WO2007085463A1

WO2007085463A1 - Process for making nanostructured metal catalysts and their use in catalytic reactions.

Info

Publication number: WO2007085463A1
Application number: PCT/EP2007/000666
Authority: WO
Inventors: Glauco Sbrana; Carlo Carlini; Anna Maria Raspolli Galletti; Simone Giaiacopi
Original assignee: Universita Di Pisa
Priority date: 2006-01-26
Filing date: 2007-01-26
Publication date: 2007-08-02
Also published as: ITPI20060011A1

Abstract

The present invention relates to a new easy, cheap and reproducible process for preparing nanostructured metal catalysts and their use in catalytic reactions. One or more metal precursors are reduced by heating in the presence of a low boiling alcohol (employed as solvent or co-solvent) under an overpressure, eventually in the presence of a stabilizing agent. A support can be directly added in the reduction step or in a successive one. The obtained catalysts present good characteristics in terms of metal particles average diameter and dimensions distribution. The said catalysts can be advantageously employed in many peculiar catalytic reactions, such as the selective hydrogenation of organic substrates. In particular, these ruthenium nanocatalysts have shown higher performances respect to commercial ones in the selective hydrogenation of benzene to cyclohexene and of phenol to cyclohexanone.

Description

TITLE

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.

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.

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.

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. This type of process is, for instance, described in FR 2537898. 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. 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.

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.

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 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.

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.

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.

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.

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:

— a halogenide

— a carboxylate — a substituted carboxylate

— a hydroxide

— a carbamate

— an aldiminate

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.

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 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.

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, 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.

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.

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.

The present invention will be described in further detail by the following examples 1-16; however they should not be construed as limiting the scope of the present invention.

In particular, the nanosized ruthenium metal catalyst described in Example 1 has been prepared according the known process of the reduction in glycol , in order to have a comparison with the ruthenium catalysts described in the Examples 2-11 , prepared according to the present invention.

Example 1

123 mg of RuC13^»nH2O (50.26 mg Ru) and 2.57 g of PVP were introduced under nitrogen in a 250 ml two neck round bottom flash equipped with a condenser and a magnetic stirrer. 280 ml of ethylene glycol were successively added and the resulting reaction mixture was refluxed at 198 ⁰C by heating with a thermostatted oil bath for 3 h. At the end, after cooling, 4.17 g of γ-Al203 (surface area 110 m2/g) were added and the stirring was continued for 12 h. Then ethylene glycol was removed by evaporation at reduced pressure and the catalyst was washed with acetone and dried up to constant weight.

TEM analysis revealed the presence of metal particles with an average diameter of 3.33 nm and with a standard deviation of 0.59 nm.

In the Examples 2-16 some preparations according the present invention of the nanosized metal catalysts are described.

The morphologies of the prepared catalysts have been compared with those of the commercial catalysts and of other prepared according the known process of reduction in polyols.

Example 2

48.8 mg of RuC13»nH2O, 0.60 g of PVP and 100 ml of ethanol were introduced in a 300 ml autoclave equipped with a magnetic stirrer and a manometer for pressures up to 200 bar.

The autoclave is then pressurized with 60 bar of nitrogen and the stirring started when the temperature of 200 ⁰C was reached; the stirring was maintained for 3 h. Then the autoclave was cooled up to room temperature and the gas discharged. Successively 1.98 g of γ-A12O3 were added under stirring. The dispersion was filtered, washed with acetone and dried.

TEM analysis revealed metal particles with an average diameter of 2.00 nm and a standard deviation of 0.28 run.

Example 3

The catalyst was synthesized analogously to that of Example 2, but using as support basic Al₂O₃ with a surface area of 150 m²/g.

Example 4

The catalyst was synthesized analogously to that of Example 2, but adopting 220 ⁰C as reaction temperature. TEM analysis revealed metal particles with an average diameter of 2.05 nm and a standard deviation of 0.25 nm.

Example 5

The catalyst was synthesized analogously to that of Example 2, but using as support 1.95 g of carbon (surface area 900 m2/g) . Metal particles have an average diameter of 2.16 nm with a standard deviation of 0.33 nm.

Example 6

The catalyst was synthesized analogously to that of Example 2, but using as support 4.1 g of γ-alumina. Metal particles have an average diameter of 2.07 nm with a standard deviation of 0.31 nm.

Example 7

The catalyst was synthesized analogously to that of Example 6, but using as a stabilizer 0.57 g of a grafted copolymer polyethyleneglycol-PVP.

Metal particles have an average diameter of 2.03 nm with a standard deviation of 0.44 nm.

Example 8

The catalyst was synthesized analogously to that of Example 2, but using as a stabilizer 1.23 g of a grafted copolymer polyethyleneglycol-PVP.

Metal particles have an average diameter 3.04 nm with a standard deviation 0.78 nm. Example 9

The catalyst was synthesized analogously to that of Example 2, but γ-alumina was directly introduced in the autoclave before the reduction step.

Example 10 The catalyst was synthesized analogously to that of Example 2, but using as solvent 110 ml of isopropyl alcohol.

Example 11

The catalyst was synthesized analogously to that of Example 2, but without a stabilizing polymer and introducing in the autoclave 5.5 ml of an aqueous solution of NaOH 0. 5 M.

Example 12

The catalyst was synthesized analogously to that of Example 2, but using as a metal precursor 48 mg of RhC13.3H2O

Example 13

The catalyst was synthesized analogously to that of Example 2, but using as a metal precursor 50 mg of H₂PtCl₆.6H₂O. Example 14

51.7 mg di Pd (OAc) 2, 0.499 g di PVP, 4.177 g of carbon powder (Strem n.93-0601) and 100 ml of ethanol were premixed and then introduced in a 300 ml autoclave equipped with a • magnetic stirrer and a manometer for pressures up to 200 bar.

The autoclave was then pressurized with 60 bar of nitrogen and the stirring started when the temperature of 100 ⁰C was reached; the stirring was maintained for 3 h. Then the autoclave was cooled up to room temperature and the gas discharged. The catalyst was filtered, washed with acetone and dried.

TEM analysis revealed metal particles with an average diameter of 3.81 nm with a standard deviation of 1.22 run. Example 15

55.9 mg of Pd (OAc) 2, 0.487 g of PVP, 4.08 g of carbon powder (Strem n.93-0601) and 100 ml of methanol were premixed and then introduced in a 300 ml autoclave equipped with a magnetic stirrer and a manometer for pressures up to 200 bar.

The autoclave was then pressurized with 60 bar of nitrogen and the stirring started when the temperature of 80 ⁰C was reached; the stirring was maintained for 3 h. Then the autoclave was cooled up to room temperature and the gas discharged. The catalyst was filtered, washed with acetone and dried.

TEM analysis revealed metal particles with an average diameter of 3.53 nm with a standard deviation of 1.24 nm.

Example 16 0.2 ml of an aqueous solution of HAuC14 I M, 0.41 g of PVP, 3.9 g of carbon powder (Strem n.93-0601) , 70 ml of water and 30 ml of ethanol were premixed and then introduced in a 300 ml autoclave equipped with a magnetic stirrer and a manometer for pressures up to 200 bar. The autoclave was then pressurized with 60 bar of nitrogen and the stirring started when the temperature of 150 ⁰C was reached; the stirring was maintained for 3 h. Then the autoclave was cooled up to room temperature and the gas discharged. The catalyst was filtered, washed with acetone and dried.

Some examples of catalytic applications of the above described catalysts are below reported and the obtained results are compared with those reached with the commercial catalysts or with the catalysts prepared according the known procedure of reduction in the presence of polyols.

These examples, however, should be not construed as limiting the scope of the present invention.

Selective hydrogenation reactions As an example, the same ruthenium and palladium catalysts, prepared according to the present invention, have been employed in the hydrogenation of cyclohexene to cyclohexane and in the selective hydrogenation of benzene to cyclohexene, of phenol to cyclohexanone and of benzaldehyde to benzyl alcohol comparing the performances of the catalysts prepared according to the present invention with those of some commercial ones and of the catalysts obtained according to Example 1 by reduction in glycol. As an example some results are summarized in the Tables 1-4. In the selective hydrogenation of benzene to cyclohexene and of phenol to cyclohexanone the ruthenium nanocatalysts synthesized according to the present invention, besides involving a much easier preparative method, have offered better performances (in terms of yields in the target product) with respect not only to the commercial catalysts, but also to the nanosized metal catalyst 1, prepared according the reduction in glycol.

In fact, in the benzene hydrogenation to cyclohexene this superiority is observed either working in organic medium (n-hexane) or, more suitably, in water in the presence of zinc sulfate. The only formed by-product is cyclohexane. (Table 1)

The ruthenium based catalysts have shown a good catalytic activity also in the selective hydrogenation of phenol to cyclohexanone (Table 2) . Also in this reaction the catalysts prepared according to the present invention are better for activity and selectivity with respect to the commercial ones and to the catalyst prepared in glycol, allowing to achieve high yields in cyclohexanone. In this reaction the only formed by-product is cyclohexanol.

The palladium catalysts prepared according to the present invention result significantly more active than the commercial ones in the hydrogenation of cyclohexene to cyclohexane (Table 3) and of benzaldehyde to benzyl alcohol (Table 4) . In this last reaction a particularly higher selectivity has been also evidenced.

Table 1: Catalytic hydrogenation of benzene to cyclohexene

Runa Cat. of (mg Ru) Benzene Conv. Selectivity to Example (mol%) cyclohexene (%) 30' Ih 3h 30' Ih 3h l^a) 1 (5.2) 4.6 6 8 25.6 24.3

₂ a, 8 (4.34) 32.9 58 8 9.3 4.2

Aldrich (3.07) 0.61

3^b» 5% on C 45.5 -

Engelhard (3.7)

4 ^bl 62.41 - - 0.10 5% on C

5^b) 1 (5.86) 2.3 - 62.6

6^b) 2 (5.6) 51.4 - 36.7

7 ^b) 9 (3.70) 24.1 - 46.6 g b, 5 (5.11) 29.1 - 31.5

₉ b. 4 (4.62) 47.3 - 31.5

10 ^b) 8 (5.9) 43.1 - 52.3 ^a) Reaction conditions: benzene (25 ml); solvent: n-hexane (20 ml); T: 100 ⁰C; P hydrogen: 50 bar. ^b) Reaction conditions: benzene (25 ml); solvent: water (50 ml) with ZnSO₄ (2.4 g) ; T: 160 ⁰C; P hydrogen: 50 bar. Table 2; Catalytic hydrogenation of phenol to cyclohexanone³'

Run Cat. of (mg Ru) Phenol Conv. Selectivity to Example: (mol%) cyclohexanone <%) 30' Ih 3h 30' Ih 3h

11 Aldrich (2.0) 33.8 75. 4 - 63.1 31 1 - 5% on C

12 1 (2.2) _ 2. 4 10. 1 86 5 78 .2

13 10 (2.5) 58.8 82. 3 _ 70.0 53 6 _

14 9 (2.3) 44.1 60. 7 _. 68.6 63 3

15 6 (2.0) 28.2 38. 4 74. 2 72.8 69 6 54 .4

16 ^b) 6 (2.0) 32.6 51. 1 91. 4 76.0 70. 0 46 .9

17 5 (1.8) _ 27. 9 51. 9 65. 0 48 .8

18 3 (2.1) 74.9 _ _ 54.1

19 11 (2.2) 31.5 _ 87. 2 81.7 59 .2 ^a)Reaction conditions:: phenol (10 g) ; solvent; cyclohexane(50 ml); T: 160 ⁰C; P hydrogen: 50 bar. ^b) Water (0.46 g) has been added to the catalyst.

Table 3 - Cyclohexene hydrogenation to cyclohexane ^a)

Run Cat. (mg Pd) Cyclohexene conversion of Example (mol %)

Ih 2h 3h 4h 5h

20 Engelhard 2 % 0 .1 3.0 6.5 13.8 16.5 19 .1 on carbon

21 14 0 .1 10.7 19.4 27.4 35.5 42 .0

22 15 0 .1 14.1 27.9 36.8 46.2 53 .7 ^a)Reaction conditions: cyclohexene (5 ml); solvent: toluene (50 ml); T: 30 ⁰C; P hydrogen: 10 bar. Table 4 - Benzaldehyde hydrogenation to benzyl alcohol ^a) b)

Run Cat . (mg Pd) Benzaldehyde Selectivity of conversion (mol%) (mol %) Example 30' 60' 90' 30' 60' 90'

23 Engelhard 0 .05 22 2 38 9 58 1 42 .1 44. 4 48. 5 2 % on carbon

24 14 0 .05 28 8 65 1 91. 0 95 .0 95. 3 93. 7

25 15 0 .05 29 4 63. 6 90. 9 94 .5 96. 6 92. 1

^a)Reaction conditions: benzaldehyde (0.5 ml); solvent: methanol (38 ml); T: 35 ⁰C; P hydrogen: 1 bar. ^b)Selectivity to benzyl alcohol; main by-product: benzaldehyde dimethoxy acetal.

The foregoing description of a specific embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such an embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Claims

1. Process 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 characterised in that said reduction reaction is carried out under overpressure.

2. Process, according to claim 1, wherein said or each metal precursor has the formula: M_nX_y or H_xM_nXy, used as that or solvated, where M is a metal cation and X is an anion selected from the group comprised of: a halogenide

- a carboxylate

- a substituted carboxylate - a hydroxide

- a carbamate

- an aldiminate

3. Process, according to claim 2, wherein said metal cation is selected from the group comprised of : - rhodium;

- ruthenium;

- rhenium;

- palladium;

- platinum; - nickel;

- copper;

- iridium; iron;

- gold.

4. Process, according to claim 1, wherein said alcoholic solvent or co-solvent is an alcohol with a molecular weight below 100, preferably selected from the group comprised of: - methanol;

- ethanol;

- n-propanol;

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

5. Process, according to claim 1, wherein said said reduction reaction of said at least one metal precursor is carried out under a pressure in the from 10 to 150 bar, using an inert gas.

6. Process, according to claim 1, wherein said reduction reaction of said at least one metal precursor is carried out under a pressure from 20 to 100 bar.

7. Process, according to claim 1, wherein said reduction reaction is carried out in an autoclave filled with an amount of inert gas corresponding to said overpressure conditions.

8. Process, according to claim 1, wherein said reduction reaction of at least one metal precursor is carried out from 50 to 400 ⁰C , preferably from 50 to 250 ⁰C.

9. Process, according to claim 1, wherein a stabilizing agent is also employed selected from the group comprised of:

- poly-N-vinyl-2-pyrrolidone (PVP) , polyethylenoxide, - polypropylenoxide,

- polyacrylates, copolymers thereof.

10. Process, according to claim 1, wherein a base and/or a salt is also employed.

11. Process, according to claim 1, wherein an inert support is employed where said nanosized mono- or polymetallic catalyst is dispersed.

12. Process, according to claim 11, wherein said inert support is directly introduced in the reactor where the reduction reaction is carried out.

13. Process, according to claim 11, wherein said inert support is introduced at room temperature in a successive step after said reduction reaction.

14. Reaction of selective hydrogenation of organic compounds using a nanostructured metal catalyst according to the above claims.

15. Reaction of selective hydrogenation of organic substrates, according to claim 14, wherein said reaction is selected from the group comprised of: hydrogenation of benzene to cyclohexene, - hydrogenation of phenol to cyclohexanone hydrogenation of benzaldehyde to benzyl alcohol.

16. Use of a metal nanostructured catalyst obtained in a process according to the claims from 1 to 13, for the catalysis of organic reactions selected from the group comprised of:

- dehydrogenation reactions oxidation reactions dihydroxylation reactions

- hydroxylation reactions - hydrosilylation

- carbon-carbon bond formation - carbonylation reactions

- carbomethoxylation reactions or their combinations.