Patent Publication Number: US-2023139787-A1

Title: Process for the preparation of transition metal nanoparticles

Description:
TECHNICAL FIELD 
     The present invention relates to a process for preparing transition metal nanoparticles, a colloidal suspension of transition metal nanoparticles, transition metal nanoparticles and the use thereof as a catalyst. 
     INTRODUCTION 
     Colloidal palladium is typically prepared by using two reagents, in particular a suspending agent and a reducing agent, usually polyvinylpyrrolidone as suspending agent. As far as a reducing agent is concerned, one or more of ascorbic acid, sodium borohydride, citric acid, and formic acid/formate are typically used. 
     Generally, several different recipes can be found in the prior art which are however limited to low palladium concentrations, in particular smaller than 0.1 weight-%. Another constriction is caused by steric effects of a suspending agent hindering diffusion of an organic substrate in the colloidal palladium, when deposited on a porous support. 
     US 10,493,433 B2 relates to colloidal precious metal nanoparticles with high precious metal concentration for catalyst applications and methods for their synthesis. Further, a colloidal dispersion is disclosed therein comprising a dispersion medium, a water-soluble polymer suspension stabilizing agent, and a reducing agent. The precious metal is selected from one or more of Pt, Pd, Au, Ag, Ru, Rh, Ir, and Os. Z.-L. Wang et al disclose in Scientific reports a study on Pd nanoparticles on carbon synthesized in the presence of citric acid. M. Michaelis et al. disclose in J. Phys. Chem. preparation of a Pd colloid stabilized with polyvinylsulfate wherein citrate is used to reduce Pd(II). H. Lv et al. disclose in Nano Energy a study on the design of tailored nanomaterials for efficient oxygen reduction reaction. In particular, preparation of nanoparticles is disclosed using sodium tetrachloropalladate, citric acid, ethanol, and polyvinylpyrrolidone. M. Shao et al. disclose in Chem. Commun. a synthesis of Pd octahedra using polyvinylpyrrolidone, citric acid, and sodium tetrachloropalladate. 
     Therefore, it was an object to provide an improved process for preparing colloidal suspensions and nanoparticles of one or more transition metals, in particular a process not suffering from the above mentioned drawbacks but allowing an economical advantageous process and especially allowing use of comparatively high concentrations of a transition metal. Further, it was an object to provide transition metal nanoparticles and a suspension of a colloid of transition metal nanoparticles. 
     Surprisingly, it was found that a novel process for the preparation of transition metal nanoparticles can be provided, the process particularly comprising providing a mixture comprising, preferably consisting of, one or more salts of one or more transition metals M, one or more complexing agents C, and a solvent system S, optionally adjusting the pH of said mixture to a pH comprised in the range of from 4 to 8, and heating the obtained mixture for obtaining a colloidal suspension of transition metal nanoparticles, wherein the mixture does not comprise polyvinyl sulfate and/or polyvinylpyrrolidone. 
     Further, it was found that a novel process for the preparation of transition metal nanoparticles can be provided, the process preferably using only one reagent, wherein the reagent preferably has a low steric hindrance, and which can act as suspending agent and as reducing agent. In particular, one or more complexing agents can be used exhibiting a comparatively low steric hindrance, being able to form a colloid with a flexible structure, and which is able to generate no diffusional control or mass transport restriction to the one or more complexing agents to be bonded to a transition metal and hydrogenated. 
     Further, it was surprisingly found that the novel process for the preparation of transition metal nanoparticles according to the present invention allows for a comparatively high transition metal concentration of a colloidal suspension. In addition thereto, it was found that transition metal nanoparticles can be provided having an average particle size of up to 2 nm. 
     Therefore, the present invention relates to a process for the preparation of transition metal nanoparticles, the process comprising:
     (a) providing a mixture comprising, preferably consisting of, one or more salts of one or more transition metals M, one or more complexing agents C, and a solvent system S;   (b) optionally adjusting the pH of the mixture provided in (a) to a pH comprised in the range of from 4 to 8;   (c) heating the mixture provided in (a) or obtained in (b) for obtaining a colloidal suspension of transition metal nanoparticles;   
 wherein the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise polyvinyl sulfate and/or polyvinylpyrrolidone.
     It is preferred that the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise a polyvinyl sulfate, polyvinylpyrrolidone, a copolymer comprising vinylpyrrolidone, and/or a fatty acid-substituted or unsubstituted polyoxyethylene, more preferably does not comprise a polyvinyl and/or a polyoxyethylene compound, more preferably does not comprise a polymer compound, more preferably does not comprise a complexing agent other than the one or more complexing agents C. 
     It is preferred that the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise ascorbic acid, an ascorbate, formic acid, a formate, and/or a borohydride, wherein more preferably the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise ascorbic acid, an ascorbate, formic acid, a formate, a borohydride, hydrogen, hydrazine, hydroxyethylhydrazine, formic hydrazide, urea, formaldehyde, glucose, sucrose, xylitol, meso-erythritol, sorbitol, glycerol, maltitol, oxalic acid, methanol, ethanol, 1-propanol, iso-propanol, 1-butanol, 2-butanol, 2-methyl-propan-1-ol, allyl alcohol, diacetone alcohol, ethylene glycol, propylene glycol, diethylene glycol, tetraethylene glycol, dipropylene glycol, tannic acid, a tannate, gallic acid, and/or a gallate, wherein more preferably the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise a reducing agent other than the one or more complexing agents C. 
     It is preferred that the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) consists of the mixture comprising, preferably consisting of, the one or more salts of one or more transition metals M, the one or more complexing agents C, the solvent system, and optionally one or more compounds employed for adjusting the pH of the mixture in (b). It is particularly preferred that the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) consists of the one or more salts of one or more transition metals M, the one or more complexing agents C, the solvent system, and optionally one or more compounds employed for adjusting the pH of the mixture in (b). 
     It is preferred that the one or more complexing agents C are selected from the group consisting of carboxylates and salts thereof, 
     preferably from the group consisting of optionally protonated monocarboxylates, dicarboxylates, tricarboxylates, tetracarboxylates, and polycarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated dicarboxylates, tricarboxylates, and tetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated oxalates, malonates, succinates, glutarates, adipates, pimelates, suberates, azelaiates, sebacates, undecanoates, dodecanoates, citrates, isocitrates, aconitates, propane-1 ,2,3-tricarboxylates, trimesates, furantetracarboxylates, methanetetracarboxylates, and ethylenetetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated malonates, succinates, glutarates, adipates, citrates, isocitrates, aconitates, propane-1 ,2,3-tricarboxylates, methanetetracarboxylates, and ethylenetetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated succinates, glutarates, citrates, isocitrates, propane-1,2,3-tricarboxylates, and methanetetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated succinates, citrates, isocitrates, and propane-1 ,2,3-tricarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated citrates and isocitrates, including mixtures of two or more thereof,   wherein more preferably the one or more complexing agents C comprise optionally protonated citrates, including mixtures of two or more thereof, preferably citrates, including mixtures of two or more thereof   wherein more preferably the one or more complexing agents C consist if optionally protonated citrates, including mixtures of two or more thereof, preferably of citrates, including mixtures of two or more thereof,   wherein the counterion of the unprotonated form or forms of the one or more complexing agents C is selected from the group consisting of alkali metals, alkaline earth metals, ammonium, and combinations of two or more thereof, wherein preferably the counterion is Na +  and/or K + , preferably Na + .   

     It is preferred that the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise a complexing agent other than the one or more complexing agents C. 
     It is preferred that in (b) the pH of the mixture provided in (a) is adjusted to a pH comprised in the range of from 5 to 7, more preferably from 5.5 to 6.5, more preferably from 5.9 to 6.3, more preferably from 6.0 to 6.2, and more preferably to about a pH of 6.1. 
     It is preferred that in (a) the one or more transition metals M are selected from the group consisting of Cu, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, and Au, including mixtures of two or more thereof, more preferably from the group consisting of Pd, Pt, Ru, Rh, Au, and Ag, including mixtures of two or more thereof, more preferably from the group consisting of Pd, Pt, Au, and Ag, including mixtures of two or more thereof, wherein more preferably the one or more transition metals M comprise Pd and/or Pt, preferably Pd, wherein more preferably the one or more transition metals M consist of Pd and/or Pt, preferably of Pd. 
     It is preferred that in (a) the counterion of the one or more salts of one or more transition metals M is selected from the group consisting of halides, hydroxide, nitrate, phosphate, sulfate, and combinations of two or more thereof, more preferably from the group consisting of chloride, bromide, hydroxide, nitrate, sulfate, and combinations of two or more thereof, wherein more preferably the counterion is chloride and/or nitrate, preferably chloride. 
     It is preferred that in (a) the one or more salts of the one or more transition metals M are provided as a halide complex, more preferably as a chloride complex, and more preferably as a tetrachloro complex, wherein the counterion of the complex is preferably selected from the group consisting of H + , alkali metals, alkaline earth metals, ammonium, and combinations of two or more thereof, more preferably from the group consisting of Na + , K + , H + , and combinations of two or more thereof, wherein more preferably the counterion is Na +  and/or H + , preferably Na + . 
     It is preferred that in the mixture provided in (a), the C : M molar ratio of the one or more complexing agents C to the one or more transition metals M is comprised in the range of from 1 to 200, more preferably from 5 to 100, more preferably from 10 to 50, more preferably from 15 to 40, more preferably from 20 to 30, more preferably from 23 to 27, and more preferably from 25 to 26. 
     It is preferred that the solvent system S employed in (a) comprises one or more polar solvents, more preferably one or more polar protic solvents, more preferably one or more polar protic solvents selected from the group consisting of C1-C4 alcohols, water, dimethylformamide, and mixtures of two or more thereof, more preferably from the group consisting of n-propanol, isopropanol, methanol, ethanol, water, and mixtures of two or more thereof, more preferably from the group consisting of methanol, ethanol, dimethylformamide, water, and mixtures of two or more thereof, wherein more preferably the solvent system comprises ethanol and/or water, preferably water, wherein more preferably the solvent system consists of ethanol and/or water, preferably of water. 
     It is preferred that in the mixture provided in (a), the S : C molar ratio of the solvent system S to the one or more complexing agents C is comprised in the range of from 1 to 200, more preferably from 5 to 100, more preferably from 10 to 50, more preferably from 15 to 40, more preferably from 20 to 30, and more preferably from 24.2 to 26.2. 
     It is preferred that in the mixture provided in (a), the S : M molar ratio of the solvent system S to the one or more transition metals M is comprised in the range of from 50 to 3500, more preferably from 60 to 2000, more preferably from 75 to 1000, more preferably from 300 to 900, preferably from 400 to 800, preferably from 500 to 700, more preferably from 620 to 665, and more preferably from 642 to 643. 
     It is preferred that in (b) the pH of the mixture provided in (a) is adjusted using a base, more preferably using a salt of a hydroxide, more preferably using an alkali metal hydroxide, wherein the alkali metal is preferably selected from the group consisting of Na, K, Rb, and Cs, wherein more preferably the alkali metal hydroxide is sodium hydroxide and/or potassium hydroxide, wherein more preferably the alkali metal hydroxide is sodium hydroxide. 
     It is preferred that in (b) the pH of the mixture provided in (a) is adjusted using an acid, more preferably using a mineral acid, more preferably using a mineral acid selected from the group consisting of HCl, HBr, HI, HClO, HClO 2 , HClO 3 , HClO 4 , H 2 SO 4 , HSO 3 F, HNO 3 , H 3 PO 4 , HSBF 6 , HBF 4 , and HPF 6 , including mixtures of two or more thereof, more preferably from the group consisting of HCl, HBr, HClO 4 , H 2 SO 4 , HSO 3 F, HNO 3 , and H 3 PO 4 , including mixtures of two or more thereof, more preferably from the group consisting of HCl, HBr, H 2 SO 4 , HNO 3 , and H 3 PO 4 , including mixtures of two or more thereof, more preferably from the group consisting of HCl, H 2 SO 4 , and HNO 3 , including mixtures of two or more thereof, wherein more preferably the mixture provided in (a) is adjusted using HCl. 
     In the case where in (b) the pH of the mixture provided in (a) is adjusted using an acid, it is preferred that the acid is employed as an aqueous solution, more preferably as an aqueous solution comprising from 5 to 60 wt.-% of the acid, more preferably from 10 to 50 wt.-%, more preferably from 20 to 40 wt.-%, more preferably from 25 to 39 wt.-%, more preferably from 29 to 37 wt.-%, more preferably from 31 to 35 wt.-%, and more preferably from 32 to 34 wt.-%. 
     It is further that in (c) the mixture provided in (a) or obtained in (b) is heated to a temperature comprised in the range of from 80 to 120° C., more preferably from 90 to 110° C., more preferably from 95 to 105° C., more preferably from 98 to 102° C., and more preferably from 99 to 101 ° C. 
     It is preferred that in (c) the mixture provided in (a) or obtained in (b) is heated to a temperature comprised in the range of from 70° C. to the boiling point of the mixture provided in (a) or obtained in (b), more preferably from 90° C. to the boiling point of the mixture provided in (a) or obtained in (b), wherein more preferably the mixture provided in (a) or obtained in (b) is heated to the boiling point of the mixture provided in (a) or obtained in (b), wherein the boiling point refers to the boiling temperature at 100 kPa. 
     It is preferred that in (c) the mixture provided in (a) or obtained in (b) is heated for a duration comprised in the range of from 1 to 360 minutes, more preferably from 5 to 60 minutes, more preferably from 10 to 40 minutes, more preferably from 10 to 20 minutes, more preferably from 12 to 18 minutes, and more preferably about 15 minutes. 
     It is preferred that in (c) the mixture provided in (a) or obtained in (b) is heated under a pressure comprised in the range of from 90 to 110 kPa, more preferably of from 95 to 105 kPa, more preferably of from 98 to 102 kPa, and more preferably from 99 to 101 kPa, wherein more preferably the mixture provided in (a) or obtained in (b) is heated under atmospheric pressure. 
     It is preferred that the process further comprises: (e) contacting the colloidal suspension of transition metal nanoparticles obtained in (c) or the transition metal nanoparticles obtained in (d) with a support material for supporting the transition metal nanoparticles on the support material. 
     In the case where the process further comprises (e), it is preferred that the support material in (e) comprises carbon and/or a metal oxide and/or a metalloid oxide, more preferably activated carbon and/or an oxide selected from the group consisting of oxides of Si, Al, Ti, Zr, Hf, La, Ce, Pr, Nd, and mixtures and/or mixed oxides of two or more thereof, more preferably from the group consisting of oxides of Si, Al, Ti, Zr, and mixtures and/or mixed oxides of two or more thereof, wherein more preferably the support material is selected from the group consisting of activated carbon, silica, alumina, silica-alumina, aluminosilicates, titanosilicates, and mixtures of two or more thereof, wherein more preferably the support material comprises activated carbon and/or silicalite, preferably activated carbon, wherein more preferably the support material consists of activated carbon and/or silicalite, preferably of activated carbon. 
     Further in the case where the process further comprises (e), it is preferred that the support material in (e) is a monolith substrate and/or is in the form of granules and/or is in the form of a powder. 
     Further in the case where the process further comprises (e), it is preferred that contacting in (e) is achieved by impregnation, more preferably by incipient wetness impregnation and/or by vacuum impregnation. 
     Further in the case where the process further comprises (e), it is preferred that contacting in (e) is performed in a solvent system, wherein the solvent system preferably comprises, more preferably consists of, a polar solvent, more preferably water, more preferably demineralized water, wherein contacting in (e) is more preferably performed under stirring. 
     Further in the case where the process further comprises (e), it is preferred that contacting in (e) is performed at a temperature in the range of from 15 to 30° C., preferably in the range of from 20 to 25° C. 
     Further in the case where the process further comprises (e), it is preferred that contacting in (e) is performed for a duration in the range of from 0.1 to 5 h, preferably in the range of from 0.5 to 1.5 h. 
     Further in the case where the process further comprises (e), it is preferred that contacting according to (e) is performed in a solvent system, and wherein the process further comprises (f) separating the solvent system from the transition metal nanoparticles supported on the support material, preferably by filtration. 
     In the case where the process further comprises (f), it is preferred that the process further comprises (g) washing the transition metal nanoparticles supported on the support material obtained according to (f) with a solvent system, wherein the solvent system comprises, preferably consists of, a polar solvent, more preferably water, more preferably demineralized water. 
     Furthermore, the present invention relates to a colloidal suspension of transition metal nanoparticles obtainable and/or obtained according to the process of any one of the embodiments disclosed herein. 
     It is preferred that the concentration of the transition metal nanoparticles in the colloidal suspension is comprised in the range of from 0.01 to 5 wt.-%, more preferably from 0.1 to 4 wt.-%, more preferably from 0.2 to 3 wt.-%, more preferably from 0.4 to 2 wt.-% and more preferably from 0.5 to 1 wt.-%, wherein the total weight of the transition metal nanoparticles corresponds to the total weight of the transition metal contained in the colloidal suspension calculated as the element. 
     It is preferred that the weight-based average particle size D50 of the transition metal nanoparticles is in the range of from 0.2 to 20 nm, more preferably from 0.4 to 10 nm, more preferably from 0.6 to 5 nm, more preferably from 0.8 to 4 nm, more preferably from 1 to 3, nm more preferably from 1.2 to 2.5 nm, more preferably from 1.4 to 2.3 nm, more preferably from 1.6 to 2.1 nm, more preferably from 1.65 to 1.9 nm, and more preferably from 1.7 to 1.8 nm, wherein the weight-based particle size D50 is preferably determined according to Reference Example 1. 
     It is preferred that the molecular weight of the transition metal nanoparticles is in the range of from 3000 to 50000 Dalton, preferably in the range of from 4000 to 10000 Dalton, more preferably in the range of from 5000 to 8000 Dalton, preferably determined according to Reference Example 2. 
     It is preferred that the transition metal of the nanoparticles is selected from the group consisting of Cu, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, Au, and alloys of two or more thereof, more preferably from the group consisting of Pd, Pt, Ru, Rh, Au, Ag, and alloys of two or more thereof, and more preferably from the group consisting of Pd, Pt, Au, Ag, and alloys of two or more thereof, wherein more preferably the transition metal of the nanoparticles comprises Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd, wherein more preferably the transition metal of the nanoparticles is Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd. 
     Yet further, the present invention relates to transition metal nanoparticles obtainable and/or obtained according to the process of any one of the embodiments disclosed herein, in particular according to the embodiments for the process for the preparation of transition metal nanoparticles as disclosed herein and comprising (d). 
     It is preferred that the weight-based average particle size D50 of the transition metal nanoparticles is in the range of from 0.2 to 20 nm, more preferably from 0.4 to 10 nm, more preferably from 0.6 to 5 nm, more preferably from 0.8 to 4 nm, more preferably from 1 to 3, nm more preferably from 1.2 to 2.5 nm, more preferably from 1.4 to 2.3 nm, more preferably from 1.6 to 2.1 nm, more preferably from 1.65 to 1.9 nm, and more preferably from 1.7 to 1.8 nm, wherein the weight-based particle size D50 is preferably determined according to Reference Example 1. 
     It is preferred that the molecular weight of the transition metal nanoparticles is in the range of from 3000 to 50000 Dalton, more preferably in the range of from 4000 to 10000 Dalton, more preferably in the range of from 5000 to 8000 Dalton, preferably determined according to Reference Example 2. 
     It is preferred that the transition metal of the nanoparticles is selected from the group consisting of Cu, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, Au, and alloys of two or more thereof, more preferably from the group consisting of Pd, Pt, Ru, Rh, Au, Ag, and alloys of two or more thereof, and more preferably from the group consisting of Pd, Pt, Au, Ag, and alloys of two or more thereof, wherein more preferably the transition metal of the nanoparticles comprises Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd, wherein more preferably the transition metal of the nanoparticles is Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd. 
     Furthermore, the present invention relates to a catalyst comprising transition metal nanoparticles obtainable and/or obtained according to the process of any one of the embodiments disclosed herein, in particular according to the embodiments for the process for the preparation of transition metal nanoparticles as disclosed herein and comprising (e). 
     It is preferred that the catalyst comprises from 0.01 to 10 wt.-% of transition metal nanoparticles calculated as the transition metal and based on 100 wt.-% of the support material, more preferably from 0.05 to 6 wt.-%, more preferably from 0.1 to 4 wt.-%, more preferably from 0.3 to 3 wt.%, more preferably from 0.5 to 2.5 wt.-%, more preferably from 0.6 to 2 wt.-%, more preferably from 0.7 to 1.5 wt.-%, more preferably from 0.8 to 1.3 wt.-%, and more preferably from 0.9 to 1.1 wt.-%. 
     It is preferred that the catalyst has a metal surface area in the range of from 1 to 200 m 2 /g, more preferably in the range of from 5 to 150 m 2 /g, more preferably in the range of from 10 to 120 m 2 /g, wherein the metal surface area is preferably determined according to Reference Example 5. 
     It is preferred that the catalyst has a moisture content in the range of from 40 to 60 weight-%, more preferably in the range of from 45 to 55 weight-%, more preferably in the range of from 47 to 53 weight-%, based on the total weight of the catalyst, wherein the moisture content is preferably determined according to Reference Example 4. 
     It is preferred that the catalyst comprises particles, and wherein the particles have a volume-based particle size D10 in the range of from 1 to 10 micrometer, more preferably in the range of from 2 to 7 micrometer, more preferably in the range of from 3 to 5 micrometer, wherein the particle size distribution is preferably determined according to Reference Example 6. 
     It is preferred that the catalyst comprises particles, preferably is in the form of particles, and wherein the particles have a volume-based particle size D50 in the range of from 10 to 50 micrometer, more preferably in the range of from 25 to 35 micrometer, more preferably in the range of from 28 to 32 micrometer, wherein the particle size distribution is preferably determined according to Reference Example 6. 
     It is preferred that the catalyst comprises particles, preferably is in the form of particles, and wherein the particles have a volume-based particle size D90 in the range of from 50 to 160 micrometer, more preferably in the range of from 90 to 120 micrometer, more preferably in the range of from 100 to 110 micrometer, wherein the particle size distribution is preferably determined according to Reference Example 6. 
     Furthermore, the present invention relates to a use of a catalyst according to any one of the embodiments disclosed herein as a hydrogenation catalyst, preferably as a hydrogenation catalyst in the production of hydrogen peroxide, and more preferably as a hydrogenation catalyst in the anthraquinone process for the production of hydrogen peroxide. 
     The unit bar(abs) refers to an absolute pressure of 10 5  Pa and the unit Angstrom refers to a length of 10 -10  m. 
     The present invention is further illustrated by the following set of embodiments and combinations of embodiments resulting from the dependencies and back-references as indicated . In particular, it is noted that in each instance where a range of embodiments is mentioned, for example in the context of a term such as “any one of embodiments (1) to (4)”, every embodiment in this range is meant to be explicitly disclosed for the skilled person, i.e. the wording of this term is to be understood by the skilled person as being synonymous to “any one of embodiments (1), (2), (3), and (4)”. Further, it is explicitly noted that the following set of embodiments is not the set of claims determining the extent of protection, but represents a suitably structured part of the description directed to general and preferred aspects of the present invention. 
     According to an embodiment (1), the present invention relates to a process for the preparation of transition metal nanoparticles, the process comprising:
     (a) providing a mixture comprising, preferably consisting of, one or more salts of one or more transition metals M, one or more complexing agents C, and a solvent system S;   (b) optionally adjusting the pH of the mixture provided in (a) to a pH comprised in the range of from 4 to 8;   (c) heating the mixture provided in (a) or obtained in (b) for obtaining a colloidal suspension of transition metal nanoparticles;   (d) optionally isolating the transition metal nanoparticles obtained in (c), preferably by centrifugation and/or evaporation to dryness of the colloidal suspension obtained in (c); wherein the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise polyvinyl sulfate and/or polyvinylpyrrolidone.   

     A preferred embodiment (2) concretizing embodiment (1) relates to said process, wherein the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise a polyvinyl sulfate, polyvinylpyrrolidone, a copolymer comprising vinylpyrrolidone, and/or a fatty acid-substituted or unsubstituted polyoxyethylene, preferably does not comprise a polyvinyl and/or a polyoxyethylene compound, more preferably does not comprise a polymer compound, more preferably does not comprise a complexing agent other than the one or more complexing agents C. 
     A further preferred embodiment (3) concretizing embodiment (1) or (2) relates to said process, wherein the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise ascorbic acid, an ascorbate, formic acid, a formate, and/or a borohydride, wherein preferably the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise ascorbic acid, an ascorbate, formic acid, a formate, a borohydride, hydrogen, hydrazine, hydroxyethylhydrazine, formic hydrazide, urea, formaldehyde, glucose, sucrose, xylitol, meso-erythritol, sorbitol, glycerol, maltitol, oxalic acid, methanol, ethanol, 1-propanol, iso-propanol, 1-butanol, 2-butanol, 2-methyl-propan-1-ol, allyl alcohol, diacetone alcohol, ethylene glycol, propylene glycol, diethylene glycol, tetraethylene glycol, dipropylene glycol, tannic acid, a tannate, gallic acid, and/or a gallate, wherein more preferably the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise a reducing agent other than the one or more complexing agents C. 
     A further preferred embodiment (4) concretizing any one of embodiments (1) to (3) relates to said process, wherein the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) consists of the mixture comprising, preferably consisting of, the one or more salts of one or more transition metals M, the one or more complexing agents C, the solvent system, and optionally one or more compounds employed for adjusting the pH of the mixture in (b), wherein more preferably the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) consists of the one or more salts of one or more transition metals M, the one or more complexing agents C, the solvent system, and optionally one or more compounds employed for adjusting the pH of the mixture in (b). 
     A further preferred embodiment (5) concretizing any one of embodiments (1) to (4) relates to said process, wherein the one or more complexing agents C are selected from the group consisting of carboxylates and salts thereof, 
     preferably from the group consisting of optionally protonated monocarboxylates, dicarboxylates, tricarboxylates, tetracarboxylates, and polycarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated dicarboxylates, tricarboxylates, and tetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated oxalates, malonates, succinates, glutarates, adipates, pimelates, suberates, azelaiates, sebacates, undecanoates, dodecanoates, citrates, isocitrates, aconitates, propane-1 ,2,3-tricarboxylates, trimesates, furantetracarboxylates, methanetetracarboxylates, and ethylenetetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated malonates, succinates, glutarates, adipates, citrates, isocitrates, aconitates, propane-1 ,2,3-tricarboxylates, methanetetracarboxylates, and ethylenetetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated succinates, glutarates, citrates, isocitrates, propane-1,2,3-tricarboxylates, and methanetetracarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated succinates, citrates, isocitrates, and propane-1 ,2,3-tricarboxylates, including mixtures of two or more thereof,   more preferably from the group consisting of optionally protonated citrates and isocitrates, including mixtures of two or more thereof,   wherein more preferably the one or more complexing agents C comprise optionally protonated citrates, including mixtures of two or more thereof, preferably citrates, including mixtures of two or more thereof   wherein more preferably the one or more complexing agents C consist if optionally protonated citrates, including mixtures of two or more thereof, preferably of citrates, including mixtures of two or more thereof,   wherein the counterion of the unprotonated form or forms of the one or more complexing agents C is selected from the group consisting of alkali metals, alkaline earth metals, ammonium, and combinations of two or more thereof, wherein preferably the counterion is Na +  and/or K + , preferably Na + .   

     A further preferred embodiment (6) concretizing any one of embodiments (1) to (5) relates to said process, wherein the mixture provided in (a) and heated in (c) or obtained in (b) and heated in (c) does not comprise a complexing agent other than the one or more complexing agents C. 
     A further preferred embodiment (7) concretizing any one of embodiments (1) to (6) relates to said process, wherein in (b) the pH of the mixture provided in (a) is adjusted to a pH comprised in the range of from 5 to 7, preferably from 5.5 to 6.5, more preferably from 5.9 to 6.3, more preferably from 6.0 to 6.2, and more preferably to about a pH of 6.1. 
     A further preferred embodiment (8) concretizing any one of embodiments (1) to (7) relates to said process, wherein in (a) the one or more transition metals M are selected from the group consisting of Cu, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, and Au, including mixtures of two or more thereof, preferably from the group consisting of Pd, Pt, Ru, Rh, Au, and Ag, including mixtures of two or more thereof, more preferably from the group consisting of Pd, Pt, Au, and Ag, including mixtures of two or more thereof, wherein more preferably the one or more transition metals M comprise Pd and/or Pt, preferably Pd, wherein more preferably the one or more transition metals M consist of Pd and/or Pt, preferably of Pd. 
     A further preferred embodiment (9) concretizing any one of embodiments (1) to (8) relates to said process, wherein in (a) the counterion of the one or more salts of one or more transition metals M is selected from the group consisting of halides, hydroxide, nitrate, phosphate, sulfate, and combinations of two or more thereof, more preferably from the group consisting of chloride, bromide, hydroxide, nitrate, sulfate, and combinations of two or more thereof, wherein more preferably the counterion is chloride and/or nitrate, preferably chloride. 
     A further preferred embodiment (10) concretizing any one of embodiments (1) to (9) relates to said process, wherein in (a) the one or more salts of the one or more transition metals M are provided as a halide complex, preferably as a chloride complex, and more preferably as a tetrachloro complex, wherein the counterion of the complex is preferably selected from the group consisting of H + , alkali metals, alkaline earth metals, ammonium, and combinations of two or more thereof, more preferably from the group consisting of Na + , K + , H + , and combinations of two or more thereof, wherein more preferably the counterion is Na +  and/or H + , preferably Na + . 
     A further preferred embodiment (11) concretizing any one of embodiments (1) to (10) relates to said process, wherein in the mixture provided in (a), the C : M molar ratio of the one or more complexing agents C to the one or more transition metals M is comprised in the range of from 1 to 200, preferably from 5 to 100, more preferably from 10 to 50, more preferably from 15 to 40, more preferably from 20 to 30, more preferably from 23 to 27, and more preferably from 25 to 26. 
     A further preferred embodiment (12) concretizing any one of embodiments (1) to (11) relates to said process, wherein the solvent system S employed in (a) comprises one or more polar solvents, preferably one or more polar protic solvents, more preferably one or more polar protic solvents selected from the group consisting of C1-C4 alcohols, water, dimethylformamide, and mixtures of two or more thereof, more preferably from the group consisting of n-propanol, isopropanol, methanol, ethanol, water, and mixtures of two or more thereof, more preferably from the group consisting of methanol, ethanol, dimethylformamide, water, and mixtures of two or more thereof, wherein more preferably the solvent system comprises ethanol and/or water, preferably water, wherein more preferably the solvent system consists of ethanol and/or water, preferably of water. 
     A further preferred embodiment (13) concretizing any one of embodiments (1) to (12) relates to said process, wherein in the mixture provided in (a), the S : C molar ratio of the solvent system S to the one or more complexing agents C is comprised in the range of from 1 to 200, preferably from 5 to 100, more preferably from 10 to 50, more preferably from 15 to 40, more preferably from 20 to 30, and more preferably from 24.2 to 26.2. 
     A further preferred embodiment (14) concretizing any one of embodiments (1) to (13) relates to said process, wherein in the mixture provided in (a), the S : M molar ratio of the solvent system S to the one or more transition metals M is comprised in the range of from 50 to 3500, preferably from 60 to 2000, more preferably from 75 to 1000, more preferably from 300 to 900, preferably from 400 to 800, preferably from 500 to 700, more preferably from 620 to 665, and more preferably from 642 to 643. 
     A further preferred embodiment (15) concretizing any one of embodiments (1) to (14) relates to said process, wherein in (b) the pH of the mixture provided in (a) is adjusted using a base, preferably using a salt of a hydroxide, more preferably using an alkali metal hydroxide, wherein the alkali metal is preferably selected from the group consisting of Na, K, Rb, and Cs, wherein more preferably the alkali metal hydroxide is sodium hydroxide and/or potassium hydroxide, wherein more preferably the alkali metal hydroxide is sodium hydroxide. 
     A further preferred embodiment (16) concretizing any one of embodiments (1) to (15) relates to said process, wherein in (b) the pH of the mixture provided in (a) is adjusted using an acid, preferably using a mineral acid, more preferably using a mineral acid selected from the group consisting of HCl, HBr, HI, HClO, HClO 2 , HClO 3 , HClO 4 , H 2 SO 4 , HSO 3 F, HNO 3 , H 3 PO 4 , HSBF 6 , HBF 4 , and HPF 6 , including mixtures of two or more thereof, more preferably from the group consisting of HCl, HBr, HClO 4 , H 2 SO 4 , HSO 3 F, HNO 3 , and H 3 PO 4 , including mixtures of two or more thereof, more preferably from the group consisting of HCl, HBr, H 2 SO 4 , HNO 3 , and H 3 PO 4 , including mixtures of two or more thereof, more preferably from the group consisting of HCl, H 2 SO 4 , and HNO 3 , including mixtures of two or more thereof, wherein more preferably the mixture provided in (a) is adjusted using HCl. 
     A further preferred embodiment (17) concretizing embodiment (16) relates to said process, wherein the acid is employed as an aqueous solution, preferably as an aqueous solution comprising from 5 to 60 wt.-% of the acid, more preferably from 10 to 50 wt.-%, more preferably from 20 to 40 wt.-%, more preferably from 25 to 39 wt.-%, more preferably from 29 to 37 wt.-%, more preferably from 31 to 35 wt.-%, and more preferably from 32 to 34 wt.-%. 
     A further preferred embodiment (18) concretizing any one of embodiments (1) to (17) relates to said process, wherein in (c) the mixture provided in (a) or obtained in (b) is heated to a temperature comprised in the range of from 80 to 120° C., preferably from 90 to 110° C., more preferably from 95 to 105° C., more preferably from 98 to 102° C., and more preferably from 99 to 101° C. 
     A further preferred embodiment (19) concretizing any one of embodiments (1) to (18) relates to said process, wherein in (c) the mixture provided in (a) or obtained in (b) is heated to a temperature comprised in the range of from 70° C. to the boiling point of the mixture provided in (a) or obtained in (b), preferably from 90° C. to the boiling point of the mixture provided in (a) or obtained in (b), wherein more preferably the mixture provided in (a) or obtained in (b) is heated to the boiling point of the mixture provided in (a) or obtained in (b), wherein the boiling point refers to the boiling temperature at 100 kPa. 
     A further preferred embodiment (20) concretizing any one of embodiments (1) to (19) relates to said process, wherein in (c) the mixture provided in (a) or obtained in (b) is heated for a duration comprised in the range of from 1 to 360 minutes, preferably from 5 to 60 minutes, more preferably from 10 to 40 minutes, more preferably from 10 to 20 minutes, more preferably from 12 to 18 minutes, and more preferably about 15 minutes. 
     A further preferred embodiment (21) concretizing any one of embodiments (1) to (20) relates to said process, wherein in (c) the mixture provided in (a) or obtained in (b) is heated under a pressure comprised in the range of from 90 to 110 kPa, preferably of from 95 to 105 kPa, more preferably of from 98 to 102 kPa, and more preferably from 99 to 101 kPa, wherein more preferably the mixture provided in (a) or obtained in (b) is heated under atmospheric pressure. 
     A further preferred embodiment (22) concretizing any one of embodiments (1) to (21) relates to said process, wherein the process further comprises: (e) contacting the colloidal suspension of transition metal nanoparticles obtained in (c) or the transition metal nanoparticles obtained in (d) with a support material for supporting the transition metal nanoparticles on the support material. 
     A further preferred embodiment (23) concretizing any one of embodiments (1) to (22) relates to said process, wherein the support material in (e) comprises carbon and/or a metal oxide and/or a metalloid oxide, preferably activated carbon and/or an oxide selected from the group consisting of oxides of Si, Al, Ti, Zr, Hf, La, Ce, Pr, Nd, and mixtures and/or mixed oxides of two or more thereof, more preferably from the group consisting of oxides of Si, Al, Ti, Zr, and mixtures and/or mixed oxides of two or more thereof, wherein more preferably the support material is selected from the group consisting of activated carbon, silica, alumina, silica-alumina, aluminosilicates, titanosilicates, and mixtures of two or more thereof, wherein more preferably the support material comprises activated carbon and/or silicalite, preferably activated carbon, wherein more preferably the support material consists of activated carbon and/or silicalite, preferably of activated carbon. 
     A further preferred embodiment (24) concretizing any one of embodiments (1) to (23) relates to said process, wherein the support material in (e) is a monolith substrate and/or is in the form of granules and/or is in the form of a powder. 
     A further preferred embodiment (25) concretizing any one of embodiments (1) to (24) relates to said process, wherein contacting in (e) is achieved by impregnation, preferably by incipient wetness impregnation and/or by vacuum impregnation. 
     A further preferred embodiment (26) concretizing any one of embodiments (1) to (25) relates to said process, wherein contacting in (e) is performed in a solvent system, wherein the solvent system preferably comprises, more preferably consists of, a polar solvent, more preferably water, more preferably demineralized water, wherein contacting in (e) is more preferably performed under stirring. 
     A further preferred embodiment (27) concretizing any one of embodiments (1) to (26) relates to said process, wherein contacting in (e) is performed at a temperature in the range of from 15 to 30° C., preferably in the range of from 20 to 25° C. 
     A further preferred embodiment (28) concretizing any one of embodiments (1) to (27) relates to said process, wherein contacting in (e) is performed for a duration in the range of from 0.1 to 5 h, preferably in the range of from 0.5 to 1.5 h. 
     A further preferred embodiment (29) concretizing any one of embodiments (1) to (28) relates to said process, wherein the process comprises contacting according to (e), wherein contacting according to (e) is performed in a solvent system, and wherein the process further comprises (f) separating the solvent system from the transition metal nanoparticles supported on the support material, preferably by filtration. 
     A further preferred embodiment (30) concretizing embodiment (29) relates to said process, wherein the process further comprises (g) washing the transition metal nanoparticles supported on the support material obtained according to (f) with a solvent system, wherein the solvent system comprises, preferably consists of, a polar solvent, more preferably water, more preferably demineralized water. 
     An embodiment (31) of the present invention relates to a colloidal suspension of transition metal nanoparticles obtainable and/or obtained according to the process of any of embodiments (1) to (21). 
     A preferred embodiment (32) concretizing embodiment (31) relates to said colloidal suspension of transition metal nanoparticles, wherein the concentration of the transition metal nanoparticles in the colloidal suspension is comprised in the range of from 0.01 to 5 wt.-%, preferably from 0.1 to 4 wt.-%, more preferably from 0.2 to 3 wt.-%, more preferably from 0.4 to 2 wt.-% and more preferably from 0.5 to 1 wt.-%, wherein the total weight of the transition metal nanoparticles corresponds to the total weight of the transition metal contained in the colloidal suspension calculated as the element. 
     A further preferred embodiment (33) concretizing embodiment (31) or (32) relates to said colloidal suspension of transition metal nanoparticles, wherein the weight-based average particle size D50 of the transition metal nanoparticles is in the range of from 0.2 to 20 nm, preferably from 0.4 to 10 nm, more preferably from 0.6 to 5 nm, more preferably from 0.8 to 4 nm, more preferably from 1 to 3, nm more preferably from 1.2 to 2.5 nm, more preferably from 1.4 to 2.3 nm, more preferably from 1.6 to 2.1 nm, more preferably from 1.65 to 1.9 nm, and more preferably from 1.7 to 1.8 nm, wherein the weight-based particle size D50 is preferably determined according to Reference Example 1. 
     A further preferred embodiment (34) concretizing any one of embodiments (31) to (33) relates to said colloidal suspension of transition metal nanoparticles, wherein the molecular weight of the transition metal nanoparticles is in the range of from 3000 to 50000 Dalton, preferably in the range of from 4000 to 10000 Dalton, more preferably in the range of from 5000 to 8000 Dalton, preferably determined according to Reference Example 2. 
     A further preferred embodiment (35) concretizing any one of embodiments (31) to (34) relates to said colloidal suspension of transition metal nanoparticles, wherein the transition metal of the nanoparticles is selected from the group consisting of Cu, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, Au, and alloys of two or more thereof, preferably from the group consisting of Pd, Pt, Ru, Rh, Au, Ag, and alloys of two or more thereof, and more preferably from the group consisting of Pd, Pt, Au, Ag, and alloys of two or more thereof, wherein more preferably the transition metal of the nanoparticles comprises Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd, wherein more preferably the transition metal of the nanoparticles is Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd. 
     An embodiment (36) of the present invention relates to transition metal nanoparticles obtainable and/or obtained according to the process of any one of embodiments (1) to (21), in particular according to any one of embodiments (1) to (21) for the process for the preparation of transition metal nanoparticles comprising (d). 
     A further preferred embodiment (37) concretizing embodiment (36) relates to said transition metal nanoparticles, wherein the weight-based average particle size D50 of the transition metal nanoparticles is in the range of from 0.2 to 20 nm, preferably from 0.4 to 10 nm, more preferably from 0.6 to 5 nm, more preferably from 0.8 to 4 nm, more preferably from 1 to 3, nm more preferably from 1.2 to 2.5 nm, more preferably from 1.4 to 2.3 nm, more preferably from 1.6 to 2.1 nm, more preferably from 1.65 to 1.9 nm, and more preferably from 1.7 to 1.8 nm, wherein the weight-based particle size D50 is preferably determined according to Reference Example 1. 
     A further preferred embodiment (38) concretizing embodiment (36) or (37) relates to said transition metal nanoparticles, wherein the molecular weight of the transition metal nanoparticles is in the range of from 3000 to 50000 Dalton, preferably in the range of from 4000 to 10000 Dalton, more preferably in the range of from 5000 to 8000 Dalton, preferably determined according to Reference Example 2. 
     A further preferred embodiment (39) concretizing any one of embodiments (36) to (38) relates to said transition metal nanoparticles, wherein the transition metal of the nanoparticles is selected from the group consisting of Cu, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, Au, and alloys of two or more thereof, preferably from the group consisting of Pd, Pt, Ru, Rh, Au, Ag, and alloys of two or more thereof, and more preferably from the group consisting of Pd, Pt, Au, Ag, and alloys of two or more thereof, wherein more preferably the transition metal of the nanoparticles comprises Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd, wherein more preferably the transition metal of the nanoparticles is Pd, Pt, PdAu, PdAg, PtAu, or PtAg, preferably Pd, PdAg, or PdAu, more preferably Pd or PdAu, and more preferably Pd. 
     An embodiment (40) of the present invention relates to a catalyst comprising transition metal nanoparticles obtainable and/or obtained according to the process of any one of embodiments (22) to (30). 
     A preferred embodiment (41) concretizing embodiment (40) relates to said catalyst, wherein the catalyst comprises from 0.01 to 10 wt.-% of transition metal nanoparticles calculated as the transition metal and based on 100 wt.-% of the support material, preferably from 0.05 to 6 wt.%, more preferably from 0.1 to 4 wt.-%, more preferably from 0.3 to 3 wt.-%, more preferably from 0.5 to 2.5 wt.-%, more preferably from 0.6 to 2 wt.-%, more preferably from 0.7 to 1.5 wt.%, more preferably from 0.8 to 1.3 wt.-%, and more preferably from 0.9 to 1.1 wt.-%. 
     A preferred embodiment (42) concretizing embodiment (40) or (41) relates to said catalyst, wherein the catalyst has a metal surface area in the range of from 1 to 200 m 2 /g, preferably in the range of from 5 to 150 m 2 /g, more preferably in the range of from 10 to 120 m 2 /g, wherein the metal surface area is preferably determined according to Reference Example 5. 
     A preferred embodiment (43) concretizing any one of embodiments (40) to (42) relates to said catalyst, wherein the catalyst has a moisture content in the range of from 40 to 60 weight-%, preferably in the range of from 45 to 55 weight-%, more preferably in the range of from 47 to 53 weight-%, based on the total weight of the catalyst, wherein the moisture content is preferably determined according to Reference Example 4. 
     A preferred embodiment (44) concretizing any one of embodiments (40) to (43) relates to said catalyst, wherein the catalyst comprises particles, and wherein the particles have a volume-based particle size D10 in the range of from 1 to 10 micrometer, preferably in the range of from 2 to 7 micrometer, more preferably in the range of from 3 to 5 micrometer, wherein the particle size distribution is preferably determined according to Reference Example 6. 
     A preferred embodiment (45) concretizing any one of embodiments (40) to (44) relates to said catalyst, wherein the catalyst comprises particles, preferably is in the form of particles, and wherein the particles have a volume-based particle size D50 in the range of from 10 to 50 micrometer, preferably in the range of from 25 to 35 micrometer, more preferably in the range of from 28 to 32 micrometer, wherein the particle size distribution is preferably determined according to Reference Example 6. 
     A preferred embodiment (46) concretizing any one of embodiments (40) to (45) relates to said catalyst, wherein the catalyst comprises particles, preferably is in the form of particles, and wherein the particles have a volume-based particle size D90 in the range of from 50 to 160 micrometer, preferably in the range of from 90 to 120 micrometer, more preferably in the range of from 100 to 110 micrometer, wherein the particle size distribution is preferably determined according to Reference Example 6. 
     An embodiment (47) of the present invention relates to a use of a catalyst according to any one of embodiments (40) to (46) as a hydrogenation catalyst, preferably as a hydrogenation catalyst in the production of hydrogen peroxide, and more preferably as a hydrogenation catalyst in the anthraquinone process for the production of hydrogen peroxide. 
     EXPERIMENTAL SECTION 
     Reference Example 1: TEM and Determination of Average Particle Size 
     The weight-based average particle size D50 of the colloidal suspension and of the transition metal nanoparticles was determined by analyzing the samples obtained according to the examples using a PHILIPS EM208S (FEI) transmission electronic microscope equipped with a MEGA VIEW III (OLYMPUS) acquisition camera, wherein the data was analyzed and processed using the IMAGE J software package (see also  FIG.  2   ). 
     Reference Example 2: Determination of Molecular Weight 
     The determination of molecular weight was carried out by ultrafiltration with two different semipermeable membranes: 3000 NMWL (nominal molecular weight limit) and 50000 NMWL (nominal molecular weight limit), using a Sorvall ST 16 Centrifuge unit. 
     Approximately 1 - 2 mL of a sample solution was transferred to an ultrafiltration tube and centrifuged at 5000 rpm for 10 minutes. 
     The content of Palladium in the filtrate, after separation trough the membrane, was determined by analysis by ICP-OES. 
     Reference Example 3: Determination of Pd Assay 
     The Palladium assay of a Pd colloid sample was determined via inductively coupled plasma (ICP) mass spectrometry. The Palladium assay of Pd colloid adsorbed on activated carbon powder was determined by XRF apparatus (model Zetium by Analytical). 
     Reference Example 4: Determination of Moisture Content 
     The moisture content was determined gravimetrically by the weight difference of a wet and dried sample, whereby drying was performed at a temperature of 105° C. for 16 h in air. 
     Reference Example 5: Determination of Metal Surface Area 
     The metal surface area was determined by CO chemisorption using dynamic method in a Micromeritics Chemisorb 2750. 
     Reference Example 6: Determination of Particle Size Distribution 
     The particle size distributions of the Pd colloid adsorbed on activated carbon powder, in particular the volume-based D10, D50, and D90 values, were determined by a laser scattering method using a Mastersizer 3000 apparatus. 
     Example 1: Preparation of Colloidal Pd 
     In a 100 ml flask were added 50 g of H 2 O and 35.3 g of Na 3 Ct · H 2 O (trisodium citrate dehydrate; 120 mmol). The resulting mixture was stirred until complete dissolution of the citrate. Then 2.76 g of Na 2 PdCl 4  (corresponding to 0.5 g Pd; 4.7 mmol Pd) were added. The molar ratio of Pd : Na 3 Ct was 1 : 25.5. The pH(25° C.) of the resulting mixture was determined as being 6.52. Then, the pH(25° C.) was lowered to 6.1 with aqueous HCl 33% (33 weight-% of HCl in deionized water). The color of the solution was red. Then, the solution was heated until the boiling point of 100° C. After 15 minutes of heating, the reaction was stopped. During that time the color of the solution changed to brown. 
     The resulting product was analyzed with an ultracentrifugation apparatus. The molecular weight of the resulting nanoparticles was determined as being between 3000 and 50000 Dalton. The average particle size of the resulting nanoparticles was determined as being 1.75 ± 0.44 nm. The TEM image of the nanoparticles is shown in  FIG.  1   . The particle size distribution of the colloidal Pd of Example 1 is shown in  FIG.  2   . 
     Example 2: Preparation of a Heterogeneous Catalyst by Adsorption of Colloidal Pd on Activated Carbon 
     A colloidal suspension prepared via incipient wetness impregnation according to Example 1 was used, the colloidal suspension contained 1 g of Palladium based on 100 g of the total colloid (weight/weight), i. e. 1 weight-%. 
     Based on the pore volume of high activation granular carbon, a sufficient amount of granular carbon to adsorb the total volume of Pd colloidal suspension was weighed and sprayed with the Pd colloid: the nominal assay was 0.25 weight-% Pd; the experimental assay was 0.26 weight% Pd. 
     The resulting colloid adsorbed on granular carbon showed a grey color of Pd mirror. It was found that the metal surface area was comparatively low, i. e. 15 m 2 /g Pd, which indicated an edge coated Pd distribution. 
     Example 3: Preparation of a Heterogeneous Catalyst by Adsorption of Colloidal Pd on Activated Carbon Powder 
     Activated carbon powder was slurried in demineralized water at room temperature. A colloidal Pd suspension according to Example 1 was added within 60 minutes under stirring to said slurry. The slurry was then stirred for 60 minutes at room temperature for complete adsorption. The resulting slurry was filtered via a Buchner funnel with a filtering paper for recovering the solid. The obtained wet cake was washed 3 times with demineralized water; then drained until a moisture content of approximately 50% was achieved. 
     The resulting heterogeneous catalyst had a palladium assay of 0.95 weight-% based on 100 weight-% of the heterogeneous catalyst on dry basis. The moisture content was 52.3 weight-% based on 100 weight-% of the wet (as-obtained after draining) heterogeneous catalyst. The metal surface area was 114 m 2 /g Pd. The particle size distribution of the resulting heterogeneous catalyst afforded a D10 value of 4.2 micrometer, a D50 value of 30.0 micrometer, and a D90 value of 105.0 micrometer. 
     Thus, the Pd assay was found to be close to the nominal value and the moisture content was typical for the activated carbon used as carrier. The metal surface area was comparatively low, however, in a typical range for an eggshell or edge coated catalyst. The particle size distribution was mainly related to the physical properties of the carrier. 
     Example 4: Hydrogenation of an Alpha-Beta Unsaturated Aldehyde 
     A common catalyst was tested as well as a heterogeneous catalyst according to Example 3. 
     An alpha-beta unsaturated aldehyde was used as substrate for a hydrogenation reaction. Typically, palladium shows a high affinity for double bonds, which may be easily converted from an olefinic to a paraffinic group. Further, palladium shows also a certain reactivity towards a carbonyl group, especially towards an aldehydic carbonyl group with low steric hindrance. 
     The main product of such a hydrogenation reaction is the saturated aldehyde. Additionally, primary alcohols may be obtained as by-products from saturation of carbonyl groups. In case where the unsaturated aldehyde contains one or more further non-conjugated double bonds, the fully saturated aldehyde could be a further by-product. 
     Experimental conditions:
     autoclave volume 500 cm 3 ;   alpha-beta unsaturated aldehyde as neat oil;   catalyst loading below 1% weight/weight;   reaction temperature 70 to 90° C.;   hydrogen pressure 5 to 10 bars;   number of total tests: 5, in particular a first cycle (as test cycle) with a fresh catalyst and then 4 recycles, whereby the used catalyst was recovered by filtration and loaded again into the autoclave.   

     Main reaction: Hydrogenation to double bond saturated aldehyde 
     It can be gathered from the results shown in  FIGS.  3  and  4    that the common catalyst yields about 95% of final product as fresh product. After the first cycle, the following four recycles showed a longer reaction time, but approximately the same yield. This may be due to the Pd active surface restructuring, that gives a lower reaction rate (reaction time increases from 3 to 5-6 hours), but a similar yield. 
     Thus, it was found that the colloidal Pd catalyst according to the present invention also shows a good activity at the same catalyst loading when compared to a common catalyst. Thus, the yield was about 78% after 5 hours of reaction, as fresh product. After recycling, the yield was about 50 to 60% after 7 hours of reaction. 
     Side Reaction 1: Hydrogenation to Unsaturated Alcohol (Consecutive Reaction) 
     The selectivity towards the alcohol as by-product was monitored. The results are shown in  FIGS.  5  and  6   . It can be gathered from the results that the common catalyst yielded about 0.7% of unsaturated alcohol as fresh product. After the first cycle, the following four recycles showed a lower yield in the unsaturated alcohol (0.4 to 0.5%), i.e. a better selectivity, which may be due to the Pd active surface restructuring. The colloidal Pd catalyst according to the present invention was comparatively less active, but more selective. Thus, it yielded only about 0.2% of unsaturated alcohol as fresh product. After the first cycle, the following four recycles showed an even lower yield in the unsaturated alcohol (below 0.1%), i.e. a better selectivity. 
     Side Reaction 2: Hydrogenation to Fully Unsaturated Aldehyde (Consecutive Reaction) 
     Further, the selectivity towards the fully unsaturated aldehyde as by-product was monitored. The results are shown in  FIGS.  7  and  8   . It can be gathered from the results that the common catalyst yielded about 4% of fully saturated aldehyde as fresh product. After the first cycle, the following recycles showed a constant yield after 6 hours of reaction. 
     In contrast thereto, the colloidal Pd catalyst according to the present invention was particularly more selective. Thus, it yielded about 4% of fully saturated aldehyde as fresh product for the first cycle (see upper line in  FIG.  8   ), similarly to the common catalyst. But after the first cycle, the following four recycles showed a comparatively lower yield of the unsaturated alcohol. In particular, it was surprisingly found that the selectivity towards the unsaturated alcohol was below 2% for the four recycles, representing an improved selectivity compared to the results for the common catalyst. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG.  1   : shows a TEM image of a sample colloidal suspension prepared according to Example 1. 
         FIG.  2   : shows a particle size distribution of a sample colloidal suspension prepared according to Example 1. On the abscissa, the particle size is given in nm and on the ordinate the frequency is given in %. 
         FIG.  3   : shows the yield of saturated aldehyde for a common catalyst. On the abscissa, the reaction time in hours is given, and on the ordinate, the yield in % is given. The line starting at about 1 hour with a yield of 40% relates to the first cycle, and the other four lines relate to the following four recycles. 
         FIG.  4   : shows the yield of saturated aldehyde for a heterogeneous catalyst according to Example 3 of the present invention. On the abscissa, the time in hours is given, and on the ordinate, the yield in % is given. The upper line relates to the first cycle, and the other four lines relate to the following four recycles. 
         FIG.  5   : shows the selectivity towards alcohol for a common catalyst. On the abscissa, the reaction time in hours is given, and on the ordinate, the selectivity towards alcohol in % is given. 
         FIG.  6   : shows the selectivity towards alcohol for a heterogeneous catalyst according to Example 3 of the present invention. On the abscissa, the reaction time in hours is given, and on the ordinate, the selectivity towards alcohol in % is given. 
         FIG.  7   : shows the selectivity towards fully saturated aldehyde for a common catalyst. On the abscissa, the reaction time in hours is given, and on the ordinate, the selectivity towards fully saturated aldehyde in % is given. 
         FIG.  8   : shows the selectivity towards fully saturated aldehyde for a heterogeneous catalyst according to Example 3 of the present invention. On the abscissa, the reaction time in hours is given, and on the ordinate, the selectivity towards fully saturated aldehyde in % is given. The upper line shows the first cycle. 
     
    
    
     CITED LITERATURE 
     
         
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