Study On Synthesis Of Mesoporous Titania And Its Formation Mechanism From Industrial Titanyl Sulfate Solution

The study in this thesis was focused on the synthesis of mesoporous titania in view of the current problems and difficulties comparing with mesoporous silica and aluminate, such as high cost of titanium source (titanium alkoxide, titanyl ester or purified inorganic titanium salt, etc.), poor effect of simplex template removal method such as extraction or calcinations, which often resulted in destroying the pore structure, poor crystalline of mesoporous titania, and so on. Herein, it is very important and meaningful to explore the possibility of synthesizing high performance mesoporous titania from cheap titanium source, study on the synthesis influence factors for the precursor, probe into ultrasonic and microwave irradiation acting on the precursor structure, make a search for effective template removal method, establish a new process to prepare mesoporous titania, seek after mesostructure formation and stabilization mechanism, and so forth.The hydrolysis of industrial titanyl sulfate (TiOSO₄), obtained from sulfate process of ilmenite, was induced by a supermolecule template resulted from surfactants self-assembly, which oriented the precursor structure of mesoporous TiO₂. Then template in the precursor was removed by the combination of oxidation, extraction and calcinations, and anatase-phase mesoporous titania with high specific surface area and steady structure was obtained.The as-prepared mesoporous titania and precursors were studied through systematic experiments and characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared test (FT-IR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), nitrogen adsorption-desorption isothermos and so on. The technical conditions and influence factors for synthesis of mesopoorus titania were investigated, especially the outfield effects and template removal methods on the meso-structure. And its catalytic activities were evaluated by photo-catalytic decomposition of Methylene Blue and acid catalytic synthesis of ethyl acetate and Mafeng oil. Based on these results, formation procedure and synthesis mechanism of mesoporous titania precursor prepared under high acidic aqueous solution were analyzed and discussed. And the effects of template removal on the crystal structure were elementarily explored by using Rietveld profile refinement method. Experimental studies and analyzing results can be concluded as follows.1. The synthesis conditions have very important effect on formation of mesostructure and crystal phase for mesoporous titania precursors when industrial TiOSO₄ solution hydrolyzing is induced by self-assembly supermolecular template which orients the mesostructure. (1) Surfactants such as N-Dodecyl amine, Cetyltrimethyl Ammonium Bromide (CTAB) and triblock copolymer P123 have better structural directing ability than others, especially composite template (CTAB/P123). (2) By adding industrial TiOSO₄ solution to surfactant solution, selecting proper template dosage and titanium concentration and keeping the reaction system pH value at about 1.5, it is convenient for controlling hydrolysis and condensation rate of titanium ion and its interface assemble with template, which results in better structural mesoporous precursor and avoiding impurities such as iron separating out. (3) Elevating synthesis temperature (above 75℃) and prolonging synthesis time are beneficial to crystallizing pore wall and enhancing pore order degree. (4) Increasing aging temperature and adopting post hydrothermal treatment are favourable for pore wall crystallization and structural stability. On the basis of these factors, the mesoporous precursor, which structure is directed by composite template (CTAB/P123), is prepared via hydrolysis route from industrial TiOSO₄ solution. Then anatase mesoporous titania is obtained after template removal by step-by-step calcination. It is with narrow pore size distribution, stable mesostructure, average pore size of 3.0 nm and specific surface area of 132.6 m²/g.2. Introducing outfield effect into the synthesis of mesoporous precursor can promote the crystal structure and the pore formation. The products are obviously with higher specific surface area and stability. Ultrasonic and microwave irradiations are helpful to form higher pore order, and the product under microwave irradiation is with the highest specific surface area, 146.6 m²/g. While hydrothermal synthesis strengthens the interaction between the inorganic source and surfactants, condenses and crystallizes the pore wall, resulting in improving the mesoporous stability.3. Proper template removal method is also important for stabilizing mesoporous titania structure. Composite template (CTAB/ P-123) is entirely removed by compositive template removal route, i.e. ozone oxidizing first, then multistage extracting, and step-by-step calcining at last. After these treatments, mesoporous titania with anatase phase is prepared, with high specific area (SBET=133m²/g), larger average pore size (D_BJH=4.65nm). Template is oxidized to short-chain and small molecular organic matter after ozone oxidation, which decreases the following resistance of removing template. The extraction system of non-aqueous ethanol/hydrochloride acid reduces the surface activity of surfactants in the pore, weakenes the action with the framework, and the ratio of template removal is larger than 53% after multistage extraction, effectively reducing intensity of the following heat-treatment. Adopting step-by-step calcinations (125℃for 1h, 300℃for 1h and 450℃for 2h ) thoroughly remove the template and anatase stable mesoporous TiO₂ is obtained, minishing the side effects of inorganic network shrinkage, the framework and pore structure destroying and collapse due to breakage of electric charge equilibrium, which is induced by long time and high temperature calcinations.4. The catalytic activities of mesoporous TiO₂ are primarily influenced by its structure and surface properties. Mesoporous TiO₂, obtained from hydrolysis of industrial TiOSO₄ solution and after template removal, is with anatase phase, high specific surface area and small particle size, still with a little SO₄^2- adsorpted or bonded on the surface, forming acidic center of the catalyst. Mesoporous TiO₂ has higher performance in the photocatalytic oxidation of Methylene Blue (MB) and esterification reaction of acetic acid and ethanol, with degradation of MB at 98.4% and yield of ester at 91%. It also shows some catalytic performance for the esterification of Jatropha curcas L. seed oil in high acid value with methanol as precursors to manufacture biodiesel.5. The formation procedure of mesoporous titania is studied via hydrolysis induced by supermolecule template under high acidic aqueous solution, using industrial TiOSO₄ as titanium source and composite surfactants (CTAB/P123) as template. The product is with narrow distribution of planar hexagonal pore and high specific surface area (205.7 m²/g). The synthesis mechanism of the precursor belongs to cooperative formation mechanism, which could be summarized as follows: (1) The pH value greatly effects on cooperative action via static action( S⁺I^-) and hydrogen bond action (N⁰I⁰) on the interface. (2) Controlling pH not only makes titanium ion hydrolysis and condensation rate and surfactants self-assembling rate matching, but also avoids impurities separating out. (3) Raising synthesis temperature properly is liable to form larger pore size mesoporous material. (4) Increasing hydrophobic chain length of surfactant is helpful for synthesizing large pore size mesoporous material. (5) Adding inorganic cationic and anionic strong electrolytes to the synthesis system could enlarge the pore size. (6) The mol ratio of Ti/H₂O and H⁺/Ti has great effect on the hydrolysis rate (H) and restraining rate (p) of inorganic titanium ion, lager ratio of H/p led increasing hydrolysis rate, diminishing micelle size, resulting in smaller mesopore size.6. In the template removal procedure, the changes of interface action had some effect on the crystal structure and lattice strain, simultaneously produce atomic occupancy lacking. Template removal by extraction has weaker effect on crystal structure than calcination, and increasing template removal by extraction could effectively reduce calcinations intensity, improve the stability of mesoporous titania.