Composite hollow fiber membranes useful for CO

1. A composite hollow fiber polymer membrane, comprising:

a porous core layer comprising a polyamide-imide polymer; and

a selective sheath layer on the core layer comprising a polyimide polymer prepared from monomers A, B, and C wherein

A is a dianhydride of the formula:

wherein X 1 and X 2 are independently halogenated alkyl group, phenyl or halogen and R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 are independently H, alkyl, or halogen;

B is a diamino cyclic compound without a carboxylic acid functionality; and

C is a diamino cyclic compound with a carboxylic acid functionality; and

wherein the polyimide polymer further comprises covalent ester crosslinks.

2. The composite hollow fiber polymer membrane according to claim 1 , wherein the polyamide-imide polymer and the polyimide polymer form a homogeneous blend.

3. The composite hollow fiber polymer membrane according to claim 1 , wherein A is 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA).

4. The composite hollow fiber polymer membrane according to claim 1 , wherein B is 2,4,6-trimethyl-m-phenylenediamine (DAM).

5. The composite hollow fiber polymer membrane according to claim 1 , wherein C is 3,5-diaminobenzoic acid (DABA).

6. The composite hollow fiber polymer membrane according to claim 1 , wherein A is 6FDA, B is DAM, and C is DABA.

7. The composite hollow fiber polymer membrane according to claim 6 , wherein the polyimide polymer prior to crosslinking to form the covalent ester crosslinks is represented by the formula:

8. The composite hollow fiber polymer membrane according to claim 1 , wherein the membrane is a dual layer hollow fiber membrane having a selective outer layer comprising about 1/10 th by volume of the polyimide polymer that would be employed in a dual layer hollow fiber membrane comprising both a porous substructure layer and selective outer layer made of polyimide polymer.

9. The composite hollow fiber polymer membrane according to claim 1 , wherein the polyamide-imide polymer of the core layer is

10. A method of making a composite hollow fiber polymer membrane, comprising:

extruding a porous core layer comprising a polyamide-imide polymer and

a selective sheath layer, wherein the selective sheath layer comprises a polyimide polymer prepared from monomers A, B, and C, wherein A is a dianhydride of the formula:

wherein X 1 and X 2 are independently halogenated alkyl group, phenyl or halogen and R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 are independently H, alkyl, or halogen;

B is a diamino cyclic compound without a carboxylic acid functionality; and

C is a diamino cyclic compound with a carboxylic acid functionality.

11. A process for purifying natural gas, comprising:

feeding a natural gas stream comprising at least one impurity to at least one membrane unit comprising composite polymer membranes to provide an impurity-rich permeate and an impurity-depleted product gas stream; and

recovering the impurity-depleted product gas stream,

wherein the composite polymer membranes comprise the composite hollow fiber membrane of claim 1 .

12. The process according to claim 11 , wherein the at least one impurity is selected from the group consisting of CO 2 , H 2 O, H 2 S, and mixtures thereof.

13. The process according to claim 11 , wherein the at least one impurity comprises CO 2 and H 2 O.

14. The process according to claim 11 , wherein the impurity-depleted product gas stream comprises ≦2 mol % CO 2 .

15. A composite hollow fiber polymer membrane, comprising:

a porous core layer comprising a polyamide-imide polymer which has a Tg greater than 230° C.; and

a selective sheath layer on the core layer comprising a polyimide polymer prepared from monomers A, B, and C wherein

A is a dianhydride of the formula

wherein X 1 and X 2 are independently halogenated alkyl group, phenyl or halogen and R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 are independently H, alkyl, or halogen;

B is a diamino cyclic compound without a carboxylic acid functionality; and

C is a diamino cyclic compound with a carboxylic acid functionality;

wherein the polyimide polymer further comprises covalent ester crosslinks; and,

with the core layer polymer compatible with the sheath layer polyimide polymer.

16. The composite hollow fiber polymer membrane according to claim 15 , wherein the core layer polymer and the sheath layer polyimide polymer are compatible as determined by a lack of delamination upon SEM observation of the cross-section of the hollow fiber after spinning.

17. The composite hollow fiber polymer membrane according to claim 15 , wherein the core layer polymer and the sheath layer polyimide polymer are compatible as determined by a lack of delamination upon SEM observation of the cross-section of the hollow fiber after crosslinking.

18. The composite hollow fiber polymer membrane according to claim 15 , wherein A is 6FDA, B is DAM, and C is DABA.

19. A process for purifying natural gas, comprising:

feeding a natural gas stream comprising at least one impurity to at least one membrane unit comprising composite polymer membranes to provide an impurity-rich permeate and an impurity-depleted product gas stream; and

recovering the impurity-depleted product gas stream,

wherein the composite polymer membranes comprise the composite hollow fiber membrane of claim 15 .