METALLIC COLLOIDAL DISPERSIONS

Abstract: A process is disclosed for producing a metallic colloidal dispersion which comprises reducing a salt of a metal of Group VIII of the Periodic Table in a mixed solvent system comprising a lower alcohol and an aprotic polar compound. A reducing catalyst comprising the dispersion thus produced, and its use in non-electrolytic plating processes, is also disclosed.

Description: The present invention relates to a colloidal dispersion containing a metal of Group VIII of the Periodic Table and to a process for producing the dispersion. Metals of Group VIII of the Periodic Table, such as Ni, Co, Rh, Pd and the like, are well known to have activity as reducing catalysts. Raney nickel, for instance, is used as a catalyst for reductive reactions, such as hydrogenation of double bonds and reduction of nitrobenzene into aniline. This will also hold true for Pd, Rh and the like. The present invention is designed to provide stable colloidal dispersions of these metals having catalytic activity. The colloidal dispersions of the present invention are useful, for instance, as catalysts for the hydrogenation reduction of organic compounds, and are also very highly valuable as non-electrolytic plating initiating catalysts of Cu, Ni, Co and the like. US-A-4,339,476 discloses a process for the electroless deposition of a metal onto a non-conductor substrate, which comprises (a) contacting the substrate with a colloidal dispersion of a non-precious catalytic metal capable of electroless metal initiation, and (b) immersing the treated substrate in an electroless plating bath. Raney nickel is unstable in air and entails the danger of ignition upon contact with air. Because of this, more safe nickel type reducing catalysts have been studied. It is mentioned in IEC, vol. 44, No. 5, pp. 1006-1010, for instance, that finely divided Ni₂B (nickel boride) obtained by reducing the salt of nickel with NaBH₄ in ethanol under H₂ stream is a strong reducing catalyst. Further, in US-A-4,339,345 protective colloids, such as polyvinyl pyrrolidone, soluble nylon and the like, are disclosed as being effective for stabilizing nickel in colloidal form. However, although it is true that these nickel type fine powders or colloids, unlike Raney nickel, are not ignitable, they are rapidly inactivated upon contact with air and water. Because of this, they entail the defect that reducing operations must be conducted at an atmosphere from which oxygen and water are strictly removed off. On the other hand, Pd is stable to air and water and widely used as the reducing catalyst. For instance, it is widely carried out to precipitate colloidal Pd in the PdCl₂-SnCl₂ system for making it a nickel plating initiating catalyst for non-conductors. It is also proposed to reduce PdCl₂ in aqueous solution in the presence of a protective colloid for making it a Pd colloidal dispersion to thereby make it a nickel plating initiating catalyst. However, these Pd colloid type catalysts entail an defect that activation treatment with strong acids (such as sulfuric acid) is finally required for imparting them with catalytic activity. The object of the present invention is to provide colloidal dispersions of metals of Group VIII of the Periodic Table having reducing catalytic activity being stable, very tractable and economically very advantageous, overcoming said problems with prior art. The present inventors found that a very stable metallic colloidal dispersion can be obtained by reducing a salt of a metal of Group VIII of the Periodic Table, particularly of nickel, with a reducing agent, such as NaBH₄ or KBH₄, dissolved in a mixed solvent system of at least one lower alcohol and an aprotic polar liquid compound. Thus, according to the present invention there is provided a process for producing a metallic collodial dispersion which comprises reducing a salt of a metal of Group VIII of the Periodic Table dissolved in a mixed solvent system consisting essentially of at least one lower alcohol and an aprotic polar liquid compound. By "lower alcohols" we mean C₁-C₆, for example C₁-C₄, alcohols. The metallic colloidal dispersion produced in the process of the invention is characterized by being stabilized even upon contact with air, being free of ignitability, and what is more, maintaining strong reductive catalytic function. Thus, the invention includes a colloidal dispersion obtainable by the above process. In the present invention the use of the mixed solvent system is important, and when the metal salt is reduced by the use of lower alcohols alone, such as methanol and ethanol, the product obtained is immediately coagulated and precipitated and can no longer be obtained as a uniform dispersion. Moreover, the same result is obtained when reduction is carried out by the use of an aprotic polar liquid compound alone, such as dimethylformamide (DMF). As metal salts usable in the present invention, salts of metals of Group VIII of the Periodic Table, such as nickel, cobalt, ruthenium, rhodium, platinum and palladium, may be used, and there are cited, for instance, nickel chloride-6H₂O, nickel acetate-4H₂O, nickel nitrate-6H₂O, cobalt chloride-6H₂O and cobalt acetate-6H₂O, and of these, one using the salt of nickel is suitable since a very stable colloidal dispersion is obtained. These metal salts are dissolved in the mixed solvent system of the alcohol and aprotic polar liquid compound. As the alcohol there is used, for instance, methanol, ethanol, propanol, isopropanol or butanol, and particularly methanol, ethanol or or a mixture thereof is suitable. As the aprotic polar liquid compound there is used, for instance, a lower molecular weight one, such as dimethyl sulphoxide or an amide compound, such as formamide, dimethylformamide, dimethylacetamide, α-pyrrolidone, vinylpyrrolidone, N-methylformamide, N-methylpyrrolidone or tetramethylurea. Of these, dimethylformamide and dimethyl sulfoxide are preferred. In the present invention, the weight ratio of the alcohol and aprotic polar liquid compound in this mixed solvent system is also important to some extent. The amount of aprotic polar liquid compound used should suitably fall in the range of 200 parts to 0.01 part, preferably 50 parts to 0.02 part, and more preferably 30 parts to 0.03 part, per 100 parts of alcohol by weight. When the aprotic polar liquid compound is used in an amount of more than 200 parts or less than 0.01 part, the metal tends to be coagulated and precipitated at the time of reduction, and there the tendency that it becomes difficult to obtain a stabilized colloidal dispersion. Further, the solution concentration of the metal salt in the mixed solvent system is not critical, but it can be set as falling generally in the range of 0.01 weight % to 5 weight %, preferably 0.05 weight % to 3 weight % and more preferably 0.1 weight % to 1 weight %. Reduction is carried out by addition of a reducing agent to this metal salt-containing solution. As the reducing agent NaBH₄, KBH₄, LiBH₄, (CH₃)₂NBH₃, formaldehyde, stannous chloride and sodium hypophosphite can advantageously be used, and these are added as 0.1 to 5 weight % solution of said alcohol, aprotic polar liquid compound or their mixed liquid solvent system. The amount of the reducing agent added should be set as the amount corresponding to the reducing electric equivalent which is usually 0.5 to 10 times, particularly 1 to 3 times, the reduced electric equivalent of reduced metal. The reduction temperature should preferably be set as falling generally in the range of about 10°C to about 30°C, viz., a temperature of about room temperature. Stirring is not always necessary at the time of reduction reaction, but it is preferable because stirring accelerates a uniform reaction. Low speed stirring is sufficient. By these operations a very stable colloidal dispersion can be obtained in several seconds to scores of minutes. It is not always economical, but the colloidal dispersion of the present invention can be made even under a non-oxygen atmosphere. That is, when metal salts are reduced with NaBH₄ and the like in the mixed solvent system under H₂ stream, for instance, by shutting off air, a colloidal dispersion is obtained. This one has a strong activity, but it is unstable as colloid and it begins to coagulate and precipitate in several hours to several days, but its catalytic activity remains unchanged even after its coagulation. However, when it is stirred under contact with air, the coagulated particles are once again dispersed and it turns into a stable colloidal dispersion. The colloidal dispersion can also be concentrated. When the colloidal dispersion is concentrated until it reaches 1/10 volume or less at a relatively low temperature, such as about 20 to about 50°C, under reduced pressure, a slurried residue is obtained. When alcohol is once again added to this slurry, a colloidal dispersion is obtained and it is quite the same as the untreated dispersion in both catalytic activity and stability. This concentration method is very advantageous in the case of transporting the colloidal dispersion to a remote place. The colloidal dispersion has excellent activity as a reducing catalyst and can be used as a non-electrolytic plating or a hydrogenation catalyst. The colloidal dispersion is very stable and tractable in air. In the case of handling in air, it is high in activity as a non-electrolytic plating catalyst, in particular, and the one which is prepared in a non-oxygen atmosphere, in particular, is higher not only in activity as a non-electrolytic plating catalyst but also in activity as a hydrogenation catalyst. It is not necessary to use colloid protective agents which were heretofore needed for conversion to stabilized colloids. What is more, it can advantageously be carried or transported since it can be concentrated. The invention will be explained in more detail by way of Examples. Example 1 2.5 g of nickel acetate (tetrahydrate) was dissolved in a mixed solution (497.5 g) of 100 g of dimethylformamide and 397.5 g of ethanol to make a metal salt solution (500 g). Separately 0.8 g of sodium boronhydride was dissolved in ethanol to make a reducing solution (500 g). When this reducing solution was added dropwise to said metal salt solution and stirred, it was turned to black colloidal form (metallic colloidal dispersion). This colloidal dispersion was stable without causing coagulation and precipitation for more than 2 months in sealed air at room temperature and without causing a lowering of activity. 50 g of Kevlar® pulp was immersed in this metallic colloidal dispersion and then a surface activated product (amount of catalyst adhered 1.3 wt. %/Kevlar pulp) was obtained passing through filtration and rinsing (ethanol) steps. When this surface activated product was immersed in commercially available nickel plating solution (S680, a product of Nippon Kanigen company) at 50°C, plating proceeded with foarming occurring in 1-2 seconds. The amount of it plated after 1 minute was 100 wt. %/Kevlar pulp. The plated product obtained can be used as electro-conductive fibers. Example 2 A polyester spun fabric was immersed in the same metallic colloidal dispersion as in Example 1 and it was rinsed with ethanol and then immersed in commercially available copper plating solution (made by Okuno Seiyaku company) thereby the fabric obtained gave off copper lustre from its surface. Example 3 The same metallic colloidal dispersion as in Example 1 was spray-coated on one side of filter pater and it was immersed in commercially available nickel plating solution thereby paper which was metallized only on one side was obtained. Example 4 Glass fibers (strand mat or the like) were immersed in the same metallic colloidal dispersion as in Example 1, extra colloidal dispersion was taken off by squeezing and they were directly immersed in commercially available nickel plating solution thereby they were metallized in surface. Example 5 The same metallic colloidal dispersion as in Example 1 was spray coated on an ABS molded product (subjected only to defatting) or the ABS molded product was immersed in said dispersion thereby the resin surface swelled and after drying colloidal particles were fixed steadfast. When this surface activated product was immersed in commercially available nickel plating solution, metal having good adhesion was precipipated. With this it was made possible to improve adhesion without utilizing the usual anchor effect by etching. Example 6 A metallic colloidal dispersion was prepared in like manner as in Example 1 except that the same moles of nickel chloride-6H₂O or cobalt chloride-6H₂O or the like was used instead of nickel acetate-4H₂O. This dispersion was a metallic colloidal dispersion being good in stability (more than 2 months in air at room temperature) as was the case with the one of Example 1, and its activity as the plating catalyst was also of the same extent. Example 7 Using a dehydrated and deoxidized solvent, it was attempted to prepare a metallic colloidal dispersion by the same operations as in Example 1 at a hydrogen atmosphere. The dispersion obtained was not of complete colloidal form and its coagulation and precipitation occurred in part. It was used in dispersed suspension condition to investigate activity as the hydrogenation catalyst for olefins, in consequence of which it showed high activity. The following is a table for comparison in activity between the system used in Examples of this invention and the protective colloid (polyvinylpyrrolidone) system. Olefin DMF/EtOH system protective colloidal system MMA 0.36 0.42 Cyclohexene 1.8 x 10⁻² 1.2 x 10⁻² (Unit [mmol - H₂/sec . g - Ni]) Ni/olefin (molar ratio) 1/20 Example 8 2.5 g of nickel acetate and 2.5 g of dimethyl sulfoxide were dissolved in ethanol to make a metal salt solution (500 g). 500 g of the same reducing solution as prepared in Example 1 was added thereto in portions and stirred and thereby a black metallic colloidal dispersion was obtained. This dispersion was good in stability.

Claims: A process for producing a metallic colloidal dispersion which comprises reducing a salt of a metal of Group VIII of the Periodic Table dissolved in a mixed solvent system consisting essentially of at least one lower alcohol and an aprotic polar liquid compound with a reducing agent A process according to claim 1 in which the lower alcohol is methanol, ethanol or a mixture thereof. A process according to claim 1 or 2 in which the aprotic polar liquid compound is an amide. A process according to claim 1 or 2 in which the aprotic polar liquid compound is dimethylformamide or dimethyl sulfoxide. A process according to any one of the preceding claims in which the metal of Group VIII of the Periodic Table is Ni. A process according to any one of the preceding claims in which the weight ratio of aprotic polar liquid compound to lower alcohol in the mixed solvent system is from 2:1 to 0.0001:1. A process according to any one of the preceding claims in which the solution concentration of the metal salt in the mixed solvent system is from 0.01 % to 5 % by weight. A process according to any one of the preceding claims in which the reducing agent used is NaBH₄ or KBH₄. A reducing catalyst which comprises a metallic colloidal dispersion obtainable by the process as claimed in any one of the preceding claims. Use of a reducing catalyst according to claim 9 in a non-electrolytic plating process.

Inventors: HASEGAWA, OSAMU, MARUTSUKA, TOSHINORI
Applicants: NISSHINBO INDUSTRIES, INC.
Application year: 1989
Main classification: C23C18
IPC codes: B01J37, B01J13, B01J23, C23C18, B01J35, C03C25