Elsevier

Fuel

Volume 89, Issue 11, November 2010, Pages 3602-3606
Fuel

Short communication
Advances on the development of novel heterogeneous catalysts for transesterification of triglycerides in biodiesel

https://doi.org/10.1016/j.fuel.2010.05.035Get rights and content

Abstract

This paper describes experimental work done towards the search for more profitable and sustainable alternatives regarding biodiesel production, using heterogeneous catalysts instead of the conventional homogenous alkaline catalysts, such as NaOH, KOH or sodium methoxide, for the methanolysis reaction. This experimental work is a first stage on the development and optimization of new solid catalysts, able to produce biodiesel from vegetable oils. The heterogeneous catalytic process has many differences from the currently used in industry homogeneous process. The main advantage is that, it requires lower investment costs, since no need for separation steps of methanol/catalyst, biodiesel/catalyst and glycerine/catalyst. This work resulted in the selection of CaO and CaO modified with Li catalysts, which showed very good catalytic performances with high activity and stability. In fact FAME yields higher than 92% were observed in two consecutive reaction batches without expensive intermediate reactivation procedures. Therefore, those catalysts appear to be suitable for biodiesel production.

Introduction

Biodiesel, a renewable fuel, can be used in conventional diesel engines, pure or mixed with conventional diesel, without significant modifications of the engine. This fuel has higher oxygen content than petroleum diesel and its use in diesel engines have shown great reduction in the emission of particulate matter, carbon monoxide, sulphur, polyaromatics, hydrocarbons, smoke and noise. In addition, burning of vegetable-oil based fuel does not contribute to net atmospheric CO2 [1]. Biodiesel is mostly produced through a transesterification reaction between a lipid source (vegetable oils and animal fats) and an alcohol (low molecular weight such as methanol and ethanol) yielding a mixture of long chain esters and a valuable co-product, glycerol.

In a previous paper [2], the authors pointed out that an important drawback related with the use of homogeneous catalysts in biodiesel production is that, they have to be neutralized after the end of the reaction, thus producing salt streams. Moreover, if the oil contains free fatty acids, they react with the catalyst to form soaps as unwanted by-products, hence requiring more expensive separation processes [3]. Therefore, there is currently a drive towards the development of industrial processes for biodiesel production using solid catalysts. The key benefit of using heterogeneous catalysts is that no polluting by-products are formed and the catalysts do not mix with biodiesel and can be recovered and reused allowing the operation in continuous reactors instead batch reactors used with homogeneous catalysis. In addition to lower separation costs, less maintenance is needed as these catalysts are not corrosive [4].

Section snippets

Catalyst preparation

Based on previous work reported in the literature concerning the use of heterogeneous catalysts [2], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], a pre-screening comprising some solid catalysts was made in order to select catalysts that could show promising performances in terms of methyl ester (biodiesel) yield, reaction conversion, potentiality and sustainability, as well as simplicity and reduction of costs preparation.

This pre-screening comprised several (around 20) solid basic

Results

The methanolysis of triglycerides (TG) presents three steps, consecutive and reversible, reaction mechanism. Diglycerides (DG) and monoglycerides (MG) are intermediates species, while GL represents glycerol [19]:TG+CH3OHDG+RCOOCH3DG+CH3OHMG+RCOOCH3MG+CH3OHGL+RCOOCH3

A large methanol excess is generally used in order to increase the rate of the global reaction and to displace the equilibrium towards the reaction products.

The main catalytic performances for the prepared catalysts (Table 1) are

Conclusions

From the presented results it can be inferred that CaO and CaO modified with Li are promising catalysts. In fact, these catalysts showed high catalytic activity and stability with biodiesel yields higher than 93% for both reaction batches. So, they will be further studied in detail. However, studies using other alkaline and alkaline earths oxides catalysts unsupported and supported over materials such as alumina, magnesia, zirconia and titania and, also, over materials obtained by thermal

Acknowledgements

The authors would also like to thank to Eng. Pedro Felizardo for the calculations of the biodiesel properties from its NIR Spectrum. The authors would like, also, acknowledge to Professor José Cardoso Menezes of the Centre for Biological and Chemical Engineering of Instituto Superior Técnico for the use of the NIR spectrometer ABB BOMEM MB160.

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