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Synthesis and Properties of Metal-Substituted Aluminophosphates and Recrystallizable Metal-Organic Frameworks

  • Author(s): Citrak, Susan
  • Advisor(s): Oliver, Scott
  • Zhang, Jin
  • et al.
Creative Commons 'BY' version 4.0 license

Two projects are presented within: ionothermal synthesis of metal-substituted aluminophosphate molecular sieves for use in catalysis, and the solubility properties of silver-bipyridine coordination polymers and its effect on ion exchange kinetics and thermal properties.

Metal-substituted, nanoporous aluminophosphates (MAPOs) have been extensively studied for catalytic applications since their invention by chemists at Union Carbide in 1982. Typically MAPOs are synthesized hydrothermally, which introduces risk of pressure build-up during heating. Recently, ionic liquids (ILs) have been used for the synthesis of aluminophosphates (AlPO4-n) and MAPOs, termed ionothermal synthesis. Ionothermal synthesis has a number of advantages over hydrothermal synthesis: (i) ILs have vanishingly low vapor pressure, reducing the pressure hazard associated with hydrothermal synthesis; (ii) the IL acts as both the solvent and the structure-directing agent (SDA), thereby eliminating any potential competition between the two; (iii) new phases may be accessible that are not available via hydrothermal synthesis.

Two ionic liquids not commercially available with symmetrical cationic moieties, diisopropylimidazolium (DIPI) and diisobutylimidazolium (DIBU), were used to synthesize AlPO4-5, NiAPO-5 and MnAPO-5. The DIPI was found to template for AlPO4-5, but only when a specific reagent amount of water was added to the system. Too little water, as well as too much water both led to the condensed phase(s), namely cristobalite or a mixture of cristobalite and tridymite. The DIBU only resulted in cristobalite regardless of the amount of water added. Hydroflouric acid (HF) was used in the syntheses as a mineralizer, as it was discovered that when no HF was present, a dense-phase resulted. It was determined that 0.12 eq of HF was the ideal amount to lead to an AFI structure, with greater amounts leading to a yet-to-be-identified phase.

MnAPO-5 and NiAPO-5 were ionothermally synthesized using DIPI and DIBU. The SDAs both exhibited different behaviors when adding the metals into the syntheses. When adding varying levels of Ni2+ into the reaction, the SDAs displayed opposite behavior in terms of phase development from AFI to dense-phase. There were also differences in results between the two SDAs for manganese substitution that appear to be linked to the kinetics of the reaction. By selecting both an appropriate SDA and tuning the amount of Ni or Mn in a given reaction, a pure-phase AFI material was easily achieved, and easily reproduced. The reaction was a simple one-pot synthesis, and because of the vanishingly low vapor pressure of the ILs, the synthesis did not carry the risk of pressure build-up typical of hydrothermal synthesis. These aspects make ionothermal synthesis of aluminophosphate molecular sieves an incredibly noteworthy area of research.

Metal-organic coordination polymers have gained attention for their interesting polymer conformational patterns as well as their ion exchange properties. Development of materials to trap toxic oxo-anions such as perchlorate (ClO4-), chromate (CrO42-) and pertechnetate (TcO4-), has become incredibly important for environmental remediation and industrial waste disposal. The low dimensionality of these layered and 1-D metal-organic coordination polymers allow for different arrangements of the polymeric layers depending on the nature of the charge balancing anion, as well as tunability in anion exchange with regards to the size/shape of the intercalating and deintercalating anions. This flexibility, and the apparent selectivity for a variety of toxic anions, provides great promise for these materials to be used as a water remediation tool.

The ion exchange capabilities of [Ag(4,4’-bipy)+][CH3CO2−] was probed and found to exchange for a wide variety of toxic anions in very high capacity. Perchlorate, permanganate, perrhenate, nitrate and chromate uptake was found to be 99.9%, 98.5%, 91.5%, 64.4% and 46.4% respectively, with near record high loading capacities of 310, 351, 705, 125 and 160 g/mg respectively. Additionally, the material releases acetate, which is environmentally benign. In an effort to learn more about the properties of these types of materials, a solubility study of [Ag(4,4’-bipy)+][X−] materials (where X− = CH3CO2−, NO3−, BF4−, O3SCH2CH2SO32−, ClO4−, CrO42−, ReO4−, and MnO4−) was performed. The study led to insight on the relationship between anion hydration energy and the structure and solubility of the material. Whereas hydration energy of the charge-balancing anion was previously thought to be the key factor in the material’s solubility, it was discovered that the structure of the material was a much more likely contributor. A subset of these materials, (X− = NO3−, BF4−, ClO4−, and MnO4−), were studied for trends in ion exchange, kinetics and thermal stability, and how these properties relate to solubility. It was discovered that ion exchange capabilities can successfully be predicted based on the solubilities of the starting and ending materials, and that kinetics and thermal stability also seem to have a distinct link to the solubility of the material.

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