Ind. Eng, Chem. Prod. Res. Dev. 1981, 20, 115-122
115
Carbonylation and Homologation of Dimethyl Ether in the Presence of Ruthenium Catalysts Gluseppe Braca, Lap0 Paladlnl, Glauco Sbrana, and Glorglo Valentlnl Istituto di Chimica Organica Industrkle, University of
Plsa, v/a Rlsorgimento 35, 56100 Pisa, Ita&
Glanpaolo Andrlch and Gugllelmo Gregorlo D I E , Centro Rlcerche Bollate, Montedlson S.p.A., 20021 Bollate, Ita&
use of ruthenium catalysts in conjunction with iodide promoters and proton suppliers was investigated at a temperature of 200 O C and under CO pressure of 150-200 atm. Using CO/HP mixtures and analogous reaction conditions, dimethyl ether was simultaneously carbonylated and homologated to ethyl acetate wkh selectlvity as high as 70 % . The effect of the several reaction variables on the reaction rate and selectMty was studied. Information on the role of the promoters and proton suppliers and on the nature of the ruthenium species involved in the catalytic cycle was obtained. A reaction pathway and its accordance with the experimental results was discussed. The carbonyhtii of methanol, dimethyl ether, and methyl acetate to acetic derlvatlves by
Introduction Only a few examples are reported in the literature of the use of ruthenium compounds as catalysts for carbonylation reactions on saturated oxygenated substrates. The homologation of methanol and l-propanol with CO and Hz to high molecular weight alcohols in the presence of RuOa is reported in two early DuPont patents (Gresham, 1950; Howk and Hager, 1951); moreover, the use of ruthenium halides as promoters in the cobalt iodide-catalyzed methanol homologation to ethanol has been claimed (Butter, 1966; Pretzer et al., 1979). We wish to report here the results of our recent carbonylation and homologation experiments on oxygenated substrates (methanol, dimethyl ether, and methyl acetate) in the presence of ruthenium carbonyl iodide systems as catalysts. A preliminary communication on theae reactions has appeared (Braca et al., 1978, 1979). Results Carbonylation Reactions. Rhodium complexes with iodide promoters are well recognized as the best catalysts for alcohols carbonylation to carboxylic acids (eq 1)(Roth et al., 1971; Piacenti and Bianchi, 1977) but also Co (von Kutepow et al., 1965; Mizoroki and Nakayama, 1968), Ni (Reppe et al., 1953; Adkins and Rosenthal, 1950), and Ir (Brodzki et al., 1977; Mizoroki et al., 1979; Forster, 1979) systems were thoroughly investigated and used. Ruthenium derivatives were considered practically inactive for this reaction (Brodzki et al., 1977). Catalytic systems based on Fe, Co, Ni, and Rh derivatives are also effective catalysts for the carbonylation of noncyclic ethers and esters to give esters and anhydrides, respectively (eq 2 and 3); however, only low rates and selectivities in carbonylation products were obtained (Piacenti and Bianchi, 1977; Kuckertz, 1974; Naglieri and Rizkalla, 1977). ROH + CO RCOOH (1) ROR + CO RCOOR (2) RCOOR + CO (RC0)zO (3) In our recent carbonylations of oxygenated substrates (alcohols, ethers, and esters), we found that catalytic
--
-
systems based on ruthenium precursors such as tris(acetylacetonato)ruthenium(III) or diiodotetracarbonylruthenium(I1) with iodide promoters (CH31,HI, NaI) are effective catalysts for these reactions. Methyl acetate and acetic acid are the main carbonylation products when methanol is used as substrate; however, also homologation and hydrogenation products (ethanol, ethyl acetate, methyl ethyl ether, and methane) are formed in spite of the fact that we operate initially in the absence of hydrogen. The hydrogen necessary for the formation of these last products arises from a watewas shift (WGS) reaction based on the water formed in methanol etherification and/or in esterification reactions. Moreover, hydrogen may be formed through a partial decomposition of methanol to CO + 2Hz. Thus, in methanol carbonylation without solvent (Table I, run 1)substantial amounts of dimethyl ether (selectivity 59%) are formed due to the catalytic effect of HI present in the reaction medium. As a consequence, an important hydrogen formation by WGS reaction (-30% in the final gas mixture) was observed, thus explaining the occurrence of the homologation and hydrogenation reactions. On the contrary, using methyl acetate as solvent and NaI as promoter without added acid (run 2), the prevailing formation of simple carbonylation products (methyl acetate and acetic acid) was observed. In the absence of hydrogen or hydrogen donors, dimethyl ether and methyl acetate are not carbonylated, no matter what ruthenium iodide catalytic system, i.e., R u ( A ~ a c ) ~ / was used. CH31, or Ru(CO)~I~/CH~I, or RU(CO)~I~/HI, However, a low pressure of hydrogen (5-10 atm) or the presence of a small amount of methanol are sufficient to allow the carbonylation of these substrates. Thus, dimethyl ether was chiefly carbonylated to acetic acid (runs 3 and 5 ) and methyl acetate to acetic acid and anhydride (run 4). Homologation Reactions. Methanol homologation to ethanol and higher products has received in theae last years a renewed interest as underlined by the appearance of several papers (Albanesi, 1973; Braca et al., 1978a, 1979; Koermer and Slinkard, 1978; Dumas et al., 1979) and patenta (Dini, 1975; Slaugh, 1977;Braca et al., 1977; Taylor, 1978; Novotny and Anderson, 1978; Pretzer et al., 1979).
01964321/81~1220-0115$01.00/0 0 1981 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1 , 1981
Table I. Carbonylation Experimentsa run catalyst promoter substrate mmol p c o , initial, atm P H ~ initial, , atm solvent time, h
1
2
3
4
5
Ru(CO),I, HI MeOH 74 2 160
Ru(CO),i,b NaI MeOH AcOMe 370 187 200
Ru(Acac),C CHJ Me ,O 122 140 10 toluened 32
Ru(Acac),' CHJ AcOMe 125 140 10 toluened 32
Ru(CO),12 HI Me20 MeOH 103 30 167
77.8 4.1
27.5 6.5 4.5
23.8
2.5 15.0
9.0 2.0
10.0 4.0
22
acetyl groups ethyl groups hydrocarbons
56.0 50.0 59.0 200.0 176.0 16.0
H2
cog H2O
MeOH Me,O AcH AcOH EtOH AcOMe AcOEt heavy products CH, CO t 2 H2h substrate conversioni
3.5 products, mmol 210.0 26.6 24.0 7.4 80.0 66.0 selectivity, %e
("/.I
methyl groups accountability, %
1.1 3.7 10.2 3.7 traces 9.2 9.8 MeOH (94) 90.2
0.6
1 .o 4.6 31.0 1.2 39.0 12.0 traces 7.0
13.0 traces 81.1 5.9 traces traces
16.4 traces 5.8
MeOH AcOMe (92.5) (-1 100
Me20 (57) 100
AcOMe (32) 96
59.2
toluened 8
77.2/
5.0 traces 84.5 traces tracesg Me,O MeOH (18) (60) 99
Catalyst concentration, 1.5 g of Ru/L of solution; I/Ru, 10;T, 200 "C. Catalyst concentration, 3.6 g of Ru/L of solution. Ru(Acac), is ruthenium(II1) tris(acety1acetonato). It is known that Ru(Acac), reacts with CO and H, to give Ru,(CO),, (Braca et al., 1968). 25 mL. e Calculated as [(product, mol) (no. of methyl groups)] /I:[(reagents converted, mol) (no. of methyl groups)]; water and carbon dioxide were excluded as products. Acetic acid + acetic anhydride. g Other products: CHJ (9.5%)and HCOOMe (1%). Coming from MeOH decomposition and calculated as (MeOH not accounted, mol)/I:[(reagents converted, mol) (no. of methyl groups)]. {[(substrate, mo1)M - (substrate, m o l ) m ] / (substrate, m o l ) M } x 100.
Ruthenium iodide systems are catalytically active for methanol homologation, but the selectivity to ethanol is low (max. 60%) owing to the side-formation of ethers, acetic esters, and hydrocarbons (Braca et al., 1979). Moreover, we recently reported that systems based on ruthenium carbonyls and iodide promoters are effective catalysts, under the reaction conditions described in the previous section, for the simultaneous carbonylation and homologation of dimethyl ether (eq 4) and for the homologation of methyl acetate to ethyl acetate (eq 5) (Braca et al., 1978b, 1979). CHBOCH, + 2CO + 2Hz -.+ CH3COOC2H5 +HzO (4) CH3COOCH3 + CO + 2H2 CH3COOC2Hb + H2O ( 5 )
-
These reactions can be carried out in inert solvents such as hydrocarbons, but the best results are obtained in acetic
acid-methyl acetate medium. In this last case, however, in the presence of free HI, dimethyl ether can react with acetic acid giving methyl acetate and water (eq 6) or it can be hydrolyzed to methanol (eq 7; K473K = 0.19). CH,OCH, + 2CH3COOH -* 2CH3COOCH3 + HzO (6) CH3OCH3 + HzO 2CHBOH (7)
-
These reactions, nevertheless, do not represent a real loss of substrate since methyl acetate and methanol can be further activated for successive carbonylation and homologation reactions. It was also recognized that acetic acid can be directly hydrogenated to ethyl derivatives (ethanol, ethyl acetate) under the reaction conditions (T,
200 "C; P, 250 atm; CO/H2, 1/1; catalyst, Ru(C0)J2 1.5 X M; NaI/Ru, lo), but this reaction (conv. 4% in 5 h) accounts only for about 1&20% of the total ethyl derivatives produced starting from dimethyl ether in acetic acid-methyl acetate medium. Thus, to gain better information on the influence of the different reaction variablea on the synthesis of ethyl acetate from dimethyl ether and to clarify the mechanism of these type and concentration reactions, the effect of Pco, PHz, of iodide promoter and proton supplier on reaction rate and selectivity was investigated. Effect of PHI.The effect of the hydrogen partial pressure on the reaction rate and selectivity was investigated by carrying out a series of isochronous experiments using CHJ or NaI as promoters (I/Ru molar ratio = lo), at 200 "C in the 25-190 atm PH2range. The initial dimethyl ether/methyl acetatelacetic acid ratio was 1/1/1 and the PCOpressure was maintained constant at 70 7 2 atm. As shown from the results reported in Figures 1and 2, the catalytic activity with CH31as promoter is about three times higher than with NaI. To avoid a partial decomposition of dimethyl ether to CO + Ha, the PHzat the reaction temperature must be maintained at >lo0 atm with CH31and >150 atm with NaI; moreover, the reaction rate and the selectivity of the homologation and hydrogenation products increased in general by increasing the
PH h e behavior of the two promoters, CH31and NaI, with changing PHpdiffers in the following points.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 117 0
MepO to PeaCticn pmducts
0
MeBO to CO+H2 AcOH
!-z +
3
I
0 0
Me,O to rescticn products MepO to CO+H, AcOMe
A EtOH 0 0
AcOEt AcOH hydrocarbons
A EtOH
o AcOEt o AcOMe hydrocarbons 0
l0I
'4 .15.
o acyl derivatives o derivatives hydmcarbons
et41
4
e t i y derivatives hydmcarbons
4
Q
\
/
B 0
pH2 1atm1.
Figure 1. Effect of hydrogen pressure on carbonylation-homologation of dimethyl ether: catalytic system RU(AC~C)~/CHJ. Reaction conditions: Ru(A~ac)~, 1.5 X M; CH,I/Ru = 10; AcOMe, 119 m o l ; AcOH, 120 m o l ; Me20,120 m o l ; T, 200 O C ; Pco at the reaction temperature, 70 7 2 atm; time, 8 h.
(i) With CH81. The production of new acyl groups (simple carbonylation products) decreases with increasing PH, and practically ceases at PH,higher than 140 atm. The conversion of acetic acid strongly increases with PH, (Figure lA), indicating that the reduction of this substrate to ethanol and ethyl acetate predominates at high PH,.Thus, the production of the same amount of new acyl and ethyl groups (corresponding to the transformation of dimethyl ether to ethyl acetate) was obtained at PH,of about 30 atm. However, under these conditions, a substantial amount of dimethyl ether was both transformed into methyl acetate via reaction with acetic acid and decomposed to CO and Hz (=lo%). The hydrogenation of the substrates to hydrocarbons increases with PH,; (ii) With NaI. The production of new acyl groups increases and the formation of hydrocarbons decreases with increasing PH2.No consumption of acetic acid, but rather conversion of methyl acetate to products was observed. Substantial improvements in ethyl acetate selectivity up to 60-70% and decrease in the formation of methyl acetate and hydrocarbons are possible operating with different Me20/AcOMe/AcOH initial ratios and increasing the Pco (Braca et al., 1979). Effect of Pco. The effect of the carbon monoxide partial pressure was studied in a series of experiments in which, other conditions being equal, the Pco was varied in the range 6-135 atm at 200 "C. The PHsw a ~kept constant at 140 atm, in order to have a high homologation
o
20
40
w
m
io0
110
140
iw
1w
c
pH, ~ a t m ?
Figure 2. Effect of hydrogen pressure on carbonylation-homologation of dimethyl etheE catalytic system Ru(A~ac)~/Nd. Reaction conditions: see Figure 1; NaI/Ru = 10.
reaction rate and to avoid the dimethyl ether decomposition to CO and Hz. As shown in Figure 3, the dimethyl ether conversion and the production of homologation (ethyl groups) and hydrogenation products (methane and ethane) exhibit a maximum at 20-40 atm PCO,similar to that observed in the hydroformylation of olefins (Braca et al., 1970; Schulz and Bellstedt, 1973; Cesarotti et al., 1978) and in the synthesis of esters from olefins, CO, and alcohols (Sbrana et al., 1972) in the presence of ruthenium carbonyls as catalysts. Moreover, in the experiments with PCOC 70 atm the amount of acyl groups in the products is lower than in the charged reagents, since under these conditions the hydrogenation rate was higher than that of the carbonylation. Only by operating with Pco higher than 150 atm the carbonylation and homologation reactions took place with comparable rates and new acyl groups are produced. Unfortunately, the activity of the system for the homologation and hydrogenation reactions decreases with increasing Pco. Effect of Dimethyl Ether Concentration The effect of the dimethyl ether concentration (Figure 4) on the reaction rate and selectivity was investigated in a series of isochronous experiments in which only the initial MezO/AcOH ratio was changed. The results show that the rate of the simple carbonylation reaction increased with increasing dimethyl ether concentration, whereas the reactions which produce ethyl groups and hydrocarbons were not affected by the initial dimethyl ether concentration.
118
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 Q,
9
."E E
w9 c1
h
9
B
6 w a
'Ol
C
."a 3R C 0
D 3 0
EtGH AcOEt AcOMe hydrucarbons
e u"
Q,
5
.E o acyl derivatives
a
hydrocarbons
.15.
--
c 0,
z pCc
IatmP
Figure 3. Effect of carbon monoxide pressure on carbonylationhomologation of dimethyl ether. Reaction conditions: see Figure 1; PH2at the reaction temperature, 40 =F 2 atm.
Influence of the Type and Concentration of Iodide Promoter and Proton Supplier The effect on the rate and selectivity of the reaction of the type of iodide promoter which can be supplied as hydrogen iodide, alkyl iodide, or alkali iodide in acetic acid solution was studied operating at the same I/Ru ratio in a series of isochronous experiments (Table 11). The catalytic activity for the carbonylation and homologation reactions (formation of new acetyl and ethyl groups, respectively) increased in the order HI > CH31> NaI. The presence of HI favors the simultaneous carbonylation and homologation of dimethyl ether to ethyl acetate, whereas ita absence favors the reduction of the acetyl to ethyl groups with respect to their formation by carbonylation of methyl groups (run6). At the same time, with HI, a large amount of dimethyl ether was converted into methyl acetate by direct reaction with acetic acid (eq 6). On the contrary, in the presence of NaI, new acetic acid is produced although it could react under these conditions with dimethyl ether to give methyl acetate [at 200 "C, under nitrogen for 12 h in the presence of NaI (3.3 mmol), dimethyl ether (50 mmol) was converted into methyl acetate (18%) by reaction with acetic acid (260 mmol)]. In fact, methyl acetate in these conditions is a more reactive substrate than dimethyl ether, as shown by its higher conversion to products (run 6). The higher promoting activity of HI with respect to CH31or CH31+ HCl when used at the same I/Ru ratio (Table 11, runs 7, 8, 9, and 10, 11) for the production of new ethyl groups confirms the peculiar role in the catalytic
Ind. Eng. Chem. prod. Res. Dev., Vol. 20, No. 1, 1981 118 0
MepO
7 AcOH
50
0
AcOEt AcOMe
0
acyl derivatives
0
I
A EtOH 0 0
AcOEt AcOMe hydrocarbons
t
I ethyl derivatives A hydrocarbons 0
w
El I
P
ethyl derivatives
I ' OO
0
0.6
1.0
Me,O/
1.I
1D
20
A hydrocarbons
io
do
s
160
120
140
160-
2.6
l/Ru
AcOH
Figure 4. Effect of the initial dimethyl ether concentration. Reaction conditions: see Figure 1;P(WHa, at the reaction temperature, 160 atm; CO/H2 = 1/1.
Figure 5. Effect of I/Ru ratio on carbonylation-homologation of dimethyl ether. Reaction conditions: see Figure 1;PCOat the reaction temperature 100 7 2 atm; PH3at the reaction temperature, 140 7 2 atm.
cycle of HI which probably acts both as a proton and iodine supplier. Contrary to that observed for the rhodium-catalyzed carbonylation of alcohols and esters which are fiist order in methyl iodide concentration (Forster, 1976; Brodzki et al., 1977; Kuckertz, 1974), the rate of the simple and reductive carbonylation of dimethyl ether in the presence of ruthenium iodide catalysts did not increase proportionally with the concentration of the iodide promoter (Figure 5). In a series of isochronous experiments carried out by increasing the initial CH31/Ru ratio from 10 to 500 and by maintaining constant all other variables, the dimethyl ether conversion showed only a slight maximum at I/Ru = 50 to 100; the selectivity to ethyl acetate decreased and the productions of methyl acetate and hydrocarbons slightly increased. It is also of interest to remark that the ruthenium catalytic systems displayed the maximum catalytic activity with smaller amount of iodide promoter (I/Ru = 25-75) than the corresponding rhodium systems in the methanol carbonylation (I/Rh = 100-500); however, due to the higher activity of rhodium systems the catalyst concentration is typically about 1/3 as that used in the ruthenium experiments. The presence of an acid component was recognized as essential since in the absence of a proton donor the reaction did not take place. The acidity of the reaction medium markedly affected the conversion of dimethyl ether which increased from the 5% value obtained by operating
in the presence of acetic acid and NaI (run 6; final pH = 3.3), to 34% operating with HI (run8; final pH = 1.5). In the same manner increased also the total amount of carbonylation, homologation and hydrogenation products. However, it must be pointed out that also the formation of methyl acetate from dimethyl ether and acetic acid was favored at low pH. Ruthenium Species Involved in the Catalytic Cycle. Some insight into the ruthenium species involved in the catalytic cycle came from the study of the metallic complexes formed, under the reaction conditions, starting from different ruthenium precursors or recovered from the final reaction mixtures (Scheme I). Tris(acetylacetonato)ruthenium(III) or dodecacarbonyltrirutheniumwere treated at 200 OC, in n-heptane with CO and H2 ( l / l ; 200 atm) in the presence of CH31 (I/Ru = 10) and the resulting solutions analyzed in a high pressure-high temperature spectrophotometric cell. The IR spectra, recorded at 150 "C and 100 atm, indicated the presence of two main iodocarbonyl ruthenium species in equilibrium: cis-Ru(CO)J2 [YCO 2161 (m), 2105 (s), 2096 (s), and 2066 (m) cm-' (Calderazzo and L'Eplattenier, 1967)l and [RU(CO)~I,]-[vc0 2116 (8) and 2042 (8) cm-', comparable to the 2120 and 2050-cm-' values reported in literature for [Ru(CO),13]- in HC1 (as) (Colton and Farthing, 1971)l. Moreover, the product recovered by evaporating the solvent at room temperature contained, in addition to the above reported complexes, a third species corresponding
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Ind. Eng.
Chem. Prod. Res. Dev., Vol.
20, No. 1, 1981
Scheme I. Ruthenium Species Involved in the Catalytic Cycle CO + H2
NaI
;
-
CO
Ru (Acad3
N~~[R~Ico)~I~] Na2C03
Ru~(CO),, CH31
Nal
Ru~(CO)~~
co
;
(HI)
1
AcOH
(15 atm) ;
100°C
-
Na+
ia~-Na'[Ru(C0)~1~]
-
CO
+ H2 Nal;
i
200'~
Ru(Acac)3
AcOH
Scheme 11. Reaction Conditions and Constituents for the Dimethyl Ether and Methyl Acetate Carbonylation or Homologation. (Solvent:Toluene, 25 mL;Pco, 150 atm; T,200 'C; Catalyst Concentration, 1.5 X M; I/Ru, 10)
-
RUTHEN IUM
SUBSTRATE
PRECURSOR
E-
e
Ru (CO)4 1 2
'ROTON DONOR
-
AcOH
R U ( C O ) ~ I ~ CH I or Nai 3 HI Ru(C0)4 1 2
CH3-O-CH3
G-
Ru(C0) 4 1 2
HI
HI
Ru(C0) 4 1 2
HI
HI
(Acac)
HI
HI
Ru (Acac)
HI
HI
Ru (Acac)
CH31
(8)
Na@udCO)ioIX1
0.03
MeOH
0.03
10
MeOH
0.03
NO REACTION CARBONYLATION + HOMOLOGATION CARBONYLATION ONLY (Run 5 CARBONYLATION + HOMOLOGATION (Run 3)
( 10
N~,[RU~(CO)~~I,] + 2CO 100 "C
NO REACTION
MeOH
HI
Ru(C0)412
EtOH; I8 *C
-
10
to [Ru(CO),12], [YCO 2124 (s), 2052 (s), and 2008 (m) (Johnson et al., 1969)l. The product recovered in the final reaction mixture of dimethyl ether homologation experiments in the presence of NaI as promoter in acetic acid was isolated in diethyl by ether solution and characterized as fu~-Na[Ru(C0)~1~] elemental analysis and IR data [VCO 2114 (s), 2045 (8) cm-l a 2122 (in CHCld comparable with the IR absorptions at v (s), 2051 (8) for fu~-Cs[Ru(CO)~1~] (Cleare and Griffith, 1969)J. The single resonance in the 13C NMR (c&, 23 MHz, 6(c&)= 185 ppm) and the two carbonyl IR bands are in agreement with the C3" symmetry of the fuc structure. The Ru(C0)J; anion is present in the reaction mixtures also when either Ru(A~ac)~ or R U ~ ( C Oor) ~Ru(CO)& ~ are used as starting compounds. In order to explain the possibility of formation of Ru(I1) iodo derivatives from Ru(0) carbonyls, the reaction of R U ~ ( C Owith ) ~ ~NaI in acetic acid was studied. An analogous reaction to that reported for Ni(CO), (Fol and Cassar, 1973) occurs in two steps (eq 8 and 9) R u ~ ( C O+ ) ~NaI ~
NO REACTION
25
CH31
Hi
TYPE O F R E A C T I O N
NO REACTION
CH31
RU
ACTIVE HYDROGEN C O M P O U N D , mol
HI
Ru (CO)4 1 2
R u ( C 0 ) 4i2
CH 3COOCH3
IODIDE PROMOTER
AcOH Na[Ru(CO),I,] + H2 + AcONa (9) The product of the first step was not isolated, whereas
'
N O REACTION APPARENTLY NO REACTION ON METHYL ACETATE NO REACTION CARBONYLATION + HCMOLOGATION (Run 4 )
Na[Ru(CO),I,] was isolated and characterized and stoichiometric amounts of AcONa detected. Thus Ru(0) is oxidized to Ru(II) by acetic acid; this is reminiscent of the reaction of ruthenium(0) complexes with Schiff bases to give the corresponding ruthenium(I1) complexes (Calderazzo et al., 1969). Discussion The reaction conditions and the components of the catalytic system necessary to perform the carbonylation or homologation of dimethyl ether and methyl acetate are summarized in Scheme 11. On the basis of the results obtained the following points were ascertained. The formation of an iodocarbonyl ruthenium(I1) species is essential: this can be produced "in situ" from different ruthenium precursors according to path I or I1 of Scheme I. When Ru(A~ac)~ was used as a precursor, hydrogen was necessary to reduce the ruthenium derivative to Ru(I1) species (experiments G and M against L). Preformed neutral iodocarbonyl ruthenium derivatives (i.e., Ru(CO)&) are catalytically inactive even in the presence of hydrogen and organic acids when the iodide promoter was absent (expt A). However, reaction occurred when an iodide supplier such as NaI or HI was present. This suggests that in the catalytically active species the I/Ru(II) ratio must be higher than 2. The presence of HI as proton donor is essential: this can be initially charged or formed from CH31and H2 (expt E, F,G ) . In the absence of free HI, even starting from
Id. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 121
R u ( C O ) ~and I ~ CH31 or NaI, the reaction did not occur (expt B and D). Contrary to the Rh-catalyzed methanol carbonylation, methyl iodide is not the organic intermediate into which methanol or dimethyl ether must be transformed to interact with the iodocarbonyl ruthenium catalyst. In fact, it does not promote the reaction even operating with Ru(COl4I2in the presence of a hydrogen active component (CH30H) (expt D). However, it can act as I- and H+ supplier through a partial hydrogenolysis to HI under the reaction conditions. The presence of a hydrogen active compound (water or alcohols) or of H2 is essential: the former to cleave the Ru-acyl intermediate with formation of acid or esters (expt F and I); the latter to produce alcohols by reduction of the acyl intermediates (expt G and M). The fiiding in the reaction mixture at high temperature and pressure of the iodocarbonyl ruthenium species Ru(CO)412and [Ru(CO),IJ, the strong promoting effect of NaI, and the increase of the reaction rate in polar solvents (i.e., organic acids) suggest that the catalytic active species may really consist of an anionic iodocarbonyl ruthenium derivative [Ru(CO),I,]-. The ascertained necessity of a proton donor and the dependence of the rate on the pH of the reaction medium seem to indicate that the activation of the substrate occurs through a protonation step similar to that proposed for the cobalt-catalyzed methanol carbonylation (Mizoroki and Nakayama, 1968) and homologation (Wender, 1976) (eq 10) ROCH3
+
H+
-
I’
ROCH3
[a
(R
= H ; CH3;CH3CO)
(10)
The protonating agent can be: (i) HI initially added or formed by hydrogenolysis of CH31; (ii) an organic acid (Le., AcOH) used with NaI as promoter; (iii) a hydrido-iodocarbonyl ruthenium derivative formed by reaction of ruthenium carbonyls with HI. Recently the HI addition to a rhodium carbonyl complex with formation of a hydrido-iodocarbonyl rhodium derivative was demonstrated in a mechanistic study of the rhodium iodide catalyzed carboxylation of ethylene to propionic acid (Morris and Johnson, 1978). The protonated substrate, which may be the counterion of an anionic iodocarbonyl derivative, could produce a methyl-ruthenium intermediate according to eq 11 (Wender, 1976) [Ru(CO),I,]-
I’
ROCH3
[a
-
RuCH~(CO),-II,
+
ROH
+ CO
(11)
The ionic intermediate may also be produced by interaction of a neutral iodocarbonyl ruthenium derivative (i.e., Ru(CO)J2) with a protonated substrate containing I- as counterion (eq 12). In this way a further iodide ligand
necessary to promote the reaction can be introduced into the coordination sphere of the metal. The CO insertion into the metal-carbon bond of the methyl-ruthenium intermediate (eq 13) followed by hydrolysis or alcoholysis of the acyl derivative (water or al-
cohols are necessary even for the simple carbonylation of dimethyl ether to acetic derivatives) according to the general accepted mechanism for the carbonylation reaction (Piacenti and Bianchi, 1977), produce acetic acid and esters (eq 14) RuCH3(CO),-1IY + CO Ru(CH~CO)(CO),-~I, (13) Ru(CH~CO)(CO),-,I,
+
