Catalyst composition for the selective conversion of synthesis gas to light olefins

Abstract

A catalyst composition and process for preparing it and for using it to enhance the selectivity to light (C2 to C3) olefins in a Fischer-Tropsch conversion of synthesis gas is disclosed. The catalyst composition is an iron-based catalyst on an yttria/zirconia support. In a Fischer-Tropsch reaction the selectivity to ethylene may be enhanced by at least 20 mole percent and to propylene by at least 4 mole percent, in comparison with use of an otherwise identical catalyst that is free of yttria, in an otherwise identical Fischer-Tropsch reaction.

Claims

1. A catalyst comprising an iron compound comprising iron carbides and, optionally, an alkali metal, alkaline earth metal, or a combination thereof, on a support comprising zirconia and yttria, the iron being present in an amount ranging from 1 weight percent to 20 weight percent, based on combined weight of the iron and the support; the yttria being present in an amount ranging from 1 mole percent to 95 mole percent, based on combined moles of yttria and zirconia, and the optional alkali metal, alkaline earth metal or combination thereof being present in an amount ranging from 0 mole percent to 6 mole percent, based on moles of the iron.

2. The catalyst of claim 1, wherein the support is substantially free of silica, tungsten oxide and sulfate dopants.

3. The catalyst of claim 1 further comprising a sulfate promoter on the support in an amount ranging from 0.1 mole percent to 5 mole percent, based on the moles of the iron.

4. The catalyst of claim 1 wherein the yttria is present in an amount ranging from 1 mole percent to 75 mole percent, based on combined moles of the yttria and zirconia.

5. The catalyst of claim 1 wherein the yttria is present in an amount ranging from 1 mole percent to 20 mole percent, based on combined moles of the yttria and zirconia.

6. A process for converting synthesis gas to olefins, comprising contacting synthesis gas and the catalyst of claim 1 under reaction conditions sufficient to convert, at a selected carbon monoxide conversion percentage, at least a portion of the synthesis gas to a mixture of hydrocarbons that has an ethylene content and a propylene content, each of the ethylene content and the propylene content being greater than the ethylene content and the propylene content resulting from use of an otherwise identical catalyst that is substantially free of yttria, under otherwise identical reaction conditions and at a carbon monoxide conversion percent that is within 2 percent of the selected carbon monoxide conversion percent.

7. The process of claim 6, wherein the ethylene and propylene contents resulting from use of the catalyst of claim 1 are, respectively, at least 20 percent greater and at least 4 percent greater than the ethylene content and the propylene content resulting from use of the otherwise identical catalyst.

8. A catalyst prepared by a process comprising (1) dispersing an iron-containing compound and, optionally, an alkali metal, an alkaline earth metal, or a combination thereof, on a particulate catalyst support comprising zirconia and yttria; the amount of the iron-containing compound ranging from 1 weight percent to 20 weight percent, based on combined weight of the iron and the support; the amount of the optional alkali metal, alkaline earth metal, or combination thereof ranging from 0 mole percent to 6 mole percent, based on combined moles of the alkali metal, the alkaline earth metal, or combination thereof and the iron; and the amount of yttria ranging from 0.1 mole percent to 95 mole percent, based on combined moles of yttria and zirconia; (2) thermally at least partially decomposing the iron-containing compound to form a catalyst precursor composition comprising an iron oxide; (3) subjecting the catalyst precursor composition to at least partial carburization in a carbon monoxide-containing atmosphere to convert at least some of the iron oxides to iron carbides.

9. The catalyst prepared by the process of claim 8, wherein the iron-containing compound comprises Fe(II), Fe(III) or both.

10. The catalyst of claim 1, wherein the iron compound is in amounts from 4 weight percent to 15 weight percent, based on combined weight of the iron and the support.

11. The catalyst of claim 10, wherein the support is substantially free of silica and tungsten oxide.

12. The catalyst of claim 10, wherein the yttria is present in an amount ranging from 1 mole percent to 75 mole percent, based on combined moles of the yttria and zirconia.

13. The catalyst of claim 10, wherein the yttria is present in an amount ranging from 1 mole percent to 20 mole percent, based on combined moles of the yttria and zirconia.

14. A catalyst comprising: iron, a sulfate promoter, and optionally, an alkali metal, alkaline earth metal, or a combination thereof, on a support comprising zirconia and yttria, the iron being present in an amount ranging from 1 weight percent to 20 weight percent, based on combined weight of the iron and the support; the sulfate promoter being present in an amount ranging from 0.1 mole percent to 5 mole percent, based on the moles of the iron; the yttria being present in an amount ranging from 1 mole percent to 95 mole percent, based on combined moles of yttria and zirconia; and the optional alkali metal, alkaline earth metal or combination thereof being present in an amount ranging from 0 mole percent to 6 mole percent, based on moles of the iron.

Description

EXAMPLES

Examples 1-7 and Comparative Examples 1-7

(1) A series of catalyst precursor compositions is prepared as described. A solution containing the desired amounts of iron and alkali metals is prepared for each of Examples 1-4 and Comparative Examples 1-7, containing the constituents shown in Table 1, by dissolving the precursor salts in deionized water. Each catalyst precursor is then prepared by incipient wetness impregnation of the solution on the designated support. Each catalyst precursor is dried at 120 C. The impregnation and drying steps are repeated until all the solution has been loaded onto the supports in order to obtain the desired iron content (wt %). The resulting catalyst precursor compositions are each calcined in air at 500 C. for 4 hours (h).

(2) Test in FTO Conditions

(3) All catalyst precursor compositions are then treated for activation and then tested in a Fischer-Tropsh-to-olefins (FTO) reaction following the same methodology. For all Examples and Comparative Examples, except for Examples 5, 6, and 7, a fixed volume of 100 microliters (L) of catalyst is mixed with silicon carbide and loaded in a tubular reactor. For Examples 5, 6, and 7, 25 milligrams (mg) of catalyst is mixed with silicon carbide and loaded to a tubular reactor. The reactor is heated to 425 C. and pressurized to 0.3 MPa. After stabilization, a reduction step is initiated by flowing a stream of 5 milliliters per minute (mL/min) of hydrogen (H.sub.2) for 3 h.

(4) After that, the H.sub.2 flow is stopped and replaced by a flow of nitrogen (N.sub.2). The reactor temperature is cooled down to 340 C., the pressure is raised to 2 MPa, and a 5 mL flow consisting of 45 vol % CO, 45 vol % H.sub.2, and 10 vol % N.sub.2 is introduced. The reaction is left to proceed at these conditions for 60 h. Data used for comparison are taken after a minimum of 10 h in order to allow the system to stabilize. Special care is taken to compare data obtained for different catalyst at approximately the same conversion percent, i.e., within 2 percent of each other, as shown in Table 1 (Note: Table 1 has been broken into sub-tables, denominated Tables 1.1 through 1.6, in order to more easily illustrate comparisons.)

Example 1

(5) A commercially available yttria/zirconia containing 10 mol % Y.sub.2O3 (TOSOH TZ-10YS; TOSOH is a tradename of Tosoh Corporation) is selected as the support. A solution is prepared by dissolving the desired amount of an ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe) to achieve the desired iron concentration of 1.4 moles per liter (mol/L). The desired amount of potassium nitrate (Sigma-Aldrich) is also dissolved to achieve a Fe/K molar ratio of 20. The resulting solution is impregnated onto the support by incipient wetness impregnation until the solid support is filled with liquid. Then the sample is dried in an oven at 120 C. for 1 hour (h), and the impregnation/drying sequence is repeated until all the solution is impregnated onto the support. Finally, the support is calcined at 500 C. for 4 h.

(6) The test in FTO reaction is performed as described in the section Test in FTO conditions. The results are summarized in Table 1.

Example 2

(7) A catalyst precursor is prepared as in Example 1, but using a different commercially available yttria/zirconia support, TOSOH TZ-4YS which has 4 mol % Y.sub.2O.sub.3. No potassium nitrate is used. All other processing, testing, and results recording is the same as in Example 1.

Example 3

(8) A catalyst precursor is prepared as in Example 1, but using as the yttria/zirconia support TOSOH TZ-4YS, containing 4 mol % Y.sub.2O.sub.3. The desired amount of cesium sulfate (Cs.sub.2SO.sub.4, Sigma-Aldrich) is also dissolved to achieve a Fe/Cs molar ratio of 125. All other processing, testing, and results recording are the same as in Example 1.

Example 4

(9) A catalyst precursor is prepared as in Example 3, but using TOSOH TZ-10YS, containing 10 mol % Y.sub.2O.sub.3. Both ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe), to achieve the desired iron concentration of 1.4 mol/L, and Cs.sub.2SO.sub.4, to achieve a Fe/Cs molar ratio of 125, are dissolved to form the precursor solution. All other processing, testing, and results recording are the same as in Example 1.

Example 5

(10) A catalyst precursor is prepared as in Example 3, but using TOSOH TZ-10YS, containing 10 mol % Y.sub.2O.sub.3. Both ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe), to achieve a desired iron concentration of 1.4 mol/L, and K.sub.2SO.sub.4 and Na.sub.2SO.sub.4, to achieve a Fe/alkali molar ratio of 17 and a K/Na molar ratio of 3, are dissolved to form the precursor solution. All other processing, testing, and results recording are the same as in Example 1.

Example 6

(11) A catalyst precursor is prepared as in Example 3, but using TOSOH TZ-10YS, containing 10 mol % Y.sub.2O.sub.3. Both ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe), to achieve the desired iron concentration of 1.4 mol/L, and K.sub.2SO.sub.4 and Na.sub.2SO.sub.4, to achieve a Fe/alkali molar ratio of 17 and a K/Na molar ratio of 3, are dissolved to form the precursor solution. All other processing, testing, and results recording are the same as in Example 1.

Example 7

(12) A catalyst precursor is prepared as in Example 3, but using TOSOH TZ-10YS, containing 10 mol % Y.sub.2O.sub.3. Both ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe), to achieve the desired iron concentration of 1.4 mol/L, and K.sub.2SO.sub.4, Na.sub.2SO.sub.4 and Rb.sub.2SO.sub.4, to achieve a Fe/alkali molar ratio of 17 and a K/Na/Rb molar ratio of 1/3.25/0.75, are dissolved to form the precursor solution. All other processing, testing, and results recording are the same as in Example 1.

Example 8

(13) A catalyst precursor is prepared as in Example 1, but using TOSOH TZ-18YS, which contains 18 mol % Y.sub.2O.sub.3, as the support. The solution contains the ammonium iron citrate precursor and also potassium nitrate (Sigma-Aldrich) to achieve a Fe/K molar ratio of 125. No Cs.sub.2SO.sub.4 is included. All other processing, testing, and results recording are the same as in Example 1.

Example 9

(14) An yttria/zirconia support containing 74 mol % Y.sub.2O.sub.3 is prepared as follows. A solution containing 0.6 mol/L of yttrium is prepared by dissolving the desired amounts of yttrium nitrate hexahydrate (Sigma-Aldrich) into demineralized water. Another solution containing 0.49 mol/L of zirconium is obtained by dissolving the desired amount of zirconyl nitrate hydrate in demineralized water. The two solutions are co-precipitated by adding dropwise the desired amounts of each solution to an excess ammonia solution (2 mol/L) to achieve the final molar ratio. After aging 3 h at 70 C., the precipitate is filtered and washed several times with demineralized water. The resulting solid is dried in an oven overnight at 120 C. and finally calcined at 1200 C. for 4 h resulting in the yttria/zirconia material serving as support.

(15) A solution is then prepared by dissolving the desired amount of an ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe) to achieve the desired iron concentration of 1.4 mol/L. The desired amount of potassium sulfate (Sigma-Aldrich) is also dissolved to achieve a Fe/K molar ratio of 50. The resulting solution is impregnated onto the support by incipient wetness impregnation until the solid support is filled with liquid. Then the sample is dried in an oven at 120 C. for 1 h, and the impregnation/-drying sequence is repeated until all the solution is impregnated onto the support to achieve a final loading of 5 wt % Fe based on combined weights of iron and the support. Finally, the catalyst precursor is obtained by calcination at 500 C. for 4 h. The Test in FTO Conditions is performed as described hereinabove and results are summarized in Table 1.

Example 10

(16) The commercially available yttria/zirconia support containing 10 mol % Y.sub.2O.sub.3 (TOSOH TZ-10YS) is selected as the support. A solution is prepared by dissolving the desired amount of an ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe) to achieve the desired iron concentration of 1.4 mol/L. The desired amounts of potassium sulfate and potassium nitrate (Sigma-Aldrich) are also dissolved to achieve a Fe/K molar ratio of 50, so that an equimolar amount of potassium is introduced from each precursor. Subsequent processing is then carried out as in previous examples, with the impregnation/drying sequence being repeated until all solution is impregnated onto the support to achieve a final loading of 5 wt % Fe based on combined weights of iron and the support. Calcination, testing, and results recording also are as carried out for previous examples.

Comparative Example 1

(17) This comparative example is the same as Example 1 except that the support is a commercially available zirconia without yttria (TOSOH TZ-0).

Comparative Example 2

(18) This comparative example is the same as Example 2 except that the support is the same as in Comparative Example 1, i.e., TOSOH TZ-0.

Comparative Example 3

(19) This comparative example is the same as Example 3 except that the support is the same as in Comparative Example 1, i.e., TOSOH TZ-0.

Comparative Example 4

(20) This comparative example is the same as Example 4 except that the support is the same as in Comparative Example 1, i.e., TOSOH TZ-0.

Comparative Example 5

(21) This comparative example is the same as Example 9 except that the support is the same as in Comparative Example 1, i.e., TOSOH TZ-0.

Comparative Example 6

(22) This comparative example is the same as Example 1 except that the support is a commercially available sulfated zirconia (NORPRO SZ61192; NORPRO is a tradename of Saint-Gobain NorPro Corporation) having a sulfur content of 4.7 wt %. The support is impregnated by a solution containing a mixture of ammonium iron citrate and potassium sulfate having a Fe/K molar ratio of 50 to achieve an iron loading of 5 wt % based on combined weights of iron and support. All other synthesis steps are the same as described in Example 1. The FT test is performed under the same conditions as described in Example 1, but this catalyst displays a very low activity with less than 5% CO conversion, indicating that a sulfate/zirconia support is not desirable for use with iron to prepare a catalyst that is active in the production of light olefins from synthesis gas.

Comparative Example 7

(23) An all-yttria support is prepared as follows. A solution containing 0.6 mol/L of yttrium is prepared by dissolving the desired amounts of yttrium nitrate hexahydrate (Sigma-Aldrich) into demineralized water. The solution is precipitated by adding it dropwise to an excess ammonia solution (2 mol/L). After aging 3 h at 70 C., the precipitate is filtered and washed several times with demineralized water. The resulting solid is dried in an oven overnight at 120 C. and finally calcined at 1200 C. for 4 h resulting in the yttria material serving as support. A solution is prepared by dissolving the desired amount of an ammonium iron citrate precursor (Sigma-Aldrich, 16.2 wt % Fe) to achieve the desired iron concentration of 1.4 mol/L. The desired amounts of potassium sulfate and potassium nitrate (Sigma-Aldrich) are also dissolved to achieve a Fe/K molar ratio of 50, so that an equimolar amount of potassium is introduced from each precursor. The resulting solution is impregnated onto the support by incipient wetness impregnation until the solid support is filled with liquid. Then the sample is dried in an oven at 120 C. for 1 h, and the impregnation/drying sequence is repeated until all the solution is impregnated onto the support to achieve a final loading of 5 wt % Fe based on combined weights of iron and the support. Finally, the catalyst precursor is obtained by calcination at 500 C. for 4 h. Testing and recording of results are carried out as in previous examples and comparative examples.

(24) TABLE-US-00001 TABLE 1 Table 1.1 Yttria Fe Alkali content content Promoter content Conver- C2 O/(O + P) C3 O/(O + P) (mol %) (wt %) salt % mol/molFe sion (%) (%) (%) C. Ex. 1 0 5 KNO.sub.3 5 70 41 85 Ex. 1 10 5 KNO.sub.3 5 71 57 89 Table 1.2 Yttria Fe content content Promoter Alkali Conver- C2 O/(O + P) C3 O/(O + P) (mol %) (wt %) salt content sion (%) (%) (%) C. Ex. 2 0 5 NA NA 51 23 71 Ex. 2 4 5 NA NA 52 31 77 Table 1.3 Yttria Fe Alkali content content Promoter content Conver- C2 O/(O + P) C3 O/(O + P) (mol %) (wt %) salt % mol/molFe sion (%) (%) (%) C. Ex. 3 0 5 Cs.sub.2SO.sub.4 0.8 89 13 59 Ex. 3 4 5 Cs.sub.2SO.sub.4 0.8 89 23 74 Ex. 4 10 5 Cs.sub.2SO.sub.4 0.8 88 30 80 Ex. 5 10 10 K.sub.2SO.sub.4 + 6 87 32 82 Na.sub.2SO.sub.4 (4.5 + 1.5) Ex. 6 10 10 K.sub.2SO.sub.4 + 6 88 42 85 Na.sub.2SO.sub.4 (4.5 + 1.5) Ex. 7 10 15 K.sub.2SO.sub.4 + 6 89 43 85 Na.sub.2SO.sub.4 + (3.9 + 1.2 + 0.9) Rb.sub.2SO.sub.4 Table 1.4 Yttria Fe Alkali content content Promoter content Conver- C2 O/(O + P) C3 O/(O + P) (mol %) (wt %) salt % mol/molFe sion (%) (%) (%) C. Ex. 4 0 5 KNO.sub.3 2 50 23 70 Ex. 8 18 5 KNO.sub.3 2 50 50 86 Table 1.5 Yttria Fe Alkali content content Promoter content Conver- C2 O/(O + P) C3 O/(O + P) (mol %) (wt %) salt % mol/molFe sion (%) (%) (%) C. Ex. 5 0 5 K.sub.2SO.sub.4 2 25 50 84 Ex. 9 74 5 K.sub.2SO.sub.4 2 25 65 90 C. Ex. 6 0 5 K.sub.2SO.sub.4 2 5* Table 1.6 Yttria Fe Alkali content content Promoter content Conver- C2 O/(O + P) C3 O/(O + P) (mol %) (wt %) salt % mol/molFe sion (%) (%) (%) Ex. 10 10 5 K.sub.2SO.sub.4 + 2 26 64 92 KNO.sub.3 (1 + 1) C. Ex. 7 100 5 K.sub.2SO.sub.4 + 2 25 52 86 KNO.sub.3 (1 + 1) O/(O + P) means percentage of the named (C2 or C3) olefin per combined percentage of all olefins and paraffins. *indicates poor suitability of the 4.7 wt % sulfur-containing support with iron. indicates no data obtained.