Chromium-on-alumina dehydrogenation catalysts and methods for preparing and using them

Abstract

The present disclosure relates to chromium-on-alumina dehydrogenation catalyst materials, to methods for making such catalysts, and to methods for dehydrogenating hydrocarbons using such catalysts. In one aspect, the disclosure provides a method for preparing a dehydrogenation catalyst material, the method comprising impregnating a chromium-on-alumina material with ascorbic acid, one or more of sodium, lithium and potassium (e.g., sodium), and chromium; and calcining the impregnated material to provide the dehydrogenation catalyst material comprising chromium in the range of 2.5 wt. % to about 35 wt. % and having no more than 100 ppm chromium(VI).

Claims

1. A method for preparing a dehydrogenation catalyst material, the method comprising providing a porous chromium-on-alumina material having at least 1000 ppm Cr(VI); impregnating the chromium-on-alumina material with an aqueous impregnating solution comprising ascorbic acid, one or more of sodium, lithium and potassium, and chromium; and calcining the impregnated material to provide the dehydrogenation catalyst material having chromium in the range of 10-35 2.5 wt. % and having no more than 100 ppm chromium(VI).

2. A method according to claim 1, wherein the chromium-on-alumina catalyst material before the impregnating step comprises at least 50 wt. % alumina, calculated as Al.sub.2O.sub.3.

3. A method according to claim 1, wherein chromium is present in the chromium-on-alumina catalyst material before the impregnating step in an amount within the range of about 2.5 wt. % to about 35 wt. %, calculated as Cr.sub.2O.sub.3.

4. A method according to claim 1, wherein the chromium-on-alumina catalyst material before the ascorbic acid impregnating step comprises at least about 5,000 ppm chromium(VI).

5. A method according to claim 1, wherein the impregnation is performed by impregnating the porous chromium-on-alumina material with an aqueous impregnation solution comprising ascorbic acid, present in an amount within the range of about 1 wt. % to about 30 wt. %, sodium, lithium and/or potassium, present in an amount within the range of about 0.1 wt. % to about 5 wt. % calculated on an oxide basis, and chromium, present in an amount within the range of about 0.01 wt. % to about 5 wt. % calculated as Cr.sub.2O.sub.3.

6. A method according to claim 5, wherein the ascorbic acid is present in the aqueous impregnation solution in an amount within the range of about 5 wt. % to about 25 wt. %.

7. A method according to claim 5, wherein chromium is present in the aqueous impregnation solution in an amount within the range of about 0.1 wt. % to about 5 wt. %, calculated as Cr.sub.2O.sub.3.

8. A method according to claim 7, wherein chromium is present in the aqueous impregnation solution in an amount within the range of 0.1 wt. % to 3 wt. %.

9. A method according to claim 1, wherein the impregnated material is calcined at a temperature within the range of 100° C. to 600° C.

10. A method according to claim 1, wherein the dehydrogenation catalyst material has no more than 20 ppm chromium(VI) on an elemental mass basis.

11. A method according to claim 1, wherein the dehydrogenation catalyst material has no more than 10 ppm chromium(VI) on an elemental mass basis.

12. A method according to claim 10, wherein the porous chromium-on-alumina material has at least 7500 ppm Cr(VI).

13. A method according to claim 11, wherein the porous chromium-on-alumina material has at least 7500 ppm Cr(VI).

14. A method according to claim 12, wherein the porous chromium-on-alumina material has 15-35 wt % chromium calculated as Cr.sub.2O.sub.3.

Description

EXAMPLES

(1) The Examples that follow are illustrative of specific embodiments of the invention, and various uses thereof. They are set forth for explanatory purposes only, and are not to be taken as limiting the invention.

Example 1. Catalyst Preparation

(2) A conventional chromium-on-alumina catalyst, C, comprising 20 wt. % chromium (calculated as Cr.sub.2O.sub.3) was prepared according to known methods.

(3) The chromium-on-alumina material C was impregnated with an aqueous solution of 10 wt. % ascorbic acid, and calcined at 120° C. in an atmosphere of 80 vol. % N.sub.2, 19.5 vol. % O.sub.2, and 0.1 vol. % H.sub.2O, to provide catalyst B1.

(4) The chromium-on-alumina material C was impregnated with an aqueous solution of 10 wt. % ascorbic acid, 0.65 wt. % sodium (present as NaOH, calculated as Na.sub.2O), and 0.2 wt. % chromium (present as chromium ascorbate, calculated as Cr.sub.2O.sub.3), and calcined at 315° C. in an atmosphere of 95 vol. % N.sub.2, 0.2 vol. % O.sub.2, and 4.8 vol. % H.sub.2O, to provide catalyst A1.

(5) The chromium-on-alumina material C was impregnated with an aqueous solution of 15 wt. % ascorbic acid, 1 wt. % sodium (present as NaOH, calculated as Na.sub.2O), and 1.5 wt. % chromium (present as chromium ascorbate, calculated as Cr.sub.2O.sub.3), and calcined at 425° C. in an atmosphere of 0.5 vol. % N.sub.2, 0.2 vol. % O.sub.2, and 99.3 vol. % H.sub.2O, to provide catalyst A2.

(6) The chromium-on-alumina material C was impregnated with an aqueous solution of 15 wt. % ascorbic acid, 1 wt. % sodium (present as NaOH, calculated as Na.sub.2O), and 1.5 wt. % chromium (present as chromium ascorbate, calculated as Cr.sub.2O.sub.3), and calcined at 450° C. in an atmosphere of 0.5 vol. % N.sub.2, 0.2 vol. % O.sub.2, and 99.3 vol. % H.sub.2O, to provide catalyst A3.

(7) TABLE-US-00001 TABLE 1 Catalyst Properties C B1 A1 A2 A3 Cr(Vl) (ppm) 11,000 2.2 3.9 2 6.5

Example 2. Propane Dehydrogenation

(8) Catalyst materials prepared according to Example 1 were tested as prepared in a fixed-bed reactor. A feed containing 100 mol. % propane was passed over a catalyst bed at 2.0 h.sup.−1 liquid hourly space velocity (LHSV), at a temperature within the range of 540-600° C. Results are provided in Table 2, below.

(9) TABLE-US-00002 TABLE 2 Propane Dehydrogenation Property C (wt. %) B1 (wt. %) A1 (wt. %) A2 (wt. %) A3 (wt. %) Loss on Ignition (1000° C.) <0.01 5.2 5.3 4.98 4.1 1000° F. C1 to C2 2.22 3.32 2.2 1.89 2.31 Propane Conversion 32.09 33.25 31.06 34.23 31.06 Propylene Selectivity 83.06 77.77 83.67 83.89 82.63 Propylene Yield 26.62 25.8 25.91 28.69 25.57 Coke Yield 0.71 1.41 0.71 0.71 0.78 1050° F. C1 to C2 4.83 5.87 4.37 4.34 4.61 Propane Conversion 44.34 44.58 42.05 47.68 41.57 Propylene Selectivity 77.65 71.76 78.05 78.22 76.18 Propylene Yield 34.44 32.01 32.85 37.35 31.7 Coke Yield 1.1 2.77 1.09 1.41 1.25 1100° F. C1 to C2 9.13 9.62 8.47 9.14 9.44 Propane Conversion 57.44 58.01 54.69 60.59 54.49 Propylene Selectivity 70.02 62.74 69.3 68.38 66.07 Propylene Yield 40.22 36.39 37.86 41.43 35.97 Coke Yield 3.26 6.57 3.45 4.07 3.89

(10) The results show that the performance of the catalysts, each having less than 10 ppm Cr(VI), was acceptable. Dehydrogenation results of catalyst materials A1-3 further demonstrate that inclusion of one or more of an alkali metal such as sodium, lithium, or potassium in the aqueous impregnation solution can provide increased propylene selectivity.