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The Role of Amide Solvents in the Formation of Ethylene Glycol from Synthesis Gas
Journal of Molecular Catalysis, 40 (1987)
223 - 228
223
Letter
The Role of Amide Synthesis Gas YUJI OHGOMORI, WATANABE
Solvents
SHOICHIRO
Central Research Laboratory, Zbaraki 300-03 (Japan) (Received
July 14,1986;
in the Formation
MORI,
SHIN-ICHI
Mitsubishi Petrochemical
accepted
November
of Ethylene
Glycol
YOSHIDA
and
Co., Ltd.,
8-3 Chu-ou,
from
YOSHIHISA
Ami,
12, 1986)
The active species in the formation of ethylene glycol from syn gas using rhodium catalysts has been claimed to be the polynuclear clusters of rhodium carbonyl hydrides [ 11. The reaction is effected by a combination of a solvent having a high dipole moment (e.g. polyethers, sulfolane) and promoters such as amines or alkali metal salts [2]. On the other hand, dimethylimidazolidone (DMI) [ 31 and N-methylpyrrolidone (NMP) [ 41 solvents are known to yield ethylene glycol in the absence of promoters. These basic solvents are thought to act like amine promoters [5] in general. But the differences among the individual solvents have not hitherto been discussed. We have conducted the hydrogenation of carbon monoxide with Rh4(C0)i2 in DMI, NMP and tetramethylurea (TMU) under conditions of 400 bar, 200 “C and 0.04 M Rh. DMI and TMU clearly yield higher selectivities to ethylene glycol than NMP (Table 1). Decomposition of TMU has been observed at reaction temperatures higher than 220 “C, while DMI is stable up to 250 “C. Thus investigation of the DMI solvent system is an interesting problem for practical application. JR studies of the reaction solutions have shown that rhodium mainly exists as a cluster anion Rh,(CO)1s2- (2040, 1985, 1960 cm-’ [6]) with minor quantities of mononuclear Rh(CO), anion (1900 cm-‘). This suggests that DMI stabilizes an unstable H2Rh6(C0)1s species as an ion pair, such as [DMI-H] 2[ Rh,( CO) r5]. The conductance of a reaction solution was 6.6 S cm2 eq-’ (lop3 M Rh, 25 “C), while PPNCl at the same concentration gave a value of 20.4. The molar conductivities of PPNCl and [DMI-H],[Rh,(CO),,] are not yet known. But assuming similar conductivities for the two compounds, the apparent nuclearity/charge ratio of the rhodium species is estimated to be more than unity. This also supports the presence of cluster compounds. On the other hand, the rhodium species was exclusively Rh(CO), in the NMP solvent system, which yielded a lower selectivity to ethylene glycol. 0304-5102/87/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
1
TMU DMI DMId DMId*e NMP
1 2 3 4 5
2.14 1.31 2.25 1.53 0.28
0.66 1.41 4.46 5.66 2.54
1.41 1.85 10.02 8.64 0.71
0.009 0.020 0.091 0.132 0.035
EG/MeOH
MF
MeOH
EG
ratio
Product
Rates (h-‘P
73.3 70.6 49.0 42.9 72.6
MeOH/MF
“[ Rh) = 0.04 M (Rhe(CO)lz was used), solvent 20 ml, CO/H2(1/1) 403 - 395 bar, 200 ‘C, 4 h. bTurnover number; EG = ethylene glycol, MF = methyl formate. ci (2040 cm-‘), ii (1985 cm-‘), iii (1960 cm-‘), iv (1900 cm-‘). d[Rh] = 0.013 M, solvent 7.5 ml, CO/H2(1/1) 550 - 540 bar, 220 ‘C, 2 h. e [ CsO&Ph] = 6.7 x 10V3 M was employed.
Solvent
of syn gas by Rh in ureas or NMP solvent&
No.
Conversion
TABLE
0.1 0.1 0.1 0.1 -
i
ii
intensityC
1.0 1.0 1.0 1.0 -
IR relative
0.6 0.6 0.6 0.6 -
111 . ..
2.5 0.2 0.3 1.2 1.0
iv
225
The existence of Rh(CO), has been previously correlated with a high yield of methyl alcohol [7,8], which coincides with our observation, As is seen in Scheme 1, DMI yields mainly 2 while NMP reduces rhodium more intensively to give 4. Addition of cesium benzoate (a strong cation source) to the DMI system increases the Rh(C0)4-/Rh6(C0)152ratio with a decrease in the ethylene glycol/methyl alcohol ratio (Nos. 3 and 4 in Table 1). These observations imply that the active rhodium species for ethylene glycol in these solvents are probably polynuclear carbonyl hydrides.
Rh,(C0)12
CO, Hz, Solv. 400 bar, 200 “C
CO, Hz, Solv.
Solv. l HRh(C0)4
Solv.,*H2Rh6(CO)iS \
[Solv..Hl,[Rh,(CO)1~1
[ Solv. *HI [ Rh(CO),]
2 (observed)
4 (observed)
Scheme 1. Proposed equilibria of rhodium complexes in DMI or NMP.
The intermediates in the synthesis of ethylene glycol from syn gas have been assumed to be HCHO and HOCH,CHO, since trace quantities of 1,3dioxolane and 2-hydroxymethyl-1,3dioxolane have been detected as minor products [ 91. We have confirmed these dioxolanes, as well as minor products such as 1,4dioxane and 2-hydroxy-1,4-dioxane which are considered as the reduction products of glycol aldehyde (No. 2, Table 2). These byproducts were not detected in the reaction mixtures in NMP solvent under the same conditions (No. 6, Table 2). Addition of propylene glycol was then examined to investigate the reaction mechanism in detail. Dioxolanes that derive from propylene glycol are observed in high yields in DMI (No. 7, Table 2), but in quite low yields in NMP (No. 8, Table 2). The IR spectra of the reaction solutions did not change with addition of propylene glycol. This may suggest that the intermediates such as HCHO and HO~H~CHO exist in larger quantities in DMI than in NMP solvent. In the next step, the reaction of paraformaldehyde-i3C with syn gas was carried out in the presence of propylene glycol. The mass spectrum analysis is shown in Table 3. The reaction time was minimized to reduce the products from syn gas only (ca. 10 - 20%). It became clear that the obtained 2-hydroxymethyE4-methyl-l,3dioxolane-13C contains i3C completely on the hydroxymethyl fragment. This indicates that glycol
226 TABLE 2 Identified products in DMI or NMPa Productb
Yield (mmol)
No.
2
6e
7d
8C.d
ethylene glycol methyl alcohol 2-hydroxymethyl-l,3-dioxolane methyl formate ethyl alcohol 1,3-dioxolane 2-methyl-1,3-dioxolane 1,4-dioxane 2-hydroxy-1,4-dioxane
5.92 4.51 0.14 0.06 0.06 0.05 0.05 0.05 trace
2.30 8.16
1.38 1.34 1.37e
2.47 11.05 0.14e
3.4ge
trace
0.09 trace -
a[Rh] = 0.04 M, solution 20 ml, CO/H~(l/l) 400 bar, 200 OC,4 h. bStructures were confirmed by GC-MS spectroscopy, CSolvent was NMP instead of DMI. d50 mmol of 1,2-propylene glycol was employed. @q-Methyl substituted derivative. TABLE 3 Mass spectrum analysis of the products from (*CHzO), in DMIa No.
ProdUb
(M.W.)
M+l
m/e”
I
*CH30H
(33)
34(1.00)
33(0.15)
2
I$*
(89)
90(1.00)
89(0.11)
3
HO*CH2CH20H
(63)
6411.00)
63(0.19)
120(1.00)
119(0.10)
(119)
Key fragments
87(
c)+,
45c*~mi+)
aRh4(CO)rz 0.3 mg-atom, (*CHsO), (97.8 atom% r3C) 4.5 mmol (as CHzO), DMI 7.5 ml, CO/Hs( l/l) 400 bar, 200 ‘C, 30 min. bYield (mmol); 1(0.77), 2(1.96), 3(0.77), 4(0.77). CRelative intensities are shown in parentheses; M + 1 and key fragments were observed by CI and EI methods, respectively.
aldehyde is formed by hydroformylation of formaldehyde [ 10 - 121, not by dimerization of formaldehyde. Based on these results and related literature [ 5,131, the reaction pathways in amide solvents are summarized in Scheme 2. The ion-paired formyl species 5 has been isolated in iron complexes [14,15]. In the case of rhodium catalysts using amide solvents, formation of the formaldehyde intermediate would be accelerated through formation of 5 stabilized by the [Solv-H] cation. By adding HCHO to 1 or 3, 6 or 8 is formed. It is possible
227
0 lor3-
[H!
[Sol”. ‘H] [H-!-Rh=_-]
[H1 -HCHO+lor3
5
0 Solv:HOCH,-Rh=_
s
Solv:HOCH&Rh~
6
7
lor3+HCHO
,
-
HZ
HOCH,CHO H2
HOCH,CH,OH CH,OH 0 CH,OAH
Scheme 2. Proposed reaction pathways.
that methyl alcohol is obtained by reduction of 6. But the route via 8 is preferred since the ratio of methyl formate/methyl alcohol is nearly identical in both DMI and NMP (Table 1). As to the ratio of 6 and 8, the presence of basic solvent will favour 6, resulting in the improved selectivities to ethylene glycol in DMI solvent. On the other hand, NMP accelerates the reaction from 8 to methyl formate and methyl alcohol as shown in Table 2, which reduces the concentration of the formaldehyde intermediate, leading to preferential formation of 8 over 6. This may be the cause of lower selectivities to ethylene glycol in NMP solvent. Acknowledgement This work is a part of ‘C1 Chemistry Project’, a National Research and Development Program of the Agency of Industrial Science and Technology, Ministry of International Trade and Industry (M.I.T.I.), Japan. The authors are grateful to members of the Ethylene Glycol Research Group of the Project for valuable discussions. References 1 J. L. Vidal and W. E. Walker, Inorg. Chem., 19 (1980) 896. 2 U.S. Pat. 4 115428 (1978) to J. L. Vidal, Z. C. Mester and W. E. Walker (Union Carbide Corp.).
228 3 4 5 6 7 8 9 10 11 12 13 14 15
U.S. Pat. 4 302 547 (1981) to P. W. Hart (Union Carbide Corp.). W. Keim, M. Berger and J. Schlupp, J. Catal., 61 (1980) 359. B. D. Dombek,Adu. Catal., 32 (1983) 325. P. Chini, G. Longoni and V. G. Albano, Adu. Organometall. Chem., 14 (1976) 285. U.S. Pat. 3 948 965 (1976) to J. N. Cawse (Union Carbide Corp.). U.K. Pat. 1565 979 (1980) to L. Kaplan (Union Carbide Corp.). D. R. Fahey,J. Am. Chem. Sot., 103 (1981) 136. A. S. C. Chan, W. E. Carroll and D. E. Willis, J. Mol. Catol., 19 (1933) 377. A. Spencer, J. Organometall. Chem., 194 (1980) 113. T. Suzuki, K. Kudo and N. Sugita, Nippon Kagaku K&hi, (1982) 1357. L. C. Costa, Catal. Rev. Sci. Eng., 25 (1983) 325. J. P. Collmann and S. R. Winter, J. Am. Chem. Sot., 95 (1973) 4089. S. R. Winter, G. W. Cornett and E. A. Thompson, J. Organometall. Chem., 133 (1977) 339.
223 - 228
223
Letter
The Role of Amide Synthesis Gas YUJI OHGOMORI, WATANABE
Solvents
SHOICHIRO
Central Research Laboratory, Zbaraki 300-03 (Japan) (Received
July 14,1986;
in the Formation
MORI,
SHIN-ICHI
Mitsubishi Petrochemical
accepted
November
of Ethylene
Glycol
YOSHIDA
and
Co., Ltd.,
8-3 Chu-ou,
from
YOSHIHISA
Ami,
12, 1986)
The active species in the formation of ethylene glycol from syn gas using rhodium catalysts has been claimed to be the polynuclear clusters of rhodium carbonyl hydrides [ 11. The reaction is effected by a combination of a solvent having a high dipole moment (e.g. polyethers, sulfolane) and promoters such as amines or alkali metal salts [2]. On the other hand, dimethylimidazolidone (DMI) [ 31 and N-methylpyrrolidone (NMP) [ 41 solvents are known to yield ethylene glycol in the absence of promoters. These basic solvents are thought to act like amine promoters [5] in general. But the differences among the individual solvents have not hitherto been discussed. We have conducted the hydrogenation of carbon monoxide with Rh4(C0)i2 in DMI, NMP and tetramethylurea (TMU) under conditions of 400 bar, 200 “C and 0.04 M Rh. DMI and TMU clearly yield higher selectivities to ethylene glycol than NMP (Table 1). Decomposition of TMU has been observed at reaction temperatures higher than 220 “C, while DMI is stable up to 250 “C. Thus investigation of the DMI solvent system is an interesting problem for practical application. JR studies of the reaction solutions have shown that rhodium mainly exists as a cluster anion Rh,(CO)1s2- (2040, 1985, 1960 cm-’ [6]) with minor quantities of mononuclear Rh(CO), anion (1900 cm-‘). This suggests that DMI stabilizes an unstable H2Rh6(C0)1s species as an ion pair, such as [DMI-H] 2[ Rh,( CO) r5]. The conductance of a reaction solution was 6.6 S cm2 eq-’ (lop3 M Rh, 25 “C), while PPNCl at the same concentration gave a value of 20.4. The molar conductivities of PPNCl and [DMI-H],[Rh,(CO),,] are not yet known. But assuming similar conductivities for the two compounds, the apparent nuclearity/charge ratio of the rhodium species is estimated to be more than unity. This also supports the presence of cluster compounds. On the other hand, the rhodium species was exclusively Rh(CO), in the NMP solvent system, which yielded a lower selectivity to ethylene glycol. 0304-5102/87/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
1
TMU DMI DMId DMId*e NMP
1 2 3 4 5
2.14 1.31 2.25 1.53 0.28
0.66 1.41 4.46 5.66 2.54
1.41 1.85 10.02 8.64 0.71
0.009 0.020 0.091 0.132 0.035
EG/MeOH
MF
MeOH
EG
ratio
Product
Rates (h-‘P
73.3 70.6 49.0 42.9 72.6
MeOH/MF
“[ Rh) = 0.04 M (Rhe(CO)lz was used), solvent 20 ml, CO/H2(1/1) 403 - 395 bar, 200 ‘C, 4 h. bTurnover number; EG = ethylene glycol, MF = methyl formate. ci (2040 cm-‘), ii (1985 cm-‘), iii (1960 cm-‘), iv (1900 cm-‘). d[Rh] = 0.013 M, solvent 7.5 ml, CO/H2(1/1) 550 - 540 bar, 220 ‘C, 2 h. e [ CsO&Ph] = 6.7 x 10V3 M was employed.
Solvent
of syn gas by Rh in ureas or NMP solvent&
No.
Conversion
TABLE
0.1 0.1 0.1 0.1 -
i
ii
intensityC
1.0 1.0 1.0 1.0 -
IR relative
0.6 0.6 0.6 0.6 -
111 . ..
2.5 0.2 0.3 1.2 1.0
iv
225
The existence of Rh(CO), has been previously correlated with a high yield of methyl alcohol [7,8], which coincides with our observation, As is seen in Scheme 1, DMI yields mainly 2 while NMP reduces rhodium more intensively to give 4. Addition of cesium benzoate (a strong cation source) to the DMI system increases the Rh(C0)4-/Rh6(C0)152ratio with a decrease in the ethylene glycol/methyl alcohol ratio (Nos. 3 and 4 in Table 1). These observations imply that the active rhodium species for ethylene glycol in these solvents are probably polynuclear carbonyl hydrides.
Rh,(C0)12
CO, Hz, Solv. 400 bar, 200 “C
CO, Hz, Solv.
Solv. l HRh(C0)4
Solv.,*H2Rh6(CO)iS \
[Solv..Hl,[Rh,(CO)1~1
[ Solv. *HI [ Rh(CO),]
2 (observed)
4 (observed)
Scheme 1. Proposed equilibria of rhodium complexes in DMI or NMP.
The intermediates in the synthesis of ethylene glycol from syn gas have been assumed to be HCHO and HOCH,CHO, since trace quantities of 1,3dioxolane and 2-hydroxymethyl-1,3dioxolane have been detected as minor products [ 91. We have confirmed these dioxolanes, as well as minor products such as 1,4dioxane and 2-hydroxy-1,4-dioxane which are considered as the reduction products of glycol aldehyde (No. 2, Table 2). These byproducts were not detected in the reaction mixtures in NMP solvent under the same conditions (No. 6, Table 2). Addition of propylene glycol was then examined to investigate the reaction mechanism in detail. Dioxolanes that derive from propylene glycol are observed in high yields in DMI (No. 7, Table 2), but in quite low yields in NMP (No. 8, Table 2). The IR spectra of the reaction solutions did not change with addition of propylene glycol. This may suggest that the intermediates such as HCHO and HO~H~CHO exist in larger quantities in DMI than in NMP solvent. In the next step, the reaction of paraformaldehyde-i3C with syn gas was carried out in the presence of propylene glycol. The mass spectrum analysis is shown in Table 3. The reaction time was minimized to reduce the products from syn gas only (ca. 10 - 20%). It became clear that the obtained 2-hydroxymethyE4-methyl-l,3dioxolane-13C contains i3C completely on the hydroxymethyl fragment. This indicates that glycol
226 TABLE 2 Identified products in DMI or NMPa Productb
Yield (mmol)
No.
2
6e
7d
8C.d
ethylene glycol methyl alcohol 2-hydroxymethyl-l,3-dioxolane methyl formate ethyl alcohol 1,3-dioxolane 2-methyl-1,3-dioxolane 1,4-dioxane 2-hydroxy-1,4-dioxane
5.92 4.51 0.14 0.06 0.06 0.05 0.05 0.05 trace
2.30 8.16
1.38 1.34 1.37e
2.47 11.05 0.14e
3.4ge
trace
0.09 trace -
a[Rh] = 0.04 M, solution 20 ml, CO/H~(l/l) 400 bar, 200 OC,4 h. bStructures were confirmed by GC-MS spectroscopy, CSolvent was NMP instead of DMI. d50 mmol of 1,2-propylene glycol was employed. @q-Methyl substituted derivative. TABLE 3 Mass spectrum analysis of the products from (*CHzO), in DMIa No.
ProdUb
(M.W.)
M+l
m/e”
I
*CH30H
(33)
34(1.00)
33(0.15)
2
I$*
(89)
90(1.00)
89(0.11)
3
HO*CH2CH20H
(63)
6411.00)
63(0.19)
120(1.00)
119(0.10)
(119)
Key fragments
87(
c)+,
45c*~mi+)
aRh4(CO)rz 0.3 mg-atom, (*CHsO), (97.8 atom% r3C) 4.5 mmol (as CHzO), DMI 7.5 ml, CO/Hs( l/l) 400 bar, 200 ‘C, 30 min. bYield (mmol); 1(0.77), 2(1.96), 3(0.77), 4(0.77). CRelative intensities are shown in parentheses; M + 1 and key fragments were observed by CI and EI methods, respectively.
aldehyde is formed by hydroformylation of formaldehyde [ 10 - 121, not by dimerization of formaldehyde. Based on these results and related literature [ 5,131, the reaction pathways in amide solvents are summarized in Scheme 2. The ion-paired formyl species 5 has been isolated in iron complexes [14,15]. In the case of rhodium catalysts using amide solvents, formation of the formaldehyde intermediate would be accelerated through formation of 5 stabilized by the [Solv-H] cation. By adding HCHO to 1 or 3, 6 or 8 is formed. It is possible
227
0 lor3-
[H!
[Sol”. ‘H] [H-!-Rh=_-]
[H1 -HCHO+lor3
5
0 Solv:HOCH,-Rh=_
s
Solv:HOCH&Rh~
6
7
lor3+HCHO
,
-
HZ
HOCH,CHO H2
HOCH,CH,OH CH,OH 0 CH,OAH
Scheme 2. Proposed reaction pathways.
that methyl alcohol is obtained by reduction of 6. But the route via 8 is preferred since the ratio of methyl formate/methyl alcohol is nearly identical in both DMI and NMP (Table 1). As to the ratio of 6 and 8, the presence of basic solvent will favour 6, resulting in the improved selectivities to ethylene glycol in DMI solvent. On the other hand, NMP accelerates the reaction from 8 to methyl formate and methyl alcohol as shown in Table 2, which reduces the concentration of the formaldehyde intermediate, leading to preferential formation of 8 over 6. This may be the cause of lower selectivities to ethylene glycol in NMP solvent. Acknowledgement This work is a part of ‘C1 Chemistry Project’, a National Research and Development Program of the Agency of Industrial Science and Technology, Ministry of International Trade and Industry (M.I.T.I.), Japan. The authors are grateful to members of the Ethylene Glycol Research Group of the Project for valuable discussions. References 1 J. L. Vidal and W. E. Walker, Inorg. Chem., 19 (1980) 896. 2 U.S. Pat. 4 115428 (1978) to J. L. Vidal, Z. C. Mester and W. E. Walker (Union Carbide Corp.).
228 3 4 5 6 7 8 9 10 11 12 13 14 15
U.S. Pat. 4 302 547 (1981) to P. W. Hart (Union Carbide Corp.). W. Keim, M. Berger and J. Schlupp, J. Catal., 61 (1980) 359. B. D. Dombek,Adu. Catal., 32 (1983) 325. P. Chini, G. Longoni and V. G. Albano, Adu. Organometall. Chem., 14 (1976) 285. U.S. Pat. 3 948 965 (1976) to J. N. Cawse (Union Carbide Corp.). U.K. Pat. 1565 979 (1980) to L. Kaplan (Union Carbide Corp.). D. R. Fahey,J. Am. Chem. Sot., 103 (1981) 136. A. S. C. Chan, W. E. Carroll and D. E. Willis, J. Mol. Catol., 19 (1933) 377. A. Spencer, J. Organometall. Chem., 194 (1980) 113. T. Suzuki, K. Kudo and N. Sugita, Nippon Kagaku K&hi, (1982) 1357. L. C. Costa, Catal. Rev. Sci. Eng., 25 (1983) 325. J. P. Collmann and S. R. Winter, J. Am. Chem. Sot., 95 (1973) 4089. S. R. Winter, G. W. Cornett and E. A. Thompson, J. Organometall. Chem., 133 (1977) 339.