Method for the production of a catalyst containing vanadium, phosphorus, and oxygen
Disclosed is a method for producing a catalyst containing vanadium, phosphorus, and oxygen, which is used for oxidizing the gas phase of a hydrocarbon having at least four carbon atoms to maleic anhydride. According to the inventive method, a corresponding catalyst precursor which contains vanadium, phosphorus, and oxygen and is provided with particles having an average diameter of at least 2 mm is converted into a catalytically active form by means of calcination, and a flow of the catalyst precursor is transported on a conveyor belt across at least one calcination area over a distance 1 n at an essentially steady speed in order to be calcinated. The variation over time of the gas temperature in relation to the set point value amounts to ≦5° C. at each position in the area of the flow of the catalyst precursor, which lies within the second half 1 n /2 of the calcination area, while the local difference in the gas temperature between any positions in the area of the flow of the catalyst precursor, which is located within the second half 1 n /2 of the calcination area, amounts to ≦5° C.
1. A process for preparing a vanadium, phosphorus, and oxygen containing catalyst suitable for heterogeneously catalyzed gas-phase oxidation of a hydrocarbon having at least four carbon atoms to maleic anhydride, in which a corresponding vanadium, phosphorus, and oxygen containing catalyst precursor containing particles having an averaged diameter of at least 2 mm is converted into a catalytically active form by calcining, which comprises passing a stream of the catalyst precursor for calcining at substantially constant speed through at least one calcining zone n of length 1 n on a conveyor belt, the fluctuation in the gas temperature from a setpoint value over time at each point in the region of the stream of the catalyst precursor after half the length of the calcining zone, 1 n /2, being in each case≦5° C. and the local difference in gas temperature between any two points in the region of the stream of the catalyst precursor after half the length of the calcining zone, 1 n /2, being in each case≦5° C.
2. A process as claimed in claim 1 , wherein a gas-permeable conveyor belt is used and a gas stream is passed through the stream of the catalyst precursor perpendicularly to its direction of advance in the calcining zone n.
3. A process as claimed in claim 1 , wherein a gas circulation system is set up in the calcining zone n, the volume of gas circulating in the calcining zone n per unit time being greater than the volume of gas supplied to the calcining zone n per unit time.
4. A process as claimed in claim 3 , wherein a fan is used to generate the gas circulation system.
5. A process as claimed in claim 1 , wherein the fluctuation in the gas temperature from said setpoint value over time at each point in the region of the stream of the catalyst precursor after half the length of the calcining zone, 1 n /2, is in each case≦2° C.
6. A process as claimed in claim 1 , wherein the local difference in gas temperature between any two points in the region of the stream of the catalyst precursor after half the length of the calcining zone, 1 n /2, is ≦3° C.
7. A process as claimed in claim 1 , wherein the stream of the catalyst precursor for calcining is passed through at least two calcining zones and the temperatures of the individual calcining zones can be regulated independently of one another.
8. A process as claimed in claim 7 , wherein a temperature increase between successive calcining zones is up to 225° C.
9. A process as claimed in claim 7 , wherein the gas supplied to a successive calcining zone comprises nitrogen and steam, or nitrogen.
10. A process as claimed in claim 9 , wherein the gas supplied tote successive calcining zone comprises steam.
11. A process as claimed in claim 1 , wherein a catalyst precursor is used which has substantially a hollow cylindrical structure.
12. A process as claimed in claim 1 , wherein the catalyst precursor
(a) is heated to a temperature of from 200 to 350° C. in at least one calcining zone in an oxidizing atmosphere having an oxygen content of from 2 to 21% by volume and is left under these conditions until the desired average oxidation state of the vanadium is reached; and
(b) is heated to a temperature of from 300 to 500° C. in at least one further calcining zone in a nonoxidizing atmosphere having an oxygen content of ≦0.5% by volume and a hydrogen oxide content of from 20 to 75% by volume and is left under these conditions for ≦0.5 hour.
13. A process as claimed in claim 1 , wherein a catalyst is prepared which has a phosphorus/vanadium atomic ratio of from 0.9 to 1.5, an average oxidation state of the vanadium of from +3.9 to +4.4, a BET surface area of from 10 to 50 m 2 /g, a pore volume of from 0.1 to 0.5 mug, and a bulk density of from 0.5 to 1.5 kg/1.
14. A process as claimed in claim 1 , wherein the vanadium, phosphorus, and oxygen containing catalyst comprises a phosphate.
15. A process as claimed in claim 14 , wherein the vanadium, phosphorus, and oxygen containing catalyst comprises vanadyl pyrophosphate.