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Petroleum-based fuel plays an important role in the energy sector mainly as fuel for power plants, running vehicles and motor engines in the transportation sector. However, petroleum-based fuel is not sustainable and the prices are also not stable. Due to these factors, new alternative energy especially renewable energy has been widely studied to fulfil the world's insatiable demand for energy. Examples of the new alternative energy are solar, wind, geothermal and biomass. Nevertheless, most of this alternative energy is only capable of generating thermal and electrical energy, whereas more than 40% of the world energy demand is in liquid form. Therefore, the use of biomass as a new source of alternative liquid fuel has attracted a lot of attention lately.
There are different methods of biodiesel production and application such as direct use and blending, microemulsions, thermal cracking (pyrolysis) of vegetable oil and transesterification (Ma and Hanna, 1999). Among these, the most common method of biodiesel production is transesterification (alcoholysis) of oil (triglycerides) with methanol in the presence of a catalyst which gives biodiesel (fatty acid methyl esters) and glycerol (by-product). The selection of catalyst depends on the amount of free fatty acid (FFA) present in the oil.
Generally, the catalysts are alkali, acid, or enzyme. For triglyceride stock having lower amount of FFAs, alkali catalyzed reaction gives a better conversion in a relatively short time while for higher FFAs containing stock, acid catalyzed esterification followed by transesterification is suitable (Schuchardt 1998). The stoichiometric reaction requires 1 mole of triglyceride and 3 moles of alcohol. However, excess alcohol is used to drive the reversible reaction forward to increase the yields of the alkyl esters and to assist phase separation from the glycerol formed. Several aspects, including the type of catalyst (alkaline or acid), alcohol/vegetable oil molar ratio, temperature, purity of the reactants (mainly water content), and free fatty acid content, have an influence on the transesterification rates (Schuchardt 1998). Figure 1.1 shows the reaction of vegetable oil (triglyceride) with alcohol in the presence of a catalyst producing biodiesel (mixture of alkyl esters) and glycerol (Schuchardt 1998). Alkali-catalyzed transesterification is much faster than acid-catalyzed transesterification and is the most commonly used method commercially (Ma and Hanna, 1999). Putting that together with the fact that the alkaline catalysts are less corrosive than acidic compounds, industrial processes usually favour base catalysts such as alkaline metal alkoxides and hydroxides as well as sodium or potassium carbonates.
Figure 1.1 Transesterification of triglyceride to mixture of alkyl esters
1.2 Heterogeneous Transesterification
The benefits of heterogeneous transesterification process include easier and simpler separation process (as the catalyst is in different phase from the product/reactants) and elimination of soap formation. Heterogeneous transesterification is a relatively cheaper alternative to the current homogenous transesterification for biodiesel production. Heterogeneous transesterification process is also easier to perform due to moderate temperature and pressure required in the process compared to that of non-catalytic supercritical transesterification technology. There are three types of heterogeneous transesterification process; basic (alkali), acidic and enzymatic. Up until today, there are many catalysts developed and used in the heterogeneous transesterification process. Examples of the heterogeneous catalysts are metal oxides (Liu, 1994); (Noureddini and Zhu, 1997), zeolites (Xie et al., 2007); (Suppes et al., 2004) and active metals loaded on supports (Jitputti et al., 2006); (Chung and Park, 2009). Each type of heterogeneous transesterification process has its own challenges.
Enzymatic transesterification process for instance, requires activity of a living organism or cells (enzyme) to convert vegetable oil to biodiesel. This makes the enzymatic process very sensitive to disturbances. The enzyme could be easily inhibited when the conditions are unsuitable. The enzymatic reactions are also very specific due to its specific active sites. Moreover, enzymes are costlier compared to basic and acidic solid catalysts. Due to these reasons, heterogeneous transesterification processes using basic and acidic solid catalysts seem to be more attractive. However, the heterogeneous transesterification itself has other challenges. The solid catalysts for the heterogeneous transesterification process must possess good catalytic properties to produce the desirable product. This actually has become the main hurdle in the development of heterogeneous catalysts that is to synthesize a very reactive catalyst that can catalyze the heterogeneous transesterification process.
1.3 Problem Statement
Solid acid have found widespread application in the catalysis field and belong to the group of new catalytic material which are greatly develop in recent years (Leung et al., 2010), (Lee and Saka., 2010)., (Ma and Hanna., 1999) However research dealing with the acidity of solid acid catalysts for biodiesel synthesis has been limited due to low expectations about reaction rates and probable undesired side reactions. On the other hand, the acidity strength of the solid acid is not uniform and strong acidic sites are not durable makes some difficulties in practical application of the solid acid. The ability of solid acid to act as a catalyst is consequences of the presence of different types of acid sites which are provide a surface for the chemical reaction to take place on.
In general, solid acid may exhibit Bronsted (prone to donate a proton) as well as Lewis (prone to accept an electron pair) acid sites. In order for the reaction to occur, one or more of the reactants must diffuse to the catalyst surface and adsorb into it. After reaction, the products must desorb from the surface and diffuse away from the solid surface. A good catalyst need to adsorb the reactant molecules strongly enough for them to react, but not so strongly that the product molecules stick more or less permanently to the surface. According to Triwahyono et al. (2006), number and strength of acid sites play an important role in the chemical reaction and showed that acidity properties of catalyst greatly affected by the amount of sulfated ion loading. For a limited range of sulfated loading investigated in their study of sulfated zirconia, it appears that that conversion per specific surface area of catalysts increase with the sulfated ion loaded. This means that if the acidity of catalyst is low, the conversion rates of reaction will be low due to the low concentration of acid sites. And when the acidity is too high, deactivation by coking maybe occur or undesirable by-product might be formed requiring additional and expensive separation procedures.
Many research especially of a fundamental nature using model compound is required in order to better delineate esterification and transesterification reactions on solid acid catalysts (Lotero et al., 2005). The chemistry and structural simplicity of triacetin simplifies the identification and quantification of reaction products, while keeping the same chemical functionality shared by every triglycerides (TG) (Lotero et al., 2005). Transesterification of triacetin with methanol proceed via three consecutive and reversible reactions where the triacetin ligands combine with the methanol to produce an acetic acid methyl ester and a glycerol as its by-product. The stoichiometric reaction requires 1 mole of triacetin and 3 moles of methanol to produce 3 moles of fatty acid methyl ester and 1 mole glycerol. Methanol and to lesser extent alcohol are the most frequently used alcohol in biodiesel synthesis because of their availability and low cost (López et al., 2005).
The present study will investigate a fundamental conversion of triglyceride which is related to the acidity of the studied catalyst in an effort to find suitable solid catalyst that would be ideal for biodiesel synthesis and increasing the understanding of the nature of the active site for transesterification. Based on the previous research, less than a handful of proper studies have explored the catalyst acidity and without deceive conclusion about this matter.
1.4 Research Objectives
The objective of this research is to give fundamental insight into the transesterification of triglyceride on solid acid catalyst. In an effort to identify the optimum acidity and catalyst characterization that would be ideal for biodiesel synthesis, this study compare the catalytic activity of a number of solid acid catalyst in the transesterification of triacetin with methanol at 60 oC. The solid acid catalyst studies are zirconia, sulfated zirconia (SZ), sulfated zirconia-alumina (SZA), tungstated zirconia (WZ), tungstated zirconia-alumina (WZA), sulfated tin oxide (STO), and zeolite ZSM-5. The conversion result using these solid catalysts will be compared with their acidity. Therefore the objectives of this work are:
To evaluate the solid acid catalysts for transesterification.
To characterize the catalysts in terms of structure and acidity.
and its effect towards the conversion of triglyceride (TGs).
To present the optimized condition operation for triacetin conversion to biodiesel.
1.5 Scopes of Study
The development of the heterogeneous catalyst may be regarded as an interactive optimization process, basically consisting three steps; namely preparation, characterization and testing of the catalyst. Figure 1.2 shows the schematic representation of the research questions and Figure 1.3 shows schematic representation of the statement of the research problem. Therefore, in order to achieve the above objective, the research scope is dividing into several stages:
Preparation and evaluation in term of acidity and conversion of the solid acid such as zirconia, sulfated zirconia (SZ), sulfated zirconia-alumina (SZA), tungstated zirconia (WZ), tungstated zirconia-alumina (WZA), sulfated tin oxide (STO), and zeolite ZSM-5 for transesterification of triacetin
Acidity concentration and structural identified using X-Ray diffraction (XRD), Scanning Electron Microscope (SEM) and pyridine adsorption with IR spectroscopy (IR-pyridine).
Solid acid catalyst modification with various amount of sulphate ion loading.
Optimization studies on the effect of the reaction condition in term of molar ratio methanol to triacetin, temperature and reaction time using Response Surface Methodology (RSM).
Figure 1.2 Schematic representation of the research questions
Figure 1.3 Schematic representation of the statement of the research problem
1.6 Significance of Study
This study is important to the environmental and economic concerns such that a continuous process that uses a heterogeneous catalyst is much desirable. In particular, solid acid catalyst are ideal because they are able to catalyze both transesterification and esterification reaction simultaneously, which becomes important when using lower-quality feedstocks such as used deep-frying oils.
This research will lead towards advancement of knowledge and aid in the design of more active solid acid catalysts, certainly impacting in the positive way of biodiesel synthesis technologies.