If the petrochemical industry is ever to wean itself from oil and gas, it must find sustainably sourced chemicals that blend easily with existing processes used to make products such as fuels, lubricants, and plastics.
Biologically manufacturing these chemicals is the obvious option, but microbial products differ from fossil hydrocarbons in two major ways: they contain too much oxygen and have too many other atoms attached to the carbons. In order for microbial hydrocarbons to function in existing synthesis processes, they often have to be freed from oxygen – reduced in chemical usage – and freed from foreign chemical groups, all of which costs energy.
A team of chemists from the University of California, Berkeley, and the University of Minnesota have now engineered microbes to make chains of hydrocarbons that can be deoxygenated more easily and with less energy – basically just the sugar glucose the bacteria eat plus one little warmth.
The process enables the microbial production of a wide range of chemicals currently made from oil and gas – especially products like lubricants made from medium-chain hydrocarbons, which contain between eight and ten carbon atoms in the chain.
“Part of the problem with trying to move to something like glucose as a raw material to make molecules or to power the chemical industry is that petrochemical fossil fuel structures are so different – they’re usually completely reduced with no oxygen exchange. “Said Michelle Chang, professor of chemistry and chemistry and biomolecular engineering at UC Berkeley. “Bacteria know how to make all these complex molecules that have all these functional groups protruding from them, like all natural products, but making petrochemicals, which we use as precursors for the chemical industry, is a bit of a challenge for them. . ”
“This process is a step in the deoxygenation of these microbial products and allows us to make things that can replace petrochemicals by only using glucose from plant biomass, which is more sustainable and renewable,” she said. “That way we can get away from petrochemicals and other fossil fuels.”
The bacteria were engineered to make medium-length hydrocarbon chains, which has not been achieved before, although other microbial processes have evolved to make shorter and longer chains up to about 20 carbon atoms. But the process can be easily adapted to make chains of other lengths, Chang said, including short-chain hydrocarbons, which are used as precursors to the most common plastics like polyethylene.
She and her colleagues published their results in the journal this week Natural chemistry.
A bioprocess used to make olefins
Fossil hydrocarbons are simple linear chains of carbon atoms with a hydrogen atom on each carbon. But the chemical processes optimized for this, to transform these into high-quality products, do not allow them to be replaced by microbially produced precursors that are enriched with oxygen and whose carbon atoms are decorated with many other atoms and small molecules.
To get bacteria to produce something that can replace these fossil fuel precursors, Chang and her team, including co-first authors Zhen Wang and Heng Song, former UC Berkeley postdocs, searched databases for enzymes from other bacteria called medium-chain hydrocarbons can synthesize. They were also looking for an enzyme that could add a special chemical group, carboxylic acid, to one end of the hydrocarbon and convert it into something called a fatty acid.
All in all, the researchers added five separate genes in And coli Bacteria, forcing the bacteria to ferment glucose and produce the desired medium-chain fatty acid. The added enzymatic reactions were independent or orthogonal to the bacteria’s own enzyme pathways, which worked better than trying to optimize the bacteria’s complex metabolic network.
“We identified new enzymes that could actually produce these medium-sized hydrocarbon chains and that were orthogonal, i.e. separate from the fatty acid biosynthesis by the bacteria. That allows us to run them separately and they use less energy than if you did the native synthase pathway, ”said Chang. “The cells use up enough glucose to survive, but besides that, your path has to chew through all the sugar for higher sales and high yields.”
This final step in the production of a medium chain fatty acid prepared the product for simple catalytic conversion to olefins, which are precursors to polymers and lubricants.
The UC Berkeley group worked with the Minnesota group, led by Paul Dauenhauer, who demonstrated that a simple acid-based catalytic reaction called Lewis acid catalysis (after the famous UC Berkeley chemist Gilbert Newton Lewis) the carboxylic acid is easily removed from the microbial end products – 3-hydroxyoctanoic acid and 3-hydroxydecanoic acid – to produce the olefins hepten and nonene, respectively. Lewis acid catalysis uses much less energy than the redox reactions normally required to remove oxygen from natural products to produce pure hydrocarbons.
“The bio-renewable molecules that Professor Chang’s group produced were perfect raw materials for catalytic refining,” says Dauenhauer, who calls these precursor molecules bio-petroleum. “These molecules contained just enough oxygen that we could easily convert them into larger, more useful molecules using metal nanoparticle catalysts. This allowed us to adjust the distribution of the molecular products as needed, just like traditional petroleum products, only this time we used renewable energy. “Resources.”
Heptenes with seven carbon atoms and nuns with nine can be used directly as lubricants, cracked into smaller hydrocarbons and used as precursors to plastic polymers such as polyethylene or polypropylene, or linked to form even longer hydrocarbons, such as those in waxes and diesel fuel.
“This is a general process for making target compounds regardless of their chain length,” said Chang. “And you don’t have to develop an enzyme system every time you want to change a functional group or the chain length or its branching.”
Despite her metabolic masterstroke, Chang noted that the long-term and more sustainable goal would be to completely redesign processes for synthesizing industrial hydrocarbons, including plastics, so that they are optimized for the use of chemicals normally produced by microbes, instead of changing microbial products to fit into existing synthetic processes.
“The question, ‘What if we looked at completely new polymer structures?’ She says. “Can we use fermentation to produce monomers for plastics from glucose that have similar properties to the plastics used today, but not the same structures as polyethylene or polypropylene, which are not easy to recycle.”
The work was supported by the Center for Sustainable Polymers, a National Science Foundation-supported center for chemical innovation (CHE-1901635). Other co-authors include Edward Koleski, Noritaka Hara and Yejin Min from UC Berkeley and Dae Sung Park, and Gaurav Kumar from the University of Minnesota.