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.