Hydrogen activating niobium

Uranium enhances the hydrogenation activity, but the selectivity is very low. Niobium gives somewhat improved results while the lanthanides do not show any effect... 

For niobium and cobalt clusters structures have been proposed based upon the elements behavi or (71). Niobium s specific inertness has been associated with structures that are analogous to close-packed surface of W(110) which also has an activation barrier for hydrogen chemisorption. Since the IPs are also expected to be higher for closed packed structures these two sets of observations are in agreement. This model at its current stage of development requires different structures for each system and as yet has not been useful in making predictions. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

Figure 7 shows the activity and selectivity towards sulfur compounds of H-NbMCM-41 with various Si/Nb ratio H-NbMCM-41 seems to be a good catalyst as far as the formation of methanethiol is concerned. All hydrogen forms of niobium-containing MCM-41 materials are very stable in the production of sulfur compounds. The reason for that is the low acidity of the material, the absence of dissociative adsorption of H2S, and very easy formation of methoxy species on the surface as demonstrated earlier on the basis of IR measurements [3,10] If the mesoporous matrix possesses A1 instead of Nb, the MeOH conversion is higher, but the selectivity to methanothiol is lower. 

The pentoxides of vanadium, niobium, and tantalum react with hydrogen peroxide to produce per-acids of the general formula HR04. H20. These per-acids increase in stability with increase in atomic weight. Pertantalic acid is a white solid which can be heated to 100° C. without undergoing decomposition. The oxyfluorides of these metals also take up active oxygen to yield peroxyfluorides, which are much better defined in the case of niobium and tantalum than with vanadium. 

Niobic acid displays a much less pronounced tendency than vanadie acid to form heteropoly-compounds with other acids, but oxaloniobates are known. It reacts with hydrogen peroxide to form perniobic add, HNb04.a,H20, salts of which are known. The double niobium oxy-fluorides also take up active oxygen. 

Niobium Peroxyfluorides or Fluoroxyperniobates.—Like niobic acid, the alkali niobium oxyfluorides have the property of taking up active oxygen by reaction with hydrogen peroxide. 

Hydrogenation of aromatic rings will not be described in detail in this review since it is mostly concerned with isolated double bonds. This is however an area of active research, as recently demonstrated by the use of catalytic niobium complexes for the selective hydrogenation of aryl phosphines (Scheme 3)56. 

Hydrides. According to X-ray and neutron diffraction and metallographic studies of the Nb-H system,524 the H may be considered a lattice gas with phase transitions. In the a-, a -, j6-, and -phases of the system, H occupies tetrahedral interlattice positions. Whereas direct reaction between niobium metal and hydrogen occurs only after repeated activation of the metal by hydrogen absorption at ca. 7 atm and 350 °C, NbH2 is formed at temperatures as low as 22 °C in mixtures of LaNi5H6 7 and Nb.525 The extraordinary catalytic effect of the lanthanum-nickel complex is attributed to the presence of surface-absorbed atomic hydrogen species which are able to diffuse into the niobium lattice. There has been a review of the T a-H system.526... 

Preparation of metal oxide thin film by means of stepwise absorption of metal alkoxide has been carried out in the past for the activation of heterogeneous catalysts [13]. For example, Asakura et al. prepared one-atomic layer of niobium oxide by repeating chemisorption of Nb(OEt)5 on silica beads. The catalyst obtained by immobilizing platinum particles on a niobum oxide layer showed improved reactivity for hydrogenation of ethylene in comparison with... 

A few years ago Smalley and coworkers were able to obtain detailed experimental information about the reactivity of specific transition metal clusters with hydrogen molecules (1). The results for copper and nickel clusters were essentially as expected from the known results for surface and metal complex activities. For copper no clusters were able to dissociate whereas for nickel all clusters were active with a slow, steady increase of activity with cluster size. For the other transition metals studied, cobalt, iron and niobium, a completely different picture emerged. For these metals a dramatic sensitivity of the reactivity to cluster size was detected. No convincing explanation for these surprising results has hitherto been suggested. It should be added that there are no dramatic differences in the activity towards Hg for the metal surfaces (or the metal complexes) of nickel on the one hand and iron, cobalt and niobium on the other. 

It is noteworthy that the transition metals that serve as hydroaluminating catalysts are also active in establishing the equilibrium between aluminum alkyls and their decomposition products, aluminum, hydrogen and alkene (equation 12). Accordingly, these metals, in addition to hafnium, niobium, vanadium, scandium and lanthanum, have found use as activators for the direct synthesis of aluminum alkyls (equation 12, to the left). Probably most of these metal salts will also be capable of accelerating the hydroalumination reaction. 

Some metal oxide catalysts are activated by thermal reduction with hydrogen or carbon monoxide. For example, the catalytic activity of molybdenum oxide and tungsten oxide for the metathesis reaction of olefins is very much enhanced by their slight reduction (1). The catalytic activity for butene isomerization and ethene oligomerization appears on niobium oxide by its... 

The catalytic activities for ethene oligomerization, induced by the photoreduction of niobium oxide with ethene and other various compounds at room temperature, are listed in Table 1. The catalysts photoreduced with H2 at room temperature and reduced thermally with H2 at 823K show comparable activity, but with much lower values than the catalyst photoactivated with ethene. Not only ethene but also other olefins are effective for the photoactivation. Low activation ability of 2,3-dimethyl-2-butene, however, has been observed, suggesting that vinyl hydrogen in olefins may play a role in the photoactivation. 

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