Enhancing the conversion of waste motor oil into diesel-like fuels using mineral-impregnated biochar catalysts

Abstract This study shows how the presence of metal-impregnated biochar influences the kinetics of the catalytic conversion of waste motor oil (WMO) into a diesel-like fuel (DLF). First, the initial metal concentration in the wet impregnation process was optimized, showing that high metal concentrations tend to clog the biochar pores, reducing the reaction’s conversion and speed. Zinc and calcium showed higher kinetic constants at the optimized conditions than nickel, while the yield- and selectivity-to-liquid products were unaffected. Then, a reduction method was developed for impregnating metal particles of calcium oxide to enhance the metal dispersion at the biochar surface. This material showed the best performance in the reaction due to the presence of small particles instead of metal agglomerates. An increase of about 280% in the kinetic constant at 420 °C and a much lower activation energy (246 kJ mol−1) compared to the thermal cracking (308 kJ mol−1) was obtained. Finally, the DLF from the best catalytic system was analyzed. Interestingly, the fuel showed similar rheological properties to commercial diesel with a similar hydrocarbon distribution but with the presence of smaller hydrocarbon chains.


Introduction
Biochar is a carbon-rich material obtained by the thermal decomposition of biomass in a limited oxygen environment, the so-called pyrolysis process.Biochar has found various applications, such as soil amendment, adsorber of contaminants, and energy production (Hazman et al. 2023;Xie et al. 2015).Recently, it has shown potential as a low-cost and sustainable catalyst support material due to its unique physicochemical properties.Thus, using biochar in catalysis is an active research area, focusing on developing novel activation and impregnation methods to enhance biochar-based catalytic activity and stability (Lee et al. 2017;Xiong et al. 2017;Zhou et al. 2021).
One method for activating biochar is through physical or chemical methods, such as heat treatment or acid/alkali treatment, which can modify the surface chemistry and increase the surface area of the material.Chemical impregnation methods, such as incipient wetness impregnation, have also been used to deposit metal or metal oxide nanoparticles onto the biochar surface to enhance its catalytic activity (Sakhiya et al. 2020).Examples of biochar-based catalysts include using iron-impregnated biochar for the catalytic oxidation of pollutants in water (Rubeena et al. 2018) and the use as basic catalysts for biodiesel production from oil (Chi et al. 2021;Dehkhoda et al. 2010), among many others.
Recycling residues is essential for reducing waste generated and preserving natural resources.It can help to conserve energy and raw materials by reusing resources that would otherwise be discarded.One example of a recyclable waste material is waste motor oil (WMO).Improper disposal of WMO can have negative environmental impacts, including contamination of water and soil, and can pose health risks to humans and wildlife.WMO can be recycled and re-refined into new lubricants through pyrolysis, co-pyrolysis, and cracking (Demirbas 2008;Maceiras et al. 2017;Mishra et al. 2021;Sarkar et al. 2023;Tekin et al. 2019;Zerdane et al. 2023).Cracking reactions, in particular, are a point of interest, as they convert WMO into secondary diesel-like fuels (DLFs), which have similar properties as commercial diesel.Metal-supported amorphous silica, zeolites, and clays have been widely used as heterogeneous catalysts for enhancing reaction rates and yields to lighter products.It is known that metal (e.g., Ni and Zn) incorporation might implement defects and oxygen vacancies, improving the materials' activity (Tamunaidu and Bhatia 2007).
Previous contributions have demonstrated the feasibility of using aluminosilicates (Al/Si) as active heterogeneous catalysts for WMO transformation into DLFs (Vargas et al. 2016).Al/Si reduced the activation energies and increased the yield-to-liquid products.Then, the investigation was extended to metal-doped aluminum silicates.Active metals in cracking reactions, including Zn, Ni, Cu, etc., were used.The presence of metals affected the cracking reaction positively, despite an overall reduction in the material's surface area.Among them, materials doped with Ni revealed the best catalytic performance (Rodriguez et al. 2023).Recently, in our aim to replace synthetic materials with biomass-based catalysts for WMO catalytic cracking, superior alkali-treated rice husk biochar activity was demonstrated, showing a remarkable activity increment of the kinetic constant, implying lower activation energy, outperforming our previous synthetic materials (i.e., metal-doped Al/Si) (Juiña et al. 2023;Rodriguez et al. 2023).There, it was shown that the ashes content of rice husk and surface functionalization of biochar are more relevant than its surface area for catalytic activity.Thus, the possibility of enhancing rice husk biochar activity via metal impregnation into the biochar surface in the catalytic cracking of WMO has to be explored.
In this sense, in this contribution, alkali-treated biochars are impregnated with different metal ions: Ni, Zn, and Ca.Its kinetic performance in the reaction system was studied.Metal ions' initial concentration and impregnation methods are assessed.Liquid product characterization and a complete kinetic study (i.e., activation energy) for the best catalyst were developed.Complete materials characterization was also done to correlate properties to its catalytic activity.

Materials
WMO was primarily recovered from automobile maintenance facilities in Quito.Rice husk was collected from a local farm.The biomass pyrolysis was developed under an inert nitrogen atmosphere (N 2 N5.0 purity).Potassium hydroxide was used for biochar activation (KOH, Fischer Chemical).

Cracking process
The cracking process was based on the method of Rodriguez et al. (2023).Before the reaction, the WMO was heated to remove the presence of water (24 h of stirring at 100 � C).A Precision Scientific Petroleum Herzog distiller (Max.power of 1100 W, see picture in Figure 1(a)) was used for the cracking reaction.The recovered oil was heated and sieved to eliminate water and solids residues before the reaction.For a thermal reaction, 40 g of pretreated waste oil was added to the glass reactor.For the catalytic cracking reaction, 0.4 wt.% of catalyst was added to the glass batch reactor together with the WMO.The equipment was preheated to 260 � C for 10 min before placing the reactor inside the equipment (Figure 1(a).i); then the temperature was increased to 420 � C. The condensate from the cracking reaction was collected in the cylinder (Figure 1(a).ii); and weighted.Gas product was collected in a Tedlar bag (Figure 1(a).iii) and weighted.The liquid product is registered as a function of temperature and time; thus, these values are used to calculate the concentration of WMO as a function of time.Coke residues were settled at the bottom of the reactor.The reaction was carried out for both feed conditions until a significant equilibrium volume was recorded.

Biochar metal-doped synthesis
For biomass pyrolysis, rice husk was dried in an oven at 60 � C for 48 h.The dried rice husk was then pyrolyzed in a horizontal tubular reactor for 1 h at 700 � C (heating rate: 5 � C min −1 ), with a continuous inert nitrogen atmosphere.The produced biochar (RH) was then ground to a � 250mesh sieve to homogenize the particle size.Then, biochar and 3 M KOH solution (500 mL per 5 g biochar) were stirred for 2 h at 70 � C. The activated biochar was filtered and cleaned with distilled water until it reached a filtrate with a neutral pH.Finally, it was dried for 20 h in an oven at 105 � C.
Biochar metal impregnation followed a simple wet-impregnation method by Nguyen et al. (2016).Metal salt solutions of 0.03, 0.07, and 0.3 M (100 mL) were prepared.Then 2 g of biochar was put in each metal solution.The solutions had their pH value raised to 10 using 1 M NaOH.Finally, the solutions were stirred for 48 h at 80 � C with a laboratory-scale condenser apparatus.The impregnated biochar solutions were filtered and dried at 105 � C for 20 h.
Moreover, Ca oxide nanoparticles were impregnated using the reduction method shown by Kim et al. (2021).Of 3 g of activated biochar and 6 g of citrate were added to 250 mL of distilled water.The mixture was stirred for 0.5 h at 1000 rpm to disperse the biochar.Subsequently, 30 mL of 0.03 M Ca(NO 3 ) 2 � 4 H 2 O was added to the solution.After an additional stirring for 0.5 at 1000 rpm, NaBH 4 (0.9 g NaBH 4 in 180 mL of distilled water) was added to the principal solution.The resulting mixture was stirred for 0.5 h.Then, the solution was filtered and washed using 500 mL of distilled water.Finally, the solution was dried for 20 h at 105 � C. Please see Table 1 for all materials synthesized in this study.

Kinetic analysis
For all experiments, the mass balance was calculated according to Equation (1).
where m o denotes the WMO initial amount, m g is the amount of gas product, m l is the liquid product in the cylinder, and m r the WMO left at the reactor after reaction.Conversion (Equation ( 2)), Yield (Equation (3)), and Selectivity (Equation ( 4)) were calculated.These equations were based on the analyses of Rodr� ıguez Lamar et al. (2021).
where i represents the products (liquid or gas).
As previously reported, an elementary reaction was defined for the kinetic model.For a detailed explanation of the system, please refer to Vargas et al. (2016).In summary, the reaction system was shortened to an average oil molecule of C 30 , where the WMO produces the liquid products (2 P) and lighter fractions (B) (see Equation ( 5)).Thus, the rate of WMO consumption can be calculated according to Equation ( 6).The activation energy, E a J mol −1 and pre-exponential factor, k 0 min −1 were calculated from the linear expression of the Arrhenius law (Equation ( 7)).
where R ¼ 8.314 J mol K −1 , and T is the temperature in K.

Analytical methods
A Tescan Mira 3 microscope equipped with a Schottky Field Emission Gun (Schottky FEG-SEM) was employed to investigate the material's morphology.To prepare the samples for imaging, they were affixed to SEM stubs and coated with a 20 nm layer of 99.99% pure gold using a sputtering evaporator.A Bruker X-Flash 6j30 detector for Energy Dispersive Spectroscopy (EDS) was used.Specific surface area analysis via a singlepoint superficial analysis using an Auto Chem II micrometrics was conducted.A Rigaku diffractometer model MiniFlexll, was used for X-ray diffraction (XRD) patterns.The samples were scanned in the angular 2theta (2h) range of 10-90 � with steps of 0.02 � .
Liquid products were analyzed in a gas chromatograph (Clarus 590) coupled with a mass spectrometer (Clarus SQ8S).Samples were cooled down before injection.Typically, a GC-MS liquid phase analysis identified over 100 compounds in the samples.To assess rheological behavior, an MCR 72 rheometer (Anton Paar) was used.We introduced 30 mL of the liquid sample into the cylinder, and all operational parameters, including shear rate and temperature, were defined within the equipment's software.Heating cycle: shear rate range: 10-500 s −1 , Temperature range: 15-90 � C (steps 15 � C).Cooling cycle: same procedure was repeated but in a decreasing sequence.

Cracking reaction
Rice-husk metal-doped biochars were prepared and tested for the catalytic cracking of WMO (see Table 1).Each catalytic cracking experiment provided a measurable liquid and gaseous product quantity, later used for general analysis.
Figure 1(a) shows a typical experiment for thermal cracking of WMO developed in the setup, showing the reactor temperature and recovered liquid product as a function of time.With the defined parameters, the reaction temperature stabilizes after 20 min, at approximately 420 ± 1 � C. At this point, starts the recovery of the liquid and gas products.Previous contributions showed that a pseudo-first-order behavior concerning WMO consumption could be calculated.Figure 1 For all experiments, a complete kinetic study was developed for comparison.It means conversion, yield, selectivity, and kinetic constants were calculated.In preliminary experiments, low concentrations of initial metal salt (<5 wt.%) with respect to the initial amount of biochar did not show a significant improvement in the catalytic activity.Thus, as a first insight, the effect of metal salt initial concentration during the wet impregnation process under catalytic performance was conducted for higher metal loadings.Kinetic investigation using Nickeldoped biochar is summarized in Figure 2.
Figure 2(a) shows that the conversion of WMO is slightly increased by the presence of the metal at the material's surface (Ni_0.07M, a 12% increment compared to thermal reaction); however, at much higher Ni initial concentrations, the reaction seems to be limited, showing lower conversion values.Similar behavior is observed for the yieldto-liquid product values (see Figure 2(b)), showing that highly loaded materials might interfere with the cracking process, producing more gaseous products.Even though, conversion and Yield values seem to be affected, the selectivity-to-liquid product values are close to 90% for all systems.Interestingly, a notable difference in the kinetic constant for Ni-doped biochar is observed.Low concentration Ni-doped (0.03 M) catalyst shows a considerable increment in the kinetic constant compared to the thermal and RH-KOH reaction systems (190 and 40% increment, respectively), showing the effect of Ni for improving the catalytic activity.This improvement is by the literature where nickel has been shown to rise both carbon diffusion and carbon adsorption capacity making it a highly active catalyst for hydrocarbon cracking (Amin et al. 2011;Hamdan et al. 2022).Interestingly, while metal impregnation did not alter the inherent activity of activated rice husk biochar, it notable augmented its catalytic activity.
As can be inferred from the above-shown dimensionless numbers, this outstanding increment seems to be affected if the concentration of initial metal salt increases during the wet impregnation process.The kinetic constant radically decreased for higher amounts (just a 150 and 77% increment compared to thermal cracking, for Ni_0.07M and Ni_0.3M, respectively) (see Figure 2(d)).Thus, higher reaction rates are related to higher conversions and yield-to-liquid products.It is well known that coke and gases formation in reduced by faster catalytic cracking (Speight, 2015).On the other hand, higher-loaded materials might not provide a suitable surface area, thereby potentially impeding reaction kinetics and resulting in diminished catalytic activity.
SEM images and EDS spectra of Ni-impregnated materials are shown in Figure 3.As shown, the alkali-treated biochar shows a considerable increment in the presence of free pores, principally due to the extraction of silicon from the biochar matrix after treatment.Silica content decreased from around 20-7% after KOH activation.After metal impregnation, the presence of heterogeneously impregnated nickel in the porous biochar surface is evident (Figure 3(c)), confirmed by the detected Ni via EDS analysis (Figure 3(f)).XRD spectra of materials were also developed (see Figure 4).XRD pattern shows the amorphous structure expected for raw and alkalitreated biochar.A characteristic peak at 2Ɵ ¼ 22 � is also observed, which is typical for silica present in the rice husk (Shen et al. 2014).After treatments, slight differences in the patterns are observed.First, a slight increment in intensity between 42 and 47 � is observed, which is related to the carbon (100) plane, showing an increment in the degree of carbon order.Moreover, the presence of new peaks around 58 and 60 � and the vanishing of the silica peak (see dash lines in Figure 4) might be related to incorporating amorphous Ni 2þ cations into the surface of the biochar bonding network (Nguyen et al. 2016).These changes become more significant while increasing the initial Ni concentration, thus resulting in a higher presence of Ni at the biochar surface.Interestingly, as shown in Figure 3(e) in the SEM image, the free porous biochar seems to be covered by the metal.This clogging of pores could be related to the lower activity shown by high-metal-loaded materials, which show a decrement while increasing the Ni initial concentration.Clogging is confirmed by the specific surface area reduction measured by a singlepoint BET analysis (please see Table 2).Initially, impregnated materials increased their specific surface area due to the wet impregnation process.It is possible that the salts and temperature during the process promote the development of meso and macropores within the biochar structure.However, once the metal loading is increased, a steady decrement of the specific surface area was observed while increasing the initial Ni content, 210.69, 151.20, and 139.98 m 2 g −1 , for Ni_0.03M,Ni_0.07M, and Ni_0.3M, respectively.Thus, in this study, 0.03 M initial metal salt concentration was used as the optimum value for further metal impregnation.
Zinc and calcium oxide have also been successfully used for the catalytic cracking of fatty acid methyl esters and vegetable oils into green biofuels (Zaher et al. 2017;Zhang et al. 2022).Thus, biochars were also impregnated with these two compounds to develop a complete screening.AS mentioned before, the lowest salt initial concentration value (0.03 M) was used as an optimized parameter from the above experiments with nickel.More or less expected, conversion, yield, and selectivity-to-product values are not affected by the presence of Zn and Ca at the biochar surface (see Figure 5).However, Ca and Zn considerably increase the catalytic activity of the metal-doped biochars (221 and 247% increment compared to the thermal reaction).It is assumed that metal doping does not significantly affect the reaction pathway to produce gas or liquid products.However, it might have an incidence in the carbon distribution of produced DLF.Ca and Zn are the most suitable metals for WMO catalytic cracking, as they dramatically impact the reaction speed, which might reduce the production cost of DLFs from WMO.
Finally, as calcium seems to be a more affordable metal substrate for producing biomass-based catalysts in the future, optimization of the impregnation method for calcium into the biochar matrix was developed and tested.Unlike the previous method, a reduction method for obtaining better-defined particles was developed using the same metal initial concentration.The   reduction method involves reducing metal ions on the surface of the support material using a reducing agent.This method can be more efficient than wet impregnation, leading to higher metal dispersion and stronger metal-support interactions (Mehrabadi et al. 2017).Interestingly, a higher kinetic constant was obtained by using this method for the material preparation, obtaining a kinetic constant of 0.016 min −1 compared to 0.012 m −1 from the wet impregnation method.It is, by now, the fastest reaction rate measured for this reaction.Finally, temperature variation experiments were developed for this catalyst compared to the thermal reaction for calculating the activation energy and the pre-exponential factor (see Figure 6).As expected, a lower activation energy for the catalytic cracking of WMO using Ca_red (246 kJ mol −1 ) compared to the thermal reaction (308 kJ mol −1 ) is obtained (see Figure 6 and Table 3), indicating a reduction of approximately 20% in the activation energy requirement.SEM images of materials impregnated with calcium with both methods are shown in Figure 7.Both materials show a characteristic porous morphology.Moreover, Ca was detected for both materials by EDS characterization (Figure 7(d)).However, for Ca_red, the clear presence of CaO particles at the nanometric size is observed instead of agglomerates observed in wet-impregnated materials.This behavior is also confirmed by XRD characterization (see Figure 4), where the spectrum is slightly changed by the presence of calcium at the biochar surface compared to metal-doped wet impregnation methods.This better distribution of CaO active sites might be related to the better activity of the optimized materials.

Liquid-products characterization
GC-MS characterization was developed under commercial diesel and DLF from Ca_red for comparison.As shown in Figure 8 top, a typical chromatogram for diesel is obtained.Numbers show the retention time of the most significant peaks in the chromatogram, whose corresponding data is shown in Table S1 in the Supporting Information (SI).As expected, commercial diesel shows organic molecules with hydrocarbon chains between C-10 and C-23, such as Aromatics, Olefins, isoparaffins, naphthenes, and oxygenates.DLF resembles a similar carbon distribution in this range (see Table S2 in SI).However, it is also noticeable a higher presence of shorter hydrocarbon chains ranging from 6 to 9 typically present in commercial gasoline, such as aliphatic (e.g., n-hexane and n-heptane) and some aromatic hydrocarbons (Nishiwaki et al. 2018).As mentioned before, it is expected as catalytic cracking enhances the heterolytic instead of hemolytic bond-breaking process in thermal cracking, which might imply the presence of shorter hydrocarbon chains.
The rheological behavior of diesel fuel, which refers to its flow and deformation characteristics under different conditions, is important for diesel engine operation and fuel system design.Thus, rheological experimentation was also developed.Both commercial diesel and DLF showed similar behavior.As depicted in Figure 9(a), before 40 s −1 , the viscosity gradually decreases until it reaches, more or less, a constant value (2.6 and

Conclusion
This study demonstrates the effect of metal impregnation of rice husk biochar in the kinetic behavior of the catalytic cracking process of WMO into DLF.It has been shown that around 10 wt.% of initial metal concentrations with respect to biochar during the wet impregnation method is the most optimum value for improving the catalytic effect.Higher metal concentrations were shown to block the biochar pores, reducing their activity.From the metals tested, the presence of zinc and calcium at the biochar surface showed the highest kinetic constants without affecting product yield and selectivity.An optimized reduction method was developed for impregnating calcium oxide particles.This material showed a more significant catalytic activity with a 290% increment in the kinetic constant compared to the thermal reaction and a 25% reduction in the activation energy value, thus, reducing the energy requirements for the WMO cracking.
Liquid cracking products showed the presence of short hydrocarbon chains (C 6 -C 9 ) and a distribution like commercial diesel for C 10 -C 27 .Moreover, DLF showed similar rheological behavior to commercial diesel, an important factor in avoiding engine modifications.
In summary, this study reaffirms the potential application of materials based on biomass wastes as suitable green catalysts for chemical reforming in countries where synthetic materials are hardly available and the shift to greener processes and materials is desired.Nevertheless, bigger efforts must be put into developing viable cleaning methods for achieving catalyst reusability in larger-scale applications.

Figure 1 .
Figure 1.(a) Picture of the setup used for the cracking reactions, (b) exemplary catalytic cracking experiment, (c) exemplary experimental vs. predicted values curves.Inset: fitting of WMO catalytic cracking to a first reaction kinetic model.Reaction conditions: Temperature: 410 � C, 40 g of WMO, 160 mg of Zn_0.03M catalyst.
(b) perfectly depicts the experimental vs. predicted values curves, fitting to a first-order kinetic model (see inset in Figure 1(b)).

Figure 8 .
Figure 8.The catalytic cracking obtained the GC-MS of commercial diesel (top) and DLF (bottom).

Table 1 .
Synthesized catalyst in this study

Table 2 .
Single-point BET surface area of materials synthesized in this study