Biogenic Silica Nanoparticles Derived from Rice Husk Biomass and their Applications
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According to the Food and Agriculture Organization of the United Nations (FAO), the global paddy rice production in 2013 is estimated to be 746 million tons. Based on this, the amount of rice husks (RHs) are estimated to be ca. 160 million tons. Applications of RHs have been very limited. Therefore, RHs are often considered as a biowaste. RHs could be a suitable candidate of feedstock for silica based materials because of their high silica content (15−28 wt %) and large availability. In recent years environmental demand and sustainable development have become increasingly important. It is important to study and utilize RH biowaste, and convert RHs into valued materials. This is the focus of this research. The work is reported and summarized in seven chapters in this dissertation.
Chapter 1 is an overview of the preparation of silicon based materials from rice husk biomass. Researches have been conducted on using RHs as a raw material to synthesize a number of silicon compounds, including silica, silicon carbide, silicon nitride, silicon tetrachloride, zeolite, and elementary silicon. The applications of such materials are very comprehensive. Synthesis of these silicon based materials from RHs and their applications are reviewed in this chapter.
In Chapter 2, efforts on comprehensive utilization of RH have been made to obtain both lignocellulose and high quality porous silica nanoparticles. RHs are mainly composed of lignocellulose (ca. 72-85 wt %) and silica (ca. 15-28 wt %). The majority of previous explorations focused on the preparation of silica or other silicon based materials from RHs. The lignocellulose in RHs was usually burnt and wasted. In this study, most of the lignocellulose in RHs was extracted by ionic liquids which are environmentally benign solvents. The dissolved lignocellulose was separated and collected. The RH residue after extraction contains a high concentration of silica and was thermally treated to synthesize amorphous porous silica nanoparticles with a high purity and surface area. During the extraction of lignocellulose using ionic liquids, some metal cations (e.g., K+) that can compromise the synthesis of silica are removed, which is synergistic for this comprehensive approach to make full use of RH biomass.
Chapter 3 studied the effects of different sources of silica on synthesizing lithium aluminum silicate (LAS) powders via a sol-gel method. Silica from the ash of untreated rice husk (RHA), silica from HCl treated rice husk (Silica-RH-HCl), and fumed silica were used to prepare LAS powder with various calcination temperatures, i.e., 400, 600, 800 and 1000 °C. X-ray diffraction (XRD) characterization showed that, in terms of apparent reactivity in the LAS synthesis, fumed silica has the highest reactivity. The silica from HCl treated rice husk has a similar but slightly lower reactivity when compared to fumed silica. RHA has the lowest reactivity among the three silica sources. This apparent reactivity is mainly determined by the intrinsic structure, including surface area and crystallinity of silica, which can be characterized by XRD and BET measurement. Overall, a higher surface area and low crystallinity are favored for the proceeding of reactions. More details about the intrinsic structure of silica are discussed in this chapter.
In Chapter 4, platinum nanoparticles (Pt-NPs) supported on silica derived from HCl treated RHs were prepared. Pt-NPs based catalysts have received attention in the past decades because of their unique catalytic properties in many important industrial processes. Experimentally and theoretically the size of Pt-NPs plays a crucial role in governing the catalytic activity. Smaller Pt-NPs exhibit higher catalytic activity. Silica has been extensively used as a support to synthesize metal nanoparticles for heterogeneous catalysis applications. However, the silica used was mostly prepared from silanes, such as tetraethoxysilane (TEOS), via a sol-gel process, which is expensive and non-environmentally friendly. Moreover, the silica particles from the sol-gel process typically possess a smooth surface, which is not ideal for the supporting of metal nanoparticles. In Chapter 4, we report a facile method to synthesize Pt-NPs based heterogeneous catalysts using the silica from RHs as the support. The biogenic silica from RHs offers a rough surface, which appeared to be much more ideal for supporting Pt-NPs than the TEOS derived silica particles.
In Chapter 5, fluorescent silica was prepared from acid treated RHs with various conditions. The fluorescence intensity of some samples was strong enough to be viewed by naked eyes in daylight at room temperature when irradiated by 365 nm UV light. The fluorescence intensity of the silica samples was roughly proportional to its carbon content. The higher carbon content resulted in stronger the fluorescence intensity. The fluorescence mechanism of the silica from RHs was discussed based on the experimental data in this chapter and the results from the literature.
In Chapter 6, four colored ceramic pigments green, blue, yellow, and red/coral, were prepared using three different silica precursors which are fumed silica, the silica from HCl treated RHs, and commercial crystalline silica. Studies showed that the silica from HCl treated RHs has a similar apparent reactivity to fumed silica, and can be a suitable substituent material for the pigment preparations. Among the three silica precursors, commercial crystalline silica exhibited the lowest apparent reactivity in terms of ZrSiO4 conversion rate. The apparent reactivity of the three silicas has a similar pattern to that of the silicas studied in Chapter 3. Overall, higher degree of silica crystallinity results in a lower reactivity for the pigment synthesis.
In Chapter 7, a brief summary and the outlook are discussed. Categorization of biomass is defined and discussed. Biomass was divided into three categories, which are food competitor, non-food competitor, and biowaste. RHs are a “true biowaste” based on our definition. Compared to the other two categories of biomass, true biowaste would be the most economical resource to take advantage of since the investment on the raw material would be virtually “zero”. As such, RH biomass is still worthy of further investigation and exploration, because it has great potential in terms of both sustainability and business profit.