Catalysis of the Carbonylation of Alcohols to Carboxylic Acids Including Acetic Acid Synthesis from Methanol Denis Fwster and Thomas W. Dekleva Central Research Laboratories, Monsanto Company, 800 North Lindbergh Boulevard, St. Louis, MO 63167
Acetic acid has a long history as an industrially important chemical; however, over the years there have been many changes in the manufacturing process. During the 19th century, ethanol fermentation was the principal process. The advent of coal-derived acetvlene in the early decades of this century led to the development of the acetaldehyde route to acetic acid based on the mercurv-catalyzed acetylene hydration reaction. Subsequently, the availability of low-cost ethylene as a by-product of oil refining led to its exploitation via the Wacker process to give acetaldehyde.
With the development of natural gas as a major fuel following the Second World War, we have seen the evolution of large scale chemical processes based on synthesis gas derived from natural gas reforming. CH,
+ H,O
-
CO
+ BH,
(2)
In particular, methanol is one of the largest-volume organic chemicals produced in the world today, and it is made via the heterogeneously catalyzed reaction of carbon monoxide and hydrogen. CO
+ 2H,
-
CH,OH
(3)
The availability of methanol in large volumes and its relatively low price make i t an attractive raw material for other chemicals. In particular it is hecoming the feedstock of choice for acetic acid manufacture by utilizing carbonylation technology. CH,OH
+ CO
-
CH,CO,H
(4)
The technology that was initially commercialized to conduct this reaction involved a homogeneous cobalt catalyst and rather severe reaction conditions (-230°C and-600 atm ( 1 ) . The severe reaction conditions have limited attempts to achieve an understanding of this reaction, and, while there has been considerable speculation about mechanistic pathways, it is probably fair to say that a t this time we have no definitive information about the basic outline of the reaction. In 1970, Monsanto unveiled a commercial plant in which the methanol carbonylation was conducted using a homogeneous rhodium catalyst in conjunction with an iodide promoter (2). The reaction conditions are much milder than those for the cobalt-catalyzed reaction (Table 1). This reaction is much more amenable to mechanistic study than the cobalt system, and a combination of in situ spectroscopic studies and kinetic measurements has led to a good understanding of the reaction,at the molecular level (3). The reaction is characterized by kinetics that are independent of CO pressure and are first:order in hoth rhodium-and methyl iudide. A variety of rh(xliutn-containingcomplexes can bk used as the catalyst precursor, but in most cases the same reaction is achieved since a common active species is formed. I n situ spectroscopic studies show that under 204
Journal of Chemical Education
Table 1. Comparlson of Coban- and Rhodlum-Catalyzed Methanol Carbonvlatlon Reactions Cobalt process Metal concentration Reaction temperature Reaction pressure Selectivity (on methanol) Hydrogen effect
-lo-'
M -230'C 500-700 atm 90% CHI. CHSCHO. C2HrOH formed as by-products
Rhodium process
-lo@
M -180°C 30-40 atm >99% no effect
stmdv-slate cooditions the predominant rhodium species observed is IHhtCO)J~l-.The catalytic cycle can be represented ns indicated in F i"n r e I . Critical to thecatalvsis is the formation of methyl iodide from methanol since the oxidative addition of methyl iodide to the rhodium(1) center is the rate-determining step. Because of this, iodide sources incapable of generating this organic intermediate in significant quantities (e.g., alkali metal iodides) are ineffectual promoters for this reaction. Another key feature of the metal complex cycle in which the iodide acts most effectively is the nature of the active catalyst itself. The oxidative addition step is considered to be nucleophilic in nature, based on activation parameters and relative rate data ( 4 ) , and the presence of a negative charge on the metal center appears to significantly enhance the nucleonhilicitv (and hence reactivitv towards methvl iodide) of ihis metal ielative to neutral rhodium(1) specie; Another sienificant feature of the catalvtic phenomenon . . involvingrh&ium is the ease of methyl migration to form an acetyl-rhodium species (3). This occurs so rapidly that the alkyl-rhodium species in Figure 1has never heen detected. The short life of this species probably contributes t o the high selectivity in the system since a long-lived alkyl-metal spe-
Figure 1. Pmposed mechanism far Me Monsanto acetic acid synthesis.
cies in a catalytic cycle could allow its diversion into nonproductive pathways. Another reaction can be catalyzed with the reactants present under the acetic acid process conditions, namely the water gas shift reaction. CO+H20-CO,+H,
(5)
The role of rhodium in catalyzing this reaction has been studied by workers a t Monsanto (5)and Rochester University (6). The basic steps in the cycle are illustrated in Figure 2 and it can be seen that the [Rh(C0)212]- ion also plays a key role in this catalytic cycle. The facile reaction between [Rh(CO&- and HI in which HZis liberated and the rhodium(1) species is oxidized to rhodium(II1) is of critical importance in prolonging the life of the rhodium catalyst for acetic acid synthesis. Thus, under conditions when a deficiency of CHJ exists, HI will oxidize the rhodium and initiate a water gas shift cycle. By contrast, under neutral or basic conditions involving rhodium halide and carbon monoxide, reduction to rhodium cluster carbonyls and free metal occurs (7), and the catalyst would no longer he active. Carbonylatlon of Other Primary Alcohols The general reaction scheme used to describe the rhodium-catalyzed methanol carbonylation has been extended to include the carbonvlations of benzvl . (8). . . . ethvl(4.9) . . and npropyl(4) alcohol. The relative rate data (Table 2) obtained for these alcohols suggest very convincingly that the oxidative addition of the corresponding alkyl halide to the rhodium(1) center is uucleonhilic in n a t u r e . ~ ~ hkineticorofiles e for these svstems again ~ndicatedthat the reactions are first-order in both rhodium and alkvl iodide and indeoendent of CO nressure. Labelling studies(4) indicated that'there was no si'gnificant kinetic isotope effect when the substrate (EtOH system) or protic solvent (n-PrOH system) was replaced by deuteriumsubstituted species, again consistent with the SNZ-typereactivity. The study with n-propyl alcohol made i t necessary to exoand the orininal scheme to account for the production of isdmeric prod;cts. Over the pressure range examined (-20-130 atm), the carbonylation of n-propyl alcohol generated mixtures of n- and isobutyric acids. Increasing the CO pressure resulted in mixtures that contained decreasing amounts of isobutyric acid. Again, since the reaction rate exhibited rate parameters consistent with SNZ-typereactivitv. - , it was concluded that the oroduct selectivitv was determined after this single rate-determining step. This was also borne out bv labelline studies. in which the oroducts. n- and isohutyric acids, hotg bad the same isotopic composition as the recovered n-PrI. indicating a similar historv. The data are most consistent w ~ t hthe reactions shown in Figure 3. In this model, nucleouhilic oxidative addition is race-determin. ing and gives riseto a short-lived alkyl dicarbonyl rhodium(II1) species. By all accounts, cis-dicarbonyl rhodium(II1) species are very unstable. This instability is relieved, at least in this case, by two possible "decomposition" routes. The first involves the familiar migratory insertion reaction to generate an acyl monocarbonyl rhodium(II1) center, which
CO, H20
Table 2.
Relatlve Rate Data for the Iodide-Promoted, RhodlumCatalyzed Carbonylatlon of a Varlety of Alcohols S N Displacement ~ Rate
Alcohol
Relative Rate
for Organic Halides (ll)
Methanol Ethanol IPropanol 2-Propanol
21 1.0 0.47 1.23.8
30 1.0 0.4 0.02
then proceeds to regenerate the RhI2(C0)2- and n-hutyric acid.. hv,a route identical t o that orooosed . . for the methanol system il.ide supra). Alternatively, the alkyl dicarhonyl rhodium(lll) intermediate ran dissociate a CO lirand to form a more stable, but formally coordinatively unsakrated, monocarbonyl derivative. Facile @-hydrideelimination generates an intermediate hydrido-olefin complex, which can then reinsert to form either the same n-propyl or new isopropyl moiety. Further reaction of the isopropyl rhodium(II1) species gives rise to the isohutyric acid. Theretention of isotopic integrity discussed earlier also indicated that the intermediate hydride was not rapidly exchanged with the solvent. Since the reaction responsible for isomerization is the 8hydride elimination to form the hydrido-olefin complex, it is likely that the same sort of process occurs during the carbonylation of all linear alcohols higher than methanol.
Carbonylatlon of Secondary Alcohols T o date, mechanistic studies into the carbonylation of secondary alcohols with the same type of rhodiumm1 catalyst haveused isopropanol as a modeisubstrate. The carbonylation of isopropanol gives a mixture of n- and isobutyric ~ acids. The most recent study (10) shows that the S Npathway prevalent with the primary alcohols is not a major con-
[Rh(CO)&-
I R ~ ( C O ) ~ W ~ ~ ~ (trans) Figure 2. Proposed mechanism for lhe rhodium-catalyzed water gas shin reaction.
Figure 3. Mechanism for the iodidepromoled. rhcdium-catalyzad carbonylation of ~propanolto account for me fwmation of isomeric b w i c acids.
Volume 63 Number 3 March 1986
205
trihutor with the secondary alcohol. In fact, there appear to be two significant pathways with the major one heing hydrocarboxylation (eq 5). R-CH=CH2
+ CO + H20
-
R-CHp-CHp-C02H
or R-CH-CH3
I
(6)
COpH The propylene is formed via acid-catalyzed dehydration of the isopropyl alcohol and is present a t significant concentratioins during the reaction. ~h~ active catalyst for the hydrocarhoxylation reaction is almost certainly a rhodium(II1) hydride, The most likely for this species is [HRhI3(CO)]- which a r i s e s by a d d i t i o n of H I t o [RhI2(C0)2]-. Reaction of this hydride species with propylene will lead to the same rhodium(III) species discussed above with respect to the n-propyl alcohol carhonylation and hence to a mixture of primary and secondary propylrhodium species.
206
Journal of Chemical Education
Summary The rhodium-catalyzed reaction of alcohols with carbon monoxide has led t o a new general route to carhoxylic acids with one maior commercial a~olication(acetic acid from methanol) hiing in widespread"se today. One of the major limitations of the approach is that the necessity for the generation of alkyl halide from primary alcohols or olefin from secondary alcohols requires that the medium he of relatively high acidity. Literature Cited
,,,
(,) H.he.,,h,,, . , ,, , ,: , N.; Himmel., W. Hydddorbon 1966, ~ ( 1 1 )141. . (2) Fa". J. F.; Cmddoek. J.H.; Herahman. A,: Paulik, F. E. C k m . Tech. 1971.6W. (3) For8ter.D.J. Amer. C k m . Sor. 1976. SB,846. (4) ~ekieua,T. w . : F ~ ~ ~J~ A . D~. cW k m . s o c . 1985,107,3565. (5) Singleton, T C.; Park, L. J.; Price, J. L.: Forater. D. P e p . Diu Pet. Chom., Am,. Chem Soc. 1973.24.329. (6)
(7) (8) (9) (10) (11)
E.~~,E.C.:H~~~~~~*~~~,D.E.:E~~~~~~~,R.J.A~~~. (1980). Hughe8,RP. Cam~rohomiv~Orgonometol. Chem. 1982,5.277. Mssuda, A.; Mitani, H.; Oku, K.; Yamauaki, Y. Nippon Kogoku Koiahi 1982,2,249. Hjortkjaer, J.; ~orgensen,J. C J . ~ o lcotolyxi~ . I978,4,199. Dek1e"a.T.W-;Fomer.D. J . A m e r Chem.Soc. 198S,JO7,3568.
Hendricbn.J.B.:Cram.D.J.:Hammond,G.S."OrganicChemia~y";McCraw-Hill, NFW Y O T ~ .1970; p
393.
