ethylene glycol,

• = ethylene glycol mono-formate, • = ethylene carbonate.

(entries 3-6). The corresponding complexes of Ni, Pd, Co exhibit lower activity for the formation of CO and ethylene glycol under the same conditions (entries 9-11), and the Rh complex is much less active (entry 12). When either the tfinuclear cluster Ru3(CO)12 or the strong base triethylenediamine are used, the yields of ethylene glycol mono-formate become significant (entries 5, 8).

The actual temperature dependence of this catalysis is shown in Fig. 2. Although the reaction itself is favored thermodynamically (Fig. 1), the yield of CO decreases steeply at temperatures below 120°C, probably because the formation rate of ethylene carbonate becomes slower at lower temperatures. Instead, the formation of mono-formate becomes favorable around 70°C. The yields of CO and ethylene glycol increase significantly at the expense of mono-formate and carbonate over 100°C. A slight decrease in the yield of ethylene glycol is observed at higher temperatures, which may be caused by its condensation. Its maximum yield is thus reached around 140°C.

The products obtained at lower temperatures suggest two possible reaction pathways for the formation of CO; one is based on the decarbonylation of formate esters, and the other on the hydrogenolysis of ethylene carbonate [Eq. (4)]. The time course of this reaction (Fig. 3) shows that the contribution of the former path is rather small, because ethylene carbonate formed during the initial stage decreases with increases in CO and ethylene glycol, while the yield of the formate initially increases and remains constant after 3 h of reaction.

H2 + (]02 + /O - ~

O

" o

H 2 + O ~ O L__/

C"O + HO Oil (4)


t m R

20t.

0 2 4 6 8 10 12 14 16

Time, h Fig. 3. Time course of the hydrogenation of CO2 in the presence of ethylene oxide. The reaction conditions were RuCI2(PPh3)3 (0.1 mmol), N-methyl-pyrrolidine (1.0 mmol), NMP (8.0 ml), ethylene oxide (30 retool), CO2 (30 mmol), H2 (80 arm at room temperature), 140°C, • = CO, • = ethylene glycol, • = ethylene glycol

mono-formate, ~1, = ethylene carbonate.

The intermediate creation of ethylene carbonate indicated in this reaction led us to attempt its direct hydrogenation as an effective alternative for the low temperature RWGSR. This seems feasible because ethylene carbonate is one of the important products in large-scale industrial CO2 fixation processes/° Actually, it can be prepared under relatively mild conditions in good yields (see Table 1, entry 1 ), and is considered suitable as a CO2 carder because of its stability and low toxicity.

The results of the direct hydrogenation of ethylene carbonate are summarized in Table 2. As expected, ethylene carbonate is effectively hydrogenated with RuCI2(PPh3)3 and N-methyl-pyrrolidine; CO and ethylene glycol are formed with 82% and 79% yields, respectively (entry 2). Ethylene glycol mono- formate is also formed with a 7% yield which is almost the same yield obtained in the ethylene oxide- mediated RWGSR. In the absence of amines, however, ethylene carbonate is recovered mostly in unreacted form and only 1% CO is formed (entry 1 ). The other tertiary amines tested are also effective for this reaction except triethylamine which is slightly less effective (entries 3-5).

It is noteworthy that the rhodium complex shows over four times higher activity for the hydrogenation of ethylene carbonate than that observed for the ethylene oxide-mediated RWGSR, whereas the corresponding complexes of Pd, Ni, and Co are slightly less effective for this reaction (entries 7-10). The observed difference in the reactivity of the rhodium complex toward the hydrogenation of ethylene carbonate and ethylene oxide-mediated RWGSR may be related to the equilibria between the hydride complex and formate complex as is shown by:

HCOOM ~-- HM(CO2) ~ HM + CO2 • (5)

Under CO2 pressure, the equilibrium shifts to the left, and the resulting formate complex may be less active for the hydrogenation of ethylene carbonate. In fact, much more ethylene carbonate is recovered in unreacted form in the reaction using the rhodium complex in the presence of CO2 (entry 12). On the other hand, no ethylene carbonate is recovered when using the ruthenium complex even in the presence of excess CO2 (entry 11), probably because the equilibrium in Eq. (4) does not shift signifi-


Table 2. Hydrogenation of ethylene carbonate:

Entry Complex

Yield 1%1

Base OH OH CO ( ~ CH,CHO FAOH Carbonate/%

OH OEHO

1 RuCI2(PPh3)3

2 RuCl2(PPh3)3

3 RuCI2(PPh3)3

4 RuCI2(PPh3)3

5 RuCI2(PPh3)3

6 Ru3(CO)12

7 RhCl(PPh3)s

8 PdO2tPPh3h

9 CoCl(PPh3)3

I0 NiCl~?Phsh

11 i RuC12(pph3)3

12 i RhCi(PPh3)3

none 1 10 8 7 0 78

N- CH 3 82 79 7 0 0 0

N- CH 3 70 77 9 0 1 0

l q ~ 63 69 15 0 0 2 M.__/

Et3N 46 37 4 0 1 21

N- CH 3 54 50 20 0 0 0

C N - CH3 43 46 1 0 4 8

N- CH 3 24 43 1 0 4 22

N- CIt 3 1 3 0 0 0 39

N- CH 3 14 9 0 0 0 36

N- CH 3 31 53 24 0 0 0

CN - CH 3 14 20 0 0 2 53

tConditions: oomplex (0.1 retool), base ( 1.0 retool), NMP (8.0 ml), ethylene carbonate (30 mmol),H2 (80 aim at r.t.),

140"C, 15h. I B u e d o a e t h y l e n e ~ t e . iInthepreseneeofCO2(30mmoi).

cantly to the left in the case of the ruthenium complex. However, e x c e s s C O 2 leads to a significant formation of the mono-formate which may be caused by the reaction of the formate complex with ethylene glycol.

As shown in Fig. 4, the hydrogenation of ethylene carbonate proceeds above 60°C and is completed at 140°C with the formation of CO and ethylene glycol. The formation rates at these temperatures are slightly lower than those in the ethylene oxide-mediated RWGSR (Fig. 2). This result is based on the time courses of these reactions at 140°C (Figs. 3 and 5). The difference may be caused by the difficulty in activating ethylene carbonate; in the former reaction, ethylene carbonate formed from ethylene oxide and CO2 may be immediately hydrogenated without being liberated from the ruthenium complex, while in the latter reaction, oxidative a&fition of ethylene carbonate to the ruthenium complex must be activated.

Acknowledgements--The present work was partly supported by the New Energy and Industrial Technology Development Organi- zation (NEDO).


100

0'

0.

0 .

0'

0 40 200

• "If" 1-- ' I I • • " "IP '

60 80 100 120 140 160 180

.lff

.,o

Temperature, "C

Fig. 4. Effect of the reaction temperature on the hydrogenation of ethylene carbonate. The reaction conditions were RuCIz(PPh3)3 (0.1mmol), N-methyl-pyrrolidine ( l .0mmol) , NMP (8.0ml), ethylene carbonate (30 mmol), H2 (80 arm at room temperature), 15 h, • = CO, • = ethylene glycol, • = ethylene glycol mono-

formate, • = ethylene carbonate.

100

8O

20

0 | I I I I I I I

0 2 4 6 8 10 12 14 16

Time, h

Fig. 5. Time course of the hydrogenation of ethylene carbonate. The reaction conditions were RuCl2(PPh3) 3 (0.1 mmol), N-methyi-pyrrolidine (1.0 mmol), NMP (8.0 ml), ethylene carbonate (30 mmol), H2 (80atm at room temperature), 140°C, • = CO, • = ethylene glycol, • = ethylene glycol mono-formate, • = ethylene

carbonate.


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ethylene oxide-mediated reverse water-gas shift reaction catalyzed by ruthenium complexes

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