Catalytic application of niobium compounds
Introduction
Niobium compounds and materials are now interesting and important catalysts for various reactions. Although there are few differences in electronegativity and ionic radius between Nb and its neighbors (V, Zr, Mo) in the periodic table of elements (Fig. 1), it is intriguing that the catalytic behaviors of niobium compounds are quite different from those of the surrounding elements’ compounds. Thus, the research and development on the catalytic application of niobium compounds have been very active for the last 20 years.
The characteristic features of niobium compounds are the promoter effect and the support effect. Niobium oxides remarkably enhance catalytic activity and prolong catalyst life when the small amounts are added to known catalysts. Moreover, niobium oxides exhibit a pronounced effect as supports of metal and metal oxide catalysts. Mixed oxides containing niobium and niobium oxide supported on the other oxides also show the above effect.
Generally speaking, niobium oxide and the mixed oxides including Nb2O5–SiO2, Nb2O5–Al2O3, Nb-zeolite, mesoporous Nb-MCM-41, Nb-layer compounds, etc. have redox property, acidic property and/or photosensitivity, which cause interesting catalytic behaviors.
Hydrated niobium pentoxide, Nb2O5·nH2O, which is usually called niobic acid, and niobium phosphate (NbPO) have strong acidic properties on the surfaces and are used as solid acid catalysts. In particular, niobic acid containing large amounts of water exhibits high catalytic performances for acid-catalyzed reactions in which water molecules participate or are liberated.
In this paper, the most salient examples of catalytic application of niobium compounds are reviewed and discussed for their significance. For details of various reactions catalyzed by niobium compounds, refer to a review article published in 1995 [1] and for those of preparation, characterization, and catalytic application of niobium compounds, a recent review published in 1999 is useful [2]. Some other reviews on “Niobium, catalyst repair kit” [3], “Application of niobium oxides as catalysts” [4], and “Niobic acid as an unusual acidic materials” [5] are also given in references in this paper.
There are some reports on catalytic application of organometallic compounds of niobium in homogeneous phase. However, this paper is limited to heterogeneous catalysis by solid niobium compounds, since the heterogeneous catalysis is becoming even more important for establishing less cost and environmentally clean processes.
Section snippets
Oxidative dehydrogenation of alkanes
The potential use of cheap alkanes such as ethane and propane as sources of the corresponding olefins such as ethylene and propylene from which various important chemicals, drugs, polymers are obtained is becoming important.
Union Carbide disclosed an invention that ethane is oxidatively dehydrogenated to ethylene at high conversion and selectivity using mixed metal oxide catalysts containing Mo, V, and Nb [6]. As shown in Table 1, the catalytic activity (the temperature at which 10% conversion
Hydrogenation
Both activity and selectivity of a metal catalyst supported on metal oxides for the formation of hydrocarbons from CO and H2 are sensitive to the support oxide. A pronounced support effect of Nb2O5 on Rh catalyst for activity and selectivity towards high hydrocarbons was found in CO hydrogenation [25]. Fig. 5 shows that the support effect is in order of Nb2O5>ZrO2>Al2O3>SiO2>MgO. The conversion of CO over Rh/Nb2O5 is almost 76% at 220 °C in contrast to the negligible conversion (1.4%) over the
Solid acid catalysts
Hydrated niobium pentoxide (Nb2O5·nH2O) which is called niobic acid has a high acid strength (Ho=−5.6) corresponding to the acid strength of 70% H2SO4 when calcined at relatively low temperatures (100–300 °C), though the surface of niobic acid calcined at 500 °C is almost neutral [3], [5]. Since any kinds of acidic metal oxides show acidic property on calcination at about 500 °C and the acidic property is lost or decreased by absorbing water, niobic acid which shows high acid strength in spite of
Photocatalyst
It was found by Domen et al. [59] that water is decomposed to give hydrogen and oxygen in the presence of a Ni–K4Nb6O17 catalyst under UV irradiation. A catalyst with the layered structure of potassium niobate containing ultrafine particles of nickel metal in the interlayer spaces shows a stable and high photocatalytic activity for water decomposition.
The mechanism of H2 and O2 evolution from H2O is shown in Fig. 8 [60], where H2 and O2 form separately in the interlayers I and II, respectively.
Conclusion
Interesting and important examples of catalytic application of niobium compounds and materials reported up to 2001 have been demonstrated.
The promoter effect and the support effect of niobium on catalytic performances for diversified reactions are really surprising, where redox property and acidic property of niobium compounds play important roles. The characteristic features of niobic acid, NbPO, and mixed oxides containing niobium as solid acid catalysts and niobium compounds having layered
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