Process for manufacturing an eggshell catalyst

Abstract

A method for preparing an eggshell catalyst, comprises: (i) adding a first aqueous solution to a catalyst support, in an amount corresponding to 90-140 % of the absorption volume of the support; (ii)

Description

Field of the Invention The present invention relates to a process for manufacturing eggshell catalysts. Background In heterogeneous catalysis it is common to support an active metal on a catalyst support. Whilst the active metal may be distributed evenly throughout the support, it can be advantageous to distribute the active metal unevenly throughout the support, especially as an outer region containing the majority (or all) of the active metal and an inner region containing the minority (or none) of the active metal. Such catalysts are often referred to as “shell” or “eggshell” catalysts. Eggshell catalysts are often used when the reaction is diffusion limited because in such reactions the reaction takes place within the outer region of the catalyst particle, and a catalytic metal is not required in the inner region. Platinum group metals, particularly Ru, Pd and Pt are commonly formulated as eggshell catalysts because of the high cost of these metals and the need to place the metal at the location most favourable to catalysis. Various methods for the production of supported catalysts are known and a review is provided in Advances in Catalysis, Volume 61, 2017, pages 1-35 Chapter 1 “A Review of Preparation Methods for Supported Metal Catalysts” (Mehrabadi et al.). Impregnation techniques which involve adding a solution comprising a metal salt to the catalyst support are particularly simple and widely used. Impregnation techniques can be divided into those where the impregnation solution is added in an amount to just fill the pore volume of the support, termed dry impregnation or incipient wetness impregnation. Alternatively, the impregnation solution may be added in excess of the pore volume of the support, termed wet impregnation. A variant of the impregnation technique, which is less widely practiced, involves partially pre-filling the pores of the support with a metal-free solution followed by a solution comprising a metal salt. For example, WO2011 / 113881A2 describes treating a catalyst support with a first liquid medium comprising a reducing agent in an amount corresponding to 10 to 90% of the total pore volume of the catalyst support, followed by a second liquid medium comprising the transition metal(s) so that at least 91 % of the total pore volume of the catalyst support is filled, followed by drying and calcination. The reference includes examples of catalysts having both Pd and Ag distributed on an alumina support where both the Pd and Ag are distributed as an eggshell. The reason for pre-filling the pores of the support with a reducing agent appears to be so that the metal(s) added in the second step are reduced quickly on contact with the reducing agent and are therefore unable to migrate further into the catalyst support, thereby ensuring a thin eggshell. After adding the solution of metal(s) the material is dried and calcined (steps (d) and (e)). The catalyst obtained after drying and calcination is presumably oxidic due to the high temperature and oxidising environment used during the calcination step (in examples, 5 hours at 630 °C), and would probably need to be reduced prior to use, either in situ or ex situ. This method is therefore not ideal because it suffers from the additional cost and hazards of using a liquid phase reducing agent, but ultimately produces an oxidic catalyst which requires an additional reduction step prior to use. As another example, WO2019 / 195088A2 describes a method for preparing a CO to CO2 combustion promoter such as those used within a fluid catalytic cracking unit. In the examples a y-alumina support was impregnated in a first step with water to fill 90% of the pore volume, followed in a second step by the addition of palladium nitrate, followed by drying and calcining. The examples do not state explicitly what volume of solution is added in the second step, but the reference suggests that in the second step the noble metal salt solution is added in an amount equal to the remainder of the pore volume. For example, when 80 vol.% of the pores are filled with the liquid medium, at most 20 vol% of the pores can be filled with the noble metal solution. A disadvantage of the method described in WO2019 / 195088A2 is the difficulty in homogeneously distributing the liquid medium, especially when high (> 80%) pore filling is required. There is a need for alternative techniques for producing eggshell catalysts which are simple to implement and which avoid the need of using reducing agents during formation of the eggshell. Summary of the Invention In a first aspect the invention relates to a method for preparing an eggshell catalyst, comprising the steps of: (i) producing a pre-filled catalyst support by adding a first aqueous solution to a catalyst support, in an amount corresponding to 90-140 % of the absorption volume of the support, to produce a pre-filled catalyst support; (ii) adding a second aqueous solution comprising a transition metal to the prefilled catalyst support, in an amount corresponding to 100-500% of the absorption volume of the support, to produce an eggshell catalyst; (iii) separating the eggshell catalyst from the residual metal solution; (iv) drying the eggshell catalyst to produce a dried eggshell catalyst; and (v) optionally calcining the dried eggshell catalyst. It will be understood by those skilled in the art that the surface properties of the support impact the thickness of an eggshell formed during impregnation method. An advantage of the present method, which is demonstrated by the examples herein, is that it can be used to form a narrow eggshell on supports which would otherwise produce a diffuse eggshell using a conventional incipient wetness impregnation technique. Without wishing to be bound by any theory, it is thought that the step (i) of pre-filling of the pores of the catalyst support helps to reduce the rate of metal transfer into the pores, thereby leading to a narrow eggshell. Unlike the techniques described in WO2011 / 113881A2 or WO2019 / 195088A2, this method does not require the use of reducing agents during synthesis of the eggshell catalyst nor does it require careful control of the degree to which the pores are pre-filled with water before adding the metal-containing solution. In a second aspect the invention relates to an eggshell catalyst obtained or obtainable by a method according to the first aspect. Detailed Description Any sub-headings are for convenience only and are not intended to limit the invention. Step (i) In step (i) a first aqueous solution is added to a catalyst support, in an amount corresponding to 90-140 % of the absorption volume of the support. The purpose of step (i) is to fill the majority of the pores of the support prior to commencing step (ii). A preferred amount of first aqueous solution is 100-140% of the absorption volume of the support, preferably 100-120% of the absorption volume of the support. An advantage of this method compared to the technique described in WO2011 / 113881A2 is that reducing agents are not required in order to synthesise the eggshell and therefore in preferred embodiments the first aqueous solution is free of reducing agent. The first aqueous solution is preferably free of transition metals. Preferably the first aqueous solution is water, such as deionised water. The support may be a metal oxide or a carbon support, as these types of supports are most widely used for heterogeneous catalysts. Particularly preferred metal oxide supports are those selected from the group consisting of alumina, silica, titania and zirconia, because these supports are the most commonly used metal oxide supports in heterogeneous catalysis. Alumina is particularly preferred. The support preferably has a smallest dimension which is at least 1.0 mm. At these dimensions there is typically a significant amount of internal volume where no reaction is taking place and it is therefore unnecessary and therefore undesirable to have metal present. For instance, a sphere with a diameter of 1.0 mm or a cylinder with a diameter and length which are at least 1.0 mm. It is preferred that the support has a smallest dimension which is at least 2.0 mm. Step (ii) In step (ii) the second aqueous solution (comprising a transition metal) is added to the prefilled catalyst support from step (i). The transition metal may be provided in any form which is soluble in aqueous solution, for example as a transition metal salt or a transition metal complex, preferably a transition metal salt. The method may be applied for manufacturing eggshells of metals in general, but is preferably applied to manufacturing eggshells of transition metals, but the metal is preferably a transition metal, more preferably a platinum group metal. For the avoidance of doubt a “platinum group metal” is a metal selected from ruthenium, rhodium, palladium, osmium, iridium and platinum. The platinum group metal is preferably selected from platinum, palladium or ruthenium because these metals are more widely used in heterogeneous supported catalysts compared to rhodium, osmium and iridium. The second aqueous solution may comprise one or more platinum group metals, but preferably comprises a single platinum group metal. Palladium is widely used in heterogeneous catalysis and therefore in a preferred embodiment the metal is palladium. The palladium may be provided by dissolving a salt of palladium in aqueous solution. Palladium chloride and palladium nitrate are particularly preferred sources of palladium because of their wide availability. Palladium chloride produced thinner eggshells than palladium nitrate, but catalysts prepared using palladium chloride cannot easily be reduced using H2 because of the formation of HCI which can damage equipment. The use of palladium nitrate is therefore particularly preferred. The inventors have found that the more dilute the solution added in step (ii), the thinner the eggshell formed. However, there is a disadvantage towards having ever more dilute, and therefore high volume, solutions because of the increasingly large volumes of solution which need to be treated following step (iii). An amount corresponding to 100-500% of the absorption volume of the support is a compromise between forming an eggshell with the desired thinness and avoiding generating excessive volumes of liquid. An amount corresponding to 150-350% of the absorption volume of the support is particularly preferred. The second aqueous solution preferably includes an overload of 10-30% of the metal that required for the desired loading in the catalyst, i.e. if the amount of metal required to produce the desired loading is 100% then the metal-containing solution added in step (ii) comprises 110-130% of that amount. This is particularly important at commercial scales where there tend to be more opportunities for metal attrition e.g. losses to equipment and during handling. The duration of the contact time between the second aqueous solution and the pre-filled support has an impact on the eggshell thickness up to a point, i.e. the shorter the contact time the narrower the eggshell, after which the eggshell thickness remains reasonably constant. In the case of palladium nitrate there was minimal difference in eggshell thickness for durations longer than 40 minutes. Some contact time is needed to ensure that the support takes up the majority of the metal within the second aqueous solution. A contact time of 10-40 minutes at room temperature is generally appropriate, and this can readily be optimised for a desired catalyst. Step (Hi) In step (iii) the eggshell catalyst is separated from the residual metal solution. Any suitable technique may be used. At small scale the liquid can be decanted and any residue remove by pipette. At larger scale the liquid can be drained. Step (iv) In step (iv) the eggshell catalyst separated in step (iii) is dried. The role of the drying step is to drive off residual moisture. Drying is typically carried out at a temperature of 100-200 °C, preferably 100-150 °C. Studies by the present inventors have shown that the longer the duration between step (iii) and step (iv), the thicker the eggshell. It is therefore preferred that step (iv) is commenced as soon as possible after step (iii), preferably within 4 hours after step (iii), more preferably within 2 hours after step (iii). Optional step (v) In step (v) the dried eggshell catalyst is calcined. The role of the calcination step is to help oxidise any counterions remaining from the solution used in step (ii). Calcination may be carried out under inert conditions or in an atmosphere comprising oxygen content, ideally air for reasons of cost and simplicity. It will be appreciated that if the support is carbon then calcination must either be carried out under inert conditions, or no calcination step is carried out. Preferred calcination conditions involve gradually increasing the temperature to 400-600 °C and holding for a duration of 1-5 hours. Calcination conditions can readily be optimised for a desired catalyst. Examples Absorption volume of the support The catalyst support (10 g) was weighed into a dry beaker. Demineralised water was added until the support was fully submerged. The mixture was stirred to remove air pockets and then left to stand at room temperature for 60 minutes. The excess water was decanted and the wet support was tipped onto filter paper. The beaker was dried and the wet support was returned to the beaker. The difference in weight corresponds to the water uptake by 10 g of support, from which the pore volume (volume / gSUpport) was calculated by dividing by the initial mass of catalyst support. The procedure was repeated using a second 10g batch of catalyst support and the average value taken. In the above procedure, the contact time between the water and the support was 60 minutes. This was because in the catalyst preparation (described below) the contact time between the impregnation solution and the catalyst support was 60 minutes. The contact time used in the pore adsorption measurement should be the same as the contact time used in step (ii). The trials were carried using two different alumina supports A and B. Both supports were 4 x 4 mm cylindrical pellets. In each case comparative examples were carried out by adding the impregnation solution directly to the alumina support in an amount corresponding to 110%, 200% or 400% of the absorption volume of the support. In the comparative examples, an impregnation solution comprising the palladium salt was added to the support in one go. Volumes reported in Tables 1-3 are given as a percentage relative to the absorption volume of the support. After addition of the impregnation solution was complete, the mixture was allowed to stir for 60 mins. The remaining liquid was decanted and the impregnated support was dried at 120 °C in a pre-heated oven, then calcined to 540 °C (ramp rate 5 °C / min, dwell time 4 hours). In the examples according to the invention (E4 and E8) deionised water was first added to the catalyst support in an amount corresponding to 110% of the absorption volume of the support. An impregnation solution was then added in an amount corresponding to 100% of the absorption volume of the support. The resulting catalyst was separated from the solution, dried at 120 °C in a pre-heated oven, then calcined to 540 °C (ramp rate 5 °C / min, dwell time 4 hours). Measurement of eggshell thickness Eggshell depth was measured by a visual method involving staining. A representative 5 sample of catalyst intermediate pellets was collected following calcination. Thirty (30) pellets were horizontally cracked in half individually using a pestle and mortar. One half of each pellet was sieved to screen out dust and then placed in a beaker. About 10 ml of 0.2 wt% N,N-Dimethyl-4-Nitrosoaniline solution in ethanol (99.7%) were added to the beaker with the pellets and left to soak for 5 minutes. The excess solution was pipetted out, and the 10 pellets placed on paper towel. After patting dry, the pellets were spread cracked face up on the paper towel for 30 minutes. The stained pellets that had undergone the 30 minutes development were used to measure the palladium eggshell thickness using a calibrated optical microscope. The thickest part of all four pellets sides was recorded. Results 15 Results of eggshell thickness are reported in Table 1 (for Support A) and Table 2 (for Support B). The impact of palladium salt on eggshell thickness is shown in Table 3. Example CE1 CE2 CE3 E4 Pd source chloride chloride chloride chloride Support A A A A Volume step (i) (%) - - - 110 Volume step (ii) (%) 110 200 400 100 Pd eggshell depth (urn) 60-120 - - - - 120-180 - - 15 24 180-240 4 22 14 6 240-300 5 8 1 - 300-360 6 - - - 360-420 4 - - - 420-540 5 - - - 540-600 2 - - - 600+ 4 - - - Mean depth 410 228 189 162 Table 1. Example CE5 CE6 CE7 E8 Pd source chloride chloride chloride chloride Support B B B B Volume step (i) (%) - - - 110 Volume step (ii) (%) 110 200 400 100 Pd eggshell depth (urn) 60-120 2 - 10 30* 120-180 4 - 20 - 180-240 20 27 - - 240-300 4 3 - - 300-360 - - - - 360-420 - - - - 420-540 - - - - 540-600 - - - - 600+ - - - - Mean depth 203 159 128 <120* Eggshell was too thin to measure accurately. Table 2. The results in Tables 1 and 2 show that catalysts prepared by incipient wetness impregnation (CE1, CE5) had a thicker eggshell than those prepared by wet impregnation 5 using an impregnation solution in an amount equal to 200% (CE2, CE6) or 400% (CE3, CE7) of the absorption volume of the support. The catalyst prepared according to the method of the invention (E4, E8) overall used approximately the same volume of solution as CE2 / CE6, but had a much thinner eggshell. When typical incipient wetness impregnation conditions were applied to Support A (CE1) 10 the eggshell formed was diffuse. However, it was possible to possible to produce a narrow eggshell on Support A using the method according to the present invention (E4). This method may therefore enable the production of eggshells on supports where conventional techniques fail. Example CE3 CE9 CE7 CE10 Pd source chloride nitrate chloride nitrate Support A A B B Volume step (i) (%) - - - - Volume step (ii) (%) 400 400 400 400 Pd eggshell depth (um) 60-120 - - 10 - 120-180 15 - 20 1 180-240 14 - - 5 240-300 1 17 - 23 300-360 - 8 - - 360-420 - 4 - 1 420-540 - 1 - - 540-600 - - - - 600+ - - - - Mean depth 189 312 128 257 Table 3. 5 The results in Table 3 show that, for a given support, the use of palladium chloride resulted in a thinner eggshell compared to when palladium nitrate was used.

Claims

1. A method for preparing an eggshell catalyst, comprising the steps of:(i) producing a pre-filled catalyst support by adding a first aqueous solution to a catalyst support, in an amount corresponding to 90-140 % of the absorption volume of the support, to produce a pre-filled catalyst support;(ii) adding a second aqueous solution comprising a transition metal to the prefilled catalyst support, in an amount corresponding to 100-500% of the absorption volume of the support, to produce an eggshell catalyst;(iii) separating the eggshell catalyst from the residual metal solution;(iv) drying the eggshell catalyst to produce a dried eggshell catalyst; and(v) optionally calcining the dried eggshell catalyst.

2. A method according to claim 1, wherein the first aqueous solution is added in an amount corresponding to 100-140% of the absorption volume of the support3. A method according to claim 1, wherein the first aqueous solution is added in an amount corresponding to 100-120% of the absorption volume of the support.

4. A method according to any of claims 1 to 3, wherein the first aqueous solution is free of transition metals.

5. A method according to any of claims 1 to 4, wherein the first aqueous solution is free of reducing agent.

6. A method according to any of claims 1 to 5, wherein the first aqueous solution is deionised water.

7. A method according to any of claims 1 to 6, wherein the support is carbon.8.A method according to any of claims 1 to 6, wherein the support is a metal oxide.

9. A method according to claim 8, wherein the support is a metal oxide selected from the group consisting of alumina, silica, titania and zirconia.

10. A method according to claim 8, wherein the support is alumina.

11. A method according to any of claims 1 to 10, wherein the support has a smallestdimension which is at least 1.0 mm.

12. A method according to any of claims 1 to 10, wherein the support has a smallest dimension which is at least 2.0 mm.

13. A method according to any of claims 1 to 12, wherein the second aqueous solution and the pre-filled support are contact for a duration of 10-40 minutes at room temperature.

14. A method according to any of claims 1 to 13, wherein the second aqueous solution comprises a transition metal salt or transition metal complex.

15. A method according to any of claims 1 to 14, wherein the transition metal is a platinum group metal.

16. A method according to claim 15, wherein the platinum group metal is platinum, palladium or ruthenium.

17. A method according to claim 15, wherein the platinum group metal is palladium.

18. A method according to claim 17, wherein the metal solution comprises palladiumchloride or palladium nitrate.

19. A method according to any of claims 1 to 18, wherein step (iv) is commenced within 4 hours after step (iii).

20. An eggshell catalyst obtained or obtainable by a method according to any of claims 1 to 19.15