Catalytic Deoxygenation of Lipids for Bio-Jet Fuel: Advances in Catalyst Design and Reaction Pathways

Abstract

To address global climate change and the energy crisis, there is an urgent need to meet human demands through utilizing renewable energy sources. The deoxygenation of lipids to produce liquid biofuels has emerged as a promising alternative, particularly for carbon emission reduction in the aviation industry. This review critically examines recent progress in catalyst development and reaction control strategies for lipid deoxygenation. Emphasis is focused on the design of different kinds of catalysts to meet the requirements, including noble metal catalysts, non-noble metal catalysts, and non-noble metal compound catalysts, with strategies such as morphology control, utilization of metal support interactions, and constructing synergistic effects between metal acid centers and metal oxygen vacancies. The reaction networks, mechanisms, and selectivity control strategies for lipid deoxygenation, cracking, isomerization, and aromatization are comprehensively discussed. Finally, we propose that it requires focusing on the precise regulation of multiple active sites to optimizing deoxygenation performance and reusability. It is essential to integrate in situ characterization to deepen the study of structure–active relationships and explore the reaction mechanisms within complex reaction systems.

1. Introduction

Currently, the global environment and energy system are facing unprecedented challenges and opportunities. The Copernicus Climate Change Service measured that the global surface air temperature in 2023 is nearly 1.5 °C higher than the level before industrialization in 1850, which is the warmest year in the world since records began [1]. At the same time, in recent years, extreme weather events have occurred frequently around the world, and the earth’s ecosystem has been seriously threatened, which reminds us that the global climate is changing, and we need to make efforts to reduce emissions as soon as possible and try to limit the degree of warming caused by human activities. Despite the rapid development of renewable energy, fossil energy consumption still dominates. According to the World Energy Outlook 2024 [2] reported by the International Energy Agency, from 2013 to 2023, energy demand has increased by 15%; even though 40% of the growth was covered by clean energy, the dependence on fossil fuels in the global energy structure has only decreased from 82% to 80%. Renewable electricity and renewable fuels are the key objectives to achieve net zero emissions. Benefited from the rapid development and deployment of renewable electricity, the global energy economy is increasingly electrified, replacing fossil fuels to provide heat, mobile, and industrial energy demands. Renewable fuels are also crucial to energy transformation, but the growth is still lagging behind [3]. For example, in the aviation industry, the development of electrification is still facing challenges by the current battery energy and power density and is almost entirely dependent on fossil fuels [4]. Therefore, it is necessary to increase the use of renewable fuels to promote the aviation decarbonization process.

At present, the renewable and alternative jet fuel used in commercial aviation is known as Sustainable Aviation Fuel (SAF), which features similar chemical properties and combustion characteristics with petroleum jet fuel and can be directly applied in existing aircraft engines. SAF is produced from renewable resources, including waste fats, oil, non-food corps, agricultural and forestry waste, etc. As the CO₂ emitted amount from SAF combustion is equal to that absorbed by biomass during growth, CO₂ emissions in the life cycle can be reduced by up to 80%. In addition, the use of SAF can significantly reduce the emissions of Particulate Matter (PM), including incomplete combustion hydrocarbons, CO, NO_x, and other pollutants [5,6,7]. Figure 1 shows that SAF produced from HEFA (Hydroprocessed Esters and Fatty Acids) technology effectively reduces PM emissions compared to diesel and RP3 across all load conditions. SAF has attracted increasing attention in achieving carbon neutrality in the aviation industry. Compared with 2023, the global production of SAF doubled in 2024, which reached above 1 million tons, and it is expected to grow rapidly in the future. However, the output of SAF is still low, accounting for only 0.3% of the global aviation fuel production in 2024, and the cost of producing SAF is 3.1 times that of traditional jet fuel [8]. In 2021, member airlines of the International Air Transport Association passed the resolution of Flying Net Zero, promising to achieve net zero carbon emissions in operation by 2050. It is estimated that SAF is likely to be required to achieve 65% of the required emission reductions [9]. Many countries and regions have issued relevant policies to promote the growth of the SAF industry and set ambitious SAF mixing targets. In Europe, the EU’s ReFuelEU aims to achieve 6% SAF by 2030, and the UK and Japan also plan to achieve 10% SAF by 2030 [10], which means more efforts is required to optimize the SAF production process to reduce costs and increase production.

At present, seven technical pathways of SAF production have been certified by the American Society of Testing and Materials (ASTM), including Hydroprocessed Esters and Fatty Acids (HEFA); Fischer–Tropsch (FT); Fischer–Tropsch containing aromatics (FT-SKA); direct sugars to hydrocarbons producing Synthetic Iso-Paraffins (SIP); Alcohol-to-Jet (ATJ); Catalytic, Hydrothermolysis Jet fuel (CHJ), and Hydroprocessed Hydrocarbons (HH-SPK or HC-HEFA) [11]. HEFA technology is highly mature and is the only fully commercialized technology at present [12]. A wide range of raw materials can be used in HEFA, including animal and vegetable oil, waste cooking oil, non-edible oil, and algae oil. The use of waste oil can reduce the dependence on food crops and farmland. HEFA feedstock availability raises concerns due to limited waste oil supplies and ethical risks from over-reliance on vegetable oils, including food-versus-fuel conflicts and land-use disputes. The rational cultivation of non-edible oil crops has the potential to overcome the problems faced by biodiesel raw materials. Inedible jatropha oil is a promising alternative fuel source. Widely grown in tropical and subtropical regions, the drought-tolerant Jatropha curcas thrives in arid, nutrient-poor soils without competing with agricultural land or food crops. The molecular structure of bio-oil and its derivatives is similar to that of long-chain alkanes in jet fuel, and the target product component can be obtained by catalyzing the removal of oxygen with low conversion difficulty and high technical feasibility via HEFA. The main difference between CHJ and HEFA is the reaction conditions. CHJ utilizes hydrothermal liquefaction to decompose fatty acid (ester) biomass in supercritical water and catalyze hydrogenation, enabling the treatment of water-containing biomass (such as algae). However, requirements on equipment are stricter and the catalysts are more prone to deactivation. The feedstock of HH-SPK technology includes biomass-based hydrocarbons, lipids, and fatty acids. However, due to insufficient data, currently recognized biomass only includes specific algae (Botryococcus braunii), and this type of SAF is blended with fossil fuels in low proportions when used in aviation fuel.

The main components of bio-oil are triglycerides and a small amount of free fatty acids. During the hydrogenation process, ester bonds break to produce propane and fatty acids. Fatty acids generate corresponding alkanes through Hydrogenation Deoxygenation (HDO), Decarbonylation (DCX), or Decarboxylation (DCX) mechanisms, and oxygen is removed as H₂O, CO, and CO₂, respectively. Alkanes may undergo further hydrocracking, isomerization, and aromatization reactions. The catalyst plays an important role in this process. In the past few years, researchers have made great efforts to design catalysts with high efficiency, high selectivity, and high fuel yield that can realize lipid conversion under milder conditions. At present, the catalysts commonly used for hydrodeoxygenation can be divided into noble metal catalysts, non-noble metal catalysts, and noble metal like catalysts. Noble metal catalysts mainly include Pt, Pd, Ru, etc., while non-noble metal catalysts mainly include Ni, Co, Cu, etc. Non-noble metal compound catalysts are mainly sulfides, phosphates, nitrides, and carbides.

In recent years, some researchers have reviewed the catalytic production of biofuels from lipids, as shown in

Table 1. Summary of review papers published in the past five years.

Year	Review Title	Ref.
2020	Recent advancement in deoxygenation of fatty acids via homogeneous catalysis for biofuel production	[13]
2021	Catalytic hydrothermal deoxygenation of lipids and fatty acids to diesel-like hydrocarbons: a review	[14]
2021	Hydroconversion of fatty acids and vegetable oils for production of jet fuels	[15]
2022	Hydrocracking, hydrogenation, and hydro-deoxygenation of fatty acids, esters, and glycerides: Mechanisms, kinetics, and transport phenomena	[16]
2022	Production of jet biofuels by catalytic hydroprocessing of esters and fatty acids: A review	[17]
2022	Biosynthesis of alkanes/alkenes from fatty acids or derivatives (triacylglycerols or fatty aldehydes)	[18]
2022	Advances in catalytic decarboxylation of bioderived fatty acids to diesel-range alkanes	[19]
2022	Recent advances in the catalytic deoxygenation of plant oils and prototypical fatty acid models compounds: Catalysis, process, and kinetics	[20]
2023	Hydroprocessing of lipids: An effective production process for sustainable aviation fuel	[21]
2023	Deoxygenation of vegetable oils and fatty acids: How can we steer the reaction selectivity towards diesel range hydrocarbons?	[22]
2024	Emerging catalysis in solvent-free hydrodeoxygenation of waste lipids under mild conditions: A review	[23]
2024	Dilemma and strategies for production of diesel-like hydrocarbons by deoxygenation of biomass-derived fatty acids	[24]
2025	A mini review on catalytic hydrodeoxygenation for biofuels production: catalyst, mechanism, and process	[25]

. Some studies focused on specific reaction conditions (e.g., hydrothermal, solvent-free), while others focused on particular lipid reactants (e.g., fatty acids). This study provides a narrative review on the catalytic deoxygenation of lipids for Sustainable Aviation Fuel (SAF) production over the past decade, prioritizing catalyst performance. More importantly, this review emphasizes the controllable synthesis of targeted catalysts via considering reaction features. This study focuses on the research progress on lipid deoxygenation, hydrocracking, isomerization, and aromatization into bio-jet fuel components and expounds the mechanism of the reaction. The design of multifunctional catalysts, including the construction of active sites, synergy, and morphology control are discussed. We have summarized several reaction mechanisms and control methods involved in the conversion of lipid deoxygenation to bio-jet fuel components. Finally, the future development of catalysts for deoxygenation to prepare bio-jet fuel components is prospected.

2. Construction of Efficient Multifunctional Catalysts

The deoxidation of lipids depends on catalysts, which are generally divided into homogeneous catalysts, heterogeneous catalysts, and enzymes. Considering the reusability of catalysts, product separation efficiency, economic cost, and catalytic activity, solid catalysts designed for multiple catalytic processes have been studied more extensively. As the production of bio-jet fuel involves complex reactions, the catalysts need to be carefully designed to suit both the reactants and the reaction conditions. The deoxygenation reaction is substantially influenced by the metal type, support properties, and the presence of promoters.

2.1. Design of Active Sites

2.1.1. Noble Metal Catalysts

Noble metals (Pt, Pd, Ru) have been used in the deoxidation of lipid biomass, showing excellent activities.

Table 2. The activity of various noble metal catalysts in the conversion of lipids.

Catalysts	Feed	Reaction Conditions	Activity Performance	Ref.
Pt/C	Palmitic acid	Batch reactor, T = 370 °C,	76% conversion, 90% C15 selectivity	[26]
		P = 0.1 MPa N2, t = 1 h
		Rea/Cat (wt/wt) = 5/3
Pt/C	Stearic acid	Batch reactor, T = 350 °C,	100% conversion, 90% alkanes selectivity	[27]
		P = 0.1 MPa N2, t = 3 h,
		Rea/Cat (wt/wt) = 20
Pt/ZIF-67/zeolite 5A	Palmitic acid	Batch reactor,	95% conversion, 91.7% C15 selectivity	[28]
		T = 300 °C, P = 2 MPa CO2, t = 5 h,
		Rea/Cat (wt/wt) = 1
Pt/ZIF-67/zeolite 5A	Lauric acid	Batch reactor,	95% conversion, 93.5% C11 selectivity	[28]
		T = 320 °C, P = 2 MPa CO2, t = 2 h,
		Rea/Cat (wt/wt) = 1
Pd/C	Palmitic acid	Batch reactor,	85 wt% Hydrocarbons yield	[29]
		T = 270 °C, P = 2 MPa H2, t = 10 h,
		Rea/Cat (wt/wt) = 5
Pd@Al3-mSiO2	Methyl palmitate	Batch reactor,	95.6% conversion, 99% alkanes selectivity	[30]
		T = 260 °C, P = 3 MPa H2, t = 5 h,
		Rea/Cat (wt/wt) = 10/3
Ru@TpPON	Stearic acid	Batch reactor,	100% conversion, 96.0% alkanes selectivity	[31]
		T = 180 °C, P = 3 MPa H2, t = 8 h,
		Rea/Cat (wt/wt) = 5
Pt/TiO2	Stearic acid	LED photoreactor,	96% conversion, 92% C17 yield	[32]
		365 nm, 18 W,
		T = 30 °C, P = 0.1 MPa H2, t = 2 h,
		Rea/Cat (wt/wt) = 1.5
1.5Au-0.8Pd/TiO2	Hexanoic acid	Xe lamp photoreactor, 300 W,	94.7% conversion, 100% pentane selectivity	[33]
		T = 20 °C, P = 0.5 MPa H2, t = 4 h,
		Rea/Cat (wt/wt) = 1.5

summarized the activity performance of noble metal catalysts for lipid conversion. It is worth mentioning that their loading capacity is generally lower than 5% in view of the high cost of noble metals.

Pt exhibits superior catalytic activity for the deoxygenation of oxygen-containing compounds under hydrothermal conditions, and the conversion of palmitic acid can reach 76% after 1 h at 370 °C [26,34]. In addition to investigating the decarboxylation of model compounds, Na et al. [27] found that 5 wt% Pt/C was advantageous for the hydrothermal decarboxylation of microalgae pyrolysis oil to hydrocarbon fuels. The reaction was carried out at 300 °C for 3 h, and the deoxygenation rate was 80.3%. Yang et al. [28] carefully regulated the structure of the catalyst. Pt nanoparticles were loaded onto a ZIF-67 membrane-coated zeolite 5A bead, and the loading amount of Pt was very low, 0.5 wt%. Under the atmosphere without H₂, conversion rates of 95% for palmitic acid and lauric acid were achieved, with pentadecane and undecane selectivity of 91.7% and 93.5%, respectively.

Pd-based catalysts have also been proven to be effective in the deoxygenation of lipids. Silva et al. [29] investigated the catalytic deoxygenation of 5 wt% Pd/C, and under the optimal reaction conditions the highest content of hydrocarbons (85 wt%) was obtained in the reaction of macauba almond oil. Cao et al. [30] developed a stable and efficient Pd@Al₃-mSiO₂ catalyst. Under the conditions of 260 °C and 3.0 MPa H₂, 98% conversion and 99% alkane selectivity were obtained, while the conversion and selectivity of traditional Pd/γ-Al₂O₃ were 35% and 70%, respectively. The modulation of the support and the design of the core–shell structure promoted the hydrodeoxygenation activity and stability of the catalyst.

Similarly, ruthenium-based catalysts displayed excellent deoxygenation performance. Mondal et al. [31] designed a porous organic network modified by Ru nanoparticles. A porous organic grid (TpTON) was generated by the acid-catalyzed condensation of 1,3,5-triformyl phloroglucinol (Tp) and triphenylamine (TPA). Ru nanoparticles were successfully prepared on the porous surface of TpTON to obtain Ru@TpPON. TpPON features a highly cross-linked and three-dimensional rigid framework interconnected at the molecular level, which serves as an excellent platform to anchor catalytically active metal nanoparticles, thereby endowing the system with enhanced leach resistance, high thermal stability, and robust thermomechanical stability.

In addition to traditional thermal catalysis, some noble metal catalysts also exhibit excellent photocatalytic activities. In general, TiO₂ was selected as a carrier due to its photoresponsive activity. Huang et al. [32] reported a Pt/TiO₂ catalyst that can efficiently convert fatty acids into long-chain alkanes under light conditions. During the photoredox cycle on Pt/TiO₂, fatty acids are decarboxylated and converted into C_n−1 alkyl groups on photogenerated holes, which then combines with hydrogen generated by the electron-mediated reduction of carboxyl protons to produce C_n−1 alkanes. The formation of a hydrogen-rich surface constructed by the interactions between H₂ and the Pt/TiO₂ catalyst promotes the rapid free radical termination of alkyl radicals and surface hydrogen species, resulting in the production of alkanes, effectively suppressing free radical dimerization and oxidation reactions. Under extremely mild conditions (30 °C, H₂ pressure ≤ 0.2 MPa, 365 nm LED, 18 W), C_n−1 alkanes can be obtained in high yield (≥90%) from bio-derived C_12-18 fatty acids. Yang et al. [33] deposited Au (core)-Pd (shell) on TiO₂, demonstrating broad-spectrum catalytic capability for converting diverse fatty acids and raw bio-oils into hydrocarbons. This compelling activity is ascribed to the synergistic effect between Au and Pd in the nanostructured Au (core)–Pd (shell) alloy, which is used to achieve more effective charge separation efficiency under visible light excitation. Compared with traditional thermal catalytic reactions, the photocatalytic decarboxylation of these fatty acids to alkanes occurs under extremely mild conditions, requiring only room temperature and lower hydrogen pressure, thus exhibiting great potential for development.

Although noble metal catalysts perform excellent activity, their rare reserves cause high prices. In addition, the pollution caused by noble metal smelting cannot be ignored either.

2.1.2. Non-Noble Metal Catalysts

In general, the cost of non-noble metal is much lower than that of noble metal due to their high abundance in the earth. However, increased activity of non-noble metals usually requires higher metal loading, which may reduce the dispersion and affect the stability.

Table 3. The activity performance of non-noble metal catalysts in the conversion of lipids.

Catalysts	Feed	Reaction Conditions	Activity Performance	Ref.
Ni-FSM-16	Oleic acid	Round-bottom glass flask	85.7% conversion, 87% hydrocarbons selectivity	[35]
		T = 350 °C, t = 0.5 h,
		Rea/Cat (wt/wt) = 10
Ni/ZSM-5.SAPO-11	Palmitic oil	Batch reactor,	51% jet fuel yield	[36]
		T = 350 °C, P = 2.7 MPa H2, t = 2 h,
		Rea 10 mL, Cat 2 g
Ni/TS-1	Palmitic acid	Batch reactor,	100% conversion, 91.6% C15 selectivity	[37]
		T = 260 °C, P = 4 MPa H2, t = 10 h,
		Rea/Cat (wt/wt) = 5
Ni-Al0.33Ox	Stearic acid	Batch reactor, T = 250 °C, t = 8 h,	99% conversion, 93.2% C18 selectivity	[38]
		Rea/Cat (wt/wt) = 2.5
		isopropanol as hydrogen source
Ni/HUSY-4	Stearic acid	Batch reactor,	100% conversion, 96% C18 selectivity	[39]
		T = 260 °C, P = 4 MPa H2, t = 1 h,
		Rea/Cat (wt/wt) = 10
Ni-ZrO2	Methyl laurate	Batch reactor,	100% alkanes yield, 87.6% C11 selectivity	[40]
		T = 280 °C, P = 2 MPa H2, t = 8 h,
		Rea/Cat (wt/wt) = 2
Ni/CeO2-NR	Palmitic acid	Batch reactor,	99.5% conversion, 84.0% C15 selectivity	[41]
		T = 270 °C, P = 2 MPa H2, t = 10 h,
		Rea/Cat (wt/wt) = 5
Ni/B2O3-ZrO2	Methyl palmitate	Batch reactor,	100% conversion, 84.4% biofuel yield 65% C15 selectivity	[42]
		T = 375 °C, P = 4 MPa H2,
		Rea/Cat (wt/wt) = 10
Ni/BZ-Al50	Palmitic oil	Trickle bed,	75% conversion, 52% n-C16-15 selectivity	[43]
		T = 375 °C, P = 3 MPa H2, t = 6 h,
		LHSV = 1.2 h−1, H2/oil = 400 (v/v)
15Co/ZrO2	Ethyl palmitate	Batch reactor,	100% conversion, 82% alkanes selectivity	[44]
		T = 240 °C, P = 2 MPa H2, t = 8 h,
		Rea/Cat (wt/wt) = 2
Co@SiO2	Palmitic acid	Batch reactor,	100% conversion, 100% alkanes selectivity	[45]
		T = 300 °C, P = 2 MPa H2, t = 4 h,
		Rea/Cat (wt/wt) = 3
Ni1Mo1/ZrO2	Methyl palmitate	Trickle bed,	99.4% conversion, 95.0% alkanes selectivity	[46]
		T = 270 °C, P = 3 MPa H2,
		H2/oil = 400 (v/v),
		contact time = 71.3 min
Co5Ni5/HAP	Methyl stearate	Batch reactor,	99.4% conversion, 98.2% C17 selectivity	[47]
		T = 290 °C, P = 0.1 MPa N2, t = 8 h,
		Rea/Cat (wt/wt) = 1.5,
		methanol as hydrogen source
10%Ni-5%Fe/γ-Al2O3	Palmitic acid	Batch reactor,	100% conversion, 83.7% C16 + C15 selectivity	[48]
		T = 270 °C, P = 1.5 MPa H2, t = 6 h,
		Rea/Cat (wt/wt) = 2
Ni-Er/50S-Al	Jatropha oil	Trickle bed,	64.7% biofuel yield 99.3% deoxygenation ratio	[49]
		340 °C, LHSV = 0.8 h−1, P = 3 MPa
		Flow = 200 mL/min (H2/N2 = 1:1)
Ni3Fe1Re/HZSM-5	Stearic acid	Batch reactor, T = 260 °C,	100% conversion, 94.8% C17 selectivity	[50]
		P = 3 MPa H2, t not mentioned,
		Rea/Cat (wt/wt) = 10/3

summarizes the studies on lipid deoxygenation using non-noble metal catalysts. The typical active components are Ni, Co, and Mo.

Among heterogeneous catalysts, Ni-based catalysts have emerged as the most promising due to their exceptional capability for H₂ activation and remarkable activity in cleaving C–C and C–O bonds. Jiraroj et al. [35] investigated the decarboxylation reaction of oleic acid under H₂ free conditions. The catalytic activity of Ni-FSM-16 is superior to that of Pd-FSM-16, as well as to catalysts loaded with Ni on other supports. Ni-FSM-16 was characterized as having a more uniform distribution of metals, a more acidic silanol-group, and an electron rich Ni. Khan et al. [36] achieved 51% jet fuel yield from palm oil on 10 wt% Ni/ZSM-5 (50) SAPO-11. Chen et al. [37] showed that, under the optimal experimental conditions (260 °C, 4 MPa, 10 h), the palmitic acid conversion and pentadecane selectivity of Ni/TS-1 were 100% and 91.6%, respectively.

The sintering and/or carbon deposition of nickel-based catalysts are important reasons for the decrease of activity. Papageridis et al. [51] synthesized Ni-based catalysts supported on different oxides, that is, Ni/Al₂O₃, Ni/ZrO₂, and Ni/SiO₂. The running time experiments showed that all catalysts were inactivated after about 6 h, which was due to the sintered Ni particles and/or their coverage by thin graphitic carbon shells.

Cobalt-based catalysts also exhibit excellent activity. Çakan et al. [52] investigated the catalytic activity and product selectivity of cobalt-based catalysts loaded with different metal oxides for the hydrogenation deoxygenation reaction of safflower oil. The Co/ZnO catalyst performed high catalytic activity, with a hydrocarbon selectivity of up to 65.96% in the jet fuel range, and the coke content of Co/ZnO was significantly lower than other catalysts. Zhou et al. [44] synthesized a highly active 15Co/ZrO₂ catalyst for the conversion of ethyl palmitate to diesel alkanes. Under mild reaction conditions (240 °C, 2 MPa H₂, 8 h), the yield of alkane reached 82%, and the deoxygenation of ethyl palmitate was catalyzed by the Co site and the oxygen vacancy of ZrO₂. Lin et al. [45] prepared an amorphous cobalt-based catalyst with a porous structure and large specific surface area by the chemical grafting method (Co@SiO₂ Nanocolumn), at 300 °C, 4 h. Under the condition of 2 MPa H₂, the conversion of palmitic acid was 100%, and the total alkane selectivity was 99.7%. Multiple characterizations have shown that Co@SiO₂ exists in amorphous form with large specific surface area, abundant pore structure, and surface acidity.

The utilization of bimetallic catalysts enables synergistic effects to enhance catalytic performance, where the strategic modulation of electronic properties at active sites facilitates the efficient deoxygenation of lipid-based substrates. Chen et al. [53] introduced a second transition metal into the Ni/γ-Al₂O₃ catalyst to form an alloy, which significantly improved the H₂ adsorption and desorption performance of the catalyst at low temperature by adjusting the d-band central hole of the metal. Zhao et al. [46] observed that, in Ni₁Mo₁/ZrO₂ catalysts, the electron transfer from Mo to Ni promoted the formation of the active center Ni⁰ and the activation of H₂ molecules, thereby improving the hydrodeoxygenation activity of the catalyst for fatty acid methyl esters. Yan et al. [47] prepared a CoNi/HAP bimetallic catalyst for the hydrogenation of methyl stearate to heptadecane. The introduction of Ni enhanced the electronic coupling effect of CoNi alloy nanoparticles, promoted the adsorption of methyl stearate or stearic acid, and thus improved the catalytic activity. Meanwhile, the C–C bond of oxygen-containing intermediate (RCH₂CO) tends to be activated on the Ni active site of CoNi alloy nanoparticles, resulting in high heptadecane selectivity. Under the optimized conditions (290 °C, 8 h), the conversion of methyl stearate and heptadecane selectivity on Co₅Ni₅/HAP catalyst were 99% and 98%, respectively. In addition to bimetallic catalysis, a third metal is also added to the catalyst to improve the stability of the catalyst. Guo et al. [50] dropped rhenium (Re) into the Ni₃Fe₁/HZSM-5 catalyst, which significantly improved the stability of the catalyst. On the catalyst without Re dropping, the conversion rate decreased rapidly, and a large number of oxygen-containing products appeared after the third cycle. In contrast, the Re-dropped catalyst maintained stable activity through the eighth cycle, with only gradual deactivation observed thereafter. The addition of Re led to a new Ni-Fe, Fe₃Re₂ alloy phase, which in turn prevented the accumulation of active components and metal loss during the catalyst operation. In order to ensure the feasibility of industrial use, the catalyst needs to be stable and renewable for longer reuse.

2.1.3. Non-Noble Metal Compound Catalysts

Non-noble metal compounds, including metal oxides, phosphides, sulfides, carbides, and nitrides, have also been widely investigated and exhibit superior catalytic activity in the deoxygenation of lipid derivatives.

Table 4. The activity performance of non-noble metal compound catalysts in the conversion of lipids.

Catalysts	Feed	Reaction Conditions	Activity Performance	Ref.
CaO	Waste cooking oil	Batch reactor,	20.9% acid selectivity, 45.4% alkane selectivity, 18.9% alkene selectivity, 3.0% aromatic selectivity,	[54]
		T = 300 °C, P = 2.5 MPa N2, t = 1 h,
		Rea/Cat (wt/wt) = 20
TiO2	Waste cooking oil	Batch reactor,	5.4% acid selectivity, 32.8% alkane selectivity, 3.2% alkene selectivity, 48.5% aromatic selectivity,	[54]
		T = 300 °C, P = 2.5 MPa N2, t = 1 h,
		Rea/Cat (wt/wt) = 20
Sulfide NiMo/γ-Al2O3	Degummed palmitic oil	Batch reactor,	70% diesel yield	[55]
		T = 260 °C, P = 3 MPa H2, t = 0.5 h,
		Rea 2 mL, Cat 0.1 g
Ni1.5P/AC	Palmitic acid	Trickle bed,	100% conversion, 56% oil yield, 64% C11-15 selectivity	[56]
		T = 350 °C, P = 0.1 MPa, 5% H2/Ar,
		gas/feed = 15 (v/v), WHSV = 0.25 h−1
Ni12P5/SiO2	Palmitic acid	Batch reactor,	100% conversion, 59.8% C15 yield 33.7% C16 yield	[57]
		T = 270 °C, P = 1.2 MPa H2, t = 6 h,
		Rea/Cat (wt/wt) = 2
Mo2C	Palmitic acid	Batch reactor,	100% conversion, 96.6% n-C16 selectivity	[58]
		T = 275 °C, P = 2 MPa H2, t = 8 h,
		Rea/Cat (wt/wt) = 5
Mo2.56CN0.50	Palmitic acid	Trickle bed,	99.6% conversion, 99.2% alkanes selectivity	[59]
		T = 300 °C, P = 4 MPa H2,
		H2/feed= 600 (v/v),
		contact time = 1.18 min
Ni-Mo2C/MCM-41	Palmitic oil	Trickle bed,	83.3% biofuel yield 95.2% C15-18 selectivity	[60]
		T = 340 °C, LHSV = 1.4 h−1, P = 3 MPa
		Flow = 200 mL/min, (H2/N2 = 1)
Ni-Mo2C/MCM-41	Palmitic acid	Batch reactor,	100% conversion, 96.8% alkanes selectivity	[61]
		T = 270 °C, P = 2 MPa H2, t = 7 h,
		Rea/Cat (wt/wt) = 5
W-1000 (W2C)	Stearic acid b	Batch reactor,	81% conversion, 83% deoxygenation products selectivity	[62]
		T = 350 °C, P = 0.5 MPa H2, t = 5 h,
		Rea/Cat (wt/wt) = 4
Ni-W2C-WC/AC	Jatropha oil	Trickle bed, T = 340 °C,	99.7% deoxygenation rate 94.5% C15-18 selectivity	[63]
		P = 3 MPa, WHSV = 55.2 h−1,
		Flow = 200 mL/min (H2/N2 = 1)
Ni-Mo2N/γ-Al2O3	Jatropha oil	Trickle bed,	100% conversion, 80.1% C15-18 selectivity	[64]
		T = 320 °C, LHSV = 0.8 h−1, P = 3 MPa
		Flow = 200 mL/min (H2/N2 = 1)
Ni3Mo3N@600	Palmitic acid	Batch reactor,	100% conversion, 90% alkanes selectivity	[65]
		T = 270 °C, P = 2 MPa H2, t = 10 h,
		Rea/Cat (wt/wt) = 10

summarizes the activity performance of some non-metallic compounds in the deoxygenation of lipids.

Cheap non-supported metal oxide catalysts (CaO, TiO₂, Mn(IV)O, and ZnO) are used as catalysts for the deoxidation of waste cooking oil in the “one-pot” method, which obtained a liquid fuel with a Higher Heating Value (HHV) comparable to that of jet fuel. Processed at 300 °C with 5 wt% loading, CaO produced oil with the highest HHV but retained substantial acidic groups, while TiO₂ generated oil containing higher alkanes and lower acidity, yet elevated benzene/aromatic levels [54].

Metal sulfides, such as sulfide CoMo and sulfide NiMo, have been used as Hydrodesulfurization (HDS) catalysts for fossil fuels for a long time. Kiatkittipong et al. [55] found that NiMo/γ-Al₂O₃ showed better activity for the hydrodeoxygenation of triglycerides than Pd/C. Coumans et al. [66] studied the effect of supports on the hydrodeoxygenation of methyl oleate over the sulfide NiMo catalyst. When activated carbon is used as a support, the HDO activity of methyl oleate is higher and more stable, with higher C₁₈ hydrocarbon selectivity. Adding a sulfurizing agent [67,68] to the raw material could maintain the activity of the catalyst. However, conventional sulfiding agents such as Hydrogen Sulfide (H₂S) and Dimethyl Disulfide (DMDS) are challenged by their inherent toxicity and propensity to introduce sulfur contamination into products, thereby impeding the production of low-sulfur liquid fuels that comply with stringent environmental regulations. Vlasova et al. [69] developed a co-processing process of rapeseed oil and sulfur-containing straight run diesel mixture. The sulfur CoMo and NiMo catalysts were in situ sulfurized in sulfur-containing raw materials to maintain the sulfurized state of the sulfide catalyst while increasing the cetane number of the deoxygenated triglyceride product. Thongkumkoon et al. [70] introduced Re into the sulfide NiMo catalyst and formed a Re-Ni-Mo/γ-Al₂O₃ catalyst, which enhanced hydrodeoxygenation activity for palm oil feedstocks and catalyst stability.

Yang et al. [71] compared the catalytic performance of Ni₂P/SBA-15 and Ni/SBA-15 for the deoxygenation of methyl oleate and found that Ni₂P/SBA-15 exhibited higher hydrodeoxygenation selectivity than Ni/SBA-15 and reduced undesirable cracking reactions. Therefore, Ni₂P/SBA-15 is considered as a promising catalyst to produce green diesel. Xin et al. [56] demonstrated that the Ni/P molar ratio critically regulates the formation of Ni₂P and/or Ni1₁₂P₅ phases on the Activated Carbon (AC) surface. The Ni_1.5P/AC catalyst exhibited optimal deoxygenation activity for palmitic acid, attributed to the synergistic coexistence and high dispersion of Ni₂P and Ni1₁₂P₅ phases within the composite. Under atmospheric pressure and solvent-free conditions, the conversion of palmitic acid was 100%, with 56.0% oil yield, and the HHV was 46.5 MJ kg⁻¹. Comparative analysis of SiO₂-supported Ni, Ni₂P, and Ni₁₂P₅ catalysts demonstrated that their Turnover Frequencies (TOF) for palmitic acid conversion descended in the order of Ni₁₂P₅/SiO₂ > Ni/SiO₂ > Ni₂P/SiO₂ [57].

Transition metal carbides, typically molybdenum carbides, exhibit noble metal-like electronic structures and catalytic behaviors due to the insert of carbon atoms into the transition metal lattice, which induces d-band contraction and results in an elevated density of states near the Fermi level [72,73]. They show excellent activity in a variety of chemical reactions and are a promising alternative to noble metal catalysts. Zhou et al. [58] reached 100% conversion of palmitic acid and 96.6% selectivity of hexadecane on Mo₂C that was prepared by one-step carbonization of an amine molybdenum oxide precursor formed with 1,12-diaminododecane. The doping of non-metallic or metallic compounds can alter the properties of molybdenum carbide. Chen et al. [59] generated N-doped molybdenum carbide nanowires using a carbon source with higher nitrogen content (3-amino-1,2,4-triazole). N doping caused electron transfer in Mo, C, and N, resulting in alterable Mo_xCN_y properties. The conversion rate of palmitic acid on Mo_2.56CN_0.50 is 99.6%, and the selectivity of alkanes is 99.2%. Du et al. [60] adjusted the surface properties of Ni-Mo₂C/MCM-41 catalysts by changing the content of CH₄ in the carbonization atmosphere. The Ni-Mo₂C/MCM-41 catalyst exhibited a volcano-shaped trend in catalytic activity, with increasing CH₄ content in the conversion of Jatropha Oil (JO). Ni-Mo₂C/MCM-41 prepared with 25% CH₄ performed excellently, with a biofuel yield of 83.9% and a C_15-18 alkanes selectivity of 95.2%. This is attributed to the most exposed active centers and the smallest particle size on the surface of the Ni-Mo₂C/MCM-41 catalyst and the lowest amount of Lewis acid from Ni⁰. Further studies used palmitic acid as a model compound, which showed that the doping of Ni improved the dispersion of active particles and the coordination number of Mo species (Mo–C and Mo–O). The graphitization degree was improved, which promoted electron transfer. The Ni doping enhanced the cleavage of both C–O and C–C bonds, driven by the increased abundance of electron-rich Mo active sites [61].

Gosselink et al. [62] found that W₂C had superior hydrodeoxygenation efficiency for stearic acid than that of WO₃; octadecane was mainly obtained by the HDO pathway on W₂C. Zhou et al. [63] developed a 10Ni10W/AC-700 catalyst, in which the active phases were identified as Ni, W₂C, and WC. The synergistic interaction among these phases enhanced catalytic performance, attaining 99.7% jatropha oil deoxygenation and 94.5% C_15-18 alkanes selectivity.

Transition metal nitrides also exhibit similar properties. Lei et al. [64] used Ni-Mo₂N/γ-Al₂O₃ as a catalyst to completely remove oxygen from jatropha curcas oil at 320 °C and 3 Mpa to obtain C_15-18 alkanes as high-quality biofuels with high selectivity. As a promoter, Ni is conducive to the cleavage of H–H bonds, while Mo₂N is conducive to the cleavage of C–O bonds. The synergistic effect of Ni and Mo₂N endows the catalyst with excellent activity. Du et al. [65] synthesized a bimetallic NiMo nitride catalysts via the reductive decomposition of tris(ethylenediamine)nickel molybdate, resulting in the formation of a Ni₃Mo₃N phase (other research suggests Ni₃Mo₃N as the co-existence of Ni and Ni₂Mo₃N phases under similar synthetic conditions) [74]. The Ni₃Mo₃N phase showed exceptional performance in the deoxygenation of palmitic acid, achieving complete conversion (100%) with over 90% selectivity toward C₁₅/C₁₆ alkanes.

Table 5. Advantages and disadvantages of various catalysts for the deoxygenation of lipids.

Catalyst Type	Advantages	Disadvantages
Noble metal catalysts	Wide application and excellent activity. The potential for developing photocatalysts is enormous	Expensive and not environmentally friendly Sensitive to impurities of feedstocks
Non-noble metal catalysts	Most of them are cheap and easy to obtain. High catalytic activity	Stability is insufficient for industrialization
Non-noble metal compound catalysts	Most of them are cheap and easy to obtain. Metal sulfides, phosphides, carbides, and nitrides exhibit excellent activity.	The activity of metal oxides is limited There is a risk of contamination in sulfurized catalysts Metal carbides and nitrides are prone to oxidation and deactivation

compares the advantages and disadvantages of various types of catalyst in terms of cost, activity, stability, and scalability.

2.2. Morphology and Texture Properties

The morphology and texture properties are of great significance in the design and optimization of nano-catalytic materials. Specific surface area, porous structure, and pore size distribution constitute critical influence of their catalytic performance by modulating active site accessibility, reactant diffusion kinetics, and mass transfer efficiency.

From the perspective of active metal loading, the larger specific surface area and ordered pore structure are conducive to the regular array and spatial separation of metal nanoparticles and enhance the catalytic stability [75]. From the perspective of reaction, large specific surface area can provide rich active sites for the adsorption activation and surface reaction of reactants, while suitable pore structures can promote the diffusion of reactants and products, thereby improving the catalytic efficiency. Ma et al. [39] prepared a Hierarchical H-style ultra-stable Y (HUSY) zeolite with high specific surface area and interconnected hierarchical mesopores (10, 25, and 40 nm) using the sequential post synthesis strategy of water vapor dealumination and mixed alkali desilication. The mesoporous surface area of the HUSY zeolite was increased by water vapor treatment. The effects of different mixed bases—propylammonium hydroxide (TPAOH)/sodium hydroxide (NaOH) and pyridine (PI)/NaOH—on desilication were studied. The (PI)/NaOH-treated zeolite further produced more mesopores, as the smaller pyridine (0.54 nm) could pass through the super-cage (0.74 nm) to protect the zeolite framework from deep desilication. Chavarría-Escamilla et al. [76] prepared a NiO-ZrO₂ catalyst with silica gel as a hard template. After alkali washing away of the hard template, 10–20 nm mesopores were generated, and the specific surface area of the catalyst increased significantly. Compared with the catalyst without a hard template, the yield of linear alkanes was significantly increased, and higher pentadecane selectivity was obtained.

The morphological and structural features of catalyst surfaces, including the selective exposure of the lattice plane, particle geometry, and topological configurations, modulate the electronic properties of active sites, thereby governing catalytic activity. Xin et al. [41] found that the morphology of CeO₂ made a great impact on the catalytic activity and density of the double active centers formed on it. CeO₂ nanorods (CeO₂-NR) and CeO₂ nanoparticles (CeO₂-NP) were synthesized by changing the amount of alkali in the hydrothermal synthesis. CeO₂-NR exhibits higher surface area and O_v density, restricting Ni nanoparticle growth to smaller sizes. The CeO₂-NP was on the contrary. The small-sized Ni and abundant O_v showed better H₂ and palmitic acid activation abilities, respectively. In addition, abundant O_v and highly dispersed Ni increase the closeness of the dual active centers on CeO₂-NR, which makes the adsorption activation hydrogenation synergistic catalytic performance of palmitic acid to alkanes better than that of the Ni/CeO₂-NP catalyst. Zhou et al. [77] revealed that the crystalline structure of ZrO₂ governs the exposed lattice planes of cobalt. The Co/Zr-3 catalyst, featuring a mixed-phase of tetragonal (t-ZrO₂) and monoclinic (m-ZrO₂) zirconia with high O_v concentration and active β-Co (102) facets, significantly reduced the activation energy for C–C and C–O bond cleavage. This catalyst demonstrated exceptional hydrodeoxygenation activity for methyl laurate under mild conditions (240 °C, 2 MPa H₂, 8 h), achieving near 100% liquid alkane yield (Figure 2).

2.3. Metal Support Interaction

The interaction between different carriers and active species will lead to the generation of different active species. Xin et al. [48] loaded 10%Ni-5%Fe on γ-Al₂O₃ and HZSM-5, respectively. It was found that, on 10%Ni-5%Fe/γ-Al₂O₃, the strong interaction between Ni and γ-Al₂O₃ produced NiAl₂O₄ species, but there was no Ni–Fe alloy; for 10%Ni-5%Fe/HZSM-5, the weak interaction between Ni and HZSM-5 induces the formation of FeNi₃ alloy, Ni, and NiAl₂O₄. NiAl₂O₄ species are beneficial to the formation of pentadecane via the palmitic acid DCO or DCX pathway, while the FeNi₃ alloy is beneficial to the formation of hexadecane from palmitic acid HDO. The type of support directly affects the dispersion of active metals.

The loading amount of metal may also in turn affect the surface of the support. Yang et al. [40] prepared Ni/ZrO₂ catalysts with different Ni loadings by the one-step urea hydrothermal method. Their study found that the complexity of the exposed crystal face of the support was affected by the amount of Ni loading. In the 5Ni-ZrO₂ and 7Ni-ZrO₂ catalysts, only the t-ZrO₂ (011) crystal planes were exposed. In contrast, the 8.5Ni-ZrO₂ and 10Ni-ZrO₂ catalysts exposed both t-ZrO₂ (011) and (112) crystal planes. For catalysts with higher Ni loadings (12, 15, and 20 wt%), an additional ZrO₂ (002) crystal plane was detected. Hydrodeoxygenation (HDO) activity tests for methyl laurate revealed that the yield of n-undecane gradually increased as the Ni loading rose from 5 to 10 wt%. However, further increases in Ni loading resulted in a decline in undecane selectivity. It was observed on 20 Ni-ZrO₂ that, although the initial reaction rate of methyl laurate was very fast, with the extension of reaction time, the selectivity of undecane decreased, and the selectivity of C_8-10 n-alkanes gradually increased, which may be due to the exposure of the ZrO₂ (002) crystal plane promoting the cracking reaction.

2.4. Synergetic Effects Between Metal and Acid Sites

The deoxygenation activity of lipid derivatives is affected by the spatial distribution and acidic strength of catalytic sites, coupled with the synergistic interplay between metallic and acidic centers within the catalyst architecture. One strategy to adjust the type of acid sites and the amount of acid is to use composite supports. Yu et al. [42] introduced B₂O₃ into the Ni/ZrO₂ catalyst, and the amount of Lewis and Brønsted acid sites increased from 5 µmol/g and 31 µmol/g to 10 µmol/g and 73 µmol/g, respectively. The integration of boron atoms into the ZrO₂ support framework through B–O–Zr bonding significantly enhances the density of acidic sites within the catalyst [78]. The presence of moderate acidic centers of B₂O₃ and Ni species played a key role in the excellent performance of the deoxidation (especially the DCO pathway) of lipids. Dabbawala et al. [43] introduced Al₂O₃ into β-zeolite (BZ) in their study, which reduced the Brønsted to Lewis acid ratio of the support and performed better catalytic performance in the HDO of palm oil. The Brønsted acidity of zeolite molecular sieves is derived from protons attached to the negatively charged aluminosilicate framework; as a result, the Brønsted acidity of zeolite molecular sieves can be adjusted by the silicon aluminum ratio. Feng et al. [79] modulated the acidic properties and nanosheet thicknesses of ZSM-5 supports by systematically varying the silica-to-alumina ratio and adjusting the dosage of structure-directing agents (Gemini surfactants). Their study demonstrated that the synergistic interplay between metallic centers and acidic sites significantly enhanced the HDO activity of the catalyst and the intrinsic deoxygenation efficiency of Ni species. Initially, the C−O bond is activated by the acid center and hydrogenated by the active H generated on the adjacent metal Ni. The DCO_x reaction is mainly dominated by metallic Ni because of its high C−C hydrogenolysis activity. The phenomenon that metal sites and acid sites synergize to promote the adsorption of oxygen-containing substrates and hydrodeoxygenation has also been observed in other reactions [80]. Furthermore, the spatial distribution of acidic centers and the zeolitic framework can synergistically regulate the cracking and isomerization pathways of long-chain alkanes derived from deoxygenation processes. Low acid concentration, low Brønsted to Lewis acid ratio, and low reaction temperature inhibit the deep cracking of deoxygenated products but are conducive to the selective cracking of deoxygenated products into alkanes of bio-aviation fuel components. Wang et al. [81] showed that the acid center on the ZSM-5 support promoted the dispersion of Ni through metal support interaction, thereby increasing the active sites for the hydrogenolysis of methyl laurate and the hydrogenation of lauric acid.

2.5. Synergetic Effects Between Metal and Oxygen Vacancy

Defect engineering, especially the design of catalysts with rich oxygen vacancies (O_v), is widely used to promote the hydrodeoxygenation of biomass and its derived oxygenated materials. Many researchers have concluded that Ov promote the hydrodeoxygenation activity by enhancing the adsorption and activation of oxygen-containing functional groups [82,83,84]. Therefore, oxygen vacancies are widely used to convert bio-oil and its derivates. The abundasnt Ov on the surface of catalyst is conducive to the adsorption of fatty acids, thus contributing to the activation of carboxyl groups, thus promoting the hydrodeoxygenation process of fatty acids.

Wang et al. [38] synthesized Ni-Al_0.33O_x catalyst with rich O_v via hydrothermal and reduction treatment. Ni-Al_0.33O_x catalyst displayed high conversion of stearic acid with 93.2% n-heptane yield under mild conditions (T = 250 °C), where isopropanol was used as solvent as well as hydrogen donor to promote hydrodeoxygenation. DFT calculation showed that the Ni-AlO_x interface with rich O_v was more conducive to fatty acid adsorption than both Ni/Al₂O₃ interface and pure Ni surface with lower O_v. In addition, the selectivity of products can be controlled by the alternative adsorption of carbon oxygen bonds on O_v sites. For example, molybdenum oxide catalysts preferentially adsorbed carbon–oxygen bonds than C=C bonds, thereby achieving the high selectivity of unsaturated hydrocarbons [85]. Yu et al. [86] synthesized a MoO₂/Mo₂C catalyst for the selective hydrodeoxygenation of methyl oleate, which obtained product containing unsaturated hydrocarbons for 41.9%. Furthermore, some studies have shown that rich O_v can promote the dispersion of active metals [87]. Zeng et al. [88] prepared Ni/H-CeO₂ nanoparticles with rich O_v by hydrothermal methods. Under mild reaction conditions (270 °C, 2 MPa H₂, 10 h), palmitic acid was completely converted to pentadecane with a selectivity of 94.8%. DFT calculations confirmed that the rich O_v in Ni/H-CeO₂ improved the interaction between Ni metal and CeO₂ support. Therefore, enriched oxygen vacancies are more conducive to the formation of Ni nanoparticles and the dispersion of Ni. Ni nanoparticles were responsible for activating hydrogen while abundant O_v adsorbed palmitic acid effectively, and the synergetic effect between metal Ni and O_v contributes to improving the hydrodeoxygenation activity of the catalyst (Figure 3). Similarly, Song et al. [89] prepared a nitrogen functionalized oxide catalysts (NiO−MoO₃@N/SAPO-11) with abundant O_v, which showed the highest catalytic activity among other catalysts, obtaining nearly 100% conversion of methyl palmitate and palm oil, with 93.14% and 87.8% selectivity of aviation fuel under 280 °C, respectively. Significant electron interactions between the lone pair electrons of pyridine-N and metals were observed, which promotes the formation of oxygen vacancies. Ni and O_v effectively activate H₂ and C−O groups, leading to superior deoxygenation activity.

2.6. Catalyst Stability

As the stability of catalysts is directly related to their industrial potential, catalyst deactivation is a significant challenge in the catalytic deoxygenation of lipid biomass for sustainable bio-jet fuel production. During long-time operation, several processes may contribute to catalyst performance decline, including metal leaching, sintering, coking, poisoning, and skeleton collapse, resulting in a reduction in catalytic active sites. These deactivation pathways are influenced by catalyst structure, feedstock composition, reaction conditions, etc.

Metal leaching and sintering are usually irreversible. One strategy is to incorporate elements to form a more stable phase. Guo et al. [50] found that new Ni-Fe, Fe₃Re₂ alloy phases can be formed by introducing rhenium to prevent metal leaching and sintering. SiO₂ can be used as a structural stability promoter to avoid the leaching and sintering of active phases. The core–shell structure catalyst Pd@Al₃-mSiO₂, developed by Cao et al. [30], with an external SiO₂ shell, can effectively protect the metal from sintering and leaching. Liu et al. [90] introduced SiO₂ as a structural promoter into the Ni₂PPd/a-Al₂O₃ catalyst, which increased the stability of the catalyst by three times.

Coking, or carbon deposition, is one of the most common deactivation pathways during the deoxygenation of lipids. On the one hand, the feed containing more unsaturated double bonds is more prone to depositing carbon [91,92]. Ojagh et al. [93] found that rosin acid impurities in crude tall oil caused faster and higher amounts of carbon deposition, which reduced the HDO activity of sulfided NiMo/Al₂O₃. Furthermore, it is essential to rationally design the catalyst structure since many studies have found that there is an inseparable relationship between carbon deposition and catalyst surface acidity [94,95]. Yang et al. [96] showed that, during the hydrodeoxygenation of oleic acid and jatropha oil, excessive Brønsted acid sites on the catalyst surface would cause olefin polymerization and induce coke deposition. Based on this, they adopted an effective strategy using composite support to adjust the acidity of the catalyst. They developed a 2%Pt/50%SAPO-11-50%Al₂O₃ composite support catalyst, with a moderate Brønsted acid center, achieved a high yield of bio-aviation fuel (63.5%), and showed excellent coke resistance compared to other single carriers. The catalyst could run stably for 60 h under reaction conditions. On the other hand, improving the interaction between carrier and metal could also contribute to inhibiting coke. As reported by the same team, doping rare earth metals to Ni-based catalysts could adjust the interaction between metal and carrier, and as a result the active Ni metals could be better dispersed [49]. They found that the doped lanthanide elements, especially Er, could effectively improve the coke resistance of the catalyst in the hydrodeoxygenation of jatropha oil to biofuel due to the smaller Ni particle sizes and newly formed surface oxygen vacancies. However, the addition of Y was counterproductive since stronger acidity and larger Ni particles were formed, which reduced coking resistance. In addition, the doping of rare earth elements may inhibit coking through the mutual transformation of carbonate and oxide. Yan et al. [97] added lanthanum into Ni/SiO₂. The mutual transformation of La₂O₃ and La₂O₂CO₃ crystal phases effectively inhibited the carbon deposition on the catalyst surface and significantly improved the stability and reusability of the catalyst.

Carbon monoxide could also lead to the poisoning of metal catalysts. Due to the toxic effect of CO produced in the deoxidation process on the metal components of the catalyst, a significant imbalance between the metal and acid functions in the hydrocracking step was caused, resulting in a rapid decline in the activity of the catalyst [98]. The solution is to reduce the adsorption strength of poisons and promote their transformation into substances harmless to the system. Lee et al. [99] developed a PtRe/USY catalyst to adapt one-pot deoxidation, hydrocracking, and isomerization reactions. However, this strategy sacrifices some activity.

The collapse of catalyst structure also affects the diffusion, thereby reducing the activity. Rabaev et al. [100] added amine surfactant hexadecylamine to SAPO-11 crystallization gel to inhibit hydrothermal desilication and significantly improve the hydrothermal stability of SAPO-11.

To mitigate deactivation, catalyst regeneration strategies are employed. Thermal regeneration through oxidative treatment can remove coke deposits but may promote sintering or damage the catalyst structure. Chemical or reductive treatments can help restore metal functionality, but repeated cycles may gradually deteriorate catalyst performance. Therefore, developing catalysts with enhanced thermal stability, coke resistance, and regeneration capability remains a key focus in advancing sustainable bio-jet fuel production technologies.

3. Reaction Regulation of Lipid Conversion to Alternative Jet Fuels

A profound comprehension of reaction networks within complex systems constitutes a fundamental prerequisite for the optimization of catalyst design and reaction conditions. To elucidate these mechanisms, investigations have been conducted across diverse catalytic systems, focusing on the reaction pathways of oils, animal fats, and their model compounds (fatty acid esters and fatty acids). As shown in Figure 4, a reaction network for preparing bio-jet fuel components from triglycerides and fatty acid esters in a hydrogen-rich atmosphere was plotted, which mainly includes steps such as ester bond cleavage, deoxygenation of fatty acid and cracking, and isomerization and/or aromatization. The process typically begins with the hydrolysis or hydrogenolysis of triglycerides (or fatty acid methyl/ethyl ester), yielding free fatty acids as primary intermediates. Fatty acids undergo deoxygenation through distinct pathways, including Hydrodeoxygenation (HDO, producing H₂O), Decarboxylation (DCX, producing CO₂), and Decarbonylation (DCO, producing CO), to generate long-chain n-alkanes or n-olefin (typically C_8-18). Alkanes undergo cracking and isomerization to produce short-chain alkanes and hydrogen and can also undergo cyclization and aromatization. Alkenes may be saturated with hydrogen to form alkanes, and Diels–Alder addition may also occur to form cyclic structure. In addition, there are also some unwanted side reactions, such as coking, which can cause catalyst deactivation. Excessive cracking reaction can reduce the yield of liquid fuel; methanation reaction and reverse water gasification reaction increase hydrogen consumption and occupy hydrogenation active sites.

3.1. Cleavage of Ester Bond

The initial reaction pathway for triglycerides involves cleavage via hydrogenolysis [101] or β-elimination [102], yielding propane and fatty acids. In the case of unsaturated triglycerides, divergent mechanistic pathways may operate depending on reaction conditions: (1) prior hydrogenation saturation of double bonds may precede hydrogenolytic cleavage or (2) direct hydrogenolysis may generate unsaturated fatty acid derivatives, wherein the retained double bonds subsequently participate in isomerization, hydrogenation, or other transformations [103]. There are two parallel pathways for the conversion of fatty acid methyl esters; one is C–OCH₃ bond cleavage, which generates methanol and fatty aldehydes [104]. Fatty aldehydes can be hydrogenated to produce fatty alcohols or decarbonylated to produce alkanes with reduced carbon numbers. Another approach is the cleavage of O–CH₃ bonds to generate methane and fatty acids, which is more favorable to thermodynamics [105]. Similarly, as for fatty acid ethyl esters, the ester bond also cleavage like this [76].

3.2. Deoxygenation

In the majority of reactions involving the deoxygenation of lipids, fatty acids serve as critical intermediates, with their deoxygenation constituting the rate-determining step. Consequently, fatty acids were often used as model compounds to elucidate mechanistic details. It is universally acknowledged that there are three principal deoxygenation pathways: (1) Hydrodeoxygenation (HDO), removing oxygen in the form of water; (2) Decarbonylation (DCO), entailing C–C bond cleavage with CO elimination; and (3) Decarboxylation (DCX), characterized by the extraction of carboxyl groups to release CO₂.

The Hydrodeoxygenation (HDO) of fatty acids is a tandem reaction, in which carboxylic acids are sequentially reduced to corresponding aldehydes and alcohols at metal sites, followed by alcohol dehydration and conversion to olefins, and finally hydrogenated to produce saturated hydrocarbons. The absence or limited detection of olefins in most studies suggests two possibilities: alcohol intermediates undergo direct hydrogenation to alkanes without the olefin formation stage, or the hydrogenation of C=C bonds occurs rapidly due to their significantly lower activation energy. Alcohol intermediates may also undergo esterification reactions with reactant acids to form fatty acid esters, or etherification reactions with themselves [106]. But under appropriate reaction conditions, the thus formed esters or ethers can be reversibly converted into alcohols or acids for further hydrogenation and deoxygenation reactions.

Decarbonylation (DCO) is the process by which fatty acids are deoxygenated to produce C_n−1 alkanes and CO. Some studies consider that CO is generated by the decarbonylation of partially hydrogenated aliphatic aldehydes [107,108]. C_n aldehydes and alcohols were observed in the experiment as intermediate products and were investigated as a substrate, which performed on the DCO pathway to generate C_n−1 alkanes. The calculated results also matched their opinions [109]. Wagenhofer et al. [110] proposed the mechanism of direct DCO on a Ni-MoS₂ catalyst (Figure 5). When using C_n aldehydes and alcohols as reactants, HDO products were mainly observed and were hardly affected by competitive fatty acid compounds. It can be considered that HDO and DCO occur at different reaction sites on this catalyst, and the alcohol or aldehyde of C_n is not an intermediate product of DCO. Fatty acids produce C_n−1 alkanes through a continuous decarbonization–dehydration process through an adsorbed ketene intermediate. Recently, Rao et al. [111] proposed a revised mechanistic interpretation based on Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies of propionic acid adsorption on Ni/Al₂O₃. Their findings indicate that propionic acid predominantly adopts a bidentate binding configuration on the catalyst surface. The proposed pathway involves initial dehydroxylation to form a CH₃CH₂CO* intermediate, followed by sequential α-C dehydrogenation steps yielding CH₃CCO*, which ultimately undergoes CO elimination and hydrogenation to produce ethane. The decarbonylation pathway has also been used to prepare long-chain alkenes from fatty acids. The difference is that the product is desorbed from the catalyst before saturation [112,113].

In the Decarboxylation (DCX) of fatty acids, oxygen is eliminated as CO₂. The reaction initiates with fatty acids adsorbing onto the catalyst surface as carboxylate species, followed by the cleavage of the C–O bond to release CO₂ and generate an alkane with one fewer carbon atom [114]. While the DCX pathway does not stoichiometrically require hydrogen to proceed, empirical studies have demonstrated that moderate hydrogen partial pressure significantly enhances DCX activity [115,116].

From the perspective of carbon atom utilization, HDO is a preferable reaction since DCO and DCX result in hydrocarbon products with one carbon atom less than the reactants. The distribution of metallic and acid sites is an important factor affecting the deoxygenation pathway. Ding et al. [117] designed a strategy to separate metal active sites from acidic sites in three-dimensional hierarchical porous zeolites to improve the reaction activity of hydrodeoxygenation derived fatty acids to generate alkanes. They first treat ZSM-5 with NaOH to alkaline desilication to create a hierarchical mesoporous-microporous support (denoted as MZSM-5). Leveraging size exclusion effects, Pd nanoparticles were selectively deposited within the mesopores. Concurrently, a bulky Diethylenetriaminepentaacetic Acid (DTPA) chelating agent was employed to mask acidic sites in the mesopores and on the external support surface by complexing with aluminum species, thereby preserving only the intrinsic acidity of the microporous channels. In contrast, conventional impregnation–calcination methods failed to achieve this spatial segregation of active sites. This strategy of spatial separation of active sites significantly improved the activity of the catalyst for hydrodeoxygenation of lauric acid to generate alkanes and HDO selectivity. As shown in Figure 6, the reduction of carboxylic acids to alcohols occurs on Pd in the mesopores, followed by alcohol dehydration to form olefins in the micropores over acid sites. Subsequently, the olefins are hydrogenated on Pd within the mesopores, and the resulting alkane products diffuse into the bulk reaction medium. The cascade intermediates remain confined within the zeolite framework and are progressively consumed until the final products are generated. Li et al. [118] doped MoO₂ species into Ni/SiO₂, which significantly improved the conversion of hydrodeoxygenation of lauric acid and the yield of alkanes. Further study using the intermediate product lauryl alcohol as substrate showed that the introduction of molybdenum promoted the hydro-dehydration path of lauryl alcohol and inhibited the DCO reaction of lauraldehyde. This phenomenon can be attributed to hydrogen bonding interactions that enhance the adsorption of lauryl alcohol, facilitating the activation of the C–OH bond and thereby increasing the yield of dodecane. Similarly, Wang et al. [119] modified the surface of a Ni/red-mud (Ni/RM) catalyst with MoO_x species to significantly improve the selectivity of the direct HDO pathway. The presence of hydrogen will also affect the deoxidation path, and the hydrogen consumption in the three pathways is HDO > DCO > DCX. On the Pt/C catalyst for the hydrothermal deoxygenation of palmitic acid, it was observed that HDO and DCX pathways formed competitive adsorption on the active site of the catalyst. In the presence of formic acid as an in situ hydrogen source, more HDO intermediates tend to form [120].

3.3. Cracking, Isomerization, and Aromatization

The direct deoxygenation of lipids primarily yields long-chain alkanes. However, jet fuels derived from fossil sources contain measurable quantities of aromatics and isomeric alkanes, which critically regulates physicochemical properties such as low-temperature fluidity, viscosity, and other essential properties. Therefore, it is equally imperative to produce these components from biomass feedstocks. Current commercial bio-aviation fuel production employs a two-stage hydrodeoxygenation–hydrocracking process. This two-stage configuration is necessitated by the rapid deactivation of noble metal catalysts in one-pot systems due to CO poisoning, where CO originates from the decarbonylation pathways of fatty acids [98]. In recent years, researchers have been committed to developing highly active and stable catalysts for one-pot deoxygenation and cracking, which can effectively simplify the production process of biofuels and have stronger economic feasibility. Lee et al. [99] reported a bimetallic PtRe/USY catalyst system, where the incorporation of Re significantly enhanced CO tolerance and accelerated CO conversion, thereby solving the inhibitory effects of deoxygenation byproducts on catalytic activity. In parallel studies involving complex Waste Cooking Oil (WCO) systems, Verma et al. [121] optimized reaction conditions for a commercial Ni-Mo/Si-Al hydrotreating catalyst in a fixed-bed reactor. Under optimized conditions (420 °C, 10 MPa H₂, low Liquid Hourly Space Velocity (LHSV)), the selectivity for kerosene-range Hydrocarbons (HCs) comprised 43.6% alkanes, 8.2% cycloalkanes, and 3.73% aromatics. The final product met all specifications of the ASTM D1655 standard for aviation fuel, with no observable catalyst deactivation over 590 h of operation, demonstrating prolonged operational stability. However, the requirement for extreme reaction conditions (high temperature and pressure) necessitates elevated capital expenditure and operational risks. To achieve aviation fuel-compatible compositions from bio-lipids via single-pot synthesis, more deliberate catalyst design strategies—such as modulating acid site distribution, enhancing metal-support interactions, or introducing sacrificial CO scavengers—are imperative to circumvent current limitations.

The synergistic effect of metal and acid sites plays an important role in one-pot conversion. Liu et al. [122] developed a multifunctional NixP/HZSM-22 catalyst that could convert palmitic acid into diesel-like hydrocarbons under solvent-free conditions at atmospheric pressure. Ni_xP promotes deoxygenation sites, Brønsted acid sites on HZSM-22 play a key role in isomerization, and Lewis acid sites are responsible for cracking reactions. Liu et al. [123] developed a Nb-doped Pt/AlSiONb catalyst, in which Nb doping increased the number of weak and moderate acidic centers, particularly the increased Brønsted acid sites providing abundant sites for alkane isomerization. Under optimized reaction conditions, the catalyst achieved a liquid product yield of nearly 70% and an iso-paraffins selectivity of 80% for the conversion of palm oil.

The microarchitecture of zeolites can modulate the selectivity for iso-alkanes by influencing product diffusion. Feng et al. [79] investigated the impact of nanosheet thickness in Ni/ZSM-5 supports on the HDO of oleic acid. Their study revealed that thinner nanosheets, characterized by shorter micropore channels, preferentially yielded centrally branched isomers, whereas thicker nanosheets favored the formation of 2-position branched isomers. Weakened spatial confinement on thicker nanosheets allows external acid sites to dominate primary cracking and the central C–C bond cleavage of deoxygenated intermediates. Conversely, the shape-selective nature of internal acid sites within the zeolitic framework promotes deeper cracking and terminal C–C bond scission. Ding et al. [124] used an ionic liquid [C8mim]₂MoO₄ and SAPO-11 support to in situ generate catalysts with layered structures, highly dispersed MoS₂, and isomerization acidic centers (mainly Brønsted acidity) under reaction conditions. The conversion rate of methyl stearate on MoS₂/C8min-15% SA was 100%. The Brønsted acidic cites provided abundant sites for isomerization, thus reaching 20.5 mol% i-C_15-16 yield, and the yield of n-C_15-16 was 75 mol%.

Ishihara et al. [125] investigated the dehydrocyclization cracking reaction of methyl oleate on NiMo/ZnZSM-5/γ-Al₂O₃ catalyst. Methyl oleate undergoes decarboxylation, decarbonylation, and hydrogenation deoxygenation to produce C₁₇ and C₁₈ hydrocarbon fragments. Then, continuous hydrocracking and partial cracking on ZnZSM-5 are carried out to generate small molecule alkanes and alkenes, ultimately resulting in C_2-4 alkenes and gasoline fractions. The selective aromatization of C_2-4 olefins on the Zn/ZSM-5 catalyst to produce benzene, toluene, and xylene. Although the yield of aromatic hydrocarbons is high (20%) and more gasoline components are obtained, the 33 wt% gaseous products and catalyst carbon deposition cannot be ignored. Khan et al. [126] obtained C_11-29 alkanes and C_18-24 aromatics in one step using a 5% Ni-Fe/ZSM-5/SAPO-11 bifunctional catalyst. The innovation lies in finding suitable catalysts and obtaining yields of alkanes and aromatics. When using the S4 catalyst, the maximum selectivity of palm oil as a substitute for jet fuel is 81.74%, and the selectivity for aromatic hydrocarbons is 18%. The high calorific value of liquid fuel is 49.9 MJ/kg, and its freezing point (−60 °C) meets the low-temperature performance requirements of aviation fuel.

3.4. Side Reactions

The hydrodeoxygenation of bio-oil to produce bio-jet fuel is extremely complex. As mentioned above, reactions such as hydrodeoxygenation, hydrocracking, and hydro-isomerization would occur simultaneously in this process, producing heavy hydrocarbons that deposit on the catalyst surface, leading to the formation of coke. The deposited coke on the catalyst surface can obscure active sites, block the pores, and affect the adsorption and activation of reactant molecules, thereby reducing the activity and shortening the catalyst’s lifespan.

A large amount of gas products would be produced by over cracking, which reduces the utilization rate of carbon atoms. This unexpected side reaction can be suppressed by regulating the properties of the active metal. The study by Li et al. [45] showed that amorphous Co is more inclined towards the HDO pathway, while crystalline Co leads to over hydrogenation of the product, resulting in a higher proportion of cracking products. The balance between insufficient hydrogenation and excessive hydrogenation is extremely important. Asikin-Mijan et al. [127] suggested that the over cracking of deoxygenated products appears to be sensitive to metal concentration, which can be controlled at lower metal loading to avoid over cracking. The highest hydrocarbon yield (69%) and excellent jet fuel selectivity (60%) were achieved when 5 wt% Co and 10 wt% W were loaded.

3.5. The Influence of Reactants

There are mainly three forms of lipids involved in the research of catalytic deoxygenation for the preparation of bio-jet fuel: fatty acids, fatty acid methyl (ethyl) esters, and triglycerides. The type and chemical nature of lipids significantly influence both deoxygenation rate and product selectivity. As one of the most abundant fatty acids in vegetable oils, oleic acid and its esters are frequently used as model compounds to represent lipid feedstocks [128]. Coumans et al. [129] reported that, on a sulfided NiMo/Al₂O₃ catalyst, triolein exhibits notably higher HDO activity than methyl oleate, which is attributed to the easier hydrolysis of the triolein. Beyond lipid type, molecular characteristics such as carbon chain length and saturation degree also impact deoxygenation activity. Hachemi et al. [130] found that the reaction rate constants for both fatty acid methyl esters and triglycerides depend on the carbon chain length of the fatty acids, though the underlying mechanisms were not elaborated. Zeng et al. [131] conducted a study on the universality of raw materials for CuO@NiO nanoparticles, revealing > 99% conversion for all tested fatty acids. Saturated fatty acids such as stearic, palmitic, and lauric acids showed >90% selectivity toward C_8-18 alkanes, whereas unsaturated oleic acid only achieved 76.7% selectivity. High degrees of unsaturation in feedstocks have also been linked to coke formation, which deteriorates catalytic performance. Asikin-Mijan et al. [91] demonstrated that unsaturated feedstocks tend to generate more carbonaceous residues. Similarly, Kim et al. [92] investigated the effect of fatty acid saturation on the stability of Pt/γ-Al₂O₃ catalysts. They observed that increasing unsaturation in triglycerides led to more heavy by-products, which subsequently transformed into coke species, causing catalyst deactivation. To mitigate this, they proposed a two-step strategy combining pre-hydrogenation and deoxygenation, which significantly suppressed heavy product formation and extended catalyst life. However, this approach increases hydrogen consumption and process complexity, necessitating a thorough evaluation of its economic feasibility.

4. Summary and Outlook

Biofuels derived from biomass occupy an important position in the global energy landscape due to their low carbon emission footprint, non-toxic nature, environmental friendliness, and renewability. Converting inedible animal and vegetable oils, waste oils, and microalgae oils into liquid fuels can mitigate pollution and reduce the use of fossil resources. Researchers have made significant efforts to develop efficient deoxygenation catalysts, gaining a deeper understanding of active site construction, catalytic synergistic effects, and reaction mechanisms. However, biofuel production still faces challenges such as high costs, harsh reaction conditions, and insufficient catalyst stability. These shortcomings hinder the further application of biofuels in aviation decarbonization. To address these issues, the development of new, efficient, and multifunctional catalysts can be considered from the following aspects (Figure 7):

• It is necessary to accurately design the active center of the catalyst to improve performance. The rational design of the electronic properties and modulation of the coordination environment at active centers significantly enhances their intrinsic catalytic activity. Synergistic catalysis between active sites is pivotal for achieving efficient transformations, where structural engineering can be strategically employed to tailor reaction pathways. The distance between the two active centers (metal acid and metal oxygen vacancies) may be important parameters, but there are few pieces of research regarding that, which might be caused by the lack of effective and feasible in situ characterization methods.
• The stability and regeneration of catalysts require attention. Some strategies have been used to enhance the catalyst life, such as utilizing strong metal support interactions to improve metal dispersion and avoid catalyst sintering. Modification technologies for optimizing surface properties, especially for the catalysts of hydrothermal reactions, can maintain the hydrothermal stability of the catalyst while keeping the accessibility of the active site. Carbon deposition (coke) is usually the dominated reasons for catalyst deactivation during the bio-refining of lipids to produce bio-jet fuel. Accordingly, promoters such as both metal or non-metal species can be introduced to realize the in situ conversion and removal of carbon deposition, or integrated regeneration technology to oxidatively remove coke and then reduce the catalyst. It is necessary to study the recovery ability of regeneration technology on the physical and chemical properties of the catalyst.
• With the help of advanced in situ characterization methods and theoretical calculations, the structure–activity relationship of catalysts is constructed. The production of alternative jet fuels from bio fats involves complex reaction mechanism, while the persuasion of only indirectly inferring from the macroscopic properties of the catalyst and the distribution of products needs to be improved. For example, in situ FT-IR could be used to explore the functional group changes after the adsorption of reactants on specific structural sites on the catalyst surface, so as to speculate the adsorption surface reaction desorption behavior of raw materials. Furthermore, the combination of various in situ characterization techniques will contribute to monitoring the reaction process in real time, capture key intermediates, and finally clarify the reaction mechanism. A thorough understanding of the mechanism of the deoxidation, cracking, isomerization, and aromatization reactions involved in the process of oil conversion can provide theoretical guidance for the design of catalysts and the control of reaction conditions.
• The catalytic deoxygenation reaction under mild reaction conditions is always the goal of researchers. Other types of reaction have gradually attracted more attention from researchers. For example, photocatalytic conversion technology is favored due to its mild reaction conditions and low energy consumption. In particular, it is very attractive because it is expected to use natural sunlight as the final energy. Research on various reaction types in the process of bio lipid deoxygenation will further contribute to achieving the goal of net zero emissions. On the other hand, coupling thermal catalysis with both enzyme or photocatalysis is possibly promising, which would upset the thermal dynamic equilibrium to achieve high yields of targeted biofuels.
• The production of bio jet fuel from lipids is considered as a complex reaction system. Real bio-oils (animal and vegetable oils, microalgae oils, waste cooking oils, etc.) contain complex raw materials. There are significant differences between various lipid sources, which requires catalysts to have strong adaptability to different raw materials. During the reaction process, multiple intermediates coexist and involve various reaction types (deoxygenation, cracking, isomerization, and aromatization) and different reaction mechanisms. These together form a complex reaction network for the catalytic conversion of lipids. The deoxygenation of model compounds often resulted in one or two components of alkanes as products, while we hope to obtain hydrocarbon mixtures that meet strict fuel standards through the simple conversion of complex biomass. It is necessary to conduct in-depth research on the interactions between different substances and reactions in the complex reaction network of lipid deoxygenation reactions in order to develop more suitable catalytic systems.