Economical and environmentally friendly synthesis of porous cation-exchange resins
Strongly acidic cation-exchange resins are widely used for water softening, water demineralization, metal separation, and acid catalysis. Such resins are often made by incorporating functional groups onto porous styrene-divinylbenzene (St-DVB) base copolymer beads. The copolymer beads are made porous by diluting their constituent monomers with some inert organic liquid at the time of copolymerization. Acetone is then used to wash the beads and is usually employed for this purpose because it is miscible with water and the organic compounds. And being highly volatile, acetone itself is easier than water to remove from the copolymer by thermal treatment. However, serious drawbacks of acetone use include its high cost and toxicity. Here, we show that the acetone washing step can be eliminated from the synthesis of strongly acidic cation-exchange resins. We also describe a quick, inexpensive, and accurate method for monitoring the porosity of the copolymers when making tailored products.
Porous St-DVB and other related copolymer beads are produced by polymerizing droplets of a mixture of monomers diluted with some inert organic liquid (diluent) suspended in water (see Figure 1).1,2 The diluent is removed at the completion of the polymerization, leaving behind a porous copolymer. After the polymerization reaction, the copolymer beads are filtered out and washed. The filtered beads are treated with hot water to remove the suspending agents, such as gum arabic or gelatin, on their surface. They are then treated with acetone, which extracts the diluents and oligomers/homopolymers from the pores. This is the most expensive step, as acetone accounts for ~80% of the cost of chemicals involved in the synthesis of the copolymers. Further, being a toxic volatile organic compound, acetone poses risk to the workers and the environment.
The macroporosity characteristics of the copolymer—pore volume, surface area, and pore size distribution in the dry state—are critically important for synthesis of tailored products. These parameters are usually analyzed using nitrogen sorption or mercury intrusion methods, both of which require expensive equipment, long analysis time, and (in the latter case) handling of toxic mercury. Bulk density, defined as the mass of many copolymer beads divided by the total volume they occupy, can be measured using a graduated cylinder and a balance. Since bulk density varies proportionately with the variation in macroporosity, they can be correlated statistically to estimate one value from the other.
The bulk density (d) is related to the pore volume (V) and surface area (A) in the dry state through the following statistical relationships:3, 43, 5 4-vinylpyridine– divinylbenzene,3,6 glycidylmethacrylate–ethyleneglycoldimethacrylate,3 4-vinylpyridine–ethylene glycoldimethacrylate,3 n-vinylcarbazole– divinylbenzene,3 methylmethacrylate–divinylbenzene,7 methylmeth- acrylate–ethyleneglycol dimethacrylate.8
Our study shows that washing of St-DVB beads with acetone is not necessary for their conversion to strongly acidic cation-exchange resins, provided a suitable hydrocarbon like n-heptane or toluene is employed as a diluent.9 We show this by comparing the resins obtained from acetone-washed copolymers with those from unwashed copolymers. They have identical ion-exchange capacities for up to 10 cycles of resin loading/regeneration, UV spectra of the effluents of the resins loading/re-generation, and Fourier transform IR spectra of the resins. When the study was later repeated using xylene as a diluent, similar results were observed (see Figures 2 and 3).
The similar results can be attributed to the diluents n-heptane, toluene,9 and xylene (in our most recent work, as yet unpublished), which remain in the polymerizing droplets during the polymerization reaction carried out at 80°C. They distill out during the copolymer curing at 98°C. Washing the copolymers with hot water removes gum arabic, gelatin, and any residual diluent from the surface of the copolymers. Oligomers and homopolymers in the pores are sulfonated just like the copolymer. As the sulfonating agent is usually used in great excess of the stoichiometric proportions, there is no adverse effect of the homopolymers on the sulfonation of the copolymer. The sulfonated
homopolymers are extracted by water from the resins just like the homopolymers from the copolymers are extracted with acetone.
We conclude that the bulk density of porous copolymers can be used to estimate pore volume, surface area, and pore size distribution in the dry state in the porous copolymers. This is a simple, fast, economical, and accurate alternative to the mercury penetration method for macroporosity characterization. Furthermore, porous St-DVB copolymers synthesized using hydrocarbon diluents can be sulfonated without washing with acetone with no adverse effects on the ion-exchange capacity or other characteristics of the acidic cation-exchange resins. Elimination of the acetone washing step significantly reduces the cost of the chemicals, health risks to the workers, and environmental pollution. Further studies using a broad range of diluents—including other hydrocarbons, ketones, esters, and alcohols—to synthesize porous St-DVB copolymers and their sulfonation without acetone washing are in progress and we will report these findings in the near future.
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