Niobium catalyst

Sumitomo Chemical Co. (98—100) and Mitsubishi Kasei Co. (101) have patented single-step catalysts containing niobium and palladium. A Sumitomo example reports 93.5% MIBK selectivity at 41.8% acetone conversion and conditions of 160°C and 2 MPa. Other significant processes have been reported (60,102—110). 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

A process for the production of ethylenimine [151 -56-4] a suspect carcinogen, by the vapor phase dehydration of monoethanol amine has been developed (128—132). By using an alkyleneamine co-feed with the alkan olamine, higher alkylene amines are made in situ (133). The catalysts are tungsten-, niobium-, or phosphate-based. 

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

The extraction technology is applied for scrap recycling of tantalum and niobium and for the treatment of related materials. Recovery of tantalum and niobium from secondary material, such as Sn slag, Ti slag, W slag, catalyst wastes, used cemented carbide and used tantalum capacitors was reviewed by Dai, Zhong, Li and Li [483]. 

In retrospect it is not surprising that the niobium and tantalum alkylldene complexes we prepared are not good metathesis catalysts since these metals are not found in the "classical" olefin metathesis systems (2). Therefore, we set out to prepare some tungsten alkylidene complexes. The first successful reaction is that shown in equation 6 (L = PMe3 or PEt3) (11). These oxo... 

Figure 1.3 Left. Detailed view of the Nb K-edge XANES data of a pyridine salt of niobium-exchanged molybdo(vanado)phosphoric acid (NbPMo fVJpry) as a function of temperature [31]. A change in niobium oxidation state, from Nb5+ to Nb4+, is identified between 350 and 420°C by a relative increase in absorption about 19.002 keV, and can be connected with the activation of the catalyst for light alkane oxidation. Right. Radial Fourier-transform EXAFS function for the NbPMo (V)pyr sample heated to 420°C [31 ]. The two peaks correspond to the Nb-O (1.5 A) and Nb-Mo (3 A) distances in the heteropolymolybdate fragments presumed to be the active phase for alkane oxidation. (Reproduced with permission from Elsevier.)...

Datka, J. Turek, A.M. Jehng, J.M. Wachs, I.E. Acidic properties of supported niobium oxide catalysts An infrared spectroscopy investigation. J. Catal. 1992, 35, 186-199. 

To examine how and why the surface ethanol reaction is assisted by the gas-phase ethanol, the following experiments were conducted in a closed circulating reactor. Ethanol vapor was first admitted onto the dioxoniobium monomer catalyst (1), Si0 2Nb(=0)2, to form the niobium ethox-ide (2), Si0 2Nb(=0)(0H)(0C2H5), at 373 K, followed by evacuation, and then the system was maintained at 523 K for 10 min, where no H2 evolution was observed because the niobium ethox-ide (2) was stable up to 600 K in vacuum. After the confirmation of no H2 formation from the preadsorbed ethanol (2), tert-butyl alcohol was introduced to the system at 523 K, which led to a stoichiometric evolution of H2 and CH3CHO. As the fert-butyl alcohol molecule has no extractable a-hydrogen, it is evident that both H2 and CH3CHO were produced from the preadsorbed ethanol by the assistance of the postdosed tert-butyl alcohol. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

Some researchers investigated multiphase additives. Kojima et al. [101] ball-milled MgHj with a nano-Ni/Al Oj/C composite catalyst. The mixture decomposed at a really low temperature of 200°C but in vacuum. A complex composite catalyst BCN/Ni/Pd/SWNT (where BCN is barium-calcium niobium high-temperature proton conducting compound and SWNT is single-wall-nanotubes) was also used by Yoo et al. [102] high-temperature proton conducting for ball-milled MgH. It desorbed 3 wt.%H2 in about 3,600 s at 230-250°C in vacuum. 

VV Bhat, A. Rougier, L. Aymard, G.A. Nazii, J.-M. Tarascon, High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH, J. Alloys Compd. 460 (2008) 507-512. 

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. 

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