Niobium oxalate

We wish to thank E.M. Kool, P.H.H. in t Panhuis and A. Toebes for carrying out a substantial part of the experimental work. The hydrated niobia and the niobium oxalate used in the preparation of the catalysts were kindly provided by the Niobium Products Company, Inc., USA. One of the authors (R.H.H. Smits) thanks the Dutch Foundation for Chemical Research (SON) for financial assistance. 

Niobium ethoxide or niobium oxalate was used to graft niobium on calcined as well as exchanged mesoporous MCM-41 and MCM-48 molecular sieves. Similar procedure was used as described for titanium grafting using titanium butaoxide. 

The Raman investigation of niobium species in aqueous solutions of niobium oxalate (Jehng and Wachs, 1991) nicely showed the dependence of their constitution on pH and concentration. The PZC theory was successfully applied to predict the hydrated, molecular structures of multicomponent supported metal oxide species, such as iron-molybdenum, iron-vanadium, molybdenum-vanadium, tungsten-vanadium, and sodium-vanadium oxide species (Vuurman et al., 1991 Wachs et al., 1993). 

The aqueous preparation oT supported niobium oxide catalysts was developed by using niobium oxalate as a precursor. The molecular states oT aqueous niobium oxalate solutions were investigated by Raman spectroscopy as a -function o-f pH. The results show that two kinds o-f niobium ionic species exist in solution and their relative concentrations depend on the solution pH and the oxalic acid concentration. The supported niobium oxide catalysts were prepared by the incipient wetness impregnation technique and characterized by Raman, XRD, XPS, and FTIR as a -function o-f niobium oxide coverage and calcination temperature. The Raman studies reveal that two types o-f sur-face niobium oxide species exist on the alumina support and their relative concentrations depend on niobium oxide coverage. Raman, XRD, XPS, and FTIR results indicate that a monolayer oT sur-face niobium oxide corresponds to 19%... 

Niobium oxalate was supplied by Niobium Products Company with the Tollowing chemical analysis 20.5% Nb20s, 790... 

Figure 3 Raman spectra ot niobium oxalate in oxalic acid solution as a -function o-f pH -from 0.5 to 5.00.

For the synthesis of Mo/V/Te/Nb/O catalysts, ammonium paramolybdate, vanadyl sulfate, ammonium niobium oxalate and tellurium oxide or telluric acid are mixed as aqueous solutions or slurries and dried at about 150°C. These mixed slurries are then calcined at 500-650°C in a N2 stream [39]. In addition to the dominant Ml and M2 phases, impurity phases, for example M0O3, Mo,5V9O40 and Mo3Nb20ii, are also observed [28c]. It should be noted that Mo-V-Te-O catalysts cannot be obtained by the dry-up method. 

Niobium solution, 1 mg of Nb in 1 ml. Heat 0.145 g of Nb205 in a platinum crucible with 5 ml of cone. HF until the oxide dissolves. Evaporate the solution to 1 ml, add 2 ml of H2SO4 (1 + 1), and heat to white fumes. Let cool, rinse the walls of the crucible with water, and heat until white fumes appear. Let cool, rinse the walls of the crucible with water and heat to fumes again. Repeat the operation once more to remove HF completely. Pour the niobium solution in cone. H2SO4 into 30 ml of 5% aqueous ammonium oxalate solution, dilute the clear solution of niobium oxalate complex to 100 ml, with water, and mix well. 

Kim et al. [208] synthesized perovskite-type Pb(Mgi/3Nb2/3)03 powders by an emulsion method with (a) an aqueous solution of lead nitrate, magnesium nitrate hexahydrate and niobium oxalate as the water phase, (b) Span 80 as the surfactant, (c) paraffin oil as an emulsifying agent and (d) kerosene as the oil phase. The emulsion of short stability, prepared by high speed mixing, was added drop by drop into petroleum heated at 170 C. The product particles were washed in toluene and cleaned at 150"C. Careful calcination yielded dominantly a pyrochlore phase at 600 C, but only the desired perovskite-type niobate at 800 C. 

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