WO2019198093A1 - Acid and oxidative resistant homogenous cation exchange membrane and its method of preparation thereof - Google Patents

Acid and oxidative resistant homogenous cation exchange membrane and its method of preparation thereof Download PDF

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WO2019198093A1
WO2019198093A1 PCT/IN2019/050043 IN2019050043W WO2019198093A1 WO 2019198093 A1 WO2019198093 A1 WO 2019198093A1 IN 2019050043 W IN2019050043 W IN 2019050043W WO 2019198093 A1 WO2019198093 A1 WO 2019198093A1
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membrane
cation exchange
exchange membrane
acid
poly
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PCT/IN2019/050043
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French (fr)
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Arindam Kumar DAS
Geetanjali SHUKLA
Murli MANOHAR
Vinod Kumar Shahi
Baharat BHARGAVA
Puneet KISHORE
Dmaraju PARVATALU
Chandra Sekhar SARMA
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Council Of Scientific And Industrial Research
Ongc Energy Centre Oil And Natural Gas Corporation Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/76—Macromolecular material not specifically provided for in a single one of groups B01D71/08 - B01D71/74
    • B01D71/82—Macromolecular material not specifically provided for in a single one of groups B01D71/08 - B01D71/74 characterised by the presence of specified groups, e.g. introduced by chemical after-treatment
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/20—Manufacture of shaped structures of ion-exchange resins
    • C08J5/22—Films, membranes or diaphragms
    • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
    • C08J5/2218—Synthetic macromolecular compounds
    • C08J5/2225—Synthetic macromolecular compounds containing fluorine
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/20—Manufacture of shaped structures of ion-exchange resins
    • C08J5/22—Films, membranes or diaphragms
    • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
    • C08J5/2218—Synthetic macromolecular compounds
    • C08J5/2231—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds
    • C08J5/2237—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds containing fluorine
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/20—Manufacture of shaped structures of ion-exchange resins
    • C08J5/22—Films, membranes or diaphragms
    • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
    • C08J5/2218—Synthetic macromolecular compounds
    • C08J5/2231—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds
    • C08J5/2243—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds obtained by introduction of active groups capable of ion-exchange into compounds of the type C08J5/2231
    • C08J5/225—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds obtained by introduction of active groups capable of ion-exchange into compounds of the type C08J5/2231 containing fluorine
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/15—Use of additives
    • B01D2323/218—Additive materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/30—Polyalkenyl halides
    • B01D71/32—Polyalkenyl halides containing fluorine atoms
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/30—Polyalkenyl halides
    • B01D71/32—Polyalkenyl halides containing fluorine atoms
    • B01D71/34—Polyvinylidene fluoride

Definitions

  • the present invention relates to acid and oxidative resistant homogenous cation exchange membrane and its method of preparation by using 2-acrylamido-2-methyl-l-propanesulfonic acid grafted poly(vinylidene fluoride-co-hexafluoropropylene) or any other sulfonic acid functionalized monomer grafted with poly(vinylidene fluoride-co-hexafluoropropylene) with at least one polymer, in which (3 -mercaptopropyl)trimethoxy silane or any other sulphonic acid functionalized silica precursor is incorporated by acid catalysed sol-gel process to improve the functional group density in the membrane matrix.
  • These acid and oxidative resistant cation exchange membranes have practical application in electrodialysis, electrolysis or any other electrochemical processes employing cation exchange membrane as a separator.
  • Cation exchange membranes contain negatively charged acidic functional groups such as -SO 3 H, - PO 3 H 2 etc fixed to the polymer matrix cation exchange membrane are critical components for diversified electro-membrane processes including: electrodialysis (water desalination, separation of inorganics from organic molecules, separation of specific inorganic ion, etc), electrolysis ( used as separator for chlor-alkali process, production of HI by Bunsen reaction of iodine-sulfur (I— S) process, electrochemical catalytic water splitting), electro-electrodialysis for in-situ ion exchange and ion substitution, electro-deionization for producing ultrapure water, and polymer electrolyte membrane for fuel cell applications.
  • electrodialysis water desalination, separation of inorganics from organic molecules, separation of specific inorganic ion, etc
  • electrolysis used as separator for chlor-alkali process
  • ion exchange membranes depend on the density of the polymer network, hydrophobic and hydrophilic properties of the membrane matrix, the distribution of the charge density, and membrane morphology. The most desired properties for ion-exchange membranes are:
  • the membrane should be mechanically strong and should have a low degree of swelling or shrinking in transition from dilute to concentrated ionic solutions.
  • High chemical stability - the membrane should be stable over a pH range from 0 to 14 and in the presence of oxidizing agents.
  • Electrodialysis process using cation exchange membranes showed advantages over other membrane processes for allowing separation of targeted ions. In case scale formation or fouling on the membrane surfaces, unlikely to filtration membrane these can easily be avoided by acid wash of the cation-exchange membrane. Thus, cation exchange membranes should be capable of withstanding highly acidic solutions. Further, during electrolysis cation exchange membranes are placed between the electrodes (anode and cathode). The environment of electrode chambers especially anode chamber is oxidative in nature due to formation of free radicals.
  • Nafion ® is a copolymer of tetrafluroethylene (TFE) and sulfonyl fluoride vinyl ether (PSEPVE) and displays good conductivity ( ⁇ 0. l S/cm), along with chemical and mechanical stability.
  • ETS4508852 wherein chlorosulfonic acid showed disadvantage of side reactions.
  • ETS3709841 wherein sulfonation of polyether sulfones was successfully achieved by chlorosulfonic acid or oleum in an inert solvent (chlorinated hydrocarbons).
  • Macromolecular modification of polymer offers an efficient route to modify wettability, morphology, and performance of the resultant blend or composite systems due to its simplicity of operation and cost.
  • conducting polymer in non-conducting polymer matrix, hydrophilicity and conductivity of the resultant material can be improved.
  • PVDF-co-HFP partially fluorinated polyvinylidene fluoride co-hexafluoropropylene
  • PVDF-co-HFP based cation exchange membranes are based on continuous organic polymer matrices with sulfonic-acid, aromatic and/or acid-base functional groups or containing inorganic particles additives, such as zeolites and silica.
  • inorganic particles additives such as zeolites and silica.
  • PVDF-co-HFP hydrophobic fluorinated polymer
  • the present invention relates to acid and oxidative resistant homogenous cation exchange membrane.
  • the Process for the preparation of homogeneous cation exchange membrane having formula 1 as repeating unit comprising the following steps:
  • Formula 1 i. poly(vinylidene fluoride-co-hexafluoropropylene) and dimethyl acetamide are taken in a vessel;
  • step (i) solution obtained in step (i) is charged with 0.5 molar NaOH solution until the solution becomes dark brown colour followed by continuous stirring for 2 hours;
  • step (iii) solution obtained in step (ii) is poured in water and dehydroflourinated polymer is separated out and dried; iv. dehydroflourinated polymer obtained in step (iii) and 2-acrylamido-2-methyl-l- propansulfonic acid are mixed in dimethylacetamide solvent in a vessel and kept at above temperature range 50° C in the presence of radical initiator azobisisobutyronitrile (AIBN) for more than 8 hours to obtain sulphonated dehydrofluorinated poly(vinylidene fluoride-co- hexafluoropropylene) ;
  • AIBN radical initiator azobisisobutyronitrile
  • v. (3-mercaptopropyl) trimethoxy silane as silica precursor is added to sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) followed by casting the solution in a glass plate;
  • vi. cation exchange membrane obtained in step (v) is dried and dipped into hydrogen peroxide solution followed by washing with water.
  • ratio of sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) to silica precursor ranges from 15-22% (w/w).
  • the membrane has an ion exchange capacity of 1.30 to 1.40 meq/g.
  • the membrane has conductivity of 4.0 to 4.25 xlO 2 S cm 1 .
  • membrane swelling ratio (under treatment of hot water at 60°C for 24 hours) ranges from 7 to 12%.
  • membrane weight, ion exchange capacity and conductivity loss is less than 4% under highly acidic (10 M H 2 SO 4 at 30°C for 120 hours) or harassed oxidative environment (3 ppm FeS0 4 + 3% H 2 O 2 at 80°C for 3 h).
  • acid and oxidative resistant cation exchange membrane prepared by chemical grafting of 2-acrylamido-2-methyl-l-propanesulfonic acid or any other functionalized monomer with poly(vinylidene fluoride-co-hexafluoropropylene). Further, density of sulphonic acid groups in the membrane forming material followed by oxidation of functional group by hydrogen peroxide.
  • silica precursor may be functionalized with -SO3H, or -PO3H2 or both groups for significantly improvement in membrane conductivity.
  • copolymer (DHPVDF-co-HFP) with following chemical structure was subjected to the functionalization by grafting of sulphonic acid functionalized monomer (for example AMPS):
  • homogeneous cation exchange membrane made by sulphonated DHPVDF-co-HFP copolymer and functionalized silica precursor attached via strong hydrogen bonding.
  • cation exchange membrane with high functional charge density exhibited reduced cell voltage and improved current efficiency during electrolysis because of grafted functional (-SO 3 H or -PO 3 H 2 ) groups with fluorinated polymer.
  • cation exchange membrane according to the present invention showed high efficiency during electrolysis, electrodialysis, or other electro membrane applications, and superior stabilities (thermal and acid).
  • the industrial significance of the cation exchange membrane of the present invention is extremely high.
  • FIG. 1 is an illustration of reaction scheme for the preparation of acid and oxidative resistant cation exchange membrane.
  • FIG. 2 depicts electrodialysis process for desalination of brackish water.
  • FIG. 3 depicts Bunsen process (iodine-sulfur cycle) for preparing HI by membrane electrolysis.
  • This invention relates to the development of polymeric cation exchange membranes for electrolysis, separation of inorganic salts in aqueous media by electrodialysis and also useful for other electro membrane processes.
  • the cation exchange membrane of this invention showed good stabilities (oxidative and acid), excellent conductivity, permselectivity, and other physicochemical properties such water content and ion exchange capacity, which are essential requirement for high performance during diversified electro-membrane applications in aqueous medium.
  • the methods used to produce reported cation exchange membrane is quite simple and comparably less expensive, which contribute to the overall economy of the process using these ion exchange membranes.
  • the invention also encompasses the use of the reported membrane for variety of electro-membrane applications.
  • the experimental conditions adopted for the preparing cation exchange membrane include the introducing vinyl bond in PVDF-co-HFP by dehydro-fluorination under treatment with NaOH.
  • free radical copolymerization of DHPVDF-co-HFP and AMPS was achieved in the presence of initiator 2,2-azobis(2-methylpropionitrile) (AIBN).
  • AIBN 2,2-azobis(2-methylpropionitrile)
  • desired content of silica precursor (3- mercaptopropyl) trimethoxysilane
  • HC1 hydrochloric acid
  • the quotient of the total number of sulfonic acid in the polymer can be readily controlled by fine adjustment of AMPS concentration in DHPVDF-co-HFP matrix.
  • the prepared sulphonated DHPVDF-co-HFP copolymer membrane showed restricted conductivity and other electrochemical properties, essential for a high performance cation exchange membrane. These properties were further improved by incorporating silica precursor in the membrane forming polymeric material, where mercapto groups were oxidized to sulphonic acid group by hydrogen peroxide.
  • Reported cation exchange membrane showed homogenous nature because of hydrogen bonding between sulphonated DHPVDF-co-HFP copolymer and 3-mercaptopropyl) trimethoxy silane.
  • the conditions used in the present invention to produce the cation exchange membrane are completely non-hazardous or not required any sophisticated equipment or facilities in compare with methodologies currently used.
  • the production cost of cation exchange membrane in present case is considerably low.
  • highly acidic and oxidative stable cation exchange membrane was prepared by co-polymeriztion DHPVDF-co-HFP and varied content of AMPS (20-38% w/w) using AIBN as free radical initiator.
  • the process for the preparation of homogeneous cation exchange membrane having formula 1 as repeating unit comprising the following steps:
  • Formula 1 vii poly(vinylidene fluoride-co-hexafluoropropylene) and dimethyl acetamide are taken in a vessel;
  • step (i) solution obtained in step (i) is charged with 0.5 molar NaOH solution until the solution becomes dark brown colour followed by continuous stirring for 2 hours; ix. solution obtained in step (ii) is poured in water and dehydroflourinated polymer is separated out and dried;
  • x. dehydroflourinated polymer obtained in step (iii) and 2-acrylamido-2-methyl-l- propanesulfonic acid are mixed in dimethylacetamide solvent in a vessel and kept at above temperature range 50° C in the presence of radical initiator azobisisobutyronitrille (AIBN) for more than 8 hours to obtain sulphonated dehydrofluorinated poly(vinylidene fluoride-co- hexafluoropropylene) ;
  • AIBN radical initiator azobisisobutyronitrille
  • xi. (3-mercaptopropyl) trimethoxysilane as silica precursor is added to sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) followed by casting the solution in a glass plate;
  • xii. cation exchange membrane obtained in step (v) is dried and dipped into hydrogen peroxide solution followed by washing with water.
  • reported process for the preparation of stable cation exchange membrane represents a novel method with several advantages over the previously reported method of lower cost, without any use of hazardous chemicals.
  • the strategy adopted to increase the functional group (sulphonic acid) molality in the membrane matrix by incorporating silica precursor in the sulphonated DHPVDF-co-HFP copolymer matrix, responsible for significantly improvement in membrane performance is also a novel step.
  • prepared cation exchange membrane may be widely used for electrolysis in aqueous medium, different electrochemical membrane reactors for any desired chemical synthesis, electro-deionization process for producing ultra-pure water, electrodialysis for the removal of inorganic electrolyte such as desalination of sea and brackish water, separation and removal of metal ions from the industrial effluent, de-acidification of fruit juice of dashing and sugar cane juice even at high temperature (70-80 °C) for better quality of sugar, improved yield and reduced molasses production, purifications and amino acids, vitamins, vaccines and other biochemical purification and in situ ion substitution and biomolecules obtained from fermentation broth during down-stream processing. Further, electrolysis in aqueous medium (water splitting), chlor-alkali process and fuel cell processes based on selective proton-transport,
  • optimum water content in the membrane phase govern hydration of functional groups (sulphonic acid group in this case) and provide the necessary water molecules surrounded the functional group for the formation hydrophilic ion conducting channels, is a critical parameter for the high performance cation exchange membrane. Further, high water content in the membrane phase is also responsible for the membrane dimensional instability. Thus, to achieve the stable high performance cation exchange membrane, an optimized water content (20-30%, w/w) is essential. In the present invention, care was rendered for the proper balancing of hydrophobic and hydrophilic segments in the membrane forming martial, which will enables the desired water content in the membrane matrix.
  • the ion exchange capacity represents a measure of the hydrophilic character or concentration density of ionic groups, can be given by meq./g or dry membrane. 1 meq./g means that per gram of polymer responsible for the 1 mmol exchange of ions or protons. IEC may again be determined by acid-base titration. Density of exchangeable groups in the membrane matrix is also an important factor in membrane quality and controls the membrane conductivity and thus performance. However, membrane durability depends upon the environmental conditions and polymer backbone nature. All these factors were judicially considered during membrane synthesis. One skilled in the art based upon prior knowledge and description provided above should easily determine the membrane preparation procedure and parameters with specifically desired cation exchange membrane.
  • the membrane permselectivity is a measure of the characteristic difference in the membrane permeability for counter-ions and co-ions. Counter-ion transport number across the membrane was estimated by membrane potential measurement for the estimation of membrane permselectivity.
  • Membrane conductivity of cation and anion exchange membrane was determined in equilibration with 0.10M NaCl solution using a potentiostat/galvanostat impedance analyzer. The membrane resistance was determined in terms of membrane conductivity by Nyquist plots by Fit and Simulation method considering membrane thickness and area.
  • cation exchange membrane should be highly permeable to cation, but should be impermeable to anion; high membrane conductivity (> 4.0x1 O 2 S cm 1 )— cation exchange membrane should have high membrane conductivity and thus there will be less potential drop during electrodialysis or electro-membrane processes; good mechanical stability— the membrane should be mechanically strong and should have a low degree of swelling or shrinking in transition from dilute to concentrated ionic solutions; high chemical stability— the membrane should be stable in strong acidic or alkaline environment even in presence of oxidizing agents.
  • PVDF-co-HFP 10 gram of PVDF-co-HFP was dissolved in 150 ml of dimethylacetamide under constant stirring, and saturated solution NaOH (0.50 M) in isopropanol (10 ml) was added to the mixture drop-wise with vigorous stirring at 30 °C for 30 minutes. The colorless resultant solution was turned to light brown.
  • Dehydrofluorinated PVDF-co-HFP DUVDF-co-HFP
  • PVDF-co-HFP (%,w/w)
  • Brackish water may or may not be chlorine dosed is provided as drinking water (total dissolved solid: ⁇ 500 ppm) after desalination by electrodialysis.
  • the brackish water may obtained either from earth surface or from ground, passed through chlorine-dosing or other treatment and to the electrodialysis desalination unit to remove excess of salt.
  • An electrodialysis (ED) unit containing 10 cell pairs of cation exchange membrane (functionalized PVDF: 20% (11.8 g functionalized) + silica (MPS) 2.95 g) and anion-exchange membrane (AEM) (Neosepta AMX supplied by Tokuyama Soda Co.
  • Example 11 The iodine-sulfur (I-S) process is a promising thermochemical process due to C0 2 -free hydrogen production.
  • Two- compartment membrane electrolysis cell separated by cation exchange membrane functionalized PVDF: 20% (11.8 g functionalized) + silica (MPS) 2.95 g was assessed for the Bunsen process.
  • Teflon made two- compartments cell separated by cation exchange membrane (effective area: 1000 cm 2 ) and fitted with two electrodes (precious metal oxide coated titanium sheets (Ti0 2 sheet coated with a triple precious metal oxide (titanium-ruthenium-platinum), of 6.0 pm thickness) was used for membrane electrolysis (FIG. 3).
  • the gap between the electrode and the membrane was kept 2.0 mm to reduce the cell resistance minimum.
  • a known constant current density (30 mA cm or 3.0 A) was applied across the electrodes using DC power supply.
  • the initial feed solution for anode compartment was 5.0 L solution containing 6.0 M H 2 S0 4 bubbled with S0 2 gas maintained at 1 bar (g) pressure using a S0 2 cylinder.
  • the present process serves as an improved acid/oxidative stability and assessed as a potential cation exchange membrane for practical application in electrodialysis, electrolysis or any other electrochemical processes employing cation exchange membrane as a separator.
  • This invention also encompasses that cation exchange membrane in harassed acidic/oxidative environment such as iodine-sulphur cycle (Bunsen process) or any other electro-membrane process.

Abstract

The present invention relates to acid and oxidative resistant homogenous cation exchange membrane having repeating unit of formula I and its method of preparation. This membrane is prepared by grafting of 2-acrylamido-2-methyl-1-propanesulfonic acid or any other sulfonic acid functionalized monomer with poly(vinylidene fluoride-co-hexafluoropropylene), in which silica precursor such as 3-mercaptopropyl trimethoxysilane is incorporated by acid catalysed sol-gel process to improve the functional group density in the membrane matrix. Sulfonated polymeric membrane showed improved acid and oxidative stability and assessed as a potential cation exchange membrane for practical application in electrodialysis, electrolysis or any other electrochemical processes employing cation exchange membrane as a separator. This invention also encompasses that cation exchange membrane useful in harassed acidic/oxidative environment such as iodine-sulphur cycle (Bunsen process) or any other electro-membrane process.

Description

ACID AND OXIDATIVE RESISTANT HOMOGENOUS CATION EXCHANGE MEMBRANE AND ITS METHOD OF PREPARATION THEREOF
FIELD OF THE INVENTION
The present invention relates to acid and oxidative resistant homogenous cation exchange membrane and its method of preparation by using 2-acrylamido-2-methyl-l-propanesulfonic acid grafted poly(vinylidene fluoride-co-hexafluoropropylene) or any other sulfonic acid functionalized monomer grafted with poly(vinylidene fluoride-co-hexafluoropropylene) with at least one polymer, in which (3 -mercaptopropyl)trimethoxy silane or any other sulphonic acid functionalized silica precursor is incorporated by acid catalysed sol-gel process to improve the functional group density in the membrane matrix. These acid and oxidative resistant cation exchange membranes have practical application in electrodialysis, electrolysis or any other electrochemical processes employing cation exchange membrane as a separator.
BACKGROUD OF THE INVENTION
Cation exchange membranes contain negatively charged acidic functional groups such as -SO3H, - PO3H2 etc fixed to the polymer matrix cation exchange membrane are critical components for diversified electro-membrane processes including: electrodialysis (water desalination, separation of inorganics from organic molecules, separation of specific inorganic ion, etc), electrolysis ( used as separator for chlor-alkali process, production of HI by Bunsen reaction of iodine-sulfur (I— S) process, electrochemical catalytic water splitting), electro-electrodialysis for in-situ ion exchange and ion substitution, electro-deionization for producing ultrapure water, and polymer electrolyte membrane for fuel cell applications.
Stability (thermal, mechanical and chemical) along with properties of cation exchange are important parameters, to assess their suitability for proposed applications. Properties of ion exchange membranes depend on the density of the polymer network, hydrophobic and hydrophilic properties of the membrane matrix, the distribution of the charge density, and membrane morphology. The most desired properties for ion-exchange membranes are:
• High permselectivity - an ion-exchange membrane should be highly permeable to counter-ions, but should be impermeable to co-ions. • Low electrical resistance - the permeability of an ion-exchange membrane for the counter-ions under the driving force of an electrical potential gradient should be as high as possible.
• Good mechanical and form stability - the membrane should be mechanically strong and should have a low degree of swelling or shrinking in transition from dilute to concentrated ionic solutions. · High chemical stability - the membrane should be stable over a pH range from 0 to 14 and in the presence of oxidizing agents.
Electrodialysis process using cation exchange membranes showed advantages over other membrane processes for allowing separation of targeted ions. In case scale formation or fouling on the membrane surfaces, unlikely to filtration membrane these can easily be avoided by acid wash of the cation-exchange membrane. Thus, cation exchange membranes should be capable of withstanding highly acidic solutions. Further, during electrolysis cation exchange membranes are placed between the electrodes (anode and cathode). The environment of electrode chambers especially anode chamber is oxidative in nature due to formation of free radicals.
Reference may be made to US7790837 wherein E.I. duPont DeNemours & Co (Dupont) under the trade name NAFION have disclosed perflurosulfonic acid membranes. This is used for elecrodialysis, electrolysis or fuel cell applications. Nafion® is a copolymer of tetrafluroethylene (TFE) and sulfonyl fluoride vinyl ether (PSEPVE) and displays good conductivity (~0. l S/cm), along with chemical and mechanical stability.
Reference may be made to US4324636 wherein commercial cation exchange membrane, supplied by Asahi Chemical Industry, based on either styrene-divinyl benzene copolymer or perflurocarboxilic acid polymeric structure (using high density polyethylene, polyvinylidene fluoride or polyvinylfluoride as plasticizer. During electrochemical processes (electrodialysis, electrolysis or fuel cell application) low fluid (mass) transfer and good conductivity of the cation exchange membrane are extremely desirable characteristics. Reference may be made to an article J. Membrane Science 2012, 409-410, 137 wherein it is disclosed that during I-S cycle (electrochemical water splitting by Bunsen reaction), an appropriate H2S04 concentration (14-20 N) is required. At a high H2SO4 concentration, the conductivity of Nafion(R) membrane deteriorates due to dehydration. Further, during fuel cell operation at high temperature, dehydration of Nafion membrane is a serious problem responsible for deterioration in membrane conductivity. Thus, to avoid these problems, it is quite urgent and significant to design highly conductive, acid, oxidative resistant cation exchange membrane.
References may be made to articles Desalination, 1988, 46, 327 and J. Membrane Science, 1999, 156, 61 wherein efforts were made to prepare poly sulfone and polyethersulfone based cation exchange membrane after sulfonation, because of their excellent chemical and thermal resistant nature.
Reference may be made to US4413106 wherein sulfonation procedure was described by the slurry method. Reference may be made to US4625000 wherein sulfonation of poly(arylene ether sulphone) Udel P- 1700 (PSU) and Vitrex PES 5200P (PES) was achieved on the aromatic ring system using chlorosulfonic acid as a sulfonating agent.
Reference may be made to ETS4508852 wherein chlorosulfonic acid showed disadvantage of side reactions. Reference may be made to ETS3709841 wherein sulfonation of polyether sulfones was successfully achieved by chlorosulfonic acid or oleum in an inert solvent (chlorinated hydrocarbons).
Reference may be made to ETS5013765 wherein sulfonation of aromatic polymer was achieved by sulfur trioxide in concentrated sulfuric acid, using a solvent. The disadvantages of using these solvents are, inter alia, toxicity and further removal. In the above described methods, sulfonation of poly sulfone was achieved by either chlorosulfonic acid or concentrated sulfuric acid. Prepared cation exchange membranes were often unstable in strong oxidative, acid, base and environment. Temperature and mechanical instabilities of these membranes limit their durability and practical applicability. Because of the inherent limitations of these membranes, and the entire complicated preparation procedures responsible for costlier and cumbersome ion exchange membrane preparation useful to electrodialysis and other electro-membrane processes.
References may be made to an article Journal of New Material Electrochemical System, 1998, 1, 47 wherein they have disclosed the preparation of fluorinated cation exchange membranes by the polymerization of monomers and introduction of cationic functional groups by further treatment. These membranes were found suitable for the chlor-alkali industries and fuel cell application.
References may be made to articles Progress Polymer Science, 2004, 29, 75; and 2005, 30, 644 wherein they have disclosed development of perfluorinated membranes by the co-polymerization of tetrafluoroethylene with the vinylether monomer by Dow Chemical Company. A perfluorinated ion exchange membrane has also been developed by the Asahi Glass Company and commercialized as “Flemion”.
Macromolecular modification of polymer offers an efficient route to modify wettability, morphology, and performance of the resultant blend or composite systems due to its simplicity of operation and cost. By inclusion of conducting polymer in non-conducting polymer matrix, hydrophilicity and conductivity of the resultant material can be improved.
Reference may be disclosed to an article J. Membrane Science, 2007, 294, 8 wherein they have disclosed good chemical, mechanical, and thermal stability of partially fluorinated polyvinylidene fluoride co-hexafluoropropylene (PVDF-co-HFP). Due to low cost, PVDF-co-HFP was used to fabricate the solid electrolyte or proton exchange membrane, after inclusion of acidic functionalities to impart the conductivity.
References may be made to the articles Fuel Cells 2006, 6, 331 and Eur. Polym. J. 2008, 44, 932 wherein they have disclosed chemical grafting of acidic functional group to PVDF-co-HFP. References (J. Power Sources 2003, 117, 14; J. Power Sources 2006, 154, 51) disclosed blending of PVDF-co-HFP with various hydrophilic conducting materials such as Nafion™ or hydrophilic inorganic particles.
The majority of PVDF-co-HFP based cation exchange membranes are based on continuous organic polymer matrices with sulfonic-acid, aromatic and/or acid-base functional groups or containing inorganic particles additives, such as zeolites and silica. Reference may be made to the article ECS Transactions, 2009, 25 (1), 1091 wherein they have disclosed sol-gel derived strategy for incorporation of -SO3H functionalized mesoporous S1O2 network in a hydrophobic fluorinated polymer (PVDF-co-HFP). Membranes were prepared by spray- coating, but mechanical instability of the membrane was serious problem in spite of good micro-structured network and conductivity.
Reference may be made to an article Solid State Ionics, 2001, 145, 141 wherein they have disclosed preparation of cation exchange membrane by dispersing silica and phosphoric acid in PVDF-co- HFP matrix. But high porosity and low conductivity of the membrane was problem.
Reference may be made to an article Applied Energy 2014, 118, 183 wherein they have disclosed preparation of cation exchange membrane by blending PVDF-co-HFP with partially sulfonated polyaniline to reduce the mass transfer across the membrane without any deterioration in membrane conductivity, during electrochemical applications. But, good interaction between two components and relatively low membrane conductivity were serious challenges.
Reference may be made to an article ACS Appl. Mater. Interfaces 2015, 7, 28524 wherein they have disclosed chemical grafting of 2-acrylamido-2-methyl-l-propanesulfonic acid with PVDF-co-HFP to prepare the cation exchange membrane by radical polymerization in presence of initiator. But, relatively low conductivity of reported membranes limit their application. Generally, cation exchange membranes were often unstable under strong oxidative, acid, and base environment, similar to operational conditions of fuel cell, membrane electrolysis or electrodialysis. Temperature and mechanical instabilities of these membranes also limit their durability and practical applicability. Because of the inherent limitations of these membranes, and the entire complicated preparation procedures responsible for costlier and cumbersome ion exchange membrane preparation useful to electrodialysis and other electro-membrane processes.
Reference may be made to an article J. Membrane Science, 2011, 380, 13 wherein they have disclosed sulphur-iodine (SI) thermochemical cycle is promising for hydrogen production water splitting using iodine and sulphur-dioxide as recycling agents and based Bunsen reaction. Membrane electrolysis was achieved using Nafion 117 cation exchange membrane as a separator. Aqueous solution of 10 N sulphuric acid saturated with dissolved S02 was used as anolyte solution.
Reference may be made to an article J. Membrane Science, 2001, 192, 193 wherein they have disclosed electrolysis of iodine with molality of ca. 10 mol/kg was examined in the presence of hydriodic acid using a commercial cation-exchange membrane as the separator at 80 °C. Effect of temperature on electro-electrodialysis of HT-I2-H20 mixture using Nafion 212 cation exchange membrane was also explored to investigate the temperature dependence of the membrane performance for increase in HI concentration (/. Membrane Science, 2012, 411- 412, 99). Huge water permeation was reported to be a serious problem to achieve the good current efficiency. Therefore, an urgent need exists for the development the stable cation exchange membranes coupled with excellent thermal, mechanical and acid (20 N H2SO4) stabilities with excellent electrochemical properties. Also, the membrane preparation procedure must involve minimum hazardous chemicals for achieving good level of functionalization (sulfonation) for cation exchange membrane useful to electrodialysis, electrolysis or fuel cell applications. SUMMARY OF THE INVENTION
1. The present invention relates to acid and oxidative resistant homogenous cation exchange membrane. The Process for the preparation of homogeneous cation exchange membrane having formula 1 as repeating unit comprising the following steps:
Figure imgf000007_0001
Formula 1 i. poly(vinylidene fluoride-co-hexafluoropropylene) and dimethyl acetamide are taken in a vessel;
ii. solution obtained in step (i) is charged with 0.5 molar NaOH solution until the solution becomes dark brown colour followed by continuous stirring for 2 hours;
iii. solution obtained in step (ii) is poured in water and dehydroflourinated polymer is separated out and dried; iv. dehydroflourinated polymer obtained in step (iii) and 2-acrylamido-2-methyl-l- propansulfonic acid are mixed in dimethylacetamide solvent in a vessel and kept at above temperature range 50° C in the presence of radical initiator azobisisobutyronitrile (AIBN) for more than 8 hours to obtain sulphonated dehydrofluorinated poly(vinylidene fluoride-co- hexafluoropropylene) ;
v. (3-mercaptopropyl) trimethoxy silane as silica precursor is added to sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) followed by casting the solution in a glass plate;
vi. cation exchange membrane obtained in step (v) is dried and dipped into hydrogen peroxide solution followed by washing with water.
In yet another embodiment of the present invention, wherein poly(vinylidene fluoride-co- hexafluoropropylene) and dimethyl acetamide are mixed in the ratio 1 : 10 (w/v).
In yet another embodiment of the present invention, wherein the ratio of sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) to silica precursor ranges from 15-22% (w/w).
In yet another embodiment of the present invention, wherein sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) copolymer moiety and silica precursor both are grafted by -SO3H group.
In yet another embodiment of the present invention, wherein the membrane has an ion exchange capacity of 1.30 to 1.40 meq/g.
In yet another embodiment of the present invention, wherein the membrane has conductivity of 4.0 to 4.25 xlO 2 S cm 1.
In yet another embodiment of the present invention, wherein membrane swelling ratio (under treatment of hot water at 60°C for 24 hours) ranges from 7 to 12%.
In yet another embodiment of the present invention, wherein membrane weight, ion exchange capacity and conductivity loss is less than 4% under highly acidic (10 M H2SO4 at 30°C for 120 hours) or harassed oxidative environment (3 ppm FeS04 + 3% H2O2 at 80°C for 3 h).
In yet another embodiment of the present invention, wherein acid and oxidative resistant cation exchange membrane prepared by chemical grafting of 2-acrylamido-2-methyl-l-propanesulfonic acid or any other functionalized monomer with poly(vinylidene fluoride-co-hexafluoropropylene). Further, density of sulphonic acid groups in the membrane forming material followed by oxidation of functional group by hydrogen peroxide.
In yet another embodiment of the present invention, wherein the co-polymerization of sulfonated copolymer of dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) (DHPVDF-co- HFP) and 2-acrylamido-2-methyl-l-propanesulfonic acid (AMPS) or any other sulfonic acid functionalized monomer by free radical copolymerization in the presence of initiator.
In yet another embodiment of the present invention, wherein dehydrofluorination of PVDF-co-HFP dissolved in dimethyl acetamide (DMAc) using saturated solution of NaOH in isopropanol to create the double bonds in PVDF-co-HFP responsible for chemical grafting. In yet another embodiment of the present invention, wherein sulphonated copolymer prepared by chemical grafting of AMPS or any other sulfonic acid functionalized monomer with DHPVDF-co- HFP by free radical copolymerization in the presence of initiator.
In yet another embodiment of the present invention, wherein improvement in functional group (sulphonic acid group) density in the polymer matrix by incorporating functionalized silica precursor (3-mercaptopropyl) trimethoxy silane or any other) by acid catalysed sol-gel process followed by oxidation of mercapto group in to sulphonic acid group by any oxidising agent (such as hydrogen peroxide).
In yet another embodiment of the present invention, wherein homogeneous dispersion of silica precursor within DHPVDF-co-HFP matrix by strong hydrogen bonding. In yet another embodiment of the present invention, wherein chemical grafting of 2-acrylamido-2- methyl-l-propanesulfonic acid or any other phosphonic (-PO3H2) acid functionalized monomer with DHPVDF-co-HFP co-polymer.
In yet another embodiment of the present invention, wherein silica precursor may be functionalized with -SO3H, or -PO3H2 or both groups for significantly improvement in membrane conductivity. In yet another embodiment of the present invention, wherein copolymer (DHPVDF-co-HFP) with following chemical structure was subjected to the functionalization by grafting of sulphonic acid functionalized monomer (for example AMPS):
Figure imgf000010_0001
In yet another embodiment of the present invention, wherein cation exchange membrane based on DHPVDF-co-HFE copolymer grafted with AMPS having the chemical structure:
Figure imgf000010_0002
In yet another embodiment of the present invention, wherein sulphonated DHPVDF-co-HFP copolymer moiety containing at least one -SO3H or -PO3H2 group, while other functional group were attached with silica precursor, as described in the following chemical structure:
Figure imgf000010_0003
In yet another embodiment of the present invention, wherein homogeneous cation exchange membrane made by sulphonated DHPVDF-co-HFP copolymer and functionalized silica precursor attached via strong hydrogen bonding.
In yet another embodiment of the present invention, wherein cation exchange membrane with high functional charge density exhibited reduced cell voltage and improved current efficiency during electrolysis because of grafted functional (-SO3H or -PO3H2) groups with fluorinated polymer. In yet another embodiment of the present invention, wherein cation exchange membrane according to the present invention showed high efficiency during electrolysis, electrodialysis, or other electro membrane applications, and superior stabilities (thermal and acid). Thus, the industrial significance of the cation exchange membrane of the present invention is extremely high. BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of reaction scheme for the preparation of acid and oxidative resistant cation exchange membrane.
FIG. 2 depicts electrodialysis process for desalination of brackish water.
FIG. 3 depicts Bunsen process (iodine-sulfur cycle) for preparing HI by membrane electrolysis. DETAILED DESCRIPTION OF THE INVENTION
This invention relates to the development of polymeric cation exchange membranes for electrolysis, separation of inorganic salts in aqueous media by electrodialysis and also useful for other electro membrane processes. The cation exchange membrane of this invention showed good stabilities (oxidative and acid), excellent conductivity, permselectivity, and other physicochemical properties such water content and ion exchange capacity, which are essential requirement for high performance during diversified electro-membrane applications in aqueous medium. The methods used to produce reported cation exchange membrane is quite simple and comparably less expensive, which contribute to the overall economy of the process using these ion exchange membranes. The invention also encompasses the use of the reported membrane for variety of electro-membrane applications.
In present invention, the experimental conditions adopted for the preparing cation exchange membrane include the introducing vinyl bond in PVDF-co-HFP by dehydro-fluorination under treatment with NaOH. In second step, free radical copolymerization of DHPVDF-co-HFP and AMPS was achieved in the presence of initiator 2,2-azobis(2-methylpropionitrile) (AIBN). Followed by, to the obtained viscous solution, desired content of silica precursor (3- mercaptopropyl) trimethoxysilane) was drop-wise added in presence of few drops of hydrochloric acid (HC1) to achieve acid catalysed sol-gel process. The degree of sulfonation i.e. the quotient of the total number of sulfonic acid in the polymer can be readily controlled by fine adjustment of AMPS concentration in DHPVDF-co-HFP matrix. In spite of high degree of AMPS grafting, the prepared sulphonated DHPVDF-co-HFP copolymer membrane showed restricted conductivity and other electrochemical properties, essential for a high performance cation exchange membrane. These properties were further improved by incorporating silica precursor in the membrane forming polymeric material, where mercapto groups were oxidized to sulphonic acid group by hydrogen peroxide. Reported cation exchange membrane showed homogenous nature because of hydrogen bonding between sulphonated DHPVDF-co-HFP copolymer and 3-mercaptopropyl) trimethoxy silane. The conditions used in the present invention to produce the cation exchange membrane are completely non-hazardous or not required any sophisticated equipment or facilities in compare with methodologies currently used. The production cost of cation exchange membrane in present case is considerably low.
In present invention, highly acidic and oxidative stable cation exchange membrane was prepared by co-polymeriztion DHPVDF-co-HFP and varied content of AMPS (20-38% w/w) using AIBN as free radical initiator. The process for the preparation of homogeneous cation exchange membrane having formula 1 as repeating unit comprising the following steps:
Figure imgf000012_0001
Formula 1 vii. poly(vinylidene fluoride-co-hexafluoropropylene) and dimethyl acetamide are taken in a vessel;
viii. solution obtained in step (i) is charged with 0.5 molar NaOH solution until the solution becomes dark brown colour followed by continuous stirring for 2 hours; ix. solution obtained in step (ii) is poured in water and dehydroflourinated polymer is separated out and dried;
x. dehydroflourinated polymer obtained in step (iii) and 2-acrylamido-2-methyl-l- propanesulfonic acid are mixed in dimethylacetamide solvent in a vessel and kept at above temperature range 50° C in the presence of radical initiator azobisisobutyronitrille (AIBN) for more than 8 hours to obtain sulphonated dehydrofluorinated poly(vinylidene fluoride-co- hexafluoropropylene) ;
xi. (3-mercaptopropyl) trimethoxysilane as silica precursor is added to sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) followed by casting the solution in a glass plate;
xii. cation exchange membrane obtained in step (v) is dried and dipped into hydrogen peroxide solution followed by washing with water.
To the sulphonated DHPVDF-co-HFP copolymer solution in DMAc (15-17%, w/v), silica precursor (20%. w/w of DHPVDF-co-HFP) was added along with 2 drops of HC1 under constant stirring at room temperature (30 °C), to achieve the sol-gel process. Resultant polymer was transformed in to thin film of desired thickness and dried under vacuum at 60°C for 24 hours to achieve the membrane. Obtained membrane was dipped in H202 (30% aqueous) for 24 hours. To oxidize the mercapto groups in to sulphonic acid group. Such a reaction scheme for the preparation of cation exchange membrane is illustrated in FIG. 1.
In a preferred embodiment, reported process for the preparation of stable cation exchange membrane represents a novel method with several advantages over the previously reported method of lower cost, without any use of hazardous chemicals. Further, the strategy adopted to increase the functional group (sulphonic acid) molality in the membrane matrix by incorporating silica precursor in the sulphonated DHPVDF-co-HFP copolymer matrix, responsible for significantly improvement in membrane performance is also a novel step.
In the present invention, it is reported polymer thin film cation exchange membrane without any fabric (woven) support, since the reported polymer solution possessed the thin film forming capacity and resultant membrane showed high mechanical stability and burst strength. Further, reported cation exchange membrane can be prepared with non-woven support. Additionally, unlike radiation grafting techniques, the present chemical grafting between DHPVDF-co-HFP copolymer and AMPS was achieve at 60 °C in the presence of free radical initiator, and thus membrane production technique does not involve any high-energy radiation source.
Diversified electro-membrane processes utilizing cation exchange membrane, are expensive and require low electricity consumption due to its high conductive and stable nature even in strong acidic or oxidative environment. Further, prepared cation exchange membrane may be widely used for electrolysis in aqueous medium, different electrochemical membrane reactors for any desired chemical synthesis, electro-deionization process for producing ultra-pure water, electrodialysis for the removal of inorganic electrolyte such as desalination of sea and brackish water, separation and removal of metal ions from the industrial effluent, de-acidification of fruit juice of dashing and sugar cane juice even at high temperature (70-80 °C) for better quality of sugar, improved yield and reduced molasses production, purifications and amino acids, vitamins, vaccines and other biochemical purification and in situ ion substitution and biomolecules obtained from fermentation broth during down-stream processing. Further, electrolysis in aqueous medium (water splitting), chlor-alkali process and fuel cell processes based on selective proton-transport, are also preferred electro-membrane processes for the utilization of reported cation exchange membrane.
Generally, optimum water content in the membrane phase govern hydration of functional groups (sulphonic acid group in this case) and provide the necessary water molecules surrounded the functional group for the formation hydrophilic ion conducting channels, is a critical parameter for the high performance cation exchange membrane. Further, high water content in the membrane phase is also responsible for the membrane dimensional instability. Thus, to achieve the stable high performance cation exchange membrane, an optimized water content (20-30%, w/w) is essential. In the present invention, care was rendered for the proper balancing of hydrophobic and hydrophilic segments in the membrane forming martial, which will enables the desired water content in the membrane matrix.
The ion exchange capacity (IEC) represents a measure of the hydrophilic character or concentration density of ionic groups, can be given by meq./g or dry membrane. 1 meq./g means that per gram of polymer responsible for the 1 mmol exchange of ions or protons. IEC may again be determined by acid-base titration. Density of exchangeable groups in the membrane matrix is also an important factor in membrane quality and controls the membrane conductivity and thus performance. However, membrane durability depends upon the environmental conditions and polymer backbone nature. All these factors were judicially considered during membrane synthesis. One skilled in the art based upon prior knowledge and description provided above should easily determine the membrane preparation procedure and parameters with specifically desired cation exchange membrane.
The membrane permselectivity is a measure of the characteristic difference in the membrane permeability for counter-ions and co-ions. Counter-ion transport number across the membrane was estimated by membrane potential measurement for the estimation of membrane permselectivity. Membrane conductivity of cation and anion exchange membrane was determined in equilibration with 0.10M NaCl solution using a potentiostat/galvanostat impedance analyzer. The membrane resistance was determined in terms of membrane conductivity by Nyquist plots by Fit and Simulation method considering membrane thickness and area.
For these wide applications, the most desired properties required for successful cation exchange membranes are: high permselectivity ( close to unity)— cation exchange membrane should be highly permeable to cation, but should be impermeable to anion; high membrane conductivity (> 4.0x1 O 2 S cm 1)— cation exchange membrane should have high membrane conductivity and thus there will be less potential drop during electrodialysis or electro-membrane processes; good mechanical stability— the membrane should be mechanically strong and should have a low degree of swelling or shrinking in transition from dilute to concentrated ionic solutions; high chemical stability— the membrane should be stable in strong acidic or alkaline environment even in presence of oxidizing agents. Many previous membranes have either exhibited poor stabilities (thermal, chemical and mechanical) or have obtained it at expense of electrochemical properties. For, example cross-linking of the membrane film improves thermal and mechanical properties, but associated with deterioration in functional groups (acidic or basic) concentration thus electrochemical properties. Mechanical strength can be further increased by supporting the membrane by woven fabric (PVC, PE, glass or Teflon). But woven fabric contributes towards the non-conduction phase of the membrane matrix and reduces membrane physicochemical and electrochemical properties. Although only preferred embodiments of the invention are specifically described above and in the following examples, it will be appreciated that compositions and other variations of the invention are possible without departing from the spirit and intended scope of the invention.
EXAMPLES Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.
Examples 1-3
General procedure for the preparation of Acid and Oxidative Resistant Silica Composite Cation Exchange membrane by Molecular Modification of Poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-co-HFP)
10 gram of PVDF-co-HFP was dissolved in 150 ml of dimethylacetamide under constant stirring, and saturated solution NaOH (0.50 M) in isopropanol (10 ml) was added to the mixture drop-wise with vigorous stirring at 30 °C for 30 minutes. The colorless resultant solution was turned to light brown. Dehydrofluorinated PVDF-co-HFP (DPVDF-co-HFP) was precipitated in water, filtered, washed 3-4 times with deionized water, and dried in vacuum at 60 °C. In second step, co polymerization of DPVDF-co-HFP and 2-acrylamido-2-methyl-l-propansulfonic acid (AMPS) was achieved in the presence of 2,2'-azobis(2-methylpropionitrile) (AIBN) initiator. In a typical procedure, DPVDF-co-HFP (31 mmol) was dissolved in 50 ml of dimethylacetamide, followed by AMPS (11 mmol) was added to the reaction vessel. To initiate the copolymerization reaction, 1.0 ml AIBN was added to the reaction mixture in nitrogen atmosphere at 60 °C under stirred conditions for 8 hours, until the formation of viscous copolymer (functionalized PVDF-co-HFP). Afterwards, different compositions of (3-mercaptopropyl) trimethoxysilane (MPS) (according to Tab. 1) was added to the reaction mixture in presence of few drops of hydrochloric acid under continuous stirred conditions. Resultant viscous solution was poured onto a glass plate, and a film of constant thickness was formed by means of a doctor-blade. Then, the solvent was evaporated at 80-90°C in an oven under air-circulation. After the evaporation of solvent, glass plate with the polymer film was put into deionized water and membrane was peeled off from the glass plate. Obtained membrane was washed with deionized water and dipped in hydrogen peroxide solution (30%, v/v) for 24 hours at 30°C. Resultant membrane was post-treated with HC1 (1.0 M), and subsequently in deionized water. Their properties have been characterize and included in Table 1.
Table-1
Figure imgf000017_0001
* Membrane turns brittle in nature Examples 4-6
General procedure for Acid Stability of Cation Exchange membrane
For electro-separation in high acidic medium by electrodialysis or electrochemical Bunsen process (iodine-sulfur cycle) for hydrogen production by water splitting, separating cation exchange membrane is operated high acidic medium, thus its acid stability is an urgent characteristics. Acid stability of functionalized PVDF-silica cation exchange membrane was assessed by treating the membrane in H2SO4 (10 M) environment at 30°C for 120 hours and acid stability was confirmed by recording percentage loss in membrane weight, ion exchange capacity, and conductivity data after the acid treatment and included in Table 2 for cation exchange membranes with different silica content.
Table-2
Percentage loss due to acid treatment in membrane weight (W ) , ion exchange capacity (IEC- loss), and membrane conductivity ( K "SS ) for cation exchange membranes with different silica content
Example Silica content in WiOSs IE C ioss
Figure imgf000018_0001
loss
functionalized PVDF-co- (meq/g)
(%)
HFP (%,w/w) (x lO 2 S cm 1)
4 15% (11.8 g functionalized 1.5 1.9 3.6
PVDF+2.07 g MPS)
5 20% (11.8 g functionalized 1.4 2.3 3.4
PVDF+2.95 g MPS)
6 25% (11.8 g functionalized 1.6 2.2 3.9
PVDF+3.93 g MPS)
Examples 7-9
General procedure for Oxidative Stability of Cation Exchange membrane
Under strong oxidative water splitting environment, membrane degradation by oxy active radicals is serious problem. Thus, oxidative stability of cation exchange membranes was assessed under simulated oxidative conditions (3 ppm FeS04 + 3% H2O2 at 80 °C for 3 h). The oxidative stability for different functionalized PVDF-silica cation exchange membranes was confirmed by recording percentage loss in membrane weight, ion exchange capacity, and conductivity data after the treatment and included in Table 3 for cation exchange membranes with different silica content. Table-3
Percentage loss due to oxidative treatment in membrane weight (W;0ii) , ion exchange capacity ( lECioss ), and membrane conductivity ( A "' ) for cation exchange membranes with different silica content
Example Silica content in functionalized W loss IECioss (meq/g)
Figure imgf000019_0001
PVDF-co-HFP (%,w/w)
Figure imgf000019_0002
(xlO 2 S cm 1)
7 15% (11.8 g functionalized 1.9 2.7 3.9
PVDF+2.07 g MPS)
8 20% (11.8 g functionalized 2.5 2.4 4.1
PVDF+2.95 g MPS)
9 25% (11.8 g functionalized 2.8 2.8 3.9
PVDF+3.93 g MPS)
Example 10 Desalination of Brackish Water Using Cation Exchange Membrane by Electrodialysis
Brackish water may or may not be chlorine dosed is provided as drinking water (total dissolved solid: < 500 ppm) after desalination by electrodialysis. The brackish water may obtained either from earth surface or from ground, passed through chlorine-dosing or other treatment and to the electrodialysis desalination unit to remove excess of salt. An electrodialysis (ED) unit containing 10 cell pairs of cation exchange membrane (functionalized PVDF: 20% (11.8 g functionalized) + silica (MPS) 2.95 g) and anion-exchange membrane (AEM) (Neosepta AMX supplied by Tokuyama Soda Co. Ltd., Japan); structure properties: anion poly(styrene)/divinyl benzene; ion exchange capacity: 1.5 meq/g; water uptake: 26%, area resistance: 3.2 W cm2) with 100 cm2 effective membrane area was used desalination of brackish water. There was four compartments in ED unit, namely two electrode wash (EW), concentrated stream (CS) and desalinated stream (DS) (FIG. 2). Precious metal oxides (titanium-ruthenium- platinum) coated Ti02 sheets of 6.0 mm thickness, obtained from Titanium Tantalum Products (TITAN, Chennai, India) were used as electrodes fitted in the ED unit. Flow arrangement of each compartments was monitored by parallel-cum-series pattern. Na2S04 solution (0.10M) was recirculated in both interconnected EW compartments. Initially, brackish water (total dissolved solid (TDS): 2000-5000 ppm) was fed into DS (flow rate: 2.0 LPH) and CS ((flow rate: 0.6 LPH), both using peristaltic pumps. ED experiments were performed under influence of constant voltage (15.0 V) using a direct current power supply and resultant current was recoded. With progress of experiment TDS of DS was reduced to < 500 ppm (drinking water as per World’s Health Organization), while TDS of CS was significantly increased. The recovery of desalinated water was about 65%.
Example 11 The iodine-sulfur (I-S) process is a promising thermochemical process due to C02-free hydrogen production. Two- compartment membrane electrolysis cell separated by cation exchange membrane (functionalized PVDF: 20% (11.8 g functionalized) + silica (MPS) 2.95 g) was assessed for the Bunsen process.
Teflon made two- compartments cell separated by cation exchange membrane (effective area: 1000 cm2) and fitted with two electrodes (precious metal oxide coated titanium sheets (Ti02 sheet coated with a triple precious metal oxide (titanium-ruthenium-platinum), of 6.0 pm thickness) was used for membrane electrolysis (FIG. 3). The gap between the electrode and the membrane was kept 2.0 mm to reduce the cell resistance minimum. A known constant current density (30 mA cm or 3.0 A) was applied across the electrodes using DC power supply. The initial feed solution for anode compartment was 5.0 L solution containing 6.0 M H2S04 bubbled with S02 gas maintained at 1 bar (g) pressure using a S02 cylinder. While, initial feed solution for cathode compartment was 5.0 L solution containing HI (3.0 M) + I2 (0.78 M). Both, anode and cathode compartments were continuously recirculated with respective feed solution using peristaltic pumps. Under influence of 30 mA cm applied current density (corresponding voltage: 1.0 V), the concentration H2S04 in the cathode compartment and concentration of HI in the anode compartment was increased with the time. Current efficiency close to 100%, and about 328.05 kJ mol 1-H2 electrical energy consumption was observed for the electrochemical Bunsen process.
ADVANTAGES OF THE PRESENT INVENTION The various advantages of the present process are given below.
1. The present process serves as an improved acid/oxidative stability and assessed as a potential cation exchange membrane for practical application in electrodialysis, electrolysis or any other electrochemical processes employing cation exchange membrane as a separator.
2. This invention also encompasses that cation exchange membrane in harassed acidic/oxidative environment such as iodine-sulphur cycle (Bunsen process) or any other electro-membrane process.

Claims

We Claim:
1. A homogeneous cation exchange membrane having following chemical structure (formula 1) as repeating unit:
Figure imgf000022_0001
Formula 1
2. A process for the preparation of a homogeneous cation exchange membrane having Formula 1 as claimed in claim 1 comprising the following steps:
i. poly(vinylidene fluoride-co-hexafluoropropylene) and dimethyl acetamide are taken in a vessel;
ii. solution obtained in step (i) is charged with 0.5 molar NaOH solution until the solution becomes dark brown colour followed by continuous stirring for 2 hours;
iii. solution obtained in step (ii) is poured in water and dehydroflourinated polymer is separated out and dried;
iv. dehydroflourinated polymer obtained in step (iii) and 2-acrylamido-2-methyl-l- propanesulfonic acid are mixed in dimethylacetamide solvent in a vessel and kept at above temperature range 50° C in the presence of radical initiator azobisisobutyronitrile (AIBN) for more than 8 hours to obtain sulphonated dehydrofluorinated poly(vinylidene fluoride-co- hexafluoropropylene) ;
v. (3-mercaptopropyl) trimethoxy silane as silica precursor is added to sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) followed by casting the solution in a glass plate; vi. cation exchange membrane obtained in step (v) is dried and dipped into hydrogen peroxide solution followed by washing with water.
3. The process as claimed in claim 2, wherein poly(vinylidene fluoride-co-hexafluoropropylene) and dimethyl acetamide are mixed in the ratio 1 : 10 (w/v).
4. The process as claimed in claim 2, wherein the ratio of sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) to silica precursor ranges from 15-22% (w/w).
5. The process as claimed in claim 4, wherein sulphonated dehydrofluorinated poly(vinylidene fluoride-co-hexafluoropropylene) copolymer moiety and silica precursor both are grafted by -SO3H group.
6. The cation exchange membrane as claimed in claim 1, wherein the membrane has an ion exchange capacity of 1.30 to 1.40 meq/g.
7. The cation exchange membrane as claimed in claim 1, wherein the membrane has conductivity of 4.0 to 4.25xl0 2 S cm 1.
8. The cation exchange membrane as claimed in claim 1, wherein membrane swelling ratio ranges from 7 tol2% under treatment of hot water at 60°C for 24 hours.
9. The cation exchange membrane as claimed in claim 1 , wherein membrane weight, ion exchange capacity and conductivity loss is less than 4% under highly acidic (10 M H2SO4 at 30°C for 120 hours) or harassed oxidative environment (3 ppm FeS04 + 3% H2O2 at 80°C for 3 h).
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CN111871071A (en) * 2020-06-17 2020-11-03 吉祥三宝高科纺织有限公司 High-temperature-resistant and corrosion-resistant microporous membrane material for processing mask filter layer
CN114914508A (en) * 2022-05-13 2022-08-16 宿迁时代储能科技有限公司 Application of porous membrane in neutral water system organic flow battery

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