GB2605508A - Palladium catalysts for the conversion of acetylene to ethylene - Google Patents

Abstract

A catalyst suitable for conversion of acetylene to ethylene comprising palladium, silver, and tin. The catalyst support comprises alumina. The ratio of Pd:Sn is between 1:1 and 1:8 by weight. Preferably the catalyst has a Pd: Ag ratio of between 1:12 and 1:20 by weight. Preferably the loading of silver is between 0.1 and 1 wt%. Optionally the ratio of Pd: Sn may be from 1:1 to 1:4. The loading of Sn is between 0.01 and 0.1 wt%. The catalyst may have a spherical morphology. Preferably the catalyst is prepared by first co-impregnating an alumina support with a solution containing a palladium salt and a silver salt, carrying out a reduction using 4-24 equivalents of hydrazine based on the total amount of Pd and Ag, drying the product, impregnating a tin salt as a dopant before finally drying the catalyst. Preferably the quantity of hydrazine used is between 4 and 14 equivalents. Preferably the tin salt contains the stannate ion. The catalyst formed may be an eggshell type catalyst.

Description

Palladium catalysts for the conversion of acetylene to ethylene

Field of the Invention

The present invention relates to catalysts for the conversion of acetylene to ethylene. Background Ethylene and propylene are important monomers for the production of plastics, such as for example polyethylene or polypropylene. Ethylene and propylene are primarily derived from petroleum and petroleum products by means of thermal or catalytic cracking of hydrocarbons. The ethylene or propylene derived with the aid of the cracking process does, however, contain an undesirably high proportion of acetylenic compounds such as 7_0 acetylene or methyl acetylene (propyne), which can negatively influence downstream ethylene or propylene polymerization. Therefore prior to polymerization the ethylene or propylene must be freed from acetylenic compounds as far as possible.

Typically for the polymerization of ethylene the acetylene concentration must, for example, be reduced to a value of below 1 ppm. For this the acetylene is selectively hydrogenated into ethylene. High requirements are placed on the catalyst and the hydrogenation process. On the one hand, the acetylene must be removed as completely as possible by transformation into ethylene, while the hydrogenation of ethylene into ethane must be prevented, hence the term "selective hydrogenation". In order to ensure this result, the hydrogenation is carried out within a temperature range that is delimited by the so-called "clean-up" temperature and the so-called "run-away" temperature. In the present context the "clean-up" temperature is understood as the temperature from which an appreciable hydrogenation of acetylene into ethylene is observed, while "run-away" temperature is understood as the temperature at which an appreciable hydrogenation of ethylene into ethane commences. The said temperatures can be determined in that the hydrogen consumption of a defined gas mixture containing acetylene, ethylene, and hydrogen is, for example, measured depending on the temperature.

Palladium catalysts, often using silver as a promoter, are primarily used as commercial 30 catalysts for the selective hydrogenation of acetylene into ethylene in hydrocarbon streams. The palladium and the silver are supported on an inert, temperature-resistant substrate.

The production of these catalysts is carried out in such a way that suitable salts of palladium and silver, for example palladium nitrate and silver nitrate, are applied to a substrate in form of an aqueous solution (impregnation). The impregnation can take place during separate steps with a palladium compound solution and a silver compound solution. It is, however, 5 also possible to apply the solution of palladium compounds and the solution of silver compounds to the substrate simultaneously during a single impregnation step. The impregnated substrate is then calcined to transform the silver into silver oxide, or the palladium into palladium oxide, and is then subjected to a reduction in order to transfer the catalyst into the active form. During the reaction the silver and palladium are assumed to 10 be transferred into the oxidation state "zero".

EP 0 064 301 Al (Phillips Petroleum Company) offers a catalyst for the selective gaseous phase hydrogenation of acetylene. The palladium content is 0.01 to 0.025 wt% and the weight percent of silver is at least twice that of palladium. The palladium is present as an eggshell.

US Patent 4 409 410 (Institut Francais du Petrole) describes a method for the selective hydrogenation of a diolefin with at least 4 carbon atoms in a hydrocarbon mixture. Hydrogenation takes place with hydrogen on a catalyst containing palladium and silver. The palladium content of the catalyst is 0.05 to 0.5 wt%, the silver content is 0.05 to 1 wt%, and the silver/palladium weight ratio of the catalyst is 0.7: 1 to 3: 1.

US Patent 5 648 576 (Institut Francais du Petrole) describes a method for the selective gaseous phase hydrogenation of acetylenic hydrocarbons (C2 or C3) into the corresponding 25 ethylenic hydrocarbons. The process uses a catalyst comprising palladium and at least one Group IB metal, with a palladium/Groupl B metal ratio of 0.05 to 0.25.

An example of a commercial acetylene conversion catalyst is PRICATT" PD 608 from Johnson Matthey. While this catalyst shows excellent activity and selectivity on a lab and pilot scale, it has not performed as expected on a plant scale. The primary reason for this is believed to be that the catalyst is too active. For some plant designs it is undesirable to have a catalyst that is too active because catalyst activity can come at the expense of catalyst selectivity.

There is a need for alternative acetylene for the conversion of acetylene to ethylene, having a higher inherent selectivity and which shows good activity and high selectivity for ethylene at relevant plant temperatures. This invention addresses this problem.

Description of the Figures

Figure 1 shows the impact of feed temperature on acetylene conversion for the catalyst PRICAT PD 608.

Figure 2a shows the impact of reducing the equivalents of hydrazine used in the reduction step on acetylene conversion.

1 0 Figure 2b shows the selectivity at a given conversion for the results shown in Figure 2a. Figure 3a shows the impact of increasing the Ag content.

Figure 3b shows the selectivity at a given conversion for the results shown in Figure 3a. Figure 4 shows the impact of changing the alumina support.

Figure 4b shows the selectivity at a given conversion for the results shown in Figure 4a. 15 Figure 5a shows the impact of doping with Sr, Ba or Sn.

Figure 5b shows the selectivity at a given conversion for the results shown in Figure 5a.

Figure 6 shows the impact of reducing the equivalents of hydrazine used in the reduction step and doping with Sn.

Figure 6b shows the selectivity at a given conversion for the results shown in Figure 6a. 20 Figure 7a shows the conversion vs time on-line for an extended run.

Figure 7b shows the selectivity vs time on-line for the results shown in Figure 7a.

Figure 8 shows the atomic distribution of Al, Pd, Ag and Sn in a catalyst according to the invention measured using an electron probe microanalyzer.

Summary of Invention

US Patent 5 648 576 (Institut Francais du Petrole) suggests that in addition to Pd and Ag, an alkali or alkaline-earth metal may be included as an "enhancer" in an acetylene conversion catalyst. Contrary to the suggestion in this reference, the present inventors found that doping of PR ICATTm PD 608 with either Sr or Ba did not enhance the activity or selectivity.

Surprisingly, the present inventors found that the inclusion of tin (Sn) as a dopant in a catalyst containing Pd and Ag results in a catalyst which has high selectivity under relevant acetylene conversion conditions. We are not aware of Sn having been suggested previously as a dopant for acetylene conversion catalysts. The promoting effect of Sn was even more appreciable when the catalyst manufacturing route was altered by reducing the number of equivalents of reducing agent (hydrazine) used in the manufacturing method.

Catalysts containing Pd, Ag and Sn are known. W02014/037300 (Solvay SA) describes a process for manufacturing hydrogen peroxide using a catalyst comprising 0.1 to 0.9 wt% palladium, 0.0050 to 0.0375 wt% tin and optionally up to 0.06 wt% silver, on a silicon oxide or aluminosilicate support. The catalysts described in this reference have an excess of palladium relative to tin, whereas in the present invention the content of tin is equal to or greater than the content of palladium. This reference does not describe the use of catalysts containing Pd, Ag and Sn for acetylene hydrogenation.

US3957688A and US4191846A (Phillips Petroleum Company) describe a dehydrogenation catalyst comprising 0.1 to 5 wt% of nickel, palladium, platinum, iridium, osmium of mixtures thereof; 0.01 to 5 wt% of tin; and 0.1 to 5 wt% of gold or silver on a support selected from alumina, silica, magnesia, zirconia, alumino-silicates and Group II aluminate spinels. CN102039130A (China Petroleum) describes a hydrogenation catalyst comprising 0.001 to 1.0 wt% palladium, 0.001 to 1.0 wt% gold, 0.001 to 10 wt% silver, and 0.001 to 10 wt% of a co-active component selected from Bi, Zr, Ce, Zn, Ni, Cu, K, Mg, Ba, Ca, Sn, Pb, Mn, La, Ti, Sr and Na. There are no examples in any of these references of a catalyst containing palladium, silver and tin.

In a first aspect the invention provides a catalyst comprising: palladium, silver and tin; on a 30 support comprising alumina, wherein the weight ratio of Pd: Sn is from 1: 1 to 1: 8.

The catalysts according to the first aspect are suitable for the selective hydrogenation of acetylene to ethylene, especially in a tail-end configuration acetylene conversion process.

In a second aspect the invention provides a method for manufacturing a catalyst according to the first aspect, comprising the steps of: (i) co-impregnating an alumina support with an impregnation solution comprising a palladium salt and a silver salt; (ii) carrying out a reduction using 4-24 molar equivalents of hydrazine based on the combined equivalents of Pd + Ag; (iii) drying the product of step (fi); (iv) impregnating the product of (iii) with a tin salt; (v) drying the product of step (iv).

In a third aspect the invention provides a process for the conversion to acetylene to 15 ethylene, wherein a catalyst according to the first aspect is used as the catalyst.

Detailed Description

Any sub-headings are included for convenience only, and are not to be construed as limiting the disclosure in any way.

Catalyst In a first aspect the invention provides a catalyst comprising: palladium, silver and tin; on a support comprising alumina.

The commercially available catalyst PRICAT PD 608 is an acetylene conversion catalyst which is commercially available from Johnson Matthey. The catalyst of the present invention 25 differs from this known commercial catalyst in that it includes Sn as a dopant The present inventors have surprisingly found that doping the catalyst with Sn results in a slightly more active catalyst at low reaction temperatures. A Sn-doped catalyst prepared using low hydrazine equivalents (described later) provides a catalyst with a high selectivity and a stable selectivity profile.

The weight ratio of Pd: Sn is from 1: 1 to 1: 8. Without wishing to be bound by any theory, it is thought that Sn has an electronic effect which reduces the activity of the Pd. When the catalyst is used for acetylene hydrogenation, the unwanted ethylene hydrogenation reaction is hindered, which improves the catalyst selectivity for ethylene. It is preferred that the weight ratio of Pd: Sn is from 1: 1 to 1: 4, preferably from 1: 1 to 1: 3, more preferably from 1: 1.5 to 1: 2.5.The loading of Sn in the catalyst is preferably from 0.01 to 0.2 wt%, preferably from 0.01 to 0.1 wt%, preferably from 0.03 to 0.07 wt%, more preferably from 0.04 to 0.06 wt%.

As used herein, wt% refers to the content of metal relative to the weight of catalyst as a 15 whole.

The weight ratio of Pd: Ag in PRICAT PD 608 is 1: 8. The present inventors have found that increasing the loading of Ag in the catalyst reduces the catalyst activity at temperatures below 60 °C (see examples), particularly when combined with the use of Sn as a dopant.

Without wishing to be bound by theory, it is thought that the additional Ag atoms offer a selectivity benefit by blocking active Pd sites, thereby reducing catalyst activity.

It is preferred that the weight ratio of Pd: Ag is from 1: 12 to 1: 20, preferably from 1: 14 to 1: 18. These ratios provide a catalyst which is less active but which has improved selectivity in the hydrogenation of acetylene to ethylene. A Pd: Ag ratio of 1: 16 is particularly preferred.

The loading of Ag in the catalyst is preferably from 0.1 to 1 wt%, such as 0.25 to 0.75 wt%, preferably from 0.3 to 0.75 wt%, more preferably from 0.3 to 0.5 wt%.

The loading of Pd in the catalyst is preferably from 0.01 to 0.05 wt%, such as 0.02 to 0.04 wt%.

The support comprises alumina. The support is preferably predominantly alumina, i.e. at least 90 wt% alumina, preferably at least 95 wt% alumina, preferably at least 98 wt% alumina. The support may include low levels of impurities, such as silica.

As the person skilled in the art will be aware, alumina takes on various different forms depending on the temperature to which it has been calcined. Different forms of alumina include a-alumina, y-alumina and 0-alumina. The form of alumina does not appear to have any significant impact on catalyst activity or selectivity in the present invention and therefore any type of alumina can be used. The alumina may be a single form or a mixture of different forms.

Typical supports will be alumina having a surface area of 5-60 m2/g, measured by BET. It is thought that catalysts based on higher surface area alumina supports may be more prone to side reactions such as oligomerisafion compared to lower surface area aluminas. For this reason, it is preferred that the alumina has a surface area of 5-40 m2/g, measured by BET.

It is preferred that the catalyst contains 0 to 0.01 wt% of any metal other than Pd, Ag, Sn 15 and Al, preferably 0 to 0.005 wt% of any metal other than Pd, Ag, Sn and Al. The presence of other metals may interfere with the selectivity and/or activity of the catalyst. It is preferred that no metals are present other than Pd, Ag, Sn and Al.

The catalyst may have a variety of different shapes, for example powder, spheres or extrudates (e.g. trilobes or tetralobes). It is preferred that the catalyst is in the form of spheres.

In a preferred embodiment the catalyst comprises: 0.01 to 0.05 wt% palladium; 0.2 to 1.0 wt% silver; 0.01 to 0.1 wt% tin; on a support comprising alumina. The catalyst is preferably in the form of spheres.

In a further preferred embodiment the catalyst comprises; 0.015 to 0.03 wt% palladium; 0.3 to 0.5 wt% silver; 0.03 to 0.07 wt% tin; on a support comprising alumina. The catalyst is preferably in the form of spheres.

Manufacturing method The catalysts described herein may be prepared by a method according to the second 5 aspect of the invention comprising the steps of: (i) co-impregnating an alumina support with an impregnation solution comprising a palladium salt and a silver salt; (ii) carrying out a reduction using 4-24 molar equivalents of hydrazine based on the combined equivalents of Pd + Ag; (iii) drying the product of step (ii); (iv) impregnating the product of (iii) with a tin salt; (v) drying the product of step (iv).

Step (i) is a co-impregnation. The choice of palladium salt and silver salt used in step (i) is not particularly limited. It is preferred that the counteranion of the Pd and Ag salts is the same; this has the advantage that the Pd and Ag salts can be dissolved in the same solution rather than requiring sequential impregnations. It is particularly preferred that the palladium salt is palladium nitrate and the silver salt is silver nitrate as these are both relatively soluble in aqueous solution and can be dissolved together to produce a co-impregnation solution.

Co-impregnation techniques will be well known to those skilled in the art. Typically, a co-impregnation involves preparing an impregnation solution having a volume approximately equal to the absorption volume of the support. In step (i) to co-impregnation solution is preferably added gradually to the alumina support, typically over a period of 15 minutes with tumbling. After addition, the mixture will typically be tumbled for a further 15 minutes to ensure full take-up of the impregnation solution.

In step (ii) the Pd and Ag ions are at least partially reduced to Pd and Ag metal using hydrazine. Hydrazine is commonly used as a reducing agent in catalyst manufacture. A reduction in the equivalents of hydrazine used is beneficial from several standpoints including cost and safety. Reducing the equivalents of hydrazine slightly also decreased the catalyst activity at low temperatures, and the effect was particularly notable when combined with Sn doping. Without wishing to be bound by theory, the reduction in equivalents of hydrazine is believed to allow the Pd/Ag crystallites to grow larger. This has the effect of reducing the active metal surface area for a given metal loading, thereby reducing catalyst activity.

In step (h) 4-24 molar equivalents of hydrazine are added based on the total moles of Pd + Ag. This has the advantage of producing a less active catalyst. Reducing the quantity of hydrazine used is also advantageous from a process safety standpoint due to risks associated with hydrazine. Reducing the quantity of hydrazine used is also advantageous from an environmental standpoint, because ammonia is by-product from hydrazine reduction. Therefore, reducing hydrazine equivalents results in a decrease in ammonia levels on the plant during catalyst manufacture. Preferably 4-18 molar equivalents of hydrazine are added in step (h), preferably 4-14 equivalents.

Step (ii) is typically carried out by adding an aqueous solution of hydrazine to the product of step (i). The volume of hydrazine solution added in step (ii) is typically approximately equal to the adsorption volume of the support. The mixture of impregnation solution and product from step (i) will be typically be tumbled during step (ii) to ensure thorough mixing.

In step (iii) the product from step 00 is dried. The skilled person will readily be able to 20 determine drying conditions, which may vary depending on scale. Typical drying conditions are 105 °C for 12 hours.

The process of the invention involves a step (iv) of impregnating the reduced Pd/Ag catalyst with a tin salt. A separate impregnation step with Sn was found to be necessary because the basic conditions associated with sodium stannate used as the tin salt resulted in precipitation of the Pd and Ag nitrates when co-impregnation of Pd, Ag and Sn was attempted.

It is preferred that the Sn salt is a Sn (IV) salt. Preferably the tin salt is a salt containing the stannate ion [Sn(OH)6]2-. A preferred salt is sodium stannate (Na2Sn(OH)6).

Step (iv) is typically carried out by adding an aqueous solution of the tin salt to the product of step 00. The volume of impregnation solution added in step (iv) is typically equal to the adsorption volume of the support. In step (iv) the impregnation solution is preferably added gradually to the product from step (iii), typically over a period of 15 minutes with tumbling.

After addition, the mixture will typically be tumbled for a further 15 minutes to ensure full take-up of the impregnation solution.

In step (v) the product from step (iv) is dried. The skilled person will readily be able to determine drying conditions, which may vary depending on scale. Typical drying conditions are 105 °C for 12 hours.

Hydrogenation process The catalysts described herein are particularly suitable for the conversion of acetylene to ethylene. As the skilled person will be aware, acetylene to ethylene conversion processes may be run in front-end configuration or tail-end configuration.

In front-end configuration the acetylene converter is located upstream of the cold box prior 15 to the separation of hydrogen from the olefins. In addition to acetylene and ethylene, the feed to a front-end configuration process typically contains various amounts of hydrogen, carbon monoxide and heavier hydrocarbons, depending on the distillation configuration.

In tail-end configuration the acetylene converter is located downstream of the cold box and de-ethanizer. The feed contains acetylene, ethylene, ethane and only a trace of lighter and heavier compounds. A controlled amount of hydrogen is added to affect the selective hydrogenation of acetylene. A tail-end catalyst should ideally ensure complete acetylene removal whilst avoiding significant ethylene hydrogenation and minimising "green oil" formation. The catalysts described herein are particularly suitable for tail-end configuration.

Examples

The invention will now be illustrated by the following non-limiting examples. General procedure for the manufacture of PdAg and PdAgSn catalysts A co-impregnation solution was prepared by dissolving silver nitrate in a palladium nitrate solution. The resulting solution was diluted to the absorption volume of the alumina support to be impregnated. The quantities of silver nitrate and palladium nitrate were chosen so as to achieve the desired loading of Pd and Ag in the final catalyst. The co-impregnation solution was added to the support (over approximately 5 minutes) with tumbling to ensure a good dispersion. Unless otherwise specified, the support was an alumina in the form of 2-4 mm diameter spheres having a mixture of a-and 0-phases. Once addition was complete, tumbling was continued for a further 15 minutes. A hydrazine solution (5 wt% hydrazine in water) was diluted with deionized water to twice the absorption volume of the support. NaOH is added to the hydrazine solution (0.1 g NaOH / 100 g support). The reducing solution was then added to the catalyst in a single portion. The yellow/brown pellets turn black. The mixture was tumbled for 15 minutes. After tumbling, the excess hydrazine solution was decanted. The catalyst was washed three times with deionized water to removed excess hydrazine and any ammonia-based by-products. The catalyst was then dried at 105°C overnight.

PdAgSn catalysts were made according to the above procedure but with the following additional step. A solution of sodium stannate was prepared by dissolving the required amount of sodium stannate in demineralized water and diluting the solution to the absorption volume of the support. The solution was added over 5 minutes to the PdAg catalyst with tumbling, followed by tumbling for a further 15 minutes. The catalyst was discharged and dried at 105 °C overnight.

General procedure for measuring acetylene conversion and selectivity at varying 25 temperature A reactor was charged with catalyst in the form of three separate beds separated by silicon carbide and secured with glass beads and glass wool. The reactor was then heated to the required temperature and an inlet gas mixture comprising of 1% acetylene and trace amounts of ethane in ethylene was passed over the catalyst at a GHSV in the range of 5000-10000 h-1 at pressures of 15-20 bar. Samples of the exit gas were analysed using online GC analysis. The peaks for acetylene, ethylene and ethane were calibrated using known gas mixtures and were used to quantify the gas composition and calculate conversion and selectivity as follows. () x 100

Acetylene conversion -acetylene inlet -ace tlyene exit acetylene inlet ( ethane make Ethylene selectivity = 100 ( 1. x 00) acetylene inlet -acetylene exit) Where ethane make = ethane exit -ethane inlet Baseline testing The activity profile of PRICATTm 608/1 (Johnson Matthey) was measured and the results are shown in Figure 1.

Role of hydrazine equivalents Catalysts were prepared according to the general procedure as follows: Hydrazine Pd loading Ag loading Pd: Ag (molar equivs) (wt%) (wt%) (w/w) CE1 24 0.025 0.2 1: 8 CE2 6 0.025 0.2 1: 8 The results are shown in Figures 2a and 2b. Reducing the hydrazine concentration gave a similar activity profile, with only a minor reduction in activity at low temperatures (Figure 2a). Reducing the hydrazine concentration resulted in only a small improvement in selectivity (Figure 2b) Role of Ag content Catalysts were prepared according to the general procedure as follows: Hydrazine Pd loading Ag loading Pd: Ag (molar equivs) (wt%) (wt%) (w/w) CE1 24 0.025 0.2 1: 8 CE3 24 0.025 0.4 1: 16 The results are shown in Figures 3a and 3b. Increasing the Ag loading significantly decreases activity at temperatures below 60 °C (Figure 3a) Increasing the Ag loading slightly drops selectivity (Figure 3b).

Comparative Examples 1-2 show that catalyst activity is impacted by decreasing hydrazine equivalents used in the reduction step or by increasing Ag content. However, neither modification gave a step change improvement in selectivity.

1 0 Role of alumina Catalysts were prepared according to the general procedure as follows: Alumina Hydrazine Pd loading Ag loading Pd: Ag phase (molar equivs) (wt%) (wt%) (w/w) CE1 ale mix 24 0.025 0.2 1: 8 CE4 a 24 0.025 02 1: 8 The alumina used was an a-alumina in the form of spheres having a diameter of 2-4 mm.

The results are shown in Figures 4a and 4b. The use of an alumina had no impact on the catalyst conversion (Figure 4a) or selectivity (Figure 4b).

Role of dopants Catalyst CE5 was prepared according to the general procedure with strontium nitrate included in the co-impregnation step with silver nitrate and palladium nitrate.

Catalyst CE6 was prepared according to the general procedure with a co-impregnation using silver nitrate and palladium nitrate followed by an impregnation using barium acetate.

Catalyst E7 was prepared according to the general procedure for PdAgSn catalysts.

Hydrazine Pd loading Ag loading (wt%) Dopant Pd: Dopant (molar (wt%) wt% (w/w) equivs) CE1 24 0.025 0.2 - n/a CE5 24 0.025 0.2 Sr 0.05 1: 2 CE6 24 0.025 0.2 Ba 0.05 1 2 E7 24 0.025 0.2 Sn 0.05 1 2 The results are shown in Figures 5a and 5b.

Doping with Sr (CE5) or Ba (CE6) gave a conversion profile identical to the reference 608/1. 10 Doping with Sn (E7) gave a slight increase in conversion at low temperatures (Figure 5a). Doping with Sn gave higher selectivity at moderate conversion (70-80% conversion), but a drop in selectivity at high conversion (Figure 5b).

Low hydrazine, high Ag with Sn doping Catalyst E8 was prepared according to the general procedure as follows: Hydrazine Pd loading Ag loading Dopant Pd: Dopant (molar (wt%) (wt%) wt% (w/w) equivs) E8 6 0.025 0.4 Sn 0.05 1: 2 The results are shown in Figures 6a and 6b.

The combination of fewer equivalents of hydrazine, high Ag loading and Sn doping resulted in a catalyst which was less active (Figure 6a) but showed much higher selectivity than 608/1 (Figure 6b).

Deactivation profiles The deactivation profiles of 608/1 and E8 were studied. A first run was carried out at an initial temperature. The temperature was chosen so that the conversion was approx. 6080% throughout the run. After a run was complete, the temperature was raised slightly for the subsequent run. The results are shown in Figure 7a. The catalysts 608/1 and E8 showed similar conversions but the selectivity of E8 was higher throughout the course of the run and had a stable stability profile.

Catalyst morphology The metal distribution of Ag, Pd and Sn was analysed for E8 using an electron probe microanalyzer. Both Pd and Sn were present as a tight eggshell. Ag was also present as an eggshell, albeit less tight than either Pd or Sn (Figure 8).

Claims (17)

1. Claims 1. A catalyst for the conversion of acetylene to ethylene comprising: palladium; silver; and tin; on a support comprising alumina; wherein.the weight ratio of Pd: Sn is from 1: 1 to 1 8.
2. 2. A catalyst as claimed in claim 1, wherein the weight ratio of Pd: Ag is from 1: 12 to 1: 20.
3. 3. A catalyst as claimed in claim 1 or claim 2, wherein the loading of Ag in the catalyst is from 0.1 to 1 wt%.
4. 4. A catalyst as claimed in claim 1 or claim 2, wherein the loading of Ag in the catalyst is from 0.25 to 0.75 wt%.
5. 5. A catalyst as claimed in any of claims 1 to 4, wherein the weight ratio of Pd Sn is from 1: 1 to 1: 4.
6. 6. A catalyst as claimed in any of claims 1 to 5, wherein the loading of Sn in the catalyst is from 0.01 to 0.2 wt%.
7. 7. A catalyst as claimed in any of claims 1 to 6, wherein the loading of Sn in the 25 catalyst is from 0.01 to 0.1 wt%.
8. 8. A catalyst as claimed in any of claims 1 to 7, wherein the loading of Pd in the catalyst is from 0.01 to 0.05 wt%.
9. 9. A catalyst as claimed in any of claims 1 to 8, wherein the catalyst contains at most 0.01 wt% of any metal other than Pd, Ag, Sn and Al.
10. 10. A catalyst as claimed in any of claims 1 to 9, wherein the catalyst comprises: 0.01 to 0.05 wt% palladium; 0.2 to 1.0 wt% silver; 0.01 to 0.1 wt% tin; on a support comprising alumina.
11. 11. A catalyst as claimed in any of claims 1 to 9, wherein the catalyst comprises: 0.015 to 0.03 wt% palladium; 0.3 to 0.5 wt% silver; 0.03 to 0.07 wt% tin; on a support comprising alumina.
12. 12. A catalyst as claimed in any of claims 1 to 11 in the form of spheres.
13. 13. A method for manufacturing a catalyst according to any of claims 1 to 12, comprising the steps of: co-impregnating an alumina support with an impregnation solution comprising a palladium salt and a silver salt; (ii) carrying out a reduction using 4-24 molar equivalents of hydrazine based on the combined equivalents of Pd + Ag; (iii) drying the product of step (ii); (iv) impregnating the product of (iii) with a tin salt; (v) drying the product of step (iv).
14. 14. A method as claimed in claim 13, wherein 4-14 molar equivalents of hydrazine based on the combined equivalents of Pd + Ag are used in step (ii).
15. 15. A method as claimed in claim 13 or claim 14, wherein the tin salt contains the stannate ion.
16. 16. A process for the conversion to acetylene to ethylene, wherein a catalyst as claimed in any of claims 1 to 12 is used as the catalyst.
17. 17. A process as claimed in claim 16, wherein the catalyst is used in the tail-end configuration