No description available.

No description available.

A study of solid base catalysis was conducted in three main categories, namely condensation reactions of primary alcohols, alkylation reactions of dihydroxybenzene and hydrogenation reactions of phenol. Ethanol was converted into 1-butanol over alkali earth metal oxides and modified MgO catalysts (1-20 % yield). The MgO catalyst exhibits the highest reaction activity and 1-butanol selectivity amongst the catalysts studied. Reaction of various possible intermediates (acetaldehyde, crotonaldehyde, crotylalcohol, butanal) and ethanol over MgO (1 bar, 450 oC) revealed that the dimerisation reaction does not proceed primarily through the aldol condensation reaction. The reaction is proposed to proceed through a mechanism, previously proposed by Yang and Meng, in which a C-H bond in the -position in ethanol is activated by the basic metal oxide, and condenses with another molecule of ethanol, by dehydration to form 1-butanol. iv A self-condensation reaction of propanol was carried out at atmospheric pressure over MgO as catalyst. The reaction gave 2-methylpentanol and propionaldehyde as the major products. The introduction of hydrogen before and during the reaction enhanced the catalyst selectivity to 2- methylpentanol. The effect of possible reaction intermediates on the catalyst selectivity and the mechanism of the reaction were investigated over MgO and indicated the important role of hydrogen transfer in the reaction. The highest selectivity (69 %) was achieved in the presence of hydrogen at 450 oC with propanol conversion of 24 %. Condensation reactions of ethanol with butanol or propanol to higher alcohols were carried out at atmospheric pressure over various solid-base catalysts. The ability of ethanol to be activated at either the CH2 or CH3 units played a significant role in the formation of a wide range of long chain alcohols. The major products observed during the reaction between ethanol and butanol were 2-ethylbutanol, 1-hexanol and to a lesser extent 2-ethyhexanol. The main products in the reaction of ethanol and propanol were 2-methylbutanol, 1-pentanol and butanol. Trace amounts of 2- methylpentanol were also observed. The vapour phase alkylation of catechol over supported cesium catalysts gave good selectivity to guaiacol formation. The TPD studies of the catalysts used indicated that the results can be correlated with the presence of weak basic sites on the catalyst. The gas phase selective hydrogenation of phenol to cyclohexanone was investigated over palladium supported catalysts in order to clarify the influence of the support on products distribution. High selectivities towards cyclohexanone (about 86 %) were observed on palladium supported on high surface area titanium oxide supports. On the basis of the TPD studies, it has been suggested that the basic properties of the support strongly influence the adsorption-desorption of phenol and products, and are therefore responsible for the selectivity towards the reaction products

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No description available.

No description available.

No description available.

No description available.

Compounds of the form RuO2(Y-py)2CI2 1 (py = pyridine, Y = H, 4-t-Bu, 3-CN, 4-CN, 2-Br, 3-Br, 4-Br, 4-Cl, 4-Me, 4-C(O)C6H5, 3-COOH, 4-COOH), RuO2ZCI2 2(Z = bipyridine, phenanthroline) and RuO2(OH)2(Y-py)2 3 (py = pyridine, Y = H, 4-CN, 4-t-Bu) were prepared and the effects of the various substituents on the properties of the compounds were investigated. The position of the v(Ru=O) bands of the complexes 1 correlate with the Hammett σ1 (or Taft σ*) and Pauling-type electronegativity values of the substituents on the pyridine ring. A comparative study of most of these complexes as both stoichiometric and catalytic oxidants for the oxidation of 1-hexanol to hexanal was carried out. The known dioxoruthenium(VI) compound [PPh4][RuO2(OCOCH3)CI2] (a) and a range of new compounds [PPh4][RuO2(OCOR)CI2] (R = CF3 b, C6H5 c, C6F5 d, C5H11 e) were synthesized and characterized. High synthetic yields were obtained for the fluorinated complexes b and d relative to their protonated equivalents a and c, possibly due to the varying charge distributions within such complexes. No clear trend in catalytic activity was noticed on varying the ligand on the ruthenium for the oxidation of 1-hexanol and 2-hexanol to hexanal and 2-hexanone respectively, with a range of co-oxidants. The complexes c and d were used very successfully in the catalytic oxidations of a variety of alcohols with the co-oxidants tetrabutylammonium periodate and 4-methylmorpholine N-oxide. The novel compounds [PPh4][RuO2(OCOC6H5)CI2 ] and [PPh4][RuO2(OCOC6F5)CI2] were supported on poly-4-vinylpyridine and used catalytically in oxidizing cyclohexanol to cyclohexanone, using 4-methylmorpholine N-oxide and tetrabutylammonium periodate as co-oxidants. / Thesis (M.Sc.)-University of Natal, Durban, 2000.

L'objectiu fonamental d'aquesta tesi és l'estudi mitjançant dinàmica molecular del comportament de líquids sobrerefredats. S'ha escollit líquids amb pont d'hidrogen, concretament els alcohols de baix pes molecular etanol i metanol. En els dos sistemes s'ha trobat una temperatura crítica per a la qual apareix una divergència en els temps de relaxació i en diferents coeficients de transport. En totes les funcions de correlació analitzades, la relaxació presenta dues etapes. La dinàmica de translació del centre de massa molecular està d'acord amb les prediccions de la teoria mode coupling ideal. S'ha demostrat que la desviació respecte a un comportament gaussià és important en els dos sistemes a baixes temperatures i temps intermedis, i que aquesta desviació està relacionada amb la presència d' heterogeneïtats dinàmiques. S'ha analitzat també el paper dels ponts d'hidrogen en els resultats obtinguts. Hom ha estudiat un model idèntic al metanol excepte pel que fa a la capacitat d'establir ponts d'hidrogen. Per aquest s'ha trobat que, per temperatures decreixents, el ritme d'augment en el temps de relaxació de translació és superior al de reorientació. La supressió de l'enllaç d'hidrogen produeix una temperatura crítica més petita per la difusió i pels temps de relaxació. El comportament no gaussià de la dinàmica per a temps intermedis és menys important quan se suprimeix l'enllaç d'hidrogen, però la seva dependència temporal per a temps grans és independent de l'existència de l'enllaç.En el sistema format per molècules diatòmiques s'han investigat les heterogeneïtats dinàmiques. S'ha comprovat la correlació entre els desplaçaments consecutius d'una molècula per temps intermedis i a temperatures baixes, que tendeix a desaparèixer per a temps molt grans (relaxació ). La correlació és màxima quan la desviació del comportament gaussià de la dinàmica arriba també al seu màxim. La direccionalitat dels moviments de translació ha estat també estudiada, tot observant-se un comportament anisòtrop especialment a temps intermedis (relaxació ). Han estat també observades heterogeneïtats dinàmiques en la reorientació, de manera que són les molècules amb gran desplaçament les que presenten un gran moviment de reorientació. Aquest acoblament augmenta a mesura que la temperatura decreix. La relació entre heterogeneïtat dinàmica i espacial s'ha estudiat en el sistema de molècules diatòmiques. Al final d'un interval temporal s'han seleccionat les molècules que presenten el desplaçament més gran i més petit. S'ha demostrat que les molècules d'aquests dominis dinàmics s'agreguen formant agrupacions més grans que les resultants en un sistema estadísticament aleatori. En refredar el sistema, el nombre mitjà de molècules constituents d'una agrupació ràpida, així com la seva escala de longitud, creixen notòriament. Ambdues quantitats resulten ser màximes quan l'interval de monitorització és proper a l'instant per al qual la dinàmica del sistema més se separa de la dinàmica gaussiana. Les agrupacions de molècules ràpides tenen una geometria allargada. Les agrupacions de molècules lentes tenen una grandària més petita que les ràpides i tenen una geometria més compacta. / The aim of this thesis is the study of supercooled liquids by means of molecular dynamics simulations. We have investigated the behavior of hydrogen bonded liquids, specifically the low weight alcohols methanol and ethanol. In both systems, relaxation times and transport coefficients diverge at a critical temperature. For all correlators under study, a two-step relaxation behavior has been encountered. Translation dynamics of molecular centers-of-mass agrees reasonably well with Mode Coupling predictions. At low temperatures and intermediate time scales, a remarkable deviation from Gaussian dynamics is found. This deviation is related to the existence of dynamical heterogeneities. The role of hydrogen bonding in supercooled methanol has also been analyzed. The results obtained for methanol have been compared with those of an ideal methanol-like system whose diatomic molecules have the same dipole moment as the methanol ones but lack sites for hydrogen bonding. Upon cooling, it has been found that translational relaxation times increase more rapidly than reorientational ones, being this effect more important in the non-associated system. Suppression of hydrogen bonds also results in lower critical temperatures for diffusion and for several characteristic relaxation times. Deviations from Gaussian dynamics are more relevant in methanol, but the long time behavior does not depend on the existence of hydrogen bonds.Dynamic heterogeneities have been analyzed in the supercooled system of diatomic molecules. Correlations between molecular displacements performed during consecutive time intervals are apparent at low temperatures and intermediate times, whereas they tend to disappear at long times (-relaxation). These correlations maximize when the deviation from Gaussian dynamics takes a maximum. The molecular displacement has an important anisotropic component at intermediate time scales (-relaxation). Dynamic heterogeneities in reorientation have been observed at the beginning of the -relaxation regime, and it has been obtained that molecules that perform faster translational motions experience faster reorientational motions too. This coupling increases as temperature decreases.Finally, the connection between dynamic and spatial heterogeneities has been studied. It has been encountered that molecules belonging to the same dynamical domain are spatially correlated. They form clusters bigger than those resulting from random statistics. Clusters composed by mobile molecules are string-like shaped, and both their associated size and lenght scale grow notoriously upon cooling. Both quantities maximize when non-Gaussian dynamics maximizes too. Clusters made up by slow molecules have a smaller size and a more compact geometry than fast clusters.