Technical Field
The present invention relates to a metal complex having a skeletal structure in which a disiloxane group is coordinated as a chelate to a central metal. Further, the present invention relates to a supported metal complex obtained by fixing the metal complex on an inorganic oxide while retaining a skeletal structure thereof, and a method for producing the same.
Also, the present invention relates to a method for producing a supported metal catalyst by calcining the supported metal complex.
Background Art
A reaction system in which a catalyst containing a metal component as a catalytically active component is used can be roughly classified into a homogeneous catalytic system and a heterogeneous catalytic system. In the homogeneous catalytic system, a catalyst such as a metal salt, a metal complex and the like is dissolved or dispersed in a solvent or a reactant and the catalyst is brought into contact with the reactant in a liquid phase to allow a catalytic reaction to proceed. On the other hand, in the heterogeneous catalytic system, a supported metal catalyst prepared by carrying a metal component on a solid carrier is brought into contact with a gas phase or a liquid phase containing a reactant to allow a catalytic reaction to proceed.
In a homogeneous catalytic system, utilization efficiency of the catalyst is usually high, and a catalytic reaction can be allowed to proceed with high activity and high selectivity, but involved therein is the problem that it takes time to separate and recover the catalyst from the reaction product after the reaction.
On the other hand, in a heterogeneous catalytic system, the catalyst is readily separated from the reaction product, and it can be easily applied as well to a flow reaction system in which a catalyst is used in a fixed bed system and in which a reactant is supplied thereto to remove the reaction product in order from the system. Accordingly, heterogeneous catalysts, especially supported metal catalysts, are widely used in the broad fields ranging from petroleum refineries to synthesis of chemicals and environmental cleanup.
In general, a method in which a carrier such as an inorganic oxide is impregnated with a solution containing a metal component such as metal salts and in which the metal is fixed on the carrier after the steps of drying, calcining, reducing and the like has so far been carried out as the production method for a supported metal catalyst as described above. In the foregoing method, however, the metal forms large particles, and therefore it is very difficult to obtain a catalyst in which the metal is evenly dispersed on the carrier at a high dispersion rate.
In order to solve the foregoing problems of the homogeneous catalyst and the supported metal catalyst, it has been considered to fix the metal complex on a solid carrier. If the metal complex having a catalytic activity can be fixed as it is on the carrier, a catalyst having the foregoing advantage involved in the heterogeneous catalyst while maintaining a catalytic activity and a selectivity of the metal complex in a homogeneous system is expected to be obtained. Also, the metal component is expected to be dispersed more evenly on the carrier and fixed thereon by fixing the metal component on the carrier in the form of the metal complex, and calcining the carrier described above allows metal particles having a smaller particle size to be more evenly dispersed and formed on the carrier, whereby a supported catalyst having a higher catalytic activity is expected to be obtained.
In general, a method for fixing a metal complex on an inorganic oxide includes (1) a method in which a metal complex is fixed by making use of a reaction of a hydroxyl group and the like on the surface of an inorganic oxide with a metal and a ligand of a metal complex and (2) a method in which the surface of an inorganic oxide is modified with an organic group such as an amino group, a phosphino group and the like to fix a metal complex using the above organic group as a ligand.
However, in the fixing method of (1), a change (isomerization, clustering, decomposition and the like) in the skeletal structure of the metal complex takes place due to the action of a hydroxyl group and the like on the surface of the inorganic oxide in fixing the metal complex on an inorganic oxide carrier to cause a central metal of the metal complex to be combined directly with the surface of the carrier, and it is difficult to fix the metal complex while maintaining an inherent skeletal structure of the metal complex. Thus, it is difficult to maintain a catalytic activity of the metal complex. Further, it is very difficult to produce a supported metal catalyst in which metal particles having a small particle size are evenly carried on the carrier at a high dispersion rate by calcining the carrier described above.
In connection with the fixing method of (1), Tilley et al. report a method in which in fixing a metal complex on a silica carrier, a metal complex compound having two siloxy groups having three organic leaving groups (R) is utilized in order to fix the metal on the carrier in a mononuclear form (non-patent documents 1 to 2).
However, in the above method, as shown in the following Equation 1, it is reported that the siloxy groups combined with the metal are released instead of the organic leaving groups (R) included in the metal complex and the metal is combined directly with the carrier via an oxygen atom on the surface of the carrier. That is, the skeletal structure of the metal complex is not maintained in the method described above. Further, since the metal is combined with the carrier only by one covalent bond, the bond strength is not sufficiently high, and a metal-metal bond is liable to be formed in a calcining step, so that the metal may be carried on the carrier in the form of metal particles having a large particle size.

Also, the same change in the skeletal structure can take place as well in the fixing method of (2), and in addition thereto, the carrier itself of the inorganic oxide modified with an organic group such as an amino group, a phosphine group and the like is less resistant to high temperatures, so that it is very difficult to produce a metal catalyst in which metal particles are evenly carried on the carrier by calcining the carrier described above.
As described above, the inherent skeletal structure of the metal complex cannot be maintained in the conventional methods for fixing a metal complex on an inorganic oxide carrier, and a change (isomerization, clustering, decomposition and the like) in the skeletal structure takes place, so that it is difficult to maintain a catalytic activity of the metal complex. Further, involved therein is the problem that a metal-metal bond is readily formed by calcining the carrier described above and that it is very difficult to obtain a supported metal catalyst in which fine particles of the catalyst metal are evenly carried on the carrier at a high dispersion rate.