High-Efficiency Biodiesel Production Using ZnO-Modified Starfish-Based Catalysts

Abstract

This study introduces a novel approach to biodiesel production by repurposing starfish, an abundant marine waste, as a sustainable catalyst material. Starfish, primarily composed of Ca-Mg carbonate, were calcined to produce calcium oxide (CaO) and magnesium oxide (MgO), which were subsequently doped with varying zinc loadings through hydrothermal treatment. This innovative use of marine waste not only addresses environmental concerns but also provides a cost-effective catalyst source. Among the tested compositions, the catalyst doped with 10 wt% Zn achieved the highest biodiesel yield of 96.6%, outperforming both lower and higher Zn loadings. Zinc incorporation significantly improved the catalyst’s surface area, pore volume, and active site density, as confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller (BET) surface analysis. These enhancements facilitated a biodiesel yield of 96.6% within 10 h, a substantial increase compared to the undoped catalyst (86.5%) under identical conditions. Reusability tests further confirmed the catalyst’s high activity over three consecutive cycles, with yields of 96.6%, 94.2%, and 86.5%, respectively, while SEM-EDS analysis demonstrated effective Zn retention after repeated use. This study demonstrates a pioneering strategy for transforming marine waste into a high-performance catalyst, paving the way for sustainable biodiesel production.

1. Introduction

The growing global energy demand and increasing environmental concerns, particularly regarding greenhouse gas emissions from fossil fuels, have accelerated the search for sustainable and renewable energy sources [1]. Biodiesel has emerged as a promising alternative due to its biodegradability, reduced carbon footprint, and compatibility with existing diesel engines [2]. Transesterification is the primary method for biodiesel production, in which triglycerides react with methanol in the presence of a catalyst to form fatty acid methyl esters (FAMEs) and glycerol [3,4]. For this reaction, the choice of feedstock and catalyst plays a crucial role in optimizing biodiesel production efficiency and sustainability.

Grapeseed oil, a by-product of the wine industry, has gained attention as a biodiesel feedstock due to its high content of polyunsaturated fatty acids and its contribution to agricultural waste reduction [5,6,7,8]. While such feedstocks promote sustainability, the use of catalyst remains a critical challenge. Homogeneous catalysts such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) are widely used due to their high catalytic activity [3]. However, they present drawbacks, including difficulties with separation, wastewater generation, and corrosion issues, which increase operational costs and environmental concerns [9].

Heterogeneous catalysts used in biodiesel production can be broadly classified into acid and base catalysts, each with distinct advantages and limitations. Acid heterogeneous catalysts, such as SO₄/Mg–Al-FeO₃ [10] and SO42-/ZrO₂ [11], have demonstrated strong catalytic activity, particularly by simultaneously catalyzing both the esterification of free fatty acids (FFAs) and the transesterification of triglycerides. Their ability to tolerate high FFA content makes them suitable for low-quality feedstocks, reducing the need for pre-treatment steps [10,11]. However, acid catalysts generally require higher reaction temperatures, longer reaction times, and may promote unwanted side reactions, while exhibiting lower catalytic efficiency in transesterification compared to base catalysts [10]. Additionally, many solid acid catalysts suffer from active site leaching, leading to deactivation and limiting their long-term reusability [10]. In contrast, base heterogeneous catalysts such as CaO, MgO, and ZnO, are highly effective in transesterification reactions, offering faster reaction rates and higher conversion efficiencies under mild conditions [12,13]. Among these, CaO has been widely studied due to its strong basicity and high catalytic activity [12]. Therefore, this study focuses on repurposing starfish as a catalyst precursor for transesterification. Starfish are abundant, naturally contain both Ca and Mg, and require minimal pretreatment—making this species an ideal candidate for catalyst synthesis. Moreover, repurposing ecologically disruptive species like starfish supports circular economy principles and contributes to the restoration of marine ecosystems.

However, conventional CaO catalysts suffer from critical drawbacks such as active site leaching, CO₂ adsorption, and structural degradation into CaCO₃, which ultimately lead to performance loss and poor reusability over multiple cycles [13]. To address these limitations, several studies have attempted to improve CaO catalyst stability through metal oxide doping, composite material synthesis, and the use of biomass-derived precursors [14,15,16,17,18]. While these approaches offer partial improvements, challenges such as leaching, inconsistent catalytic activity, and long-term structural instability remain inadequately addressed. Moreover, many studies have focused on one-time catalytic performance, with limited assessment of material behavior under repeated use. While numerous strategies have been proposed to improve CaO-based catalysts, few studies have simultaneously addressed long-term reusability and dopant retention, particularly under repeated transesterification conditions. In addition, the reliance on agricultural waste as a precursor often involves extensive processing steps. Thus, there is still a need to develop a sustainable, structurally stable catalyst from low-impact marine biomass.

To overcome these persistent limitations of biomass-derived CaO catalysts, the present study explores Zn doping as a strategic modification. Zn has been widely reported to enhance both the basicity and surface area of oxide catalysts, while also mitigating critical issues such as active site leaching and particle sintering during transesterification reactions. Furthermore, Zn incorporation contributes to the stabilization of active sites under repeated operational conditions, significantly improving the long-term durability and catalytic performance of the system [19]. Building on prior developments in biomass-derived catalysts, the present study develops a 10 wt% Zn-loaded ZnO-modified CaO/MgO catalyst (abbreviated as CMZ) using starfish-derived marine biomass as a sustainable and cost-effective source of calcium and magnesium. This work aims to enhance catalytic activity and structural stability through Zn incorporation and to evaluate the catalyst’s performance over multiple reuse cycles.

2. Results

2.1. Structural and Surface Characterization

2.1.1. XRD Analysis

The crystalline structures of the catalysts were analyzed using X-ray diffraction (XRD), and the corresponding patterns are shown in Figure 1. The diffraction peaks at 32.2°, 37.3°, and 53.8° were assigned to the cubic phase of CaO, while those at 42.9° and 62.3° corresponded to MgO. The CMZ catalyst exhibited additional diffraction peaks at 31.8°, 34.4°, and 36.3°, which are characteristic of ZnO, indicating that Zn incorporation altered the crystalline structure of the catalyst and facilitated the formation of ZnO phases.

2.1.2. XPS Analysis

X-ray photoelectron spectroscopy (XPS) was employed to determine the chemical states of the catalyst surface (Figure 2 and Figure 3). The CM900 exhibited Ca 2p peaks at 347 eV and 351.4 eV [20], and Mg 1s peak at 1304 eV [21], indicating the presence of CaO and MgO. For CMZ, additional peaks appeared at 1022.0 eV and 1045.0 eV, corresponding to Zn 2p₃/₂ and Zn 2p₁/₂, respectively [22]. The O 1s spectrum of CM900 displayed three components: 531.2 eV (lattice oxygen), 533.2 eV (adsorbed water or surface hydroxyl groups), and 530.4 eV (MgO) [23].

2.1.3. Morphological Analysis by SEM

The surface morphology of the catalysts was examined by scanning electron microscopy (SEM), and the results are displayed in Figure 4. The CM900 catalyst exhibited a highly porous structure with relatively large, interconnected pores. In contrast, the CMZ catalyst displayed a more compact and granular morphology, suggesting that Zn doping affected the structural properties of the catalyst.

2.1.4. BET Surface Area and Porosity Analysis

The Brunauer–Emmett–Teller (BET) method was used to determine the surface area and porosity of the catalysts (

Table 1. Textural properties of catalysts.

Catalyst	SBET (m2·g−1)	Vp (cm3·g−1)	Pore Size (Å)
CM900	12 ± 5%	0.031 ± 5%	106 ± 5%
CMZ	19 ± 5%	0.047 ± 5%	101 ± 5%

). The surface area of the CM900 catalyst was measured as 12 m²/g, whereas that of the CMZ catalyst increased to 19 m²/g. The pore volume also increased from 0.031 cm³/g to 0.047 cm³/g after Zn doping, indicating that Zn incorporation modified the textural properties of the catalyst by altering the pore structure and surface area.

2.2. Catalytic Performance in Biodiesel Production

2.2.1. Basicity Measurement by HCl Titration

HCl titration revealed that the basicity of the CMZ catalyst was 2.68 mmol/g, whereas CM900 exhibited a basicity of 2.21 mmol/g (

Table 2. Basic strength and site density of CM900 and CMZ calculated by HCl titration.

	Basic Strength (mmol·g−1)	Site Density (Sites·g−1)
CM900	2.48	1.49 × 1021
CMZ	2.68	1.61 × 1021

).

2.2.2. Biodiesel Yield and Reaction Efficiency

The catalytic performance of the synthesized catalysts was evaluated by measuring the biodiesel yield at different reaction times (Figure 5). CMZ achieved 31.5%, 39.8%, 70.9%, 96.6%, and 95.3% for 4 h, 6 h, 10 h, and 12 h, respectively. The performance of CMZ catalyst reached a biodiesel yield of 96.6% after 10 h, whereas the CM900 catalyst only yielded 56.4% under identical conditions.

To assess the reusability of the CM900 and CMZ catalysts, recycling experiments were performed over three consecutive reaction cycles. The biodiesel yields of CM900 decreased progressively from 56.4 ± 1.8% in the first cycle to 51.2 ± 2.1% in the second cycle, and further to 24.1 ± 0.7% in the third cycle, as shown in Figure 6. In contrast, the CMZ catalyst maintained significantly higher yields of 96.6 ± 1.1%, 94.2 ± 0.6%, and 86.5 ± 1.2% for the first, second, and third cycles, respectively (n = 3), demonstrating superior catalytic stability during repeated use.

3. Discussion

3.1. Effect of Zn Doping on Catalyst Structure

The presence of ZnO-specific peaks in CMZ confirms successful Zn doping. Compared to the undoped catalyst, the crystalline structure of CMZ appears more complex, suggesting multiphase formation. Zn incorporation has been shown to reduce CaO crystallite size and enhance stability in other studies [19], supporting its potential role in improving catalyst durability under transesterification conditions.

3.2. Surface Chemistry and Catalytic Activity

X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the chemical states of the catalyst surface and the impact of Zn incorporation. Figure 2 and Figure 3 illustrates the XPS analysis of CM900 and CMZ, respectively, showing characteristic peaks for Ca 2p, Mg 1s, and Zn 2p. In the O 1s spectrum of CM900, three distinct peaks were observed, and these peaks indicate that CM900 primarily consists of CaO and MgO phases [23].

For the CMZ catalyst, the Zn 2p₃/₂ and Zn 2p₁/₂ peaks appeared at 1022.0 eV and 1045.0 eV, respectively, slightly higher than the typical values for pure ZnO (1021.2 eV and 1044.2 eV) [24]. This shift aligns with previous reports on CaO-ZnO composite systems, where Zn 2p peaks are commonly observed at approximately 1022.0 eV and 1045.3 eV due to interactions between ZnO and the CaO matrix [13]. Such a binding energy shift suggest that Zn is incorporated into the catalyst lattice rather than merely existing as a surface species. Additionally, the slight shift in the O1, 531.0 eV, compared to CM900, further supports the structural integration of Zn within the catalyst framework.

The higher basicity observed for the CMZ catalyst likely arises from the introduction of additional Lewis basic sites associated with Zn species. Zn incorporation is known to increase the electron density on the catalyst surface, thereby enhancing its basic character [19]. This improvement in basicity facilitates the deprotonation of methanol, promoting the formation of methoxide ions essential for the transesterification reaction.

3.3. Role of Surface Area and Porosity in Catalysis

The SEM and BET results (Figure 4 and Table 1) indicate that Zn doping altered the textural properties of the catalyst, leading to an increase in surface area and pore volume. The CMZ catalyst exhibited a higher BET surface area and increased pore volume compared to CM900. Higher surface area and optimized porosity are well known to improve catalytic activity by facilitating better reactant adsorption and diffusion [19]. The structural modifications observed in CMZ further confirm that Zn doping enhances catalyst performance by providing a more favorable surface for transesterification reactions.

3.4. Catalytic Performance and Reusability

The increased basicity observed in CMZ (Table 2) suggests that Zn incorporation contributed to an increase in the density of Lewis base sites, which are essential for facilitating the transesterification process by providing active sites for the reaction. Prior studies have reported that Zn doping enhances the basic strength of the catalyst [19]. ZnO incorporation likely contributed to the formation of additional basic sites, thereby enhancing catalytic performance. Similar findings have been reported in the literature, where basicity plays a crucial role in determining the catalytic efficiency of heterogeneous catalysts [19]. To evaluate the zinc-doping effect on the catalyst, the amount of zinc loading was differentiated to 5 (abbreviated as CMZ 5), 10, and 15 wt.% (abbreviated as CMZ 15). The transesterification condition was set as 10:1 methanol/oil molar ratio, 1 wt.% of catalyst loading, and 68 °C of reaction temperature. Figure S9 shows the biodiesel yield for different zinc loading for zinc-doped CaO/MgO catalyst. The biodiesel yields of CMZ 5, CMZ, and CMZ 15 were 88.1 $\pm 0.7$%, 96.6 $\pm 1.4$%, and 90.2 $\pm 1.1$%, respectively (n = 3). Although direct experimental comparison with pure ZnO catalyst was not performed, published results offer a useful reference. Salim et al. (2022) reported a maximum biodiesel yield of 75% using ZnO nanoparticles (4.7 wt.% catalyst, 70 °C, 20:1 methanol/oil ratio, 3 h reaction time), but also noted that extending the reaction time beyond 3 h reduced yield due to side reactions [25]. In contrast, the CMZ catalyst achieved a much higher yield of 96.6 % under milder conditions (1 wt.% catalyst, 68 °C, 10:1 ratio, 10 h) and maintained performance over a prolonged reaction time, highlighting its superior catalytic efficiency and stability over pure ZnO systems.

The catalytic performance results (Figure 5) demonstrate that the CMZ catalyst achieved a biodiesel yield of 96.6% after 10 h, whereas the CM900 catalyst only reached 84.2% under identical conditions. This enhancement can be attributed to the combined effects of increased basicity, improved surface area, and better porosity. These findings highlight the importance of Zn doping as a strategy for optimizing CaO-based catalysts for biodiesel production.

The reusability test was performed for CM900 and CMZ, whose catalytic performances are shown in Figure 6. The biodiesel yield for CM900 decreased drastically after the recycling test, whereas CMZ maintained its performance with a slight decline. As the catalytic performance analyzed, the zinc doping increased its active sites and the structural stability of the catalyst.

To further understand the structural stability of the catalyst during reuse, SEM-EDS analysis was carried out before and after each cycle.

Table 3. Ca, Mg, Zn atomic percentage analyzed via SEM-EDS.

Catalysts	Ca (%)	Mg (%)	Zn (%)
CMZ	$92 \pm$ 2%	$8 \pm$ 1%	$2 \pm$ 0.5%
CMZ 1st reuse	$84 \pm$ 2%	$14 \pm$ 1%	$2 \pm$ 0.5%
CMZ 2nd reuse	$86 \pm$ 2%	$11 \pm$ 1%	$3 \pm$ 0.5%
CMZ 3rd reuse	$84 \pm$ 2%	$13 \pm$ 1%	$3 \pm$ 0.5%

shows the atomic percentages of Ca, Mg, and Zn. The atomic percentages of Ca and Zn were not drastically decreased after multiple recycling tests. Zn was consistently detected in all EDS spectra, indicating negligible Zn leaching and confirming the retention of the active dopant within the catalyst matrix. The corresponding EDS spectra are provided in the Supplementary Materials (Figures S5–S8). These stable atomic percentages correlate well with the consistent catalytic performance observed in the reusability tests. The minimal variation in Ca and Zn contents suggests that the catalyst framework remained intact, and no significant leaching of active species occurred during the transesterification reactions, thereby preserving catalytic activity over multiple cycles.

Table 4. Comparison of heterogeneous catalysts used for biodiesel production.

Catalysts	Reaction Temperature (°C)	Catalyst Amount (wt %)	Methanol/Oil Molar Ratio	Reaction Time (h)	Biodiesel Yield (%)
CMZ	68	1	10:1	10	94.7
CaO/MgO [14]	64.5	10	18:1	3.5	92
CaO [15]	65	4	15:1	6	85
CaMgO [15]	65	4	15:1	6	83
CaZnO [15]	65	4	15:1	6	81
CaO (0Si5Ca) [16]	65	9	15:1	3	90.2
CaO-SiO2 (2Si5Ca) [16]	65	9	15:1	8	80.1
CaO (Ostrich eggshell) [17]	60	8	9:1	1	92.7

compares the performance of various heterogeneous catalysts used in biodiesel production, focusing on reaction temperature, catalyst amount, methanol/oil ratio, reaction time, and biodiesel yield. CMZ, a biomass-based catalyst, demonstrated the highest biodiesel yield at 68 °C with 1 wt.% catalyst loading and a 10:1 methanol/oil ratio over 10 h, highlighting its superior catalytic efficiency.

In comparison, CaO/MgO [14] achieved a similar yield (92%) but required a significantly higher catalyst amount and methanol/oil ratio, resulting in a shorter reaction time. Other CaO-based catalysts (CaO, CaMgO, CaZnO) [15] showed moderate yields, with variations based on composition, while silica-modified CaO-SiO₂ [16] exhibited around 80% yield.

In contrast, CaO derived from ostrich eggshells [17] achieved a high initial biodiesel yield (92.7%) at 60 °C with a reaction time of 1 h. However, prolonged reaction times resulted in a gradual decline in yield, likely due to reverse reactions leading to the decomposition of FAMEs (fatty acid methyl esters). This suggests that while biomass-derived CaO catalysts can be highly efficient under optimal conditions, their long-term stability may be a limiting factor in extended reaction processes. These findings emphasize the influence of catalyst composition and reaction conditions on biodiesel yield, with CMZ emerging as a promising and sustainable option, offering high catalytic efficiency under relatively mild conditions while maintaining stability over extended reaction times.

4. Materials and Methods

4.1. Materials

Grapeseed oil (100% grade, Beksul, Seoul, Republic of Korea) was used as the biodiesel feedstock. Methanol (HPLC grade) was obtained from Duksan Pure Chemicals Co., Ltd. (Asan, Republic of Korea). Zinc nitrate hexahydrate (98% purity) was purchased from Sigma Aldrich (St. Louis, MO, USA) as the zinc precursor. Ammonia solution was sourced from Junsei Chemical Co., Ltd. (Tokyo, Japan). Starfish (Asterias rubens) were purchased from Bei Dai He, China, and utilized as the catalyst bioresource. Additional solvents, including isopropyl alcohol (99.7%, GR) and n-hexane (96.0%), were procured from Daejung Chemical & Metals Co., Ltd. (Ansan, Republic of Korea) and Samchun Pure Chemical Co., Ltd. (Seoul, Republic of Korea), respectively.

4.2. Catalyst Preparation

4.2.1. Calcination of Starfish-Derived Catalyst (CM900)

The starfish samples were soaked in deionized (DI) water for 48 h to remove impurities. The cleaned samples were then dried and pulverized using a ball mill. The resulting starfish powder was calcined at 900 °C for 2 h to convert magnesium calcium carbonate (MgCaCO₃) into calcium oxide (CaO) and magnesium oxide (MgO) [18,19]. This calcined catalyst was labeled CM900.

4.2.2. Zinc Doping via Hydrothermal Treatment (CMZ)

To prepare the Zn-modified catalyst, 1 g of CM900 and 0.53 g of zinc nitrate hexahydrate (corresponding to a 10:1 molar ratio of CaO to Zn) were separately stirred in 100 mL and 60 mL of DI water, respectively, at 600 rpm for 2 h. The solutions were then combined, and 2 mL of ammonia solution was added [26]. The mixture was stirred for an additional 4 h at 600 rpm. The resulting solution was hydrothermally treated at 120 °C for 12 h in a box oven. The precipitate was filtered, dried at 80 °C for 12 h, and subjected to calcination at 850 °C for 2 h to obtain the Zn-doped CMZ catalyst [19,26].

4.3. Catalyst Characterization

The synthesized catalysts were characterized using various analytical techniques:

• X-ray diffraction (XRD): Structural analysis was conducted using an X-ray diffractometer (Rigaku, Ultima IV, Tokyo, Japan) with Cu Kα radiation (40 kV, 40 mA).
• Field Emission Scanning Electron Microscopy (FE-SEM): The morphology of the catalysts was examined using a MIRA3 SEM (TESCAN, Brno, Czech Republic).
• X-ray photoelectron spectroscopy (XPS): The surface chemical composition was analyzed with an X-ray photoelectron spectrometer (JEOL, JPS-9010 MC, Tokyo, Japan) at the Korea Basic Science Institute (Busan, Republic of Korea), using Al Kα monochromatic radiation (hν = 1486.6 eV).
• Brunauer–Emmett–Teller (BET) Surface Area Analysis: The surface area and pore volume were measured to assess catalyst textural properties.

4.4. Transesterification of Grapeseed Oil

The biodiesel production process involved the transesterification of grapeseed oil with methanol in the presence of the synthesized catalyst. All transesterification was repeated 3 times (n = 3). The reaction was as shown in Equation (1) [19]:

Grapeseed Oil + Methanol ⇄ Fatty Acid Methyl Ester (FAME) + Glycerol

(1)

Reaction conditions:

• Methanol/oil molar ratio: 10:1.
• Catalyst loading: 1 wt.% (based on the total weight of oil and methanol).
• Reaction temperature: 68 °C.
• Stirring speed: 1500 rpm.
• Reaction time: 4, 6, 8, 10, and 12 h.

The composition of grapeseed oil fatty acids is summarized in

Table 5. Fatty acids composition of 1 g grapeseed oil [27].

Linoleic Acid (mg)	Oleic Acid (mg)	Palmitic Acid (mg)	Stearic Acid (mg)
694.55	139.38	61.71	35.69

.

4.5. Catalytic Performance Evaluation

4.5.1. Basicity Measurement via HCl Titration

The basicity of the catalysts was evaluated using HCl titration [26,28]. A 300 mg sample of catalyst was dispersed in 10 mL of 1% phenolphthalein solution and stirred for 6 h. The sample was titrated with HCl solution, and the consumed volume was used to determine the basicity.

4.5.2. Biodiesel Yield Analysis via HPLC

High-Performance Liquid Chromatography (HPLC, Shimadzu LC-20A, Tokyo, Japan) equipped with a Refractive Index Detector (RID-20A, Tokyo, Japan) and a Pursuit 5 C18 column (250 × 4.6 mm, 5 µm particle size) was used to analyze biodiesel yield. The mobile phase consisted of 85% methanol, 10% isopropyl alcohol, and 5% n-hexane. The HPLC conditions were as follows:

• Oven temperature: 40 °C.
• Flow rate: 0.5 mL/min.
• Injection volume: 1 µL.

Biodiesel yield was determined using Equation (2) [23]:

Yield of Biodiesel (%) = (Mass of Methyl Ester/Mass of Grapeseed Oil) × 100

(2)

5. Conclusions

In this study, a Zn-doped CaO/MgO catalyst (CMZ) was successfully synthesized using starfish-derived marine biomass as a sustainable source of calcium and magnesium. Systematic evaluation of Zn loading identified 10 wt% as the optimal composition, which significantly enhanced the catalyst’s surface area, basicity, and catalytic activity, resulting in a biodiesel yield of 96.6% under mild reaction conditions. Reusability tests confirmed sustained performance over three cycles, while SEM-EDS analysis demonstrated Zn retention and compositional stability. Compared to the reported pure ZnO catalysts, which exhibited lower yields and reduced stability at extended reaction times, the CMZ catalyst maintained superior catalytic efficiency and durability. These findings underscore the potential of marine waste-derived catalysts for sustainable biodiesel production and provide valuable insights for biomass-based catalyst development.