Abstract
The continuous increase of the world’s population results in an increased demand for energy drastically from the industrial and domestic sectors as well. Moreover, the current public awareness regarding issues such as pollution and overuse of petroleum fuel has resulted in the development of research approaches concerning alternative renewable energy sources. Amongst the various options for renewable energies used in transportation systems, biodiesel is considered the most suitable replacement for fossil-based diesel. In what concerns the industrial application for biodiesel production, homogeneous catalysts such as sodium hydroxide, potassium hydroxide, sulfuric acid, and hydrochloric acid are usually selected, but their removal after reaction could prove to be rather complex and sometimes polluting, resulting in increases on the production costs. Therefore, there is an open field for research on new catalysts regarding biodiesel production, which can comprise heterogeneous catalysts. Apart from that, there are other alternatives to these chemical catalysts. Enzymatic catalysts have also been used in biodiesel production by employing lipases as biocatalysts. For economic reasons, and reusability and recycling, the lipases urged to be immobilized on suitable supports, thus the concept of heterogeneous biocatalysis comes in existence. Just like other heterogeneous catalytic materials, this one also presents similar issues with inefficiency and mass-transfer limitations. A solution to overcome the said limitations can be to consider the use of nanostructures to support enzyme immobilization, thus obtaining new heterogeneous biocatalysts. This review mainly focuses on the application of enzymatic catalysts as well as nano(bio)catalysts in transesterification reaction and their multiple methods of synthesis.
Highlights
Biocatalysts based on enzymes have been investigated for decades due to their catalytic power, their high degree of specificity, and stereoselectivity [1,2,3]
[44] performed factor analysis concerning the enzymatic transesterification of immobilized ion-exchange hydrophilic resin having an irregular shape, dimensions waste oil aiming at biodiesel production
They observed a maximum point for lipase immobilization (250 mg/g) on this support at pH 6 and the Katiyar and Ali [47] studied the preparation of molecular sieve MCM-41 as effective support concerning the immobilization of Candida rugosa lipase using the physical adsorption technique
Summary
Biocatalysts based on enzymes have been investigated for decades due to their catalytic power, their high degree of specificity, and stereoselectivity [1,2,3]. In comparison with the transesterification of triglycerides using chemical catalysts, biodiesel production by lipase-catalysis offers some benefits, such as ease of product separation (and ease of glycerol separation), wastewater treatment requirement is reduced to a minimum and the complete lack of side reactions [26,27,28]. Another major benefit lays in the possibility to process raw materials such as waste cooking oils (WCO), having high free fatty acid (FFA) content. Lipases used in biodiesel production have to be non-stereospecific, so that all compounds of different molecular lengths, such as tri-, di-, monoglycerides, and FFA of different lengths, can be efficiently converted into FAAE [28,31,32,33]
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