Advanced nanostructured metal - Polymerization catalysts
Polymerization catalysts,Analyzing of Materials,Heterogeneous catalysis, Nanostructured metals,Homogeneous catalysis,Production of Nanostructured metal catalysts,Catalysis by Small Metal Clusters,Hydrogenation catalysts,Metal vapors technique,Nanostructured metal catalysts,Very active and selective catalysts,Advanced nanostructured metal,Nanostructured metal powders,Advanced Catalysts,Hetero Nanostructured metal,Oxidation catalysts,Bimetallic catalytic systems,Carbon-Carbon coupling reactions catalysts,Nanostructured Metal,New generation catalysts,Metal vapors technology
 

 

The main known methods concern processes of electrochemical reduction of metal salts, chemical reduction of metal salts, the "metal vapors technique" and the reduction, or decomposition, of organometallic precursors.
In particular, nanostructured metal catalysts offer excellent catalytic properties and many advantages in comparison with corresponding catalysts based on metal particles with larger dimensions. Their main advantage is represented by a significant increase of the surface activity which allows to greatly improve the performances of catalytic processes where they are employed. As a consequence, different synthetic methods have been studied in order to produce nanostructured metal catalysts.

   
PROCESS FOR MAKING NANOSTRUCTURED METAL CATALYSTS AND THEIR USE IN
CATALYTIC REACTIONS.

DESCRIPTION

Field of the invention

The present invention relates to a process for preparing nanostructured metal catalysts and their use in catalytic reactions. In particular, the present invention relates to the preparation of heterogeneous or homogeneous nanostructured metal catalysts for peculiar reactions, mainly involving the selective hydrogenation of organic substrates.


Description of the prior art Metal catalysts are generally employed in many catalytic reactions for the production of different organic products.



Very active and selective New generation catalysts, Homogeneous Heterogeneous Oxidation Polymerization catalysts,Advanced nanostructured metal powders Catalysts by Small Metal Clusters,Bimetallic catalytic systems,Hydrogenation catalysis, Production of Hetero Nanostructured metals catalysts, Metal vapors technology technique
Otherwise, not very often, high boiling alcohols such as n-octanol are employed, as described in WO 9604088. Both polyols and high molecular weight alcohols present the obvious drawback of a difficult removal from the final product and thus are not suitable for a large scale production of nanocatalysts. Another method for the production of nanostructured metal catalysts is based on the "metal vapors technology" . For this approach very expensive and rarely available reactors are necessary, thus appearing not suitable for large scale preparations. Finally, a further method for the production of nanostructured metal catalysts involves the reduction, or decomposition, of organometallic precursors.

 

In particular, the or each metal precursor has the formula: MnXy or HxMnXy, used as that or solvated, where M is a metal cation and X is an anion selected from the group comprised of:

  1. — a halogenide
  2. — a carboxylate — a substituted carboxylate
  3. — a hydroxide
  4. — a carbamate
  5. — an aldiminate

 

The main known methods concern processes of electrochemical reduction of metal salts, chemical reduction of metal salts, the "metal vapors technique" and the reduction, or decomposition, of organometallic precursors.

 

In particular, the up to now known catalysts employed in the selective hydrogenation of benzene to cyclohexene and of phenol to cyclohexanone are not able to perform in high yield the target product and imply high costs, as when palladium based catalysts are required.

Otherwise, the nanosized catalyst can be employed as not supported catalyst . Advantageously, the support can be directly introduced in the reactor where the reduction of the metal precursor or precursors is carried out or, otherwise, at room temperature in a successive step after the reduction reaction. The process of synthesis of metal nanostructured catalysts as above described allows to avoid on the one
hand the use of expensive and difficult to be synthesized organometallic precursors and on the other hand the employment of high boiling solvents such as glycols and polyglycols, in particular diethylene- and triethylene- glycol, hardly removable from the final product after the
reduction of the starting metal precursor.

Advantageously, a stabilizing agent can be also employed, such as a polymer or a copolymer, for instance poly-N-vinyl-2-pyrrolidone (PVP) , polyethylenoxide, polypropylenoxide, polyacrylates, or their copolymers.

Moreover, a base and/or an inorganic or organic salt, such as alkali or alkali earth hydroxides and/or their salts as acetates, oxalates, formates, amines etc. can be employed as a stabilizer. In this case the
use of the polymer as stabilizing agent is not necessary.
In particular, nanostructured metal catalysts offer excellent catalytic properties and many advantages in comparison with corresponding catalysts based on metal particles with larger dimensions. Their main advantage is represented by a significant increase of the surface activity which allows to greatly improve the performances of catalytic processes where they are employed. As a consequence, different synthetic methods have been studied in order to produce nanostructured metal catalysts.


In particular, the above described reduction reaction of the or each metal precursor is carried out at a pressure in the range from 10 and 150 bar, produced by an inert gas, such as, for instance, nitrogen.

 

Preferably, the metal cation is selected from the group comprised of:

— rhodium;

— ruthenium;

— rhenium;

— palladium; — platinum;

— nickel;

— copper;

— iridium;

— iron; — gold.

In particular, the employed alcoholic medium can have a molecular weight
below 100.

Advantageously, the alcoholic solvent or co-solvent is selected from the
group comprised of: — methanol;

— ethanol;

— n-propanol;

— n-butanol; — n-pentanol and their isomers.

Preferably, the alcoholic medium is selected from the group comprised of:

— methanol — ethanol;

— propanol;

— isopropanol.

 

Summary of the invention It is therefore a first feature of the present invention to realize a simple and cheap process for the preparation of nanosized metal catalysts on a large scale resolving the drawbacks of the conventional methods.

 

It is a particular feature of the present invention to produce a nanosized metal catalyst that is particularly efficient, in terms of activity and selectivity for specific catalytic reactions.

 

It is another feature of the present invention to give a process for the preparation of nanosized metal catalysts which allows to easily separate any by-products present in the reaction mixture from the final product.
Also this type of process appears not suitable for large scale applications. This procedure, in fact, employs as precursors organometallic derivatives which are very difficult to be synthesized and very expensive. Moreover, this process does not often enable to reach a good control of the particles sizes and their morphology. Moreover, frequently, this method is scarcely reproducible.

Preferably, the said reduction reaction of the or each metal precursor is carried out at a pressure from 20 and 100 bar. In particular, the reduction of one or more metal precursors together at the same time can be carried out at a temperature from 50 to 400 0C and preferably from 50 to 250 0C.

 

It is a further feature of the present invention to give a process for the preparation of nanosized metal catalysts which allows to obtain a final product with improved characteristics, in terms of average
diameter and size distribution of the metal particles, with respect to the known nanostructured catalysts.

According to another aspect of the present invention, the nanosized
metal catalyst as above described can be advantageously employed in
hydrogenation, dehydrogenation, oxidation, hydroxylation,
cis-dihydroxylation and in C-C bond formation reactions.

In particular, the nanosized metal catalyst as above described can be
advantageously employed in the selective hydrogenation of organic
substrates, in particular in the reaction of selective hydrogenation of
benzene to cyclohexene, of phenol to cyclohexanone and of benzaldehyde
to benzyl alcohol.





In particular, the nanosized catalyst, mono- or poly- metallic, can be deposited on an inert support, such as — © ?" alumina, silica, magnesia, zirconia, ceria and other metal oxides.

 

The method of the chemical reduction of the metal salts is based on the use of reducing agents such as metal hydrides or hydrogen itself, and of a stabilizing agent, generally a polymer. Otherwise, in order to avoid the eventual poisoning of the product by the reducing agent and to perform the reduction under higher control, a method of reduction in an alcohol has been proposed. Because the reduction in the presence of an alcohol generally needs high temperature to be efficient and complete, the reduction in the presence of polyols such as ethylene glycol, but mainly diethylene glycol and triethylene glycol, has been preferred.

 

In particular, the electrochemical reduction of metal salts is extremely expensive for large scale applications and frequently does not allow to control the particles size. Moreover, this process is scarcely suitable for important transition metals such as Pt, Rh, Ru and Mo, due to low solubility of their cations when employed as an anode.

 

Nanosized metal catalysts can be employed in many catalytic reactions such as the selective hydrogenation of organic molecules.

 

These and other features are accomplished with one exemplary process, according to the present invention, for the preparation of nanosized metal catalysts through the reduction reaction of at least one metal precursor carried out by heating in an alcoholic solvent or co- solvent, whose main characteristic is that the above reduction is carried out under an overpressure. In this way it is possible to employ a low molecular weight alcoholic solvent which is easily removable from the target product.


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