Nano Nickel-Zirconia: An Effective Catalyst for the Production of Biodiesel from Waste Cooking Oil

Abstract

The utilization of heterogeneous catalysts during the production of biodiesel potentially minimize the cost of processing due to the exclusion of the separation step. The (X wt%)Ni–ZrO₂ (where X = 10, 25 and 50) catalysts prepared through a hydrothermal process were tested for the production of biodiesel by the transesterification of waste cooking oil (WCO) with methanol. The influences of various reaction parameters were systematically optimized. While the physicochemical characteristics of the as-synthesized catalysts were examined using numerous techniques such as FTIR, XRD, TGA BET, EDX, SEM, and HRTEM. Among all the catalysts, (10 wt%)Ni–ZrO₂ exhibited high surface area when compared to the pristine ZrO₂, (25 wt%)Ni–ZrO₂ and (50 wt%)Ni–ZrO₂ nanocatalysts. It may have influenced the catalytic properties of (10 wt%)Ni–ZrO_2, which exhibited maximum catalytic activity with a biodiesel production yield of 90.5% under optimal conditions. Such as 15:1 methanol to oil molar ratio, 10 wt% catalysts to oil ratio, 8 h reaction time and 180 °C reaction temperature. Furthermore, the recovered catalyst was efficiently reused in several repeated experiments, demonstrating marginal loss in its activity after multiple cycles (five times).

1. Introduction

Over the last few decades, the consumption of petroleum products has incremented significantly because of excessive industrialization and rapid increase in the world population. As a result, the major portion of the energy used in these processes comes from fossil fuels. However, the petroleum reservoirs are depleting quickly, which could lead to the attrition of many fossil fuel-based stocks [1]. Particularly, the increasing demand for fossil fuels in the transportation sector in recent years has seriously affected the rate of depletion of fossil fuels. Therefore, developing alternative technologies based on renewable resources is inevitable [2]. Recently, many research efforts have been devoted to finding a suitable alternative to fossil fuels. The alternative energy option should be competitive from energy efficiency and cost-effectiveness viewpoints [3]. In this pursuit, biofuels/biodiesels are a promising substitute with high energy efficiency, abundant renewable raw materials, and green environmental impact [4,5].

Biodiesel is derived from triglycerides like plant or animal oils or fats [6]. These constituents are mainly comprised of fatty acid alkyl esters (FAAEs), which are typically generated by the transesterification of triglycerides of lipids with alcohols (low molecular weight) [7]. Compared to mineral diesel, biodiesel is usually considered a better alternative due to its extraordinary properties, including high renewability, effective biodegradability, environment-friendliness, and excellent lubricity. Besides, biodiesel is portable, non-hazardous, safe to use, readily available and free from aromatics and sulfur contents, significantly increasing its demand in transportation and other relevant sectors [8]. Additionally, biodiesel has a high quality of exhaust gas emission because organic carbon is photosynthetic in origin. It also reduces the percentage of CO₂ emission into the atmosphere and has a negative greenhouse effect [9]. Moreover, due to the close resemblances among the properties of biodiesel and petrodiesel, such as similar energy content, cetane number, viscosity, and phase changes, these two can be easily blended and used as effective alternatives in conventional diesel engines.

For the cost-effective production of biodiesel, various economic and ecological parameters need to be considered. Particularly, the feedstock price has a notable effect on the resulting cost of biodiesel. Thus, local feedstocks are usually considered to reduce the cost of transportation. Biodiesel is primarily produced using various vegetable oils, cooking oils, and animal grease. So far, various vegetable oils (edible or non-edible), such as soybean, rapeseed, cotton, peanut, corn, olive, sesame, and sunflower, have been extensively used as feedstocks for biodiesel production. Apart from this, the synthesis of biodiesels can also be achieved by other non-conventional feedstocks, such as waste cooking oils, microalgae, or animal fats, which have been extensively studied by several investigators. Notably, about 350 types of oil-containing crops are potentially used as precursors to synthesize biodiesel [10]. Among these resources, palm, soybean, rapeseed, moringa, coconut, pongamia, neem, jatropha, sunflower, castor oil, jojoba, cotton, rice bran, and microalgae has received great attention from various researchers globally [11,12,13]. Since these raw materials are rich in triglyceride contents and mainly consume environmental CO₂ during their growth, this renewable source could help in the remediation of environmental pollution [14]. Due to the increased knowledge and advancements in processing the feedstock, universal biodiesel production is predicted to continue to extend in the upcoming years to a significant amount, i.e., increasing from 29.7 × 10⁶ m³ in 2014 to 39 × 10⁶ m³ in 2024 [15].

The formation of biodiesel through the transesterification of oil or fat with alcohol is demonstrated in Scheme 1. This reaction produces fatty acid methyl ester, while the ref. [16] glycerol is formed as a byproduct. Mechanistically, transesterification is a reversible process. Therefore, it requires an excess of alcohol which facilitates the shifting of the equilibrium reaction towards the product side. At the same time, chemical catalysts are applied to enhance the reaction kinetics and product yield. During the transesterification reactions, the types and contents of alcohol play a major role. Short-chain alcohols like ethanol, methanol, butanol, and propanol are mostly preferred for this process. Among these alcohols, methanol is the maximum selected alcohol due to its less expensive and physicochemical (polar, short carbon chain) properties [17]. The transesterification reaction could be catalyzed using an alkaline or acidic medium. The alkaline medium with better kinetic is mostly utilized on a commercial scale, even under low amounts of fatty acid [18].

The commercialization of biodiesel is predominantly inhibited by the price of production. So far, biodiesel is still costly when compared to conventional fossil fuels. The cost of raw feedstock oils and the synthesis of catalysts are still considered the main cost of biodiesel production (60–80%) [19]. The former problem could be solved by choosing less expensive and locally available feedstock which are low costs like, waste cooking oil (WCO), animal fats [20], palm fatty acid distillate (PFAD) [21], Jatropha curcas oil [22] and grease etc. [23,24]. Whereas extensive scrutiny is still needed to minimize the cost of catalyst production. In this regard, WCO is a suitable feedstock which is abundantly available and largely considered an environmental pollutant and harmful to health. WCO is a growing menace to the environment and thus needs to be disposed of safely, but it often requires a large amount of management resources for safe disposal. The leftover WCO usually becomes part of sewerage leaching and significantly contaminates the water resources, affecting human health, animals, plants and most aquatic life [25].

Conventionally, biodiesel production is achieved by employing base catalysts such as sodium hydroxide or potassium hydroxide under homogeneous catalytic conditions. However, this method suffers from various limitations, such as separation, recycling, and regeneration of the catalyst. Moreover, the product should be neutralized with strong acids like hydrochloric acid, phosphoric acid, or sulfuric acid, followed by washing of product with plenty of water. Altogether, the process becomes cost-intensive and environmentally hazardous due to the applications of many inorganic acids and water for post-reaction work-up [26]. Therefore, heterogeneous catalysis qualifies better to solve these problems [27]. The heterogeneous catalysts operate under benign conditions that help in the catalyst’s easy separation, recycling, and regeneration.

Moreover, it doesn’t require washing and neutralization of reaction contents [28]. Therefore, the process could be scaled-up to meet the market demands. Accordingly, heterogeneous catalysts have become popular for producing biodiesel via transesterification reactions.

Typically, biodiesel production through transesterification of WCO has been reported to be performed by using diversification of heterogeneous catalysts, including mixed metal oxides. For instance, Mohadesi et al. [29] have generated biodiesel in the existence of catalyst (KOH) and small-chain alcohol (methanol). Detailed analysis of the reaction has been performed by varying parameters, including alcohol-to-oil ratio, catalyst contents, reaction duration and temperature. The results have revealed that a methanol to oil ratio of 9.4:1, 1.16 wt% of catalyst, duration of 120 s, and a temperature of 62.4 °C have resulted in the formation of high yield of transesterification product (methyl ester, 98.2%). In another study, Mansir et al. [16] utilized WCO to produce biodiesel in high yield (92.1%) under optimized reaction conditions. They have used ~3 wt% of Mn-Zr (2:3)/CaO as a catalyst, a CH₃OH to oil ratio of 15:1, a temperature of 80 °C and a time of ~3 h. Similarly, Putra and co-workers [30] have reported the preparation of CaO/SiO₂ catalyst using cost-effective raw materials obtained from waste eggshells. When the catalyst was applied for biodiesel production using WCO under appropriate conditions (1 h and 60 °C), a high yield of transesterification product was obtained (~91%). While in another study, Sadaf et al. [31] successfully transformed WCO to biodiesel in high yield (~94%) using KOH (1 wt%), CH₃OH to oil molar ratio of 1:3 at 60 °C. Furthermore, CaO/KI/γ-Al₂O₃ catalyst prepared by Asri et al. has demonstrated efficient catalytic activity during the WCO transesterification by facilitating the formation of a high yield of product (83.08%) under optimized conditions, such as ref. [32] 6% of catalyst, 1:15 WCO to CH₃OH ratio, a temperature of 65 °C and a 5 h reaction time.

Apart from these, other commonly applied heterogeneous catalysts are metal oxides [33], zeolites [34] and hydrotalcites etc. [35]. This report presents the synthesis of nickel-zirconium mixed metal oxides (X%)Ni–ZrO₂ with different ratios of Ni(X = 10, 25 and 50 wt%) and its application in the transesterification of WCO as a heterogeneous catalyst. Furthermore, nickel-zirconium-based catalysts have also been used as heterogeneous catalysts in many other organic transformations, such as the hydrogenation of acetonitrile [36], hydrogenation of carbon oxides [37], hydrogenation of tetralin [38], hydrogen production [39] and oxidation of alcohols [40].

The process of transesterification of WCO to biodiesel using nickel-zirconium-based mixed metal oxide catalysts is depicted in Scheme 1. The as-prepared catalyst was identified by using a variety of techniques, including Fourier transform infrared spectra (FT-IR), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and Brunauer Emmett Teller (BET) surface area analysis. Detailed analysis of the catalytic transesterification of WCO was performed to optimize the reaction conditions to obtain the maximum yield of the product. Furthermore, several experiments were performed under similar conditions to test the reusability of the catalysts for the same reaction.

2. Experimental Section

2.1. Catalyst Preparation

Ni–ZrO₂ with different % ratio of Ni was synthesized using the solvothermal method. For example, to synthesize Ni (10 wt%)–ZrO₂, a clear solution of nickel acetylacetonate (0.166 g) in tert. butanol (10 mL) and oleylamine (2 mL) were made in a conical flask by heating at 80 °C. In another flask, a clear solution of zirconium isopropoixde (1.66 g) in benzyl alcohol (15 mL) was taken. Both solutions were mixed and sealed in a Teflon-lined autoclave at 200 °C for 48 h. After the required time, the reaction vessel was brought down to room temperature, and ethanol (50 mL) was added to precipitate the product, which was separated using centrifugation and washed three times by dissolving the precipitates in dichloromethane (10 mL) and precipitating using ethanol (50 mL).

2.2. Transesterification Experiment

To test the catalytic activity of the as-prepared catalyst, WCO (3 g), methanol (15 times molar; molecular weight of oil was considered as 890) and catalyst (10 wt% of oil) were taken into a 100 mL Teflon cup, which was later placed in a stainless-steel autoclave. To complete the reaction, the autoclave was kept in a muffle furnace at a high temperature (180 °C) for 8 h. After this, the autoclave was taken out and allowed to cool down to room temperature. The catalyst was isolated from the reaction mixture using centrifugation (4000 rpm), while the unreacted alcohol was isolated on a rotary evaporator. Subsequently, the reaction mixture only contained trans-esterified products (fatty acid alkyl esters), unreacted oil, and glycerol as a side product. This reaction mixture added 5 mL of hexane to separate the glycerol, which ultimately precipitated out at the bottom of the vessel. The oil and the transesterified product readily went into the organic layer, and glycerol persisted as a discrete layer. The organic layer was separated using a separating funnel, and hexane was distilled using a rotary evaporator. The conversion of the products (fatty acid alkyl esters) was analyzed by ¹H NMR spectroscopy.

3. Results and Discussion

X-ray diffraction was used to confirm the catalyst’s crystal structure and phase purity. Figure 1 displays the XRD patterns of the synthesized (10 wt%)Ni–ZrO₂, (25 wt%)Ni–ZrO₂ and (50 wt%)Ni–ZrO₂ catalysts. The characteristic reflections observed in the X-ray diffractograms of the samples with different compositions of Ni were situated at 2θ = 44.44°, 51.71° and 76.41° belonged to planes (111), (200) and (220), respectively. This indicates that the prepared samples contain nickel in a face-centred cubic (fcc) form (JCPDS file no. 04-0850). [41] Whereas the reflections at 2θ = 40.00° and 46.58° could appear due to the mixed hexagonal phase of metallic Ni [42]. Besides, the prepared materials exhibited a series of reflections at 2θ = 30.32°, 50.49° and 60.23°, corresponding to (111), (220) and (311) planes for the cubic ZrO₂ structure, which is in high accordance with (JCPDS file no. 27-0997) [43].

To recognize the functional groups existing on the surface of synthesized nano-catalysts, we characterized the catalysts using FT-IR. FTIR spectra of (10 wt%)Ni–ZrO₂, (25 wt%)Ni–ZrO₂ and (50 wt%)Ni–ZrO₂ are presented in Figure 2. All three samples with different amounts of Ni showed very similar surface functional groups. The major peaks appeared at 3450, 1632, 1560 and 1440 cm⁻¹. These could be assigned to the stretching υ(–OH) and bending δ(–OH) vibration modes of the OH group due to the occurrence of benzyl alcohol which acts as both solvent and surface stabilizing ligand [44]. Meantime, the CH₂ stretching vibration illustrates the sharp absorption bands of nearly 2940 cm⁻¹ [45]. The absorption peak situated at 1040 cm⁻¹ demonstrates the existence of the stretching vibrations of υ(C−O) [46]. These absorption bands suggest the presence of benzyl alcohol on the surface as stabilizing ligands. The stretching vibration band υ(Zr–O) of zirconia displays the peaks centered approximately at 630 and 740 cm⁻¹ [47]. Besides, the FT-IR peaks in the 460–600 cm⁻¹ could be due to the stretching vibration modes of υ(Ni–O) [48].

To determine the catalysts’ thermal stability, the samples were subjected to Thermal gravimetric analysis at various temperatures. The thermal behavior of the as-synthesized catalyst with compositions (10 wt%)Ni–ZrO₂, (25 wt%)Ni–ZrO₂ and (50 wt%)Ni–ZrO₂ catalysts were studied up to 800 °C with a heating rate of 10 °C min⁻¹ under N₂ gas flow, and the results are shown in Figure 3. The TGA thermogram of (50 wt%)Ni–ZrO₂ catalyst was stable up to 340 °C with a slight mass loss of <3%, which can be attributed to the evolution of adsorbed moisture and surface stabilizing ligand on the as-prepared catalyst [47]. After this, a gradual weight loss of ~9% was observed in the temperature range of 400–600 °C. Upon further increasing the temperature to 800 °C, an overall weight loss of ~18% was observed. These analyses have revealed that, upon decreasing the weight percentage of Ni in the composition of the catalyst, i.e., (10 wt%)Ni–ZrO₂ and (25 wt%)Ni–ZrO₂, the thermal stability was considerably decreased. The total weight loss for (10 wt%)Ni–ZrO₂ and (25 wt%)Ni–ZrO₂ was approximately 28% and 40%, respectively.

The synthesized samples’ morphology was studied using scanning electron microscopy and transmission electron microscopy. Figure 4 shows the representative SEM images of the (10 wt%)Ni–ZrO₂ (Figure 4a), (25 wt%)Ni–ZrO₂ (Figure 4b), (50 wt%)Ni–ZrO₂ (Figure 4c) and transmission electron microscopy image of (10 wt%)Ni–ZrO₂ (Figure 4d). Although the SEM images show some spherical shape nanoparticles, overall, these particles are beyond the resolution limits of SEM. However, TEM shows small spherical nanoparticles with very well crystalline domains. Additionally, the elemental mapping of (10 wt%)Ni–ZrO₂ catalyst was confirmed using SEM at the rectangular area in Figure 5. The data disclosed the presence of homogeneously distributed O (turquoise), Ni (yellow) and Zr (red) atoms over the 20 μm sized cross-section area of the image. Moreover, the elemental composition analysis of the as-prepared (10 wt%)Ni–ZrO₂ catalyst is also studied using the EDX technique, as viewed in Figure 5. The existence of zirconium, nickel and oxygen has been indicated in the EDX spectrum.

The catalysts’ surface area was analyzed by BET by taking the surface areas of the pure ZrO₂ as a reference. The BET of (10 wt%)Ni–ZrO₂, (25 wt%)Ni–ZrO₂ and (50 wt%)Ni–ZrO₂ catalysts measured with N₂ adsorption are presented in Figure 6. The transesterification process is highly dependent on the surface area of the heterogeneous catalysts [49,50]. The surface area of pristine ZrO₂ was about 59.72 m² g⁻¹, whereas the surface areas of the catalysts, i.e., (10 wt%)Ni–ZrO₂, (25 wt%)Ni–ZrO₂ and (50 wt%)Ni–ZrO₂ catalysts were 117.83, 96.46, and 85.07 m² g⁻¹, respectively. The catalytic activity of the catalysts with Ni NPs exhibited higher catalytic efficiency than pure ZrO₂ NPs-based catalysts. The surface area of all the studied catalysts was reduced upon increasing the weight percentage of Ni NPs. The trend of the surface area reduction upon increasing the Ni content can be attributed to the limited obstruction of the catalytically active catalyst sites by the Ni NPs. This, in turn, affects the activity of the prepared catalysts. Interestingly, among various catalysts, the catalyst with the highest surface area, i.e., (10 wt%)Ni–ZrO_2, has exhibited the superior conversion of WCO to biodiesel. Therefore, it can be concluded that the catalytic behavior of the fabricated catalysts is powerfully affected by the specific surface area of the material.

3.1. Catalytic Investigation

(10 wt%)Ni–ZrO₂ catalyst was investigated for transesterification using WCO as a feedstock. The product formed during this transesterification reaction was analyzed by using ¹H-NMR spectroscopy. A schematic illustration of the process is described in Scheme 2. The NMR spectrum displays two proton peaks at δ = 4.10–4.30 and 5.10 ppm, which can ascribe to the –CH₂– and –CH– of the glycerol moiety in triglyceride, respectively. In this case, the mono and the -di glycerides of the glyceride moieties were not considered, as these compounds are soluble in MeOH and might have segregated during the separation process. Notably, triglyceride exhibits various chemical shifts due to the effect of couplings from –CH₂– constituents under different chemical environments (δ = 1.30, 1.60, 1.95, 2.30 and 2.63 ppm). These types of all coupling peaks are commonly found in fatty acid methyl ester (FAME) and triglycerides, in addition to the existence of other couplings which occurred due to the non-conjugated and conjugated proton resonances (δ = 5.30, 5.35 and 6.03 ppm). The ¹H-NMR spectrum of FAME exhibited a prominent peak at δ 3.65 ppm due to the presence of the methoxy group. This peak was utilized to assess the conversion percentage of biodiesel during the reaction using the equation below [51,52].

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)=100\times 2\times \raisebox{1ex}{${\mathrm{A}}_{{\mathrm{C}\mathrm{H}}_3}$}\!\left/ \!\raisebox{-1ex}{$3\times {\mathrm{A}}_{{\mathrm{\alpha }-\mathrm{C}\mathrm{H}}_2}$}\right.$$

(1)

where ${\mathrm{A}}_{{\mathrm{C}\mathrm{H}}_3}$—area of methoxy protons in the methyl esters, ${\mathrm{A}}_{{\mathrm{\alpha }-\mathrm{C}\mathrm{H}}_2}$—area of the CH₂ protons adjacent to the carbonyl group.

Several experiments were performed to examine the influence of different reaction parameters on the transformation of WCO to biodiesel, such as catalyst loading, molar ratio of MeOH to WCO, duration of the reaction, and temperature of the reaction. These parameters were systematically varied to find the optimum reaction conditions for the production of biodiesel in high yield. The products produced from the transesterification process were analyzed using ¹H-NMR spectra by calculating the WCO conversion to biodiesel, which was performed per the outcomes computed from glycerol attained. The conversion obtained from glycerol quantity will be lesser, by 1–5 mol% compared with the NMR results, since the higher glycerol viscosity prohibits the 100% regaining of it from the reaction mixture [53].

3.2. Impact of Catalyst Loading

The catalytic activity of the (X wt%)Ni–ZrO₂ catalysts (where X = 0, 10, 25 and 50) was studied while other catalytic parameters were kept unchanged, i.e., the temperature at 180 °C, MeOH-to-oil ratio15:1, 10 wt% catalysts and reaction temperature of 8 h. The conversion of transesterification of WCO by MeOH was monitored using ¹H-NMR spectroscopy. Pure ZrO₂ was also used in the conversion of WCO with MeOH to produce biodiesel; the biodiesel yield was approximately 7.43% under the above-mentioned conditions. However, the presence of 10 wt% of Ni–ZrO₂ catalyst could improve the conversion of WCO up to 90.5%. The enhanced biodiesel yield can be ascribed to the significant improvement in the specific surface area of the catalyst, as illustrated in the BET data [54]. The high surface area of the catalyst allowed better adsorption of the reactants on its surface, which ultimately enhanced the efficiency of the reaction [55]. As shown in Figure 7a, the (10 wt%)Ni–ZrO₂ catalyst exhibits the highest conversion among all three catalysts. The conversion of WCO was gradually decreased by loading more weight percentages of Ni on the ZrO₂ catalyst. The catalysts (25 wt%)Ni–ZrO₂ and (50 wt%)Ni–ZrO₂ yielded 79.2% and 75.0% conversion, respectively. This could be due to the decline in the surface area or blocked active sites upon the addition of Ni, as mentioned earlier. However, the surface area of the synthesized catalyst plays a pivotal role in the catalytic efficiency towards biodiesel production. Therefore, the (10 wt%)Ni–ZrO₂ catalyst was found to be the optimum catalyst, and hence it is employed in future investigations to enhance other catalytic variables.

3.3. Impact of Molar Ratio

Among various parameters the methanol amount concerning the oil. i.e., the molar ratio of MeOH to oil is regarded as one of the most crucial factors in the transesterification process, which can potentially inhibit the formation of biodiesel. The common stoichiometric ratio of the transesterification process requires 3 moles of MeOH to generate 3 moles of methyl ester and 1 mole of glycerol. However, as the transesterification process of triglyceride is reversible, an excess volume of MeOH can potentially drive the equilibrium towards the right. Therefore, in these reactions, the methanol to oil molar ratio is higher than that of the stoichiometric ratio of 3:1is maintained. The process was performed by changing methanol to oil molar ratios from 6:1 to 18:1 under unchanged operational conditions (180 °C, 8 h and 10 wt% catalysts to oil ratio), as displayed in Figure 7b. Upon raising the MeOH to oil molar ratio from 6:1 to 15:1, the biodiesel yield was enhanced considerably from 28.8% to 90.5%.

Nevertheless, a further increase in alcohol to oil molar ratio from 15:1 to 18:1 decreased the biodiesel formation. This is probably attributed to the dilution catalyst effect, which led to the insolubility of the MeOH [56]. Therefore, in this study, the prime molar ratio of methanol to WCO was established to be 15:1.

3.4. Impact of Reaction Time

Figure 8 a represents the conversion of WCO at 180 °C for numerous reaction times. Reaction time plays a fundamental role in biodiesel yield, especially for the catalytic transesterification processes. Typically, the rate of reaction is highly dependent on the existence of the catalyst; furthermore, the participation of the mass transfer rate happened during the reaction utilizing a heterogeneous catalyst [57]. As the reaction duration increases, the biodiesel yield increases significantly. A lower WCO conversion of 13.5% was obtained after 2 h. The optimal WCO conversion of 90.5% was achieved when the reaction time was increased to 8 h. Upon further prolonging the reaction period to 10 h, there was an insignificant decline in the yield of biodiesel. This is typically feasible when the equilibrium is obtained at the optimal biodiesel yield, and the reverse reaction considerably occurs. Besides, some side reactions occur after the optimal reaction time [58,59]. Based on this data, the ideal reaction time for the transesterification procedure in the present study is 8 h.

3.5. Impact of Reaction Temperature

Apart from this, the temperature of the reaction is also a significant factor which can influence the transesterification reaction. Therefore, evaluation of the catalytic performance at different temperatures was carried out under optimized conditions, such as a reaction time of 8 h, 15:1 as MeOH-to-oil ratio and the number of catalysts as 10 wt% of WCO. The increment in the reaction temperature favored the formation of methyl ester, as displayed in Figure 8b. The influence of temperature was also examined on the esterification reaction by varying the temperature from 120 °C to 200 °C. The plotted results showed that the conversion of WCO to methyl ester increased from 6.3% to 90.5% when the upsurge reaction temperature increased from 120 to 180 °C, respectively.

Further reaction temperature upsurge from 180 to 200 °C led to a slight decrease in methyl ester yield. This behavior is per several other published studies which proposed that the saponification process of triglyceride may take place at high temperatures, which subsequently form an enormous number of bubbles that inhibit the reaction on the three phases (alcohol/oil/catalyst) interphase; therefore, decreased the biodiesel yield [60]. Thus, the optimal biodiesel yield can be attained at a temperature of the reaction 180 °C.

3.6. Impact of Catalyst to Oil Ratio

Usually, catalyst concentration remarkably affects the transformation of WCO to biodiesel. An insufficient quantity of the catalyst led to an incomplete transesterification reaction [61]. To assess the influence of catalyst dosage, the quantity of the catalyst was varied from 6 to 14 wt% while keeping the other operational conditions persistent. Upon increasing the catalyst amount, the yield of the biodiesel also upsurges. Since with increasing the quantity of the catalyst, the amount of O²⁻ anion sites also increases, which facilitates the adsorption of more hydrogen ions (H⁺) from CH₃OH, leading to the formation of a higher number of active sites [62]. This will ultimately lead to an increased number of contacts between the reactants and the active sites, enhancing the resulting yield of biodiesel. Overall, the increase in catalyst percentage from 6 to 10 wt% resulted in the conversion from 44.6% to 90.5% (Figure 9a). The results in Figure 9a demonstrate that the conversion of WCO to biodiesel is negatively impacted upon further increasing the amount of catalysts to more than 10%.

In some cases, a high amount of catalyst in the identical ratio of oil and methanol led to a lower mixing producing higher viscosity of the mixture of reactants and catalyst. The resultant increase in mass transfer resistance could reduce the biodiesel yield [63]. Therefore, the optimal catalyst dosage utilized for biodiesel production was selected to be 10 wt%.

3.7. Catalyst Recovery Evaluation

The recyclability tests were carried out by employing the (10 wt%)Ni–ZrO₂ catalyst for several reactions (up to five cycles) under the optimized conditions (Figure 9b). After the accomplishment of every reaction, the residual catalyst was isolated by centrifugation and washed away with hexane. Furthermore, to clean the catalysts from other impurities, the catalysts were also washed with methanol [64]. The results showed that the WCO conversion was reduced by almost 6% after 5 reactions. The reduced catalytic activity can presumably be credited to the inevitable catalyst weight loss during purification. However, the fabricated catalyst was active and recyclable even after five cycles of the transesterification process.

4. Conclusions

In this paper, we have synthesized (X%)Ni–ZrO₂ (X = 10, 25 and 50 wt%) catalysts using the green and scalable solvothermal procedure. The prepared materials were employed for the transesterification reaction of WCO with methanol to obtain biodiesel. The structures and compositions of the as-synthesized materials were characterized by different microscopic and spectroscopic techniques. The influence of several reaction factors, including catalyst loading, MeOH to oil molar ratio, reaction time, the catalyst to oil ratio, and reaction temperature, are thoroughly optimized. The optimal conditions for biodiesel production are a MeOH to oil molar ratio of 15:1, a catalyst to oil ratio of 10 wt%, a reaction temperature of 180 °C and a reaction time of 8 h. under these conditions. The transesterification resulted in a high FAME yield of 90.5%. The (10 wt%)Ni–ZrO₂ catalyst possesses the highest surface area compared to the un-doped catalyst (i.e., pure ZrO₂), and the higher Ni-containing catalyst exhibited the maximum conversion of WCO to biodiesel. In addition, the (10 wt%)Ni–ZrO₂ catalyst was stable and could be successfully recycled five subsequent times with a marginal decline in its effectiveness. Additionally, the easy separation and recyclability of the catalyst after the reaction added another advantage to the prepared catalyst.