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published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph.D.)--The Ohio State University, 2008. / Advisers: Yann Guezennec, Vish Subramaniam. Includes bibliographical references.

This work has focused on showing the differences among four different NMHC oxidation mechanisms: GT Lurmann, CBIV, RACM, and SAPRC mechanisms. This study was carried out to characterize these mechanisms using both specified NOx/NMHC gas mixtures and observational data from NASAs TRACE-P campaign. The differences among these mechanisms were found to be mainly driven by the use of different kinetic data and the specifics of each oxidation scheme. In the test runs, the differences between mechanisms were shown to be dependent on the levels of NOx and NMHC, as well as the reactivity of NMHC species used. Typically, propane had the smallest impact on all product species, whereas propene had the largest. Differences in the predicted levels of OH and HO2 were much smaller compared to those for CH3O2 and CH2O due to the fact that HOx species were generally less sensitive to the presence of NMHCs. During TRACE-P, which involved flights over only marine areas that were slightly polluted by the inflow of pollutants, the alkanes were the dominant NMHC family. Thus, most of the model runs involved relatively low levels of NMHCs and NOx. Therefore, the levels of OH, HO2, CH3O2, and CH2O predicted by the four mechanisms were not dramatically different. A net O3 increase was found only in areas where the NMHC reactivity was high. Because of the similar O3 destruction rates given by all four mechanisms, the difference in O3 tendency among these mechanisms was mainly determined by the O3 formation rate. A significantly higher (e.g., ~30%) O3 formation was found in the Lurmann mechanism than in CBIV due to the stronger contribution from the NO/RO2 channel in this mechanism. This resulted in a difference in the O3 tendency of a factor of 1.5. A major need in terms of future studies will be that of examining these same four mechanisms with a data set that enfolds observations in more polluted regions.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2006. / This work focuses on the effects of hydrogen peroxide concentration on the catalytic activity and product selectivity in the liquid-phase hydroxylation of phenol over titanium-substituted zeolites Al-free Ti-Beta and TS-1 in water and methanol solvents. Hydroquinone is typically the desired product, and these solvents employed have previously been shown to be of importance in controlling the selectivity of this reaction. Different volumetric quantities of an aqueous 30 wt-% peroxide solution were added to either water or methanol solutions containing the catalyst and phenol substrate, and the reaction monitored by withdrawing samples over a period of 6-8 hours. For Al-free Ti-Beta catalysed reactions, the peroxide concentration affects the selectivity and activity differently in water and methanol solvents. Using methanol solvent, the selectivity to hydroquinone formation is dominant for all peroxide concentrations (p/o-ratio > 1), and favoured by higher initial peroxide concentrations (> 1.27 vol-%), where p/o-ratios of up to can be reached; in water solvent, increasing the peroxide concentration above this level results in almost unchanging selectivity (p/o-ratio of ca. 0.35). For lower peroxide concentrations in water, the p/o-ratio increases slightly, but never exceeds the statistical distribution of ca. 0.5. Using water as a solvent, higher phenol conversion is obtained as the initial peroxide concentration increases; in methanol the phenol conversion is largely independent of peroxide concentration. As expected for the smaller pore TS-1, higher hydroquinone selectivity is obtained in methanol than for Al-free Ti-Beta, which is consistent with shape-selectivity effects enhanced by the use of this protic solvent. Interestingly, with TS-1 the p/o-ratio is higher at lower phenol conversions, and specifically when the initial peroxide concentration is low (p/o-ratio exceeding 3 were obtained at low phenol conversion), and decreases to a near constant value at higher conversions regardless of the starting peroxide concentration. Thus, low peroxide concentrations favour hydroquinone formation when TS-1 is used as the catalyst. Comparing the performance of the two catalysts using methanol solvent, the phenol conversion on TS-1 is more significantly influenced by higher hydrogen peroxide concentrations than Al-free Ti-Beta. However, with higher initial concentrations the unselective phenol conversion to tars is more severe since the hydroquinone selectivity is not higher at these high peroxide concentrations. The increased tar formation, expressed as tar deposition on the catalyst or as the tar formation rate constant, confirms that the greater amount of free-peroxide present is mainly responsible for the non-selective conversion of phenol. Kinetic modelling of the reaction data with an overall second-order kinetic model gave a good fit in both solvents, and the phenol rate constant is independent of changing hydrogen peroxide concentration for the hydroxylation over Al-free Ti-Beta using water as the solvent (kPhenol = 1.93 x 10-9 dm3/mmol.m2.s). This constant value suggests that the model developed to represent the experimental data is accurate. For TS-1 in methanol solvent the rate constant is also independent of peroxide concentration (kPhenol = 1.36 x 10-8 dm3/mmol.m2.s). The effect of the method of peroxide addition was also investigated by adding discrete amounts over a period of 4.5 hours, and was seen to improve hydroquinone selectivity for reaction on both catalysts, and most significantly for Al-free Ti-Beta in methanol solvent. With TS-1, the mode of peroxide addition had little influence on phenol conversion, but the initial selectivity to hydroquinone was ca. 1.6 times higher than for an equivalent single-portion addition (at a similar phenol conversion). Discrete peroxide addition for hydroxylation in methanol over Al-free Ti-Beta gave greatly improved hydroquinone selectivities compared to the equivalent single-dose addition. Compared to TS-1, the initial selectivity was not as high (p/o-ratios of 0.86 and 1.40 respectively at 10 mol-% phenol conversion), but this can be explained on the basis of geometric limitations in the micropores of TS-1 favouring hydroquinone formation. The final selectivity, however, is marginally higher (using the same mode of peroxide addition, and at the same phenol conversion). Discrete peroxide addition has an additional benefit in that it also reduces the quantity of free-peroxide available for product over-oxidation, and consequently reduces the amount of tars formed. Thus, the interaction of the effects of peroxide concentration and the solvent composition and polarity on the product selectivity and degree of tar formation is important. Particularly with TS-1, lower peroxide concentrations in bulk methanol solvent are highly beneficial for hydroquinone formation, because of the implicit geometric constraints in the micropores, the lower water concentration, and the decreased tar formation associated with high methanol concentrations. This could have significant reactor design implications, as the results obtained here suggest that the reaction should be terminated after approximately 30 minutes to maximise hydroquinone production (under the conditions evaluated in these experiments), even though the corresponding phenol conversions are low (ca. 10 mol-%). The higher hydroquinone selectivities reached at low phenol conversions for the discrete peroxide addition experiments also confirm this. Practically, to enhance the hydroquinone selectivity for reaction over TS-1, the initial phenol-peroxide molar ratio should be ca. 10, methanol should constitute not less than 90 vol-% of the reaction volume, and the peroxide should be added in discrete amounts. For reaction over Al-free Ti-Beta, methanol solvent also enhances the hydroquinone formation as expected. At low phenol conversions (ca. 10 mol-%) hydroquinone is still the preferred product, although in contrast to TS-1 the selectivity increases with phenol conversion, and is higher with higher initial peroxide concentrations. Under the best conditions evaluated here for optimal hydroquinone formation, the initial phenol-peroxide molar ratio should be ca. 2.5, with methanol making up at least 90 vol-% of the total volume. Discrete peroxide addition in methanol solvent for the Al-free Ti-Beta catalysed hydroxylation gives excellent improvements in hydroquinone selectivity (2.5 times higher than water solvent), and the addition in more discrete portions might further improve hydroquinone formation, and should therefore be examined.

This diploma thesis is focused on the study of conditions of benzo[a]pyrene, one of the major contaminant of foods, photodegradation under different conditions (solvents with different polarity, light sources, presence of antioxidants). In another part of the thesis, the degradation process of benzo[a]pyrene at various concentrations was studied, in order to characterise the kinetic aspects of photoinduced degradation. The attempt to identify the products of benzo[a]pyrene photodegradation was performed involving methods of gas chromatography and high performance liquid chromatography coupled with mass detectors, as well.