Acid catalysed synthesis of mesoporous silica and its applications as adsorbents, catalyst support and micromotors

Micron-sized mesoporous silica particles have been used in various applications such as adsorption, separation, catalysis, and drug delivery due to their high specific surface area, pore size, high thermal/chemical stability. However, controlling the properties of mesoporous silica particles is difficult because of the need for different chemicals and additional steps required in the synthesis method. In this thesis , the properties of mesoporous silica particles have been investigated and tuned by controlling the process parameters of sol-gel synthesis method carried out by mixing Pluronic F127 surfactant and tetra ethyl orthosilicate (TEOS) in highly acidic solution at room temperature. Ternary phase diagram of mesoporous silica morphologies was determined by adjusting the compositions of three reactants in the synthesis solutions. The ternary phase diagram was an important development for the investigation of all possible morphologies that can be obtained with a simple and low-cost synthesis method. After analysis of over 150 silica samples, regions with different morphologies (sphere, monolith and polyhedron) and variable surface areas were determined. Mesoporous spherical silica particles (MSPs) with average particle sizes ranging from approximately 1.00 µm to 5.00 µm and pore sizes from 3 nm up to 15 nm were successfully synthesized. Furthermore, hydrothermal treatment process was used to improve the specific surface area (up to 1000 m2/g) and pore size (up to 15 nm) of MSPs. With hydrothermal treatment, the surface area of MSPs was increased by up to a factor of 1.76. The synthesized mesoporous silica structures were used in the purification of wastewater from textile dyes, photocatalysis and self-propelled micromotor applications and their performances were studied. The changes in the morphology and pore structures of the particles were successfully identified by using different acids (nitric acid (HNO3) and sulfuric acid (H2SO4)) compared to the hydrochloric acid (HCl) in the investigation of ternary phase diagram. Pore size of the particles was increased from 3 nm to 14 nm by using sulfuric acid without changing the morphology of MSPs. Wastewater treatment performance of mesoporous silica particles synthesized with different acids was tested with Rhodamine 6G as a model molecule and the performances were compared. The maximum absorption capacity of about 20 mg/g was reached by silica particles synthesized with sulfuric acid. The capacity of the particles for removal of dye molecules from water was investigated by changing the process parameters (pH, temperature, time and initial concentrations). Adsorption processes were analyzed with linearized Langmuir and Freundlich isotherms, to estimate the mechanism of adhesion of dye molecules to particles. In addition, the suitability of the adsorption processes to the pseudo-first-order and pseudo-second-order kinetic models was also investigated by varying the contact time. In addition, the changes in thermodynamic parameters (ΔG°, ΔH°, and ΔS°) of the adsorption processes were calculated with the experimental results obtained as a function of temperature at which adsorption took place. The results showed that mesoporous silica particles synthesized with a simple and cost-effective process can satisfy the requirements for adsorbents in wastewater treatment applications. The synthesis of metal (iron and copper) loaded porous silica particles as catalysis material has been successfully completed. The catalytic activity of the metal loaded particles was investigated by degradation of Methylene Blue (MB) textile dye selected as the model pollutant. The degradation efficiency observed by Fenton and photo-Fenton reactions was compared depending on the type of metals in the particles and the amount of loading. It was also observed that the iron-loaded mesoporous silica particles cleaned the MB solution at 100 mg/L concentration by 99% in 20 minutes under UV light. The experimental results showed that the use of catalyst particles produced by a simple synthesis method in advanced oxidation processes will provide high efficiency. Self-propelled mesoporous silica particles have been successfully synthesized by CaCO3 loading into the structure. The loading process was carried out by the formation reaction of the CaCO3 structure by the reaction of CaCl2 and NaCO3 and proved by EDX and SEM analyses. Optimization of the CaCO3 loading process has been completed and the synthesis parameters required for the highest loading yield have been determined. The ability of the loaded mesoporous silica particles to move in acidic aqueous solution (acetic acid) by bubble propulsion was observed under optical microscope. In addition, the velocities of silica particles with different particle size, surface area and pore size were compared. It has been observed that similar size particles having smaller pore diameters move faster. Moreover, it has been understood that pore size is as effective as particle size and surface area. The highest velocity was observed as 1.6 μm/s in particles with a diameter of about 1.5 μm and a surface area of 789 m2/g. Self-propelled mesoporous silica particles have the potential to be converted to Janus particles and used in biological applications as micromotors.