Elsevier

Fuel

Volume 87, Issues 10–11, August 2008, Pages 2286-2295
Fuel

Transesterification of Jatropha curcas oil glycerides: Theoretical and experimental studies of biodiesel reaction

https://doi.org/10.1016/j.fuel.2007.12.006Get rights and content

Abstract

Vegetal oil, also known as triglycerides, is a mixture of fatty acid triesters of glycerol. In the triglycerides alkyl chains of Jatropha curcas oil, predominate the palmitic, oleic and linoleic fatty acids. The process usually used to convert these triglycerides to biodiesel is called transesterification. The overall process is a sequence of three equivalent, consecutive and reversible reactions, in which di- and monoglycerides are formed as intermediates. Semi-empirical AM1 molecular orbital calculations were used to investigate the reaction pathways of base-catalyzed transesterification of glycerides of palmitic, oleic and linoleic acid. The most probable pathway and the rate determining-step of the reactions were estimated from the molecular orbital calculations. Our results suggest the formation of only one tetrahedral intermediate, which in a subsequent step rearranges to form the products. The rate determining-step is the break of this tetrahedral intermediate.

Introduction

Fatty acid methyl or ethyl esters derived from renewable sources, such as vegetable oils, have gained importance as an alternative fuel for diesel engines. Edible oils such as soybean oil in USA, rapeseed oil in Europe and palm oil in countries with tropical climate, such as Malaysia, are being used for the production of biodiesel to their compression ignition engines. In other countries, the use of edible oils for engine fuel is not usual; however, there are several non-edible oil seed species which could be utilized as a source for oil production. Among these, Jatropha curcas is a multipurpose species with many attributes and considerable potential. The oil from the seeds is potentially the most valuable end product, with properties like: low acidity, good oxidation stability as compared to soybean oil, low viscosity as compared to castor oil and better cold properties as compared to palm oil. In addition, viscosity, free fatty acids and density of the oil and the biodiesel are stable within the period of storage [1].

Jatropha curcas is a drought-resistant tree belonging to the Euphorbiaceae family, which is cultivated in Central and South America, South-east Asia, India and Africa. This highly drought-resistant species is adapted to arid and semi-arid conditions. It grows almost anywhere, even on gravelly, sandy and saline soils and is often used for erosion control [2].

The fatty acid profile of Jatropha curcas oil, determined by chromatography analysis, is listed in Table 1[2]. Typically 1% of the vegetable oils are unsaponifiable compounds (carotenoids, phospholipids, tocopherols or tocotrienols and oxidation products).

The commonly used method for production of biodiesel is the transesterification of vegetable oils. Transesterification, also called alcoholysis, is the reaction of triglycerides with alcohols to produce for examples methyl or ethyl esters and glycerol as a by-product. A catalyst is usually used to improve the reaction rate and yield. The reaction requires excess of alcohols to increase the efficiency of the transesterification process [3].

The transesterification reaction is represented by the general equation show in Scheme 1. It consists in three equivalent, consecutive and reversible reactions. The triglyceride is converted stepwise to diglyceride, monoglyceride and finally glycerol. At each reaction step, one molecule of methyl or ethyl ester is produced for each molecule of methanol or ethanol consumed. The three stages of the transesterification reaction are indicated in Scheme 2.

The base-catalyzed transesterification has a long story of development. Biodiesel fuel produced by this method is in the market in some countries such as North America, Brazil and mainly in some European countries like Germany and France. However, although base-catalyzed transesterification represents the best alternative to produce biodiesel, it still has some disadvantages. Low free fatty acid content and anhydrous reagents are required due to the saponification possibility. This soap formation lowers the ester yields and can hinder the stages of separation and purification of ester and glycerol as well as the washing stage. Additional disadvantages are that the catalyst recovery process is slow and expensive, increasing the operating costs. Therefore, the process still needs to be optimized.

In order to understand and control the transesterification reaction it is necessary to know the reaction mechanism. The several steps of the mechanism of the transesterification reaction are poorly understood. In particular, doubts still exist about the pathway of the process involving formation and breaking of the tetrahedral intermediate. Two types of mechanisms describing the base-catalyzed transesterification reactions have been proposed by various research groups:

  • The first mechanism proposes the formation of two tetrahedral intermediates [3], [4].

  • Other mechanism suggests the formation of only one tetrahedral intermediate which, in a subsequent step, rearranges to form the products [5], [6].

However, the lack of detailed information on the actual molecular species, the rate determining-step in the overall reaction mechanism, and the configuration of the transition state complex have severely hampered the quantitative understanding of the reaction kinetics.

Computational chemistry methodologies have been used as a powerful tool to study the mechanisms and kinetics of several chemical reactions. Therefore, by means of theoretical calculations it may be possible to better understand the mechanism and the kinetics of the transesterification reactions, with the objective of relating the computer simulations with the results obtained experimentally [7], [8], [9], [10].

Such a detailed knowledge would provide valuable guidelines for a deeper understanding of the role played by the catalyst in the formation and breaking of the tetrahedral intermediates in the base-catalyzed transesterification.

The purpose of this paper is to study the reaction mechanism of base-catalyzed transesterification of the glycerides of the Jatropha curcas oil, to provide relevant results of kinetic properties, rate determining-step and catalytic effects, to define the catalyst and other experimental conditions that allow maximizing the reaction yield. The results obtained in the computational simulations are compared with experimental Jatropha curcas oil transesterification data.

Section snippets

Materials and methods

The procedures and methods used in the quantum studies and in the transesterification experiments are given in this section. The methods used in each stage, as well as the equipment, reagents and methods of characterization are also described.

Mechanism with formation of two tetrahedral intermediates [3,4]

This reaction mechanism for the base-catalyzed transesterification was formulated as three steps. In a pre-step the basic catalyst (NaOH or KOH) reacts with the alcohol, producing an alkoxide anion. The first step is a nucleophilic attack of the alkoxide anion on the carbonyl group of the glyceride to form a tetrahedral intermediate (intermediate I). In the second step, the tetrahedral intermediate reacts with a second alcohol molecule (methanol) to regenerate the anion of the alcohol

Conclusions

  • 1.

    Based on the semi-empirical calculations the detailed molecular mechanism for the base-catalyzed transesterification of glycerides was discussed. The catalytic effect of the OH- ion in promoting the reaction has been specifically addressed here. Finally, the actual molecular trajectories involved in the reaction (e.g. the formation of the transition state and reaction intermediate and the tetrahedral intermediate breakdown to form the product) were obtained.

  • 2.

    The theoretical results, which are

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