AbstractsChemistry

The project concerns with design, synthesis and characterization of molecular and extended structure materials composed of vanadium oxides and evaluation of their catalytic oxidative dehydrogenation (ODH) properties. A long-term objective is to develop an understanding of the structure-property relationships in ODH catalysts. Chapter 1 provides an overview of the field of metal oxides, polyoxometalates, and catalytic oxidative dehydrogenation of propane. This chapter also defines the research problem concerning this thesis and its fundamental and practical significance. Chapters 2-4 describe the synthesis, characterization and ODH properties of a select number of structurally correlated vanadium oxide based materials. The ODH catalysts studied in this work includes molecular compounds - (NH4)8[VIV 12VV 6O42(SO4)0.85(VO4)0.15]·10H2O (NH4-POV) and NH4V10O28, layered structure materials - MgVV 2VIV 2O10·4H2O (MV4) and V2O5, a chain structure - NH4VO3, and a nanostructured three-dimensional framework material - [Co3V18O42(H2O)12 (XO4)]·24H2O (X = V, S) (Co-POV). Their catalytic activities for ODH of propane to propylene - an important industrial feedstock material, were studies and compared in an attempt to get an insight of the structure-property relationships. Chapter 2 discusses synthesis and characterization of a new mixed-valence vanadate, MgVV 2VIV 2O10·4H2O (MV4), an extended structure solid, synthesized hydrothermally and characterized by single-crystal X-ray diffraction, spectroscopic methods, thermogravimetric analyses and temperature dependent magnetic measurements. MV4 represents the first model compound for the naturally occurring mineral melonovanadite, Ca2VV 4VIV 4O20·10H2O. MV4 is an important material with xv structural and electronic properties that are attractive for making it a potential promising ODH catalyst. The framework structure in MV4 consists of vanadium oxide layers crosslinked by {Mg(H2O)4} groups. The vanadium oxide layers are composed of edge shared {VIVO5} square pyramids, forming {V2O8} dimers, which share corners with {VVO4} tetrahedral units. Chapter 2 also discusses synthesis of a novel mixed-valence molecular polyoxovanadate - (NH4)8[VIV 12VV 6O42(SO4)0.85(VO4)0.15]·10H2O (NH4-POV) and its characterization by single-crystal X-ray diffraction, spectroscopic and thermogravimetric analyses. Structure consists of {V18O42} shell composed of 18 edge sharing {VO5} square pyramids, hosting a tetrahedral species {XO4} (X = S, V). Highly reduced molecular structure of NH4-POV makes it a promising ODH catalyst. Chapter 3 describes the ODH properties of a polyoxovanadate based openframework material - [Co3V18O42(H2O)12 (XO4)]·24H2O (X = V, S) (Co-POV). It is composed of {V18O42(XO4)} (X = S, V) building units, which is the molecular cluster present in NH4-POV, interconnected by {-O-Co-O-} bridging groups. Alternatively, since the building unit clusters {V18O42XO4} can be viewed as derived from sheets of V2O5, the 3-D structure of Co-POV can be…