Process for preparing hydroxyaromatics

Details Process for preparing hydroxyaromatics Process for preparing hydroxyaromatics

2001-07-24

2002-11-05

Shippen, Michael L. (Department: 1621)

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

Organic compounds

Oxygen containing

C558S424000, C568S705000, C568S730000, C568S774000, C568S775000, C568S780000

Reexamination Certificate

active

06476277

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for preparing hydroxyaromatics by oxidizing aromatics with dintrogen monoxide in the gas phase in the presence of nanocrystalline zeolites.
Hydroxyaromatics are valuable intermediates in organic chemistry that are used for synthesizing numerous further intermediates and end products. Hydroxybenzenes are used in photography, as antioxidants, and as stabilizers in plastics. Phenol, for example, is further processed to form phenol resins, caprolactam, bisphenol A, adipic acid, or alkylphenols. Cresols are used for preparing antioxidants, herbicides, insecticides, dyes, and also odorants and flavorings. Among the three cresol isomers, p-cresol is, from economic aspects, the methylphenol of most interest. p-Cresol is used for synthesizing plastics, lubricating oils, medicaments, and nutraceuticals and for preparing perfumes and flavorings. However, it is predominantly used for producing BHT (2,6-di-tert-butyl-4-hydroxy-toluene), an important anti-aging agent and antioxidant.
A number of processes exist for preparing hydroxyaromatics. Thus, there currently exist, for example, four important industrial processes for synthesizing cresol, which are based on two synthesis routes: either toluene is hydroxylated or phenol is alkylated.
Cresol is produced from alkaline melts of toluenesulfonates in four reaction steps. Toluene is first sulfonated by concentrated sulfuric acid, and the resultant sulfonic acid mixture is neutralized with sodium sulfite or sodium hydroxide solution and then fused with sodium hydroxide at approximately 300° C. An aqueous solution of the melt is then acidified with sulfur dioxide or sulfuric acid, which releases the cresols. Generally, a cresol mixture is produced comprising 6 to 12% o-cresol, 6 to 12% m-cresol and 80 to 85% p-cresol. The p-cresol can be separated off by means of fractional crystallization. However, this process has the disadvantage that large amounts of sodium sulfite are produced that must be disposed of.
Chlorotoluene hydrolysis is used especially for m-cresol production. In the first step, toluene is chlorinated in the presence of iron chlorides and disulfur dichloride, an o/p-chlorotoluene mixture in a ratio of 1:1 being formed. If the hydrolysis with sodium hydroxide solution is carried out directly thereafter, o-, m-, and p-cresols are produced in a ratio of 1:2:1. After rectification, an o-cresol fraction is obtained therefrom together with a poorly separable m/p-cresol mixture in a ratio of 3:1. For this reason, the chlorotoluene isomers are usually isolated and separated and only then hydrolyzed. From o-chlorotoluene, after hydrolysis and subsequent distillation, pure o- and m-cresol can be produced but p-chlorotoluene still forms a 1:1 mixture of m- and p-cresol. Disadvantages in this process are, in addition to the low yield of p-cresol, the production of by-products, such as tolyl cresols and tolyl ethers.
In the three-stage cymene hydroperoxide cleavage, toluene is alkylated with propene in the first step, forming an isomeric mixture of the cymenes. These are then oxidized with oxygen to cymene hydroperoxides that are cleaved in a third step by acid catalysis to form m/p-cresol in a ratio of 3:2 and acetone. In addition a multiplicity of by-products are formed. This process produces a distribution of isomers of 3% o-cymene, 64% m-cymene, and 33% p-cymene. The yields are low, which is principally due to the low oxidation rates of the cymenes, which only permit oxidation rates of 20% for a reasonable production of by-products. In addition, the requirements for separation and for wastewater treatment are high. In addition, the acetone formed in equivalent amounts to the cresol greatly affects the economic efficiency of the process.
Cresol synthesis by methylation of phenol is carried out both in the gas phase and in the liquid phase. Compared with toluene, phenol is a relatively expensive starting material and gives only a very low yield, to none at all, of p-cresol. A further disadvantage of the single-stage alkylation of phenol is the very high separation requirement for the product mixtures, since their components have boiling points that are very close together. In addition, high capital costs for corrosion-resistant plants considerably impair the economic efficiency of the process.
Since currently the industrially implemented processes give an unfavorable isomeric distribution of the cresols with respect to p-cresol, and this may only be shifted by multistage processes in the direction of higher p-cresol contents, a single-stage, selective p-cresol synthesis would be desirable. In this case the heterogeneously catalyzed selective oxidation of toluene with dinitrogen monoxide in the gas phase is an interesting alternative. The use of dinitrogen monoxide as oxidizing agent is a new synthesis route that, since the beginning of the 1980s, has been pursued for the direct synthesis of phenol from benzene. The reaction is catalyzed by zeolites.
DE-A-196 34 406 describes a process for reacting aromatics with dinitrogen monoxide to give the corresponding hydroxyaromatics, the catalyst used being a zeolite of the pentasil or &bgr; type that is subjected to a hydrothermal pretreatment with water vapor. In the hydroxylation of toluene, a selectivity with respect to cresol of 27% is achieved at a conversion rate of 24%. A disadvantage of this process is that the p-cresol content in the cresol fraction is only 16%.
U.S. Pat. No. 5,110,995 describes a process for toluene hydroxylation using dinitrogen monoxide that is carried out in the presence of special iron-containing zeolites in a narrow temperature range of 275 to 450° C. At a conversion rate of 48%, a selectivity with respect to cresol of 20% is achieved. The p-cresol content in the cresol fraction is 33%.
EP-A 889,081 describes a process for preparing hydroxyaromatics that is carried out in the presence of zeolites that have passed through a special two-stage calcination process. In the hydroxylation of toluene, a conversion rate of 25% and a yield with respect to cresol of 22% were achieved. No information is provided on the distribution of isomers.
There was therefore a need for a process for hydroxylating toluene with dinitrogen monoxide that gives p-cresol in high yield and selectivity.
SUMMARY OF THE INVENTION
A process has been found for preparing hydroxyaromatics comprising reacting aromatics of the formula (I)
Ar—R
n
  (I)
where
Ar represents benzene or naphthalene,
R represents Br, Cl, F, NO
2
, CN, NH
2
, OH, C
1
-C
6
-alkyl, or phenyl, and
n denotes zero, 1, or 2,
with dinitrogen monoxide in the gas phase in the presence of zeolites selected from the group consisting of pentasils, ferrierite, and zeolite-&bgr;, wherein the zeolites have a crystallite size <100 nm and are calcined at temperatures of 500 to 1350° C. before use.
DETAILED DESCRIPTION OF THE INVENTION
In the inventive process, aromatics of the formula (I) are used. These are unsubstituted or substituted benzene or unsubstituted or substituted naphthalene. Preferably, benzene, C
1
-C
6
-alkylbenzene, chlorobenzene, fluorobenzene, benzonitrile, naphthalene, or biphenyl are used. Particularly preferably, C
1
-C
6
-alkylbenzene is used, very particularly preferably toluene is used.
Hereinafter, the inventively used zeolites are described in more detail. In general, zeolites are crystalline aluminosilicates that have a highly ordered structure having a rigid three-dimensional network of SiO
4
and AlO
4
tetrahedra, which are joined by shared oxygen atoms. The electrovalency of the aluminum-containing tetrahedra is equalized by including cations in the crystal, for example, by those of the first, second, or third main group of the Periodic Table of the Elements, or by hydrogen ions. Cation exchange is possible. The spaces between the tetrahedra are occupied by water molecules before dehydration by drying or calcination.
In the inventive process, nanocrystalline zeolites from the group consisting of pentasils, &bgr;-zeolite, and ferrie

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