Hydrothermal synthesis of organically modified transition metal vanadates
by Law Sui Chun
xxiii, 315 leaves : ill. (some col.) ; 30 cm
Microporous oxide solids such as the zeolites are commercially important for their selective ion exchange, absorption and catalytic properties. The incorporation of REDOX active metals in these has led to expanded applications including the selective oxidation of hydrocarbons. The aim of this research is to develop new microporous solids which are complementary to zeolitic phases and which possess frameworks that incorporate REDOX metal centers such as vanadium and manganese....[ Read more ]
Microporous oxide solids such as the zeolites are commercially important for their selective ion exchange, absorption and catalytic properties. The incorporation of REDOX active metals in these has led to expanded applications including the selective oxidation of hydrocarbons. The aim of this research is to develop new microporous solids which are complementary to zeolitic phases and which possess frameworks that incorporate REDOX metal centers such as vanadium and manganese.
The design of new porous frameworks can best be achieved using the hydrothermal method in which organic groups, such as amines, are reacted with inorganic framework components and may serve as structure directing or 'templating' agents. Frequently the organics are left as counter cations, though use of milder conditions may leave them as an integral part of the framework. The resulting new class of hybrid solids may also have interesting and unique materials properties due to the novel component combination. The background and existing literature on these subjects are reviewed in Chapter 1.
The synthetic work described in this thesis begins in Chapter 2 with a study of new vanadoborate cluster solids, compounds l-7. Three goals were attempted, the first was to use transition metal ions to coordinate the cluster exteriors and serve as cross-linking agents in order to form new porous cluster frameworks. This was partially achieved using Mn2+ in the formation of [enH2]4[Mn2V12B16O58H8(H2O)4].H2O 1, which is a 2-D network of connected clusters. This demonstrated the possible success of this approach, which has since been fully realized using Cd2+ ions as the cross-linking ions.
The second goal was the incorporation of metal ions into the [VB] cluster itself to form new cluster types. This was achieved using Mn2+ with the formation of [Mn 4V10B28] cluster solids 6 and 7. These clusters are notable in having [Mn2] units which resemble the active sites of hydrolase enzymes. The hydroxonium salt [H3O]10[Mn4(B2O4)V10B28O74H8]. 11H2O 7 is water soluble and study of the stability, REDOX and catalytic properties of the cluster are now underway. Finally, attempts to incorporate larger units such as trangulo-[Mn3O] trimer unit into the vanadoborate clusters were unsuccessful. However they led to the discovery of an organically templated microporous manganese vanadate, or [MnVOx],[enH2][Mn3(V2O7)2(H2O)2], 8. The failure to incorporate borate is linked to the lower temperature of synthesis which changes the vanadate chemistry to V5+ from V4+. The structure of 8 is thermally stable to 250℃, capable of ion-exchange and exhibits a low temperature anti-ferromagnetic transition (TN = 10K).
The hydrothermal synthesis conditions for formation of organic [MnVOx] phases was then explored in depth in Chapter 3. Through variation of the reaction conditions, such as temperature, reaction time, pH and the types of aliphatic diamines used, a total of 16 further new compounds 9-24 were synthesized and characterized by single crystal X-ray diffraction. Most of them are made in good yield and are phase pure. They exhibit a wide range wide of structure diversity and functionality. All have new [MnVOx] architectures not seen previously.
The mineral chemistry of nickel and cobalt vanadate often parallels that of manganese. In Chapter 4 exploration of the [NiVOx] and [CoVOx] systems using the aliphatic diamines as organic structure-directing agents reveals quite different chemistry from Chapter 3. A total of 12 new compounds 25-36 were synthesized including several novel salts and hybrid solids. The difference in the solids produced lies in the preference of Co and Ni for diamine chelation.
Finally in Chapter 5 the transition metal vanadates phases formed using planar aromatic amines such as imidazole and pyridine are studied. A further 10 new compounds 37-46 were made and characterized. Among them, a novel 3-D microporous phase [ImH][Mn3(OH)2(V4O13)], 37 is found. This contains mixed valence MnII and MnIII centers, is found to be thermally stable up to 350℃ and capable of ion-exchange. The compound is thus a potential candidate for both acid and REDOX catalysis.
The new transition metal vanadates are architecturally and compositionally diverse yet the wide range of structures observed can often be rationalized on the basis of supra-molecular principles. The vanadate components are usually tetrahedral with V5+ ions, but exist in various forms including (VO3)n chains, (V2O7) dimers, linear (V4O13) tetramers, and as cyclic tetramers and hexamers [V4O12] and [V6O18]. One example with trigonal bipyramidal V5+ ions is also found [Mn2(Im)3(V2O6)2] 39. The Mn, Co and Ni components are octahedral, may be found as discrete metal centers, as dinuclear or trimeric units, or as ladder chains as in [ImH][Mn3(OH)2(V4O13)] 37 and [Mn3(pnH)2(V2O7)2] 22 they also may have variable N/O coordination environments.
In this thesis, apart from appreciating the structural diversities of the compounds, their thermal stability, magnetic, and ion-exchange properties are evaluated. Low temperature ferro-magnets as well as anti-ferromagnets are observed for the [MnVOx] solids.
The factors affecting coordination of amines to the metal oxide framework forming a true hybrid solid, rather than their segregation in the structure as separate counter cations to a metal oxide framework ('templated' solid) are of interest. In this work we see that both alternatives are possible in a given synthetic system and that conditions dictate the result. The use of lower temperature, higher pH or loading of amine, as well as higher affinity for N-ligand binding by the metal, all increase the likelihood of forming hybrid solids. Conversely if 'templated' solids are sought then higher temperatures, lower pH, as well as a choice of non-chelating ligand should assist their formation.