Catalysis by Small Metal Clusters
New generation catalysts
Advanced Catalysts
Nanostructured metal catalysts
Homogeneous catalysis
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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.
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.
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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.
CATALYTIC REACTIONS. DESCRIPTION |
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| Advanced nanostructured metal powders Catalysts by Small Metal Clusters,Very active and selective New generation catalysts,Bimetallic catalytic systems, Production of Hetero Nanostructured metals catalysts,Hydrogenation catalysis, Homogeneous Heterogeneous Oxidation Polymerization catalysts, Metal vapors technology technique |
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Nanosized metal catalysts can be employed in many catalytic reactions
such as the selective hydrogenation of organic molecules.
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.
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.
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.
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.
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:
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.
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.
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.
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. 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.
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, 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.
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.
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. 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.
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 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. |