RHENIUM CATALYST SUPPORTED ON MODIFIED ALUMINA AND USE THEREOF IN THE METATHESIS REACTION OF OLEFINS The present invention relates to a process for the preparation of a heterogeneous catalyst containing alumina as inert carrier, rhenium as the first active catalytic component and an inorganic halide, introduced either before the supporting of the rhenium or contemporaneously therewith, as the second active catalytic component. The activation of the heterogeneous catalyst thus pre¬ pared is effected by means of..thermal" treatment followed-- ,by a rapid final cooling. The present invention also relates to the use of said catalyst in the metathesis reaction of olefins. A metathesis reaction, also known as dismutation or disproportionation of olefins, is a reaction of great prac¬ tical interest which can be used, for example, for balanc¬ ing the weight of olefins resulting from steam cracking. When olefins are treated in the presence of suitable catalysts, they are converted to other olefins in a reac- tion in which the alkylidene groups (R1R2C=) are inter- exchanged with a process schematically represented by the following equation:

R1R2C=CR1R2 R1R2C=CR3R4

R3R4C=CR3R4 R1R2C=CR3R4

Heterogeneous catalysts essentially consisting of rhe¬ nium derivatives supported on inert materials (for example silica or alumina) are known to be active in the metathesis of olefins. For example, US 3,641,189 and US 3,676,520 de¬ scribe the preparation of these materials and their use in the metathesis of olefins. In the preparation of this catalyst, the active compo- nent is normally introduced onto the surface of the carrier through ■impregnation. In this., reaction, the carrier §»s mixed with a solution in which the active component has been dissolved. When the solvent is removed by evaporation, the active component remains inside the carrier particles . With these catalysts, however, it is necessary for the active component to be present in amounts ranging from 5 to 7% and, in spite of this, not particularly high yields have been observed, whereas", in the case of higher olefins, there is also a poor selectivity, often due to secondary isomerization reactions of double bonds (J. MoI. Cat: 46, 1988, 119-130 and App. Catal . , 70, 1991, 295-306) . With the aim of overcoming the above limits, it has been found that by treating alumina with HCl, the catalyst activity is improved, even if secondary isomerization reac- tions still remain (J. Catal. 150, 46-55, 1994) . It has now been found that it is possible to overcome the above-mentioned drawbacks and to obtain opti¬ mum catalyst performances, using decidedly lower amounts of active component, by means of the catalyst of the present invention containing alumina as inert carrier, rhenium as the first active component and a suitable inorganic halide, introduced before the rhenium is supported or contemporane¬ ously therewith, as the second active component. The acti¬ vation of the heterogeneous catalyst thus prepared is ef- fected by thermal treatment followed by a rapid final cool¬ ing. Said catalyst is active in metathesis reactions even when used in the absence of a conventional co-catalyst and allows problems due to. the formation of isomers or side- reactions to be reduced, obtaining a high selectivity. In accordance with the above, an object of the present invention therefore relates to a heterogeneous catalyst ac¬ tive in the metathesis reaction of olefins, comprising alu¬ mina as inert carrier and a rhenium compound as active com- ponent, characterized in that rhenium is present in an amount of less than 5% by weight with respect to the total, preferably from 1 to 4% by weight, and the inert carrier is impregnated with at least one inorganic halide selected from FeCl3, CuCl2, TiCl4, RuCl3, ZnCl2 and NH4Cl, and/or the corresponding bromides or iodides, and subsequently treated at a high temperature. According to the present invention, alumina is preferably used as inert carrier, with a surface area > 50 iriVg, preferably from 100 to 200 m2/g, and a total cumula- tive pore volume higher than 0.1 ml/g, preferably from 0.3 to 0.8 ml/g. The rhenium compound can be introduced into the car¬ rier, possibly pretreated at a temperature ranging from 100 to 6000C, in the presence of a stream of air, either con- temporaneously with the halide or separately, through pre¬ cipitation or impregnation starting from precursors con¬ sisting, for example, of solutions of its salts or soluble complexes. The rhenium precursors are selected from rhenium hep- toxide, ammonium perrhenate, tetra-alkyl ammonium perrhen- ate, perrhenic acid, or from other compounds known to ex¬ perts in the art. Impregnation of the inert carrier using a saturated solution of the rhenium compound, in a solvent selected from water or an organic solvent, for example a hydrocar- bon, an alcohol or an ether, is generally preferred. The impregnation is preferably carried out at a tem¬ perature ranging from 10 to 9O0C in order to increase the solubility of the rhenium salt; in this case, the carrier is also heated to the same temperature. The inorganic halide is introduced by using aqueous or organic solutions having a salt concentration ranging from 1% by weight to saturation. The compound containing rhenium can also be dissolved in this solution, or not. Alumina is maintained in the presence of the halo- genated compound solution for a period of time ranging from 0.5 to 24 hrs, preferably from 8 to 15 hrs at a temperature ranging from 10 to 900C. After the impregnation of the carrier with the rhenium precursor and inorganic halides, the catalyst is activated by- means of a pre-calcination at a temperature; oranging 'from 100 to 2000C under a flow of dry air and a subsequent cal¬ cination at a temperature ranging from 300 to 6000C, under a flow, first of dry air and subsequently of nitrogen. The cooling is carried out in a flow of nitrogen for a time ranging from 5 to 30 minutes, preferably from 10 to 20 min¬ utes. To obtain a further improvement of the catalyst, it is possible to wet it, after the above-mentioned treatment, with an amount of water equal to the porosity of the car- rier, and calcine it again according to the method de¬ scribed above. The catalysts of the present invention can be used in metathesis reactions of olefins. Said reactions can be homo-metathesis (when -the two olefins are the same) or co-metathesis (when the two ole¬ fins are different) . The olefins which can be subjected to metathesis reac¬ tions are mono-olefins having from 2 to 30 carbon atoms, such as, for example, ethylene, propylene, butene, pentene, hexene; cycloolefins having from 5 to 20 carbon atoms, for example cyclopentene, cyclooctene, norbornene; olefins hav¬ ing two or more unsaturations, containing from 5 to 30 car¬ bon atoms, for example 1,4-hexadiene, 1, 7-octadiene, cyclopolyolefins containing two or more unsaturations and having from 5 to 30 carbøB~.atoms, for example 1,5-cyβlo- octadiene, norbordiene, dicyclopentadiene. Other olefins are mono-olefins or olefins containing several unsaturations, linear or cyclic, carrying func- tional groups, such as, for example, halogens or ester groups such as methyl oleate. The metathesis reaction can be carried out both in batch and in continuous operations, by feeding the substrates into a fluid bed or fixed bed reactor. The reac- tion conditions, such as temperature, pressure and flow rates are selected in relation to the feed stream and the end-product to be obtained. The metathesis reaction is normally carried out at a temperature ranging from 0 to 1000C, preferably from 25 to 6O0C, and a pressure of up to 10 MPa, preferably from 0.1 to 6 MPa and can be carried out in gaseous or liquid phase, with or without an organic solvent . When a solvent is used, this is selected from ethers, aliphatic and aromatic hydrocarbons. Examples of these sol- vent are: ethyl ether, hexane, heptane, toluene, etc. The catalyst is normally dispersed in the reaction medium at a concentration ranging from 1 to 50% by weight, preferably from 1 to 10% by weight, of the total composition. The metathesis reaction can be optionally carried out in the presence, of co-catalysts, selected from. alky-It mefeals such as, for example, tin tetraalkyls (tin tetramethyl, tin tetraethyl, tin tetrabutyl) , or other' alkyl metals such as. lead tetramethyl, lead tetraethyl, aluminum triethyl, chloro-aluminum diethyl, as described in US patent 3,855,338. The following examples are illustrative but non- limiting of the invention described. Example 1 Preparation of catalyst A 10 g of γ-alumina with a specific surface of 180 m2/g and a porosity of 0.5 ml/g, are pre-calcined in a muffle at HO0C for 1 hour in a flow of air and subsequently at 5500C for 4 hours in a flow of air. The carrier is then treated with 5 ml of a hexane so¬ lution containing 80 μl of TiCl4, and is maintained for 18 hrs at 250C. The liquid phase is then evaporated maintain¬ ing the sample in an oven for 2 hrs at 6O0C. The carrier is subsequently wetted with 5 ml of an aqueous solution containing 0.5 g of NH4ReO4, and is main¬ tained for 18 hrs at 25°C. The liquid phase is then evapo¬ rated maintaining the sample in an oven for 2 hrs at 600C. The carrier is then calcined first at 1100C for 1 hour in a flow of dry air and subsequently at 55O0C for 3 hours in a flow of dry air and 1 hour in a flow of nitrogen and is cooled for 15 minutes in a flow- of argon. The catalyst thus prepared has a rhenium content of 3.5% by weight. Example 2 Use of catalyst A in metathesis 360 mg of catalyst A prepared as in example 1 and 40 ml of a solution consisting of 10 μl of co-catalyst SnMe4 in 100 ml of hexane are charged into a 200 ml tailed flask, in an argon atmosphere. The resulting mixture is maintained under gently stir- ring, at 250C for 10 minutes and 50 ml of 1-hexene are sub¬ sequently added. The reaction mixture is analyzed, after 10 minutes, by means of gas chromatography, using an internal standard. The following results are obtained: conversion of 1-hexene 70% selectivity of 5-decene 100% Example 3 Preparation of catalyst B 10 g of γ-alumina with a specific surface of 180 m2/g and a porosity of 0.5 ml/g, are pre-calcined in a muffle at HO0C for 1 hour in a flow of air, then at 5500C for 4 hours in a flow of air. The carrier is then wetted with 5 ml of an aqueous so- lution containing 0.2 g of CuCl2 and 0.50 g of NH4ReO4, and is maintained-»-€or 18 hours at 600C. -The carrier thu& treated is calcined first at 1100C for 1 hour in a flow of dry air and subsequently at 55O0C for 3 hours in a flow of dry air and 1 hour in a flow of argon The catalyst thus prepared has a rhenium content of 3.5% by weight. Example 4 Use of catalyst B in metathesis 360 mg of catalyst B prepared as in example 3 and 40 ml of a solution consisting of 2.5 μl of co-catalyst SnMe4 in 100 ml of hexane, are charged into a 200 ml tailed flask, in an argon atmosphere. The resulting mixture is maintained under gently stir¬ ring, at 25°C for 10 minutes and 50 ml of 1-hexene are sub- sequently added. The reaction mixture is analyzed, after 10 minutes, by means of gas chromatography using an internal standard. The following results are obtained: conversion of 1-hexene 65% - selectivity of 5-decene 100% Example 5 (comparative) Preparation of catalyst C 10 g of γ-alumina with a specific surface of 180 rrr/g and a porosity of 0.5 ml/g, are pre-calcined in a muffle at HO0C for 1 hour in a flow of air and subsequently at 5500C for 4 hours In a flow of air. The carrier is then wetted with 5 ml of an aqueous so¬ lution containing 1.12 g of NH4ReO4. The water is subse¬ quently evaporated maintaining the sample in an oven at 600C. The catalyst is then calcined first at 1100C for 1 hour in a flow of dry air and subsequently at 5500C for 3 hours in a flow of dry air and 1 hour in a flow of nitro¬ gen. Then the reactor is get out the muffle and is cooled for 15 minutes in a flow of argon. The catalyst thus prepared has a rhenium content of 7.5% by weight. Example 6 (comparison) Use of catalyst C in metathesis 360 mg of catalyst C prepared as in example 3 and 23 ml of a solution consisting of 10 μl of co-catalyst SnMe4 in 100 ml of hexane, are charged into a 150 ml tailed flask, in an argon atmosphere. The resulting mixture is maintained under gently stir- ring, at 250C for 10 minutes and 26 ml of 1-hexene are sub¬ sequently added. The reaction mixture is analyzed, after 30 minutes, by means of gas chromatography, using an internal standard. The following results are obtained: - conversion of 1-hexene 3% selectivity of-^-5-decene 100%. Example 7 (comparison) Preparation of catalyst D 10 g of γ-alumina with a specific surface of 180 m2/g and a porosity of 0.5 ml/g, are calcined in a muffle at HO0C for 1 hour in a flow of air and subsequently at 5500C for 4 hours in a flow of air. The carrier is then wetted with 5 ml of an aqueous so¬ lution containing 0.5 g of NH4ReO4, the water is evaporated by maintaining the sample in an oven at 600C. The catalyst is calcined first at HO0C for 1 hour in a flow of dry air and subsequently at 55O0C for 3 hours in a flow of dry air and 1 hour in a flow of nitrogen. The re¬ actor is get out the muffle and is cooled for 15 minutes in a flow of nitrogen. The catalyst thus prepared has a rhenium content of 3.5% by weight. Example 8 (comparison) Use of catalyst D in metathesis 360 mg of catalyst D prepared as in example 5 and 23 ml of a solution consisting of 10 μl of co-catalyst SnMe4 in 100 ml of hexane are charged into a 150 ml tailed flask, in an argon atmosphere. The resulting mixture is maintained under light stir- ring, at 250C for 10 minutes and 26 ml of 1-hexene are sub¬ sequently added. The reaction mixture is analyzed, after 10 minutes, by means of gas chromatography, using an internal standard. The following results are obtained: - conversion of 1-hexene 15% selectivity of 5-decene 85%. Example 9 (comparison) Preparation of catalyst E 10 g of γ-alumina with a specific surface of 180 mVg and a porosity of 0.5 ml/g, are pre-calcined in a muffle at HO0C for 1 hour in a flow of air and subsequently at 5500C for 4 hours in a flow of air. The carrier is then wetted with 5 ml of an aqueous so¬ lution containing 57 mg of HCl and 0.5 g of NH4ReO4, and is maintained for 18 hours at 6O0C. The carrier thus treated is calcined first at 1100C for 1 hour in a flow of dry air and subsequently at 5500C for 3 hours in a flow of dry air and 1 hour in a flow of argon. The catalyst thus prepared has a rhenium content of 3.5% by weight. Example 10 (comparison) Use of catalyst E in metathesis 360 mg of catalyst E prepared as in example 3 and 20 ml of a solution consisting of 10 μl of co-catalyst SnMe4 in 100 ml of hexane are charged into a 150 ml tailed flask, in an argon atmosphere. The resulting mixture is maintained under light stir¬ ring, at 250C for 10 minutes and 20 g of 1-hexene are sub¬ sequently added. The reaction mixture is analyzed, after 10 minutes, by means of gas chromatography, using an internal standard. The following results are obtained: conversion of 1-hexene 47% ' selectivity of 5~decene 98%. Example 11 (comparison) Preparation of catalyst F 10 g of γ-alumina with a specific surface of 180 m2/g and a porosity of 0.5 ml/g, are pre-calcined in a muffle at HO0C for 1 hour in a flow of air and subsequently at 55O0C for 4 hours in a flow of air. The carrier is then wetted with 5 ml of an aqueous so¬ lution containing 200 mg of MnCl2 and 0.5 g of NH4ReO4, and is maintained for 18 hours at 600C. The carrier thus treated is calcined first at 1100C for 1 hour in a flow of dry air and subsequently at 5500C for 3 hours in a flow of dry air and 1 hour in a flow of argon. The catalyst thus prepared has a rhenium content of 3.5% by weight. Example 12 (comparison) Use of catalyst F in metathesis 360->mg of catalyst E., prepared as in example 3- and 20 ml of a solution consisting of 10 μl of co-catalyst SnMe4 in 100 ml of hexane, are charged into a 150 ml tailed flask, in an argon atmosphere. The resulting mixture is maintained under gently stir¬ ring, at 25°C for 10 minutes and 20 g of 1-hexene are sub¬ sequently added. The reaction mixture is analyzed, after 10 minutes, by means of gas chromatography, using an internal standard. The following results are obtained: conversion of 1-hexene 50% selectivity of 5-decene 88%.