No description available.

No description available.

A number of ligands that belong to the salen-type family were synthesized in this thesis. These ligands were synthesized from salicylaldehyde and 1,2-phenylenediamine, 1,3- diamino-2-hydroxypropane, 1,2-diamino-ethane, N-(3-aminopropyl)-1,3-propanediamine, diethylenetriamine, diaminomaleonitrile, 2,2-dimethyl-1,3-propanediamine and 1,3- diaminopropane. From this range of ligands, H2salophen was chosen as the ligand for further studies. This work is aimed primarily at elucidating the structures and spectroscopic properties of [Co(salophen)(amine)2](OAc) derivatives, where salophen is N,N’-disalicylidene-1,2- phenylenediamine and the amines used were butylamine, benzylamine, a- methylbenzylamine, dibutylamine, N-methylpiperazine and piperidine. Three novel crystal structures of [CoIII(salophen)L2]Cl derivatives, where L = butylamine, benzylamine, and piperidine, with Co-N distances that range from 1.901 Å to 2.024 Å, have been reported in this thesis. The novel crystal structure of [Co(salophen)(N-MePipz)(OAc)] is also reported in this thesis. These cobalt complexes have been analysed by 1H, 13C and 59Co NMR as well as electronic and IR spectroscopy. A 59Co NMR spectrum was obtained for the [Co(salophen)(BuNH2)2]CH2Cl2×Cl complex. The spectrum exhibits a single line at 8504 ppm. The binding constants of all [Co(salophen)(amine)2](OAc) complexes, where amine = butylamine, benzylamine, a-methylbenzylamine, dibutylamine, N-methylpiperazine and piperidine, were determined by spectroscopic titrations. The titrations were carried out at various concentrations of the amine and at temperatures ranging from 25°C to 45°C. It was found that the primary amines had much larger values of K1 and K2 compared to the secondary amines. Typical values of K1 and K2 were 8000 M-1 and 63.6 M-1 respectively at 25°C, for a-methylbenzylamine. Of the primary amines, it was found that a- methylbenzylamine had the largest value of K1 and K2 compared to the other two amines. For the secondary amines, it was found that N-methylpiperazine had the bigger value of K1 compared to that of dibutylamine. / Thesis (M.Sc. )-University of KwaZulu-Natal, Pietermaritzburg, 2007.

As ~ 50% of global photosynthesis occurs in marine environments, the factors regulating this process e.g. trace metal availability, have an impact on the global carbon cycle. The key cyanobacteria genera Prochlorococcus and Synechococcus have an absolute requirement for Co. Dissolved cobalt (dCo) concentrations in the open ocean are extremely low (5–120 pM). A sensitive flow injection technique using chemiluminescence detection (FI-CL) was developed (detection limit 4.5 pM dCo, RSD ≤ 4%). Seawater samples must be UV-irradiated prior to analysis, in order to liberate organically-bound Co. A field study in the Sargasso Sea, demonstrated that aerosol Co was significantly more soluble than aerosol Fe over a range of aerosol dust deposition fluxes (1–1040 μg Fe m-2 d-1) (8-100% for Co versus 0.44-45% for Fe). The dry deposition flux of aerosol Co was of the same order of magnitude as the advective upwelling flux (47-1540 pmol m-2 d-1 and 1.7-1430 pmol m-2 d-1 respectively). Wet deposition, dominated the total aerosol flux (~ 85%). The vertical distribution of dCo influenced Prochlorococcus abundance. A regional study in the eastern North Atlantic gyre demonstrated that the highest rates of N2 fixation occurred with the highest dFe concentrations (9.8 nM N L-1 h-1, 0.6 nM respectively). No increase in primary production following additions of trace metals (Co, Cu, Fe, Zn) was observed. The addition of N resulted in an increase in primary production. However, there was no synergistic effect of trace metal plus N addition, suggesting that alleviation of N-limitation shifted the system to P-limitation. On a meridional transect from ~ 50°N–50°S in the Atlantic Ocean, the highest concentrations of dCo (> 80 pM) coincided with low-O2 (< 150 μM) upwelled water. The lowest dCo (< 20 pM) was observed in the eastern North Atlantic gyre. Lateral advection of continental Co and upwelling were identified as important sources of Co. The highly efficient recycling of Co in the euphotic zone is an important additional source.

No description available.

Modern organic chemistry strives to achieve rapid molecular complexity from simple achiral substrates. One method by which this may be achieved is with enolate formation followed by attack on an electrophile which can generate one, two or even more new stereocentres in one step. However regioselective generation of an enolate in the presence of several enolisable sites has always proved problematical. A partial answer to this problem has been provided by the development of the reductive aldol reaction. The first part of this thesis is concerned with describing a highly diastereoselective Co(II)-catalysed reductive aldol reaction between α,β-unsaturated amides and ketones. The reaction proceeds using substoichiometric quantities of cobalt(II) in the presence of a stoichiometric quantity of the reductant diethylzinc. Using both N,Ndimethyl and morpholine amides, the reactions are tolerant of substituted aromatic ketones as well as aliphatic ketones. The reaction also proceeded well when the β-carbon was substituted with both aromatic and aliphatic groups resulting in improved diastereoselection. The racemic work is followed by the development of an asymmetric version of the reaction using oxazolidinone chiral auxiliaries that impart high levels of diastereofacial selectivity. The reaction was found to proceed with a variety of aromatic ketones and once again, substitution of the β-carbon resulted in improved diastereoselectivity. Finally work on formal homo aldol cyclisations using substoichiometric quantities of Ni(II) also in the presence of a stoichiometric quantity of diethylzinc is described. This work aims to develop methodology that involves double cyclisations with the formation of up to five contiguous stereocentres. Although unsuccessful, useful conclusions for future work were made as well as the serendipitous discovery of a apparent base catalysed alternative cyclisation pathway that successfully generated two new rings and four contiguous stereocentres.

Cette thèse porte sur l’étude des différents phénomènes qui se produisent au cours de l’activation des catalyseurs industriels à base de cobalt pour la synthèse Fischer-Tropsch et lors de la réaction dans des conditions opératoires réalistes. Une première approche a consisté à étudier l’impact de différents paramètres qu’ils soient opératoires ou structuraux sur les performances catalytiques. Diverses techniques de caractérisations ex-situ combinées à des tests catalytiques ont permis d’établir des corrélations entre la structure de catalyseur et les conditions opératoires. Néanmoins, en raison de limitations de la caractérisation ex-situ, il subsistait un manque d’informations sur la structure du catalyseur au cours de la réaction.De fait, une étude operando innovante de l'évolution de la structure de catalyseur en conditions de réaction Fischer-Tropsch, représentant un défi expérimental significatif en raison des conditions de réaction sévères (haute température, haute pression, milieu multiphasique…), a permis de combler ce manque d’informations.Ces travaux ont finalement mis en évidence la versatilité notable des solides étudiés lors du procédé Fischer-Tropsch, et permis d’apporter des réponses quant aux phénomènes de l’activation et la désactivation des catalyseurs tels que la réduction et l’oxydation du cobalt, le frittage ou encore la carburisation de la phase métallique parfois suggérés dans la littérature. / This PhD thesis focuses on the investigation of different phenomena which occur during activation of the industrial catalysts for Fischer-Tropsch synthesis and during the reaction under realistic conditions. The first approach was to study the impact of different operational and structural parameters on the catalytic performances. Various ex-situ characterization techniques combined with catalytic tests uncovered a few correlations between the catalyst structure and operating conditions. Nevertheless, because of limitations of ex-situ characterization, there remained a lack of information on the evolution of catalyst structure during the reaction. Then, an innovative operando study of the evolution of catalyst structure under reaction conditions in the Fischer-Tropsch process, which represents a significant experimental challenge due to the severe reaction conditions (high temperature, high pressure, multiphase environment ...), has fill this information gap.This work has finally revealed the significant versatility of studied solids in the Fischer-Tropsch process, and provided new insights into understanding catalyst activation and deactivation phenomena such as cobalt reduction/oxidation, sintering or carbidization of the metallic phase often discussed in the literature.

The tricobalt cluster PhCCo₃(CO)₉ (1) reacts with the bidentate phosphine ligand 2,3-bis(diphenylphosphino)maleic thioanhydride (bta) with added Me₃NO to yield PhCCo₃(CO)₇(bta) (2), which upon heating overnight yields Co₃(CO)₆(μ₂-η²,η¹-C(Ph)C=C(PPH₂)C(O)SC(O)) (μ₂-PPh₂) (3). Cluster (3) has been isolated and characterized by FT-IR and ³¹P NMR spectroscopy. Structural determination of the cluster has been demonstrated by X-ray diffraction analysis. Cluster (3) is analogous to the cluster synthesized by Richmond and coworkers. The redox properties of (3) have been examined by cyclic voltammetry and the data are reported within.

Call number: LD2668 .T4 1950 H5 / Master of Science

PhD, Faculty of Engineering and the Built Environment, University of the Witwatersrand / Fischer-Tropsch is a process that converts synthesis gas (especially H2 and CO) into hydrocarbons by the mean of metal catalysts (such as Fe, Co, Ru, and Ni). Its success depends strongly on the catalyst used for the reaction, the reactor where the reaction is taking place, and some parameters such as the operating temperature, the reactor pressure, and the gas purity, composition (ratio H2:CO) and flow rate. Besides the above parameters, other factors, such as the degree of reduction of the catalyst, also play an important role for a successful FT reaction. Water can deactivate (by re-oxidation) the catalyst and carbon deposit can reduce the catalyst’s activity. It is well known that FT is a complex reaction because of the range of products that it produces as well as the reactions that occur during the process. A good choice or combination of catalysts, reactor and operating conditions can help to control the product spectrum. 2 In this thesis we develop a simple graphical technique to represent the mass, energy balance and thermodynamic constraints that affect both the catalyst and the reactor. This graphic model is shown to be capable of opening up insights into reactor operations and indicating preferred operational regions. The diagrams make it possible to visualize operations and understand the interactions between the catalysts and the reactor. The mass and energy balances also provide information about the best possible region in which the FT reactor system can be designed and operated. A few catalysts (Fe/TiO2, Co/TiO2 and Fe:Co/TiO2) were prepared for the completion of this work. Some of them were tested separately and others were mixed in the same reactor. The results showed that the physical mixture (of Fe/TiO2 and Co/TiO2) and bimetallic catalysts behave differently from one another. The addition of Fe Fe/TiO2 to a constant amount of Co/TiO2 results in an increase of CO hydrogenation activity, WGS activity and CH4 selectivity. However, the position of the two catalysts in the reactor (one followed by another) shows little effect on the rate of hydrogenation of CO and the CO conversion.