Study On Preparation And Mechanism Of Fe-doped Sulfated Mesoporous Titania From Industrial Titanyl Sulfate Solution

The rapid development of modern society led to the excessive consumption offossil energy （including various mineral resources）, which caused the energy crisis andenvironmental pollution. Both of them have become two important constraints onsustainable human development. Traditional pollution control technologies （such asactivated carbon adsorption, incineration, ultrafiltration, biological treatment, etc.） cannot effectively or economically address the environmental contamination problems. Atthe same time, it has become a common strategic choice to seek renewable energy formany countries. Heterogeneous photocatalysis as an efficient depth oxidationtechnology, not only can directly convert solar power into clean energy such as electricalenergy （solar cells） and chemical energy （hydrogen production by photochemical watersplitting）, but also for sewage treatment and air purification. It is expected to solvefundamentally the issues of clean energy production and environmental pollutionremediation.Fe-doped sulfated mesoporous titania catalysts were prepared by one step thermalhydrolysis of industrial titanyl sulfate without using organic template reagent andcharacterized using X-ray diffraction （XRD）, scanning electron microscopy （SEM）,Fourier transform infrared spectroscopy （FT-IR）, UV-vis diffuse reflectancespectrophotometer （UV-vis DRS）, X-ray Photoelectron Spectroscopy （XPS）,Thermogravimetry analysis-differential scanning calorimeter （TGA-DSC）, N2adsorption-desorption techniques. The photocatalytic activity of Fe-doped sulfatedmesoporous titania was evaluated using the photocatalytic degradation of methyleneblue and phenol aqueous solutions under UV and visible light irradiation, respectivelyand the solid acid catalytic performance of it was determined through thepre-esterification reaction of free fatty acid in high acid value Jatropha curcas L. oil withmethanol as well. The effects of the industrial titanyl sulfate solution composition, thermal hydrolysis conditions, calcining regime, and reaction parameters on thecatalytic activity of Fe-doped sulfated mesoporous titania were systematicallyinvestigated. The adsorption thermodynamics and kinetics of methylene blue onto werealso studied. In addition, the structure formation, catalytic mechanism, deactivation andregeneration of Fe-doped sulfated mesoporous titania catalysts were discussed. Thespecific conclusions are as follows:（1） The total titanium concentration （counted as TiO₂） affects the boiling point andviscosity of industrial titanyl sulfate solution, and the mass transfer resistance in titaniacrystal growth process. The m（Fe）/m（TiO₂） can affect the density, viscosity and ionicstrength of industrial titanium titanyl sulfate solution, and the nuclei formation in theinitial stage of hydrolysis process. The acidity coefficient will adjust the stability oftitanyl sulfate solution and the hydrolysis rate and consequently the crystallinity,specific surface area and pore structure of Fe-doped sulfated mesoporous titaniacatalysts. For the photocatalytic degradation of methylene, the optimal industrial titanylsulfate solution compositions are as follows: TiO₂of175g/L, m（Fe）/m（TiO₂） of0.35and acidity coefficient of1.77. The volume ratio of pre-adding water to TiOSO4affectsthe number of in the nucleation stage. The feeding speed of TiOSO4solution impacts theformation time, composition, activity and consistency of nucleus. The heating ratebefore the first boiling stage prevailingly influences the secondary nucleation process ininduction period and the ups and downs of structure and energy in the nucleation stage.For the photocatalytic degradation of methylene, the optimal thermal hydrolysisconditions are as follows: The volume ratio of pre-adding water to TiOSO4of23%, Thefeeding speed of TiOSO4solution of8.55mL/min and the heating rate before the firstboiling stage in the range of0.8-1.0℃/min.Furthermore, the calcination regime is animportant factor for Fe-doped sulfated mesoporous titania catalyst. For example,calcination temperature can control the crystalline phase, grain size, specific surfacearea, pore volume, pore size and its distribution, sulfur-containing species and thenumber of surface hydroxyl groups in the surface, and the light absorption properties.While the impact of calcining time is relatively small. For the photocatalytic degradationof methylene blue and phenol, and the pre-esterification reaction of free fatty acid inhigh acid value Jatropha curcas L. oil with methanol, the optimal calcination regime are:500℃,1.5h;600℃,2.5h and500℃,2h; respectively.（2） Reaction parameters have a significant impact on the catalytic properties ofFe-doped sulfated mesoporous titania. For instance, as a photocatalyst, the Reactionparameters （the initial substrate concentration, catalyst dosage and pH value of thereaction system） can distinctly affect the photocatalytic efficiencies of Fe-doped sulfated mesoporous titania. For the photocatalytic degradation of the colored methyleneblue dye, the optimal catalyst concentration and the pH values are1.0g/L and8.0respectively when the initial substrate concentration was selected as6mg/L. while1.2g/L and4.0for the photocatalytic degradation of colorless phenol organic compoundsat the same conditions. When Fe-doped sulfated mesoporous titania used as a solid acidcatalyst, the reaction variables such as the reaction temperature, the molar ratio ofmethanol to free fatty acid, the catalyst dose and the reaction time have an effect uponthe catalytic activity of Fe-doped sulfated mesoporous titania. For the pre-esterificationreaction of free fatty acid in high acid value Jatropha curcas L. oil with methanol, theoptimal reaction temperature is90℃, the optimal molar ratio of methanol to free fattyacid is20:1, the optimal catalyst dose is4wt.%of oil and the optimal reaction time is3h.（3） The adsorption of methylene blue onto Fe-doped sulfated mesoporous titania isconformed to the Langmuir model, and adsorption thermodynamic parameters have thesame variation trend as the photocatalytic efficiencies with increasing calcinationtemperature, which provide a reasonable explanation for the change of thephotocatalytic activity of Fe-doped sulfated mesoporous titania with calciningtemperature from the viewpoint of adsorption thermodynamics. In addition, theadsorption kinetic experimental data of methylene blue onto Fe-doped sulfatedmesoporous titania appropriately correlate with the pseudo-second order model, andadsorption activation energy and the adsorption rate constant have also the samevariation trend as the photocatalytic efficiencies with increasing calcinationtemperature. The adsorption process is controlled by intraparticle diffusion, but in theinitial stage, the boundary layer diffusion has a significant influence on adsorptionkinetic.（4） The structure formation of Fe-doped sulfated mesoporous titania includes thefollowing three steps: nucleation and growth, agglomeration, and aggregation. In thisstructure, Ti4+is partly substituted by Fe3+in titania lattice, which induced theformation of oxygen vacancies. The oxygen vacancies are conducive to the dissociationadsorption molecular water and formation of surface hydroxyl. The sulfates arecovalently bounded to titania lattice in chelating bidentate coordinative model, which isfavorable to stabilize the anatase crystalline phase and oxygen vacancies, and toincrease the solid acidity of Fe-doped sulfated mesoporous titania. In addition, Fedopant forms impurity energy levels above the valence band and oxygen vacancygenerates defect levels below the conduction band of titania. Both two internal energylevels are beneficial to the absorption of visible light and consequent to enhancing the photocatalytic activity under the visible light irradiation. At the same time, Fe dopantcan also promote the separation of photo-induced carriers and thus improve thephotocatalytic efficiencies of Fe-doped sulfated mesoporous titania, but too many Fedopant will facilitate the recombination of the photogenerated electron-hole pairs andthen reduce the photocatalytic efficiencies of Fe-doped sulfated mesoporous titania.Furthermore, the solid acid photocatalytic mechanism of Fe-doped sulfated mesoporoustitania for the pre-esterification of free fatty acids in high acid value Jatropha curcas L.oil with methanol is proposed based on homogeneous acid catalytic mechanism for theesterification reaction. More specifically, the mechanism includes the following steps:protonated fatty acid, nucleophilic addition with methanol, and dehydrogenation to formesters while the solid acid sites to be restored.（5） FTIR study results evinced that the deactivation of Fe-doped sulfatedmesoporous titania used as photocatalyst is mainly ascribed to the irreversible chemicaladsorption of intermediate product onto the surface of it. while direct calcination of thedeactivated photocatalyst in air atmosphere at500℃for1.5h is a simple and effectiveregeneration method.