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• A series of chitosan-derived ultramicroporous carbons were prepared using activating reagents-free method. • CNC-700 possesses excellent C 2 H 4 /C 2 H 6 molecular sieving performance. • High purity of C 2 H 4 can be produced via one-step adsorption–desorption process. • The good stability and scalability make CNC-700 a promising adsorbent for industrial applications. The selective separation of C 2 H 4 /C 2 H 6 pair is of paramount importance in the petrochemical industry, which is yet challenged by their similar polarities, shapes, and kinetic diameters. The use of carbonaceous adsorbents with molecular sieving capability holds great promise in achieving the desired selectivity and economically feasibility. However, such progress has been hindered by the typically wide pore size distribution observed in traditional carbons. Herein, we report a series of chitosan-based ultramicroporous carbons with C 2 H 4 /C 2 H 6 molecular sieving performance through an activating reagents-free method. The hydrothermal process of chitosan was initially employed to prepare condensed carbonaceous hydrochar rich in heteroatoms. The subsequent thermal etching led to the formation of sieving-based ultramicropores, stemming from the decomposition of heteroatoms and hot-shrinkage of pores at high temperature. The as-synthesized optimal CNC-700 sample achieved a high C 2 H 4 uptake of 32.3 cm3 (STP) g−1 and negligible C 2 H 6 adsorption at 298 K and 1.0 bar. The isosteric heat of C 2 H 4 on CNC-700 reached 34.5 kJ mol−1, significantly lower than other thermodynamically driven carbonaceous adsorbents. The fixed-bed breakthrough experiments further validated the efficient separation of equimolar C 2 H 4 /C 2 H 6 mixture under dynamic conditions. This work provides an important guideline to fabricate carbons with adjustable ultramicropore size derived from biomass for the highly effective separation of light hydrocarbons via molecular sieving. [ABSTRACT FROM AUTHOR]