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Hydrothermal synthesis of organically modified metal phosphates and borates

Authors Sung, Ho Yung
Issue Date 2001
Summary Crystalline inorganic oxides are an important class of materials with many technological uses. Chapter 1 gives a brief history of this area with special focus on the formation of microporous metal oxides such as the zeolites and related AlPO family of aluminophosphates, as well as the fundamental parameters of hydrothermal synthesis used and the key methods used for characterization of these types of solid. In this work we have sought to extend the range and composition of such open framework materials, which have commercial importance in catalysis and separation technologies, to include examination of indium phosphates, mixed-metal phosphates, zirconium phosphates, vanadophosphates, vanadomolybdates, and vanadoborate cluster solids. The rationale is to find new stable oxide frameworks or clusters, for which REDOX active centers can be incorporated and leading to new catalytic applications complementary to existing phases. The experimental work in this thesis describes the formation and isolation of over 35 new crystalline solids from these above families in which the structure and arrangement of inorganic oxide components, such as metal phosphate or borate, is varied through their synthesis in the presence of organic structure directing groups. In general all compounds reported have been isolated in phase-pure form and have been fully characterized. This includes a single crystal X-ray structure determination in the majority of cases. The physio-chemical properties of certain selected compounds are also reported. Chapter 2 describes the synthesis of 19 new organically modified group 13 phosphates. Initial studies show that indium phosphates vary considerably from the lighter group 13 congeners the AlPOs and GaPOs, in that indium is always octahedrally (rather than tetrahedrally) coordinated. Hydrothermal crystallization is effected usually at low pH conditions and use of fluoride mineralizer is effective, though often non-innocent as in formation of 3[pnH2]3[H3O][In9(PO4)8H2F16]3[H2O]. The crystal chemistry is quite rich with a wide range of In:P:F ratio possible. Several new open framework compounds, such as [hopipzH2][In4(PO4)4F2(H2O)4], have been formed although they appear relatively fragile to removal of organic 'guest' counter ions. Attempts to incorporate transition metals into discrete framework sites were partially successful, with the formation of [en][CoInH(PO4)2F2(H2O)2] which is a 2-D sheet phosphate. More promising was the formation of [FeM(PO4)2(enH)] M = Al, Ga which are novel mixed metal phosphates with pendant organic ions. These can be ion-exchanged and the framework is stable to 200°C. Chapter 3 describes our parallel studies on group IV phosphate phases of Zr and Ti and five new phases are reported. These metals have chemical and coordination similarities to indium. Phases of variable dimensionality, including a 3-D open framework, have been formed using the same organic template (en) but differing hydrothermal conditions. The 1-D and 2-D phases are difficult to crystallize and the X-ray structures obtained are rare examples of this class of phosphate solid. Chapter 4 describes our attempts to isolate group V and VI mixed metal phosphate solids and five new compounds are reported. The chemistry here, especially that of vanadium, is highly varied and is oxidation state dependent. Vanado(III) and vanadium (IV) phosphates are well established as open framework solids. We have found new phases of these, as well as new reduced cluster solids based on the new cluster anions [Mo8V6PO42]8-, in which the Mo and V sites are distinct. Initial electrochemical studies on these are highly promising and show reversible REDOX activity. Chapter 5 describes our work on vanadium (IV) borates, which again typically take the form of cluster sub units. A wide range of cluster types and stoichiometries are found, which are influenced by organic groups. We have succeeded in isolating six new heterometallic vanadoborate clusters and cluster solids. The use of secondary metals such as zinc and cadmium is shown to be a successful strategy for cross-linking the [VBOx] clusters to form a new class of thermally stable microporous solids. In particular a 3-D cubic phase with the anionic framework [Cd3V12B18O60H6]n6- has been synthesized which shows reversible sorption / desorption of water, cation exchange and is stable to 250°C. Finally the use of low aquation levels in these syntheses which may be regarded as boric acid flux' synthesis is shown to yield distinct results from hydrothermal approach. First a phase containing the 1-D chain polymer [V6B22O53H8]8- is found instead of salts of the discrete [V6B20H8]8- ions. When copper is used instead of vanadium a novel borate phase [Cu(en)2][B7O13H3] is found in which the borate forms a specific porous framework around the metal-organic templating cations. In summary we have explored several relatively undeveloped areas of metal oxide solid and in each case have found that depending on synthesis conditions, the inorganic component can be co-crystallized with organic co-components which modulate the structure through supra-molecular effects. In the case of cluster work less obvious dependence is seen for the counter ion in vanadoborate chemistry, however for the vanado-molybdo-phosphate system the selective precipitation of particular clusters from a complex 'soup' of similar cluster ions appears to be possible through variation of the organic cation. This synthesis approach looks extremely promising for further development of designed metal oxide species.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2001
Language English
Format Thesis
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