As the global aviation industry strives to reduce its environmental impact, the race is on to develop sustainable aviation fuels (SAFs) that can replace traditional petroleum-based jet fuel. One promising solution lies in an unlikely source – the abundant biopolymer lignin. Researchers have now demonstrated a novel process to directly convert lignin into high-quality jet fuel hydrocarbons, paving the way for a more sustainable future in air travel.

The Challenge of Sustainable Aviation Fuels

The aviation sector is a major contributor to global greenhouse gas emissions, accounting for around 2-3% of the worldwide total. With air travel expected to rebound strongly in the coming decades, the pressure is on to find ways to decarbonize the industry. SAFs have emerged as a key part of the solution, offering the potential to slash emissions compared to conventional jet fuel.

Most current SAFs are produced from plant and animal lipids, such as vegetable oils and animal fats. However, the limited availability of these feedstocks means they cannot be scaled up to meet the projected surge in future jet fuel demand. “It is commonly agreed that a multitude of new pathways will be necessary to meet the 2050 goal of net-zero aviation emissions,” says Bin Yang, a professor at Washington State University and lead author of the new study.

Another challenge is that many SAFs lack the desired properties of conventional jet fuel. “Many airlines limit the direct use of these SAFs and balance their fuel with conventional jet fuel to rectify concerns around density, seal swell, and other components,” explains Yang. This is largely due to the lack of certain hydrocarbon classes, such as aromatics and cycloalkanes, which are important for fuel compatibility and performance.

Tapping into Lignin’s Potential

Enter lignin, a complex aromatic polymer that is the second most abundant natural polymer on Earth after cellulose. Lignin is a byproduct of various biorefining processes, with around 300 million tonnes produced globally each year. “Lignin has the capacity to produce an aromatic structure-based polymer that can act as the base to produce fuels, chemicals, and materials,” says Yang.

Previous research from Yang’s group and others has shown the potential of using catalytic depolymerization and hydrodeoxygenation (HDO) to convert lignin into jet fuel-range hydrocarbons. These processes can selectively produce a mixture of cycloalkanes – a hydrocarbon class that can potentially substitute for aromatics in jet fuel, providing comparable density and seal swell properties.

However, the challenge has been developing a continuous flow process that can handle the complexity of technical lignin feedstocks. “Addressing the challenges posed by the heterogeneous nature of lignin is crucial for successful technical lignin processing in continuous flow reactors,” says Yang.

A Breakthrough in Continuous Lignin Conversion

In their latest work, published in Fuel Processing Technology, the researchers demonstrate a novel “simultaneous depolymerization and hydrodeoxygenation” (SDHDO) process that can continuously convert lignin into high-quality jet fuel hydrocarbons.

The key to their approach is an engineered bifunctional catalyst, dubbed Ru-HY-60-MI, which combines an acidic zeolite support with highly dispersed ruthenium nanoparticles. “The design and synthesis of efficient and engineered bifunctional catalysts for the direct lignin HDO is challenging,” explains Adarsh Kumar, the paper’s first author and a researcher at the National Renewable Energy Laboratory.

“Competition between depolymerization, ring saturation and C-O hydrogenolysis creates difficulty to originate ring saturated hydrocarbons. An excellent combination of these properties can be an effective catalyst for saturated hydrocarbon production.”

The researchers packed the Ru-HY-60-MI catalyst into a continuous flow reactor and fed in a solution of alkali-treated corn stover lignin. At optimized reaction conditions of 250°C and 1,150 psi hydrogen pressure, they were able to achieve a maximum carbon yield of 17.9% for the lignin-based jet fuel (LJF) product.

Detailed analysis revealed that the LJF consisted of 60.2% monocycloalkanes and 21.6% polycycloalkanes – hydrocarbon classes that are crucial for fuel compatibility and performance. “Tier α fuel property testing indicates that LJF production using SDHDO chemistry can produce SAF with high compatibility, good sealing properties, low emissions, and high energy density for aircraft,” says Yang.

A key aspect of the research was the detailed characterization of the Ru-HY-60-MI catalyst before and after use. This provided valuable insights into the catalyst’s structure and behavior during the SDHDO process.

The team found that the HY zeolite structure and crystallinity were largely preserved after the catalyst engineering steps, with small ruthenium nanoparticles (average size of 2.9 nm) dispersed throughout the support. Temperature-programmed reduction analysis revealed the presence of two different types of ruthenium oxide species, suggesting good metal-support interactions.

However, the spent catalyst showed some significant changes. Inductively coupled plasma analysis detected a substantial increase in potassium content, likely due to ion exchange between the lignin solution and the HY zeolite. There was also evidence of carbon deposition on the catalyst surface, which contributed to its deactivation over time.

“Catalyst deactivation due to contaminants and coke formation is one of the main challenges we need to overcome to make this approach commercially viable,” says Kumar. “Further work is needed to develop more robust catalysts and reactor designs that can withstand the harsh conditions of lignin conversion.”

The researchers believe their insights into the catalyst structure-activity relationships will help guide the development of next-generation catalysts for continuous lignin upgrading. “Catalyst characterization methodologies suggested that Ru-HY-60-MI has small Ru size with good dispersion over HY-60 support,” notes Kumar. “However, a catalyst deactivation was observed due to exchange of K+ ion from lignin solution to HY-60 and carbon deposition on Ru-HY-60-MI surface during the experiment.”

The successful demonstration of the SDHDO process in continuous flow is a significant step forward for the commercialization of lignin-based jet fuels. Previous studies on LJF had relied on batch reactors, which are less scalable and economically viable for large-scale production.

“Commercial scale production will help to realize the environmental benefits of sustainable aviation fuel,” says Yang. “To achieve the commercial production goal of LJF, a continuous process is required.”

The researchers note that their approach addresses several key challenges associated with continuous lignin conversion. “We overcame the complexities of direct lignin conversion and the preservation of cyclic structures in the product by employing innovative reaction engineering techniques,” explains Yang.

Looking ahead, the team is working to further optimize the process and address remaining hurdles. “We will continue advancing the process towards deployment by overcoming three main challenges: decreasing the cost of the hydrotreating catalyst, improving the efficiency of solid catalysts due to the polymeric structure of lignin, and hindering catalyst deactivation due to contaminants and coke formation,” says Yang.

If successful, lignin-based jet fuel could open up new possibilities for sustainable aviation. “LJF can offer complementary properties to existing SAF pathways, providing the required density and seal swell that n- and isoalkanes cannot achieve, while boosting the derived cetane number of the blended fuel,” explains Yang.

The potential environmental benefits are also significant. “Economical production of lignin-based jet fuel can improve the sustainability of sustainable aviation fuels as well as can reduce the overall greenhouse gas emissions,” says Yang.

A Promising Path to a Greener Future in Aviation?

The development of lignin-based jet fuel represents an exciting breakthrough in the quest for more sustainable air travel. By leveraging the abundant and underutilized resource of technical lignin, researchers have demonstrated a viable pathway to produce high-quality jet fuel hydrocarbons that can complement existing SAF options.

The continuous SDHDO process, enabled by an innovative bifunctional catalyst, overcomes several key challenges associated with lignin conversion and sets the stage for potential commercial-scale production. While challenges remain, the promise of lignin-derived jet fuel is clear – a future where air travel can be powered by renewable, low-emission fuels derived from plant waste.

As the global aviation industry continues to grapple with its environmental impact, solutions like lignin-based jet fuel offer a glimpse of a more sustainable path forward. With further research and optimization, this technology could play a crucial role in helping the sector achieve its ambitious decarbonization goals.

Reference(s)

1. https://doi.org/10.1016/j.fuproc.2024.108129

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