Synthesis of Bipolar Membrane Using Pyridine Functionalized Anion Exchange Layer

Synthesis of Bipolar Membrane using
Pyridine Functionalized Anion Exchange Layer

T. Jeevananda, Kyeong-Ho Yeon, Seung-Hyeon Moon, South Korea

Journal of Membrane Science 283 (2006) 201–208

1.INTRODUCTION/ABSTRACT

The main objective of the present study is to prepare inexpensive Bipolar Membrane(BPM) exhibiting low electrical resistance, high water dissociation capacity with stable anion exchange functional group. In this study an Anion exchange membrane (AEM) with stable aromatic pyridine as functional group is prepared. Then BPM is prepared by a monomer paste method utilizing cation exchange layer and poly(NBR-DVB-PVC-Py) anion exchange layer is formed. To enhance the water dissociation efficiency,

2.EXPERIMENT

2.1. Materials

Polyvinylchloride (PVC) and Divinyl benzene (DVB) were used without further purification. Benzoyl peroxide (BPO) was purified by recrystallizing in a methanol/water mixture. Analytical grade pyridine (Py) was used to introduce quaternary pyridinium group to the base membrane. Acrylonitrile butadiene rubber (NBR) was added to monomer paste. PVC powder was ground using mortar and pestle and screened through 100 m standard testing sieve

MECHANISM/PROCESS

1. ANION EXCHANGE MEMBRANE:

• A paste is created consisting of monomers of GMA, DVB, PE/PVC, NBR and BPO and mixed homogeneously for around 24hrs and then casted on the PET reinforcing net/sheet.

• Composition:

            GMA/DVB = 99.5/0.5 wt.%

            NBR/paste = 1/6 as weight ratio

            BPO = 0.04 parts

            PE or PVC powder = 3 wt.%

• After casting the monomer paste it is placed between two glass plates and sealed with cloth tape to prevent evaporation of monomers during polymerization. Polymerization is carried out at 80 ◦C for 4 h.

• After drying for 24hrs (room temp), the base membranes are treated with 0.5 mol dm-3 pyridine aqueous solution to introduce quaternary pyridinium group for 2 h at 50 ◦C. Then washing is carried out.

1. BIPOLAR MEMBRANE

• Initially, the surface of CM1 membrane is slightly roughened with fine sandpaper to increase the contact area and thereby, the strength of the bonding. To immobilize metal hydroxide on one side of the CM1 membrane, the other side is protected by polycarbonate sheet.
• Then the membrane was successively immersed in 1 or 5% (w/v) of iron(III) chloride solution at room temperature for 24 h and in 1N NaOH solution at 600C for 1 h to convert iron salt to its hydroxide form.
• The optimized monomer paste (GMA-DVB-NBR-PVC) is casted on CM1 membrane, fixed on a glass plate, and then another glass plate was placed on the top. The glass plates are then sealed with a cloth tape to prevent any leakage during copolymerization.
• The monomers are copolymerized at 800C for 4 h. Then the
  prepared membranes are immersed in a 0.5 mol dm-3 pyridine
  aqueous solution to introduce the quaternary pyridinium groups.
  During the introduction of functional group CEL is protected
  by polycarbonate sheet.

PROPERTIES/ADVANTAGES

• Anion exchange and bipolar membranes containing pyridinium anion exchange functional groups showing reasonable flexibility. The prepared membranes are free from voids and the thickness is in the range (110–140) micro m (AEM) and (220-240) micro m(BPM).
• More alcohol and ion exchangeable pyridinium groups in the base membrane leads to a higher degree of swelling (48.6%), because alcohol groups readily attract water molecules through hydrogen bonding. With the addition of PE or PVC powder the WSR of the base membrane is reduced due to hydrophobic nature of these polymers.
• The electric resistances (2.5-2.76 Ω cm2) of the prepared membranes are lower than that of a commercial membrane AMX. Ion exchange capacity and transport number of the prepared AEMs are comparable to those
  of a commercial membrane AM1.
• Among the prepared membranes, membrane with PVC34 shows
  lower R3rd/R1st and ∆V values than the other prepared and commercial membranes. Also it shows higher LCD (35 Am-2) than the other membranes.
• Electroconvection effect on the prepared membranes containing pyridinium groups were diminished because it exhibits relatively weaker
  positive charge intensity than quaternary aliphatic ammonium group due to the nature of the aromatic resonance stability.
• With the immobilization of iron(III) hydroxide in the intermediate layer, water dissociation flux is increased and is comparable to that of commercial membrane.
• Low cost of pyridinium functionalized bipolar membranes is economically favourable.
• The aliphatic quaternary ammonium group degrades to give tertiary amine group on the cathode side of the membrane surface under an electric field. The unshared electron pairs of the tertiary amine group attack the hydrogen atoms of the pre-polarized water molecules, because the tertiary amine is an H-affinity group, while the quaternary ammonium group is an OH-affinity group.

• Limitation: The aromatic pyridinium groups are thermally and chemically stable due to their resonance structure, avoiding the decomposition of quaternary ammonium to tertiary amine groups. But the reaction of metal species with the quaternary pyridinium group of the AEL may not be suppressed completely. The reaction model implies that in the presence of strong electric field, the immobilized metal species in the BPM interface as a hydroxide will react with the water molecules and quaternary pyridinium groups.

• Remedy 1: In my opinion and based upon my study till now, the problems of swelling and effect of alkali or metal salts on pyridinium groups can be minimized if we use Sepiolite/Palygorskite with FeCl3 as these clays have crystalline structure and anti-swelling property and being insensitive to salts. Good binding (adhesive) property and catalytic property (high surface area) and wide pH range. Chemically inactive and provide protection for the loss of Fe.
• Remedy 2: Instead of using GMA, we can use styrene (with different compositions) as it has no epoxy groups and thus water swelling ratio can be minimised. Hence, use of PVC is also prevented as hydrophobic agent.

We can use chlorinated PE (low cost) blended in CEM or AEM (0.1-10% by wt.) to enhance adhesion properties thus reducing the voltage (high softening point) and it can also reduce the tendency of peeling of membranes under extreme conditions. It exhibits a high degree of compatibility to various polar solvents and also exhibits a high degree of affinity to the monomer component.

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