Homogeneous Transition Metal Catalyzed Reactions - American

26 Rhodium-Catalyzed Carbonylation of Methyl Acetate Joseph R. Zoeller , James D . Cloyd , Norma L . Lafferty , Vincent A. Nicely , Stanley W. Polichnowski , and Steven L . Cook 1

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Downloaded by TROY UNIV on October 16, 2014 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch026

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'Research Laboratories and Tennessee Eastman Acid Division, Eastman Chemical Company, Kingsport, T N 37662 2

In principle, the rhodium-catalyzed carbonylation of methyl acetate involves the types of transformations found in the conversion of methanol to acetic acid, namely the activation of a normally unreactive methyl group, insertion of carbon monoxide, and recombination with the initial leaving group. However, several significant differences become apparent upon switching from the well-documented aqueous methanol carbonylation to the anhydrous methyl acetate carbonylation to acetic anhydride. A mechanism based primarily upon high-pressure kinetic and infrared data is proposed. The operational and mechanistic similarities and differences involved in the carbonylation of methanol to acetic acid and the carbonylation of methyl acetate to acetic anhydride will be discussed, as well as the historical development of the process.

XJLCETIC ANHYDRIDE HAS BEEN GENERATED COMMERCIALLY by the thermal cracking of acetic acid to ketene and subsequent reaction with acetic acid to render acetic anhydride. The Eastman Chemical Company successfully used this process in Kingsport, Tennessee, for more than 60 years. However, our development of a commercial process for the carbonylation of methyl acetate to acetic anhydride (I) has replaced this older technology as the method of choice in the last decade. The concept of carbonylating methyl acetate to acetic anhydride is not new; it was initially demonstrated almost 40 years ago at B A S F (2). However these processes, which used C o , N i , or Fe, were never adequate for commercial purposes. Following the success of the Rh-catalyzed carbonylation 0065-2393/92/0230-0377$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.


378

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

of methanol described by Monsanto (3-8), processes demonstrating com mercially viable rates finally began to appear in the patent literature in the early 1970s. Several patent applications were published in very rapid succes sion by Halcon (9), Ajinomoto (10), Showa Denko (11, 12), and Hoechst (13). The claims in these patents covered all Group VIII metals, but clearly favored Rh. In a similar program we had narrowed our catalyst choices to N i , Pd, and Rh. Negotiations with Halcon to combine our technologies ultimately resulted in the merger of research and development efforts in 1980. This agreement was followed within a year by the announcement of Eastmans intentions to construct a plant in Kingsport, Tennessee, for the carbonylation of methyl acetate to acetic anhydride. The plant bore a nameplate capacity for 225 thousand metric tons (KMT) per year of acetic anhydride. Production began in March 1983. We have successfully operated this process, generally in excess of nameplate capacity, for more than 8 years. The plant, which can coproduce in excess of 70 K M T per year of acetic acid, uses coal as its sole source of carbon. The operation has been a resounding success, and an expansion of the facility doubled its capacity in mid-1991. These complex plants for the conversion of coal to acetic anhydride required a myriad of creative chemical and engineering innovations to make the process commercially viable. Among these innovations was the devel opment of an iodine-promoted rhodium catalyst system. Although it bears significant similarities to the well-known Monsanto methanol carbonylation to acetic acid (3-9), several unique requirements distinguish it from the earlier Monsanto system. Unlike the earlier rhodium-catalyzed carbonylation of methanol, the carbonylation of methyl acetate required the addition of a salt (or salt pre cursor) and the addition of a reducing agent to achieve and maintain com mercially viable rates (I, 14). These additional requirements, when applied to the commercially practiced aqueous methanol carbonylation, are reported to have little effect upon the catalysis (5, 15). We felt that a clear under standing of the factors affecting this catalytic process was imperative. There fore we undertook a detailed mechanistic investigation with high-pressure kinetics and in-line high-pressure infrared spectroscopy as probes.

Experimental Procedures Kinetic Measurements. The following procedure is typical for a kinetic run. A solution of consisting of 676.5 g (9.14 mol) of methyl acetate, 220.5 g (3.67 mol) of acetic acid, and 57 m L (130 g, 0.92 mol) of methyl iodide was added to a Hastelloy Β autoclave equipped with a high-pressure condenser and a liquid sampling loop. To this mixture was added 0.62 g of R h C l · X H 0 (2.53 mmol of Rh) and 25.39 g (0.190 mol) of anhydrous L i l . The autoclave was sealed, flushed thoroughly with nitrogen, then pressurized to 100 psi of 5% H in C O , 3

2

2


26.

Z O E L L E R ET AL.

Rhodium-Catalyzed Carbonylation of Methyl Acetate

379

and a flow rate of 2.0 mol h was established through the condenser. The reaction was heated to 190 °C. Upon reaching the desired temperature, the mixture was pressurized to 750 psi with 5% H in C O and a sample was removed immediately via the sampling loop. Thereafter, the pressure was maintained by using 5% H in C O as feed gas, and samples were removed every 30 min. The samples were analyzed by gas chromatography (GC). Because this reaction is reversible, rates were determined by using the method of initial rates with data up to 30% of completion. _ 1

2


2

High-Pressure Infrared Spectroscopy. A Hastelloy Β autoclave with Has telloy Β plumbing throughout and heat-traced lines was equipped with a gas inlet and an outlet for a pump suitable for use under high pressure. A small portion of the reaction mixture was pumped through a loop containing a heated IR flow cell constructed from 6-mm polycrystalline M g F (Irtran 1; Eastman Kodak) or polycrystalline ZnS (Irtran 2; Eastman Kodak) windows. IR spectra were recorded with a Fourier transform infrared (FTIR) spectrometer. Results were analyzed by using a linear regression technique to remove solvent inter ferences (16). Equilibrium Measurements for Metal Salts and Methyl Acetate. An acetic acid solution was prepared to attain a 1 M concentration of all of the following components: methyl acetate, acetic anhydride, p-dichlorobenzene (internal stan dard), and either lithium iodide or sodium iodide. A series of samples was placed in an aluminum heating block that was filled with oil to increase thermal transfer and maintained at 190 °C. Samples were removed and quenched by chilling in a cold-water bath every 30 s for the first 5 min, then every 1 min thereafter for an additional 10 min. Several samples were retained in the bath for 3 h to positively ascertain the equilibrium constant. The heating rate was determined and corrected for in all kinetic measurements. The rate constants were deter mined by using a published method (17).

Results and Discussion The rhodium-catalyzed carbonylation of methyl acetate to acetic anhydride gave a selectivity of >95%

after accounting for recovered methyl acetate.

Modifications made after this study achieved a selectivity well in excess of 99%. The product is accompanied by the formation of low levels of ethylidene diacetate (1,1-diacetoxyethane), acetone, carbon dioxide, and methane. A typical reaction profile for the carbonylation of methyl acetate is shown in Figure 1 along with a statement of typical reaction conditions. The thermodynamic parameters for the carbonylation of methyl acetate are much less favorable than those for the carbonylation of methanol. Cal culated values of the free energy (AG 2%) and heat of reaction ( Δ Η β ) at 298 29

Κ for the methyl acetate carbonylation are A G Δ// 8 29

AH298

= -12.1 = :

kcal mol" as compared to A G 1

2 9 8

2 9 8

= -2.5 = -17.8

kcal mol

and

1

kcal mol" and 1

-28.8 kcal mol \ respectively, for methanol carbonylation. Because

of this low thermodynamic driving force, the reaction does not go to com pletion. It reaches an equilibrium that is a function of temperature and carbon monoxide pressure.



Figure? i . Typical reaction profile for the carbonylation

of methyl acetate to acetic


anhydride.

C/5

z

3

50 PI >

m

N

>

O

r

PI

z

H C

C/)

Z

H

C

pi z P5 o

C

o

33

CO

oc o

26.


Rhodium-Catalyzed Carbonylation of Methyl Acetate 381

Our mechanistic studies utilized both high-pressure kinetics and highpressure infrared spectroscopy. Under the conditions in Figure 1, the highpressure infrared spectrum displayed bands at 2055 and 1984 cm" that are consistent with the formation of R h ( C O ) I · This catalytically active species was identified by Monsanto during mechanistic examinations of the methanol carbonylation (3). 1


2

2

Our initial working mechanistic hypothesis for the methyl acetate car bonylation was a minor modification of the well-studied and well-accepted catalytic cycle for the carbonylation of methanol presented by Forster and others (3-8). The catalytic cycles for the Rh-catalyzed carbonylation of meth anol and the modifications needed for adaptation to methyl acetate carbon ylation are presented in Scheme I. (The required changes are indicated in parentheses.) We successfully repeated the stoichiometric reactions de scribed by Forster (3, 6—8) for the Rh-induced conversion of methyl iodide to acetyl iodide.

Temperature Dependence.

Our examination of temperature de

pendence strengthened the relationship between methanol carbonylation and methyl acetate carbonylation. By using the reaction composition and gas mixtures shown in Figure 1, we examined the effect of temperature between 170 and 210 °C. A log rate vs. inverse temperature (Arrhenius) plot indicated that the reaction had an energy of activation Ε = 15.4 kcal mol" , with an enthalpy of activation (ΔΗ*) = 14.4 kcal mol" and entropy of ac tivation (AS*) = -27 eu. These values compared remarkably well with the reported parameters for the carbonylation of methanol: £ = 14.7 kcal mol" , ΔΗ* = 13.6 kcal m o l , and A S = -32 eu (18). These results indicate that the rate-determining step (the oxidative addition of methyl iodide to Rh(CO) I ~) is probably common to both processes. 1

Λ 1

1

a

1

2

1

2

The methyl acetate and methanol carbonylations apparently share a common catalytically active species- and rate-limiting step. The interesting portions of this investigation were not the similarities, but understanding the differences between the two systems. Unlike the methanol carbonylation, the methyl acetate carbonylation requires a reducing agent, such as hydro gen, and a salt (or salt precursor) to attain and sustain high reaction rates. Hydrogen Effect. Figure 2 shows the rate of formation of acetic anhydride, with pure carbon monoxide and with 5% hydrogen in carbon monoxide as feed gas. Clearly, the addition of hydrogen removes a significant induction period and maintains a more consistent reaction rate. The induc tion period can vary considerably from run to run. In continuous processes the reaction rate steadily deteriorates with pure C O . With the addition of hydrogen, the reaction rate remains constant. The high-pressure infrared spectrum using 5% hydrogen displays bands at 2055 and 1984 cm" that are consistent with the presence of Rh(CO) I ~. 1


2

2


Scheme I. Mechanism for the carbonylation of methanol. The proposed changes necessartj to adapt this mechanism to the carbonylation of methyl acetate are noted in parentheses.


ζ

>

50

d

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992. Figure 2. Effect of hydrogen on rate.


GO OC GO

«•>•»

s

o'

a"

s

a-

6

s*

8-

r

H >

P5

P5 33

r

?5

C

N

to

384


When pure C O is used, the high-pressure infrared spectrum displays the same bands with an additional band at 2087 cm" . This band indicates Rh(CO) I ~, which is inactive in the carbonylation. This peak increases as the reaction progresses. If small amounts of hydrogen are introduced into the reactor while a pure C O run is underway, the peak at 2087 cm rapidly disappears and the rate is accelerated. 1

2

4

1


The application of high-pressure infrared spectroscopy has been reported in the related triphenylphosphine-promoted system with a pure C O feed (19). Our results with pure C O feeds are consistent with the published work. However, the earlier investigators were unaware of the hydrogen effect and consequently did not report any spectra under these conditions. Iodocarbonyl complexes of rhodium are excellent catalysts for the water-gas shift reaction (5). Traces of water were reported to be responsible for the presence of the reduced species. The water would generally be added with the catalyst, and small quantities of water are likely to be present in the reagents. We agree with this interpretation. Addition of small amounts of water at the beginning of the reaction, before any acetic anhydride is formed, will reduce the induction period and produce a temporary rate acceleration. Once acetic anhydride formation begins, water is effectively scavenged and the accelerated rate is not maintained. The water-gas shift accounts for the lack of any required auxiliary reducing agent in methanol carbonylation because it is run in an aqueous media. The addition of hydrogen extracts a penalty, however, as acetic anhydride is slowly hydrogenated to acetaldehyde and acetic acid. The acetaldehyde subsequently reacts with a mole of the acetic anhydride product to generate ethylidene diacetate, which constitutes the major impurity in this process. The rate of ethylidene diacetate formation was found to be a direct function of the hydrogen level. Cationic Promoter. The second significant difference between the methanol and methyl acetate carbonylations is that the methyl acetate carbonylation requires the presence of a cation. At the outset of our efforts to identify active catalyst systems, we examined innumerable promoters. However, much of this investigation and the external reports (19-22) preceded our knowledge of the need for hydrogen in achieving and maintaining acceptable catalyst activity. A representative list of promoters that we reexamined after this knowledge was attained appears in Table I. We recognized the need for a cationic promoter early in our studies. However, an understanding of the role of the cation was very slow to emerge. Our mechanistic study focused on the alkali metals, particularly lithium, because they demonstrated fast rates and were structurally simple. Our initial kinetic measurements using lithium iodide as a promoter indicated that the reaction was a complex function of lithium iodide, methyl iodide, and rhodium. Like the Monsanto carbonylation of methanol, the rate


26.


Rhodium-Catalyzed Carbonylation of Methyl Acetate

385

Table I. Comparison of Selected Cationic Promoters Cationic Promoter None Li Na Bu,N Bu.P Al Zn Mg

Relative Rate 1.0 9.2 6.3 4.9 6.0 7.4 1.4 5.5

+

+

+

4

3 +

2 +


2 +

NOTE: Conditions: temperature, 175-190 °C; pressure, 750 psig; feed gas composition, 5% H 2-95% C O ; initial concentrations of feed materials: [Rh], 4.9 x 10^ M ; promoter concentration, 0 . 1 9 M ; [Mel], 0.92 M ; [MeOAc], 9.14 M ; [AcOH], 3.63 M . The promoter was added as the iodide, except in the case of Al, where the acetate was used.

of methyl acetate carbonylation is independent of carbon monoxide pressure above 500 psi. This complex behavior contrasts with the aqueous methanol carbonylation, which is a kinetically simple reaction demonstrating firstorder dependencies on rhodium and methyl iodide. The source of this complex behavior was most clearly seen when the reaction rate was examined as a function of added lithium iodide. This comparison, using two different rhodium levels, is represented in Figure 3. The reaction was characterized by an initial surge in the reaction rate as lithium was added, followed by a period in which the addition of more lithium iodide produced a negligible response. Next we examined the effect of the remaining two rate-determining factors at two levels of lithium iodide: 0.19 M (a region of little response to lithium) and 0.02 M (a region of high lithium response). Graphic depictions of the rate dependency on rhodium and methyl iodide at these two lithium iodide levels appear in Figures 4 and 5, respectively. Figure 4 clearly shows a first-order reaction in rhodium at the higher lithium levels. However, at the lower lithium levels the rate is nearly independent of rhodium between 1.25 and 5.0 m M Rh. The rate dependency upon methyl iodide, displayed in Figure 5, showed a similar pattern. At 0.02 M lithium, the reaction was nearly independent of the methyl iodide level between 0.4 and 2.0 M C H I . However, the rate was slightly less than first order (~0.86 when calculated from the plot in Figure 4) and had a nonzero intercept with respect to methyl iodide at the higher lithium levels. 3

Changes in Rate-Determining Step.

Initially, this behavior was

rather puzzling. The results at the higher lithium levels seemed to support our original assumption that this reaction rate was related to methanol carbonylation, even though we were disturbed by the slight deviation from


In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992. Figure 3. Effect of lithium iodide level on rate.


oc

CO

C/5

ζ

C

> 4

5C

σ

Ν

Η > Γ

>

η

pϊ Η > Γ

£

Ζ

C

Η

CA

> Ζ

Η S3

C/Î

C

C

pi ζ π

C

Χ

99% in favor of acetyl iodide and that acetic acid is a poor scavenger of acetyl iodide. However, when lithium acetate was added to a solution of


390


acetyl iodide, the lithium acetate quantitatively consumed the acetyl iodide and the equilibrium lies well in favor of acetic anhydride. We concluded that, in the absence of lithium, the iodine promoter was likely to accumulate as acetyl iodide. However, the lithium salt efficiently converted the acetyl iodide to acetic anhydride.


This conversion did not completely satisfy the kinetic picture with respect to lithium. The reaction of lithium acetate with acetyl iodide, essentially instantaneous at room temperature, was too fast to account for the dependence on lithium. The sole process left was the reaction of methyl acetate with lithium iodide. We examined the equilibrium between methyl acetate and lithium iodide, shown in reaction 2, at 190 °C.

M e O A c + L i l

Homogeneous Transition Metal Catalyzed Reactions - American

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