Background of the Invention Ethylene is a monomer that is used in preparing a number of olefin polymers. Ethylene is generally made by the pyrolysis or catalytic cracking of refinery gas, ethane, propane, butane, and the like. Ethylene, so produced, usually contains small quantities of acetylene. In polymer grade ethylene, it is generally preferred that the acetylene content be less than 5 parts per million by weight (ppmw).

One technique that has been used in the past for reducing the amount of acetylene in a stream containing ethylene is to selectively hydrogenate the acetylene using a catalyst comprising an active alumina carrier, a palladium component, and a silver component. In such a hydrogenation process, it is desirable for substantially all of the acetylene to be converted to other hydrocarbons, preferably ethylene, while only an insignificant amount of the ethylene is converted.

The selective hydrogenation of acetylene from a feed containing ethylene is promoted by the presence of carbon monoxide in the feed. Although a variety of palladium-silver catalysts have proven to be effective for selectively hydrogenating acetylene in feeds containing relatively high concentrations of carbon monoxide, many of these same catalysts are ineffective when the feed contains low concentrations of carbon monoxide (for example, less than 5 ppmw).

Because certain commercial hydrocarbon streams that contain low concentrations of carbon monoxide and high concentrations of ethylene also contain acetylene, there exists a need to develop a catalyst and process for selectively hydrogenating acetylene in such a feedstream.

Summary of the Invention It is desirable to provide a method, and catalyst, for the treatment of a hydrocarbon stream comprising ethylene and acetylene whereby the acetylene is selectively and substantially consumed.

Again it is desirable to provide such a method and catalyst whereby acetylene in admixture with ethylene is hydrogenated to form a further amount of ethylene, without the concurrent consumption of a significant portion of ethylene.

Once again it is desirable to provide a method and catalyst suitable for selectively hydrogenating acetylene in a feedstream which contains high concentrations of ethylene and low concentrations of carbon monoxide.

In accordance with a first embodiment of the present invention a catalyst composition is provided. Such catalyst composition comprises an inorganic support material, a palladium component, a silver component, and a promotor component. The promotor component has a formula XYFn, wherein X is an alkaline metal, Y is an element selected from the group consisting of antimony, phosphorus, boron, aluminum, gallium, indium, thallium, and arsenic, and n is an integer which makes Yin a monovalent anion.

In a second embodiment of the present invention a process is provided. Such process comprises contacting, under acetylene hydrogenation conditions, a feed comprising ethylene and acetylene with the catalyst described in the first embodiment of the present invention.

In a third embodiment of the present invention a process for making a catalyst composition is provided. Such process comprises incorporating a palladium component, a silver component, and a promotor component, described in the first embodiment of the present invention, into an inorganic support.

Description of the Drawings FIG. 1 is a plot of Weight Percent Acetylene Conversion versus Weight Percent Selectivity to Ethylene for the catalysts prepared in Example I and employed in the acetylene hydrogenation process of Example II.

FIG. 2 is a plot of Reaction Temperature versus Weight Percent Selectivity to Ethylene for the catalysts prepared in Example I and employed in the acetylene hydrogenation process of Example II.

Detailed Description of the Invention The inventive composition of the first embodiment of the present invention comprises an inorganic support, a palladium component, a silver component, and a promotor component.

The inorganic support can be any inorganic solid support material suitable for use in a selective hydrogenation catalyst. Preferably, the inorganic support is selected from the group consisting of alumina, titania, zirconia, and mixtures thereof. The presently more preferred support material is alumina, most preferably alpha-alumina.

The palladium component can be palladium metal or palladium oxide. The amount of the palladium component present in the inventive composition can be any amount which provides a suitable acetylene hydrogenation catalyst. Preferably, the palladium component is present in the inventive composition in an amount which is from about 0.001 weight percent to about 1 weight percent by weight of the total catalyst composition. More preferably, the palladium component is present in an amount which is from about 0.005 weight percent to about 0.2 weight percent by weight of the total catalyst composition. Most preferably, the palladium component is present in an amount which is from 0.01 weight percent to 0.1 weight percent by weight of the total catalyst composition.

The silver component can be silver metal or silver oxide. The silver component can be present in any amount which provides a suitable acetylene hydrogenation catalyst. Preferably, the silver component is present in the inventive composition in an amount which is from about 0.001 weight percent to about 5 weight percent by weight of the total catalyst composition.

More preferably, the silver component is present in an amount which is from about 0.005 weight percent to about 1 weight percent by weight of the total catalyst composition. Most preferably, the silver component is present in an amount which is from 0.01 weight percent to 0.5 weight percent by weight of the total catalyst composition.

The weight ratio of the silver component to the palladium component can be any ratio which provides a suitable acetylene hydrogenation catalyst. Preferably, the weight ratio of the silver component to the palladium component in the inventive composition is from about 1: 1 to about 10: 1. More preferably the weight ratio of the silver component to the palladium component is from 2: 1 to 6: 1.

The promotor component is a compound having the formula XYFn, wherein X is an alkaline metal, Y is an element selected from the group

consisting of antimony, phosphorus, boron, aluminum, galladium, indium, thallium, and arsenic, and n is an integer that makes Yin a monovalent anion.

Preferably, X is an element selected from the group consisting of potassium, rubidium, and cesium, Y is an element selected from the group consisting of antimony and phosphorus, and n is an integer from 2 to 8 that makes YFn a monovalent anion. Most preferably, X is potassium, Y is antimony, and n is 6.

Examples of suitable promotor components include KPF6, KSbF6, KAsF6, ILBF4, KA1F4, KPF6 and KSbF6, with KSbF6 being the most preferred promotor component.

The amount of the promotor component present in the inventive catalyst composition can be any amount which provides a suitable acetylene hydrogenation catalyst. Preferably, the promotor component is present in the inventive composition in an amount such that the weight of X as a percentage of the total weight of the catalyst is from about 0.001 percent to about 10 percent, more preferably from about 0.01 percent to about 2 percent, and most preferably from 0.05 percent to 1 percent. The amounts of Y and F in the catalyst composition can be calculated using the amount of X, described above, and the structure of the promoter component, described above.

It is preferred for the palladium component to be distributed on the inorganic support as a"skin", with a high concentration of palladium being present near the surface of the support and a low concentration of palladium being present towards the center of the support. The silver and promotor components can be distributed evenly throughout the catalyst.

The inventive process of the second embodiment of the present invention comprises contacting a feed comprising ethylene and acetylene with the catalyst of the first embodiment of the present invention.

The feed preferably contains a high concentration of ethylene and a low concentration of acetylene. Preferably, the weight of ethylene as a percentage of the total weight of the feed is more than about 50 percent, more preferably more than about 80 percent, still more preferably more than about 90 percent, and most preferably more than 95 percent. Preferably, the weight of acetylene as a percentage of the total weight of the feed is less than about 50 percent, more preferably less than about 10 percent, still more preferably less than about 5 percent, and most preferably less than 2 percent.

An important feature of the present invention is its ability to selectively hydrogenate acetylene in a feedstream which contains small quantities of carbon monoxide. The amount of carbon monoxide present in the feed is preferably less than about 100 ppmw, more preferably less than about 20 ppmw, still more preferably less than about 5 ppmw, and most preferably less than 1 ppmw by weight of the total hydrocarbon feed. Typically, a minimum concentration of about 0. 001 ppmw carbon monoxide is present in the feedstream.

Hydrogen can be present in the feed to promote the hydrogenation reaction. The amount of hydrogen employed can be an amount which provides a molar ratio of hydrogen to acetylene of from about 0.1: 1 to about 100: 1, more preferably from about 0.05: 1 to about 20: 1, most preferably from 1: 1 to 5: 1.

The feed may contain small amounts of various compounds other than ethylene, acetylene, hydrogen, and carbon monoxide (such as, for example, methane, ethane, propane, propene, butane, butenes, and hydrogen sulfide) ; however, it is preferred that the total amount of compounds other than ethylene, acetylene, hydrogen, and carbon monoxide in the feed is minimized such that the total weight of such other compounds as a percentage of the total weight of the feed is less than about 50 percent, more preferably less than about 10 percent, still more preferably less than about 5 percent, and most preferably less than 2 percent.

The reaction conditions employed in carrying out the acetylene hydrogenation process of the present invention can be any conditions which are effective to selectively hydrogenate acetylene. The temperature employed in the selective hydrogenation process of the present invention depends largely on the activity of the catalyst composition. Generally, a reaction temperature in the range of from about 40°C to about 200°C is employed. Preferably, the reaction temperature is from about 60°C to about 150°C. Any suitable reaction pressure can be employed. Generally, the total pressure is in the range of from about 100 psig to about 1000 psig. The gas hourly space velocity (GHSV) of the feed can also vary over a wide range. Typically, the GHSV will be in the range of from about 1000 to about 10,000 m3 of feed per m3 of catalyst per hour, more preferably from about 2000 to about 8000 m/m/hr.

It is desirable for the hydrogenated product stream produced by

the selective hydrogenation process of the present invention to be substantially acetylene-free. Preferably, the hydrogenated product stream contains acetylene in an amount such that the weight of acetylene as a percentage of the total weight of the hydrogenated product stream is less than about 1 percent, preferably less than about 0.5 percent, still more preferably less than about 0.1 percent, and most preferably less than about 0.05 percent.

The substantially acetylene free hydrogenated product stream is produced by converting substantially all the acetylene present in the feed into other compounds. Preferably, the weight of acetylene converted to other compounds as a percentage of the total weight of the acetylene present in the feed is more than about 50 percent, more preferably more than about 80 percent, still more preferably more than about 90 percent, and most preferably more than about 95 percent.

It is preferred for a substantial amount of the acetylene present in the feed to be converted to ethylene, as opposed to other compounds.

Preferably, the weight of acetylene converted to ethylene as a percentage of the weight of the total acetylene converted is greater than about 20 percent, more preferably greater than about 40 percent, and most preferably greater than 50 percent.

It is also preferred in carrying out the process of the present invention that the amount of ethylene consumed by hydrogenation be minimized. Preferably, less than 10 weight percent of the ethylene in the feed is consumed. More preferably, less than 3 weight percent of the ethylene is consumed. Still more preferably, less than 1 weight percent of the ethylene is consumed. Most preferably, less than 0.5 weight percent of the ethylene is consumed.

The inventive process of the third embodiment of the present invention is a method for making a catalyst composition comprising an inorganic support, a palladium component, a silver component, and a promotor component. The quantities and identity of the inorganic support, palladium component, silver component, and promotor component in the catalyst produced by the process of the present embodiment are those described in the first embodiment of the present invention.

The palladium, silver, and promotor components can be

incorporated into the inorganic alumina support using any suitable method known in the art that will yield a catalyst composition meeting the above-described parameters. The presently preferred technique involves impregnating the alumina support using an aqueous solution or solutions of the components. The alumina support can be simultaneously impregnated using an aqueous solution containing a palladium component, the silver component, and the promotor component. It is preferred, however, for the impregnation of the support with each component to be performed separately.

The preferred aqueous solution used to impregnate the alumina support with the palladium component comprises palladium chloride. The preferred aqueous solution used to impregnate the alumina support with the silver component comprises silver nitrate. The preferred aqueous solution used to impregnate the alumina support with the promotor component comprises a promotor component selected from the group consisting of potassium hexafluorophosphate, potassium hexafluoroantimonate, potassium hexafluoroarsenate, potassium tetrafluoroborate, potassium tetrafluoroarsenate, and potassium hexafluoroaluminate. Most preferably, the aqueous solution used to impregnate the alumina support with the promotor component comprises potassium hexafluoroantimonate.

The catalyst may be dried or calcined, or both, after each impregnation, or after all impregnations have been performed. The calcining can be performed at temperatures of from about 100°C to about 600°C, preferably from about 150°C to about 450°C.

The calcining is preferably followed by a reduction step. The reduction step can be accomplished using the feed for the selective hydrogenation process and/or hydrogen. Preferably, the reduction step is practiced in accordance with the wet reduction process taught in U. S. Patent No.

5,510,550, the disclosure of which is incorporated by reference herein.

The following examples are provided to help illustrate the invention, but they are not intended to limit the scope of the invention.

Example I This example illustrates the preparation of supported palladium/silver catalysts to be used for acetylene hydrogenation.

Catalyst A, which was a PdlAg/Al203 catalyst containing

0.02 weight percent palladium, 0.13 weight percent silver, 0.33 weight percent potassium as potassium fluoride, and about 99 weight percent alumina, was prepared by reducing 50 grams of the catalyst in hydrogen flowing at 200 cc/m at 1 atmosphere and 200°C for 3 hours.

Catalyst B was prepared by reducing 50 grams of a commercial Pd/Ag/A'203 catalyst in hydrogen flowing at 200 cc/m at 1 atmosphere and 194°C for about 3 hours. The commercial catalyst was provided by United Catalysts Incorporated (UCI), Louisville, Kentucky, under the product designation of"G-83C"and contained 0.023 weight percent palladium, 0.065 weight percent silver, and about 99 weight percent alumina.

Catalyst C (invention) was prepared by impregnating 25.02 grams of G-83C (UCI) commercial catalyst with a solution of 0.0621 grams KPF6 in 7.18 grams of water, followed by drying at 105°C for several hours and calcining in air at 377°C for 1 hour.

Catalyst D (invention) was prepared by impregnating 50.16 grams of G-83C (UCI) commercial catalyst with a solution of 0.1844 grams KSbF6 in 13.4 grams of water, followed by drying at ambient temperature, and calcining in air at 400°C for about 4 hours.

Example II This example illustrates the effectiveness of Catalysts A-D for selectively hydrogenating acetylene in feeds containing ethylene.

The acetylene hydrogenation tests were carried out as follows.

About 20 cc of Catalysts A-D was placed in a stainless steel reactor tube having a 0.5 inch inner diameter and a length of about 18 inches. Prior to charging the hydrogenation feed to the reactor, Catalysts A-D were reduced in hydrogen flowing at 50 cc/m. Catalyst A was reduced at 1 atmosphere and 136°F for 1 1/2 hours. Catalyst B was reduced at 1 atmosphere and 188°F for 1 hour.

Catalyst C was reduced at 100 psig and 131°F for 2 hours. Catalyst D was reduced at 1 atmosphere and 143°F for 1 1/2 hours.

After reduction with hydrogen, a feed containing ethylene and acetylene was introduced into the reactor tube at a rate of about 900 cc/min.

The compositions of the feeds used to test Catalysts A and D were the same, and the compositions of the feeds used to test Catalysts B and C were the same.

Hydrogen was added to the feed in an amount such that the molar ratio of

hydrogen to acetylene was 1.8. The reactor temperature was gradually increased to the desired reaction temperature, and samples of the formed product were analyzed by means of gas chromatograph at various time intervals.

Tables A-D present reaction temperatures, feed compositions, acetylene conversions and selectivities for the selective acetylene hydrogenation tests employing Catalysts A-D.

Table A<BR> CATALYST A Product Product Product Product Product Product Product Product Product Product Stream Feed Run #1 Run #2 Run#3 Run #4 Run #5 Run #6 Run #7 Run #8 Run #9 Run #10 Reaction Temp. (F) - 88 88 91 94 100 101 101 107 111 116 Wt% Acetylene 1.26 0.18 0.17 0.15 0.10 0.9 0.07 0.05 0.03 0.03 0.02 Wt% Ethlyene 95.22 95.13 95.14 95.10 94.894 94.90 94.79 94.80 94.77 94.71 94.30 Wt% Ethane 0 0.87 0.95 0.96 1.19 1.125 1.31 1.36 1.36 1.39 1.32 Wt% C1 3.50 3.56 3.46 3.52 3.49 3.52 3.52 3.50 3.52 3.55 3.54 Wt% C4 0 0.3 0.14 0.14 0.15 0.15 0.15 0.16 0.16 0.16 0.16 Wt% C4+ 0 0.26 0.28 0.28 0.29 0.24 0.30 0.29 0.31 0.32 0.32 %Acet. conversion - 85.51 86.86 88.12 92.40 92.64 94.62 96.12 97.62 97.39 98.18 %Sel. to Ethylene - -8.65 -7.14 -11.38 -24.40 -27.73 -35.86 -34.96 -36.44 -41.64 -33.74

Table B<BR> CATALYST B Stream Product Product Product Product Product Product Product Product Product Product Product Feed Run #1 Run #2 Run #3 Run #4 Run #5 Run #6 Run #7 Run #8 Run #9 Run#110 Run#11 Reaction Temp. (F) - 118 121 128 128 129 129 132 136 138 142 144 Wt% Acetylene 1.13 0.31 0.42 0.11 0.9 0.08 0.09 0.12 0.04 0.03 0.02 0.01 Wt% Ethylene 92.67 98.01 97.96 97.74 97.60 97.66 97.77 97.71 97.59 97.55 97.52 97.60 Wt% Ethane 0 0.30 0.30 0.71 0.86 0.82 0.69 0.74 0.92 0.97 1.00 0.96 Wt% Cl 1.17 1.14 1.14 1.15 1.16 1.15 1.15 1.15 1.15 1.15 1.15 1.15 Wt% C4 0 0.11 0.09 0.13 0.13 0.13 0.13 0.13 0.12 0.14 0.14 0.13 0.14 Wt% C4+ .04 0.24 0.19 0.28 0.29 0.29 0.29 0.27 0.29 0.29 0.29 0.28 %Acet conversion - 72.49 62.73 89.97 92.46 93.08 92.28 89.80 96.81 97.34 98.05 99.33 %Sel. to Ethlyene - 41.63 40.46 6.95 -6.60 =0.41 10.15 4.39 -6.98 -11.05 -13.33 -5.75

Table C CATALYST C (Invention) Stream Product Product Product Product Product Product Feed Run #1 Run #2 Run #3 Run #4 Run #5 Run #6 Reaction Temp. (F)-135 140 151 158 164 178 Wt % Acetylene 1. 13 0. 26 0. 25 0. 05 0. 03 0. 01 0. 00 Wt % Ethlylene 97. 67 98. 08 98. 08 97. 94 97. 86 97. 78 97. 63 Wt % Ethane 0 0. 28 0. 28 0. 59 0. 69 0. 80 0. 96 Wt % C1 1. 17 1. 15 1. 15 1. 15 1. 15 1. 15 1. 15 Wt % C4 0 0. 10 0. 11 0. 12 0. 13 0. 14 0. 14 Wt % C4+. 04 0. 23 0. 24 0. 26 0. 26 0. 27 0. 27 % Acet. e conversion-76. 93 77. 73 95. 30 97. 52 99. 11 99. 65 % Sel. to Ethlyene 47. 41 46. 81 24. 99 17. 47 10. 28-3.52 Table D<BR> CATALYST D (Invention) Product Product Product Product Product Product Product Product Stream Feed Run #1 Run #2 Run #3 #4 Run #5 #6 #7 Run #8 Reaction T emp. (F) - 126 1289 137 140 144 146 151 152 Wt% Acetylene 1.26 0.25 0.27 0.13 0.12 0.01 0.01 0.01 0.01 Wt% Ethylene 95.22 95.90 95.90 95.65 95.76 95.45 95.38 95.36 95.29 Wt% Ethane 0 0.00 0.00 0.31 0.22 0.61 0.67 0.69 0.74 Wt% Cl 3.50 3.61 3.60 3.61 3.61 3.64 3.61 3.62 3.64 Wt% G4 - 0.12 0.11 0.15 0.15 0.17 0.17 0.17 0.17 Wt% C4+ 0 0.25 0.23 0.30 0.29 0.33 0.32 0.32 0.33 %Acet. conversion - 80.36 78.38 89.39 90.66 99.13 99.21 99.45 99.29 %Sel. to Ethylene - 66.60 68.59 37.90 46.57 18.33 12.62 10.95 5.39

Figures 1 and 2 plot data from Tables A-D. Figure 1 illustrates that for a given acetylene conversion value the inventive catalysts provide superior selectivity to ethylene. Figure 2 illustrates that for a given reaction temperature the inventive catalyst provides superior selectivity to ethylene.

While this invention has been described in detail for the purpose of illustration, it should not be construed as limited thereby but intended to cover all changes and modifications within the spirit and scope thereof.