Study on the derived based catalysts from Theobroma cacao pod husks for the conversion of beef tallow blend with waste used vegetable oil for fatty acid ethyl ester synthesis: burnt, submerged fermented, calcination, hybrid design, catalyst rening and reusability

Billions of dollars paid by industries on catalysts used as feedstocks to obtain their end products are increasing at a geometrical rate, the report revealed that the global marketplace price of catalysts stood at USD 26.1 billion in 2019, and is anticipated to increase by 4% in 2020, and 4.5% progress rate in 2025. To salvage the world from extravagance spending, there is an urgent need for biomass wastes consideration and utilization. In this paper, three novel CaO-based catalysts derived from Theobroma cacao pod husks were tested based on ecacy for the production of biodiesel (fatty acid ethyl ester: FAEE) from the blend of beef tallow-waste use oil in the ratio of 5:95 (BTO 5 ) , 10:90 (BTO 10 ) , 15:85 (BTO 15 ) , 20:80 (BTO 20 ) , ….., 95:5 (BTO 95 ) , respectively. Process optimization of the transesterication reaction was carried out using a hybrid design to determine the effects of catalyst on the FAEE yield. The eciencies of the catalyst were tested via the rening and reusability test. Results revealed the oil blend ratio of BTO 60 : WUO 40 suciently produced low viscous oil that was easily converted to biodiesel. Catalysts' characterization revealed the three catalysts produced high CaO-based of 68.20, 81.46, and 87.65 (wt.%), which accounted for the high yield of FAEEs. Mathematical optimization showed that the catalyst amount (F-value between 14159.69-3063.24 with P-value between 0.0053-0.0115), played the most signicant role in oil conversion to biodiesel among the constraint factors considered (reaction time, catalyst amount, reaction temperature, and EtOH/OMR). Based on Box-Cox transformation, the values of the lambda obtained indicated a normal data results with an inverse function of Y 2 and Y 3 and normal function of Y 3 for polynomial model accuracy. Optimum validated FAEEs yields of 92.81, 93.02, and 99.64 (%wt.), respectively, with high R 2 . The qualities of the FAEEs were within the standard specication and the produced Process optimization Experimental Catalysts regeneration and carried and the biodiesel established physicochemical for transesterication process as the reaction time of 78.58 min, catalyst amount of 3. 37 (wt.%), reaction temperature of 79.23 o C, and EtOH/OMR of respectively. on catalyst BET adsorption analysis, the percentage CaO-based obtained from the developed catalysts showed Theobroma cacao pod husks could be used as industrial feedstock, and the quality of the FAEEs proved are within the ASTM D-6751 and EN 14214 biodiesel recommended standards.

developed from solid wastes as heterogeneous catalysts and applied it to the synthesis of biodiesel from vegetable oil. Vadery et al. (2014) [3] synthesized biodiesel from Jatropha oil through methanolysis of a developed based catalyst from coconut husk ash, while Bazargan et al. (2015) [4] adopted the used of palm kernel shell gasi cation as a heterogeneous biocatalyst for biodiesel synthesis. Chouhan et al. (2013) [5] converted Jatropha curcus oil to biodiesel through the help of Lemna perpusila Torrey ash, but, Razaei et al. (2013) [6]used waste mussel shell as a catalyst to synthesized biodiesel. The work of Ikbal et al. (2018) [7] reported the practice of discarded snail shells as a heterogeneous catalyst for the synthesis of biodiesel from soybean oil, while in 2019, Subramaniapillai [8] and co used Donax delltoides shell as heterogeneous catalyst. The use of pearl spar as a heterogeneous catalyst was employed in the study reported by Adepoju et al. (2018) [9], while in another work, Betiku et al. (2015) [10] developed heterogeneous catalyst from plantain peel, applied it to biodiesel synthesis from yellow oleander. The study conducted in another work by Betiku et al. (2017) [11] reported the use of heterogeneous catalyst developed from cocoa pod husks, while Nath et al. (2019) [12] developed heterogeneous catalyst from waste Brassica nigra plant for biodiesel production. In the work of Balajii et al. (2020) [13], the heterogeneous catalyst was made from the Banana peduncle, meanwhile, Minakshi et al. (2020) [14] used Carica papaya stem as a bio-based catalyst. Further study by Hadiyanto et al. (2016) [15] utilized Anadara granosa as a heterogeneous CaO-based catalyst, but, Trisupakitti et al. (2019) [16] used golden apple cherry snail as a heterogeneous catalyst to synthesized biodiesel from the vegetable. Falowo et al. (2020) [17] developed a new base catalyst from a combination of biomass wastes. All these reports developed catalysts from mixture or single solid wastes, and applied it to the synthesis of biodiesel from vegetable oil or its blend, except in the work of Adepoju, (2020) [18] where two derived base catalysts were tested on the synthesized biodiesel from the blended oil.
Therefore, to cover the gap between the e ciency of the developed catalysts derived from single or the mixture of solid wastes, and to introduce a novel blend ratio through the BTO5, BTO10, BTO15,………., BTO95 in an interval of 5 between the Beef Tallow Oil (BTO) and Waste Used Vegetable Oil (WUVO) for process industry (bio-fuel or margarine), this study developed three CaO-based catalysts from a burnt Theobroma cacao pod husk (BTCPH), calcined Theobroma cacao pod husk (CTCPH), and submerged fermented calcined Theobroma cacao pod husk (SFCTCPH), applied each for the synthesis of biodiesel from the blended of the oil obtained from Beef tallow-vegetable used oil. The catalysts prepared were characterized via Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray diffraction analysis (XRD), Fourier transforms infrared spectroscopy (FTIR), BET adsorption analysis, and Hammett indicator. Process modeling and optimization of biodiesel synthesis was carried out by considering ve levels-four factors (reaction time: X 1 , reaction temperature: X 2 , catalyst amount: X 3 , and ethanol to oil molar ratio (EtOH/OMR): X 4 ) via Box Behnken Experimental Design (BBED). Catalysts regeneration and reusability were carried out, and the suitability of the biodiesel in an internal combustion engine was established by determining the physicochemical properties.

Materials
WUVO was gotten from University cafeteria, Akwa Ibom State University; beef tallow was obtained from a slaughterhouse in Ikot Akpaden, Akwa Ibom State, Nigeria. The WUVO was thermally heated in a reactor pot at 120 o C placed on the temperature regulated hot plate for 25 min, and allowed to at 27 -28 o C, and then sieved to eliminate impurities. The ltered oil made cleaned was kept in a 10-litre drum tightly covered for further used.
Meanwhile, the beef tallow was thoroughly washed in an open vessel with half litre of 1.0 N disodium tricarbonate, and stirred for 15 min via mechanical mean. The resulting mix was centrifuged at 3000 revolution per minute for 15 min at a temperature of 30 o C using a polypropylene tube.
The supernatant was separated by ltration, and 40 g of anhydrous disodium sulphate was added, stirred for 8 min, and was re-centrifuged again for 6 min at a temperature of 25 o C (Adepoju, 2020) [18]. The puri ed beef tallow oil (PBTO) acquired was saved in a tightly covered container for further use.
Cocoa pod husk was obtained from Cocoa processing factory in Ondo State, Nigeria. The pod husk was cleaned by washing with ionized water, and was decanted, kept overnight to allow proper draining. The drained cleaned cocoa pod husk was separated into three-samples: Sample A was burnt in the open air until its completely formed ash; the ash was separated into smaller unit sizes (0.30 mm) by sifting, labelled as BTCPH. Sample B was milled into powder, divided into a smaller unit size of 0.3 mm by mesh sieve, and then calcined at 750 o C for 4 h in a mu e furnace. Sample C was fermented in distilled water anaerobically (submerged) for 10 days, and then the fermented sample was separated from fermented water by decantation, dried in an oven at 120 o C until a constant weight was achieved (bone dried). The dried fermented sample was milled and sieved into powder of 0.33 mm particle size before calcined at 750 o C for 4 h in a furnace. Both samples B (CTCPH) and C (SFCTCPH) after calcination were left in the furnace for 24 h for proper cooling, and then placed in cleaned containers as well as BTCPH for further characterization to determine their potential as a heterogeneous catalyst for industrial application (biofuel production).
All chemicals used in this study were of analytical graded and need no further puri cations, and were supplied by Sigma Aldrich.

Catalysts calcination and characterization
Samples BTCPH, CTCPH, and SFCTCPH were characterized by SEM to study the high spatial resolution (surface morphology) of the samples, XRD forti ed thru Kά and Cu radiation source, enhanced at 20 mA and 40 kV, to institute the angular scanning electron implemented in the range of 10 o <2θ <80 o at speed of 2.5 o C min -1 and to con rm the elemental analysis of the samples and the quantitative structure of the samples. FTIR was used to check the presence of functional group and validate the presence of characteristic absorption bands of major elements present within the crystals powder structures. BET isothermal adsorption and Hammett indicator was used to establish the pore volume, the surface area, the basic density site, and the total basic density.

Oil blend and its physicochemical properties
For proper oil mixed, it is necessary to know the synergy behind the oil mixed, a probability guesses mixed adopted in reported studies might result in fuel-engine problems. The key factors to be considered in mixing oil are the oil low viscosity, high volatility, and moderate acid value. Since there is no single report on the blend/mix ratio of BTO and WUVO, therefore this study adopted the following blend/mix ratios for BTO:WUVO in volumetric ratios as; 5:95 (BTO 5 ) , 10:90 (BTO 10  respectively. These ratios were chosen to ascertain oil with low viscous, high volatility, and low acid value that enhanced higher biodiesel yield using the derived CaO-based heterogeneous catalysts. 5:95 (BTO 5 ) is an abbreviation for 5 ml of BTO and 95 ml of WUVO.
The blended oil in different ratios was properly mixed by heating at 35 o C on a hot plate for easy miscibility owing to the uncertainty in the nature fat.
The viscosity, the acid value, and the speci c gravity of the resultant mix were examined. The mix with low acid value, low speci c gravity, and low viscosity was used for methylester (biodiesel) production. Additional properties such as iodine value, saponi cation, peroxide, cetane number, moisture content, etc. of the mixed oil were further determined via the o cial standard methods (AOAC 1997 andAOAC 1990) [19,20].

Biodiesel production
Production of biodiesel was carried out through the use of derived heterogeneous based CaO-catalyst synthesized from samples. The reaction process took placed in a 1000 ml reactor with three-necked, 200 ml of the oil mixed was rst heated at 100 o C for 60 min using a hot plate equipped with a magnetic stirrer. 2.5 (wt.%) of CaO catalyst was measured in a 250 ml dried-cleaned ask, and 50 ml of ethanol was measured and added to the ethanol ask to achieved EtOH/OMR of 1:4. The mix was positioned on a shaker for 15 min, and then introduced to the heated oil in the reactor.
The resulting dilution displayed two separated layers which contained the ethanol-catalyst layer and the oil layer, then the stirrer was introduced, and the reaction process was observed at 70 o C for 65 min.
At the end of reaction process, the non-soluble CaO-catalyst was removed by decanting, and the resultant product (ethanol-based-diesel) was distinguished through density separation. The biodiesel (fatty acid ethyl ester: FFEE) with leached catalyst was separated by washing with warm mixture of 1.0 g NaCO 3 and 20 ml methanol. The washed mixed was separated by ltration, and the ltrate-diesel was washed with ionized water, and separated via gravity settling. The water wet-diesel was dried over anhydrous sodium sulphate (Na 2 SO 4 ), and was separated by liquid-solid separation (decantation) to obtain the pure FAEE as liquid. The solid residual catalyst puri ed and reused. The step by step reaction process was conducted based on number of experimental runs generated by design expert.

Experimental design for FAEE
Experimental design software like Box Behnken Design (BBD), Central Composite Design (CCD), Factorial Design (FD), as well as Tanquchi design (TD) contains a lack of ts in the model analysis. Lack of ts only exist when the experimental design is poor, a poor model of the data, or poor choice of variables. This then results in high residual/error value, experimental replications, and a high number of experimental runs (between 28 -45 runs) when considering four-variable-ve level-factors. Therefore, to avoid these problems, considering four factors (reaction time:X 1 , catalyst amount:X 2 , reaction temperature:X 3 , and ethanol/oil molar ratio (EtOH/OMR):X 4 ), a Hybrid Design (HD) with a minimal point design for 3, 4, 6, and 7 factors with 5 level levels each was employed. This design has no replication to avoid time wastage; it has no lack of ts because the residual or error values are so small (10 -10 ) due to better experimental design, and a low number of experimental runs (four factors produced 16 runs) to avoid repetitions. These rotatable designs are better than BBD, CCD, FD, and TD, but are highly sensitive to outlier (missing data). Table 1 displayed the four-ve level-four variable-factors, the units, and the symbol used for the HD design.

Optimization analysis of FAEE
The experimental results were used for the process optimization analysis of the FAEE production. The response variable was the FAEE yield, the input variables were the factors at ve levels evaluated by mean of t summary. In the second-order model, the effects of variable signi cant and preferred terms were appraised by model effects. The ANOVA analysis (Analysis of Variance) was adapted to elucidate the results while diagnostic was used to estimate the t of the model, and model transformation, the graphical plots were used to interpret and evaluates the model. The p-value called the probability value, the f-value called the factor value, the df known as degree of freedom, and the VIF called the variance in ation factor, were used for model signi cance. The regression parameters such as the coe cient of determination: the predicted coe cient of determination, the adjusted coe cient of determination: and the adequate precision: Adeq. Prec., respectively, were used to check the model aptness.
Tri-dimensional space (three-dimensional plots) was used as a geometric setting to express the relationship between three variables (two factors and FAEE yield), while the second-order differential equation that further elucidates the connection between FAEE yield and the four factors is expressed arithmetically as Eq. (1). (see Supplementary Files)

Catalyst puri cation and reusability
The recovered catalysts were tried for its effectiveness by carried out the reusability tests however the catalyst was rst re ned. Puri cation stage was carried out by the method used by Adepoju et al. (2020a) [21] with modi cations as; the obtained catalyst was washed with an alcohol to eliminate the contaminate adhere at the catalyst interface as a result of transesteri cation processes. The catalyst puri ed with alcohol was centrifuged at 3500 rpm using an inbuilt heating vacuum centrifuge, and separated by decantation. The wet catalyst was dried in oven at 80 o C for 60 min so as to make free of the alcohol before cooled temperature of 27 o C and then reused.

Properties of FAEE
Properties of the FAEE such as density, viscosity, moisture content, mean molecular mass, acid, saponi cation, iodine, peroxide, higher heating value, cetane number, API gravity, and diesel index were determined so as to determine its aptness as an substitute for conventional fuel used in an diesel engine, via AOAC, 1997 [19]. The qualities were likened with ASTM D6751 [22] and EN 14214 [23] recommended standard.

Physicochemical properties of the blended oil
Presented in Table 2 are the datasets of the physicochemical qualities of the blended oil in a different ratio. However, since the major key factor in the selection of any oil is the viscosity and the oil speci c gravity, therefore the BTO 60 with the low viscosity of 22. 30 mm 2 /s and a speci c gravity of 0.890 were selected as blended oil for FAEE production. This blended oil produced lighter oil with low acid value and high API gravity (Adepoju, 2020b) [24].  shapes and a high surface area. This could be due to submerged fermentation which involves the growth of the microorganism in a homogeneous medium (inter-particle space and surface area) of the substrate and moisture content. However, it was observed that the release of the CaO from

FTIR analysis
The results of FTIR analysis of the burnt and calcined sample powder catalysts are displayed in Fig. 2(a-c). The spectral showed a sinusoidal waveform at different peaks con rming the effects of heat on the thermal degradation of the catalysts. The vivid descriptions of the wavelengths at different peaks for each catalyst which showed the stretches and the bending vibration of organic-inorganic functional groups present are presented in Table 4. The wavelength bands noticed between 693.3 -913.2 cm -1 for CTCPH, and the bands between 752.9 -913.  [24]. This showed that fermentation increased the presence of functional groups and the surface area of the sample, but the calcined fermented submerge Theobroma cacao pod husk SFCTCPH showed more functional groups.

Brunauer-Emmett-Teller (BET) and XRD analysis
Displayed in Table 5 Fig. 3(a) showed the relationship between the experimental results and the predicted values by the design software, the identi er of an appropriate exponent (Lambda = 1) to transform data into a normal shape due to residual error was as indicated in Fig. 3(b).
Usually, the Lambda values between -5 and +5 showed the transformation data has the highest likelihood of normal data. The value of lambda of 0.47 indicated the data obtained in this study were normal and have the function of Y 1 con rming the polynomial model choice accuracy. In this case, all the model terms were signi cant except X 1 2 and X 1 X 3 when FAEE1 was considered, the non-signi cant quadratic terms found in FAEE2 production were X 3 2 , X 1 X 2, X 1 X 3, and X 1 X 4, while only X 4, X 4 2 , and X 1 X 4 were found non-signi cant when the analysis of the signi cant factors was carried out on FAEE3 production. Based on the results of Fit statistics datasets, the coe cient of determination obtained showed the model's predictions interaction was good (>98%) with low standard deviations (<0. 20). The values also showed that there is a certainty above 98% that the model generated explained the data variability (Arjun et al., 2019) [26]. Meanwhile, the mean values obtained depicted the high accuracy of the data obtained for the variable factors.  , X 1 X 2, X 1 X 4, X 2 X 3, X 2 X 4, and X 3 X 4 X 1 2 and X 1 X 3 AEE2 X 1, X 2, X 3, X 4, X 2 2 , X 1 2 , X 4 2 , X 2 X 3, X 2 X 4, and X 3 X 4 X 3 2 , X 1 X 2, X 1 X 3, and X 1 X 4, AEE3 X 1, X 2, X 3, X 1 2 , X 2 2 , X 3 2 , X 1 X 2, X 1 X 3, X 2 X 3, X 2 X 4, and X 3 X 4 X 4, X 4 2 , and X 1 X 4 Meanwhile, the second-order mathematical differential equation that correlated the response variables (FAEE1, FAEE2, and FAEE3) with the constraint variables (X 1 : reaction time, X 2 : catalyst amount, X 3 : reaction temperature, and X 4 : EtOH/OMR) are presented in Eqn.   Table 6 displayed the results of the qualities of the blended oil (BTO 60 ) and the FAEEs produced with the references to ASTM and EN. Observation from the table indicated that there were signi cant changes as the oil was converted to biodiesel due to transesteri cation with developed catalysts, the effect of alcohol, reaction time, and reaction temperature. However, there were no distinct differences in the values of the properties of the three biodiesel (FAEE1, FAEE2, and FAEE3), except a slight difference noticed in the iodine and acid value of the FAEE1 produced by the BTCPH, this may be attributed to the powder preparation processes. Burning could result in the loss of the volatile nutrient, making the ash more acidic and increase the unsaturation level of the biodiesel when used during the transesteri cation process (Betiku et al., 2017) [11]. The physicochemical properties of the FAEE2 and FAEE3 remain almost the same; this could be attributed to the calcination process involved in the sample preparation, causing the gaseous evolution of CO 2 from the CaCO 3 at a controlled temperature more than burning. The properties of biodiesel produced were in the range of recommended standard stated by ASTM D6751 [22] and EN 14214 [23].

Conclusion
The blending ratio of oil 60:40 (BTO 60 ) effectively produced a low oil acid value. Three novel catalysts derived using Theobroma cacao pod husks were demonstrated as the potential catalysts for FAEEs production and their e cacy might be ascribed to the high proportion of calcium existing.