Evaluating the properties of a novel diesel-like fuel produced by biochar-assisted catalytic cracking of waste motor oil

Abstract In light of the need for sustainable and eco-friendly alternatives to conventional fuels, waste-based diesel-like fuels have emerged as a promising solution. This study explores the chemical, physical, and rheological properties of a diesel-like fuel (DLF) produced via chemical recycling of waste motor oil (WMO) using an alkali-treated rice husk biochar as a catalyst. DLF from biochar-assisted cracking (DLFB) resembles commercial diesel hydrocarbon distribution better than DLF from thermal cracking, which shows to many molecules in the gasoline range. DLFB meets the minimum requirements for commercial diesel as per ASTM standards. The study also presents the rheological properties of the DLFB and its blends with commercial diesel, assessing their flow behavior under various operating conditions. The results indicate that all samples exhibit Newtonian behavior. The shear stress rises with the shear rate in a linear manner. Moreover, rheograms indicate that viscosity gradually decreasing with temperature. The obtained DLFB resembles commercial diesel in chemical composition when analyzed via through FTIR and GC-MS analysis, though with a small presence of low-molecular-weight hydrocarbons. In summary, these results demonstrate the potential of this novel DLFB as a sustainable fuel, given its favorable properties and the circular approach applied to the valorization of WMO. Highlights Biomass-based catalytic recycling of waste motor oil to obtain diesel-like fuels (DLFs). DLFB meet commercial diesel standards and exhibit favorable rheological properties. DLFB potential as sustainable fuel with a similar composition to commercial diesel.


Introduction
Energy production still relies on fossil-based sources, yet these fuels are rapidly depleting due to increasing demand worldwide.Reducing this dependency has been one of the most challenging tasks, which has led researchers to seek new technologies for sustainable development.Renewable sources have been extensively studied, including biomass conversion, solar, geothermal, and wind energy, among many others.However, following another approach to a circular economy, waste from products manufactured by humans can be incorporated into the value chain by recycling them and upgrading their properties toward an alternative biofuel production [1][2][3].Even though alternative fuels allow the valorization of specific residues, fuel properties and the potential environmental impact during combustion must be evaluated.
One example of alternative fuel is biodiesel, which can be produced from waste cooking oils via transesterification, as demonstrated by many research groups in the last decade [4][5][6].Though, biodiesel might have problems concerning feedstock availability, the presence of chemical traces (i.e.methanol), reaction by-products (e.g.glycerin and soaps), and limited rheological behavior, namely, properties that are usually not assessed in studies [7].Biodiesel usually presents higher viscosity than petroleum diesel, which can cause issues with fuel flow, fuel injection, and engine performance, resulting in delayed combustion [8].On the other hand, many researchers have explored technologies for converting waste motor oil (WMO) via pyrolysis and copyrolysis, hydrogenation, and thermal distillation by breaking large molecules of WMO to obtain a diesel-like fuel (DLF), which, due to its nature, seems more suitable for using it in a typical diesel engine [9][10][11][12].In general, DLF production is not limited to WMO, but can be obtained from other sources like industrial oily waste [13], waste cooking oil [14], methyl palmitate [15], crude oil sludge [16], hydrotreated tire pyrolytic oil [17], etc.
Going further on WMO, the feasibility of chemically recycling WMO via catalytic cracking for producing a diesel-like fuel was already evaluated.Demonstrating the potential to conduct reactions at decreased temperatures, all while retaining viable reaction rates, showcases the influence of catalyst.A first contribution demonstrated that metaldoped aluminum silicates are prone to speed up the cracking reaction [12].Recently, it was showed the possibility of replacing those catalysts with biochar from agro-residues [18].There, it was revealed that biomass origin and postpyrolysis chemical activation methods affect the catalytic activity of the materials.A kinetic study showed that alkalitreated rice husk biochar (RH-KOH) is a highly active material, opening an exciting opportunity for a double valorization of residues.Nevertheless, the liquid product from the catalytic cracking reaction was not studied in detail.Specifically, rheological characteristics at different operating temperatures, molecule composition, and carbon number distribution must be assessed to demonstrate its utilization in diesel engines.
In this sense, a complete physicochemical characterization of a diesel-like fuel obtained from the catalytic cracking of WMO using an alkali-modified rice husk biochar as a catalyst is shown.Its blends with commercial diesel are also evaluated.First, rheological properties are studied for showing dynamic viscosity and shear stress behavior at diverse service temperatures (15 to 90 � C) in heating and cooling cycles at different shear rates (10-500 s À 1 ) for covering the typical operating conditions and ambient temperatures that diesel engines experience during operation.Then, a complete composition analysis by FTIR and GC/MS is also presented.

Materials
A collection station in Quito-Ecuador supplied WMO.Rice husks were collected from a local farm.The catalytic cracking reactions were developed using alkali-modified rice husk biochar (RH-KOH), which was synthesized following the method presented in a previous contribution [18].In summary, first, moisture was removed by drying RHs at 60 � C for 48 h.The pyrolysis was carried out in a horizontal tubular reactor controlled by Carbolite Gero equipment (700 � C with a 5 � C/min ramp rate and a 1-h dwell time under a nitrogen atmosphere at 40 mL/min).The resulting biochar was ground to � 250-mesh sieve.For chemical activation, 2 g of ground biochar was treated with a 3 M KOH solution, stirred at 600 rpm, and heated to 70 � C for 2 h.
Biochar morphology was carried out with a Scanning Electron Microscope (SEM) and a Tescan Mira 3 microscope with a Schottky Field Emission Gun (Schottky FEG-SEM).An Energy Dispersive Spectroscopy (EDS) was used for the elemental analysis on the SEM chamber at 30 kV with a Bruker X-Flash 6j30 detector.FTIR spectroscopy was developed using Cary 630 FTIR Spectrometer from Agilent Technologies from 600 to 4000 cm À 1 wavenumber range.In summary, RH-KOH shows a porous structure due to the pyrolysis process followed by the alkali treatment.Chemical modification removed ashes, liberating the pore cavities as depicted in Figure 1(a).FTIR spectrum showed typical peaks, which infer the amorphous structure of a turbostratic carbon, and Si-O-Si stretching vibrations (1100 cm À 1 ) due to rice husk silica content (see Figure 1(b)).EDS characterization confirmed the carbonization and removal of Si after chemical activation of the raw biochar (see Figure 1(c)).For a complete material characterization, please refer to this previous study [18].

Waste motor oil cracking process
Before the cracking process, WMO was stirred at 100 � C for 24 h and filtered to remove solid residues.The equipment used for the cracking process was a Precision Scientific Petroleum Herzog distiller (see Figure 2(a)).40 g of pretreated WMO was placed in the batch reactor, and 0.4 wt% of RH-KOH was added (for the biochar-assisted experiments).The equipment was pre-heated and then set to 420 � C. The liquid product (i.e.DLF) as indicated in Figure 2(c) was recovered for further characterization.In summary, reaction temperature stabilizes after a 20 min induction period, and liquid product recovery starts.An 86%-conversion of initial WMO was calculated with a 90% of selectivity to liquid products.Products from thermal and catalytic cracking are denominated as DLFT, and DLFB, respectively.For subsequently comparison, DLFB and commercial diesel were mixed in different blends, as shown in Table 1.

Viscosity measurements
The flow behavior of the samples was investigated using an MCR 72 rheometer (Anton Paar, Graz, Austria).Acknowledging the inherent fluidic nature of the specimens, a revolving viscometer with a coaxial cylinder was used.30 mL of liquid sample was placed in the cylinder.The rheometer had its own software, where all operating parameters (shear rate, temperature, etc.) were defined.The software also calculated output variables (shear stress and dynamic viscosity).
Two different experiments were developed for obtaining the dynamic viscosity and shear stress values for all samples: i) a shear rate range from 10 to 500 s À 1 was defined.Dynamic viscosity started at 15 � C, the sample was heated to 15 � C (15!30!45!60!75!90 � C), once the temperature was constant, the dynamic viscosity was measured.The procedure was repeated until reaching 90 � C to simulate a heating cycle; ii) the same procedure was developed, but starting at 90 � C and decreasing temperature, simulating a cooling cycle (90!75!60!45!30!15 � C).All experiments were developed two times.

Composition measurements and calculations
FTIR spectra were recorded on regimes between 650 and 4000 cm À 1 using an Agilent Cary 630 (Agilent Technologies, Santa Clara, CA, USA).
The reaction products were analyzed by a gas chromatograph model Clarus 590 with a mass spectrometer model Clarus SQ8S, both manufactured by Perkin Elmer.All the reaction liquid products (C4-C25) were lumped by carbon number and were analyzed with this gas chromatograph-mass spectrometer (GC-MS) to identify all the isomers in the liquid phase.Before the analysis, all reaction products were cooled to 0 � C to ensure all C5þ hydrocarbons were in the liquid phase.A GC-MS liquid phase analysis typically determined that the samples had over 100 compounds.
The calorific values of commercial diesel and DLF samples were measured by a Parr 6400 bomb calorimeter, following the ASTM D 4809 standard [19].The sample was about 1 g, and after ignition, the temperature increased by about 13 � C because of the heat of combustion.The equipment is automatic and, using the internal constant of the equipment records the heat of combustion of the sample in J g À 1 .

Distribution of hydrocarbon groups
Figure 3 compares the GC-MS chromatograms of both commercial diesel and DLFs obtained through thermal cracking (DLFT) and biochar-assisted cracking (DLFB), providing a comparative analysis to assess the catalyst influence on the hydrocarbon composition.A list of identified compounds is detailed in the SI section in Tables S1-S4.The GC-MS results revealed an extraordinary difference between both DLFs.Namely, DLFT shows many small hydrocarbon groups in the gasoline range and lower concentration of molecules in the diesel range without a defined distribution.Thus, products from thermal cracking, solely, might not be recommendable for use in a diesel engine as the combustion characteristics of small hydrocarbon molecules may differ significantly from diesel fuel,   potentially leading to incomplete combustion, increased emissions, and reduced engine efficiency.Remarkably, when using the biochar catalyst, not only the speed of reaction increases, but also DLFB resembles more the distribution of commercial diesel.It shows that catalytic cracking affects the characteristics of the distilled product, improving the formation of a diesel-type fuel.Ultimately, the utilization of DLFB derived from catalytic cracking, with its enhanced resemblance to commercial diesel, appears as a potential component for formulation in blends.Thus, in the next sections, a further characterization will be developed under DLFB and its blends with commercial diesel.

Composition study
After the first insights showed above, a more detailed composition experimentation was developed.Furthermore, in addition to examining commercial and DLFB, particular attention was given to blend_75-25, as this blend ratio represents the upper limit commonly utilized in diesel engines.First, superior caloric power is the amount of heat that would be released when performing complete combustion.Similar values are obtained for commercial diesel, DLFB, and blend_75-25 (45,563.13 ± 6.21, 45,554.76± 84.91, and 45,643.05± 113.31, respectively).These values and those presented in Table 2 must indicate the similar composition of commercial diesel and DLFB.
FTIR spectra are well-known for identifying characteristic compounds in commercial diesel [20].Figure 4 shows the FTIR spectra of commercial diesel, DLFB, and blends for comparison.Table 2 lists the most representative peaks and possible vibrations assigned to them.According to the literature [14], commercial diesel shows representative peaks in the range of 2854-2954 cm À 1 , typical of aliphatic carbons.Peaks between 177 and 1457 cm À 1 can be identified as the deformation of the C-H bond, and peaks at lower values (722-805 cm À 1 ) represent double bond C ¼ C stretches.
Interestingly, DLFB and blend_75-25 spectra look like commercial diesel, showing its suitability as a possible replacement.Even though they have similar curves, the DLFB spectrum shows two peaks at 880 and 915 cm À 1 , which are not present in commercial diesel.Both can be related to -CH ¼ CH 2 , -C ¼ CH 2 groups of shorter hydrocarbons not present in commercial diesel.It is known that catalytic cracking might follow a hemolytic bond-breaking process, which increases the presence of aromatic and isoparaffin constituents (short-carbon chains), reducing the formation of coke and gases [21].It is thus confirmed when comparing the thermal and catalytic cracking processes conversion (i.e.73 and 86.1%, respectively), which means less coke and gases remain after the DLFB production [18].
Moreover, the carbon number distribution was studied based on the available hydrocarbon types in each sample to understand whether the obtained diesel-like fuel resembles commercial diesel.Figure 5 shows that commercial diesel samples contain heavier hydrocarbons than DLFB samples.It can be noticed by first analyzing the highest peaks for commercial diesel and DLFB samples found at C13 and C7, respectively.It is confirmed by comparing the average carbon numbers (ACN), which are 15.8 for commercial diesel and 12 for DLFB.Typically, diesel fuels contain hydrocarbons ranging from C12 to C20, mostly between C16 and C20 [22].Another difference lies in the hydrocarbon type since commercial samples show higher concentrations of isoparaffins.
In contrast, DLFB samples mainly contain naphthenes (low carbon numbers) and isoparaffins (high carbon numbers).These differences might be related to the catalytic cracking mechanism mentioned above and resemble the additional peaks observed in the FTIR spectra in Figure 4.The increased naphthene concentrations in DLFB samples seem to be the main difference in composition between these two fuels.C7-based naphthenes like 1,3-dimethyl cyclopentane were found in gasoline-range hydrocarbons obtained from isopropanol conversion over HZSM-5 catalysts [23].Despite this significant amount of C7 naphthenes found in DLFB, the ACN remains in the diesel carbon range.However, Mart� ınez et al. [24] indicated that low cetane numbers could be identified in high aromatic and naphthene concentrations of diesel fractions.These authors have successfully addressed this issue by applying selective ring opening (SRO) of naphthenes to convert them into linear or mono-branched paraffins, as also investigated in other works: SRO of decalin (i.e. the most abundant polycyclic naphthene in diesel) using Pt/CNT-USY catalysts [25]; SRO of methylcyclohexane and decalin employing Rh-Pd catalysts supported on Al 2 O 3 and SiO 2 [26].
Based on this background, an alternative to properly use the obtained DLFB could be to prepare commercial diesel/DLFB blends.Blend_75-25 was evaluated to determine how similar this blend's characteristics are to commercial diesel.When comparing Figure 5(a,c), it can be noticed that ACN are very close, and the most significant peak of the blend carbon number distribution (i.e. at C13 and C21) resembles the peak for commercial diesel.Furthermore, regarding blend composition, although naphthene concentration is still significant, isoparaffins were . FTIR spectra of the studied samples.
found to contribute the most among all hydrocarbon types.After assessing these parameters, it can be confirmed the commercial diesel/DLFB blend properties are closer to the pure diesel ones than DLFB, which gives good insights into the potential commercialization of this blend to be used in a standard diesel engine.In the race to waste valorization, it is expected first to test blends containing the converted waste and its corresponding commercial counterpart.The aim of using blends is to avoid possible engine modifications required to operate pure DLFB [27].
There is even the possibility of producing DLFB/biodiesel mixtures, which reflect similar properties to commercial diesel [14].

Physicochemical properties
Basic physicochemical properties were determined for DLFB and compared to the commercial diesel properties.As indicated in Table 3, DLFB lies within the values defined by ASTM norms for a commercial diesel.Regarding API gravity, the obtained DLFB can be classified as light oil, resulting in better fuel quality [28].On the other hand, the kinematic viscosity of DLFB is very close to the minimum limit for commercial diesel guidelines.As it is well-known, fuel viscosity increases when higher-molecular-weight chemical compounds are present [29].Thus, the lower measured kinematic viscosity of DLFB can probably be attributed to low-molecular-weight hydrocarbons.
In the case of the flash point, higher values reflect higher fuel volatility resulting in flammability risk during storage and transportation.According to ASTM norm, commercial diesel should have a minimum value of 51 � C. Nevertheless, in literature, values around 63 � C can be found [30].The slightly higher value for DLFB flash point (66 � C), might be associated with fewer oxygenated compounds in DLFB samples than in commercial diesel [29].

Rheological study
After the preliminary experiments, a complete rheological study was developed.The viscosity profiles in Figure 6 and Figure S1 in Supporting Information (SI) depict the behavior of commercial diesel, DLFB, and blends versus the assigned shear range (10-500 s À 1 ) in heating (15 to 90 � C) and cooling cycles (90 to 15 � C).This temperature range was assigned to allow observation of the flow at different operating conditions.Figure 6(a,b) shows the behavior of samples at the assigned shear rate under two representative operating conditions (30 and 70 � C, respectively) in a heating cycle (please refer to Figures S1a and S1b for the cooling cycle).For commercial diesel, at shear rates lower than 40 s À 1 , the viscosity decreased gradually with the increasing shear rate until reaching a constant value.This flow behavior is similar to the DLFB sample at lower viscosity values (commercial ¼ 3.9 mPa s, diesel-like ¼ 1.7 mPa s, both at 30 � C).The same curve behavior was observed at a higher temperature (70 � C) with a lower average viscosity value.The decrease in average viscosity can be attributed to the deterioration of constituent intermolecular forces of the fluids.
Interestingly, for different commercial-DLFB blends, the shape of the curves was not affected.Nevertheless, the average viscosity value increases with the amount of diesel in the blend.Regarding the curve trend, for most all samples, at shear rates over 100 s À 1 , the viscosity value tends to decrease to the average value.A similar trend was observed for experiments developed in a cooling cycle.Thus, flow characteristics might not be affected in different working conditions.
Figure 7 shows the rheogram profiles of the shear stress vs. shear rate curves for all samples at different temperatures.Commercial diesel, DLFB, and its blends exhibited a Newtonian behavior in all temperatures, evidencing the linear relationship between shear stress and shear rate.This tendency is not affected by increasing or decreasing temperatures.It is a considerable advantage compared to biodiesel from cooking oil.Biodiesel generally exhibits a non-Newtonian fluid behavior, which might limit fluid performance during actual operation in a diesel engine [7,8].
Finally, the dynamic viscosity as a function of temperature is depicted in Figure 8. Diesel, DLFB, and the blends followed the same trend.As indicated, viscosity decrease as temperature increases.In summary, all the blends showed the same trend as pure components.As exhibited already before, dynamic viscosity values tend to rise by increasing the commercial diesel content.In summary, DLFB's rheological behavior might not be a limitation when using it in a typical diesel engine.

Conclusion
This contribution demonstrates the possible use of a diesel-like fuel obtained from the chemical recycling of waste motor oil catalyzed by an alkali-treated rice husk biochar as an alternative fuel in diesel engines without modification.Firstly, DLFB from biochar-assisted cracking seems to be more suitable for its use as diesel fuel compared to DLFT   obtained from thermal cracking, as the latter shows more hydrocarbon chains in the gasoline range.Density, API gravity, kinematic viscosity, and flash point values of DLFB were within the minimum values established by ASTM norms.A rheological analysis showing the behavior of shear stress and dynamic viscosity against the applied shear rate of the DLF and blends compared with commercial diesel simulating different operating conditions (i.e.heating and cooling cycles) showed a Newtonian flow behavior of all samples.Moreover, FTIR and GC-MS analysis confirmed this similarity by comparing the average carbon numbers, which were 15.8 and 12 for commercial diesel and DLF, respectively.Aromatics, isoparaffins, naphthenes, oxygenates, etc., showed to be the major constituents of all samples.Nevertheless, due to the cracking process, DLF showed shorter hydrocarbon chains and more oxygenated compounds than commercial diesel, making their blends a more suitable product.
Future perspectives lie in testing DLFB or the DLFB/commercial diesel blends during real operation in a diesel engine.It is essential to evaluate the engine performance as well as the pollutant emissions to determine whether this novel DLFB is able to reduce emissions.If not, elucidating the required modifications of the WMO cracking process to do so.Furthermore, with regards to the cracking process, additional investigation is imperative to comprehensively grasp the catalytic impact in attaining diesel-like fuels that closely emulate the characteristics of commercial diesel.Then, the next step is to focus on techno-economic analysis and life cycle analysis regarding DLFB production, distribution, and subsequent combustion.It will allow assessing the potential cost-effectiveness of producing and utilizing the so-called DLFB at industrial scale and the corresponding carbon footprint.

Figure 1 .
Figure 1.(a) FTIR spectra, (b) EDS analysis, and (c) SEM imaging of the RH/KOH catalyst used in this study.

Figure 2 .
Figure 2. Experimental equipment (a) glass batch reactor heating: (i) heating, (ii) glass Flask, (iii) temperature control; (b) condenser: (iv) cooling, (v) condensed liquid hose; (c) graduated cylinder for the liquid product collection; and (d) liquid product and temperature vs. time diagram for a typical WMO catalytic recycling experiment.

Table 3 .
Physical properties of commercial diesel and diesel-like fuel samples.