Synthesis of (meth) acrylic anhydride by transanhydrization

1. A process for preparing an anhydride of formula A-C(═O)—O—(O═)C-A in which A is a group —CH═CH 2 or —C(CH 3 )═CH 2 , said process comprising:

a) a step of reacting an anhydride B—C(═O)—O—(O═)C—B with an acid A-COOH, A being as defined above, this step leading to the formation of an anhydride A-C(═O)—O—(O═)C—B and of an acid B—COOH, A and B being such that said acid B—COOH is more volatile than said acid A-COOH; and

b) a step of reacting said anhydride A-C(═O)—O—(O═)C—B with the acid A-COOH under conditions such that the amount of acid B—COOH is less than the amount of acid A-COOH, resulting in the formation of the anhydride A-C(═O)—O—(O═)C-A,

in which:

said steps (a) and (b) are performed in the presence of triflic acid in hydrated form

the anhydride A-C(═O)—O—O(O═)C-A formed in step (b) is isolated from the reaction medium derived from step (b) according to the following successive or concomitant separation steps:

e1) the “heavy” compounds, having a volatility less than or equal to that of the anhydride A-C(═O)—O—O(O═)C-A, are separated from said reaction medium,

these heavy compounds including the anhydride A-C(═O)—O—(O═)C-A and triflic acid in hydrated form;

e2) the anhydride A-C(═O)—O—O(O═)C-A is separated from the heavy compounds by difference in volatility.

2. The process as claimed in claim 1 , in which the acid B—COOH is removed during the reaction steps a) and b).

3. The process as claimed in claim 1 , wherein the process is performed continuously.

4. The process as claimed in claim 1 , wherein the molar mass of B is less than the molar mass of A.

5. The process as claimed in claim 1 , wherein B is a methyl or ethyl group.

6. The process as claimed in claim 1 , in the reaction steps a) and b) are performed at a temperature of from 60° C. to 120° C.

7. The process as claimed in claim 1 , in which the reaction steps a) and b) are performed at a pressure of from 0.01 bar to 3 bar.

8. The process as claimed in claim 1 , comprising a step of extracting the acid catalyst and the acid B—COOH formed, in order to separate out the acid catalyst and the acid B—COOH.

9. The process as claimed in claim 8 , in which the reaction steps are performed in a reactor and the extraction step is performed in one or more successive distillation columns separate from the reactor.

10. The process as claimed in claim 9 , in which the reaction steps and the extraction step are performed in one or more successive distillation columns.

11. The process as claimed in claim 1 , wherein, in step e1), the “heavy” compounds, having a volatility less than or equal to that of the anhydride A-C(═O)—O—O(O═)C-A, are separated from said reaction medium by fractional distillation.

12. The process as claimed in claim 5 , wherein B is a methyl group.

13. The process as claimed in claim 6 , in which the reaction steps a) and b) are performed at a temperature of from 70° C. to 110° C.

14. The process as claimed in claim 13 , in which the reaction steps a) and b) are performed at a temperature of from 90° C. to 100° C.

15. The process as claimed in claim 7 , in which the reaction steps a) and b) are performed at a pressure of from 0.5 bar to 1.5 bar.

16. The process as claimed in claim 15 , in which the reaction steps a) and b) are performed at atmospheric pressure.

17. The process as claimed in claim 8 , wherein the acid catalyst and the acid B—COOH formed is extracted by distillation.