The present invention relates to an improved process for the preparation of adipic acid. More particularly, the present invention relates to an environmental-friendly, clean process for the preparation of adipic acid through oxidation of cyclohexanol, cyclohexanone or a mixture thereof with oxygen or oxygen-containing gas, in the presence of an oxidation initiator, a polar solvent and an organometallic xcexc3-oxo-bridged Co/Mn cluster complex catalyst.
Adipic acid is a large volume commodity chemical and is used in the manufacture of synthetic fibers (e.g, Nylon-6,6), polyurethane resins. plasticisers, food additives, lubricants, electronics, soil conditioners, glass protection agents and leather tanning agents. More than two million tons of adipic acid is produced worldwide per year, mostly, by a two-step oxidation process (Du Pont U.S. Pat. Nos. 2,233,494 (1940); 2,439,513 (1948); 2,825,742 (1958); 3,035,092 (1962); 3,390,174; BASF British Pat. Nos. 852,523 (1958); 918,900 (1963); British Pat. 1,304,855) wherein cyclohexane, in the first-step, is oxidized with oxygen, at 423-433 K, using a cobalt catalyst to form a cyclohexanone and cyclohexanol mixture (KA oil). In the second step the mixture is further oxidized to adipic acid with nitric acid, in the presence of V and Cu-containing catalysts. Conversion levels of cyclohexane, in the first step, are to be maintained below 10% to achieve acceptable selectivity of cyclohexanol-cyclohexanone. The second-step of the reaction (nitric acid oxidation) produces significant toxic nitrogen oxide effluents, and hence, is not environmental-friendly. The oxidation of cyclohexane is one of the least efficient of all the major industrial processes. Methods of improving the selectivity have been the focus of research in this area over the decades. Asahi Chelmical Industry (Japan Pat. No. 45-16,444 (1970)) and Gulf (U.S. Pat. No. 4,263,453 (1979)) have developed a direct single-step oxidation process of cyclohexane to adipic acid. In this process cyclohexane is oxidized with air or oxygen, in acetic acid medium and cobalt salt as catalyst, at reaction temperature of 343-373 K and residence time of 2-6 h. Cyclohexane conversion of the order of 50-94% and adipic acid selectivity of 70-80% have been reported in the single step process. Alternative single-step synthetic routes have been reported by Talsi et al (J. Mol. Catal. 81, 215 (1993)) and Maschmeyer et al (Angew. Chem. Int. Ed. Engl. 36, 1639 (1997)). However, the results on single-step oxidation to adipic acid have been less than satisfactory.
Kulsrestha et al in U.S. Pat. No. 5,547,905 (1996) have reported the preparation of adipic acid using Co and Fe containing homogeneous catalysts. U.S. Pat. No. 6,147,256 (2000) reports oxidation of cyclohexane in liquid phase using Co and Cr containing salts. Sato et al (Science 281, 1646 (1998)) have reported a process for the synthesis of adipic acid by oxidation of cyclohexene with H2O2 and solid NaWO4 catalyst.
Kamath and Chandalia (J. Appl. Chem. Biotechnol. 23, 469-478 (1973)) have reported the preparation of adipic acid by the oxidation of cyclohexanone with air (atmospheric pressure), in the presence of acetic acid and cobalt acetate or manganese acetate salt, at 323-373 K. But the cyclohexanone conversion and adipic acid selectivities were very low, S. H. A. Zaidi (Appl. Catal. 42, 247 (1988)), E. T. Crisp and G. H. Whittfield (Great Britain Patent 2,818,807 (1978)) and T. N. Antonova, G. N. Koshel, and M. I. Farberov (Uch. Zap. Yaroslav, Tekhnol. Inst. 27, 100 (1971), CA 78(11): 71339p) have also reported the oxidation of cyclohexanone to adipic acid, but with lower conversions and adipic acid yields.
Raja and Ratnasamy in U.S. Pat. No. 5,767,320 (1998) have reported dioxygen oxidation of cyclohexane using pure and zeolite-Y-encapsulated substitute phthalocyanine catalysts; cyclohexanone and cyclohexanol were the selective products of the oxidation reaction Thomas et al have recently reported the preparation of adipic acid by aerial oxidation of cyclohexane or n-hexane using metal containing aluminophosphate molecular sieve catalysts (Nature 398, 227-230 (1999); Angew. Chem. Int. Ed. 39(13), 2310-2313 (2000); Angew. Chem. Int. Ed. 39(13), 2313 (2000)). U.S. Pat. Nos. 2,223,493; 2,589,648; 3,390,174; 3,649,689; 3,987,100; 4,263,453; 4,158,739; 4,902,827; 5,321,157 (1994); 5,981,420 (1999), 6,160.183 (2000). 6,258,981(2001) and EP-A-0,694,333 also describe other methods for the preparation of adipic acid. Comprehensive reports on the state-of the-art of adipic acid preparation are available in the reviews by K. Tanaka in CHEMTECH 555-559 (1974) and Hydrocarbon Process 53(11), 114-120 (1974), Castellan et al in Catal. Today 9(3), 237-322 (1991), Schuchardt et al in SYNLETT 713-718 (1993) and Appl. Catal. A. General 211, 1-17 (2001); Partenheimer in Catal. Today 23, 69-158 (1995); Suresh et al in Ind. Eng. Chem. Res. 39, 3958-97 (2000). However, in these reports adipic acid yields are lower than the commercial process and limit their applicability.
The commercial adipic acid manufacturing process has the following disadvantages: (1) the commercial process is not an environmentally benign or xe2x80x9cgreenxe2x80x9d approach. (2) nitrous oxide is an inevitable by-product, which has been implicated in global warming and ozone depletion, (3) substantial amount of nitric acid is consumed in the process, (4) decarboxylation to lower mono- and dicarboxylic acids is inevitable, and (4) 0.25 kg of by-products is produced per kg of the product,
The present invention is an environmental-friendly green process. It does not use nitric acid, but utilizes cleaner oxidants like air or oxygen-containing gas. The method of the present invention utilizes an organometallic xcexc3-oxo-bridged Co/Mn cluster complex or a solid catalyst containing the organometallic xcexc3-oxo-bridged Co/Mn cluster complex as catalyst in a polar solvent medium like acetic acid-water in the presence of an oxidation initiator. Examples of such solid catalysts include micro and mesoporous materials like, aluminosilicate zeolites, aluminophosphates, carbon molecular sieves, silica and the like, containing an organometallic cluster complex wherein the chemical composition of each molecule of the organometallic cluster complex includes cobalt/manganese.
It is a surprising discovery of the present invention that when an organometallic xcexcl3-oxo-bridged Co/Mn cluster complex or the solid catalyst containing xcexc3-oxo-bridged cluster complex was used as catalyst the activity and adipic acid selectivity were significantly higher. In the experiments with the solid catalysts containing the organometallic cluster complex, the solid catalyst can be easily separated from the reaction mixture by filtration. Moreover the reaction conditions like temperature and pressure were moderate and the process was atom-efficient. The synergistic effect of a cobalt and manganese combination, and facile redox behavior in cluster complexes are perhaps responsible for high yields of adipic acid in the present invention.
Charvan et al in J. Mol. Catal. A. Chemical 161, 4964 (2000) and Chem. Commun. 1124-1125 (2001) teach that solid, encapsulated oxo-bridged metal cluster complexes are efficient catalysts in the aerial oxidation of para-xylene to terephthalic acid, an yet another large volume commodity chemical used in polyester industry. These novel solid catalysts while retaining all the advantages of the homogeneous catalysts, like high yield of adipic acid, are easily separable from the reaction products by a simple filtration process. This not only avoids the tedious process of catalyst recovery characteristic of the prior art processes, but also eliminates the presence of toxic metal ions and nitrous oxide in the effluents from the process. Processes utilizing these novel solid catalysts are, hence, environmentally more beneficial. Representatives of the organometallic cluster complexes of cobalt and manganese of the present invention are Co3(O)(CH3COO)6(py)3, Mn3(O)(CH3COO)6(py)3, CoMn2(O)(CH3COO)6(Py)3, Co2Mn(O)(CH3COO)6(py)3, CoMn2(O)(CH3COO)y(py)z, and Co2Mn(O)(CH3COO)y(py)z, where y+z=9 and py=pyridine. It is also found that the organic ligands in the above mentioned organometallic cluster complex, namely the acetate and pyridine ligands, can be replaced by other suitable organic moieties. The critical active site ensemble responsible for the high yields of adipic acids in the oxidation reaction was the heterometallic cluster complex containing both cobalt and manganese. While the exact origin of this enhancement effect is not known in detail, it may be speculated that the multimetallic clusters of transition metal ions are better able to activate oxygen, than the monometallic and monomeric ions. The common prevalence of such heteronuclear, multimetallic clusters in the oxygen activating enzymatic oxygenase catalyst systems supports such a suggestion.
The major object of the present invention is to provide an improved process for the preparation of adipic acid which an environmental-friendly and atom-efficient.
Another objective of the present invention is provide a process for the production of adipic acid wherein nitric acid is not used as an oxidant; cleaner oxidants like oxygen or oxygen-containing gas and a oxo-bridged Co/Mn cluster complex either in its neat form or contained in a solid matrix are used as catalyst.
Yet another objective of the present invention is to prepare adipic acid from cyclohexanol, cyclohexanone or a mixture thereof.
Accordingly, the present invention provides an improved process for the preparation of adipic acid which comprises oxidizing a cyclic compound selected from the group consisting of cyclohexanol, cyclohexanone and a mixture thereof in a polar solvent with oxygen in the presence of an xcexc3-oxo-bridged Co/Mn cluster complex catalyst and an oxidation initiator, at a pressure of at least 130 psig, at a temperature ranging between 353 and 403 K, for a period ranging from 0.5 to 8.0 hrs, bringing the temperature of reaction mixture to an ambient temperature and recovering the adipic acid from the reaction mixture by conventional methods.
In an embodiment of the present invention the cluster complex has a general formula
[CoxMny(O)(OOCR)n]
wherein x and y can take values of 0, 1, 2 or 3 and x+y=3, R is selected from the group consisting of alkyl, substituted alkyl group containing I or more carbon atoms, awl and substituted aryl group and n is in the range of 4 to 6.
In yet another embodiment the xcexc3-oxo cluster complex used is contained in a solid matrix selected from the group consisting of aluminosilicate zeolite, aluminophosphates, carbon molecular sieves and silica.
In yet another embodiment the polar solvent used is a mixture of alkyl or aryl carboxylic acid and water.
In yet another embodiment the oxygen used is selected from the group consisting of pure oxygen, air and a mixture of inert gases and oxygen.
In still another embodiment wherein the oxidation initiator used is selected from the group consisting of methyl ethyl ketone (MEK), acetaldehyde, hydrogen peroxide, alkyl hydroperoxide and halide ion preferably bromide ion.