Reaction‐controlled kinetic sieving: Mechanistic insights into selective SO ₂ / CO ₂ absorption using alkaline‐wetted membrane contactor

Membrane contactors for gas separation typically employ hydrophobic membranes to prevent pore wetting, a phenomenon considered detrimental to performance. This work disruptively demonstrates that deliberately wetted hydrophilic membranes enable ultrahigh selective and fast SO 2 /CO 2 separation, outperforming hydrophobic membranes. We unfold the underlying mechanism through three causal and hierarchical layers: interface advancement, microenvironment creation, and asymmetric compression of reaction fronts, formally defining it as Reaction‐Controlled Kinetic Sieving (RCKS). To transform it into a quantitative theory, we defined an intrinsic parameter, RCKS sieving factor (SF), which quantifies the relative positioning of reaction fronts between fast and slow components. Each front is defined by the condition where its local Damköhler number equals one, indicating the region of strongest reaction–diffusion competition. By establishing a two‐dimensional mechanism variable space (intrinsic SF vs. external fast/slow component interfacial concentration ratio), we provide a universal framework for designing high‐performance, process‐centric gas separation systems that transcend material‐centric limitations.