GB1604082A

GB1604082A - Production of catalysts by impregnation of a support

Info

Publication number: GB1604082A
Application number: GB2352778A
Authority: GB
Original assignee: Gallaher Ltd
Current assignee: Gallaher Ltd
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-12-02

Description

(54) PRODUCTION OF CATALYSTS BY IMPREGNATION OF A SUPPORT (71) We, GALLAHER LIMITED, of 65, Kingsway, London, WC2B 6TG. do hereby declare the invention, which we pray that a patent may be granted to us and the method by which it is to be perfomred, to be particularly described in and by the following statement: One of the standard ways of making a catalyst comprises depositing a substance providing a catalytically active material from solution on a support material. Often the support material is microporous, in which event it is desirable to deposit the substance within the pores. The solution used has generally been aqueous although proposals have been made to use wholly organic solutions. However they tend to suffer from the disadvantages of requiring solvent recovery processes.

The most common method of depositing a catalytically active material on a support comprise dissolving a substance providing the catalytically active material in cationic form in water and contacting the resultant solution with the support material, whereupon ion exchange occurs between the cations of the solution and cations in the support.

We are concerned with catalysts for gas phase reactions and it is accepted that increased dispersion of the catalytically active material tends to increase the activity of the catalyst.

Whilst the activity of many catalysts in high temperature reactions is considered to be adequate, although improvement in this may be desirable in many instances, the activity of catalysts in low temperature gas phase reactions tends to be too low for many purposes. For instance attempts have been made to devise a catalyst for the low temperature (e.g. below 100"C and preferably 15 to 800C) oxidation of carbon monoxide to carbon dioxide, for instance for incorporation in a cigarette filter, but such catalysts have been too inactive to be commercially useful.

We have now discovered than when impregnating microporous supports having an average pore size less than 30 Angstrom units it is possible to obtain a large increase in catalytic activity by appropriate control of the impregnation conditions.

A method according to the invention comprises physically adsorbing a substance providing a catalytically active metal of groups 6, 7 and 8 of the Periodic Table into a microporous support having an average pore diameter less than 30 A by contacting the support with a solution of the substance in a solvent which is a mixture of water and 10 to 90% by volume of an organic liquid which reduces the surface tension of the solution and which is inert to the substance, and evaporating at least some of the solvent and thereby depositing the substance in the micropores. Preferably an aqueous methanol solution is used, generally containing 20 to 50% water and 50 to 80% methanol. This is particularly suitable when the catalytic metal is platinum.

We have surprisingly found that the improved activity which arises when physical adsorption is conducted, in the manner described, generally does not arise when the similar solvent system is for ion exchange deposition. In ion exchange deposition techniques the use of an organic solvent tends to reduce the activity of the catalyst.

To ensure adequate physical adsorption it is normal for contact between the solution and the support to be maintained while at least 50%, and in some instances even 100%, of the solvent evaporates. Preferably the contact between the solution and the support is conducted over a prolonged period, eg at least 6 hours and usually at least 10 hours, so as to give optimum time for the metal to be deposited within the pores.

The organic liquid preferably is inert to the catalytically active material, should reduce the hydrogen bonding within the solution and between the solution and the support, and should be wholly miscible with the water in the solution. Often it is preferred that it should have molecular dimensions smaller than the pore size of the support material. Preferred organic liquids are selected from alcohols, cyclic ethers and amines, most preferably being selected from tetrahydrofuran, methanol, ethanol, dioxan and furan. The liquid is generally aliphatic or alicyclic. Preferably the solvent in the solution consists of 20 to 50% water and 50 to 80% of the organic liquid.

The support material has a re size of below 30 , preferably below 16 A, for example 4 to 16 A and thus the organic liquid must have relatively small molecular size, in order that the solution can migrate into the pores It appears that the reduction in surface tension, and reduction in hydrogen bonding, is in some way related to the Hammett Acidity Function theory. Under this the acidity of a mixture of water and certain organic liquids varies according to the proportions of the liquids, generally being highest with pure organic liquid and pure water and lowest with certain mixtures in between. In the invention best results are obtained when the Hammett Acidity of the aqueous mixture is near the minimum possible value.

The organic liquid must not react with the substance that is dissolved in the solution. In some instances it may be found that, for instance, ethanol might react with a platinum-containing substance, in which event ethanol should be avoided as the organic liquid for that solution. Methanol however is satisfactory for platinum-containing substances. Suitable solvent mixtures can be found by routine experimentation. Preferably they are such as to reduce the surface tension of the solution from the value of 73 dynes per cm at 25"C (for pure water) to a value of from 20 to 30 dynes per cm at 25"C.

The solution may initially have a concentration of catalytically active material ranging from a trace to quite high percentages, but preferably contains less than 2.5% based on the weight of the support. In particular the concentration is generally less than 0.2% and preferably less than 0.1% of the catalytically active material. Best results are obtained with 0.01 to 0.10% of the metal (e.g. about 750 parts per million metal). If the solution is too concentrated there seems to be a tendency for it to deposit the catalytically active metal on the external surface and not substantially within the pores.

After depositing the catalytically active material from solution it is generally necessary to reduce it before use. Whilst reduction can be conducted with any reducing gas including hydrogen or hydrogen carbon monoxide mixtures, or with an organic reducing, e.g.

formaldehyde, best results are obtained if reduction is with carbon monoxide, preferably substantiallyure. Reduction is preferably at a temperature of 100 to 450"C, most preferably 300 to 400"C.

The catalytically active material is preferably a metal that is a transition metal, most preferably of groups 6, 7 and 8 of the Periodic Table. Preferred metals are Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cr, Mo, Re and W.

Particularly preferred for low temperature oxidation of carbon monoxide to carbon dioxide are catalysts containing platinum, palladium, rhodium, rhenium and tin and nickel (although nickel catalysts may be more suitable for industrial uses of the catalysts than in smoking products). Mixtures of metals are often useful, especially mixtures of platinum or palladium with rhodium, rhenium or tin. Especially preferred are catalysts based on platinum, palladium, rhodium, rhodium mixed with palladium or any of these together with tin. Whilst the palladium or platinum are generally present in metallic form the tin may be present as stannous oxide. Such catalysts have particularly stable activity in the presence of moisture.

Catalysts containing two or more metals may be made by contacting the powdered support with a solution containing compounds of both metals or by contacting the powdered support sequentially wit solutions of different metal compounds.

The total amount of catalytic metal on the support is preferably from 0.1 to 5% by weight, most preferably 0.5 to 2%.

Compounds that provide the catalytic material in the desired anionic form are readily available. For instance when the catalytic material is to be platinum or a compound of platinum chloroplatinic acid can be used as the source of platinum.

The support material may be any support material having a microporous structure and suitable for use as a catalyst either in low temperature or high temperature processes.

Generally it will be a refractory material. It may be, for instance, carbon. Typical materials, that may be used in the amorphous state, are alumina, silica, titania, magnesia, zirconia, and silico-aluminates that contain some hydroxylic group. An example of an aluminium silicate clay that can be used is montomorillonite. Preferably however the support material is a zeolite. The zeolite may be, for example, a zeolite of the A, X or Y series with best results generally being obtained with the A and X series. Preferred support materials are zeolite 3A, 4A, 5A, 10X and 13X with zeolite 13X, 4A and 5A being preferred. The support material may be utilised in the form of pellets, for example containing a clay binder and having a particle size of 1.3 to 3 mm and which are then crushed to powder, for example less than 0.1 mm, most preferably less than 50 microns, so as to generate the active surfaces or the support material may be in the form of powder, for example less than 0.1 mm and preferably less than 50 microns, and then heated in this form to generate the active surfaces.

Most preferably the particle sizes of the powder, especially when it is being heated to activate it, is from 5 to 15 microns. The powder particles, whether produced by crushing or by heating, preferably have a substantially uniform diameter, for example with substantially none of the articles having a diameter more than 3 times the diameter of a significant proportion of any of the other particles. If crushing produces over size particles they are preferably sieved away and rejected.

Although it is necessary to activate the support material on which the catalytic substance is actually deposited it is of course possible for this support material itself to be carried by a second support material, in which event this second support material of course may not have to be activated in this manner. For instance the final catalyst made by the invention may comprise catalytic substance deposited on, for instance, alumina that has been activated in the described manner and which itself is present as a coating on a honeycomb or other macroporous refractory material which serves as the second support. This refractory material may be a ceramic substance or a metal support, for example stainless steel.

It is particularly preferred that the microporous support material into which the substance is physically adsorbed should be one that has been activated by creating a deficiency of hydroxyl roups in its surfaces. Methods of doing this are described in our copending application 2 91/78 to which reference should be made for full details. (Serial No 1604081). Broadly the methods generally comprise crushing pellets of a support material such as zeolite or alumina and which have generally been made by calcination or by heating powder. Generally activation is achieved by heating either powder or material that is being pelleted to a temperature at least 200C above the temperature at which chemisorbed water is substantially driven off.

Many catalysts made according to the invention are of value for low temperature oxidation of carbon monoxide to carbon dioxide, for example in the oxidation of stack gases or in motor car or other engine exhausts (especially when carried on a refractory macroporous support) but they are of particular value distributed through smoking products or included in a filter for a smoking product. Preferably they are included in a filter. The filter may be a triple filter, with catalytic powder, either by itself or mixed with absorbents such as granular carbon, in a central component between fibrous end portions.

The powder may be loose or may be bonded into a porous plug. The powder may also be bonded to fibres that form the central portion of a triple filter or that are distributed throughout some or all of any filter construction or may be bonded to a sheet which is crumpled or spirally wound to form part or all of a filter.

The following are examples of the invention. In each of Examples 1, 2, 3 and 4 zeolite 13X pellets were used, different commercial grades of zeolite 13X being used in each of the four examples. In each instance the pellets were ground in a domestic grinder and were sieved to leave a fraction having a particle size of 30 to 60 mesh. In Example 5 zeolite 4A powder was used.

In each instance the powder was mixed with an impregnating solution containing sufficient chloroplatinic acid solution (5% w/v) to give a total pick-up of platinum of 0.5% by weight onto the catalyst. The mixture was left for 12 hours at about 4 0C by which time the solution had evaporated to dryness to leave a free flowing powder. As shown in the Table below, the solvent for the solution consisted of water or various mixtures of water and methanol.

After drying each catalyst was reduced by carbon monoxide and its activity in converting carbon monoxide to carbon dioxide was determined by forming a gas mixture of 3% CO, 10% CO2, 13% 2 and 74% N2 (the percentages being by volume) and puffing this over 500 mg of the catalyst and analysing the resultant gas mixture, each puff constituting 35 ml of the gas mixture, at atmospheric pressure and being passed for 2 seconds over the catalyst at the rate of 1 puff per minute. The degree of conversion in the first 10 puffs, and sometimes also in the second 10 puffs was recorded. Naturally the highest possible degree of conversion is desirable, 100% being optimum. The following results were obtained.

Example Zeolite Volume of Composition of Solvent Activity Solution (ml) 1st 10 2nd 10 puffs puffs 1A 13X 20 100% H20 64 1B 13X 20 50% H2O/50% MeOH 95 70 2A 13X 20 100% H2O 82 59 2B 13X 20 50% H2O/50% MeOH 100 71 3A 13X 5 100% H2O 75 - 3B 13X 10 50% H2O/50% MeOH 100 73 4A 13X 20 85% H2O/15% MeOH 93 4B 13X 20 50% H2O/50% MeOH 100 70 4C 13X 20 30% H2O/70% MeOH 94 76 4D 13X 20 20% H2O/8O% MeOH 100 70 4E 13X 20 100% MeoH 81 5A 4A 100% H2O 45 - 5B 4A - 50% H2O/50% MeOH 88 WHAT WE CLAIM IS: 1. A method of making a catalyst comprising physically adsorbing a substance providing a catalytically active metal of groups 6, 7 or 8 or the Periodic Table into a microporous support having an average pore diameter less than 30 by contacting the support with a solution of the substance in a solvent which is a mixture of water and 10 to 90% by volume of an organic liquid which reduces the surface tension of the solution and which is inert to the substance, and evaporating at least some of the solvent and thereby depositing the substance in the micropores.

2. A method according to claim 1 in which the amount of solution that is evaporated is at least 50% by volume of the amount of solution that is contacted with the support and the amount of the substance present in the solution that is contacted with the support is less than about 2.5%, calculated as the metal, based on the weight of the support.

3. A method according to claim 1 or claim 2 in which the micropores have an average pore diameter of 4 to 16 .

4. A method according to any preceding claim in which the catalytically active material is present in the substance in anionic form.

5. A method according to any preceding claim in which the organic liquid is methanol.

6. A method according to claim 5 in which the solution is formed of 20 to 50% by weight water and 80 to 50% by weight methanol.

7. A method according to any preceding claim in which the catalytically active metal is platinum.

8. A method according to any of claims 1 to 4 in which the amount of the organic liquid is 10 to 90% by volume of the mixture and the organic liquid is inert to the catalytically active material, reduces the hydrogen bonding within the solution and between the solution and the support and is wholly miscible with the water in the solution.

9. A method according to claim 8 in which the organic liquid is selected from alcohols, cyclic ethers and amines.

10. A method according to claim 9 in which the organic liquid is selected from tetrahydrofuran, methanol, ethanol, dioxan and furan.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

Example Zeolite Volume of Composition of Solvent Activity Solution (ml) 1st 10 2nd 10 puffs puffs 1A 13X 20 100% H20 64 1B 13X 20 50% H2O/50% MeOH 95 70 2A 13X 20 100% H2O 82 59 2B 13X 20 50% H2O/50% MeOH 100 71 3A 13X 5 100% H2O 75 - 3B 13X 10 50% H2O/50% MeOH 100 73 4A 13X 20 85% H2O/15% MeOH 93 4B 13X 20 50% H2O/50% MeOH 100 70 4C 13X 20 30% H2O/70% MeOH 94 76 4D 13X 20 20% H2O/8O% MeOH 100 70 4E 13X 20 100% MeoH 81 5A 4A 100% H2O 45 - 5B 4A - 50% H2O/50% MeOH 88 WHAT WE CLAIM IS: 1. A method of making a catalyst comprising physically adsorbing a substance providing a catalytically active metal of groups 6, 7 or 8 or the Periodic Table into a microporous support having an average pore diameter less than 30 by contacting the support with a solution of the substance in a solvent which is a mixture of water and 10 to 90% by volume of an organic liquid which reduces the surface tension of the solution and which is inert to the substance, and evaporating at least some of the solvent and thereby depositing the substance in the micropores.
2. A method according to claim 1 in which the amount of solution that is evaporated is at least 50% by volume of the amount of solution that is contacted with the support and the amount of the substance present in the solution that is contacted with the support is less than about 2.5%, calculated as the metal, based on the weight of the support.
3. A method according to claim 1 or claim 2 in which the micropores have an average pore diameter of 4 to 16 .
4. A method according to any preceding claim in which the catalytically active material is present in the substance in anionic form.
5. A method according to any preceding claim in which the organic liquid is methanol.
6. A method according to claim 5 in which the solution is formed of 20 to 50% by weight water and 80 to 50% by weight methanol.
7. A method according to any preceding claim in which the catalytically active metal is platinum.
8. A method according to any of claims 1 to 4 in which the amount of the organic liquid is 10 to 90% by volume of the mixture and the organic liquid is inert to the catalytically active material, reduces the hydrogen bonding within the solution and between the solution and the support and is wholly miscible with the water in the solution.
9. A method according to claim 8 in which the organic liquid is selected from alcohols, cyclic ethers and amines.
10. A method according to claim 9 in which the organic liquid is selected from tetrahydrofuran, methanol, ethanol, dioxan and furan.
11. A method according to any preceding claim in which the catalytically active material

is selected from platinum, palladium and rhodium.
12. A method according to any preceding claim in which the support is zeolite or alumina.
13. A method according to claim 1 substantially as herein described with reference to any of the examples.
14. A catalyst made by a method according to any preceding claim.
15. A smoking product incorporating a catalyst according to claim 14.
16. A catalytic converter for an automobile exhaust, the converter incorporating a catalyst according to claim 14.