Transesterification of vegetable oil to biodiesel using heterogeneous base catalyst
Introduction
Air pollution is one of the most serious environmental problems all over the world. Since diesel engines of buses and trucks exhaust a huge amount of NOx and particulates, a clean alternative fuel is highly demanded. For the recent few decades, many efforts to develop a clean fuel have been under way in many countries. Among many possible sources, biodiesel fuel derived from vegetable oil (VOs) attracts attention as a promising one to be substituted for conventional diesel fuels [1], [2]. Continuously increasing use of petroleum will intensify local air pollution and accelerate the global warming problems caused by CO2. If pure or blend biodiesel is used as fuel, the net production of CO2 can be highly suppressed. Sharmer et al. have estimated that in the case of using 1 kg of pure biodiesel instead of the fossil fuel, 3.2 kg of CO2 production could be reduced [3].
Biodiesel can be blended at any level with petroleum diesel to create a biodiesel blend. It can be used in compression-ignition (diesel) engines with little or no modifications. Biodiesel not only has proper viscosity, boiling point, and high cetane number [4], but also is simple to use, biodegradable, nontoxic, and essentially free of sulfur and aromatics [5].
One hundred years ago, Rudolf Diesel tested VOs as fuel for his engine. With the advent of cheap petroleum, appropriate crude oil fractions were refined to be used as fuel, and diesel fuels and diesel engines were evolved together. In the 1930s and 1940s, VOs were used as diesel fuels from time to time, but usually only in emergency situations. Recently, because of rise in crude oil prices, limited resources of fossil oil, and environmental concerns, there has been a renewed focus on VOs to make biodiesel fuels.
Biodiesel has been produced by transesterification of triglyceride (VOs) to methyl esters with methanol using sodium or potassium hydroxide dissolved in methanol as catalyst, as represented by the following equation.
In this conventional method, a large amount of waste water was produced to separate and clean the catalyst and the products. Therefore, for the development of an environmentally benign process and the reduction of the production cost, a new process using heterogeneous catalyst should be introduced.
In this work, Na/NaOH/γ-Al2O3 heterogeneous base catalyst was firstly adopted for the production of biodiesel. Na/NaOH/γ-Al2O3 catalyst was prepared by the successive treatment of γ-Al2O3 with sodium hydroxide and sodium at 320 °C under nitrogen following the method proposed by Suzukamo et al. [6]. In their reports, Na/NaOH/γ-Al2O3 catalyst had basic sites stronger than H_ = 37 and exhibited high activity on olefin isomerization. The formation of the basic sites was studied with XRD, XPS and TPD analysis. And then, a correlation between the basic strength and the activity towards transesterification was proposed.
Section snippets
Preparation of the catalyst
To eliminate chemical species adsorbed on the surface, γ-Al2O3 was pretreated at 550 °C for 12 h. The pretreated γ-Al2O3 was introduced into a stainless steel vessel equipped with a continuous stirring system, a nitrogen flow line, and two small solid reagent holders cooled to ambient temperature with cold circulating water flow. The vessel was heated up to 320 °C, into which predetermined amount of sodium hydroxide had kept in the reagent holder was added (0–30 wt.%). The stirring was continued
Catalyst characterizations
The measured BET surface area, pore volume, and pore diameter are shown in Table 2. The BET surface areas as well as the pore volumes decreased with loading sodium and sodium hydroxide, and this tendency was more outstanding in the case of sodium. As shown in the XRD analysis presented in Fig. 2, sodium aluminate was formed by the introduction of the sodium hydroxide. The crystal structure of sodium aluminate seems to be formed by the reaction with γ-Al2O3 according to the following equation:
Conclusions
Na/NaOH/γ-Al2O3 heterogeneous base catalyst was firstly used for the production of biodiesel from the soybean oil. Both the sodium aluminate formed by loading sodium hydroxide on γ-Al2O3, and the ionization of sodium, originated the strong basic sites of the catalysts. The activities of the heterogeneous base catalysts correlated with their basic strengths. The reaction conditions for the system were optimized to maximize the biodiesel production yield. A utilization of a co-solvent was found
Acknowledgements
The authors appreciate the support by research grants from the Korea Science and Engineering Foundation (KOSEF) through the Applied Rheology Center (ARC) at Korea University.
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