3.1 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) Granite-potassium 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, 0.3 and 0.6 %. The biodiesel yield increased on increasing catalyst amount from 0.15 to 0.30 %. The maximum biodiesel yield for all composite catalysts was obtained at 0.30 %. 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 (Table 1). The effect of reaction times on biodiesel yield was evaluated from to 2 to 6h (Table 2) at methanol to oil ratio 10:1, 0.3% catalyst, and 60˚C reaction temperature. Biodiesel yield increased with increasing the transesterification reaction time from 2 h up to 4 h. A further increase in reaction time (6 h) resulted in the loss of biodiesel yield. 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 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 (Table 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 (Giuffrè et al., 2017).
3.2 Determination of fuel properties
The estimated values of various fuel quality parameters are tabulated in Table 4. Biodiesel density is important parameters as the fuel working in the fuel injector system and engine is strongly related to density value (Górnaś et al., 2017). 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/lit. The recommended range of density lies between 0.86-0.90 kg/L by EN 14214:2003 for a B100 type biodiesel. 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 (Kostić et al., 2016Ndukwe and Ugboaja, 2020)Sajjadi et al., 2016). The iodine values measured during the present study ranged from 140.2 to 168.3 kg/L. 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 1g of oil sample and is inversely proportion to the molecular weight of fatty acid of the biodiesel” (Górnaś et al., 2017). 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 45-60 (Górnaś et al., 2017).
3.3 Gas chromatographic analysis (GC-MS Analysis)
The fatty acid composition of plum seed oil is given in the table 5. 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 susceptible to oxidation than the fatty acid having single bond (Churchill and Srinivasan, 2017). 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 (Azeem et al., 2016).
3.4 Characterization of composite supports.
FTIR spectra of various composites used in the present study including (a) Bentonite-potassium ferricyanide composite (b) Calcinized bentonite -potassium ferricyanide composite (c) White pocha-potassium ferricyanide composite (d) Granite-potassium ferricyanide composite (e) Sindh clay-potassium ferricyanide composite (f) Kolten-potassium ferricyanide composite are presented in the figure 1. The peaks in functional group region (4000-1500 cm-1) are characteristic of specific kinds of bonds, and therefore can be used to identify whether a specific functional group is present. The peaks in fingerprint region (1500-400 cm-1) arise from complex deformations of the molecule. They may be characteristic of molecular symmetry, or combination bands arising from multiple bonds deforming simultaneously. It can be seen from figure 1 that overall FTIR spectra of all composites significantly vary from each other although peaks in some regions matches too. However, frequencies of absorbing functional groups were different in all cases. A broad band of different intensities seen in the all-composite catalytic materials in the range of 3550-3200 cm-1 is due to O-H stretching vibrations. A clear C≡C band in the range of 2300-2100 cm-1 was observed for Bentonite-potassium ferricyanide composite, Calcinized bentonite -potassium ferricyanide composite, White pocha-potassium ferricyanide composite and Granite-potassium ferricyanide composite. Sindh clay-potassium ferricyanide composite and Kolten-potassium ferricyanide composite did not show any clear band. However, a weak band was present for C≡C. The bands in the region of 1650-1600 cm-1 (C=C stretching), 1650-1580 cm-1 (N-H bending), 1550-1500 cm-1 (N-O stretching) and 1000-1200 cm-1 (C-O stretching) 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 can be seen in FTIR spectra (Fig. 1). 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 (Rashid et al., 2018).
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-1 (Ndana et al., 2013). Two infrared bands are observed in the spectrum from 1650-1630 cm-1 indicates that the composite materials contained highly hydrogen bonded water (Lima et al., 2008). 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 (Murakami et al., 2016).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 (Atabani et al., 2019; Tang et al., 2011; Wang et al., 2012).
Scanning electron microscopy (SEM) images were recorded to study surface morphology of different catalytic materials (Fig. 2). 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 (Mainhardt-Gibbs, 2011).
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. 3. 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 Fe3O4 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 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 Al13 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 percent and 60 percent. 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 (Bazargan et al., 2015).