The present invention relates to a method for producing adamantanes.  More precisely, the invention relates to a method of selectively producing adamantanes in a production apparatus made of an inexpensive material, not having any negative influence on the natural environment. 
Adamantanes are useful for intermediates of various chemicals such as lubricants and medicines.  For producing such adamantanes, generally employed is a method of isomerizing tricyclic saturated hydrocarbons having at least 10 carbon atoms each.  For the isomerization, used is aluminium chloride for the catalyst. 
However, in the method of using aluminium chloride for the catalyst for the isomerization, a large amount of aluminium chloride must be used relative to the starting compound.  In addition, in the method, the aluminium chloride catalyst forms a complex with a heavy component during the isomerization, and therefore it cannot be recycled.  Accordingly, the method gives a large amount of aluminium waste and its serious problem is that the aluminium waste to be processed for its disposal has some negative influence on the natural environment.  Another problem with the method is that the aluminium chloride catalyst used discolors the adamantanes produced therein, and therefore the method requires recrystallization of the products or decoloration thereof with activated charcoal, but the post-treatment is troublesome. 
In Japanese Patent Publication Nos.  2909/1977 and 35944/1978, proposed is a method of using a catalyst of various active metals such as platinum, rhenium, nickel or cobalt carried by cation-exchanged zeolite, for that isomerization.  In the method, however, since hydrogen chloride is used along with the metal-carrying catalyst for increasing the yield of adamantanes to be produced through isomerization, a production apparatus made of an expensive corrosion-resistant material must be used.  This is a problem with the method. 
Given that situation, it is desired to develop a method of selectively producing adamantanes in a production apparatus made of an inexpensive material, not having any negative influence on the natural environment. 
The object of the present invention is to provide a method of selectively producing adamantanes in a production apparatus made of an inexpensive material, not having any negative influence on the natural environment. 
We, the present inventors have assiduously studied so as to solve the problems noted above, and, as a result, have found that, in a method for producing adamantanes by isomerizing a tricyclic saturated hydrocarbon having at least 10 carbon atoms in the presence of a metal-carrying solid acid catalyst, when an unsaturated bond-having compound is made to coexist along with the catalyst during the isomerization, then it attains the object as above.  On the basis of this finding, we have completed the present invention. 
Specifically, the invention is summarized as follows:
(1) A method for producing adamantanes by isomerizing a tricyclic saturated hydrocarbon having at least 10 carbon atoms in the presence of a metal-carrying solid acid catalyst, wherein an unsaturated bond-having compound is made to coexist along with the catalyst during the isomerization. 
(2) The method for producing adamantanes of above (1), wherein the metal in the metal-carrying solid acid catalyst is a metal of the Groups 8 to 10 of the Periodic Table. 
(3) The method for producing adamantanes of above (2), wherein the metal of the Groups 8 to 10 of the Periodic Table is platinum. 
(4) The method for producing adamantanes of any of above (1) to (3), wherein the metal-carrying solid acid catalyst is a catalyst of a metal of the Groups 8 to 10 of the Periodic Table carried by zeolite. 
(5) The method for producing adamantanes of any of above (1) to (4), wherein the metal-carrying solid acid catalyst is a solid acid catalyst of platinum carried by Y-type zeolite. 
(6) The method for producing adamantanes of any of above (1) to (5), wherein the unsaturated bond-having compound is a carbon-carbon double bond-having compound. 
(7) The method for producing adamantanes of above (6), wherein the carbon-carbon double bond-having compound is an unsaturated hydrocarbon. 
(8) The method for producing adamantanes of above (7), wherein the unsaturated hydrocarbon is an aromatic hydrocarbon. 
(9) The method for producing adamantanes of above (7), wherein the unsaturated hydrocarbon is an acyclic unsaturated hydrocarbon. 
(10) The method for producing adamantanes of above (8), wherein the aromatic hydrocarbon is benzene. 
(11) The method for producing adamantanes of above (9), wherein the acyclic unsaturated hydrocarbon is 1-hexene. 
Embodiments of the invention are described hereinunder. 
The invention is a method for producing adamantanes by isomerizing a tricyclic saturated hydrocarbon having at least 10 carbon atoms in the presence of a metal-carrying solid acid catalyst, wherein an unsaturated bond-having compound is made to coexist along with the catalyst during the isomerization.  The adamantanes to be produced in the invention are hydrocarbons having an adamantane structure, including unsubstituted adamantane and alkyl-substituted adamantanes having a lower alkyl group such as methyl or ethyl group. 
The tricyclic saturated hydrocarbon having at least 10 carbon atoms, which is the stating material in the invention, is especially preferably a tricyclic saturated hydrocarbon having from 10 to 15 carbon atoms, including, for example, trimethylenenorbornane [tetrahydrodicyclopentadiene], perhydroacenaphthene, perhydrofluoren, perhydrophenalene, 1,2-cyclopentanperhydronaphthalene, perhydroanthracene, perhydrophenanthrene.  Also preferred for use herein are alkyl-substituted derivatives of these compounds, for example, 9-methylperhydroanthracene. 
These tricyclic saturated hydrocarbons having at least 10 carbon atoms each can be readily obtained by hydrogenating starting compounds such as dicyclopentadiene or acenaphthene in the presence of a known hydrogenation catalyst such as Raney nickel or platinum. 
The catalyst to be used in the invention is a metal-carrying solid acid catalyst having at least one type of metal.  Preferred examples of the metal in the metal-carrying solid acid catalyst are metals of Groups 8 to 10 of the Periodic Table, concretely iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum.  Of those metals, more preferred is a solid acid catalyst with platinum.  Preferred examples of the solid acid that carries the metal are various types of zeolite such as A-type zeolite, L-type zeolite, X-type zeolite, Y-type zeolite, ZSM-5; and metal oxides such as silica-alumina, alumina, heteropoly-acids.  Of those solid acids, more preferred are X-type zeolite and Y-type zeolite. 
For producing the metal-carrying solid acid catalyst with a carrier of zeolite, for example, at least one type of metal is applied to zeolite through ion-exchanging or dipping.  For producing it through ion-exchanging, for example, an aqueous solution of a salt or complex of the metal is kept in contact with zeolite so as to exchange the cation site (e.g., H+, NH4+) in zeolite with the metal, and the thus-processed zeolite is dried and baked.  For producing the catalyst through dipping, for example, an aqueous solution of a salt or complex of the metal is mixed with zeolite, and then the resulting mixture is dried up in a rotary evaporator or the like whereby the metal is infiltrated into the carrier, zeolite.  Regarding its morphology, the catalyst thus obtained may be powdery or granular. 
In the method of the invention, a tricyclic saturated hydrocarbon having at least 10 carbon atoms is isomerized in the presence of the metal-carrying solid acid catalyst that is prepared in the manner as above, and the isomerization is effected in the co-existence of an unsaturated bond-having compound.  For the unsaturated bond-having compound, preferred is a carbon-carbon double bond-having compound.  The carbon-carbon double bond-having compound includes, for example, aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene, anthracene; oxygen-containing aromatic compounds such as phenol, benzaldehyde, benzoic acid, benzyl alcohol, anisole; nitrogen-containing aromatic compounds such as aniline, nitrobenzene; halogen-containing aromatic compounds such as chlorobenzene, bromobenzene; acyclic unsaturated hydrocarbons such as propylene, butenes, butadiene, pentenes, hexenes; and alicyclic unsaturated hydrocarbons such as dicyclopentadiene, cyclopentadiene, cyclohexene, norbornene. 
Of the carbon-carbon double bond-having compounds, preferred are aromatic hydrocarbons and acyclic unsaturated hydrocarbons as they are more effective for improving the selectivity of adamantanes in isomerization of tricyclic saturated hydrocarbons.  Of the aromatic hydrocarbons, more preferred is benzene; and of the acyclic unsaturated hydrocarbons, more preferred is 1-hexene. 
The ratio of the unsaturated bond-having compound to the starting compound, tricyclic unsaturated hydrocarbon having at least 10 carbon atoms preferably falls between 1:1000 and 2:1, more preferably between 1:100 and 1:1, even more preferably between 1:30 and 1:3 in terms of (unsaturated bond-having compound):(tricyclic saturated hydrocarbon) by weight.  The reason is because, if the ratio of the unsaturated bond-having compound to the starting compound is smaller than 1:1000, the selectivity of adamantanes in the reaction product could not increase; and if it is larger than 2:1, the reactivity of the starting compound will lower. 
Next described is the reaction condition in isomerization of the tricyclic saturated hydrocarbon having at least 10 carbon atoms in the presence of the metal-carrying solid acid catalyst along with the unsaturated bond-having compound.  The reaction temperature may fall between 150 and 500xc2x0 C., but preferably between 200 and 400xc2x0 C.; and the reaction pressure may be normal pressure or elevated pressure.  Regarding the reaction style, employable are any of flow reactors or batch reactors.  In case where the reaction is effected in a batch reactor, the reaction time may fall between 1 and 50 hours.  Preferably, the reaction is effected in the presence of hydrogen for improving the yield of the product, adamantanes.  For regenerating the catalyst used in the reaction, employable is a method of reactivating the used catalyst by baking it in air at a temperature falling between 350 and 550xc2x0 C.
The invention is described more concretely with reference to the following Examples and Comparative Examples.