The almost identical molecular sizes and volatilities of acetylene and carbon dioxide make their separation extremely challenging in industry. Here, we report the efficient separation of acetylene and carbon dioxide (v/v=2/1, which is relevant to that in the industrial cracking stream) in faujasite zeolites decorated with atomically-dispersed copper(II) sites under ambient conditions. In situ neutron powder diffraction and inelastic neutron scattering confirm that the confined copper(II) site displays chemo-selective yet reversible binding to acetylene, whereas adsorbed carbon dioxide molecules are stablished by weak host-guest supramolecular interactions with the framework oxygen centres, thus resulting in the efficient separation of these two gases under flow conditions. A designed adsorption-purging-desorption system based upon Cu@FAU is established for the recovery of high purity acetylene (98-99%) from the mixture of acetylene and carbon dioxide, offering an unprecedented separation factor of 22.2 with an effective dynamic uptake of acetylene of 1.51 mmol g -1 at 298 K.