Página 1 dos resultados de 224 itens digitais encontrados em 0.002 segundos

Propriedades de blendas ternárias de polipropileno

Tedesco, Adriana
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Tese de Doutorado Formato: application/pdf
POR
Relevância na Pesquisa
17.66%
Os copolímeros heterofásicos ou blendas de reator formam um sistema bastante complexo constituído de um copolímero randômico de eteno-propeno (EPR), um copolímero de eteno-propeno com diferentes teores e seqüência de propeno, gerando uma fase rica em PE (co-PE), e homopolímero de propileno (PP). Estudaram-se três copolímeros heterofásicos com EPR de diferentes composições e teores diferentes do co-PE. Os copolímeros heterofásicos foram misturados com homopolímero de propileno (PP), com peso molecular similar ao da matriz polimérica, para obter blendas com diferentes proporções e composições de EPR. As composições das blendas foram determinadas pelas técnicas de fracionamento em xileno, FTIR e DSC. Os resultados da caracterização evidenciaram que as amostras são constituídas por homopolímero de propileno (PP), copolímero de eteno-propeno (borracha EPR) e copolímero de eteno-propeno rico em eteno (co-PE). A composição da amostra com maior teor de eteno na EPR e do co-PE resultou na formação de co-PE com dois perfis de composição. A morfologia dos sistemas estudados foi avaliada por microscopia de força atômica (MFA) e microscopia eletrônica de varredura (MEV). Os resultados mostraram uma grande distribuição do tamanho das partículas da EPR na matriz e a adição do PP nas amostras resultou no aumento dos domínios da EPR. As amostras com alto teor de co-PE e de eteno na EPR...

Estudo da influência do ambiente química e textual do suporte sobre as propriedades catalíticas dos sistemas híbridos Ni(MeCN)6(BF4)2/SiO2, [Si,AL]-MCM-41 ou [Si]-MCM-41 na reação de oligomerização de eteno e propeno

Rodrigues, Larissa Ribeiro
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Tese de Doutorado Formato: application/pdf
POR
Relevância na Pesquisa
17.74%
Este trabalho trata do estudo das reações de oligomerização de eteno e propeno catalisada pelo complexo Ni(MeCN)6(BF4)2 imobilizado por adsorção seletiva nos suportes inorgânicos [Si]-MCM-41, [Al]-MCM-41 e SiO2 associados aos agentes alquilantes AlEt3 (oligomerização de eteno) e Al2Et3Cl3 (oligomerzação de propeno). Os sistemas suportados são avaliados em suspensão de diclorometano e clorobenzeno em um reator semicontínuo com alimentação à pressão constante de eteno ou propeno. A caracterização por a DRIFT e XPS dos sistemas suportados em [Si] - MCM-41 e [Si, Al] - MCM-41 demonstra a presença de espécies ativas diferentes e mostra que a esfera da coordenação de níquel é modificada pela natureza do suporte. As análises de TEM e XRD demonstram que a organização dos poros dos sistemas MCM-41 é mantida após a imobilização do complexo. A análise da adsorção do N2 demonstra que a imobilização do complexo ocorre dentro dos poros dos sistemas suportados em MCM-41 e SiO2. Os resultados que referem os testes catalíticos demonstram que estes novos sistemas são ativos na dimerização e na trimerização do eteno e do propeno. Os resultados que referem os testes catalíticos demonstram que estes novos sistemas são ativos na dimerização e na trimerização do eteno e do propeno. A concentração dos agentes de alquilação revela ser um parâmetro muito sensível em relação à produção seletiva de 1-buteno e à atividade das reações de oligomerização do propeno. São apresentados...

Mechanistic insights into the reaction between VO2+ and propene based on a DFT study

Gracia, L.; Sambrano, JR; Andres, J.; Beltran, A.
Fonte: Amer Chemical Soc Publicador: Amer Chemical Soc
Tipo: Artigo de Revista Científica Formato: 1643-1653
ENG
Relevância na Pesquisa
27.66%
Calculations based on density functional theory have been carried out to investigate the free energy profiles at singlet and triplet electronic states associated with the gas-phase ion/molecule reactions of VO2++ ((1)A(1)/(3)A) with propene. The complex potential energy Surfaces, including Six reaction pathways (three dehydrogenation and three oxygen transfer processes), have been explored and analyzed. Along dehydrogenation reactive channels, three final products can be obtained: V(OH)(2)(+) ((1)Sigma(+)/(3)Sigma(-)) and allene (path Dehl), being the most kinetically and thermodynamically favorable reaction pathway, V(OH)(2)(+) ((1)Sigma(+)/(3)Sigma(-)) and propyne (path Deh2),and VO2+ ((1)A(1)/(3)A) and H-2 plus allene (path Deh3). The oxyoenation processes can yield its final products Vo(+) ((1)Delta/(3)Sigma) and acetone (path Ox1), VO+ ((1)Delta/(3)Sigma 2) and propanaldehyde (path Ox2), and VO+ ((1)Delta/(3)Sigma) and H-2 and propenaldehyde (path Ox3). Both paths Deh1 and Deh2 are associated with two consecutive hydrogen transfer processes from carbon atoms of the propene fragment to vanadyl oxygen atoms, while in path Deh3 the second hydrogen migration takes place to the vanadiurn atorn followed by the formation ola hydrogen molecule. Both paths Ox1 and Ox2 comprise an intramolecular hydrogen transfer between the ethylenic moiety of the propene fragment...

Polyfunctionalized carbohydrate-derived scaffolds for the production of libraries of bioactive compounds

Silva, Ana Catarina de Araújo, 1979-
Fonte: Universidade de Lisboa Publicador: Universidade de Lisboa
Tipo: Tese de Doutorado
Publicado em //2010 ENG
Relevância na Pesquisa
17.53%
Tese de doutoramento, Química (Química Orgânica), Universidade de Lisboa, Faculdade de Ciências, 2010.; Inspired by the role of carbohydrates as natural scaffolds, we exploited the sugar skeleton to generate new libraries of polyfunctionalized compounds as GABAA receptor ligands. Hybrids of benzodiazepines, γ-butyrolactone and -lactam derivatives, and a GABA analogue were developed. The incorporated sugar moiety offered the possibility of diverse and controlled functionalization, modulating physicochemical and structural properties, namely solubility and rigidity, and consequently biological activity of the synthesized scaffolds. 1,4-Benzodiazepine-2,5-dione scaffolds (i and iv) derived from spiro bicyclic D- or Lproline analogues (ii and v), containing a D- or a L-fructose moiety were synthesized in the present work. The D-proline analogue ii was prepared in a sixteen-step synthesis in 24% overall yield, adopting a methodology which used D-fructose as starting material and 3-C-(3,4,6-tri-O-benzyl-α-D-fructofuranos-2-yl)propene iii as key intermediate. Instead, the L-fructose moiety, required for the preparation of the corresponding L-proline analogue v, was obtained using a new synthetic pathway. The key intermediate of this synthesis...

Continuous degradation of trichloroethylene by Xanthobacter sp. strain Py2 during growth on propene.

Reij, M W; Kieboom, J; de Bont, J A; Hartmans, S
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /08/1995 EN
Relevância na Pesquisa
27.91%
Propene-grown Xanthobacter sp. strain Py2 cells can degrade trichloroethylene (TCE), but the transformation capacity of such cells was limited and depended on both the TCE concentration and the biomass concentration. Toxic metabolites presumably accumulated extracellularly, because the fermentation of glucose by yeast cells was inhibited by TCE degradation products formed by strain Py2. The affinity of the propene monooxygenase for TCE was low, and this allowed strain Py2 to grow on propene in the presence of TCE. During batch growth with propene and TCE, the TCE was not degraded before most of the propene had been consumed. Continuous degradation of TCE in a chemostat culture of strain Py2 growing with propene was observed with TCE concentrations up to 206 microns in the growth medium without washout of the fermentor occurring. At this TCE concentration the specific degradation rate was 1.5 nmol/min/mg of biomass. The total amount of TCE that could be degraded during simultaneous growth on propene depended on the TCE concentration and ranged from 0.03 to 0.34g of TCE per g of biomass. The biomass yield on propene was not affected by the cometabolic degradation of TCE.

Low stereoselectivity in methylacetylene and cyclopropene reductions by nitrogenase.

McKenna, C E; McKenna, M C; Huang, C W
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /10/1979 EN
Relevância na Pesquisa
17.74%
The stereochemistry of reductions catalyzed by nitrogenase in 2H2O has been investigated by using allene, methylacetylene, and cyclopropene as substrates. Deuterium labeling patterns in the reduction products were determined by mass spectroscopy, infrared spectroscopy, 2H-decoupled 220-MHz 1H NMR, and 1H-decoupled 30.7-MHz 2H NMR. Reduction of allene gave pure [2,3-2H2]propene. Reduction of methyl acetylene gave a 1.8:1.0 mixture of [cis- and [trans-1,2-2H2]propene. (Similar reduction of acetylene reportedly gave virtually all [cis-1,2-2H2]ethylene.) Reduction of cyclopropene gave [cis-1,2-2H2]cyclopropane and a mixture of [2H2]propenes. The major propene 2H2 isomers formed were [trans-1,3-2H2]-propene (approximately 2), [cis-1,3-2H2]propene (approximately 1) and [2,3-2H2]propene (approximately 1). Cyclopropene appears to be unique as a nitrogenase substrate in that it simultaneously undergoes parallel reductions, one of which proceeds with high stereoselectivity while the other proceeds with low stereoselectivity. The weakly selective stereochemistry observed in these reductions is not consistent with a completely concerted dual proton-dual electron transfer mechanism. The results provide a basis to probe stereochemical effects in nitrogenase and in biomimetic model systems.

Reduction of cyclopropene by NifV- and wild-type nitrogenases from Klebsiella pneumoniae.

Gemoets, J P; Bravo, M; McKenna, C E; Leigh, G J; Smith, B E
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 01/03/1989 EN
Relevância na Pesquisa
17.53%
The nitrogenase from wild-type Klebsiella pneumoniae reduces cyclopropene to cyclopropane and propene in the ratio 1:2 at pH 7.5. We show in this paper that the nitrogenase from a nifV mutant of K. pneumoniae also reduces cyclopropene to cyclopropane and propene, but the ratio of products is now 1:1.4. However, both nitrogenases exhibit the same Km for cyclopropene (2.1 x 10(4) +/- 0.2 x 10(4) Pa), considerably more than the Km for the analogous reaction with Azotobacter vinelandii nitrogenase under the same conditions (5.1 x 10(3) Pa). Analysis of the data shows that the different product ratio arises from the slower production of propene compared with cyclopropane by the mutant nitrogenase. During turnover, both nitrogenases use a large proportion of the electron flux for H2 production. CO inhibits the reduction of cyclopropene by both K. pneumoniae proteins, but the mutant nitrogenase exhibits 50% inhibition at approx. 10 Pa, whereas the corresponding value for the wild-type nitrogenase is approx. 110 Pa. However, H2 evolution by the mutant enzyme is much less affected than is cyclopropene reduction. CO inhibition of cyclopropene reduction by the nitrogenases coincides with a relative increase in H2 evolution, so that in the wild-type (but not the mutant) the electron flux is approximately maintained. The cyclopropane/propene production ratios are little affected by the presence of CO within the pressure ranges studied at least up to 50% inhibition.

The negative effect of ceria on the propene selectivity for isopropanol decomposition over phosphated and phosphate-free ceria/alumina catalysts

Khalaf, Hussein A
Fonte: Springer International Publishing Publicador: Springer International Publishing
Tipo: Artigo de Revista Científica
Publicado em 20/11/2013 EN
Relevância na Pesquisa
27.06%
Consequences of loading level of ceria (5, 10 and 20-wt%) on the structural, textural and surface acid–base properties as well as the impacts on catalytic activity towards isopropanol decomposition on pure and phosphated alumina is described. The obtained results indicated that composites mainly kept γ–alumina structure and the addition of ceria and/or phosphate has no effect on the bulk structure of alumina. Textural characteristics show that the phosphated composites have higher surface area than phosphate free samples. Pyridine adsorption as followed by FTIR indicated the presence of Lewis acid sites with different strengths. Moreover, results of gas phase decomposition of isopropanol show a systematic inhibition in propene selectivity based on ceria ratio.

Identification and Environmental Distribution of dcpA, Which Encodes the Reductive Dehalogenase Catalyzing the Dichloroelimination of 1,2-Dichloropropane to Propene in Organohalide-Respiring Chloroflexi

Padilla-Crespo, Elizabeth; Yan, Jun; Swift, Cynthia; Wagner, Darlene D.; Chourey, Karuna; Hettich, Robert L.; Ritalahti, Kirsti M.; Löffler, Frank E.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /02/2014 EN
Relevância na Pesquisa
27.53%
Dehalococcoides mccartyi strains KS and RC grow with 1,2-dichloropropane (1,2-D) as an electron acceptor in enrichment cultures derived from hydrocarbon-contaminated and pristine river sediments, respectively. Transcription, expression, enzymatic, and PCR analyses implicated the reductive dehalogenase gene dcpA in 1,2-D dichloroelimination to propene and inorganic chloride. Quantitative real-time PCR (qPCR) analyses demonstrated a D. mccartyi cell increase during growth with 1,2-D and suggested that both D. mccartyi strains carried a single dcpA gene copy per genome. D. mccartyi strain RC and strain KS produced 1.8 × 107 ± 0.1 × 107 and 1.4 × 107 ± 0.5 × 107 cells per μmol of propene formed, respectively. The dcpA gene was identified in 1,2-D-to-propene-dechlorinating microcosms established with sediment samples collected from different geographical locations in Europe and North and South America. Clone library analysis revealed two distinct dcpA phylogenetic clusters, both of which were captured by the dcpA gene-targeted qPCR assay, suggesting that the qPCR assay is useful for site assessment and bioremediation monitoring at 1,2-D-contaminated sites.

A study of the thermal decomposition of allyl t-butyl peroxide and 3-hydroperoxy-1-propene (allyl hydroperoxide) in toluene /|nby Krishnankutty Nair V. G. -- 260 St. Catharines [Ont. : s. n.],

Nair, Krishnankutty V. G.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
ENG
Relevância na Pesquisa
27.06%
Kinetics and product studies of the decompositions of allyl-t-butyl peroxide and 3-hydroperoxy- l-propene (allyl hydroperoxide ) in tolune were investigated. Decompositions of allyl-t-butyl peroxide in toluene at 130-1600 followed first order kinetics with an activation energy of 32.8 K.cals/mol and a log A factor of 13.65. The rates of decomposition were lowered in presence of the radical trap~methyl styrene. By the radical trap method, the induced decomposition at 1300 is shown to be 12.5%. From the yield of 4-phenyl-l,2- epoxy butane the major path of induced decomposition is shown to be via an addition mechanism. On the other hand, di-t-butYl peroxyoxalate induced decomposition of this peroxide at 600 proceeded by an abstraction mechanism. Induced decomposition of peroxides and hydroperoxides containing the allyl system is proposed to occur mainly through an addition mechanism at these higher temperatures. Allyl hydroperoxide in toluene at 165-1850 decomposes following 3/2 order kinetics with an Ea of 30.2 K.cals per mole and log A of 10.6. Enormous production of radicals through chain branching may explain these relatively low values of E and log A. The complexity of the reaction is indicated a by the formation of various products of the decomposition. A study of the radical attack of the hydro peroxide at lower temperatures is suggested as a further work to throw more light on the nature of decomposition of this hydroperoxide.

Emulsion Copolymerization of alpha-Olefins with Carbon Monoxide Using Water-Soluble Palladium(II) Complexes; Emulsionscopolymerisation von alpha-Olefinen und Kohlenmonoxid mit wasserlöslichen Palladium(II)-Komplexen

Sunjuk, Mahmoud Shaker Ahmed
Fonte: Universidade de Tubinga Publicador: Universidade de Tubinga
Tipo: Dissertação
EN
Relevância na Pesquisa
17.94%
The objective of this thesis was the development of stable dispersions for aqueous polyketone latices on the basis of the copolymers propene/undecenoic acid/CO, butene/undecenoic acid/CO, and propene/butene/undecenoic acid/CO. Therefore, attention has been focused on the design of new water-soluble palladium(II) catalysts for the production of stable polymer latices with (i) high solids contents, (ii) high molecular weights, (iii) particle sizes in the range between 50 and 500 nm, and (iv) glass transition temperatures with values below room temperature. In the first chapter of this thesis the synthesis and characterization of novel diphosphine ligands and their palladium(II) complexes is described. The diphosphines R2P(CH2)3PR2 [R = (CH2)nP(O)(OEt)2, n = 2-6, 8; (CH2)nOH, n = 6; (CH2)nNH2, n = 3] were obtained by heating P(OEt)3 with 1,3-dibromopropane according to a Michaelis-Arbuzov reaction, followed by reduction of the resulting diphosphate with LiAlH4 in diethyl ether to give H2P(CH2)3PH2. An excess of the corresponding olefin CH2=CH.(CH2)n.X (X = OH, n = 4; P(O)(OEt)2, n = 0-4, 6; NH2, n = 1) was photochemically hydrophosphinated with the diprimary phosphine H2P(CH2)3PH2 over night. This convenient synthesis is nearly quantitative and simplifies the purification of the products. Purification is achieved by removing excess alkene under reduced pressure. The alkenyl phosphonates CH2=CH.(CH2)n.P(O)(OEt)2 were obtained either from commercial suppliers (n = 0...

A MATRIX ISOLATION SPECTROSCOPIC INVESTIGATION INTO THE REACTION PRODUCTS OF VANADIUM METAL ATOMS WITH PROPENE

Walker, Stephen
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado Formato: 1482706 bytes; application/pdf
EN; EN
Relevância na Pesquisa
38.05%
The products of vanadium metal atom reactions with propene and some propene isotopomers (propene-d6 and propene-3,3,3-d3) are investigated using FT-IR matrix isolation. The major product from the condensation of V atoms with propene at elevated mole ratios is found to be propane (C3H8), the production of which is seen to increase as concentration of propene increases. Additionally a matrix isolated product formed after metal atom insertion into the C-H bond of propene at low propene mole ratios is isolated and identified. The location of the insertion site is identified as one of the methyl hydrogen carbon bonds. The structure of the product is identified as an allyl vanadium hydride complex, through a FT-IR matrix isolation study of propene isotopomers. It is also shown that this primary product acts as an intermediate in the formation of propane. A full mechanism for the proposed formation of propane from sacrificial hydrogenation is proposed and compared with the reported mechanism for the similar reaction involving ethene. The mechanistic identification of the hydrogenation of propene is shown as a generalization of the previous reaction involving ethene. Photochemistry of reactants and intermediates trapped in the matrix are investigated. Irradiation of matrices with several different UV-visible wavelength ranges indicate that no further chemistry occurs after formation of the matrix and further irradiation has no effect on intermediates or reactants. Additionally the reactivity of water with vanadium and propene under low propene concentration conditions is also studied. Results from this study show that under all conditions studied no incorporation of water into the propene molecule is found.; Thesis (Master...

Lignin peroxidase initiates O2-dependent self-propagating chemical reactions which accelerate the consumption of 1-(3',4'-dimethoxyphenyl)propene.

ten Have , R; Franssen, M C; Field, J A
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 15/04/2000 EN
Relevância na Pesquisa
27.06%
Lignin peroxidase (LiP) has been used to study the C(alpha)-C(beta) cleavage of the propylene side chain in 1-(3',4'-dimethoxyphenyl)propene (DMPP) to 3,4-dimethoxybenzaldehyde (veratraldehyde, VAD). Under an air atmosphere, LiP oxidized DMPP to VAD (27.8%) and 1-(3',4'-dimethoxyphenyl)propan-2-one (DMPA, 8.7%), after 10 min of incubation. Dissolved O(2) was rapidly consumed during DMPP conversion, of which one-third was converted into superoxide. The remaining two-thirds of the consumed O(2) was involved in C(alpha)-C(beta) cleavage of DMPP to VAD and in self-propagating chemical reactions stimulating the consumption of DMPP. The involvement of peroxyl radicals, in the chemical consumption of DMPP, was confirmed by using the well-known peroxyl radical reductant Mn(2+). This metal ion severely inhibited the DMPP consumption rate under air, but did not affect the lower enzymic DMPP consumption rate under N(2). The substoichiometric requirement of LiP for H(2)O(2) during DMPP oxidation could be explained in part by dismutation of superoxide, but more importantly by direct chemical reactions of DMPP-derived peroxyl radicals with fresh DMPP. Another VAD-producing route was found by incubating the DMPP oxidation product, DMPA, with LiP. Under air the molar yield of VAD was 29.7%. In the absence of O(2)...

Cloning, Expression, and Site-Directed Mutagenesis of the Propene Monooxygenase Genes from Mycobacterium sp. Strain M156

Chan Kwo Chion, Chan K.; Askew, Sarah E.; Leak, David J.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /04/2005 EN
Relevância na Pesquisa
27.06%
Propene monooxygenase has been cloned from Mycobacterium sp. strain M156, based on hybridization with the amoABCD genes of Rhodococcus corallinus B276. Sequencing indicated that the mycobacterial enzyme is a member of the binuclear nonheme iron monooxygenase family and, in gene order and sequence, is most similar to that from R. corallinus B-276. Attempts were made to express the pmoABCD operon in Escherichia coli and Mycobacterium smegmatis mc2155. In the former, there appeared to be a problem resolving overlapping reading frames between pmoA and -B and between pmoC and -D, while in the latter, problems were encountered with plasmid instability when the pmoABCD genes were placed under the control of the hsp60 heat shock promoter in the pNBV1 vector. Fortuitously, constructs with the opposite orientation were constitutively expressed at a level sufficient to allow preliminary mutational analysis. Two PMO active-site residues (A94 and V188) were targeted by site-directed mutagenesis to alter their stereoselectivity. The results suggest that changing the volume occupied by the side chain at V188 leads to a systematic alteration in the stereoselectivity of styrene oxidation, presumably by producing different orientations for substrate binding during catalysis. Changing the volume occupied by the side chain at A94 produced a nonsystematic change in stereoselectivity...

Synthesis and properties coming from the copolymerization of propene with alpha-olefins using different metallocene catalysts

Galland, Griselda B.; López Majada, Juan M.; Guevara, Juan Luis; Rabagliati, Franco; Quijada Abarca, Raúl
Fonte: ELSEVIER SCI LTD Publicador: ELSEVIER SCI LTD
Tipo: Artículo de revista
EN
Relevância na Pesquisa
27.06%
Copolymerization of propene with two alpha-olefins (1-hexene and 1-octadecene) using iso- and syndioselective metallocene catalysts (EtInd(2)ZrCl(2), Et(2-MeInd)(2)ZrCl2, Me(2)SiInd(2)ZrCl(2), Ph(2)CFluCpZrCl(2) and Me(2)CFluCpZrCl(2)) activated with methylaluminoxane (MAO) is reported. The so-called comonomer effect was seen in the catalytic activity of the Me(2)SiInd(2)ZrCl(2)/MAO system. Incorporation of syndiotactic copolymers was greater than that of isotactic copolymers. The molecular weight of the isotactic copolymers was not affected significantly by the presence of the comonomer, but the molecular weights of the copolymers obtained with the syndioselective catalysts decreased with increasing comonomer concentration in the medium. Tensile properties were studied. Syndiotactic copolymers with incorporation of the order of 6 mol% of 1-octadecene presented elastomeric properties.

Electron and positron scattering cross sections for propene and cyclopropane

Makochekanwa, Casten; Hoshino, Masamitsu; Kato, H; Sueoka, O; Kimura, Mineo; Tanaka, H
Fonte: American Physical Society Publicador: American Physical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
27.06%
In this paper we investigate electron and positron total and electron vibrational excitation scattering cross sections for propene and cyclopropane molecules. The electron and positron total cross sections were measured over the energy range 0.2-1000 eV u

Detection of Propene in Titan's Stratosphere

Nixon, Conor A.; Jennings, Donald E.; Bezard, Bruno; Vinatier, Sandrine; Teanby, Nicholas A.; Sung, Keeyoon; Ansty, Todd M.; Irwin, Patrick G. J.; Gorius, Nicolas; Cottini, Valeria; Coustenis, Athena; Flasar, F. Michael
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
27.06%
The Voyager 1 flyby of Titan in 1980 gave a first glimpse of the chemical complexity of Titan's atmosphere, detecting many new molecules with the infrared spectrometer (IRIS). These included propane (C3H8) and propyne (CH3C2H), while the intermediate-sized C3Hx hydrocarbon (C3H6) was curiously absent. Using spectra from the Composite Infrared Spectrometer (CIRS) on Cassini, we show the first positive detection of propene (C3H6) in Titan's stratosphere (5-sigma significance), finally filling the three-decade gap in the chemical sequence. We retrieve a vertical abundance profile from 100-250 km, that varies slowly with altitude from ~2 ppbv at 100 km, to ~5 ppbv at 200 km. The abundance of C3H6 is less than both C3H8 and CH3C2H, and we remark on an emerging paradigm in Titan's hydrocarbon abundances whereby: alkanes > alkynes > alkenes within the C2Hx and C3Hx chemical families in the lower stratosphere. More generally, there appears to be much greater ubiquity and relative abundance of triple-bonded species than double-bonded, likely due to the greater resistance of triple bonds to photolysis and chemical attack.; Comment: 16 pages; 4 figures; 1 table. Withdrawn due to journal policy

Mechanism of Selective Ammoxidation of Propene to Acrylonitrile on Bismuth Molybdates from Quantum Mechanical Calculations

Pudar, Sanja; Oxgaard, Jonas; Goddard, William A. III
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 23/09/2010
Relevância na Pesquisa
27.35%
In order to understand the mechanism for selective ammoxidation of propene to acrylonitrile by bismuth molybdates, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for the various steps involved in converting the allyl-activated intermediate to acrylonitrile over molybdenum oxide (using a Mo_3O_9 cluster model) under conditions adjusted to describe both high and low partial pressures of NH_3 in the feed. We find that the rate-determining step in converting of allyl to acrylonitrile at all feed partial pressures is the second hydrogen abstraction from the nitrogen-bound allyl intermediate (Mo−NH−CH_2−CH═CH_2) to form Mo−NH═CH−CH═CH_2). We find that imido groups (Mo═NH) have two roles: (1) a direct effect on H abstraction barriers, H abstraction by an imido moiety is (~8 kcal/mol) more favorable than abstraction by an oxo moiety (Mo═O), and (2) an indirect effect, the presence of spectator imido groups decreases the H abstraction barriers by an additional ~15 kcal/mol. Therefore, at higher NH_3 pressures (which increases the number of Mo═NH groups), the second H abstraction barrier decreases significantly, in agreement with experimental observations that propene conversion is higher at higher partial pressures of NH_3. At high NH_3 pressures we find that the final hydrogen abstraction has a high barrier [ΔH‡_(fourth-ab) = 31.6 kcal/mol compared to ΔH‡_(second-ab) = 16.4 kcal/mol] due to formation of low Mo oxidation states in the final state. However...

Reaction Mechanism for Ammonia Activation in the Selective Ammoxidation of Propene on Bismuth Molybdates

Pudar, Sanja; Goddard, William A., III
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 15/10/2015
Relevância na Pesquisa
27.35%
In this paper, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for various pathways of ammonia activation on bismuth molybdates, a process required for ammoxidation of propene to acrylonitrile. Using a Mo_3O_9 cluster to model the bulk surface, we examined the activation of ammonia by both fully oxidized (MoIV) and reduced (Mo^(IV)) molybdenum sites. Our results show that ammonia activation does not take place on the fully oxidized Mo(VI) sites. Here the net barriers for the first hydrogen transfer (ΔE‡ = 44.6 kcal/mol, ΔG‡_(673K) = 44.2 kcal/mol) and the second hydrogen transfer (ΔE‡ = 54.5 kcal/mol, ΔG‡_(673K) = 51.7 kcal/mol) are prohibitively high for the reaction temperature of 400 °C. Instead, our calculations show that the reduced Mo(IV) surface sites are far more suitable for this process. Here, the calculated barrier for the first hydrogen transfer from a Mo(IV)–NH_3 to an adjacent Mo(VI)═O is 18.2 kcal/mol (ΔG‡_(673K) = 15.4 kcal/mol). For the second hydrogen transfer step, we explored three pathways, and found that the H transfer from a Mo–NH_2 to an adjacent Mo(V)–OH to form water is more favorable (ΔE‡ = 26.2 kcal/mol (ΔG‡_(673K) = 24.0 kcal/mol) than transfer to an adjacent Mo(VI)═O or Mo(V)═O group. These studies complement previous studies for activation and reaction of propene on these surfaces...

Monomolecular alkane activation on Al-- and Fe--containing zeolites

Yun, Jang Ho
Fonte: University of Delaware Publicador: University of Delaware
Tipo: Tese de Doutorado
Relevância na Pesquisa
27.06%
Lobo, Raul F.; Zeolites are crystalline aluminosilicate materials with an open micro-porous three-dimensional framework structure and are used as a solid acid catalyst in oil refining and petrochemical industries, in processes such as fluidized catalytic cracking (FCC). FCC catalysts constitute more than 95% of zeolite catalyst consumption. Other zeolite-catalyzed processes include hydroxylation, alkylation, and epoxidation. Even though synthetic zeolites have been used in the industry for decades, prospects are still bright for recent new challenges and applications. Synthetic and post-synthetic modifications of zeolites can be used in the improvement of catalysts; the modification of zeolites can lead to new catalytic chemistry. In this thesis, we first investigated the effect of thermal treatments of SSZ-13 (CHA) with Si/Al ratios of 6 and 12 for the monomolecular propane conversion. SSZ-13 with Si/Al of 12 exhibited a gradual change in selectivity and activation energy with treatment temperature, indicating generation of different active sites. Second, Fe3+ was isomorphously incorporated in the zeolite structure instead of Al3+ initially to compare the difference in acidity on alkane activation. A novel redox catalytic cycle was proposed and a mechanistic study with H-[Fe]ZSM-5 was explored to test the proposed mechanism. In the next section...