Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene. Thus, a process that does not require propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol.
One such process involves the catalytic hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513, in which the hydroalkylation catalyst is a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family.
However, one problem in producing phenol via cyclohexylbenzene is that the oxidation of cyclohexylbenzene is considerably more difficult than that of cumene. Thus, whereas cumene oxidation is normally conducted in the absence of a catalyst, cyclohexylbenzene oxidation typically requires the presence of a catalyst containing a nitroxyl radical, such as N-hydroxyphthalimide (NHPI), to provide commercially acceptable levels of conversion. However, even using NHPI as a catalyst, the selectivity to cyclohexylbenzene hydroperoxide decreases with increasing conversion. Thus the product of the cyclohexylbenzene oxidation step typically contains large amounts (of the order of 80 wt %) of unreacted cyclohexylbenzene as well as significant quantities of impurities. If these materials are allowed to pass to the ensuing cleavage step, not only can they generate expensive separation problems but also some of the impurities may be converted in the cleavage process to produce tars and thereby reduce the yield of phenol and cyclohexanone.
There is therefore significant incentive to concentrate/isolate the cyclohexylbenzene hydroperoxide after the oxidation step. Thus, using pure or high concentrated of cyclohexylbenzene hydroperoxide in the cleavage step may reduce tar/by-product formation during cleavage and will also reduce the volume of cyclohexylbenzene that needs to be treated and handled after the cleavage step. However, whereas cumene hydroperoxide can readily be concentrated by distillation, the high boiling point of cyclohexylbenzene (240° C.) means that concentration of cyclohexylbenzene hydroperoxide by distillation requires high temperatures and/or high vacuum, which can lead to thermal decomposition of the hydroperoxide. Thus alternative methods of concentrating and/or purifying cyclohexylbenzene hydroperoxide would be desirable.
According to the present invention, it has now been found that high purity cyclohexylbenzene hydroperoxide can be recovered from the reaction product of cyclohexylbenzene oxidation by selective crystallization. After separation from the mother liquor, the resultant cyclohexylbenzene hydroperoxide crystals can be redissolved in a solvent, such as acetone, and then fed to the cleavage step.
U.S. Pat. No. 3,821,314 discloses a process for the separation of cyclohexylbenzene hydroperoxide from a mixture of cyclohexylbenzene and its oxidation products by adsorption of the oxidation products with a polyurethane foam. The oxidation products can then be separated selectively from the foam by elution with a non-polar hydrocarbon solvent, such as pentane.
U.S. Pat. No. 3,959,381 discloses a two-stage process for purifying cyclohexylbenzene hydroperoxide produced by the oxidation of cyclohexylbenzene, in which the oxidation effluent is initially subjected to vacuum distillation at a temperature between ambient and 90° C. and a pressure between about 0.1 and 0.5 mm Hg to remove at least part of the unreacted cyclohexylbenzene. The distillation residue is then treated with a lower (C5 to C10) liquid alkane to separate the cyclohexylbenzene hydroperoxide as extract from the oxidation by-products.
U.S. Pat. No. 7,285,684 discloses a process for separating a reaction product and an imide compound having an imide unit represented by the following formula:
wherein X represents an oxygen atom, a hydroxyl group, or an acyloxy group, which process comprises separating the imide compound from the reaction mixture by solvent-crystallizing the imide compound with water or a mixture of water and at least one solvent selected from the group consisting of a hydrocarbon and a chain ether, or, when the reaction mixture is obtained by an oxidation reaction of a monocyclic C4-C16 cycloalkane substrate, with at least one solvent selected from the group consisting of a hydrocarbon and a chain ether.