Active Razor Shell CaO Catalyst Synthesis for Jatropha Methyl Ester Production via Optimized Two-Step Transesterification

Calcium based catalysts have been studied as promising heterogeneous catalysts for production of methyl esters via transesterification; however a few were explored on catalyst synthesis with high surface area, less particle size, and Ca leaching analysis. In this work, an active Razor shell CaO with crystalline size of 87.2 nm, SBET of 92.63m /g, pore diameters of 37.311 nm, and pore volume of 0.613 cc/g was synthesized by a green technique “calcination-hydro aeration-dehydration.” Spectrographic techniques TGA/DTA, FTIR, SEM, XRD, BET&BJH, and PSA were employed for characterization and surface morphology of CaO. Two-step transesterification of Jatropha curcas oil was performed to evaluate CaO catalytic activity. A five-factor-five-level, two-block, half factorial, central composite design based response surface method was employed for experimental analysis and optimization of Jatropha methyl ester (JME) yield. The regression model adequacy ascertained thru coefficient of determination (R: 95.81%). A JME yield of 98.80% was noted at C (3.10 wt.%),M (54.24mol./mol.%), T (127.87min),H (51.31C), and R (612 rpm). The amount of Ca leached to JME during 1st and 4th reuse cycles was 1.43 ppm ± 0.11 and 4.25 ppm ± 0.21, respectively. Higher leaching of Ca, 6.67 ppm ± 1.09, was found from the 5th reuse cycle due to higher dispersion of Ca; consequently JME yield reduces to 76.40%. The JME fuel properties were studied according to biodiesel standards EN 14214 and comply to use as green biodiesel.


Introduction
The sustainability "strengthening the mechanisms for redistribution from the present to the future" has become a motto of all nations around the world, for promoting intrinsic scientific research methodologies into agriculture, materials, energy, economy, and even urban planning [1].Despite multiphase research in energy, rising global warming and air pollution issues instigated by fossil fuel combustion besides limited petroleum fuel reserves have led to research for sustainable renewable energy sources.Fatty acid methyl ester (FAME), commercially known as biodiesels, introduced in the 1980s as a sustainable fuel energy resource for reducing greenhouse emissions [2].FAME comprises monoalkyl ester of long fatty acids, typically produced by the transesterification of biologically produced feedstocks such as vegetable oil (VO), animal fats, and microalgae oils in the presence of methyl alcohol (MeOH) and a suitable catalyst [3,4].Transesterification reaction is a combination of three sequential catalyzed reactions (1)- (3) in which triglycerides (TG) of a VO were transformed to diglycerides (DG) and monoglyceride (MG) and finally as glycerol and methyl ester (ME) known as biodiesel [5].Jatropha curcas Linnaeus, an euphorbia family member, has attained the researcher's attention as one of the best suited nonedible VO feedstock types due to its agromedical tangible interests as well as pro human food cycle nature, for biodiesel fuel (BF) production via transesterification [4].Acid or base catalyzed transesterification of VO was largely reported in literature and also concluded on the use of heterogeneous catalysts for sustainable BF production [6,7].
(20 ∘ C-75 ∘ C) and catalyst concentration (0.2-1.8 wt.%), using sunflower oil in the presence of various homogeneous and heterogeneous catalysts.Goyal et al. [23] reported the impact of four reaction parameters including the methanol-oil ratio, reaction time, NaOH catalyst concentration, and temperature over transesterification of JCO.A CCD based four-factorfive-level was utilized to analyze the parametric impact on the FAME yield.An optimal FAME field of 98.3% was reported at 11 : 1 molar ratio, 1 wt.% catalyst, 54 ∘ C temperature, and 110 min of reaction time, while Dhingra et al. investigated Karanja [29] and Jatropha [30] biodiesels production optimization by considering five reaction parameters using the RSM coupled with GA [29].Many studies reported that the choice of reaction parameters directly contributes in biodiesel conversion process and influences FAME yield.Also, it is noted that reaction parameters, both stirring speed and reaction time, were less studied.
The literature studies emphasize suitability of calcium catalyst for biodiesel production via transesterification.Besides, higher surface area, large pores, and lesser particle size of a catalyst enhance their catalytic efficacy of transesterification reaction [11][12][13][14][15]. Ensis arcuatus has been commonly found to be fishery food source in the sandy coastal locations of Pacific and Antarctic areas and regionally referred to as Razor clams.The study reports of Kanakaraju et al. [31], Akmar and Rahim [32], and Hossen et al. [33] demonstrate potential of Razor clams marine life and abundance in the state of Sarawak as well as in the west Malaysian provinces.The previous works on synthesis of calcium catalysts are very limited and have been focused on increasing catalyst surface area and lowering the particle size from naturally available renewable wastes such as aquatic seashells [18,20,21,[34][35][36][37][38].However, according to the best of our broad literature survey, no work has reported on activated CaO nanocatalyst synthesis using Razor shells.Moreover, a very scant work was accounted on the study on kinetics of transesterification reaction parameters that include catalyst loading, methanol to oil ratio, heating temperature, stirring speed, and reaction time, which need to be undertaken.
In this study, CaO of high surface area and pores as well as lower particle size was synthesized from a seashell so as to result in an activated heterogeneous catalyst.Besides, the direct and interaction impacts of five reaction parameters on the FAME yield were proposed to be investigated over JCO two-step transesterification process.Activated CaO was synthesized using locally available seashell species, Razor shells.A laboratory scale experimental protocol "calcinationhydro aeration-dehydration" was developed for Razor shell CaO synthesis; besides, structural and morphological characteristics were analyzed.The transesterification reaction kinetics was studied by employing a CCD based on the fivefactor-five-level, two-block, half factorial, RSM model.JME production optimization protocol using active Razor shell CaO catalyst is shown in "Scheme 1."The catalyst stability, reusability, and leaching were investigated.The synthesized JME fuel properties were tested according to the biodiesel standards EN14214.Scheme 1: Jatropha methyl ester production optimization using active Razor shell CaO catalyst.

Material and Method
The Jatropha curcas seeds were obtained from a Jatropha farm, Wahaba Sdn.Bhd., situated in Sarawak, Malaysia.Jatropha curcas oil (JCO) was extracted mechanically using an oil expelling machine from their dried seeds.The crude JCO was used as extracted without any processing like purification or refining.JCO fatty acid profile was determined by the gas chromatography and the results are as indicated in Table 6.

Preparation of Active
Razor Shells CaO Catalyst.Razor shells were collected from a local community market at Muara Tebas, Sawarak, Malaysia.The CaO catalyst was synthesized following "calcination-hydro aeration-dehydration" of the Razor shell.The synthesis protocol is adopted as described before by Niju et al. [40,41] and modified to get a fine Razor shell powder.In a nutshell, about 500 gm of Razor shells was thoroughly water washed and dried overnight at 105 ∘ C in a hot air oven (IMPACT, Scotland, UK) [42].The dried Razor shell was finely crushed using a heavy duty professional blender (OmniBlend V, imbaco, Australia) and then sieved using an 80 m mesh.The micron-sized Razor shells powder was calcinated in a high temperature muffle furnace (KSL-1700X-A4, MTI Corporation, USA), at a target temperature of 850 ∘ C for 2.5 h.At a calcine temperature of 850 ∘ C, CaCO 3 of Razor shell decomposes as CaO and CO 2 .The fine CaO powder was refluxed in water for 6 h at 60 ∘ C. Simultaneously, the mixture was aerated using spherical and cylindrical air stone bubbler setup fitted with a Hi-blow air pump (HAP-60, 0.01 Mpa, 60 w Hailea) and then the mixture was allowed to settle for 2 h.Continuous aeration oxygenates refluxing mixture and precipitates heavy dirt and solid particles which results in refined highly pores Razor shell CaO particles.The catalyst was filtered and dried overnight at 120 ∘ C in an oven [42].The refined Razor shell solid powder was grounded for high grade fine catalyst particles utilizing a Planetary Ball mill (PM 400, Retsch, Germany) at 250 rpm for 2 h.Calcination of fine Razor shell CaO was carried out for 3 h at 600 ∘ C so as to transform hydroxides to oxide form through dehydration.Accordingly, an active Razor shell CaO catalyst was synthesized following "calcination-hydro aeration-dehydration" protocol.

Jatropha Methyl Ester
Production.Jatropha methyl ester (JME) production was carried out by following an in-house laboratory protocol.The high free fatty acids (FFA) and moisture contents of JCO strongly influence JME production through the catalyzed transesterification reaction [45].JCO of FFA value > 3% and moisture content > 1 wt.% which lead to saponification and oil hydrolysis besides reducing rate of biodiesel yield [46].Hence, a two-step transesterification process was adopted [47].The first step is acid esterification in which FFA of JCO reduced to a suitable minimum level followed by the second step which is base catalyzed transesterification [48].The successive steps which involved acid esterification and catalyzed transesterification are explained in the following sections.

Acid Esterification.
A digital hot plate magnetic stirrer mixer (MS300-BANTE make 300 ∘ C, 2 L, 220 V, 1250 rpm) and a 500 ml two-neck flat bottom glass reactor are attached to a water cooled reflex condenser used for the acid esterification of JCO.In order to improve the JCO conversion process and enhance the JME yield, esterification was carried out using methanol and concentrated H 2 SO 4 as a catalyst as mentioned in ( 4).An amount of 100 ml JCO was preheated at 110 ∘ C for 30 min to get moisture-free oil.A mixture of 60% (v/v) methanol and 1% (v/v) catalyst to JCO was prepared and added to the JCO.The mixture was vigorously stirred at 200 rpm and then heated to 50 ∘ C for 1 h.After the reaction, the reactant mixture was allowed to settle for 1 h and then washed successively thrice using double distilled deionized water.Finally, the oil portion was duly dried at 110 ∘ C to prepare moisture-free oil.As a result of acid esterification, FFA of JCO was noted all in a minimum of 1% with an esterification rate of 98.4%.Thus, the esterified JCO was suitable for the transesterification with base catalyst [48].

Design of Experiments for
Optimal JME Yield.The flow of the JME yield optimization was broadly executed in four sequential operations starting with the review, experimental design, modeling and optimization, and results validation.The intrinsic actions followed in each operation stage together with decision flow are shown in Figure 2.
In this study, five significant independent transesterification reaction parameters which influenced the JME yield include Razor shell CaO catalyst loading (), methanol to oil ratio (), reaction time (), heating temperature (), and the stirring rpm () which have been considered for the optimization of % JME yield.The effects of five significant reaction parameters together with intrinsic interactions were devised by utilizing a central composite design (CCD) based on the half-fraction model response surface statistical model.The transesterification experimental data obtained were then fit to a full quadratic equation as specified in ( 5) to analyze the (3) response variable, % JME yield, through the response surface regression procedure.
The symbolic notations of (5) are as follows: "" is the predicted response for % JME yield; " 0 " is the constant coefficient; "  ,   and   " are the regression coefficients of intercept, linear, quadratic interactions;   and   are the coded independent process parameters; and "" is the residual of the predicted and experimental value, known as standard error.The five significant parameters were set independently within the following ranges: 1 ≤  (wt.%) ≤ 5, 30 ≤  (mol./mol.%)≤ 70, 60 ≤  (min) ≤ 180, 40 ≤  ( ∘ C) ≤ 60, and 500 ≤  (rpm) ≤ 700.Considering each factor at five levels, 33 base experimental reaction iterations were concluded from a CCD of two-block, half-fraction model, which corresponds to sixteen cubic points, six center points in the cube, ten axial points, and one center point at the axial level.For a CCD the numerical value of alpha,  ± 2, is a measure of the distance that keeps each of the axial design points from the center in evolution of the factorial levels [27].In Table 1, the actual levels of individual parameters were tabulated representing their coded and uncoded values.The redundancy in datasets and results was minimized by performing all the experiments in a random run order.The Minitab5 16.2.1 software was employed for performing the analysis of variance (ANOVA) of the model together with CCD based response surfaces generation and % JME yield optimization.A Minitab 16.2.1 software was utilized for the response surface regression analysis and ANOVA of the designed model.The regression model was presented in (5) and validation was ensured through the confirmatory experiments, besides analysis of corresponding contour and surface plots.

Study of Razor Shell CaO Reusability, Leaching Analysis, and Transesterification with Reference
Catalyst.The Razor shell CaO catalyst was recovered by centrifuging the transesterification reactant samples at 4000 rpm for 60 min.The precipitated CaO was then washed in four sequential repeats with -hexane to remove Jatropha oil residues and then dried overnight in a forced air convection oven followed by recalcination at 850 ∘ C for 2 h before reuse.The oil portion separated was washed using double distilled deionized hot water successively for four times to clear away the traces of methanol and catalyst from the samples followed by drying over 110 ∘ C in order to obtain a pure JME.Furthermore, all the experiments were conducted using the variant operating parameters to investigate their specific impact on the JME yield.The biodiesel obtained from each reusability cycle was tested using an atomic absorption spectrometer-AAS (AA-7000, Shimadzu, calcium "422.7 nm" hollow cathode lamps as radiation source) and to measure the amount of calcium ions (Ca 2+ ) concentration that leached into the JME samples.The newly synthesized Razor shell CaO catalytic activity was evaluated in comparison with a lab grade CaO (CAS.NO.0001305788).Before being used, the lab grade CaO catalyst was dehydrated at 105 ∘ C for 2 h in a hot air oven.Further, the reused Razor shell CaO catalyst surface morphology was analyzed using SEM (TM3030, Hitachi, Japan).Five successive cycles of relative transesterification were carried out using esterified JCO under similar experimental conditions and standard ratios.

Analysis of Razor Shell CaO Characteristics.
Thermogravimetric analysis (TGA) and differential thermal analysis Figure 2: Schematic of optimal JME yield using experimental, modeling, and optimization protocol (customized based on [22]).The absorption bands over wavelength < 700 cm −1 were strong besides broad medium absorption bands at 990 cm −1 , 1430 cm −1 , and 3640 cm −1 which attributes presence of Ca-O stretching.According to the reports of Tan et al. [14], McDevitt and Baun [51], Nasrazadani and Eureste [52], and Zaki et al. [53], the strong IR spectral absorption over wavelengths 400 cm −1 and 290 cm −1 signifies Ca-O which confirms CaO presence.Further, weak absorption wavelengths > 3700 cm −1 specifically 3722 cm −1 and 3807 cm −1 were present due to carbonyl "C=O" and hydroxyl "O-H" groups asymmetric bending.As reported by De Sousa et al. [21], Tan et al. [14], and Margaretha et al. [19], the hygroscopic nature of catalyst is highly prone to absorb carbon dioxide and moisture from the atmosphere and subsequently forms CaCO 3 and Ca(OH) 2 .
The crystalline structural information of Razor shell synthesized CaO and lab grade CaO obtained from XRD analysis is shown in Figure 5 6(c) and 6(d).The Razor shell CaO SEM monograms encompass a number of particles in regular sizes of 145 nm-371 nm together with agglomerates observed.This can be attributed to structural changes of isolated isotropic calcium oxide particles after hydro aeration followed by calcined at high temperatures while CaCO 3 constituents decompose into CaO and CO 2 , as a result of decreases in particle sizes.These results are fully in accordance with investigation reports of Tan et al. [14] and Buasri et al. [10].
The BET and BJH analysis of Razor shell synthesized, hydro aerated, and nonaerated calcined CaO and uncalcined Razor shell together with reference catalyst samples had determined  BET of 92.63 m 2 /g, 85.27 m 2 /g, 5.21 m 2 /g, and 36.6 m 2 /g; pore diameters of 37.311 nm, 33.342 nm, 11.355 nm, and 13.861 nm; and also total pore volume of 0.613 cc/g, 0.423 cc/g, 0.0121 cc/g, and 0.126 cc/g, respectively.Compared to literature reports, Tan et al. [14] derived calcium catalyst from chicken-eggshells (54.6 m 2 /g) and ostricheggshells (71.0 m 2 /g); Buasri et al. [10] synthesized mussel shells (89.91 m 2 /g), cockle shells (59.87 m 2 /g), and scallop shells (74.96 m 2 /g), while Margaretha et al. [19] reported CaO of 17 m 2 /g.The  BET of Razor shell CaO is relatively high; moreover, the resulted pore diameter ranges within mesopores (2 nm-50 nm) demonstrate high value of the catalyst surface area together with their suitability for adsorption, catalytic, and energy storage applications [57].Hence, the CaO of Razor shells synthesized through "calcination-hydro aeration-dehydration" is an active catalyst owing to high pores and external surface area.Further, the Razor shell synthesized CaO particles demonstrated bimodal particle sizes (Figure 7).The particle sizes of hydro aerated calcined Razor shell CaO are in the range of 0.82 m-5.55m with a mean particle size of 2.97 m; meanwhile the nonaerated calcined Razor shell CaO showed particle sizes over 0.8 m-9.25 m and mean size of 4.37 m.The difference in mean and median particle sizes of hydro aerated and nonaerated calcined Razor shell CaO is significant.Therefore the "calcination-hydro aeration-dehydration" caused a mix of micro-and nanocatalyst particles together with few large agglomerates.The results are comparable and consistent with the literature reports of Tan et al. [14] and Kesić et al. [58].By adding Hammett indicators to Razor shell synthesized CaO, the catalyst samples successfully changed the color of phenolphthalein (H_ = 8.2) from being colorless to pink, thymolphthalein (H_ = 10.0) from being colorless to blue, and 2,4-dinitroaniline (H_ = 15.0) from yellow to mauve, respectively.Conversely, the catalyst was unsuccessful to change the color of 4-nitroaniline (H_ = 18.4).Hence, the newly synthesized CaO catalyst basic strength was labeled as 15 < H_ < 18.4 and therefore the basic strength of newly synthesized CaO catalyst was a strong basicity for JCO.The basicity of Razor shell CaO agreed with the published results for CaO, synthesized from waste materials [15,43,44].The Razor shell CaO catalyst was synthesized following "calcination-hydro aeration-dehydration" protocol which resulted in smaller particle size as well as greater surface area.According to Thiele [59], Buasri et al. [10], and Teo et al. [8] catalyst of higher surface area and lesser particle size improves its catalytic activity and reaction kinetics rapidly by refining the particle diffusion drawbacks.Henceforth the Razor shell CaO is labeled as "an active catalyst."

Jatropha Methyl Ester Production Analysis.
The JME was produced by transesterification of esterified JCO using methanol and Razor shell CaO catalyst and, in addition, three controlled parameters such as reaction time, heating temperature, and stirring rpm.The minimum FFA condition from the acid esterification was opted for Razor shell CaO catalyzed transesterification of Jatropha triglycerides.The standard protocol comprising experimental, modeling and optimization, and validation operation processes as presented in Figure 2 was followed in order.The selection of optimal experimental parametric values was determined on the basis of their relative impact evaluations while optimizing the JME yield, which is discussed in the following sections.

Regression Model Analysis.
The experimental results of second step pf transesterification reaction were analyzed using a two-block, half-fraction CCD statistical model as tabulated in Table 2.The estimated response surface regression coefficients for optimal JME using the uncoded units are as listed in Table 3.A regression modal equation that was fitting the JME yield as a response parameter to the other five significant reaction parameters in terms of their uncoded values is as given in The sign of regression coefficient is an indication of the effect of the corresponding term being synergetic or antagonistic, on the response parameter.Referring to the mathematical equation ( 6) all the five parameters in their linear form show a positive effect, whereas the quadratic terms of these parameters have a negative effect on the JME yield.However, the ten interaction terms have a mix of both positive and negative effects on the JME yield, which will be discussed in the later part of this section, in detail.
The regression modal analysis of variance (ANOVA) was utilized to study the adequacy of the model [60].Table 4 summarizes the ANOVA for the model designed.The value of the coefficient of determination  2 for the model was noted as 95.81%, which implies the fitness of the regression model in attributing good correlation between the percentage of JME yield and five transesterification parameters studied [61], and also to confess the successful integrity of the regression equation among the experimental data, appropriately.The difference between the coefficients of determination ( 2 ) and the correlation ( 2 adj = 87.81%) at large discloses the presence of the nonsignificant parametric terms in the designed regression model.The comparative graph of Figure 9 shows coherence between JME predicted and experimental yields.Besides, the -test and  values were the indications of modal and each regression coefficient significance, respectively [30].

Transesterification Reaction Parameter Effects Analysis on JME Yield.
Each of the five transesterification parameters' significance and their effect on JME yield were tested and analyzed in terms of their parametric interactions through the regression model ( 6) as well as the ANOVA.From the data reported in Table 4, the linear form of the parametric terms of methanol to oil ratio (), reaction time (), and heating temperature () was having relatively higher impact on the JME yield owing to the high -values as compared to the other parameters catalyst loading () and stirring rpm ().The interaction between any two selected parameters on % JME yield was analyzed by holding the remaining three reaction parameters at their median levels.

Catalyst Loading (C).
Catalyst loading is one of the key parameters that influence the production of JME in terms of both JME yield percentage and its quality [62][63][64].The effect of the Razor shell CaO catalyst loading interactions with other reaction parameters of the reaction time, methanol to oil ratio, heating temperature, and stirring speed was plotted, respectively, in the contour plots shown in Figures 8(a), 8(b), 8(c), and 8(d).Though the catalyst loading was carried through 1 wt.%-5 wt.%, JME yield was significant, ">98%" over 2 wt.%-4 wt.%.From the regression equation  (6), it is evident that the catalyst loading has a positive impact on the JME yield; that is, JME yield was proportional to the utilization of 1 wt.%-4 wt.% catalyst.It was noted that the lower catalyst loading (<2 wt.%) does not result in a productive JME yield.Furthermore, the higher catalyst loading (>4 wt.%) leads to the catalyst secondary reactions with other reactants which result in the reduction of JME yield [65].The calcium based catalysts have demonstrated leaching tendency to the biodiesels during transesterification [12,46,47].According to the biodiesel standards, EN 14214:2008, allowable concentration of group-I (Na + K) and group-II (Ca + Mg) is max.5.0 mg/kg in the biodiesel samples while the residues of excess metal oxides act as impurities in the biodiesel fuel produced [66].The catalyst loading interactions with methanol to the oil ratio ( × ) and transesterification reaction time ( × ) resulted in positive interactions.In contrast, interactions with reactants heating temperature ( × ) and stirring speed rpm ( × ) have alleviated the JME yield.Among all, ( × ) with regression coefficient "+4.379" demonstrated the highest effect impact in accelerating the transesterification reaction kinetics for the catalyst loading 2 wt.%-4 wt.% and about 46-65mol./mol.%, Figure 8(b), leading to ">98%" JME yield [67,68].

Methanol to Oil Ratio (M).
Methanol to oil ratio is a primary reactant parameter in the transesterification of VO.The methanol to oil ratio is determined in correlation with a type of VO, catalyst, and other reaction conditions.Figures 8(b), 8(e), 8(f), and 8(g) present the methanol to oil molar ratio with catalyst loading, reaction time, heating temperature, and stirring speed interactions, respectively.The JME yield of ≥98% was reported on to 46 ∘ C ± 1 ∘ C-56 ∘ C ± 1 ∘ C operating temperatures.According to the stoichiometric estimations, 30 : 1 mole/mole% methanol to JCO ratio is required [69]; however, in actual esterification experiments a critical methanol to oil ratio which ranges from 30 : 1 to 210 : 1 mol./mol.%was reported to carry out complete reaction, that is, transformation of JCO triglycerides into the JME and glycerin.The linear regression coefficient of methanol to oil ratio () in the regression equation ( 6) "+6.707" refers to its greater influence on transformation reaction process as well as on optimal JME yield%.Although experiments were performed over a range 30 ≤  (mol./mol.%)≤ 70, lower JME yield% results were noted for < 50 mol./mol.% of methanol to oil ratio; this may be due to the insufficient amount of methanol under the reaction conditions set forth, while 50-60 mol./mol.%methanol maximum transformation was achieved relatively compared to the molar ratio which was 60-70 mol./mol.%which indicates keeping the excessive amount of methanol hampers in achieving the reaction equilibrium as well as biodiesel cost economics [62].The methanol to oil ratio interaction with the stirring speed ( × ) resulted as a positive response.This may be due to increase in the catalytic and transesterification kinetics as a whole with the stirring speed and hence there is a proportional increase in the % JME yield together with methanol to oil ratio.On the other hand, methanol interactions with parameters heating temperature ( × : −4.479) and reaction time ( × : −2.021) were marked as highly negative on the response surface, that is, % JME yield.This is due to the reason that the reactant heating temperatures are close to the methanol boiling point (64.7 ∘ C) leading to continuous and rapid evaporation of methanol and hence lesser availability of the methanol for completing the transesterification [70].

Transesterification Reaction Time (T).
The requisite duration of reaction time for catalyzed transesterification is governed by factors such as catalyst type, degree of temperature, and also reactants mixing rpm [70,71].The effects of transesterification reaction time on JME yield% were investigated in conjunction with catalyst loading, methanol to oil ratio, heating temperature, and stirring speed parameter interactions.The analysis of experimental results is showed graphically in Figures 8(a), 8(e), 8(h), and 8(i) and statistically from ( 6), the highest significance for interactions of the reaction time and stirring speed ( × ) over interactions ( × ), ( × ), and ( × ) confesses through their regression coefficients "+3.753," "+0.493," "−1.393," and "−2.021," respectively.At the beginning of the transesterification, a passive rate of reaction phase was observed due to the gradual dispersion of methanol into the glycerides in JCO [70,72].As the reaction time increases, the amount of triglycerides conversion into ME besides JME yield% was also raised.Normally, across the operational reaction time, the "110 ± 5 min-150 ± 5 min" transesterification iterations results revealed a JME yield of "≥98%."Beyond these reaction duration levels a notable decline in JME yield% was reported.This may be due to the fact that extended reaction timings lead to reversible reactions (1)-(3) which result in continual ester content fall besides soap and diglyceroxide formation [70,73,74].Both methanol to oil ratio and temperature were showing unfavorable interactions with reaction time.Hence, an appropriate reaction time was maintained.

Heating Temperature (H).
The transesterification reaction kinetics was significantly affected by the reactants heating temperature [3].Figures 8(c), 8(f), 8(h), and 8(j) present the interactions of heating temperature over 40 ∘ C-60 ∘ C with the catalyst loading, methanol molar ratio, reaction time, and stirring speed.The study of temperature effect is very crucial since a JME yield of "≥98%" was reported on to 46 ∘ C ± 1 ∘ C-56 ∘ C ± 1 ∘ C. Both higher and lower temperature ranges showed distinct negative impacts on the reaction process.The use of heterogeneous catalysts suffers from their poor rates of reaction compared to homogeneous catalysts; however, higher reaction pressures and temperatures can contribute in accelerating the reaction rate [75].The reaction time gets shorter with higher temperatures and accelerates the rate of reaction but then rises the viscosity of reactants and reduces methyl esters conversion [3,76].Also, the increase in reaction temperature eventually results in rapid evaporation of methanol which can greatly promotes saponification besides leading to side reactions and soap formation [3,76].Despite a proper water cooling condenser attachment, if the operating temperature approaches close to boiling point of methanol, 64.7 ∘ C, evaporation of methanol accelerates [70,72].This effect was evident from the experimental results tabulated in Table 2. Likewise, the interaction ( × ) bears the relatively highest negative regression coefficient of "−4.479" besides the fact that other interactions include ( × ), ( × ), and ( × ).All these predominantly indicate that an increase in heating temperature does not appreciate the JME yield rather than reducing the whole reaction activity.Therefore, the heating temperature ranges are lower than methanol boiling point which minimizes its exponential vaporization and allows maintaining sufficient methanol levels during the transesterification.

Stirring Speed rpm (R).
Stirring speed of reactants is one of the key reaction parameters in the biodiesel production through heterogeneous catalyzed transesterification process [3,44,77].In Figures 8(d), 8(g), 8(i), and 8(j) the stirring speed interaction effects on JME yield together with the catalyst loading, methanol molar ratio, reaction time, and heating temperature were plotted.From the graphical results JME yield of "≥99 ± 0.5%" can be seen.Besides, the stirring speed interactions on the reaction time ( × ) and methanol to oil ratio ( × ) were comparably significant, with regression coefficients "+3.759" and "+0.721" as noted from (6).Even though the other two interactions ( × ) and ( × ) are indicated with negative regression coefficients, Figures 8(d) and 8(j) contour plots demonstrate a JME yield of "99.5%" and "98.5%," respectively.This emphasizes that a stirring speed is a productive reaction parameter in achieving a maximum JME yield.

Optimization of Reaction Parameters and Results
Validation.Optimal % JME yield was obtained by comprehensive integration analysis of linear and interaction impacts of all five reaction parameters.The comparative analysis of both statistical and experimental results along with the interaction contour plots reveals a significant effect of a reaction paramat a specific parameter levels.However, the minimum levels of any parameter were not intended to obtain optimal JME yield.The optimization of transesterification reaction parameters was carried out using a response surface optimizer tool and the results were validated for ten sets through confirmatory experiments successively.Data validation for ten sets together with the predicted and experimental results were tabulated as listed in Table 5.Among the ten validation test results 98.80% of JME yield was noted at optimal conditions  (3.10 wt.%),  (54.24 mol./mol.%), (127.87 min),  (51.31 ∘ C), and  (612 rpm) for the data set-9.A similar result with a simple variation of "−0.18%"JME yield was achieved for data set-10.The parametric optimal results are ascertained with previously published results [22][23][24]49].Further, the Razor shell CaO performance as a catalyst in converting the triglycerides of JCO to JME is comparable with literature reports by Tan et al. [14]  The reduction in JME yield may be due to the gradual loss of active sites on the catalyst surface, particle agglomerations with other reactant molecules, and/or calcium catalyst leaching to the biodiesel [13,14].An averaNge JME yield of 93.6% was noted up to the 4th catalyst reuse cycle.However, during the 5th catalyst reuse cycle JME yield was 76.40%, which is remarkably less by 7.93% from the 4th reuse cycle.Besides, the leaching of Razor shell CaO catalyst was carried out successively using AAS for measuring calcium ion (Ca 2+ ) leached to JME.The test results indicate Ca 2+ dispersion range of 1.43 ppm ± 0.11 to 4.25 ppm ± 0.21 during the 1st and 4th reuse cycles, which indicate compliance with EN14214 fuel standards.However, from the 5th reuse cycle, Ca 2+ of 6.67 ppm ± 1.09 was reported and subsequently dropped in JME yield besides increased leaching.Hence, it can be concluded that the Razor shell CaO is stable for four reuse cycles.Transesterification of JCO using lab grade CaO together with catalyst reuse tests was conducted at optimal conditions as obtained.Results of % JME yield using from both Razor shell CaO and lab grade CaO are shown in Figure 10.The JME yield achieved from Razor shell CaO is higher besides good catalytic performance over its reusability.Moreover, the JME yield obtained from present investigations is comparably higher than other researchers, Tan et al. [14], reported.This variation in JME yield can be attributed to the higher catalyst surface area, particle size, and pore volume of Razor shell CaO compared to catalysts used by Tan et al. [14].

Conclusions
Synthesis of heterogeneous CaO catalyst using Razor shells conformed by spectral characterization results of FTIR, SEM, XRD, BET&BJH, and PSA with a crystalline size of 87.2 nm,  BET of 92.63 m 2 /g, pore diameters of 37.311 nm, and pore volume of 0.613 cc/g which signifies CaO as an active catalyst.CaO was derived using a green synthesis protocol "calcination-hydro aeration-dehydration." The catalyst yielded an optimal Jatropha methyl ester (JME) of 98.80% via two-step transesterification of Jatropha curcas oil (JCO) at  (3.10 wt.%),  (54.24 mol./mol.%), (127.87 min),  (51.31 ∘ C), and  (612 rpm).The reaction kinetics and JME yield optimization were investigated utilizing a five-factorfive-level, two-block, half factorial, central composite design (CCD) based response surface method (RSM) design.The atomic absorption spectroscopic characterization of JME showed minimal leaching of calcium to JME until catalyst of 4th reuse cycle.The amount of Ca leaching increases for each type of reuse and subsequently reduces JME yield.The fuel properties tested according to biodiesel standards EN 14214 comply with JME's suitability as a green biodiesel and offers sustainable benefits.

C
Contour plot of JME versus M, C

Figure 4 : 3 Figure 5 :
Figure 4: IR spectra of calcined lab grade CaO and Razor shell CaO.
Figures 10(b)-10(f)) depict changes in surface morphology of reused Razor shell CaO catalyst over five cycles successively.

3. 7 .
JME Fuel Properties Analysis.The JME fuel property test results such as density at 15 ∘ C, calorific value, flash point, cetane value, specific gravity at 15 ∘ C, viscosity at 40 ∘ C, water content, ash content, acid value, monoglyceride,

Figure 10 :
Figure 10: (a) JME yield study over Razor shell CaO and lab grade CaO catalysts reuse at optimal transesterification reaction conditions.(b-f) SEM images of Razor shell CaO catalyst presenting catalyst surface structure variations after reuse.
+ CH 3 OH [22][23][24]49]ut the reaction process.The schematic of the experimental setup is shown in Figure1.A mix of methanol of 30 to 70 mol./mol.%andRazorshell CaO catalyst of 1 wt.% to 5 wt.% was thoroughly mixed for one hour and then transferred to the glass reactor for each batch of experiments.The transesterification reaction was performed over discrete operating parameters that include heating temperature of 40 ∘ C to 60 ∘ C, stirring speed of 500 rpm to 700 rpm, and a reaction time of 60 min to 180 min.These parametric ranges were ascertained with published literature[22][23][24]49].
Ester Fuel Properties Analysis.The specific JME fuel properties that include density at 15 ∘ C, calorific value, flash point, cetane value, specific gravity at 15 ∘ C, viscosity at 40 ∘ C, water content, ash content, acid value, mono-

Table 1 :
Factors and their uncoded levels.

Table 3 :
Estimated response surface regression coefficients for optimal JME yield using uncoded units.

Table 4 :
of variance for optimal JME.

Table 5 :
Predicted JME yield % data sets validation at optimal conditions with experimental results.

Table 6 :
Fatty acid composition analysis of JCO.

Table 7 :
JME properties., triglyceride, free glycerine, total ester content, and total glycerine contents are summarized in Table7.The results are compliance with the European biodiesel standard EN14214; thus JME indicates its suitability as a green biodiesel source. diglyceride