Nanocomposites Comprising Cellulose and Nanomagnetite as Heterogeneous Catalysts for the Synthesis of Biodiesel from Oleic Acid

A nanocomposite comprising cellulose and nanomagnetite based on rice husk cellulose was used as the catalyst for the formation of methyl esters from oleic acid as an alternative method for biodiesel production. The resulting nanocomposite properties supported by FTIR, XRD, SEM and TEM characterization revealed that nanomagnetite Fe₃O₄ had impregnated the acetylated nanocellulose. The nanomagnetite Fe₃O₄ obtained had an average size of 30 nm. The best conversion of oleic acid to methyl esters for the catalytic application of the nanocomposite was 89.21%, which was achieved at a reaction temperature of 60°C, reaction time of 5 hours, catalyst concentration of 1.5 wt.%, and ratio of oleic acid to methanol of 3:1. Kinetic analysis at different temperatures (40, 50, 60 and 70°C) was performed, and a low activation energy of 16.56 kJ/mole was obtained. These results indicate that the biopolymer-based nanocomposite utilizing nanocellulose from rice husks composited with inorganic Fe₃O₄ nanoparticles has good potential for use as a green biocatalyst, and the proposed reaction can be used as an innovative new method to produce biodiesel in the future.

Catalyst; Cellulose; Magnetite; Methyl esters; Nanocomposite

Recently, the catalysts used have been replaced with more environmental friendly heterogeneous ones (Caetano et al., 2013; Mendonça et al., 2019). The catalysis process using heterogeneous catalysts is great interest because it has beneficial characteristics such as high selectivity, long catalyst life, easy recovery, repeatability, and temperature stability; the catalyst can also be easily separated from the reaction mixture (Climent et al., 2012; Santos et al., 2015). Among such heterogeneous catalysts, considerable research has been conducted using inorganic substances such as Fe₃O₄, CaO, Al₂O₃ and MgO. However, if the inorganic catalyst is not modified, it will have low thermal stability and solubility in water; for example, an unmodified Fe₃O₄ catalyst is reversible, causing an unstable reaction. Therefore, several studies have focused on developing procedures for nanocomposite synthesis using biopolymers as a support (Sabaqian et al., 2016). Biopolymers such as cellulose have the potential to be used as supporting substances in heterogeneous catalysis (Eyley et al., 2014; Arantes et al., 2017).

Cellulose is one of the most abundant and renewable natural polymers; approximately 10¹¹-10¹² tons per year can be obtained from plants and it has been widely studied worldwide in academic and industrial research (Ummartyotin & Manuspiya, 2015). It is a carbohydrate polymer consisting of repeating units of ?-D-glucopyranose, comprising three hydroxyl groups in each of its anhydro-d glucose units, meaning the cellulose molecule has great OH functionality (Lavoine et al., 2012). Cellulose can be converted into nanocellulose, which acts as a sustainable nanomaterial because of its availability, biodegradability and biocompatibility, because the materials produced from nanocellulose can be highly porous. Nanotechnology involving cellulose substrates has become a major focus of research because of the exceptional physical and chemical properties of nanocellulose. It has the potential to be used as an efficient support material because it can form bonds with several functional groups to produce heterogeneous catalysts based on biopolymers (Ummartyotin & Manuspiya, 2015; Jabasingh et al., 2016).

However, the larger the number of OH functional groups, the greater the number of inter-or intramolecular hydrogen bonds, making cellulose less attractive as a catalyst-supporting substance. This can be avoided by modifying the cellulose surface so that the number of active sites will increase and efficiency will be higher (Habibi, 2014; Fatona et al., 2018). Modification of the cellulose surface functionalization can be made by acetylation (Sun et al., 2016) or phosphorylation (Wanrosli et al., 2013), among other processes. Functionalized nanocellulose combined with inorganic nanoparticles can form superior nanocomposites (El-Nahas et al., 2017)

In this study, cellulose is derived from rice husks, which have a large cellulose content (Helmiyati et al., 2017). We converted the cellulose to nanocellulose by mechanical ball milling, whereas previous studies used chemical methods (Nahas et al., 2017), and then acetylated it with anhydrous acetate to functionalize the surface. Subsequently, the acetylated nanocellulose was impregnated with magnetite iron oxide nanoparticles (Fe₃O₄) to form a nanocomposite. The aim of the study is to evaluate the catalytic efficiency of these cellulose-magnetite nanocomposites as applied to the synthesis of methyl ester biodiesel from oleic acid. The reaction kinetics of the methyl ester synthesis from oleic acid were studied by observing the unreacted oleic acid concentration, and in order to determine the activation energy value the effect of temperature on the reaction was observed.

A cellulose-nanomagnetite Fe₃O₄ nanocomposites was successfully synthesized, as evidenced by the FTIR, XRD, SEM and TEM characterization, which revealed that nanomagnetite Fe₃O₄ had impregnated the acetylated nanocellulose as the support material. Nanomagnetite Fe₃O₄ with an average size of 30 nm in the nanocomposites was obtained. The nanocellulose-Fe₃O₄ nanocomposites was applied as a catalyst for the synthesis of methyl esters from oleic acid. A molar ratio of methanol to oleic acid of 3:1, catalyst amount of 1.5 wt.%, reaction time of 5 hours and reaction temperature of 60°C were employed as the optimum reaction parameters, with a conversion yield of 89.21%. The type of methyl ester formed as the product of the GC-MS characterization was 9-octadecenoic acid methyl ester, with a relative molecular mass of 296.1 g/mol. In the kinetics study, a low activation energy of 16.56 kJ/mol was obtained. The synthesis of biopolymer cellulose-magnetite nanocomposites using nanocellulose from rice husks with Fe₃O₄ nanoparticles can therefore be used as an effective catalyst for biodiesel synthesis from fatty acids such as vegetable oil.

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