Development of heterogeneous catalyst from assorted periwinkle snail shells for sustainable biodiesel synthesis

. Owing to the environmental friendliness of biodiesel compared to fossil fuels, its application in internal combustion engines has gained wide recognition. The biodiesel industry requires effective heterogeneous catalysts developed from agricultural biomass/residues due to their overall cost-effectiveness, recoverability, and reusability. Scientific reports on developing active heterogeneous catalysts from periwinkle snail shells for application in transesterification reactions are limited, as there is no evidence of stability test results for the catalyst’s structural and elemental composition analysis. Also, there is a deficit of information on the catalytic effect on biodiesel yields. This research developed a heterogeneous catalyst derived from periwinkle snail shells (PSS) for biodiesel production. Pulverized PSS were sieved through several apertures to separate the samples into different particle sizes, viz. 250 mm, 500mm, and 1000 mm. The samples were calcined in an electric furnace at 500 o C for 3 and 5 hrs. The calcined catalyst samples were characterized using XRD, SEM, and EDS. Purified WCO was transesterified using the CPSS with the highest metallic oxides percentage under different experimental levels. All the CPSS samples exhibited a change in colour and weight loss after the calcination period. XRD observations revealed that calcium oxide (CaO) is the principal constituent of the calcined PSS (CPSS). The results showed that the highest (93%) traces of CaO was found in PSS 500 mm size calcined at 500 o C for 5 hrs. EDS results showed that the same sample had the highest calcium (Ca) composition with a weight percent of 57.38%. The highest biodiesel yield of 94.6% was obtained at optimum conditions of 70 o C reaction temperature, 9:1 molar ratio, 60 minutes reaction period, and catalyst content of 3 wt%. It was observed that biodiesel yields decreased when the catalyst concentration increased beyond 3wt%. Compared to the same amount of homogeneous catalyst, KOH, an average biodiesel yield of 90% was recorded from the same transesterification reaction conditions. The development of heterogeneous catalysts from PSS was established as a cost-effective means of catalyzing transesterification reactions and obtaining high biodiesel yields from WCO with the prospects of possible catalyst recovery and reuse.


Introduction
Over the years, efforts have been made to obtain more environmentally friendly alternative energy sources to replace traditional transportation fuels.Due to the adverse impacts of fossil fuel combustion on the human-environment scenario, most countries have noted the importance of utilizing alternative cleaner fuel sources, especially in running transportation diesel engines [1].Biodiesel is one of the possible solutions to overcome oil shortage and environmental issues [2].In recent times, biodiesel has gained more popularity due to the fleeting fossil fuel reserves and threat to human wellbeing, which is occasioned by gaseous pollutants released from the combustion of fossil fuels in industrial and vehicular engines.Many countries consider biodiesel production a potential alternative fuel source due to its combustion similarity to diesel [3].Biodiesel has similar chemical properties compared to conventional diesel fuel besides a higher flash point, higher viscosity, upgraded cetane number, and lower greenhouse emissions such as carbon monoxide and sulphur aromatic compounds [4].Owing to its environmental friendliness to fossil fuels, the production and use of renewable biodiesel would mitigate the excessive release of greenhouse gases into the atmosphere.Biodiesel is produced through a chemical process called transesterification.The feedstock (vegetable oil) is reacted with an alcohol (methanol, ethanol, propanol butanol) in the presence of a catalyst to produce Fatty Acid Methyl Esters (FAME) and other side reactions such as soap, depending on the type of catalyst used [5].When producing biodiesel, the catalysts used can be categorized into homogeneous and heterogeneous based on the phases in which they operate with the reactants.Most common transesterification processes utilize sodium hydroxide () and potassium hydroxide () as the primary catalyst due to their high catalytic activity.Sodium hydroxide and potassium hydroxide are sensitive to reacting to Free Fatty Acids (FFAs) but can produce some by-products that make it difficult to purify the biodiesel product [6].The homogeneous catalysts can either be acidic, enzymatic, or alkaline.Acidic catalysts are not cost-effective for biodiesel manufacturers because they possess corrosive properties and generally require higher process parameters such as oil to methanol ratio, temperature, reaction time, and pressure than others [7].Enzymatic catalysts are known for their high purchase costs, which hinder the total cost of biofuel production.Enzymatic catalysts react slowly, meaning the reaction rate of the whole production process would be stalled; hence they are only utilized for lower-scale production [7].Alkaline catalysts produce better FAME conversion and formation than acid catalysts at modest reaction conditions [8].They also have a long-lasting life span that will yield more reactions, higher catalytic activity, and lower solubility in methanol [9].To overcome the limitations of homogeneous catalysts, heterogeneous catalysts are introduced.Heterogeneous catalysts are favoured for their non-corrosive nature and can be utilized in batch or continuous system production [10].Furthermore, the heterogeneous catalyst can easily be separated and recovered for repeated use after the transesterification process, cutting production costs [10].Heterogeneous catalysts operate in different phases from the reactants [11].Biodiesel production processes that use heterogeneous catalysts are environmentally friendly as they do not lead to side reactions that promote the formation of soap and glycerol.Associated benefits, such as catalyst recoverability, reusability, shorter reaction durations, and high catalytic activities, have established wider applications of heterogeneous catalysts in transesterification reactions.Compared to conventional homogeneous catalysts, heterogeneous catalysts can initiate the formation of higher and quality biodiesel [12], [13].Some examples of heterogeneous catalysts include Vanadyl phosphate ( 4 ) [14], Zirconia ( 2 )supported tungsten oxide ( 3 ) [15], Sodium molybdate ( 2  4 ) [16], Sulphated Zirconia ( 2 ) [17], copper vanadium  [18], and calcium methoxide ( 2  6  2 ) [19].However, the abovementioned catalysts require a lot of complex steps in their synthesis, which makes them expensive to produce [7].It is recorded that the world generates approximately 1 billion tons of agricultural waste annually from all agricultural activities [20], [21].This indicates the abundance of raw material that can be utilized to develop effective heterogeneous catalysts.Agricultural waste is any leftover material or substance from agricultural products such as damaged crops, waste vegetables, spoilt fruits, inedible plant materials, animal remains, and harvest residues [22].Utilizing biomass in bioenergy production is one of the advantages that has received significant attention due to its natural abundance and renewability [23].Existing literature on the lifecycle analysis of biomass valorization into biofuels has shown that biomass is a potent way of carbon sequestration.Since the plants absorb carbon dioxide through photosynthesis during their life span, this denotes that carbon dioxide is not created nor destroyed during the production and use of bioenergy [24].Catalysts derived from agricultural waste have received much attention lately due to their practical environmental friendliness.The use of mollusks shells [25], [26], fruit peels [27], plant pod husk [28], fruit seeds and cobs [29], crop peels, animal bones [30], and biomass ashes [31], [32] have been applied to biodiesel production from different sources of oil feedstock.Since these catalysts are derived from agricultural waste/biomass, the raw materials are abundant and renewable.Furthermore, biomass is non-toxic to the environment, easy to handle, and biodegradable [7].Agricultural wastes accumulate natural organic compositions throughout their lifespans, such as high carbon, oxygen, and metals such as potassium, sodium, calcium, magnesium, and other elements [33].If the agricultural waste were to be burnt, this would reduce the amount of carbon and oxygen content available, leaving metal oxides and carbonates to form ashes [31], [34].These characteristics, as mentioned earlier, indicate that an agricultural waste-derived catalyst would be the most costeffective catalyst in the industry [7].Calcination is one of the processes applied when producing a heterogeneous catalyst derived from agricultural waste [7].The calcination period and the temperature at which calcination occurs are critical since they both affect the catalyst's formation and surface morphology development [7].This research studied the development of an optimized heterogeneous catalyst utilized in biodiesel production.A heterogeneous catalyst was developed from periwinkle snail shells via calcination in an electric furnace at different temperature and time conditions.The developed heterogeneous catalyst was tested in a transesterification process for biodiesel production.Periwinkle is a particular species of sea snail identified by its distinct shape and marks.They are commonly found on riverbanks and mostly in coastal areas.The periwinkle snail has a robust shell.The mantle contains cells responsible for producing an organic matrix mineralizing with calcium carbonate providing the shell with complex structure properties [35].Many industries are already utilizing the calcium carbonate property such as the glass industry, construction industry to enhance the properties of concrete, water treatment, and the farming industry has managed to convert the calcium carbonate into poultry feed which provides calcium to the poultry.Orji et al. [36] conducted a study that determined the amount of calcium carbonate and magnesium carbonate present in the periwinkle shells they obtained from their local rivers.They reported that the characterized shells contained 38.40% calcium carbonate and 18.70% magnesium oxide.This showed that a calcium-based catalyst could be synthesized from the shells since it contains high traces of calcium.
To the authors' best knowledge, no researcher has reported the development of active heterogeneous catalysts from periwinkle snail shells for application in transesterification reactions.Hence, the need for this research.Although the reports from Orji et al. [36] may suggest the periwinkle shell sample as a prospective catalyst for transesterification owing to the composition of calcium carbonate, there is no evidence of standard stability test results available to analyze the structural and elemental compositions of the developed catalyst.Also, their report is deficient in facts regarding the catalytic effect on biodiesel yields during transesterification reactions.In contrast to our study, their work aimed to analyze the chemical content of the periwinkle shell using complexometric titration and determine its suitability in thin layer chromatography.In addition, their study is devoid of high-temperature calcination of the raw periwinkle shells.To establish the calcined periwinkle shells as a potential catalyst for transesterification reactions, plans were formulated to develop an active catalyst and characterize the contents.The shells were ground into different particle sizes and calcined for different hours at 500 oC.The calcined samples were then characterized using XRD, SEM, and EDS techniques to determine their crystallographic structures, chemical compositions, and physical properties.

Equipment and materials
The equipment used in this research includes sample bottles, pestle, and mortar, sieves, electric furnace, X-ray diffraction microscope, filtration medium, separation funnel, and magnetic stirrer.The materials used include periwinkle shells, ethanol.

Catalyst preparation and characterization
The selected agricultural waste, assorted Periwinkle Snail Shells (PSS) (Figure 1), were crushed and divided into two categories to investigate the effects of surface area and calcination period.The first and second categories consist of three samples of crushed PSS sieved through different aperture diameter sizes of 250 mm, 500 mm, and 1000 mm (Figure 2).The first category samples were labelled Periwinkle Snail Shell Quantity (PSSQ) 1.1, 1.2, and 1.3, while the second category samples were labelled PSSQ 2.1, 2.2, and 2.3, respectively.The two categories of samples were calcined at different temperature and time conditions.The sequential steps involved in the heterogeneous catalyst synthesis are highlighted as follows: • Assorted periwinkle snail shells were acquired and cleaned to remove potential contamination materials.• The shells were pulverized and sieved through different aperture sizes.
• The developed heterogeneous catalyst samples were characterized using XRD and SEM/EDS.Relative to preliminary laboratory experimentation, PSSQ samples were calcined at a constant temperature of 500 ℃ for 3 hrs and 5 hrs, respectively.The characterization results will aid in characterizing the developed heterogeneous catalyst's similar chemical compositions, crystallographic parameters, name, and formula of the obtained dominant compounds and elements.The crystallinity parameters and compound classification for the developed heterogeneous catalyst were done utilizing an X-ray diffraction pattern.The diffraction pattern was obtained from 2 degree angle utilizing (Copper) Cu radiation with a wavelength of 0.154nm.The XRD generator was operated at a voltage of 40kV (kilovolts) and a current of 40mA.The samples were referenced from several authors within the database, such as J. Terada with Physics society Japan (1953) with reference code 04-006-6528.The powder diffraction reference code, 01-086-4274, by Lui Shankei and Lui Tainming (2014) was used.Additionally, reference code 01-083-0578 by Rudolf Wartchow (1989) in Krystallography was used.The SEM equipment was used to provide microscopic images of the samples required to analyse the surface area of the tested samples.The SEM test was conducted according to the procedures described by Kohli and Mittal [37].For the electron beam to yield feasible data, the conductivity of the samples had to be enhanced.Through, coating the samples utilizing a carbon coating thickness machine providing at least 250 Armstrong.Once, the samples' conductivity was enhanced, they were then placed inside an SEM machine for testing.The SEM initiates by creating a vacuum chamber for the specimens by introducing Nitrogen with a baseline of 5.0, to a pressure of 4.8 −002 .Then finally applies a voltage of 20Kv to a filament of electrons to create a beam that will scan the samples.

Purification and transesterification of waste cooking oil (WCO)
Biodiesel synthesis was done using the developed heterogeneous catalyst derived from assorted PSS.Some quantity of WCO was obtained from local food outlets within the Johannesburg municipality.The acquired WCO contained impure particles or chemical elements that may hinder the process and quality of the biodiesel.Therefore, it is vital to remove the waste oil's impurities.This research used some of the guidelines provided by Mannu et al. [38].The following steps were used for purifying the WCO: • Separation of insoluble material: The initial step was to remove any solid material suspended within the cooking oil while it was in use.Once all the insoluble solid materials had been removed from the waste cooking oil, the second step was the separation of soluble materials.• Separation of soluble material: This was the step where soluble contaminants that had been acquired during its operation were removed by washing them off with water.Once the impurities were removed, the cooking oil and excess water were separated using a separating funnel and rotary evaporator to remove the water.The purified WCO was converted into clean biodiesel fuel using ethanol in the presence of the developed heterogeneous catalysts using the reaction process flow presented in Figure 3.

Fig. 3. Biodiesel production process flow diagram
Optimized conditions from the transesterification of WCO using non-synthetic caustic potash were chosen for the applied reaction parameters [39].The reaction setup is presented in Figure 4.The biodiesel product was measured and computed using Eqn 1 [40], [41].
To determine the performance of the developed catalyst on biodiesel yield, different amount (2wt%, 3wt%, 5wt%, and 7 wt%) of the catalyst with the highest amount of CaO was used in

Results and discussion
The results obtained and discussed in this work include the data from PSS calcination, XRD/SEM/EDS results of CPSS, and biodiesel yields from the WCO transesterification using the optimal catalyst sample.

Developed heterogeneous catalyst
The calcination data for the developed catalyst samples are presented in Table 1.It was observed that all the calcined PSS samples experienced a change in colour and loss of weight after the calcination time.The PSSQ 1.1 was found to experience the largest percentage of weight loss due to its size and lower calcination period.A corresponding significant reduction in weight was observed in the same particle size (250 mm) in the second category samples.The colour changes in all the samples were observed to reflect a shade of grey after calcination.The colour variation in each sample after a calcination period of 3 hrs and 5 hrs is shown in Figure 5.The samples calcined for 3 hrs had a darker colour than those calcined for 5 hrs.Comparing the colour change in the first category samples, PSSQ 1.1 had the lightest shade of grey, followed by PSSQ 1.3, which had larger white particles sieved through the 1000 aperture.Sample PSSQ 1.2 appeared to have the darkest shade of grey compared to PSSQ 1.1 and PSSQ 1.3.Regarding the second category samples, PSSQ 2.1 had the lightest shade of grey of the three samples, followed by sample PSSQ 1.2.Sample PSSQ 2.3, which consists of larger white particles sieved through the 1000mm aperture, had the darkest shade of grey.The colour variations of the samples were affected by the calcination period since the calcination temperature was kept constant.As the calcination period increased, some chemical elements were broken down and emitted from the samples.According to Kaur and Bhattacharya [42], lower calcination conditions, such as temperature and time, result in lighter and brighter pigment shade but with less strength, whereas higher temperature results in deeper shade.This can be confirmed by the XRD and SEM results.

Characterization results
The XRD and SEM/EDS results are presented in the following sections.

XRD results
The XRD characterization results are presented in Figure 6.From the XRD characterization tests, the scanned images are black and white due to the carbon coating process, which enhances the samples' surface visual properties when exposed to the electron beam.The sample with the highest similarity of calcium oxide shall be used as an illustration of the test data obtained.All the samples contained high calcium oxide traces, referenced from the XRD database.However, sample PSSQ2.2 contained 93% similarity of calcium oxide at the peak of 29.4546 on the position [2] (Cu) with a count well over the 4000 mark.All the samples indicated a dominant similarity of natural mineral, calcite, a calcium carbonate compound.Furthermore, a calcium oxide compound was referenced from the XRD database 04-012-0489.From the X-ray diffraction tests, the obtained results are summarized as follows: • PSSQ1.1 indicated that 76% of the sample was calcium oxide, similar to reference 04-006-6528.
, 01 approximately 88% similarity to calcium oxide.The database further indicated the application of calcium oxides and carbonate within various industries, from enhancing cement in construction to pharmaceutical, superconducting material, and ceramic enhancement.The indicated structure of all the samples is rhombohedral.

SEM/EDS results
The EDS charts for samples PSSQ1.1,PSSQ1.2,PSSQ1.3,PSSQ2.1,PSSQ2.2, and PSSQ2.3 are presented in Figure 7 a, b, c, d, e, and f, respectively.In addition, the elemental compositions of the catalyst samples are presented in Table 2.The elemental traces within each sample are presented in Figure 5.As shown in Table 2, the results indicated calcium oxide being the dominant compound within the samples.The SEM images for the CPSS samples are shown in Figure 8.

(7A)
, 01   The relationship between the change in density of the grain and pore size in the microstructure of PSS samples calcined at 500 o C is shown in the SEM analysis.The calcined PSSQ 1.3 and 2.3 particles possess a large, flat, and dense structure with dispersed pores after calcination, which may result from particle melting at high temperatures [43].The CPSS samples appear smooth and less dense at a higher calcination period forming low inter-particle packing distances with smaller pore sizes.Calcination also led to thermal decomposition of PSS particles, thereby changing the structure to form a high inter-particle packing structure associated with larger pore sizes than the raw PSS.This is in agreement with the report of Trisupakitti et al. [43] and Laskar et al. [44] that concluded that heat leads to the degradation of organic substance in the snail shell, leaving the inorganic intact (CaO), which is brittle and more easily grounded.The EDS showed the elemental composition of the CPSS.The calcined PSS samples comprise oxygen, sodium, silicon, potassium, and calcium.The results showed that PSSQ 2.1 had the highest calcium composition (57.38%) while PSSQ 2.3 had the least calcium composition (47.36%).After calcination, it was noticed that the calcium percentage composition decreased as the particle size increased.This observation was common to the two categories.Calcium and oxygen were found to be the major in all the CPSS samples.Calcium is a dominant element in mollusk shells which can be set free by applying heat.Silicon and potassium were observed to have very low compositions, as observed in the reports of Checa [45].

Biodiesel yields from WCO
The biodiesel yields obtained from the transesterification of WCO using CPSS are recorded in Table 3.Compared to the non-synthetic caustic potash [39] and other homogeneous catalysts [46]- [48], higher biodiesel yields were obtained from all the reactions catalyzed with the CPSS samples.A catalyst concentration of 3wt% was found to be the optimal amount among the different concentrations, with an average biodiesel yield of 97.6%.It was also noticed that biodiesel yields decreased when the catalyst concentration increased beyond 3wt%.

Conclusion
This research was based on developing an active heterogeneous catalyst derived from PSS through calcination at 500 o C for biodiesel synthesis.All the CPSS samples exhibited a change in colour and weight loss after the calcination period.The SEM results obtained from the characterization of the CPSS indicated calcium oxide being the dominant compound within the samples.It was found that the highest similarity for calcium oxide compound was 93% from sample PSSQ2.2 referenced with 04-012-0489.The second highest calcium oxidedominated sample was PSSQ2.3, with approximately 88% similarity to calcium oxide.An average biodiesel yield of 97.60% was obtained from a catalyst concentration of 3wt%.This was found to be the optimal amount amongst the different concentrations considered.It was also noticed that biodiesel yields decreased when the catalyst concentration increased beyond 3wt%.Compared to homogeneous catalysts, higher biodiesel yields were obtained from all the reactions catalyzed with the CPSS samples.The development of heterogeneous catalysts from PSS has been established as a costeffective means of catalyzing transesterification reactions and obtaining high biodiesel yields from WCO with the prospects of possible catalyst recovery and reuse.The development of the heterogeneous is being targeted through experimental variation of the quantity of raw PSS, calcination time, and calcination temperature.The application of conventional statistical methods and AI algorithms can also be considered for optimal catalyst quantity and quality.
The CPSS catalyst can also be considered a potential constituent in synthesizing biomassderived heterogeneous hybrid catalysts for sustainable biodiesel production.

Table 1 .
Calcination data for catalyst samples.

Table 3 .
Biodiesel yields from WCO transesterification using CPSS PSSQ2.2 is an active heterogeneous catalyst that can be used in place of homogeneous catalysts for transesterification of plant seed oils.
The physicochemical properties of the highest biodiesel yields, compared with ASTM D6751 standards, are presented in Table4.The properties were found to be within the acceptable standard of being used as an engine fuel.The biodiesel physicochemical characterization results show that low-cost heterogeneous catalysts derived from PSS can be applied in the conversion of WCO to biodiesel.This also indicates that the synthesized catalysts sample

Table 3 .
Fuel properties of WCO biodiesel compared with ASTM D6751 standard