Breaking the ASF Constraint: Catalytic Surface Resonance as a Route to Controlled Chain-length Selectivity

The Anderson–Schulz–Flory (ASF) distribution provides a statistical description of Fischer–Tropsch (FT) product selectivity under steady-state conditions. While optimization of FT product selectivity through control of the ASF chain-growth parameter α has been widely studied, this approach inherently relies on steady-state assumptions. In this work, we introduce the first application of the nonlinear frequency response (NFR) framework to catalytic surface resonance, to analyze and predict how periodic modulation of surface kinetic parameters reshapes FT product chain-length selectivity beyond steady-state ASF descriptions. A mechanistic chain-growth kinetic model, formulated in a continuous stirred-tank reactor (CSTR), is employed, in which a periodic modulation of the propagation rate parameter kp is imposed to represent nominal catalytic surface resonance operation. The resulting nonlinear periodic reactor response is analyzed using the nonlinear frequency response method, which yields an explicit expression for the time-averaged shift in the FT product distribution as a function of forcing amplitude and modulation frequency. Steady-state analysis confirms an optimum jet-fuel-range selectivity at α = 0.85. Under dynamic operation, the FT product distribution departs systematically from steady-state ASF predictions and exhibits a pronounced resonance band between 4 and 15 Hz, within which the jet fuel carbon selectivity increases sharply and reaches a maximum near 14 Hz. Parametric analysis further reveals that the optimal forcing frequency depends on intrinsic kinetic timescales, with higher propagation rates shifting the resonance to higher frequencies while reducing the attainable enhancement. Compared with steady-state operation at the optimal static α, catalytic resonance enables time-averaged FT product distributions that not only achieve higher selectivity toward the desired jet-fuel range fraction, but also avoid undesirable increases in light hydrocarbons, demonstrating the potential of dynamic catalysis to extend selectivity control beyond the limits implied by steady-state ASF descriptions.