Renewable diesel—often called green diesel or hydrotreated vegetable oil (HVO)—has become a leading drop-in replacement for petroleum diesel. Unlike biodiesel (fatty acid methyl esters), renewable diesel is chemically identical to fossil diesel, giving it superior blending capabilities, higher energy density, and better cold-flow performance. The production of renewable diesel from triglycerides (vegetable oils, animal fats, used cooking oils) and lignocellulosic biomass relies heavily on heterogeneous catalysis—solid catalysts that operate in a different phase than the liquid or gaseous reactants. This article examines the role of heterogeneous catalysis in renewable diesel manufacturing through detailed case studies, highlighting catalyst chemistries, process configurations, and ongoing innovation.

Fundamentals of Heterogeneous Catalysis in Renewable Diesel Production

Heterogeneous catalysis is the backbone of several key upgrading steps in renewable diesel production. The most common routes involve treating oxygen-rich feedstocks with hydrogen at elevated temperatures and pressures over solid catalysts. The catalysts serve multiple functions: removing oxygen (deoxygenation), breaking large molecules (cracking), rearranging hydrocarbons (isomerization), and saturating double bonds (hydrogenation). Because the catalyst is in a separate solid phase, it can be easily separated from the product stream, regenerated, and reused—a critical advantage for continuous, large-scale operations.

Typical catalyst materials include sulfided metals (NiMo, CoMo) supported on alumina, noble metals (Pt, Pd) on acidic supports like zeolites, and oxide-based catalysts (e.g., Ni/Al₂O₃, Ni/ZrO₂). The choice of catalyst dictates the reaction pathway and, ultimately, the quality of the final fuel. For example, sulfided catalysts favor hydrodeoxygenation (HDO) to produce water and hydrocarbon, whereas noble-metal catalysts can also facilitate decarboxylation and decarbonylation. Understanding these mechanisms allows process engineers to tailor the fuel composition for maximum cetane number and minimal aromatic content.

Case Study 1: Catalytic Hydrotreatment of Triglycerides

Catalysts and Reaction Pathways

Hydrotreatment of triglycerides—the primary component of vegetable oils and animal fats—is the most commercially mature route to renewable diesel. In this process, triglycerides react with hydrogen over a solid catalyst at temperatures of 300–400°C and pressures of 30–80 bar. The catalyst typically consists of sulfided nickel-molybdenum (NiMo) or cobalt-molybdenum (CoMo) supported on gamma-alumina. These catalysts remove oxygen via hydrodeoxygenation (HDO), yielding straight-chain alkanes with 15–18 carbon atoms, water, and minor amounts of CO₂ and CO from decarboxylation/decarbonylation side reactions.

Sulfided NiMo catalysts are known for their high HDO activity and resistance to poisoning by sulfur and nitrogen compounds present in waste-derived feedstocks. However, the need to maintain a sulfided state requires continuous addition of a sulfiding agent (e.g., dimethyl disulfide), adding operational complexity. Recent research has explored non-sulfided alternatives such as reduced Ni/Al₂O₃ and Co/ZrO₂, which show comparable activity with fewer environmental concerns.

Industrial Implementation: Neste and the UOP/Eni Ecofining Process

The most prominent industrial example is Neste’s NExBTL process, which uses a proprietary heterogeneous catalyst to convert vegetable oils and waste fats into high-quality renewable diesel. Neste has scaled the process to multiple refineries with combined capacity exceeding 3 million tons per year. The catalyst system operates in fixed-bed reactors with periodic regeneration. Another well-known technology is the UOP/Eni Ecofining process, which also uses a dedicated catalyst to hydrotreat triglycerides. Ecofining units have been deployed worldwide, including a 400,000-ton-per-year plant in Venice, Italy. These case studies demonstrate that heterogeneous catalysts can achieve >95% conversion with high selectivity for diesel-range hydrocarbons (C15–C18) while meeting ASTM D975 specifications for renewable diesel. For a detailed review of catalyst design in these processes, see this recent study on hydrodeoxygenation catalysts.

Case Study 2: Catalytic Cracking of Biomass-Derived Oils

Zeolite-Based Upgrading

Fast pyrolysis of lignocellulosic biomass produces a complex bio-oil that is high in oxygen (35–50 wt%), acidic, and thermally unstable. Upgrading this bio-oil to renewable diesel requires simultaneous deoxygenation, cracking, and hydrogenation—a tall order for any single catalyst. Zeolite-based catalysts, particularly HZSM-5 and H-Y, have been extensively studied due to their acidic sites and shape-selective micropores. At temperatures of 400–500°C and atmospheric pressure (no external hydrogen), zeolites can catalyze dehydration, decarboxylation, and cracking reactions, converting bio-oil into a hydrocarbon-rich organic phase with a boiling range similar to diesel.

However, severe coking (carbon deposition) is a major challenge. Zeolites rapidly deactivate due to pore blockage and active site coverage. To mitigate this, researchers have developed mesoporous zeolites (e.g., hierarchical ZSM-5) that combine micropores with mesopores to improve mass transfer and accommodate larger molecules. Additionally, metal-modified zeolites (e.g., Ni/HZSM-5, Ga/HZSM-5) show improved hydrogen transfer and reduced coking. A pilot-scale study at the National Renewable Energy Laboratory (NREL) demonstrated that a Ga/ZSM-5 catalyst could produce a diesel-like product with a yield of 35–40 wt% from pine-derived fast pyrolysis oil, although significant process optimization was needed to extend catalyst lifetime beyond several hours.

For a comprehensive review of zeolite-catalyzed cracking of biomass-derived oils, this article in ChemSusChem provides an excellent overview of recent advances and remaining hurdles.

Case Study 3: Hydroisomerization for Improved Cold-Flow Properties

Bifunctional Catalysts

Renewable diesel produced by hydrotreatment of triglycerides consists primarily of n-alkanes (n-C15 to n-C18). While these straight-chain hydrocarbons give excellent cetane numbers (>80), they also have high melting points, leading to poor cold-flow properties (pour point often above 20°C). To make renewable diesel usable in cold climates, the n-alkanes must be isomerized into branched iso-alkanes, which have significantly lower melting points. This is achieved via catalytic hydroisomerization using bifunctional catalysts that combine a hydrogenation/dehydrogenation metal function (Pt, Pd, Ni) with an acidic support (zeolite, SAPO, or amorphous silica-alumina).

For example, Pt/HZSM-22 catalysts are highly selective for mono-branching without excessive cracking, producing a renewable diesel with cloud points below -10°C while maintaining a cetane number above 65. The process is typically operated at 250–350°C and 30–70 bar hydrogen pressure. Industrial implementation is often integrated with the hydrotreatment step: after deoxygenation, the product stream passes directly to a hydroisomerization reactor. Neste’s NExBTL process includes such an isomerization step, and many new plant designs incorporate it. A detailed case study of a Pt/ZSM-22 catalyst for hydroisomerization of model n-alkanes can be found in this Journal of Catalysis paper.

Process Integration

Integrating hydroisomerization with hydrotreatment requires careful management of catalyst compatibility and operating conditions. The sulfided hydrotreating catalyst (NiMo/Al₂O₃) is typically intolerant to the low sulfur environment needed for noble-metal isomerization catalysts. Hence many processes use dedicated reactors with intermediate sulfur removal, or they employ base-metal isomerization catalysts (e.g., Ni/W on alumina) that can tolerate low levels of sulfur. The recent development of sulfur-tolerant isomerization catalysts is a significant enabler for single-step processes.

Advantages of Heterogeneous Catalysis in Renewable Diesel Production

  • Ease of separation and reuse: Solid catalysts can be recovered by simple filtration or remain in a fixed bed, allowing continuous operation and reducing waste.
  • High selectivity: Catalyst composition (metal, support, promoters) can be tuned to favor specific reactions—e.g., hydrodeoxygenation over cracking—resulting in high yields of diesel-range hydrocarbons.
  • Energy efficiency: Heterogeneous catalysts operate at moderate temperatures (200–450°C) and pressures (10–80 bar), reducing energy input compared to thermal processes.
  • Process flexibility: The same catalyst family can be adapted to different feedstocks (virgin oils, waste fats, algal oil, tall oil) by adjusting process parameters.
  • Lower environmental impact: Heterogeneous catalysis eliminates the need for stoichiometric reagents and produces fewer byproducts, simplifying wastewater treatment and reducing emissions.
  • Scalability: Fixed-bed and fluidized-bed reactor technologies are well-established, allowing straightforward scale-up from pilot to commercial plants.

Challenges and Future Directions

Catalyst Deactivation

Despite their advantages, heterogeneous catalysts in renewable diesel production face significant deactivation challenges. Coking—the deposition of carbonaceous species on active sites—is the most common cause, especially in cracking and hydroisomerization. Regeneration by controlled oxidation can restore activity but reduces catalyst lifetime over multiple cycles. Poisoning by impurities in the feedstock (e.g., phosphorus, alkali metals, sulfur in waste oils) also deactivates catalysts irreversibly. Developing catalysts with higher resistance to coking and poisoning is a top priority. Promising approaches include using supports with optimized pore architecture (e.g., hierarchical zeolites, core-shell structures) and adding promoters that enhance hydrogen transfer.

Novel Catalyst Materials

Research is exploring non-sulfided metal catalysts to avoid the need for sulfur addition. Supported noble metals (Pt, Pd, Ru) are active but expensive and prone to sintering. Base metals like Ni, Co, and Fe on reducible supports (CeO₂, ZrO₂, TiO₂) show promise but require careful control of reduction conditions. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are emerging as tunable supports for single-atom catalysts, potentially offering exceptional selectivity. Additionally, this review in Chemical Society Reviews highlights the potential of bifunctional catalysts with spatially separated metal and acid sites for cascade reactions in biomass upgrading.

Process Intensification

Integrating catalytic reaction with separation (e.g., membrane reactors, reactive distillation) can reduce energy consumption and improve yields. For instance, catalytic membrane reactors allow continuous removal of water and other byproducts, shifting equilibrium toward desired products. Microchannel reactors provide excellent heat and mass transfer, enabling faster reaction rates and safer operation with exothermic hydrotreatment reactions. These innovations, combined with advanced catalyst design, are expected to lower capital and operating costs for renewable diesel plants.

Conclusion

Heterogeneous catalysis is the transformative technology that makes renewable diesel an economically viable and environmentally superior fuel. The three case studies examined—catalytic hydrotreatment of triglycerides, zeolite-catalyzed cracking of biomass oils, and hydroisomerization for cold-flow properties—demonstrate the breadth and depth of catalytic science required to produce a fuel that matches petroleum diesel in performance while reducing greenhouse gas emissions. Continued progress in catalyst materials (non-sulfided, hierarchical, single-atom) and process intensification will further enhance yields, extend catalyst lifetimes, and enable the use of low-cost waste feedstocks. As global demand for sustainable fuels grows, heterogeneous catalysis will remain at the heart of renewable diesel production, driving the energy transition toward a cleaner future.