1 Introduction

Continuous reduction in petroleum fuel, concerns over global warming and climate change coupled with environmental pollution caused by burning of conventional fuels has generated interest for biodiesel as alternative fuels [1]. Principally, biodiesel is produced by transesterification reaction using oil and alcohol in the present of a catalysts [2]. It is renewable, biodegradable, has low emission profiles, high flash point and environmentally friendly [2, 3]. The application of heterogeneous catalysts in biodiesel production is highly recommended as a result of its high catalytic efficiency and mild reaction conditions [4]. Heterogenous catalysts can potentially reduce the expenses involved in producing biodiesel through elimination of the excess alcohol as well as many other downstream processing steps [5]. Presently, the most common heterogeneous catalysts used for the production of biodiesel include oxides of metals [6, 7], metal complexes [8], active metals loaded on supports [9], resins [10, 11], lipases [12] zeolites [13] etc. Majority of these catalysts exist in powder form and thus making separation hard [14].

Transitional metals which composed of various acids and bases are now commonly used. Typical examples of those transitional metals commonly used include in biodiesel production include MnO, ZnO, TiO, WO and ZrO [15]. Amongst the transitional metals, Lanthanum contains complexes that produces many doles with high surface area, small crystal size and high number of basic sites [16]. The metal composition and acidity or basicity of the supported catalysts plays a critical role in the catalyst activity [17]. ZnO-Al2O3-La2O3 [18], CaO-La2O3 [19], La-CaO, La/Mn, [16] have however, all been tested as heterogeneous catalyst and have exhibited very good transesterification activity [20]. However, high reaction temperature (200–588 ℃) is required for producing their biodiesels. In some instances, supporting these transitional metals with acid or base compounds which has well distributed pores have proven suitable for improving the catalytic activity [21].

Studies have shown that Molybdenum compounds supported with Potassium Carbonate (K2CO3) can be very good heterogeneous catalyst for transesterification of vegetable oil to biodiesel [22, 23]. This is because most molybdenum compounds have low solubility in water. However, when molybdenum bearing minerals get in contact with oxygen and water, the resulting molybdate ion \({\text{MoO}}_{4}^{2 - }\) is quite soluble.

Industrially, molybdenum compounds (about 14% of world production of the element) are used in high-pressure and high-temperature applications [15]. In this study sodium molybdate (Na2MoO4) has been synthesized and investigated as a heterogeneous catalyst for producing biodiesel from Camelina sativa seed oil.

Camelina sativa is a broadleaf oilseed flowering plant of the Brassicaceae family and grows optimally in temperate climates. It originated from Germany in around 600 B.C. and later spread to Central Europe. From the beginning of 20th century up to the 1930s, Camelina sativa was grown sporadically in France, Belgium, Africa and Holland. In Ghana, it is indigenous to the Volta Region. Camelina sativa grows very well in less rainfall and less fertile lands. It also thrives in cool, arid climates and is adapted to the northern regions of North America and Asia. The yield of Camelina sativa is somewhere from 336 to 2240 kg of seeds per hectare at maturity.

In Camelina sativa industrial oil production in Ghana located at Ho, Volta Region, the oil is considered a by-product and has not been exploited for various uses. This provides practical foundation for developing the Camelina sativa seed oil to biodiesel. This study is therefore, aimed at producing biodiesel from Camelina sativa oil using modified sodium molybdate as an efficient high yielding heterogeneous catalyst. Even though catalysts based on molybdenum compounds have shown very efficiency in the epoxidation of cyclic olefins [15], the catalyst that has been synthesized in this study has not been investigated for transesterification of oils to biodiesel. It is expected that the outcome of this study will go a long way to enhance the cultivation of Camelina sativa plant in Ghana and also sell out the oil which is currently treated as a by-product. The catalyst will also contribute to reducing cost of biodiesel production since catalyst is one of the main sources of biodiesel cost.

2 Experimental

2.1 Materials

The oxide of molybdenum (MoO3) and sodium molybdate dihydrate (Na2MoO42H2O) were bought from Ryte Aid Pharmacy (Ho, Ghana). Unless specified otherwise, all other reagents were used as received without further purification. The Camelina sativa seeds were obtained from Ryte Aid Chemicals (Ghana).

2.2 Extraction of Camelina sativa oil

The seeds of Camelina sativa were subjected to drying at 110 ºC for a period of 9 h in oven for the purpose of removing the extra moisture. The dried seeds were then grounded and weighed. The process of extraction was done by means of soxhlet extractor using petroleum ether (60–90 ºC) for a period of 8 h. 6 L of petroleum ether for every kilogram of Camelina sativa seeds was applied. The extracted oil was recovered using rotary evaporator and the amount determined using Li et al. [24] recommended procedure (Fig. 1).

Fig. 1
figure 1

Indigenous Ghanaian Camelina sativa (a) plant and (b) seeds

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2.3 Catalysts synthesis

MoO3 (0.1450 mol) was first dried at 500 °C for 3 h and then dissolved in 11 mol L1 NaOH. Methanol (70 mL) was added to the mixture and the recovery of Na2MoO42H2O was done through filtration, then washed with methanol and acetone and finally dried for 3 h at 120 °C. For comparative purposes, a commercial sample of Na2MoO42H2O (Synth) as reference was obtained and used after drying at 120 °C for 3 h.

2.4 Catalyst characterization

The characterizations of the basic strength of the catalysts were done using Hammett indicator procedure [25, 26]. Typically, this was done by keeping 300 mg of the sample in a 1 ml solution of Hammett indicators, then followed by dilution using a 10 ml of methanol. The reaction proceeded for about 2.5 h to allowed equilibrium to be reached. The basic strength was therefore defined as being stronger than the weakest indicator which exhibits a color change but weaker than the strongest indicator that produced no color change. The Hammett indicators that were used included; bromothymol blue (H_7.2), phenolphthalein (H_9.8), 2,4-dinitroaniline (H_15) and 4-nitroaniline (H_18.4). Hammett indicator benzene carboxylic acid (0.02 mol/L anhydrous ethanol solution) titration method was used in the determination of the basicity of the solid catalyst.

The shapes and surface characteristics of the samples were examined on a field emission scanning electron microscope (S-4800, HITACHI Corp., Tokyo, Japan) at the accelerating voltage of 20-kV.

Thermogravimetric analysis (TGA) was carried out on a Netzsch instrument (STA 449C, Netzsch, Seligenstadt, Germany). The heating programme ranged from room temperature to 800 ºC with the rate of heating being 10 ºC/min under a nitrogen atmosphere. The measurement was taken for 5–10 mg samples.

The nitrogen adsorption and desorption isotherms were measured at – 196 ºC using NDVA2000e analytical system (Quntachrome Corporation, USA). The surface areas were calculated by means of Brunauer–Emmett–Teller (BET) method. The size of the pore and its distribution as well as the volumes of the pores were determined using Barrett–Joyner–Halenda (BJH) method.

Intermediate compounds of the catalyst were analyzed by X-ray diffractometer using a reflection scan with nickel-filtered Cu Ka radiation (D8, Bruker-AXS, Germany) diffractometer. The XRD measurements were done at 2θo values of between 20 and 80º.

The Fourier transform infrared (FT-IR) of the catalyst samples was done using potassium bromide (KBr) powder method on a FTIR spectrometer (AVATAR 360, Nicolet, Madison, USA) with a resolution of 2/cm in the 4000–500/cm range. 32 scans were carried out.

2.5 Transesterification experiments

Transesterification reaction of triglycerides to fatty acid methyl esters was done using 30 g of the oil with varied molar ratio (9:1–17:1) of methanol to oil and catalyst in various amounts (2–10%,); with reference to the weight of the oil. The mixture was refluxed in a 250 ml three-neck reaction flask equipped with a condenser and magnetic stirrer (600 rpm) at different times (1–3 h) and temperatures (40–70 ºC). After the reaction, the mixture was washed with n-hexane to remove any absorbed fatty acid methyl esters (FAMEs) out of the solid catalyst. The mixture was subsequently centrifuged at 5000 rpm for 10 min to ensure separation of the solid catalyst from the liquid layer containing the biodiesel and n-hexane. The decanted liquid phase was then transferred to a rotary vacuum evaporator to remove the n-hexane and any other by-products from the biodiesel. Each biodiesel sample was then kept in separatory funnel and allowed to stand for 24 h before washing with water three times. Drying and analyses of the biodiesel then followed.

The biodiesel samples were analysed using 7890A gas chromatograph (Agilent Technology Inc. USA), equipped with a flame-ionization detector (FID) and a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm). Helium was used as the carrier gas. The oven temperature ramp program was ran using Li et al. [27] method with little modification (typically the oven temperature was held at 160 ºC for 1 min, then increased to 210 ºC at 20 ºC/min for 2 min then further increased from 210 ºC to 213 ºC at 0.3 ºC/min for 3 min, and finally increased to 250 ºC at 30 ºC/min for 1 min). Hydrogen gas’s flow rate was 40 mL/min and that of air was 400 mL/min. Temperature of the injector and detector were set at 250 ºC. The injection was done in split mode with a split ratio of 30:1. Analysis of the biodiesel samples was done by dissolving 1 mL of the biodiesel with 5 mL of petroleum ether (30–60 ºC) in the presence of tetradecane as the internal standard and 0.5 μL of the solution injected into GC. The yield of each biodiesel sample was calculated from the content of esters using Qiu et al. [28] method. All data in this study are presented as mean (values) of triplicate of experimental and GC determinations with standard deviations.

3 Results and discussion

3.1 Extraction of oil

Having extracted the oil from the seeds, the amount was calculated using Li et al. [24] method. The content of oil in the seeds (41%) was however, comparable to those of other oils and this indicated that the seeds contain relatively high amount of oil [29, 30]. High amount of free fatty acid and water content in oils leads to soap formation and this can lower the yield of biodiesel [31]. The free fatty acids and water content in the oil in this case was less than one thus, closer to other oil(s) [32]. Table 1 shows the composition of oil in the seeds.

Table 1 Physico-chemical properties of Camelina sativa oil
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3.2 Catalyst characterization

SEM photographs of sodium molybdate dihydrate (Na2MoO42H2O) is shown in Fig. 2. The SEM image of sodium molybdate dihydrate (Na2MoO42H2O) sample shows crystallites of 1 μm size. The crystallites shape of sodium molybdate dihydrate (Na2MoO42H2O) image suggest that there is a good dispersion of NaOH onto Molybdenum oxide (MoO3).

Fig. 2
figure 2

SEM photograph of sodium molybdenum oxide (Na2MoO42H2O)

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The BET surface areas of sodium molybdate dihydrate (Na2MoO42H2O) was 5.2 m2g−1 while the pore volume of sodium molybdate dihydrate (Na2MoO42H2O) was 0.01 cm3g−1. From the BET data, the sodium compounds on sodium molybdate dihydrate (Na2MoO42H2O) could generate the catalytic activity for the transesterification reaction. The low pore volume value of the catalyst (cm3g−1) indicates that the triglyceride molecules can easily penetrate through the pores of the catalyst as indicated by Granadose et al. [33]. This pre-supposes that most of the active sites of the catalyst would be utilized during the transesterification.

The UV–vis spectrum of sodium molybdate dihydrate (Na2MoO42H2O) shows a broad band between 400 and 700 nm which is attributed to π=π charge transfer transitions in the C=O group of OH. This result indicates that the absorbed NaOH onto Molybdenum oxide (MoO3) did fully decompose at 600 ºC (Fig. 3).

Fig. 3
figure 3

UV spectra of sodium molybdate dihydrate (Na2MoO42H2O)

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Figure 4 shows the XRD patterns of pure A (raw) and C [sodium molybdate dihydrate (Na2MoO42H2O)]. There were peaks at 2θ = 8.4°, 20.9°, 28.1° and 31.9°. Upon modifying the sodium molybdate dihydrate (Na2MoO42H2O) typical Na2MoO42H2O peaks appear at 2θ = 29.1°, 31.2°, 36.6° and 45.4°. New phases associated with Na2O also appeared at 11.1°, 32.3° and 48.4°.

Fig. 4
figure 4

XRD of A (raw) and C [calcined sodium molybdate dihydrate (Na2MoO42H2O)]

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FT-IR spectra of the sodium molybdate dihydrate (Na2MoO42H2O) is shown in Fig. 5. In the spectrum of the sodium molybdate dihydrate (Na2MoO42H2O), 2 strong bands at 3428 and 1028 cm–1 can be attributed to the symmetric and asymmetric stretching vibrations of Na2=O bond of Na2MoO42H2O. The results confirm the XRD data in Fig. 4.

Fig. 5
figure 5

FTIR of sodium molybdate dihydrate (Na2MoO42H2O)

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3.3 Optimization of transesterification reaction parameters

3.3.1 Effect of methanol to oil molar ratio

Efficiency of triglyceride conversion is strictly dependent on methanol amount used for the transesterification reaction. The molar ratio of methanol to vegetable oil is very important parameter which affect the conversion of triglycerides to methyl esters [34, 35]. The stoichiometric molar ratio of methanol to Camelina sativa oil is 3:1. However, excess methanol (typically methanol/oil > 6) can be used to increase the yield of methyl esters and facilitate the separation of glycerine [30]. To investigate the effects of methanol to oil ratio for the Na2MoO42H2O catalyst, the transesterification experiment was conducted by systematically changing the molar ratio of methanol to Camelina sativa oil, i.e., from 9:1 to 19:1 whilst keeping the reaction temperature, catalyst amount and reaction time constant at 60 ºC, 6% and 2 h, respectively (Fig. 6). The yield increased as the methanol to oil molar ratio was increased (from 9:1 to 17:1). The results suggest that altering the methanol/oil molar ratio in the 9:1 to 17:1 range did affect the methyl esters yield. The highest methyl ester yield of 90.3% was reached at the molar methanol/oil ratio of 17:1. The reaction at methanol to oil molar ratio of 9:1 took a longer time to obtain substantial yield indicating that at low molar ratio, the reaction is slow. These data, reported in terms of the standard error of mean according to Origin 8.1, confirmed that methanol/oil ratios that are > 6:1 can increase the yield of methyl ester substantially. Comparatively, methanol to oil molar ratio of 19:1 also provided relatively higher yield as 17:1, but this ratio should be avoided for the purpose of cost minimization. To ensure the optimum biodiesel yield, methanol to oil molar ratio of 17:1 in this case was selected.

Fig. 6
figure 6

Effects of methanol: oil ratio (reaction temperature 60 ºC, catalyst amount 6% and reaction time of 2 h)

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3.3.2 Effect of catalyst amount

In order to examine the influence of catalyst amount on the conversion efficiency of triglyceride to methyl esters, series of reactions were performed by varying the amount of catalyst. Since catalyst accelerates transesterification reactions [30], it will be important to determine the influence of this variable in the conversion of triglyceride to esters. Earlier studies on Cynara cardunculus L. indicates that the conversion of oil to its methyl ester by transesterification without catalyst (0% NaOH) [36] was impossible. In this study, the effects of catalyst amount on methyl ester yield was investigated at the mass ratio of Na2MoO42H2O to Camelina sativa seed oil at a varied range of 2 to 10% using 17:1 methanol to oil molar ratio at reaction time of 2 h and reaction temperature of 60 ºC (Fig. 7). The percentages of the Na2MoO42H2O were weight fractions of the oil used for the reaction. Figure 7 indicate that, increase in catalyst amount (from 2 to 6%) leads to increase in methyl ester yield till a plateau value (90.3) was reached. This findings agree with other studies [31, 36]. At the lowest catalyst amount (2%) the reaction was slow, indicating insufficient amount of the catalyst to catalyze the process to completion. At 6% of Na2MoO42H2O, 90.3% yield of methyl esters was generated. This is consistent with earlier discussion on screening of catalyst on the effect of basicity on biodiesel yield. Similar conclusions were also reached in the 2011 study of Qiu et al. [28] when Na2MoO4 salts were loaded on ZrO2 to increase the basicity of ZrO2 surfaces. It is however, noted that when the catalyst amount was increased further from 6 to 8%, the yield however, dropped slightly from that of 6% [31, 36], which was due to the addition of a higher amount of the catalyst, causing slight soap formation thereby inhibiting the reaction process which consequently increased the viscosity of the reactants [31]. Considering economic reasons, 6% catalyst was proposed to be used for further reactions.

Fig. 7
figure 7

Effect of catalyst amount (methanol to oil molar ratio of 17:1, reaction temperature of 60 ºC and reaction time of 2 h)

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3.3.3 Effect of reaction temperature

Transesterification of Camelina sativa oil was analyzed at temperatures of 40–70 ºC and the obtained results are shown in Fig. 8. Studies indicate that reaction temperature has an important effect on base-catalyzed transesterification [31, 36]. The effect of reaction temperature on the transesterification of crude Camelina sativa seed oil was investigated at different temperatures (40, 50, 60, 65 and 70 ºC) with methanol to oil molar ratio of 17:1 and catalyst amount of 6% in 2 h (Fig. 8). As shown in Fig. 8, the reaction rate was slow at low temperatures. The reaction however, proceeded till a yield of 90.3% was reached. Evidently, the methyl esters yield increased with increasing reaction temperature till maximum at 60 ºC (90.3%). The maximum temperature studied i.e., 60 ºC however, coincides with the boiling point of methanol. Further increase in temperature does have a little impact at the yield of the biodiesel while the foremost advantage of higher temperatures takes less reaction time. Henceforth, the optimum reaction temperature for the transesterification reaction was 60 ºC.

Fig. 8
figure 8

Effect of reaction temperature (reaction time 2 h, methanol to oil molar ratio of 17:1 and catalyst amount of 6%)

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3.3.4 Effect of reaction time

Duration of reaction also have significant influence on transesterification efficiency as triglycerides in oil starts degrading after long time. The reaction time was varied in the range of 1–3 h. Figure 9 revealed that the transesterification reaction was strongly dependent on reaction time. At the beginning (1 h), the reaction was slow due to the mixing and the dispersion of methanol into oil. The reaction however, proceeded with increase in yield till the yield (95.2%) was reached in 2.5 h. After that (2.5 h), further increase in the time led to a reduction in the yield possibly due to reversible reaction [37]. On the basis of the optimum yield (95.2%), 2.5 h was selected as optimum reaction time.

Fig. 9
figure 9

Effect of reaction time (methanol to oil molar ratio of 15:1, reaction temperature of 60 ºC and catalyst amount of 6%)

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3.4 Catalyst regeneration

A stable catalyst should have resistance toward impurities. A fresh reaction mixture of Na2MoO42H2O, methanol and Camelina sativa seed oil were used at each cycle of the stability tests. The typical results of methyl ester yield of the number of cycles are shown in Fig. 10. For all the five runs, the rate of reaction was very high at the end of the 2.5 h reaction period (with the methyl esters yield exceeding 80%). The range of reduction of the yield of methyl ester was from 95.2% (fresh use), to 81.1% (fifth reuse) (Fig. 10). In all, about 14.1% of total esters was reduced at the end of the fifth run, suggesting that the catalyst particles were stable. The reduction in the yield after each cycle of reuse was possibly due to leaching of Na [38,39,40]. Figure 11 show GC–MS spectrum of the biodiesel at the optimum reaction condition.

Fig. 10
figure 10

Catalyst regeneration (methanol to oil molar ratio of 15:1, reaction temperature of 60 ºC, catalyst amount of 6% and reaction time of 2.5 h)

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Fig. 11
figure 11

GC–MS of the biodiesel at the optimum reaction conditions

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3.5 Comparison of Camelina sativa biodiesel properties to international standards

The Camelina sativa biodiesel produced at the optimum reaction conditions were tested for their quality and the standards compared with those of European Union-EN 14214 (2008) and American ASTM D 6751 (07b) (Table 2). The properties including cetane number, kinematic viscosity, flash point, acid value, copper strip corrosion and water content among others were comparable to the international standards. The cloud point of biodiesel reflecting its cold weather performance indicate that the methyl esters of Camelina sativa seed oil can be used as fuel in relatively cold-weather conditions without much problem [33]. The flash point of the biodiesel (148 °C) was however, higher than that of diesel and when blended with diesel (67.5 °C), could be reduced [41]. In all, the properties of the methyl esters were comparable to international standards.

Table 2 Comparison of properties of the Camelina sativa biodiesel and the standards of Europe and the United States
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4 Conclusions

In this study sodium molybdate (Na2MoO42H2O) has been prepared and investigated as efficient heterogeneous solid catalyst for transesterificating Camelina sativa seed oil to biodiesel. The transesterification reaction was very efficient, with the optimum yield higher than 95% at methanol to oil molar ratio of 17:1, catalyst amount of 6%, reaction temperature of 60 °C and reaction time of 2.5 h. The catalyst was characterised using SEM, TGA, UV, XRD and FTIR. The catalyst indicated sustained reusability of 5 times with yield of 81.1% at the 5th reuse. The catalyst was easily recovered and after being washed showed capacity of recyclability for another catalytic reaction of five cycles with similar activity. The properties of the biodiesel were comparable to international standards.