Rhodium-Catalyzed Reductive Carbonylation of Methanol - Advances

22 Rhodium-Catalyzed Reductive Carbonylation of Methanol

Downloaded by DICLE UNIV on November 6, 2014 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch022

Kenneth G . Moloy and Richard W. Wegman Union Carbide Corporation, P.O. Box 8361, South Charleston, WV 25303-0361

Rhodium catalyzes the reductive carbonylation of methanol to acetaldehyde if the appropriate diphosphine ligands are employed. The reductive carbonylation of methanol has been studied for nearly 50 years. Cobalt catalysis has dominated this area from its beginning, and significant improvements were made through the use of various promoters and cocatalysts. However, in all cases the reaction conditions are extreme (4000-8000 psi, 175-220 °C). The new rhodium catalyst gives rates and selectivities comparable to the best cobalt catalysts (100-200 turnovers per hour, 80-90%) but at much lower temperature and pressure (140 °C, 1000 psi). Addition of ruthenium to this catalyst results in the in situ hydrogenation of acetaldehyde and production of ethanol. The catalyst is very robust, and crystalline acetyl complexesRh(diphosphine)(COCΗ )(I) are isolated quanti tatively after catalysis. These complexes can be reused as catalysts with no loss in catalyst performance and again be isolated in very high yield. Mechanistic studies suggest that the acetyl complexes are important intermediates in the catalytic reaction. 3

2

V ^ O B A L T - B A S E D C A T A L Y S T S H A V E D O M I N A T E D research activity in the area

of reductive carbonylation of methanol for nearly 50 years (1-8). Although other metals also catalyze these reactions, they are generally inferior to cobalt-based catalysts. CO/H

2

CH OH % CH3CHO + H 0 (methanol hydroformylation) 3

2

0065-2393/92/0230-0323$06.00/0 © 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

(1)

324

H O M O G E N E O U S TRANSITION

CO/H CH3OH

METAL CATALYZED

REACTIONS

2

i CH CH OH 3

2

+ H 0

(2)

2

(methanol homologation) Improvements in cobalt-based catalysts have been made through the use of various cocatalysts and promoters such as iodide, phosphines, and transition metals. Iodide, by far the most important of this group, is almost always employed. In spite of significant advances in catalyst performance, high pressures and temperatures are required. These traditional catalysts are usually operated at pressures of 4000-8000 psi and temperatures of 175-200 ° C . The high-pressure requirement poses obvious difficulties. T h e


high temperatures employed often result in the formation of heavy byproducts via aldol condensation reactions of acetaldehyde. A catalyst exhibiting high activity at pressures below 1000 psi and at lower temperature would be of significant practical interest. We

became interested in the homologation of methanol as part of a

Department of Energy contract to investigate the synthesis of fuel alcohols from

synthesis gas. The drawbacks of the traditional cobalt catalysts

prompted us to consider other metals. An obvious choice is rhodium. Both rhodium and cobalt catalyze a variety of carbonylation reactions, such as olefin hydroformylation and the carbonylation of alcohols to acids (9, 10). Rhodium catalysts are significantly more active than their cobalt counterparts, allowing reactions to be conducted at much lower extremes of temperature and pressure. Previous studies of the use of rhodium in the reductive carbonylation of methanol were not encouraging (II). The reason is quite simple. Iodide is a standard promoter in the cobalt-based catalysts. In the presence of iodide and C O , rhodium is an extremely proficient catalyst for the carbonylation of methanol to acetic acid. O f course, this property forms the basis for the well-known Monsanto acetic acid process (12-15). To devise a strategy to divert the carbonylation to reductive carbonylation, it is instructive to consider the mechanism of the carbonylation reaction. Fortunately, the mechanism of the Monsanto chemistry has been studied extensively (12-15). It is summarized in Scheme I. A

key step

Rh(CO) (COCH )I 2

3

is the reductive 3

elimination of acetyl

iodide from

. This elimination is facile, occurring rapidly at tem-

peratures as low as 25 °C (oxidative addition of C H I to Rh(I) is rate-limiting). 3

If rhodium is to be employed in a reductive carbonylation scheme, it would seem that acyls much more stable with respect to reductive elimination of acid iodides or other nonproductive reactions would be required. In this way interception of the acyl by hydrogen or other reductant might be possible. Work by Slack et al. (16) and McGuiggan et al. (17) suggests a ligand environment to achieve this interception. They showed that chelating diphosphines [e.g., P h P ( C H ) P P h ] impart high stability to rhodium acyl 2

2

3

2

complexes. Thus, the five-coordinate complexes (1) resist reductive elimi-


22.

MOLOY & WEC;MAN

325

Reductive Carbonylation of Methanol

Rh(CO) l 2 2

Rh(CO) (CH )l 2

Rh(CO)(COCH )l -


3

3

3

J

3

+ CH C0 CH • HI

CH3COI + CH3OH

3

CH3OH + HI

2

3

**CH UH 0 3

2

Scheme I. Mechanism of the carhonyhtion reaction. (Reproduced from reference 22. Copyright 1989 American Chemical Society.) nation and decarbonylation even at elevated temperatures. To our knowledge, the reactivity of these acyls with respect to reducing agents has not been investigated, although they have been reported to catalyze methanol carbonylation (18). The stability of these complexes prompted us to attempt the reductive carbonylation of methanol with rhodium, iodide, and diphosphine ligands (19-22).

COR R = phenyl, alkyl

1 Experimental Procedure The following is a general procedure for conducting catalysis experiments. In a nitrogen-filled glove box a 100-mL Parr autoclave was charged with Rh(CO) (acac) (0.26 g, 1 mmol; acac is acetylacetonate) and methanol. P h P ( C H ) P P h (0.41 g, 1 mmol) was slowly added. When gas evolution ceased, R u C l ( H 0 ) (0.82 g, ~4 mmol) was added; the reactor was sealed and removed from the glove box. Next the reactor was connected to a stirrer and high-pressure gas manifold. The apparatus was flushed with synthesis gas (2:1 H : C O ) by pressurizing to 30 psig and venting three times. C H I (2.5 mL, 40 mmol) was added via syringe; the reactor was sealed and pressurized to 400 psig. The reactor was heated to 140 °C and then pressurized to 975 psig. The reaction was monitored by gas uptake. After each 50-psig drop, the reactor was repressurized to 975 psig. After 2.5 h the autoclave was cooled to 18 °C and the gas was vented through a trap cooled by dry ice. 2

2

2

3

2

3

2

t

2

3


326

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

Analysis of the liquid products (gas chromatography) showed the presence of C H 3 C H O , C H C H O H , and C H C 0 H , in addition to unreacted methanol. The selectivity to reductive carbonylation products was 80%. This calculation takes into account the many ether-, ester-, and acetal-producing equilibria that are established in these reaction mixtures, as is traditional in studies of this chemistry (IS). Table I shows the composition of a typical product mixture resulting from these reactions. 3

2

3

2

Table I. Representative Product Distribution Product

Condition A"

Condition B

1.9 0.3 0.2 68.9 6.1 1.2 4.5 6.2 10.2

12.1 1.7 0.4 55.5 8.1 0.7 2.3 4.3 14.5

CH3CHO

CH CH(OCH ) CH CH(OCH )(OCH CH ) CH CH OH CH CH OCH (CH CH ) 0 CH C0 CH CH CH C0 H CH C0 CH


3

3

3

2

3

3

2

3

2

3

2

3

3

2

3

2

3

2

3

2

2

2

3

3

b

NOTE: All values are given in mole percents. All experiments were conducted with 40 mL of C H , O H and 40 mmol of CH,I, at 100 psi of 2:1 H - C O and 140 °C. "Condition A: catalyst, Rh, Rh(CO) (acac) (2.0 mmol); ligand, PhaPiCH^PP^ (2.0 mmol); Ru, RuCl, (4.0 mmol); time, 2.5 h. 'Condition B: catalvst: Rh, Ph P(CH»WPPh (2.0 mmol); no ligand; Ru, (CHJ NRu(CO).J., (4.0 mmol); time, 2.0 h. 2

2

2

2

4

The rate of methanol conversion to these products was 3 mol of C H O H per liter of catalyst per hour, which corresponds to a turnover frequency of ~ 120 per hour. Analysis of the gaseous products showed small amounts of C H and trace amounts of C 0 . The autoclave also contained an orange crystalline material, which was shown by N M R and IR spectroscopy and elemental analysis to be Rh(Ph P(CH ) PPh )(COCH )(I) . IR analysis also showed the presence of Ru(CO) (I) " (23-25). This species is formed regardless of the ruthenium source employed and therefore is a thermodynamic sink under these reaction conditions. The most likely counterion is H (23-25). Further details regarding experimental procedures and characterization of the complexes described here may be found elsewhere (22). 3

4

2

2

3

2

3

2

3

2

3

+

Results and Discussion Our initial experiments involved the addition of diphosphine ligands to methanol suspensions of Rh(CO) (acac). When gas evolution ceases (displacement of C O by diphosphine), the reactor is charged with C H I and synthesis gas. The resulting solution contains a very active catalyst for the hydroformylation of methanol to acetaldehyde. More important, the catalysis occurs at much lower temperatures and pressures than those required for cobalt catalysts. Thus, for the diphosphine P h P ( C H ) P P h , acetaldehyde is produced at rates of 4-6 mol L " h " and 80+% selectivity at 1000 psi total pressure and 2

3

2

1

2

3

2

1


22.

MOLOY & WEGMAN

327


140 °C. The remaining liquid products are composed entirely of acetic acid (as the free acid and its methyl ester). Small amounts of methane are also produced, but generally no more than 5 mol %. Methanol can be hydrogenated to methane in the presence of rhodium and iodide (26). Reaction Selectivity. As our initial interest in this topic was the production of higher alcohols via synthesis gas chemistry, we investigated methods to hydrogenate the acetaldehyde produced in the hydroformylation reaction to ethanol. Addition of ruthenium to the rhodiumdiphosphine-iodide catalyst results in the homologation of methanol to ethanol (27-31). As shown in Figure 1, the relative amount of C H C H O and C H C H O H is directly related to the amount of ruthenium employed. Moreover, the total selectivity (defined as the sum of C H C H O and C H C H O H ) remains constant with changing ruthenium concentration. This result shows that rhodium governs the overall reaction selectivity and ruthenium serves as an in situ reductant.


3

3

2

3

3

2

The reaction selectivity is highly dependent on the diphosphine ligand. Although a wide spectrum of diphosphines has been examined (Table II), to date we have found little correlation between ligand structure and catalyst performance. The best results are obtained with diphosphines related to P h P ( C H ) P P h , where selectivities greater than 80% can be achieved. In contrast, shortening or lengthening the diphosphine bridge or replacing the phenyl groups with alkyls results in significant losses in selectivity. As expected, monodentate phosphines (e.g., PPh ) give very poor results. 2

2

3

2

3

80

0^ 0.00

,

, 0.02

,

, , j i , 0.04 0.06 0.08 [Ru(CO)3l3-], M

1

1 0.10

1

1

0.12

Figure 1. Product distribution as a function of ruthenium concentration. Key: •, CH CHO; • , CH3CH2OH; ·, CH CHO + CH CH OH. Conditions for all experiments: 1000 psi of 2:1 H -CO, 140 °C, 0.0188 M 2a. (Reproduced from reference 22. Copyright 1989 American Chemical Society.) 3

3

3

2

2


328


Table II. Selectivity as a Function of Diphosphine Ligand Diphosphine

Selectivity (mmol)"

Ph P(CH ) PPh (CH ) P(CH ) P(CH ) (p-tol) P(CH ) P(p-tol) (p-ClC H ) P(CH ) P(p-ClC H ) (CH )PhP(CH ) PPh(CH ) (c-hex) P(CH ) P(c-hex) (CH CH ) P(CH ) PPh Ph P(CH ) PPh Ph P(CH ) PPh (p-tol) P(CH ) P(p-tol) Ph P(CH ) CH(CH )PPh Ph P(CH ) C(CH ) PPh 2PPh 2

2

3

3

2

2

3

2

6

5

3


2

2

2

2

2

3

2

3

3

3

6

3

2

2

4

2

2

2

2

2

2

2

2

2

3

2

2

2

2

2

2

2

2

2

5

3

2 3

2

80 35 54 29 24 34 54 7 39 26 65 71 6

2

2

2

2

3

3

2

2

2

3

NOTES: All experiments were conducted at 140 ° C , 1000 psig of 2:1 H - C O . Ph, phenyl; toi, tolyl; and c-hex, cyclohexyl. "Selectivity to C H C H O - C H C H O H ; liquid products only. 2

3

3

2

Stability. For most diphosphines, an orange crystalline product is typically present at the end of these experiments. W e have unequivocally identified a number of these complexes as the acetyl complexes 2a-2f. These same complexes led us to investigate the use of diphosphine ligands. The fact that they are isolated in quantitative yield at the end of these experiments further demonstrates their stability. The complexes so isolated, or prepared by alternative synthetic routes, can be employed as catalysts with no change in rate or selectivity and again be isolated unchanged in essentially quantitative yield. Some phosphine ligands are quaternized by C H I under the reaction conditions, particularly monodentate phosphines such as P P h . This situation 3

3

COCH

3

Ph PCH C(CH3)2CH PPh2

2c

Ph P(CH ) CH(CH )PPh

2

2d

2

2e

2

2

2

2

2

2

3

(p-tol) P(CH ) P(p-tol) 2

2

3

(p-CIC H ) P(CH ) P(p.CIC H ) 6

5

2

2

3

6

5

2

2f


22.

MOLOY & WEGMAN


329

is easily detected by the presence of R h ( C O ) I (IR) or C H P R ( P NMR) in the product solutions. Diphosphine complexes 1 are very resistant to this degradation, showing no evidence of quaternization under reaction conditions after the longest times we have investigated to date (11 h). 2

2

3

3

+

31

Reactivity of Complexes 2. The stability and isolation of what may be a key reactive intermediate in the catalysis provided us with a unique opportunity to investigate the mechanism of this chemistry. A reasonable assumption is that the acyls are converted to acetaldehyde. This possibility was tested by investigating the reaction chemistry of complex 2a. We first examined the reaction of 2a directly with H (120 psi, 120 °C); it does indeed yield acetaldehyde. The hydride complex 3 is also produced in this reaction, as shown in eq 3. Spectroscopic data indicate that this complex adopts a square-based pyramidal structure with an apical hydride, analogous to acyl complexes 2. X-ray crystallography, done in collaboration with J. L . Peterson (West Virginia University), shows that the structure of complex 3 is a centrosymmetric dimer with bridging iodides. Both the hydride and acetaldehyde are formed in essentially quantitative yield.


2

CH OH

2a + H — * - * Rh(PPh (CH ) PPh )(H)I + C H C H ( O C H ) 2

2

2

3

2

2

3

3

(3)

2

3

Other pathways to convert the acetyl ligand of 2 to acetaldehyde were also investigated. One likely possibility is via the protonation of 2 (32). Given the acidic nature of these reaction solutions, this possibility seemed reasonable. However, treating 2a with the powerful acids H I or C F S 0 H does not produce acetaldehyde. In fact, no reaction at all occurs. IR monitoring of C H C 1 solutions of 2a in the presence of C F S 0 H shows no shift in the acetyl carbonyl vibration at 1701 cm" . This result is perhaps not surprising in that little backbonding from Rh(III) to the acetyl ligand is expected. Thus the contribution of a resonance form such as 5 is minimized. This result seems to rule out the involvement of acid in the production of acetaldehyde. 3

2

2

3

3

3

1

4

5

Reaction of 2 with hydride (e.g., R h - H ) is also a potential route to acetaldehyde. However, treatment of 2 with a variety of hydridic ( R S n H , R B H L i ) reagents does not lead to liberation of C H C H O . Complexes 2 are also unreactive with hydride complex 3. Further, bimolecular reactions of 2 with a rhodium hydride is inconsistent with kinetic results (vide infra). 3

3

3


330


In addition to the possibility that the acyl complexes are directly re sponsible for the production of acetaldehyde, the cis arrangement of acetyl and iodide ligands in complexes 2 suggests that reductive elimination of acetyl iodide is also possible. In the presence of C H O H or H 0 this reaction would immediately result in the formation of acetate and therefore explain the formation of acetic acid. In view of the preceding discussion, however, it was not surprising to find that thermolysis of complex 2a in C H O H (140 °C, Ν , 45 min) leads to only trace amounts of C H C 0 H (as the free acid or its methyl ester) and recovery of 2a. This rate is far too slow to account for the formation of C H C 0 H during catalysis. If the reaction is conducted under a C O atmosphere, however, C H C 0 H is produced much more rapidly. In fact, this reaction leads to the catalytic carbonylation of C H C 0 H , and complex 2a is again recovered intact (18). No additional C H I was added. This result not only confirms the possibility that 2 can be used to generate C H C 0 H , but also demonstrates that 2 is a competent catalyst for this transformation. The fact that C H C 0 H is observed only upon treatment of 2 with C O suggests that C O coordination is required prior to reductive elimination of C H C O I . 3

2

3

2

3

2


3

3

2

2

3

2

3

3

2

3

2

3

Kinetic Studies. These studies suggest a reaction sequence in which complexes 2 are intimately involved in the selectivity-determining step. Thus, conversion of the acetyl ligand to acetaldehyde has been demonstrated by reaction with H . Alternatively, the catalytic formation of C H C 0 H is possible if 2 is treated with C O in the presence of C H O H . The stability of complexes 2 (relative to other reaction intermediates) further indicates that they are thermodynamic sinks. This conclusion predicts that complexes 2 are directly involved in the overall rate-determining step. 2

3

2

3

The reaction studies discussed suggest that the rate- and selectivitydetermining step involves the reaction of 2 with either H or C O . If this is true, then the reaction rate should be first-order in 2. No other chemistry should appear in the rate law. Thus, the reaction should be zero-order in iodide. A brief kinetic analysis was undertaken to test these predictions. 2

The rate dependence on 2a was first investigated. The kinetic analyses were performed by monitoring gas uptake as a function of time. Uptake is typically linear for the first 40-50 min; at longer times significant curvature occurs. This time span corresponds to ~5% C H O H conversion and is there fore suitable for determining reaction orders by the method of initial rates (33). Reaction orders were determined by measuring the initial gas uptake rate vs. time at different concentrations of 2a. Concentrations were varied by 1 order of magnitude. 3

Figure 2 illustrates the results from these experiments. The data show that the reaction is first-order in 2a over the concentration range 0-0.03 M . This result is consistent with the reaction scheme discussed. However, the rate law abruptly changes to zero-order in [Rh] at concentrations greater


22.

331


MOLOY & WEGMAN

30

Ε CO


Q.

0.00

0.02

0.04

[Rh],

0.08

0.06

0.10

M

Figure 2. Initial gas uptake rate dependence on rhodium concentration. Con ditions for all experiments: 1000 psi of 2:1 H -CO, 140 °C, 0.10 M (CHahNRuiCOhh, 10 M CH I. (Reproduced from reference 22. Copyright 1989 American Chemical Society.) 2

3

than ~0.03

M . The explanation for this observation is straightforward: at

concentrations greater than 0.03 M the solubility limit of 2a is exceeded. We measured the solubility of 2a at 140 °C in C H O H ; it corresponds exactly 3

with the change in rate law. This observation is very significant. Although 2a was employed as the catalyst charge for these experiments, the experiments in reality only measure the rate as a function of rhodium concentration and do not distinguish among the many possible rhodium species that could be responsible for the observed kinetics. The observation that the rate behavior depends on a distinct physical property of 2 is con sistent with a model wherein this species is involved in the rate-determining step. However, the possibility cannot be ruled out that another species with similar solubility properties is actually responsible for the observed rate behavior. The reaction order with respect to iodide was also measured. Figure 3 shows that over the concentration range 0-1.0

M the rate exhibits a zero-

order dependence on iodide. This result is exactly as predicted. In these experiments rhodium and ruthenium are charged as the iodo complexes 2a and [ ( C H ) N ] [ R u ( C O ) 3 l ] , respectively. Thus, there is an ample reservoir 3

3

4

of iodide available for catalysis. Gas chromatographic analyses of the final reaction solutions show that experiments employing little or no C H I initially 3

have only trace amounts of C H I present at the end of the experiment, and 3

the metal-iodo complexes are recovered intact. This analysis further confirms that very little free C H I is required for catalysis. 3

Although this latter result may further suggest that C H I is not involved 3

in the reaction chemistry whatsoever, labeling studies indicate otherwise.


332


25

c

Ε CL


0

[CH3I],

Μ

Figure 3. Initial gas uptake rate dependence on iodide concentration. Con ditions for all experiments: 1000 psi of 2:1 H - C O , 140 ° C , 0.0188 M 2a, 0.1 M (CH ) NRu(CO) l . (Reproduced from reference 22. Copyright 1989 Amer ican Chemical Society.) 2

3

4

3

3

Thus, a typical reductive carbonylation experiment was conducted with l 3

C H I as promoter. Analysis of the reaction products showed incorporation 3

of the label in both the C H C H O - C H C H O H and C H C 0 H produced. 3

3

2

3

2

The C H I in the final reaction solution was found to be almost entirely C . l 2

3

C H I is apparently converted to products and regenerated under the reaction 3

conditions. This result is consistent with the involvement of iodide as a promoter in this chemistry, although it has no influence on the reaction rate.

Isotopic Tracer Studies.

An additional pathway to C H C H O that 3

must be considered is via hydrogénation of C H C 0 H or its esters. The 3

diphosphine catalyst

2

has been shown to catalyze the carbonylation of

C I I O H . The selectivity to reductive carbonylation products may simply 3

reflect the different ability of various catalysts to hydrogenate the acid or ester. To test this possibility a homologation run was spiked with labeled acetic acid ( C H

3

1 3

C 0 H ) . The resulting products, analyzed by gas chro2

matography-mass spectroscopy, showed no detectable amounts of label in the C H C H O or C H C H O H produced. Diluted label was detected in 3

CH C0 H 3

2

3

2

and its methyl and ethyl esters.

Hydrogénation of acetic

acid-acetate ester is therefore not a viable pathway to the reductive carbonylation products. This result is not surprising in that hydrogénation of esters and acids is difficult. It requires much more extreme conditions than those employed here (34-37).

The Catalytic Cycle.

The catalytic cycle shown in Scheme II is

consistent with the available kinetic, mechanistic, and labeling results. The



22.

MOLOY & WEGMAN


333

Scheme 11. The catalytic cycle.

first step is oxidative addition of C H I to Rh(I); a reasonable possibility is 3

Rh(diphosphine)(CO)I. We tested this theory by preparing R h ( P h P ( C H ) 2

2

n

PPh )(CO)I (n = 2 or 3) and examining their reactivity with C I I I . Oxidative 2

3

addition occurs under mild conditions (25 °C, several hours) and produces a mixture of the two isomeric methyl complexes, as shown in eq 4.

ÇH

C > < J ,

CH I 3

25°C

3

d>Rh

Rhodium-Catalyzed Reductive Carbonylation of Methanol - Advances

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