The production of biodiesel from plum waste oil using nano-structured catalyst loaded into supports

The present study was undertaken with aims to produced catalyst loaded on low-cost clay supports and to utilize plum waste seed oil for the production of biodiesel. For this purpose, Bentonite–potassium ferricyanide, White pocha-potassium ferricyanide, Granite-potassium ferricyanide, Sindh clay-potassium ferricyanide, and Kolten-potassium ferricyanide composites were prepared. Transesterification of plum oil under the different conditions of reactions like catalysts concentrations (0.15, 0.3 and 0.6 g), temperature (50, 60, 70 and 80 °C), reaction time (2, 4 and 6 h) and oil to methanol ratio (1:10) was conducted. The maximum biodiesel yield was recorded for Bentonite–potassium ferricyanide composite. This composite was subjected to calcination process to produce Calcinized bentonite–potassium ferricyanide composite and a further improvement in biodiesel amount was recorded. The fuel quality parameters of all biodiesel samples were in standard range. Gas chromatographic mass spectrometric analysis confirmed the presence of oleic and linoleic acids in the plum seed oil. The characterization of composite was done using FTIR, SEM and EDX. Two infrared bands are observed in the spectrum from 1650 to 1630 cm−1 indicates that the composite materials contained highly hydrogen bonded water. The presence of surface hydroxyls groups can also be confirmed from FTIR data. SEM image clearly show the presence of nano-rods on the surface of Granite-potassium ferricyanide and Kolten-potassium ferricyanide composites. Another interesting observation that can be recorded from SEM images is the changes in surface characteristic of Bentonite–potassium ferricyanide composite after calcination (at 750 °C, 1 atm for 4 h). Calcinized bentonite–potassium ferricyanide composite found to contain more nano rod like structures at its surface as compared to Bentonite–potassium ferricyanide composite which contained spherical particles. EDX data of Bentonite–potassium ferricyanide composite and Calcinized bentonite–potassium ferricyanide composite show that after calcination carbon and oxygen was reduced. The other lost volatile compounds after calcination were of Na, Mg, Al, Si, and S. The XRD spectrum of pure bentonite showed the average crystal size of 24.46 nm and calcinized bentonite of 25.59 nm. The average crystal size of bentonite and potassium ferricyanide composite and its calcinized form was around 33.76 nm and 41.05 nm, respectively.

Preparation of catalyst and transesterification of oil. Bentonite potassium ferricyanide composite was prepared by mixing 25 g of potassium ferricyanide and 25 g of bentonite clay in 250 mL distilled water. The mixture was stirred for 5 min at 100 rpm and filtered. The obtained solid material was dried at 60 °C in an electric oven. By following similar procedure potassium ferricyanide catalyst was mixed with different support materials including White pocha, Granite, Sindh clay, and Kolten. Bentonite-potassium ferricyanide composite have shown maximum biodiesel yield during the present study. To improve catalytic activity of Bentonite-potassium ferricyanide composite further, this composite was subjected to at 750 °C for 4 h.
Transesterification of plum oil under the different conditions of reactions like catalysts concentrations (0.15, 0.3 and 0.6 g), temperature (50, 60, 70 and 80 °C), reaction time (2, 4 and 6 h) and oil to methanol ratio (1:10) was conducted. Magnetic stirring was maintained at 300 rpm during all experiments. Glycerol was formed as a byproduct during biodiesel production. The upper biodiesel was separated from glycerol and washed with hot water until clear biodiesel layer was obtained. The biodiesel quality was accessed by the determination of density, specific gravity, pH, saponification value, acid value, cetane number, iodine value, free fatty acids contents and acid value 13,18-20 . Characterization of plum oil. GC-MS (gas chromatographic-mass spectrometric analysis) was conducted for the quantification of methyl esters present in the biodiesel. For this purpose, three samples were selected. GC-MS analysis was performed on a Perkin Elmer Clarus 600 GC System, fitted with an Elite-5MS capillary column (30 m × 0.25 mm i.d. × 0.25 μm film thickness; maximum temperature, 350 °C), coupled to a Perkin Elmer Clarus 600C MS. Ultra-high purity helium (99.999%) was used as a carrier gas at a constant flow of 0.2 ml/min. The injection, transfer line and ion source temperatures were 220, 200 and 200 °C, respectively. At the ionizing energy of 70 eVthe data was collected from 10 to 600 m/z by using 0.1 μL of sample with 50:1 spilt ratio. The temperature program for oven was as follows: 35 °C holds for 10 min, 10 °C/min 200 °C hold for 10 min. The unknown compounds were identified by the use of NIST 2011 (v.2.3 and Wiley, 9th edition).

Results and discussion
Optimization of biodiesel yield. The plum oil was transesterified into fatty acid methyl esters in the presence of following composite materials: (a) Bentonite-potassium ferricyanide composite (b) Calcinized bentonite-potassium ferricyanide composite (c) White pocha-potassium ferricyanide composite (d) Granitepotassium ferricyanide composite (e) Sindh clay-potassium ferricyanide composite (f) Kolten-potassium ferricyanide composite. Effect of catalyst concentration on yield of biodiesel produced from plum waste oil was studied at three catalyst concentrations 0. 15 A further increase in the catalyst amount decreased the biodiesel yield. The decrease in the biodiesel yield at increased catalyst concentrations might be due to increase in the viscosity of reaction mixture (Fig. 1). The effect of reaction times on biodiesel yield was evaluated from to 2 to 6 h ( Fig. 2) at methanol to oil molar ratio 8:1, 0.3% catalyst, and 60 °C reaction temperature. The maximum biodiesel yield for (a) Bentonite-potassium ferricyanide composite (b) Calcinized bentonite-potassium ferricyanide composite (e) Sindh clay-potassium ferricyanide composite and (f) Kolten-potassium ferricyanide composite was obtained after 4 h. A further increase in the reaction time resulted in the loss of biodiesel yield may be due to breakdown of fatty acid methyl esters. However, (c) White pocha-potassium ferricyanide composite and (d) Granite-potassium ferricyanide composite have produced maximum of biodiesel after 6 h of reaction time. The determination of an optimum time to produce biodiesel is essential as it contributes to calculate the cost on pilot and commercial scales. The impact of transesterification reaction temperature was studied on three temperatures (50, 60, 70 and 80 °C) by keeping other variables constant as follows: reaction time of 4 h, catalyst amount 0.3%, and methanol to oil ratio of 10:1 (Fig. 3). The rate of transesterification reaction increased as the reaction temperature increased from 50 °C to 60 °C. On increasing reaction temperature from 60 to 80 °C, a decrease in the biodiesel yield was observed. Although, it is expected that biodiesel yield increases with the temperature, however, increasing temperature above 60 °C could result in the decrease of biodiesel yield due gasification of methanol 21 . Among all used composites as catalysts, the maximum biodiesel yield was obtained for Bentonite-potassium ferricyanide. The calcination of Bentonite-potassium ferricyanide composite have further increased the biodiesel yield. The highest biodiesel yield observed using calcinized Bentonite-potassium ferricyanide composite was due to increase in the average crystal size (as supported by XRD results) that has provided comparatively greater surface area for reactants for successful conversion into products. The XRD spectrum of pure bentonite showed the average crystal size of 24.46 nm and calcinized bentonite of 25.59 nm. The average crystal size of bentonite and potassium ferricyanide composite and its calcinized form was around 33.76 nm and 41.05 nm, respectively.
Determination of fuel properties. The estimated values of various fuel quality parameters are tabulated in Table 1. Biodiesel density is important parameters as the fuel working in the fuel injector system and engine is strongly related to density value 22 . Amount of weight comprised in a unit volume is referred as density. Denser the oil more the energy it contains. Standards used to measure density of biofuels are 3675/12185 in European Union and D1298 in USA and are measured at reference temperature of 15 or 20 °C. Density which is measured by comparing with water's density is called relative density. The density of biodiesel measured relatively necessary for calculating the conversion of mass to volume and for determination of flow rate (Sanford et al., 2009). The densities of different biodiesel samples determined in the present study was between 0.856 to 0.877 kg/L. The recommended range of density lies between 0.86 and 0.90 g/cm 3 by EN 14214:2003 for a B100 type biodiesel.  www.nature.com/scientificreports/ All biodiesel samples have density in the recommended range. These results reveal that produced biodiesel may be suitable for optimal performance. Acid value is defined "as the number of milligrams of potassium hydroxide (KOH) required to neutralize the free fatty acid in oil". The acid values of all biodiesel samples produced in the present study were in the standard range. Iodine value is "the measure of the total degree of unsaturation, and it provides useful guidance for preventing various problems in engines". The iodine value tells about stability and the presence of double bonds in the fatty acid methyl esters [23][24][25] . The iodine values measured during the present study ranged from 140.2 to 168.3 g I 2 /100 g. Catalyst concentration is an important factor that affect the iodine value of produced biodiesel samples.
Saponification value is "the amount of alkali required to saponify a given quantity of oil sample, which is expressed as the number of milligrams of KOH required to saponify 1 g of oil sample and is inversely proportion to the molecular weight of fatty acid of the biodiesel" 22 . The saponification value of plum oil biodiesel ranged from 103.78 to 186.23 mg/g. Cetane number is a fuel quality parameter related to the ignition delay time and combustion quality. According to UNE-EN 14214 (2003) specification, biodiesel should have minimum Cetane number of 51, while ASTM D6751-02 assigns 47 as the minimum cetane number for biodiesel. The cetane number of all biodiesel sample were greater than 50. Cetane values obtained in the present study has higher value as compared to the previous study on soya bean oil that ranged from 45 to 60 22 .

Gas chromatographic analysis (GC-MS analysis).
The fatty acid composition of plum seed oil is given in the Table 2. The chemical composition of biodiesel determines its fuel stability, while the stability of the fatty acid methyl ester depends on its number of double bonds, polyunsaturated fatty acids, which are more suscepti- www.nature.com/scientificreports/ ble to oxidation than the fatty acid having single bond 26 . The major fatty acids present in the plum oil were oleic acid and linoleic acid. According to a previous study, the oils having fatty acids with more than 15 carbon atoms as major components could be explored to produce good quality biodiesel 13 .
Characterization of composite supports. were observed in all composite materials, however, they have shown variable transmittance intensities. A clear difference between bentonite-potassium ferricyanide composite and calcinized bentonite-potassium ferricyanide composite band intensity can be seen in FTIR spectra (Fig. 4). Calcination, which refers to the heating of inorganic materials to remove volatile components. The release of volatile matter during calcination minimizes internal shrinkage in later processing steps that can lead to the development of internal stresses and, eventually, cracking or warping. Calcination treatment is an integral part during fabrication and activation of the heterogeneous catalysts 27 . The OH-bending shows vibrations of the inner surface OH groups were observed at 913 cm −1 and that of the surface OH groups near 936 cm −1 ; the surface hydroxyls are associated with additional bands near 701 and 755 cm −1 . Iron-bearing composite materials show typical of bands due to Fe(AlFeOH) at 865-875 cm −1 and compressing at 3607 cm −128 . Two infrared bands are observed in the spectrum from 1650-1630 cm −1 indicates that the composite materials contained highly hydrogen bonded water 29 . The band just above 3600 cm −1 (at 3620 cm −1 ) corresponds to the "inner hydroxyls" located on the plane common to octahedral and tetahedral sheets. The vibration of the "outer hydroxyls" located on the surface and along the broken edges of composites may be attributed to bands recorded at 3668 and 3652 cm −1 . Adsorption of ions and complexes on clay minerals is considered to occur as a result of surface complexation, ion exchange, electrostatic and hydrophobic interaction 30 . The adsorption capacity modes on mineral surfaces are primarily divided into complexes of the outer sphere and inner-sphere surface. In general, in the inner-sphere complexes, chemical interactions are stronger than in the outer-sphere complexes. The mobility of ionic species in the environment influences these differences in binding strengths [31][32][33] .
Scanning electron microscopy (SEM) images were recorded to study surface morphology of different catalytic materials (Fig. 5). SEM image clearly show the presence of nano-rods on the surface of Granite-potassium ferricyanide composite and Kolten-potassium ferricyanide composite. Another interesting observation that can be recorded from SEM images is the changes in surface characteristic of Bentonite-potassium ferricyanide composite after calcination. Calcinized bentonite-potassium ferricyanide composite found to contain more nano rod like structures at its surface as compared to Bentonite-potassium ferricyanide composite which contained spherical particles. In broader sense, SEM images show that catalyst loaded composite materials surface particles were different in size and of variable shape 34 .
Energy-dispersive X-ray spectroscopy (EDX) is a powerful tool for the analysis of fine-grained clay mineral components, and Al-pillared clays in particular. It can be seen that Al, Si, Co, Ni and Fe are the primary elements of the untreated clay. The exchange process resulted in an increase in the composite's aluminum, iron and nickel content. EDX spectra for all composite materials were recorded and is presented in Fig. 6. The S, Al, S, Mg, P, Ca and O were recorded in the element analysis EDX, and various peaks of Fe and C of carbon with Fe 3 O 4 elements were also observed. Oxygen was present as a major element and shows the presence of most other compounds as oxygen derivatives. Bentonite-potassium ferricyanide composite was calcinized to produce Calcinized bentonite-potassium ferricyanide composite. EDX data of Bentonite-potassium ferricyanide composite and Calcinized   www.nature.com/scientificreports/ bentonite-potassium ferricyanide composite show that after calcination carbon and oxygen was reduced. The other lost volatile compounds after calcination were of Na, Mg, Al, Si, and S. Within the clay structure, the content of these elements was not constant, and the pillaring process impacted it. The increase of Al-Si is due to presence of Al 13 poly cation in the Al-pillared clay particle. Which can be seen in the curves, the reflectance of the non-treated clay is lower than that of the particles containing Al-integrated clays and in the considered spectral region varies between 50 and 60%. The findings also showed that there are more EM waves reflected from the Al-exchanged clays than those from the Al-pillared ones. This could be due to the contribution of electrical dipoles or multiple reflection phenomena in Al-exchanged clays from the front face of the first layer 35 .
The optimized catalytic materials that have shown highest transesterification ability were subjected to XRD studies (Fig. 7) including bentonite, calcinized bentonite, bentonite, and potassium ferricyanide composite and calcinized bentonite and potassium ferricyanide composite (Tables 3, 4 36,39 . In the present study, the combination of calcinized bentonite and potassium ferricyanide showed no significant variations as compared to the un-calcinized modified bentonite clay. Only the slight differences appeared in the clarity and sharpness of peaks as calcination improves the purity of sample

Conclusions
Following important conclusions can be withdrawn from the present study. Plum seed oil is of toxic nature and can be added to list of those waste oils which can be further explored to produce biodiesel. The present study reported the use of Bentonite-potassium ferricyanide, White pocha-potassium ferricyanide, Granitepotassium ferricyanide, Sindh clay-potassium ferricyanide, and Kolten-potassium ferricyanide composites to produce biodiesel from plum seed oil. The maximum biodiesel yield for all composite catalysts was obtained at 0.30% catalyst concentration and 60 °C. The maximum biodiesel yield was recorded for Bentonite-potassium ferricyanide composite which further increased after calcination of the composite. The fuel quality parameters of all biodiesel samples were found in the standard range. The calcination process was remarkably effective in the removal volatile compounds from composite materials to generate further active sites to enhance biodiesel yield.    FTIR indicated the presence of surface hydroxyl groups and bonded water on the surface of composite materials. Calcinized bentonite-potassium ferricyanide composite found to contain more nano rod like structures at its surface as compared to Bentonite-potassium ferricyanide composite which contained spherical particles. EDX data of Bentonite-potassium ferricyanide composite and Calcinized bentonite-potassium ferricyanide composite show that after calcination carbon and oxygen was reduced. The other lost volatile compounds after calcination were of Na, Mg, Al, Si, and S.