967 resultados para Conversion catalytique
Resumo:
La gazeification est aujourd'hui l'une des strategies les plus prometteuses pour valoriser les dechets en energie. Cette technologie thermo-chimique permet une reduction de 95 % de la masse des intrants et genere des cendres inertes ainsi que du gaz de synthese (syngaz). Le syngaz est un combustible gazeux compose principalement de monoxyde de carbone (CO), d'hydrogene (H2) et de dioxyde de carbone (CO2). Le syngaz peut etre utilise pour produire de la chaleur et de l'electricite. Il est egalement la pierre angulaire d'un grand nombre de produits a haute valeur ajoutee, allant de l'ethanol a l'ammoniac et l'hydrogene pur. Les applications en aval de la production de syngaz sont dictees par son pouvoir calorifique, lui-meme dependant de la teneur du gaz en H2. Laugmentation du contenu du syngaz en H2 est rendu possible par la conversion catalytique a la vapeur deau, largement repandu dans le cadre du reformage du methane pour la production d'hydrogene. Au cours de cette reaction, le CO est converti en H2 et CO2 selon : CO + H2O CO2 + H2. Ce processus est possible grace a des catalyseurs metalliques mis en contact avec le CO et de la vapeur. La conversion catalytique a la vapeur deau a jusqu'ici ete reserve pour de grandes installations industrielles car elle necessite un capital et des charges dexploitations tres importantes. Par consequent, les installations de plus petite echelle et traitant des intrants de faible qualite (biomasse, dechets, boues ...), n'ont pas acces a cette technologie. Ainsi, la seule utilisation de leur syngaz a faible pouvoir calorifique, est limitee a la generation de chaleur ou, tout au plus, d'electricite. Afin de permettre a ces installations une gamme dapplication plus vaste de leurs syngaz, une alternative economique a base de catalyseur biologique est proposee par lutilisation de bacteries hyperthermophiles hydrogenogenes. L'objectif de cette these est d'utiliser Carboxydothermus hydrogenoformans, une bacterie thermophile carboxydotrophe hydrogenogene comme catalyseur biologique pour la conversion du monoxyde de carbone en hydrogene. Pour cela, limpact d'un phenomene de biomineralisation sur la production dH2 a ete etudie. Ensuite, la faisabilite et les limites de lutilisation de la souche dans un bioreacteur ont ete evaluees. Tout d'abord, la caracterisation de la phase inorganique predominante lorsque C. hydrogenoformans est inocule dans le milieu DSMZ, a revele une biomineralisation de phosphate de calcium (CaP) cristallin en deux phases. Lanalyse par diffraction des rayons X et spectrometrie infrarouge a transformee de Fourier de ce materiau biphasique indique une signature caracteristique de la Mg-whitlockite, alors que les images obtenues par microscopie electronique a transmission ont montre l'existence de nanotiges cristallines sapparentant a de lhydroxyapatite. Dans les deux cas, le mode de biomineralisation semble etre biologiquement induit plutot que controle. L'impact du precipite de CaP endogene sur le transfert de masse du CO et la production dH2 a ensuite ete etudie. Les resultats ont ete compares aux valeurs obtenues dans un milieu ou aucune precipitation n'est observee. Dans le milieu DSMZ, le KLa apparent (0.22 0.005 min-1) et le rendement de production dH2 (89.11 6.69 %) etaient plus eleves que ceux obtenus avec le milieu modifie (0.19 0.015 min-1 et 82.60 3.62% respectivement). La presence du precipite n'a eu aucune incidence sur l'activite microbienne. En somme, le precipite de CaP offre une nouvelle strategie pour ameliorer les performances de transfert de masse du CO en utilisant les proprietes hydrophobes de gaz. En second lieu, la conversion du CO en H2 par la souche Carboxydothermus hydrogenoformans fut etudiee et optimisee dans un reacteur gazosiphon de 35 L. Parmi toutes les conditions operationnelles, le parametre majeur fut le ratio du debit de recirculation du gaz sur le debit d'alimentation en CO (QR:Qin). Ce ratio impacte a la fois l'activite biologique et le taux de transfert de masse gaz-liquide. En effet, au dessus dun ratio de 40, les performances de conversion du CO en H2 sont limitees par lactivite biologique alors quen dessous, elles sont limitees par le transfert de masse. Cela se concretise par une efficacite de conversion maximale de 90.4 0.3 % et une activite specifique de 2.7 0.4 molCOg1VSSd1. Malgre des resultats prometteurs, les performances du bioreacteur ont ete limitees par une faible densite cellulaire, typique de la croissance planctonique de C. hydrogenoformans. Cette limite est le facteur le plus contraignant pour des taux de charge de CO plus eleves. Ces performances ont ete comparees a celles obtenues dans un reacteur a fibres creuses (BRFC) inocule par la souche. En depit dune densite cellulaire et dune activite volumetrique plus elevees, les performances du BRFC a tout le moins cinetiquement limitees quand elles netaient pas impactees par le transfert de masse, l'encrassement et le vieillissement de la membrane. Afin de parer a la degenerescence de C. hydrogenoformans en cas de penurie de CO, la croissance de la bacterie sur pyruvate en tant que seule source de carbone a ete egalement caracterisee. Fait interessant, en presence simultanee de pyruvate et de CO, C. hydrogenoformans na amorce la consommation de pyruvate quune fois le CO epuise. Cela a ete attribue a un mecanisme d'inhibition du metabolisme du pyruvate par le CO, faisant ainsi du pyruvate le candidat ideal pour un systeme in situ de secours.
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Le premier volet de ce travail portera sur lexprience acquise lors dun stage dtude Tokyo, au Japon, dans le groupe de recherche du Pr. Makoto Fujita, une sommit denvergure internationale dans le domaine de lauto-assemblage. En continuit avec les plus rcents travaux du Pr. Fujita, des systmes poreux auto-assembls prsentant des cavits fonctionnalises ont t dvelopps dans le but dencapsuler des acides gras afin den dterminer la structure cristalline. Ces ponges ont t caractrises par des techniques courantes telles que la spectroscopie rsonance magntique nuclaire 1H, 13C{1H} et Cosy, la spectromtrie de masse, lanalyse lmentaire, la microscopie optique infrarouge ainsi que la diffraction des rayons X. Une autre approche employe pour obtenir de meilleures proprits spectroscopiques fut la synthse de dendrimres mtalliques de gnration 0. Un nouveau ligand de type 1,3,5-triazine a t synthtis par une raction typique de cyclisation de nitrile en prsence catalytique dhydrure de sodium. Des espces mono-, bis- et trinuclaire de Ru(II) furent synthtiss ainsi que deux espces htromtalliques de Ru(II)/Pt(II) et de Ru(II)/Os(II). Tous les complexes obtenus furent caractriss par spectroscopie rsonance magntique nuclaire (1H, 13C{1H} et Cosy) ltat liquide, par spectroscopie de masse haute rsolution et par analyse lmentaire. La gnration de dihydrogne partir de lespce htromtallique a t tudie. Les proprits optiques et lectroniques ont t analyses par spectroscopie UV-Vis, par analyse de la luminescence, du temps de vie de luminescence, par des analyses de rendement quantique ainsi que par des analyses de voltampromtrie cyclique balayage. Finalement, dans le but damliorer les proprits spectroscopiques dabsorption de complexes mtalliques, nous avons synthtis une srie de polymres homo- et htromtalliques, intgrant des ligands de type bis(2,2:6,2-terpyridine). Les complexes gnrs furent caractriss par diverses techniques tel que la spectroscopie rsonance magntique nuclaire (1H, 13C{1H} et Cosy) ltat liquide, par spectroscopie de masse haute rsolution ainsi que par analyse lmentaire. Les proprits optiques et lectroniques ont t analyses par spectroscopie UV-Vis, par analyse de la luminescence, du temps de vie de luminescence, par des analyses de rendement quantique ainsi que par des analyses de voltampromtrie cyclique balayage.
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The aim of this study was to evaluate the degree of conversion (DC) and the cytotoxicity of photo-cured experimental resin composites containing 4-(N,N-dimethylamino)phenethyl alcohol (DMPOH) combined to the camphorquinone (CQ) compared with ethylamine benzoate (EDAB). The resin composites were mechanically blended using 35 wt% of an organic matrix and 65 wt% of filler loading. To this matrix was added 0.2 wt% of CQ and 0.2 wt% of one of the reducing agents tested. 5x1 mm samples (n=5) were previously submitted to DC measurement and then pre-immersed in complete culture medium without 10% (v/v) bovine serum for 1 h or 24 h at 37 C in a humidifier incubator with 5% CO2 and 95% humidity to evaluate the cytotoxic effects of experimental resin composites using the MTT assay on immortalized human keratinocytes cells. As a result of absence of normal distribution, the statistical analysis was performed using the nonparametric Kruskal-Wallis to evaluate the cytotoxicity and one-way analysis of variance to evaluate the DC. For multiple comparisons, cytotoxicity statistical analyses were submitted to Student-Newman-Keuls and DC analysis to Tukey's HSD post-hoc test (=0.05). No significant differences were found between the DC of DMPOH (49.9%) and EDAB (50.7%). 1 h outcomes showed no significant difference of the cell viability between EDAB (99.26%), DMPOH (94.85%) and the control group (100%). After 24 h no significant difference were found between EDAB (48.44%) and DMPOH (38.06%), but significant difference was found compared with the control group (p>0.05). DMPOH presented similar DC and cytotoxicity compared with EDAB when associated with CQ.
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To evaluate the influence of light-activation of second, third and fourth increments on degree of conversion (DC) and microhardness (KHN) of the top (T) and bottom (B) surface of the first increment. Forty samples (n = 5) were prepared. In groups 1-4, after each increment light-activation (multiple irradiation), T and B of the first increment were measured in DC and KHN. In groups 5-8, only the first increment was made (single irradiation) and measurements of DC and KHN were taken at 15 min intervals. The light-activation modes were (XL) 500 mW/cm(2) 38 s (G1/G5); (S) 1000 mW/cm(2) 19 s (G2/G6), (HP) 1400 mW/cm(2) 14 s (G3/G7); (PE) 3200 mW/cm(2) 6 s (G4/G8). Data for DC and KHN were analyzed separately by using PROC MIXED for repeated measures and Tukey-Kramer test ( = 0.05). For KHN, B showed lower values than T. PE resulted in lower values of KHN in B surface. For single and multiple irradiations, T and B of first measurement showed the lowest KHN and the fourth measurement showed the highest, with significant difference between them. For single irradiation, first and second increments presented similar KHN, different from the third and fourth increment, which did not differ between them. For multiple irradiations, the second light-activation resulted in KHN similar to first, third and fourth increments. For DC, except QTH, T presented higher DC than B. The light-activation of successive increments was not able to influence the KHN and DC of the first increment.
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Very low intensity and phase fluctuations are present in a bright light field such as a laser beam. These subtle quantum fluctuations may be used to encode quantum information. Although intensity is easily measured with common photodetectors, accessing the phase information requires interference experiments. We introduce one such technique, the rotation of the noise ellipse of light, which employs an optical cavity to achieve the conversion of phase to intensity fluctuations. We describe the quantum noise of light and how it can be manipulated by employing an optical resonance technique and compare it to similar techniques, such as Pound - Drever - Hall laser stabilization and homodyne detection. (c) 2008 American Association of Physics Teachers.
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The conversion of red excitation light into blue emission light (uphill energy conversion) using unstable 1,2-dioxetanes is described. The method is based on 1,2-dioxetane formation by red-light sensitized photooxygenation of adequate alkenes and subsequent blue-light emission due to thermal 1,2-dioxetane cleavage. The energy gain resulting from the chemical energy obtained in the transformation of an alkene into two carbonyl compounds transforms a red-light excitation laser beam into a blue-light chemiluminescence emission, producing thereby a formal anti-Stokes shift of 200-250 nm, opening up a whole spectrum of possible applications.
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Large-scale soy agriculture in the southern Brazilian Amazon now rivals deforestation for pasture as the region`s predominant form of land use change. Such landscape-level change can have substantial consequences for local and regional hydrology, but these effects remain relatively unstudied in this ecologically and economically important region. We examined how the conversion to soy agriculture influences water balances and stormflows using stream discharge (water yields) and the timing of discharge (stream hydrographs) in small (2.5-13.5 km2) forested and soy headwater watersheds in the Upper Xingu Watershed in the state of Mato Grosso, Brazil. We monitored water yield for 1 year in three forested and four soy watersheds. Mean daily water yields were approximately four times higher in soy than forested watersheds, and soy watersheds showed greater seasonal variability in discharge. The contribution of stormflows to annual streamflow in all streams was low (< 13% of annual streamflow), and the contribution of stormflow to streamflow did not differ between land uses. If the increases in water yield observed in this study are typical, landscape-scale conversion to soy substantially alters water-balance, potentially altering the regional hydrology over large areas of the southern Amazon.
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This work addressed the production of carbon nanomaterials (CNMs) by catalytic conversion of wastes from the bioethanol industry, in the form of either sugarcane bagasse or corn-derived distillers dried grains with solubles (DDGS). Both bagasse and DDGS were pyrolysed at temperatures in the range of 600-1000 degrees C. The pyrolyzate gases were then used as CNM growth agents by chemical vapor deposition on stainless steel meshes, serving as both catalysts and substrates. CNM synthesis temperatures of 750-1000 degrees C were explored, and it was determined that their growth was most pronounced at 1000 degrees C. The nanomaterials produced from pyrolysis of bagasse were in the form of long, straight, multi-wall nanotubes with smooth walls and axially uniform diameters. Typical lengths were circa 50 mu m and diameters were in the range of 20-80 nm. The nanomaterials produced from pyrolysis of DDGS were in the form of long, entangled, rope-like structures with rugged walls, and axially non-uniform diameters. Typical diameters were in the range of 100-300 nm and their lengths were in the tens of microns. This process also produces a bio-syngas byproduct that is enriched in hydrogen. (C) 2011 Elsevier B.V. All rights reserved.
One-step biomimetic conversion of a furanoheliangolide into an eremantholide using Stryker`s reagent
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The conversion of a furanoheliangolide structure (15-deoxygoyazensolide) into an eremantholide one (eremantholide C) was achieved by tandem hydride conjugate addition-intramolecular carbanion addition using Stryker`s reagent. (c) 2008 Elsevier Ltd. All rights reserved.
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The acid-mediated transformation of syn and anti methylene interrupted cis,cis and cis,trans bisepoxides to tetrahydrofurans is high yielding, and demonstrates both regioselectivity and stereoselectivity. Trans,trans methylene interrupted bisepoxides do not yield tetrahydrofurans under the same conditions.
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Catalytic conversion of N2O to N-2 over Cu- and Co-impregnated activated carbon catalysts (Cu/AC and Co/AC) was investigated. Catalytic activity measurements were carried out in a fixed-bed flow reactor at atmospheric pressure. The catalysts were characterized by N-2 adsorption, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). This study aimed to provide insights into the following aspects: the metal dispersion, changes in pore structure, influence of catalyst loading on reaction, and reaction mechanism. Increasing loading of Co or Cu led to decreasing dispersion, but 20 wt % loading was an upper limit for optimal activities in both cases, with too high loading causing sintering of metal. Co exhibited a relatively better dispersion than Cu. Impregnation of metal led to a large decrease in surface area and pore volume, especially for 30 wt % of loading. 20 wt % of loading has proved to be the optimum for both Cu and Co, which shows the highest activity. Both N2O-Co/AC and -Cu/AC reactions are based upon a redox mechanism, but the former is limited by the oxygen transfer from catalysts to carbon, while N2O chemisorption on the surface of Cu catalyst controls the latter. The removal of oxygen from cobalt promotes the activity of Co/AC, but it is beneficial for Cu/AC to keep plenty of oxygen to maintain the intermediate oxidation of copper-Cu1+. The different nature of the two catalysts and their catalytic reaction mechanisms are closely related to their different electronegativities.
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Catalytic conversion of N2O to N-2 With potassium catalysts supported on activated carbon (K/AC) was investigated. Potassium proves to be much more active and stable than either copper or cobalt because potassium possesses strong abilities both for N2O chemisorption and oxygen transfer. Potassium redispersion is found to play a critical role in influencing the catalyst stability. A detailed study of the reaction mechanism was conducted based upon three different catalyst loadings. It was found that during temperature-programmed reaction (TPR), the negative oxygen balance at low temperatures (< 50 degrees C) is due to the oxidation of the external surface of potassium oxide particles, while the bulk oxidation accounts for the oxygen accumulation at higher temperatures (below ca. 270 degrees C). N2O is beneficial for the removal of carbon-oxygen complexes because of the formation of CO2 instead of CO and because of its role in making the chemisorption of produced CO2 on potassium oxide particles less stable. A conceptual three-zone model was proposed to clarify the reaction mechanism over K/AC catalysts. CO2 chemisorption at 250 degrees C proves to be an effective measurement of potassium dispersion. (C) 1999 Academic Press.
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The effect of acidic treatments on N2O reduction over Ni catalysts supported on activated carbon was systematically studied. The catalysts were characterized by N-2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). It is found that surface chemistry plays an important role in N2O-carbon reaction catalyzed by Ni catalyst. HNO3 treatment produces more active acidic surface groups such as carboxyl and lactone, resulting in a more uniform catalyst dispersion and higher catalytic activity. However, HCl treatment decreases active acidic groups and increases the inactive groups, playing an opposite role in the catalyst dispersion and catalytic activity. A thorough discussion of the mechanism of the N2O catalytic reduction is made based upon results from isothermal reactions, temperature-programmed reactions (TPR) and characterization of catalysts. The effect of acidic treatment on pore structure is also discussed. (C) 1999 Elsevier Science B.V. All rights reserved.
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Reports on the use of sirolimus (SRL) in pancreas transplantation are still limited. The aim of this study was to evaluate the outcome of SRL conversion in pancreas transplant patients. Among 247 patients undergoing simultaneous kidney-pancreas or solitary pancreas transplantation, 33 (13%) were converted to SRL. The reasons for conversion were calcineurin inhibitors (CNI) nephrotoxicity (n = 24; 73%), severe neurotoxicity owing to CNI (n = 1; 3%), severe and/or recurrent acute rejection episodes (n = 7; 21.%), gastrointestinal (GI) side effects of mycophenolate mofetil (MMF; n = 5; 15%), and hyperglycemia (n = 4; 12%). Before conversion, all patients were maintained on a CNI, MMF, and low-dose steroids. They were gradually converted to SRL associated with either CNI or MMF withdrawal. Sixty-three percent (n = 15) of patients who were converted owing to CNI nephrotoxicity, showed stable or improved renal function. At 12 months after conversion, serum creatinine levels were significantly decreased in this group (2.2 +/- 0.5 vs 1.6 +/- 0.3 mg/dL; P = .001) and C-peptide values increased (2.9 +/- 1.1.1 vs 3.1 +/- 1.3 nmol/L; P = .01.8). The only patient with leucoencephalopathy showed improved neurologic status after SRL conversion. All patients converted to SRL because of GI side effects of MMF showed improvements, and none of those converted because of hyperglycemia experienced improvement. There were no episodes of acute rejection after conversion. We concluded that conversion to SRL in pancreas transplantation should be considered an important alternative strategy, particularly for CNI nephrotoxicity and neurotoxicity, and in cases of severe diarrhea due to MMF.
