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. 2023 Jun 19;8(26):23720–23732. doi: 10.1021/acsomega.3c01892

Biodiesel Production from Waste Cooking Oil via β-Zeolite-Supported Sulfated Metal Oxide Catalyst Systems

Basiru O Yusuf †, Sulayman A Oladepo †,‡,*, Saheed A Ganiyu †,§
PMCID: PMC10324085  PMID: 37426238

Abstract

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Waste cooking oil (WCO) is a readily available and cheap feedstock for biodiesel production. However, WCO contains high levels of free fatty acids (FFAs), which negatively impact the biodiesel yield if homogeneous catalysts are used. Heterogeneous solid acid catalysts are preferred for low-cost feedstocks because the catalysts are highly insensitive to high levels of FFA in the feedstock. Therefore, in the present study, we synthesized and evaluated different solid catalysts, pure β-zeolite, ZnO-β-zeolite, and SO42–/ZnO-β-zeolite for the production of biodiesel using WCO as feedstock. The synthesized catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), pyridine-FTIR, N2 adsorption–desorption, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, while the biodiesel product was analyzed using nuclear magnetic resonance (1H and 13C NMR) and gas chromatography–mass spectroscopy. The results revealed that the SO42–/ZnO-β-zeolite catalyst showed excellent catalytic performance for simultaneous transesterification and esterification of WCO, with a higher percentage conversion than the ZnO-β-zeolite and pure β-zeolite catalyst, due to the large pore size and high acidity. The SO42–/ZnO-β-zeolite catalyst exhibits 6.5 nm pore size, a total pore volume of 0.17 cm3/g, and high surface area of 250.26 m2/g. Experimental parameters such as catalyst loading, methanol:oil molar ratio, temperature, and reaction time were varied in order to establish the optimal parameters. The highest WCO conversion of 96.9% was obtained using the SO42–/ZnO-β-zeolite catalyst under an optimum reaction condition of 3.0 wt % catalyst loading, 200 °C reaction temperature, and 15:1 molar ratio of methanol to oil in 8 h reaction time. The WCO-derived biodiesel properties conform to the ASTM6751 standard specification. Our investigation of its kinetics revealed that the reaction follows a pseudo first-order kinetic model, with an activation energy (Ea) of 38.58 kJ/mol. Moreover, the stability and reusability of the catalysts were evaluated, and it was found that the SO42–/ZnO-β-zeolite catalyst exhibited good stability, giving a biodiesel conversion of over 80% after three synthesis cycles.

1. Introduction

Fossil fuels have been major fuel sources for domestic and industrial uses, and they meet the majority of the world’s energy demands and use in petrochemicals.1,2 Nevertheless, fossil fuels are linked to major environmental issues such as greenhouse gas emissions, air pollution, oil spillage, and degradation of aquatic and arable land.3−5 Biofuels have a high priority among fuel alternatives because of their renewable nature. Biodiesel is regarded as the most promising renewable fuel among the various biofuels, presumably owing to its nontoxic, biodegradable, and renewable nature, as well as its low carbon monoxide and sulfur emissions during combustion.6 Biodiesel is produced by either transesterification of triglycerides or esterification of free fatty acids (FFAs) with methanol using a suitable catalyst, leading to fatty acid methyl ester (biodiesel) and glycerol.7 Vegetable oil-based biodiesel typically costs more than 0.50 USD L–1,8,9 and depending on the feedstock oils, the price can be about 1.5 times that of fossil-based diesel.8 Use of cheap starting materials like non-edible oil and waste cooking oil (WCO) rather than fresh vegetable oils and the development of highly active, cost-effective, and easily recyclable catalysts can lower the production costs of biodiesel.8,9 Moreover, using WCO offers three possible advantages: environmental sustainability, waste management, and good economics. Detailed economic analyses have shown that use of WCO is more economically feasible and profitable because of the lowest manufacturing cost, attractive rate of return, and reduction in selling prices.10,11

Owing to the high FFAs and water content in WCO, using a homogeneous base catalyst will result in saponification, which will cause significant difficulties with product separation and eventually, a significant reduction in biodiesel yield. When using feedstocks that contain a high level of FFAs, a two-step approach is typically employed.12,13 Pre-esterification of FFAs with alcohol is the first step, in which the FFAs are catalyzed using liquid acids (liquid acid catalysis, which reduces the FFAs content in oil by more than 99%). In the second step, the products from the first step are converted into biodiesel and glycerol by base-catalyzed transesterification. Unlike base catalysts, FFAs have no effect on homogenous acid catalysts, and homogenous acid catalysts are capable of catalyzing both transesterification and esterification simultaneously.14 Nevertheless, separation and regeneration of the catalyst, corrosion-related problems, and serious environmental issues limit the utilization of homogeneous acid catalysts. The use of heterogeneous catalysts to produce biodiesel is gaining popularity due to the fact that they are recyclable and offer a simple catalyst removal step.15 Heterogeneous acid catalysts may be less active but they are more stable than heterogeneous base catalysts, allowing them to be employed for feedstocks containing a high quantity of FFAs without catalyst deactivation.16

Many scientists have made substantial progress toward producing biodiesel through solid acid catalysts.17 Heterogeneous acid catalysts employed for triglyceride transesterification, including zeolite, Amberlyst-15, and sulfated tin oxide (SO42–/SnO2), exhibited high catalytic activity.18 Solid acid catalysts have also been used to esterify FFAs.19 Incorporation of the sulfate group into metal oxides primarily improves acidity without compromising the catalytic activity.19−21 Sulfated titanium oxide (SO42–/TiO2), ion-exchange resin, sulfated zirconium oxide (SO42–/ZrO2), and sulfated tin oxide are some examples of typical catalysts with high catalytic activity.19−21 When utilized to catalyze both transesterification and esterification simultaneously, some of these heterogeneous acid catalysts, like sulfated titanium oxide, demonstrated remarkable catalytic activity and stability.21,22

Latest developments in solid acid catalysts for biodiesel production are predicated on the fact that such catalysts have better tolerance for high contents of FFA and water present in low quality oils such as WCO. Solid acid catalysts are so FFA-tolerant that several researchers have demonstrated their suitability for esterification of fatty acids.23,24 In a recent report, Zhang and Xie synthesized ZrMo oxides catalyst supported on hierarchical porous structure made of polystyrene and used the resulting catalyst system for biodiesel production from acidic oils.23 The catalyst is recyclable and gave 93.8% oil conversion. The authors claimed that incorporation of transition metal oxides into porous support materials is a viable way to significantly increase the catalytic surface area, heterogeneity, and catalytic performance.23 In a similar vein, Lin and co-workers used polymerization and ion-exchange to prepare a series of poly(divinylbenzene) polymer-based solid acid catalysts using various p-styrene sulfonate/divinylbenzene loading ratios. The resulting catalyst was used for the transesterification of waste palmitic oils, giving a biodiesel yield of 83.3%.24 This work shows that their acidic catalyst can efficiently convert waste oils containing FFA from triglycerides and FFA to biodiesel.24

Several research groups have also reported the use of heterogeneous nanocatalysts for biodiesel production, including zinc oxide, calcium oxide, magnesium oxide, zirconium oxide, iron oxide, and cobalt oxide nanocatalysts.25−28 For instance, Saravanan et al. used a solvent-free approach to prepare 11 nm sulfated zirconia nanoparticles by the precipitation technique and then used them as nanocatalysts for transesterification of palmitic acid, which gives a 90% yield.27 Other types of solid acid catalysts have also been used for biodiesel production.29,30 For example, Brønsted acidic ionic liquids have also been exploited for the catalytic transesterification of coconut oil.29 A biodiesel yield of 98.3% was obtained, and there was no significant reduction in catalytic activity even after 5 reaction cycles.

Perhaps in line with the claim of Zhang and Xie that incorporation of solid acid catalysts in porous support materials will increase their surface area and catalytic performance, several researchers have harnessed metal–organic frameworks (MOFs) as a catalyst support.31−34 Using zeolitic imidazolate framework (ZIF-90), Mao and co-workers modified the MOF with sulfamic acid, thereby increasing the number of Brønsted and Lewis acid sites of the framework. They used the catalyst for microbial biodiesel production and obtained 98.3% conversion.31 Similarly, a novel bismuth-based MOF impregnated with phosphomolybdic acid (PMA@Bi-BTC) was used by Zhang et al. for the conversion of oleic acid to biodiesel.32 They reported that the catalyst gave a conversion of 92.5%, which was attributed to textural property of the catalyst, the presence of more acid sites, and the synergistic catalytic effects between the duo of phosphomolybdic acid and Bi-BTC.32 An efficient and stable heterogeneous acid catalyst has also been synthesized by impregnation method, combining UiO-66 MOFs and ammonium sulfate.34 The resulting solid acid catalyst was put through a two-stage calcination process under a nitrogen atmosphere in order to enhance the catalyst stability, and a maximum conversion of 96.2% was obtained for oleic acid conversion to biodiesel. All the studies show that MOF-supported solid acid catalysts also constitute efficient and stable heterogeneous catalysts for biodiesel production.

We wish to state that the use of porous and stable catalyst support is also crucial for catalyst performance and stability. Owing to the challenges of filtering the tiny catalyst particles and the high catalyst cost, heterogeneous acid catalysts such as sulfated titanium oxide have not been widely used in commercial biodiesel production operations. Zinc oxide and lead oxide are promising catalysts for both transesterification and esterification, but they are prone to leaching and have negative reuse effects.35 This problem is also common with several homogeneous and heterogeneous acid catalysts.36 Therefore, incorporation of proper support is also very important to improve the reusability of the catalyst and reduce leaching problems. Pure oxides are mostly supported by activated carbon, silica, and alumina.35 In many cases, conventional support materials (such as activated carbon, alumina, and silica) are inefficient.35 Therefore, zeolites, which have high porosity, large surface area, high acidity, and high hydrothermal stability than alumina and silica, may display improved metal–support interaction, resulting in improved catalyst stability.

β-zeolite is a form of zeolite that is rich in silica with a three-dimensional structure of intersecting 12-membered ring channel.37 Several acid-catalyzed reactions can be carried out efficiently due to the relatively large channel structure.37 Other factors influencing reaction efficiency of β-zeolite are the high thermal stability, large surface area, and tunable acidity in the protonic form.38 It has been found that β-zeolite is an effective catalyst for transesterification, which could be utilized in the production of a vast array of products.18,38 Transesterification occurs at the β-zeolite Brønsted acidic sites, which can be slightly altered with cation modifications, resulting in modified catalysts with appropriate acidity to fit diverse transesterifications.38−40

In the present study, we prepared β-zeolite, zinc oxide, and sulfated-zinc oxide supported on β-zeolite (ZnO-β-zeolite and SO42–/ZnO-β-zeolite, respectively) and investigated their catalytic performance in the production of biodiesel using WCO. The parameters that affect reactions such as reaction time, the ratio of methanol to oil, catalyst loading, and temperature were optimized. A kinetic study was also conducted at different reaction temperatures to evaluate the rate of reaction and determine the activation energy (Ea). The surface acidity (Lewis or Brønsted) and its influence on the activity of pure β-zeolite, ZnO-β-zeolite and sulfated/ZnO-β-zeolite were also investigated. A sulfated/ZnO-β-zeolite catalyst was found to exhibit a significant amount of acidity relevant to high biodiesel conversion. The superiority of the SO42–/ZnO-β-zeolite catalyst over ZnO-β-zeolite and pure β-zeolite was attributed to a number of physico-chemical characteristics such as pore size, acidity, and so forth, which resulted from the dispersion of the active phase caused by sulfate incorporation into the catalyst matrix.

2. Experimental Section

2.1. Materials

WCO was obtained from staff residence at King Fahd University of Petroleum and Minerals, Saudi Arabia. All chemicals were of analytical quality and utilized just as they were received, without additional purification. Methanol (purity >99%), zinc nitrate hydrate, tetraethylammonium hydroxide (40 wt % TEAH), sodium chloride (reagent grade), ammonium nitrate, sulfuric acid, and potassium chloride (reagent grade) were purchased from Sigma-Aldrich (USA). Also, fumed silica (amorphous silicon dioxide particles produced in an oxygen-hydrogen flame), sodium hydroxide (98%), and sodium aluminate (56 wt % Al2O3, 37 wt % Na2O) were purchased from Sigma-Aldrich (USA).

2.2. Synthesis of Catalysts

2.2.1. Synthesis of β-Zeolite

The hydrothermal method described by Camblor et al. was used to produce β-zeolite,40 using tetraethylammonium hydroxide as a structure-directing agent. In a typical procedure, 59.4 g of deionized water and 89.60 g of tetraethylammonium hydroxide (40%) were mixed with 0.53 g of sodium chloride and 1.44 g of potassium chloride and agitated until dissolved. Subsequently, fumed silica (30.0 g) was added to the mixture and agitated until homogenized. Then, sodium hydroxide (0.33 g) and sodium aluminate (1.79 g) were dissolved in deionized water (20.0 g) and stirred till completely dissolved. Following that, the two resulting liquids were mixed and stirred for 10 min. After stirring for 10 min, the thick gel formed was heated for 20 h at 135 °C in an autoclave. Following a hydrothermal reaction, the mixture was cooled in cold water and centrifuged. After centrifugation, the resulting mixture was dried at 77 °C after being washed with distilled water. The solid product that was produced after drying was calcined in a muffle furnace at 550 °C for 6 h. The sodium form of β-zeolite was thereby produced (Na-β-zeolite).

Using an ion-exchange method, Na-β-zeolite obtained was transformed into H-β-zeolite. Na-β-zeolite was impregnated with 1 M ammonium nitrate solution (1.0 g sample vs 20 mL 1 M NH4NO3) for 4 h at 80 °C. After two cycles of Na+/NH4+ ion-exchanged β-zeolite, the resulting product was subjected to filtration, followed by washing with distilled water before drying at 100 °C overnight. The calcination of NH4-β-zeolite for transformation to H-β-zeolite was carried out at 550 °C for 6 h in a muffle furnace. H-β-zeolite is the name given to the final sample obtained (Scheme 1).

Scheme 1. Schematic Showing Synthesis of H-β-Zeolite.

Scheme 1

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Corresponding schematics for ZnO-β-zeolite and SO42–/ZnO-β-zeolite are shown in Figure S1.

2.2.2. Synthesis of ZnO-β-Zeolite

In a typical synthesis, zinc oxide supported on β-zeolite (ZnO-β-zeolite) was prepared by adding a predetermined amount of zinc nitrate hydrate (equivalent to 10 wt % of β-zeolite) and an appropriate amount of β-zeolite to 15 mL of distilled water and agitated at a temperature of 60 °C for 3 h at 600 rpm. After stirring for 3 h, the resulting precipitated solid was dried for 8 h at 100 °C. Finally, the solid obtained was calcined at 550 °C for 6 h, as shown in Scheme S1a.

2.2.3. Synthesis of SO42–/ZnO-β-Zeolite

SO42–/ZnO-β-zeolite was prepared by slowly adding a predefined amount of zinc nitrate hydrate and an appropriate amount of β-zeolite to 20 mL of 0.5 M sulfuric acid solutions and vigorously stirring at 60 °C for 3 h at a speed of 600 rpm. After stirring, the resultant solid precipitate was filtered, oven-dried for 8 h at 100 °C, and calcined for 6 h at 550 °C in a muffle furnace [Scheme S1b].

2.3. Catalyst Characterization

The synthesized materials were thoroughly characterized in order to determine their structural, physical, and chemical characteristics. The crystalline structures of the catalytic materials were examined using various techniques. A Rigaku Ultima IV X-ray diffractometer was used to perform XRD on the catalysts, with a scan range of 5°–80° (2-theta) and a speed of 5.0°/min. The textural properties of the as-synthesized materials were measured utilizing Micrometrics ASAP 2020 equipment. (Micrometrics, USA). Pore size distribution of the catalytic materials was investigated utilizing the Barrett, Joyner, and Halenda technique. The surface area was determined using the Brunauer, Emmett, and Teller (BET) technique. Prior to the analysis, a vacuum pretreatment process at 250 °C with nitrogen gas flow was carried out for 3 h. Fourier transform infrared (FTIR) was utilized to determine the functional groups present in the catalysts. A Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific, USA) was utilized to obtain FTIR spectra (wavenumber ranging from 400–4000 cm–1). Pyridine-FTIR was used to measure the catalyst’s acidity. In a typical procedure, the sample was put into a Specac cell and pretreated at 500 °C for 1 h under vacuum, and then the temperature was reduced to 150 °C. Pyridine vapor was flowed for 30 min. Following that, the samples were degassed under vacuum conditions at 150 °C in order to remove excess pyridine, and then at 150 °C, the acidity owing to Brønsted acid and Lewis acid site was measured. We examined the surface characteristics of the catalyst using a scanning electron microscope: Thermo Scientific Quattro ESEM equipped with an electron gun, a secondary electron mode, and a backscattered electron mode. An SDT Q600 thermogravimetric analyzer was used for the thermogravimetric analysis (TGA) (Thermal analysis instrument, USA), for which the samples were heated from 26 to 800 °C with an air flow rate of 10 °C/min.

2.4. Biodiesel Synthesis

Typically, a 50 mL autoclave made of stainless steel and fitted with a magnetic stirrer was used to perform the reaction. The reactor was filled with the required quantities of WCO, methanol, and catalysts. Afterward, the reaction system was agitated and heated until the desired temperature was reached. Following the mixture’s cooling, the catalyst was centrifuged. The resulting sample mixture was allowed to settle (separated into two distinct layers: oil phase and aqueous phase). Fatty acid methyl esters (FAMEs) constituted the main component of the oil phase, while glycerol, methanol, and water were the components of the aqueous phase.

2.5. Determination of Biodiesel Yield

1H NMR spectroscopy was utilized to determine the conversion of WCO to FAMEs. The biodiesel product was evaluated using a Bruker NMR 400 MHz spectrometer with deuterated chloroform (CDCl3) as a solvent, in which case 10 mg of biodiesel product was dissolved in 1 mL of deuterated chloroform with 0.05% tetra-methyl silane.

The percent conversion of biodiesel to methyl ester was calculated according to eq 1:

2.5. 1

where AME represents the integrated peak area of the methoxy proton in the methyl ester, and ACH2 represents the integrated peak area of the methylene proton adjacent to the carbonyl group. The percentage of oil converted to biodiesel is denoted by C.41

3. Results and Discussion

3.1. Characterization of the Catalyst

3.1.1. XRD Analysis

One of the most common characterizations for heterogeneous catalysts is XRD analysis.23,31−33Figure 1 shows the XRD patterns of the three catalysts prepared in this work. Figure 1 shows the XRD patterns of the catalysts. It was observed that the synthesized H- β-zeolite has the XRD pattern exhibiting representative Bragg reflection conforming to the β-zeolite structure and a well-preserved crystalline structure, which is comparable to the findings that were previously reported in the literature.42,43 The diffraction peak at 2θ = 6°–8°, ascribed to (101) reflection of β-zeolite topology, is generally seen as an evidence of a highly distorted structure resulting from various isomorphs in the zeolite structure.44 The narrow main distinct signal at 2θ = 22°–23°, assigned to (302) reflection of β-zeolite, indicates zeolite structure lattice expansion/contraction.45 Following the incorporation of Zn species and sulfonate group, the typical β-zeolite diffraction peak at 22°–23° slightly decreased in intensity, indicating a slight compression of the β-zeolite crystalline structure in the presence of zinc species without changing the framework of the zeolite and this is consistent with previous reports.46,47 As illustrated in Figure 1, the zinc oxide is well incorporated into the β-zeolite support. The peaks at 2θ = 31.7°, 34.4°, 36.2°, 56.5°, and 62.9° corresponding to (100), (002), (101), (110), and (103) lattice planes were attributed to the phase of zinc oxide, suggesting the formation of the crystals.48 In addition, no diffraction peaks for ZnSO4 were detected in SO42–/ZnO-β-zeolite, which may be due to uniform distribution of sulfate species over the zeolite crystals’ surface, in agreement with the literature.49 These results, which are in agreement with previous literature reports on solid acid catalysts,29−36 provide evidence of the crystallinity of the catalysts and the incorporation of zinc and sulfonate species into the β-zeolite structure.

Figure 1.

Figure 1

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XRD patterns of the three catalysts. Peaks corresponding to lattice planes of ZnO are shown.

3.1.2. FTIR Analysis

FTIR is another important analytical technique used for the characterization of catalysts, given that it has the capability to delineate the different functional groups present in the catalyst system.50−53 Thus, we have employed this technique to characterize the molecular composition of the catalysts. The FTIR spectra of synthesized catalysts are depicted in Figure 2. The band around 575 cm–1 is characteristic of β-zeolite since the structure contains six-membered rings.50 The bands at about 800 and 1080 cm–1 represent the T–O–T (T = Al or Si) symmetric stretching vibrations, which are susceptible to the aluminum composition of the framework.51 The band that appears at 1225 cm–1 is related to asymmetric stretching of the T–O–T.51 Based on previous reports on different siliceous zeolites, the 950 cm–1 band represents the stretching vibrations of Si–O and it belongs to uncoupled SiO4 tetrahedron.52,53 As seen in Figure 2, the structural vibration is comparable for all samples under investigation, confirming that the incorporation of metal oxide into β-zeolite support does not affect the structure. The band at 425 cm–1 is related to the zinc oxide stretching mode, suggesting that ZnO is incorporated into the zeolite framework, which is consistent with the literature.48 In addition, the intensity of the peak at 1225 cm–1 is stronger (sharper) in SO42–/ZnO-β-zeolite than in H-β-zeolite and ZnO-β-zeolite because of the incorporation of the sulfate species, which is related to S=O bonding in SO42–.49 Given the spectral signatures obtained from our FTIR measurements, we can say that the metal and sulfonate species are well incorporated into the β-zeolite support framework.

Figure 2.

Figure 2

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FTIR spectra of the three catalysts.

3.1.3. Pyridine-FTIR Analysis

Pyridine-FTIR involves the use of pyridine as a probe molecule, and it is a useful method for quantitative analysis of the Brønsted and Lewis acid sites present on a catalyst surface. It is widely applied in industrial and laboratory settings for quantifying acidic sites on the surface of a catalyst. Therefore, we performed pyridine-FTIR analysis on the catalysts in order to determine acidity, which is related to catalytic performance. Figure 3 shows the pyridine-adsorbed FTIR spectrum of the catalysts measured at 150 °C between 1430 and 1800 cm–1. As seen in the figure, there are sharp adsorption bands resulting from the stretching of C–C bond of pyridine. Pyridine molecules interacting with Lewis (L) acid sites have sharp adsorption bands at 1456 and 1630 cm–1, while pyridine molecules adsorbed on Brønsted (B) acid sites have a band at 1550 cm–1. A pyridine molecule interacting with both L and B acid sites was assigned to the band at 1490 cm–1, in agreement with a previous study.54 Also, we observe that the intensity of all the bands is substantially higher in SO42–/ZnO-β-zeolite than in H-β-zeolite and ZnO-β-zeolite, indicating an increase in total acid sites. Conversely, the lower FTIR intensities in H-β-zeolite and ZnO-β-zeolite indicate very low acidity in both cases,23 which is responsible for lower conversion of the WCO to biodiesel when these two catalysts were used (vide infra). The deposition of sulfated groups in the case of the SO42–/ZnO-β-zeolite catalyst is responsible for the increase in total acid sites, which in turn is responsible for the more intense absorption bands shown for this catalyst (Figure 2). Thus, the result suggests that the prepared SO42–/ZnO-β-zeolite has high acidity and can be utilized as a heterogeneous acid catalyst in biodiesel production.

Figure 3.

Figure 3

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Pyridine-FTIR spectra of the catalysts.

3.1.4. Textural Properties Analysis

The N2 adsorption–desorption isotherms of the catalysts are shown in Figure 4. A type-1 isotherm is observed in the H-β-zeolite.55 The type-1 isotherm pattern suggests a micro-meso hierarchical porous textural characteristics.55 The type-1 isotherm remains the same after zinc oxide loading on the β-zeolite support. The pore size, volume, and specific surface areas of the catalysts are shown in Table 1. The surface area of β-zeolite was 469.21 m2/g, while ZnO-β-zeolite and SO42–/ZnO-β-zeolite had surface areas of 198.01 and 250.26 m2/g, correspondingly. The reduction in the surface area could be attributed to pore blockage caused by metal oxide deposition on the β-zeolite. The catalyst is mostly found on the surface of zeolite support. As a result, the decrease in the support surface area is unimportant in this case because the primary role of zeolites is holding metal oxide particles together and preventing metal leaching.56 Nevertheless, when the metal oxide is incorporated into the zeolite support, the pore size changes, and it increases upon incorporating both metal oxide and sulfate groups. The active phase dispersion caused by sulfate incorporation in the SO42–/ZnO-β-zeolite catalyst matrix was also attributed to the high surface area and pore size of SO42–/ZnO-β-zeolite compared to ZnO-β-zeolite. In a study involving PMA@Bi-BTC catalyst for biodiesel production from oleic acid,32 the surface area and pore volume of the resulting catalyst increased on impregnation of PMA into Bi-BTC due to the interaction of the PMA with the Bi-BTC framework. Thus, in the present work, the incorporation of the metal oxide and sulfate species in the β-zeolite resulted in the observed increase in surface area and pore size. When producing biodiesel with a heterogeneous acid catalyst, this phenomenon is important. This could be because catalysts with larger pores reduce diffusion restrictions, particularly for long alkyl chain molecules.47

Figure 4.

Figure 4

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N2 adsorption–desorption isotherms plot of H-β-zeolite support and the catalysts.

Table 1. Textural Properties of Supporting H-Beta Zeolite and the Catalysts.
sample SBET (m2/g) Smicro (m2/g) Smeso (m2/g) Vmicr (cm3/g) Vtotal (cm3/g) PS (nm)
H-β-zeolite 469.21 317.72 151.48 0.14 0.28 4.52
ZnO-β-zeolite 198.01 140.72 57.27 0.06 0.13 6.46
SO42–/ZnO-β-zeolite 250.26 180.27 69.99 0.08 0.17 6.50
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3.1.5. Thermogravimetric Analysis

The thermal stabilities of H-β-zeolite, ZnO-β-zeolite, and SO42–/ZnO-β-zeolite were established with TGA. In general, the TGA curve depicts the stages of weight loss in the samples (Figure 5). According to the literature, weight loss below 350 °C can be ascribed to the removal of physically adsorbed water and the thermal degradation of noninteracting tetraethylammonium hydroxide (TEAOH) species, whereas weight loss above 350 °C can be attributed to the pyrolysis of tetraethylammonium cation (TEA+) interacting with the framework of zeolite.40 This effect leads to a weight loss of about 10–15%. The extra weight loss in this temperature range could be related to lower temperature decomposition residues. As seen in Figure 5, the weight loss of about 5–15% observed in the three catalysts in the 100–200 °C temperature range is ascribed to physically adsorbed water, while the weight loss of about 5% observed between 200 and 300 °C may be due to loss of water trapped within the composite framework and thermal degradation of TEAOH.32,33 Further weight loss observed for H-β-zeolite in the 400–500 °C temperature range may be due to the pyrolysis of TEA+.40 After the weight loss around 100–300 °C, the catalysts are essentially stable. Accordingly, we can argue that the catalysts show high thermal stability, as the main decomposition of the zeolite support started at high temperature, which indicates the high stability of β-zeolite. Thus, the results showed that the synthesized catalysts are thermally stable and can be utilized to produce biodiesel.

Figure 5.

Figure 5

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TGA analysis of the catalysts.

3.1.6. SEM Analysis

Figure 6 shows the SEM images depicting the morphology of the catalysts. The morphology of the prepared H-β-zeolite revealed uniform and spherical particles with sizes varying between 0.5–2 μm (Figure 6a), which is congruent with published research findings.56 After zinc oxide had been loaded onto H-β-zeolite (ZnO-β-zeolite) and incorporation of both zinc oxide and sulfate groups in H-β-zeolite (SO42–/ZnO-β-zeolite), there is no substantial change in the shape of the particles (Figure 6b,c). These results suggest that zinc oxide particles were uniformly dispersed on the surface of the β-zeolite, such that the three catalysts have similar morphology and appearance. The similar morphology displayed by all three catalysts reported in this work may not be out of place. Recently, Mao et al. also reported SEM images of their sulfamic acid (SA)-modified ZIF-90 catalysts that are every similar to each other despite different SA/ZIF-90 weight ratios being used.31

Figure 6.

Figure 6

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SEM images of the catalysts: (a) H-β-zeolite; (b) ZnO-β-zeolite; and (c) SO42–/ZnO-β-zeolite.

3.2. Catalytic Performance of the Catalysts

The performance of the synthesized catalysts was investigated for the methanolysis of WCO for biodiesel production. The catalytic activities of H-β-zeolite, ZnO-β-zeolite, and SO42–/ZnO-β-zeolite were investigated at fixed process parameters such as reaction temperature of 200 °C, WCO/methanol molar ratio of 1:12, and catalyst loading of 2.0 wt % for 4 h. Figure 7 shows the biodiesel conversion trend when the catalysts were used. The highest WCO conversion was achieved by using ZnO-β-zeolite and SO42–/ZnO-β-zeolite. This increase in catalytic activity is most likely due to the appropriate particle size. It should be emphasized that triglyceride access to β-zeolite internal sites may be limited due to their sizes; consequently, catalytic activity originates primarily from sites on the β-zeolite external surfaces. Due to the large size of zinc species, Zn ion exchange is expected to occur on the β-zeolite external surface, which can significantly increase the number of sites on the β-zeolite exterior surfaces. Thus, it can be inferred that the ZnO-β-zeolite and SO42–/ZnO-β-zeolite has a greater number of external active sites available for the reactant. As depicted in Figure 7, ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalysts were more appropriate in terms of external accessibility to promote WCO conversion, which appears to be guided primarily by the improvement of characteristics of the β-zeolite external surface. Therefore, ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalysts were further compared for biodiesel production, and the process parameters (such as methanol to WCO molar ratio, catalyst loading, time, and temperature) were optimized.

Figure 7.

Figure 7

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Catalytic performance of H-β-zeolite, ZnO-β-zeolite, and SO42–/ZnO-β-zeolite catalysts related to conversion of WCO [A = H-β-zeolite, B = ZnO-β-zeolite, and C = SO42–/ZnO-β-zeolite].

3.3. Biodiesel Analysis

Following biodiesel synthesis as described in the experimental section, the conversion of WCO to FAME and biodiesel characterization were carried out using 1H NMR and 13C NMR spectroscopy, respectively. The 1H NMR spectrum of the biodiesel sample obtained after WCO conversion to FAME using the SO42–/ZnO-β-zeolite catalyst is shown in Figure S1. The presence of a triplet peak at 2.31 ppm and a single peak at 3.67 ppm corresponding to α-CH2 protons (ACH2) and methoxy protons (AME), respectively, confirms biodiesel formation.28 These two peaks indicated oil conversion to biodiesel. Other peaks include signals at 0.88, 1.26, and 1.61 ppm for end-of-chain methyl protons, carbon methylene proton, and carbonyl methylene protons, respectively. The 5.35 ppm signal is attributed to an olefinic proton.41,57,58Eq 1 describes the oil conversion into biodiesel.41 Using eq 1, the percentage conversion of WCO to equivalent FAMEs was found to be 96.9%.

Figure S2 depicts the 13C NMR spectra of WCO biodiesel. The ester carbonyl carbon (−COO−) peak was observed at 174.31 ppm, and the C–O carbon appears at 51.42 ppm in the biodiesel spectrum. The two signals at 127.90 and 130.19 ppm indicate the presence of unsaturated fatty acids. The 14.11 ppm signal corresponds to the end-chain methyl carbon, while the signals in the range of 22.69–34.10 ppm are related to aliphatic methylene groups (−CH2-S) of FAMEs.57,58 The composition of the WCO biodiesel was investigated using GC–MS analysis. The GC–MS chromatogram of the biodiesel sample is shown in Figure S3. Table S1 lists the components of the biodiesel. The biodiesel sample comprised both saturated and unsaturated FAMEs.

The performance property of the biodiesel product is compared with ASTM standards. The fuel properties such as flash point, kinematic viscosity, density, specific gravity, and acid value of the WCO-based biodiesel are shown in Table 2. The data conform to the recommended values specified in ASTM D6751. Therefore, the biodiesel product obtained in the present study can be utilized either as a pure fuel or blended with conventional diesel fuel.

Table 2. Performance Properties of the WCO Biodiesel in Comparison with ASTM Standards.

fuel properties ASTM method used ASTM D6751 biodiesel WCO biodiesel
kinematic viscosity (40 °C, mm2/s) D445 1.9–6.0 5.04
density (40 °C, g/cm3) D5002 0.86–0.90 0.871
specific gravity (40 °C, g/cm3) D287 0.88 0.876
flash point (°C) D93 100–170 132
acid value (mg KOH/g) D664 0.5 0.15
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3.4. Effect of Different Reaction Parameters

To investigate the activity of the catalyst in biodiesel’s production, WCO was reacted with methanol over ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalysts. Thus, different parameters affecting methyl ester conversion were examined here. The effects of experimental parameters such as the molar ratio of methanol to oil (MRMO), catalyst loading, time, and temperature were investigated. Figures 8 and 9 depict the results of the parametric investigation for biodiesel production, while varying the experimental conditions.

Figure 8.

Figure 8

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Effects of (a) catalyst loading and (b) molar ratio of methanol to oil.

Figure 9.

Figure 9

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Effects of (a) reaction temperature and (b) reaction time.

3.4.1. Effect of Catalyst Loading and Methanol/Oil Ratio

In order to investigate the impact of catalyst loading on biodiesel conversion, the amount of catalyst used was varied from 1.0 to 4.0 wt % (Figure 8a). At a constant MRMO of 15:1, 8 h reaction time, and 200 °C, increase in catalyst loading from 1.0 to 3.0 wt % steadily enhanced biodiesel conversion, reaching a maximum of 84.1% for ZnO-β-zeolite and 96.9% for SO42–/ZnO-β-zeolite. Increased catalyst loading above 3.0 wt % reduces biodiesel conversion due to a mixing problem that involves solid catalyst, reactant, and product.39,59 Moreover, if catalyst loading exceeds the optimum amount, biodiesel products can adsorb on the catalyst surface, reducing conversion.41,60 Thus, the optimum catalyst loading for WCO conversion to biodiesel in this study is 3.0 wt %, and other reaction parameters were optimized at this loading.

Another factor that affects biodiesel conversion is the MRMO. Transesterification reaction requires one mole of oil and three moles of methanol to produce one mole of glycerol and three moles of FAMEs. Since the reaction is reversible, it is preferable to use excess methanol to favor the forward reaction. We varied the MRMO from 9:1 to 18:1 at 8 h reaction time, 200 °C and 3.0 wt % catalyst loading. We found that increasing the MRMO increases biodiesel conversion. When the MRMO rose to 15:1, the biodiesel conversion increased to 84.1 and 96.8% for ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalyst, respectively. However, beyond 15:1 MRMO, biodiesel conversion decreases. Increasing the MRMO might lead to incomplete transesterification and mass transfer limitation caused by dilution.61 Therefore, optimum MRMO was kept at 15:1 in this study.

3.4.2. Effect of Reaction Temperature and Time

Temperature has a significant impact on reaction rate, and so one of the most important parameters to optimize is the reaction temperature. Also, the formation of three immiscible phases consisting of oil, methanol, and solid acid catalyst at the beginning of the reaction limits mass transfer; therefore, high temperature is required when a heterogenous acid catalyst is employed. Most studies reported that high temperature favor biodiesel conversion from low-grade material/WCO feedstock.19,21Figure 9a depicts biodiesel conversion as reaction temperature changes. The reaction temperature was varied from 140 to 230 °C, at 8 h reaction time, MRMO of 15:1 and 3 wt % catalyst loading. At lower reaction temperatures, the biodiesel conversion was very low (Figure 9a). However, the reactants acquired sufficient kinetic energy at higher temperatures to enhance the system’s mass transfer rate, resulting in the optimum conversion of 84.1 and 96.9% for ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalyst, respectively, at 200 °C. When the temperature rose beyond 200 °C, the biodiesel conversion remained unchanged. Hence, 200 °C is considered the optimal reaction temperature in this study.

The impact of reaction time on biodiesel conversion was also investigated by varying the reaction time from 2 to 10 h at 2-hour intervals with a catalyst loading of 3.0 wt % and a 15:1 MRMO (Figure 9b). The result showed that the heterogeneous catalyst had not yet been fully activated in the first 2 h of reaction, resulting in a slow reaction rate and low biodiesel conversion. However, increasing the reaction time to 8 h increases biodiesel conversion to maximum values of 84.2 and 96.9% for the ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalysts, respectively. After 8 h, there was a decrease in biodiesel conversion. This behavior could be attributed to reverse transesterification between FAMEs and glycerol as reaction time increases.62,63 The optimal reaction time for biodiesel production was thus taken to be 8 h. We note that the activity of the SO42–/ZnO-β-zeolite catalyst was higher than that of the ZnO-β-zeolite catalyst, and this behavior may be attributed to the highly acidic nature of the SO42–/ZnO-β-zeolite catalyst.

3.5. Kinetic Studies of Biodiesel Synthesis

The kinetics of WCO biodiesel production was studied utilizing the SO42–/ZnO-β-zeolite catalyst at varying temperatures (140 to 230 °C) under optimum reaction conditions, and the reaction activation energy (Ea) was thereby computed. The reverse reaction can be ignored since an excess amount of methanol was employed, and the pseudo-first-order model is assumed to govern the reaction.64,65 Therefore, eq 2 fits the reaction rate constant,66 while the activation energy (Ea) of the process is computed using the Arrhenius eq 3.66

3.5. 2
3.5. 3

where R = universal gas constant (8.314 J mol–1 K–1); A = Arrhenius constant; T = reaction temperature (Kelvin); X = conversion of WCO at time t; k = reaction rate constant.

According to eq 2, a plot of −ln (1 – X) versus time is linear, as depicted in Figure 10a. The plots show that the model is acceptable for pseudo-first order kinetics due to its good linearity and high regression coefficients. A linear plot of ln k against 1/T that shows a high regression coefficient is depicted in Figure 10b. According to the Arrhenius plot, the activation energy (Ea) of the reaction was 38.58 kJ/mol using the SO42–/ZnO-β-zeolite catalyst. The activation energy obtained from the WCO is consistent with the literature and is comparable to the activation energy achieved for soybean oil transesterification (i.e., 33.6–84 kJ/mol).67,68 Moreover, a value of Ea greater than 15 kJ/mol indicates that the reaction is controlled chemically.54

Figure 10.

Figure 10

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(a) Graph depicting the relationship between −ln (1 – X) and time; and (b) Arrhenius graph showing the relationship between k and 1/T.

3.6. Plausible Mechanism

Figure 11 shows a plausible mechanism for simultaneous transesterification and esterification of WCO. The carbonyl group of fatty acid/triglyceride is typically adsorbed on the SO42–/ZnO-β-zeolite catalyst to produce the protonated carbonyl group, which is then attacked by the nucleophilic oxygen atom from methanol’s hydroxyl group to form a tetrahedral intermediate. Finally, in the esterification reaction, the tetrahedral intermediate removes water to produce methyl ester. This intermediate removes glycerol during the transesterification reaction to produce a new methyl ester.

Figure 11.

Figure 11

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Plausible mechanism for concurrent esterification and transesterification reactions.

3.7. Reusability of Catalysts

Solid catalysts have the primary advantage of being reusable. The reusability of the catalyst was examined in this work (Figure 12). To evaluate catalyst reusability, both ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalysts were centrifuged from the reaction mixture, rinsed with methanol, oven-dried at 100 °C for 12 h, and then used immediately after each run. Both transesterification and esterification reaction of WCO were carried out five times under the optimal conditions of 3.0 wt % catalyst loading at 200 °C and 15:1 MRMO for 8 h, and the results are shown in Figure 12. We found that the catalysts maintained fairly good stability even after three experiments retaining 67.49 and 80.89% for ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalyst, respectively, of its original activity. Furthermore, XRD analysis was conducted to determine the stability of the catalyst (see Figure S4). According to Figure S4, the XRD spectra of the five-recovery catalyst were nearly identical to that of the fresh one, except for a slight decrease in peak intensity, indicating the better recyclability of the two catalysts. The partial loss in activity was most likely due to the leaching part of the active component into the reaction media during repeated reactions. Therefore, these findings clearly show that the catalysts are highly recyclable.

Figure 12.

Figure 12

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Reusability potential of the ZnO-treated catalysts.

Table 3 compares the performance of different heterogeneous catalysts in simultaneous transesterification and esterification reactions using low-grade/waste oils as feedstock. Our catalysts’ performance is comparable to other studies.

Table 3. Different Solid Catalysts’ Performance in Biodiesel Production Utilizing Low Grade Material/Waste Oils as Feedstock.

catalyst operating conditionsa performance reference
Mg–Al–CO3 hydrotalcite 3 h, 200 °C, 1 wt %, 6:1 99% oil conversion (68)
SO4–2/TiO2–SiO2 6 h, 200 °C, 3 wt %, 16:1 92% FAME yield (19)
Zn3La1 3 h, 200 °C, 2.3 wt %, 36:1 96% FAME yield (69)
zinc stearate immobilized on silica gel 10 h, 200 °C, 3 wt %, 18:1 98% FAME yield (70)
12-tungstophosphoric acid supported on zirconia 10 h, 200 °C, 3 wt %, 9:1 90% FAME yield (71)
H-ZSM-5 4 h, 100 °C, 10 wt %, 45:1 55% oil conversion (72)
H-ZSM-5 (1.0 M citric acid modified) 4 h, 100 °C, 10 wt %, 45:1 84% oil conversion (73)
La2O3/Na-Y-800 1 h, 70 °C, 10 wt %, 15:1 80% FAME yield (61)
S-La2O3/Na-Y-800 (S = surfactant modified) 1 h, 70 °C, 10 wt %, 15:1 84% FAME yield (74)
CaO/zeolite 1.25 h, 8 wt %, 30:1 90.85% FAME yield (75)
ZnO-β-zeolite 8 h, 200 °C, 3.0 wt %, 15:1 84.1% conversion this work
SO42–/ZnO-β-zeolite 8 h, 200 °C, 3.0 wt %, 15:1 96.9% conversion this work
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a

The operating conditions (reaction time, temperature, catalyst loading, and MRMO).

4. Conclusions

ZnO-β-zeolite and SO42–/ZnO-β-zeolite catalysts, prepared by the wet impregnation approach, proved to be effective catalysts for simultaneous transesterification and esterification of WCO. The SO42–/ZnO-β-zeolite catalyst exhibited remarkable catalytic activity in contrast to ZnO-β-zeolite. Characterization analyses show that when the sulfonate group is incorporated to the surface of the catalyst, Lewis/Brønsted acid sites are formed. These acid sites contribute to the large surface area, high acidity, and high dispersion of the active phase of the SO42–/ZnO-β-zeolite. The highest WCO conversion of 96.9% was obtained using the SO42–/ZnO-β-zeolite catalyst under the optimum reaction condition of 3.0 wt % catalyst loading, 200 °C reaction temperature, and 15:1 MRMO for 8 h. Kinetic studies show that simultaneous transesterification and esterification exhibit a pseudo-first order kinetic model, with an apparent activation energy of 38.58 kJ/mol. This result suggests that the reaction is chemically controlled and not by mass transfer and diffusion constraints. Furthermore, the SO42–/ZnO-β-zeolite showed good stability, and after three cycles of use, biodiesel conversion was about 84% (indicating no significant loss of activity). The characteristics of WCO-derived biodiesel satisfy the requirements of the ASTM6751 standard. When compared with the performance of some heterogeneous acid catalysts used for biodiesel synthesis, the SO42–/ZnO-β-zeolite reported in this work exhibits promising activity for biodiesel production from WCO. Regardless, given the catalysts operate at an optimal temperature of 200 °C, the activity and stability of the catalysts may be further improved by using the equilibrium adsorption method of catalyst preparation. This may reduce pore blockage of the β-zeolite support, improve WCO conversion, and enhance the stability of the catalyst. A future study should investigate this method of catalyst preparation for WCO conversion to biodiesel and compare the performance and stability of the resulting catalysts with the present work. Overall, the catalyst system reported in the present work has the potential to be used in industrial biodiesel production since it is capable of converting waste oil, low-grade oil, and inexpensive oil with a high FFA content into biodiesel.

Acknowledgments

The authors gratefully acknowledge Dr. Khalid Alhooshani, the Chemistry Department, and Deanship of Research Oversight and Coordination at King Fahd University of Petroleum and Minerals for making available funding and other resources used for this work.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c01892.

  • Schematic of ZnO-β-zeolite and SO42–/ZnO-β-zeolite synthesis, 1H NMR spectrum of biodiesel obtained with SO42–/ZnO-β-zeolite catalyst, 13C NMR spectrum of biodiesel obtained with the SO42–/ZnO-β-zeolite catalyst, GC/MS chromatogram of FAMEs obtained with SO42–/ZnO-β-zeolite catalyst, constituent FAMEs of biodiesel obtained with the SO42–/ZnO-β-zeolite catalyst, XRD patterns of fresh and used SO42–/ZnO-β-zeolite catalyst (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c01892_si_001.pdf (533.1KB, pdf)

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