• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 632
  • 170
  • 81
  • 68
  • 36
  • 19
  • 14
  • 10
  • 7
  • 6
  • 5
  • 4
  • 3
  • 2
  • 2
  • Tagged with
  • 1276
  • 193
  • 181
  • 169
  • 118
  • 116
  • 113
  • 110
  • 106
  • 99
  • 95
  • 89
  • 86
  • 85
  • 84
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
211

Oxidation catalyst studies on a diesel engine

Ye, Shifei January 2010 (has links)
In this thesis, the experimental test facilities consisted of a well instrumented live Ford 2.0 litre turbocharged diesel engine connected to a specially made exhaust can, which contained a diesel oxidation catalyst (DOC). Experiments were performed on DOCs, which were specially prepared by Johnson Matthey, and had thermocouples mounted in their walls to measure axial temperature profiles. These DOCs consisted of a Pt catalyst dispersed in an alumina washcoat on a cordierite monolith supports, and were representative of a commercial application. Experiments were performed on Full-scale DOCs (o.d. = 106 mm, length = 114 mm), and also on Thin-slice DOCs (length = 5 and 10 mm), which generate some interesting data, and enabled a technique that is normally only used in laboratory bench-top experiments to be applied to a live engine. A number of different methodologies were developed based on (a) the operation of the engine at pseudo-steady-state operating conditions, and (b) transient experiments (e.g. a pulse of CO was injected into the exhaust gas just before the DOC). For example, it was shown how experiments on a live engine can be used to explore: (a) the hysteresis between light-off and extinction curves, (b) how catalyst temperature rise during warm-up of a DOC, (c) the promotion effect that hydrogen has on the conversion of CO, (d) the extent of competition for active catalytic sites, e.g. between CO, THCs, propane or hydrogen. The main findings are: (a) the hysteresis between light-off and extinction curves are mainly caused by CO inhibition, (b) the promotion effect of hydrogen on CO oxidation is largely attributed to thermal effect, (c) LHHW form rate expression is not adequate for catalytic converter modelling under transient conditions, (d) the competition for active catalytic sites is not apparent at the test conditions performed in this thesis. Moreover, a number of case studies were also used to illustrate how the experimental results/techniques developed in this thesis, may be used to support modelling studies. iii
212

Carbon Dioxide as a Benign Solvent for Homogeneous Catalyst Recovery and Recycle

Jones, Rebecca S. 19 July 2005 (has links)
We have successfully investigated the use of CO2 as a miscibility switch to create an environment in which we can run a homogeneously catalyzed reaction while maintaining a heterogeneous separation. We explored the use of this technique with fluorous biphasic systems, a fluorous solid support, and aqueous biphasic systems. In the case of the fluorous systems, CO2 was added to induce solubility of the fluorous catalyst. When the reaction was completed, CO2 was vented and the system returned to a biphasic state, making the separation easy. For the aqueous biphasic systems, the organic phase is chosen such that it is fully miscible with water at ambient conditions. Examples include acetonitrile, THF, and dioxane. The addition of CO2 reduces the polarity of the solvent and causes a phase split. The recovery of the water-soluble catalyst is once again heterogeneous. The application to aqueous biphasic systems is the most exciting studied. Aqueous biphasic systems are used industrially in the hydroformylation of propylene. With our technique, these systems can be extended to more hydrophobic substrates. We have shown a rate increase of 65 fold and 99% product recovery at modest pressures for the hydroformylation of 1-octene. These aqueous biphasic systems also show much promise in the arena of enzyme catalyzed reactions. We can create an environment in which the enzyme kinetics will no longer be mass transfer limited.
213

Single-site polymerization catalysts: branched polyethylene and syndiotactic poly(alpha-olefins)

Schwerdtfeger, Eric Dean 15 May 2009 (has links)
Utilization of methylaluminoxane (MAO) activated metallocene and constrained geometry (CGC) olefin polymerization catalysts containing fluorenyl or octamethyloctahydrodibenzofluorenyl (Oct) moieties has yielded three series of syndiotactic copolymers of propylene with higher a-olefins. The melting temperatures of these polymers were analyzed, and found to correspond directly with the mole percent incorporation of comonomer, as well as with the frequency of stereoerrors in the polymers. Further analysis indicated that rmrr stereoerrors, a result of site epimerization, occur in close proximity to the incorporated comonomers. The MAO-activated fluorenyl/Oct-containing metallocene and CGC catalysts were further utilized to produce syndiotactic samples of poly(1-butene) (s-PB) and poly(1- pentene) (s-PPe). The syndiotacticity of the samples was quantified by 13C NMR and the melting temperatures determined by DSC. The samples of s-PB and s-PPe produced by Me2Si( h1-C29H36)( h1-N-tBu)ZrCl2·OEt2 (Oct-CGC) were found to melt at higher temperatures (55.9 and 43.1 °C, respectively) than any previously reported samples. The MAO-activated Oct-CGC was also used to produce polyethylene samples at a variety of polymerization temperatures and pressures. All of the samples were found to contain an unprecedented degree of branching (13-65 total branches per 1000 carbon atoms) for an early transition metal single-site catalyst. The branches were found to be almost exclusively of two or greater than five carbon atoms in length, and the levels of the longer branches could be controlled by varying the polymerization conditions. The number of ethyl branches was roughly 5 per 1000 carbon atoms for all samples. Finally, a binary catalyst system comprising the Oct-CGC and a chromium-based ethylene trimerization catalyst, ((tBuSCH2CH2)2NH)CrCl3, was developed. This MAOactivated catalyst system could be tuned to produce polyethylene samples with 17-49 total branches per 1000 carbon atoms. Between 4 and 16 of these branches were found to arise from incorporation of 1-hexene produced by the chromium oligomerization catalyst. Adjusting the ratios of oligomerization catalyst, polymerization catalyst, and activator was found to allow rational control over the branch content of the polymers. The branching levels could also be varied by altering the time between injection of the oligomerization and polymerization catalysts into the system.
214

Synthesis of vinyl acetate on palladium-based catalysts

Kumar, Dheeraj 02 June 2009 (has links)
Vinyl acetate (VA) is an important monomer used in the production of paints, surface coatings and adhesives. Synthesis of VA is usually carried out over supported Pd alloy catalysts with a selectivity as high as 96% and described as C2H4 + CH3COOH + ½ O2 -> C2H3OOCCH3 + H2O Although the VA synthesis reaction has been industrially carried out for many years, the nature of the active sites and the reaction mechanism is still unclear. The goal of this study was to acquire a fundamental understanding of the VA reaction mechanism by carrying out detailed kinetic and spectroscopic investigations on single crystals and supported Pd catalysts, and to detail the role of alloying in optimizing the selectivity of this important industrial reaction. A combination of surface science techniques and kinetic measurements has been used to address the mechanism. Supported catalysts, 1 wt% Pd/SiO2 and 5 wt% Pd/SiO2, and 1 wt% Pd-0.5 wt% Au/SiO2, were prepared by an incipient wet-impregnation method and characterized using XRD and TEM. On Pd-only catalysts the reaction rates were found to be: Pd(100) < 5 wt% Pd/SiO2 (dpd = 4.2 nm) < 1 wt% Pd/SiO2 (dpd = 2.5 nm). Particle size-dependence of the reaction rates is evident for the Pd-only catalysts, which suggests a degree of structure sensitivity of the reaction. There is an increased availability of uncoordinated, edge atoms on small particles. With a Pd single crystal, fewer less-coordinated surface sites are present compared to a comparable area on a small Pd particle on a supported Pd catalyst. The formation of Pd carbide (PdCx) during the synthesis of VA was investigated over Pd/SiO2 catalysts with two different Pd particle sizes, as well as over a Pd-Au/SiO2 mixed-metal catalyst. XRD data indicate that smaller Pd particles show greater resistance to the formation of PdCx. The alloying of Au with Pd is apparently very effective in preventing PdCx formation in Pd-based catalysts for VA synthesis. Addition of Au to Pd/SiO2 catalysts significantly enhances the VA formation rate and selectivity. Infrared reflection absorption spectroscopy (IRAS) of CO on Pd/Au(100) and Pd/Au(111) confirms the presence of Pd as isolated monomers on a Au-rich surface. A pair of Pd monomers is the most favorable active site for the formation of VA. The spacing between the two active isolated Pd atoms is critical and is demonstrated by the relative rates of VA formation on Pd/Au model catalysts, i.e. Pd/Au(111) < Pd/Au(100). The role of Au is to isolate the surface Pd atoms and thus suppress the formation of by products, CO and CO2. A pair of Pd monomers required for VA synthesis is further confirmed by the results from model studies of Sn-Pd.
215

Novel hybrid organic/inorganic single-sited catalysts and supports for fine chemical and pharmaceutical intermediate synthesis

Gill, Christopher Stephen 06 February 2009 (has links)
The study of catalysis is a fundamental aspect of chemical engineering, as its implications affect all chemical transformations. Traditionally, catalysis has been subdivided into two areas: homogeneous and heterogeneous catalysis. Homogeneous catalysis refers to single-sited catalysts that exist in the same phase as the reaction media. These catalysts tend to be highly active and selective but often difficult to recover and reuse. In contrast, heterogeneous catalysts are typically multi-sited catalysts that exist in a different phase from the reaction media. These catalysts tend to be less active and selective than their homogeneous counterparts. However, the vast majority of industrial scale catalysts are heterogeneous because they can be easily separated, making them easily implemented in continuous processes, allowing for efficient, large scale operations. Due to the limitations of traditional homogeneous and heterogeneous catalysts, researchers have increasingly investigated hybrid catalysts that incorporate aspects of homogeneous and heterogeneous catalysis. This is accomplished via immobilization of homogeneous catalyst analogues onto solid-phase supports, thereby preserving the activity and selectivity of homogeneous catalysts while allowing for facile recovery and reuse from the insoluble, heterogeneous support. A variety of systems is presented here including organic and organometallic catalysts immobilized on organic and inorganic supports. Five cases are included. The first discusses utilization of supported acid and base catalysts for use in one-pot cascade reactions. The second example illustrates use of silica-coated magnetic nanoparticle supported acid catalysts for organic transformations. The third case presents novel polymer brush supported Cobalt-salen catalysts for the enantioselective, hydrolytic kinetic resolution of epoxides. A fourth case presents novel, magnetic polymer brush supported organic and organometallic catalysts for organic transformations. The fifth example illustrates polymer and silica supported ruthenium-salen catalysts for the asymmetric cyclopropanation of olefins. The overall goal of this thesis work is to develop novel supports and immobilization techniques to advance the field of hybrid organic/inorganic catalysts for the production of fine chemical and pharmaceutical intermediates.
216

Avaliação de catalisadores do tipo Pt-Re-Au/Al2 O3 na reforma de n-heptano.

Ferreira, Márcio Luís Oliveira January 2011 (has links)
127f. / Submitted by Ana Hilda Fonseca (anahilda@ufba.br) on 2013-04-04T14:19:14Z No. of bitstreams: 11 SUMÁRIO.pdf: 387928 bytes, checksum: 97572aa6a0c6f9e1603c58e13445c4b0 (MD5) SUGESTÕES PARA TRABALHOS FUTUROS.pdf: 185948 bytes, checksum: a4cf4835add63bfe3410965e6ed99060 (MD5) REVISÃO BIBLIOGRÁFICA.pdf: 613262 bytes, checksum: 899ef505497cce7f776561ff2903c41b (MD5) RESUMO.pdf: 90910 bytes, checksum: e6ff997e49241b2db759eb772d78d856 (MD5) RESULTADOS E DISCUSSÃO.pdf: 1410244 bytes, checksum: 0fbf4fad7b7711daf673de9304838f4d (MD5) REFERÊNCIAS BIBLIOGRÁFICAS.pdf: 300838 bytes, checksum: 62907154b73fa8bcd4beeee4fd354b30 (MD5) MATERIAIS E MÉTODOS.pdf: 429719 bytes, checksum: 9268517f88f142b52bb2143340091ae0 (MD5) INTRODUÇÃO.pdf: 119594 bytes, checksum: 2fcf034b8b72f623889327317c5a1acf (MD5) FICHA CATALOGRÁFICA.pdf: 138900 bytes, checksum: 286ccabdd5fdfac3f47751be3a687469 (MD5) CONCLUSÕES.pdf: 116706 bytes, checksum: 36835ca113d33c96d635431e3f74e4b5 (MD5) ANEXO.pdf: 295248 bytes, checksum: 636ae63d87a78c28a1aa23421f0cc552 (MD5) / Rejected by Ana Hilda Fonseca(anahilda@ufba.br), reason: Pedir para o aluno colocar em um só arquivo. on 2013-04-29T15:49:36Z (GMT) / Submitted by Ana Hilda Fonseca (anahilda@ufba.br) on 2013-05-21T16:00:42Z No. of bitstreams: 1 Tese Márcio Luís O Ferreira.pdf: 3909041 bytes, checksum: c22e89a198f2b436130cf2b453fd102b (MD5) / Approved for entry into archive by Ana Hilda Fonseca(anahilda@ufba.br) on 2013-05-21T16:02:17Z (GMT) No. of bitstreams: 1 Tese Márcio Luís O Ferreira.pdf: 3909041 bytes, checksum: c22e89a198f2b436130cf2b453fd102b (MD5) / Made available in DSpace on 2013-05-21T16:02:17Z (GMT). No. of bitstreams: 1 Tese Márcio Luís O Ferreira.pdf: 3909041 bytes, checksum: c22e89a198f2b436130cf2b453fd102b (MD5) Previous issue date: 2011 / Neste trabalho, foi estudado o efeito da adição de ouro em diferentes concentrações (0,01; 0,1 e 0,3%) na atividade, seletividade e estabilidade de catalisadores de platina (0,3 %) e rênio (0,3 %) suportados em alumina clorada, destinados à reforma catalítica de nafta de petróleo. As amostras foram preparadas por co-impregnação dos sais metálicos (ácido hexacloroplatínico, perrenato de amônia e ácido tetracloroaúrico) em alumina comercial e caracterizados por difração de raios X, redução termoprogramada e dessorção à temperatura programada de piridina. Além disso, os catalisadores foram avaliados na desidrogenação de cicloexano e isomerização de n-pentano, reações consideradas modelo das funções metálicas e ácidas, respectivamente. O desempenho catalítico das amostras foi avaliado na reforma de n-heptano. Após a reação de isomerização de n-pentano e reforma de n-heptano, os catalisadores foram analisados por oxidação termoprogramada. Observou-se que a adição de ouro facilita a redução da platina e este efeito aumentou com a diminuição do teor desse metal; um efeito inverso foi observado com os catalisadores de rênio. Nos catalisadores trimetálicos, a presença de 0,3% de ouro causa um aumento da interação dos metais entre si e com o suporte. A adição de ouro no catalisador monometálico de platina promoveu um decréscimo da atividade catalítica dos sítios metálicos, na reação de desidrogenação do cicloexano. Esse efeito aumentou com o teor de ouro e foi atribuído à provável formação de ligas com a platina, inativas da reação ou ao recobrimento de átomos de platina pelo ouro. A natureza e a quantidade dos sítios ácidos também foram alterados pela presença do ouro, mas não se observou uma relação simples essa propriedade e o teor desse metal. A atividade do catalisadores mono e bimetálicos de platina na reação de isomerização de n-pentano foi diminuída devido à adição de ouro. Por outro lado, a seletividade a isopentano dos catalisadores bimetálicos de platina e rênio foi pouco alterada com a adição de ouro. A adição de ouro ao catalisador de platina, associada ou não ao rênio, suportada em alumina conduziu a uma diminuição na conversão do n-heptano. Por outro lado, a adição de ouro influenciou pouco a seletividade a isômeros e a compostos aromáticos. A presença de ouro, de maneira geral, diminuiu a formação de coque, tanto na isomerização do n-pentano como na reação de reforma de n-heptano. Os catalisadores com teores mais elevados de ouro apresentaram menores quantidades de coque. A amostra trimetálica com 0,01% de ouro apresentou atividade similar à amostra bimetálica de platina e rênio na reação de reforma e n-heptano. Entretanto, a presença de 0,3% de ouro no catalisador bimetálico de platina e rênio inibiu a formação de coque durante as reações de isomerização de n-pentano e de reforma de n-heptano tornando-o, dessa forma, um sistema promissor na reação de reforma de nafta para a produção de gasolina com elevado índice de octanas. / Salvador
217

Poisoning of automotive exhaust gas catalyst components:the role of phosphorus in the poisoning phenomena

Kröger, V. (Virpi) 31 October 2007 (has links)
Abstract The aim of this thesis project was to gain new knowledge on the effect of phosphorus on the catalytic activity and characteristics of automotive exhaust gas catalyst components. The simultaneous roles of phosphorus and calcium were also studied. The first test series of powdery catalyst samples contained Rh and oxide (Test series 1) and the second, Pt and oxide or ZSM-5 (Test series 2). The catalysts were analyzed when fresh and after two ageing and phosphorus poisoning procedures developed in this work. The procedures consisted of adding poison via impregnation in an aqueous solution (for Test series 1) and in the gaseous phase under hydrothermal conditions (for Test series 2). The poison compounds formed and the changes in the washcoat were studied by using physisorption analyses, SEM, TEM, XRD, and FTIR-ATR. The poison content of the samples was determined by ICP-OES and XRF. Laboratory-scale activity measurements were done to investigate the catalytic activity. Thermodynamic calculations were used to obtain information about ageing conditions and phosphorus compounds formed during ageing. Phosphorus decreased the catalytic activity and the characteristic surface areas of the catalysts. Addition of calcium to a phosphorus-poisoned catalyst was found to have even a regenerating effect on the catalysts' activity. The poisoning methods developed in this study resulted in the same phosphorus compounds as can be found in vehicle-aged catalysts. Phosphorus was identified as cerium, zirconium, aluminium, and titanium phosphates. Phosphorus was detected in zeolites, but phosphorus-containing compounds were not observed. Phosphorus poisoning takes place in the gas phase at high operating temperatures and with high oxygen and water contents. It was also shown that the role of phosphorus poisoning was more pronounced than the role of hydrothermal ageing alone. Phosphorus poisoning mainly affects the oxide components used in this study, not the noble metals. The results can be utilized in the development of catalytic materials and catalyst compositions that can better tolerate phosphorus poisoning under hydrothermal conditions. The results can also be applied in evaluating the effects of phosphorus on different catalyst compositions and in estimating the age of commercial catalysts.
218

Catalytic oxidation of volatile organic compounds and malodorous organic compounds

Ojala, S. (Satu) 11 November 2005 (has links)
Abstract This thesis describes efforts made on the development of an existing catalytic incinerator. The development work, called process characterization, consists of four general parts. These are the development of measurement methodology, the studying of construction materials, the selection of suitable catalysts and the testing of the effects of process operation conditions. The two application areas for catalytic incineration considered in this thesis are solvent emission abatement (VOC, volatile organic compounds) and chip bin emission abatement (SVOC, sulphur-containing volatile organic compounds). As a baseline, the process characterization is started with the development of measurement methodology. In general, the methodology will decrease costs and simplify the carrying out of the actual measurements and thereby make the measurement time more effective. In the methodology it is proposed that continuous total concentration measurement should be used in connection with qualitative sampling to obtain reliable measurement data. The selection of suitable construction materials for the application is very important. As shown in this thesis, the end conversions in solvent emission abatement may even be improved through the selection of the proper construction materials. In chip bin emission abatement, the problem arises from corrosive oxidation products that set limits on the construction materials used as well as on oxidation conditions. Catalyst selection is based on the following catalytic properties: activity, selectivity and durability. These catalytic properties are studied either at the laboratory or on an industrial scale. The catalytic materials tested are Pt, Pd, Pt-Pd, Cu-Mn oxides, MnO2-MgO, CuxMg(1-x)Cr2O4 and CuxCr2O4. The most important selection criteria in solvent emission abatement are proposed to be activity and selectivity. In the case of chip bin-SVOC-abatement, these are selectivity and durability. Based on these criteria, catalysts containing Cu-Mn oxides and Pt were demonstrated to be the best catalysts in VOC oxidation, and catalyst containing MnO2-MgO was shown to be best catalyst in SVOC oxidation. A study on the effect of process operation parameters (temperature, concentration and gas hourly space velocity (GHSV)) and moisture was carried out with the aid of factorial design. In VOC (n-butyl acetate) oxidation, the most influential process parameter was GHSV, which decreased the end conversion when it was increased. In SVOC (DMDS) oxidation, the effect of temperature was most significant. The end conversions increased as the temperature increased. Moisture slightly decreased the formation of by-products in n-butyl acetate oxidation. In DMDS oxidation, moisture slightly increased the end conversions at a lower temperature level (300°C). At the end of the thesis, these process parameters are also discussed from the standpoint of the catalysts' activity, selectivity and durability. Finally, proposals for process improvements are suggested.
219

Studies on Site-Selective C-H Borylation Reactions of Arenes and Heteroarenes / アレーンおよびヘテロアレーンのサイト選択的C-Hホウ素化反応に関する研究

Yang, Lichen 23 March 2020 (has links)
京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22455号 / 工博第4716号 / 新制||工||1737(附属図書館) / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 中尾 佳亮, 教授 松原 誠二郎, 教授 杉野目 道紀 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM
220

Development and Synthetic Application of N-Boc-Protected Aminals as the Precursors of N-Boc-Protected Imines / Boc保護イミン前駆体としてのBoc保護アミナールの開発と合成反応への応用

Yurino, Taiga 23 May 2013 (has links)
京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第17773号 / 理博第3896号 / 新制||理||1562(附属図書館) / 30580 / 京都大学大学院理学研究科化学専攻 / (主査)教授 丸岡 啓二, 教授 大須賀 篤弘, 教授 時任 宣博 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Page generated in 0.0671 seconds