TECHNOLOGY
New Route to Adipic Acid Developed at Monsanto Dicarbonylation of 1,4-disubstituted 2-butenes with palladium catalysts containing chloride ligands is basis of new chemistry
National Meeting
StSLows M o n s a n t o is d e v e l o p i n g a new chemical route to adipic acid, a principal raw material for nylon 66 polymers and for hexamethylenediamine, a widely used organic intermediate. The new route may not immediately replace existing routes to adipic acid but it does offer an a l t e r n a t i v e that may be exploited as economics permit. The new chemistry was developed at Monsanto's central research laboratories by Albert S. Chan, now a research chemist at Monsanto Industrial Chemicals Co. It is based on dicarbony lation of 1,4-disubstituted 2-butenes with palladium catalysts containing chloride ligands. Chan told the Division of Industrial & Engineering Chemistry that most of the previous attempts to dicarbonylate 1,4-disubstituted 2butenes have not been successful. The usual problem is the preferred formation of unwanted by-products from isomerization and/or hydrogenolysis. The idealized reaction is
t h e isomerization and h y d r o g e n olysis was the protic solvents that had been used. Polar, aprotic, and nonbasic solvents are preferred, although particular solvents may have drawbacks on occasion. In fact, Chan says, the dicarbony lation of 1,4-dimethoxy-2-butene actually can be carried out in the absence of solvents. The explanation is that the substrate itself and the products act as effective solvents. Contrary to some published reports on the carbonylation of aryl halides, the presence of bases actually inhibited the PdC^-catalyzed carbonylation of 1,4-disubstituted 2-butenes. The inference made by Chan is that there is a unique catalytic species involved. His explanation is that Lewis acids increase rates but decrease selectivity. Trials with several Lewis acids tend to bear out this observation. Another difference from the general experience with similar systems
PdCl 2
XCH 2 CH=CHCH 2 X + CO * XCOCH 2 CH=CHCH2COX where X = CH3COO, CH3O, OH, or CI, for example. Chan determined that the source of 28
April 30, 1984 C&EN
Chan: unique catalytic species
is that although triphenylphosphine ligands stabilize the palladium catalysts against decomposition to the metallic form, they also inhibit catalysis. Phosphites, arsines, and stibines have shown similar effects, but to a lesser extent than phosphines. There is no doubt about the importance of the halide ligands; they are necessary. Without them the palladium catalysts are inactive for carbonylation of the dimethoxy2-butenes. Chlorides, bromides, and iodides are most effective in that order. Fluoride ligands are ineffective. Stannous chloride, which promotes the catalytic carbonylation of allylic compounds, inhibits carbonylation of the substituted 2-butenes. An optimum temperature for the reaction appears to be about 100 °C. Catalyst deactivation becomes severe at higher temperatures, and the rates fall to unacceptable levels at lower temperatures. No other catalyst species has been found comparable to PdCl 2 for the present application. However, there is some interest in certain nickel catalysts because of the economics involved in the process. The preferred butène found thus far is l,4-dimethoxy-2-butene, because it provides the highest yields. The prospects for a new route to adipic are almost tantalizing. Virtually all of the adipic acid produced is consumed by makers of nylon polymers. The present U.S. capacity for adipic acid is about 1.9 billion lb per year, and less than 10% is sold on the open market. Two major routes to adipic acid are now in production, according to Chan. One uses oxidation of cyclohexane with air to a mixture of cyclohexanone and cyclohexanol, which is further oxidized to adipic acid with nitric acid. The second
route obtains the cyclohexanol from hydrogénation of phenol, and this is followed by nitric acid oxidation. There have been some minor variants in the oxidation of cyclohexane, but the nitric acid oxidation accounts for virtually all the production of adipic acid. Chan thinks there are two drawbacks to the present processes. One is low conversion in the air oxidation of cyclohexane. The other is that in the near future the hydrocarbon source will be based on benzene, and that will be expensive. Chan suggests that his process will be preferable on both counts. D
Catalytic activation of carbon dioxide clarified J
National Meeting
Kf»wrma The chemical activation of carbon dioxide proceeds along two general lines of investigation. One is concerned with the basic mechanisms of photosynthetic fixation. The other is concerned with insertion of carbon dioxide into metal-carbon, carbon-carbon, and hydrogen-carbon bonds. In both lines of investigation, catalysis is the key element, and there are some similarities among the many fundamental differences in a p p r o a c h . Examples of b o t h approaches to the problem of C 0 2 activation were presented in a Division of Inorganic Chemistry symposium on CO2 activation. Chemistry professor Shohei Inoue of the University of Tokyo has found only a few potential reactions of interest for C 0 2 fixation, but these offer considerable promise in opening the door to greater understanding. They utilize the catalytic effects of aluminum and zinc porphyrins. Aluminum porphyrin with an aluminum alkoxide group traps carbon dioxide easily and reversibly at room temperature in the presence of 1-methylimidazole. The trapped
CO2 is sufficiently activated to react with epoxide, also at room temperature, to yield the corresponding alkylene carbonate. A second reaction is the lightinduced fixation of CO2 by alkylzinc porphyrin in the presence of a secondary amine or alcohol to yield zincoporphyrin, containing zinc carbamate or the carbonate group. Of special interest, Inoue says, is the accelerating effect of visible light on the reaction of iV-methyltetraphenylporphinatozinc ethyl (NMTPPZnEt), CO2, and amine or alcohol. If the a m i n e is d i i s o p r o p y l a m i n e , for example, and the reactants are irradiated with a xenon lamp, the conversion increases from 29% to 100%. Inoue also described the discovery of a novel six-coordinate alumin u m p o r p h y r i n complex that is formed by mixing a porphyinatoaluminum carboxylate and a tetraalk y l a m m o n i u m carboxylate. This finding permitted the Japanese investigators to look into the possibility of trapping CO2 as a carbamate group on the "reverse" side of the porphyinatoaluminum carboxylate. This has been confirmed by NMR using CO2 tagged with 13 C. Another approach to the problem of fixing carbon dioxide is that of chemistry professor Donald J. Darensbourg of Texas A&M University. One aspect of his program is centered on obtaining more definitive knowledge of what factors affect CO2 insertion into metal-metal
(M-M) a n d metal-carbon (M-C) bonds. Specifically, he has shown that alkyl and aryl derivatives, M(CO) 5 R- (M = Cr or W; R = CH 3 or C6H5) smoothly undergo insertion reactions with CO2 to provide the corresponding carboxylates M(CO) 5 R- + C 0 2 -* M(CO) 5 OC(0)RA significant acceleration of CO2 insertion was noted in the presence of alkali metal counterions, which neutralized the buildup of negative charge on the incipient carboxylate ligand. Unfortunately, subsequent reactions result in removal of the carboxylate ligand from the metal center. The rate of enhancement from the alkali metal ions allowed investigation of the effects of added CO2 on the rate of insertion. Since the interaction of CO2 with metal centers is expected to intensify at electron-rich metal centers, Darensbourg has synthesized alkyltungsten carbonyl derivatives with phosphorus donor ligand substituents. One derivative contains the anion ris-CH3W(CO)4P(CH3)3-. Substitution at the metal center by the p h o s p h o r u s donor ligands greatly expedites the C 0 2 insertion. A similar e n h a n c e m e n t was noted for CO2 insertion into the W-C bond in the chelating derivative (CO) 4 WCH2CH2CH2P(C 6 H 5 )2-. C-H bond insertion of CO2 is catalyzed by anionic group 6b metal hydrides and formates in the près-
Anionic group 6b metal hydrides catalyze C0 2 , H2 reactions HCOJH(CO)§-
-v \
7
Γ °°
H[M,(CO)10r Ξ = Ξ [MtCOJJ + HM(CO)s-
) V
( L = COorPR3)
/Ht
HCOjMtCOl, I V
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: : s
γ
CH.OH v HCOOH
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