In situ modification of molybdenum-based catalysts

1. A process for the conversion of a hydrocarbon selected from the group consisting of propylene, isobutylene, propane, isobutane or mixtures thereof, to acrylonitrile, methacrylonitrile, or mixtures thereof, the process comprising contacting in a reactor and reacting in the vapor phase at an elevated temperature and pressure said hydrocarbon with a molecular oxygen-containing gas and ammonia, in the presence of a molybdenum-based ammoxidation catalyst and a catalyst modifier, wherein said molybdenum-based ammoxidation catalyst comprises an oxide represented by the following formula:

A a B b C c D d Bi e Mo f O x

where

A is one or more of Li, Na, K, Cs, Rb, Sm, In, Ca, Sr, Ba and Tl,

B is one or more of Fe, Co, Mg, Mn, Ni, V and Zn,

C is one or more of Cr, Ce, Eu, P, Sb, Ge, Te and W,

D is one or more of Sn, B, As, Pt, Pd, Ga, Nd, Nb, Pr and Pb, and

a is 0.05 to 3.0,

b is 0.1 to 14.0,

c is 0.0 to 5.0,

d is 0.0 to 2.0,

e is 0.2 to 6.0,

f is 8.0 to 18.0,

and x is a number determined by the oxidation states and amounts of the other elements present in the catalyst (i.e. x a number determined by the valence requirements of the other elements present); and

wherein said catalyst modifier comprises a molybdate or a polymolybdate of at least one element M selected from the group consisting of cesium, rubidium, potassium, sodium, thallium, lithium, nickel, cobalt, iron, chromium, copper, magnesium, manganese, cerium and phosphorus.

2. The process of claim 1 , wherein the catalyst modifier is impregnated onto a support and calcined.

3. The process of claim 1 , wherein catalyst modifier comprises a molybdate or a polymolybdate of at least one element M selected from the group consisting of cesium, rubidium, potassium, sodium, thallium, and lithium.

4. The process of claim 1 , wherein the catalyst modifier is selected from the group consisting of Li 2 MoO 4 , Li 2 Mo 2 O 7 , Li 6 Mo 7 O 24 , Li 4 Mo 8 O 26 , Na 2 MoO 4 , Na 2 Mo 2 O 7 , Na 6 Mo 7 O 24 , Na 4 Mo 8 O 26 , K 2 MoO 4 , K 2 Mo 2 O 7 , K 6 Mo 7 O 24 , K 4 Mo 8 O 26 , Rb 2 MoO 4 , Rb 2 Mo 2 O 7 , Rb 6 Mo 7 O 24 , Rb 4 Mo 8 O 26 , Cs 2 MoO 4 , Cs 2 Mo 2 O 7 , Cs 6 Mo 7 O 24 , Cs 4 Mo 8 O 26 , or mixtures thereof.

5. The process of claim 1 , wherein the catalyst modifier comprises Na 2 MoO 4 .

6. The process of claim 1 , wherein catalyst modifier is added to the molybdenum-based catalyst while the catalyst is operating in the reactor.

7. The process of claim 1 , wherein the catalyst modifier is coated onto an internal structure of the reactor.

8. The process of claim 1 , wherein the catalyst modifier is coated onto a solid substrate and mounted or placed in a reactor.

9. The process of claim 1 , wherein the process further comprises the steps of:

combining said molybdenum-based ammoxidation catalyst and said catalyst modifier to form a catalytic mixture; and

adding said catalytic mixture to an ammoxidation reactor.

10. The process of claim 9 , wherein said step of combining comprises mixing a solid catalyst modifier with fresh molybdenum-based catalyst.

11. The process of claim 9 , wherein said step of combining comprises contacting said molybdenum-based catalyst with an aqueous solution or suspension of said catalyst modifier and drying.

12. The process of claim 1 , wherein the ratio of the M elements to Mo in the molybdates and polymolybdates is greater than the ratio for these M elements to Mo in the molybdenum-based ammoxidation catalyst.

13. The process of claim 1 , wherein the molybdates and polymolybdates are produced by reacting a base, comprising at least one of cesium, rubidium, potassium, sodium, thallium, lithium, nickel, cobalt, iron, chromium, copper, magnesium, manganese, cerium and phosphorus or mixtures thereof, with a molybdenum oxide represented by the formula Mo x O y , wherein the y/x ratio is in the range of 1 to 3.