2.1 Synthesis of nickel oxide nanoparticles

The method of sol–gel was used to synthesize nickel oxide nanoparticles. The precursor used was Ni(NO₃)₂·6H₂O (nickel nitrate hexahydrate). First, we prepared a 0.1 M solution of nickel nitrate hexahydrate and, second, a 0.02 M solution of citric acid (C₆H₈O₇). Then, the two solutions were mixed under constant stirring at 500 rpm and 80 °C temperature, leading to gel formation. The gel was first dried at 80 °C in an oven, then it was calcined for two hours at 500 °C in a muffle furnace, and last, it was pulverized (Adiba et al., 2020).

2.2 X-ray (XRD) diffraction of the nanoparticles

XRD is a widely used technique to investigate the basic crystallographic properties of solids. For this purpose, the SHIMADZU 6000 diffractometer was used. For maintaining the current at 30 mA and voltage at 40 kV, the diffractometer was equipped with a CuKα (K = 1.54 A°). The data was collected at 2θ within the range of 10°-90°. The nanoparticle size was measured from step size and scan rate at a speed of 10° per minute. Between 10 °C and 60 °C, all measurements were taken. With the help of XRDA31 software, the Peak broadening and peak positions were determined. A heterogeneous average diameter of nanoparticles was obtained by completing the calculation using the Scherer equation (Nath et al., 2020).

k = 0.9 representing the shape factor.

λ = 1.54 Å representing the radiation wavelength.

β = representing the intensity of FWHM (full width of half of the maximum) radians.

2.3 Nanoparticle scanning electron microscopy (SEM)

For studying the surface morphology of the nanoparticles, SEM was conducted. For this purpose, the JEOL JSM-5910 & HT7800 Ruli model of scanning electron microscope was used. The scanned images were captured using the field emissions from an SEM microscope under a 20 kV accelerating voltage. It made it possible to evaluate catalyst surfaces qualitatively and helped explain events that occurred during pretreatment and calcination.

2.4 Determination of oil contents in the feedstock

The collected seeds were washed thrice with distilled water and then desiccated in a hot air oven at 50 °C. About 25 gm of dried seeds were powdered. 10 gm of seed powder was taken in the thumbnail. As an internal solvent, we used ether. The soxhlet containing the seed powder was run for 8 h at a temperature of 65 °C. The given equation was used for calculating the total oil content (Jan et al., 2022b).

Where W₁ = represents the empty flask weight, W₂ = quantity of the ground seed before the oil is extracted, W₃ = the weight of the flask after the extraction of the oil, and W₄ = represents the amount of oil contents in the feedstock.

2.5 Determining free fatty acid (FFA)

The method of acid-base titration (Jan et al., 2022b) was followed to measure the free fatty acid (FFA) content. With the help of the given equation, the FFA value was determined.

A = Used quantity of KOH.

B = Used amount of KOH in blank titration

C = KOH ^g/_L concentration of.

V = Oil sample volume.

2.6 Method of biodiesel synthesis

Due to the low FFA of the feedstock, the transesterification method was followed for the biodiesel synthesis. The percent yield of biodiesel was calculated using the equation given below (Birla et al., 2012).

2.7 Evaluation of the fuel properties

Following ASTM standards, the fuel properties of the synthesized biodiesel were evaluated (Table 1) (Fadhil et al., 2017).

2.8 Analyzing the physicochemical characteristics of biodiesel

The synthesis of AAB and examination of its physicochemical characteristics have been confirmed by FT-IR, GC–MS, and NMR spectroscopy.

3.1 X-ray diffraction of nickel oxide nanoparticles

XRD is used to determine a material's crystalline phase (Dawood et al., 2021). Monochromatic X-ray interference on crystalline material is a phenomenon that XRD is investigating (Dawood et al., 2021). Fig. 1 clearly shows that the nanoparticles are crystalline. The strong diffraction peaks obtained at 2θ angles of 37.22°, 43.34°, and 62.81° that correspond to the 111, 200, and 220 Miller indices, respectively, confirm the synthesis of nickel oxide nanoparticles and crystalline nature of it (Ghazal et al., 2021). The average diameter of nanoparticles is 24 nm.

Close

Fig. 1

XRD patterns confirming the NiO nanoparticles synthesis.

Export to PPT

3.2 Surface and morphological characteristics study of nanoparticles

We studied the surface and morphological characterizations of the nanoparticles with the help of the SEM Jeol Model JSM-6390LV model. It is clear from Fig. 2 that the particles have spherical to cuboidal symmetry with a smooth surface. The nanoparticles have uniform sizes (Dharmaraj et al., 2006; Ariyanta et al., 2021).

Close

Fig. 2

SEM analysis of NiO nanoparticles to confirm the synthesis of nanoparticles and size, a) 0.5 µm X5,000b) 0.5 µm X10,000; c) 1 µm X10,000; d) 1 µm X20,000.

Export to PPT

3.3 Determination of oil and FFA contents

The soxhlet method confirmed that the feedstock has 22 % oil content. The acid-base titration method confirmed that the feedstock has an FFA of 0.47 mg KOH/g, less than the ASTM D-6751 standard (Kuepethkaew et al., 2017).

3.4 Parametric optimization of biodiesel yield through transesterification

Because of its simplicity and being economically viable, transesterification is the most convenient method (Rezania et al., 2019). Transesterification is an important step since it may bring the viscosity of the feedstock or vegetable oils closer to that of conventional fuel (Mumtaz et al., 2017). During the transesterification process, a number of variables have been reported to be crucial for obtaining the maximum biodiesel yield, including the oil-to-methanol ratio, catalyst concentration, reaction time, and reaction temperature (Rezania et al., 2019). Therefore, we also conducted a parametric transesterification study to get the optimum biodiesel production (Takase et al., 2014).

3.4.1

3.4.1 Oil to methanol ratio

The yield of biodiesel is significantly affected by the methanol-to-oil ratio, and previous studies show that as methanol concentration increases, so does the yield of biodiesel (Takase et al., 2014; Ullah et al., 2020). Therefore, we also varied the oil-to-methanol ratios as 1:12, 1:18, 1:24, 1:30, and 1:36 to attain the maximum yield. The finding of the current work demonstrates clearly that the maximum biodiesel yield was attained at a ratio of 1:24 oil to methanol (Fig. 3a). Due to the reversibility of the transesterification process, a higher stoichiometric ratio improves the solubility as well as the reactivity of the Me-OH and triglyceride molecules (Tan et al., 2019). Furthermore, a significant quantity of Me-OH is needed to break the bonds between glycerin and fatty acids (Kumar, 2017). Conversely, a high methanol concentration cannot enhance biodiesel output by not only hindering the ester recovery but also raising costs (Ullah et al., 2020).

Close

Fig. 3

A) oil to methanol ratio has a positive effect on biodiesel yield and a negative on glycerin formation; b) catalyst concentration has a positive effect on biodiesel yield, but a high concentration of catalyst also favours the formation of more glycerin.

Export to PPT

3.4.3

3.4.3 Reaction temperature

In order to determine the minimum temperature at which the highest biodiesel production should be attained, we performed transesterification operations at five distinct temperatures of 40, 45, 50, 55, and 60 °C while maintaining the other variables constant. Due to its direct effect on the production of biodiesel as well as the duration of time it takes to react (Yu et al., 2021). In addition, to reduce energy consumption and, therefore, product cost, the transesterification reaction temperature for industrial-scale biodiesel synthesis must be as low as possible. According to our study, the biodiesel yield jumped to 95 % when the temperature was raised from 55 to 60 °C. (Fig. 4a). Since high temperatures reduce the solution’s viscosity, increase the reactants' solubility, and speed up the rate at which reactants are transferred to the product, it is clear that the reaction temperature, biodiesel yield, and reaction time all have a significant relationship (Silva and Oliveira, 2014; Barros et al., 2020).

1. Reaction time

Close

Fig. 4

A) increase in reaction temperature increases the biodiesel yield and reduces the glycerin formation; b) increase in reaction time increases the biodiesel yield and reduces the glycerin formation.

Export to PPT

As the other factors addressed earlier, the reaction duration is important to biodiesel yield, especially on an industrial scale, since it raises production costs owing to higher energy consumption (Gebremariam and Marchetti, 2018; Dhawane et al., 2018; Barros et al., 2020). The reaction durations were kept at 30, 60, 90, 120, and 150 min in order to establish the minimal optimal reaction time for maximum biodiesel output. 90 min of reaction time resulted in the highest yield (95 %), whereas longer reaction times resulted in no change in yield (Fig. 4b). The same findings have been reported by more studies (Niju et al., 2016; Gebremariam and Marchetti, 2018; Barros et al., 2020; Ullah et al., 2020).

3.5 Fuel properties of biodiesel

The fuel characteristics of biodiesel are mentioned in Table 1. We determined the kinematic viscosity for AAB 4.21 mm²/s at 40 °C, which is near to normal diesel and below the ASTM D-6751 required limit (Ullah et al., 2020). The density of 0.893 kg/L is within the parameters set by ASTM D-6751 (Patchimpet et al., 2020). Moreover, the calorific value of 27763 kJ/kg falls below the ASTM D-6751 range. It has a reduced calorific value compared to conventional diesel because of its increased oxygen concentration (Kaisan et al., 2017; Ullah et al., 2020).

AAB has a flash point of 78 °C. It is dependent on the methanol level of the biodiesel. Adding 0.5 % methanol decreases biodiesel's flash point to 50 % (Table 1). Some studies have suggested a flash point for biodiesel of 160–202 °C (Dias et al., 2008; Refaat et al., 2008).

The AAB has a cetane value of 46, which is slightly above the ASTM standard (Table 1). Nitric acid Iso-octyl, in small amounts, can lower the cetane number (Khan et al., 2009; Knothe, 2009). Moreover, the PP and CP values of the synthesized biodiesel are −8.6 °C and −10.9 °C, respectively (Mofijur et al., 2015). Synthetic biodiesel has sulphur contents of 0.0007 parts per million. It is smaller than ASTM's recommended value. So, the AAB is environmentally friendly biodiesel (Kumar, 2017; Ullah et al., 2020).

The ASTM D-6751 minimal requirement is met by the oxidative stability of AAB, which is 2.91 h. The biodiesels can react with oxygen even at room temperature. It demonstrates the relative sensitivity of fuel the oxidative degradation (Pullen and Saeed, 2012). Thus, the oxidation susceptibility of biodiesel is advantageous for the environment (Chrysikou et al., 2022); however, it is certainly a significant problem as well as an obstacle in the industrialization of biodiesel. The physicochemical and tri-biological properties of biodiesel are altered by oxidation during storage or usage. The effects of this phenomenon are immediately felt by the characteristics of biodiesel. However, adding different antioxidants may lessen biodiesel's sensitivity to oxidation (Bannister et al., 2011; Kumar, 2017).

According to our analysis, AAB has a water content of 0.085, which is only slightly more than the ASTM standard. A critical component of fuel quality is its water content. Biodiesel has a higher potential to absorb moisture than conventional fuels since it is more hygroscopic and hydrophilic. High moisture levels in biodiesel encourage the growth of algae in gasoline tanks, which corrodes fuel tanks and produces slime and sludge to clog engine filters and fuel pipes, damaging the vehicle's fuel injection system (Fregolente et al., 2012).

The iodine value was determined to be 81, which is less than the ASTM D-6751 standard. This demonstrates biodiesel's oxidative stability, saturation, and unsaturation level (Cardoso et al., 2019). AAB has a refractive index of 1.458. It demonstrates that converting crude oil to biodiesel was successful (Lugo-Méndez et al., 2021). Additionally, we examined the carbon residue in the synthetic biodiesel that falls below the ASTM D-6751 standard.

According to this study, the HHV is 41.83, which is within the acceptable limit given by ASTM D-6751. A crucial aspect of biodiesel is its high heat value (HHV), which, like other fuel qualities, reveals details about the energy contents of the fuel as well as efficiency (Demirbaş, 2003; Marzban et al., 2022). The biodiesel's fatty acid composition, iodine concentration, and saponification value are used to compute it (Anand et al., 2010). Biodiesel has a lower HHV (39 to 43 MJ/kg) than fossil fuels (49.65 MJ/kg) (Demirbaş, 2009).

Numerous studies have shown that the fatty acid, as well as the chemical composition of non-edible biodiesels, affects the fuel properties. The fuel properties of other feedstocks are given in Table 2 (Ashraful et al., 2014).

3.6 ¹H NMR spectroscopy of AAB

The singlet peak at 3.498 ppm showed that the biodiesel sample included methoxy proton (–OCH₃). It verifies the process of converting crude oil to biodiesel (Adewuyi et al., 2014). The existence of alpha-methylene proton (α-CH₂) in the synthesized biodiesel was confirmed from the peaks between 2.015 and 2.062 ppm. These two peaks demonstrated that FAMEs were produced from triglycerides. The presence of terminal methyl protons (CH₃) in the biodiesel was confirmed from the peaks obtained at 0.843–0.951 ppm. Peaks obtained at 1.275–1.638 ppm confirm the presence of beta-carbonyl methylene protons. At 5.189–5.426 ppm, the olefinic hydrogen indicator peaks were attained (Fig. 5a) (Knothe, 2000). The successful conversion of triglycerides to biodiesel can be confirmed by these peaks (Portela et al., 2016; Ullah et al., 2020).

Close

Fig. 5

Spectroscopy confirms the successful occurrence of the transesterification process a) ¹H NMR and b) ¹³C NMR.

Export to PPT

3.7 ¹³C NMR study of AAB

23.92–34.21 ppm confirms the presence of long-chain ethylene carbons (–CH₂-) in the biodiesel. Furthermore, the presence of carbonyl carbon (–CO) was confirmed from the singlet peak received at 173.98 ppm. The olefinic carbons had confirmation peaks that ranged from 128.82 to 130.09 ppm. The presence of vinylic (C═H) substituent was confirmed by the peaks at 126.54 and 127.41 ppm (Fig. 5b). The peaks in this range for the same function groups have also been reported in other studies (Ullah et al., 2015).

3.8 Qualitative and quantitative analysis of AAB

An examination of the FAMEs produced by successful transesterification was done using gas chromatography and mass spectroscopy on both a qualitative and quantitative level. According to the GC–MS data, there are 14 different types of FAMEs in the AAB (Table 3). The peak of each methyl ester of fatty acid was verified using the NIST 02 library match programme. Based on the retention time, each methyl ester of a fatty acid was identified. Furthermore, it is clear from the spectrum of GC–MS that the majority of FAMEs are unsaturated and thus suggest better fuel efficiency (Asci et al., 2020).

3.9 Fourier transform infrared analysis of AAB

FT-IR spectroscopy was used to confirm the presence of different functional groups in the biodiesel sample (Table 4 & Fig. 7). It is an effective analytical instrument for identifying the pool of macromolecular pools (such as lipids, proteins, and carbohydrates) and tracking biochemical changes in those pools (Miglio et al., 2013). FAMEs are capable of absorbing electromagnetic radiation of infrared wavelengths (Atabani et al., 2019). Two principal absorption bands, the carbonyl, with an absorption band from 1725 cm⁻¹ to 1750 cm⁻¹, and C–O, having an absorption band range of 1000–1300 cm⁻¹_, are used to confirm the synthesis of esters (Ullah et al., 2020). The presence of an additional prominent distinctive absorption band at 1225 cm⁻¹ verifies aromatic ether (Siatis et al., 2006; Taufiq-Yap et al., 2014). The absorption bands detected in FT-IR spectroscopy show that the transesterification process was effective (Andrade et al., 2017; Elango et al., 2019).

Close

Fig. 7

FT-IR spectroscopy confirms the biodiesel formation by showing peaks for important functional groups and compounds.

Export to PPT