Synthesis of Fe3O4 Nanocatalyst Capped Citric Acid (Fe3O4-CA) from Sargassum filipendula

The nanocatalyst Fe3O4 capped citric acid (Fe3O4-CA) was successfully synthesized using brown seaweed Sargassum filipendula. Sulfated polysaccharides in Sargassum filipendula extract contain sulfate, hydroxyl, and aldehyde groups which cause Fe reduction and nanoparticle stabilization. The FT-IR results of Sargassum filipendula extract showed the presence of C-O-SO3 stretching vibrations at 1040 cm , sulfate groups at 1241 cm, aromatic C-C at 1413 cm, carbonyl at 1604 cm, C-H stretching vibrations at 2932 cm, and the hydroxyl group at 3316 cm-1. Meanwhile, citric acid was used as capping to prevent agglomeration of the synthesized nanocatalyst. Fe3O4-CA nanocatalyst were characterized using XRD, PSA, and SEM-EDX. The XRD results were processed using the Debye-Scherrer equation and the crystal size of Fe3O4CA was 8.5 nm. PSA results show that Fe3O4-CA particles has an average radius of 45.09 nm. This nanocatalyst was also tested for the synthesis of pyrimidine-derivative compound at optimum conditions using 7.5% mol of Fe3O4-CA catalyst, 50 °C for 6 hours, in order to obtain a yield of 83.2%.


Introduction
Nanocatalyst have a size of 1-100 nm. Reducing the dimensions of the catalyst to nano size will certainly increase the surface area of the catalyst and consequently increase the activity of the catalyst in certain reactions. The fineness and size of the nanocatalyst formed is influenced by at least 3 factors, namely the heating temperature in the nanocatalyst manufacturing process, continuous media, and the time used [1]. These three things need to be controlled and varied in the process of making nanocatalyst. The higher the temperature used in the nanocatalyst manufacturing process, the better the interaction of the nanocatalyst base material with the continuous media used [2]. However, as the reaction temperature increases, this means that more energy is needed and equipment that is more capable of withstanding higher heat will result in higher costs [3].
Nanocatalyst of varied shapes and sizes can be synthesized by using physical, chemical, or biological pathways. However, exploiting physical and chemical routes lead to high energy consumption, low yield, high cost, and environmental damage by employing harsh reducing agents. The biological pathways involve the use of microorganisms (bacteria, fungi, yeast, algae, etc.) or plants, and using microorganisms is riskier because of the pathogenicity issue. It also requires maintenance of large cultures. Therefore, synthesis of nanocatalysts with greener methods is preferred because a cost-effective, simpler, and eco-friendly [4]. Nano Fe 3 O 4 which is ferrimagnetic has wide application opportunities. The application of Fe 3 O 4 in nanoparticle size is an alternative that is needed to meet the needs of industrial raw materials which are in development and their needs are increasing. Nanomagnetic particles have varied physical and chemical properties and can be applied in various fields [5]. These nanoparticles can be used as materials for drug delivery systems (Drug Delivery System = DDS), catalysts, Magnetic Resonance Imaging (MRI), and cancer therapy [6] [7]. In order to be applied in these various fields, it is very important to consider the particle size, magnetic properties, and surface properties of the nanoparticles themselves [5] [8].
Surface functionalization of the particles and proper selection of solvents are important factors to prevent aggregation between particles and produce a thermodynamically stable colloidal solution. The surface of Fe 3 O 4 can be stabilized by aqueous dispersion through the adsorption of citric acid. This happens because citric acid forms a coordination in the presence of a carboxylate. Carboxylates have an important influence on the growth of Fe 3 O 4 nanoparticles and their magnetic properties. Increasing the concentration of citric acid causes a significant decrease in the crystallinity of Fe 3 O 4 formation. In addition, the presence of citrate causes changes in the surface geometry. Its stability is highly dependent on the pH and concentration of the acid being adsorbed [9]. Fe 3 O 4 coated with citric acid is shown in Figure 1.
Brown seaweed with Sargassum filipendula species shown in Figure 2 is a potential source of bioactive compounds that are very useful for the development of the pharmaceutical industry such as antibacterial, antitumor, anticancer or as a reversal agent and the agrochemical industry, especially for antifeedants, fungicides and herbicides [10]. Alginate is a constituent of cell walls in seaweed which is commonly found in brown seaweed [11]. This compound is a heteropolysaccharide from the formation of monomer chains of mannuronic acid and gulunoric acid. Alginate content in seaweed depends on the type. The highest content of alginate (30-40% dry weight) can be obtained from Laminariales species while Sargassum only contains 16-18% dry weight [12].  In a previous study, the synthesis of Fe 3 O 4 nanoparticles was successfully carried out through bioreduction of FeCl 3 using the crude extract from brown seaweed Sargassum muticum which was characterized by a colour change from light yellow to blackish brown. It was assumed that the role in this reduction process is the sulfated polysaccharides found in seaweed. Sulfated polysaccharides contain hydroxyl, aldehyde, and sulfate functional groups [13] [14]. This is evidenced from the FT-IR results where the sulfate group appears at wave number 1233 cm -1 , aldehyde at 1610 cm -1 , and a hydroxyl group at 3348 cm -1 . It begins with the involvement of the hydroxyl group in the hydrolysis of FeCl 3 to form Fe (OH) 3 . Then in the presence of aldehydes, Fe(OH) 3 is reduced to form Fe 3 O 4 while at the same time the aldehyde is oxidized to form the corresponding acid [15]. In this study, it will be observed how the role of Sargassum filipendula in the synthesis of nanocatalyst Fe 3 O 4 capped citric acid (Fe 3 O 4 -CA).

Preparation of Sulfated Polysaccharide Extract from Sargassum filipendula
The sample of Sargassum filipendula was taken from Binuangeun, Banten, Indonesia. Starting with the drying process using indirect sunlight for 10 hours, then aerated again at room temperature for 24 hours so that the water content is reduced making it easier to grind. Sargassum filipendula was crushed to obtain a dark brown powder. The powder was put into an Erlenmeyer then added with distilled water with a concentration of 5% by weight and boiled for 2 hours at a temperature of 100 °C. The suspension formed is filtered. The filtrate was added with 96% ethanol in a ratio of 1: 3 [15].

Preparation of Fe 3 O 4 Nanoparticles
A total of 1 mmol FeCl 3 and 0.25 mmol FeCl 2 were added to 5 mL of Sargassum filipendula extract and 50 mL of distilled water as a medium for making nanoparticles. The mixture is stirred until homogeneous. Then 6 M NaOH was added little by little until it reached pH 10. The mixture was stirred for 2 hours and put in the oven [2].

Synthesis of Fe 3 O 4 -CA Nanocatalyst
A total of 2 grams of Fe 3 O 4 was added with 250 mL of 1% citric acid. Heated to 60 °C and stirred for 60 minutes. The mixture was cooled at room temperature. The product was washed with distilled water and dried at 60 °C. The formed Fe 3 O 4 -CA nanocatalyst was characterized by PSA, SEM-EDX, and XRD [16].

Results and Discussion
The involvement of Sargassum filipendula in the synthesis of Fe 3 O 4 nanoparticles is caused this seaweed contains various types of polysaccharides that can be used as metal reducing agents. Sargassum filipendula extract was taken by boiling seaweed (5% by weight) at 100 °C with distilled water for 1 hour, then filtered. The use of distilled water as a solvent is due to all the polysaccharides dissolved in it. The filtrate obtained was then added with ethanol (1:3 v/v), because ethanol can attract sulfated polysaccharides [15]. Figure 3 is a sulfated polysaccharide extract after being baked, in the form of a solid with blackish-brown colour. The presence of sulfated polysaccharides contained in this solid was confirmed by the appearance of elements C, O, and S in the analysis using EDX as shown in Table 1. While other elements such as Na, Mg, Al, and so on are minerals that are commonly contained in the seaweed. Sulfated polysaccharides contain sulfate, hydroxyl, and aldehyde groups which cause Fe 3+ reduced and nanoparticle stabilized.  Figure 5.
The purpose of Fe 3 O 4 capping using citric acid (Fe 3 O 4 -CA) in addition to preventing agglomeration between particles, can also catalyze substrates in the synthesis of organic compounds through the carboxylic groups on the surface of Fe 3 O 4 [1]. Fe 3 O 4 -CA was made using a simple method by adding 1% citric acid to Fe 3 O 4 and stirring at 60 °C. The formation schema of Fe 3 O 4 -CA is shown in Figure 6, where MNP is a magnetic Fe 3 O 4 nanoparticle and MNP-CA is Fe 3 O 4 which has been capped with citric acid.
The crystal size was determined using the Debye-Scherrer equation with the formula D = k.λ / β cos θ. Where k is the crystal factor, λ is the wavelength used, β is the FWHM value in radians (1 radian = 57.3°), and θ is the Bragg diffraction angle. Table 2 shows that the β value of Fe 3 O 4 in radian is 1.745 x 10 -2 , the value of θ is 17.83, so that the D value is 8.34 nm. Table 3 shows that the β value of Fe 3 O 4 -CA in radian is 1.710 x 10 -2 . The value of θ is 17.52. So that the D value of 8.5 nm is obtained. Crystal size obtained in this study smaller than without the use of Sargassum filipendula as previously reported with a crystal size of 10 nm [17].   Particle size was characterized using the Particle Size Analyzer (PSA). The appropriate liquid medium used as a dispersion is 99% ethanol. Measurements were carried out seven times. The study results have obtained that the average particle radius is 30.41 nm for Fe 3 O 4 and 45.09 nm for Fe 3 O 4 -CA. The graph of the relationship between the particle radius and intensity shown in Figure 7 and Figure 8.   Table 3. XRD Data of Fe3O4-CA The SEM results in Figure 9 shows the morphology of Fe 3 O 4 is granular and Fe 3 O 4 -CA in the form of granular and fiber. The morphological differences indicate that in Fe 3 O 4 -CA, the citric acid has capped Fe 3 O 4 although in Figure 10 there's still a defect caused the uneven size of particles and the distribution is not homogenous. SEM-EDX presents qualitative and quantitative information of material components as in Table  4.
Characterized Fe 3 O 4 -CA nanocatalyst was tested on the synthesis of pyrimidine-derivative compound with optimum conditions at 7.5% mol catalyst and temperature of 50 °C for 6 hours with 83.2% yield obtained. Figure 11 shows this compound is yellow powder and plausible reaction mechanism with Fe 3 O 4 -CA nanocatalyst is in Figure

Conclusion
Fe 3 O 4 -CA nanocatalyst from Sargassum filipendula polysaccharide extract had a crystal size of 8.5 nm and an average particle radius of 45.09 nm. This nanocatalyst was also tested on the synthesis of pyrimidine derivative compounds and obtained a yield of 83.2%. Fe 3 O 4 -CA nanocatalyst is heterogeneous catalyst that can remove easily after reaction using magnetic bar.