By Y.H. Taufiq-Yap, A.N. Fitriyah and H.V. Lee

The search for alternative and sustainable fuels has gained more importance due to several current situations i.e. (i) gradual depletion of world petroleum reserves, (ii) an extremely high increase in the price of petroleum and (iii) environmental concerns about air pollution caused by the combustion of fossil fuels. Biodiesel, a renewable fuel, consists of fatty acid methyl esters (FAME) derived through the transesterification of vegetable oils, animal fat and also recycles oil from the food industry with methanol. Biodiesel is recognized as “green fuel” with several advantages, i.e. safe, non-toxic and biodegradable compared to petroleum diesel. It is oxygenated and essentially free of sulfur and aromatics making it a cleaner burning fuel with reduced emission of SOx, CO, unburnt hydrocarbons and particulate matter.

Moreover, using biodiesel as a fuel in a car can give many advantages such as it is not harmful to the environment, does not require an engine modification, cheap, makes the vehicle perform better, last longer, reduces the environmental effect of a waste product, and is energy efficient. But, when compared with the cost of diesel obtained by petroleum, the high cost is the main drawback in biodiesel production. However, the cost of biodiesel could certainly be lowered by improving the production process. The first step in this direction could be represented by the use of heterogeneous catalysts instead of homogeneous ones.

Current technology for biodiesel manufacturing employs transesterification of triglycerides with methanol using homogeneous acid or base catalysts. The homogeneous acid-catalyzed process often uses hydrochloric acid and sulfonic acid as a catalyst. The problem with this process is the costly separation of the catalyst, is corrosive to the equipment and toxic. Moreover, the reaction time is very long and a high molar ratio of methanol to oil is needed (30-150:1 mol %). Potassium hydroxide and sodium hydroxide are usually used as a homogeneous base-catalyst. These base catalysts show higher performance for obtaining biodiesel but, they also react with free fatty acids (FFA) to form unwanted soap by products in which an expensive separation is required.

Since the homogeneous catalysts can cause many problems and increased the production cost, they can be replaced by environmentally friendly heterogeneous catalysts because of environmental constraints and simplifications in the existing processes. Moreover, heterogeneous catalysts can be more easily separated and give place to a higher quality ester products and glycerol by product. In this case, pure high grade glycerol can be obtained without expensive refining operations. Most heterogeneous catalysts employed for transesterification are solid bases. Solid base catalysts have a higher activity and faster reaction rate as compared to solid acid catalysts. But solid base catalysts are very sensitive to the presence of FFA and water.

In contrast to solid bases, solid acid catalysts can be applied to feedstock with high FFA and water. Furthermore, by using solid acid catalyst there is no polluting by-products formed and the catalysts do not have to be removed since they do not mix with the product. When compared to liquid acids that possess well-defined acid properties, solid acid may contain a variety of acid sites. However, transesterification using solid acids is not yet well established in the industry, as it is more difficult to find a suitable solid acid catalyst for long-chain acids compared to shorter acids such as acetic acid.

Transesterification or alcoholysis is the displacement of alcohol from an ester by another in a process similar to hydrolysis, except that alcohol is used instead of water. The reaction is represented by the general equation below:

Transesterification is a reversible reaction and proceeds essentially by mixing the reactants. However, the presence of a catalyst accelerates the conversion. Transesterification process is affected by the mode of reaction condition, molar ratio of alcohol to oil, type and amount of catalysts, reaction time and temperature and purity of reactants.

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