Modified Polymetallic Zeolite-Based Catalysts for Hydroprocessing Diesel Oil Fraction and Tetradecane

According to international standards, motor fuels require limits on the content of sulfur, benzene, aromatic and olefinic hydrocarbons. To achieve this goal, it is necessary to create new, active and selective catalysts and improve technologies for deep hydrotreating and hydroprocessing. Tetradecane and diesel oil fraction hydroprocessing on modified zeolite-containing Ni(Co)-Mo(W)/Al₂O₃-ZSM-5 catalysts were studied under varying process conditions. The novelty of the work lies in the synthesis of new catalysts based on ZSM-5 type zeolite, modified with metals Ni(Co)-Mo(W) and active additives of rare earth metals and phosphorus. After hydrotreating the diesel oil fraction on the catalyst KT-3, the pour point decreases from -27 to -57°C, the sulfur level decreases from 0.141 to 0.059%, and the yield of the liquid phase is 92.2%. The lowest content of residual sulfur (0.005%) is observed on the KT-4 catalyst at 400°C. Electron microscopic examinations have demonstrated that the modified zeolite-containing catalysts are distributed in a highly dispersed manner, and the metal components in the active phase are primarily in the oxidized state, generating associate clusters on the surface, the dispersion, structure and state of which is influenced by the catalyst components' characteristics. The developed modified zeolite-containing catalysts have multifunctional properties and simultaneously carry out the reactions of hydrocracking, hydroisomerization, hydrodesulfurization and dehydrogenation.

Catalyst; Diesel fraction; Hydroprocessing; Motor fuel; Tetradecane

The efficient use of petroleum feedstock is potentially achieved by catalytic processing of hydrocarbons into practical important products. In this context, a key source of energy is diesel fuel, which is economical, environmentally friendly, practical, and generally effective in many industrial processes requiring large amounts of energy. Diesel engines are capable of providing stable and reliable power for a variety of industrial applications.

The growing trend of performance requirements for motor fuel underscores the need to develop new technological and environmental solutions for oil refining. This fuel is the main component of plastics, synthetic fibers, resins, rubbers, dyes, surfactants, and pharmaceutical products.
    Hydrotreating stage in petroleum raw material processing ensures the production of clean motor fuel that meets environmental standards. Further intensification of production relies on developing new, more active, and selective catalysts. In this context, an important and topical issue is the formation of efficient catalytic systems for hydroprocesses. Zeolite catalysts, particularly when based on highly silica zeolite of the pentasil family, are a promising material in the petrochemical and oil refining industry (Suhartono et al., 2023). These catalysts are widely used due to the unique microporous structure and acid-base properties, which enable the conversion of light alkanes into valuable products during petrochemical synthesis (Zhang et al., 2023).
    Various studies have synthesized hydrocracking catalysts with different zeolites to investigate the role of modified zeolite-Y in crude oil hydroconversion (Ding et al., 2021). The results showed that catalysts with high acidity, strong acid sites, and mesopores could improve crude oil conversion.
     A previous study (Huseynova et al., 2022) provided an overview of zeolite-based catalysts used in the alkylation of benzene and toluene with olefins, isobutane, and butenes with butane-butene fractions, gasoline and oil fractions with olefins, as well as propane-propylene and butane-butylene fractions of catalytic cracking. Developing novel technologies in this field requires a new generation of catalysts for the processing of hydrocarbon raw materials. International standards mandate that motor fuel contains a significant amount of sulfur, benzene, aromatic hydrocarbons, and olefin hydrocarbons.
    Catalytic hydroprocessing of sulfurous and paraffinic oils, comprising hydrotreatment, hydroisomerization, and hydrogenation, achieves a high degree of quality for commercial products. Hydrodesulfurization, a chemical change that removes sulfur from gas and refined crude products such as gasoline, jet fuel, kerosene, alongside diesel and fuel, upgrades the octane number of the resolvent streams (Sikkander et al., 2022).
     A study by (Majodina et al., 2023) compared traditional hydroprocessing and recently improved catalysts, detailing the chemical considerations underlying the selection of mineral materials used in both. Furthermore, investigations into the electronic interactions of more economical and abundant metals including Nb, V, and Fe with other elements and supporting materials have led to a better understanding of the synergistic effects that help access noble metal-like properties.
    To increase the production of petroleum products, expand the range, and improve quality, it is necessary to change the existing oil refining technology using highly efficient catalysts. A highly promising CoMo catalyst for hydrotreating low-pressure diesel fuel on carriers made of thermally activated aluminum hydroxide was investigated in comparison with a commonly imported analog (Salnikov et al., 2023). Previous studies also developed technologies suitable for processing rehydrated pseudoboehmites from flash-calcined aluminum hydroxide to prepare catalysts for hydrotreating light and heavy oil fractions (Bayanov et al., 2023). Modification with metals enhances catalyst acidity, improving catalytic performance, conversion, and selectivity/yield toward the product (Mavai, 2022). To advance the field of study, new multifunctional catalysts are needed to effectively hydrotreat diesel fractions in a single stage. This entails removing sulfur-containing compounds, hydrogenating unsaturated and aromatic compounds, hydroisomerization, and selective hydrocracking of n-paraffin hydrocarbons (Tuktin et al., 2021).
     The development of new and highly efficient catalysts for hydroimprovement of diesel fractions is crucial (Janardhan, Shanbhag and Halgeri, 2014; Zhang et al., 2010; Saih and Segawa, 2009; Rodriguez-Castellon, Jimenez-Lopez, and Eliche-Quesada, 2008). A previous study (Fitri et al., 2022) used citronella oil as a dietary supplement to diesel fuel. Citronella fractions and oil have shown great potential as bio-additives to diesel fuel, evidenced by acceptable density and viscosity in tested various concentrations (0.1 - 0.5%). Co-Ni/HZSM-5 catalyst with a hierarchical porous structure was tested for hydrocracking of vegetable oils at a temperature of 400°C, and a pressure of 20 bar for 2 hours. The liquid product had similar hydrocarbon compounds to petroleum diesel fuel, with the most common being pentadecane and heptadecane (Marlinda et al., 2022).
     The influence of technological regimes on the yield and hydrocarbon composition of products formed during the cracking of commercial and M-100 fuel oil in the presence of air in the reactor was studied by (Shakiyeva et al., 2022). Natural Taizhuzgen zeolite and Narynkol clay were used to prepare catalysts.
       Sulfide catalysts were reportedly obtained through mechanochemical combination of commercial powders including molybdenum, cobalt, and nickel (Fedushchak et al., 2019). The study discussed the activity of catalysts in model hydrodesulfurization reactions of dibenzothiophene and 4,6-dimethyldibenzothiophene as well as in hydrotreating S-components of diesel fraction. Based on previous reports, the higher activity of Ni-based catalysts is due to the superior hydrogenizing capacity. Studies by (Altynov et al., 2023; Aleksandrov, Buhtiyarova and Reshetnikov, 2019) examined the behavior of CoMo/Al₂O₃ catalyst in hydrotreating a straight-run diesel fraction with a high sulfur content (more than 2 wt.%) mixed with light gas oil in the temperature range of 335 - 365°C at a volumetric feed rate of 0.8 - 2.5 h^-1. Adding gas oil to the straight-run diesel fraction during hydrotreating has diverse effects based on the temperature and feed rate of the raw materials (Tanimu et al., 2022).
     A study examined catalytic abilities of trimetallic NiMoWS catalysts supported on aluminum oxide during hydrotreating of straight-run diesel fraction (Nikul’shina et al., 2019). The results showed that the oxide precursor’s nature had a significant effect on catalytic activity. Mixing n-heptane, n-dodecane, tetralin, and decalin with diesel fuel linearly changes density, viscosity and improves atomization (Wei et al., 2022). The study by (Jaroszewska et al., 2021) showed that catalysts containing titanium (HTiMCM-41 and NiMo/HTiMCM-41) improved textural properties, as well as acidity and binding energy with the metal substrate than Al-based analogs (HAlMCM -41 and NiMo/HAlMCM-41). Replacing aluminum or titanium in modifying MCM-41 zeolite significantly affects the properties and activity of Ni catalysts.
     Typical transition metal sulfides Ni/Co-promoted Mo, as well as W-based bi- and tri-metallic catalysts, are used for selective removal of sulfur from refractory compounds. The review (Prihadiyono et al., 2022; Leon et al., 2019) examined three very specific topics of catalysts to produce ultra-low sulfur diesel. Furthermore, (Winarto et al., 2024) explored a biphasic hydrogenation approach using solid NR dissolved in a solvent and a hydrogen source (hydrazine hydrate and hydrogen peroxide) mixed in water. The choice of solvents, catalysts, and the water-to-solvent volume ratio were examined for the impact on hydrogenation.    
    Studies on developing effective catalysts and technologies to process diesel fractions into valuable chemical products and motor fuel are crucial (Laredo et al., 2020; Kar, Göksu and Yalman, 2018). The production of high-quality commercial petroleum products can be achieved using catalysts prepared based on aluminosilicates with a zeolite structure of the ZSM-5 type for hydrorefining diesel oil fractions (Tuktin et al., 2022).

    This study investigated hydroprocessing of tetradecane and diesel oil fraction oil on zeolite catalysts modified using metals of variable valence. A series of new catalysts with matrix structure Ni(Co)-Mo(W)/Al₂O₃-ZSM-5 were synthesized as shown in Table 1.

Table 1 Composition of the developed catalysts

The synthesis of modified zeolite-based catalysts was performed by saturating a mixture of peptized aluminum hydroxide and zeolite ZSM-5 (China) with aqueous solutions of metal salts. These include (NH₄)₁₀W₁₂O₄₁×5H₂O (Cherkasy chemical reagents plant, Russia), Ni(NO₃)₂×6H₂O (Ural Chemical Plant, Russia), Co(NO₃)₃×6H₂O (Novosibirsk Rare Metals Plant, Russia), (NH₄)₆M_O7O₂₄×4H₂O (Cherkasy chemical reagents plant, Russia), La(NO₃)₃×6H₂O (Novosibirsk Rare Metals Plant, Russia) and modifying additives. After molding, catalysts were dried at 150°C and calcined at 550°C for 5 hours. Before the experiments, catalysts were subjected to preliminarily sulphidation to increase activity in the reactor. The synthesized catalysts were tested through hydroprocessing tetradecane and diesel oil fraction using flow-through installation (Figure 1) with a stationary catalyst bed. The testing conditions included temperature ranging from 320-400°C, hydrogen pressure of 4.0 MPa, and a volumetric feed rate of 2 h^-1.

Hydrocarbon composition of reaction products was analyzed using Khromatek-Kristall and Khromatek-Kristall 5000 chromatographs (Russia). The chromatograph calculates the fractional composition automatically. For the analysis of hydrocarbons, a glass column 3 m long, 4 mm in diameter, filled with g-Al₂O₃ was used. The integral selectivity (S) of aromatization, dehydrogenation, and cracking was calculated using the formula S = Y/X, where Y is the yield of products; and X is the feedstock degree of conversion.

The analysis of sulfur content, pour point, as well as cloud point of diesel oil fraction and products of hydroprocessing was carried out at Oilsert International LLP (Almaty, Kazakhstan). Various methods were used to analyze the physical and chemical characteristics of the developed catalysts. Surface area and porosity were measured by BET method on an AccuSorb unit manufactured by Micromeritics (USA), while electron microscopy was performed using an EM-125K transmission electron microscope (Williams and Carter, 2009). Microdiffraction images were interpreted using standard ASTM tables. To examine the number of acid sites and their strength distribution, the method of temperature-programmed desorption of ammonia was used (Mustafayeva, 2012).

Figure 1 Scheme of a laboratory installation of hydroprocessing, where 1 – burette, 2 – pump; 3, 7, 12, 14 – valves; 4, 8 – pressure gauges; 5 – hydrogen cylinder; 6 – reducer; 9 – reactor; 10 – refrigerator; 11 – rotameter; 13 – separator; 15 – flask

    The development of new catalysts effective for processing low-solidification diesel oil fractions into high-octane products is currently a major concern. The original diesel oil fraction has the following characteristics: pour point -27°C, cloud point -18°C, and a high content of n-alkanes, which solidifies at higher temperatures compared to branched analogs. In addition, the sulfur content is approximately 0.141%.

Tetradecane, an n-alkane, and a component of diesel oil fraction was used as a hydrocarbon model. The conversion of tetradecane on the developed modified zeolite-based catalysts was studied by varying the technological parameters of the process.

During hydroprocessing of n-tetradecane on KT-1 catalyst (Figure 2), the liquid catalysate formed n-alkanes, isoalkanes, olefins, aromatic hydrocarbons, and naphthenes. With an increase in temperature from 320 to 400°C, the degree of conversion increased from 41.2 to 74.5%. The yield of the liquid phase decreased from 92.3 to 54.1% with an increase in temperature. In addition, the yield of isoalkanes and aromatic hydrocarbons on KT-1 catalyst increased from 12.9 to 22.3% and 5.5 to 18.3%, respectively. Based on the results, the content of olefins and naphthenic hydrocarbons varied insignificantly.

On KT-2 catalyst, hydroprocessing of tetradecane led to an increased content of isoalkanes, rising from 15.0 to 35.3% as the temperature rose from 320 to 400°C, while the amount of aromatic hydrocarbons decreased from 5.1% to 6.3%. During hydroprocessing of tetradecane on synthesized catalysts, the highest yield of isoalkanes was observed on KT-3 catalyst at 400°C. Based on the results, a sharp rise was observed in the yield of isoalkanes from 17.1 to 37.7% with an increase in temperature.

Compared to other catalysts, KT-4 produced a smaller quantity of olefinic hydrocarbons as reaction products. A rise in the temperature from 320 to 400°C led to a change in the conversion from 38.8 to 83.4%. Under these conditions, isoalkanes and aromatic hydrocarbons increased from 13.4 to 30.5% and 6.2 to 19.2%, respectively, while the naphthenes content decreased from 3.0 to 2.3%.

For all catalysts studied, the degree of tetradecane conversion increased with rising reaction temperature. Based on the results, the developed modified zeolite-based catalysts showed polyfunctional properties, simultaneously facilitating isomerization of n-alkanes, dehydrogenation, and dehydrocyclization.

Figure 2 Results and composition of tetradecane hydroprocessing products using catalysts CoO-MoO₃-P₂O₅-La₂O₃/Al₂O₃-ZSM-5 (KT-1), CoO-WO₃-P₂O₅-La₂O₃/Al₂O₃-ZSM-5 (KT-2), NiO-MoO₃-P₂O₅-La₂O₃/Al₂O₃-ZSM-5 (KT-3) and CoO-NiO-MoO₃-P₂O₅-La₂O₃/Al₂O₃-ZSM-5 (KT-4)

Catalysts were evaluated during hydroprocessing of diesel oil fraction (Figure 3). At a temperature of 400°C, the lowest sulfur content (0.005%) was observed on KT-4 catalyst. The pour point of diesel oil fraction decreased by 15 - 30°C compared to the feedstock after hydroprocessing on the developed catalysts.

Figure 3 Results of diesel oil fraction hydroprocessing on KT-3 and KT-4 catalysts

KT-3 catalyst produced the most significant drop in pour point. After hydroprocessing of diesel fraction at 400°C, the pour point dropped to -57°C, and the yield of the liquid phase was 92.2%.

During hydroprocessing of diesel fraction on the developed catalysts, simultaneous reactions of hydroisomerization, hydrotreatment, and hydrocracking were observed. For example, after hydroprocessing of diesel oil fraction on KT-3 catalyst at 400°C, the sulfur content decreased from 0.141 to 0.059%, and the pour point reduced from -27 to -57°C. On KT-4 catalyst at 400°C, the reduction reached 0.005% and -55°C respectively.

Catalyst activity is influenced by surface structure, composition, and state of active sites. Different methods can be used to investigate the physicochemical traits of catalysts including BET, temperature-programmed desorption of ammonia, and electron microscopy.

BET analysis showed that the surface area of the developed catalysts ranged from 221.0-285.0 m²/g and the pore diameter was between 1.5-3.5 nm. Furthermore, the acid-base characteristics of catalysts are essential for hydrocarbon processing process. In a previous study (Wei et al., 2020), Ni-Mo catalysts supported on Ni-modified ZSM-5 zeolites prepared by co-impregnation method showed higher stability and isomerization selectivity in n-octane hydroconversion. This enhanced property was attributed to the synergistic effect between Brønsted acid sites and Lewis acid sites on catalysts. The acid-base properties of catalysts were determined through temperature-programmed desorption of ammonia (Table 2).

Table 2 Acid-base properties of catalysts

The data showed that acid centers with a desorption temperature of 195°C predominated on the surface of KT-1 The data showed that acid centers with a desorption temperature of 195°C predominated on the surface of KT-1 catalyst, with a concentration of 23.4×10^-4 mole/g. The amount of ammonia desorbed from the surface of KT-2 catalyst was 28.0×10^-4 mole/g and the concentration of acid sites on KT-3 catalyst was 32.5×10^-4 mole/g. On the surface of KT-4 catalyst, ammonia was adsorbed in two forms with T_max equal to 185 and 225°C. KT-3 catalyst, characterized by the highest concentration of acid sites (32.5×10^-4 mole/g) and an average binding energy corresponding to desorption temperature of 220°C, demonstrated high hydroisomerization activity in hydroprocessing of tetradecane and diesel oil fraction. Metals with different degrees of oxidation can be found in the composition of acid sites, attached both inside and on the outer surface of the zeolite cavities, ensuring the multifunctionality of catalytic system.

Catalyst activity is influenced by the surface structure, phase composition, and the state of modifying additives. In the study by (Abdullaev et al., 2021), the surface of SSZ-13 zeolite was modified with varying amounts (1-15%) of tungsten oxide, demonstrating significantly improved selectivity and yield of propylene from ethylene. This enhancement was attributed not only to softer and reduced strong acid sites but also to limited diffusion of bulky products, as confirmed by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (STEM-EDS) data. An electron microscopy study was carried out to examine the structure and state of active centers in catalysts promoted by Co, Ni, Mo, and W, among others.

Electron microscopic studies showed that on the surface of KT-1 catalyst (Figure 4a), extensive accumulations of small particles were observed with diameter 3.0 - 5.0 nm, corresponding to a mixture of MoOPO₄, La₂O₃, MoO₅, and La₂MoO₃ phases. Small accumulations of highly dispersed particles with 8.0 - 10.0 nm in size were also found, which could be attributed to La₄(P₂O₇)₃. Furthermore, there were small transparent aggregates with d 20.0 nm related to LaAlO₃ in the La° mixture. The emergence of La° could be connected to redox processes occurring between the active phase components.

The nature of the components in complex polymetallic catalysts for hydroprocessing of hydrocarbons has a significant effect on the dispersion and state of active centers. Formations with diameter 4.0 - 6.0 nm, consisting of AlLa₃, Co₂O₃, Co₂SiO₃, and La₆O₁₁, were prevalent on the surface of KT-1 catalyst (Figure 4b). Cobalt formed single structures of metallic Co° with a diameter of 2.5 nm on the surface. Additionally, there were accumulations (d  15.0 nm) of small particles (d  2.5 nm) consisting of CoSi, CoSi₂, and MoPCo₂; lamellar particles (d  15.0 nm) such as MoPCo₂Si, CoO, and Mo₃Si; semitransparent structures (d  2.5 nm) namely MoO(OH)₂, Co₂O₃, MoP₂, Co₃Al₃Si₄, and SiP₂O₇; alongside particles with d  6.0 - 10.0 nm, including LaP, MoO₃, and Mo₃Si. Accumulations containing Co₂Mo₃O₈, LaP₂O₇, AlPO₄, CoOOH, MoOPO₄, Co(P₂O₇), and La(MoO₄)₂ had particles with different shapes in the range of 50-200 nm in diameter. In addition, there were particles 4-5 nm in size including La₄(P₂O₇)₃, CoSi, MoP, MoSi₂, La₆O₁₁, and CoMoP₂

Highly dispersed structures of AlNi, Ni₂O₃, Mo₃O₅, and Mo₅Si₃ with d  2.0 nm were predominant on the surface of KT-3 catalyst (Figure 5). There were well-spaced small accumulations of MoNiSi, Ni₂O₃, MoSi₂, and M₆O₁₁ particles with diameter 6.0 - 10.0 nm. In addition, the oxidized states of nickel Ni₂O₃ formed single islands, with a size ranging from 5.0 to 10.0 nm (Figure 5a). KT-3 catalyst was defined by clusters measuring 3.0 - 4.0 nm, formed by fine particles with d = 0.05 nm, containing NiSi₂ and Ni₂O₃. Particles with hexagonal faceting and diameters between 15.0 and 30.0 nm were also found, composed of AlNi₂Si, AlNi, La₂O₃, MoO(OH)2)₂, AlMo₃, MoSi₂, and Al₃La (Figure 5b). The discovered structures of AlNi₂Si, AlMo₃, AlNi, MoSi₂, and LaAlO₃ NiSi₂ indicated the incorporation of metal components from the active phase into the zeolite structure with the formation of new centers that could work as Lewis acid centers.

During hydroprocessing on polyfunctional catalysts with dehydrogenating, hydrogenating, and acidic abilities, n-alkane dehydrogenation occurs first on the metal centers, and the olefin produced on the acid center is converted into a carbonium ion, which is easily isomerized (Tuktin et al., 2019). Mo₅O₇(OH)₂, MoO₃, NiMO₄, CoOOH, NiOOH, AlLa₃, P and La, MoNiP, Ni₂O₃ particles with diameter ranging from 10.0 - 15.0 nm along the edge of dense and large aggregates were discovered on the surface of KT-4 catalyst (Figure 6a). Additionally, numerous loose formations of Mo₃O₅, Mo₅Si₃, Mo₆O₁₁, Al₃Ni, Ni₂O₃, and PLa₂ were observed with diameter of 3.0 - 5.0 nm. There were also clusters of AlNi₂Si and Mo₃O₅ particles with diameters of 3.5 - 4.0 nm (Figure 6b).

Electron microscopy studies showed that particles on the surface of the developed catalysts were highly dispersed, with sizes ranging from 2.0 to 10.0 - 20.0 nm depending on the nature of the modifier metal and the amount of zeolite introduced. All the studied catalysts were characterized by the incorporation of modifier metals into the matrix structure with the formation of CoSi, CoSi₂, MoPCo₂Si, Co₃Al₃Si₄, Mo₃Si, Co₂Si, Mo₅Si₃, AlNi, MoNiSi, MoSi₂, AlNi₂Si, and W(Si, Al)₂. These structures can function as Lewis acid sites, while the phosphorus-containing centers including SiP₂, PLa₂, MoNiP, LaP, and Co₂P are also quite strong acidic centers. The modification of catalysts leads to the formation of highly dispersed heteronuclear clusters of complex composition, which are active in hydrotreatment and hydroisomerization reactions of diesel oil fraction (Gackowski et al., 2020).

Studies have identified several types of surface structures found on the surface of these catalysts, differing significantly in size and chemical state of components. For instance, a small amount of hierarchical Y zeolite (10 %) mixed with alumina, nickel nitrate, and molybdenum oxide in the light diesel fuel hydrocracking catalyst, played a key role in selectively hydrogenating naphthalene and further ring-opening activity. The mesoporous structure of zeolite provided an effective interface and improved the accessibility of acid sites for bulk reagents (Zhang et al., 2020).

According to electron microscopy data, the active phase catalyst components are mostly in the oxidized state, creating associate clusters on the surface. The dispersion, structure, and nature of the modifying additives in catalyst determine the active state.

In conclusion, this study evaluated hydroprocessing of tetradecane and diesel oil fraction using new modified zeolite-based Ni(Co)-Mo(W)/Al₂O₃-ZSM-5 catalysts under varying process conditions. The developed catalysts used in hydroprocessing diesel oil fraction showed polyfunctional properties, simultaneously facilitating hydrocracking, hydroisomerization, hydrodesulfurization, and dehydrogenation reactions. These results suggest that modification using metals with variable valence allows proper control of hydroisomerization, hydrodesulfurization, and hydrocracking activity of catalysts in hydroprocessing of diesel oil fraction, facilitating the production of fuel with low sulfur and pour point.

The authors are grateful to the Committee of Science, the Ministry of Science and Higher Education of the Republic of Kazakhstan for funding this study through the project “ARO8857065 Creation of scientific foundations for the development of new efficient catalysts and technology for deep hydroprocessing of vacuum gas oil to produce high-quality motor fuel” (2020-2022).

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