Composite carbon molecular sieve membranes having anti-substructure collapse particles loaded in a core thereof

Abstract

A carbon molecular sieve (CMS) membrane is made by pyrolyzing, to a peak pyrolysis temperature TP, a hollow fiber membrane having a polymeric sheath surrounding a polymeric core, anti-substructure collapse particles present in pores formed in the polymeric core help prevent collapse of pores formed in the hollow fiber membrane before pyrolysis. The anti-substructure collapse particles are made of a material or materials that either: i) have a glass transition temperature TG higher than TP, ii) have a melting point higher than TP, or ii) are completely thermally decomposed during said pyrolysis step at a temperature less than TP. The anti-substructure collapse particles are not soluble in a solvent used for dissolution of the polymeric material of the core.

What is claimed is: 
     
         1 . A method for producing a CMS membrane fiber, comprising the steps of:
 forming a composite precursor polymeric hollow fiber having a sheath covering a hollow core, the core being solidified from a core composition comprising a polymeric core material dissolved in a core solvent and anti-substructure collapse particles insoluble in the core solvent, the anti-substructure collapse particles being disposed within pores formed in the polymeric core material, the sheath being solidified from a sheath composition comprising a polymeric sheath material dissolved in a sheath solvent, the anti-substructure collapse particles having an average size of less than one micron; and   pyrolyzing the composite precursor polymeric hollow fiber up to a peak pyrolysis temperature T P , wherein the anti-substructure collapse particles are made of a material or materials that either:
 i) have a glass transition temperature T G  higher than T P , 
 ii) have a melting point higher than T P , or 
 ii) are completely thermally decomposed during said pyrolysis step at a temperature less than T P . 
   
     
     
         2 . The method of  claim 1 , wherein the material or materials of the anti-substructure collapse particles are selected from the group consisting of: polymer, glasses, ceramics, graphite, silica and mixtures of two or more thereof. 
     
     
         3 . The method of  claim 2 , wherein the material of the anti-substructure collapse particles is polybenzimidazole. 
     
     
         4 . The method of  claim 2 , wherein the material of the anti-substructure collapse particles is silica. 
     
     
         5 . The method of  claim 1 , wherein the material or materials of the anti-substructure collapse particles are selected from cellulosic materials and polyethylene. 
     
     
         6 . The method of  claim 1 , wherein the polymeric sheath material and the polymeric core material are a same polymer or copolymer. 
     
     
         7 . The method of  claim 6 , wherein a wt % of the polymer or copolymer in the core composition is lower than a wt % of the polymer or copolymer in the sheath composition. 
     
     
         8 . The method of  claim 1 , wherein the polymeric sheath material is different from the polymeric core material. 
     
     
         9 . The method of  claim 8 , wherein the polymeric sheath material comprises a major amount of a first polymer or copolymer and a minor amount of second polymer or copolymer and the polymeric core material comprises a minor amount of the first polymer or copolymer and a major amount of the second polymer or copolymer. 
     
     
         10 . The method of  claim 8 , wherein the polymeric sheath material is a first polymer having a first coefficient of thermal expansion, the polymeric core material is a second polymer having a second coefficient of thermal expansion, and the first and second coefficients of thermal expansion differ from one another by no more than 15%. 
     
     
         11 . The method of  claim 10 , wherein first coefficient of thermal expansion is greater than the second coefficient of thermal expansion. 
     
     
         12 . The method of  claim 10 , wherein a wt % of the anti-substructure collapse particles in the core composition is selected such that the polymeric sheath material shrinks along a length of the fiber no more than +1-15% than that of the polymeric core material, but in any case is at least 5 wt %. 
     
     
         13 . The method of  claim 8 , wherein the polymeric sheath material is a first polymer exhibiting a first coefficient of thermal shrinkage above a temperature at which the first polymer starts to thermally degrade, the polymeric core material is a second polymer having a second coefficient of thermal shrinkage above a temperature at which the second polymer starts to thermally degrade, and the first and second coefficients of thermal shrinkage differ from one another by no more than 15%. 
     
     
         14 . The method of  claim 8 , wherein the polymeric sheath material is a first polymer, the polymeric core material is a second polymer, and the second polymer has a glass transition temperature equal to or greater than 200° C. 
     
     
         15 . The method of  claim 14 , wherein the second polymer has a glass transition temperature equal to or greater than 280° C. 
     
     
         16 . The method of  claim 1 , wherein the polymeric core material is a polyaramide consisting of repeating units of diamino mesitylene isophthalic acid. 
     
     
         17 . The method of  claim 1 , wherein the polymeric core material is the condensation product of 2,2-bis[4-(2,3-dicarboxyphenoxy)phenyl]propane dianhydride and m-phenylenediamine or p-phenylenediamine. 
     
     
         18 . The method of  claim 1 , wherein the polymeric core material is polybenzimidazole 
     
     
         19 . The method of  claim 1 , wherein each of the polymeric core and sheath materials is made of a polymer or copolymer independently selected from the group consisting of polyimides, polyether imides, polyamide imides, cellulose acetate, polyphenylene oxide, polyacrylonitrile, and combinations of two or more thereof. 
     
     
         20 . The method of  claim 19 , wherein the polymeric sheath material is made of a polyimide. 
     
     
         21 . The method of  claim 20 , wherein the polyimide consists of the repeating units of formula I: 
       
         
           
           
               
               
           
         
       
     
     
         22 . The method of  claim 20 , wherein the polyimide is 6FDA:BPDA/DAM. 
     
     
         23 . The method of  claim 20 , wherein the polyimide is selected from the group consisting of: 6FDA:mPDA/DABA and 6FDA:DETDA/DABA. 
     
     
         24 . The method of  claim 19 , wherein the polymeric sheath material is poly (4,4′-oxydiphenylene-pyromellitimide). 
     
     
         25 . The method of  claim 19 , wherein the polymeric sheath material consists of the repeating units of formulae II and III: 
       
         
           
           
               
               
           
         
       
     
     
         26 . The method of  claim 20 , wherein the polyimide consists of repeating units of formula IV: 
       
         
           
           
               
               
           
         
       
     
     
         27 . A CMS membrane fiber produced according to the method of  claim 1 . 
     
     
         28 . A CMS membrane module including a plurality of the CMS membrane fibers of  claim 27 . 
     
     
         29 . A method for separating a gas mixture, comprising the steps of feeding a gas mixture to the CMS membrane module of  claim 28 , withdrawing a permeate gas from the CMS membrane module that is enriched in at least one gas relative to the gas mixture, and withdrawing a non-permeate gas from the CMS membrane module that is deficient in said at least one gas relative to the gas mixture.