Process for the preparation of an aromatic carboxylic acid

Details Process for the preparation of an aromatic carboxylic acid Process for the preparation of an aromatic carboxylic acid

2001-06-28

2003-11-18

Richter, Johann (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

C562S412000, C502S324000

Reexamination Certificate

active

06649791

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for the preparation of an aromatic carboxylic acid. More particularly, the present invention relates to a process for the preparation of an aromatic carboxylic acid of the general formula
R—COOH
wherein R comprises an aryl group having 1 to 3 benzene rings, or a substituted aryl group. Still more particularly, it relates to the preparation of aromatic carboxylic acids by oxidation of alkyl aromatic compounds having a general formula
R
1
—R
2
wherein R
1
=alkyl or substituted alkyl having 1 to 3 carbon atoms, and R
2
=aryl having 1 to 3 benzene rings, using solid catalysts containing organometallic cluster complexes of cobalt and manganese.
BACKGROUND OF THE INVENTION
Aromatic carboxylic acids such as benzoic acid, phthalic acid, terephthalic acid, trimethyl benzoic acids, naphthalene dicarboxylic acids and the like are used widely as intermediates in the chemical industry. They are usually prepared from the corresponding alkyl aromatic compounds by oxidation with air in the presence of liquid phase, homogeneous catalysts like cobalt acetate, manganese acetate etc. Terephthalic acid, for example, is prepared, as described in U.S. Pat. No. 2,833,816 issued to Mid Century Coloration in 1958, by the oxidation of para-xylene by air in acetic acid solvent, at around 200° C. and 200 psig pressure, in the presence of homogeneous, liquid phase catalysts comprising of cobalt, manganese and bromine. Various modifications and improvements of this process are utilised for the manufacture of terephthalic and many other aromatic carboxylic acids. These processes are described in U.S. Pat. Nos. 5,693,856; 3,562,318A; 5,760,288; 6,160,159; 4,329,493; 4,593,122; 4,827,025; 4,835,307; 5,087,741; 5,112,992; and EP 0 754,673 A. Comprehensive reviews of the oxidation of alkyl aromatic compounds to aromatic carboxylic acids are available by Suresh et al in Industrial Engineering Chemistry Research, volume 39, pages 3958-3997, year 2000 and W. Partenheimer in Catalysis Today, volume 23, pages 69-158, year 1995. Phthalic acid is manufactured by the aerial oxidation of ortho-xylene in the vapor phase over vanadia-based catalysts. In the oxidation of para-xylene to terephthalic acid, for example, the process begins with hydrogen atom extraction from a methyl group by the bromine atom. The resultant benzyl radical adds to O
2
and proceeds through the hydroperoxide to para-tolyl alcohol, para-tolyl aldehyde and para-toluic acid. Hydrogen atom abstraction from the methyl group of toluic acid generates a secondary benzyl radical, which follows the same pathway to yield, eventually, terephthalic acid.
There are many improvements that are desirable in the presently used homogeneous, liquid phase processes for the manufacture of aromatic carboxylic acids; (1) Replacement of the homogeneous by solid heterogeneous catalysts, (2) replacement of the corrosive bromine promoters which require the use of expensive titanium steel, by non-corrosive compounds, (3) elimination or reduction of the significant acetic acid oxidation to CO and carbon dioxide (5-10 wt. % of the carboxylic acid); this can, perhaps, be achieved by the use of more efficient radical promoters which allow oxidizer temperatures to be lowered without reducing reaction rates, and (4) lowering of the concentration, in the reaction product, of intermediates which are difficult to be removed from the final, aromatic carboxylic acid product: 4-carboxy benzaldehyde is a typical example of such an intermediate which necessitates elaborate hydrogenation and recrystallisation procedures in the manufacture of purified terephthalic acid required for the polyester industry.
Jacob et al in the journal Applied Catalysis A: General, volume 182, year 1999, pages 91-96 described the aerial oxidation of para-xylene over zeolite-encapsulated salen, saltin and salcyhexen complexes of cobalt or manganese in the absence of added halogen promoters and using tertiary butyl hydroperoxide, instead of bromide ions, as the initiator at low temperatures. Significant conversion levels of para-xylene (upto 50-60%) were attained. However, the main product was para toluic acid. The yields of terephthalic acid were negligible. The feasibility of using a solid, non-Br-containing catalyst in the absence of any solvent including acetic acid for the para-xylene oxidation to toluic acid, which is the first stage in the oxidation of para-xylene to terephthalic acid, was claimed to be established.
In prior art processes for the manufacture of aromatic carboxylic acid from alkyl aromatic compounds, the alkyl aromatic compound is dissolved in acetic acid along with the homogeneous catalysts, usually the acetates of cobalt and manganese, and oxidized by an oxygen containing gas, usually air or oxygen, in the presence of a promoter like NaBr or HBr at temperatures around 200° C. and pressures of about 200 psig. Other metal acetates have also been used. U.S. Pat. No. 4,786,753, for example teaches the use of nickel, manganese and zirconium acetates in place of the acetates of cobalt and manganese. The commercial processes have been optimized to the point where typical crude aromatic acid yield is around 96-98% weight. However, as mentioned herein before, there is scope for improvement in the practice of the process one of them being the replacement of the homogeneous catalysts by solid catalysts since the latter can be more easily separated from the reaction products. In the investigations leading to the present invention, it was found that when complexes of cobalt, manganese, nickel, zirconium or any of their combinations were supported or encapsulated or grafted in solid supports, the yields of the aromatic carboxylic acids were always low in accord with the findings of Jacob et al published earlier and mentioned herein above. Hence, the prior art catalysts, like the acetates of cobalt, manganese, nickel or zirconium, while active in the homogeneous oxidation of alkyl aromatic compounds are not sufficiently active in the solid state. It is a surprising discovery of the present invention that when the solid catalyst contains certain organometallic cluster complexes of cobalt and manganese wherein each molecule of the cluster complex contains both cobalt and manganese, then their activity in the oxidation of alkyl aromatic compounds to aromatic carboxylic acids is enhanced significantly. These novel solid catalysts while retaining all the advantages of the homogeneous catalysts, like high yields of the desired aromatic carboxylic acids in the range of 96 to 98% weight, are easily separable from the reaction products by simple filtration processes, thereby not only avoiding the tedious process of catalyst recovery characteristic of prior art processes, but also eliminating the presence of toxic elements, like cobalt, manganese, nickel etc., in the waste effluent from the process. Processes utilizing these novel solid catalysts are, hence, environmentally more beneficial. Representative of the organometallic cluster complexes of cobalt and manganese of the present invention are CoMn
2
(O)(CH
3
COO)
6
, Co
2
Mn(O)(CH
3
COO)
6
, CoMn
2
(O)(CH
3
COO)
y
(pyridine)
z
, Co
2
Mn(O)(CH
3
COO)
y
(pyridine)
z
, where y+z=9, etc. It was 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 aromatic carboxylic acids in the oxidation of the alkyl aromatic compounds was the heterometallic cluster complex containing both cobalt and manganese. While the exact origin of this enhancement effect is not known in detail, it is believed that multimetallic clusters of transition metal ions are better able to activate dioxygen, O
2
, than monometallic and monomeric ions. The common prevalence of such heteronuclear multimetallic clusters in the O
2
activating enzymatic oxygenase catalyst systems supports such a suggestion. Processe

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