1 Introduction

Currently, the increasing energy demand, depleting fossil fuel reserves, and several environmental issues have search lead for a renewable, eco-friendly alternative resource [1,2,3]. Among sundry renewable energy, biodiesel was considered a promising energy solution to the energy problem. Moreover, biodiesel proposes several excesses compared with diesel fuel such as non-toxic, renewable, and biodegradable [4,5,6]. Biodiesel, also familiar as fatty acid methyl ester, is generated through the transesterification reaction of vegetable oil/animal fats with an alcohol (methanol/ethanol) in the existence of a catalyst support [7,8,9]. In this stage, the catalyst becomes a very crucial part of biodiesel production. The catalyst utilized in the transesterification process can be either homogeneous or heterogeneous [10,11,12].

Nowadays, heterogeneous catalysts are preferred and more considerably used for biodiesel production than homogeneous catalysts. This is due to several reasons such as the homogeneous catalyst’s lower biodiesel yield, difficult separation, non-reusability, and can pollute the environment [13,14,15]. These deficiencies of homogeneous catalysts could be overcome using heterogeneous catalysts because the catalysts could be recovered, easy to separate, environmentally friendly, as well as recycled [16,17,18]. Several heterogeneous catalysts that have been widely reported for biodiesel production include carbon-based [19], agricultural wastes [20], graphene-based metal oxide [21], modified resin [22], rare-earth metal [23], and shell waste [24]. Among the most examined heterogeneous catalyst, transition metal oxides such as titanium dioxide (TiO2) have unique advantages in biodiesel production due to their higher basicity, easier to obtain, and inexpensive.

TiO2 is one of the superior transition metal oxides that can be applied for biodiesel production as it is non-toxic and eco-friendly [25,26,27,28,29]. However, undoped TiO2 has various limitations in its wide application. Furthermore, the selection of dopants also greatly affects the performance of TiO2. Doping with 3d elements such as Fe and Ni metal can enhance the surface area, reduce the bandgap, increase the catalytic activity, and also reduce the particle size. The choice of Fe metal is caused by its stability at high temperatures, multivalent, and effectively reduces the band gap energy [30, 31]. Moreover, Fe metal is a magnetic oxide owning the ability to transesterify free fatty acid to achieve biodiesel products. The presence of Ni metal is also expected to increase the efficiency of TiO2 photocatalysts in the visible region [32,33,34]. In addition to creating a new band gap, Ni doping can also accelerate photocatalytic activity and improve the transesterification process for biodiesel production [35]. The catalyst was synthesized by the sol-gel method and its catalytic performance was studied. At present, there are no studies reported that combined co-doped TiO2 using 3d elements (Fe and Ni metal).

In the present work, we investigated Fe/TiO2–Ni mixed nanocomposite as a heterogeneous catalyst for transesterification reaction that was efficiently utilized in biodiesel production from coconut oil. We also evaluated the effect of co-doped Fe and Ni against catalyst performance. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy-energy dispersive X-ray (SEM-EDX), and gas chromatography and mass spectroscopy (GCMS) were conducted for the characterization of prepared catalysts and biodiesel product.

2 Materials and Method

2.1 Materials

The materials used in the present study were sodium hydroxide (NaOH) 0.1 N, ethanol (C2H5OH), iron(III) nitrate (Fe(NO3)3) 0.1 M, nickel(II) nitrate (Ni(NO3)2) 0.1 M, sodium thiosulfate (Na2S2O3), titanium tetraisopropoxide (TTIP), iodide solution (I2), chloroform (CHCl3), acetyl acetonate (C5H8O2), methanol (CH3OH), distilled water, sulfate acid (H2SO4) (97 %), acetic acid (CH3COOH) 0.5 M, phenolphthalein (PP) indicator, starch indicator, and n-hexane (C6H14). The coconut oil was pretreated by separating the meat from the shell. Furthermore, the meat was grated and squeezed to get coconut milk. The coconut milk was silenced to remove the water content. After pretreatment, the coconut milk was heated at 45 °C and for 5 h with vigorous stirring. After heating was completed, the coconut milk was cooled and filtered to achieve oil.

2.2 Catalyst Preparation

Fe/TiO2-Ni nanocomposite was synthesized by the sol-gel method adapted from previous research: 4 mL of titanium tetraisopropoxide, 0.5 mL of acetyl acetonate, and 15 mL of ethanol were put into a beaker glass as solution A. Solution B was prepared by mixing 15 mL of ethanol, 2 mL distillate water, and 0.5 mL acetate acid. Furthermore, solutions A and B were refluxed for 3 h at 50 °C. Moreover, followed with added of 1 mL Fe(NO3)3 and Ni(NO3)2 0.1 M and stirred using a magnetic stirrer for 1 h at 50 °C. Finally, the catalyst was calcined for 1 h at 400 °C.

2.3 Characterization

Fe/TiO2-Ni nanocomposite was characterized using SEM-EDX analysis to observe the surface morphology, compositions, and also the micropore. Crystal shape and pore size were also observed using XRD analysis. The averaged diameter of the nanocomposite was determined using the Scherrer equation at 2θ between 10 and 80 °C. The crystal phases that exist in the samples were compared with the JCPDS database files. In addition, the structural composition and functional groups of the nanocomposite were also investigated using FTIR. FTIR studies for all the prepared samples were conducted in the 400–4000 cm−1 region.

2.4 Transesterification Reaction Studies

The transesterification reaction was conducted at the reactor system equipped with a hotplate stirrer and beaker assembled. The transesterification condition was diverse including the catalyst concentration (0.5, 1.0, and 1.5 b/v), oil:methanol mol ratio (1:6 mol: mol), reaction time (3 h), and under UV light. After the reaction was completed, the mixture was separated into the two-phase (biodiesel and glycerol) by using centrifugation for 5 min at 1600 rpm. The biodiesel obtained was placed in a bottle for methyl ester analysis. In the final stage, the methyl ester component was characterized using gas chromatography-mass spectroscopy.

3 Results and Discussion

3.1 SEM-EDX Analysis

SEM images at different magnifications (1000 and 7500 times) of the Fe/TiO2-Ni nanocomposite are displayed in Fig. 1. The preparation of the catalyst for the SEM analysis was conducted by the sol-gel method and calcined at 400°C for 1 h. SEM images (Fig. 1a) present the formation of agglomeration on the catalyst. The agglomeration suspectly originated from the calcination temperature and the existence of dopant ions distributed on the TiO2 surface. Figure 1b illustrates the SEM images of the Fe/TiO2-TiO2 nanocomposite with magnification 7500 times. The catalyst exhibits TiO2 nanoparticles on the surface material. TiO2 particles are grainy in shape and irregular. In general, Fe/TiO2-Ni catalysts own an average size of 9.16 nm (colors of gray), suitable with Scherrer assessment. Therefore, the Fe/TiO2-Ni nanocomposite could be classified as a heterogeneous catalyst reason the particles on a nano-size. The Fe/TiO2-Ni composite also analyzed by EDX (Fig. 1c) indicates that it presents compositions such as Ti, O, Ni, and Fe elements. This result was supported by XRD and FTIR analyses and become an excuse for why the Fe/TiO2-Ni catalyst showed the most excellent activity during the experiment.

Fig. 1
figure 1

SEM image of Fe/TiO2-Ni nanocomposite (a) 1000 times, (b) 7500 times magnifications, and (c) EDX spectra of Fe/TiO2-Ni

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3.2 XRD Analysis

The X-ray diffraction patterns of TiO2 pure and Fe/TiO2-Ni nanocomposite are shown in Fig. 2. Specifically, the pattern exhibits different peaks at 2θ of 21,43° and 45,69° was associated with the [101] and (004) fields of Fe-TiO2 bond, correspond JCPDS No. 96-900-6336 [30]. The peaks at 2θ of 35,12° and 64,48° were associated with the [110] and [300] fields of the Ni-TiO2 bond. These results are in accordance with the report by Gabal et al. [36]. Moreover, the peaks at 2θ of 25,24°; 37,79°; 47,48°; and 54,21°, 62,46°, and 69,70° are also visible and associated with the TiO2 structure (JCPDS file no. 00-21-1272) with the crystal plane [101], [200], [004], [105], [211], and [220] [37]. The Fe/TiO2-Ni nanocomposite crystalline size determined using Scherrer’s equation was discovered approximately 9.16 nm.

Fig. 2
figure 2

XRD pattern of the (a) TiO2 pure and (b) Fe/TiO2-Ni catalyst

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3.3 FTIR Analysis

The FTIR spectrum of both Fe-TiO2 and Fe/TiO2-Ni nanocomposites is exhibited in Fig. 3. The wide band around 3417 cm−1 is associated with O–H group stretching and bending vibration, which is pertained to H2O molecules. Moreover, the IR band is visible from 400 to 800 cm−1 according to the Ti–O stretching vibrations [38]. The absorption band at 636.51 cm−1–659.66 cm−1, typical for the Ni-O stretching vibration, ensures due to the presence of nickel oxide. The identical band at 600 cm−1–700 cm−1 was also investigated by Ahmed et al. [39], which is shown that the (NO3−) group was always present when (NaNO3) are utilized as precursor. Furthermore, the absorption band at 1039,62 cm−1 and 578,64 cm−1 was characterized as a vibration of Fe-O, which more convinces the existence of Fe in nanocomposite [40].

Fig. 3
figure 3

FTIR spectra of (a) Fe-TiO2 (red line) and (b) Fe/TiO2-Ni (blue line)

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3.4 GC-MS Analysis

GC-MS analysis was determined to observe the presence of several kinds of fatty acid methyl ester from biodiesel products. The fatty acid methyl esters analysis characterized by GC-MS is displayed in Fig. 4 and visible in Table 1. The primary component discovers in biodiesel products such as methyl laurate (C13H26O2), methyl octanoate (C9H18O2), methyl caprate (C11H22O2), and methyl myristate (C14H28O2). The high peak at a retention time of 10.77 min in Fig. 4 and Table 1 presents the existence of methyl laurate with 51.38% of total content.

Fig. 4
figure 4

Chromatogram of methyl ester

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Table 1 Composition of biodiesel conforming to the result of GC-MS analysis
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Figure 5a shows the primary peak in all the mass spectra of saturated methyl ester was observed at m/z 74 which is a rearrangement of McLafferty the well-known [41]. Three other peaks i.e., a peak at m/z 171, m/z 143, and m/z 129 due to the removal of a propyl radical (carbon 2 to 4). Moreover, methyl ester identified in coconut oil has characteristic fragmentation pattern peaks at m/z 55, [M-]+ due to the removal of C3H6O2. Figure 5b shows the mass spectrum of methyl caprate with a retention time of 8.127 min providing peaks with m/z of 43, 55, 74, and 101 respectively. Peaks at m/z 143 is due to the deletion of C8H15O2+. The other peaks at m/z 43, 101, and 143 are fragmentation patterns due to the cleavage of each C-C bond and are known as fragmentation patterns of the CnH2n-1O2+ ion series [42].

Fig. 5
figure 5

GC-MS chromatogram of (a) methyl laurate and (b) methyl caprate

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The transesterification reaction using the Fe/TiO2-Ni heterogeneous catalyst was performed in a UV reactor system. The mechanism of the Fe/TiO2-Ni photocatalyst reaction was initiated with the process of transferring methanol and triglycerides to the surface of TiO2. This process was accelerated through vigorous stirring. Methanol and triglycerides will be adsorbed on the surface of the photocatalyst. The proposed photocatalytic reaction mechanism can be seen in Fig. 6 (scheme 1). Based on Fig. 6 (scheme 1), electrons will react with triglycerides and produce RCO•OR radicals while holes react with methanol to form CH3O• radicals. The two radicals produced will react to form methyl ester, as can be seen in Fig. 6 (scheme 2).

Fig. 6
figure 6

The photocatalytic reaction mechanism using Fe/TiO2-Ni heterogeneous catalyst in transesterification reaction

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Moreover, numerous studies have previously documented a variety of comparisons involving the various feedstock in biodiesel production. These studies examined various heterogeneous catalysts, and their findings have been compiled and presented in Table 2.

Table 2 Comparison of various feedstock in biodiesel production using heterogeneous catalysts
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4 Conclusions

The purpose of the present study was to increase the performance of TiO2 in biodiesel production by adding 3d elements (such as Fe and Ni) using the sol-gel method and calcining at 500 °C. The surface morphology of nanocomposite depicted the grainy shape and irregular with an average size of 9.16 nm. The study indicated that the addition of 3d elements (Fe and Ni) has enhanced the TiO2 surface area and activity against biodiesel production. GC-MS analysis of methyl ester for coconut oil revealed the existence of diverse compounds containing carbon atoms from C7 to C19. Its main components include methyl laurate (51.38%), methyl octanoate (18.75%), methyl caprate (10.21%), and methyl myristate (8.80%). On the whole, the TiO2 modified of 3d elements (such as Fe and Ni) became a promising candidate as the heterogeneous catalyst for biodiesel production from coconut oil.