Technology
Synthesis gas route to carboxylases developed Texaco's ruthenium-catalyzed process uses synthesis gas to homologize carboxylic acids, removing dependence on petroleum feed stocks A synthesis gas conversion program under way at Texaco Chemicals Inc., Austin, Tex., has resulted in a novel thrust in Ci chemistry research. The company has developed a process for ruthenium-catalyzed syngas (carbon monoxide and hydrogen) homologization of carboxylic acids. Details of the process were dis closed at the recent national meeting of the American Chemical Society in Atlanta by John F. Knifton, supervi sor of syngas research at Texaco. Lower carboxylic acids, particularly acetic acid, are reacted using iodide promoters such as hydrogen iodide and alkyl iodides. A general reaction can be written as
nium-catalyzed homologization of carboxylic acids is not only novel but perhaps unique. In any event, it rep resents some significant progress in developing a nonpetroleum-based organic chemicals industry in the fu ture. Effective catalyst precursors in the homologization process are ruthenium(IV) oxide and hydrate, ruthenium(III) acetyl acetonate, triruthenium dodecacarbonyl, and ruthenium hydrocarbonyls. The highest yields of the higher-molecular-weight acids
occur with Ru02-methyl iodide combinations where the total selec tivity to C3 and higher acids is about 45%. The catalysts remain in solution, suggesting that catalyst recovery may not be a problem in any process based on the reactions. The principal competing reactions to the carboxylic acid homologization are water gas shift to carbon dioxide, the formation of hydrocarbons (chiefly ethane and propane in the case of acetic acid homologization), and small amounts of esterification
Acetic acid homologization typifies new Texaco process CHX 3
N
+ Hi ^ = £ CH3C
CH3CH2C 3
H
2
x
OH
N
-f H 2 0 I
+ HI OH
l>
Ru(co) x i y
Λ
^\^
2°^_,©'
CH3COOH + CO/H2 - C n H 2 n + 1 COOH with acetic acid as the reactant, al though the synthesis isn't restricted to acetic acid. When acetic acid is used, the homologization yields the corresponding C3 aliphatic carboxylic acid. Because acetic acid itself is derived from syngas, the synthesis disclosed by Texaco suggests that all of the higher-molecular-weight carboxylic acids can be derived from syngas alone, eliminating dependence on petroleum feedstocks. Recently there has been a redirec tion of interest in Ci synthesis into several channels (C&EN, Feb. 23, page 39). These include renewed in terest in Fischer-Tropsch chemistry, hydroformylation, and the opportu nities afforded by zeolitic conversion of methanol to higher hydrocar bons. Higher carboxylic acids have been reported as by-products of methanol carbonylation and there have been some successes with cobalt-catalyzed homologization of benzyl and alkyl al cohols. But Knifton believes the current Texaco work with ruthe
CH 3 CH 2 C-Ru(CO) x l v t
Θ CO CH 3 CH 2 -Ru(CO) x . y
CH3CH3
A possible mechanism for acetic acid homologization involves successive hy drogénation of the acetyl group while coordinated to the iodoruthenium carbonyl (1) through intermediate species (2 and 3). This would allow the rationale attributed to the observed product array. By-product ethanol formed from the hydrogénation of one intermediate subsequently is esterified to ethyl acetate. Similarly, hydrogenolysis of the next intermediate produces by-product ethane. The insertion of carbon monoxide to form Ru-acyl (4) agrees with evidence from carbon-13 tracer studies. Elimination of propionyl iodide from the Ru-acyl intermediate is accompanied by the immediate hydrolysis of the acyl iodide to the propionic acid product homolog (5). The original ruthenium carbonyl complex is then regenerated for reuse.
April 27, 1981 C&EN
27
Technology and the formation of ethyl acetate. The iodide promoters do not appear to carry over into the products. According to Knifton, acetic acid homologization appears to be sensitive to at least four operating parameters: ruthenium and methyl iodide promoter concentrations, syngas composition, and operating pressure. In general, under the experimental conditions the reactions are first order with respect to the initial ruthenium concentration. Yields of higher acids are increased with higher promoter (methyl iodide) concentrations, but there are complications introduced by a tendency for the corresponding esters to form, particularly at low promoter/catalyst ratios. In the case of acetic acid homologization, ethyl acetate and ethyl propionate become the principal products at low iodide concentrations. Despite stoiohiometry to the contrary, the maximum yield of propionic acid from acetic acid is achieved with a carbon monoxide/ hydrogen ratio of 1:1. No acetic acid homologization is observed without hydrogen; homologization generally increases with increased hydrogen concentration. There is also a parallel increase in the generation of carbon dioxide, ethane, and propane in the off-gases. The maximum yield and selectivity of propionic acid from acetic acid occur at an operating pressure of about 100 atm. Within the pressure limitations of the equipment, yields of butyric and valeric acids continue to rise at higher pressures. The development work at Texaco has concentrated on acetic acid reactants. But, Knifton says, the process is generally applicable to the higher acid reactants. For straight-chain acids, the homologs generated are the corresponding acids containing one more carbon atom in the chain. There are traces of branched-chain isomers in the product mixture but the straight-chain isomers dominate. Increasing the chain length does not drastically affect conversion or yield with a given catalyst/promoter combination. Homologization with branchedchain acids often is accompanied by structural rearrangements. In particular, there is a tendency to generate tertiary acids in which the alphacarbon atom is bonded to three alkyl groups. In all cases, straight and branched chains, the by-products are carbon dioxide, water, and the corresponding hydrocarbons. Knifton explains t h a t labeling studies with carbon-13 and deuteri28
C&EN April 27, 1981
um were used to understand the mechanism better and to confirm the carbon source for the higher-molecular-weight products. The results appear to be consistent with the notion that carbon monoxide addition to the carbonyl carbon of the acid substrate is the dominant step in the reaction sequence. The mechanism for the homologization of carboxylic acids with ruthenium catalysts and iodide promoters has not been established definitely. However, Knifton offered a leading candidate that has been supported by independent research outside Texaco. The ruthenium-catalyzed homologization suggested by Knifton is probably in competition with at least four other alternative reaction pathways leading to the formation of hydrocarbons, aliphatic alcohols, higher-molecular-weight acids, and other rearranged products. As had been noted in previous research dealing with syngas generation of longer-chain products, the effectiveness of ruthenium catalysts is somewhat dimmed by their great ability to promote hydrogénation. From the viewpoint of producing hydrocarbons this is a desirable characteristic. However, if the higher acids are desired it is a handicap. Alternative mechanisms for the reaction system are being considered by Knifton's group using other substituted acids. An area of synthesis closely related to the homologization of higher carboxylic acids is the generation of vicinal glycol esters from syngas. Knifton also described work on this synthesis, particularly with respect to ethylene glycol acetate esters prepared with ruthenium catalysts. The aliphatic carboxylic acids, such as acetic acid, are simultaneously the reactant and reaction medium for reactions with carbon monoxide and hydrogen. The general synthesis is believed to be represented by the equation 2CO + 3H2 + 2RCOOH—• H2OOCR + 2H 2 0 H2OOCR Acetic acid solutions of ruthenium(III) salts combined with large cationic species such as quaternary phosphonium and ammonium salts are the preferred catalytic precursors. The major by-products of the synthesis are methyl and ethyl acetate. Thermodynamic considerations
favor the formation of the acetate esters from synthesis gas in acetic acid media over direct synthesis of ethylene glycol by rhodium catalysis. The formation of glycol acetate esters is believed to be the result of catalysis by a solubilized anionic ruthenium carbonyl species. The reaction is influenced considerably by cocatalyst species, pressure, and other factors. In particular, the ruthenium hydrocarbonyl cluster [HRu3(CO)n]"~ has been detected by NMR spectra. Knifton says that the presence of such large cationic species as RiuP"1" should help stabilize the ruthenium cluster, but substitution of such solvents as acetic anhydride, which has a high dielectric constant, does not noticeably increase selectivity to glycol diacetate. D
Hypochlorite process eliminates discharges Separating inorganic compounds by the process of fractional crystallization is a slow, time-consuming experiment when practiced in the laboratory. But when an inorganic chemicals producer uses it in a manufacturing process to avoid discharge of effluents, it is an advance in chemical technology. That's just what Potasse et Produits Chimiques, a subsidiary of Rhône-Poulenc located in Thann, France, is doing. The company has developed a novel "clean technology" process that produces neutral calcium hypochlorite with a 75 to 80% active chlorine content and with total recycling of the reaction medium. It is a nonpolluting process in which there is no systematic discharge of wastes. The reaction medium and mother liquors are totally recycled. The process thus eliminates discharge of hypochlorite into landfills, and leaching into groundwater is avoided. Before the process was put into use, more than 50% of the byproduct materials—sodium and calcium chlorides, chlorates, and hypochlorites—were placed in landfills or discharged as effluents. Neutral calcium hypochlorite is a solid, stable product with a high concentration of active chlorine that is widely used as a germicide for eliminating undesirable microbial activity in swimming pools. Discharges from the production of calcium hypochlorite can result, for example, from accidental or periodic formation of suspensions of calcium
