الفهرس 
Chapter 1 : Introduction and object of investigation.Only 14 pages are availabe for public view
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Abstract
DME is produced by dehydration of methanol using acidic porous materials, e.g. zeolites, alumina and silica–alumina, as the catalysts [8].
Among the several solid-acid porous catalysts studied for methanol dehydration reaction, γ-Al2O3 is mainly used as a catalyst for this reaction.
This is due to its large surface area, large pore volume, uniform pore size distribution and excellent mechanical properties (attrition, resistance and crush strength) [9]. In addition, γ-Al2O3 has an excellent thermal stability and high catalytic activity for DME formation due to its surface acidity [10].1.1.4.1 Types of Alumina Aluminium oxide, alumina (Al2O3), is one of the most interesting ceramic materials for its numerous applications and its varied physical properties [11]. Because of its thermal, chemical and mechanical stability [11, 12], alumina has numerous applications. For example, it is used as a catalyst support, adsorbent, ion exchangers, in electronic-device fabrication, as a cutting-tool material, as a protective barrier against corrosion on aluminaforming alloys, or as an alternative for surgical material for implants.
Alumina has the particularity to exist in a variety of metastable structures, the so-called transition aluminas, e.g. χ, k, γ, δ, η, Ө as well as its stable α-Al2O3 phase. This polymorphism can be classified in terms of the oxygen sublattice structure and the distribution into this sublattice of aluminium ions in tetrahedral and octahedral interstitial sites [11]. Thus, in α-Al2O3, the oxygen sublattice is hexagonal-close-packed (hcp) structured with 2/3 of octahedral sites occupied with cations, while γ, δ, η, Ө have a face-centered cubic (fcc) arrangement of oxygen atoms and cations present in various proportions in both octahedral and tetrahedral sites. It has been established that boehmite transforms into stable α-Al2O3 via the following sequence [11].
AlO(OH) γ δ Ө α -Al2O3 6 Aluminium Oxide Al2O3 can be produced in a variety of solid form. For example dehydration of boehmite at 300-500°C yields γ-Al2O3, at 700-800°C it produces δ-Al2O3, at 900-1000°C it gives Ө-Al2O3, and at 1000-1200 °C, α-Al2O3 is produced [13].Zabeti et al. [14] studied the activity of solid catalysts for biodiesel production. They reported some important properties of alumina such as its use as a support, aluminium oxide can exist in the form of porous γ-Al2O3 and η-Al2O3. Nonporous crystalline α-Al2O3 has been widely used as a support in catalysis processes, such as polymerization, reforming, steam reforming, dehydration and hydrogenation. It is also used in the synthesis of biodiesel. Finally they concluded that effective factors on catalytic activity of solid catalysts are specific surface area, pore size, pore volume and active site concentration on the surface of catalyst. Moreover, the type of precursor of active materials has significant effect on the catalyst activity of supported catalysts. However, active site concentration was found to be the most important factor for solid catalyst performance. The use of catalyst supports such as alumina, silica and zinc oxide could improve the mass transfer limitation of the three phase reaction. Furthermore, by anchoring metal oxides inside pores, catalyst supports could prevent active phases from sintering in the reaction medium.
Al2O3 is used as a common catalyst in the production of DME [15, 16] because of its fine particle size, high surface area, surface catalytic activity, excellent thermal stability, high mechanical resistance and wide range of chemical, physical and catalytic properties. Among the various crystalline phases of alumina, γ-Al2O3 has received much attention due to less generation of by-product [15]. Therefore, it is used as a support in heterogeneous catalysis, it allows the dispersion of active phases due to its high specific surface area. The unique properties of nanoparticles make them of interest for applications in catalysis [17]. Commercially, γ-Al2O3 is used as