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Article

Biodiesel Synthesis from Milk Thistle (Silybum marianum (L.) Gaertn.) Seed Oil using ZnO Nanoparticles as a Catalyst

by
Hammad Ahmad Jan
1,
Igor Šurina
2,*,
Ahmed S. Al-Fatesh
3,
Abdulaziz M. Almutlaq
3,
Sher Wali
4 and
Anton Lisý
2
1
Department of Botany, University of Buner, Buner 19290, Pakistan
2
Department of Wood, Pulp and Paper, Institute of Natural and Synthetic Polymers, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, 812 37 Bratislava, Slovakia
3
Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
4
Department of Botany, Islamia College Peshawar, Peshawar 19002, Pakistan
*
Author to whom correspondence should be addressed.
Energies 2022, 15(20), 7818; https://doi.org/10.3390/en15207818
Submission received: 17 July 2022 / Revised: 3 October 2022 / Accepted: 19 October 2022 / Published: 21 October 2022
(This article belongs to the Special Issue Plant Biomass for Chemicals and Biofuels Applications)
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Abstract

:
Biodiesel is considered valuable to reduce dependency on petrofuels. This work aimed to synthesize biodiesel from Silybum marianum using synthesized ZnO nanoparticles as a catalyst. The synthesized ZnO nanoparticles were examined by scanning electron microscopy and X-ray diffraction for confirmation. The synthesized biodiesel was confirmed by ASTM D-6751, H and C-NMR, GC-MS, and FT-IR spectroscopy. The optimum biodiesel yield of 91% was obtained with an oil-to-methanol ratio of 1:24, 15 mg of catalyst concentration, 60 °C temperature, and 45 min of reaction time. Fuel properties were determined according to the ASTM-defined methods and found within the defined limits of ASTM D-6751. 1H-NMR and 13C-NMR showed characteristic peaks at 3.667 ppm, 2.000–2.060 ppm, 0.858–0.918 ppm, 5.288–5.407 ppm, 24.93–34.22 ppm, 172.71, 173.12, 130.16 ppm, and 128.14 ppm, respectively, which confirm biodiesel synthesis. The FAMEs composition of biodiesel was determined by GC-MS, which recognized 19 peaks for different types of FAMEs. FT-IR spectroscopy showed two main peaks, first in the range of 1725–1750 cm−1 and second in the range of 1000–1300 cm−1, which confirmed that the transesterification process had completed successfully. The physicochemical characteristics of Silybum marianum confirm that it is a suitable source to produce biodiesel on an industrial scale.

1. Introduction

Currently, the world is confronting an intense shortage of petrofuels. In this scenario, the demand for alternative energy resources is increasing, whilst also being time consuming. Biodiesel is considered valuable because of the reduction in demand for petrofuels and is economically important. Furthermore, biodiesel is environmentally friendly because it reduces greenhouse gas emissions, and it is also important socially, due to the generation of new employment and income opportunities [1]. It can be synthesized from animal fats or vegetable oils using the transesterification technique. In this technique, alcohol and fats or oil are reacted in the presence of a catalyst at a specific temperature, resulting in the formation of fatty acid methyl esters (FAME) [2,3]. However, the use of oil obtained from plants is a competitor source for biodiesel production with regard to animal fats [4].
In the presence of either a base or an acid catalyst, the biodiesel synthesis process is carried out efficiently. There are two main disadvantages which make acid catalysts not suitable for commercial purposes: equipment corrosion and a slower reaction rate. Base catalysts are much more efficient due to the higher rate of reaction (4000 times more) compared to acidic catalysts [5,6]. Although base catalysts are widely available and cheap, base catalysts still have one main drawback: these catalysts work best when the oil has a free fatty acid (FFA) content below 0.5%; at higher concentrations of FFA, their catalytic activity is reduced [7]. Biodiesel synthesis is commonly carried out on an industrial scale in the presence of homogeneous catalysts (sodium hydroxides or alkoxides). However, homogeneous catalysts have some limitations; for example, homogeneous catalysts are not recyclable and are not environmentally friendly due to forming biodiesel polluted with potassium or sodium ions [8]. Furthermore, these catalysts also favor more soap formation during biodiesel synthesis. Moreover, homogenous catalysts become less active when the feedstock has a humidity greater than 0.3 wt.% [8,9]. Thus, these catalysts need pure raw materials from feedstock to synthesize biodiesel.
Animal fats, cooking oils, non-edible plant oils, etc., have been used as feedstock to synthesize biodiesel [10]. However, the main problems with these feedstocks are their high humidity (~3% by weight) and FFA content (up to 12%); in the synthesis of biodiesel, these factors do not favor the use of homogeneous catalysts. To solve this problem, researchers have synthesized nanocatalysts, which have many advantages over homogeneous catalysts, such as their good performance as a catalyst and recyclability at the end of reactions. Furthermore, these catalysts maintain their activity even at high FFA content and humidity of the feedstock [7]. Furthermore, compared to ordinary catalysts, nanocatalysts are more effective because of their ultra-small size (10–80 nm), which provides a substantial surface area-to-volume ratio, and nano-sized materials have characteristics that those same materials do not have when they are macroscopic sized [11]. Furthermore, the catalytic activity of a nanoparticle is significantly influenced by its size and distance between the nanoparticles [7]. The methods of catalyst preparation are also important [6,11,12,13,14,15,16].
Silybum marianum (L.) Gaertn., commonly known as milk thistle, is an annual herb of the family Asteraceae. The plant is native to the Mediterranean region of Europe, but is commonly found in the subtropical and temperate region of Pakistan. The plant grows in the wild and also as a weed in wheat crop fields. The plant grows to a height of 1–2 m. The stem of the plant is glabrous. The leaves are oblong to lanceolate and shiny green, with milky white veins and spines on the edges. The flower heads are 4–12 cm long and wide and have a red or purple color. The flower heads are surrounded by bracts, which are hairless and with triangular spiny edges. The seeds of the plant start germination in October and continue through December. The plant produces seeds from February to March [17]. This study aimed to synthesize biodiesel from milk thistle (Silybum marianum (L.) Gaertn.) using green synthesized ZnO nanoparticles as a catalyst.

2. Materials and Methods

2.1. Extract Preparation

Fresh plant materials from Silybum marianum were taken and washed three times with double distilled water to eradicate dust and pollution. The cleaned materials were kept in an oven at 60 °C for 30 min to dry. The dried leaves were ground to a fine powder in a Panasonic miller grinder. A 15 g amount of powdered plant material was then boiled (100 °C) in 120 mL of deionized water for one hour with constant stirring on a hot plate. The color of the extract obtained was dark green. The mixture was then cooled to room temperature. This solution was filtered three times through Whatman No. 42 filter paper. The pH of the extract was 5.9. The extract was stored in a refrigerator at 4 °C for future use.

2.2. ZnO Catalyst Synthesis

Hydrated zinc nitrate Zn (NO3)2.6H2O of analytical grade (Merck, Kenilworth, NJ, USA) was used as a zinc precursor. Approximately 100 mL of the plant material extract was placed in a conical flask and 6 g of zinc nitrate was added to it. The mixture was boiled (100 °C) under constant stirring on hotplate until a light-yellow color suspension was formed. The precipitate was washed three times with deionized water and centrifuged at 1000 rps for 10 min. Subsequently, it was calcined in a furnace at 450 °C for two hours. A pale white crystalline powder of ZnO was obtained after calcination. The powder was stored carefully for further analysis. The JCPDS file number (Joint Committee on Powder Diffraction Standards) of synthesized ZnO is 800075 [18].

2.3. Catalyst Characterizations by XRD and SEM

XRD (Model No. D8 Advance Bruker) was used for the characterization of catalysts used in the present work to safeguard the establishment of a desired crystal-like assembly of all nanoparticles. With the help of the Scherrer equation [19], the calculation was completed, providing a heterogeneous ordinary diameter of the nanoparticles.
The Scherrer equation can be written as
τ = K λ β cos θ
where
τ is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size, which may be smaller or equal to the particle size;
K is a dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about 0.9, but varies with the actual shape of the crystallite;
λ is the X-ray wavelength;
β is the line broadening at half the maximum intensity (FWHM) after subtracting the instrumental line broadening (in radians);
θ is the Bragg angle.
All measurements were made between 10 and 60 °C XRD analysis was performed at 2θ with a range of 10–90° using a SHIMADZU RF-6000 diffractometer equipped with a Cukα (K = 1.54 A°) source, maintaining an applied voltage of 40 kV and a current of 30 mA. The detector used has an SNR (signal-to-noise Ratio) of 1000 or more. The detector enables high-speed scanning of 60,000 nm/min to minimize the scan time.
SEM was achieved using SEM, Model JEOL JSM-5910, and HT7800 Ruli. Scanned images were obtained by operating-field emissions from a SEM microscope with 20 kV accelerating voltage. The chemical composition of the synthesized nanoparticles was determined using Energy Dispersive X-ray analysis, which aided in the interpretation of the phenomena that occurred during calcination and pretreatment and permits qualitative characterization of the surface of catalysts.
The magnitude of the XRD points of the sample reveals that the designed nanoparticles remain crystalline, and wide-ranging diffraction peaks specify the very small crystallite [20].
D = k λ β cos θ
where k is the shape factor = 0.9, λ is the radiation wavelength (1.54 Å), and β is the full width of half of the maximum intensity (FWHM; in radians).

2.4. Oil Extraction from Feedstock

The seeds were collected from various parts of the Tehsil Khadukhel district in Buner, northern Pakistan. The collected seeds were washed with half-warm distilled water to remove the dust and then dried in an oven at 50 °C. A 10 g amount of dried seeds was ground to a fine powder using a mortar and pestle. This fine powder was then subjected to a Soxhlet extractor for the chemical extraction of oil using ether as the solvent. After extraction, the solvent was recycled with the help of a rotary evaporator at 55 °C under moderate vacuum. The total oil content of the feedstock was then calculated with the help of the mathematical formula given below [21]:
O i l   c o n t e n t s   p e r c e n t a g e   W 4 = W 3 + W 1 W 2
where W1 is the weight of the empty flask, W2 is the weight of the seed powder before extraction, W3 is the weight of the flask and oil extracted, and W4 is the weight of oil extracted through the Soxhlet extractor from the seed powder of the feedstock.
The chemical extraction was followed by the mechanical extraction of oil from the feedstock to extract oil in a bulk amount. The oil was extracted using an electrical expeller Model YZS-130A/C. The crude oil was filtered three times using Whatman No. 42 filter paper and stored in a glass jar for further experimental work.

2.5. Free Fatty Acid Content Determination

The crude oil was analyzed for FFA content to make an appropriate adaptation of the method for the synthesizing of biodiesel. The FFA content was determined using the Ullah et al. titration method [22]. After titration, the FFA content of the feedstock was calculated to be 0.74 mg KOH/g with the help of the formula given below:
P e r c e n t a g e   o f   F F A = ( A − B ) × C V × 100
where
  • A = potassium hydroxide (KOH) volume used in the sample titration;
  • B = potassium hydroxide (KOH) volume used in blank titration;
  • C = potassium hydroxide (KOH) concentration (g/L);
  • V = volume of oil sample.

2.6. Biodiesel Synthesis Process

To synthesize biodiesel, we adopted the transesterification method, as the FFA content of the feedstock was 0.74 mg KOH/g.
Energies 15 07818 i001
The biodiesel yield in percent after the end of the experiment was calculated by using the following formula [23]:
P e r c e n t a g e   y i e l d   o f   B i o d i e s e l = B i o d i e s e l   p r o d u c e d O i l   s a m p l e   u s e d   i n   r e a c t i o n × 100

2.7. Fuel Property Determination

The following fuel properties (as defined by ASTM for biodiesel) were determined for biodiesel synthesized from Silybum marianum: acid value (KOH mg/kg), flash point PMCC (°C), density at 15 °C (kg/L), kinematic viscosity at 40 °C CST, pour point (°C), cloud point (°C), sulphur (wt%), calorific value kJ/kg, cetane no., oxidative stability at 110 °C (h), refractive index at 20 °C, iodine number (mg I2/100), higher heating value (MJ/kg), and distillation temperature for 90% recovery. These were all analyzed according to the ASTM methods (Table 1) [24,25].

2.8. Chemical Assessment of Biodiesel Synthesis

FT-IR, GC-MS, and NMR spectroscopic analyses were performed to confirm the synthesis and to study of the chemical properties of SMB.

2.9. Fourier Transform Infrared Analysis of SMB

FT-IR spectroscopy was performed using a VARIAN AA280Z atomic absorption spectrometer with a GTA-120 graphite tube atomizer in the range of 400–4000 cm−1 for confirmation of the successful occurrence of the transesterification process and biodiesel synthesis.

2.10. NMR Spectroscopy of SMB

1H and 13C NMR spectroscopic analyses were performed at 21 °C on 11.75 T using an Avance NEO Bunker 600 MHz spectrometer equipped with a 5 mm BBFO smart probe using chloroform-D and Si(CH3)4 solvents as internal standards for authentication. To record the 1H NMR (300 MHz) spectrum, the pulse duration was set at 30 °C, with a recycle delay of 1.0 and 8 scans. Similarly, the 13C NMR spectrum (75 MHz) was recorded with a pulse duration of 30°, with a recycle delay of 1.89 and 160 scans. All chemical transformations were reported in ppm relative to the residual solvent peak. The following formula was used to calculate the conversion percentage [26]:
Percentage of Biofuel, C = 100 × 2AMe/3ACH2
where
  • C = oil-to-biodiesel conversion percentage;
  • AMe = methoxy proton integration value in biodiesel;
  • ACH2 = α-methylene proton integration value in biodiesel.

2.11. GC-MS Determination of FAMEs

The synthesized biodiesel was analyzed both qualitatively and quantitatively for various FAMEs by GC-MS spectroscopy; for this purpose, a QP 2010 Plus (Shimadzu, Japan) spectrometer was used. To analyze the biodiesel, 1 μL of sample was injected into GC-MS. Hexane was used as a solvent, and helium was used as a carrier gas. The gas chromatographic separation was carried out at a column temperature of 50–300 °C. Injector and detector temperature were maintained at 250 °C.

3. Results and Discussion

3.1. X-ray Diffraction of ZnO Nanocatalysts

The XRD diffractogram obtained for ZnO nanoparticles (Figure 1) indicates strong deflection heights at 2 θ angles of 31.77, 34.35, 35.86, 47.25 56.50 62.53 66.18, 67.69, 68.76, 72.42, and 76.50, corresponding to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes, respectively.
In the XRD pattern, no additional peaks were observed for any other compound of zinc or metallic zinc. Furthermore, the sharpness and intense diffraction of obtained peaks are indications of crystallization of the nanoparticles. These characteristics show that the synthesized substance is pure ZnO nanoparticles [27].

3.2. Scanning Electron Microscopic Study of ZnO

The surface and morphological characterizations of the ZnO nanoparticles were studied by SEM, using a Jeol Model appliance (JSM-6390LV). The synthesized nanoparticles are spherical in shape and densely packed, with a size of 43 nm (Figure 2). The size of the nanoparticles was determined with the help of the Scherrer equation [19].
Furthermore, the particles have less agglomeration, good connectivity, and homogeneity among the particles. Moreover, the surface of the particles is porous, which may be due to the release of hot gases from the reaction mixture during the combustion process [27,28]. ZnO-ZnO nanoparticles bond to each other due to the presence of nodule formation [29]. The ordered and regular shape of particles is a sign of a better-interconnected regular pore distribution system [30].

3.3. Oil Extraction and FFA Content Determination

We used two approaches (chemical through Soxhlet extractor and mechanical) for oil extraction to compare our result with the international standard techniques stated by various researchers in their work. The Soxhlet extraction technique is specified by the European Union for the extraction of oil from feedstock. Furthermore, these techniques are cost effective and easy to carry out [22]. The oil content in the feedstock used in this work was 45.2%. Other researchers have reported similar results [24,30]. Furthermore, the FFA contents were also determined through the acid titration method prior to the biodiesel synthesis process. The FFA content is reported as 0.74mg of KOH/g, which is below the internationally specified limit. It has been reported previously that when the FFA content of feedstock exceeds 3%, the oil-to-biodiesel conversion efficacy declines gradually [15].

3.4. Biodiesel Synthesis and Optimization

To determine conditions suitable for maximum biodiesel yield, a series of experiments were accomplished considering four main reactions variables: the oil-to-methanol ratio, catalyst concentration, reaction time, and temperature.

3.4.1. Oil-to-Methanol Ratio

The oil-to-methanol ratio has a profound effect on the biodiesel yield; increasing the amount of methanol with respect to oil increases the biodiesel yield [22,31]. Therefore, we varied this ratio, using values of 1:6, 1:12, 1:18, 1:24, and 1:30. The reported results clearly show that the highest biodiesel yield was obtained at 1:24 oil/methanol ratio. (Figure 3). Other researchers have reported similar results [22,24,32]. Furthermore, according to the literature, the minimum stoichiometric molar ratio for the transesterification of alcohol to oil is 3:1.
The transesterification reaction is reversible; therefore, a higher molar ratio enhances the miscibility as well as the interaction among the triglycerides and alcohol molecules. According to the literature, for the completion of the reaction or maximum biodiesel yield, the molar ratio would be kept higher than the stoichiometric ratio [33]. To break the links among glycerin and fatty acids during the transesterification reaction, the presence of an adequate amount of methanol is desirable [34]. However, a very high amount of methanol must be avoided because higher oil-to-methanol ratios cannot optimize the biodiesel yield and ester content; it also complicates the ester recovery process and increases the cost [22]. Furthermore, in the biodiesel synthesis process, a high amount of methanol of more than 1:70 molar ratio slows the separation of glycerol and ester [35].

3.4.2. Catalyst Concentration

The catalyst plays an important role in biodiesel yield; the efficient conversion of oil to methyl ester during transesterification is only possible in the presence of a suitable catalyst [31,36]. In this work, we studied the influence of catalyst concentration by using values of 5 mg, 10 mg, 15 mg, 20 mg, and 25 mg to achieve the optimum yield of biodiesel. The result clearly shows that the optimal yield was obtained at 15 mg of catalyst concentration (Figure 4).
According to Bojan and Durairaj [37] and Leung and Guo [38], a high concentration of catalyst causes soap formation due to emulsification. Furthermore, the large amount of catalyst increases the viscosity of the reactants, which results in a reduction in the biodiesel yield [22].

3.4.3. Reaction Temperature

To synthesize biodiesel on a large scale in industries, a low temperature is needed for transesterification because a high temperature leads to an increase in energy cost. Therefore, during the transesterification reaction, we used reaction temperatures of 45, 50, 55, 60, and 65 °C, respectively. Our findings clearly show the influence of temperature on biodiesel yield; it was observed that when the temperature was increased from 55 °C to 60 °C, the biodiesel yield rose to 91% (Figure 5).
The reaction temperature markedly influenced the biodiesel yield, as a high reaction temperature increases the reaction speed and minimizes the duration of the reaction because it lowers the viscosity of the reaction medium, increases the solubility of alcohol, and accelerates the transfer rate of the reagent [22,39,40]. Other researchers have reported similar results [22,40,41,42]. The decrease in biodiesel amount at high temperatures (more than 65 °C) may be due to the high miscibility, which results in a reduction in phase separation as well as yield [22].

3.4.4. Reaction Time

To produce biodiesel on a large scale for industry, time is a primary parameter and should be minimized because extended reaction time increases energy expenditure and thus the cost of the product [40,41,42,43]. Therefore, we varied the duration of the reaction—using values of 20, 40, 60, 80, and 100 min while keeping other conditions constant—to determine the ideal time for optimum biodiesel yield. The result of this study clearly shows that an optimal yield (91%) of biodiesel was obtained at 45 min, and no change in yield was observed at 60 or 75 min (Figure 6).
Similar results were reported by Ahmad [16], Ullah [22], Barros [40], and Gebremariam [42].

3.5. Physical and Fuel Properties of Biodiesel

The SMB biodiesel prepared in this study had a 0.74 mg KOH/g acid value, which was below the standard range defined by ASTM D-644 (0.80 mg KOH/g). The acid value of SMB was more or less similar to the values reported by Vázquez [44]. The acid value of biodiesel is also influenced by its purity quality. Furthermore, the kinematic viscosity of SMB was calculated as 4.69 mm2/s at 40 °C, which is within the limit of the ASTM D-6751 standard. The kinematic viscosity of SMB was also found to be close to that of fossil diesel [22]. Moreover, the density of obtained SMB was 0.857 kg/L, which is within the ASTM D-6751 standard range [22,45]. According to Kaisan [46] and Ullah [22], due to high oxygen content, SMB biodiesel has a lower calorific value than fossil diesel. The calorific value of SMB is also within the limit of ASTM D-6751 (Table 1).
The EN 14214 standard states that biodiesel must have a flash point of more than 120 °C, while the ASTM D-6751 allows the range to be below 130 °C. The methanol content of any biodiesel has a pronounced effect on the flash point [22]. According to Ullah [22], when the methanol content of biodiesel increases by 0.5%, its flash point decreases by 50 °C. The SMB flash point in this study was calculated as 87 °C (Table 1); which is within the EN 14214 and ASTM D 6751-02 standard range. According to previous studies, the range of flashpoints for biodiesel is 160–202 °C [43,44].
The cetane number of SMB in this study was determined as 49 (Table 1), which is just above the ASTM D-6751 standard. If the cetane number is above the limit, it is possible to adjust it by adding a small amount of nitric acid iso-octyl to achieve the conformation quality [11,47,48,49]. It has been indicated that there is an inverse proportionality between the degree of unsaturation of fatty acids and the cetane number [22].
The CP and PP of SMB in this study were determined as −2 °C and −8 °C, respectively. There is no limit for CP and PP according to the ASTM D 6751 standard. However, Mofijur et al. [50] specified values for CP and PP. The sulfur content in this study was determined to be 0.00018 ppm, which is in the range limit of ASTM D 4294 (0.05 ppm). Therefore, SMB can be considered as an environmentally friendly fuel [22,34].
In the current study, we also determined the oxidative stability of SMB to be 2.8 min, which is within the limit defined by ASTM D-6751 (3 min). From an environmental perspective, susceptibility to oxidation is desirable [51]; however, oxidative stability is the main drawback impeding commercial use in vehicles. During storage or use, oxidation alters the tri-biological and physicochemical properties of biodiesel. The adverse effects of this phenomenon are incensement in insoluble deposits, acid number, density, iodine value, kinematic viscosity, polymer content, and peroxide value. However, the oxidative instability of biodiesel can be minimized by adding various antioxidants [52,53]. Other researchers have reported similar results [24].
Another important aspect of fuel quality is its water content. Biodiesel is composed of fatty acid methyl esters (FAMEs), which are more hygroscopic, thus making the biodiesel more hydrophilic than fossil diesel. Therefore, biodiesel absorbs more moisture than fossil fuel. Free water favors biological growth in fuel tanks, which may result in fuel tank corrosion (made up of iron and steel) as well as slime and sludge formation, and may consequently lead to blockage of engine filters as well as fuel pipes. This may damage the vehicle fuel injection system [54]. In this study, the water content for SMB was determined as 0.039, which is below the limit specified by ASTM D-6751.
The degree of saturation and unsaturation and the oxidative stability of biodiesel can be estimated from its iodine content value [55]. For the current biodiesel, the iodine content was determined as 124 (mg I2/100), which is within the limit specified by ASTM D-6751. In this work, the refractive index was obtained as 1.437, which confirms the fact that crude oil was transformed into biodiesel during the transesterification reaction [56].
Another important fuel property of biodiesel is the higher heating value (HHV) that defines the fuel efficiency and energy content [57,58]. HHV can be estimated from the fatty acid composition, the iodine content, and the saponification value of biodiesel [58,59]. Biodiesel has an HHV of 39 to 43 MJ/kg, which is slightly lower than that of diesel (49.65 MJ/kg) [58,60]. The HHV of SMB in the current study was determined as 40.86 MJ/kg.

3.6. H-NMR Study of SMB

The characteristic singlet peak obtained at 3.667 ppm confirmed the presence of methoxy protons (-OCH3). Furthermore, alpha-methylene proton (α-CH2) peaks were observed from 2.000 to 2.060 ppm. These two peaks confirmed the formation of FAMEs from triglycerides. The other important peaks observed were for terminal methyl protons (CH3) at 0.858–0.918 ppm. The next strong indication was the peak obtained from 1.254 to 1.682 ppm for beta-carbonyl methylene protons (Figure 7).
The presence of olefinic hydrogen [61] was confirmed by the multiple peaks obtained at 5.288–5.407 ppm. The obtained peaks confirm the successful transformation of triglycerides into FAMEs [22,62].

3.7. 13C-NMR Study of SMB

The presence of long-chain ethylene carbons (-CH2-) in this study was confirmed by the occurrence of peaks at 24.93–34.22 ppm. Peaks at 172.71, 173.12, and 130.16 ppm confirmed carbonyl carbon (-CO) and unsaturation position in the SMB. Furthermore, the peak at 128.14 ppm is an indication of vinylic (C=H) substituent (Figure 8).
Peaks for these functional groups in the specified range have also been reported in other studies [63].

3.8. Qualitative and Quantitative Analysis of SMB

Gas chromatography and mass spectroscopy were performed to study the composition of FAMEs produced during the transesterification process of biodiesel synthesis. The results of gas chromatographic and mass spectrometric analysis show 19 peaks of different types of fatty acids methyl esters. Every single peak represents a specific FAME and was confirmed by comparison of measured mass spectra with the mass spectrometry database NIST. The retention time data identify each FAME after mass spectrometric analysis (Table 2). The analysis shows that linoleic acid methyl ester, palmitic acid methyl ester, oleic acid methyl ester, and arachidic acid methyl ester are the major FAMEs identified in the SMB. The mass fragmentation patterns and retention time of the eluted components were used for the confirmation of various FAMEs. It is evident from the GC-MS data that the SMB is primarily composed of various FAMEs (Figure 9).
In the present study, 19 different types of FAMEs in SMB have been reported for the first time (Table 2). The presence of long-chain fatty acids in the FAME profile indicates improved properties of biodiesel and, therefore, high fuel efficiency [64]. Other researchers using the same feedstock reported lower numbers of FAMEs; for example, Takase et al. [24] reported five FAMEs, Ullah et al. [63] reported seven FAMEs, and Fadhil et al. [25] reported six FAMEs.

3.9. FT-IR Analysis of SMB

In the biodiesel samples for the identification of the various functional groups and types of binding linkages, corresponding to various vibrations of stretching and bending, FT-IR spectroscopy was used. Oils and esters are known to be fairly strong absorbers in the infrared region of the electromagnetic spectrum [65,66]. Fourier transform infrared (FT-IR) spectroscopy has been proven to be a powerful analytical tool for identifying macromolecular pools (e.g., proteins, lipids, and carbohydrates) and monitoring biochemical changes [67]. According to Soon et al. [68], the position of the carbonyl group is sensitive to the molecular structure as well as substituent effects. In the FT-IR spectroscopy of the SMB, the stretch for terminal (vinyl) C-H was observed at 3011 cm−1. Stretching bands for methyl and methylene were attained at 2856 cm−1 and 2926 cm−1, respectively. The methoxycarbonyl (methyl ester) group was observed at 1742 cm−1. Stretching for alkenyl was obtained at 1648 cm−1. Peaks for methyl C-H bending were obtained at 1436 cm−1 and 1463 cm−1. The peak for aromatic tertiary amine C-H stretching was obtained at 1360 cm−1. The peak for aromatic ether (aryl–O) stretching was obtained at 1245 cm−1. Stretching for organic sulfates was obtained at 1198 cm−1. The stretching peak obtained at 1172 cm−1 is for the secondary amine (C-N). The peak for the cyclohexane ring C-O was obtained at 1018 cm−1. The peak for methine skeletal C-C vibration was obtained at 913 cm−1. The peak for vinylidene C-H was obtained at 883 cm−1. The peak for peroxide C-O-O stretching was observed at 843 cm−1. The peak for methylene –(CH2)n– rocking was obtained at 722 cm−1. The peak obtained at 831.51 cm−1 confirmed the presence of C-O-O stretching, and the last peak obtained at 603 cm−1 is for disulfide stretching (S-S) (Figure 10) [69].
Through Fourier-Transform Infrared Spectroscopy (FTIR), it was possible to confirm the synthesis of biodiesel. Various functional groups were confirmed during the transesterification process. Two main peaks confirm the ester formation; one is carbonyl, for which the peak range is 1725–1750 cm−1, and the other is C-O, for which the peak range is 1000–1300 cm−1 [22]. Another strong characteristic peak obtained at 1245 cm−1 also confirms the formation of methyl esters [70,71]. The peaks obtained in FT-IR spectroscopy confirm that the transesterification process occurred successfully, and that it is also a suitable method for the conversion of triglyceride to fatty acid methyl esters [72,73].

4. Conclusions

In the near future, due to the increase in demand for energy and the depletion of fossil fuels reservoirs, the demand for biodiesel is expected to increase rapidly. Silybum marianum (L.) Gaertn., commonly known as milk thistle, is an annual herb of the family Asteraceae. In this study, the transesterification technique was used to synthesize biodiesel using Silybum marianum as feedstock and ZnO as a catalyst. The oil content of the feedstock was 45.2%, with an acid value of 0.74 mg KOH/g. To achieve the optimum yield of biodiesel, five variables were explored: oil-to-methanol molar ratio, catalyst concentration, reaction temperature, and reaction time. The optimum yield of 91% was obtained at 1:24 oil-to-methanol ratio, 15 mg catalyst concentration, 60 °C temperature, and 45 min reaction time. The fuel properties were tested through standard methods of ASTM and were found within the limits specified by ASTM D-6751. 1H-NMR and 13C-NMR, GC-MS, and FT-IR spectroscopies were performed to confirm the successful occurrence of the transesterification process. Monitoring the quality of manufactured biodiesel and its potential for reliable commercialization was the major goal of numerous analytical method applications and fuel characteristic tests. All physicochemical characteristics in this study clearly show that Silybum marianum could be a potential non-edible feedstock for the biodiesel industry because it is economical, indigenously available, and environmentally friendly and can be easily grown under various environmental conditions.

Author Contributions

All authors declare to have made equal, direct, and intellectual contributions and have approved the current work for publication in this journal. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially supported by the Slovak Research and Development Agency (APVV) under the contracts APVV-14-0393 and APVV-16-0088, and the Researchers Supporting Project (number RSP-2021/368), King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are thankful to the Slovak Research and Development Agency under the contracts APVV-14-0393 and APVV-16-0088, and by the Slovak Scientific Grant Agency VEGA by the contract VEGA 1/0012/19, and the Researchers Supporting Project number (RSP-2021/368), King Saud University, Riyadh, Saudi Arabia.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

ASTMAmerican Society for Testing and Materials
CPCloud Point
FAMEFatty Acid Methyl Esters
FFAFree Fatty Acid
GC-MSGas Chromatography–Mass Spectrometry
FT-IR spectroscopyFourier Transform Infrared Spectroscopy
H and C-NMRNuclear Magnetic Resonance
HHVHigher Heating Value
PMCCFlash Point (Pensky-Martens Closed Cup)
PPPour Point
SEMScanning Electron Microscopy
SMBSilybum Marianum Biodiesel
XRDX-ray Diffraction
ZnOZinc Oxide

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Figure 1. XRD analysis spectrum patterns confirming the synthesis of ZnO nanoparticles.
Figure 1. XRD analysis spectrum patterns confirming the synthesis of ZnO nanoparticles.
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Figure 2. SEM analysis of ZnO nanoparticles to confirm their synthesis and determine their size: (a) 1 µm × 5000; (b) 1 µm × 10,000; (c) 1 µm × 20,000; (d) 1 µm × 30,000.
Figure 2. SEM analysis of ZnO nanoparticles to confirm their synthesis and determine their size: (a) 1 µm × 5000; (b) 1 µm × 10,000; (c) 1 µm × 20,000; (d) 1 µm × 30,000.
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Figure 3. Oil-to-methanol ratio has a positive effect on biodiesel yield and a negative effect on glycerin formation.
Figure 3. Oil-to-methanol ratio has a positive effect on biodiesel yield and a negative effect on glycerin formation.
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Figure 4. The effect of catalyst concentration on biodiesel yield.
Figure 4. The effect of catalyst concentration on biodiesel yield.
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Figure 5. The influence of reaction temperature on biodiesel yield and on glycerin formation.
Figure 5. The influence of reaction temperature on biodiesel yield and on glycerin formation.
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Figure 6. The influence of reaction time on biodiesel yield and on glycerin formation reduction.
Figure 6. The influence of reaction time on biodiesel yield and on glycerin formation reduction.
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Figure 7. 1H-NMR spectroscopy confirming the synthesis of biodiesel through various important peaks.
Figure 7. 1H-NMR spectroscopy confirming the synthesis of biodiesel through various important peaks.
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Figure 8. 13C-NMR spectroscopy confirmation of transesterification process.
Figure 8. 13C-NMR spectroscopy confirmation of transesterification process.
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Figure 9. GC-MS analysis of SMB composition of different FAMEs formed in transesterification reaction.
Figure 9. GC-MS analysis of SMB composition of different FAMEs formed in transesterification reaction.
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Figure 10. FT-IR spectroscopy result confirming SMB formation through peaks for important functional groups.
Figure 10. FT-IR spectroscopy result confirming SMB formation through peaks for important functional groups.
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Table
Table 1. Fuel properties of the Silybum marianum biodiesel and ASTM standard method values.
Table 1. Fuel properties of the Silybum marianum biodiesel and ASTM standard method values.
Fuel PropertyASTM Methods1 ASTM D67512 SMB
Acid value (KOH mg/kg)ASTM D-6640.800.74
Flash Point PMCC (°C)D-93>9387
Density at 15 °C (kg/L)D-40520.820–0.9000.857
Kinematic Viscosity at 40 °C (mm2/s)D-4451.9–64.69
Pour Point (°C)D-97-12−15 to 16−8
Cloud Point (°C)D-2500-11−3 to 12−2
Sulphur (wt%)D-54530.0070.00018
Calorific Value (kJ/kg)D-586535,00026,984
Cetane no.D-6134549
Oxidative stability at 110 °C (min)EN-1411232.8
Water content (mg/kg)ASTM D-6304≤0.050.039
Refractive index at 20 °CASTM D-1747----1.437
Iodine number (mg I2/100)ASTM D-4607≤120124
Higher heating value (MJ/kg)ASTM D-24039–4340.86
Distillation temperature for 90% recoveryASTM D-1160-06360345
1 ASTM D6751—Value Test Limit; 2 SMB—Silybum marianum biodiesel.
Table
Table 2. FAME composition of Silybum marianum biodiesel determined by GC-MS.
Table 2. FAME composition of Silybum marianum biodiesel determined by GC-MS.
No.Identified FAMEsFormulaCAS*RT*C*
1Caprylic acid methyl esterC9H18O2111-11-54.7390.16
2Myristic acid methyl esterC15H30O2124-10-710.1680.34
3Pentadecanoic acid methyl esterC16H32O27132-64-111.2671.08
4Palmitic acid methyl esterC17H34O2112-39-013.42416.11
5Palmitoleic acid methyl esterC17H32O21120-25-813.8780.12
6Margaric acid methyl esterC18H36O21731-92-615.5000.21
7Heptadecenoic acid methyl esterC18H36O21731-92-615.9430.27
8Stearic acid methyl esterC19H38O2112-61-817.8972.33
9Oleic acid methyl esterC19H36O2112-62-918.3758.52
10Linoleic acid methyl esterC19H34O2112-63-019.65962.23
11Octadecenoic acid methyl esterC19H36O21937-62-820.8170.25
12Linolenic acid methyl esterC19H34O2112-63-021.7430.31
13Arachidic acid methyl esterC21H42O21120-28-124.6614.47
1411, 14-Eicosadienoic acid methyl esterC21H38O261012-46-226.9480.39
1511, 14, 17-Eicosenoic acid methyl esterC21H40O21120-28-129.2590.81
16Behenic acid methyl esterC23H46O2929-77-131.9940.83
17Erucic acid methyl esterC23H44O21120-34-933.9100.41
1813, 16-Docosadienoic acid methyl esterC23H42O261012-47-335.0300.19
19Nervonic acid methyl esterC25H48O22733-88-238.6110.97
CAS*—Chemical Abstracts Service; RT*—retention time (min); C*—concentration (%).
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Jan, H.A.; Šurina, I.; Al-Fatesh, A.S.; Almutlaq, A.M.; Wali, S.; Lisý, A. Biodiesel Synthesis from Milk Thistle (Silybum marianum (L.) Gaertn.) Seed Oil using ZnO Nanoparticles as a Catalyst. Energies 2022, 15, 7818. https://doi.org/10.3390/en15207818

AMA Style

Jan HA, Šurina I, Al-Fatesh AS, Almutlaq AM, Wali S, Lisý A. Biodiesel Synthesis from Milk Thistle (Silybum marianum (L.) Gaertn.) Seed Oil using ZnO Nanoparticles as a Catalyst. Energies. 2022; 15(20):7818. https://doi.org/10.3390/en15207818

Chicago/Turabian Style

Jan, Hammad Ahmad, Igor Šurina, Ahmed S. Al-Fatesh, Abdulaziz M. Almutlaq, Sher Wali, and Anton Lisý. 2022. "Biodiesel Synthesis from Milk Thistle (Silybum marianum (L.) Gaertn.) Seed Oil using ZnO Nanoparticles as a Catalyst" Energies 15, no. 20: 7818. https://doi.org/10.3390/en15207818

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