Niobium phosphate

The niobium phosphate complex ( Pr0)4Nb[02P(0 Bu)2] 2 was produced by the reaction of Nb(0 Pr)5 with H0P(0)(0 Bu)2 in pentane [71]. Although crystals of sufficient quality for an X-ray crystallographic analysis were not obtained, spectroscopic evidence suggests that the phosphate ligands bridge the two Nb centers. 

In 1999, Carlini et al. investigated the ability of niobium-based phosphate to catalyze the selective dehydration of fructose, sucrose, and inulin to HMF (Scheme 7) [74]. Starting from fructose and using a column reactor packed with niobium phosphate catalyst, 67% selectivity to HMF was obtained at 38% conversion. This catalyst was stable in the presence of water and was successfully reused without notable change of activity. Interestingly, from sucrose and inulin, the niobium-based catalysts afforded HMF with 66% selectivity at 47% conversion. A significant improvement of both the catalyst activity and the HMF selectivity was achieved when the HMF was continuously extracted from the water phase with methylisobutylketone (MIBK). Indeed, under these conditions, HMF was produced with 98% selectivity at 60% conversion of fructose. Using the same procedure, but from inulin, HMF was obtained with 72% selectivity at 70% conversion. 

Armaroli, T., Busca, G., Carlini, C., Giuttari, M., Galletti, A. M. R., and Sbrana, G., Acid sites characterization of niobium phosphate catalysts and their activity in fructose dehydration to 5-hydroxymethyl-2-furaldehyde. J Molecular Catalysis A-Chem 2000, 151 (1-2), 233-243. 

Tungsten, molybdenum, and vanadium interfere in the determination of niobium. In contrast to the corresponding tungsten complex, the niobium-thiocyanate complex is decomposed by oxalic acid. Fe(ni), U, Ti, and Ta do not interfere if they are present in no greater than hundred-fold amounts relative to niobium. Phosphate and fluoride interfere, but the latter can be masked with aluminium ions [37]. 

Vapor phase catalytic alkylation of phenols with methanol was carried out on various phosphates as catalysts. The best activity and selectivity was observed on boron, rare-earth and niobium phosphate. With boron phosphate, the reaction is very selective for O-alkylation even at high temperature. On this catalyst o-methoxy-phenol is selectively obtained from 1-2-dihydroxybenzene. With rare-earth phosphate calcinated at 400°C and with niobium phosphate, O-alkylation selectivity decreases with an increase of reaction temperature. For rare-earth phosphates it is possible to improve the selectivity by calcination at higher temperature or by a wetness impregnation of cesium hydrogenophosphate. An explanation of these results is proposed. 

Zirconium and niobium phosphates may be coprecipitated to give a product which has ion exchange properties (233). 

Finally, calorimetric measurements can also be used to monitor adsorption phenomena on the surface of solid catalysts in contact with a liquid phase (in a solvent). For example, the so-called cal-ad method [30-33] has been used to measure the adsorption heats evolved upon addition of dilute solutions of pyridine in -hexane to a solid acid catalyst (TS, H-ZSM-5) in a slurry wifli -hexane. The amount of free base in solution is measured separately using a UV-Vis spectrophotometer [30,31]. A similar technique has been used to determine the acidic character of niobium oxide and niobium phosphate catalysts in different solvents [34,35], using aniline and 2-phenyl-ethylamine as probe molecules. 

The catalytic properties of niobic acid (Nb205 H20) and niobium phosphate (Nb0P04) surfaces have been studied in the reaction of fiiictose dehydration carried out in water. The reaction was performed in a continuous reactor at different temperatures (363-383 K) and pressures (from 2 to 6 bar). Niobium phosphate e diibited a superior activity and selectivity to 5-hydroxymethyl-2-furaldehyde (HMF) compared to niobic acid. The higher catalytic performance of niobium phosphate compared to niobic acid could be related to the higher effective acidity of its surface, as evidenced by the calorimetric study of acid-base titrations carried out in different polar liquids [34,35]. 

Fructose, one of the most common ketohexoses, readily dehydrates to afford HMF in the presence of Br0nsted acids in polar solvents. A variety of aprotic polar solvents, including DMSO, DMF, N,N-dimethylacetamide (DMA), and sulfolane, are used for these liquid-phase reaction because of the solubility of carbohydrates. A variety of solid acids, such as ion-exchange resins [156], zeolites [157, 158], metal oxides, and heteropoly acid salts, have been examined for HMF production from fructose [159,160]. Niobic acid, niobium phosphate, vanadium phosphate, sulfated zirconia, Amberlyst-15, and acid-functionalized mesoporous silicas are also found to exhibit high catalytic activity for fructose dehydration [161-167]. Moreover, soHd acid catalysts have also been examined in ionic liquids [168-175]. 

Sun P, Long X, He H, Xia C, Li F (2013) Conversion of cellulose into isosorbide over bifunctional ruthenium nanoparticles supported on niobium phosphate. ChemSusChem 6(11) 2190-2197... 

Immobilization is also a current research activity. The proposal is to immobilize the resulting high level waste, or the remaining compound, using glass matrices based on niobium phosphate glasses, which can be melted in microwaves or in electrical furnaces. Three different samples of niobium phosphate glasses have been produced and sent for vapour... 

Chai, Z., Dong, D., Wang, C., Zhang, H., Webley, P.A., Zhao, D., and Wang, H. (2010) Nanoporous niobium phosphate electrolyte membrane for low temperature fuel cell. / Memb. Set, 356, 147-153. 

Pholjaroen B, Li N, Wang Z, Wang A, Zhang T. Dehydration of xylose to furfural over niobium phosphate catalyst in biphasic solvent system. J Energy Chem 2013 22 826-32. 

Weng, W., Davies, M., Whiting, G., Solsona, B., Kiely, C. J., Carley, A. F., and Taylor, S. H. Niobium phosphates as new highly selective catalysts for the oxidative dehydrogenation of ethane. Phys. Chem. Chem. Phys. 13,17395-17404 (2011). 

Useful feature of this method is possibility to change solvent and investigate the influence of solvent on acid strength. Niobium oxide and niobium phosphate were investigated by this technique, using aniline and 2-phenyl-ethylamine as probe molecules, in order to understand the effective acidity of these solids measured in various... 

P. Camiti, A. Gervasini, S. Biella, A. Auroux et al.. Intrinsic and effective acidity study of niobic acid and niobium phosphate by a multitechnique approach. Chem. Mater. 17,6128-6136 (2005)... 

Various papers have recently been published concerning the liquid-phase titration of acid solids, such as acidic polymeric resins [107-109]. Example is here presented on the study of the intrinsic and effective acidities of two catalysts based on niobium niobium oxide (NBO) and niobium phosphate (NBP) [110] which found application in reaction of acid transformation of monosaccharides (fructose, glucose, in particular) to useful chemicals, like 5-hydroxymethyl-2-furaldehyde (HMF) [111, 112]. 

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