Producing ethylene via photosynthesis
Scientists at the U.S. Department of Energy (DOE)'s National Renewable Energy Laboratory (NREL) have demonstrated a better way to use photosynthesis to produce ethylene, a breakthrough that could change the way materials, chemicals, and transportation fuels are made, and help clean the air.
NREL scientists introduced a gene into a cyanobacterium and demonstrated that the organism remained stable through at least four generations, producing ethylene gas that could be easily captured. Research results were published in Energy & Environmental Science.
The organism—Synechocystis sp. PCC 6803—produced ethylene at a high rate and is still being improved. The laboratory demonstrated rate of 170 mg of ethylene per liter per day is greater than the rates reported for the photosynthetic production by microorganisms of ethanol, butanol, or other algae biofuels.
The process does not release carbon dioxide into the atmosphere. Conversely, the process recycles carbon dioxide, a greenhouse gas, since the organism uses the gas as part of its metabolic cycle.
Ethylene is the most widely produced petrochemical feedstock in the world. But currently it is produced only from fossil fuels, and its production is the industry’s largest emitter of carbon dioxide. Steam cracking of long-chain hydrocarbons from petroleum produces 1.5 to 3 tons of carbon dioxide for every ton of ethylene produced.
The NREL process, by contrast, produces ethylene by using carbon dioxide, which is food for the bacteria. That could mean a savings of six tons of carbon dioxide emissions for every ton of ethylene produced—the three tons that would be emitted by tapping fossil fuels and another three tons absorbed by the bacteria.
NREL principal investigator, Jianping Yu, says it’s the difference between using old photons and new photons. Ethylene from old photons is the ethylene produced from fossil fuels, derived from photosynthetic organisms that captured the sun’s energy millions of years ago. The NREL process uses new photons that are currently hitting plants, algae, and bacteria capable of producing fuels directly.
Ten years ago, a group of Japanese scientists led by Takahira Ogawa at Sojo University was the first to try to produce ethylene via photosynthetic conversion in the cyanobacterium Synechococcus 7942. But by the fourth generation, the bacteria were defunct, producing no ethylene at all, Yu says.
NREL turned to a different cyanobacterium, Synechocystis 6803, which scientists had been researching for a long time, knowing how to change its DNA sequences. They manipulated the sequence to design an ethylene-producing gene to be more stable and more active than the original version.
This process resulted in an organism that uses carbon dioxide and water to produce ethylene, but doesn’t lose its ability to produce ethylene over time. The product ethylene is non-toxic to the producing microorganisms and is not a food source for other organisms that could potentially contaminate an industrial process.
"Our peak productivity is higher than a number of other technologies, including ethanol, butanol, and isoprene," Yu says. "We overcame problems encountered by past researchers. Our process doesn't produce toxins such as cyanide and it is more stable than past efforts. And it isn't going to be a food buffet for other organisms."
After the culture reaches maximum growth, it's possible that it could keep producing for months at a time, says Rich Bolin, who is a member of NREL's partnerships group. The ethylene gas it produces naturally leaves the organism, spurring the organism to keep producing more.
The ethylene would be produced in an enclosed photobioreactor containing seawater enriched with nitrogen and phosphorous. The ethylene gas would rise and be captured from the reactor’s head space. It could then undergo further processing, including a catalytic polymer process to produce fuels and chemicals. The continuous production system improves the energy conversion efficiency and reduces the operational cost.
NREL is initiating discussions with potential industry partners to help move the process to commercial scale. Interested companies include those in the business of producing ethylene or—transportation fuels, as well as firms that build photobioreactors.
"Separations in biotechnology are complicated and costly," says Jim Brainard, director of NREL's Biosciences Center. "The nice thing about this system is that it is a gas that just separates from the culture media and rises to the head space. That's a huge advantage over having to destroy the valuable culture that is taking carbon dioxide and light and water to make your product. It's much easier than a liquid-liquid separation like in ethanol."Source: National Renewable Energy Laboratory