BACKGROUND TO THE INVENTION Cyclohexylbenzene can be used to produce phenol, which is one of the most important industrial chemicals in the world. As of December 1995, more than 88% of world phenol capacity was based on cumene peroxidation with acetone coproduction. One of the primary economic difficulties of the cumene peroxidation route is that it requires the existence of an available market for the co-produced acetone. Currently, the growth of market demand for phenol exceeds that for acetone, and hence there exists an acetone oversupply problem. It is expected that this unbalanced growth will continue for some time.

Hydroperoxidation of cyclohexylbenzene (analogous to cumene peroxidation) could offer an alternative route for phenol production without the problem of acetone co-production. This alternative route co-produces cyclohexanone, which is a much more valuable and desirable by-product than acetone. Thus cyclohexanone is useful for the manufacture of caprolactam and nylon.

Dehydrogenation of cyclohexylbenzene also offers a low cost alternative to produce diphenyl from benzene. Diphenyl is used mainly for heat-transfer applications. Currently the main source of diphenyl is as a by-product (1 g diphenyl/100 g benzene) in benzene production by toluene dealkylation. The crude diphenyl is refined to 93-97% purity by distillation. High purity diphenyl can also be produced by direct thermal dehydrocondensation of benzene at 700-

800°C in gas or electrically heated tubular reactors. This process is energy intensive and produces by-products of terphenyl, higher polyphenyls and tars.

It is known that cyclohexylbenzene can be produced from benzene by the process of hydroalkylation or reductive alkylation. In this process, benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce cyclohexene which then alkylates the benzene starting material. Thus US Patent Nos 4,094,918 and 4,177,165 disclose hydroalkylation of aromatic hydrocarbons over catalysts which comprise nickel-and rare earth-treated zeolites and a palladium promoter. Similarly, US Patent Nos 4,122,125 and 4,206,082 disclose the use of ruthenium and nickel compounds supported on rare earth-treated zeolites as aromatic hydroalkylation catalysts. The zeolites employed in these prior art processes are zeolites X and Y. More recently, US Patent No. 5,053,571 has proposed the use of ruthenium and nickel supported on zeolite beta as an aromatic hydroalkylation catalyst.

In our co-pending U. S. Patent Application Serial No. 09/112546 filed July 9,1998 we have described a process for the hydroalkylation of aromatic hydrocarbons over a catalyst comprising a hydrogenation metal and a crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima at 12.4i0.25,6.90.15,3.570.07 and 3.420.07 Angstrom. Preferably the oxide material is MCM-22.

However, known methods for the hydroalkylation of benzene produce significant quantities of cyclohexane and dicyclohexylbenzene as byproducts.

Cyclohexane can be readily recovered by dehydrogenation to benzene.

However, there is need for an effective process for converting the dicyclohexylbenzene by-product to useful products, preferably the desired monocyclohexylbenzene. One possible solution is transalkylation of the dicyclohexylbenzene with benzene to form additional cyclohexylbenzene. To date, little work has been reported on dicyclohexylbenzene/benzene transalkylation. Moreover, although certain catalysts, such as rare earth exchanged faujasite (U. S. Patent 3,984,490), aluminum chloride and acid clay, have been proposed for this reaction, the existing catalysts suffer from significant shortcomings. Accordingly, a need exists for an improved process for the

transalkylation of polycycloalkyl aromatic compounds to their monocycloalkyl analogs.

SUMMARY OF THE INVENTION According to the invention there is provided a process for the transalkylation of a polycycloalkyl aromatic compound comprising the step of contacting the polycycloalkyl aromatic compound with benzene in the presence of a catalyst selected from the group consisting of an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIBA metal oxide, TEA-mordenite, zeolite beta and a porous crystalline material having an X-ray diffraction pattern including d-spacing maxima at 12.40.25,6.90.15,3.570.07 and 3.420.07 Angstrom.

Preferably, the polycycloalkyl aromatic compound is dicyclohexylbenzene.

Preferably, said acidic solid is a WOx/ZrO2 material.

Preferably, said porous crystalline material is MCM-22.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process for the transalkylation of a polycycloalkyl aromatic compound, particularly dicyclohexylbenzene, with benzene to produce the monocycloalkyl aromatic analog, particularly monocyclohexylbenzene. The process is conducted in the presence of a catalyst comprising a Group IVB metal oxide modified with an oxyanion of a Group VIBA metal oxide, TEA-mordenite, zeolite beta or a porous crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima at 12.4_0.25,6.90.15,3.570.07 and 3.420.07 Angstrom.

Mixed metal oxide catalysts of the type comprising a Group IVB metal oxide modified with an oxyanion of a Group VIBA metal oxide are described in, for example, U. S. Patent No. 5,510,309, the entire contents of which are incorporated herein by reference. Preferably, the Group IVB metal oxide is an oxide of zirconium or titanium and the Group VIB metal oxyanion is an oxyanion of tungsten or molybdenum. Most preferably, the mixed metal oxide is woX/zro2-

Zeolite beta is described in U. S. Patent No. 3,308,069.

TEA-mordenite, i. e., synthetic mordenite prepared from a reaction mixture comprising a tetraethylammonium directing agent, is disclosed in U. S. Patent Nos. 3,766,093 and 3,894,104. Small crystal TEA-mordenite having a crystal size less than 0.5 micron is disclosed in U. S. Patent Application Serial No.

09/305019 filed May 4, 1999.

Alternatively, the catalyst may comprise a porous crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima at 12.4t0.25,6.90.15,3.570.07 and 3.420.07 Angstrom. The X-ray diffraction data used throughout this specification were obtained by standard techniques using the K-alpha doublet of copper as the incident radiation and a diffractometer equipped with a scintillation counter and associated computer as the collection system.

Suitable inorganic oxide materials are MCM-22 (described in U. S. Patent No. 4, 954,325), PSH-3 (described in U. S. Patent No. 4,439,409), SSZ-25 (described in U. S. Patent No. 4,826,667), MCM-36 (described in U. S. Patent No.

5,250,277), MCM-49 (described in U. S. Patent No. 5,236,575) and MCM-56 (described in U. S. Patent No. 5, 362, 697), with MCM-22 being particularly preferred.

The preferred catalyst is WOx/ZrO2.

The process of the invention is carried out by admixing benzene with the polycycloalkyl aromatic compound in a weight ratio of about 1: 1 to 5: 1, preferably 2: 1 to 4: 1. The mixture is then contacted with a catalyst as described above under reaction conditions including a temperature of about 100 to 300°C, a pressure of about 800 to 3500 kPa and a WHSV of about 1 to 10. Preferably, the reaction conditions including a temperature of about 130 to 250°C, a pressure of about 1500 to 2800 kPa and a WHSV of about 2 to 8.

The invention will now be more particularly described with reference to the accompanying Example.

Example 1 A general procedure described here was foliowed to test catalysts for transalkylation of dicyclohexylbenzene with benzene. A catalyst containing 65 wt % zeolite Beta and 35 wt % alumina binder as 1/16"cylindrical extrudate was chopped to 1/16"length prior to use. One gram of this catalyst was diluted with sand to 3.0 cc and the catalyst/sand mixture was charged to a fixed-bed micro reactor with 3/8"outside diameter. The catalyst was dried with 100 cc/min of flowing nitrogen for 2 hours at 125°C and 1 atm pressure. Nitrogen was turned off and the reactor pressure was set to 300 psig. A feed containing 75% of benzene and 25% of p-dicyclohexylbenzene by weight was introduced to the reactor through a syringe pump at 60 cc/hour for 1 hr while the reactor was ramped to the desired temperature. The feed rate was then reduced to 4.0 WHSV for the test run. Liquid products were collected in a cold product trap and analyzed off-line. The results are summarised in Table 1.

The above procedure was repeated with catalysts containing WOx/ZrO2, TEA-mordenite and MCM-22. Again the results are summarised in Table 1.

Table 1 Catalyst WOx/ZrO2 TEA-Mordenite Beta MCM-22 Binder None 35% alumina 35% alumina 35% alumina Days on Stream 1. 1 3. 7 2. 8 3. 8 Temperature, °C 150 170 180 195 <BR> <BR> p-Dicyclohexylbenzene 63. 0 46. 8 47. 0 41. 1<BR> Conversion,63. 0 46. 8 47. 0 41. 1 Selectivity, wt % Lights 0.227 0.030 0.047 0.039 Methylcyclopentane 0.442 0.022 0.024 0.205 Cyclohexane 0.091 0.138 0.428 0.086 Toluene 0.028 0.000 0.028 0.000 Other Cl2 compounds 1.609 5.927 5.852 6.497 Phenylcyclohexane 96.640 93.437 92.769 92.675 Other C, compounds 0.963 0.446 0.853 0.498 Sum 100.00 100. 00 100. 00 100.00 It will be seen that the WOx/ZrO2 catalyst was very active for this reaction and gave phenylcyclohexane with high selectivity.