Impact of Impregnation pH on NiMo Surface Species in Al₂O₃-Supported Catalysts for Green Diesel Production

Abstract

Green diesel is a high-quality biofuel obtained through the transformation of triglycerides into linear alkanes. In order to obtain green diesel, this study investigates the impact of impregnation pH on the surface species of NiMo/Al₂O₃ catalysts in the hydroprocessing of soybean oil. NiMo catalysts supported on Al₂O₃ were synthesized at different pH values (pH = 7 and 9). In the oxide state, solids were characterized by UV-Vis diffuse reflectance, Raman, and FT-IR spectroscopies, and, in the sulfide state, they were characterized by HR-TEM. The results show that the pH of impregnation significantly determines the surface species formed. An impregnation at pH = 7 favors the formation of Ni²⁺_(Oh) and Ni²⁺_(Oh-dis) interacting with non-crystalline molybdenum trioxide, while the formation of Ni²⁺/Al₂O₃, Ni_2+(Oh-dis), and MoO₃ species is favored at pH = 9. These surface species play a fundamental role in the hydrogenolysis and deoxygenation steps. Catalyst impregnated at pH = 7 shows higher activity due to the formation of shorter MoS₂ slabs. This study emphasized the importance of controlling impregnation conditions for optimizing catalyst performance.

1. Introduction

Biofuels represent a promising solution to meet the increasing global energy demand [1]. Recently, biofuels derived from biomass have gained significant attention, as biomass is an energy source capable of replacing fossil fuels [2,3]. Additionally, biomass is a renewable and sustainable resource [4]. Green diesel has gained recognition as one of the most advantageous biofuels because of its higher oxidative stability, high cetane number, energy density, and reduced corrosiveness [5,6]. Green diesel is a mixture of hydrocarbons that is chemically identical to petroleum diesel [7,8]. It is considered a high-value-added product because it can exhibit similar, or even superior, properties compared to fossil fuels [9,10].

Biofuel is typically produced through the transesterification of vegetable oils [11]. However, this process generates large amounts of glycerol, which can impact the quality of the final product [12]. The hydroprocessing of vegetable oils (HVO) can convert the oils into linear chain alkanes, similar to those found in diesel fuels [13,14]. This process is carried out using heterogeneous catalysts (usually NiMo or CoMo), in a hydroprocessing reactor, at high hydrogen pressures (30–80 bars) and temperatures (320–410 °C) [15,16]. The main goal of HVO is to convert triglycerides into products compatible with fuel supply and storage systems. HVO proceeds through two consecutive reaction mechanisms shown in Figure 1: hydrogenolysis (k₁), which transforms triglycerides into fatty acids (e.g., palmitic linolenic, linoleic, and oleic acids), and deoxygenation (k₂), which converts these fatty acids into linear hydrocarbons such as pentadecane, hexadecane, heptadecane, and octadecane (C₁₅–C₁₈) [17].

Alumina is a material widely employed as a support for catalysts because of its textural properties, high thermal stability, and moderate Lewis acidity. Catalysts such as NiMo/γ-Al₂O₃ and CoMo/γ-Al₂O₃ have been used in HVO reactions to obtain green diesel [12]. Green diesel has been obtained using oils from palm [18], castor [19], colza [20], jatropha [21], sunflower [12], and soybean [22]. In this regard, high-quality diesel can be produced from sunflower oil using a NiMoS/γ-Al₂O₃ catalyst, as mentioned by Huber et al. [12]. On the other hand, Liu et al. [23] reported that sulfurized NiMo catalysts supported on SiO₂-Al₂O₃ can produce pentadecane, hexadecane, heptadecane, and octadecane from Jatropha oil. However, the quality of the resulting green diesel depends on the type of vegetable oil used and the active phases formed on the catalyst [24]. Therefore, the development of more active catalysts able to produce high-quality green diesel is necessary.

The development of more active catalysts largely depends on the precise control of parameters during the catalyst preparation steps. The main steps include (1) preparation of the impregnation solution; (2) impregnation, where species are deposited onto the support; (3) drying and calcination; and (4) sulfurization [25]. In NiMo catalysts, the structure of the Ni-Mo oxide precursors determines the formation of active or nonactive surface species (such as NiAl₂O₄) [26,27]. For this reason, proper interaction between superficial Ni and Mo needs to be maintained in each step of catalyst preparation. This interaction can be influenced by factors such as additives, salts, and the pH of impregnation [28,29]. The pH plays a critical role in determining the type of Ni-Mo species deposited on the surface [30]. Nickel nitrate and ammonium heptamolybdate are commonly used to prepare the conventional impregnation solution, where Mo₇O₂₄⁶⁻/Ni²⁺_(Oh) interaction predominates at pH = 7, while the formation of MoO₄²⁻/Ni²⁺_(Oh) interaction is observed at pH = 9 [31]. These different pH values enable the study of the different surface Ni-Mo interactions. The surface species can be further modified after their deposition onto the support and during the drying, calcination, and sulfidation steps.

In this work, the effect of impregnation pH on the Ni and Mo species deposited on the surface was studied by synthesizing NiMo catalysts supported on Al₂O₃ sol–gel at pH = 7 and 9. The solids were characterized after the drying, calcination, and sulfidation steps to identify the surface species deposited on the support. The as-prepared catalysts were evaluated in a hydrotreating reactor in the hydroprocessing of soybean oil. Finally, the hydroprocessing results were correlated with the species formed as a function of the impregnation pH.

2. Materials and Methods

2.1. Synthesis of the Support (SG)

The Al₂O₃ support was obtained through the sol–gel process. To achieve this, 0.1 mol of aluminum isopropoxide (Sigma-Aldrich, >98%, St. Louis, MO, USA) was added to 0.18 L of 1-propanol (Sigma-Aldrich, >99.5%, St Louis, MO, USA) and stirred until dissolved. Then, 20 mL of deionized water was added to promote the hydrolysis and obtain the xerogel. The xerogel was thermally treated at 393 K for 720 min and then at 823 K for 300 min (5 K min⁻¹). This support is referred to as SG.

2.2. Synthesis of the NiMo Catalyst Supported on SG

Catalysts were synthesized applying the co-impregnation technique, employing an aqueous solution of Ni and Mo to obtain a metallic content of 14 wt% of MoO₃ and 3.1 wt% of NiO. For the impregnation with a solution at pH = 7, ammonium heptamolybdate tetrahydrate (Sigma-Aldrich, >99.98%, St Louis, MO, USA) and nickel nitrate hexahydrate (J.T. Baker, >99%, Phillipsburg, NJ, USA) were dissolved in deionized water. For the impregnation with a solution at pH = 9, a solution of NH₄OH was used to adjust the pH. The impregnated samples were thermally treated at 373 K for 240 min, followed by treatment at 823 K (5 K min⁻¹) for 240 min. Catalysts were activated by sulfidation at 673 K for 240 min in a continuous reactor and a gas flow of 3.5 × 10⁻² L/min (P_H2 = 684 mmHg and P_H2S = 76 mmHg). The catalysts were labeled NiMo/SG7 and NiMo/SG9 for the samples impregnated at pH = 7 or 9, respectively. The endings -D, -C, or -S were employed to denote dried, calcined, or sulfided samples, respectively.

2.3. Characterization Techniques

Samples were analyzed through zeta potential technique (Malvern Zeta90 instrument, Malvern Instruments, Worcestershire, UK). For this purpose, 10 mg of catalyst was suspended in 0.1 L of an aqueous electrolytic solution (0.01 mol L⁻¹ NaNO₃) and treated with ultrasound for 20 min. For N₂ physisorption analysis (ASAP 2020 Micromeritics instrument, Norcross, GA, USA), samples were thermally pretreated at 573 K for 240 min under vacuum at P = 3 × 10⁻⁵ mm Hg. Additionally, the solids were characterized by UV-vis diffuse reflectance spectroscopy using an integration sphere coupled to a spectrophotometer (Lambda 35 PerkinElmer, Waltham, MA, USA), with data presented using the Kubelka–Munk expression as reported previously by [25]. FT-IR (PerkinElmer frontier apparatus with ATR, Waltham, MA, USA) and Raman (BWTEK iRamanPlus spectrometer, B&W TEK, Newark, DE, USA, equipped with an HQE-CCD detector, 532 nm laser, and microscope) spectroscopies were also conducted. Sulfided catalysts were characterized through high resolution electronic microscopy (Thermo Fisher Tecani G2 microscope, Thermo Fisher Scientific, Eindhoven, The Netherlands, 300 kV). For statistical analysis, 10–12 micrographs (2.356 × 10⁻¹⁵ m² each) from several regions of the samples (700 counted particles) were analyzed. The average stacking number was obtained by Equation (1) and the average slab length was obtained by Equation (2) as follows:

$$\overline{N}={\displaystyle \frac{{\sum }_{i=1}^n{n}_i{S}_i}{{\sum }_{i=1}^n{n}_i}}$$

(1)

$$\overline{L}={\displaystyle \frac{{\sum }_{i=1}^n{n}_i{l}_i}{{\sum }_{i=1}^n{n}_i}}$$

(2)

where S_i represents the stacking number, l_i is the length of the MoS₂ slab (both determined from micrographs), and n represents the number of particles measured within a stacking number of index i.

2.4. Catalytic Evaluation

HVO tests were performed in a microreactor of the fixed-bed type. For this, 0.1 g of the sulfided sample was packed between two beds of inert Al₂O₃ (Sigma Aldrich, 99%, St Louis, MO, USA). A liquid stream of 6.7 × 10⁻⁶ L s⁻¹ (10 wt% soybean oil and 0.05 wt% dimethyl disulfide (Sigma-Aldrich, >99%, St Louis, MO, USA) dissolved in n-heptane (Sigma-Aldrich, 99%) and a hydrogen (INFRA, 99.9%) gas stream of 5.8 × 10⁻⁴ L s⁻¹ were mixed and fed to the reactor. Catalytic evaluation was conducted at 40 bar H₂ and 390 °C for 720 min to achieve stable activity. Reactions aliquots were analyzed via FT-IR (PerkinElmer, Frontier, Waltham, MA, USA) and gas chromatography (PerkinElmer, Autosystem XL, Shelton, CT, USA). The quantification of triglyceride and fatty acid concentrations was calculated following the procedure reported by Rivera-Guasco [17].

The hydrogenolysis constant rate (k₁) was calculated using Equations (3) and (4) as follows:

$$x_1={\displaystyle \frac{{Tg}_0-{Tg}_x}{{Tg}_0}}$$

(3)

$$k_1=-{\displaystyle \frac{F_0}{m_c{Tg}_0}}ln(1-x_1)$$

(4)

where x₁ represents the triglycerides conversion, Tg₀ the initial molar concentration (mol L⁻¹) of triglycerides, Tg_x the molar concentration (mol L⁻¹) of triglycerides, F₀ the initial molar flow (mol s⁻¹) of triglycerides, and m_c the catalyst mass (g).

The deoxygenation constant (k₂) was determined using Equations (5) and (6) as follows:

$$x_2={\displaystyle \frac{{FA}_0-{FA}_X}{{FA}_0}}$$

(5)

$$k_2=-{\displaystyle \frac{F_{F_0}}{m_c{FA}_0}}ln(1-x_2)$$

(6)

where x₂ represents the fatty acids conversion, FA₀ the initial molar concentration (mol L⁻¹) of fatty acids, FA_X the molar concentration (mol L⁻¹) of fatty acids, and $F_{F_0}$ the initial molar flow (mol s⁻¹) of fatty acids.

A commercial NiMo/Al₂O₃ catalyst served as the reference for catalytic activity: MoO₃ content = 14 wt% NiO content = 3 wt%, Vp = 0.48 cm³ g⁻¹, and specific surface area = 250 m² g⁻¹.

In the green diesel obtained, the selectivity of hydrocarbons ($S_i$) was calculated as follows:

$$S_i={\displaystyle \frac{A_i}{\sum A_i}}$$

(7)

where $A_i$ is the area of the peak corresponding to the obtained i alkane (n-C₁₈, n-C₁₇, n-C_16, or n-C₁₅).

3. Results

3.1. Surface Characterization

The Al₂O₃ support was characterized by using zeta potential and N₂ physisorption, and the results are shown in Figure 2. The amphoteric character of the Al₂O₃ surface was analyzed by the zeta potential, Figure 2a. A zeta potential value of zero indicates equilibrium between the solution charges and the surface charge of the support [32]. At pH values between 2 and 7, the positive zeta potential values observed indicate that the Al₂O₃ surface possesses a positive charge (Al-OH²⁺) in the presence of an acidic solution (H⁺). At pH values between 8 and 12, the negative zeta potential values indicate that the surface becomes negatively charged (Al(OH)²⁻ or AlOO⁻) due to the presence of OH⁻ ions from the solution [31,32]. The net surface pH of the Al₂O₃ support is 7.8, as shown in Figure 2a. This value is comparable to the one reported by [33]. Based on this, it is suggested that in an impregnation solution at pH = 7, the Al₂O₃ surface is positively charged, allowing interaction with the negative ions in the solution. In contrast, in an impregnation solution at pH = 9, the surface is negatively charged, favoring interaction with the metallic cations in the solution.

Figure 2b shows the N₂ physisorption results of the Al₂O₃ sol–gel. The isotherm presents a type IV profile and an H1 hysteresis loop, usually associated with mesoporous solids composed of cylindric channels or agglomerates of spheroidal particles [34]. From the adsorption isotherm, the following textural properties were obtained: porous diameter: 18 nm, porous volume: 1.7 cm³/g, and specific surface area: 377 m²/g.

3.2. Catalyst Characterization at the Oxide State

3.2.1. UV-Vis DRS Analysis

Alumina was impregnated with two solutions at pH = 7 and 9 to examine the interactions between Ni, Mo, and the alumina surface. The solution at pH = 7 mainly contains a mixture of heptamolybdate ions (Mo₇O₂₄⁶⁻) and Ni complexes with octahedral local symmetry [Ni²⁺6O²⁻], located between 650 and 756 nm, and commonly referred to as Ni²⁺_(Oh). On the other hand, in the solution at pH = 9, the predominant species are MoO₄²⁻ and nickel with tetrahedral local symmetry [Ni²⁺4O²⁻], located at 623 nm, referred to as Ni²⁺_(Td) [27,31].

Figure 3 shows the UV-vis DR spectra of the catalysts after the drying and calcination steps. The alumina support shows no bands. A broad band with a maximum at 300 nm is observed in the NiMo/SG7-D catalyst, corresponding to the O²⁻ → Mo⁶⁺ charge transfer [35], Figure 3a. The band at 370 nm corresponds to the surface species [Ni²⁺6H₂O] [36]. The 650 nm band is related to the Ni²⁺_(Oh) surface species. According to the zeta potential results at pH = 7, the alumina possesses a positive surface charge (+2 mV), which is sufficient for the deposition of ions.

After calcination, the NiMo/SG7-C solid exhibits bands at 320 and 360 nm, corresponding to the O²⁻ → Mo⁶⁺ charge transfer, while the band at 390 nm can be associated with the Ni²⁺/Al₂O₃ species formed from the calcination of [Ni²⁺6H₂O]/Al₂O₃ [35,36,37], as shown in Figure 3a. The formation of nickel with octahedral distorted symmetry, Ni²⁺_(oh-dis), is indicated by the presence of bands at 720 and 820 nm. These bands can be associated with the Ni-Mo interaction [31,32]. For the impregnation at pH = 9, alumina shows a zeta potential value of −10 mV (Figure 2), indicating that the surface is negatively charged with Al(OH)²⁻ or AlOO⁻ species.

The RD-UV-vis spectrum of the NiMo/SG9-D catalyst shows a band at 290 nm due to the O²⁻ → Mo⁶⁺ charge transfer (Figure 3b). The band at 402 nm corresponds to the Ni²⁺/Al₂O₃ spinel, which is favored by the presence of Al(OH)²⁻ or AlOO⁻ surface species. The band at 680 nm is associated with Ni²⁺_(Oh). The calcined catalyst NiMo/SG9-C shows an intense band with a maximum of 336 nm, as shown in Figure 3b. This band can be attributed to both the O²⁻ → Mo⁶⁺ charge transfer and the Ni²⁺/Al₂O₃ spinel. The bands at 750 and 820 nm suggest that the Ni²⁺_(oh-dis) interacts with Mo species [38,39].

3.2.2. FT-IR Analysis

In the supported metal oxide catalysts, spectroscopy analysis provides complementary information regarding the surface species formed [40]. FT-IR spectroscopy allows the analysis of the vibrational modes associated with chemical interactions within the catalyst structures [41], particularly the vibrations of the terminal Mo=O and bridging Mo-O-Mo. The FT-IR spectra of the NiMo/SG7 and NiMo/SG9 catalysts after the drying and calcination steps are shown in Figure 4. All catalysts show a band at 540 cm⁻¹, associated with the vibrations of Mo-O-Mo bridges [42]. Additionally, all samples exhibit a wide band between 820 and 1000 cm⁻¹, which is associated with the Mo=O and Mo-O vibrations of surface metal oxide species [40]. Both dried samples, NiMo/SG7-D and NiMo/SG-9-D, exhibit two broad bands at 1320 and 1410 cm⁻¹, corresponding to ionic nitrates from the precursors [43]. After calcination, these bands are no longer detected in the NiMo/SG7-C and NiMo/SG9-C samples.

3.2.3. Raman Analysis

Catalysts were also characterized by Raman spectroscopy, as shown in Figure 5. The NiMo/SG7-D catalyst shows a band at 942 cm⁻¹, corresponding to the Mo=O_t vibration of the Mo₇O₂₄⁶⁻ species [27,31,44], as shown in Figure 5a. This suggests that the positive charge of Al₂O₃ during the impregnation at pH = 7 facilitates the adsorption of these species. The NiMo/SG9-D catalyst shows a band at 935 cm⁻¹, attributed to the Mo=O_t vibration of the Mo₇O₂₄⁶⁻ species, as shown in Figure 5a. It is important to note that the impregnation solution at pH = 9 mainly contains MoO₄²⁻ species, which polymerize into Mo₇O₂₄⁶⁻ species upon deposition onto the support.

After calcination, the NiMo/SG9-C sample exhibits two bands at 820 and 995 cm⁻¹, which are associated with Mo-O-Mo and Mo=O_t vibrations of the orthorhombic MoO₃ cluster [44,45], as shown in Figure 5b. This result suggests that during impregnation at pH = 9, the formation of the molybdenum cluster is favored by the negative charge on the alumina surface. The NiMo/SG7-C catalyst shows the band at 820 cm⁻¹, which is associated with an amorphous molybdenum trioxide cluster [46], as shown in Figure 5b. In this sample, the band at 995 cm⁻¹ has low intensity due to the formation of a Ni-O-Mo interaction.

Based on the UV-vis, FT-IR, and Raman results, impregnation at pH = 9 induces a negative charge on the Al₂O₃ surface, resulting in the formation of Ni²⁺/Al₂O₃, Ni²⁺_(Oh-dis), and MoO₃ surface species. In contrast, impregnation at pH = 7 results in a positive charge of the Al₂O₃ surface, leading to the formation of surface species such as Ni²⁺_(Oh) and Ni²⁺_(oh-dis), which interact with non-crystalline molybdenum trioxide.

3.3. Catalyst Characterization at the Sulfide State

TEM Analysis

The influence of the impregnation pH on the morphology of MoS₂ particles was analyzed by TEM. Figure 6 shows the micrographs of the NiMo/SG7-S and NiMo/SG9-S catalysts. The dispersion of MoS₂ particles on both catalysts is relatively homogeneous, consisting of one to two slabs and showing the characteristic fringes of the MoS₂ phase. The number of stacked particles per nm², stacking degrees, and average slab lengths are shown in

Table 1. Density of stacks per 1000 nm², average stacking number (N), and average stack length (L) of catalysts.

Catalyst	Stacks per 1000 nm2	N	L (nm)
NiMo/SG7-S	10.9	1.56	2.91
NiMo/SG9-S	10.0	1.54	2.79

.

According to TEM analysis, the number of stacks per 1000 nm² is very similar between both samples, with values of 10.9 and 10.0 for the NiMo/SG7 and NiMo/SG9 catalysts, respectively. This result suggests that the impregnation pH does not significantly affect the stacking density in these catalysts. The stacking degree of MoS₂ particles is also not influenced by the impregnation pH, remaining within a narrow range: 1.56 for NiMo/SG7-S and 1.54 for NiMo/SG9-S. The most significant change observed is in the average slab, where impregnation at pH = 7 favors the formation of longer slabs (2.91 nm) compared to those formed at pH = 9 (2.79 nm).

3.4. Hydroprocessing of Soybean Oil

The impact of the impregnation pH on the activity of sulfided catalysts was analyzed in the hydroprocessing of soybean oil. The results for the hydrogenolysis (k₁) and deoxygenation (k₂) rate constants are summarized in Figure 7. In the first step, the NiMo/SG7-S catalyst exhibits the higher hydrogenolysis rate constant (k₁), with a value of 48.2 × 10⁻⁵ Ls⁻¹g⁻¹, as shown in Figure 7a. This catalyst exhibits a K₁ value 13% higher than the reference catalyst (42.6 Ls⁻¹g⁻¹). One can notice that the NiMo/SG7-S catalyst shows a k₁ value 17% higher than the NiMo/SG9-S catalyst (41.1 × 10⁻⁵ Ls⁻¹g⁻¹). This result suggests that the presence of the Ni²⁺/Al₂O₃ species reduces the activity of the NiMo/SG9-S solid. On the other hand, deoxygenation rate constants (k₂) exhibit values higher than those obtained in the hydrogenolysis step, as shown in Figure 7b. These results suggest that green diesel production is mainly controlled by the hydrogenolysis mechanism. Figure 7b shows that the NiMo/SG7-S catalyst exhibits the highest deoxygenation rate constant (k₂), with a value of 16.3 × 10⁻⁴ Ls⁻¹g⁻¹. This value is even higher than the one observed for the NiMo/SG9-S catalyst (12.8 × 10⁻⁴ Ls⁻¹g⁻¹). In the deoxygenation step, the NiMo/SG7-S catalyst shows a k₂ value higher than the one observed for the reference catalyst (11.8 × 10⁻⁴ Ls⁻¹g⁻¹), suggesting that the NiMo/SG7-S sample has the potential to serve as a catalyst in green diesel production.

The green diesel yield of each catalyst is shown in Figure 8. The NiMo/SG9-S catalyst exhibits a similar yield (27.1%) to the reference catalyst (25.4%). However, the NiMo/SG7-S catalyst shows the highest yield value (37.1%). This result is consistent with the rate values observed previously.

Reaction products were analyzed by gas chromatography. The selectivity results at 390 °C are shown in

Table 2. Hydrocarbon selectivity at 390 °C.

Catalyst	n-C18 (mol%)	n-C17 (mol%)	n-C16 (mol%)	n-C15 (mol%)
NiMo/Al2O3-S	7.3	77.3	4.8	10.3
NiMo/SG7-S	8.3	75.5	5.3	10.6
NiMo/SG9-S	10.3	74.3	5.9	9.2

. All samples exhibit very high selectivity to n-C₁₇, with values of 77.3, 75.5, and 74.3 mol% for NiMo/Al₂O₃-S, NiMo/SG7-S, and NiMo/SG9-S samples, respectively. Notably, both NiMo/SG7-S and NiMo/SG-9-S catalysts show higher selectivity to n-C₁₈ (8.3 and 10.3% mol, respectively) compared to the reference sample (7.3%mol). In HVO, n-C₁₈ is considered the most valuable product [47]. Furthermore, the NiMo/SG7-S sample exhibits higher selectivity to n-C₁₅ (10.6%mol) than the NiMo/SG9-S catalyst (9.2%mol).

4. Discussion

4.1. Effect of NiMo Surface Species on the Hydroprocessing of Soybean Oil

In this work, the Ni and Mo surface species generated during the catalyst synthesis were analyzed. Impregnations with solutions at pH = 7 and 9 allowed for the identification of the Ni-Mo-Al₂O₃ interactions. These interactions are crucial for understanding the behavior of the catalysts during hydroprocessing reactions. The results show that impregnation at pH = 7 causes the Al₂O₃ sol–gel to become positively charged, leading to the formation of the Ni²⁺_(Oh)/Mo₇O₂₄⁶⁻ surface species. These species are transformed into Ni²⁺_(Oh)/molybdenum trioxide (non-crystalline), after the calcination step. In contrast, impregnation at pH = 9 results in a negatively charged alumina surface, leading to the formation of Ni²⁺_(Oh) and Mo₇O₂₄⁶⁻. However, after calcination, the formation of Ni²⁺_(Oh)-crystalline molybdenum trioxide and the Ni²⁺/Al₂O₃ species becomes evident.

In the sulfide state, the activity of the catalysts is influenced by the formation of the so-called NiMoS active phases [9,48], where Ni interacts with the MoS₂ phase. According to HRTEM results, similar dispersion and length of the MoS₂ slabs were observed for both catalysts. However, a deeper analysis of the spectroscopy results suggests that the NiMo/SG9-S catalyst exhibits the formation of the Ni²⁺/Al₂O₃ spinel after the drying and calcination steps. These species limit the availability of Ni to form the NiMoS active phases because the nickel interacts with the support instead of with Mo. Consequently, a decrease in HVO activity is observed, as shown in Figure 7a.

Finally, the green diesel produced through the combination of sol–gel alumina and impregnation at pH = 7 results in the development of a catalyst that is more active than the reference catalyst.

4.2. Future Research Directions

The hydroprocessing of vegetable oil (HVO) is performed in a hydroprocessing unit, which involves multiple variables. As a result, understanding and optimizing all the parameters involved are continuous challenges. Future research should focus on several aspects to improve efficiency:

• Catalyst design and optimization: A crucial aspect of improving HVO production is the development of more efficient catalysts. This includes investigating metal support interactions, optimizing metal loading, and exploring the use of other types of promotors or alternative active phases.
• Alternative feedstocks: The use of alternative feedstocks, such as canola or palm oils, may improve the quality of green diesel. Moreover, using lignocellulosic biomass for biofuel production offers the potential for increasing sustainability.
• Waste feedstocks: The use of waste feedstocks, such as used cooking oils, could provide a more cost-effective alternative for HVO.

5. Conclusions

The impact of impregnation pH on the surface species of Ni and Mo was studied during both the catalyst synthesis and the hydroprocessing of soybean oil. Although different types of catalysts have been used for biofuel production, this study emphasizes that a deeper understanding of the surface species formed provides the basis for proposing and designing strategies for catalyst optimization, favoring the efficient production of green diesel.