Ultra dispersed particles of Fe(III) compounds in the strongly basic crosslinked ionic polymer-precursors for new sorbents and catalysts
Highlights
► Fe3+ containing particles in strongly basic cross-linked polymer. ► Modifying the morphology and sizes particles. ► The metal containing polymer is stable up to 120 °C.
Introduction
The cross linked ionic polymers containing strongly basic functional groups are largely used in different branches of science and industry, especially in water treatment at thermo and nuclear power station. The mean property of such type of polymers is their ability to adsorb anions from solutions. The sorption of inorganic anions on strongly basic cross linked ionic polymers is not a selective process because is conditioned by Coulomb's electrostatic interactions. But, the modern technologies of purification of various categories of fluids and gases, concentration and separation of substances required new selective sorbents and catalysts.
The strongly basic crosslinked ionic polymers theoretically are unable to adsorb metallic cations because they do not contain in their matrix negatively charged or electron donor atoms. However in our previous study [1], [2], [3], [4], it was shown that strongly basic crosslinked ionic polymers in certain conditions are able to adsorb metallic cations from M2(SO4)3 solutions, where M = Fe3+,Cr3+or Al3+. From MCl3 or M(NO3)3 solutions sorption of cations do not takes place. The sorption of metallic cations from sulphate solutions takes place through the formation in the polymer phase of the jarosite(alunite) mineral type compounds: R4N[M3(OH)6(SO4)2] and H3O[M3(OH)6(SO4)2 [5], where R4N+ is a functional group of the polymer.
The jarosite mineral type compounds are formed as layers of 3 or 6 octahedral cycles [6]. The OH– groups are located in the equatorial plane, forming a bridge between metal ions and SO42− groups are located in axial position, each coordinate 3 metal ions of 3 octahedra. Between the jarosite polymer layers there are mobile R4N+, H3O+ and other cations retained by Coulomb's electrostatic interactions. The jarosite mineral type compounds in the polymers phase change essentially their physical–chemical properties. The R4N+ and H3O+ ions from jarosite-type compounds can be exchanged with different cations and the anions SO42− – with different anions or molecules capable to form coordination bonds with the central metal ions.
On heating in water (t ≥ 80 °C) synthetic Na[Fe3(OH)6(SO4)2] is converted into α-FeOOH [7]. But jarosite type compounds in the polymer phase on boiling in water are converted into β-FeOOH ultradispersed particles in a superparamagnetic state [5].
The synthesis and transformation of metallic compounds in the polymer phase could expand the use of crosslinked ionic polymers, particularly in obtaining selective sorbents [8], [9], catalysis [10], modeling of biochemical processes [11] and as models with peculiar magnetic properties [12], in redox processes. The strongly basic crosslinked ionic polymers containing Fe(III) compounds are selective for sorption of NCS−, NCO−, CN− and other anions and molecules [9]. Selective sorption of ions or molecules takes place through the ligand exchange of SO42− groups of jarosite type compounds.
In the previous work [5], it was shown that on heating in air up to 200 °C of strongly basic crosskicked ionic polymer containing a little amount of jarosite-type compounds (about 12 mg Fe/g), in the polymer phase take place redox processes with forming of stable radicals and Fe2+ ions. To identify the nature of Fe(III)-containing compounds in the polymer phase it was necessary to make many “Fe3+ sorption – heating in water” cycles. This requires an additional loss of reagents, energy and time.
For obtaining of effective and selective sorbents and catalysts the polymers must contain much more amount of metallic compounds in their phase. It is also necessary to know and have possibility to modify the morphology and composition of structural units of ultra fine particles, their distribution on the surface and in the volume of polymer granule, method to modify particles sizes. To apply Fe(III)-containing polymers in catalysis, in liquids or gases purification also it is necessary to know their thermal behavior. These are the main objectives of this investigation.
Section snippets
Experimental procedures
The commercial strongly basic anion exchanger AV-17 in Cl− form (AV-17(Cl)) has been used. The exchanger is gel-type crosslinked polystyrene–divinylbenzene polymer with –N+(CH3)3 functional groups. Its full anion-exchange capacity is 3.5–4.0 mg of equiv g−1 [13].
The process of obtaining of the jarosite-containing polymer takes place according to Ref. [8]. A 5 g sample of the dried AV-17(Cl) was put in contact with 0.5 L of solution containing 7 g Fe2(SO4)3 L−1 at 50 ± 1 °C for 10 h. The pH of solution
Behavior of the Fe(III) containing compounds in the polymer phase on heating in water
In the previous investigation [5], the Fe(III)-containing cations sorption on AV-17(Cl) took place at the room temperature and iron content in the polymer phase has constituted only about 12 mg Fe g−1. In the present investigation the samples of the AV-17 polymer contained much higher amount of Fe (Table 1, Table 2).
The Mössbauer spectra of the AV-17 polymer samples after contacting with Fe2(SO4)3 solution are doublets both at 300 and 80 K. The isomeric shift of these spectra is 0.70 and 0.83 mm s−1
Conclusions
The crosslinked ionic polymers containing strongly basic functional groups adsorb Fe(III)-containing cations from sulphate solution through the formation in the polymer phase of high dispersed particles of jarosite mineral type compounds. Using different procedures the particles sizes and morphology, their repartition on the surface and in the volume of the polymer granule, the composition of the “polymer-inorganic compound” structural units may be modified that is important in obtaining
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2013, Thermochimica ActaCitation Excerpt :Strongly basic anion exchangers do not contain negatively charged or electron donor atoms in their matrix and therefore, theoretically, they cannot interact with metallic cations. However, in certain conditions, these polymers may interact with cations in M2(SO4)3 solutions, where M is Fe3+, Al3+ or Cr3+ ion [1–4]. The interaction of the polymer with the metallic cations results in the formation of the jarosite (alunite) mineral type compounds dispersed within the polymeric phase: R4N[M3(OH)6(SO4)2] and H3O[M3(OH)6(SO4)2], where R4N+ are functional groups of the polymer [1,2].
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