1. Introduction
The global energy sector is under increasing pressure to reduce reliance on fossil fuels and mitigate greenhouse gas emissions. Biofuels derived from vegetable oils have gained attention as renewable alternatives to petroleum-based fuels [6, 9]. Palm oil, due to its high oil content, low cost, and abundant availability in tropical regions, represents a promising feedstock for biofuel production.
However, direct use of vegetable oils as fuel leads to issues such as high viscosity, poor volatility, and incomplete combustion. Hydrocracking, a catalytic process involving hydrogenation and cracking reactions, can convert triglycerides in palm oil into shorter-chain hydrocarbons similar to diesel [2, 8, 10]. Developing effective catalysts is crucial to enhancing the conversion efficiency and selectivity of this process.
2. Catalyst Design and Rationale
Table 1. Properties of components used in the Ni–Al₂O₃–bentonite catalyst
| Component | Function | Key Properties |
| Nickel (Ni) | Hydrogenation metal | High hydrogenation activity, promotes deoxygenation |
| Alumina (Al₂O₃) | Support | High surface area, thermal stability, moderate acidity |
| Bentonite | Acidic co-support | High surface area, enhances acidity, low cost |
Figure 1. Schematic of the bifunctional nature of Ni–Al₂O₃–bentonite catalyst
The Ni–Al₂O₃–bentonite system combines these advantages:
- Ni sites facilitate hydrogenation of unsaturated bonds and deoxygenation of triglycerides [5, 8].
- Al₂O₃ serves as a robust support material with high mechanical strength.
- Bentonite contributes additional Bronsted and Lewis acid sites, promoting C–C bond cracking and improving catalyst porosity [4].
3. Catalyst Preparation
The Ni–Al₂O₃–bentonite catalyst can be synthesized via the wet impregnation method [2, 3]:
- Support preparation: Bentonite is purified, dried, and mixed with Al₂O₃ powder. The mixture is calcined at 500–600 °C.
- Metal impregnation: A nickel nitrate [Ni(NO₃)₂·6H₂O] solution is added dropwise to the Al₂O₃–bentonite mixture.
- Drying and calcination: Dried at 110 °C and calcined at 500 °C for 4 h.
- Reduction: Reduced in hydrogen at 450–500 °C to obtain metallic Ni.
4. Catalyst Characterization
Table 2. Typical characterization results of Ni–Al₂O₃–bentonite catalyst
| Technique | Purpose | Typical Observation |
| XRD | Crystal phases, Ni dispersion | NiO peaks, well-dispersed Ni particles |
| BET surface area | Surface area, porosity | 150–250 m²/g, mesoporous structure |
| SEM/TEM | Morphology and particle distribution | Uniform Ni nanoparticles (5–15 nm) |
| TPR | Reducibility of Ni species | Main reduction peak at 400–500 °C |
| NH₃-TPD | Surface acidity | Strong and moderate acid sites observed |
Figure 2. Representative SEM/TEM images of Ni–Al₂O₃–bentonite catalyst
These properties confirm the presence of active Ni particles and the mesoporous structure that enhances accessibility of reactant molecules [2, 4, 5].
5. Hydrocracking Process
Hydrocracking of palm oil is typically carried out in a fixed-bed or batch reactor under high hydrogen pressure (30–80 bar) and 350–450 °C [1, 2].
Process steps:
- Feed preparation (filtering and preheating palm oil)
- Reaction under hydrogen flow over Ni–Al₂O₃–bentonite catalyst
- Product condensation and separation into gaseous, liquid, and solid fractions
- Analysis using GC and GC–MS for hydrocarbon distribution
Figure 3. Process flow diagram of palm oil hydrocracking
6. Reaction Pathways
Palm oil mainly composed of triglycerides undergoes:
- Hydrogenation: Saturation of double bonds
- Hydrodeoxygenation (HDO): Removal of oxygen as water
- Decarboxylation/decarbonylation: Removal of oxygen as CO₂ or CO
- Cracking: Breaking of C–C bonds to produce C₅–C₂₀ hydrocarbons [5, 8, 10].
7. Catalyst Performance and Factors Affecting Yield
Table 3. Effect of process variables on biofuel yield over Ni–Al₂O₃–bentonite
| Parameter | Typical Range | Effect on Yield |
| Ni loading | 5–15 wt% | Higher loading improves hydrogenation, too high causes sintering |
| Temperature | 350–450 °C | Higher temperature increases cracking and diesel-range yield |
| Pressure | 30–80 bar | High H₂ pressure promotes deoxygenation |
| H₂/oil ratio | 400–800 Nm³/m³ | Prevents coking, enhances conversion |
| Reaction time | 1–5 h | Longer time improves conversion but may cause over-cracking |
Ni–Al₂O₃–bentonite typically achieves >90% conversion of palm oil with selective diesel-range hydrocarbon production, and shows good reusability over multiple cycles [2, 3, 8, 10].
8. Environmental and Economic Considerations
- Renewable feedstock reduces reliance on fossil fuels [6, 9].
- Produced biofuel is sulfur-free and cleaner-burning.
- Bentonite is inexpensive and widely available [4].
Challenges include:
- Ensuring sustainable palm oil cultivation [9].
- Efficient regeneration of spent catalysts.
- Process cost vs. conventional diesel production.
9. Conclusion
The Ni–Al₂O₃–bentonite catalyst is a promising system for hydrocracking palm oil into biofuels. Its bifunctional nature—combining hydrogenation and cracking activities—enables efficient deoxygenation and conversion of triglycerides to hydrocarbons. Continued optimization of catalyst formulation and reaction conditions could make this renewable route commercially viable for green fuel production.
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