NO149632B - PROCEDURE FOR THE PREPARATION OF AROMATIC ISOCYANATES FROM SIMILAR NITRODE DERIVATIVES - Google Patents

Abstract

Process for the preparation of aromatic isocyanates from corresponding nitro derivatives.

Description

The present invention relates to a method for the synthesis of organic isocyanates from nitro compounds and more particularly to a method for producing aromatic isocyanates in the liquid phase by reacting aromatic nitro derivatives with carbon monoxide in the presence of new catalysts consisting of metal complexes of porphyrins.

Aromatic isocyanates are organic products of

big interest. Two of these are particularly important in the industries. These are toluene-2,4-diisocyanate and diphenyl-methane-4,4'-diisocyanate, which are used in the synthesis of polyurethanes. Industrialized processes for the preparation of these products all involve a phosgenation reaction of an amine originating from the catalytic hydrogenation of a nitro derivative. The inconveniences of these processes are numerous, they necessitate the synthesis and treatment of phosgene, a very dangerous product; they produce hydrochloric acid in significant quantities, which entails the use and maintenance of a special electrolysis plant for this acid to recover chlorine.

The interest shown in a process which avoids the use of phosgene is obvious and numerous patents claim catalytic preparations which allow, at elevated temperature and pressure, to carry out a synthesis of isocyanates with the reaction of an "organic--nitrofer or bind Ise with carbon monoxide. This is for example the case with FR-PS 1,600,529 which describes the use of a catalyst in the form of a noble metal halide in the presence of an amine base of aromatic type; DE-PS 1,910,303 which requires catalysts consisting of chlorides or oxides of Ru, Rh, Pd, Os, Ir, Pt, Ag, Au and a heteroaromatic sulfur compound, optionally in the presence of an oxide of Cr, Mo, Nb, W, V; FR-PS 1,567,321 which describes the use of a catalytic system consisting of a noble metal halide and an organic phosphorus compound, eg a triarylphosphine or a phosphite FR-PS 2,155,242 requires a catalytic system consisting of one or more halides of palladium and/or rhodium,

one or more heteroaromatic nitro bases and a co-catalyst

sator consisting of one or more iron borates; in FR-PS 2.120.110 the catalytic compound comprises, in addition to a palladium halide and heteroaromatic nitro bases,

a co-catalyst consisting of one or more molybdates.

and iron and/or manganese. All these systems give isocyanates from nitro compounds with variable selectivity and productivity.

It has now been discovered that the direct reduction reaction of aromatic nitroderivatives to isocyanates with the aid of carbon monoxide takes place with good yields with the aid of new catalysts consisting of metallic complexes of porphyrins. These complexes have good chemical and thermal stability

and does not, like the main case of the previous systems, lead to the formation of azo derivatives. The reaction equation can be written according to the following form:

Accordingly, the present invention relates to a process for the synthesis of aromatic isocyanates of the gene where x is 0 or 1 and R^, and R^, which are the same or different, are hydrogen, halogen or an alkyl or alkoxy group with from 1 to 10 carbon atoms, by reaction of a compound with the general formula

where x, R-^, R 2 and R^ have the meaning given above, with carbon monoxide in the presence of a catalyst, and this method is characterized by the fact that one or more pre-prepared catalysts are used

metal complexes of porphyrin where the metal is from group VIII and/or Ib in the periodic table of the elements, as the reaction

is carried out at a temperature between 100 and 500°C, preferably between 150 and 300°C, and at a pressure between 20 and 500 bar, preferably between 150 and 350 bar, using compounds such that the ratio between the number of gram atoms of metal and an --4 the number of nitro groups to be converted is between 10 and 1 and preferably between 5 x 10 ^ and 10

It is possible to work in the presence of an organic solvent, by a discontinuous technique in an apparatus of the autoclave type or in a continuous way by a working method that allows the isocyanate product to be removed after its formation.

Examples of aromatic compounds with one or two nitro functions that can be used according to the invention are nitrobenzene, orthonitrotoluene, paranitrotoluene, 1,2-dinitrobenzene, 1,3-dinitrobenzene, 1,4-dinitrobenzene, 2,4-dinitrotoluene, 2,6 -dinitrotoluene, 1-methoxy-2,4-dinitrobenzene, 1-chloro-2-nitrobenzene and 1-chloro-2,4-dinitrobenzene.

The metallic porphyrins used as catalysts are obtained by reacting a salt or a metal complex on a porphyrin ligand which has been previously prepared according to the synthesis methods indicated and described by K.M. Smith, in "Porphyrins and Metalloporphyrins," Elsevier (1975). The porphyrins that can be used to catalyze the reaction can optionally carry different substituents on the carbon ring, e.g. halogen atoms, alkyl groups, aryl groups, alkoxy groups, acid chlorides, amides, acetyls and nitriles; preferably a tetraphenylporphyrin or octaethylporphyrin is used..

The metals which are particularly suitable according to the invention are metals from groups VIII and Ib in the periodic table and in particular iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium

um, copper and silver.

The concentration of catalysts expressed in the ratio of the number of g atoms of metal and the number of nitro groups to be re--4

is formed, can vary between 10 and 1 and preferably between 5. 10-3 and 10<_1>.

The reaction can be carried out in the absence of solvent, but the presence of a chemically inert solvent usually favors isocyanate selectivity. Solvents that are preferably used are saturated or aromatic hydrocarbons, such as hexane, heptane, benzene, toluene or xylene, and halogenated aromatics, such as chlorobenzene and dichlorobenzene. The proportion of solvent is not significant, but usually work with solutions containing 5 to 50% by weight nitro derivative ± the solvent.

The metal portyrins can be used as such as catalysts, but they can also be deposited on a support to disperse the catalyst and facilitate recovery. Among the possible carriers, mention can be made of silicon oxide, aluminum oxide, silicon oxide-aluminum oxide, activated carbon, magnesium oxide, zirconium oxide and silicon carbide.

The reaction temperature is between 100 and 500°C and more particularly between 150 and 300°C, depending on the nature and stability of the reactants subjected to these operating conditions.

The reaction pressures are between 20 and 500 bar, and preferably between 150 and 350 bar and must be sufficient to keep a significant part of the reactants in liquid phase and to introduce a total amount of carbon monoxide corresponding to one mole of CO

ratio which is usually between 3 and 100,

N02~groups

and preferably between 10 and 65.

The tests carried out in the following examples were carried out in a discontinuous manner in a stainless steel autoclave and with a volume of 500 ml, equipped with a magnetic piping capable of working under pressures up to 500 bar and temperatures up to 300 °C. The reactor, which was charged with the various reactants in solution, was then flushed with nitrogen before being pressurized with carbon monoxide at room temperature. The insulated autoclave was then heated to the selected temperature and the progress of the reaction was regulated by recording the pressure. The first tests were carried out on mononitrated aromatic derivatives. After the reaction, the amount of isocyanate was determined by chemical dosing of dibutylamine and the amount of remaining nitro derivatives and possibly of azo derivatives by chromatography in the vapor phase.

In the following examples, the results are indicated by the following definitions:

Example 1.

A palladium tetraphenylporphyrin was prepared in the following manner, described by D.W. Thomas and A.E. Martell in "J. Amer.

Chem. Soc", vol. 81, page 51 1 1 (1959). 10 g of purified tetraphenylporphyrin and 7.7 g of palladium chloride were placed in 385 ml of acetic acid. The whole was brought to reflux temperature for 10 minutes and then 23 g of sodium acetate was added, after which the whole was allowed to reflux for 1 hour and another 23 g of sodium acetate was added. After a further 2 hours under reflux, the mixture was allowed to cool and poured into 800 ml of water. The precipitate was recovered by filtration, washed with water and after drying, obtained 12.3 g of the palladium complex with a light brown color.

The chemical analysis indicated a palladium amount of 14.8% and a chlorine amount of less than 0.5%. 2.0 g of the palladin porphyrin, 30 g of nitrobenzene were charged to the previously described autoclave and the volume was made up to 100 ml with orthodichlorobenzene. A stream of nitrogen under atmospheric pressure was passed through the autoclave before carbon monoxide was forced in up to a pressure of 200 bar at 20°C. The autoclave was insulated and heated to 230°C, which brought the pressure to 340 bar. The reaction was allowed to proceed for 7 hours with stirring, then the whole was allowed to cool and the mixture was analysed. The TTG for nitrobenzene was 25.4% and the phenylisocyanate selectivity was 94.8%. No azobenzene was formed.

Example 2.

In the autoclave described above, 2 g of palladium tetraphenylporphyrin, 30 g of nitrobenzene were added and the volume was adjusted to 100 ml using orthodichlorobenzene. The reactor was closed and after flushing with nitrogen, carbon monoxide was added up to a pressure of 200 bar at 20°C. The autoclave was isolated and stirring was maintained while heating to 235°C for 3 hours and then 240°C for 4 hours and 30 minutes. After cooling, the mixture was analyzed and a TTG value for nitrobenzene of 94.7% and a phenylisocyanate selectivity of 60.2% was obtained. The formation of azobenzene was 0.

Example 3.

1.4 g of palladium tetraphenylporphyrin was added to the autoclave. 20 g of nitrobenzene were added and the volume was brought to 200 ml by means of orthodichlorobenzene. Nitrogen was flushed and carbon monoxide was added to a pressure of 200 bar at ^° C. The autoclave was isolated and kept under stirring and the whole was heated to 240° C. for 4 hours and then 250° C. for 4 hours. After cooling, the analysis showed a nitrobenzene TTG of 1.00% and a phenylisocyanate selectivity of 69.3%. No trace of azobenzene was found.

Example 4.

0.7 g of palladium tetraphenylporphyrin, 10 g of nitrobenzene were added to the autoclave and the volume was made up to 100 ml with

orthodichlorobenzene. The autoclave was closed and after flushing with nitrogen, carbon monoxide was added until the pressure was 200 bar at 20°C. After isolating the reactor and starting the stirring, the whole thing was heated to 250°C for 1 hour and then 243°C for another 1 hour. After cooling, the analysis showed a TTG value for nitrobenzene of 100% and a phenylisocyanate selectivity of 69.3%. The formation of azobenzene was 0.

Example 5.

0.7 g of palladium tetraphenylporphyrin, 10 g of nitrobenzene were added to the autoclave and the volume was made up to 100 ml with orthodichlorobenzene. After flushing with nitrogen, it was

introduced carbon monoxide until the pressure was 200 bar at ordinary ) temperature. The autoclave was isolated, the stirring started and the whole was heated to 240°C for 2 hours and then held at 235°C for another 1 hour. After cooling, the mixture was analyzed and a TTG value of 100% was obtained for nitrobenzene, while the phenylisocyanate selectivity was 81.2%. There were no traces of azobenzene.

Example 6.

0.7 g of palladium tetraphenylporphyrin, 10 g of orthonitrotoluene were added to the autoclave and the volume was brought to

100 ml using orthodichlorobenzene. After flushing with nitrogen, carbon monoxide was added until the pressure was 200 bar at 20°C. The autoclave was isolated, the stirring started and the whole thing was heated to 240°C for 3 hours and then to 245°C for 2 hours and 30 minutes and finally at 250°C

for 2 hours. After cooling, the analysis showed a TTG value for orthonitrotoluene of 17.5% and an orthotoluene isocyanate selectivity of 55%.

Example 7.

Palladium tetraphenylporphyrin was prepared according to the method described by A.D. Adler, F.R. Longo,

F. Kampas and J. Kim in "J. Inorg. Nucl. Chem.", vol. 32, page 2443, (1970). To 300 ml of dimethylformamide under reflux was added but 3 g of tetraphenylporphyrin. After dissolving this, 0.9 g of palladium chloride was added. The whole was refluxed for a further 10 minutes, after which the mixture was cooled in an ice bath and poured into 300 ml of distilled water. The precipitate formed was filtered off, washed with water and dried.

The chemical analysis indicated a palladium amount of 14.3% and a chloride amount of 2.4%. A solution of 100 ml containing 10 g of nitrobenzene, 0.7 g of palladium porphyrin, while the remainder was orthodichlorobenzene, was added to the autoclave. A stream of nitrogen was passed through the atmosphere before carbon monoxide was added under pressure until the pressure was 200 bar at 20°C. The autoclave was isolated and heated to 240°C for 3 hours while stirring, after which it was cooled and analyzed. The TTG value for nitrobenzene was 100% and the selectivity for the phenyl isocyanate was 60.2%. No azobenzene was formed.

Example 8.

Iron tetraphenylporphyrin was prepared according to the method described in example 7. 2 g of tetraphenylporphyrin were dissolved in 200 ml of dimethylformamide under reflux, after which 1.6 g of FeC~[ 2 • 4H 2 O was added. The whole was refluxed for 20 minutes, after which the mixture was cooled in an ice bath and the complex was precipitated by the addition of 200 ml of oxygenated water. It was filtered under nitro-

gene, washed with water and dried under vacuum.

2.3 g of complex were obtained, one part of which was kept under vacuum and the other part in a desiccator. To the previously described autoclave, 0.7 g of iron tetraphenylporphyrin which had been stored under vacuum, 10 g of nitrobenzene were added and the volume was completed to 100 ml by means of orthodichlorobenzene. The autoclave was closed and after flushing with nitrogen, carbon monoxide was added until the pressure was 200 bar at 20°C. After isolating the reactor and starting the stirring, it was heated to 240°C for 6 hours and 30 minutes. After cooling, the mixture was analyzed and the TTG value for nitrobenzene was 88.1% and the phenyl isocyanate selectivity was 80.6%. No azobenzene was formed.

Example 9.

To the autoclave was added 10 g of nitrobenzene, 0.7 g of iron tetraphenylporphyrin stored in air, prepared as described above, and the volume was made up to 100 ml using orthodichlorobenzene. After working in the same way as in Example 8, the analysis showed a TTG value for nitrobenzene of 92.2%, a selectivity for phenyl isocyanate of 59.7%. There was no azobenzene.

Claims (2)

1. Process for the production of aromatic isocyanates of the general formula: where x is 0 or 1 and R-^, R 2 and R^, which are the same or different, are hydrogen, halogen or an alkyl or alkoxy group having from 1 to 10 carbon atoms, by reaction of a compound of the general formula where x, R^, R2 and R^ have the above meaning, with carbon monoxide in the presence of a catalyst, characterized in that one or more pre-prepared metal complexes of porphyrin are used as catalyst, where the metal is from group VIII and/or Ib in the periodic table of the elements, the reaction being carried out at a temperature between 100 and 500°C, preferably between 150 and 350°C, and at a pressure between 20 and 500 bar, preferably between 150 and 350 bar, using compounds such that the ratio between the number of gram atoms of metal and the number of -4 nitro groups to be converted is between 10 and 1 and -3 preferably between 5 x 10 and 10 2. Process according to claim 1, characterized in that palladium-tetraphenylporphyrin is used as catalyst.