No description available.

Infra-Red thermography and spatially-resolved capillary inlet mass spectrometry (SpaciMS) have been used to characterize propylene oxidation along a Pt/Al2O3 monolith-supported catalyst, before and after heterogeneous deactivation. The combined techniques clearly show reaction location, and therefore catalyst use, and how these change with thermal and sulphur degradation. Following the heterogeneous thermal aging, the reaction zones at steady state were broader and located farther into the catalyst relative to those observed with the fresh catalyst. As well, the time for the temperature and concentration waves to travel through the catalyst during back-to-front ignition increased. These effects were more pronounced with 1500 ppm propylene relative to 4500 ppm propylene. Such trends could not be detected based on standard catalyst-outlet measurements. The light-off behaviour was also impacted by the aging, resulting in complex changes to the temperature front propagation, depending on the propylene concentration. With each sulphur exposure step, light-off temperatures increased and the time for back-to-front ignition during temperature programmed oxidation changed pattern. With 1500 ppm propylene fed, the reaction zones established during steady-state operation shifted farther into the catalyst and increased slightly in width following sulphur treatment; at very high temperature and for 4500 ppm propylene, the reaction zones were very close to the catalyst inlet and virtually indistinguishable between catalyst sulphation states. However, at lower steady-state temperatures for the higher propylene concentration, the catalyst did experience delays in reaction light-off and light-off position moved downstream in the catalyst with sulphur damage.

Infra-Red thermography and spatially-resolved capillary inlet mass spectrometry (SpaciMS) have been used to characterize propylene oxidation along a Pt/Al2O3 monolith-supported catalyst, before and after heterogeneous deactivation. The combined techniques clearly show reaction location, and therefore catalyst use, and how these change with thermal and sulphur degradation. Following the heterogeneous thermal aging, the reaction zones at steady state were broader and located farther into the catalyst relative to those observed with the fresh catalyst. As well, the time for the temperature and concentration waves to travel through the catalyst during back-to-front ignition increased. These effects were more pronounced with 1500 ppm propylene relative to 4500 ppm propylene. Such trends could not be detected based on standard catalyst-outlet measurements. The light-off behaviour was also impacted by the aging, resulting in complex changes to the temperature front propagation, depending on the propylene concentration. With each sulphur exposure step, light-off temperatures increased and the time for back-to-front ignition during temperature programmed oxidation changed pattern. With 1500 ppm propylene fed, the reaction zones established during steady-state operation shifted farther into the catalyst and increased slightly in width following sulphur treatment; at very high temperature and for 4500 ppm propylene, the reaction zones were very close to the catalyst inlet and virtually indistinguishable between catalyst sulphation states. However, at lower steady-state temperatures for the higher propylene concentration, the catalyst did experience delays in reaction light-off and light-off position moved downstream in the catalyst with sulphur damage.

Over the past 15 years, ruthenium-catalyzed olefin metathesis has emerged as a cornerstone synthetic methodology in academia. Applications in fine-chemicals and pharmaceutical manufacturing, however, are just beginning to come on stream. Industrial uptake has been impeded by economic constraints associated with catalyst costs. These are due both to direct costs (exacerbated by intellectual property issues), and to further pressure exerted by the low turnover numbers attainable, and the need for extensive purification to remove ruthenium residues. From another perspective, however, these difficulties can be seen as arising from our rudimentary understanding of the fundamental organometallic chemistry of the Ru=CHR bond. In particular, we know little about the nature and reaction pathways of the Ru-methylidene unit present in the active species that propagates metathesis, and in the catalyst resting state. We know slightly more about the ruthenacyclobutane species, but still too little to guide us as to their non-metathetical reaction pathways, their contribution to deactivation relative to the methylidene species, and potential work-arounds. This thesis work was aimed at improving our understanding of the reactivity, speciation, and decomposition of key ruthenium intermediates in olefin metathesis. A major focus was the behaviour and deactivation of species formed from the second-generation Grubbs catalyst RuCl2(H2IMes)(PCy3)(=CHPh) (S-GII), which dominates ring-closing metathesis. Also studied were derivatives of the corresponding IMes catalyst A-GIIm, containing an unsaturated Nheterocyclic carbene (NHC) ligand. The methylidene complexes RuCl2(NHC)(PCy3)(=CH2) (GIIm) represent the resting state of the catalyst during ring-closing and cross-metathesis reactions: that is, the majority Ru species present during catalysis. Mechanistic studies of these key intermediates have been restricted, however, by the low yields and purity with which they could be accessed. Initial work therefore focused on designing a clean, high-yield route to the second-generation Grubbs methylidene complexes S-GIIm and A-GIIm. These routes were subsequently expanded to develop access to isotopically-labelled derivatives. Locating a 13C-label at the key alkylidene site, in particular, offers a powerful means of tracking the fate of the methylidene moiety during catalyst deactivation. Access to GIIm enabled detailed studies of the behaviour and decomposition of the Grubbs catalysts. First, the long-standing question of the impact of saturation of the NHC backbone (i.e. IMes vs. H2IMes) was examined. Dramatic differences in the behaviour of the two complexes were traced to profound differences in PCy3 lability arising from the diminished π-acidity of the IMes ligand. Secondly, the vulnerability of GIIm to nucleophiles was examined. This is an important issue from the perspective of decomposition by adventitious nucleophiles in the reaction medium during catalysis, but also reflects on substrate scope. For amine additives, the dominant deactivation pathway was shown to typically involve attack on the resting-state methylidene complex, not the metallacyclobutane, which has often been regarded as the most vulnerable intermediate. In addition, the sigma-alkyl intermediate formed by nucleophilic attack of displaced phosphine at the methylidene carbon was trapped by moving to the first-generation complex, and using a nitrogen donor (pyridine) that cannot promote decomposition via N–H activation pathways. Interception of this long-suspected species led to the proposal of “donorinduced” deactivation as a general decomposition pathway for Grubbs-class catalysts. Finally, the capacity of phosphine-free catalysts to overcome the shortcomings of the secondgeneration Grubbs catalysts was demonstrated, in a case study involving application of crossmetathesis (CM) to the synthesis of a high-value antioxidant. An efficient CM methodology was developed for the reaction of renewable essential-oil phenylpropenoids with vinyl acrylates. This work illustrates a new paradigm in sustainable metathesis. Rather than degrading unsaturated feedstocks via metathesis (a process that can be termed “metathe[LY]sis”), it demonstrates how metathesis with directly-functionalized olefins can be used to augment structure and function. From the perspective of organometallic chemistry and catalyst design, key comparisons built into this thesis are the effect of the NHC ligand (IMes vs. H2IMes) and its trans ancillary ligand on the efficient entry into catalysis; the susceptibility to nucleophilic attack of the alkylidene ligand (benzylidene vs. methylidene) vs. the metallacyclobutane; and the effect of replacing a phosphine ancillary ligand with a non-nucleophilic donor. From a practical standpoint, Chapter 2 brings new life to metathesis with the high-yield synthesis of the resting state species, Chapters 3 and 4 examine the deactivation, or death, of the methylidene complexes, and Chapter 5 establishes a new paradigm for olefin metathesis within the context of sustainable synthesis.

Stickoxide NOx werden von Menschenhand in verschiedenen Verbrennungsprozessen emittiert. Die selektive katalytische Reduktion mit Ammoniak (NH3-SCR) hat sich weltweit als wichtigste Methode zur Minderung von NOx-Emissionen etabliert. Derzeit erhältliche Katalysatoren für die NH3-SCR werden bei Temperaturen unterhalb von 473 K stark in Gegenwart von Wasser desaktiviert, welches unvermeidbar in Abgasen aus der Verbrennung von organischen Stoffen enthalten ist. In dieser Arbeit werden drei verschiedene Arten der Modifikation von SCR-Katalysatoren diskutiert, die eine gesteigerte H2O-Resistenz bewirken. Eine Methode ist die Verwendung von mischoxidischen Trägermaterialien, eine Andere ist eine mischoxidische aktive Komponente und schließlich eine postpräparative Oberflächenmodifikation mit Organosilylgruppen. Die Katalysatoren wurden sowohl auf ihre katalytische Aktivität als auch auf ihre adsorptiven, redox und andren Oberflächeneigenschaften untersucht. Die Wechselwirkungen zwischen H2O und der Katalysatoroberfläche wurden mittels temperaturprogrammierter Desorption (TPD), isothermaler Adsorption bei erhöhtem Druck und einer gravimetrischen Methode untersucht. Besonders die H2O-TPD hat sich als eine leistungsstarke Methode für diesen Zweck herausgestellt. Jede der drei Modifikationen bewirkte eine Verminderung der Wechselwirkungen zwischen H2O und der Katalysatoroberfläche. Neben einer allgemeinen Erhöhung der Aktivität eines SCR-Katalysators, wird die gezielte Verminderung dieser Wechselwirkungen als Schlüsselrolle in der Entwicklung von Katalysatoren mit verbesserter H2O-Resistenz angesehen. Jedoch gibt es zur Zeit kaum Publikationen, die diesen Zusammenhang behandeln. Daneben wurde auch die Bildung von N2O als ungewünschtes Nebenprodukt bei der SCR-Reaktion untersucht. Dessen Treibhauspotential entspricht ungefähr dem 300-fachen von CO2. Die Verwendung von einem mischoxidischem Trägermaterial kann die Freisetzung von N2O während der SCR verringern, was größtenteils auf die Unterdrückung der Bildung nach einem ER-Mechanismus zurückgeführt wurde. Auch die N2O-Bildung wird in vielen Publikationen über die Entwicklung von SCR-Katalysatoren nicht betrachtet.:0.1 Abbreviations 0.2 Symbols 1 Introduction and Objectives 2 Literature Overview 2.1 NH3-SCR 2.1.1 NH3-SCR Catalysts 2.1.2 Mechanisms of NH3-SCR Reaction 2.1.3 N2O-Formation under SCR-Conditions 2.2 Deactivation of NH3-SCR Catalysts 2.2.1 Deactivation by H2O 2.2.2 Deactivation by SO2 2.3 Low-Temperature NH3-SCR 2.3.1 Requirements and Challenges of LT-SCR 2.3.2 LT-SCR Catalysts 2.4 Silylation of Metal Oxide Surfaces 3 Experimental Section 3.1 Catalyst Preparation 3.1.1 Support Modification with Different Metal Oxides 3.1.2 Deposition of Active Component 3.1.3 Catalyst Modification with Organosilyl Groups 3.2 Catalyst Characterization 3.2.1 Texture Analysis 3.2.2 Phase Analysis 3.2.3 Elementary Analysis 3.2.4 Adsorption Properties 3.2.5 Surface Spectroscopy 3.2.6 Redox Properties 3.3 Catalytic Experiments 4 Results and Discussion 4.1 Impact of Mixed-Oxide Support on Catalyst Activity 4.1.1 Impact in Dry Gas-Flow: Reduced N2O-Emission 4.1.1.1 Catalytic Activity 4.1.1.2 Catalyst Characterization 4.1.1.3 Discussion 4.1.2 Impact in Wet Gas-Flow: Higher H2O-Resistance 4.1.2.1 Catalytic Activity 4.1.2.2 Catalyst Characterization 4.1.2.3 Discussion 4.1.3 Summary of SiO2-Impact 4.2 Mn-Ce Mixed-Oxide as Active Component 4.2.1 Catalytic Activity 4.2.2 Catalyst Characterization 4.2.3 Discussion and Summary 4.3 Catalyst Modification with Organosilyl Groups 4.3.1 Stability of Organosilyl Groups 4.3.2 Impact of Organosilyl Modification on H2O-Adsorption 4.3.3 Impact of Organosilyl Modification on Catalytic Activity in Pre- and Absence of H2O 4.3.3.1 Catalytic Activity 4.3.3.2 Catalyst Characterization 4.3.3.3 Discussion 4.3.4 Summary of Organosilyl Modification 4.4 Discussion on the Investigation of H2O-Adsorption 5 Conclusions and Outlook 5.1 Conclusions 5.2 Outlook 6 References 7 Appendix 7.1 Evaluation of H2O-Sorption Data through BET-Theory 7.2 Evaluation of Kinetic SCR Investigation 7.3 Calculation of the Average Oxidation State of Mnz+ from H2-TPR 7.4 Calculation of the Surface-Density of Mn 7.5 Supplementary Data 7.6 Scientific Contributions 7.7 Curriculum Vitae 8 Summary (german) 8.1 Einleitung 8.2 Experimentelles 8.3 Ergebnisse und Diskussion 8.3.1 Einfluss eines mischoxidischen Trägermaterials auf die katalytische Aktivität 8.3.2 Mn-Ce-Mischoxide als aktive Komponente 8.3.3 Modifikation von Katalysatoren mit Organosilyl-Gruppen 8.4 Schlussfolgerungen / Nitrogen oxides NOx were anthropogenically emitted by various combustion processes. The selective catalytic reduction with ammonia (NH3-SCR) has been established worldwide as the most important technique for the abatement of NOx . Currently available catalysts for NH3-SCR become strongly deactivated at temperatures below 473 K in presence of H2O which is unavoidable present in the exhaust gas arising from the combustion of organic matter. In this work three different kinds of a modification of an SCR-catalyst were discussed that cause a higher H2O-resistance. One is the application of a mixed-oxide support material, the other is a mixed-oxide active component and finally a post-preparative surface modification with organosilyl-groups. The catalysts were assessed for their catalytic activity as well as their adsorptive, redox and other surface properties. The interactions between H2O and the catalyst surface were investigated by means of temperature programmed desorption (TPD), isothermal adsorption at elevated pressure and a gravimetric method. Especially the H2O-TPD turned out to be a powerful method for this purpose. Each of the three modifications caused a reduction in the H2O-catalyst interactions. Beside a general increase of the activity of an SCR-catalyst, the purposeful reduction of these interactions is considered to play a key role in the development of catalysts with an enhanced H2O-resistance. However, there is a lack of publications that deal with this correlation. Also the formation of the unwanted by-product N2O was investigated. Its global warming potential is about 300-times that of CO2. The application of a mixed-oxide support can reduce the release of N2O during SCR which was attributed mainly to the suppression of the ER-type formation pathway. Also the N2O-formation is not considered in many publications dealing with the development of SCR-catalysts.:0.1 Abbreviations 0.2 Symbols 1 Introduction and Objectives 2 Literature Overview 2.1 NH3-SCR 2.1.1 NH3-SCR Catalysts 2.1.2 Mechanisms of NH3-SCR Reaction 2.1.3 N2O-Formation under SCR-Conditions 2.2 Deactivation of NH3-SCR Catalysts 2.2.1 Deactivation by H2O 2.2.2 Deactivation by SO2 2.3 Low-Temperature NH3-SCR 2.3.1 Requirements and Challenges of LT-SCR 2.3.2 LT-SCR Catalysts 2.4 Silylation of Metal Oxide Surfaces 3 Experimental Section 3.1 Catalyst Preparation 3.1.1 Support Modification with Different Metal Oxides 3.1.2 Deposition of Active Component 3.1.3 Catalyst Modification with Organosilyl Groups 3.2 Catalyst Characterization 3.2.1 Texture Analysis 3.2.2 Phase Analysis 3.2.3 Elementary Analysis 3.2.4 Adsorption Properties 3.2.5 Surface Spectroscopy 3.2.6 Redox Properties 3.3 Catalytic Experiments 4 Results and Discussion 4.1 Impact of Mixed-Oxide Support on Catalyst Activity 4.1.1 Impact in Dry Gas-Flow: Reduced N2O-Emission 4.1.1.1 Catalytic Activity 4.1.1.2 Catalyst Characterization 4.1.1.3 Discussion 4.1.2 Impact in Wet Gas-Flow: Higher H2O-Resistance 4.1.2.1 Catalytic Activity 4.1.2.2 Catalyst Characterization 4.1.2.3 Discussion 4.1.3 Summary of SiO2-Impact 4.2 Mn-Ce Mixed-Oxide as Active Component 4.2.1 Catalytic Activity 4.2.2 Catalyst Characterization 4.2.3 Discussion and Summary 4.3 Catalyst Modification with Organosilyl Groups 4.3.1 Stability of Organosilyl Groups 4.3.2 Impact of Organosilyl Modification on H2O-Adsorption 4.3.3 Impact of Organosilyl Modification on Catalytic Activity in Pre- and Absence of H2O 4.3.3.1 Catalytic Activity 4.3.3.2 Catalyst Characterization 4.3.3.3 Discussion 4.3.4 Summary of Organosilyl Modification 4.4 Discussion on the Investigation of H2O-Adsorption 5 Conclusions and Outlook 5.1 Conclusions 5.2 Outlook 6 References 7 Appendix 7.1 Evaluation of H2O-Sorption Data through BET-Theory 7.2 Evaluation of Kinetic SCR Investigation 7.3 Calculation of the Average Oxidation State of Mnz+ from H2-TPR 7.4 Calculation of the Surface-Density of Mn 7.5 Supplementary Data 7.6 Scientific Contributions 7.7 Curriculum Vitae 8 Summary (german) 8.1 Einleitung 8.2 Experimentelles 8.3 Ergebnisse und Diskussion 8.3.1 Einfluss eines mischoxidischen Trägermaterials auf die katalytische Aktivität 8.3.2 Mn-Ce-Mischoxide als aktive Komponente 8.3.3 Modifikation von Katalysatoren mit Organosilyl-Gruppen 8.4 Schlussfolgerungen

The goal of this project was to advance adhesion science and technology related to the Australian hardwood spotted gum (Corymbia citriodora). Plantation-grown spotted gum exhibits poor adhesion properties in comparison with similar woods, such as Gympie messmate (Eucalyptus cloeziana). To better understand adhesion differences between these two woods, this research compared and contrasted the surface chemistries of plantation-grown spotted gum and Gympie messmate with a particular focus on sensitivity to thermal deactivation. Wetting measurements were performed using the sessile drop method. Initial and equilibrium contact angles, time-dependent wetting, and surface energy were determined. Time-dependent wetting and equilibrium contact angles were most informative. Initial contact angles and surface energy calculated with them were misleading and often generated anomalous results. Heating water-saturated wood to mild surface temperatures (105 deg C, directly after evaporative cooling) severely deactivated spotted gum but not Gympie messmate. This suggests conventional kiln drying appears unsuitable for spotted gum while amenable for Gympie messmate. Spotted gum likely requires non-aqueous, low surface tension adhesives or aqueous adhesives formulated with surface active wetting agents. Water-saturation (followed by room-temperature vacuum drying) substantially altered the surface chemistries of both woods, making them more hydrophilic. Consequently, the question was raised of whether a water-spray onto the wood surface prior to adhesive application could improve bonding. If so, this simple, industrially-feasible treatment could prove very beneficial to the wood composites industry. Water-saturation also revealed differences in the two wood's water permeability, which has implications for adhesive penetration and wood drying and may additionally help explain adhesion differences. Analysis of the plantation-grown heartwood (inner, middle, and outer heartwood regions) revealed significant wetting differences on spotted gum with only minor differences on Gympie messmate. The Australian woods were compared to two North American woods-loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii). Examining water wetting measurements, the Australian and North American woods exhibited some interesting similarities. However, methylene iodide wetting measurements revealed that the Australian woods were quite different from the North American samples studied here. / Master of Science

Trace amounts of carbon monoxide, typically as low as 10 ppm CO, have a deleterious effect on the activation overpotential losses in proton exchange membrane (PEM) fuel cells. This is because CO preferentially adsorbs on the Pt electrocatalyst at the anode at typical PEM fuel cell operating temperatures, thereby preventing the absorption and ionisation of hydrogen. The inability of current preferential oxidation steps to completely remove CO from hydrogen-rich gas streams has stimulated research into CO tolerant anodes. As opposed to other CO oxidation catalysts, metal oxide supported gold catalysts have been shown to be active for the afore mentioned reaction at low temperatures, making it ideal for the 80°C operating temperatures of PEM fuel cells. The objective of this study was to investigate the viability of incorporating titanium dioxide supported gold (Au/TiO2) catalysts inside a PEM fuel cell system to remove CO to levels low enough to prevent poisoning of the Pt-containing anode. Two distinct methods were investigated. In the first method, the incorporation of the said Au/TiO2 catalyst inside the membrane electrode assembly (MEA) of a PEM fuel cell for the selective/preferential oxidation of carbon monoxide to carbon dioxide in hydrogen-rich gas fuels, facilitated by the injection of an air bleed stream, was investigated. It was important for this study to simulate typical fuel cell operating conditions in an external CO oxidation test rig. Factors such as gold loading, oxygen concentration, temperature, pressure, membrane electrode assembly constituents, water formation, and selectivity in hydrogen-rich gas streams, were investigated. The Au/TiO2 catalysts were prepared via deposition-precipitation, a preparation procedure proven to yield nano-sized gold particles, suggested in literature as being crucial for activity on the metal oxide support. The most active catalysts were incorporated into the MEA and its performance tested in a single cell PEM fuel cell. The catalysts proved to yield exceptional activity for all test conditions inside the CO oxidation test rig. However, no significant improvement in CO tolerance was observed when these catalysts were incorporated into the MEA. It was concluded that the thin bilayer configuration resulted in mass transfer and contact time limitations between the catalysts and the simulated reformate gas mixture. Other factors highlighted as possible causes of deactivation included the deleterious effect of the acidic environment in the fuel cell, the formation of liquid water on the catalyst’s surface, and the adverse effect of the organic MEA constituents during the MEA production procedure. The second method investigated was the incorporation of the Au/TiO2 catalyst in an isolated catalyst chamber in the hydrogen feed line to the fuel cell, between the CO contaminated hydrogen gas cylinder and the anode humidifier. Test work in a CO oxidation test rig indicated that with this configuration, the Au/TiO2 catalysts were able to remove CO from concentrations of 2000 ppm to less that 1.3 ppm at a space velocity (SV) of 850 000 ml.gcat -1.h-1 while introducing a 2 per cent air bleed stream. Incorporation of this Au/TiO2 preferential oxidation system into a Johnson Matthey single cell PEM fuel cell test station prevented any measurable CO poisoning when 100 and/or 1000 ppm CO, 2 per cent air in hydrogen was introduced to a 0.39 mg Pt.cm-2 Pt/C anode. These results were superior compared to other state of the art CO tolerance technologies. An economic viability study indicated that the former can be achieved at a cost of gold equal to 0.8 per cent of the USDoE target cost of $45/kW. This concept might allow fuel cells to operate on less pure hydrogen-rich gas, e.g. from H2 that would be stored in a fuel tank/cylinder but that would have some CO contamination and would essentially be dry. The use of less pure H2 should allow a cost incentive to the end user in that less pure H2 can be produced at a significantly lower cost.

O controle da superfície química em colunas capilares abertas é de fundamental importância para atingir um alto desempenho na análise cromatográfica, sendo muito estudada ao longo das décadas de 70 e 80, mas tendo pouco destaque nos últimos anos por parte dos pesquisadores. A atividade da superfície de uma coluna capilar é causada por grupos silanol (Si-OH) e impurezas como metais, por exemplo. A presença destes grupos silanol faz com que alguns compostos sofram adsorção, principalmente pela formação de pontes de hidrogênio, originando picos com longas caudas no cromatograma. Por isso, a eliminação dos sítios ativos faz-se necessária quando uma amostra apresenta interação com a superfície da coluna. Esta desativação pode ser feita por meio de agentes silanizantes que reagem com os grupos hidroxila. Visto que a maioria dos artigos publicados sobre este assunto envolve colunas de vidro, o presente trabalho estudou os efeitos de cada fase do pré-tratamento de tubos aplicado para as colunas atuais de sílica fundida, verificando os agentes silanizantes HMDS, DPTMDS e TPDMDS que seriam mais adequados para suportar as fases estacionárias OV-73 e OV-17. Também se avaliou o uso de filmes finos de 0,01 , 0,05 e 0,10 ?m de polietileno glicol (PEG) como alternativa para a desativação da parede interna do capilar, o qual mostrou resultados satisfatórios de estabilidade e inatividade. / The control of the inner surface in open tubular capillary columns has fundamental importance to reach a high performance in gas chromatography analysis. It was very studied during 1970´s and 1980´s, but has not interested the researches along last years, not excluding also, the possibility of commercial secrets regarding column preparation. The activity on the surface of capillary column is mainly caused by silanol groups (Si-OH) and impurities such as metal ions and water. These groups favours reversible and irreversible adsorption of polar compounds, which results in tailed peak shapes and incomplete elution from the column. Therefore, the elimination of the active sites is necessary. The deactivation procedures are based on blocking of silanol groups by means of chemical reaction. This present work studied the effects of the surface pretreatment steps applied to fused silica capillary columns. Some silylating agents such as HMDS, DPTMDS and TPDMDS were valued. The coating behavior using OV-73 and OV-17 stationary phases was studied too. Film thickness of 0,01 , 0,05 and 0,10 ?m of poly(ethylene glycol) (PEG) was applied as alternative procedure, showing good stability and column deactivation.

The alkylation of benzene with ethane was studied at 370 oC over two Pt-containing ZSM-5 catalysts with SiO2/Al2O3 ratios of 30 and 80. While the benzene and ethane conversion decreased with time-on-stream for the PtH-ZSM-5(30) catalyst, the PtH-ZSM-5(80) catalyst demonstrated a stable performance. The deactivation of the PtH-ZSM-5(30) catalyst was found to be more significant, when compared to the PtH-ZSM-5(80) catalyst as a result of differences in the formation of coke. Results from gas sorption and x-ray diffraction experiments showed that coke is preferentially formed within the channel segments of the PtH-ZSM-5(30) catalyst as opposed to coke deposition on the outside surface of the PtH-ZSM-5(80) crystallites, subsequently blocking entrance to the zeolite channels. <br /> The location of the coke deposition was found to affect the product selectivity of the two PtH-ZSM-5 catalysts. The accessibility functions, derived from nitrogen and argon sorption data, suggested that, with prolonged time-on-stream, the coke molecules build up from the middle of the zeolite crystallites outwards towards the surface, as the reaction was carried out over the PtH-ZSM-5(30) catalyst. Partial blockage of the internal pore structure of the PtH-ZSM-5(30) catalyst decreased the diffusion length within the crystallites. In contrast to the typical effect of coking, where the selectivity of para- isomers would be enhanced with coke deposition, the effect of the reduction in the diffusion length of the PtH-ZSM-5(30) crystallites is consistent with the decreasing para-selectivity of the diethylbenzene (DEB) isomers with time-on-stream. <br /> n investigation of the causes of coke locations was conducted, and the results suggested that, the spatial distribution of Pt metal was responsible for the different locations of coke observed. Surface reactions initiated coking in the intercrystalline region of the PtH-ZSM-5(80) catalyst, as the Pt particles on the surface of the PtH-ZSM-5(80) crystallites increased the difficulty of access for reactants to the interior of the crystallites. Within the PtH-ZSM-5(30) catalyst, ethane dehydrogenation and benzene alkylation took place in the micropore network as a result of preferential intracrystalline deposition of Pt metal. Further conversions on the active sites within the PtH-ZSM-5(30) crystallites thus lead intracrystalline coking.

Modeling of fluid flow, heat transfer and reaction in fixed beds is an essential part of their design. This is especially critical for highly endothermic reactions in low tube-to-particle diameter ratio (N) tubes, such as methane steam reforming (MSR) and alkane dehydrogenation as two important commercial reactions. The modeling of fixed bed reaction is available in literatures with lots of assumptions. However, there is a need for implementing the reaction conditions with diffusion aspects on a real fixed bed reactor without assuming any pseudo conditions. Computational fluid dynamics (CFD) has been found as a suitable tool by many researchers to simulate fixed beds. CFD can simulate complex geometry of randomly-packed tubes, and provides us with more fundamental understanding of the transport and reaction phenomena in reactor tubes. CFD can be used to obtain detailed three-dimensional velocity, species and temperature fields that are needed to improve engineering approaches. Previously, the geometry of 120-degree wall segment (WS) of the whole reactor tube has been studied in our group. Previous works have introduced the coupling of gas flow and resolved species and temperature gradients inside pellets by CFD for methane steam reforming (MSR) and propane dehydrogenation (PDH) without considering deactivation. The deactivation of catalysts due to carbon formation is an important problem in industry, such as steam reforming and the catalytic dehydrogenation of alkanes, which are both strongly endothermic reactions. Many researches were carried out to study the effect of carbon formation and catalyst deactivation on the reactor performance. The local carbon deposition on catalysts can cause particle breakage and strongly decrease reaction rates. Catalyst deactivation in heated tubes removes the heat sink and can result in local hot spots that weaken the reactor tube. This is particularly a problem for a low tube-to-particle diameter ratio fixed bed reactor. A 3D resolved CFD model simulation was used to study the local details of carbon deposition in which the reactions and deactivation take place inside the catalytic solid particles. CFD simulations of flow, heat transfer, diffusion and reaction were carried out using the commercial CFD code FLUENT/ANSYS 6.3 in a 3D 120-degree periodic wall segment with N=4. The mesh used boundary layer prism cells at both the inside and outside particle surfaces and at the tube wall. These reactions were represented in the solid particles using user-defined scalars to mimic species transport and reaction, with user-defined functions supplying reaction rates. Diffusion in the particles was modeled by Fick's law using an effective diffusivity, given by Hite and Jackson's approximation of the Dusty Gas Model. The transient developments of particle internal gradients and carbon accumulation have been studied for the early stages of deactivation. Carbon concentration is initially strongest close to the surface and in the high temperature regions of the catalysts and affected by the wall heat flux. Deactivation of the endothermic reactions causes a slow increase in the average catalyst temperature. The second stage of the research was the verification of our CFD reaction model with experimental data under reacting conditions. The highly endothermic commercial methane steam reforming (MSR) reaction was studied experimentally in a fixed bed reactor. The temperature contributions inside catalyst particles were measured. The MSR reaction showed strong effects on the temperature profile along the reactor. Then, a CFD model was used to predict temperature profiles under MSR reaction conditions. Comparison of CFD and experimental data showed very good qualitative as well as quantitative agreement for temperature inside catalyst particles at different inlet gas temperatures. The last stage was to develop a fundamental energy equation without introducing new adjustable parameters to study heat transfer in fixed beds. In the past, many researchers have been carried out to simulate the heat transfer in fixed bed reactors by using kr (effective thermal conductivity) and hw (heat transfer coefficient). But the classical model with kr and hw cannot give a correct T(r) near tube wall, where deactivation is strongest. Therefore we need a better model which can represent the near wall heat transfer more accurate. CFD modeling was used to develop pseudo-continuum model for T(r) using Vr(r,z) and Vz(r). To get better temperature at the wall vicinity close to the physical reality. In this model radial thermal conductivity was obtained from Zehner-Schlünder model. The convection heat transfer was calculated in the 2D flow fluid from the CFD run. Results were obtained for Reynolds numbers in the range 240€“1900. The accuracy of the new model has been validated by analytical solution. The temperature calculated by the new velocity field pseudohomogenous energy equation showed reasonable quantitative agreement with values predicted by the CFD model.