Towards sustainable biofuel production: Design of a new biocatalyst to biodiesel synthesis from waste oil and commercial ethanol
Graphical abstract
Introduction
The consciousness of sustainable technologies is gradually gaining ground. The need to replace the non-renewable fossil fuel by an alternative energy has been the objective of extensive research in recent years (Araújo, 2014). Biodiesel has been identified as one of the most notable options to replace or at least to complement conventional fuels consume (Aransiola et al., 2014). As biodiesel is derivate from vegetable oil or animal fats, it is renewable and biodegradable. Its use reduces the energy dependence on petroleum and most exhaust emissions (with the exception of nitrogen oxides, NOx) resulting in an environmental benignity. Biodiesel presents a higher flash point, leading to safer handling and storage; therefore, it also can be stored and transported using diesel tanks and equipment. Since biodiesel molecule is oxygenated, it is a better lubricant than diesel fuel, increasing the life of engines, and is combusted more completely (Knothe et al., 2005, Vasudevan and Briggs, 2008).
Biodiesel can be produced by different kinds of raw materials (refined, crude, non edible or frying oils) and with different types of catalysts, such as basic compounds (sodium or potassium hydroxides), acids (sulfuric acid, ion exchange resins), heterogeneous solids (zeolites, CaO), enzymes (lipases) and supercritical fluids.
Currently, the homogeneous catalysis with sodium hydroxide and methanol is the most used for the biodiesel production (Vasudevan and Briggs, 2008). However, the alcohol must be substantially anhydrous and vegetable oil must have a low free fatty acid (FFA) content, because the presence of water lowers the activity of catalyst and FFA reacts with the catalyst to produce soaps. The formation of soaps reduces the biodiesel yield, and difficulty the product separation and purification (Freedman et al., 1984). In fact, the purification process represents between 60 and 80% of the biodiesel production cost (Atadashi et al., 2011, Wassell and Dittmer, 2006). Ineffective biodiesel separation and purification cause severe diesel engine problems such as plugging of filters, coking on injectors, more carbon deposits, excessive engine wear, oil ring sticking, engine knocking, and thickening and gelling of lubricating oil (Demirbas, 2009).
Then, the high consumption of energy and separation costs in the homogeneous process requires the development of heterogeneous catalysts for transesterification reaction, which can be easily separated from the reaction mixture and recycled. Several authors have used heterogeneous catalysts in order to eliminate the neutralization and washing steps needed in the homogeneous processes but they were faced with major problems such as higher temperature of transesterification reaction or catalyst deactivation (Atadashi et al., 2011, Chew and Bhatia, 2008, Dias et al., 2013).
On the basis of the above and in order to achieve a truly environmentally-friendly production of biodiesel, the enzyme immobilization appears as a promising technology to obtain a solid as biocatalyst. The use of enzyme-catalyzed transesterification reactions avoids drawbacks such as feedstock pretreatment, catalyst removal, continuous use, prevention of product contamination, reduction of effluent problems, material handling and high-energy requirement with respect to conventional homogeneous catalysts (Stoytcheva et al., 2011). The enzymes that present this activity are called lipases (EC 3.1.1.3 triacylglycerol acylhydrolase) and represent a group of water soluble enzymes that originally catalyze the hydrolysis of ester bonds in water insoluble lipid substrates. A disadvantage of use the free enzyme in non-aqueous media, is that these free enzymes tend to form aggregates that difficult the optimum subtract flux to inner of the same and therefore only the enzyme molecules present on the aggregate surface can work (Lawson et al., 2004). These lipases can be immobilized on several materials to improve enzyme stability and reusability obtaining biocatalysts with high selectivity, efficiency and yield into methyl/ethyl esters in milder reaction conditions (Lima et al., 2015, Liu et al., 2010, Salis et al., 2008). Moreover, the glycerol recovered via this enzymatic process has a higher grade of purity compared to that one obtained from the conventional alkaline process.
On the other hand, to achieve the maximum of active sites working and the full catalytic power, it is convenient to disperse the enzyme on high specific areas like those of mesoporous materials. These usually affect the enzymatic performance given its specific properties such as the pore size and the surface nature. To exploit all surface of the material and to avoid the enzyme leaching, the pore size of material has to be similar to the enzyme size. This allows preserving enzyme activity and specificity (Salis et al., 2010, Salis et al., 2009, Tran and Balkus, 2011). SBA-15 is one of the most popular supports to enzyme immobilization because they are highly ordered materials, with pore sizes in the mesopore range, high areas, and large pore volumes (Alam et al., 2010, Hudson et al., 2005). Moreover, the possibility of surface modification with different species confers new properties to the support that can affect the enzyme performance (Kim et al., 2006, Salis et al., 2009, Tran and Balkus, 2011), opening the gates to a very interesting area to explore.
The present work is aimed at evaluating the role of the support surface on the loading and the activity of Pseudomonas fluorescens lipase. Enzyme immobilization on pure SBA-15 material and modified with calcium (Ca/SBA-15) was evaluated in the ethanolysis of sunflower, soybean and waste frying oil as a high industrially interested reaction to obtain biodiesel. Interestingly, the biocatalyst worked efficiently with commercial ethanol and the mentioned oil sources, which represent important economic and ecologic advantages over homogeneous catalysts.
Section snippets
Chemicals
The lipase from Pseudomonas fluorescens (PFL, ≥20,000 IU/g at 55 °C, pH 8.0) was acquired from Sigma-Aldrich Co. (St. Louis, USA). This is an enzyme with a high lipolytic activity produced by “Amano Labs”. This enzyme was characterized by the Amano researchers and has a molecular weight of about 33 kD, an isoelectric point of pI = 4, a pH stability range 4 < pH < 10, and an optimum pH of activity in the range 8 < pH < 10 (Amano, 2008).
Sunflower oil used (‘‘Vicentin’’ brand) was acquired in a
Support characterization
The structural and textural characterization of the mesoporous supports was obtained by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and power XRD patterns in the high angle region. The scattering vector q is employed in SAXS analysis and it is related to the diffraction angle (θ) by using the Bragg equation, λ = 2d θ, as followed: q = 4π sin θ/λ, where λ is 1.5418 Å. The corresponding spectra of pure SBA-15 and modified with a
Conclusion
Pseudomonas fluorescens lipase was successfully immobilized on mesoporous SBA-15 by physisorption in order to obtain active hybrid catalysts for the transesterification reaction of vegetables oils. Firstly, an optimum enzyme loading of 400 mgprotein/gsupport was determined. Meanwhile, the suitable water concentration to optimize the enzymatic activity was 4 wt% with respect to oil mass. In this work, we could demonstrate how the nature of support can improve the enzymatic activity and how the
Acknowledgements
G.O.F, H.J.R, C.E.A and G.A.E area members of CONICET. The authors are grateful to CONICET and UTN for the financial support. The authors acknowledge beam time to the Brazilian Synchrotron Light Laboratory (LNLS) at Campinas, Brazil, experimental stations SAXS-1 and SAXS-2 and Dr. R. Oliveira for helpful discussions. The authors are gratefully to Ph.D Vaschetto E.G. for the TEM and SEM images.
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