Jet Fuel from CO₂? How United and Twelve Are Pioneering the Future of Clean Aviation

A bold bet on photosynthesis and limestone

In a world racing toward net-zero, United Airlines is staking its future on two powerful tools: direct air capture and synthetic fuels. Backed by its $200 million Sustainable Flight Fund, the airline is investing in technologies that don’t just offset emissions—they transform them.

One of its most promising partners is Twelve, a California-based startup that mimics photosynthesis to convert CO₂ into drop-in sustainable aviation fuel (SAF). At the same time, United is also teaming up with Heirloom, whose limestone-based capture system pulls carbon from the sky and sequesters it—or uses it to make fuel.

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From waste gas to jet fuel: Twelve’s breakthrough

Founded in 2015, Twelve has developed a novel process that splits CO₂ into carbon monoxide (CO) using a metal catalyst. Water is also split into hydrogen, and when CO and hydrogen are combined, they form synthesis gas, a key ingredient for creating SAF.

All of this is powered entirely by renewable electricity. The result? An ultra-clean aviation fuel that can cut lifecycle greenhouse gas emissions by up to 90% compared to fossil-derived jet fuel.

According to Nicholas Flanders, co-founder and CEO of Twelve, “We’re not just envisioning the decarbonization of air travel—we’re actively building it.” Twelve’s first commercial facility, AirPlant One in Moses Lake, Washington, is set to begin production this year, delivering 50,000 gallons of SAF annually.

United’s long-term play on SAF and infrastructure

United’s investment in Twelve isn’t just about science—it’s a strategic push toward scalable solutions. Andrew Chang, managing director of United Airlines Ventures, said, “Scaling the SAF industry is the major hurdle air travel needs to clear in order to increase the supply and reduce the price of lower carbon fuels.”

The commitment is more than symbolic. Twelve recently signed a 14-year supply deal to provide 260 million gallons of SAF to a European airline group—one of the largest SAF agreements to date. That commercial traction signals Twelve’s shift from innovation to industrial scale.

United itself used 14 million gallons of SAF in 2024, which still accounted for just 0.3% of its total fuel use. The challenge now is scaling up those numbers while reducing production costs.

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Heirloom: turning rocks into climate solutions

While Twelve tackles synthetic fuel production, Heirloom addresses another part of the puzzle: pulling CO₂ directly out of the atmosphere. Their process starts by heating crushed limestone in kilns powered by cleaner energy. This releases carbon dioxide and leaves behind calcium oxide.

The calcium oxide is mixed with water to form calcium hydroxide, which is spread on trays and exposed to air. In just a few days, it reabsorbs atmospheric CO₂ and turns back into limestone - creating a loop that can be repeated over and over.

This captured CO₂ can either be permanently stored or converted into SAF, offering a dual-use advantage. United’s fund has the option to purchase 500,000 tons of CO₂ from Heirloom, equivalent to offsetting emissions from a 737 MAX flying 33 million miles.

Why this matters for the industry

United’s partnerships reflect a deeper industry trend: decarbonization strategies are shifting from tree-planting offsets to engineered, measurable removals. Technologies like SAF and direct air capture are being recognized as the backbone of long-term climate plans, especially in hard-to-abate sectors like aviation.

Beyond tech, policy also plays a role. United’s Lauren Riley, Chief Sustainability Officer, helped shape the SAF provisions in the U.S. Inflation Reduction Act, including the 45Z Clean Fuel Production Tax Credit. This provides 35 cents per gallon in incentives for SAF, giving companies like Twelve a critical runway for scaling.

However, with political shifts underway, the industry is watching closely. The continuation of the 45Z credit, and other SAF-supportive policies, will determine how fast these innovations move from niche to norm.

A path that cuts through politics

Interestingly, many new SAF projects are landing in traditionally conservative U.S. states. Riley notes that these areas are benefiting economically from the job creation and infrastructure investment that SAF facilities bring.

“This is good for the economy. This is creating jobs in a transitioning industry,” she said in a recent interview. With SAF demand growing and bipartisan economic benefits becoming clearer, momentum is building despite federal uncertainty.

Looking ahead

Twelve’s $83 million Series C round, major SAF contracts, and active commercial plant prove the company isn’t just a promising startup - it’s now a key player. Heirloom’s scalable capture system adds another layer to United’s integrated sustainability strategy, linking fuel innovation to permanent carbon removal.

If aviation is to fly into a net-zero future, it will require bold bets - and bigger coalitions. United’s alignment with startups like Twelve and Heirloom suggests the blueprint is already underway.

For the aviation sector, this isn’t just a test flight. It’s a full-throttle launch into cleaner skies.