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Processing of mesoporous silica materials (MCM-41) from coal fly ash
Halina Misran, Ramesh Singh, Shahida Begum, Mohd Ambar Yarmo
1. Introduction Mesoporous silica with high surface area and pore volumes has gained considerable attention since the discovery of mesoporous molecular sieves (M41S) by scientist at Mobil Oil [1–4]. The main attracting feature of this material is its unique chemical structure that consists of functional silanol (Si–OH) group couple with pore sizes and shapes that can be tailor-made through careful processing to suit a host of function where molecular recognition is needed [5,6]. Indeed, mesoporous silica has found wide range of applications such as adsorbents, reaction catalysts, catalysts supports and chemical sensors [1–4,7–10]. One of the most investigated members of the M41S family is the MCM-41 which is an amorphous silica comprising of an array of unidirectional, 2-D hexagonal mesopore structure with an extremely high surface area of >1000m2/g−1 [1,11]. Mesoporous MCM-41 material can be synthesized using a variety of silica precursors such as n-alkoxysilanes, n-alkylamines, sodium water glass and aerosil. However, a major drawback of these precursors is the high starting costs of the raw material that result in high production cost. An alternative silica source, the coal fly ash (CFA) that is formed as a by-product in coal-fired power plant during combustion of pulverized coal and air. Coal combustion method contributes approximately37%of the total electricity production in theworld [12]. This in turn generates a huge amount of fly ash, up to 5.5 million tonnes/year [13]. However, the utilization rate of the fly ash is rather low, approximately 15% of the generated amount [14]. CFA is utilized in industry as a substitute for fine aggregates in cement and concrete, in bricks and ceramic tiles,
as filler in plastics and paints. In general, CFA has pozzolanic properties and phase minerals comprise of major components of silica (60–70 wt%) and alumina (16–20 wt%) in the form of quartz and mullite, with traces of transition metal oxides. Owing to the high content of useful silica and alumina in CFA, it makes perfect economical sense to recover these minerals for useful industrial applications. Several studies based on the hydrothermal method were employed to convert CFA to mesoporous silica such as faujasite, zeolite P, zeolite X, hydroxysodalite and gismodinee [15–18]. Although this effort was successful, the abundance generation of liquid waste during hydrothermal treatment and the high ratio of solution to CFA as well as the difficulties to obtain single-phase zeolite, make this method less favourable. Other chemical techniques using different silica precursors were subsequently investigated by many researchers to produce mesoporous silica, particularly MCM-41. For instance, Gruen et al. [19] reported a novel route in the synthesis of MCM41 by hydrolysis of tetraethoxysilane (TEOS) with ammonia as a catalyst. Chen et al. [20] prepared MCM-41 based on a simplified hydrothermal synthesis method in an alkali-free medium using cetyltrimethylammonium hydroxide(CTMAOH) as a template. They found that MCM-41 material disintegrated readily in a strong basic aqueous medium (pH > 11) but were relatively stable in an acidic environment. Shigemoto et al. [21] reported that by alkali fusion, silica in the form of quartz and mullite can be converted into more soluble form of sodium silicate and other aluminosilicate phases to increase the yield of MCM-41 and zeolites. By using the surfactant method, Simonutti et al. [22] prepared high quality MCM-41 materials. The surfactant used was cetyltrimethylammonium chloride (C16TMACl) whereas TEOS was used as the silica precursor. The synthesis was carried out with a surfactant weight concentration of 15% corresponding to the molar ratio of (1.46:1:5.4:125.3 =C16TMACl:TEOS:HCl:H2O). The surfactant was mixed with deionized water at 50 ◦C. HCl (32% concentration, 10 M) was added dropwise under vigorous stirring
condition for 1 week. The resultant material was filtered, washed repeatedly with deionized water and allowed to dry at room temperature or at 130 ◦Cfor 8 h under a nitrogen flux. After removal of the templating molecules by calcination, accessible mesopores were produced. The pore size, however, depends basically on the surfactant employed in the synthesis (i.e. the pore size increases with the chain length of the surfactant), the initial pH of the synthesis mixture, mixture composition and other process parameters such as temperature, post-synthesis treatment, etc. [23–25]. Although all these processes are feasible, they are rather intensive, requiring highly pure starting materials. Furthermore, the high toxicities of the preferred silica precursors and the structure directing agents used during processing require careful attention and handling so as not to pollute the environment. Thus, more effort is required to develop economical processes that utilize not only cheap starting materials but also one that is simple, yet effective in producing mesoporous MCM-41 material. This paper reports a methodology to extract silica from CFA and discusses the usage of the recycle silica sources in green synthesis process of MCM-41 mesoporous silica material.
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