Green Heterogeneous Base Catalyst from Ripe-Unripe Plantain Peels for the Transesterification of Waste Cooking Oil


 Economical feedstocks such as agricultural wastes, food wastes, and waste cooking oil were used for biodiesel production to expand their application. Thus, a solid base catalyst was synthesized from a mixture of ripe and unripe plantain peels at a calcination temperature of 500 oC for 4 h. The catalyst was characterized using Scanning Electron Microscope (SEM), X-ray Diffraction (XRD) analysis, Fourier Transform Infrared (FT-IR) spectroscopy, Energy dispersive X-ray (EDX) analysis, and Brunauer-Emmett-Teller (BET) method. The waste cooking oil (WCO) used in this study was first pretreated with 3% (v/v) of H2SO4 via esterification reaction due to its high acid value. The esterified WCO was converted to biodiesel via transesterification reaction, and the process was then modeled and optimized using Taguchi L9 orthogonal array design method considering reaction temperature, reaction time, catalyst amount, and methanol/WCO molar ratio as the input variables. Based on the results, the synthesized catalyst predominantly contained potassium phases with 45.16 wt.%. The morphology of the catalyst revealed a crystalline mesoporous nanocomposite. At the end of WCO esterification, the acidity of the oil decreased from 5 to 1 mg KOH/g. The optimal conditions established for the transesterification process were catalyst amount of 0.5 wt.%, methanol/WCO molar ratio of 6:1, reaction temperature of 45 oC, and reaction time of 45 min with a corresponding biodiesel yield of 97.96 wt.%. The quality of the biodiesel produced satisfied the specifications (ASTM D6751 and EN 14241) recommended for biodiesel fuels. Hence, a blend of ripe and unripe plantain peels could serve as an efficient heterogeneous base catalyst in producing biodiesel from WCO.


Introduction
The petroleum oil price and the environmental pollution generated from fossil fuels have made biofuel production a developing area. Other alternative non-polluting technologies such as hydrogen-fueled vehicles, hybrids, plug-in electrics, compressed natural gas, and solar enable devices are possible solutions to energy and environmental problems (Kumar and Sharma, 2011). However, biofuels are renewable energy sources with vast potentials to replace fossil fuels in the present and future energy mix (Deora et al., 2021). Biomass sources from which biofuels are naturally abundant and can ensure the stability of the ecosystem. Biodiesel, a form of biofuel, is a renewable source of energy with several environmental bene ts.
The methods involved in producing biodiesel depend on the catalyst used in the process. Different catalysts that have deployed in producing biodiesel are base catalysts (e.g., KOH or NaOH), acid catalysts (e.g., H 2 SO 4 ), and enzymes (e.g., lipases) (Marchetti et al., 2007). Currently, investment in biodiesel production is adversely affected by several factors including, production technology type, nature of raw materials, and market feedstocks price (Gebremariam and Marchetti, 2018). Biodiesel price doubles that of mineral diesel in the market, due to its production cost. Biodiesel production cost involves two parts, viz., raw materials (feedstocks) cost and processing cost (Huang et al., 2010).
It is argued that the biodiesel production cost depends on the technological routes; however, the cost of feedstock, regardless of the technology employed, accounts for the major production costs (Akella et al., 2009;Ahmad et al., 2011). To produce economical biodiesel would require a cheap source of feedstock such as inedible oil, used cooking oil, and animal fats (Gebremariam and Marchetti, 2018). Generally, non-edible oils are not consumed by humans due to the presence of toxic compounds. Also, their availability and the low cost of plantation of non-edible oil crops position them as economically viable feedstocks. However, many non-edible oils are not suitable for biodiesel synthesis and require pretreatment (Gebremariam and Marchetti, 2018;Deora et al., 2021). Several developed countries, due to climate change threats, are embracing alternative energy sources. The increasing interest in biofuel is due to its intrinsic advantages over fossil fuels. To this effect, the growth projection of the biodiesel industry may place huge pressure on nature and biodiversity in emergent nations (Kumar and Sharma, 2011). One way to address this pending crisis is to adopt total biomass-dependent fuel. Waste cooking oil is a good raw material for biodiesel development since it is cheap and abundant in several countries. Its usage in biodiesel synthesis will prevent competition from using edible oils (Soji-Adekunle et al., 2018; Sivarethinamohan et al., 2022). Apart from the converting waste cooking oil (WCO) to fuel, the problems associated with its disposal are equally solved (Predojević, 2008). It is estimated that a minimum of 16.54 million tons of WCO is produced every year, covering major countries in Europe, Asia, and North America Conventionally, biodiesel is industrially produced using homogeneous catalysts. The use of homogeneous catalysts for the transesteri cation of triglycerides is not friendly to the environment and equipment, while the enzymatic transesteri cation process is expensive Bhatia et al., 2020). The use of solid catalysts for biodiesel production has several advantages: minimum energy consumption, lower material costs, and minimum water usage. This class of catalysts can easily be separated from the product and recycled many  (Balajii and Niju, 2020). Also, a mixture of plant biomass has been used to develop solid catalysts; cocoa pod husk-plantain peel (Olatundun et al., 2020) and cocoa pod husks-plantain peel-kola nut husk .
This study reports the synthesis of a green solid catalyst from agricultural wastes and its subsequent application in the transesteri cation reaction. The focus was to produce biodiesel from WCO via transesteri cation using the base catalyst developed from ripe and unripe plantain peels mixture. The heterogeneous base catalyst was synthesized from a mixture containing an equal proportion of ripe and unripe plantain peels ashes. The biodiesel yield and the process parameters (reaction time, molar ratio of oil to methanol, reaction temperature, and catalyst loading) in uencing the process were investigated using the Taguchi approach. The optimization of the transesteri cation of WCO was carried out to maximize the utilization of raw materials, and hence, lessen the production cost.

Materials And Methods
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Materials
The WCO, together with the ripe and unripe plantain peels used in this study, were obtained from the Cafeteria Kitchen of Landmark University, Omu-Aran, Kwara State, Nigeria. Analytical grade reagents and chemicals were used for the study.

Synthesis of catalyst from plantain peels
The two types of plantain peels used in this study were treated separately before calcination. The ripe and unripe plantain peels collected were cut into small pieces, then washed in distilled water to remove the dirt and stones. The washed peels were oven-dried at 100 o C until a constant weight was observed. The plantain peels were burnt, and the resultant ashes were mixed in equal proportion (ripe:unripe of 50:50). The mixed powdered ash was loaded in the porcelain crucible and calcinated in the mu e furnace at 500 o C for 4 h, according to Betiku and Ajala (2014). After calcination, the calcined ripe-unripe plantain ash (CRUPA) was sieved using 80 mesh size, and the obtained ash was stored for further work.

Characterization of CRUPA
The synthesized catalyst was characterized to investigate its features. Energy-dispersive X-ray spectroscopy (EDS) of CRUPA was performed to determine the elemental contents using a Thermo Fisher Nova NanoSem, and the diffractogram generated was recorded with an Oxford X-max 20mm 2 detector (Oxfordshire, U.K.) The functional groups on the active site of the CRUPA surface were determined by Fourier Transform Infrared (FTIR) analysis using a Perkin Elmer UATR Two spectrophotometer (Llantrisant, U.K.). The spectra generated were recorded over the range of 4000 to 400 cm −1 . The crystalline phases in CRUPA were determined using X-ray diffractometer D8-Advance from Bruker AXS (Germany). The measurement involves a continuous scan in locked coupled mode with Cu-K a radiation with LynxEye detectors. The sample measurements run within a range of 2θ with a typical step size of 0.034° in 2θ. The surface morphological features of CRUPA were taken using Tescan MIRA3 RISE scanning electron microscopy (SEM). The Brunauer-Emmett-Teller (BET) method of adsorption/desorption on nitrogen gas was used to determine the surface area and porosity of CRUPA using a Micromeritics instrument (Tristar II 3020). The catalyst pore size distribution, width, and adsorption pore volume were determined by Barret-Joyner-Halenda (BJH) method.

Pretreatment of WCO
WCO was centrifuged to remove the solid residues and particles left after being used. Thereafter, the oil was moderately heated to evaporate any water moisture that might be trapped. After cooling, the physicochemical properties of WCO (acid value, density, kinematic viscosity, saponi cation value, and iodine value) were measured according to the standard methods (AOAC, 1990). The physicochemical characterization of the WCO had an acid value of 5 mg KOH/g oil. Hence, WCO was subjected to the esteri cation process to reduce its acid value . The esteri cation of WCO was carried out in a two-neck glass reactor placed on a hotplate with a magnetic stirrer. The esteri cation conditions were methanol/WCO molar of 6:1, temperature of 65 o C, reaction time of 45 min, and catalyst amount of 3% (v/v). A known volume of WCO was measured, and methanol of known volume was added. The esteri cation process proceeds accordingly. After the process, the esteri ed oil was separated using a centrifuge (TMT-XC-450-PRP) machine. The esteri ed oil was dried over a hotplate and stored for further use.

Transesteri cation of WCO using CRUPA
The transesteri cation of WCO and methanol using a synthesized catalyst (CRUPA) was carried out according to the conditions stipulated by Taguchi orthogonal array (L9) design method. The L9 design was chosen for its simplicity and ability to provide exhaustive information about the process parameters and the yield using a few experiments (Oraegbunam et al., 2020). The parameters considered in the modeling of WCO transesteri cation are methanol-to-WCO molar ratio, temperature, time, and catalyst amount. The chosen parameters were based on our previous studies Fadara et al., 2021). The different levels of each parameter used in this study are methanol-to-WCO molar ratio (6:1, 9:1 and 12:1), reaction temperature (45,55 and 65 o C), reaction time (50, 70 and 90 min) and catalyst loading (0.5, 1.5 and 2.5 wt.%).
Biodiesel production from WCO was conducted in a three-necked glass reactor placed on a hotplate with a magnetic stirrer. A glass re ux condenser was mounted on the reactor to prevent methanol evaporation from the reaction mixture. Initially, a known volume of WCO was charged into the glass reactor, a known amount of methanol was added to it depending on the molar ratio speci ed. The mixture was allowed to heat to the set temperature under continuous stirring. Then, a known amount of CRUPA was added, and the reaction continued for a time suggested by the model. After the transesteri cation process, the mixture was centrifuged to separate methanol, biodiesel, and the residual catalyst. The biodiesel was washed three times with warm distilled water (65 o C) to remove impurities and residual methanol. The biodiesel obtained (waste cooking oil methyl esters -WOME) was dried, and the yield was gravimetrically determined using Eq. (1).

Physicochemical properties of WOME
The quality of WOME produced was ascertained by determining the physicochemical properties of the fuel. The speci c gravity, saponi cation value, acid value, density, iodine value, and kinematic viscosity were determined using the standard method (AOAC, 1990). Other fuel parameters such as cetane number (Krisnangkura, 1986) and calori c value (Demirbaş, 1998) were also determined. The FTIR analysis of WOME was carried out using an FTIR-UATR spectrophotometer.

Elemental composition of CRUPA
The chemical composition obtained using EDX analysis at ve different sites is shown in Figure 1. The mean (wt.%) of the elemental composition of CRUPA is presented in Table 1, and the elements present are C, O, Mg, Si, and K. The results show a considerable presence of potassium (K) in the form of K 2 O and K 2 CO 3 as well as other traces of metallic oxides. This indicates that the developed CRUPA has high basicity due to the presence of metallic oxide. As a result, the synthesized catalyst could be an e cient heterogeneous catalyst in expediting the transesteri cation of biomass oil. The K content (45.16 wt.%) in this study was more than the K value (41.37 wt.%) found in the calcined banana peel ash (Gohain et al., 2017). However, a higher K content was earlier reported in previous studies that used either ripe plantain peels or unripe plantain peels. The K content (54.73 wt.%) was reported for a calcined sample (temperature: 500 o C) developed from unripe plantain peels (Betiku and Ajala, 2014). Balajii

XRD results of CRUPA
The bulk phases of the CRUPA were identi ed using XRD analysis. The XRD peak of the synthesized catalyst is depicted in Figure 3. According to the results, the two phases identi ed in this catalyst are potassium chloride (KCl) and potassium carbonate hydrate (K 2 CO 3 1.5H 2 O).
The lattice structure of KCl was face-centered cubic while that of potassium carbonate hydrate was monoclinic. The surface morphology of CRUPA was examined by scanning electron microscope. The SEM images of the catalyst taken at different nano-sizes are shown in Figure 4(a-d), which revealed the microstructure similar to the morphology earlier reported (Betiku et al., 2016;Olatundun et al., 2020). Figure 4a shows a large aggregate of spherical particles clumped together. The SEM image at (4b) shows increasing agglomerated particles with regular size distribution. The SEM image becomes clearer in (4c) with a highly ordered surface with large cracks across the image. At 10µm (4d), the porosity of CRUPA was highly noticeable, having a glossy-like surface.
3.1.5 Surface area, pore-volume, and pore width of CRUPA The synthesized catalyst (CRUPA) had a BET surface area of 1.10 ± 0.0050 m 2 /g, pore volume of 0.002366 cm 3 /g, and pore size width of 13.15 nm. The surface area compared favorably to the earlier reported results; 1.487 m 2 /g from Musa Balbisiana trunk (Deka and Basumatary, 2011). The BET isotherm of CRUPA is displayed in Figure 5a. The adsorption isotherm exhibited by CRUPA is a typical Type IV isotherm. This means that adsorbate would condense in the tiny capillary of the catalyst at a pressure below saturation pressure of the gas (Hwang and Barron, 2011). At the lower pressure, the in ection region represents monolayer formation while capillary condensation or multilayer formation occurs after monolayered surface coverage of the pore walls (Hwang and Barron, 2011;Naderi, 2015). The adsorption of the adsorbate on the pore surface steadily increases until it jumps at the relative pressure (P/P 0 ) equals 0.9. The BJH pore-size distribution of CRUPA is depicted in Figure 5b. The plot exhibits a strong interaction between adsorbate and calcined sample surface between the pore diameters of 5-25 nm. The synthesized catalyst in this study can be classi ed as a mesoporous material, that is, material or particles having a pore diameter within 2-50 nm (Naderi, 2015; Falowo et al., 2020).

Results of WCO transesteri cation
The pretreatment of high acid value oil is essential in producing biodiesel (Olagbende et al., 2021). The esteri cation reaction of WCO using H 2 SO 4 as a catalyst proved effective in reducing the %FFA of the oil 5 mg KOH/g oil to 1 mg KOH/g oil, which meant that 80% FFA reduction was achieved within the experimental conditions. The esteri ed WCO was directly transesteri ed to biodiesel since its acid value was within an allowable range. The actual yields and predicted biodiesel yields obtained from the transesteri cation of WCO under 9 experimental conditions are presented in Table 2. It was observed that the actual and predicted biodiesel yields are comparable, demonstrating the accuracy of the model in predicting the responses. Run 5 at the lowest level of the operating parameters gave the highest biodiesel yield of 96.39 wt.%. The lowest biodiesel yield (60.35 wt.%) was recorded at run 3, with the lowest reaction time, the upper level of catalyst loading, and the middle level of temperature and methanol/oil molar ratio. The results obtained from the transesteri cation of WCO were tted into the Taguchi L9 orthogonal array model response. An analysis of variance (ANOVA) was used to optimize the experimental results to determine the relative in uence of process parameters considered. The p-value of the model and each parameter at a 95% con dence level (p-value ≤ 0.05) was used to determine the signi cance of these terms. From the results (Table   3), the model is signi cant, including the methanol/WCO molar ratio, temperature, and reaction time. The impact of catalyst loading on the transesteri cation process cannot be captured statistically; hence, it is insigni cant within the range investigated. It was observed that there is a 0.01% chance of getting this high F-value (1342.92) due to noise. The relative contribution of the individual parameter was quanti ed by the ratio of the pure sum of squares to the total sum of squares. The methanol/WCO molar ratio is the most in uential parameter, followed by temperature. Also, reaction time had minimal effect on the biodiesel yield since it has the lowest F-value among the parameters investigated. The statistical tness of this model was evaluated (Table 4). The coe cient of determination (R 2 ) in this study was 0.9998, which indicates that the experimental data obtained from the chosen model tted perfectly. The R 2 value is close to 1, an indication that 99.98% of the variation observed in biodiesel yields with respect to the process parameters can be explained by the model. The adjusted R 2 and predicted R 2 values are 0.9993 and 0.9990, respectively. These values are in reasonable agreement with one another and less than the maximum allowable difference of 0.2. The standard deviation of this analysis was 0.1804, which shows that the results obtained from this study are reliable. The coe cient of variation (C.V. %) was 0.2301; the lower the value, the better the capability of the model in optimizing the process (Dhawane et al., 2016). The adequate precision of the model was 227.82; a value greater than 4 is considered adequate (Olagbende et al., 2021). Meanwhile, the value obtained is very high, which indicates that the model has su cient signal to optimize the transesteri cation of WCO.

Effect of methanol/WCO molar ratio on WOME yield
The in uence of methanol/WCO molar ratio on WOME yield at a temperature of 45 o C, reaction time of 50 min, and 0.5 wt.% catalyst loading is displayed in Figure 7. Generally, transesteri cation reaction according to the stoichiometric ratio requires a methanol/oil molar ratio of 3:1. However, excess methanol is needed to shift the equilibrium position to favour forward reaction. In this study, maximum biodiesel yield (96.39 wt.%) was reached at a methanol/oil molar ratio of 6:1. Oladipo and Betiku (2020) obtained 96.97 wt.% of Hevea brasiliensis oil methyl ester using a methanol/oil molar ratio of 6:1. Increasing the molar ratio to 9:1 led to a sharp reduction in biodiesel yield. The reason could be due to excessive dilution of oil by the methanol, thereby decreasing the rate of transesteri cation reaction (Zhang et al., 2019). A marginal increase in biodiesel yield was observed when the molar ratio was later increased to 12:1. A similar observation was reported in Oraegbunam et al. (2020), the maximum biodiesel yield was reached at a methanol/oil molar ratio of 6:1, and excess methanol to the reaction mixture led to a reduction in biodiesel yield. Excess methanol was reported to have increased glycerol solubility, which interferes with the ester phase and subsequently lowers biodiesel yield (Predojević, 2008).

Effect of reaction temperature on WOME yield
The in uence of temperature on WOME yield at methanol/WCO molar ratio of 6:1, catalyst loading of 0.5 wt.%, and reaction time of 50 min is depicted in Figure 7. The biodiesel yield was at the peak at the lowest level of the temperature (45 o C). It can be seen that the biodiesel yield decreased when the temperature was increased, and

Effect of reaction time on WOME yield
The impact of reaction time on the WOME yield at methanol/WCO molar ratio of 6:1, a temperature of 45 o C, and 0.5 wt.% CRUPA is shown in Figure 8. was su cient to produce 96.53 wt.% of biodiesel from moringa oil. Since transesteri cation is a reversible reaction (Ma and Hanna, 1999), produced biodiesel can be converted to triglycerides at a longer reaction time. This could be the reason the biodiesel yield decreases with increasing reaction time.

Establishment of optimal condition and model validation
The optimal condition for the transesteri cation of WCO to WOME by CRUPA was established using the numerical optimization method of solving the regression model. the optimum values was repeated thrice, and the average value of biodiesel yield obtained was 97.52 wt.%. The average biodiesel yield is close to the predicted biodiesel yield, which signi es that the model used in this study is capable of predicting the optimum values and adequately describes the process.

Results of CRUPA reusability
One of the advantages of the heterogeneous catalyst over the homogeneous catalyst is recycling the catalyst and reapplying it in another production cycle. The reusability test was conducted to determine the number of production cycles this catalyst can be reused. This test was conducted at the optimum conditions of the model. At the end of each run, the solid catalyst was separated and applied to another transesteri cation set-up without any treatment. A plot of biodiesel yields against production run is presented in Figure 9. It was observed that the synthesized catalyst could be reused for ve production cycles without losing a substantial amount of biodiesel. At the end of the 5th cycle, the biodiesel yield was reduced by 13.32%. The loss of catalytic activity of the catalyst can be attributed to partial blocking of catalyst pores by reacting molecules which subsequently prevent other triglyceride molecules from accessing the active catalyst surface (Konwar et al., 2014).

FT-IR spectrum of WOME
The FT-IR spectrum of the WOME is shown in Figure 10. The absorption bands present and the corresponding vibrational types are depicted in Table 5 obtained indicates that the transesteri ed WCO possessed characteristics signals of fatty acid methyl esters, which con rms that the transesteri cation process was successful.

Physicochemical properties of WOME
The physical and chemical properties of WCO and biodiesel produced using CRUPA were characterized, and the results, along with the recommended standards (ASTM D6751 and EN 14214) for biodiesel, are presented in Table 6. Based on the results, there was an improvement in the physicochemical properties of the biodiesel over WCO. This further highlights the importance of the transesteri cation in the conversion of triglycerides to methyl esters. The acid value of WOME synthesized in this study compared with the value reported in other studies (Soji-Adekunle et al., 2018; Gohain et al., 2020) indicates that the biodiesel is stable and will minimize the deposit rate in the fuel system. The kinematic viscosity, cetane number, calori c value, and density of the produced WOME show that the biodiesel has a good ignition quality and su cient energy capacity. The values also compared favorably with the biodiesel produced from WCO using different catalysts and the values recommended for biodiesel standards.

Conclusion
The utilization of biodiesel in the energy mix has been reported to guarantee energy security and ensure ecological balance. Biodiesel production on a large scale, if to be economical, would require inexpensive, abundant biomass as feedstock. A mixture of ripe and unripe plantain peels used in synthesizing solid base catalyst is a viable feedstock. The catalyst (CRUPA) had high catalytic properties to transesterify WCO to biodiesel. The synthesized solid base catalyst predominantly contains potassium compounds; the K content was 45.16 wt.%. Factors such as temperature, reaction time, catalyst loading, and methanol/oil molar ratio affect the response of the transesteri cation process. The optimal condition established for WOME production from WCO using CRUPA was catalyst loading of 0.5 wt.%, methanol/oil molar ratio of 6:1, a temperature of 45 o C, and reaction time of 45 min, with a corresponding biodiesel yield of 97.96 wt.%. The biodiesel produced (WOME) had physicochemical properties which met biodiesel standards (ASTM D6751 and EN 14241). Thus, a blend of calcined ripe-unripe plantain peels could be utilized as an e cient heterogeneous catalyst for biodiesel production.

Declarations
The research results presented in this manuscript are the original works of the authors. This paper has not been published elsewhere in any form or language. The manuscript has not been submitted to another journal for consideration. Authors of this work have presented their results clearly without any falsi cation or misrepresentation of data. All references used in this study were properly acknowledged and all rules guiding a good scienti c practice were maintained to ensure professionalism.

Consent to participate
Not applicable Consent to publish Not applicable

Funding
The authors declare that no funds, grants and other support were received during the preparation of this manuscript.

Competing Interest
The Authors have no relevant nancial or non-nancial interest to disclose.

Author Contributions
Falowo Olayomi Abiodun and Olaiya Tomiwa Ayomiposi contributed to the study conception and design. Material preparation, data collection and analysis were performed by Falowo Olayomi Abiodun, Oladipo Babatunde, Oyekola Oluwaseun, Taiwo Abiola Ezekiel and Betiku Eriola. The rst draft of the manuscript was written by Falowo Olayomi Abiodun and Oladipo Babatunde, and Oyekola Oluwaseun and Betiku Eriola commented on and proof-read the previous versions of the manuscript. All authors read and approved the nal manuscript.   Plot of methanol/WCO molar ratio effect on WOME yield (wt.%) Figure 7 In uence of temperature on WOME yield (wt.%) Figure 8 In uence of reaction time on WOME yield (wt.%) Figure 9 Reusability results of CRUPA Figure 10 FT-IR spectrum of WOME