Method for producing aromatic alcohols, especially phenol

Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing

Reexamination Certificate

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C568S573000

Reexamination Certificate

active

06720462

ABSTRACT:

The invention relates to a process for preparing aromatic alcohols, in particular phenol, by catalytic oxidation of aromatic hydrocarbons to the corresponding hydroperoxides and subsequent cleavage of the hydroperoxides.
To obtain hydroxyl-containing aromatic compounds such as phenol, it is not possible to convert benzenes directly in one stage and in high yields into the phenols by selective oxidation using atmospheric oxygen. Either the aromatic ring is not attacked at all or the oxidation continues to carbon dioxide since the aromatic ring which has been functionalized by oxygen atoms is more reactive than the starting benzene.
For this reason, the hydroxyl group has to be introduced into the aromatic system via intermediates.
The cumene process is frequently employed for preparing phenol derivatives from benzene derivatives, i.e. aromatic hydrocarbons. In this process, for example, cumene prepared from benzene and propene by alkylation is peroxidized and the oxidation product is then cleaved into the two products of value phenol and acetone (“Hock process”). Owing to its good economics, this process has become established worldwide for the production of phenol.
Both the preparation of the starting material prepared by alkylation, e.g. cumene, cyclohexylbenzene (in the Hock process, cyclohexylbenzene gives not acetone but cyclohexanone) and cyclododecylbenzene (in the Hock process, cyclododecylbenzene gives cyclododecanone), and the acid-catalysed cleavage and rearrangement to phenol which follow the oxidation step generally proceed with high selectivity to give high yields.
The economics of the Hock process are therefore particularly critically dependent on the selectivity of the oxidation of the tertiary carbon atom and on the reaction rate and the conversion.
For this reason, a great deal of effort has been directed, in particular, at the preparation of the peroxide. In practice, the oxidation of the starting material by means of atmospheric oxygen has proven useful. Additives such as free-radical initiators or the use of other oxidants, e.g. the compounds KMnO
4
, CrO
3
and HNO
3
frequently used for the oxidation of hydrocarbons, adversely affect the selectivity, lead to disposal problems, produce ecologically unacceptable by-products and corrode the plant.
The use of metal redox catalysts makes it possible to utilize molecular oxygen for the oxidation of organic compounds. A series of industrial processes are based on the metal-catalysed autooxidation of hydrocarbons. Thus, for example, the oxidation of cyclohexane to cyclohexanol or cyclohexanone by means of O
2
is carried out using cobalt salts. This process is based on a free-radical chain mechanism. The diradical oxygenreacts with a hydrocarbon radical to form a peroxy radical and subsequent chain propagation by abstraction of an H atom from a further hydrocarbon. Apart from metal salts, it is also possible for organic molecules to function as free-radical initiators.
A disadvantage of this process is that the selectivity drops very severely with increasing conversion and the process therefore has to be operated at a low conversion level. Thus, for example, the oxidation of cyclohexane to cyclohexanol/cyclohexanone is carried out at a conversion of from 10 to 12% so that the selectivity is from 80 to 85% (“Industrielle Organische Chemie” 1994, 261, VCH Verlagsgesellschaft mbH, D-69451 Weinheim). In a further important industrial autooxidation process for cumene oxidation, the conversion is about 30% as a cumene hydroperoxide sensitivity of about 90% (loc. cit. p. 495 ff).
An alternative to metal salt catalysts is provided by the use of catalyst systems or mediator systems such as N-hydroxyphthalimide (NHPI). However, the reaction rate in the processes described in the literature is not satisfactory despite the high amount of mediator (up to an equimolar amount based on the substrate) (J. Mol. Catalysis A. 1997, 117, 123-137). Thus, U.S. Pat. No. 5,030,739 describes the use of N-hydroxydicarboximides for the oxidation of isoprene derivatives to the corresponding acrolein compounds. Combined oxidation/dehydration of cyclohexadienes or six-membered ring systems such as &agr;-terpenes leads to the cumene derivative which is, however, not oxidized further. This process is therefore unsuitable for the conversion of cumene into cumene hydroperoxide.
In general, amounts of mediator of at least 10 mol % based on the substrate are used, with larger amounts of mediator being used to increase the reaction rate (
J. Org. Chem.
1995, 60, 3934-3935). The product selectivity is not satisfactory for industrial use. Thus, oxidation of cumene using NHPI gives a product mixture comprising acetophenone as main product, but the desired oxidation product cumene hydroperoxide was not able to be isolated (
J. Org. Chem.
1995, 60, 3934-3935).
A further development of the system is the use of cocatalysts. Cocatalysts which can be used are metal compounds, in particular heavy metal salts, enzymes or strong Brönsted acids. Thus, Ishii et al., demonstrated that NHPI in combination with metal salts as cocatalyst can display advantages over the oxidation using NHPI without cocatalyst (e.g. EP 0878234, EP 0864555, EP 0878458, EP 0858835, JP 11180913, J. Mol. Catalysis A. 1997, 117, 123-137). However, a disadvantage of these systems is, apart from the undesirable heavy metal content, the large amount of NHPI used here, too. To ensure a satisfactory reaction rate, at least 10 mol % of mediator has to be used. A further disadvantage is that some of the redox metals used catalysed further reactions of the products and thus reduce the selectivity of the reaction.
Processes which use only a mediator without a cocatalyst have also become known. However, these are restricted to the oxidation of particularly activated substrates such as ethers, esters or isoprene derivatives.
Thus, the oxidation of cumene using the system NHPI/cobalt acetate gives a product mixture comprising acetophenone (selectivity: 54%), 2-phenyl-2-propanol (10%) and phenol (17%) (J. Mol. Catal. A 1997, 117, 123-137). The desired product cumene hydroperoxide is formed only as an intermediate and is not stable under the prevailing process conditions. The phenol wanted as final product is obtained in relatively minor amounts compared to the primary oxidation product acetophenone.
A further process variant comprises the use of NHPI in combination with alcohols or aldehydes (Chem. Commun, 1999, 727-728, Tetrahedron Letters 1999, 40, 2165-2168, Chem. Commun. 1997, 447-448). Disadvantages of these processes are the formation of coproducts and the high mediator/substrate ratio employed (10 mol %). DE 19723890 describes an oxidation system comprising an organic mediator and the redox enzyme laccase for the preparation of aromatic and heteroaromatic aldehydes and ketones. Here too, the amount of mediator used is very high. In addition, due to the use of an enzyme, this process involves a complicated reaction system with a biologically necessary buffer system which restricts broad applicability of the system.
It is an object of the present invention to develop a heavy-metal-free or metal-free process for preparing aromatic alcohols, in particular phenols, by catalytic oxidation of hydrocarbons to the hydroperoxides with subsequent cleavage of the hydroperoxides, which process displays high selectivities at high conversions.
It has surprisingly been found that compounds of the type
can be used even without heavy metals or strong acids as cocatalysts for the oxidation of aromatic hydrocarbons to the corresponding hydroperoxides.
The present invention accordingly provides a process for preparing phenol derivatives by catalytic oxidation of an aromatic hydrocarbon to the hydroperoxide and subsequent cleavage of the hydroperoxide to give the phenol derivative and a ketone, wherein a compound of the formula I
where
R
1
, R
2
=H, aliphatic or aromatic alkoxy radical, carboxyl radical, alkoxycarbonyl radical or hydrocarbon radical, in each case having from 1 to 20 carbon atoms, SO
3
H, NH

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