Method for producing linear pentenenitrile

1. A process for preparing 3-pentenenitrile comprising the following process steps:

(a) isomerizing a reactant stream which comprises 2-methyl-3-butenenitrile over at least one dissolved or dispersed isomerization catalyst to give a stream 1 that comprises the at least one isomerization catalyst, 2-methyl-3-butenenitrile, 3-pentenenitrile and (Z)-2-methyl-2-butenenitrile,

(b) distilling stream 1 to obtain a stream 2 top product that comprises 2-methyl-3-butenenitrile, 3-pentenenitrile and (Z)-2-methyl-2-butenenitrile, and a stream 3 bottom product that comprises the at least one isomerization catalyst, wherein a pressure and temperature conditions in process step (b) are 1 mbar to 6 bar, and 40 to 180° C. at the bottom of the distillation apparatus,

(c) distilling stream 2 at a bottom temperature of 40 to 180° C. and a pressure of 10 mbar to 500 mbar to obtain a stream 4 top product which, compared to stream 2 is enriched in (Z)-2-methyl-2-butenenitrile, based on the sum of all pentenenitriles in stream 2,

hydrocyanating 1,3-butadiene over at least one hydrocyanation catalyst using hydrogen cyanide to obtain a stream 8 which comprises the at least one hydrocyanation catalyst, 3-pentenenitrile, 2-methyl-3-butenenitrile, 1,3-butadiene and residues of hydrogen cyanide,

distilling stream 8 one or more times to obtain a stream 9 which comprises 1,3-butadiene, a stream 10 which comprises the at least one hydrocyanation catalyst, and a stream 11 which comprises 3-pentenenitrile and 2-methyl-3-butenenitrile, and

distilling stream 11 to obtain a stream 12 bottom product which comprises 3-pentenenitrile, wherein the distilling of the stream 11 is carried out in the step (c) distillation column almong with stream 2 from process step (b) to provide the stream 4 as top product comprising (Z)-2-methyl-2-butenenitrile, stream 12 as bottom product comprising 3-pentenenitrile, and stream 13 as a side draw stream comprising 2-methyl-3-butenenitrile, which is recycled to the reactant stream of step (a).

2. The process according to claim 1 , wherein the at least one isomerization catalyst obtained in stream 3 in process step (b) is recycled to process step (a).

3. The process according to claim 1 , wherein process steps (b) and (c) are carried out together in one distillation apparatus, in which case stream 3 which comprises the at least one isomerization catalyst is obtained as the bottom product, stream 4 which comprises (Z)-2-methyl-2-butenenitrile as the top product, and stream 5 which comprises 3-pentenenitrile and 2-methyl-3-butenenitrile at a side draw of the column.

4. The process according to claim 1 , wherein process steps (a), (b) and (c) are carried out together in one distillation apparatus, in which case stream 4 which comprises (Z)-2-methyl-2-butenenitrile is obtained as the top product, and stream 5 which comprises 3-pentenenitrile and 2-methyl-3-butenenitrile at a side draw of the distillation apparatus, and the isomerization catalyst remains in the bottom of the distillation column.

5. The process according to claim 1 , wherein the isomerization catalyst contains nickel, a trivalent phosphorus-containing compound which as a ligand complexes the nickel, and optionally, a Lewis acid.

6. The process according to claim 1 , wherein the hydrocyanation catalyst and the isomerization catalyst are identical.

7. The process according to claim 5 , wherein the trivalent phosphorus-containing compound is of formula (Ib)

P(O—R 1 ) x (O—R 2 ) y (O—R 3 ) z (O—R 4 ) p   (Ib)

where

R 1 : is an aromatic radical having a C 1 -C 18 -alkyl substituent in the o-position to the oxygen atom, or having an aromatic substituent in the o-position to the oxygen atom, or having a fused aromatic system in the o-position to the oxygen atom,

R 2 : is an aromatic radical having a C 1 -C 18 -alkyl substituent in the imposition to the oxygen atom, or having an aromatic substituent in the m-position to the oxygen atom, or having a fused aromatic system in the m-position to the oxygen atom, the aromatic radical bearing a hydrogen atom in the o-position to the oxygen atom,

R 3 : is an aromatic radical having a C 1 -C 18 -alkyl substituent in the p-position to the oxygen atom, or having an aromatic substituent in the p-position to the oxygen atom, the aromatic radical bearing a hydrogen atom in the o-position to the oxygen,

R 4 : is an aromatic radical which bears substituents other than those defined for R 1 , R 2 and R 3 in the o-, m- and p-position to the oxygen atom, the aromatic radical bearing a hydrogen atom in the o-position to the oxygen atom,

x: is 1 or 2, and

y, z, p: are each independently 0, 1 or 2, with the proviso that x+y+z+p=3.

8. The process according to claim 5 , wherein the trivalent phosphorus-containing compound is of formula (Ib)

P(O—R 1 ) x (O—R 2 ) y (O—R 3 ) z (O—R 4 ) p   (Ib)

where R 1 , R 2 and R 3 are each independently selected from o-isopropylphenyl, m-tolyl and p-tolyl, and R 4 is phenyl; x is 1 or 2, and y, z, p are each independently 0, 1 or 2, with the proviso that x+y+z+p=3; and mixtures thereof.

9. The process according to claim 1 , wherein step (c) is carried out at a temperature of 5 to 150° C. at the top of the distillation apparatus.

10. The process according to claim 1 , wherein step (c) is carried out at a temperature of 60 to 140° C. at the bottom of the distillation apparatus.

11. The process according to claim 1 , wherein the stream 12 as bottom product comprises a single isomer of 3-pentenenitrile or a mixture of two, three, four or five different such isomers including cis-2-pentenenitrile, trans-2-pentenenitrile, cis-3-pentenenitrile, trans-3-pentenenitrile, or 4-pentenenitrile.