A Novel Heterogeneous Superoxide Support-Coated Catalyst for Production of Biodiesel from Roasted and Unroasted Sinapis arvensis Seed Oil

Abstract

Disadvantages of biodiesel include consumption of edible oils for fuel production, generation of wastewater and inability to recycle catalysts during homogenously catalyzed transesterification. The aim of the current study was to utilize low-cost, inedible oil extracted from Sinapis arvensis seeds to produce biodiesel using a novel nano-composite superoxide heterogeneous catalyst. Sodium superoxide (NaO₂) was synthesized by reaction of sodium nitrate with hydrogen peroxide via spray pyrolysis, followed by coating onto a composite support material prepared from silicon dioxide, potassium ferricyanide and granite. The roasted (110 °C, 20 min) and unroasted S. arvensis seeds were subjected to high vacuum fractional distillation to afford fractions (F1, F2 and F3) that correlated to molecular weight. For example, F1 was enriched in palmitic acid (76–79%), F2 was enriched in oleic acid (69%) and F3 was enriched in erucic acid (61%). These fractions, as well as pure unroasted and roasted S. arvensis seed oils, were then transesterified using NaO₂/SiO₂/PFC/Granite to give biodiesel a maximum yield of 98.4% and 99.2%, respectively. In contrast, yields using immobilized lipase catalyst were considerably lower (78–85%). Fuel properties such as acid value, cetane number, density, iodine value, pour point, and saponification value were within the ranges specified in the American biodiesel standard, ASTM D6751, where applicable. These results indicated that the nano-composite catalyst was excellent for production of biodiesel from unroasted and roasted S. arvensis seed oil and its fractions.

1. Introduction

At present, fossil fuels account for approximately 80% of the total energy utilized by the transport sector. The impact of fossil fuel consumption has driven researchers to explore alternative fuels such as biodiesel. The industrial economies of many countries depend on non-renewable fossil resources like power plants, natural gas, petroleum and coal, heavy trucks, electric generators, and locomotive equipment. Current levels of energy consumption are not sustainable due to irregular distribution of petroleum throughout the world and economic, environmental and geopolitical considerations [1]. The increasing concern for environmental damage caused by fossil fuel emissions boosts the need for new sources of energy. Biofuels are gaining importance as environmentally friendly and renewable alternatives to fossil fuels around the world. Important biofuels include biogas, biodiesel, and bioethanol. As the name suggests, biodiesel is a renewable substitute or blend component for conventional petroleum diesel fuel.

Biodiesel is defined as fatty acid esters and is commonly produced catalytically by transesterification of lipids with methanol. The emissions generated by biodiesel during combustion are “recyclable” through plant photosynthesis. The CO₂ is released into the atmosphere when the biodiesel is burned but is recycled by the growing plants, which are later processed into fuel. As no sulfur components are present in biodiesel, the resulting exhaust gases contain essentially zero sulfur oxides. Biodiesel is also biodegradable and has a higher flash point (FP). However, important drawbacks include more nitrogen oxide emissions, less hydrolytic and oxidative stability and generally inferior low temperature performance [2,3,4,5,6,7,8,9,10,11,12,13].

Many different feedstocks can be used to produce biodiesel, but feedstock selection primarily depends on cost, oil content and regional availability. The price of biodiesel depends mostly on feedstock cost with minor contributions from other chemicals and solvents needed for production [14]. Commercial oil seed crops utilized for biodiesel production, such as soybean and canola, are relatively expensive and have competing food applications. Thus, several social movements and non-governmental organizations identify biodiesel production as a reason for higher edible oil prices, particularly in developing countries. Hence, non-edible oilseeds are gaining importance for biodiesel synthesis. Examples of biodiesel synthesized from vegetable oils include palm [15], rapeseed [16], jatropha [17], rubber seed, and numerous others described in the literature [2].

Wild mustard (Sinapis arvensis) is a promising potential feedstock for biodiesel production. The cost of producing S. arvensis seed oil is less than canola and Brassica napus. Moreover, the food chain is unaffected when S. arvensis seed oil is used as a biodiesel feedstock. This is because S. arvensis seed oil contains inedible erucic acid (>50%) [1]. S. arvensis seeds are hard and round and change color during maturation. Approximately 300 L of S. arvensis seed oil can be produced from 1200 kg of seed. Wild mustard yields approximately 590–875 kg of oil per ha. In addition, oil extraction costs are low. S. arvensis is thus an economically acceptable feedstock for biodiesel production [18].

The yield of biodiesel depends on various reaction conditions such as temperature, type and amount of catalyst, alcohol to oil ratio, nature of the feedstock, and reaction time [19]. Transesterification is conducted with catalysts that can be organic, acidic, or basic in nature. The economics depend on quantity and type of catalyst along with feedstock cost. Process costs are reduced due to glycerol, a co-product generated during transesterification. In fact, 66% of world-wide glycerol is from the biodiesel industry [20]. Glycerol is an important commercial starting material for food additives, surfactants, lubricants, beverages, pharmaceuticals, cosmetics, textiles, and many more [9]. The catalysts can either be homogeneous or heterogeneous [21]. Homogeneous catalysts are normally used for industrial production because of performance and economic considerations. However, recovery and reuse of dissolved homogeneous catalysts is generally not possible. Additionally, post-processing steps like neutralization, washing and dehydration are compulsory. As a result, homogeneous catalysts are not environmentally friendly [22]. Different biodiesel production methods have been extensively researched to resolve these problems, including heterogeneous catalytic processes by ion-exchange resins and metal oxides, non-catalytic supercritical methanol, and enzymatic processes [23]. Conventional heterogeneous catalysts for biodiesel production include alkali earth and transition metal oxides as well as mixed metal oxides [24] and lipase.

Heterogeneous (solid) catalysts have the general advantages of facile recoverability and reusability. As a result, aqueous treatment steps are not needed. Purification is thus much simpler and nearly theoretical yields of fatty acid methyl esters (FAME) are achieved [25]. However, heterogeneously-catalyzed transesterification requires more severe operating conditions, and catalytic activity is generally lower than homogeneous catalysts [26]. Moreover, one of the main problems with heterogeneous catalysts is their deactivation over time due to poisoning, coking, sintering, and/or leaching. The main advantages of utilizing a catalytic support are the elimination of the mass transfer limitation and low stability and high cost of precious metal catalysts in transesterification/esterification [27]. The use of easily available, low-cost waste materials for composite support preparation is highly economical. For example, approximately 250–400 tons of granite waste are generated annually from the cutting and finishing of granite blocks.

Therefore, we endeavored to prepare a novel nanosuperoxide catalyst supported on composite material in order to resolve the problems related to conventional heterogeneous catalysts. Superoxides are multifunctional inorganic compounds that are important for a broad array of technological applications, including heterogeneous catalysis. For example, superoxides are extensively used as redox catalysts [28]. Nemade and Waghuley [29], reported the synthesis of sodium superoxide (NaO₂) nanoparticles and used for gas sensing applications. Despite the considerable interest in superoxide catalytic systems, superoxides have not been studied for biodiesel production. Sodium superoxide (NaO₂) nanoparticles can be synthesized using spray pyrolysis under oxygen-rich conditions. Most importantly, NaO₂ nanoparticles exhibit higher sensing response, shorter response and recovery times, low operating temperature, and excellent stability. The nano sodium superoxide catalyst was coated onto a composite support material prepared from silicon dioxide, potassium ferricyanide and granite. For these reasons, our research focused on transesterification of S. arvensis seed oil using NaO₂/SiO₂/PFC/Granite catalyst and subsequent determination of the resulting fuel properties.

2. Results and Discussion

2.1. Effect of Roasting on Oil Yield

The mean oil yields for unroasted and roasted S. arvensis seeds were 35.4% and 39.2%, respectively (

Table 1. Oil yield from unroasted and roasted Sinapis arvensis seeds.

Seeds Type	Seed Weight (kg)	Oil Weight (kg)	Yield (%)
Unroasted seeds	5	1.77	35.4
Roasted seeds	5	1.96	39.2

). These results indicated that roasting increased oil yield significantly. This agreed with previous studies that showed increased oil yield upon roasting due to enhanced protein denaturation, which enhanced lipid extractability [30,31,32]. Although extraction of oil from roasted seeds was higher than from unroasted seeds, yields of biodiesel from unroasted and roasted seed oils were almost identical. For example, the maximum yields of biodiesel from unroasted and roasted S. arvensis seed oils were 98.4% (F2) and 99.2% (F2), respectively (

Table 2. Transesterification reaction parameters for unroasted S. arvensis seed oil. For each fraction, the catalyst amount that gave the highest yield was used for optimization of oil to methanol ratio.

Unroasted Oil	NaO2/SiO2/PFC/Granite (%)	Oil to Methanol Ratio	Biodiesel Yield (%)
Pure oil	0.25	1:0.3	87.3
	0.50	1:0.3	63.6
	1.00	1:0.3	56.0
	1.50	1:0.3	55.0
	2.00	1:0.3	45.0
	0.25	1:0.6	80.4
	0.25	1:0.9	85.0
	0.25	1:1.2	88.0
	0.25	1:1.5	72.6
Fraction F1	0.25	1:0.3	80.0
	0.50	1:0.3	86.0
	1.00	1:0.3	82.8
	1.50	1:0.3	77.6
	2.00	1:0.3	70.4
	0.50	1:0.6	85.6
	0.50	1:0.9	87.8
	0.50	1:1.2	88.6
	0.50	1:1.5	70.2
Fraction F2	0.25	1:0.3	81.8
	0.50	1:0.3	96.0
	1.00	1:0.3	90.8
	1.50	1:0.3	82.0
	2.00	1:0.3	33.6
	0.50	1:0.6	84.6
	0.50	1:0.9	81.5
	0.50	1:1.2	98.4
	0.50	1:1.5	86.8
Fraction F3	0.25	1:0.3	50.2
	0.50	1:0.3	54.4
	1.00	1:0.3	76.6
	1.50	1:0.3	60.1
	2.00	1:0.3	49.8
	1.00	1:0.6	70.1
	1.00	1:0.9	74.3
	1.00	1:1.2	88.0
	1.00	1:1.5	82.8

).

2.2. Effect of Catalyst Concentration

The concentration of NaO₂/SiO₂/PFC/Granite catalyst was varied from 0.25–2.00% while keeping all other reaction variables constant. The optimized catalyst concentrations were 0.25%, 0.50%, 0.50% and 1.00% for pure (unfractionated) unroasted S. arvensis seed oil, fraction F1, fraction F2, and fraction F3, respectively, with biodiesel yields of 87.3%, 86.0%, 96.0% and 76.6%, respectively (Table 2). The optimized catalyst concentrations were 0.50%, 1.00%, 1.50% and 0.50% for pure (unfractionated) roasted S. arvensis seed oil, fraction F1, fraction F2, and fraction F3, respectively, with biodiesel yields of 82.0%, 75.4%, 90.8% and 83.4%, respectively (

Table 3. Transesterification reaction parameters for roasted S. arvensis seed oil. For each fraction the catalyst amount that gave the highest yield was used for optimization of oil to methanol ratio.

Roasted Oil	NaO2/SiO2/PFC/Granite (%)	Oil to Methanol Ratio	Biodiesel Yield (%)
Pure oil	0.25	1:0.3	76.2
	0.50	1:0.3	82.0
	1.00	1:0.3	50.0
	1.50	1:0.3	48.0
	2.00	1:0.3	30.6
	0.50	1:0.6	80.4
	0.50	1:0.9	77.0
	0.50	1:1.2	89.0
	0.50	1:1.5	85.4
Fraction F1	0.25	1:0.3	69.6
	0.50	1:0.3	72.2
	1.00	1:0.3	75.4
	1.50	1:0.3	74.2
	2.00	1:0.3	71.3
	1.00	1:0.6	86.8
	1.00	1:0.9	80.8
	1.00	1:1.2	88.0
	1.00	1:1.5	81.1
Fraction F2	0.25	1:0.3	63.6
	0.50	1:0.3	72.2
	1.00	1:0.3	75.6
	1.50	1:0.3	90.8
	2.00	1:0.3	33.6
	1.50	1:0.6	79.2
	1.50	1:0.9	81.8
	1.50	1:0.15	82.1
	1.50	1:1.2	99.2
Fraction F3	0.25	1:0.3	57.8
	0.50	1:0.3	83.4
	1.00	1:0.3	80.8
	1.50	1:0.3	79.1
	2.00	1:0.3	65.4
	0.50	1:0.6	82.4
	0.50	1:0.9	82.0
	0.50	1:0.15	84.0
	0.50	1:1.2	87.4

). These results clearly showed that the nano-composite catalyst was efficient for biodiesel production from roasted and unroasted S. arvensis seed oils and their various fractions. The nature and characteristics of any catalyst can affect quality of the resulting biodiesel [33,34].

Enzymatic transesterification was also conducted. Immobilization of lipase facilitates low cost since it can be separated easily and reused [35]. The main drawback of immobilized lipase is mass transfer barriers owing to deactivation, enzyme entrapment and encapsulation during binding onto the surface of carriers [36]. The biodiesel yield of unroasted and roasted S. arvensis were 82.4% and 84.5%, respectively, using immobilized lipase (

Table 4. Yield of biodiesel from unroasted and roasted S. arvensis seed oil using immobilized lipase enzyme.

S. arvensis Seed Oil	Enzyme (%)	Biodiesel Yield (%)
		Pure Oil	F1	F2	F3
Unroasted	1.00	78	82.4	80.4	80.9
Roasted	1.00	84.5	81	80.7	79.3

). In summary, the newly developed nano-composite NaO₂ catalyst not only provided better yields but can also be used to overcome the aforementioned limitations related to conventional heterogeneous catalysts.

2.3. Effect of Oil to Methanol Molar Ratio on Biodiesel Yield (%)

The effect of oil to methanol molar ratio on biodiesel yield was investigated by varying the molar ratio and holding all other reaction parameters constant. The optimized catalyst amount for each respective fraction, as discussed in Section 2.2, was used for optimization of molar ratio. Five molar ratios (1:0.3, 1:0.6, 1:0.9, 1:1.2, and 1:1.5) of oil to methanol were investigated. Table 2 and Table 3 show that an optimized oil/methanol molar ratio of 1:1.2 provided the highest yields. Biodiesel yields of 98.4% and 99.2% were obtained from fraction F2 of unroasted S. arvensis seed oil and fraction F2 of roasted oil, respectively. A slight excess of methanol shifted the equilibria to the products, which enhanced conversion. Higher amounts of methanol had no major impact on yield, but negatively affected biodiesel yield by rendering glycerol separation more difficult due to increased solubility [37].

2.4. Fatty Acid Profile

The most prevalent fatty acids identified in roasted and unroasted S. arvensis seed oil (

Table 5. Fatty acid composition of unroasted and roasted S. arvensis seed oil and its fractions.

Fatty Acid	Percentage (%)
	Unroasted	F1	F2	F3	Roasted	F1	F2	F3
Palmitic acid (C16:0)	3.6	76.7	-	-	3.7	79.0	-	-
Stearic acid (C18:0)	1.1	12.1	-	-	1.2	12.7	-	-
Oleic acid (C18:1)	22.3	4.7	68.9	-	23.2	5.8	69.1	-
Linoleic acid (C18:2)	15.2	0.5	11.9	1.2	16.1	0.9	11.7	1.7
Gondoic acid (C20:1)	11.1	-	3.3	15.9	11.7	-	3.1	16.9
Erucic acid (C22:1)	38.1	-	15.4	60.7	40.1	-	15.6	61.5
Nervonic acid (C24:1)	1.1	-	0.1	10.2	1.4	-	0.2	11.2

) were erucic (C22:1), oleic (C18:1), linoleic (C18:2), and gondoic (C20:1) acids. Fatty acids identified in lower amounts were palmitic (C16:0), stearic (C18:0) and nervonic (C24:1) acids. Fractions F1, F2 and F3 of both roasted and unroasted S. arvensis oil contained C16:0, C18:1 and C22:1 as the major fatty acids, respectively. The properties of biodiesel depend on chain length, unsaturation and chain branching. In a previous study, biodiesel from shorter-chain fatty acids was superior to that of longer-chain fatty acids [38]. The longer chain length afforded higher CN but also higher viscosity, CP and PP. Fatty acids with carbon chain lengths of more than 22 usually have negative effects on biodiesel quality and yield. Shorter-chain fatty acids yield biodiesel with low CN and viscosity as well as low flash point. Thus, removal of such fatty acids from S. arvensis seed oil was carried out using HVFD to avoid negative effects on CN, CP, PP, and other fuel properties [39]. As seen in

Table 6. Comparison of fuel properties of FAME from S. arvensis seed oil with Jatropha, Karanja, and diesel.

Fuel Parameters	S. arvensis Biodiesel	Jatropha	Karanja	Diesel ASTM D975	ASTM D6751 Limits
Density (g/cm3)	0.84–0.94	0.871	0.8	0.85	Not specified
Cloud point (°C)	−3 to −1	11	19	−15–5	−15 to 10
Pour point (°C)	−9 to −12	0	15	−35–15	Not specified
Acid value (mg KOH/g)	0.06–0.14	0.20	0.5	-	0.50 max
Iodine value (g I2/100g)	105–110	91.25	86.5	-	Not specified
Saponification value (mg KOH/g)	140–149	180.33	-	-	Not specified
Cetane number	58.2–61.7	62.5	56	40–55	47 minimum

, F1 fractions were enriched in shorter-chain fatty acids (C16:1) relative to F2 and F3 and F3 was enriched in longer-chain fatty acids (C20:1, C22:1 and C24:1) relative to the others. The F2 fraction was enriched in C18:1 and C18:2 versus F1 and F3. These distributions are explained by the boiling points of the fatty acids, as boiling point generally increases with chain length and the distillation temperature range increased from F1 and F3.

2.5. Assessment of Fuel Quality Parameters

Biodiesel properties (Table 6) were determined and compared against diesel, other feedstocks and the American biodiesel standard, ASTM D6751 [40,41,42]. The density of S. arvensis FAME varied from 0.84–0.94 g/cm³. Density differs according to the nature of the feedstock and affects fuel efficiency [43]. This is because the mass of fuel is greater when density is higher [44]. A lower value for density thus decreases fuel efficiency [45]. Impurities such as FFA, partial glycerides and steryl glucosides present in unpurified biodiesel as well as high percentages of saturated FAME will raise CP and PP [46]. Low temperature behavior is important as fuels with high CP and PP will solidify in filters and cause various problems including fuel undernourishment and delayed ignition. The CP and PP of S. arvensis seed oil biodiesel was −3 to −1 °C and −9 to −12 °C, respectively (Table 6). Acid value (AV) is important because acids corrode automotive parts. For this reason, the maximum allowable AV specified in ASTM D6751 is 0.50 mg KOH/g. The AV of the starting material can play a crucial role in final biodiesel quality [47]. Biodiesel produced from pure, F1, F2, and F3 of unroasted and roasted S. arvensis seed oils had AVs that ranged from 0.06 to 0.14 mg KOH/g, which were below the maximum limit specified in ASTM D6751. The low AV indicated low amounts of FFA. Iodine value is often used to determine the amount of unsaturation in biodiesel. The IV of S. arvensis seed oil biodiesel produced from pure and fractionated (F1, F2 and F3) unroasted and roasted oils ranged from 105 to 110 g/100 g. The upper limit for IV specified in EN 14214, the European biodiesel standard, is 120 g/100 g. The American standard (ASTM D 6751) does not contain an IV specification. Unsaturation is needed to some degree to avoid fuel solidification at low temperatures. The CP and PP of biodiesel thus indirectly depend on IV, as unsaturated FAME have considerably lower melting points than saturated FAME. As a result, higher levels of unsaturation will generally result in lower CP and higher IV. However, high unsaturation causes deposition due to breakdown of double bonds caused by oxidation. Saponification is a process that involves production of soap or metal salts from lipids. The SV of biodiesel is an indicator of average molecular weight (MW) of FAME within biodiesel [48]. High SV signifies lower MW fatty acids or vice versa [49]. The SV of biodiesel produced using NaO₂ and immobilized lipase catalysts was in the range of 140–149. Neither ASTM D6751 nor EN 14214 contain limits on SV. Cetane number (CN) characterizes biodiesel ignition quality. Structural factors that decrease CN include shorter chain length, increasing unsaturation and branching. If CN is too low, then difficulty is faced in starting the engine and it will function irregularly. If CN is too high, then ignition will occur without proper mixing with air, which will result in partial combustion and increased exhaust smoke. The CN of S. arvensis biodiesels were 58.18–61.65, which were within the range specified in ASTM D6751. As seen in Table 6, the fuel properties of biodiesel produced from S. arvensis seed oil were within the limits specified in ASTM D6751.

FTIR and SEM characterization of NaO₂/SiO₂/PFC/Granite catalyst are reported in the Supplementary Materials. The future work on the detailed characterization of the produced catalysts is needed to further understand the reaction mechanism.

3. Materials and Methods

3.1. Chemicals and Reagents

Sodium sulphate (anhydrous), methanol (99%), ethanol (99.5%), sodium hydroxide (99%), potassium hydroxide, hydrochloric acid (37%), petroleum ether, sodium thiosulphate, sodium alginate, calcium chloride dihydrate, silicon dioxide, potassium ferricyanide (PFC), sodium nitrate, hydrogen peroxide (30%), potassium iodide, starch, Wijs solution, phenolphthalein, and enzyme (lipase) were of analytical grade, used as received and purchased from Sigma-Aldrich (St. Louis, MO, USA). Granite powder (GP) was collected from a nearby industrial area located in Faisalabad, Pakistan.

3.2. Materials and Oil Extraction

S. arvensis seeds were purchased from a local market in Faisalabad, Pakistan. Unhealthy seeds were manually separated from healthy seeds. The healthy seeds were divided into two equal halves. To check the difference between the oil yields extracted from the roasted and unroasted seeds, half of the seeds were roasted at 110 °C for 20 min with continuous shaking. The seeds were crushed to a smaller size by a stainless-steel mill equipped with a blade. Crushed seeds were ground into a uniform powder particle size (1 mm) using a heavy-duty grinder. Finely ground seeds were fed into an automatic screw press (Vosoco oil press machine). Oil was processed using vacuum filtration to remove solid particles. Lipase (1 g) was mixed with previously dissolved (by slow heating) 2 g of sodium alginate in 100 mL of deionized distilled water (DDW) and the mixture was cooled to room temperature before addition of lipase. The whole mixture was introduced into a solution containing 0.1 M CaCl₂·2H₂O through a 50 mL burette. The beads of immobilized lipase were then washed twice with distilled water and stored in 0.05 M CaCl₂·2H₂O.

3.3. High Vacuum Fractional Distillation (HVFD)

Roasted and unroasted S. arvensis seed oils were fractionated by boiling point using HVFD. The objective was to isolate very low (fraction F1) and very high (fraction F3) molecular weight fatty acids, which have negative effects on biodiesel quality, from the remaining fatty acids (fraction F2). The relative abundancies of unroasted and roasted S. arvensis seed oil fractions (F1, F2 and F3) separated using HVFD are given in

Table 7. Unroasted and roasted S. arvensis seed oil fractions separated using HVFD.

Fractions	S. arvensis Seed Oil (%)
	Temperature Range (°C)	Unroasted	Temperature Range (°C)	Roasted
F1	218–241	13.87	215–253	14.36
F2	248–263	48.48	219–264	49.53
F3	267–280	36.60	257–274	35.92

.

3.4. Preparation of Sodium Superoxide and Transesterification

A composite support material was prepared by mixing 1 g of silicon dioxide (SiO₂) and 1 g of potassium ferricyanide (PFC) with 8 g of granite stone powder waste (used as a substrate). DDW was then added to make a thick paste-like mixture. The paste was crushed using a mortar and pestle to a fine size. The fine paste was heated at 400 °C under continuous air flow for 7 h. Meanwhile, NaO₂ nanoparticles were prepared from NaNO₃ and H₂O₂ in an O₂-rich environment by spray pyrolysis. Spray pyrolysis was initiated by dispersing 1 M NaNO₃ in 100 mL H₂O₂ in probe sonication [29]. Finally, the prepared solution was sprayed onto the hot composite support (SiO₂/PFC/Granite) under constant O₂ flow.

$${\mathrm{NaNO}}_{3 }+{ \mathrm{H}}_2{\mathrm{O}}_2 \to {\mathrm{NaO}}_2+{ \mathrm{H}}_2\mathrm{O}+{ \mathrm{NO}}_2$$

The optimized variables for transesterification [50] were methanol to oil molar ratio (1:0.3, 1:0.6, 1:0.9, 1:1.2, and 1:1.5) and catalyst concentration (0.25, 0.50, 1.00, 1.50, and 2.00 w/w %). Biodiesel was also produced using immobilized enzyme catalyst (1 w/w % immobilized lipase). A mixture of 5 g S. arvensis seed oil, 25 g methanol and 1.98 g of immobilized lipase was mixed in a shaker flask under stirring (100 rpm) for 10 h. The reaction conditions were kept constants for all experiments, which were conducted in duplicate. Hot excess distilled water was used to remove surplus methanol from biodiesel. A detail of the production procedure is shown in Figure 1. Biodiesel yield was calculated using the following equation and presented as w/w %:

$$\mathrm{Process} \mathrm{yield} (\%)=\frac{\mathrm{Pure} \mathrm{biodiesel} \left(\mathrm{g}\right)}{\mathrm{Oil} \mathrm{used} \left(\mathrm{g}\right)}\times 100$$

3.5. Quality Parameters of Biodiesel

Different physiochemical parameters like density, pour point (PP), cloud point (CP), iodine value (IV), saponification value (SV), acid value (AV), and cetane number (CN) were determined to characterize the resulting biodiesel. Density (g/cm³) was measured by weighing the mass of 1.0 mL of sample. The degree of unsaturation was determined by IV following ISO method 3961, which is defined as the number of grams of iodine absorbed by 100 g of biodiesel and is expressed as g I₂/100 g of biodiesel. In short, 0.05 g of biodiesel was added to a 250 mL conical flask and was dissolved in 20 mL of petroleum ether and 25 mL of Wijs solution. After shaking, the contents were transferred to a dark place for 0.5 h. Then, 20 mL of KI solution (15%) was added. After addition of 100 mL of distilled water, the contents were shaken vigorously and titrated against 0.1 N sodium thiosulphate. Starch was used as an indicator. Disappearance of the yellow color was an indication of the end point. The following equation was used for calculation of IV:

$$\mathrm{IV}=\frac{\left(\mathrm{Blank} \mathrm{titration}-\mathrm{sample} \mathrm{titration}\right)\times {\mathrm{Normality} \mathrm{of} \mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3·5{\mathrm{H}}_2\mathrm{O} \times 12}{\mathrm{Sample} \mathrm{weight} \left(\mathrm{g}\right)\times 100}$$

To determine SV, 0.5 g oil and 20 mL alcoholic KOH solution were added to a 250 mL conical flask. The flask was then heated while connected to a water-cooled condenser. After 30 min a clear solution was obtained, which indicated completion of saponification. After cooling to room temperature 1 mL of phenolphthalein was added and immediately titrated against 0.5 N HCl. The end point was disappearance of the pink color. A blank sample was also analyzed following the same procedure. The SV of S. arvensis seed oil was calculated as follows:

$$\mathrm{SV} =\frac{\left(\mathrm{Blank} \mathrm{titration} – \mathrm{Sample} \mathrm{titration}\right)\times \mathrm{Normality} \mathrm{of} \mathrm{HCl} \times 56.1}{\mathrm{sample} \mathrm{wt} \left(\mathrm{g}\right)}$$

AV was determined by combining 0.5 g of oil and 10 mL of ethanol in a titration flask (250 mL). Phenolphthalein was used as an indicator and the solution was titrated against 0.1 N NaOH solution until a pink color appeared. The following formula was used to obtain the percentage of FFA in the sample:

% FFA = (V × N × 282 × 100)/W

where % FFA = percent free fatty acids (g/100 g × 100), expressed as oleic acid, V = volume of titrant (mL), N = Normality of NaOH (mol/L), 282 = molecular weight of oleic acid (g/mol), W = weight of sample (g). FFA can then be converted to AV with the following equation:

AV = 1.989 × % FFA

CN was evaluated using the following formula:

$$\mathrm{CN}=46.3+\frac{5458}{\mathrm{SV}}-0.225\times \mathrm{IV}$$

CP and PP were determined following standard methods ASTM D2500 and EN 2305, respectively. All fuel property determinations (AV, CP, CN, density, IV, PP, SV) were measured in triplicate and mean values were reported.

3.6. Characterization

The functional groups present in NaO₂/SiO₂/PFC/Granite catalyst were analyzed using an Agilent (Santa Clara, CA, USA) FTIR spectrometer. The surface morphologies were analyzed by SEM (FEI Quanta 400F electron microscope, Hillsboro, OR, USA). Gas-chromatographic mass spectrometric (GC-MS) analysis was performed to quantify FAME content. Analyses were conducted using a Perkin-Elmer Clarus 500 model GC-MS equipped with a capillary column (HP-1, 30 m × 0.25 mm × 0.25 µm), coupled to a Perkin-Elmer (Waltham, MA, USA) Clarus 500C MS. The sample injection tool was placed at an oven temperature of 50 °C and was held at that temperature for 1 min. The oven temperature was then increased to 325 °C at a heating rate of 10 °C/min and held for 2 min. Helium (99.99%) with a constant flow rate of 1.2 mL/min was used as a carrier gas. The entire process for biodiesel production and catalyst preparation is presented in Figure 1.

4. Conclusions

Seeds of S. arvensis (wild mustard) were rich in inedible vegetable oil (35.4–39.2%), which was converted to biodiesel using a novel sodium superoxide support coated catalyst and compared with the immobilized lipase catalyzed process. The following important conclusions were drawn from the present study: (1) The oil yield from S. arvensis was enhanced from 35.4% to 39.2% by roasting seeds at 110 °C for 20 min. Roasting enhanced protein denaturation and in turn improved lipid extractability. (2) The dominant fatty acids identified in unroasted and roasted S. arvensis seed oil were erucic (38.1–40.1%), oleic (22.3–23.2%), linoleic (15.2–16.1%), and gondoic (11.1–11.7%) acids. (3) High vacuum fractional distillation of S. arvensis seed oil provided three fractions that generally correlated with molecular weight. The lowest boiling fraction (F1) was highly enriched in palmitic acid (76.7–79.0%) relative to the other fractions whereas F2 was enriched in oleic (69.9–69.1%) and linoleic (11.7–11.9%) acids and F3 was enriched in gondoic (15.9–16.9%) and erucic (60.7–61.5%) acids, respectively. (4) The newly prepared sodium superoxide catalyst was readily synthesized from sodium nitrate and hydrogen peroxide under oxygen via spray pyrolysis and bound to a composite support material prepared by calcination of a mixture of silicon dioxide, potassium ferricyanide and granite. (5) The sodium superoxide support-coated catalyst gave considerably higher biodiesel yields (99.2%) than the immobilized lipase catalyst (78–85%). (6) Fuel properties such as AV, CN, CP, density, IV, and PP were within the ranges specified in ASTM D6751, where applicable. These results clearly showed that the sodium superoxide support-coated nano catalyst is not only useful in overcoming the limitations that are inherent to conventional heterogeneous catalysts but also increases biodiesel yield. In addition, the present study reports a low-cost method for preparing a novel catalyst (sodium superoxide) onto a composite support of a waste material from marble factories (granite waste powder). Lastly, the catalyst was highly effective at producing biodiesel from low-cost, inedible S. arvensis seed oil.