الفهرس 
Chapter 1 : Introduction and Objects of InvestigationOnly 14 pages are availabe for public view
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Abstract
The present thesis is concerned with the preparation, characterization and catalytic activity of zirconia-based solid acid and base catalysts. The prime target of the research work conducted is to prepare highly active and stable heterogeneous catalysts for the production of biodiesel and associated fine chemicals. Accordingly, the esterification of fatty acids, trans-rectification of tributyrin and isomerization of α-pinene on different sets of the prepared catalysts have been brought into prominence and discussed.
General Conclusions
Results presented and discussed in the present thesis may help drawing the following general conclusions:
Sulfated zirconia (SZ) catalysts with systematically increasing SO4 content are successfully prepared by wet impregnation with (NH4)2SO4 precursor. Isolated, tridentate sulfate ((ZrO)3-S=O) species dominate low S loadings <1.7 wt%, with induced Brønsted acidity arising from adjacent Zr4+–OH groups. At intermediate sulfur loadings (1.7-3.1 wt%), bisulfate-like ((ZrO)2(OH)(S=O)) species emerge, predominate, and exhibit superacidity arising from delocalized protons in S-OH moieties. At higher S content, Brønsted acidity retrogresses due to pyrosulfate/multilayer formation.
High purity tetragonal zirconia (t-ZrO2) particles of nanometric average crystallite size (7.5 nm) and high surface area (47 m2/g) are obtainable via carbonization (at 450 oC, 15 min) and subsequent calcination (600 oC, 3 h) of zirconium citrate gel. The tetragonal order of the ZrO2 withstands the metal partial substitution by either aluminum (AlxZ1-x) or magnesium (MgxZ1-x) ion dopants up to x = 0.15 gram-atom without being destructed or significantly altered.
Loaded sulfate to 2 wt %-S on t-ZrO2 is thermally resolved into comparable proportions of species-I, whose desorption is maximized at 633 oC, and species-II, which desorbs at the higher temperature of 840 oC. The high-temperature species-II is strongly bound multicentered ((ZrO)3-S=O) and bridging sulfate ((ZrO)2-(S=O)2) or hydroxo-sulfate ((ZrO)2(OH)(S=O)) species dispersed in monolayers. Whereas, the low-temperature species-I is suggestively weakly bound unidentate (ZrO-SO3) and/or polymeric sulfate species organized in overlayers.
Increasing the extent of Al substitution into the zirconia lattice results in a monotonic increase in surface area, acid site population, Lewis acid character, and decrease in acid strength, accompanied by the transformation of less stable polymeric/multilayer sulfate species to the more thermally stable isolated sulfate species.
Catalytic activity of zirconium hydroxide derived SZ catalysts for fatty acid esterification with methanol is a function of the solid acid strength, with optimum activity associated with the formation of a saturated sulfate monolayer comprising monomeric, chelating/bridging bisulfate species (which exhibit both strong Brønsted acidity and concerted Brønsted-Lewis acid sites).
The isomerization of α-pinene is optimized on surfaces exposing high population of synergetic Lewis-Brønsted acid pair-sites. Surfaces exposing no Brønsted acidity (e.g., pure zirconia) and/or reduced Lewis acidity (e.g., samples with high S loadings) exhibit poor activity.
Incorporation of Al at doping levels < 0.10 enhances the activity for α-pinene isomerization, attributed to generated super acidity. Higher Al concentrations favor weaker Lewis acid sites, resulting in lower activity and increased selectivity to polycyclic isomers.
Stability of sulfated zirconia-based catalysts depends strongly on the polarity of reaction medium. The high thermally stable monomeric bisulfate-like species are highly stable versus multiple-use of catalyst in nonpolar medium. In contrast, poly/multilayer sulfate species are easily removed from the surface after first re-use, even in nonpolar medium, leading to a detectable loss on catalytic activity.
Application of citric acid-mediated procedure for the preparation of magnesia–zirconia mixed oxide catalysts provides a high degree of homogeneity facilitating effective incorporation of magnesium ions into the crystallite lattice of zirconia, thus forming MgO-stabilized tetragonal zirconia phase.
Activity and stability of MgO-ZrO2 catalysts are greatly influenced by the preparation method. Highly dispersed MgO on the surface of ZrO2 is obtained by non-aqueous impregnation method. Dispersed MgO has high base strength and, hence, high transesterification activity, but its stability needs improvement. Whereas, homogeneous MZ-CT solid solution prepared by the aqueous citrate method exhibits both excellent activity and stability because of the strong interaction between active sites and ZrO2.
Activity of MgO-ZrO2 catalysts is also influenced by calcination temperature. The optimal calcination temperature is strongly dependent on the starting precursor and the synthetic route adopted. 650 oC and 550 oC are optimal calcination temperatures for catalysts prepared via non-aqueous impregnation and citrated method, respectively.