Team SSwEET explains their electrochemical process for converting carbon dioxide and water into sugar, which can be fed to microbes genetically engineered to produce more complex chemicals, including food. (Video by team SSwEET and Peidong Yang)
Cestellos-Blanco admits that the SSwEET team’s process is not ready for scaling up to bulk production of sugars, but he is convinced it will work as promised, with potential applications on Earth as well as on Mars and in deep space, as a possible way to reduce carbon dioxide in the atmosphere that results from fossil fuel burning.
“This started as a side project that became very involved, but it’s been a really amazing learning process,” said Cestellos-Blanco, who is in the Department of Materials Science and Engineering, where Yang has a co-appointment. “A lot of things happen in graduate school that you don’t foresee when you first start.”
Cestellos-Blanco’s main research involves a different process for turning CO2 into more complex chemicals. Invented by Yang, the biohybrid process links microbes with semiconductor nanowires to turn CO2 into the building blocks for organic molecules, like fuels or plastics. This process also would be useful for Mars colonists or during deep space missions to other planets.
But NASA’s competition specified a non-biological process to make sugar, because the goal is to feed the sugars — ideally glucose, a sugar with six carbons — to microbes so that space explorers or planetary settlers can biomanufacture organic molecules such as food, bioplastics and pharmaceuticals. Yang’s team — which included former graduate student Yifan Li, now at Lockheed Martin Advanced Technology Center, and former postdoctoral fellow Michael Ross, now an assistant professor at the University of Massachusetts, Lowell — had to go back to the chemical literature to see how others approached the problem. They found zilch.
“For converting CO2 to sugar, there is essentially zero chemistry in the literature,” said Yang, the S. K. and Angela Chan Distinguished Chair in Energy in the College of Chemistry.
A chemical reaction key to the origin of life
They did, however, come across an old chemical process from the mid-1800s that employed lime (calcium hydroxide) to convert a different compound, formaldehyde, into various types of sugars. Some scientists have proposed that this so-called formose reaction created the first organic molecules in space that eventually became the building blocks of life.
The reaction was once thought to proceed by the condensation and addition of formaldehyde alone to form sugars, but the first step of the reaction — the conversion of formaldehyde to glycolaldehyde — occurs at an undetectable rate with an uncertain mechanism. The UC Berkeley team discovered that adding a little bit of glycolaldehyde kick-starts the formose reaction, like an autocatalyst, to yield sugars.
Since both formaldehyde and glycolaldehyde are short chains of carbon, hydrogen and oxygen atoms, the scientists asked themselves, “Could these chemicals be made directly from CO2 and then fed into the formose reaction to yield sugar?”
“I asked the team to think how to reverse engineer the CO2-to-sugar reaction,” Yang said.
The team members’ proposal to demonstrate this process earned a $50,000 phase-one grant from NASA in 2019, and despite restrictions on research necessitated by the COVID-19 pandemic, Cestellos-Blanco was able to demonstrate by the end of last year that an electrochemical process involving only electricity, copper nanoparticles as catalysts and CO2 in water, produces glycolaldehyde usable in the formose reaction. In space, the electricity would be provided by solar power.
“Ultimately, in deep space or Mars applications, everything has to be electrochemical because you can power it by solar panels,” said Yang, who has previously harvested solar energy with silicon nanowires.
Yang and Cestellos-Blanco have since shown that, using a different catalyst, they can generate formaldehyde electrochemically from CO2, as well. For the competition, the team demonstrated that the formose reaction using formaldehyde, potentially from thermochemical CO2 fixation, and glycolaldehyde from CO2 electrosynthesis , generates sugars — from three-carbon sugars up to eight-carbon sugars — in about four hours, within the time frame specified in the competition.
“We’ve made a soup of sugars and have been able to identify which sugars those are, and we’ve been able to actually go ahead and use our sugars to feed E. coli and grow them in cultures,” Cestellos-Blanco said, referring to the most common lab bacteria and the workhorse bacteria for genetic engineering.
With the new funds from NASA, the researchers plan to improve the yield of formaldehyde and glycolaldehyde using their separate electrochemical processes. Currently, glycolaldehyde is a minor product of their electrochemical process, but luckily, it is needed in only trace amounts in the formose reaction. The majority ingredient required is formaldehyde. Once these chemicals are in hand, the formose reaction is very efficient in turning all the carbon atoms into sugary carbons.
“Following a cascade-like path inspired by nature, we leveraged our expertise in CO2 recycling to open a door on the abiotic production of sugars, presenting an approach to achieve renewable sugar production,” Cestellos-Blanco said. “Using electricity and CO2 and water, we believe our findings can be used to plan deep space exploration.”
Cestellos-Blanco said that he is pleasantly surprised that his team placed in the top three of the CO2 Conversion Challenge after two rounds of proposal and report submissions, interviews with a panel of judges and an on-site visit, considering that they began knowing very little about non-biological ways to fix CO2 into complex molecules, like sugars.
“Converting CO2 directly to sugar is a pretty tall task that had never been demonstrated before, and they not only wanted you to demonstrate that you could do it, but also within a few hours, a relatively short amount of time,” he said. “The individual parts of our process have been reported before, but no one knew that you could put them all together and essentially come up with a pathway to produce useful sugars from CO2.”
Cestellos-Blanco is particularly excited that the process involves an ancient chemical reaction that may have led to life in the universe.
“I think the most interesting part, to me, is that we’re combining two types of CO2 conversion — electrocatalysis of CO2 to form formaldehyde and glycolaldehyde — with something, the formose reaction, that’s mostly been thought of as important for the origin of life,” he said.