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Methanol homologation by cobalt-phosphine-iodine catalyst systems
Journal ofMofecu!ar Cdalysis,
17 (1982) 315 - 322
!WXT”HANOL HOMOLOGATION CATALYST SYSTEMS
institut
D-5100
fiir Technische Aadzen (F_R.G.)
Chemie
315
BY COBALT-PHOSPHINE-IODINE
und Pefrolchemie
der RWTH
Aachen.
Worringer
IVeg I.
and J. KORFF Union
Rheinische
.&cunkohlenkraftsCroff
AC.
Pastfach
8.
D-5047
Weaseling
(F.R.C.)
Despite numerous research efforts, the mechanism of methanol homologation and especially the role of iodine promoters remain unclear. In an approach to improve the understanding ofthisreaction,the basic properties of cobalt-iodine-phosphme catalyst systems have been studied. In addition, well-defined cobalt complexes and clusters were used as catalysts, and experiments with labelled reagents have been carried out. Based on these data a reaction mechanism via oxidative addition of methyl iodide to a cobalt(I) center is proposed.
The methanol
first reports of the cob&-catalyzed to give ethanol or acetidehyde date
CHjOkI
+
CO +
CH3CH0
+
H2
Hz -
CH$HO
+
hydrocarbonylation of back to 1941 [I] _ This
Hz0
Ul
CH$HflH
(21
homogeneously catalyzed syngas reaction pias introduced into the open literature by Wender et al, who proposed Co,(CO)s as the active catalyst precursor [ZJ _ The reaction is significantly promoted by iodine. as was shown by Berty ef a1. in 1956 [S] . bigands such as phosphines were found to enhance the stability and selectivity of the catalyst system [4 - S] . In order to improve the hydrogenation activity and thus the ethanol selectivity, ruthenium compounds have been used in recent studies as cocateiysts of cobalt iodine cadysts E7, 8J. Though economic analysts have estimated methanol homclogation more attractive for ethylene synthesis than meth.ano! dehydration with moIecular sieves or Fischer-Tropsch synthesis [9], no industrial application has been reported so far. This is due to the Iimited selectivity of the process, which gives rise to a variety of side 0
Eketier
SequoiafPrinted
in The
Netherlands
316 products_ Furthermore, the mechanism of methanol homologation is still unclear and contradictory proposals exist in the literature. In order to improve the understanding of the mechznistic aspects of this reaction, we have studied the basic properties of cobalt-iodine-ligand catalyst systems.
Experimental All experiments have been carried out in 150 ml sttinlesssteelautoclaves (No. 2.4610) equipped with a magnetic stirrer, 2 dropping funnel, and a syngas storage with a pressure-regulation system to mtintain a constant reaction pressure_ GC analysis was czrried out on a ‘LOO m Carbowax 1500 SCOT column (WC-A) using toluene as internal standard. LMethz-iol conversion was calculated from the unreacted methvlol determined by GC. Molar selectivities are based on converted methanol, and include a stoichiometric fac’cor indicating the number of methyl groups in the molecule. GC/MS measurements were carried out on a Varian MAT 112 S/SS 200 system.
Results and Discussion For our studies the catalyst system Co(OAc),4Hz0, Ph,P, and aqueous HI w.z.s chosen. With this system acetaldehyde is the prevailing product, and only minor amounts of ethanol are formed. This reflects the reduced hydrogenation ability of the catalyst due to the iodine promoter. Also formed are acetals such as 1,1-dimethoxy ethane, l-methoxy-l-ethoxy ethane, and l,l-dimethoxy butane, esters such as methyl and ethyl acetate, ethers such as dimethyl and methyl ethyl ether, and acetaldehyde condensation products such as paraldehyde, crotonaldehyde, butyraldehyde and n-butanol. All products have been-identified by GC/_MS and by comparison with authentic samples. More than 20 reaction products have been detected in a homologation mixture by GC/MS [ 101. The methanol conversion as well as protiuct distribution is highly sensitive to the catalyst composition and reaction conditions. As is shown in Table 1, methvlol conversion remtins low in the absence of iodine. Even minor amounts of HI prompte the reaction significantly, znd a maximum for conversion and acetidehyde selectivity is found at I:Co = 2:1. At higher I/Co ratios the formation of ethers and acetaldehyde condensation products increases significantly. The variation of the P/Co ratio affects conversion rates only if a high excess of phosphine is used (P/Co > 3). In this case inhibition occurs which can be interpreted by blocking of active sites by excess of ligand. Addition of triphenyl phosphine improves the selectivity to acetidehyde, acetals, and ethariol, and reduces the formation of ethers, acetaldehyde condensation products, and to some extent of acetztes. Best results are observed at a P/Co ratio of 2:1.
TABLE1 Effect
of catalyst
composition Conversion
Molar selectivity (W)
(5)
EWH
AcH
acetaLs
ace’alxs
ethers
7.2 26 i6 8.5
42 58 40 25 23
3.4 7.2 15 12. 10
2.3 2.0 1.6 3.1 5.6
20 18 26 16 -
41 29 40 52 25
20 18 15 11 1.2
6.8 3.0 1.6 1.2 -
I/Co ratio' 0
1 2
3 4 P/Co ratiob 0 1 2 3 ;
- 7.4 6% 73 62 67
69
76 73 71 -11
C~'kdyst: Co(OAs)~-4H20, = 2:1. bI/Co= 2:l.
1.2
6.1 4.4 4.0 1.6
5.9 3.9
4.4 3.1 -
-
PhxP, HI; conditions: 300 bar CO/H,
(lrl), 200°C.
2 h.
“P/CQ
The effect of reaction parameters is depicted in Table 2_ Thus, at low temperatures acetals are formed selectively. With increasing temperature the acetis are converted to acetaldehyde and ethanol. Best acetaldehyde yields are found at 180 - 200 OC. Higher temperatures largely favour side product formation such as acetidehyde condensation. Ace’& also prevail at short reaction times. After 30 min an acetal selectivity of 73% is found. The yield of ace’aldehytie and ethanol increases, at the cost of ace’&, up to re action times of 2 - 2.5 h. Longer reaction tunes lead to increased side product formation and in particular acetaldehyde condensates are found. Another important factor is syngas pressure. The catalyst system is active even at low pressures, such as 100 bar, and mainly acetals are formed at low conversion rates. Increasing the pressure up to 300 bar results in an improved methanol conversion and in enhanced yields of acetidehyde, ethanol, and acetates. At a pressure of 1000 bar, most of the acet& are converted to acetaldehyde, ethanol, and acetates, and selectivities of 36, LO and 24% are found, respectively. Though these results seem attractive, a syngas pressure of 300 bar was applied for further experiments. In accordance with earlier reports [II], a I:1 CO:K2 mixture gives best reza~1t.sfor methanol homologation. However it is interesting to note that syngas mixtures rich in hydrogen give rise only to reduced methanol conversion and not to increased ethanol yields. In contrast, syngas mixtures rich in CO result :x remarkable selectivities for acetate formation. This is further evidence For the poor hydrogenation activity of this type rlf catalyst.
318 TABLE Effect
2 of reaction
parameters
on the catalyst
Co(OAc),/Ph3P/HI
Con;_ersion
Molar
selectivity
(5)
EtOH
AcH
acetals
acetates
ethers
66 76 78 7.5 47
0.7 1.8 3.1 4.i 9.2
8.0 IS 21 24 3.2
66 44 36 35 13
14
16 1s 22 16
1.2 1.5 1.7 1.9 2.9
1.0 1.5 2.0 2.5
57 69 74 73 75
1.3 2.2 3.2 4.4 4.7
7.1 14 21 26 24
73 56 46 40 35
6.8 9.1 13 15 22
Press. (bar) 100 200 300 100 1000
21 51 73 76 82
1.9
3.7 4.4 4.4 9.7
2.8 10 26 27 36
68 40 32 9.2
15 15 24 24
1.7 2.0 1.6 2.2 2.3
CO/H2 I:0 2:l 1.1 1:2 1:3
73 76 45 24
0.1 14 20
5.1 36 38
71 30 15
1.3 1.7 1.8
Parameter
Temp. 150 170 190 200 210
(5%)
(“C)=
Time(h)
0.5
ratiob
-
29
Catalyst and conditions: see Table “Reaction time 2.5 h. bReaction temp. 190 “C.
2.0 3.7 5.1
4.6
5.0 5s
81
60 59
7.9
2.9 2.0
1.1 1.2 1.7 1.6 1.9
2.3 2.1
1.
The mechanistic role of iodine promoters is still not fuhy understood. For this reason we have examined various iodine compounds as activator; for the catalyst system Co(OAc),/Ph,P. With respect to their efficiency, they can be classified into three groups: HI = I, = CoIz = Me1 = Et1 > Phi = Hg,Il > EtfiI = Bu,NI = PhyI = Ph&ePI --1NaI = KI = CsL. Effective promoters are selected from the first group, and the highest syngas uptake is observed for HI and 12_ Clearly iess efficient are promoters from the second group, where an incubation period is observed for Phi. The ionie iodides from the !ast group are almost inactive. However, it is interesting to note that these ionic iodides are good activators in the absence of donor ligands such as Ph,P. Possibly phosphlmes prevent nucleophilic attack of I- on the cobalt center.
319
The oxidative addition of methyl iodide to Rh(I) has been shown to be the key step in methanol carbonylation !12]. A similar process must be asssmed in methanol homoIogation, since the stoichiometric hydrocarbonylation of methyl iodide has recently been reported by Ungvary and Mark0 [13]_ The use of one-component cataiysts selected from well-defined cobaltphosphineiodine complexes may shed some light on the nature of intermediates in the catalytic cycle. Table 3 contains typical results of some of the complexes examined, which were selected according to their relative I/Co and P/Co ratios. Out of a variety of complexes, CoE2(Ph3P), gave the highest conversion rate and acetaldehyde/ethanol yield. Good results were also achieved with the anionic complex CoI,(Ph$‘)-. In contrast only low conversion rates and mainly acetaI formation was found with cobalt(I)iodine complexes. These findings are in good accordance with the results obtained with Co(OAc),/Ph,P/HI. Thus it can be assumed that complexes of the type CoItLz play an important role as precursors of the catalytically active intermediates. Por several syngas reactions, clusters have been proposed as catalyticzlly active species or as catalyst precursors [14, 151. Thus it seemed attractive to examine the catalytic properties of clusters in methanol homologation. With the cobalt clusters CO&CO)~~, Co&CO),,, Co3(P,-CWCW9, Co3@3Co,@-PPh,),(CO), and Co,(p-PPh)(CO)B Co&d'PhMCO)m PPhWOL only minor methanol conversions were achieved in the absence of iodine promoters, and mainly acetals were formed. In all cases, addition of methyl iodide led to a significant increase in activity_ Table 3 contains the results obtained with Co&,-PPh),(CO) 10, which according to Pittman et aL catalyses the hydrcformylation of pentene without detectable decomposition [15]. In the absence of a promoter l,l-dimethoxy ethme is formed in high TABLE 3 C05alt c~mplexesandclustenascatalysts Conversion (5)
Cataly5.t
73
CoMPh3P)z
CoIS(Ph3P)- = CoI(Ph3PMC~lz
66
32
CoI(Ph3P)3 ~40r;-~P~~2W~ho
Co~@~-PPh)~(CO)lo
-4.6 -12
b
+ h%eIb-C
~~(P~-CHI(CW~~
63 IL
CO&~-CHKCO)~
+ MeLbsC
Conditions:seeTable "As the Bu4NCsalt.
1.
bTempeature 190 ‘C. Vo/I = L:2.
74
Molar selectivity (%) Et3H
A&H
acetals
acetates
4.8
23
41
20
1.7
3.3
27
1.6
34
68
22
2.3
trace 0.9
;race trace
62 55
trace 2.6
trace 14
4.0 8.0 5.1
13 1.0 19
60 66 34
9.0 5.7 20
12 19 5.2
I.4
3.8
&hers
1.0
320
selectivity at a low conversion rate_ -After the reaction the cluster could be recovered only partialIy, and the reaction mixkrre showed an IR absorption N-hich is typical for Co(CO),. Addition of methyl iodide at 1903 cm-’ resultAd in 63% methanol conversion and a similar product mixture as is obttined with mononuclear cobalt complexes. During the reaction the cluster decomposed completely, and only weak IR absorptions at 1899 cm-’ were observed in the reaction product. Carbon-bridged clusters such as Co&,-CH)(CO), also decomposed under reaction conditions to give the res-ults shown in Table 3. Though these results do not exclude clusters as catalyst precursors, they discourage the assumption of clusters as active species under these reaction conditions. The mechanistic proposals known for methanol homologation can be classified into two groups. Esterification according to eqn. (3) was originally proposed by Hecht and Krijper [IS] : CH3DH
“Co(COk
C H,OH;CoCCO,,
-Hzo
CH3Co(COlL
(3)
CO-insertion and hydrogenation would lead to the observed producti. In contrast, Ziesecke proposed carbene as the intermediate in me’hanol homologation which could react with hydrocobalt carbonyl to give a methyl cobalt species [ 1’7 J : CH30H
-Hz0
H Co(C01~ CH2
-
HCH2CoKDlL
The methyi grcup of reaction products formed according to eon. (4) contains a hydrogen from the cobalt hydride which originates from synthesis gas. In order to distinguish between these two reaction paths, deuterated methanol was submitted to the homologation reaction at 300 bar CO/H2 (l:l), 200 OC and 2 h. The results are depicted in eqn. (5):
CO3CHO CD30D+CO/H2
+
CD3CH,
,OCD3 OCD3
[co:PlqP1plJ_ C 03Ci-$OH
+
(5)
Ctk,COOCO3
The GC/MS analysis of the reaction products showed unequivocally that the CD,group of methanol remains intact during the course of the reaction, and can be detected in acetaldehyde, l,l-dimethoxy ethane, ethanol, and methyl acetate. These results make carbene as an intermediate highly unlikely since a hydrogen/deu+xrium exchange should be observed in this case. In order to develop a mechanistic model it is worthwhile summarizing the results observed. It has been shown that liganda such as phosphines are able to control the course of homologation reaction to a large extent. Thus it seems likely that the ligands are coordinated to the metal center during
321
Weme
1.
the catalytic cycle. Based on this assumption and on the observation that best results were obtained with a cobalt-phosphine-iodide ratio of X:2:2, the mechanism shown in Scheme 1 is proposed. Oxidztive addition of methyl iodide to the coordinatively unsaturated cobalt(I) species 1 gives the methyl compla 2 which undergoes CO insertion. Elimination of iodine from the acetyl complex 3 and oxidative addition of hydrogen gives 5. Reductive elimination of the primary product aldehyde leads to the unsaturated complex 6 which adds iodine oxidatively. Tne catalytic cycle is closed by ehmination of hydrogen iodide from 7, which is consumed by reaction with methanol to give methyl iodide. The proposed mechanism is based on cobalt(I) and cobait(III) species, which are known at least as their Rh and Ir analogues [IS - 243 _ The process is controlled by catalytic amounts of methyl iodide, hydrogen iodide, and iodine which emphasizes the muk~finctional role of the promoter. Only recently a mechanism for the rhodium-cataiyzed synthesis of acetic anhydride was published propkng intermediates corresponding to 1, 2 and 3 [251. Studies are in progress to substantiate +Ute different reaction steps by stoichiometric reactions and to isolate intermediates from the reaction mixture.
Acknowledgements
The authors gratefuLly acknowledge financial support from the Bundesministerium fir Forschung und Technologie.
322
References (1941) to G. Wietzel, AL Eider and A. Scheoermana (B&F). Chem. 1 DRP 867849 Abstr., 48 (1954) 1408a. 2 I. Wender, R. Levine and hl. Orchin, J. Am. Cfzem. Sot.. 71 (1943) 416O;I. Wender, R. A. Friedel and M. Orchin, Science. J 13 (1951) 206_ Tech. (Leipzig). 6 (1956) 260. 3 J. Berty, L. Marko and D. Kallo, Ckm to A. D. Riley and W_ 0. Eel! Jr. (Commercial Solvents); 4 Ger. Pat. I 173 075 (1964) Chem. Abstr., 59 (1963) 9793d. 5 GET. Pat. 2 625 627 (1976) to L. H. Slaugh (Shell Int. Res. Maatah.); Chem_ Abstr-. 87 (1977) 5373e. Ann. N. Y. Acad. Sci.. 333 (1980) 58. 6 W. R. Frelzer and T. P. Kobylinski, i T. Mizoroki, T. Matnrmoto and A. Ozaki, Bull. Chem. Sot. Jpn.. 52 (1979) 479. 6 H. Beuther, T. P. Kobylinski, G. M. Singerman and W. R. Pretzer, Am. Cizem. Sot.. Div. Petr. Chen., Prepr. 25 (1960) 92. 9 J. Haggin, Chcm. Eng. News. 59(20) (1981) 52; Chem. Eng_ News. 60(13) (1982) 28. 10 G. S. Koermer and W. E. Slinkard, Ind. Eng. Chem. Prod. Res. Dew.. 17 (1978) 231. 11 B. Corn&. in J. Falbe (ed.), Chcmierohstoffe QUS Xohle, Thieme, Stuttgart. 1977, p. 329. 12 J. F. Roth, J. H. Craddtik, A. Hershman and F. E. Paul&, Chem. Techn.. (1971) 600. J. Organometai. Chem.. 193 (1980) 3i9_ 13 F. Ungvary and L. MLko, 14 J. E. Vidal and W_ E. Walker, Inorg. Chem.. 19 (1980) 896. 15 C. U. Pittman Jr.. G. M. Wilemon. W. D. Wilson and R. C. Ryan. Anger;. Chem.. 92 (1980) 494. 16 0. Hecht and H. Krijper, AToturfo,schung und Medizin in Deutschhmd (FIAT Rev. German Sci.) 1939 - 46. Vol. 36, 1948. Pt. I, p_ 136. 17 K. H. Ziesecke, Brennst. Chem.. 33 (1952) 385. 18 L. Vallarino, J. Chcm. Sot.. (1957) 2287. 951. 19 D. Forster. J. Am. Chem. Sot.. 97 (1975) 20 I. C. Douek and G. Wilkinson, J_ Chem. Sot. (A). (1969) 26OC_ 21 R. F. Peck and D. S. Br&ow, J. Am. Chem. SOC., B4 (1962) 2499. 22 G. Yagupsky, C. K. Brown and G. Wilkinson, J. Chem. Sot. (A), (1970) 1392. 23 W. Hieber and I-I. Duchatsch, Chem. Ber., 98 (1965) 2933. 24 B. E. Mann. C. Masters and B. L. Shsw, J. Inorz. Nucl. Chem.. 33 (1971) 2195_ 25 M. Schrod and G. Luft, Ind. Eng. Chem. Prod. Res. Dev.. 20 (1981) 649.
17 (1982) 315 - 322
!WXT”HANOL HOMOLOGATION CATALYST SYSTEMS
institut
D-5100
fiir Technische Aadzen (F_R.G.)
Chemie
315
BY COBALT-PHOSPHINE-IODINE
und Pefrolchemie
der RWTH
Aachen.
Worringer
IVeg I.
and J. KORFF Union
Rheinische
.&cunkohlenkraftsCroff
AC.
Pastfach
8.
D-5047
Weaseling
(F.R.C.)
Despite numerous research efforts, the mechanism of methanol homologation and especially the role of iodine promoters remain unclear. In an approach to improve the understanding ofthisreaction,the basic properties of cobalt-iodine-phosphme catalyst systems have been studied. In addition, well-defined cobalt complexes and clusters were used as catalysts, and experiments with labelled reagents have been carried out. Based on these data a reaction mechanism via oxidative addition of methyl iodide to a cobalt(I) center is proposed.
The methanol
first reports of the cob&-catalyzed to give ethanol or acetidehyde date
CHjOkI
+
CO +
CH3CH0
+
H2
Hz -
CH$HO
+
hydrocarbonylation of back to 1941 [I] _ This
Hz0
Ul
CH$HflH
(21
homogeneously catalyzed syngas reaction pias introduced into the open literature by Wender et al, who proposed Co,(CO)s as the active catalyst precursor [ZJ _ The reaction is significantly promoted by iodine. as was shown by Berty ef a1. in 1956 [S] . bigands such as phosphines were found to enhance the stability and selectivity of the catalyst system [4 - S] . In order to improve the hydrogenation activity and thus the ethanol selectivity, ruthenium compounds have been used in recent studies as cocateiysts of cobalt iodine cadysts E7, 8J. Though economic analysts have estimated methanol homclogation more attractive for ethylene synthesis than meth.ano! dehydration with moIecular sieves or Fischer-Tropsch synthesis [9], no industrial application has been reported so far. This is due to the Iimited selectivity of the process, which gives rise to a variety of side 0
Eketier
SequoiafPrinted
in The
Netherlands
316 products_ Furthermore, the mechanism of methanol homologation is still unclear and contradictory proposals exist in the literature. In order to improve the understanding of the mechznistic aspects of this reaction, we have studied the basic properties of cobalt-iodine-ligand catalyst systems.
Experimental All experiments have been carried out in 150 ml sttinlesssteelautoclaves (No. 2.4610) equipped with a magnetic stirrer, 2 dropping funnel, and a syngas storage with a pressure-regulation system to mtintain a constant reaction pressure_ GC analysis was czrried out on a ‘LOO m Carbowax 1500 SCOT column (WC-A) using toluene as internal standard. LMethz-iol conversion was calculated from the unreacted methvlol determined by GC. Molar selectivities are based on converted methanol, and include a stoichiometric fac’cor indicating the number of methyl groups in the molecule. GC/MS measurements were carried out on a Varian MAT 112 S/SS 200 system.
Results and Discussion For our studies the catalyst system Co(OAc),4Hz0, Ph,P, and aqueous HI w.z.s chosen. With this system acetaldehyde is the prevailing product, and only minor amounts of ethanol are formed. This reflects the reduced hydrogenation ability of the catalyst due to the iodine promoter. Also formed are acetals such as 1,1-dimethoxy ethane, l-methoxy-l-ethoxy ethane, and l,l-dimethoxy butane, esters such as methyl and ethyl acetate, ethers such as dimethyl and methyl ethyl ether, and acetaldehyde condensation products such as paraldehyde, crotonaldehyde, butyraldehyde and n-butanol. All products have been-identified by GC/_MS and by comparison with authentic samples. More than 20 reaction products have been detected in a homologation mixture by GC/MS [ 101. The methanol conversion as well as protiuct distribution is highly sensitive to the catalyst composition and reaction conditions. As is shown in Table 1, methvlol conversion remtins low in the absence of iodine. Even minor amounts of HI prompte the reaction significantly, znd a maximum for conversion and acetidehyde selectivity is found at I:Co = 2:1. At higher I/Co ratios the formation of ethers and acetaldehyde condensation products increases significantly. The variation of the P/Co ratio affects conversion rates only if a high excess of phosphine is used (P/Co > 3). In this case inhibition occurs which can be interpreted by blocking of active sites by excess of ligand. Addition of triphenyl phosphine improves the selectivity to acetidehyde, acetals, and ethariol, and reduces the formation of ethers, acetaldehyde condensation products, and to some extent of acetztes. Best results are observed at a P/Co ratio of 2:1.
TABLE1 Effect
of catalyst
composition Conversion
Molar selectivity (W)
(5)
EWH
AcH
acetaLs
ace’alxs
ethers
7.2 26 i6 8.5
42 58 40 25 23
3.4 7.2 15 12. 10
2.3 2.0 1.6 3.1 5.6
20 18 26 16 -
41 29 40 52 25
20 18 15 11 1.2
6.8 3.0 1.6 1.2 -
I/Co ratio' 0
1 2
3 4 P/Co ratiob 0 1 2 3 ;
- 7.4 6% 73 62 67
69
76 73 71 -11
C~'kdyst: Co(OAs)~-4H20, = 2:1. bI/Co= 2:l.
1.2
6.1 4.4 4.0 1.6
5.9 3.9
4.4 3.1 -
-
PhxP, HI; conditions: 300 bar CO/H,
(lrl), 200°C.
2 h.
“P/CQ
The effect of reaction parameters is depicted in Table 2_ Thus, at low temperatures acetals are formed selectively. With increasing temperature the acetis are converted to acetaldehyde and ethanol. Best acetaldehyde yields are found at 180 - 200 OC. Higher temperatures largely favour side product formation such as acetidehyde condensation. Ace’& also prevail at short reaction times. After 30 min an acetal selectivity of 73% is found. The yield of ace’aldehytie and ethanol increases, at the cost of ace’&, up to re action times of 2 - 2.5 h. Longer reaction tunes lead to increased side product formation and in particular acetaldehyde condensates are found. Another important factor is syngas pressure. The catalyst system is active even at low pressures, such as 100 bar, and mainly acetals are formed at low conversion rates. Increasing the pressure up to 300 bar results in an improved methanol conversion and in enhanced yields of acetidehyde, ethanol, and acetates. At a pressure of 1000 bar, most of the acet& are converted to acetaldehyde, ethanol, and acetates, and selectivities of 36, LO and 24% are found, respectively. Though these results seem attractive, a syngas pressure of 300 bar was applied for further experiments. In accordance with earlier reports [II], a I:1 CO:K2 mixture gives best reza~1t.sfor methanol homologation. However it is interesting to note that syngas mixtures rich in hydrogen give rise only to reduced methanol conversion and not to increased ethanol yields. In contrast, syngas mixtures rich in CO result :x remarkable selectivities for acetate formation. This is further evidence For the poor hydrogenation activity of this type rlf catalyst.
318 TABLE Effect
2 of reaction
parameters
on the catalyst
Co(OAc),/Ph3P/HI
Con;_ersion
Molar
selectivity
(5)
EtOH
AcH
acetals
acetates
ethers
66 76 78 7.5 47
0.7 1.8 3.1 4.i 9.2
8.0 IS 21 24 3.2
66 44 36 35 13
14
16 1s 22 16
1.2 1.5 1.7 1.9 2.9
1.0 1.5 2.0 2.5
57 69 74 73 75
1.3 2.2 3.2 4.4 4.7
7.1 14 21 26 24
73 56 46 40 35
6.8 9.1 13 15 22
Press. (bar) 100 200 300 100 1000
21 51 73 76 82
1.9
3.7 4.4 4.4 9.7
2.8 10 26 27 36
68 40 32 9.2
15 15 24 24
1.7 2.0 1.6 2.2 2.3
CO/H2 I:0 2:l 1.1 1:2 1:3
73 76 45 24
0.1 14 20
5.1 36 38
71 30 15
1.3 1.7 1.8
Parameter
Temp. 150 170 190 200 210
(5%)
(“C)=
Time(h)
0.5
ratiob
-
29
Catalyst and conditions: see Table “Reaction time 2.5 h. bReaction temp. 190 “C.
2.0 3.7 5.1
4.6
5.0 5s
81
60 59
7.9
2.9 2.0
1.1 1.2 1.7 1.6 1.9
2.3 2.1
1.
The mechanistic role of iodine promoters is still not fuhy understood. For this reason we have examined various iodine compounds as activator; for the catalyst system Co(OAc),/Ph,P. With respect to their efficiency, they can be classified into three groups: HI = I, = CoIz = Me1 = Et1 > Phi = Hg,Il > EtfiI = Bu,NI = PhyI = Ph&ePI --1NaI = KI = CsL. Effective promoters are selected from the first group, and the highest syngas uptake is observed for HI and 12_ Clearly iess efficient are promoters from the second group, where an incubation period is observed for Phi. The ionie iodides from the !ast group are almost inactive. However, it is interesting to note that these ionic iodides are good activators in the absence of donor ligands such as Ph,P. Possibly phosphlmes prevent nucleophilic attack of I- on the cobalt center.
319
The oxidative addition of methyl iodide to Rh(I) has been shown to be the key step in methanol carbonylation !12]. A similar process must be asssmed in methanol homoIogation, since the stoichiometric hydrocarbonylation of methyl iodide has recently been reported by Ungvary and Mark0 [13]_ The use of one-component cataiysts selected from well-defined cobaltphosphineiodine complexes may shed some light on the nature of intermediates in the catalytic cycle. Table 3 contains typical results of some of the complexes examined, which were selected according to their relative I/Co and P/Co ratios. Out of a variety of complexes, CoE2(Ph3P), gave the highest conversion rate and acetaldehyde/ethanol yield. Good results were also achieved with the anionic complex CoI,(Ph$‘)-. In contrast only low conversion rates and mainly acetaI formation was found with cobalt(I)iodine complexes. These findings are in good accordance with the results obtained with Co(OAc),/Ph,P/HI. Thus it can be assumed that complexes of the type CoItLz play an important role as precursors of the catalytically active intermediates. Por several syngas reactions, clusters have been proposed as catalyticzlly active species or as catalyst precursors [14, 151. Thus it seemed attractive to examine the catalytic properties of clusters in methanol homologation. With the cobalt clusters CO&CO)~~, Co&CO),,, Co3(P,-CWCW9, Co3@3Co,@-PPh,),(CO), and Co,(p-PPh)(CO)B Co&d'PhMCO)m PPhWOL only minor methanol conversions were achieved in the absence of iodine promoters, and mainly acetals were formed. In all cases, addition of methyl iodide led to a significant increase in activity_ Table 3 contains the results obtained with Co&,-PPh),(CO) 10, which according to Pittman et aL catalyses the hydrcformylation of pentene without detectable decomposition [15]. In the absence of a promoter l,l-dimethoxy ethme is formed in high TABLE 3 C05alt c~mplexesandclustenascatalysts Conversion (5)
Cataly5.t
73
CoMPh3P)z
CoIS(Ph3P)- = CoI(Ph3PMC~lz
66
32
CoI(Ph3P)3 ~40r;-~P~~2W~ho
Co~@~-PPh)~(CO)lo
-4.6 -12
b
+ h%eIb-C
~~(P~-CHI(CW~~
63 IL
CO&~-CHKCO)~
+ MeLbsC
Conditions:seeTable "As the Bu4NCsalt.
1.
bTempeature 190 ‘C. Vo/I = L:2.
74
Molar selectivity (%) Et3H
A&H
acetals
acetates
4.8
23
41
20
1.7
3.3
27
1.6
34
68
22
2.3
trace 0.9
;race trace
62 55
trace 2.6
trace 14
4.0 8.0 5.1
13 1.0 19
60 66 34
9.0 5.7 20
12 19 5.2
I.4
3.8
&hers
1.0
320
selectivity at a low conversion rate_ -After the reaction the cluster could be recovered only partialIy, and the reaction mixkrre showed an IR absorption N-hich is typical for Co(CO),. Addition of methyl iodide at 1903 cm-’ resultAd in 63% methanol conversion and a similar product mixture as is obttined with mononuclear cobalt complexes. During the reaction the cluster decomposed completely, and only weak IR absorptions at 1899 cm-’ were observed in the reaction product. Carbon-bridged clusters such as Co&,-CH)(CO), also decomposed under reaction conditions to give the res-ults shown in Table 3. Though these results do not exclude clusters as catalyst precursors, they discourage the assumption of clusters as active species under these reaction conditions. The mechanistic proposals known for methanol homologation can be classified into two groups. Esterification according to eqn. (3) was originally proposed by Hecht and Krijper [IS] : CH3DH
“Co(COk
C H,OH;CoCCO,,
-Hzo
CH3Co(COlL
(3)
CO-insertion and hydrogenation would lead to the observed producti. In contrast, Ziesecke proposed carbene as the intermediate in me’hanol homologation which could react with hydrocobalt carbonyl to give a methyl cobalt species [ 1’7 J : CH30H
-Hz0
H Co(C01~ CH2
-
HCH2CoKDlL
The methyi grcup of reaction products formed according to eon. (4) contains a hydrogen from the cobalt hydride which originates from synthesis gas. In order to distinguish between these two reaction paths, deuterated methanol was submitted to the homologation reaction at 300 bar CO/H2 (l:l), 200 OC and 2 h. The results are depicted in eqn. (5):
CO3CHO CD30D+CO/H2
+
CD3CH,
,OCD3 OCD3
[co:PlqP1plJ_ C 03Ci-$OH
+
(5)
Ctk,COOCO3
The GC/MS analysis of the reaction products showed unequivocally that the CD,group of methanol remains intact during the course of the reaction, and can be detected in acetaldehyde, l,l-dimethoxy ethane, ethanol, and methyl acetate. These results make carbene as an intermediate highly unlikely since a hydrogen/deu+xrium exchange should be observed in this case. In order to develop a mechanistic model it is worthwhile summarizing the results observed. It has been shown that liganda such as phosphines are able to control the course of homologation reaction to a large extent. Thus it seems likely that the ligands are coordinated to the metal center during
321
Weme
1.
the catalytic cycle. Based on this assumption and on the observation that best results were obtained with a cobalt-phosphine-iodide ratio of X:2:2, the mechanism shown in Scheme 1 is proposed. Oxidztive addition of methyl iodide to the coordinatively unsaturated cobalt(I) species 1 gives the methyl compla 2 which undergoes CO insertion. Elimination of iodine from the acetyl complex 3 and oxidative addition of hydrogen gives 5. Reductive elimination of the primary product aldehyde leads to the unsaturated complex 6 which adds iodine oxidatively. Tne catalytic cycle is closed by ehmination of hydrogen iodide from 7, which is consumed by reaction with methanol to give methyl iodide. The proposed mechanism is based on cobalt(I) and cobait(III) species, which are known at least as their Rh and Ir analogues [IS - 243 _ The process is controlled by catalytic amounts of methyl iodide, hydrogen iodide, and iodine which emphasizes the muk~finctional role of the promoter. Only recently a mechanism for the rhodium-cataiyzed synthesis of acetic anhydride was published propkng intermediates corresponding to 1, 2 and 3 [251. Studies are in progress to substantiate +Ute different reaction steps by stoichiometric reactions and to isolate intermediates from the reaction mixture.
Acknowledgements
The authors gratefuLly acknowledge financial support from the Bundesministerium fir Forschung und Technologie.
322
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