Asymmetric membranes for use in nanofiltration

Abstract

Improved integrally skinned asymmetric membranes for organic solvent nanofiltration, and their methods of preparation and use are disclosed. Membranes are formed from polybenzimidazoles by phase inversion and are then crosslinked by addition of crosslinking agents. These stabilise the membranes and allow solvent nanofiltration to be maintained even in the solvents from which the membranes were formed by phase inversion, and in strongly acidic and strongly basic solvents.

What is claimed is: 
     
         1 . A membrane for nanofiltration of a feed stream solution comprising a solvent and dissolved solutes and showing preferential rejection of the solutes at ambient temperature, comprising an integrally skinned asymmetric polybenzimidazole membrane, wherein the membrane is impregnated with a conditioning agent that maintains the pore structure of the membrane prior to its use for nanofiltration. 
     
     
         2 . A membrane according to  claim 1  in which the polybenzimidazole membrane is crosslinked. 
     
     
         3 . (canceled) 
     
     
         4 . (canceled) 
     
     
         5 . The membrane according to  claim 1 , wherein the polybenzimidazole membrane comprises a polybenzimidazole polymer of the following formula: 
       
         
           
           
               
               
           
         
       
     
     
         6 . A membrane according to  claim 2 , wherein a discrete organic matrix is dispersed in the crosslinked polybenzimidazole asymmetric membrane at amounts up to 50% by weight of said membrane. 
     
     
         7 . A membrane according to an  claim 2 , wherein a discrete inorganic matrix is dispersed in the crosslinked polybenzimidazole asymmetric membrane at amounts up to 50% by weight of a dope solution. 
     
     
         8 . A membrane according to  claim 6 , wherein the average particle size of the discrete matrix is less than 0.1 micron. 
     
     
         9 . (canceled) 
     
     
         10 . A membrane according to  claim 1 , wherein the membrane comprises crosslinks formed from the reaction of polybenzimidazole with dibromobutane, tribromopropane, trichloropropane, pentaerythrityl tetrabromide, pentaerythrityl tetrachloride, divinyl sulfones, perfluoroglutaric acid, benzene hexacarboxylic acid, benzene pentacarboxylic acid, 1,2,3,4-benzenetetracarboxylic acid, 1,2,3,5-benzenetetracarboxylic acid, 1,2,4,5-benzenetetracarboxylic acid, 1,3,5-benzenetricarboxylic acid, dibromosuccinic acid, polyacrylic acid, 1,4,5,8-naphthalenetetracarboxylic acid, 2,6-naphthalenedisulfonic acid, aryl-sulfonic acids, aryl-sulfinic acids, aryl-phosphinic acids, aryl-phosphonic acids. 
     
     
         11 . (canceled) 
     
     
         12 . (canceled) 
     
     
         13 . Use of the membrane according to  claim 1  for nanofiltration of a feed stream solution comprising a solvent and dissolved solutes. 
     
     
         14 . (canceled) 
     
     
         15 . (canceled) 
     
     
         16 . A process for forming an integrally skinned asymmetric crosslinked polybenzimidazole membrane for solvent nanofiltration, said process comprising the steps of:
 (a) preparing a polybenzimidazole dope solution comprising:   (i) a polybenzimidazole polymer, and (ii) a solvent system for said polybenzimidazole which is water miscible;   (b) casting a film of said dope solution onto a supporting substrate;   (c) allowing the dope solution to evaporate for an evaporation period, and then immersing the film cast on the substrate into a coagulating medium;   (d) treating the resulting asymmetric membrane with a solvent comprising one or more of a multifunctional alkyl halide, a divinyl sulfone, a strong polyfunctional organic acid; and   (e) treating the asymmetric membrane with a conditioning agent.   
     
     
         17 . A process according to  claim 16 , wherein the polybenzimidazole polymer is present in amounts of 5 to 30% by weight of said dope solution. 
     
     
         18 . (canceled) 
     
     
         19 . A process according to  claim 16 , further comprising step (f) drying the membrane. 
     
     
         20 . A process according to  claim 16 , wherein the process further comprises a step of heating the membrane to about 150° C. or higher. 
     
     
         21 . A process according to  claim 16 , wherein the polybenzimidazole has the following formula: 
       
         
           
           
               
               
           
         
       
     
     
         22 . (canceled) 
     
     
         23 . A process according to  claim 16 , wherein the polybenzimidazole dope solution further comprises a viscosity enhancer in amounts of up to 10% by weight of said dope solution. 
     
     
         24 . (canceled) 
     
     
         25 . A process according to  claim 16 , wherein the polybenzimidazole dope solution further comprises a void suppressor used in amounts up to 5% of the weight of said polybenzimidazole dope solution. 
     
     
         26 . A process according to  claim 16 , wherein the polybenzimidazole dope solution further comprises a discrete organic matrix dispersed in the polybenzimidazole dope solution at amounts up to 20% by weight of said dope solution. 
     
     
         27 - 29 . (canceled) 
     
     
         30 . A process according to  claim 16  in which the solvent used to disperse the discrete matrix is removed from the polybenzimidazole dope solution by evaporation. 
     
     
         31 . A process according to  claim 16  in which the crosslinking agent is selected from the group consisting of multifunctional alkyl halides, divinyl sulfones, and strong polyfunctional organic acids. 
     
     
         32 - 37 . (canceled) 
     
     
         38 . A process according to  claim 16  in which the temperature of crosslinking solution is held between 20 and 150° C. 
     
     
         39 . (canceled) 
     
     
         40 . A process according to  claim 16  wherein the conditioning agent is selected from one or more of synthetic oils mineral oils, vegetable fats and oils, higher alcohols, glycerols, and glycols. 
     
     
         41 . (canceled)