Ruthenium eggshell catalyst

Abstract

A catalyst composition comprising particles containing ruthenium on a formed carbon support, the ruthenium is distributed on the support as an eggshell and the catalyst composition and comprises betwe

Description

Field of the Invention The present invention relates to catalysts comprising ruthenium on a carbon support, and a method for their manufacture. 5 Background Heterogeneous catalysts are often provided as a metal on a support structure. It is often desirable to have the metal distributed unevenly through the support with a higher concentration of metal at the surface of the support than in the centre. These are commonly referred to as “eggshell” catalysts. Eggshell catalysts are particularly useful where the active 10 metal is expensive, because the active metal is predominantly present in the portion of the catalyst which contacts the feed rather than being evenly dispersed throughout the support. Xl This reduces the amount of expensive metal required. Catalysts in which a platinum group C\l *• M metal (pgm) is distributed as an eggshell on a support are particularly well established LO because of the high cost of these metals. 15 CO The preparation of eggshell ruthenium catalysts has been described using a number of 1 different supports. The article “Preparation of Eggshell-Type RU / AI2O3 Catalysts for Hydrogen Production 2 0 Using Steam-Methane Reforming on PEMFC” (Catalysts, 2021, 17(8), 951) describes the preparation of eggshell catalysts using spherical y-alumina pellets by impregnation with a solution of Ru (III) nitrosyl nitrate and HNO3. The eggshell depth was found to depend on the contact time and the HNO3 / RU molar ratio. 2 5 EP1042273A1 and US2002 / 019559A1 (BASF) describe the preparation of a process for hydrogenating a benzenepolycarboxylic acid using a ruthenium-containing catalyst. In an example, a catalyst was prepared by treating an alumina support with an impregnation solution of ruthenium (III) nitrate solution, followed by drying and reducing in H2. 30 WO2006 / 136541 and US2010 / 152436A1 (BASF) describe “coated” catalysts comprising as active metal ruthenium either alone or together with at least one further transition metal, applied on a SiO2 support. The catalyst includes <1 wt% active metal and at least 60% by weight of the active metal is present in the coating up to a penetration depth of 200 pm. In an example, a catalyst was prepared by treating a SiCh support with an impregnation solution of ruthenium (III) nitrosylnitrate. 5 US2012 / 296111A1 (BASF) describes eggshell catalysts comprising 0.1 to 0.5 wt% ruthenium, rhodium, palladium, platinum and mixtures thereof on a support material comprising SiO2, where the support has certain limitations on pore volume, BET surface area and pore size. In an example, a catalyst is prepared by treating 3-5 mm SiO2 spheres with a solution of ruthenium acetate solution in four portions, followed by drying and 10 passivation. Comparative examples are prepared following the procedures in WO2006 / 136541 and EP1042273A1 (BASF). Ruthenium on carbon (hereafter Ru / C) is widely used as a heterogeneous hydrogenation catalyst. Carbon supports are complementary to oxidic supports and offer a number of 15 benefits. For example, carbon supports are generally non-acidic and non-basic, and therefore less likely to promote side reactions when compared with oxidic supports. Carbon-supported catalysts may therefore be preferred where the feed stream is acidic, which is often the case in bio-derived feedstocks. Carbon supports are also hydrothermally stable, and may be preferred over oxidic supports when the feedstock contains significant amounts 2 0 of water. Ru / C is used widely as a heterogeneous catalyst for hydrogenation reactions. One application in which Ru / C catalysts have found use is in the hydrogenation of sugars and their derivatives to the corresponding alcohol form. For example, WO2008 / 109877A1 25 (Virent) describes a process for making a C4+ compound from a C1+O1+ hydrocarbon by a deoxygenation step followed by a condensation step. The C4+ compound may be prepared from sugars by hydrogenation and Ru / C is suggested as a catalyst for this step. The reference describes that the catalyst can be prepared by the impregnation of a suitable support material with a solution comprising ruthenium (III) nitrosylnitrate or ruthenium (III) 30 chloride, drying at 120 °C in a rotary ball oven then reduced in a H2 stream at high temperature. An example is prepared by treating a carbon support (60-120 mesh = 0.25-0.125 mm) with an aqueous solution of ruthenium nitrosyl nitrate, followed by drying to produce a Ru / C with a ruthenium loading of 2.5 wt%, and used for the conversion of sucrose. EP0553815A1 (Montecatini Technologie S.p.A.) describes a catalyst comprising ruthenium supported on granulated active carbon, having a specific surface area, total pore volume, apparent specific weight (bulk density), actual specific weight, total volume of micropores 5 and ash content with specific ranges. The term “granulated active carbon” used in the reference refers to a carbon which has a particle size of between 5.7 and 0.5 mm. In the examples a carbon is used having a particle size of: 20-30 wt% 10-18 mesh and 80-70 wt% 18-35 mesh. 10 Whilst Ru / C catalysts are available commercially (e.g. 5R619, a 5% Ru / C from Johnson Matthey), they are generally powdered catalysts with small particle sizes. Powdered catalysts suffer from a number of drawbacks. Firstly, powdered catalysts are often pyrophoric and have to be carefully packaged and handled. Secondly, because powdered catalysts cannot be used in fixed bed reactors, they have to be used as a slurry which 15 requires special containment measured to prevent the catalyst being transported downstream. In some duties it is possible to replace a powdered catalyst with a formed catalyst, in which the active metal is supported on a formed support. As used herein, the term “formed 2 0 support” means a support having a regular shape (e.g. spheres, cylinders, trilobes etc...) in contrast to a powdered catalyst in which the particle shapes are generally irregular. The use of a formed catalyst with a regular shape allows greater control over the properties of the catalyst bed, such as pressure drop. Whereas powdered catalysts have small particle sizes, typically on the scale of less than 1 mm with the metal normally distributed homogeneously 25 across the catalyst, formed catalysts are generally larger and it is often desirable to provide the metal as an eggshell in order to avoid the metal being present in the centre of the support where it is less available to the reactants. This is particularly the case for catalysts containing a platinum group metal (pgm) because of the high cost of these metals. 30 Surprisingly, while many pgm catalysts are available as an eggshell on a formed carbon support, the present inventors are not aware of any commercially available catalysts having a ruthenium eggshell on a formed carbon support, or any reports thereof. The present inventors have found that there are difficulties in preparing such catalysts in an economical way, which may explain the lack of commercial offerings. There is a need for eggshell Ru / C catalysts based on formed supports, ideally not requiring the use of expensive reagents for their production. The present invention addresses this problem. Summary of the invention 5 Before arriving at the present invention the inventors trialled different techniques which had previously been used to produce eggshell catalysts on formed oxide supports. One technique involved the impregnation of ruthenium (III) nitrosyl nitrate onto a formed carbon support. While this was successful in producing a ruthenium eggshell, ruthenium (III) nitrosyl 10 nitrate is an expensive reagent. Another technique involved the impregnation of ruthenium (III) chloride onto a formed carbon support followed by reducing to ruthenium (0). However, while an eggshell was formed prior to reduction, it was found that ruthenium migrated during the reduction step which diminished the quality of the eggshell after reduction. xt 15 Ruthenium (III) chloride would be an ideal reagent for eggshell Ru / C formation because it is relatively cheap compared to ruthenium (III) nitrosyl nitrate. After extensive investigations, the inventors found that by introducing a step of treating the intermediate ruthenium eggshell (formed by impregnation of the carbon support with ruthenium (III) chloride solution) with a basic solution (e.g. hydroxide), the eggshell became fixed to the support 2 0 and ruthenium migration was not observed during the subsequent reduction step. This technique is an efficient method for producing eggshell Ru / C catalysts with formed carbon supports. In a first aspect the invention relates to a catalyst composition comprising particles 25 containing ruthenium on a formed carbon support, wherein: the ruthenium is distributed on the carbon support as an eggshell; the catalyst composition comprises 0.1 to 4.0 wt% ruthenium based on the weight of the catalyst composition as a whole; the catalyst composition has a particle size distribution in which >95 wt% of the 30 particles are retained by a US sieve size 18. The parameter “US sieve size” is well known to those skilled in the art. A US sieve size 18 refers to a mesh with openings of 1.00 mm, and is equivalent to Tyler mesh size 16. Larger sieve sizes correspond to smaller openings (US sieve size 20 is a mesh with openings of 0.841 mm) and smaller sieve sizes correspond to larger openings (US sieve size 16 is a mesh with openings of 1.19 mm). In the present invention >95 wt% of the particles in the catalyst composition are retained 5 by a US sieve size 18. The particles of such composition will have a minimum dimension of at least 1.00 mm. At this particle size and at particles sizes above this, it becomes viable and desirable to distribute the pgm as an eggshell. The catalyst composition according to the first aspect differs from that described in 10 EP0553815A1 for at least the reason that the examples of this reference are prepared using a carbon support containing 20-30 wt% particles with size 10-18 mesh and 70-80 wt% particles with size 18-35 mesh. The mesh sizes in this reference are US sieve sizes. The latter fraction corresponds to a particle size smaller than US sieve size 18 and therefore only 20-30 wt% of this composition would be retained by a US sieve size 18. CM 15 The particles of catalyst in the catalyst composition may be oxidic or reduced, as defined below. In a second aspect the invention relates to a method for producing a catalyst composition 2 0 comprising particles containing ruthenium on a carbon support, comprising the steps of: (i) adding an impregnation solution comprising ruthenium (III) trichloride to particles of a formed carbon support having a particle size distribution in which >95 wt% of the particles of formed carbon support are retained by a US sieve size 18, thereby forming a ruthenium eggshell on the support; and 2 5 (ii) treating the product of step (i) with a basic solution to fix the ruthenium eggshell to the support; (iii) drying the product of step (ii); wherein the catalyst composition after step (iii) is an oxidic catalyst composition according to the first aspect. 30 Description of the Figures Figure 1 is an XRM image showing a cross-section of a catalyst produced by impregnation of a carbon support with ruthenium (III) trichloride without “fixing” the eggshell before reduction (Comparative Example 1) 5 Figure 2 is an XRM image showing a cross-section of a catalyst produced according to the process of the invention (Example 2). The ruthenium eggshell is well defined. Figure 3a is an EPMA image showing a cross-section of catalyst 3B. 10 Figure 3b is an image of the ruthenium intensity in the EPMS image across line AB. Figure 4 is a plot showing eggshell depth as a function of ruthenium loading. Detailed Description CM 15 Any sub-headings are included for convenience only and are not to be construed as limiting the disclosure in any way. Catalyst composition 20 The ruthenium is distributed on the carbon support as an eggshell. The term “eggshell” is used herein to refer to a catalyst in which the distribution of ruthenium is uneven and is present at a higher concentration at the surface of the support compared to the centre of the support. The ruthenium eggshell can be seen in cross-section by x-ray microscopy 25 (XRM) or transmission electron microscopy (TEM). Preparation of the eggshell is described in more detail under the “manufacturing method” heading. An advantage of the method described herein is that it allows for the production of thin eggshells on formed catalysts, which in turn provide a more active catalyst for a given metal content. 30 The inventors have found that when the ruthenium content is between 0.1-4.0 wt% there is a linear correlation between ruthenium content and eggshell depth which is approximated by the equation average eggshell depth (pm) = 108 ■ (Ru loading (wt%)) - 86.9 where “average eggshell depth” is measured by the procedure in the examples section and “Ru loading (wt%)” is measured by Inductively Coupled Plasma (ICP) mass spectrometry. 5 In preferred embodiments the catalyst composition satisfies 95 ■ (Ru loading (wt%)) - 86.9 <average eggshell depth (pm) <120 ■ (Ru loading (wt%)) -86.9 preferably 0 95 ■ (Ru loading (wt%)) - 86.9 <average eggshell depth (pm) <115 ■ (Ru loading (wt%)) -86.9 It is preferred that the average eggshell depth is <200 pm, preferably <100 pm. Typical 15 average eggshell depths are 15 to 200 pm, such as 15 to 100 pm. The catalyst composition comprises 0.1 to 4.0 wt% ruthenium based on the weight of catalyst as a whole. The content of ruthenium will depend on the duty which the catalyst composition is to be used for. More typically the ruthenium content is 0.1 to 3.0 wt%, such as 0.5 to 3.0 wt%. For the avoidance of doubt, the content of ruthenium refers to the amount of ruthenium regardless of its oxidation state and includes ruthenium whether present as oxidic ruthenium or in reduced form. The support is a formed support having a particle size distribution in which >95 wt% of the 25 particles of formed carbon support are retained by a US sieve size 18. This differs from powdered catalysts which have powder sizes in the micron scale. Formed supports are suitable for fixed bed duties and generally suffer from fewer losses than slurried catalysts. It is preferred that the particles of support (and the particles of catalyst) have a particle size distribution in which >98 wt% of the particles of formed carbon support are retained by a 30 US sieve size 18, preferably >99 wt% of the particles. In one embodiment the support is in form of spheres. The spheres have a diameter which is at least 1 mm and a typical diameter is 1 to 10 mm. In one embodiment the support is in form of tablets. As used herein, “tablets” refers to a material that has been made by tabletting. The tablet has a minimum dimension which is 1 mm or more, typically 1 to 10 mm. Tablets tend to have lower pore volumes than extrudates and may therefore be preferred where it is necessary to have a support with a lower pore 5 volume. Tablets may have various shapes which will depend upon the duty. In one embodiment the tablet has a spherical cross-section. In one embodiment the support is in form of extrudates. As used herein, “extrudates” refers to a material that has been made by extrusion. The extrudate has a minimum dimension 10 which is 1 mm or more, typically 1 to 10 mm. Extrudates may have various shapes which will depend upon the duty. In one embodiment the extrudate has a spherical cross-section (i.e. is shaped as a cylinder with the radius of the cylinder being the minimum dimension). In one embodiment the support has a trilobe cross-section. It is well known that trilobe shapes offer a high geometric surface area with low pressure drop in the reactor. CM 15 The presence of metals other than ruthenium may impact the activity and / or selectivity of the catalyst. Therefore, in one embodiment the content of any metal other than ruthenium is <10,000 ppm. The content of metals can be measured by techniques known to those skilled in the art, such as Inductively Coupled Plasma (ICP) spectrometry. 20 Depending on the duty for which the catalyst will be used, the ruthenium may be in oxidic form or in reduced form, generally the reduced form. By “oxidic” we mean that the ruthenium is present in a positive oxidation state, which is usually ruthenium (III). By “reduced form” we mean that the ruthenium is present as ruthenium (0). The terms “oxidic catalyst” and 25 “reduced catalyst” will be understood accordingly. Ruthenium is readily oxidized in air and for that reason it will generally be preferred that the catalyst is supplied in the oxidic form (an oxidic catalyst), and activated prior to activation rather than being supplied in the reduced form (a reduced catalyst). Typically, the oxidic catalyst is reduced in situ in the reactor before the reaction is carried out. The preparation of oxidic and reduced form catalysts is described in more detail under the “manufacturing method” section. Manufacturing method The process of the present invention involves a first step (i) of adding an impregnation solution comprising ruthenium (III) trichloride to a formed carbon support in which >95 wt% of the particles of formed carbon support are retained by a US sieve size 18. This step 5 results in the formation of an intermediate eggshell. Ruthenium (III) trichloride is relatively cheap and available. The carbon support used in step (i) is as defined in connection with the catalyst. 10 It is preferred that step (i) is carried out using incipient wetness impregnation because this technique has been found to reliably form an eggshell when used according to the method of the present invention. The process of the present invention includes a second step (ii) of treating the product of 15 step (i) with a basic solution to fix the ruthenium eggshell to the support. The basic solution is preferably a solution containing a Group I metal hydroxide, preferably sodium hydroxide or potassium hydroxide. A further benefit of carrying out step (ii) is that it replaces chloride ions in the eggshell with hydroxide ions, which has the result that HCI production during the subsequent reduction step is reduced or eliminated. 20 The process of the present invention includes a third step (iii) of drying the product of step (ii). Drying conditions will be well known to those skilled in the art. The product of step (iii) is an oxidic catalyst. 25 Typically, the oxidic catalyst will need to be reduced before being used in a reaction. Therefore, in some embodiments the process includes a fourth step (iv) of reducing the oxidic ruthenium to ruthenium metal. Reduction may be achieved by suitable methods know to those skilled in the art, for example treatment with H2 or hydrazine. As noted above, this may be carried out as a step following production of the oxidic catalyst but will more typically 30 be carried out prior to using the catalyst for the reaction in question. Examples General procedure for measuring eggshell depth The ruthenium distribution of a catalyst pellet was measured by electron probe micro analysis. Using imaging software, the intensity of the ruthenium distribution was measured along a line perpendicular to the surface of the catalyst. The start of the eggshell was taken 5 as the point where the ruthenium intensity rose above 0. The ruthenium intensity rises to a maximum then starts to decrease. The end of the eggshell was taken as the point where the ruthenium intensity began to rise again (illustrated by the dashed line in Figure 3b). The thickness of the eggshell is the distance between the start and end of the eggshell. 10 An average eggshell depth is obtained by measuring the eggshell depth at a minimum of four points on the same catalyst particle and taking an average of these values. Example 1 (Comparative) Carbon extrudates with a minimum diameter ~1.5 mm were charged into a blender. An 15 aqueous solution of ruthenium (III) chloride having approximately the same volume as the total adsorption volume of the carbon support was added with gentle rotation. An aqueous solution of hydrazine hydrate was added and the mixture heated. Once reduction was complete the catalyst was drained, washed with aqueous ammonium bicarbonate (x3), water (x3), then dried. 2 0 The ruthenium distribution is shown in Figure 1 as an XRM image. The quality of eggshell is poor and there is significant ruthenium content throughout the support. Example 2 Carbon extrudates were charged into a blender. An aqueous solution of ruthenium (III) chloride having approximately the same volume as the total adsorption volume of the 2 5 carbon support was added with rotation. Once complete, an aqueous solution of sodium hydroxide with a volume approximately equal to the total adsorption volume of the carbon support was added with rotation. An aqueous solution of hydrazine hydrate was added and the mixture heated. Once reduction was complete the catalyst was drained, washed with aqueous ammonium bicarbonate then dried. 30 The ruthenium distribution is shown in Figure 2 as an XRM image. The eggshell is clearly defined, thin, and the content of ruthenium throughout the support is low. Example 3 The catalysts in Table 1 were prepared using the same procedure for Example 2. The catalysts were analysed for eggshell depth according to the general procedure. Ruthenium loading was measured by inductively coupled plasma (ICP) mass spectrometry. 16 05 24 Catalyst Eggshell depth* (pm) Ru loading (wt%) 3A 25 1.0 3B 52 1.1 3C 77 1.7 3D 116 2.0 3E 125 2.1 3F 153 2.1 3G 325 3.7 * eggshell depth was measured at a single location on the pellet Table 1. The calculation of eggshell depth was carried out according to the general procedure. Calculation of the eggshell depth for catalyst 2B is shown in Figures 3a and 3b. 10 There was a linear correlation between eggshell depth and loading as is shown in Figure 4.

Claims

1. A catalyst composition comprising particles containing ruthenium on a formed carbon support, wherein:5 the ruthenium is distributed on the formed carbon support as an eggshell;the catalyst composition comprises 0.1 to 4.0 wt% ruthenium based on the weight of the catalyst composition as a whole;the catalyst composition has a particle size distribution in which >95 wt% of the particles are retained by a US sieve size 18 (openings of 1.00 mm).

102. A catalyst composition according to claim 1, wherein the support is in the form of tablets.3.A catalyst composition according to claim 1, wherein the support is in the form of15extrudates.4.A catalyst composition according to claim 2 or claim 3, wherein the support has aspherical cross-section or a trilobe cross-section.5.A catalyst composition according to any of claims 1 to 4, wherein the content ofany metal other than ruthenium is <10,000 ppm.

6. A catalyst composition according to any of claims 1 to 5, wherein the catalystcomprises 0.1 to 3.0 wt% ruthenium.

257. A catalyst composition according to any of claims 1 to 6, wherein the averageeggshell depth is <200 pm.

8. A catalyst composition according to any of claims 1 to 7, wherein the ruthenium is30 oxidic.

9. A catalyst composition according to any of claims 1 to 7, wherein the ruthenium isin reduced form.16 05 2410. A method for producing a catalyst composition comprising particles containing ruthenium on a carbon support, comprising the steps of:(i) adding an impregnation solution comprising ruthenium (III) trichloride to particles of a formed carbon support having a particle size distribution in which >95 wt%5 of the particles of formed carbon support are retained by a US sieve size 18(openings of 1.00 mm), thereby forming a ruthenium eggshell on the support; and(ii) treating the product of step (i) with a basic solution to fix the ruthenium eggshell to the support;10 (iii) drying the product of step (ii);wherein the catalyst composition after step (iii) is as defined in claim 8.

11. A method according to claim 10, wherein step (i) is carried out by incipient wetness impregnation.1512. A method according to claim 10 or claim 11, wherein the basic solution is an aqueous alkali metal hydroxide.

13. A method according to any of claims 10 to 12, comprising a further step of:2 0 (iv) carrying out a reduction to reduce oxidic ruthenium to ruthenium metal;wherein the catalyst after step (iv) is as defined in claim 9.

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