ABSTRACT
The catalytically relevant Nb2Mo3O14 phase has been prepared in bulk and silica-supported forms via the so-called "multiple molecular precursors method" from water-soluble oxo-oxalato complexes of Nb and Mo, (NH4)3[NbO(ox)3].H2O, and (NH4)2[MoO3(ox)].H2O. Thermal treatment of the mixed Nb-Mo precursor has been optimized for the formation of the pure Nb2Mo3O14 phase, either as bulk oxide or a silica-supported phase with high specific surface area. A characterization of the bulk phase obtained via the conventional ceramic route has also been carried out and a comparison has been made with the precursors route. According to this route, the Nb2Mo3O14 phase is shown to be formed in a pure form at 700 degrees C (i.e., 100 degrees C below the lowest temperature reported so far for the formation of the phase by the ceramic method). The supported samples have appreciable specific surface areas of 60-70 m(2) g(-1), much larger than those reached in the previous attempts under vacuum in sealed vials. The SEM and EDX analyses reveal a high dispersion of the desired phase on the silica support.
ABSTRACT
New water-soluble bimetallic peroxo-tartrato complexes of niobium(V) and/or tantalum(V) have been prepared, characterized from the structural and spectroscopic point of view, and used as molecular precursors for Nb-Ta mixed oxides. Two new homometallic complexes, (gu)5[Nb2(O2)4(tart)(Htart)] x 4H2O (1a) and (gu)6[Ta2(O2)4(tart)2] x 4H2O (2a), and the corresponding heterometallic complex, (gu)5[NbTa(O2)4(tart)(Htart)] x 4H2O (3), have been obtained. The crystal structures of the homometallic compounds, (gu)5[Nb2(O2)4(tart)(Htart)] x 6H2O x 1H2O2 (1b) and (gu)6[Ta2(O2)4(tart)2] x 6H2O (2b), have been determined, showing, for both cases, two 8-fold-coordinated metal atoms, each surrounded by oxygen atoms belonging to two bidentate peroxides, two monodentate carboxylato, and two alkoxo groups from both bridging tartrato ligands. The coordination polyhedron around each metal atom is a dodecahedron. The thermal treatment of complexes 1a, 2a, and 3 in air at 700 or 800 degrees C, depending of the Ta content, provided Nb2O5, Ta2O5, and the solid solution TaNbO5, respectively. The thermal treatment of a 1:1 Nb/Ta molar ratio mixture of 1a and 2a has also been studied. BET and SEM measurements have been carried out and reveal these oxides possess relatively high specific surface areas and display a porous character. Comparison between the use of homo- and heterometallic precursors is discussed.
ABSTRACT
New water-soluble homoleptic peroxo complexes and heteroleptic peroxo-polyaminocarboxylato (PAC) complexes of tantalum(V) have been prepared. In the case of the peroxo-PAC complexes, the synthesis in the presence of excess H2O2 leads to the oxidation of the nitrogen atoms of the ligand into N-oxides. The compounds correspond to the general formula (gu)3[Ta(O2)2(LO2)] x xH2O (gu = guanidinium, L = edta or pdta) in which H4LO2 refers to the bis(N-oxide) derivative of the PAC ligand. The TaV complexes have been characterized on the basis of elemental and thermal analysis and by IR and 13C and 15N NMR spectroscopy. These last two spectroscopic methods have been used to suggest the coordination mode of the PAC ligand in the complexes. ESI mass spectrometry measurements have also been carried out for the peroxo-PAC compounds. The crystal structures of the homoleptic tetraperoxotantalate, (gu)3[Ta(O2)4] (1), and the heteroleptic complex, (gu)3[Ta(O2)2(edtaO2)] x 2.32H2O x 0.68H2O2 (2b), have been determined, showing, for both cases, an 8-fold-coordinated Ta atom surrounded either by four bidentate peroxides or by two peroxides and one tetradentate edtaO2 ligand.
