International Journal of Renewable Energy Development

Synthesis of the solid bifunctional vanadium-calcium mixed oxides catalyst was accomplished by application of a simple physical mixing approach. In this work, we compared the catalytic activity of CaO and 2%V2O5.CaO catalyst. Various characterization methods, such as X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), BET surface area, and temperature-programmed desorption (TPD) of CO2 and NH3, were involved in studying the newly synthesized catalysts. The presence of CaO, CaCO3, and Ca(OH)2 compounds in the synthesized catalyst were detected by XRD and FTIR analysis. The existence of 2% V2O5 on the CaO catalyst surface was demonstrated by XRF analysis. From TPD-NH3, TPD-CO2, and BET surface area analysis, it was known that the 2% V2O5-CaO catalyst had a higher total number of acid-base sites and surface area than the CaO catalyst. In the fatty acid methyl esters (FAME) production from waste cooking oil (WCO) with higher free fatty acid (FFA), CaO could only catalyze the transesterification reaction. In contrast, 2%V2O5-CaO could successfully catalyze both the esterification of FFA and the transesterification of triglyceride (TG) simultaneously in a one-step reaction process. Thus, these results prove that 2%V2O5.CaO can act as a bifunctional catalyst in the production of biodiesel from WCO. Moreover, the synthesized 2%V2O5.CaO catalyst could achieve a maximum FAME yield of 51.30% under mild reaction conditions, including a 20:1 methanol to oil molar ratio, 60°C reaction temperature, 1 wt% of catalyst loading, and 3 hours of reaction time.


Introduction
Waste cooking oil or WCO could serve as one of the potential feedstocks for biodiesel production due to its low price, abundant availability, and reducing pollutants in soil and water.Both the public sector and the business sector have recently shown interest in manufacturing biodiesel using oil extracted from WCO. Due to its limited solubility in water, WCO poses a significant disposal challenge because it pollutes land and water (Shatesh Kumar et al., 2020).It has been suggested that using WCO as a feedstock for biodiesel production could be a sustainable solution to the waste management issue (Rezania et al., 2019).When the WCO is utilized as a feedstock, biodiesel production costs can be reduced by 70 to 95%.Although WCO has some advantages in producing biodiesel, WCO also has some disadvantages, including significant levels of water and free fatty acids (FFA) (Roy et al., 2020).The high FFA content of WCO in conventional biodiesel production using a solid base catalyst causes saponification, resulting in a very viscous, emulsified, and foamy soap by product, decreasing biodiesel yield, catalyst deactivation, and purification difficulties.Meanwhile, using solid acid catalysts in biodiesel production requires excess alcohol and the reaction rate was slow (Faruque et al., 2020).Therefore, it is a challenge to find a new heterogeneous solid catalytic system that is simple to operate and can conduct moderate esterification and transesterification with low soap formation.This new system allows optimal utilization of the high FFA content WCO as a raw material for biodiesel production.The system is a bifunctional catalyst.The fact that bifunctional solid catalysts can convert FFA and triglycerides into FAME in one reaction step is a great way to cut down on the cost of biodiesel production (Asikin-Mijan et al., 2015).This integrated solid catalyst system provides a bifunctional acid-base system supporting WCO-based biodiesel production.In this catalyst, the acid sites are involved in the esterification reaction of FFA with methanol and the conjugated base sites are responsible for activating the transesterification of triglyceride simultaneously (Shobhana-Gnanaserkhar et al., 2020).
CaO base catalyst was one of the catalysts that is widely used in the development of bifunctional catalysts.Some of the advantages of the CaO catalyst include having high catalytic activity, strong base characteristics, low cost, being noncorrosive, ecologically friendly, and being affordable (Buasri et al, 2012;Li et al., 2012;Piker et al., 2016).Despite its many advantages, CaO can only enable the transesterification of low FFA (<2%) raw materials, resulting in low catalytic activity and reaction rate or even inactivity (Krishnamurthy et al., 2020).An alternative to increasing the catalytic activity of CaO in the production of WCO-based biodiesel is to add promoter metal oxides containing both Lewis and/or Brønsted acid sites.
Several studies on CaO-based bifunctional catalysts, such as Fe2O5/CaO, CaO-La2O3, W0.6-Mo0.4/CaO,and CaO/SnO2, have been found to actively convert FFA and triglycerides into FAME in one reaction step (Rahman et al., 2019).The high tolerance of FFAs and water in vegetable oil that bifunctional catalysts have is the reason behind their ability, as stated by Ezzah-Mahmudah et al. (2016).Furthermore, the Fe2O5/CaO catalyst successfully supported in producing biodiesel with high FFA feedstock in one reaction step without inducing saponification or emulsification.The incorporation of the CaO base catalyst on metal oxide La2O3 caused the appearance of an acid side on the CaO base catalyst.It causes an increase in the catalytic activity of the CaO-La2O3 catalyst in biodiesel production from lowquality oil (Lee et al., 2015).
One of the potential metal oxides to develop CaO-based bifunctional catalysts is V2O5.Metal oxides, such as V2O5 have Lewis acid sites as well as Brønsted acid sites (Kung, 1989).The addition of an acid metal oxide, like V2O5, which contains both Brønsted and Lewis acid sites, is very advantageous because it increases the strength of the acidity compared to only adding an acid metal oxide containing a Lewis acid site (Yadav & Nair, 1999).A bifunctional catalyst with high acid-base strength improves catalytic activity in the production of WCO-based biodiesel.One principal factor in the catalytic activity and selectivity of bifunctional acid-base catalysts in the production of biodiesel from feedstocks containing high FFA is the strength of the acid and base sites of the catalyst (Jeon et al., 2019).In addition, the V2O5 metal oxide has also been used in the synthesis of a silica-supported vanadium pentoxide-based catalyst (V2O5/RHA) showing Brønsted and Lewis acid sites for producing biodiesel from low quality raw materials like used cooking oil and chicken fat (Almeida et al., 2016).
This research aimed to improve the efficiency of producing biodiesel from WCO by synthesizing the bifunctional catalyst 2%V2O5.CaO via the physical mixing method.There has been no substantial research done in the biodiesel industry that explores the synthesis and characterization of a physically mixed 2%V2O5.CaO catalyst yet.The designed 2%V2O5.CaO catalyst is an acid-base bifunctional catalyst that is expected to support simultaneously esterifying FFA and transesterifying Triglycerides in the high-FFA waste cooking oil as a bifunctional catalyst, and this hypothesis has been proven.Therefore, the synthesized V2O5.CaO catalyst's physico-chemical properties and catalytic activity were studied.
WCO was collected from restaurants in Semarang city area of Indonesia.Additional physico-chemical analyses of the WCO were then carried out, i.e. acid value, FFA concentration, and molecular weight based on ASTM (American Society for Testing and Materials) standard as summarized in Table 1.Molecular weight (gmol -1 ) 898.9 ASTM (D974(00))

Catalyst Preparation
The CaO metal oxide was prepared by co-precipitation method using Ca(NO3)2.4H2Oprecursor.An amount of Ca(NO3)2.4H2Owas dissolved in deionized water.The resulted solution was adjusted to be alkaline (pH = 10) by using the dropwise addition of Na2CO3 and NaOH to Ca(NO3)2.4H2Oat room temperature until reached constant pH of 10.After that, the solid-gel mixture was stirred constantly for 24 hours at a temperature of 60ºC.After centrifugation, washing with deionized water, and drying at 100ºC, the white solid was obtained (Lee et al., 2015).The CaO metal oxide was produced after calcining the resulted white solid in air at 900 ºC for 6 hours.The purpose of the calcination stage was to produce metal oxides from insoluble precursors including metal carbonates, nitrates, oxalates, and hydroxide.High-temperature calcination decomposed loaded Ca(NO3)2 to CaO (Xie & Zhao, 2013).In order to synthesis a 2%V2O5.CaO by physically mixing process, the powdered NH4VO3 (which is a precursor of V2O5) and CaO catalyst (prepared by co-precipitation) were mixed physically and ground at a mortar (Ngaosuwan et al., 2021).The resulted mixed solid was then dried at 600ºC for 4 hours.

Catalyst Characterizations
X-ray fluorescence (XRF) (Philips Xpert MPD) was utilized in order to determine elements composition.The structure of the synthesized heterogeneous solid catalyst was examined using X-ray diffraction (XRD).Powder X-ray diffractometer (Shimadzu model XRD 6000) with specifications: Cu-Kα radiation (27.7 kW and 30 mA), 1.54 A wavelength (λ), 2° min -1 screening speed with a 0.04° step spanning the 2 range of 10-80° was used to analyze the samples.FT-IR spectra with attenuated total reflectance at room temperature between 400-4000 cm -1 were utilized to investigate surface functional groups of catalysts.Moreover, by multi-point nitrogen adsorptiondesorption isotherms (at 77 K) over Quantachrome Nova Instruments was used to determine the surface area, pore volume, and pore size distribution based on the Brunauer-Emmet-Teller (BET) theory.Temperature Programmed Desorption (TPD) CO2 and NH3 were then utilized to analyze the basicity and acidity strength, respectively, by utilizing a Thermo Finnigan TPD/R/O 1100 series apparatus equipped with a thermal conductivity detector (TCD).Pretreatment of sample consisted of heating at 350 °C for 1 hour under N2 gas flow.After 30 minutes of NH3 absorption, elimination of excess NH3 gas was carried out with He gas pretreatment.Meanwhile, TPD-CO2 was used to determine the basicity of the sample by employing CO2 as the probe gas.Both NH3 and CO2 chemisorption were performed at 30 mL.min -1 for 30 minutes.He gas of 30 mL.min -1 for 30 minutes flowed in the system to flush the excess CO2 and NH3 in the system.The desorption of NH3 and CO2 happened at 50 to 900°C with a 10°C.min - flowrate using He as a carrier gas.Thermal Conductivity Detector (TCD) was utilized to measure the amount of desorbed NH3 and CO2.

The catalytic activity of V2O5.CaO in esterification and transesterification simultaneously
Performance of the designed physically mixed 2%V2O5.CaO catalyst was measured by its ability to produce biodiesel.The 2%V2O5.CaO catalyst was used to support a simultaneous onestep esterification and transesterification reaction in a batch reactor according to Widayat et al. (2020).The biodiesel was produced in a batch catalytic reactor of round bottom flask with a capacity of 250 mL and a water-cooled reflux condenser.The reaction mixture contained 20 grams of waste cooking oil with methanol (a ratio of molar methanol to oil of 20:1) and 1 wt% of a catalyst (wt% catalyst based on the total weight of oil).The batch reactor was carried out completed with a close reflux condenser system that was heated to 60ºC using a steady magnetic stirring at a speed of 750 rpm for 3 hours.After the reaction, the catalyst was recovered by centrifuging the reaction product at 4500 rpm for 15 minutes and it was washed with nhexane.The catalyst was calcined at 550°C for 5 hours before it was loaded into the reactor.
The obtained mixture after the reaction was rich in FAME and still contained by products; hence separation process was carried out to separate glycerol, soap, excess methanol, and other undesirable products before vacuum evaporation to yield the final FAME product and biodiesel (Widiarti et al., 2021).The resulting biodiesel was heated for 15-30 minutes at a temperature of 100 ºC with a stirring speed of 150 rpm.It is required to dry biodiesel in order to get rid of any remaining water, which encourages the growth of biological organisms and raises the acidity index of the biodiesel that is produced as a result of this process (Ali et al., 2020).

FAME Yield Analysis
To identify the composition of the Biodiesel product, the Gas Chromatography-Mass Spectrometry (GCMS) of QP 2010S SHIMADZU equipped with DB-1 column was used to analyze the FAME composition.Finally, the Eq. ( 1) was used to calculate the yield of the biodiesel according to standard method (EN 14103) (Li et al., 2013): where, ∑A represents the total peak area of the methyl ester (C14-C18), AIS represents the peak area of the internal standard, CIS represents the internal standard concentration (mg.mL -1 ), VIS standards represent the internal standard volume (mL), and, the mass of the sample (mg) are represented by M. AOCS was used to quantify free fatty acids in the reaction mixture (Das et al., 2017).FFA titrimetric measurement in samples and end product determined reaction conversion.The FFA conversion is the ratio of feedstock acid value change before and after reaction to initial acid value.The Eq. ( 2) was used to calculate the FFA conversion with  f and  p representing the WCO acid value and the biodiesel product, respectively.

Catalyst Characterization
The XRF analysis was used to determine the composition of the metal oxide present in the produced catalyst, as presented in Table 2.The existence of 2%V2O5 on the CaO catalyst surface was demonstrated by the detection of V2O5 in a 2%V2O5.CaO catalyst, which was synthesized by a simple physical mixing method at a weight percentage of around 1.99% by weight.The XRD analysis determines the type and crystal structure of catalysts (see Figure 1).The CaO diffractogram exhibits 2θ peaks at 28.70º, 29.4º, 31.50º, 35.99º, 37.37º, 39.42º, 43.19º, 47.54º, 48.53º, 50.82 º, 53.92º, 57.43º, 60.70 º, 62.70º and 64,69º, that indicate the diffraction line of CaO (Ezzah-Mahmudah et al., 2016;Istadi et al., 2015).Both CaO and 2%V2O5.CaO catalysts resulting diffraction lines at 2θ = 18.01 and 34.16 indicating the presence of a certain amount of Ca(OH)2 (JCPDS No. 00-084-1267) (Chen et al., 2016).This result is consistent with the findings of Wang et al. (2018), who discovered a small amount of Ca(OH)2 in the Ca-B catalyst diffraction pattern at 2θ = 18.2º and 34.2º, due to absorption of water by the CaO catalyst (Wang et al., 2018).Calcined CaO at 900°C also produced low intensity peaks at 2θ = 29,01°(JCPDS No. 00-183-1762), corresponded to CaCO3 (Widiarti et al., 2021).CaO is an alkaline crystal, and because it is exposed to carbon dioxide at ambient temperature, its stability is limited.Calcium carbonate will be formed as a result of the reaction between calcium oxide and carbon dioxide (Hadiyanto et al 2016;Zul et al., 2021).The diffraction intensity of the 2%V2O5.CaO catalyst at 2θ = 28.70º was lower than the diffraction intensity of the CaO catalyst.It caused by partial substitution of Ca 2+ ions by the impurities (V 5+ ions) in the parent lattice (Borah et al., 2019).This fact is also in accordance with research of Ngaosuwan et al. (2021), which stated that the incorporation of La2O5 into CaO with the physical mixing method also reduced the intensity of the diffraction pattern of CaO.Mulyatun and Prasetyoko (2011) stated that the V2O5 catalyst with an orthorhombic crystal structure contributed peaks with high intensity at 2θ of 20.28º; 26.15º and 34.31º.
From Figure 1, it is known that 2%V2O5.CaO catalyst has a similar diffraction pattern to CaO catalyst.It means that the crystal character of V2O5 metal oxide is not seen in the diffractogram of the synthesized 2%V2O5.CaO using the physical mixing method.This circumstance may be due to the lower metal oxide content in the catalyst, making it hard to detect the V2O5 character.The catalyst with a V2O5 content of < 25% had no crystalline vanadia in its X-ray diffraction pattern, indicating that vanadium oxide was uniformly disseminated on its surface (Chary et al., 2004;Sun et al., 2007;Zhao et al., 2018).The FT-IR spectra of 2%V2O5.CaO and CaO catalysts are displayed in Figure 2., it known that weak absorption band at 878 cm -1 corresponding to the C-O symmetric stretching mode of carbonate ion (CO3 2-), indicating the presence of CaCO3 in the catalysts (Maneerung et al., 2016).CaCO3 could be formed due to the high affinity of the Ca-O structure, which reacted with CO2 after calcination, forming CaCO3 on the catalyst surface (Kesserwan et al., 2020).The bending vibration of the O-Ca-O group at 1430 cm -1 proves that CaO was formed (Boro et al., 2014).The spectra in the range of 3300-3600 cm -1 reveal the stretching vibration of hydroxyl group (O-H) for Ca(OH)2, both of which were formed by adsorbed interlayer of water molecules on the surface of the catalyst (Maneerung et al., 2016).Ca(OH)2 was formed during the nitrate decomposition process, followed by a reaction with water.The presence of metal-oxide bonding vibration is detected by FT-IR spectroscopy on 600-700 cm -1 wavenumber range (Syamsuddin & Hameed, 2016).Infrared spectra of 2%V2O5.CaO catalyst showed no characteristic peak of the crystalline 2%V2O5 phase.It is caused by the low content of V2O5 in 2%V2O5.CaO.Ngaosuwan et al. (2021) stated that IR spectroscopy could not detect La2O3 in the La2O3/CaO samples due to the low vibration intensity of pure La2O3 compared to CaO.The characteristic peak of V2O5 was only observed when the V2O5 content reached 30% in CaO.This result is supported by Sun et al. (2007) which stated that the peak caused by V2O5 was not observed in the infrared spectra of the V2O5/NbP catalyst with low V2O5 content (Sun et al., 2007).The characteristic peak of V2O5 was only observed when the V2O5 content reached 25%.This result suggests that the addition of V2O5 did not change the structure of CaO significantly, meaning that the CaO framework stayed stable even with the addition of V2O5.
Table 3 illustrates the physico-chemical properties of the BET surface area, average pore size and pore volume, total acid and base sites density of CaO and 2%V2O5.CaO catalysts.The surface area of the CaO is 10.367 mm 2 /g, while the 2%V2O5.CaO catalyst has a surface area of 12.382 mm 2 /g.The porosity of the CaO surface was a consequence of released CO2 in the high temperature calcination, which caused higher surface area in the 2%V2O5.CaO catalyst compared to the CaO catalyst.The addition of metal oxide (2%V2O5) to the CaO base catalyst also caused an increase in the surface area of the 2%V2O5.CaO catalyst.This is evidenced by Kaur et al. (2014) who synthesized Zr/CaO catalysts with various Zr concentration (10-20 wt.%).They found that the addition of zirconium to the CaO catalyst increased in surface area proportional to the increasing zirconium loading on the CaO catalyst (Kaur & Ali, 2014).In this study, a simple physical mixing method was used to make the 2%V2O5.CaO catalyst.It has a larger surface area than the bifunctional catalyst made by the previous researchers.
TPD-NH3 and TPD-CO2 analyses were carried out to determine the acidity and basicity degrees of a catalyst.The data could also provide the amount of acidic and basic sites on the bifunctional catalysts to help explain the relationship of catalyst activity in one-pot esterification-transesterification reaction (Lee et al., 2015).In TPD analysis, the peaks desorbed at high temperature regions indicate a high bonding strength on the active sites of the support.On the other hand, the peaks desorbed at lower temperature zones suggest a low bonding strength in the active sites, resulting in a low acidity or basicity.The peaks that are desorbed at temperatures below 300 ºC are considered to have a low acid or base strength.The peaks that are desorbed at temperatures between 400 and 500 ºC are considered to have a moderate acid or base strength, and the peaks that are observed at temperatures above 500 ºC are considered to have a high acidic or base strength (Rabiah Nizah et al., 2014).
From Figure 3, it can be observed that the prepared 2%V2O5.CaO catalysts have a higher total basicity than the CaO catalyst.Furthermore, the addition of 2%V2O5 on the CaO catalyst caused the total base site density of the catalyst to increase slightly from 4.7481 mmol/g to 5.8652 mmol/g.The dual interaction of the V2O5 surface lattice oxygen and the Ca species prompt a synergetic effect that assists strong basic sites formation.The presence of V 5+ acts as an electron donor, facilitating the interaction between the reactant molecules and the catalyst.causes the appearance of an acid side to the 2%V2O5.CaO catalyst with a total acid site density of 2.7464 mmol/g.The TPD-NH3 profile of 2%V2O5.CaO catalyst illustrates the NH3 desorbed peaks at a variety of temperature zones, including 350-450 °C and 650-800 °C, respectively.The profile peaks NH3 desorbed in the temperature zones ranging from 350-450 °C can be attributed to a medium acidity strength.In addition, the sharp peaks in the NH3 desorption curve at 600-800 °C are also indicate high acid strength (Borgna, 2015).The 2% V2O5 metal oxide contributed to the acid site formation in the CaO catalyst, which favored the esterification reaction in biodiesel production from WCO.Both the Lewis and Brønsted acid sites were found in V2O5 metal oxides (Mulyatun & Prasetyoko, 2011).This result is also obtained by Almeida et al. (2016), whom stated that V2O5 addition to SiO2 catalyst formed Brønsted and Lewis acid sites for soybean oil-based biodiesel production.Based on the TPD-NH3 and TPD-CO2, the synthesized 2%V2O5.CaO catalyst using simple physical mixing had both acidic and alkaline sides with high strength.

The Catalytic Activity of V2O5.CaO esterification and transesterification simultaneously
Catalytic activity of the physically mixed 2%V2O5.CaO catalyst was evaluated by WCO-based biodiesel production with 20:1 methanol to WCO molar ratio and 1%wt catalyst loading at 60 ºC for 3 hours.The analysis of the fatty acid methyl  (Wen et al., 2010).
This study was done in order to determine whether the 2%V2O5.CaO catalyst showed higher activity than the CaO catalyst.Figure 4 illustrates how catalytic activity of different materials affects biodiesel synthesis.It can be seen that the biodiesel yields of CaO were all less than 35.79%,In contrast, the 2%V2O5.CaO catalyst demonstrated good catalytic activity, resulting in a high biodiesel yield (51.30%) over same reaction conditions for the CaO catalyst.These findings suggest that the acidity, basicity, and surface area of the catalyst play a supportive role in the improved biodiesel yield that was achieved at a very mild reaction temperature of 60 ºC.
Figure 4 demonstrates that the total number of acid-base sites and surface area characteristics of the catalysts are the primary factors that determine the catalytic performance.It is found that the yield of FAME and FFA conversion is proportional to the total number of acid-base sites and the surface area of the catalyst.The 2%V2O5.CaO catalyst shows better catalytic activity than the CaO catalyst.This is shown by the FAME yield (%) and FFA conversion in the production of WCO-based biodiesel (Figure 4).The 2%V2O5.CaO catalyst shows a higher total number of acid-base sites and surface area compared to the CaO catalyst.It causes the yield of FAME and FFA conversion from the 2% V2O5.CaO catalyst to be higher than the CaO catalyst.The 2%V2O5.CaO catalyst is a catalyst that has both acidic and basic sides due to the addition of metal oxide V2O5, which has Lewis and Brønsted acid sites on the CaO base catalyst.The acid site (V2O5) of the catalyst is responsible for the FFA esterification reaction, while the basic site (CaO) of the catalyst helps the triglyceride transesterification reaction in the production of WCO-based biodiesel.The CaO catalyst only has a basic side, so it will be difficult to convert free fatty acids (FFA) into FAME in the production of WCO-base biodiesel.The feedstock's free fatty acids (FFA) and water content directly alter CaO catalysts' catalytic activity.CaO becomes inert when it reacts with FFA (Zul et al., 2021).Base strength and the total number of base-site catalysts affect transesterification catalytic activity.Figure 4(A) illustrates series catalytic trend with total number of base sites.Catalytic trends paralleled the total number of base sites basicity variability.The acidity of 2%V2O5.CaO catalyst may contribute to its catalytic activity, together with its basic active site.The addition of 2% V2O5 metal oxide to the CaO catalyst contributed to the appearance of the acid sides of the CaO base catalyst.The metal oxide V2O5 has both Brønsted and Lewis acid sites (Mulyatun & Prasetyoko, 2011).The inclusion of these sites at 2%V2O5.CaO would increase the selectivity and activity of the catalyst, giving greater acidity strength and more active sites, hence aiding the important free fatty acid esterification in the manufacture of biodiesel from non-food feedstocks.It is in line with the results of the TPD-NH3 analysis, which show that the 2%V2O5.CaO catalysts displayed a pronounced desorption peak of NH3 at temperatures above 500°C, indicating strong acid sites.In biodiesel synthesis, acidic materials improve esterification catalytic performance.The esterification reaction is significantly affected by the addition of acid sides (V2O5) to the surface of CaO catalysts, as shown by the lower FFA conversion of CaO (1.11%) when compared to 2% V2O5.CaO (24.42%) catalysts.FFA conversion rate also depends on the synthesized catalyst's acidity.Due to anionic surface, Brønsted acid sites can share protons, while Lewis acid sites can take electron-pairs.High total number of acid site catalysts capture more FFAs in the active site.
It was thought that the existence of acid site (V2O5) and basic sites (CaO) in the catalyst would promote its activity as a catalyst for the simultaneous esterification-transesterification of WCO containing high FFA.Furthermore, both esterificationtransesterification can be catalyzed simultaneously because of the acid and base sites in the 2%V2O5.CaO catalyst.Abukhadra et al. (2019) proposed acid and basic catalyst sites for simultaneous esterification and transesterification.These findings give further evidence that the total amount of acid-base sites and surface area of the catalyst take an extremely important part in manufacturing biodiesel from feedstock that has higher FFA.The 2%V2O5.CaO catalyst which had the great levels of both total number of acid-base sites and surface area, was the catalyst that produced the largest amount of FAME yield.In conclusion, the stability of acidic groups was extremely significant for the conversion of FFAs and assisted in the resistance of FFAs on basic groups.Both active sites (acid and base site) contributed to biodiesel production.
The catalytic activity of bifunctional catalysts in biodiesel production is strongly influenced by the total number of acid-base species and surface area of the catalyst (Ngaosuwan et al., 2021).Figure 4(B) depicts the effect of the catalyst's surface area on its catalytic activity.The 2%V2O5.CaO catalyst has a larger surface area than the CaO catalyst.It causes the catalytic activity of the V2O5.CaO catalyst to be higher than that of the CaO catalyst, both in the esterification and transesterification reaction.In catalysis applications, mesoporous catalysts' surface area has been improved via much effort.Surface area is affected by synthesis technique, support type and amount, and calcination temperature (Soltani et al., 2017).The results show that a mesoporous catalyst with a larger surface area improves catalytic activity.The catalyst's mass interaction with reactants can enhance with a larger surface area.This is also in accordance with research by Wang et al. (2018), which states that the incorporation of Ca-B (700ºC) causes the addition of an acid site and surface area to the CaO catalyst, resulting in an increase in catalytic activity.Ngaosuwan et al. (2021) synthesized La2O3/CaO catalyst with various synthesis methods, including impregnation and physical mixing.It was found that the total basicity and surface area greatly determine the catalytic activity of the La2O3/CaO catalyst (Ngaosuwan et al., 2021).The 5%La2O3/CaOphy catalyst synthesized by the physical mixing method has the best catalytic activity in biodiesel production from palm oil because this catalyst has a large surface area and total acid-base sites.According to these findings, the concentration of acid-base site and surface area catalysts has an influence, at least partially, on the yield of FAME produced from waste cooking oil.
The proposed mechanism of FFA esterification by V2O5 metal oxide in acid site and transesterification of TG by CaO catalyst in basic site is shown in Figure 5 (Wan Omar & Amin, 2011).In step 1: In order to initiate the process of physical adsorption of reactants, the carbonyl group in the free fatty acid and methanol in the bulk solution must first diffuse to the catalyst surface.Adsorption of carbonyl group belonging to the FFA in the acid site takes place, while methanol would be absorbed in the basic site, forming a methoxide.

Conclusion
The metal oxide with CaO catalyst, namely 2%V2O5.CaO catalyst has been synthesized via the physical mixing method for converting WCO to biodiesel.The obtained FAME yield when using the 2%V2O5.CaO catalyst was higher than when using the CaO catalyst.2%V2O5.CaO catalyst has a larger specific surface area, a higher total number of acid-base, a stronger basicity and acidity than CaO.It solves the saponification issue and concurrently catalyzes the FFA esterification and TG transesterification, raising the catalytic activity for biodiesel synthesis.Therefore, the prepared 2%V2O5.CaO via the physical mixing method is a suitable catalyst for supporting simultaneous esterificationtransesterification, and it is prospective for making biodiesel derived from WCO with high FFA.

Figure 3 (
Figure3(A) depicts the fundamental property of the synthesized catalyst samples (as assessed by TPD-CO2).The TPD-CO2 profiles of the samples (CaO and 2%V2O5.CaO) revealed peaks of CO2 desorption at a temperature range of 350-800ºC.This indicates that all the sample catalysts (CaO and 2%V2O5.CaO) desorb their CO2 peaks at strong and moderate basicity sites.On the catalyst 2%V2O5.CaO, strong basicity sites exhibit a desorption peak at temperatures ranging from 625-800 °C, while moderate basicity sites exhibit a desorption peak at temperatures ranging from 350-480°C.The synthesized 2%V2O5.CaO catalyst has a higher basicity strength than the CaO catalyst.It can be seen from the desorption peak of CO2 in Figure3(A), where the desorption peak of CaO at strong moderate basicity site has a lower temperature than the catalyst of 2%V2O5.CaO.Figure3(B) displays the results of the TPD-NH3 analysis, which determined the acidity strength of the 2%V2O5.CaO catalyst.Based on Figure3(B), the addition of metal oxide V2O5 causes the appearance of an acid side to the 2%V2O5.CaO catalyst with a total acid site density of 2.7464 mmol/g.The TPD-NH3 profile of 2%V2O5.CaO catalyst illustrates the NH3 desorbed peaks at a variety of temperature zones, including 350-450 °C and 650-800 °C, respectively.The profile peaks NH3 desorbed in the temperature zones ranging from 350-450 °C can be attributed to a medium acidity strength.In addition, the sharp peaks in the NH3 desorption curve at 600-800 °C are also indicate high acid strength(Borgna, 2015).The 2% V2O5 metal oxide contributed to the acid site formation in the CaO catalyst, which favored the esterification reaction in biodiesel production from WCO.Both the Lewis and Brønsted acid sites were found in V2O5 metal oxides(Mulyatun & Prasetyoko, 2011).This result is also obtained byAlmeida et al. (2016), whom stated that V2O5 addition to SiO2 catalyst formed Brønsted and Lewis acid sites for soybean oil-based biodiesel production.Based on the TPD-NH3 and TPD-CO2, the synthesized 2%V2O5.CaO catalyst using simple physical mixing had both acidic and alkaline sides with high strength.
ester (FAME) of the biodiesel component performed by GC-MS.The composition of FAME based on GCMS results is as follows: Methyl laurate, Methyl myristate, Methyl palmitoleate, Methyl palmitate, Methyl linoleate, Methyl oleate, Methyl stearate, and Methyl arachate.The WCO-based biodiesel component in this study is the same as the biodiesel component produced from waste cooking oil catalyzed by TiO2-MgO mixed oxides

Fig. 3
Fig. 3 TPD-CO2 (A) and TPD-NH3 (B) of CaO and 2%V2O5.CaO catalysts Step 2: A tetrahedral intermediate would be formed as a result of a subsequent nucleophilic attack on the acid site's carbocation (esterification) and the oxygen anions of the methanol -OH group along with the triglyceride carbonyl group at the basic site (transesterification).Step 3: Desorption of the -OH group (OH bond breakage) and alkyl triglycerides (C-O bond breakage) resulting in FAME along with water and glycerol as by-products.

Fig. 4
Fig. 4 Influence of total number of Acid-base sites (A) and surface area (B) of catalysts on the FAME yield (%) and FFA Conversion (%)

Table 2
Compound composition of CaO and 2%V2O5.CaO catalysts

Table 3
The physico-chemical properties of the BET surface area, average pore size and pore volume, total acid and base sites density of CaO and 2%V2O5.CaO catalysts ISSN: 2252-4940/© 2023.The Author(s).Published by CBIORE