Catalytic Hydrotreating of Crude Pongamia pinnata Oil to Bio-Hydrogenated Diesel over Sulfided NiMo Catalyst

Abstract

This work studied the catalytic activity and stability of Ni-MoS₂ supported on γ-Al₂O₃, SiO₂, and TiO₂ toward deoxygenation of different feedstocks, i.e., crude Pongamia pinnata oil (PPO) and refined palm olein (RPO). PPO was used as a renewable feedstock for bio-hydrogenated diesel production via catalytic hydrotreating under a temperature of 330 °C, H₂ pressure of 50 bar, WHSV of 1.5 h⁻¹, and H₂/oil (v/v) of 1000 cm³/cm³ under continuous operation. The oil yield from a Soxhlet extraction of PPO was up to 26 wt.% on a dry basis, mainly consisting of C18 fatty acids. The catalytic activity in terms of conversion and diesel yield was in the same trend as increasing in the order of NiMo/γ-Al₂O₃ > NiMo/TiO₂ > NiMo/SiO₂. The hydrodeoxygenation (HDO) activity was more favorable over the sulfided NiMo supported on γ-Al₂O₃ and TiO₂, while a high DCO was observed over the sulfided NiMo/SiO₂ catalyst, which related to the properties of the support material and the intensity of metal–support interaction. The deactivation of NiMo/SiO₂ and NiMo/TiO₂ occurred in a short period, due to the phosphorus and alkali impurities in PPO which were not found in the case of RPO. NiMo/γ-Al₂O₃ exhibited the high resistance of impure feedstock with excellent stability. This indicates that the catalytic performance is influenced by the purity of the feedstock as well as the characteristics of the catalysts.

1. Introduction

In recent years, the use of uncertain and insecure petroleum energy has been suggested as a major problem causing carbon dioxide emissions, which directly affect global climate change [1,2]. Many researchers have focused on the reduction of carbon dioxide emissions by shifting to renewable fuels. Plant biomass is an important bioresource that can reduce the environmental impact as CO₂ from the atmosphere is used for plant growth. Triglycerides in vegetable oils and animal fats, mainly consisting of C₁₆–C₁₈ fatty acids, have been utilized as important feedstocks for the production of renewable fuels due to their similar carbon chain length to diesel fuel. Generally, edible vegetable oils such as soybean oil [3], rapeseed oil [4], sunflower oil [5], and palm oil [6] have been employed as sources of triglyceride-based feedstock for biofuel production. To avoid the competitive issue of food supplies, several non-food feedstocks, such as inedible byproducts, e.g., palm fatty acid distillate [7,8], non-edible oil (e.g., Jatropha oil, rubber seed oil, and tall oil) [9,10], waste oils (e.g., waste cooking oils and waste animal fat) [11,12], and novel feedstocks (e.g., algae [13]), have received increased attention for biofuel production.

Pongamia pinnata is indigenous to the Indian subcontinent and Southeast Asia. It is a fast-growing plant that is particularly resistant to drought and salt, thus it could be cultivated on marginal land in broad tropical and subtropical areas, potentially reducing the issue of land use. Additionally, it has a high annual oil yield, so oil extracted from Pongamia pinnata seeds has been recently promoted as an energy renewable feedstock in several countries such as India and Australia. In India, it is the second-largest oil crop compared with Jatropha oil [14,15]. The advantages of Pongamia pinnata are high oil content and resistance to the drought-prone environment. The Pongamia pinnata seeds consist of 20%–42% of oil content based on dry weight [16]. In particular, the Pongamia pinnata can produce extracted oil of approximately 3600–4800 L ha⁻¹ year⁻¹ which could be competitive with palm oil of 5950 L ha⁻¹ year⁻¹ and higher than jatropha oil of 1892 L ha⁻¹ year⁻¹ [17]. The dominant fatty acids of extracted oil from Pongamia pinnata seeds are palmitic acid, stearic acid, linoleic acid, and eicosenoic acid, making them promising for biodiesel production [18]. The first-generation of biodiesel is the esterification of fatty acids and transesterification of triglycerides with alcohols, which produce the fatty acid methyl esters (FAMEs), ester-based biodiesel. The second-generation technology is catalytic hydrotreating to produce bio-hydrogenated diesel or green diesel which provides better fuel properties compared with biodiesel, such as no oxygen content, high cetane number, and, in particular, high thermal and oxidation stability [19,20,21,22]. This alkane-based renewable diesel can be produced by the catalytic hydrotreating of triglycerides and fatty acids via three main reaction pathways, including decarbonylation (DCO), decarboxylation (DCO₂), and hydrodeoxygenation (HDO), which lead to the elimination of oxygen in the form of CO and H₂O, or CO₂, or H₂O, respectively, as shown in Equations (1)–(3) [23].

R−CH₂−COOH → R−CH₃ + CO₂

(1)

R−CH₂−COOH → R=CH₂ + CO + H₂O

(2)

R−CH₂−COOH + 3H₂ → R−CH₂−CH₃ + 2H₂O

(3)

Additionally, the generated CO and CO₂ can further react with water or H₂ via water gas shift (Equation (4)) and methanation (Equations (5) and (6)) reactions.

CO + H₂O ↔ CO₂ + H₂

(4)

CO + 3H₂ ↔ CH₄ + H₂O

(5)

CO₂ + 4H₂ ↔ CH₄ + 2H₂O

(6)

The bimetallic sulfide catalysts such as NiMoS₂, CoMoS₂, and NiWS₂ supported on zeolites, carbon, Al₂O₃, SiO₂, and TiO₂ are suitable for deoxygenation of triglycerides due to high selectivity toward hydrodeoxygenation at moderate temperatures [24,25]. Among them, supported NiMoS₂ catalysts showed the highest catalytic activity for the reaction. Al₂O₃ was selected as a support material because it has moderate acidity and a high surface area [26]. SiO₂ shows a high surface area and pores volume, assisting the diffusion of the triglyceride molecules to the catalyst active sites [27]. TiO₂ can facilitate the transformation of MoO₃ to MoS₂ which acted as an active species of the catalyst [28]. The long-term stability test was previously reported using the supported NiMo catalysts. The effect of reaction time on hydrocarbon conversion was investigated in the range of 80–120 h. Gong et al. [10] investigated the lifetime of sulfided NiMoP/Al₂O₃ catalyst using Jatropha oil as a feedstock under a temperature of 350 °C and 30 bar of H₂ pressure. It was found that the catalyst was deactivated after a reaction time of 120 h due to the transformation of sulfide species to oxide species of the catalyst. Toba et al. [11] studied the addition of sulfur in feed during the deoxygenating of waste cooking oil over NiMo/B₂O₃-Al₂O₃ catalysts. The ratio of n-C₁₇/(n-C₁₇ + n-C₁₈) remained constant at 20% for 80 h on-stream. It was confirmed that the sulfur leaching in the liquid product was high at the initial stage of the reaction, meanwhile, the sulfur contamination was low and the catalyst activity remained constant after 20–80 h on-stream. However, to our knowledge, the direct comparison of the effect of support materials including Al₂O₃, SiO₂, and TiO₂ on catalytic activity and stability tests over typical NiMo bimetallic sulfide catalysts especially on unpurified crude has not been fully investigated in the literature. In addition, Pongamia pinnata is currently a feedstock of interest for ester-based biodiesel production but there are rather limited studies on its conversion to bio-hydrogenated diesel [29,30,31]. Moreover, there is a lack of study on the catalyst stability by using Pongamia pinnata oil as a feedstock.

Therefore, this research focuses on the hydrotreating of Pongamia pinnata oil to produce bio-hydrogenated diesel over sulfided NiMo supported on different industrially important support materials, i.e., γ-Al₂O₃, SiO_2, and TiO₂. Different behaviors of catalytic activity and catalyst stability among these supports are highlighted. Moreover, refined oil such as palm olein was also used as a feedstock to reveal the effect of feedstock on these supports. The physical and chemical properties of the synthesized catalysts were characterized by XRD, H₂-TPR, and N₂ sorption.

2. Materials and Methods

2.1. Materials

The γ-Al₂O₃, TiO_2, and SiO₂ supports were purchased from Alfa Aesar Chemical Co., Ltd. (Ward Hill, MA, USA). The catalyst precursors, Ni (NO₃)₂·6H₂O (purity = 97.0%) and (NH₄)₆Mo₇O₂₄·4H₂O (purity = 81.0%–83.0%) were purchased from Ajax Finechem Pty Ltd. (New South Walesm, Australia). The standard mixture for ASTM-2887 was supplied by Restek Co., Ltd. (Bellefonte, PA, USA). Hydrogen gas (99.99 vol%) was purchased from Linde Public Co., Ltd. (Bangkok, Thailand). Refined palm olein was obtained from Patum Vegetable Oil Co., Ltd. (Bangkok, Thailand).

2.2. Oil Extraction

The Pongamia pinnata seeds were crushed and sieved to a particle size of less than 1 mm and further dried in an oven overnight at 110 °C in order to remove moisture content. Subsequently, the extraction was employed using a Soxhlet extractor with n-hexane as solvent. Briefly, 400 g of dried Pongamia pinnata seeds and 3 L of n-hexane were loaded into the extraction thimble. An extraction time of 12 h was required per batch. Finally, the obtained solutions were further evaporated in order to recover n-hexane using a rotary evaporator. The percentage of oil extraction was calculated using the following equation.

$$\%\mathrm{Oil}\mathrm{extraction}=\frac{\mathrm{weight}\mathrm{of}\mathrm{the}\mathrm{extracted}\mathrm{oil}}{\mathrm{dry}\mathrm{weight}\mathrm{of}\mathrm{Pongamia}\mathrm{pinnata}\mathrm{seeds}}\times 100$$

(7)

2.3. Catalysts Preparation

The γ-Al₂O₃, SiO₂, and TiO₂ supports were crushed and sieved to 0.3–1.0 mm diameter. The catalysts were prepared by an impregnation method using nickel (II) nitrate hexahydrate and ammonium heptamolybdate tetrahydrate as the corresponding metal salt precursors. After impregnation, the resultant samples were dried at 110 °C overnight and then calcined at 500 °C for 5 h with a ramping rate of 5 °C min⁻¹ under air to obtain NiMo supported catalysts with Mo and Ni loading of 9.4 wt.% and 2.4 wt.%, respectively. The amount of metal was loaded according to our previous work [7].

2.4. Catalysts Characterization

The X-ray diffraction (XRD) patterns of prepared samples were obtained using an X-ray diffractometer (Bruker D8 Advance, Karlsruhe, Germany). The experiments were performed with CuKα radiation and over the 2θ ranges from 20° to 80°.

The specific surface area, total pore volume, and mean pore size diameter were determined from the N₂ physisorption using a BELSORP mini II device (BELJapan Inc., Osaka, Japan). Prior to the experiments, the moisture on the catalyst surface was removed by a pretreatment system at 220 °C for 3 h under a helium flow rate of 50 cm³ min⁻¹. The average pore size of samples was calculated from the desorption branch of isotherm by the BJH method.

The reduction behavior and reducibility of catalysts were measured by the H₂-temperature-programmed reduction (TPR) using Autochem 2910 (Micromeritics, Norcross, GA, USA) with 10% H₂ in N₂. Prior to the experiments, the samples were pretreated at 150 °C for 2 h with a ramping rate of 10 °C min⁻¹ under 30 cm³ min⁻¹ of a N₂ flow. The hydrogen consumption was analyzed using a thermal conductivity detector (TCD).

2.5. Catalytic Hydrotreating of Extracted Oil from Pongamia pinnata Seeds

The catalytic hydrotreating of extracted oil was carried out in a continuous fixed-bed reactor with an internal diameter of 0.9 cm. The catalyst (3.5 g) was loaded into the reactor and presulfided using a mixture of 3 wt.% carbon disulfide in n-heptane. The presulfidation was conducted under a H₂ flow rate of 100 cm³ min⁻¹ with a pressure of 40 bar. The temperature was increased from 30 to 400 °C with a ramp rate of 10 °C min⁻¹. Subsequently, the heptane/carbon disulfide solution was supplied to the reactor with a flow rate of 0.17 cm³ min⁻¹ for 3 h. In the catalytic reaction test, the reactor was heated to 330 °C and pressurized with H₂ to 50 bar controlled by a back-pressure regulator. A high-pressure pump was used to introduce the extracted oil feed, while the H₂ feed was controlled by a mass flow controller. After sulfidation, the catalysts were stabilized by flowing the feed through the reactor for 9 h before actual experiments were implemented. The conditions of the hydrotreating were temperature of 330 °C, H₂ pressure of 50 bar, WHSV of 1.5 h⁻¹, and H₂/oil ratio of 1000 cm³/cm³, which were chosen based on our previous research [32]. The liquid product was collected for analysis. The reaction test was conducted by using fresh catalysts to study the effect of catalyst deactivation during the experiments, and no extra sulfur was supplied during the reaction for all experiments.

2.6. Analysis of the Extracted Oil from Pongamia pinnata Seeds and Hydrotreating Products

The percentage of fatty acid content in extracted oil was determined following the ASTM D664. The fatty acid composition of the extracted oil and the product identifications were confirmed by gas chromatography-mass spectroscopy analysis equipped with a capillary column (TR-FAME column, 30 m × 0.25 m × 0.25 µm). The elemental analysis including carbon, hydrogen, nitrogen, and sulfur in the extracted oil was characterized by CHNS/O analysis using a Thermo Scientific Flash 2000 elemental analyzer (Thermo Fisher Scientific, Waltham, MA, USA).

The gaseous products were analyzed by gas chromatography with a thermal conductivity detector (TCD) with a Porapak Q column (30 m × 0.25 mm × 0.25 µm). The injection and detection temperatures were maintained at 150 °C. The oven temperature was increased from 40 to 100 °C with a ramping rate of 20 °C min⁻¹ and held at 100 °C for 11 min. The liquid products were analyzed by using gas chromatography (Shimadzu GC-14B, Kyoto, Japan) equipped with a flame ionization detector (FID) and a DB-2887 column (10 m × 0.53 mm × 3.00 µm). Simulated distillation (according to ASTM D2887) [33,34] based on the relationship between retention times and boiling ranges was employed to determine the boiling range distribution of the liquid products. The injector and detector temperatures were maintained at 350 °C and 320 °C, respectively. The temperature program was increased from 40 to 340 °C with a ramping rate of 15 °C min⁻¹ and held at 340 °C for 20 min. The fatty acid and triglycerides conversion, gasoline selectivity, and diesel selectivity were calculated from simulated distillation data using the following equations:

$$\mathrm{Conversion}(\%)=\frac{{\mathrm{Feed}}_{\mathrm{FA}+\mathrm{TG}}{-\mathrm{Product}}_{\mathrm{FA}+\mathrm{TG}}}{{\mathrm{Feed}}_{\mathrm{FA}+\mathrm{TG}}}\times 100$$

(8)

where

${\mathrm{Feed}}_{\mathrm{FA}+\mathrm{TG}}$ is the weight percent of fatty acid and triglyceride in the feed.

${\mathrm{Product}}_{\mathrm{FA}+\mathrm{TG}}$ is the weight percent of fatty acid and triglyceride in the product.

$$\mathrm{Gasoline}\mathrm{selectivity}(\%)=\frac{{\mathrm{Product}}_{{\mathrm{C}}_5–{\mathrm{C}}_{11} }{-\mathrm{Feed}}_{{\mathrm{C}}_5–{\mathrm{C}}_{11}}}{{\mathrm{Feed}}_{\mathrm{FA}+\mathrm{TG}}{-\mathrm{Product}}_{\mathrm{FA}+\mathrm{TG}}}$$

(9)

$$\mathrm{Diesel}\mathrm{selectivity}(\%)=\frac{{\mathrm{Product}}_{{\mathrm{C}}_{12}{\text{–}\mathrm{C}}_{22}}{-\mathrm{Feed}}_{{\mathrm{C}}_{12}{\text{–}\mathrm{C}}_{22}}}{{\mathrm{Feed}}_{\mathrm{FA}+\mathrm{TG}}{-\mathrm{Product}}_{\mathrm{FA}+\mathrm{TG}}}$$

(10)

where

${\mathrm{Feed}}_{{\mathrm{C}}_5–{\mathrm{C}}_{11}}$ is the weight percent of hydrocarbons at a boiling point in the range of C₅–C₁₁ in the feed.

${\mathrm{Product}}_{{\mathrm{C}}_5–{\mathrm{C}}_{11} }$ is the weight percent of hydrocarbons at a boiling point in the range of C₅–C₁₁ in the product.

${\mathrm{Feed}}_{{\mathrm{C}}_{12}{\text{–}\mathrm{C}}_{22}}$ is the weight percent of hydrocarbons at a boiling point in the range of C₁₂–C₂₂ in the feed.

${\mathrm{Product}}_{{\mathrm{C}}_{12}{\text{–}\mathrm{C}}_{22}}$ is the weight percent of hydrocarbons at a boiling point in the range of C₁₂–C₂₂ in the product.

$$\mathrm{Oil}\mathrm{phase}\mathrm{fraction}=\frac{\mathrm{weight}\mathrm{of}\mathrm{oil}\mathrm{phase}\mathrm{product}}{\mathrm{weight}\mathrm{of}\mathrm{feed}}$$

(11)

Gasoline and diesel yield can be calculated by:

Gasoline yield = oil phase fraction × conversion × gasoline selectivity

Diesel yield = oil phase fraction × conversion × diesel selectivity

Yield of liquid fuel = diesel yield + gasoline yield

The functional group of the Pongamia pinnata oil and liquid products was identified by the Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer spectrum GXFT-IR).

3. Results

3.1. Extraction of Pongamia pinnata Oil and Characterization of Oil Feedstock

From the extraction of oil using hexane as a solvent, the yield of the extracted oil from Pongamia pinnata seed was approximately 26% based on the dry weight of Pongamia pinnata seeds. Free fatty acids contained in the Pongamia pinnata oil were 5.7 wt.%. The fatty acid composition of Pongamia pinnata oil and refined palm olein is summarized in

Table 1. The fatty acid distributions of feedstocks.

Fatty Acid (wt.%)	Pongamia pinnata Oil	Refined Palm Olein
C12:0 (Lauric acid)	0.0	0.4
C14:0 (Myristic acid)	0.0	0.8
C16:0 (Palmitic acid)	15.1	37.4
C16:1 (Palmitoleic acid)	0.0	0.2
C18:0 (Stearic acid)	4.7	3.6
C18:1 (n-9 cis, Oleic acid)	54.4	45.8
C18:2 (n-6 cis, Linoleic acid)	13.5	11.1
C18:3 (n-3, Linolenic acid)	0.2	0.3
C20:0 (Arachidic acid)	1.1	0.3
C20:1 (n-9, Eicosenoic acid)	1.5	0.1
C22:0 (Behenic acid)	8.3	0.0
Percent of fatty acid (wt.%)	5.7	<1

. The major compositions of the fatty acids (>85 wt.%) are oleic acid, palmitic acid, and linoleic acid, making them promising for diesel application.

The elements contained in the extracted oil were characterized by CHNS-O analysis as shown in

Table 2. The elemental analysis of the feedstocks and liquid products of supported sulfide NiMo over γ-Al₂O₃, SiO_2, and TiO₂ catalysts.

Sample		C (%)	H (%)	N (%)	S (%)	O (%)
Extracted oil		76.99	12.05	0.28	2.16	8.52
Hydrotreated extracted oil	NiMoS/γ-Al2O3	81.66	14.75	0.48	2.65	0.46
	NiMoS/SiO2	79.73	12.78	0.11	2.07	5.31
	NiMoS/TiO2	81.31	15.97	0.02	2.56	0.14

. It should be noted that the extracted oil before the hydrotreating process contained high oxygen content due to the major triglyceride and free fatty acid composition of Pongamia pinnata oil.

3.2. Catalyst Characterization

The N₂ adsorption–desorption measurements were conducted to examine the textural properties of the catalysts.

Table 3. Physicochemical properties of the calcined catalysts.

Catalysts	Surface Area (m2 g−1)	Total Pore Volume (cm3 g−1)	Mean Pore Diameter (nm)
γ-Al2O3	240	0.50	9.2
NiMo/γ-Al2O3	209	0.40	8.0
SiO2	289	0.94	13.9
NiMo/SiO2	234	0.79	12.7
TiO2	155	0.38	6.2
NiMo/TiO2	147	0.33	5.4

summarizes the BET specific surface area, pore volume, and pore diameter of the bare supports and synthesized samples.

The surface area decreased in the following order: NiMo/SiO₂ > NiMo/γ-Al₂O₃ > NiMo/TiO₂ which was in a similar trend corresponding with their bare supports. The surface area, total pore volume, and mean pore diameter slightly decreased after loading with NiMo for all supports. The adsorption–desorption isotherms of the prepared samples are shown in Figure 1. It was found that the calcined NiMo/γ-Al₂O₃, NiMo/SiO₂, and NiMo/TiO₂ exhibited type IV isotherm, indicating the characteristics of a mesoporous structure. This result is consistent with the pore sizes of all the catalysts observed by the BET, which were in the range of 5–13 nm. The hysteresis loop consists of adsorption and desorption branches depending on the pore shape of the catalyst.

The phase identification and crystallinity of the synthesized catalyst were analyzed through XRD. The XRD patterns of the calcined, sulfided, and spent NiMo on γ-Al₂O₃, SiO₂, and TiO₂ catalysts are represented in Figure 2a–c. The peaks at 2θ = 32.60°, 38.49°, 56.7°, 61.37°, and 67.94° corresponding to the (220), (222), (400), (511), and (440) phase assigned to γ-Al₂O₃ were observed for the calcined and sulfided samples as shown in Figure 2a [10,11,35]. Besides, the XRD patterns of NiMo supported on TiO₂ samples exhibited the rutile phase peak at 36° (101) and 55° (211) and anatase phase peak at 25° (101) and 48° (200), suggesting the mixed-phase of TiO₂ in this present experiment [8,36].

Figure 2b shows the XRD patterns of the calcined samples. It was confirmed that the MoO₂ and Mo₄O₁₁ phases were observed in the NiMo/SiO₂ and NiMo/TiO₂ samples [37]. The XRD pattern of NiO [38,39] was found in NiMo/SiO₂. However, the MoO₃ peak could not be clearly detected for all the catalysts [40]. After the presulfidation treatment, the characteristic peaks of MoS₂ were obviously observed over NiMo/SiO₂ (Figure 3) at 33.5°and 58° corresponding to the (100) and (110) phase, respectively [41,42,43]. On the other hand, the peaks of MoS₂ could not be observed in NiMo/γ-Al₂O₃ catalyst because this peak overlaps with the support. In the case of the NiMo/TiO₂ catalyst, the XRD pattern showed highly dispersed MoS₂ on TiO₂ support.

The H₂-TPR experiments were employed to investigate the reducibility and metal-support interaction of the prepared samples. The H₂-TPR profiles of the samples are shown in Figure 3. The main reduction peak was located between 300 and 450 °C which might correspond to the simultaneous reduction of NiO and NiMoO₄ [44,45]. The TiO₂-supported catalyst exhibited a weak metal–support interaction which was the most facile to reduce (reduction temperature of 350 °C). Meanwhile, the reduction peaks of γ-Al₂O₃ and SiO₂-supported catalysts were shifted to higher reduction temperature probably due to the highly dispersed smaller-size metals on the γ-Al₂O₃ and SiO₂ support [46], leading to the strong interaction between metal and support compared to the TiO₂. The appearance of shoulder reduction peaks between 500 and 600 °C was attributed to the several reduction steps of MoO₃, which could be involved in the reduction of MoO₃ to MoO₂ and finally MoO₂ to Mo [44,45,47]. The high reduction temperatures above 650 °C were assigned to the reduction of all highly dispersed Mo species. As a result, NiMo/γ-Al₂O₃ was the most difficult one to reduce which could reach full reduction at higher 800 °C, probably due to the NiAl₂O₄ and Al₂(MoO₄)₃ formation [44].

3.3. Catalytic Hydrotreating of Pongamia pinnata Oil

The triglycerides and fatty acids conversion, diesel selectivity, and gasoline selectivity over the sulfide NiMo/γ-Al₂O₃, NiMo/SiO₂, and NiMo/TiO₂ catalysts after stabilizing the reaction for 9 h are shown in Figure 4. The conversions of Pongamia pinnata oil of the sulfide NiMo/γ-Al₂O₃, NiMo/SiO₂, and NiMo/TiO₂ catalysts were 97.2%, 52.5%, and 84.4%, respectively. Moreover, the diesel selectivity was in a range of 95–99% for all the catalysts. The higher diesel selectivity was obtained for the sulfided NiMo/γ-Al₂O₃ and NiMo/TiO₂ catalysts compared with the sulfided NiMo/SiO₂ catalyst. Besides, the gasoline selectivity was slightly higher for the sulfided NiMo/SiO₂ catalyst (approximately 5%), suggesting the cracking reaction to lighter hydrocarbons (mixture of n-C₅ to n-C₁₁) occurred over the catalyst.

Generally, the catalytic hydrotreating process could convert the triglycerides and fatty acids into a mixture of straight-chain alkanes. The oxygen atoms in the triglycerides and fatty acids were eliminated in the forms of CO, CO_2, and H₂O via decarbonylation, decarboxylation, and hydrodeoxygenation, respectively. The loss of carbon atoms by CO and CO₂ generation during deoxygenation can cause more mass loss of the yield of the liquid product than that of O removal via hydrodeoxygenation.

As seen in Figure 5, the maximum theoretical yields of deoxygenated products were calculated in order to investigate the catalyst activity by assuming no side reaction occurrs. The values would be different depending on the type of starting materials and the reaction pathways (DCO/DCO₂ and HDO). In this study, the maximum theoretical yields were calculated based on the compositions of 94.3 wt.% of triglyceride and 5.7 wt.% of fatty acid contained in the extracted Pongamia pinnata oil, which were 80.5 wt.% for DCO/DCO₂ and 85.2 wt.% for HDO. The desired product yield (diesel + gasoline) of the sulfided NiMo/γ-Al₂O₃ catalyst almost reached the theoretical value comparable to the other two catalysts, due to its high deoxygenating performance and low cracking activity.

The hydrocarbon distribution in the liquid product of sulfided NiMo/γ-Al₂O₃, NiMo/SiO₂, and NiMo/TiO₂ catalysts is summarized in

Table 4. Composition of the liquid product from the catalytic hydrotreating of Pongamia pinnata oil over the sulfided NiMo catalysts.

Catalysts	Liquid Product Composition (%)
	<n-C15	n-C15	n-C16	n-C17	n-C18	n-C19	n-C20	n-C21	n-C22	>n-C22
NiMo/γ-Al2O3	0.12	3.54	8.45	22.10	50.67	1.13	2.24	2.69	5.64	3.43
NiMo/SiO2	0.61	10.82	1.95	61.20	10.52	1.88	0.55	6.47	1.78	4.21
NiMo/TiO2	0.27	1.80	10.58	10.33	61.84	0.55	2.58	1.55	6.74	3.76

. n-C₁₇ and n-C₁₈ hydrocarbons are predominant hydrocarbon compositions corresponding to oleic acid, the main feed composition. Higher n-C₁₈ (>50 wt.%) was observed over the γ-Al₂O₃ and TiO₂ support catalysts, suggesting that HDO reaction is preferred for these two catalysts. On the other hand, DCO and DCO₂ pathways were enhanced over the sulfided NiMo/SiO₂ catalyst as evidenced by the high amount of n-C₁₇ (>60 wt.%) in the liquid product. This result can also explain the highest gas yield produced over the SiO₂-support catalyst due to the elimination of oxygen atoms in triglycerides and fatty acids in the forms of CO and CO₂.

Considering the hydrocarbon distribution above, the DCO/DCO₂ and HDO reactions occurred simultaneously over these three catalysts but with different selectivities. As obviously seen in Figure 6, the ratio of n-C_n-1/n-C_n in the liquid product was examined to represent the selectivity of DCO/DCO₂ and HDO. The value of n-C_n-1/n-C_n decreased in the following order: NiMo/SiO₂ > NiMo/γ-Al₂O₃ > NiMo/TiO₂. For the γ-Al₂O₃- and TiO₂-supported catalysts, low ratio values (less than 0.5) were achieved, while the highest value of almost 6 was obtained over the SiO₂-supported catalyst. This can be described by the influence of Ni and Mo contents. Typically, Ni catalyst is favorable for DCO/DCO₂ pathway and also enhances the cracking reaction, higher Ni offers higher DCO/DCO₂ activity, which leads to the formation of C_n-1 hydrocarbon corresponding to the carbon contained in the feedstock. Meanwhile, the activity of the HDO reaction would be enhanced by introducing Mo species [44,48,49]. Imai et al. [49] investigated the effect of Ni, Mo, and NiMo supported on alumina for the hydrotreating of methyl laurate. They found that the product distributions were different over Ni, Mo, and NiMo catalysts. However, a sulfided NiMo/γ-Al₂O₃ catalyst exhibited the highest catalytic activity due to the synergistic effects between Ni and Mo species. In addition, the NiMo/γ-Al₂O₃ with high Mo content could enhance the conversion and the HDO reaction, as well as suppress the hydrocracking and methanation reaction.

In our investigation, the reaction was carried out with the same amount of Ni (2.4 wt.%) and Mo (9.4 wt.%) loadings; however, the product distributions were rather different. This phenomenon is probably caused by the various interactions between the supports and the metal-oxide/sulfide species. As previously stated, HDO selectivity is preferable over the MoS₂ active site; however, as seen from the TPR (Figure 3), there was the formation of different types of metal oxide phase which could affect the MoS₂ slab formation during the sulfidation treatment, resulting in distinctions in HDO and DCO/DCO₂ selectivities, which have been extensively studied [50,51,52,53]. According to the literature, the most active phase, Type II, which is highly stacked and totally sulfided, can be formed at a weak metal–support interaction, while Type I, which is less stacked and partially sulfided, can be formed by strong interaction with the support. Based on our results, the TiO₂-supported catalyst exhibited less metal–support interaction, resulting in more MoS₂ active phase formation which leads to relatively high HDO activity compared to the other two catalysts. Meanwhile, the DCO/DCO₂ selectivity was highly active over the SiO₂-supported catalyst which could be ascribed to the MoS₂ suppression by the strong metal–support interaction.

Interestingly, the SiO₂-supported catalyst contains a relatively high surface area (see Table 1) which could be beneficial to catalytic activity by increasing the exposure of the active sites [46]. However, the conversion of triglyceride and fatty acid over the SiO₂-supported catalyst is relatively low compared to the value obtained from the γ-Al₂O₃ and TiO₂-supported catalysts. This result may be caused by the specific properties of the supporting materials. The deoxygenation activity also involves an acidity property of the catalyst. Mild acidity is required for the high deoxygenation activity, due to its favorable adsorption of triglycerides, fatty acid, and other oxygen-containing molecules [54,55,56], whereas the strong acidity promotes unwanted hydrocracking and increases coke formation, leading to catalyst deactivation [57]. There have been several reports indicating that high acidity support has a beneficial effect on hydrodeoxygenation activity. Kumar et al. [38] studied the impact of Ni supported on γ-Al₂O₃, SiO₂, and HSZM-5 zeolite on stearic acid deoxygenation. They observed that utilizing the HSZM-5 catalyst resulted in the maximum conversion, which was attributed to the relatively strong acidity of the HSZM-5 catalyst compared to that of γ-Al₂O₃ and SiO₂. This consequence was also observed by Peng et al. [58]. They studied the deoxygenation of palmitic acid with Ni supported over γ-Al₂O₃, SiO₂, and ZrO₂ and determined that conversion proceeded in the following order: ZrO₂ > γ-Al₂O₃ > SiO₂ due to the greater acidity of the ZrO₂ support. According to the literature, infrared spectroscopy of adsorbed CO [59] and NH₃-temperature-programmed desorption [60] were used to study the acidity of SiO₂. There were no substantial numbers of acid sites for the SiO₂ support. While the three varieties of Lewis acid sites (weak, medium, and strong) were found on γ-Al₂O₃ support [57,59,61], the medium-weak forms of Lewis acid were found on anatase TiO₂ support [54,55]_. Taking into account all of these remarks, the XRD results revealed that the TiO₂ employed in this work had a mixed-phase (anatase + rutile); consequently, the acidity of the mixed-phase TiO₂ was apparently lower than that stated in the literature. It is worth noting that bare Al₂O₃ and TiO₂ have been reported to exhibit conversion and hydrodeoxygenation activity due to their acid-based characteristics; however, their activity is much higher once loaded with active metal [62,63,64,65]. While bare SiO₂ poses very low acidity (inertness) and no significant activity has been observed when employing bare SiO₂.

As a result, it might be speculated that the acidity of the catalyst increased in the following order: NiMo/SiO₂ < NiMo/TiO₂ < NiMo/γ-Al₂O₃, which corresponds to our conversion (NiMo/SiO₂ < NiMo/TiO₂ < NiMo/γ-Al₂O₃). Consequently, we hypothesize that the observed disparities in reaction pathways are not only driven by the type of active phase but also the acidity of the supporting materials.

Figure 7 shows the gas product compositions which mainly consist of CO, CO₂, and CH₄. The CO and CO₂ could be generated via DCO and DCO₂, whereas CH₄ could be produced from the cracking of products in the liquid phase and also a methanation reaction in the gas phase. Based on the equilibrium constant for reverse water gas shift (RWGS) reaction, the calculated value at 330 °C is approximately 0.04, suggesting that the RWGS can be neglected to convert CO₂ to CO. Hence, CO would be a primary gas product which is mainly generated from the oxygen removal of triglycerides and fatty acid via DCO. Therefore, the level of CO and CO₂ content can elucidate the activity of DCO and DCO₂ reactions of the three catalysts [8]. By comparison, the CO level was higher than the CO₂, indicating that DCO has higher activity than DCO₂ for all the catalysts. Due to the low methane percentage in the gaseous product, the methanation process might be minimal. However, a small amount of methane was generated when the NiMo/γ-Al₂O₃ catalyst was in use. This might be attributed to the high acidity of γ-Al₂O₃ support, which enhances the adsorption of triglyceride and fatty acid molecules in feedstock, potentially accelerating methane generation. This finding might support our previous assumption concerning the acidity of the catalysts. It is worth noting that the amount of methane from NiMoS of all these supports is much less pronounced than single Ni [66] or single Mo [67] in various active forms.

In order to confirm the elimination of oxygen atoms in triglycerides and fatty acids, the functional groups of the liquid products were characterized by Fourier transform infrared (FTIR) spectroscopy (see Figure 8). The absorption peak at 1704 cm⁻¹ in the FTIR spectrum corresponds to the carboxylic functional groups of fatty acids. On the other hand, the absorption peaks at 1746 and 1164 cm⁻¹ in the FTIR spectrum are assigned to the carbonyl groups (–C=O stretch) and ester groups (–C–O– stretch), respectively [41]. In the hydrotreating process, triglyceride was firstly hydrogenolysed to fatty acids and further deoxygenated to n-alkane. The FTIR spectrum of Pongamia pinnata oil exhibited the absorption peaks corresponding to triglyceride and fatty acids. Considering the liquid product after the hydrotreating process, the peaks intensity of triglyceride and fatty acid in the FTIR spectrum significantly decreased over sulfided NiMo supported on γ-Al₂O₃ and TiO₂ catalysts. This result suggested that the triglyceride and fatty acid were almost completely converted to n-alkanes. On the other hand, the peak intensity of fatty acid was detected in the liquid product when the reaction was catalyzed by the sulfided NiMo/SiO₂ catalyst.

The results of the elemental composition (CHNS/O) of extracted Pongamia pinnata oil and liquid products are summarized in Table 2. Thereafter the deoxygenation activity, and percent of carbon atom content in liquid product increased with decreasing oxygen concentration. The deoxygenation degree calculated based on the oxygen content of feedstock and liquid product is increased as follows: NiMo/SiO₂ (37.7%) < NiMo/γ-Al₂O₃ (94.6%) < NiMo/TiO₂ (98.3%). These results are attributed to the various intensities of metal-support interaction, as mentioned earlier. However, the relatively high catalytic performance of the NiMo/γ-Al₂O₃ compared to NiMo/TiO₂ may be due to its higher surface area and greater mesopore size.

The H/C and O/C ratios were shown on the Van Krevelen diagram (Figure 9) to compare the elemental compositions of hydrotreated liquid products and extracted oil with petroleum fuel and hydrotreated vegetable oil (HVO) from Neste Oil. Pongamia pinnata oil had the highest O/C ratio of 0.08, which is consistent with the high amount of oxygen-containing molecules (triglycerides and fatty acid) content. In comparison to the starting feedstock, the hydrotreated products of the three catalysts displayed greater H/C and lower O/C values, particularly for the γ-Al₂O₃ and TiO₂-supported catalysts, which were close to the value of petroleum-based oil (H/C = 1.6–1.8, O/C almost nil) [68,69]. Furthermore, the H/C value of NiMo/γ-Al₂O₃ (H/C = 2.17) is comparable to the value of HVO generated from Neste oil (H/C = 2.15) [70]. The SiO₂-supported catalyst had a much higher O/C value than the other catalysts, which could be attributed to poor deoxygenation activity and strong cracking activity, as evidenced by the high gas and short hydrocarbon fraction generated. While a high H/C value ratio suggested the production of a liquid product with a high hydrogen content, this might be attributed to high hydrogenation and low cyclization/aromatization activities.

Surprisingly, slightly higher amounts of sulfur content were detected in the hydrotreated liquid products. They were 2.65%, 2.07%, and 2.56% for the NiMo/γ-Al₂O₃, NiMo/SiO₂, and NiMo/TiO₂ catalysts, respectively (Table 2). It is even higher than the value detected in the extracted Pongamia pinnata oil (2.16%). This result could demonstrate that there was some sulfur leaching from the NiMoS catalysts during the reaction, as the amount of sulfur content decreased after the reaction process (

Table 5. Sulfur content of sulfide catalysts initially and after 15 h of the reaction analyzed by energy dispersive X-ray (EDX).

Catalysts	Sulfur (%)
Sulfided NiMo/γ-Al2O3	12.922
Spent NiMo/γ-Al2O3	9.701
Sulfided NiMo/SiO2	15.130
Spent NiMo/SiO2	13.014
Sulfided NiMo/TiO2	11.965
Spent NiMo/TiO2	12.311

). In addition, the loss of sulfur in the spent catalysts, on the other hand, could be accounted for by the oxidation of the sulfide phase, which would be incorporated by oxygen atoms at the edge sulfur atoms in the MoS₂ during the reaction.

3.4. Catalyst Stability and Deactivation

The catalytic stability of the NiMo/γ-Al₂O₃, NiMo/SiO_2, and NiMo/TiO₂ catalysts was evaluated through 15 h time-on-stream using extracted Pongamia pinnata oil at a temperature of 330 °C, H₂ pressure of 50 bar, WHSV of 1.5 h⁻¹, and H₂/oil ratio of 1000 cm³/cm³. As can be seen in Figure 10a, The NiMo/γ-Al₂O₃ exhibited the highest conversion and stability for 15 h, meanwhile, the NiMo/SiO₂ and NiMo/TiO₂ catalysts gradually decreased in conversion. On the other hand, unexpectedly, the NiMo/γ-Al₂O₃ and NiMo/SiO₂ catalysts showed good catalytic activity (100% conversion) and remained stable for 42 h when using refined palm olein as feedstock (Figure 10b). This may be caused by the impurities in the Pongamia pinnata oil (

Table 6. The impurities in the feedstocks.

Sample	Concentration (ppm)
	Na	K	Ca	Mg	P
Pongamia pinnata oil	113	168	231.5	108.3	509.5
Refined palm olein	-	-	<0.1	<0.5	<2.5 [77]

) which were quantified using the inductively coupled plasma (ICP) technique. According to Kubicka and Horacek [71], an abundance of alkali content in feedstock may have a deleterious influence on the catalyst. They discovered that the presence of alkalis (Ca, K, Na, and Mg) in waste rapeseed oil increased the rate of CoMoS/γ-Al₂O₃ catalyst deactivation due to their deposition on the catalyst surface, resulting in active site blockage/poisoning. Similarly, Arora et al. [72] studied the effect of potassium-containing feedstock and discovered that when the quantity of potassium is high, the catalyst activity is inhibited. This is due to potassium atoms preferentially adsorbed on the edge vacancy sites of the MoS₂ slabs.

The effect of potassium doping on MoS₂ for CO hydrogenation has also been studied using DFT calculation by Andersen et al. [73]. Their findings revealed that doping K on MoS₂ enhances surface basicity owing to increased electron charge, and also blocks the Mo and S edge, inhibiting CO adsorption and limiting hydrogen accessing the MoS₂ surface. In agreement with May et al. [74,75], they found that modifying the CoMoS/Al₂O₃ commercial catalyst with potassium resulted in decreased hydrogenation activity for a synthetic FCC gasoline, due to changing electronic properties of sulfide phase and also decreasing the acid site of support. Furthermore, calcium has been correlated to a reduction in catalyst activity, particularly HDS and HDO activities, as well as a minor decrease in hydrogenation activity [76]. Comparing these results between using Pongamia pinnata oil and refined palm olein, it is presumed that the suppression on catalytic activity is considerably more pronounced with NiMo/SiO₂ than with NiMo/γ-Al₂O₃ and NiMo/TiO₂. These results were probably caused by the partially facilitated by the γ-Al₂O₃ and TiO₂ and catalyzed by their acid sites. Furthermore, the prolonged activity of NiMo/γ-Al₂O₃ for Pongamia pinnate oil was tested, which revealed that the catalyst activity dropped after 75 h, while the stability of the catalyst for refined palm olein was still longer, over 75 h.

Taking into account the presence of phosphorus (P) in feedstock, it was reported to promote oligomerization/polymerization reactions, resulting in increased formation of high molecular weight compounds, eventually leading to increased carbonaceous deposition on the surface of the catalyst and rapid catalyst deactivation [71,78]. As can be seen in Figure 11, significantly greater carbonaceous deposition was discovered during the hydrotreating of Pongamia pinnata oil than in the hydrotreating of refined palm olein in our study. TGA was performed in atmospheric air to quantify the formation of carbonaceous species on the surface of catalysts. The first weight loss at temperatures below 200 °C can be ascribed to evaporation of moisture adsorbed, whereas temperatures between 300 and 450 °C can be attributed to volatile substance decomposition. At higher temperatures ranging from 450 to 800 °C, it is ascribed to the decomposition of various carbonaceous species including heavy hydrocarbons accumulated inside the catalyst pores or more stable amorphous coke (450–500 °C), as well as the oxidation of graphite and graphene carbon (600–800 °C) [79,80]. The decomposition of a substrate at a temperature above 800 °C could be related to the oxidation of inorganic carbon (i.e., CaCO₃ [80] and K₂CO₃ [81]). Evidently, larger weight losses were detected in the catalyst employed for Pongamia pinnata oil, especially when compared to the SiO₂- and TiO₂-supported catalysts, showing that the presence of phosphorus content might accelerate the rate of coke formation.

Ultimately, it is possible to conclude that differences in catalytic performance are caused not only by different characteristics of metals and supporting materials but also by the purity of feedstock, which appears to have a significant impact on declining catalytic activity and catalyst deactivation. Catalyst deactivation during 42 h of time-on-stream when using refined feed (refined palm olein) could be negligible for all three catalysts. However, the difference in deactivation was more pronounced when being tested under unrefined with high metal content, e.g., P. pintnata oil. Though tested under unrefined is found to be an aggressive condition for coking, it is likely that catalyst poisoning and coking is not the main cause of the deactivation.

4. Conclusions

The Pongamia pinnata oil was extracted using Soxhlet extraction with hexane as the solvent. On a dry basis, the oil production was approximately 26 wt.%. Oleic acid, palmitic acid, and linoleic acid were the key fatty acid compositions (>85 wt.%), which made them attractive for biodiesel production. The bio-hydrotreated diesel was successfully produced through the hydrotreating of Pongamia pinnata oil in a fixed-bed reactor. To investigate the effect of different supporting materials, γ-Al₂O₃, SiO₂, and TiO₂ were all loaded with the same quantity of Ni-Mo. In terms of conversion and diesel yield, the hydrotreating activity increased in the following order: NiMo/γ-Al₂O₃ > NiMo/TiO₂ > NiMo/SiO₂. The metal–support interaction of each catalyst results in the activity and selectivity pathway due to the difference of MoS₂ active phase formation. The SiO₂-supported catalyst shows low activity with a high DCO selective pathway, leading to the formation of n-C₁₇ hydrocarbon as the main product. The HDO pathway is suppressed as a result of the strong metal–support interaction, which has a deleterious influence on the formation of the MoS₂ active phase. While the γ-Al₂O₃- and TiO₂-supported catalysts had a higher selectivity for the HDO pathway, which produces n-C₁₈ hydrocarbon as the main product. This was probably owing to the greater number of MoS₂ active sites in weak supporting interaction. The best performance of NiMo/γ-Al₂O₃ may be attributed to the synergistic effect of pore size, acidity, and support interaction. The long-term stability of the three catalysts in Pongamia pinnata oil was investigated compared to that in refined palm olein. In refined palm olein, 100% conversion was obtained using NiMo/γ-Al₂O₃ and NiMo/SiO₂. This is due to the presence of phosphorus and alkali impurities in Pongamia pinnata oil, which could accelerate the formation of coke and poisoning or blocking of the active site. As a result, excellent catalytic performance depends not only on the properties of catalyst materials but also on the quality of the feedstock.