Microbes can provide sustainable hydrocarbons for the petrochemical
industry
Engineered bacteria produce medium-chain olefins that could replace oil
and gas in syntheses

Date:
November 23, 2021
Source:
University of California - Berkeley
Summary:
The petrochemical industry turns oil and gas into precursors used
to synthesize lubricants and other critical products. Chemists
show that bacteria can be metabolically engineered to generate
similar precursors, providing a sustainable replacement for fossil
fuels and using less energy. The microbes need only glucose. The
medium-chain hydrocarbons they produce can be broken down into
shorter chains and polymerized into plastics, or lengthened to
make products such as diesel.



FULL STORY ==========================================================================
If the petrochemical industry is ever to wean itself off oil and gas,
it has to find sustainably-sourced chemicals that slip effortlessly
into existing processes for making products such as fuels, lubricants
and plastics.


========================================================================== Making those chemicals biologically is the obvious option, but microbial products are different from fossil fuel hydrocarbons in two key ways:
They contain too much oxygen, and they have too many other atoms hanging
off the carbons. In order for microbial hydrocarbons to work in existing synthetic processes, they often have to be de-oxygenated -- in chemical parlance, reduced -- and stripped of extraneous chemical groups, all of
which takes energy.

A team of chemists from the University of California, Berkeley, and the University of Minnesota has now engineered microbes to make hydrocarbon
chains that can be deoxygenated more easily and using less energy --
basically just the sugar glucose that the bacteria eat, plus a little
heat.

The process allows microbial production of a broad range of chemicals
currently made from oil and gas -- in particular, products like lubricants
made from medium-chain hydrocarbons, which contain between eight and 10
carbon atoms in the chain.

"Part of the issue with trying to move to something like glucose as a
feedstock for making molecules or to drive the chemical industry is that
the fossil fuel structures of petrochemicals are so different -- they're usually fully reduced, with no oxygen substitutions," said Michelle Chang,
UC Berkeley professor of chemistry and of chemical and biomolecular engineering. "Bacteria know how to make all these complex molecules
that have all these functional groups sticking out from them, like all
natural products, but making petrochemicals that we're used to using as precursors for the chemical industry is a bit of a challenge for them."
"This process is one step towards deoxygenating these microbial products,
and it allows us to start making things that can replace petrochemicals,
using just 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 hydrocarbon chains of medium
length, which has not been achieved before, though others have developed microbial processes for making shorter and longer chains, up to about
20 carbons. But the process can be readily adapted to make chains of
other lengths, Chang said, including short-chain hydrocarbons used as precursors to the most popular plastics, such as polyethylene.



==========================================================================
She and her colleagues published their results this week in the journal
Nature Chemistry.

A bioprocess to make olefins Fossil hydrocarbons are simple linear chains
of carbon atoms with a hydrogen atom attached to each carbon. But the
chemical processes optimized for turning these into high-value products
don't easily allow substitution by microbially produced precursors that
are oxygenated and have carbon atoms decorated with lots of 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 postdoctoral fellows,
searched databases for enzymes from other bacteria that can synthesize medium-chain hydrocarbons. They also sought an enzyme that could add a
special chemical group, carboxylic acid, at one end of the hydrocarbon,
turning it into what's called a fatty acid.

All told, the researchers inserted five separate genes into E. coli
bacteria, forcing the bacteria to ferment glucose and produce the desired medium-chain fatty acid. The added enzymatic reactions were independent
of, or orthogonal to, the bacteria's own enzyme pathways, which worked
better than trying to tweak the bacteria's complex metabolic network.



==========================================================================
"We identified new enzymes that could actually make these mid-size
hydrocarbon chains and that were orthogonal, so separate from fatty
acid biosynthesis by the bacteria. That allows us to run it separately,
and it uses less energy than it would if you use the native synthase
pathway," Chang said. "The cells consume enough glucose to survive,
but then alongside that, you have your pathway chewing through all the
sugar to get higher conversions and a high yield." That final step to
create a medium-chain fatty acid primed the product for easy conversion
by catalytic reaction to olefins, which are precursors to polymers
and lubricants.

The UC Berkeley group collaborated with the Minnesota group led by Paul Dauenhauer, which showed that a simple, acid-based catalytic reaction
called a Lewis acid catalysis (after famed UC Berkeley chemist Gilbert
Newton Lewis) easily removed the carboxylic acid from the final microbial products -- 3- hydroxyoctanoic and 3-hydroxydecanoic acids -- to produce
the olefins heptene and nonene, respectively. Lewis acid catalysis uses
much less energy than the redox reactions typically needed to remove
oxygen from natural products to produce pure hydrocarbons.

"The biorenewable molecules that Professor Chang's group made were
perfect raw materials for catalytic refining," said Dauenhauer, who
refers to these precursor molecules as bio-petroleum. "These molecules contained just enough oxygen that we could readily convert them to
larger, more useful molecules using metal nanoparticle catalysts. This
allowed us to tune the distribution of molecular products as needed,
just like conventional petroleum products, except this time we were using renewable resources." Heptene, with seven carbons, and nonene, with nine,
can be employed directly as lubricants, cracked to smaller hydrocarbons
and used as precursors to plastic polymers, such as polyethylene or polypropylene, or linked to form even longer hydrocarbons, like those
in waxes and diesel fuel.

"This is a general process for making target compounds, no matter what
chain length they are," Chang said. "And you don't have to engineer an
enzyme system every time you want to change a functional group or the
chain length or how branched it is." Despite their feat of metabolic engineering, 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 to use the
types of chemicals that microbes normally produce, rather than altering microbial products to fit into existing synthetic processes.

"There's a lot of interest in the question, 'What if we look at entirely
new polymer structures?'," she said. "Can we make monomers from glucose by fermentation for plastics with similar properties to the plastics that we
use 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 are Edward Koleski, Noritaka Hara and Yejin Min of
UC Berkeley and Dae Sung Park and Gaurav Kumar of the University of
Minnesota.

========================================================================== Story Source: Materials provided by
University_of_California_-_Berkeley. Original written by Robert
Sanders. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Zhen Q. Wang, Heng Song, Edward J. Koleski, Noritaka Hara, Dae
Sung Park,
Gaurav Kumar, Yejin Min, Paul J. Dauenhauer, Michelle
C. Y. Chang. A dual cellular-heterogeneous catalyst strategy for
the production of olefins from glucose. Nature Chemistry, 2021;
DOI: 10.1038/s41557-021-00820-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211123162806.htm

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