Unraveling Diffusion-Reaction Interplay for Rational Design of Iron-Based Fischer–Tropsch Catalysts

Industrial fixed-bed Fischer–Tropsch synthesis over large iron-based catalysts is often plagued by severe intraparticle diffusion limitations, leading to a significant loss in C5+ selectivity. Rational catalyst design has been hampered by models that fail to deconstruct the complex interplay of reaction kinetics, multicomponent transport, and the nonideal phase behavior of wax-filled pores. In this work, a rigorous multiscale framework was established by integrating detailed intrinsic kinetics with the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state to capture the thermodynamic influence on localized concentration profiles. By accurately capturing the carbon-number-dependent liquid-phase olefin concentration, this approach correctly models the enhanced olefin readsorption and secondary reactions that drive the observed deviation from ideal chain-growth polymerization. Simulation results reveal that severe CO starvation transforms the interior of large solid particles into a detrimental methane generator. This insight recharacterizes the eggshell catalyst not as a theoretical optimum, but as a pragmatic and robust compromise that effectively mitigates the diffusion limitations. Comparative analysis across structured geometries shows that the hollow cylinder effectively suppresses the internal H2/CO ratio and maintains excellent selectivity even at larger dimensions. Meanwhile, the washcoated slab demonstrates the highest kinetic potential with nearly double the conversion of eggshell spherical catalysts, though it simultaneously increases the risk of hydrothermal deactivation due to elevated water partial pressure. These findings indicate that future breakthroughs depend on the synergistic codesign of catalyst architecture and reactor configuration to manage high reaction intensities safely. This work provides the quantitative framework for such an integrated strategy.