Molecular Complexity Heuristic Suggests That Thermochemistry Will Play an Important Role in the Decarbonization of Simple Chemical Commodities

There are two competing visions for how to decarbonize
the chemical
economy. In one, renewable energy will power the chemical synthesis
of organic chemicals from small-molecule precursors such as CO2 and H2O. In the other, more complex plant-derived
compounds will fuel the bioproduction of the same commodities. This
study attempts to answer which of these visions will prevail by analyzing
the techno-economic equilibria in the current chemical industry through
the lens of chemical complexity. Our analysis suggests distinct roles
for thermochemistry and bioproduction in the decarbonization process.
For a set of the world’s most-produced molecules, we examine
the relationships among chemical price, manufacturing method, total
number of synthetic steps, and a new metric for the structural complexity,
the Böttcher number, to determine the types of molecules best-suited
to bioproduction or thermochemical synthesis. We find a stark dividing
line near a Böttcher number of ∼60. For simpler compounds
below this threshold, traditional thermochemistry is a more cost-effective
process to build compounds from small-molecule precursors, with an
average price per metric ton per step of $170/(t·step) and an
average complexity change of 6.4/step. For more complicated compounds,
bioprocesses are favored for their ability to traverse larger changes
in molecular complexity in a smaller number of steps. While the boundary
will shift with changing technological and economic conditions, the
trends are unlikely to reverse or disappear. In the absence of epochal
breakthroughs in either chemical or bioengineering, our analysis suggests
that industries should focus on decarbonizing simple feedstocks and
upgrading them through thermochemical processes for commodities with
low Böttcher complexities, which account for the majority of
greenhouse gas emissions in the chemical industry.