New sol-gel approaches for synthesis of titania-silica heterogeneous catalysts and their catalytic properties

Abstract

Heterogeneous catalysts have been extensively investigated due to their promising application in eliminating pollutants from the environment and in production of biodiesel. Meanwhile, mesoporous Titania-silica composite (MTSC) with high BET surface area and porous structure has found many applications in the field of heterogeneous photocatalysis suitable for degradation of organic contaminants which are common pollutants from textile industries. However, most of the titania-silica based photocatalysts are obtained using complicated synthetic methods, high-cost metal alkoxide precursors (M (OR)4, M=Ti or Si and R= hydrocarbon chain) and perhaps the consumption of surfactants which are unquestionably hazardous. In the present work modified sol-gel approaches to form mesoporous titania-silica composites with desirable properties for decomposition of organic contaminants are proposed. In order to achieve this goal, sodium silicate and titanium oxychloride were used as silica and titania precursors respectively; since they are more cost-effective as compared to their counterpart metal alkoxides sources. Various factors that can influence the physico-chemical properties of the final products such as calcination temperature, ageing time, medium of reaction; washing solvent and the effect of using surfactants were exquisitely investigated.

The photocatalytic experiments were performed in the degradation of methyl orange (MO) and methylene blue (MB) to evaluate the practical applicability of the synthesized samples. Moreover, raw MTSC was further modified through impregnation with sulfuric acid to yield solid acids suitable for catalyzing esterification and transesterification reactions to produce methyl esters. The possibility of producing these materials with appreciable properties from less expensive sources will promote large scale production of heterogeneous catalysts for myriads of applications.

Chapter 1 of this dissertation highlights the principle of titania photocatalysis and the attempts that have been made in improving its photocatalytic performance. Chapter 2 reports a two step condensation sol-gel method to form mesoporous TiO2-SiO2 composite with a large surface area (387m2/g) and improved textural properties in the absence of surfactants. In this particular experiment, a transparent TiO2 sol was formed at room temperature through condensation and peptization of titanium oxychloride solution using ammonium hydroxide and aqueous nitric acid respectively. Sodium silicate was used as a silica source to form a composite with the preformed TiO2 sol. The physico-chemical properties of the composite were examined and compared to those available in literature (grafting and one-pot co-condensation methods) obtained using the same precursors in the presence of Cetyltrimethylammonium bromide (CTAB). Additionally, the photocatalytic studies were performed in the photodegradation of MO in the presence of natural solar radiation.

Chapter 3 elucidates the influence of titania contents in the mesostructure of the TiO2-SiO2 composites with various Ti-to-Si ratios synthesized via the sol-gel process using less expensive precursors; sodium silicate solution as a silica source and titanium oxychloride as a titania source. The effect of varying Ti-to-Si ratios on the morphology, porosity and photocatalytic performance was examined by controlling the Ti-to-Si ratio between 0.7 and 5.6. The effect of aging was evaluated in the sample with Ti-to-Si=2.9 and eventually photodegradation experiments of methyl orange (MO) by the samples calcined at 800oC. This study will provide a criterion for selection of Ti-to-Si ratio and ageing time suitable for fabrication of composites with superior properties for various industrial applications.

Chapter 4 presents a comparative study of TiO2-SiO2 composites synthesized via the peptization sol-gel method. Typically, titanium hydroxide gel (Ti(OH)4) was formed by condensation of titanium oxychloride solution using adequate amounts of ammonium hydroxide solution. The preformed gel was peptized either in silicic acid (SA) or nitric acid (NA) to form a sol that yields a TiO2-SiO2 composite with either a single-phase (anatase phase) or a mixture of both anatase and rutile phases. Furthermore, a simple experiment of photodegradation of methyl orange in the presence of sunlight was performed to demonstrate the practical efficacy of the composites synthesized via this method. This study provides a facile and reproducible technique to control phase and ultimately the photocatalytic performance of the TiO2-SiO2 composites using these less expensive precursors.

Meanwhile, systematic synthetic approaches intended to enhance the textural properties of TiO2-SiO2 composites are being reported in chapter 5 of this dissertation. Generally, three sol-gel approaches namely; (1) washing the hydrogels with different solvents, (2) using surfactant and (3) forming the TiO2 sol in ethanol medium; were exquisitely used to form composites using cost effective precursors. It was found that in using these approaches, composites with harnessed textural properties comparable to those of aerogels can rapidly be achieved through use of cost-effective precursors while avoiding surface modification of the composites or using complicated drying processes. The photocatalytic decolorization of methylene blue by the composites was also investigated using artificial UV radiation.

Chapter 6 demonstrates the production of biodiesel from waste oil using solid acids obtained as a result of surface modification of the composites obtained by the two step method in chapter 2. Briefly, a sulfated titania-silica composites (S-TSC) was obtained through surface modification of mesoporous titania-silica composite synthesized using less expensive precursors; titanium oxychloride and sodium silicate as titania and silica sources respectively. The catalytic activity of a series of the TSC and S-TSC samples calcined at different calcination temperatures and pure H2SO4 were evaluated for esterification of oleic acid and transesterification of waste oil with methanol to yield methyl esters. The catalytic activities of the selected solid acids were comparable to that of pure H2SO4 implying that surface modification of the titania-silica composite improves the acidic properties of the composites. Since solid acid composites are heterogeneous catalysts, they can potentially be used as replacements for corrosive and inseparable inorganic acids due to their superior properties. Furthermore, chapter 7 depicts the general conclusion of this dissertation and the expected future works.