TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The catalysts (C1) and (C2) are selected from catalysts capable of alkane metathesis (preferably from supported metal clusters, especially from metal hydrides, organometallic compounds or organometallic hydrides) supported on (preferably grafted onto) a solid support. The supported metal clusters comprise at least one metal of Groups 4, 5 and/or 6 of the Periodic Table of the Elements (preferably tantalum). The catalysts selected from metal hydrides, organometallic compounds and organometallic hydrides comprise at least one metal selected from lanthanides, actinides and metals of Groups 2 to 12 (preferably transition metals of Groups 3 to 12 of the Periodic Table of the Elements). The catalysts comprise at least one metal selected from scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhenium, ruthenium, osmium, nickel, iridium, palladium, platinum, cerium and neodymium (preferably from yttrium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, rhenium, ruthenium and platinum, especially from niobium, tantalum, molybdenum, tungsten and rhenium). The catalysts comprise a solid support selected from metal or refractory oxides, sulfides, carbides, nitrides and salts, and from carbon, mesoporous materials, organic/inorganic hybrid materials, metals and molecular sieves.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The mixture (M1) obtained in stage (1) in addition comprises at least one greater than or equal to 3C alkanes (preferably 3-6C alkanes). The mixture (M2) in addition comprises unreacted ethane which is separated and isolated in the fractionating of stage (3) and recycled into the contacting of stage (3); or in addition comprises propane which is separated and isolated preferably in mixture with unreacted ethane in the fractionating of stage (3) and recycled into the contacting of stage (3); or in addition comprises propane and butanes which are separated and isolated preferably in mixture with unreacted ethane in the fractionating of stage (3) and recycled into the contacting of stage (3). Preferred Process: The contacting and the fractionating in stage (3) are simultaneously performed. The fractionating in stage (3) is performed in addition so as to separate and to isolate the methane which is recycled into stage (1). All the stages are performed continuously. In stage (1), the methane is used in the form of natural gas, preferably submitted to a purification stage prior to stage (1), for removing at least one poison capable of deactivating the metal catalysts (C1) and/or (C2) and/or at least one impurity. Prior to stage (1), a preliminary stage involving separating and isolating by fractionation the methane from natural gas and preferably the rest of the natural gas comprising a mixture of greater than or equal to 2C alkanes (preferably 2-6C alkanes) which are preferably used in mixture or in combination with the ethane in the fractionating of stage (3) is carried out. The stage (1) is performed at a temperature of 50 to 700 (preferably 150 to 700, more preferably 200 to 650, especially 100 to 600, particularly 300 to 600) degrees C. The stage (1) is performed under an absolute total pressure of 0.1 to 100 MPa (preferably 0.1 to 50 MPa, especially 0.1 to 30 MPa, particularly 0.1 to 20 MPa). In stage (2), the hydrogen is separated and isolated from the mixture (M1) and preferably used in at least one application or stage, distinct or not from the present process, particularly used as an agent of activation or regeneration of the catalyst(s) (C1) and/or (C2), or as a fuel for producing thermal and/or electrical energies (preferably as a fuel in hydrogen fuel cells for producing electrical energy, energies preferably used for running the process or as a fuel for automobile or as a reagent in chemical, petrochemical or refinery plant). In stage (2), the mixture (M1) in addition comprises unreacted methane which is separated and isolated, and then recycled into stage (1). The stage (2) involves fractionating the mixture (M1) in addition so as to separate and to isolate at least one greater than or equal to 3C (preferably 3-6C alkanes) which are preferably used in mixture or in combination with the ethane in the contacting of stage (3). The stage (3) is performed at a temperature of 20 to 400 (preferably 50 to 350, especially 70 to 300, particularly 100 to 250) degrees C; and under an absolute total pressure of 0.1 to 50 MPa (preferably 0.1 to 30 MPa, especially 0.1 to 20 MPa). EXAMPLE - Methane was introduced continuously into a reactor, heated to 475 degrees C under an absolute total pressure of 5 MPa. An organometallic tantalum hydride grafted onto silica and containing 5.5 wt.% of tantalum was added to it. After work up, a gaseous mixture comprising ethane, hydrogen and unreacted methane was obtained. The mixture was withdrawn continuously from the reactor and was then introduced into a fractionation zone to separate and isolate ethane, hydrogen and unreacted methane. The ethane thus isolated was then continuously introduced into another reactor heated to a temperature of 150 degrees C under a total absolute pressure of 2 MPa. A metal catalyst based on organometallic tungsten hydride grafted onto alumina and containing 3.9 wt.% tungsten was introduced into the reactor. After work up a gaseous mixture comprising the higher alkane mixture comprising butane, pentane, hexane, heptane and octane was obtained. The higher alkane mixture was continuously isolated in liquid form containing the butane, pentane, hexane, heptane and octane, completely free from impurities, with a high yield and with high ''carbon atom efficiency'' of 99%.