GB1574773A - Process for the preparation of supported catalysts - Google Patents

Description

(54) PROCESS FOR THE PREPARATION OF SUPPORTED CATALYSTS (71) We, STAMICARBON B.V., a Netherlands Limited Liability Company of P.O. Box 10, Geleen, the Netherlands, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates heterogeneously-catalytical processes utilizing platinum and/or palladium-containing catalysts.

Furuoya and collaborators described a method for preparing palladium-on-carbon catalysts, in which the palladium is deposited as small metal crystallites (Intern. Chemical Engineering 10, no 21, April 197u, pp 333-338). This method provides for oxidizing the active carbon with nitric acid, impregnating the oxidized carbon for 48 hours with an aqueous solution of a palladium-ammine complex, during which operation protons form the carbon surface are exchanged against positive palladium-ammihe complex ions, isolating the impreg nated solid mass by filtration, and subsequently washing and drying it. Following this, the catalyst is activated by treating it in successlon with hydrogen at 300"C for 3 hours, with air at 1500C for 1 hour and, finally, once more with hydrogen at 1500C for 10 minutes. This method has the drawback of being laborious. Moreover, carbon oxidized with nitric acid is extremely difficult to filter. The low activity suggests that coarse metal crystallites may have formed.

Catalysts in which the catalytically active metal is deposited as small crystallites can also be prepared by the method described in Applicant's Netherlands published patent application 7,502,968. This procedure yields a catalyst of high catalytic activity, by starting from an active carbon with a given favourable pore distribution to which a salt of a catalytically active metal is absorbed in an aqueous medium and by performing the hydrolysis and reduction of the resulting oxide or hydroxide in one single step with a mixture of liquid hydrolyzing and reducing agents.

It has been found that Furuoya's process can be carried out in a much simpler way, whilst yielding a catalyst of equivalent quality but of improved filtration behaviour, which, moreover, can be directly applied without requiring the fairly complicated reducing treatment.

The object of the invention is to prepare active metal-on-carbon catalyst in which the metal particles are deposited as small crystallites, whilst avoiding the cumbersome preparation method of Furuoya.

The invention provides a heterogeneous-catalytical process wherein the reaction medium includes a catalyst composition which is obtained by exchanging protons of acid groups on an active carbon support containing them with cations from a solution of a compound of platinum and/or palladium and separating the treated mass from the solution, and wherein reduction of the metal-containing component of the catalyst is effected under the reaction conditions of the said heterogeneous-catalytical process.

For the supporting material use must be made of active carbon containing acid surface groups. Some active carbons that can be used for the purpose are commercially available, and are known as hydrophilic active carbon, or as active carbon suited for treatment of liquids.

The acidity of these carbons and the number of acid groups on their surfaces can be determined titrimetrically, as described by B.R. Puri in 'Chemistry and Physics of Carbon', published by Marcel Dekker, New York, 1970, Chapter 6, pp 229 ff.

It is possible to increase the number of acid groups on the supporting surface, if a high degree of loading should be desirable, by subjecting the carbon to a known oxidative treatment with oxygen, ozone, potassium persulphate, potassium nitrate, nitrogen monoxide or nitrogen dioxide or preferably with hydrogen peroxide, the use of which when combined with an appropriate selection of time and temperature of the treatment enables the number of acid groups present to be controllably increased and ensures that the support will retain good filtration properties.

The catalysts used in the process of the invention can also contain other catalystic metals in minor proportion, starting from solutions of mixtures of salts or complex compounds of the catalytically active metals. Ion exchange with the acid surface groups results in a statistic distribution of the alloy-forming ions on the surface of the support, and formation of homogeneous alloy particles. If the compound to be exchanged, for example a complex ion, is susceptible to the action of acid, and is liable to decompose under the influence of the acid surface of the support, it is possible first to exchange the acid groups against alkali metal ions, and subsequently exchange the alkali metal ions against the positive ions of the catalytically active compound.

The amount of metal that can be deposited on the surface of the support by exchanging acid groups against positive metal ions may be within wide limits, for example from 0.1% to approximately 10%wtcalculated to the weight of the final catalyst composition. The graph of the accompanying drawing shows the titration curve for several active carbon types as a measure of the number of acid groups. The number of meq Ba(OH)2 needed for neutralizing a slurry of 100 g active carbon in distilled water is plotted on the abscissa, and the pH value on the ordinate. It can be derived from this titration curve that CX active carbon made by CECA, France, possesses a large number of acid groups. Practice has shown that up to a fairly high degree of loading this active carbon type is a suitable support for preparing catalysts for use according to the invention. However after this active carbon type has been treated with H202 at 40". for 20 hours, more acid groups appear to be present, with the consequence that exchange with a Pd(NH3)4 Cl2-solution and reduction yields a catalyst which contains approximately 5% wt of palladium, instead of 2% wt with the untreated active carbon. The quantity of catalytically active material to be deposited by exchange can be determined by means of the said curve. The degree of loading with catalytically active metal corresponds with the number of surface protons, which is determined from the number of meq.

Ba(OH)2/100 g carbon during neutralization to a pH of approximately 7.

Using the process of the invention, skin-type catalysts can be prepared from granules, pellets or other artefacts to the exchange and subsequent reduction treatments described above. The term active carbon includes carbon prepared from peat, coat or bone and graphite and carbon black, provided they carry acid groups on their surface.

The catalyst s used according to the invention can be employed in hydrogenation processes and treatment for reducing the metal ion to the free metal is carried out in situ. Such palladium-on-carbon, platinum-on-carbon or platinum-palladium-on-carbon catalysts suitable for catalyzing the preparation of hydroxylamine by reduction of nitrate ions or nitrogen monoxide with hydrogen in a strongly acid medium.

The following Examples of the invention are provided.

20 g of active carbon made by CECA, France and designated type CX, with an accessible surface area (BET area) of 1055 m2/g, was stirred with 120 ml of distilled water for 1/2 hours to expel air and other adsorbed gases. 105 ml of a palladium amine chloride Pd(NH3)4C12, solution containing 2.2 g of palladium (calculated as metal) were then added and the solution was stirred for 20 hours at room temperature to exchange surface protons against Pd(NH3)4++ ions. Subsequently, the solid mass was separated by filtration, and dried for 6 hours at 60-70"C. One fifth of the palladium in the solution deposited on the active carbon.

Because there is only exchange on the surface of the active cation, no adsorption occurs. The solution may have therefore a higher concentration of Pd(NH3)4+±ions than that necessary for the exchange.

With the aid of the catalyst prepared in this way several experiments were carried out: a. A minor portion of the catalyst was subjected to basic hydrolysis and reduction with alkaline methanol. This experiment did not yield a suitable catalyst because the dissolved palladium particles were present in colloidal form. b. Another portion of the catalyst was used, without further previous treatment, for the catalyzed preparation of hydroxylamine from nitrate ions at 30oC. To this end hydrogen gas (80 1/h) was fed into one litre of an aqueous buffer solution which contained 207 cm of phosphoric acid (85 %concentration), 82 g of sodium hydroxide and 198 g of NaNO3, whereupon 750 mg of the catalyst was added and dispersed by stirring. The activity of the catalyst was measured and expressed in g of hydroxylamine per g of metal/hour. The result of the measurements is given in the table, exp. no. 1. c. A small portion of the catalyst was reduced with hydrogen gas prior to application. To this end, nitrogen gas containing 1 vol-% of hydrogen gas was passed over it at the rate of 20 1/h in a pyrex glass tube at 3000C for 1 hour. The reduction was then continued for 1 hour at 300"C with a mixture of 50% vol. of nitrogen gas and 50% vol. of hydrogen gas, supplied at the rate of 20 1/h, after which cooling was effected to room temperature in a stream of pure nitrogen. The results obtained are shown in the accompanying Table.

For the purpose of comparison catalysts were prepared according to the process of the Netherlands patent application 7,502,968 starting from the following active carbons. CECA type CX, Carbopuron type 4N, Carbopuron type 4N without basic hydrolysis and reduction, and Norit type SX 2 (the word NORIT is a Trade Mark) in which latter case the reduction was carried out with hydrogen (experiments 4, 5, 6 and 7. Likewise for comparison, a catalyst was prepared according to the method described by Furuoya. The result obtained with this catalyst is shown in the Table, exp. no. 3.

Table Experiment Catalyst Preparation BET surface Acidity Activity area (m2/g) g NH2OH/g Pd/h 1. 1.9% wt Pd as Pd (NH3)4++ 1055 strongly acid 24.5 present on CECA CX, prepared according to the invention 2. 1.9% wt Pd on CECA CX prepared 1055 strongly acid 20.5 by ion exchange and separate reduction with H2 at 300 C 3. 10.7% wt Pd on CECA CX prepared 1055 strongly acid 2.6 according to the method described by Furuoya 4. 7% wt Pd on CECA CX prepared 1055 strongly acid 18.9 according to Neth, patent application 7,502,968 5. 9.4% wt Pd on Carbopuron 4N 895 weakly acid 19.9 prepared according to Neth. patent application 7,502,968 6. 9.4% wt Pd on Carbopuron 4N 895 weakly acid 6.3 prepared according to Neth. patent application 7.502,968 but with omission of the basic hydrolysis and reduction 7. 10.6% wt Pd on Norit SX-2 1090 weakly acid 6.7 prepared according to Neth. patent application 7,502,968 but with H2-reduction at 300 C Comparison of the results of Experiment 1 (according to the invention) with those of the other experiments (according to known processes) shows that the process of the invention yields the catalyst with the highest activity.

Claims (3)

WHAT WE CLAIM IS:

1. A heterogenous-catalytical process wherein the reaction medium includes a catalyst composition which is obtained by exchanging protons of acid groups on an active carbon support containing them with cations from a solution of a compound of platinum and/or palladium and separating the treated mass from the solution, and wherein reduction of the metal-containing component of the catalyst is effected under the reaction conditions of the said heterogeneous-catalytical process.

2. A process according to Claim 1, wherein the number of acid groups on the surface of the active carbon is increased by oxidative treatment before treatment with the said cations.

3. A process of preparing supported catalysts as claimed in Claim 1, substantially as hereinbefore described with particular reference to the Example.