Introduction

Quality standards for commercial fuels are becoming increasingly stringent. To meet these challenges, refineries must adopt innovative and efficient solutions to comply with these strict regulations, which aim to protect the environment and reduce pollutant emissions.

Investing in improved fuel quality is a key component of global strategies aimed at reducing pollution and combating climate change. Low-quality fuels can result in harmful emissions and air pollution, contributing to public health issues and environmental degradation [1,2,3]. Rising crude oil prices are pushing refineries to explore alternative methods for producing automotive fuels, including the processing of heavy fractions such as heavy cooking gas oil and light cycle oil (LCO), which are rich in sulfur and polycyclic aromatic hydrocarbons. Additionally, the use of alternative fuels is being increasingly considered.

Modern refining processes, such as catalytic cracking, delay cooking, visbreaking and hydrocracking, are essential for the valorization of heavy and residual fractions of crude oil into white products: gasoline, kerosene and diesel fuel [4].

The literature presents many studies on hydrodesulphurization and denitrification of fuels, but research on the hydrogenation of polycyclic aromatic compounds continues to arouse great interest because this method is a viable option for reducing the polycyclic aromatic hydrocarbons (PAH) content in exhaust gases [5]. The continued use of diesel engines even after the introduction of electric vehicles in transport shows that there are still requirements for diesel fuel worldwide [6].

The diesel fuels obtained in these processes have high PAH and sulfur content with a negative impact on the qualities of the diesel fuel in terms of cetane number, color, density and fuel stability. In order to comply with the requirements imposed by the EN 590 standard, all these fractions that make up the diesel fuel will be hydrotreated. Aromatic hydrocarbon saturation is the main solution to reduce the aromatic hydrocarbon content in exhaust gases, by producing less harmful fuels. Currently, environmental and public health protection requirements, determine restrictions on the sulfur content in motor fuels, up to 10 ppm and polyaromatic hydrocarbons of max. 8% [7].

PAHs are globally recognized for their potential carcinogenic and mutagenic effects [8]. The United States Environmental Protection Agency (U.S. EPA) has identified 16 PAHs as priority pollutants. These compounds are primarily released into the environment through combustion processes, with diesel engines being a major source of airborne PAH emissions [9]. Consequently, reducing the PAH content in diesel fuel can significantly lower emissions of these harmful compounds and contribute to improved air quality [10].

Hydrotreating catalysts play a crucial role, particularly in significantly reducing the levels of sulfur and aromatic hydrocarbons in diesel fuel. These reductions are driven by environmental regulations aimed at minimizing pollutant emissions, reducing environmental impact, and protecting human health. To meet these targets, refineries are continually investing in research and development to enhance the efficiency and performance of hydrotreating catalysts, thereby supporting the production of cleaner, more sustainable fuels. New noble metal catalysts, along with innovative catalyst supports that enhance catalytic properties, are being developed and actively studied. Additionally, technologies that integrate hydrotreating with other conversion processes are emerging to produce fuels with superior qualities [11].

To produce fuels with ultra-low sulfur content, various desulfurization technologies have been employed, including oxidative desulfurization (ODS), adsorption desulfurization (ADS), and biodesulfurization (BDS). These technologies are often combined with other methods to achieve effective fuel desulfurization [12,13,14,15,16].

Compared to other desulfurization technologies, the conventional HDS (hydrodesulphurization) process, when used with high-efficiency catalysts, remains the most widely applied method in refineries for producing ultra-low sulfur fuels. In modern HDT (hydrotreating), catalysts must exhibit exceptionally high activity to achieve deep sulfur removal. Noble metal-based catalytic systems are the most active in HDS reactions. However, their widespread use is limited due to high costs and low tolerance to catalytic poisons such as sulfur and nitrogen [17, 18]. Although modern HDT catalysts demonstrate excellent performance, the most commonly used catalysts are still alumina-supported TMS (transition metal sulfides), such as molybdenum or tungsten promoted by cobalt and/or nickel. These catalysts are favored for their large specific surface area and high mechanical strength [19].

Extensive research efforts have been dedicated to enhancing the activity of TMS catalyst supports, improving the properties of the active phase by optimizing the interaction between Mo/W and Co/Ni promoters, and refining the interaction between the active phase and the support. This 'active phase–support' interaction remains a central focus of current studies [20, 21].

The present experimental study investigates the performance of two types of CoMo/γAl2O3 catalysts, one industrial and one laboratory prepared with respect to their desulfurization efficiency and hydrogenation of PAHs. The goal is to produce clean fuels that meet European standard EN 590, which requires sulfur content below 10 ppm and PAH content below 8%. The feedstock used in the hydrotreating process consisted of a mixture of 65% straight-run gas oil (SRGO), 25% cooking gasoil, and 10% LCO.

Experimental

The raw material used is a mixture of 65% SRGO, 25% cooking gasoil and 10% LCO whose characteristics are presented in Table 1.

Table 1 Characteristics of raw material used experience
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The data in Table 1 are in accordance with the literature and indicate that the addition of LCO and cooking gasoil to SRGO leads to an increase in the content of bi- and tricyclic aromatic hydrocarbons, which significantly impacts the quality of the hydrotreated product. Naphthalene-type aromatics (with two benzene rings) tend to lower the cetane number, while tricyclic aromatics (with three benzene rings) can negatively affect the color of the final product. Moreover, sulfur content increases substantially, as it is primarily concentrated in the heavier fraction of LCO.

In the hydrodesulphurization process, two Co-Mo/γAl2O3 catalysts were used: one industrial and one laboratory prepared.

The bimetallic CoMo/γAl2O3 catalyst was synthesized in the laboratory using the pore-filling method (incipient wetness impregnation), a technique commonly employed for catalysts that do not contain noble metals. The catalytic support, in extrudate form (length/diameter = 1.2 mm/1 mm), was prepared by wet mixing in the presence of aqueous solutions, followed by drying at 160 °C for 8 h and calcination at 550 °C for 6 h.

The impregnation step was carried out using an experimental setup consisting of a burette containing the impregnation solution, a controlled dripping system, a rotary flask, and an electric motor. Based on a preliminary experimental program, the optimal operating conditions were established as follows: sample volume of 32 cm3, space velocity of 0.1 h⁻1, and an impregnation solution volume equal to 100% of the pore volume of the support.

The active phase precursors used were cobalt nitrate and ammonium heptamolybdate tetrahydrate. The concentrations of these precursors in the impregnation solutions were adjusted to achieve a final metal content of 3.8 wt% Co and 11 wt% Mo in the finished catalyst.

Between successive impregnation steps, the samples were reconditioned by drying at 160 °C for 4 h. Final conditioning of the catalysts included the following steps: a) drying at 160 °C for 8 h; b) calcination at 450 °C for 6 h; c) activation in a hydrogen stream at 450 °C for 6 h.

The chemical composition of the catalysts was confirmed by atomic absorption spectroscopy using the “Fast Sequential” spectrometer (Varian AA240FS), validating the targeted metal content.

Both CoMo catalysts were activated through sulfurization with thiophene dissolved in gasoil at a concentration of 1000 ppm, in a hydrogen atmosphere at 280 °C.

The textural properties of the catalysts, including specific surface area, pore volume, average pore diameter, and pore size distribution, were determined using an Autosorb 1 surface area and porosity analyzer (Quantachrome Instruments). The data were obtained by automated recording and processing of nitrogen adsorption–desorption isotherms at the boiling point of liquid nitrogen (77 K).

The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) model applied to the linear region of the adsorption isotherm. Pore size distribution and average pore diameter were evaluated from the desorption branch of the isotherm with hysteresis, using the Barrett–Joyner–Halenda (BJH) method (Fig. 1).

Fig. 1
Fig. 1
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Pore size distribution of the prepared Co-Mo/γ-Al2O3

The nitrogen adsorption–desorption isotherm for the Co-Mo/γ-Al2O3 catalyst is presented in (Fig. 2). According to the IUPAC classification, the isotherm is of type IV, characteristic of mesoporous materials, and shows an H3-type hysteresis loop. This type of hysteresis is typically associated with mesoporous structures indicating low affinity for the adsorbate molecules and a relatively narrow pore size distribution.

Fig. 2
Fig. 2
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N2 adsorption–desorption isotherms for prepared Co-Mo/γ-Al2O3

The specific surface area determined is consistent with values typically reported for γ-alumina-based catalysts.

The acid strength distribution of the catalytic sites was determined using the temperature-programmed desorption (TPD) method with diethylamine, employing a thermogravimetric analyzer over the temperature range of 20–700 °C (Fig. 3).

Fig. 3
Fig. 3
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Acid strength distribution on the prepared Co-Mo/γ-Al2O3 catalyst. A weak acid sites (150–300 °C/0.47 meq/g); B medium acid sites (300–440 °C / 0.28 meq/g); C strong acid sites (440–580 °C/0.33 meq/g)

The temperature at which desorption occurs is directly correlated with the strength of the acid sites: desorption in the range of 150–300 °C is attributed to weak acid sites, between 300 and 440 °C to medium-strength acid sites, and between 440 and 580 °C to strong acid sites. The total acidity is defined as the total mass of desorbed amine, expressed in milliequivalents per gram of catalyst (meq/g). The analyses were performed using a DuPont Instruments “Thermal Analyst 2000/2100” system coupled with the “951 Thermogravimetric Analyzer” module.

As shown in (Fig. 3), the catalyst exhibits a high concentration of weak acid sites, along with lower concentrations of medium- and high-strength acid sites. Based on this distribution, the catalyst can be classified as having predominantly weak acidity.

The characteristics of the CoMo/γAl2O3 catalysts are presented in Table 2.

Table 2 Characteristics of industrial and prepared CoMo/γAl2O3 catalysts
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A larger pore volume, as observed in the industrial catalyst, facilitates the diffusion and removal of bulky aromatic compounds and sulfur-containing aromatics present in LCO and cooker gas oil fractions. This structural feature enhances the accessibility of large molecules to the active sites within the porous network, which is essential for hydroprocessing reactions such as hydrodesulfurization and hydrogenation.

Although the industrial catalyst shows a slightly lower specific surface area and somewhat narrower pores compared to the laboratory-prepared variant, its higher pore volume contributes to a longer contact time and improved efficiency in converting voluminous molecules. Therefore, catalytic performance is influenced not only by surface area but also by pore architecture and internal diffusion capacity.

The hydrotreatment experiments were conducted in a micropilot plant equipped with a fixed-bed catalyst reactor (Fig. 4). The total reactor volume was 80 cm3, with 40 cm3 occupied by the catalyst, which was positioned between two layers of inert ceramic material to ensure uniform flow and temperature distribution.

Fig. 4
Fig. 4
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Micropilot plant. 1-Filling layer; 2-Catalyst; A-Hydrogen tank; B-Pressure regulator and safety valve; C-Admission flange with separate valves for hydrogen and feedstock; D-Burette for feedstock; E-Inlet pump for feedstock; F-Supervisory Control and Data Acquisition (SCADA) box; G-Thermocouples; H-Water inlet faucet; I-High pressure separator; J-Dripping valve; K-Collecting vessel; L-Evacuation valve with cooling jacket; M-Flowmeter; N-Electrical heater; O-Computer used for monitoring the micro-plant

The hydrotreating experiments were carried out at three temperatures: 340 °C 360 °C, 380 °C, two liquid hourly space velocities (LHSV) 1 and 1.5 h−1, a pressure of 75 bar, and an H2/feedstock = 500 cm3/cm3 in the presence of two Co-Mo/γAl2O3 type catalysts, one industrial and one prepared.

After separation, the hydrotreated product from the separator vessel is stripped with nitrogen by bubbling (40 l/h) for 1.5 h−1 to remove hydrogen sulfide and retained ammonia. The liquid product resulting after stripping was separated by PRF distillation to eliminate light fractions C5+ to 185 °C.

Product yields were determined through material balance for each experiment. The quantity of raw material was known, and the gasoil and gasoline fractions were measured by weighing, with gasoline being separated through PRF distillation. Gas yields were calculated by difference.

After each experiment, the hydrotreated product with a boiling point higher than 185 °C, corresponding to diesel, was analyzed by establishing the main properties through standardized methods as follow: density (EN ISO 12185), analysis of the composition of different types of aromatic hydrocarbons in gasoil with HPLC method on a Varian ProStar HPLC System (EN 12916/2024), sulfur content by X-ray fluorescence on a PW4025MiniPal device (EN ISO 2084–2004), diesel index ASTM D976, pour point (ISO 3016 / ASTM D97), density (EN ISO 12185), flash point (Pensky–Martens closed-cup test), and measurement for lubrication of diesel HFRR (EN ISO 12156–1:2016).

Results and discussions

The performances of the hydrotreating process of the studied gasoil mixture under the reaction conditions are presented as follows:

It is observed that hydroprocessing mixtures of SRGO, cooking gasoil and LCO results in high yields, exceeding 94%, in the hydrotreated product. At the same reaction temperature, a decrease in LHSV leads to lower yields of hydrotreated diesel, due to the increased reaction time, which favors slower hydrocracking reactions Tables 1 and 2.

For the same reaction conditions, higher yields are obtained on the industrial CoMo/γAl2O3 catalyst than on the prepared one. Lower gasoil yields obtained on the prepared catalyst are justified by its higher activity in secondary hydrocracking reactions (Fig. 5). Due to the intensification of secondary hydrocracking reactions, the yields in the hydrotreated product decrease with increasing reaction temperature, at the same LHSV, for the two catalysts studied (see Tables 3, 4, 5, 6, 7, 8).

Fig. 5
Fig. 5
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Yields in hydrotreated gasoil on two types of catalyst. (CoMo/Al2O3 industrial and prepared, LHSV = 1 h−1 and 1.5 h−1, 340 °C, 360 °C, 380 °C, p = 75 bar)

Table 3 Yields for hydrotreating gasoil mixture on CoMo/γAl2O3industrial catalyst
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Table 4 Yields for hydrotreating gasoil mixture on CoMo/ γAl2O3 prepared catalyst
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Table 5 Characteristics of the products obtained from the hydrotreatment of the gasoil mixture
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Table 6 Characteristics of the products obtained from the hydrotreatment of the gasoil mixture
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Table 7 Characteristics of the products obtained from the hydrotreatment of the gasoil mixture
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Table 8 Characteristics of the products obtained from the hydrotreatment of the gasoil mixture
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The experimental results reveal significant changes in the reaction products compared to the raw material. Notable observations include a decrease in density and molecular weight, a reduction in the content of polyaromatic hydrocarbons, an increase in the aniline point, and a decrease in the pour point. It is important to note that the pour points of the diesel fractions exceed the requirements of the EN 590 standard.

The results of hydrodesulphurization highlighted that the reduction in sulfur content in diesel fuel, imposed by environmental protection regulations, has a negative effect on the lubricity of the diesel (in the experiment, lubricity values were obtained in the range of 360–380 µm), worsening as the degree of desulfurization increases. However, this decrease in lubricity can be compensated by adding biodiesel in varying proportions to achieve values greater than 390 µm.

Regarding the chemical composition of the hydrotreated products, it was observed that increasing residence time at a temperature of 340 °C leads to a decrease in the percentage of aromatic hydrocarbons, due to the intensification of their partial hydrogenation reactions. Additionally, the diesel index increases with longer residence time in the reactor.

It is found that the reduction of sulfur content in the hydrotreating reaction is favored by higher temperatures and LHSV, which lead to an increased hydrodesulphurization reaction rate, longer residence time, and consequently higher conversion rates (Fig. 6).

Fig. 6
Fig. 6
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Variation of sulfur content with temperature on two types of catalyst. (CoMo/γAl2O3 industrial and prepared, LHSV = 1 h−1 and 1.5 h−1, 340 °C, 360 °C, 380 °C, p = 75 bar)

It is also observed that the industrial CoMo/γAl2O3 catalyst exhibits greater activity in hydrodesulphurization reactions compared to the laboratory-prepared CoMo/γAl2O3 catalyst.

For both catalysts, increasing the hydrotreating temperature at the same LHSV results in a lower degree of aromatic saturation due to thermodynamic limitations (Figs. 7 and 8).

Fig. 7
Fig. 7
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Variation of aromatic hydrocarbon content in hydrotreated products at LHSV = 1 h−1. (CoMo/γAl2O3 industrial and prepared, LHSV = 1 h−1 340 °C, 360 °C, 380 °C, p = 75 bar)

Fig. 8
Fig. 8
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Variation of aromatic hydrocarbon content in hydrotreated products at LHSV = 1.5 h−1. (CoMo/γAl2O3 industrial and prepared, LHSV = 1 h−1 340 °C, 360 °C, 380 °C, p = 75 bar)

At constant temperature, a decrease in LHSV leads to an increase in residence time, which favors the slower hydrogenation reactions of di- and tri-cyclic aromatic hydrocarbons (Figs. 7 and 8). It is observed that at higher temperatures, the conversion of di- and tri-aromatics into mono-aromatics decreases. The diesel index increases significantly, in line with the reduction in aromatic hydrocarbon content. The increase in diesel index is more pronounced at lower LHSV. Lower LHSV enhances cracking reactions, which can be attributed to the longer contact time in the catalytic bed, where hydrogenation of these hydrocarbons occurs. The saturation of aromatic compounds also contributes to a reduction in the density of the hydrotreated product. A more substantial decrease in density is observed when processing the mixture over the industrial CoMo/γAl2O3 catalyst at higher temperatures and lower LHSV.

A slight decrease in density is observed with decreasing operating temperature, more pronounced at lower LHSV values, likely due to cracking reactions. The density of diesel fuel decreases as the aromatic hydrocarbon content is reduced and the n-paraffin content increases (Fig. 9).

Fig. 9
Fig. 9
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Density of the products obtained from hydrotreating the gasoil mixture on 2 types of catalyst. (CoMo/γAl2O3 industrial and prepared, LHSV = 1 h−1 and 1.5 h−1, 340 °C, 360 °C, 380 °C, p = 75 bar)

In (Figs. 10 and 11), it can be seen that the diesel index increases with the decrease in diesel blend density for both catalysts. Meanwhile, increasing the LHSV results in a lower diesel index due to the higher aromatic hydrocarbon content.

Fig. 10
Fig. 10
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Variation of diesel index depending on density at 1 h−1. (CoMo/γAl2O3 industrial and prepared, 340 °C, 360 °C, 380 °C, p = 75 bar)

Fig. 11
Fig. 11
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Variation of diesel index depending on density at 1.5 h−1. (CoMo/γAl2O3 industrial and prepared, 340 °C, 360 °C, 380 °C, p = 75 bar)

Conclusions

The experimental study demonstrated higher gasoil yields and a more pronounced reduction in sulfur content when using the industrial CoMo/γAl2O3 catalyst compared to the laboratory-prepared variant. The industrial catalyst exhibited superior activity in hydrodesulphurization reactions, while the prepared catalyst showed greater effectiveness in secondary hydrocracking processes.

The experimental results revealed significant improvements in products characteristics compared to the feedstock, including a reduction in density and molecular weight, a decrease in polyaromatic hydrocarbon content, an increase in the aniline point, and a lower pour point. Also the pour points of the resulting diesel fractions exceeded the limits set by the EN 590 standard.

Processing the gasoil blend using both the industrial and prepared CoMo/γAl2O3 catalysts delivered notable advantages, such as substantial sulfur removal and the effective elimination of di- and tri-cyclic aromatic hydrocarbons. These transformations contributed to the production of high-quality diesel fuels that meet requirements imposed by the standards.

In conclusion, the study successfully achieved its objective of producing diesel fuels with reduced sulfur and aromatic hydrocarbon content by incorporating heavy cooking gasoil and LCO into the feedstock.