Extended Data Fig. 8: Concentration profiles of pyrolysis intermediates with different molar mass at t = 8 s, t = 19 s, and t = 36 s, obtained through COMSOL Multiphysics simulation, demonstrating the gating effect of the graded-pore reactor on higher molar mass intermediates.

Transient concentration profiles of pyrolysis intermediates with different molar mass obtained through COMSOL Multiphysics simulations. We constructed a model of the graded-pore reactor featuring the same dimensions and porosity as the experimental reactor and simulated the progression of different molar mass intermediate hydrocarbons (C₁₅₃₂, C₆₉₉, C₁₁₁, C₃₁, and C₁₁) under the reaction conditions (see Methods; detailed concentration profiles throughout the pyrolysis process can be found in Video 1). The concentration profiles of the pyrolysis intermediates demonstrate the graded-pore reactor effectively regulates the mass transport of intermediates based on their molar mass. For example, at t = 8 s into the simulation, higher molar mass intermediates, such as C₁₅₃₂, remain primarily in Zone 1 (larger pore, higher-temperature). By t = 36 s, the C₁₅₃₂ species can be found in Zone 2, but are most concentrated at the boundary between Zones 2 and 3, suggesting the decrease in pore size between these zones slows the molecular species down from progressing through the reactor, effectively ‘gating’ the oligomer and increasing its residence time at that heating zone. Higher residence time under the heating conditions could promote further pyrolysis into lower molar mass intermediates, which are able to travel through the reactor more quickly. For example, the concentration profile of C₁₁ at t = 8.0 s shows this lower molar mass intermediate is already present in Zone 3 and exiting the reactor, which could also help prevent over-pyrolysis into smaller gaseous molecules. We refer to this molecular-weight-based mass transport regulation mechanism as the "gating effect."