A series of pyrazine-interior-embodied MOF-74 composites (py-MOF-74) were successfully synthesized by a post vapor modification method, concomitant with the loading ratio of pyrazine easily controlled in this process. Here, pyrazine molecules perform as a cavity-occupant to block the wide pores of MOF-74, which accentuates the adsorption discrepancy of a pair of gases on MOFs and consequently reinforces the adsorption selectivity (typically for CO₂/N₂, CO₂/CH₄). Different from the “physical confinement” of occupants, pyrazine molecule with dual “para-nitrogen” atoms donates one N atom to bond with the open metal ion of MOF-74 for stability, and remains the other N atom available for potential CO₂ trapping site. Pyrazine-interior-embodied MOF-74 composites manifest significantly improved CO₂/N₂ and CO₂/CH₄ adsorption selectivity. Typically, py-MOF-74c with ultimate pyrazine insertion displays selectivity greatly superior to MOF-74 in the equimolar CO₂/CH₄ (598 vs. 35) and the simulated CO₂/N₂ flue gas (471 vs. 49) at 100 kPa and 298 K.