Catalytic performance in processes for preparing acetic acid

1. A process for the dehydration-hydrolysis of methyl acetate and methanol to co-produce acetic acid and dimethyl ether which process is integrated with a carbonylation process for the production of methyl acetate which integrated process comprises the steps:

a) carbonylating dimethyl ether with a carbon monoxide-containing gas in the presence of a zeolite carbonylation catalyst to produce a carbonylation reaction product and recovering methyl acetate therefrom;

b) pre-treating at least a portion of the methyl acetate recovered in step (a) to reduce the amount of acetaldehyde and 1,1 dimethoxyethane impurities therein;

c) converting methanol and methyl acetate by dehydration-hydrolysis in the presence of at least one Brønsted acid catalyst to co-produce acetic acid and dimethyl ether;

in which dehydration-hydrolysis process the performance of the at least one Brønsted acid catalyst is improved by using as a methyl acetate feed methyl acetate pre-treated in step (b) in which the total amount of acetaldehyde and 1,1 dimethoxyethane impurities is maintained at 100 ppm wt or less calculated as mass equivalents of acetaldehyde,

wherein each of the at least one Brønsted acid catalysts is a zeolite or a heteropolyacid.

2. A process according to claim 1 wherein the methyl acetate feed comprises 50 to 99 mol % methyl acetate, >0 to 45 mol % dimethyl ether and a total amount of acetaldehyde and 1,1 dimethoxyethane impurities of more than 100 ppm wt up to 1 wt % calculated as mass equivalents of acetaldehyde.

3. A process according to claim 1 wherein the methyl acetate feed is pre-treated to reduce the amount of impurities to less than 100 ppm wt calculated as mass equivalents of acetaldehyde.

4. A process according to claim 3 wherein the methyl acetate feed is pre-treated by a fractional distillation method.

5. A process according to claim 3 wherein methyl acetate comprising >0 to 80 mol % methyl acetate, a total amount of more than 100 ppm up to 1 wt % of acetaldehyde and 1,1 dimethoxyethane impurities and >0 to 50 mol % dimethyl ether is pre-treated to provide a methyl acetate feed having a total acetaldehyde and 1,1 dimethoxyethane impurity content of 0 to 100 ppm wt.

6. A process according to claim 5 wherein the methyl acetate to be pre-treated further comprises one or more of acetic acid, water, carbon oxides and hydrogen.

7. A process according to claim 1 wherein each Brønsted acid catalyst is a zeolite.

8. A process according to claim 7 wherein the zeolite of each Brønsted acid catalyst comprises at least one channel having a 10-membered ring.

9. A process according to claim 7 wherein the zeolite of each Brønsted acid catalyst has a framework type selected from FER and MFI.

10. A process according to claim 9 wherein the zeolite of each Brønsted acid catalyst has framework type FER and is selected from one or both of ferrierite and ZSM-35.

11. A process according to claim 7 wherein the zeolite of each Brønsted acid catalyst is in an exchanged form with one or more alkali metal cations.

12. A method according to claim 1 wherein each Brønsted acid catalyst is a heteropolyacid.

13. A process according to claim 1 wherein the methyl acetate feed further comprises one or both of methanol and water.

14. A process according to claim 13 wherein the methyl acetate feed further comprises one or more of carbon oxides, hydrogen and acetic acid.

15. A process according to claim 1 wherein the dehydration-hydrolysis process is carried out as a vapour phase process.

16. A process according to claim 1 wherein the process is carried out at a temperature of from 100 to 350° C.

17. A process according to claim 16 wherein the process is carried out at a temperature of 180 to 280° C.

18. A process according to claim 1 wherein the zeolite carbonylation catalyst comprises a zeolite which comprises at least one channel defined by an 8-membered ring.