Catalyst comprising dispersed gold and palladium, and its use in selective hydrogenation

1. A catalyst comprising gold, palladium, and a porous support, in the form of at least one grain, wherein:

the gold content in the catalyst is in the range 0.5% to 3% by weight with respect to the total weight of catalyst;

the mean particle size of the gold, estimated by transmission electron microscopy (TEM), is in the range 0.5 nm to 5 nm;

the gold is distributed homogeneously in said porous support;

at least 80% by weight of the palladium is distributed in an eggshell at the periphery of said porous support;

the gold/palladium molar ratio is more than 2.

2. The catalyst as claimed in claim 1 , wherein the mean particle size of the gold, estimated by transmission electron microscopy, is in the range 0.5 nm to 3 nm.

3. The catalyst as claimed in claim 1 , wherein the metallic dispersion D of the gold is in the range 30% to 100%.

4. The catalyst as claimed in claim 1 , wherein the palladium content is in the range 0.01% to 0.6% by weight with respect to the total weight of catalyst.

5. The catalyst as claimed in claim 1 , wherein the thickness of said eggshell at the periphery of the porous support is less than 300 μm.

6. A process for the preparation of a catalyst as claimed in claim 1 , comprising gold, palladium, and a porous support, in the form of at least one grain, said process comprising:

1) introducing the palladium onto the support, by:

1a) preparing an aqueous solution of palladium oxide or palladium hydroxide;

1b) impregnating said solution onto at least one grain of porous support;

1c) maturing the impregnated porous support obtained in 1b) in order to obtain a catalyst precursor;

1d) drying the catalyst precursor obtained in 1c) at a temperature in the range 50° C. to 250° C.;

1e) calcining the catalyst precursor obtained in 1d) at a temperature in the range 250° C. to 900° C.;

2) introducing gold onto the support, comprising:

2a) preparing an aqueous solution containing a precursor of gold;

2b) impregnating said porous support with said solution obtained in 2a);

2c) maturing the impregnated porous support obtained in 2b) in order to obtain a catalyst precursor;

2d) bringing the catalyst precursor obtained in 2c) into contact with a solution containing urea;

2e) drying the catalyst precursor obtained in 2d) at a temperature in the range 50° C. to 300° C.

7. The process as claimed in claim 6 wherein, in 1a), a colloidal suspension of palladium oxide or palladium hydroxide is prepared in an aqueous phase.

8. The process as claimed in claim 6 , wherein dry impregnation is carried out in 1b) and/or 2b).

9. The process as claimed in claim 6 , wherein maturation 1c) and/or 2c) is carried out for a period in the range 0.5 to 40 hours.

10. The process as claimed in claim 6 , wherein between 2c) and 2d), drying of the catalyst precursor obtained in 2c) is carried out at a temperature in the range 50° C. to 300° C.

11. The process as claimed in claim 6 , wherein the volume of the aqueous solution containing urea prepared in 2d) is in the range 0.9 to 20 times the total pore volume of the shaped porous support.

12. The process as claimed in claim 6 , wherein the urea/gold molar ratio is in the range 1 to 1000.

13. The process as claimed in claim 6 , comprising 2f) reducing the dried catalyst obtained from 2e) by contact with a reducing gas.

14. The process as claimed in claim 13 , wherein reduction 2f) is carried out at a temperature in the range 40° C. to 500° C.

15. A process for the selective hydrogenation of a C2-C4 cut comprising contacting said C2-C4 cut with a catalyst in accordance with claim 1 , at a temperature in the range 0° C. to 500° C., a pressure in the range 0.1 to 20 MPa, and an hourly space velocity in the range 0.1 to 50 h −1 for a liquid feed, and in the range 500 to 30 000 h −1 for a gaseous feed.