An environmentally friendly method for extraction of cobalt and molybdenum from spent catalysts using deep eutectic solvents (DESs)

There has been a substantially increasing demand for energy critical elements (ECEs) in recent years as energy-related technology has advanced rapidly. Spent catalysts are known as potential sources of ECCs such as Ni, Co, Mo, W, V, and rare earth elements. This study developed a novel environmentally friendly process for recovering cobalt and molybdenum from spent hydroprocessing catalysts using deep eutectic solvents (DESs). DESs based on p-toluenesulfonic acid achieved high metal extraction at 100 °C and a pulp density of 20 g/L for 48 h which 93% of cobalt and 87% of molybdenum were dissolved. FT-IR and H-NMR analyses were conducted to determine whether hydrogen bonds form between p-toluenesulfonic acid-based DES components. Leaching kinetic models were also developed for DES systems. The experimental results were well-matched with the shrinking core models. The leaching controlling step of DES-1 was determined to be the diffusion through the product layer based on kinetic studies, with an activation energy of 22.56 kJ/mol for Co and 29.34 kJ/mol for Mo in DES-1. Similarly, the mixed control reaction with an activation energy of 38.09 kJ/mol for Co and 31.48 kJ/mol for Mo in DES-2 was found to control the leaching kinetic mechanism of the DES-2 sample.


Introduction
Global population expansion, more comfortable lifestyles, technological advancements, and political decisions have changed the patterns of supply and demand for raw materials (Rezaee et al. 2022(Rezaee et al. , 2023. From the beginning of the twentieth century, the demand for energy critical elements (ECEs) has increased due to development in modern energy-related technologies (Nuss and Blengini 2018). Spent catalysts are known as a potential source of ECCs such as Ni, Co, Mo, W, V, and rare earth elements (Akcil et al. 2015). Cobalt and molybdenum are the active metal in hydroprocessing catalysts, which is encouraged by other metals on alumina support (Kim et al. 2009). The catalyst's life cycle will expire when its activity decreases below acceptable levels. After extended usage, activity reduction is typically attributed to catalyst sintering, poisoning, fouling, or mechanical damage (Lim et al. 2021). Once deactivated, catalysts are either regenerated or treated to recover usable components. Otherwise, the deactivated catalyst is discarded as waste, producing such waste in large amounts (Stanislaus et al. 1993). Spent catalysts are hazardous pollutants that create significant environmental problems, and the Environmental Protection Agency (EPA) of the United States categorized spent petroleum catalysts as hazardous waste regularly disposed of in landfills and can contaminate the environment (Le and Lee 2021).
Many studies have been conducted on metal recovery from spent catalysts by hydrometallurgy, pyrometallurgy, or a combination of the two methods (Le and Lee 2020;Meshram and Abhilash 2020). Despite the fact that traditional techniques have been widely used for recycling spent catalysts, there are still a number of significant challenges Responsible Editor: George Z. Kyzas 1 3 associated with them. The pyrometallurgical route, which involves high-temperature processing, can consume large amounts of energy (Medvedev and Malochkina 2007;Chauhan et al. 2013). However, it should be noted that the use of inorganic acidic and alkaline solutions in the hydrometallurgy route may result in low selectivity and the potential for leachates to become contaminated. (Binnemans and Jones 2017). Bioleaching, another technique used for recycling spent catalysts, has also been investigated (Beolchini et al. 2010;Amiri et al. 2011;Tayar et al. 2020), but the rate of bioleaching is slower than other methods, and it takes longer for the metal to dissolve into the leach solution (Abdollahi et al. 2021;Shahbazi et al. 2022). Furthermore, due to the participation of many leaching agents and separation phases, metal recovery in hydrometallurgical operations frequently creates a substantial volume of wastewater (Tran et al. 2021), including brine, which has a detrimental effect on environment (Panagopoulos 2022;Panagopoulos and Giannika 2022). This wastewater cannot be used directly for industrial applications. Consequently in the use of hydrometallurgical methods for catalyst recycling, alternative methods need to be developed to avoid such environmental problems.
Finding alternative techniques that would meet both efficient metal recovery and decrease environmental consequences is essential. Traditional techniques can recycle hazardous wastes, but they come with concerns for the environment. Therefore, green chemistry has received a lot of attention in an effort to lessen the negative effects that conventional solvents have on the environment. Deep eutectic solvents (DESs), a novel class of green solvents, were first introduced by Abbott et al. at the start of the twenty-first century (Abbott et al. 2003(Abbott et al. , 2004. DESs are a type of solvent composed of at least two components, namely, a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). These components work together to create a substance with a freezing point much lower than either component alone. One of the main advantages of DESs is that they are considered to be environmentally friendly or "green" solvents. DESs have a dipolar nature, meaning they have both positive and negative charges, which allows them to dissolve a wide range of polar and nonpolar compounds. Additionally, DESs are nonflammable and possess good thermal stability, making them suitable for use in a variety of applications (Abbott et al. 2003;Santana-Mayor et al. 2021). DESs have emerged as a promising alternative to traditional solvents due to their unique properties and potential to contribute to sustainable chemistry practices.
Several studies have shown promising results from the application of DESs in metal recovery from secondary resources (Abbott et al. 2009;Bakkar 2014;Bakkar and Neubert 2019). Lithium and cobalt were recovered from spent lithium-ion batteries via DES (Tran et al. 2019). Liu et al. obtained significant leaching rates by using DES to extract REEs from end-of-life NdFeB magnets . A wide range of DESs can be created using different combinations of HBA and HBD components, allowing for tailoring of DES properties to suit specific applications. For example, Jiang et al. developed a new class of DESs based on polyethylene glycol (PEG) that exhibited favorable physical properties. These DESs were found to be effective in the recovery of transition metals from metal oxides (Jiang et al. 2019).
While there has been extensive research on the use of deep eutectic solvents (DESs) for the recovery of metals from waste or secondary sources, there has been limited exploration of their effectiveness in recovering cobalt and molybdenum from spent catalysts. However, the present study demonstrated that DESs can efficiently extract cobalt and molybdenum from spent catalysts under moderate conditions. The results of this study suggest that DESs could provide a new and effective method for the recovery of valuable metals from spent catalysts, which could contribute to more sustainable practices in the chemical industry. This study investigated the effectiveness of DESs in recovering cobalt and molybdenum from spent catalysts and also examined the impact of various parameters on the leaching process involved. Furthermore, the study included a kinetic analysis to evaluate the rate of leaching in DES-based systems. This study demonstrates a novel and environmentally friendly approach to metal recovery, without the need for roasting or the use of oxidative agents. This green method involves the use of DESs as leaching agents, which could potentially reduce the environmental impact and cost associated with traditional metal recovery processes.
The spent catalysts utilized in the experiments were supplied by the Iran Petroleum Industry Research Institute and underwent acetone washing to eliminate any organic impurities, followed by overnight drying at 80 C. The catalysts were crushed with a laboratory jaw crusher and ground with a laboratory rod mill until they reached a d80 size of 75 μm. X-ray fluorescence spectroscopy (XRF) using a Philips PW 2404 apparatus was employed to chemically analyze the spent catalysts, with the findings presented in Table 1. The XRF outcomes indicated that the cobalt and molybdenum concentrations in the spent catalyst samples were 2.65% and 6.04%, respectively.

Synthesis of deep eutectic solvents
To synthesize deep eutectic solvents (DESs), a 250 mL conical flask was utilized. The flask contained the hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) compounds listed in Table 2. The synthesis process was conducted by placing the flask on a water bath with temperature control, and continuously stirring the mixture with a magnetic stirrer until a homogeneous, colorless liquid was produced. This method of synthesis is consistent with that described by (Abbott et al. 2006).
Upon completion of the synthesis, the resulting liquid was cooled to room temperature. This step is important as it allows the DES to stabilize and ensures that the properties of the liquid remain consistent over time. It is worth noting that the colorless appearance of the liquid is an indicator of the success of the synthesis process, as it suggests that the components of the DES are well mixed and have formed a uniform solution.
Leaching procedure 0.1 g of sample of spent catalyst was placed in a glass vial containing 5 mL of each deep eutectic solvent (DES), while the temperature was continually controlled at 100 °C for 48 h using an oil bath and thermometer. The solution was subsequently filtered using PVDF syringe filters (with a pore size of 0.45 μm), and inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the metal content and calculate the recovery, unless otherwise specified. The leaching temperature was kept at 100 °C, and the solid to liquid ratio was maintained at 20 g/L unless otherwise specified. Figure 1 shows the process schematic for extracting metals from spent catalyst by DES leaching.
To determine the impact of leaching parameters on Cobalt and molybdenum recovery, the solid to liquid ratio was varied from 20 g/L, 50 g/L, 80 g/L, and 100 g/L. The effect of time was studied by sampling the leachate at various leaching time of 0.5 h, 2 h, 4 h, 8 h, 24 h, and 48 h. To study the effect of temperature, the temperature was varied at 50, 75, and 100 °C.
The pregnant leach solution was diluted with 0.1 M HCl solution before being analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The leaching efficiency was computed as follows: where c is the final concentration of the metal (mg/L) in the solution, V (DES) is the volume of the initial leaching solution (liters), and m is the mass of the metal in the solid feed (mg).

Analytical methods
The FTIR spectrometry technique using KBr windows was used to obtain FTIR spectra of both DESs and leachate samples using attenuated total reflection (ATR-IR) equipment. The spectra were measured in the range of 600 to 1500 cm −1 . The components of DESs were analyzed using Solution Proton NMR spectroscopy with deuterium oxide as the external standard, using a Varian-INOVA 500 MHz instrument in the USA. The surface morphology of spent catalysts and leach residues were examined using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscope (EDAX). The morphology and surface composition of spent catalysts were studied using the same SEM technique. The X-ray diffraction (XRD) spectra of the spent catalysts were analyzed using Bruker D8-Advance with a CuKα beam.

Characterization of spent catalyst
The sample of catalyst used in the study was a pyrolysis gasoline hydrogenation spent catalyst. To investigate the structure of the spent catalyst, X-ray diffraction (XRD) was employed and the results are depicted in Fig. 2. The XRD pattern of the spent catalyst revealed that it was an amorphous sample, with the major phases identified as α-Al 2 O 3 , cobalt oxide, and molybdenum oxide. 1 3

Fourier transform infrared (FTIR)
Deep eutectic solvents (DESs) are known for their unique properties, which are largely attributed to the role of hydrogen bonding interactions. These interactions are typically observed between the hydrogen atoms in the donor compound and the oxygen, nitrogen, or sulfur atoms in the acceptor compound. The strength and nature of the hydrogen bonding interactions in DESs can significantly influence their physicochemical properties, including melting point, viscosity, and polarity (Dai et al. 2013).

The formation of hydrogen bonds between the hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs)
is crucial to the creation of DESs (Jiang et al. 2019). FTIR spectra are presented in Fig. 3a, b. In DES-1, the FTIR analysis revealed absorption peaks of significant functional groups, such as oxyalkylene -C-O-C-(1100 cm −1 ), hydroxyl groups -OH (3384 cm −1 ), and alkyl C-H (2873 cm −1 ) stretching vibrations. The observed increased bandwidth and slight shifts in the stretching vibration peaks of (-OH), (-C-O-C), and (-C = O) indicated the presence of interactions between the hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) components in DES-1.
Likewise, in DES-2, a broad band at 3322 cm −1 was observed, corresponding to the stretching vibration of the -OH hydroxyl groups. Additionally, an indicative band at 953 cm −1 that originated from ChCl was recognized for the C-N + stretching. These findings provided evidence of the formation of hydrogen bonds based on interactions between the HBA and HBD in both DESs.
The results of the FTIR analysis suggest that the hydrogen bonding interactions in DESs are a crucial factor in their stability and performance. The presence of hydrogen bonding between the HBA and HBD components enhances the interactions and, in turn, their solubility power and reactivity. Additionally, the slight shifts and increased bandwidths observed in the FTIR spectra demonstrate the ability of the DESs to modify the electronic structure of their constituent components.

Proton NMR spectroscopy
Hydrogen bonding is a type of intermolecular interaction that involves the attraction between a hydrogen atom and an electronegative atom such as oxygen or nitrogen. In H-NMR spectroscopy, hydrogen atoms are subjected to a magnetic field and the energy required to change the spin state of the hydrogen nuclei is measured (Eyman and Drago 1966).
The formation of hydrogen bonds between DES components is the main reason for the temperature-limited phase transition of DESs (Tang et al. 2022). Hydrogen bonding can affect the chemical environment of hydrogen atoms and therefore their chemical shifts in H-NMR spectra. Hydrogen atoms involved in hydrogen bonding generally shifted to higher chemical shifts. This is due to the electron-withdrawing nature of the electronegative atom involved in the hydrogen bonding interaction (Pellecchia 2005).
The strength of the hydrogen bond can also affect the degree of deshielding. Stronger hydrogen bonds generally result in larger chemical shifts for the hydrogen atoms involved in the bond. The H-NMR spectra of DES-1, DES-2, and their components are shown in Fig. 4a, b, demonstrating that the chemical shifts of both DESs are pushed downfield, giving strong evidence of hydrogen bond formation in both DESs. The increased electronegativity of the oxygen atoms from O-H and (COOH) in HBA may draw and reduce the electron cloud densities around the hydrogen atoms of OH in PEG, affecting the chemical shifts values in DESs and the creation of hydrogen bond (Chen et al. 2020).

Effect of leaching parameters on cobalt/ molybdenum recovery
The effect of leaching parameters including solid-liquid ratio, leaching duration, and temperature on the leaching efficiency of cobalt and molybdenum was investigated. By varying these parameters, it was possible to assess their individual and combined effects on the leaching process and to identify optimal conditions for achieving high leaching efficiencies.

The effect of solid-liquid ratio on cobalt/molybdenum recovery
The effects of the solid-liquid ratio on cobalt/molybdenum dissolution are depicted in Fig. 5a. The response time and temperature were held constant at 48 h and 100 °C, respectively. The results show that increasing the solid-liquid ratio led to a decrease in cobalt and molybdenum extraction efficiency, with all samples demonstrating a similar trend. This may be attributed to a decrease in the contact area between the metal-containing solids and the leaching solution, resulting in reduced dissolution rates. Moreover, a higher solid-liquid ratio led to a higher metal content in the DESs, which in turn increased the viscosity of the mixture, making it more difficult to process. However, at solid-liquid ratios ranging from 50 to 100 g/L, no significant decline in cobalt and molybdenum dissolution was observed. The best solid-liquid ratio was found to be 20 g/L, which resulted in the most cobalt/molybdenum dissolution for all samples.

The effect of time on cobalt/molybdenum recovery
The effects of leaching time on cobalt/molybdenum dissolution are shown in Fig. 5b. In the case of DES-1, the dissolution of cobalt and molybdenum increased with time during the initial phase of the reaction. After 8 h, the dissolution of cobalt and molybdenum increased at a linear rate during the second phase, followed by a slight increase during the third phase until the end of the reaction. On the other hand, DES-2 showed a slight increase in cobalt and molybdenum dissolution with time when compared to DES-1. Notably, although extremely high cobalt and molybdenum recovery rates were achieved with shorter reaction durations in DES-1, a reaction duration of 48 h is deemed appropriate for both DESs. This is because prolonged leaching times may result in diminishing returns in terms of increased metal dissolution and may also have practical implications such as increased processing time and costs.
These findings suggest that the optimal leaching time for cobalt and molybdenum dissolution can be achieved within a reasonable time frame and that the leaching process can be optimized to recover metals efficiently from spent catalysts. Although extremely high cobalt/molybdenum recovery rates were obtained with a shorter reaction duration in DES-1, 48-h reaction durations for both DES may be appropriate.

The effect of temperature on cobalt/molybdenum recovery
The effect of varying leaching temperature on cobalt/ molybdenum extraction was investigated in this study. The leaching temperature was altered from 50 to 100 °C, while maintaining a constant S/L ratio of 20 g/L and a leaching period of 48 h. The results of the study, presented in Fig. 5c, demonstrated that the dissolution of cobalt/molybdenum increased with an increase in leaching temperature. This phenomenon could be attributed to two factors. Firstly, the viscosity of DESs decreases with increasing temperature, resulting in enhanced ion mobility and mass transfer in the leaching system (Tang et al. 2022). Consequently, the collision probability between the solid fraction and DES components increased, leading to an improvement in the dissolving rate of cobalt/molybdenum. Moreover, the enhanced dissolution rates of cobalt/molybdenum due to an increase in leaching temperature resulted in faster attainment of dissolution equilibrium. Based on the findings of the study, a leaching temperature of 100 °C was identified as the most effective temperature for achieving optimal cobalt/molybdenum extraction. These findings provide valuable insights into the role of leaching temperature in the extraction process, which could be beneficial for further research in this area. Figure 6 shows SEM micrographs of the surface morphology of the sample both before and after leaching, with the EDAX results for the residues presented in Fig. 6b, c. Based on the absence of a clear peak for elemental cobalt and molybdenum in the leach residues, it can be inferred that these elements have been significantly dissolved in DES-1 and DES-2. The leach residues ICP results shown in Table S6 reveal the presence of aluminum (Al) and silicon (Si) in the leach residues. This indicates that during the DES leaching procedure, an inert layer composed of alumina covered the surface of the catalyst, which limited solvent transport to the catalyst surface and controlled the dissolution rate. Fig. 6 The SEM micrographs and EDAX analysis of samples a catalyst, leach residues b DES-1, and c DES-2 (48 h leaching time, temperature of 100 °C, particle size of 75 µm, and S/L ratio of 20 g/L)

Kinetic modeling of cobalt and molybdenum leaching
The main of kinetic modeling was to investigate the overall reaction kinetics of DES and determine its highly efficient performance. The kinetic leaching process was investigated for spent catalyst in DES under varying conditions, including temperature (ranging from 50 to 100 °C) and reaction time (ranging from 0.5 to 48 h). To accomplish this, the study utilized kinetic model equations which are provided in Table S1 (Levenspiel 1998;Fogler 2016). To calculate the DES leaching rate accurately, it was essential to consider all potential kinetic mechanisms, as it is impossible to predict the dominant mechanism in advance. By comparing the experimental results with the equation of each mechanism, the rate-controlling step was determined and the mechanism that best fits the data was selected.
The results presented in Tables S2-S5 indicate that spherical geometry models are a more accurate approximation for kinetic processes compared to other models that were studied. The models that were examined included diffusion control (Eq. 1), chemical reaction control (Eq. 2), and mixed control models (surface reaction control and diffusion via product layer) (Eq. 3), which are commonly used to manage the rate of leaching reactions by reducing core models.
The findings suggest that using spherical geometry models may be more effective for controlling the rate of leaching reactions compared to other models. This information can be useful in optimizing the conditions for leaching reactions (Meshram et al. 2015).
The leaching efficiency of cobalt/molybdenum, represented by X as a percentage, is dependent on the apparent rate constant (k) measured in hours and the leaching time (t) also measured in hours. The rate-controlling step is determined to be the mechanism with the highest correlation coefficients (R 2 ), and its model demonstrates the closest matches with the experimental data. In other words, the mechanism that has the best correlation with the experimental data is believed to be the step that controls the rate of the leaching process. Figs. S1 and S2 depict graphs that are well-fitted for DES-1. Based on the correlation coefficients presented in Table S2 for DES-1 cobalt, the experimental data was well-matched with diffusion through the product layer. A perfect linear relationship between 1 − 3(1 − X) 2 3 + 2(1 − X) and t was observed for DES-1, indicating that the leaching kinetic mechanism for DES-1 was diffusion control through the product layer. In the case of DES-2, the correlation coefficients presented in Table S3 and Fig. S2-S3 suggest that the mixed control model was well-matched during the reaction for approximately 12 h. The dissolution of cobalt/ molybdenum spent catalyst in DES typically involves several stages of a solid-liquid heterogeneous reaction. In the first stage, the reactant moves from the bulk of the liquid phase to the interface between the liquid and solid phases. This can be hindered by the presence of inert or insoluble substrates such as alumina, which can form a product layer around the reacting core. In order to access the unreacted core, the reactants must diffuse through this product layer. The porosity and thickness of the inert layer, as well as the diffusion coefficients of the ions created during the reaction, are key factors that influence the rate of diffusion through the product layer.
Once the reactant has diffused through the product layer, it moves to the major bodies of cobalt and molybdenum. At this stage, chemical interactions occur between the reactants and the elements on the surface of the reacting core. Finally, at the interface of the inert material, the products diffuse into the bulk of the liquid phase. The reaction can then proceed topochemically, meaning that the inner core of the sample progressively transforms into the unreacted particle (in this case, alumina) (Oza and Patel 2011).
The apparent activation energy (Ea) for the cobalt/ molybdenum leaching reaction in different DESs was determined using the Arrhenius equation. The apparent reaction rate constants (k) obtained from the slopes of the kinetics models at various temperatures were used in this calculation.
where k is the apparent reaction rate constant the duration of leaching (t) measured in hours (h), the apparent activation energy (Ea) expressed in kilojoules per mole (kJ/mol), the universal gas constant (R) equal to 8.314 J per Kelvin per mole (J/K·mol), the temperature (T) measured in Kelvin (K), and the preexponential factor (A).
The concept of activation energy is crucial in understanding the kinetics and thermodynamics of chemical reactions. Essentially, activation energy refers to the minimum amount of energy required to initiate a chemical reaction. The magnitude of the activation energy can provide valuable insights into the influence of temperature on the rate of the reaction, with higher activation energies indicating a greater temperature dependence (Levenspiel 1998;Fogler 2016).
Activation energies can also be used to evaluate the controlling step of the reaction. In many cases, the overall reaction rate is influenced by mass transport, which can take various forms. For instance, low-activation-energy mass transport mechanisms such as diffusion can play a significant role in determining the overall reaction rate. On the other (4) ln k = − Ea RT + ln A hand, surface chemical reactions typically have substantially higher activation energies than those of diffusion control. Activation energy is a critical parameter that can provide valuable insights into the kinetics and thermodynamics of chemical reactions, and can help to identify the controlling step of the reaction. As a result, the temperature has less effect on diffusion-controlled processes than it does on chemical-controlled processes. The activation energy is often used to estimate mass transport and response control in processes (Free 2022). Figure 7 presents a plot of ln k against 1/T for all samples. The resulting graph shows a straight line with an intercept of ln k and a slope of Ea/R. According to (Anand et al. 1988), a diffusion-controlled process typically has an activation energy ranging between 3 and 6 kcal/mol (12.55-25.08 kJ/ mol). In contrast, a chemically regulated process usually has an activation energy greater than 10 kcal/mol (41.84 kJ/mol) (Habashi 2017). A mixed control process has an activation energy in the range of 20.92-33.44 kJ/mol or 5-8 kcal/mol (Habashi 1999). After analyzing the data presented in Fig. 7a, b, it was determined that the activation energies for Co and Mo in DES-1 were 22.56 and 29.34 kJ/mol, respectively. This finding clearly indicates that the diffusion through the product layer was the controlling step in the leaching process for (DES-1). Moving on to Fig. 7c, d, the calculated activation energies were found to be 38.09 and 31.48 kJ/mol for Co and Mo in DES-2, respectively. These results suggest that the mixed control reaction could play a role in regulating the kinetic mechanism of the spent catalyst in DES-2.

Exploring recycling methods of spent cobalt and molybdenum catalysts
Spent catalysts are considered hazardous industrial waste and require proper disposal (Le and Lee 2021). The process of recovering valuable elements from spent catalysts has become an essential task for not only reducing the cost of catalysts but also preventing environmental pollution caused by the accumulation of catalyst waste. As the demand for catalysts continues to rise, it is crucial to develop sustainable practices that minimize the depletion of natural resources and mitigate the impact on the environment. Recycling spent catalysts is an effective way to achieve this objective. By extracting the valuable elements from these catalysts, it becomes possible to reuse them in the production of new catalysts, thereby reducing the reliance on raw materials. Moreover, the recovery of valuable elements from spent catalysts is a cost-effective approach that enhances the profitability of the catalyst industry. This is because recycling eliminates the need to purchase new materials, which can be expensive, and also reduces the expenses associated with the disposal of waste catalysts. However, the recovery of metals from these catalysts is also an important economic consideration. Numerous studies have been conducted on metal recovery from spent cobalt and molybdenum catalysts through hydrometallurgical techniques such as alkaline, acid, and biological leaching, as presented in Table 3. Although recycling spent cobalt and molybdenum catalysts through traditional methods has been prevalent, there are still significant challenges that need to  be addressed. The pyrometallurgical route, which involves high-temperature processing, consumes substantial amounts of energy, as highlighted by previous studies (Medvedev and Malochkina 2007;Chauhan et al. 2013). On the other hand, the hydrometallurgy route, which uses inorganic acidic and alkaline solutions, is associated with low selectivity and contamination of leachates (Binnemans and Jones 2017). Additionally, hydrometallurgical operations generate a significant amount of wastewater due to the participation of multiple leaching agents and separation phases. The recycling of spent catalysts is a critical waste management issue that demands an environmentally friendly and economically feasible approach. The process employed must be designed with the following characteristics in mind: 1. Environmental friendly: the recycling method should be designed to minimize environmental impact and ensure that it does not pose any harm to the environment or public health. 2. Cost-effectiveness: the recycling process should be economically viable, which means that it should be cost-effective and not lead to any significant financial burden on the stakeholders involved. 3. Lack of secondary pollution: the recycling process should not generate any secondary pollution or by-products that could harm the environment or public health. 4. Low energy consumption: the method should have low energy requirements to minimize its carbon footprint and reduce operating costs. 5. High kinetics: the recycling process should have a high reaction rate, ensuring that it is efficient and can be completed in a reasonable amount of time. 6. High efficiency: the recycling process should be highly efficient, ensuring that it recovers as much of the valuable metals and minerals as possible, with minimum loss or waste.
By incorporating these characteristics, the recycling of spent catalysts can be done in an environmentally sustainable and economically viable manner, making it a crucial component of responsible waste management. The emergence of alternative green DES can present a promising solution to these challenges. DES leaching methods are less energy-intensive and avoid environmental problems compared to traditional methods. Moreover, DESs are often less expensive, less toxic, and biocompatible, making them an attractive alternative for the recycling of spent catalysts (Ebrahimi et al. 2023).
Despite the development of a processing method in this study, several factors need to be considered before scaling up to an industrial level. DESs have the potential to recover various base metals from an industrial perspective, but innovative purification procedures are necessary to ensure successful manufacturing.
Therefore, while traditional methods have been the go-to approach for recycling spent catalysts, the emergence of alternative green DESs presents a promising solution. The use of DESs for leaching spent catalysts presents numerous advantages such as lower energy consumption, reduced environmental impact, and improved selectivity. However, further research is necessary to optimize the effectiveness of this approach and scale it up for industrial application.

Conclusion
This study offers a promising a sustainable approach for recycling spent cobalt/molybdenum catalysts, using deep eutectic solvents (DESs). The effectiveness of two DESs (DES-1: PEG-400: PTSA and DES-2: ChCl:PTSA) as leaching agents for recovering cobalt and molybdenum was investigated. The results show that both DESs have strong leaching capabilities. After further experimentation, the optimal leaching conditions were determined to be a pulp density of 20 g/L, a temperature of 100 °C, and a leaching time of 48 h for all samples. Kinetic studies revealed that for DES-1, the diffusion through the product layer controlled the leaching kinetics, while for DES-2, a mixed control reaction mechanism was responsible. The activation energy for cobalt and molybdenum in DES-1 was found to be 22.56 and 29.34 kJ/mol, respectively, and for DES-2, it was 38.09 and 31.48 kJ/mol, respectively. Overall, this study demonstrates the potential of using DESs as a more environmentally friendly method for recycling spent catalysts. Further research is necessary to optimize the effectiveness of DES leaching, design more selective DESs, and scale up the process to an industrial level.