Nb Catalysts Research Articles

Avoidance of flammability and temperature runaway during oxidative dehydrogenation using a distributed feed

A relatively simple model has been used to simulate the oxidative dehydrogenation of ethane to ethylene in a fixed-bed reactor. Full account of the kinetics of the main oxygenconsuming reaction have been incorporated in the model using previously published kinetics for the reaction over a Mo–V–Nb catalyst. Use of a distributed oxygen feed enables the problem of the feed flammability to be avoided and in addition, shows a significant improvement on reactor performance in terms of ethylene yield and selectivity. A distributed oxygen feed also overcomes the problem of reaction runaway and enables operation to be achieved over a wider range of reactor parameters.

Second row transition metal sulfides for the hydrotreatment of coal-derived naphtha II. Removal of individual sulfur compounds

The disappearance of individual sulfur compounds has been investigated during the hydrotreatment (simultaneous removal of sulfur, nitrogen and oxygen) of coal-derived naphtha over each of the bulk second row transition metal sulfides. The sulfur compounds in the naphtha mainly consist of thiols/sulfides, thiophene and substituted thiophenes. Thiols/sulfides are, in general, more easily converted than thiophenic compounds are. Lighter thiols/sulfides are intermediates in the conversion of higher boiling thiols/sulfides or thiophenes. Side chain alkyl CC bond breaking is predominant during the disappearance of thiophenes over the Zr and Nb catalysts while CS bond breaking is predominant over the other catalysts. Thiophenic compounds are hydrogenated prior to desulfurization over the Mo, Ru, Rh and Pd sulfides. Highly substituted thiophenes are the compounds most difficult to convert over the Mo, Ru, Rh and Pd sulfides. The substituted thiophenes exhibit different reactivity trends over molybdenum sulfide, on one hand, and the Group VIII sulfides, on the other, indicating different adsorption modes and surface mechanisms for their conversion over these catalysts. Individual sulfur compounds do not follow first order kinetics and the disappearance rate is limited by product inhibition. The overall removal of sulfur does not follow simple first or second order kinetics since the individual compounds do not react in parallel, independent or first order reactions.

Surface structures and catalytic properties of supported niobium oxides

Surface structures of SiO 2-supported niobium catalysts prepared by use of a Nb-monomer complex, a Nb-dimer complex, Nb pentaethoxides and Nb pentachlorides as precursors have been determined by means of EXAFS and XANES. These different Nb structures on SiO 2 have been characterized by ethanol dehydrogenation as a test reaction. In contrast to the case of Mo catalysts, the activity and selectivity of the Nb catalysts dramatically depended on the structure of the reaction sites at an atomic scale. Dynamic structural changes of the Nb sites are presented and local structural changes at the interface between the supported Nb atom and the SiO 2 surface are proposed to explain the role of support in catalysis.

ChemInform Abstract: Polymerization of 1‐(1‐Naphthyl)‐1‐propyne by Nb and Ta Catalysts and Polymer Properties: Effects of the 1‐Naphthyl Group.

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Oxidative coupling of methane over ternary metal oxide catalysts consisting of Groups I, III and V elements in the Periodic Table

Ternary metal oxides consisting of Group I (alkali metals)/Group III/Group V metals with an atomic ratio of 1:1:0.3 were prepared and their catalytic performance for the oxidative coupling of methane as well as their physicochemical properties were investigated. When adding Group V metals to Group I/Group III binary catalysts, a noticeable increase in the conversions of methane and oxygen was observed without any change in C 2+ selectivity. Of the ternary metal oxide catalysts, a Na/La/0.3Nb metal oxide showed the highest performance, viz. a methane conversion of 16.0% and a C 2+ selectivity of 74.1% at 1023 K under atmospheric pressure with a total gas flow-rate of 100 ml NTP/min ( CH 4/O 2molar ratio= 9 ). The stability test using a Na/La/0.2Nb catalyst, which was performed under the same conditions as above, showed that the catalyst was very stable with no decrease in methane conversion and C 2+ selectivity for more than 100 h. The crystalline structure, the surface composition and properties of base sites of Na/La/Nb oxide catalysts were investigated using X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) of carbon dioxide. The catalytic performance of the Na/La/Nb mixed oxides depended on the ratio of the two crystalline phases of La 2O 3 and La 3NbO 7. When sodium was added to La 2O 3 with a La/Na ratio of 1, new base sites were produced, resulting in an increase in the C 2+ selectivity with a significant decrease in methane conversion. The methane conversion was increased by adding a small amount of niobium to the Na/La catalyst, resulting in a change of part of the La 2O 3 to La 3NbO 7. However, when all of the La 2O 3 was changed to La 3NbO 7, the C 2+ selectivity decreased due to the lack of new base sites. A good balance of both crystals (La 2O 3 and La 3NbO 7) may be obtained at a Nb/Na atomic ratio of 0.15–0.3, which optimizes the catalytic performance of the mixed oxides.

Polymerization of 1-(1-Naphthyl)-1-propyne by Nb and Ta Catalysts and Polymer Properties: Effects of the 1-Naphthyl Group

Abstract 1-(1-Naphthyl)-1-propyne polymerized in the presence of NbCl5–cocatalyst systems to give virtually selectively a new polymer ca. 3 × 105) in high yields. Catalysts based on TaCl5 produced mixtures of the polymer and cyclotrimers. The polymer was a pale-yellow solid soluble in various organic solvents (e.g., toluene, CHCl3), and formed a free-standing film on solution casting. The onset temperature of the weight loss of the polymer in air was ca. 340 °C, which is higher than that (280 °C) of poly(1-phenyl-1-propyne) [poly(1-PP)]. The oxygen permeability coefficient at 25 °C was 80 barrers and one order of magnitude higher than that of poly(1-PP). These results are explicable in terms of the steric effects of the 1-naphthyl group.

Polymerization of Acetylenes Containing Heptadecafluorononenyloxy Group and Polymer Properties

Polymerizations of two kinds of fluorine-containing aromatic disubstituted acetylenes, 1-[p-(heptadecafluorononenyloxy)phenyl]-2-phenylacetylene (p-C9F17ODPA) and 1-[p-(heptadecafluorononenyloxy)phenyl]-1-hexyne (p-C9F17OPH) were studied with groups 5 and 6 transition-metal catalysts. p-C9F17ODPA polymerized with TaCl5-Et3SiH catalyst to give a new polymer over 70 yield and its maximum intrinsic viscosity ([η]) was as high as 2.1 dl g−1. p-C9F17OPH polymerized with Ta and Nb catalysts, but yield and [η] of the polymer were lower. Poly(p-C9F17ODPA) and poly(p-C9F17OPH) were in the form of a yellow solid and white solid, and began to lose weight at 410°C and 270°C, respectively, which indicates high thermal stability. Both polymers had a main-chain structure consisted of alternating double bonds, and were completely soluble in some fluorine-containing solvents such as m-(CF3)2C6H4 and CCl2FCClF2. Poly(p-C9F17ODPA) formed a tough film by solution casting, and showed high PO2 value [4×10−8 cm3 (STP) cm cm−2 s−1 cm Hg−1] comparable to that of poly(dimethylsiloxane).

Chain transfer reaction to terminal olefins in the polymerization of 1‐phenyl‐1‐propyne with Nb and Ta catalysts

AbstractEffects of various olefins as chain transfer agents were studied in the polymerization of 1‐phenyl‐1‐propyne catalyzed by NbCl5—Bu4Sn (1:2) (Bu: butyl) and TaCl5—Bu4Sn (1:2). In the Nb‐catalyzed polymerizations, Si‐containing terminal olefins, especially butyldimethylvinylsilane and dimethylphenylvinylsilane, proved to greatly lower the polymer molecular weight; e.g., the number‐average molecular weight (M̄n) of the polymer formed without an olefin was 2,0 · 105, whereas it decreased to 7 · 103 − 1 · 104, i.e., below 1/20 in the presence of the Si‐containing olefins (1/5 equivalent to the monomer). In the Ta‐catalyzed polymerization, in contrast, hydrocarbon terminal olefins like 1‐octene were effective; e.g., while the M̄n of the polymer reached 4,0 · 105 without an olefin, the M̄n was no more than 9 · 103 − 2 · 104, i.e., smaller than 1/25 in the presence of olefins. Poly(1‐phenyl‐1‐propyne) obtained using the Nb catalyst in the presence of butyldimethylvinylsilane possesses the terminal butyldimethylsilyl group derived from the added olefin. Thus, suitable olefins are useful to control the molecular weight of poly(1‐phenyl‐1‐propyne) by chain transfer, which verifies the metal‐carbene mechanism for the polymerization of 1‐phenyl‐1‐propyne by group 5 transition metal catalysts.

Polymerization of o-chloro- and o-bromo-substituted phenylacetylenes and polymer properties

(o-Chlorophenyl)acetylene (o-CIPA), (o-bromophenyl)acetylene (o-BrPA), and (o,o,p-trichlorophenyl)acetylene (Cl3PA) polymerized in high yields in the presence of W and Mo catalysts. Nb and Ta catalysts were also effective in producing poly(Cl3PA). Poly(o-CIPA) formed was soluble in such solvents as toluene and CHCl3, and its maximum weight-average molecular weight was 3x105. Though poly(o-BrPA) obtained with WCl6-Ph4Sn was soluble, those obtained with other catalysts were insolble in any solvent. Further, poly(Cl3PA) was insoluble irrespective of the kind of catalyst. These polymers were dark brown solids and did not lose weight below 230°C, being more stable than poly(phenylacetylene).

Polymerization of [o-(Dimethylphenylsilyl)phenyl]acetylene and Polymer Properties

o-(Dimethylphenylsilyl)phenylacetylene (o-Me 2 PhSiPA) polymerized with W catalysts in good yields to form high molecular weight polymers. Mo, Nb and Ta catalysts also provided polymers, though the polymer yields and molecular weights were lower. The resulting polymer was a dark purple solid soluble in organic solvents. Gas permeability of poly(o-Me 2 PhSiPA) was appreciably lower than that of poly(o-[trimethylsilyl]phenylacetylene)

Polymerization of [2,5-bis(trifluoromethyl)phenyl]acetylene and polymer properties

[2,5-Bis(trifluoromethyl)phenyl]acetylene [BTFPA; HC≡CC6H3-2, 5-(CF3)2]polymerized with W, Mo, and Nb catalysts to produce methanol-insoluble polymers in high yields. The poly(BTFPA) produced by the W(CO)6-based catalyst at 30 °C was soluble in p-(CF3)2C6H4, and had relatively high molecular weight ([η]=0.352 dL/g in p-(CF3)2C6H4). The main chain of the polymer was composed of alternating double bonds, and the polymer was a dark brown solid. The temperature at which the weight loss of the polymer started was higher than 300 °C. The polymerization behavior and polymer properties for BTFPA are compared with those for phenylacetylene and [o-(trifluoromethyl)phenyl]acetylene.

Spectroscopic evidence for a surface Nb carbene in a new SiO 2-attached Nb catalyst active for propene metathesis

The SiO 2-attached Nb-σ-allyl complexes (1), prepared by the reaction of Nb(η 3-C 3H 5) 4 with surface OH groups on SiO 2, have been converted efficiently by thermal treatment at 573 K under hydrogen to the Nb species (2) active for propene metathesis. The Nb-O, Nb-C and Nb-C bond distances in (2) have been shown to be 0.192, 0.195 and 0.210 nm, respectively, by EXAFS analysis. The longer Nb-C bond length is close to that of the normal single bond, while the shorter Nb-C bond length corresponds to that of a double bond, indicating the presence of NbCH 2 species. In addition, species (2) exhibited new bands at 2914 cm −1 and 2850 cm −1 in the IR spectrum which may be attributed to the C-H asymmetric and symmetric stretching modes of NbCH 2. These findings provide the first spectroscopic evidence for metal-carbene species on solid catalysts.

Copolymerization of 1‐(trimethylsilyl)‐1‐propyne with disubstituted hydrocarbon acetylenes

AbstractCopolymerization of 1‐(trimethylsilyl)‐1‐propyne (MeC ≡ CSiMe3) with several aromatic and aliphatic disubstituted acetylenes (MeC ≡ CPh, n‐BuC ≡ CPh, 2‐octyne, and 4‐octyne) were examined by using Ta and Nb catalysts. The TaCl5–Ph3Bi catalyst was effective in copolymerization with the aromatic acetylenes, whereas the NbCl5–Ph3Bi catalyst was preferable in copolymerization with the aliphatic acetylenes. The copolymerization products were not mixtures of homopolymers but copolymers. The relative reactivity of monomer tended to decrease with increasing steric effect of monomer: 2‐octyne > MeC ≡ CSiMe3 > 4‐octyne > MeC ≡ CPh > n‐BuC ≡ CPh. The copolymers of MeC ≡ CSiMe3 with MeC ≡ CPh [copoly(TMSP/PP)s] had high molecular weight (Mw > 1 × 106), and provided thermally stable tough films. With increasing MeC ≡ CPh content of copoly(TMSP/PP), the oxygen permeability coefficient (P) decreased, while the separation factor (P/P) increased.

Polymerization of 1‐( pentduorophenyl)‐1‐alkynes by Ta and Nb catalysts

Polymerization of 1‐( pentduorophenyl)‐1‐alkynes by Ta and Nb catalysts

Carbon Monoxide Hydrogenation on SiO2−, Al2O3−, or TiO2-Attached Nb-Monomer Catalysts

Abstract The new low-valent Nb monomer catalysts which were prepared by taking advantage of the reaction between Nb(η3-C3H5)4 and surface OH groups on SiO2, Al2O3, or TiO2, followed by the treatments with H2, were found to be active for the hydrogenation of carbon monoxide in which C2–C4 hydrocarbons were mainly produced in the temperature range 423–598 K. In contrast, only the disproportionation reaction of CO to form CO2 proceeded on usual impregnation Nb catalysts and Nb2O5.

Polymerization of l‐(pentafluorophenyl)‐L‐alkynes by Ta and Nb catalysts

Le catalyseur TaCl 5 −n−Bu 4 Sn donne a partir de ces composes des nouveaux polymeres qui sont blancs, insolubles, infusibles et plus stables que les poly(L-phenyl alcynes-1) correspondants

New Inorganic Oxide-attached Nb Catalysts Active for Propene Metathesis

Abstract SiO2−, Al2O3−, and TiO2-attached Nb-monomer catalysts were prepared by the reactions between Nb(η3-C3H5)4 and surface OH groups of SiO2, Al2O3, or TiO2, followed by the treatments with H2 in the temperature range 323–673 K. These catalysts showed high activities for propene metathesis at the low temperatures 273–310 K, in contrast to the known fact that traditional supported Nb catalysts show almost no activities.

Polymerization of 1‐(trimethylsilyl)‐1‐propyne homologs containing two silicon atoms by tantalum‐ and niobium‐based catalysts

AbstractPolymerization of new 1‐(trimethylsilyl)‐1‐propyne homologs containing two silicon atoms [CH3CCSi(CH3)2CH2Si(CH3)3 and CH3CCSi(CH3)2CH2CH2Si(CH3)3] was investigated by use of Ta and Nb catalysts. CH3CCSi(CH3)2CH2Si(CH3)3 was polymerized quantitatively by TaCl5 alone to provide a polymer having molecular weight over 106. CH3CCSi(CH3)2CH2CH2Si(CH3)3 was polymerized in good yield by an equimolar mixture of TaCl5 with an appropriate organometallic cocatalyst such as Ph4Sn to give a polymer with molecular weight of ca. 4 X 105. Nb catalysts were less active toward these monomers than the corresponding Ta catalysts. These two kinds of polymers had alternating double bonds along the main chain according to IR and 13C‐NMR spectra. Both polymers were white solids completely soluble in low‐polarity solvents like toluene, and solution casting afforded uniform, tough films. These polymers were thermally fairly stable, and their softening points were above 350°C. Films of these polymers showed smaller oxygen permeability coefficients [P = 4 × 10−9 – 8 × 10−9 cm3(STP) · cm/(cm2·sec·cmHg)] but larger separation factors [(P/P) = 3.4 – 3.6] than a poly[1‐(trimethylsilyl)‐1‐propyne] film.

New SiO2-supported niobium monomer catalysts for dehydrogenation of ethanol

New Nbv monomers supported on SiO2, Al2O3, or TiO2 have been prepared by reaction between Nb(η3-C3H5)4 and the surface OH groups of SiO2, Al2O3, or TiO2, followed by treatment with H2 and O2; of these the SiO2-supported Nb catalyst shows high activity and selectivity for the dehydrogenation of ethanol to give acetaldehyde at 473–573 K unlike the preferential dehydration observed over the usual Nb impregnation catalysts.