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. 2025 Nov 18;10(47):57765–57775. doi: 10.1021/acsomega.5c09787

Comparative Study of Homogeneous Heteropoly Acid Catalysts for Biodiesel Production from Canola Oil: Correlation of Acidity, Solubility, and Product Distribution

Hyungjin Kim †, Gicheon Lee ‡, Mane Rasika §, Jong Beom Lee ∥, Yu-Jin Kim ∥,⊥, Young-Pyo Jeon #, Chanmin Lee ∇, Young-Durk Park ○, Geon-Hee Kim ○, Joo-Il Park ∥,*, Yukwon Jeon †,*
PMCID: PMC12676499  PMID: 41358113

Abstract

Biodiesel, predominantly derived from canola oil, is recognized as an essential renewable and ecofriendly fuel, significantly reducing greenhouse gas emissions and fossil fuel dependency. Despite its advantages, optimizing catalytic reactions remains challenging. This research systematically evaluates the catalytic efficiency and selectivity of three homogeneous heteropoly acids (HPAs)phosphotungstic acid (PWA), phosphomolybdic acid (PMo), and silicotungstic acid (SiW)for biodiesel production using canola oil. Under the optimized homogeneous reaction conditions, the Brønsted acidity was quantitatively analyzed using UV–vis spectroscopy with 4-nitroaniline, while solvent-dependent dissociation characteristics were confirmed via FT-IR spectroscopy. Among the HPAs, PWA and PMo exhibited higher methanol solubility, correlating to significantly greater FAME yields (43.97% and 47.22%, respectively) compared with SiW (21.81%). Product analysis revealed that W-based catalysts (PWA, SiW) predominantly produced polyunsaturated esters such as C18:3 (65.9% and 67.5%, respectively), while PMo favored monounsaturated esters such as C18:1 (55.1%), reflecting intrinsic differences in acidity and catalyst configuration. Effective biphasic separation using dichloromethane and water facilitated catalyst recovery and product purification, with FT-IR confirming HPAs’ retention in the aqueous phase. This study underscores the necessity of concurrently managing catalyst solubility and acidity to optimize biodiesel production and product selectivity using homogeneous HPAs, in which the process of efficient phase separation is an advantage for effective management.

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1. Introduction

The growing demand for sustainable and environmentally friendly energy sources has accelerated due to carbon-based energy depletion and global warming. Especially, majority of research on biodiesel, a renewable alternative to petroleum-based diesel fuel, has been conducted by utilizing various feedstocks like vegetable oil, animal fat, or even waste oils. Among these feedstocks, canola oil has been widely recognized as an efficient substrate due to its high content of unsaturated fatty acids and low saturated fat content, which contribute to improved transesterification reactivity and fuel performance. , Previous studies have also emphasized that canola oil provides a consistent fatty acid composition, low sulfur content, and low viscosity. In particular, its low free-fatty-acid and moisture contents minimize side reactions such as saponification, ensuring reproducible behavior under homogeneous catalytic conditions, which makes it one of the most stable and reliable raw materials for biodiesel production. These advantages make canola oil a representative model feedstock for evaluating catalyst-dependent selectivity and product composition in biodiesel synthesis.

Compared to heterogeneous reaction systems, homogeneous catalysts offer high reaction rates and better contact with reactants, leading to higher conversion efficiency, which are also relatively easy to handle and provide consistent catalytic performance. − Various homogeneous catalyst systems have been extensively investigated (Table ); however, they still encounter significant challenges in separation from the reaction mixture, resulting in increased purification costs and waste generation. Furthermore, the presence of free fatty acids can promote soap formation, thereby decreasing the overall biodiesel yield.

1. Comparison of Homogeneous Catalysts Reported in Biodiesel Production.

Catalyst Feedstock Temperature (°C) Time (h) Conversion (%) Reference
H2SO4 Canola oil 65 3 80
H2SO4 Soybean oil 117 3 ∼99
HBr Fried soybean oil 80 0.8 94
NaOH Desert date seed kernel oil 60.5 1 93.16
NaOH Pongamia pinnata oil 60 1 96
NaOH Sunflower oil 60 1.5 94
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Biodiesel is generally synthesized through esterification and transesterification reactions, in which acid catalysts play a key role in facilitating the conversion of triglycerides and free fatty acids into alkyl esters. Among many, homogeneous acid catalysts, including Keggin-type heteropoly acids (HPAs), demonstrate good catalytic performance due to their high proton mobility, strong Brønsted acidity, and relatively lower cost compared to heterogeneous catalysts that offer advantages such as easy separation and reusability. HPA catalysts can also exhibit distinct catalytic behavior, enabling a more systematic evaluation of how molecular structure influences product selectivity, emphasizing the novelty of this study in establishing structure–selectivity correlations in biodiesel synthesis. HPAs, such as phosphotungstic acid (PWA), phosphomolybdic acid (PMo), and silicotungstic acid (SiW), provide high thermal stability and adjustable acidity, which have exhibited excellent performance in biodiesel production. However, their distinct physicochemical properties, particularly differences in solubility and acid strength, can critically affect conversion efficiency and product selectivity.

To evaluate the homogeneous HPAs, the Brønsted acidity can be quantitatively evaluated using the technique of UV–vis spectroscopy measurement, enabling fine-tuning of acid strength through concentration or structural variation. Such modulation in acidity influences catalytic selectivity in transesterification reactions under homogeneous conditions. Nevertheless, despite these advantages, systematic comparisons of different HPAs under identical reaction conditions remain limited. Furthermore, the strong hydrophilicity of HPAs often hinders efficient product separation and catalyst recovery. To address this issue, a biphasic reaction system was employed to facilitate efficient separation between the biodiesel product and the homogeneous catalyst phase, thereby improving catalyst recovery and overall process efficiency.

In this study, three representative HPAs (PWA, PMo, and SiW) were investigated as homogeneous acid catalysts for the production of biodiesel from canola oil. The quality of biodiesel is strongly influenced by its FAME composition, depending on the solubility and acid strength of the homogeneous HPAs. The Brønsted acidity of each HPA was quantified using UV–vis spectroscopy with 4-nitroaniline as an indicator. In addition, the effect of catalyst solubility on both conversion and phase separation was systematically examined. High-resolution gas chromatography–mass spectrometry (GC–MS) allowed clear identification of individual methyl esters that are closely linked to biodiesel quality. GC–MS has been widely used for FAME analysis due to its high structural specificity and reliable quantification. Compared to conventional GC–FID, GC–MS offers greater selectivity by detecting molecular fragment characteristics with the qualitative identification of each ester species. Derivatization techniques were also applied to enhance peak resolution, enabling the distinction of mono-, di-, and triglycerides and ensuring compliance with international standards such as ASTM D6751 and EN 14214. These results demonstrate that homogeneous catalyst systems, combined with advanced analytical techniques, can produce biodiesel with a well-defined and reliable product composition. Therefore, this work aims to provide a clearer understanding of how the physicochemical properties of HPAs influence biodiesel yield and product recovery, contributing to the rational design of homogeneous catalytic systems for sustainable fuel production.

2. Experimental Section

2.1. Material Preparation

All HPA catalysts used in this study were commercially obtained and used without further modification. PWA (H3PW12O40) was purchased from Junsei Chemical Co., while SiW (H4SiW12O40) and PMo (H3PMo12O40) were purchased from Sigma-Aldrich. Canola oil (food-grade, local supplier) was used as the feedstock, and methanol was purchased from Sigma-Aldrich. All of the reagents were used as received without further purification.

2.2. Material Characterization

For the structural study on the solid-state HPAs, X-ray diffraction (XRD) measurements were performed using a D2 Phaser (Bruker AXS) with a Ni-filtered Cu–Kα X-ray source (1.5418 Å) with results obtained at room temperature with angles from 5° to 40°. Thermogravimetric Analyzer (TGA) measurements were performed by a TA Instrument STA7300 (Hitachi High-Tech, Japan) at the condition of 25–500 °C and a scan rate of 5°C/min under an air atmosphere. Also, spectra of Fourier Transform Infrared (FT-IR) in the range of 4000–550 cm–1 were collected by using Spectrum Two (PerkinElmer) with Pike Technologies Miracle Universal ATR.

To quantify the acidity of each HPA in the crystallite state, NH3 adsorption analysis was performed using Bel-Cat-M (Bel Japan Inc., Toyonaka-shi, Japan). 50 mg of samples was pretreated at 100 °C in He condition for 60 min with a flow rate of 50 mL/min. The treated samples were flushed with NH3 at 80 °C with a flow rate of 50 mL/min and then cooled down to room temperature. The acidic properties of the flushed catalysts were measured with a temperature protocol the a range from 25 to 850 °CC at 5 °C/min and held at 850 °C for 10 min. NH3 gas was continuously analyzed using a thermal conductivity detector.

UV–vis spectra were recorded on a Jasco V-770 UV–vis/NIR spectrophotometer (Jasco Inc., Japan) equipped with a deuterium and halogen lamp using the Spectra Manager software. The measurements were conducted in double-beam mode over the wavelength range of 300–500 nm, with a spectral bandwidth of 0.96 nm and a scanning speed of 100 nm/min. The Brønsted acidity of the homogeneous HPA catalysts (PWA, SiW, PMo) was evaluated by UV–vis spectroscopy using 4-nitroaniline as an indicator. All absorbance measurements were performed in methanol at 370 nm. The initial absorbance (A 0) of 4-nitroaniline in methanol was measured as a reference, while the absorbance (A) in the presence of each HPA was recorded under identical conditions. Based on the simplified Hammett equation:

H0=pKI+log(AA0−A) 1

where pK I for 4-nitroaniline is 0.99, the Hammett acidity function (H 0) was calculated for each sample. This approach provides a relative comparison of the acid strength among the dissolved HPAs in a homogeneous methanolic phase.

2.3. Procedure of Biodiesel Production Test

The catalytic activity of transesterification using canola oil was evaluated with the HPA catalysts. In a typical procedure, 5 g of canola oil and 0.2 g of catalyst (4 wt %) with 2.5 mL of methanol were added into a batch reactor (HR-8200, Hanwoul, Republic of Korea) and stirred vigorously at 500 rpm. The reaction was carried out at 200 °C for 2 h under continuous stirring. After the reaction, the reaction mixture was first filtered using a pore-size filter to remove any residual solid impurities or unreacted catalyst. For product separation, dichloromethane (DCM) was added to the filtrate as an organic-phase solvent, followed by the addition of heated deionized water (75 °C). This induced phase separation, allowing for selective isolation of the FAME-rich organic layer and the glycerin-rich aqueous layer. The resulting mixture of FAME and DCM was heated at 70 °C for 24 h to completely remove the DCM, yielding purified biodiesel.

2.4. FAME Content Analysis

The FAME content of the organic layer was quantitatively measured following the standardized European procedure EN 14103. Specifically, 250 mg of the organic layer was precisely weighed and diluted in 5 mL of hexane (C6), containing an internal standard of methyl heptadecanoate at a concentration of 10 g/L (C17 ester in hexane). The prepared samples were analyzed using an Agilent 8890 GC coupled with a 5977B MSD, equipped with a DB-WAX column (60 m × 0.32 mm; film thickness: 20 μm). During the analysis, the oven temperature was programmed and gradually increased to 210 °C. Helium, at a consistent pressure of 83 kPa, was employed as the carrier gas. The following formula was applied to calculate the weight percent (wt %) of FAME in the biodiesel products, ensuring accuracy and compliance with established analytical standards. The following formula was used to calculate the FAME content (wt %) in the products.

FAMEContents(wt%)=(∑Ai−AMh)AMh×CMh×VMhm×100 2

∑Ai is the total peak area from methyl butyrate (C5:0) to methyl tetracosanoate (C25:0); A Mh is the area of methyl heptadecanoate (C18:0) for which the response factor is equal to that of FAME; C Mh is the concentration in mg/mL of the methyl heptadecanoate (10 mg/mL); V Mh is the volume (mL) of the methyl heptadecanoate solution (5 mL); m is the weight (mg) of the sample (250 mg). Quantitative concentrations were calculated based on the ratio of the total peak area of FAME (C5:0 to C25:0) to that of the internal standard (C18:0), as shown in the FAME contents equation. The ester compounds present in the biodiesel products were identified by GC–MS analysis by using a FAME standard consisting of 15 reference compounds. In this analysis, the mass spectrometer was employed for qualitative identification of individual FAME species based on the comparison of retention times and electron ionization (EI) mass spectra with NIST library data and authentic standards. Characteristic fragment ions were used to confirm the structure of each detected methyl ester. Among these, 5 major esters were detected and confirmed. The chemical names, molecular structures, and abbreviations of the identified esters are summarized in Table .

2. Identification and Classification of 15 FAMEs with Indication of Detected Species.

Short name Chemical compound Full name Classification Presence
C8:0 C9H18O2 Octanoic acid methyl ester Saturated X
C16:0 C17H32O Hexadecanoic acid methyl ester Saturated O
C16:1 C17H32O2 Methyl palmitoleate Monounsaturated X
C16:1 C17H32O (Z)-9-Hexadecenoic acid methyl ester Monounsaturated X
C18:0 C19H34O2 Octadecanoic acid methyl ester Saturated O
C18:1 C19H36O2 Methylelaidate Monounsaturated O
C18:1 C19H36O2 8-Octadecenoic acid methyl ester Monounsaturated X
C18:1 C19H36O2 Oleic acid Monounsaturated X
C18:3 C19H36O2 9,12,15-Octadecenoic acid methyl ester Polyunsaturated O
C19:0 C19H38O4 Hexadecanoic acid 2-hydroxy-1-(hydroxymethyl) ethyl ester Saturated X
C20:0 C21H42O2 Eicosanoic acid methyl ester Saturated O
C20:1 C21H40O2 11-Eicosenoic acid methyl ester Monounsaturated X
C21:1 C21H40O3 Oleic acid 3-hydroxypropyl ester Monounsaturated X
C22:0 C23H46O2 Docosanoic acid methyl ester Saturated X
C24:0 C25H50O2 Tetracosanoic acid methyl ester Saturated X
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3. Results and Discussion

3.1. Structural and Solubility Properties of Homogeneous HPA Catalysts

Figure shows the XRD and TGA analyses of the solid HPA catalysts to investigate the intrinsic physicochemical properties of crystallinity and thermal stability before being dissolved in the homogeneous reaction systems. The XRD patterns of PWA, PMo, and SiW are displayed in Figure a, where distinct diffraction peaks in the low-angle region (2θ = 6°–10°) are characteristic of the Keggin-type structure. These features are consistent with previous reports on HPA salts such as H3PW12O40, H3PMo12O40, and H4SiW12O40, which show similar diffraction behavior in this region. To evaluate the intrinsic thermal stability of the solid HPA catalysts, TGA was performed, as shown in Figure b. All HPAs show a two-step weight loss of desorption of surface water below ∼80 °C and gradual removal of structural water around ∼200 °C. No significant mass loss above this temperature confirms that the Keggin structure remains intact within the biodiesel reaction range (∼200 °C). These results are consistent with previously reported thermal behaviors of Keggin-structured HPAs. −

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(a) Normalized XRD patterns and (b) TGA profiles of solid HPA catalysts of PWA, PMo, and SiW in the temperature range of 25–500 °C.

In the FT-IR spectroscopy spectra of the crystalline solid samples displayed in Figure a and specific peak positions indicated in Table , the characteristic vibrational bands associated with the Keggin structure were clearly observed for all HPA catalysts. Specifically, peaks corresponding to P–Oa stretching (1072–1057 cm–1), terminal MOa stretching (976–957 cm–1), M–Ob–M bridging (906–873 cm–1), and M–Oc–M corner-sharing vibrations (781–740 cm–1, where M = W or Mo) confirm the preservation of the Keggin framework in the solid state. −

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FT-IR spectra of (a) pure HPA catalysts of PMo, PWA, and SiW and (b) methanol-dissolved forms, indicated as HPA­(M).

3. FT-IR Peak Summary of HPA Catalysts in Crystalline and Methanol-Solvated States.

  Summarized peak table of crystal and homogeneous HPA (cm–1)
Catalyst CH 3 OH 2 + M(OC 2 H 5 ) 6 CH 3 OH 2 + P–O a MO a M–O b –M M–O–M
PMo N.D. N.D. N.D. 1057 957 873 742
PWA N.D. N.D. N.D. 1072 974 906 781
SiW N.D. N.D. N.D. N.D. 976 906 740
PMo(M) 1738 1366 1216 N.D. 960 890 810
PWA(M) 1738 1366 1216 N.D. 972 904 820
SiW(M) 1736 1368 1212 N.D. 972 922 800
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To confirm the homogeneous state of HPA catalysts, FT-IR spectroscopy was performed for three representative HPAs, as shown in Figure b and summarized in Table , after dissolution in methanol under the optimized biodiesel production reaction conditions. Noticeable changes were observed in both the intensity and position of the characteristic Keggin bands in the solid state. In particular, the MO and M–O–M vibrations appeared significantly broadened and weakened, primarily due to the dilution effect of methanol. Additional bands appeared in the 1800–1200 cm–1 region, including peaks near 1730, 1370, and 1220 cm–1, which are associated with protonated CH3OH2 + (∼1730 cm–1), M­(OC2H5)6 (∼1370 cm–1) and protonated methanol (CH3OH2 +) or alkoxy species (M–OCH3) (∼1220 cm–1). , Basically, all three HPA catalysts (PMo, PWA, and SiW) were confirmed to be soluble in methanol. As previously reported for Keggin-type acids such as H3PW12O40 and H4SiW12O40, proton solvation with alcohol molecules was found to be responsible for the efficient transesterification reaction. However, during the catalytic cycle, the proton is primarily transferred to the carbonyl oxygen of the triglyceride rather than to methanol itself. This protonation step activates the carbonyl carbon toward nucleophilic attack by methanol, leading to the formation of a tetrahedral intermediate and the subsequent generation of FAME and glycerol.

Notably, the extent of solubility varied among the HPA catalysts. In the methanol-only spectrum, characteristic peaks such as the broad O–H stretching around 3300 cm–1 and the C–O stretching near 1050 cm–1 were clearly observed. , Upon addition of HPAs, additional peaks corresponding to Keggin-type vibrations, such as MO and M–O–M (where M = Mo or W), appeared in the 700–1100 cm–1 region. These signals were noticeably stronger in the PMo and PWA systems, suggesting that both catalysts were well-dispersed and molecularly distributed in the methanolic medium. In contrast, SiW exhibited a relatively weaker intensity in this region, implying limited solubility and less effective dispersion under the same conditions. This difference is likely to contribute to the observed variations in biodiesel conversion efficiency. Catalysts with limited solubility tend to be less dispersed in the methanol-based reaction medium, resulting in reduced contact with the reactants and a lower effective catalytic presence. Accordingly, such limitations can directly hinder the overall FAME conversion, irrespective of catalyst composition or acidity.

3.2. Acidity Assessment of HPAs in Homogeneous vs Solid State

The acid strength of the HPA catalysts was evaluated in both crystalline and homogeneous states using NH3-TPD (crystalline) and UV–vis spectroscopy (homogeneous), respectively. NH3-TPD analysis shown in Figure reveals both the strength of the acid sites, which is accounted by the temperature of the peak, and counts of acid sites in the catalyst crystallite, calculated by peak intensity. PWA exhibited the highest desorption temperature (580.9 °C), indicating the strongest acid sites in its solid form, followed by SiW with second highest desorption temperature (519.5 °C) with highest intensity of peak. PMo showed the weakest acidity (456.6 °C) with a small amount of acid sites included in the crystallite catalyst. This trend can also be confirmed by the specific number of adsorbed NH3 molecules per mass of acid sites of HPA catalysts as PWA: 0.17 mmol/g NH3, SiW: 0.18 mmol/g NH3, and PMo: 0.13 mmol/g NH3, which were calculated from the specific TPD-NH3 data of maximum peak temperature and desorbed ammonium amount and are summarized in Table .

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NH3-TPD profiles of crystallized HPA catalysts of pure PMo, pure PWA, and pure SiW.

4. NH3 Adsorption of Solid HPA Catalysts and UV–Vis Acidity (Abs and H 0) in Methanolic Solution.

Samples Temperature Total acid site Abs H 0
PWA 580.9 °C 0.17 mmol/g NH3 1.706 2.42
SiW 519.5 °C 0.18 mmol/g NH3 1.711 2.45
PMo 456.6 °C 0.13 mmol/g NH3 1.722 2.55
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Interestingly, the desorption peaks observed in NH3-TPD appear at much higher temperatures than the expected decomposition range of the Keggin framework, which is around 350 °C. This is because NH3 is not just weakly adsorbed on the surface but actually reacts with Brønsted acid sites to form stable ammonium salts such as (NH4)3PW12O40. The desorption behavior in HPA catalysts is, therefore, more closely related to the thermal stability of these ammonium salts than to the intrinsic acid strength of the Keggin structure itself. Consequently, NH3 stays bound as NH4 + ions up to very high temperatures, which is close to 900 K, that is higher than the decomposition points of the pristine Keggin unit, leading to the high-temperature desorption signals typically seen in NH3-TPD.

In the homogeneous methanol phase under our reaction composition, UV–vis analysis in Figure revealed that their Brønsted acid strength clearly differed, although both PWA and PMo showed good solubility by the broadening of MO and M–O–M bands in the FT-IR spectra displayed in Figure . However, the proton-donating ability followed the trend of PMo < SiW < PWA, indicating that PMo exhibited relatively weak acidity. This result is also consistent with previous reports, where PWA showed stronger Brønsted acidity than SiW and PMo due to its higher charge density and proton-releasing ability. , Overall, these results suggest that solubility and acidity are not directly correlated due to the different structures and that W-based HPA catalysts (PWA and SiW) exhibit intrinsically stronger Brønsted acidity in homogeneous systems.

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UV–vis spectra of methanol-based HPA samples with 4-nitroaniline-containing methanol.

This trend is generally in agreement with the acidic behavior observed in the crystalline state. However, as shown in Figure , NH3-TPD analysis revealed that SiW and PWA released ammonia at higher temperatures than PMo, suggesting a similar acidity order of PMo < SiW < PWA based on desorption strength. However, when considering the total amount of desorbed NH3, SiW exhibited the highest capacity, indicating that the NH3-TPD results reflect a combination of both acid strength and acid site density. This complexity highlights the limitations of using solid-state data alone to predict catalytic behavior. Since the actual reaction proceeds in a homogeneous methanol phase, evaluating the acidity under solution-phase conditions becomes far more relevant for understanding catalytic performance.

3.3. Correlation of Catalyst Properties with Biodiesel Conversion and FAME Composition

Figure shows the overall reaction pathway of biodiesel synthesis under acid catalysis. In Figure a, triglycerides react with methanol to produce glycerol and FAME. In Figure b, the role of the HPA catalyst is highlighted, where Brønsted acid sites originating from the M–O bonds in the Keggin unit, such as W–O in SiW and PWA or Mo–O in PMo, protonate the carbonyl groups of triglycerides and promote ester exchange with methanol. Through this stepwise conversion of triglycerides to diglycerides and monoglycerides, FAMEs are ultimately formed with glycerol as a byproduct.

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(a) Transesterification of triglycerides with methanol to form FAMEs and glycerol and (b) the catalytic role of HPA Brønsted sites from M–O bonds in the ester exchange process.

The catalytic performance of each HPA catalyst was evaluated through GC–MS analysis and FAME quantification, as displayed and summarized in Figure and Table . In this analysis, a total of 15 FAME esters ranging from C9 to C25 were initially targeted using standard compounds to broadly capture the diversity of possible products. The complete list of the 15 reference esters and the 5 identified species is summarized in Table . In Figure a, the GC–MS spectra clearly demonstrate the formation of several FAME species with a peak appearance between pure canola oil spectra and HPA-utilized FAME product spectra, with noticeable differences in signal intensity depending on the catalyst. Among the 15 reference esters, 5 esters with clearly matched retention times and prominent detection signals were finally selected as the major components in this study.

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(a) GC–MS spectra and peak positions for each ester product of canola oil, SiW, PWA, and PMo; (b) FAME conversion from 5 g of canola oil showing ester yields of 3.79 g (SiW), 3.63 g (PWA), and 3.30 g (PMo), corresponding to FAME yields of 0.82, 1.59, and 1.57 g; and (c) FAME composition percentages of C16:0, C18:0, C18:1, C18:3, and C20:0 esters included in each FAME product, calculated by GC–MS analysis of transesterification of pure SiW, PWA, and PMo.

5. Identification and Comparative Quantification of Ester Components in Biodiesel Samples Using GC–MS Spectra of Pure PWA, SiW, and PMo.

  Ester compound
Catalyst C16:0 C18:0 C18:1 C18:3 C20:0 Total FAME (%)
PWA 2.77 3.67 6.42 28.99 2.12 43.97
SiW 1.34 1.9 2.7 14.73 1.14 21.81
PMo 2.54 2.37 26.04 15.15 1.12 47.22
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Based on these spectra, the FAME conversion efficiency was calculated and is shown in Figure b. Among the three catalysts, PWA and PMo exhibited a similar conversion of over 40% (43.97% and 47.22%, respectively), while SiW showed notably lower efficiency of 21.81% indicating its limited catalytic activity under the given reaction conditions. With a constant 5 g of canola oil as feed, the total ester collected was 3.79 g for SiW, 3.63 g for PWA, and 3.30 g for PMo, which corresponds to actual FAME yields of 0.82, 1.59, and 1.57 g, respectively. Although UV–vis analysis in Figure shows that the Brønsted acidity follows the order of PMo < SiW < PWA, the actual FAME conversion trend appeared as SiW < PWA < PMo, suggesting that acidity alone does not determine catalytic performance. This result suggests that the overall FAME conversion trend corresponds well with the solubility behavior of each HPA catalyst in methanol. This is further supported by the FT-IR results in Figure . In contrast, SiW showed only minimal spectral changes, reflecting a limited interaction with the solvent and poor solubility. Despite its relatively strong acidity, the lower solubility of SiW led to the lowest FAME conversion among the three, highlighting that solubility seems to have a critical impact and play a decisive role than acidity in catalytic efficiency in homogeneous methanol systems.

Figure c presents the detailed FAME composition obtained from each catalyst with all ester species normalized to 100%. This normalization enables direct comparison across catalysts, with the most converted ester species in each case visually distinguished by hatching in the figure. The identified FAME products include hexadecanoic acid methyl ester (C16:0, saturated), octadecanoic acid methyl ester (C18:0, saturated), methylelaidate (C18:1, monounsaturated), 9,12,15-octadecenoic acid methyl ester (C18:3, polyunsaturated), and eicosanoic acid methyl ester (C20:0, saturated).

As summarized in Table , the FAME composition varied noticeably depending on the type of catalyst used. Among the 5 major esters, C18:3 was the most dominant species in the case of W-based catalysts as by PWA of 65.93% and SiW of 67.54%. This indicates that W-based catalysts promoted the formation of polyunsaturated esters more effectively, likely due to their stronger Brønsted acidity in methanol, as supported by the UV–vis result. SiW, despite being W-based, showed the weakest catalytic performance overall. It produced the lowest amounts across all FAME species, including C18:1 and C18:3. The total FAME yield for SiW was only 21.81%, which suggests that its limited solubility under methanol conditions hindered its performance, even though it has relatively strong acidity. In contrast, the Mo-based catalyst PMo showed a distinctly different product profile. The most abundant compound was C18:1, accounting for 55.15% of the total esters, clearly dominating the distribution. This indicates that PMo preferentially promotes the formation of monounsaturated esters, possibly due to its lack of acidic strength compared with W-based HPA catalysts. While C18:3 was also produced by PMo, its content (32.08%) was significantly lower than that observed for PWA.

The observed variation in the FAME composition directly reflects differences in the characteristic acidity and solubility of the respective HPA catalysts. These compositional differences are also expected to influence the overall fuel properties of the resulting biodiesel. Specifically, the high proportion of unsaturated methyl esters obtained from W-based catalysts is known to improve ignition quality and cold-flow performance, whereas a higher content of saturated esters typically enhances oxidative stability. As from the previous studies summarized in Table , canola-oil-based biodiesel produced via homogeneous transesterification exhibits a density of 0.86–0.88 g cm–3, a kinematic viscosity of 4–5 mm2 s–1, and a flash point above 180 °C, all within ASTM D6751 and EN 14214 limits. , Given that the present biodiesel contains a similar unsaturation pattern, it is reasonable to infer that the produced FAME mixture would possess comparable fuel-grade characteristics.

6. Representative Fuel Properties of Canola-Oil-Based Biodiesel.

Property Typical value Standard limit (ASTM D6751/EN14214) Reference
Density (g cm–3, 40 °C) 0.86–0.88 0.86–0.90 ,
Flash point (mm2 s–1, 40 °C) >180 >120
Cetane number >55 >47
Kinematic viscosity (°C) 4–5 1.9–6.0
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3.4. Solubility–Acidity Relationship of HPAs for Selective Biodiesel Production

The solubility–acidity relationship based on the overall performance of each HPA catalyst (SiW, PWA, and PMo) is illustrated as a scheme in Figure . There are few previous research with a similar trend comparing the selectivity between monounsaturated and polyunsaturated FAMEs using W- and Mo-based HPAs, but no studies based on the solubility–acidity relationship. W-based catalysts like H3PW12O40 have demonstrated strong reactivity toward both methyl oleate (C18:1) and methyl linoleate (C18:2) during transesterification, indicating a high ability to activate unsaturated substrates. This corresponds well with our results, where the W-based PWA catalyst produced a greater proportion of polyunsaturated esters. On the other hand, the Mo-based PMo catalyst showed a clear preference for monounsaturated esters, which likely reflects its limited ability to convert more complex species like C18:2. This is also consistent with previous findings suggesting that C18:2 generally requires higher activation energy than C18:1, and thus demands stronger and more condensed acidic environments for efficient conversion. The PMo catalyst showed lower Brønsted acidity in UV–vis analysis, and its more mobile and less localized protons may have led to nonselective reactions or rapid deactivation during the process.

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Biodiesel production using three HPA catalysts (SiW, PWA, and PMo) showing differences in solubility, acidity, and FAME conversion. W-based HPAs (SiW, PWA) produced polyunsaturated esters (C18:3), while Mo-based PMo favored monounsaturated esters (C18:1), with phase separation enabling efficient recovery of HPA and glycerin.

In this study, we found that these HPAs, despite their common Keggin structure, exhibited distinct solubility behaviors and acid strengths in the methanol-based transesterification system, which significantly influenced both the FAME conversion and product composition. PWA and PMo showed strong solubility, leading to higher FAME yields, whereas SiW, with poor solubility, resulted in a lower yield. Notably, SiW and PWA, both W-based HPAs with strong Brønsted acidity, facilitated the formation of polyunsaturated esters such as C18:3, while the Mo-based PMo, having relatively lower acidity, primarily produced monounsaturated esters like C18:1.

The degree of unsaturation in the FAME profile plays a critical role in determining the combustion quality and overall performance of biodiesel. In previous research, biodiesel fuels with higher levels of unsaturated esters, particularly C18:2 and C18:3, exhibited prolonged ignition delay, increased NO X and unburned hydrocarbon emissions, and lower overall combustion efficiency. This trend stems from the reduced cetane number and thermal stability of unsaturated molecules. Further research confirmed that cetane number is inversely correlated with the degree of unsaturation, quantified by parameters such as double bond equivalent (DBE) and allylic position equivalent (APE). As these values increase, the ignition quality deteriorates due to the molecular instability introduced by CC bonds. Complementing these findings, it was reported that biodiesels rich in saturated or monounsaturated FAMEs show superior atomization behavior and higher flash points, which enhance both combustion stability and storage performance. These results clearly show that each HPA catalyst leads to a unique FAME distribution like PWA and SiW mainly promoting polyunsaturated products like C18:3, which can be beneficial for cold-flow behavior, while PMo favors monounsaturated esters like C18:1, improving ignition and storage stability. This suggests that HPA catalysts can be purposefully selected depending on the desired biodiesel properties, offering flexibility in fuel design based on application needs.

3.5. Efficient Phase Separation for Catalyst Recovery and Product Purification

The separation of the catalyst material is crucial to improving the purity of biodiesel products, and homogeneous catalysts have been highly struggling in the industry. As shown in Figure , we achieved phase separation by adding dichloromethane (DCM) and DI water after the reaction. Due to their polarity differences, FAME preferentially dissolved in the nonpolar DCM layer, while glycerin and the HPA catalyst remained in the aqueous phase. This selective partitioning was confirmed by FT-IR analysis. In the FAME layer displayed in Figure a, no catalyst-related peaks were detected, indicating effective purification of the final product. In contrast, the water layer containing glycerin in Figure b showed weak but clear HPA-related signals, as acknowledged by the spectra of HPAs dissolved in DI water acknowledged as HPA­(W). The reduced intensity likely reflects dilution caused by water addition during the separation step, but the presence of these peaks confirms that the catalyst remained primarily in the polar aqueous phase. Such behavior is expected for HPAs, which are strongly soluble in polar solvents like water and methanol but practically insoluble in nonpolar media such as DCM or FAME.

8.

8

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FT-IR spectra showing the products obtained using different HPA (SiW, PWA, PMo) catalysts, with (a) representing the FAME phase and (b) representing the glycerin phase, acknowledged as (F)­HPA and (G)­HPA, respectively.

Supporting this, the FT-IR spectra of HPA (W) in Figure b showed only partial retention of the Keggin structure (970–800 cm–1), along with noticeable peak broadening and decreased intensity. These changes indicate effective solubilization and partial dissociation into hydrated proton species, such as H3O+, H5O2 +, and H9O4 +. In addition, δ­(H2O) bending modes observed around 1710 cm–1 and additional bands near 1360 cm–1 further suggest the presence of protonated water clusters and hydration-induced perturbations of P–O and M–O bonds. − Notably, the degree of structural dissociation and IR shift was most pronounced for SiW among the three catalysts, suggesting that SiW achieves higher solubility and protonation in water than PWA or PMo. Together with methanol-phase FT-IR/UV–vis data, where PWA and PMo showed greater solubility than SiW, these findings underscore that solvent choice strongly influences HPA solubility dynamics. Specifically, PWA and PMo behave similarly in methanol, whereas in water, SiW exhibits enhanced solubilization. Consequently, exploiting solvent-dependent solubility enables both efficient FAME purification and catalyst recovery by ensuring that the HPA remains in the aqueous phase during DCM-based separation.

4. Conclusions

This study systematically investigated the solubility and acidity of three homogeneous HPA catalysts (PWA, PMo, and SiW) and their influence on biodiesel production from canola oil. Homogeneous FT-IR analyses under the reaction conditions demonstrated that PWA and PMo had significantly higher solubility in methanol compared to SiW, resulting in enhanced FAME yields of 43.97% and 47.22%, respectively. Conversely, the limited solubility severely restricted the catalytic performance of SiW, yielding only 21.81%. Analysis of the FAME composition via GC–MS indicated distinct product selectivities: W-based catalysts (PWA and SiW), exhibiting stronger Brønsted acidity analyzed by the crystalline NH3-TPD and homogeneous UV–vis under reaction conditions, favored polyunsaturated esters such as C18:3, while PMo preferentially generated monounsaturated esters like C18:1, influenced by its comparatively lower acidity. These selectivity differences critically impact biodiesel quality, affecting the ignition behavior, cold-flow properties, and overall combustion efficiency. Furthermore, employing a biphasic separation system involving dichloromethane and water successfully achieved efficient catalyst recovery and purification of the biodiesel product, as evidenced by FT-IR spectra confirming HPAs’ retention in the aqueous phase. Thus, this research highlights that the comprehensive management of both catalyst solubility and acidity is imperative for optimizing FAME conversion efficiency and controlling product characteristics. Strategically selecting and/or designing appropriate HPAs according to desired biodiesel properties can significantly enhance sustainable fuel production processes.

Acknowledgments

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (no. RS-2025-02310648). This work was also supported by the Regional Innovation System & Education (RISE) program through the Daejeon RISE Center, funded by the Ministry of Education (MOE) and the Daejeon Metropolitan City, Republic of Korea (2025-RISE-06-002).

The authors declare no competing financial interest.

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