Scandium bearing open framework materials
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Here I report the hydrothermal chemistry of scandium, examining the behavior of the Sc³⁺ cation in various systems, including phosphates, phosphites, phosphonates and carboxylates. In total, 27 different materials, 23 of which are novel, have been synthesised and their structures solved. Seven different scandium phosphate-based materials have been successfully synthesised using amines and alkali hydroxides as structure directing agents, producing chain, layer and framework materials. Thermal analysis of these materials indicated that they were not stable upon removal of the template, because there are hydrogen bonding networks between the template and free OH groups on the phosphate groups. Certain conditions lead to the crystallization of either kolbeckite, Sc(PO₄).2H₂O, or a langbeinite-type structure, (NH₄)₂Sc₂(HPO₄)(PO₄)₂, which are dense frameworks. Investigation of scandium phosphites leads to the formation of more thermally stable frameworks. Investigation of scandium phosphite-based materials using different structure directing agents yielded three framework phosphite materials and one layered phosphite / phosphate. The use of lithium hydroxide and ethylenediamine within scandium phosphite systems resulted in the crystallization of a gainesite type framework, (LiSc(HPO₃)₂)and (H₃N(CH₂)₂NH₃)₂Sc₄(HPO₃)₈, which distorts in order to accommodate the amine, but not the lithium cation. Decreasing the potential for the formation of hydrogen bonding networks in the phosphite systems led to the formation of framework structures, however these structures did not retain their crystalline integrity upon removal of the template. In order to impart structure directing properties upon scandium-based materials but avoid the formation of hydrogen bonding networks upon which the crystalline integrity is dependent, scandium phosphonates were investigated. Seven different scandium phosphonate materials have been synthesised, two of which have been solved from powder diffraction data, and one from a combination of powder diffraction data, molecular modeling and single crystal data. Synthesis of scandium phosphonate materials yielded two thermally stable, porous materials with reversible water adsorption properties, NaSc(CH₃PO₃)₂•H₂O and Sc₂(O₃PCH₂(NHC₅H₁₀NH)₋CH₂PO₃)₃4H₂O. The success of this approach led to the examination of scandium carboxylate metal organic framework materials. The incorporation of Sc³⁺ into microporous carboxylate frameworks yielded three aliphatic scandium carboxylates and six aromatic scandium carboxylates. The scandium analogue of MIL-53 shows potential for gas adsorption studies, as well as illustrating that scandium carboxylates can be isostructural to metal carboxylate materials already published. The scandium terephthalate, Sc₂(O₂CC₆H₄CO₂)₃, is a small pore framework material with an unprecedented structure type, the adsorption properties of which have been examined using a variety of different small gas molecules and hydrocarbons, including X-ray analysis of the structures whilst adsorbing different molecules. ⁴⁵Sc MAS NMR has been performed on the materials prepared pure and characterized in this thesis, in order to establish a library of chemical shifts for scandium in different framework environments.
Thesis, PhD Doctor of Philosophy
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