Thesis (Ph. D.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains x, 174 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 155-166).

The syntheses and characterization of a series of (NHC)Pd(II) complexes (NHC = N-heterocyclic carbene) are described. A variety of architectures and precursors have been employed to lead to numerous air- and moisture-stable complexes. The use of those complexes as pre-catalysts in cross-coupling (Suzuki- Miyaura, Buchwald-Hartwig) and related (catalytic dehalogenation, £-ketone arylation) reactions is also discussed. A comparison of the activity of a variety of (NHC)Pd complexes as pre-catalysts for cross-coupling reactions was carried out. The results indicate that the activation of those pre-catalysts, leading to the catalytically active [(NHC)Pd(0)] species, was key in assuring high catalytic performance under mild reaction conditions. For the first series of complexes described, (NHC)Pd(allyl)Cl complexes, a better understanding of the process leading to the catalytically active species has permitted us to introduce simple modifications (alkyl or aryl groups at the allyl moiety) that dramatically alter the performance of the complexes by facilitating their activation, decreasing reaction times, catalyst loadings and even allowing reactions to be conducted at room temperature. Catalyst loadings as low as 0.05 mol% can be used for the Suzuki-Miyaura crosscoupling of aryl chlorides and aryl boronic acids at room temperature, leading to the synthesis of poly-ortho-substituted biaryls in excellent yields. This catalyst loading is the lowest ever used for this purpose. The system also allows for the first examples of coupling between aryl chlorides and alkenyl boronic acids at room temperature. When the temperature is raised to 80 ¢ XC for these reactions, a catalyst loading as low as 50 ppm can be used to effectively carry out Suzuki-Miyaura cross-couplings in remarkably short reaction times. As an added advantage, these complexes are air- and moisture-stable and can be prepared in a facile one-pot, multigram scale synthesis from commercially available starting materials in very high yields. The second series of complexes described revolves around the (NHC)Pd(acac)n framework. These complexes are also air- and moisture-stable and can be prepared in a one-step synthesis in high yields from commercially available materials. These complexes were tested forƒn £-ketone arylation and Buchwald-Hartwig amination reactions affording high yields of the desired products, in short reaction times and mild reaction conditions.

Steam reforming is the most economical and widely-used route for the conversion of light hydrocarbon (such as natural gas) to various valued-added products. This process is commonly carried out over a low-cost alumina-supported nickel catalyst, which often suffers from carbon deposition resulting in loss of active sites, flow and thermal maldistribution, as well as excessive pressure drop. A bimetallic catalyst with improved anti-coking properties was formulated by incorporating the nickel-based system (15% loading) with cobalt metal (5% loading). Two-level factorial design was employed to investigate the effect of major preparation variables, namely impregnation pH value (2-8), calcination temperature (873-973 K), heating rate (5-20 K min-1) and time (1-5 h). The catalysts prepared were subjected to various characterisation techniques to determine key physicochemical properties (i.e. BET area, H2-chemisorption and NH3- TPD acidity). X-ray diffraction revealed that NiO, Co3O4, NiCo2O4 and a proportion of Ni(Co)Al2O4 aluminates were transformed during H2-reduction to active Co and Ni crystallites. TEM images showed an egg yolk profile in the low-pH catalyst suggesting that main deposition site was located in the particle centre, while metal deposition occurred primarily around the particle exterior for the high-pH catalyst. Temperature programmed experiments were carried out to examine the extent of conversion, type of surface species and solid-state kinetics (using the Avrami-Erofeev model) involved during various stages in catalyst life-cycle (calcination, reduction, oxidation and regeneration). Steam reforming analysis suggested that enhanced catalyst activity may be due to synergism in the Co-Ni catalyst. Specifically, the low-pH catalyst exhibited better resistance towards carbon-induced deactivation than the high-pH formulation. The study also provided the first attempt to develop a quantitative relation between catalyst preparation conditions and its performance (activity, product selectivity and deactivation) for steam reforming reaction. Deactivation and reforming kinetic coefficients were simultaneously evaluated from propane reforming conversion-time data under steam-to-carbon ratios of 0.8-1.6 and reaction temperatures between 773-873 K. The time-dependent optimum operational policy derived based on these rate parameters gave better conversion stability despite the heavy carbon deposit. Thermal runs further showed that the catalysts regenerated via two-stage reductive-oxidative coke burn-off exhibited superior surface properties compared to those rejuvenated by a single-step oxidation.

Steam reforming is the most economical and widely-used route for the conversion of light hydrocarbon (such as natural gas) to various valued-added products. This process is commonly carried out over a low-cost alumina-supported nickel catalyst, which often suffers from carbon deposition resulting in loss of active sites, flow and thermal maldistribution, as well as excessive pressure drop. A bimetallic catalyst with improved anti-coking properties was formulated by incorporating the nickel-based system (15% loading) with cobalt metal (5% loading). Two-level factorial design was employed to investigate the effect of major preparation variables, namely impregnation pH value (2-8), calcination temperature (873-973 K), heating rate (5-20 K min-1) and time (1-5 h). The catalysts prepared were subjected to various characterisation techniques to determine key physicochemical properties (i.e. BET area, H2-chemisorption and NH3- TPD acidity). X-ray diffraction revealed that NiO, Co3O4, NiCo2O4 and a proportion of Ni(Co)Al2O4 aluminates were transformed during H2-reduction to active Co and Ni crystallites. TEM images showed an egg yolk profile in the low-pH catalyst suggesting that main deposition site was located in the particle centre, while metal deposition occurred primarily around the particle exterior for the high-pH catalyst. Temperature programmed experiments were carried out to examine the extent of conversion, type of surface species and solid-state kinetics (using the Avrami-Erofeev model) involved during various stages in catalyst life-cycle (calcination, reduction, oxidation and regeneration). Steam reforming analysis suggested that enhanced catalyst activity may be due to synergism in the Co-Ni catalyst. Specifically, the low-pH catalyst exhibited better resistance towards carbon-induced deactivation than the high-pH formulation. The study also provided the first attempt to develop a quantitative relation between catalyst preparation conditions and its performance (activity, product selectivity and deactivation) for steam reforming reaction. Deactivation and reforming kinetic coefficients were simultaneously evaluated from propane reforming conversion-time data under steam-to-carbon ratios of 0.8-1.6 and reaction temperatures between 773-873 K. The time-dependent optimum operational policy derived based on these rate parameters gave better conversion stability despite the heavy carbon deposit. Thermal runs further showed that the catalysts regenerated via two-stage reductive-oxidative coke burn-off exhibited superior surface properties compared to those rejuvenated by a single-step oxidation.

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