A square lattice topology coordination network that exhibits highly selective C2H2/CO2 separation performance

C2H2/CO2 separation is an industrially important process that remains challenging because of the similar physicochemical properties of C2H2 and CO2. We herein report that the new square lattice (sql) coordination network [Cu (bipy‐xylene)2(NO3)2]n, sql‐16‐Cu‐NO3, 16 = bipy‐xylene = 4,4′‐(2,5‐dimethyl‐ 1,4‐phenylene)dipyridine, exists in at least three forms, as‐synthesised (α), activated (α′) and hydrated (β). The activated phase, sql‐16‐Cu‐NO3‐α′, is an ultra microporous material that exhibits high selectivity towards C2H2 over CO2 as revealed by dynamic gas breakthrough experiments (1:1, C2H2/CO2) that afforded 99.87% pure CO2 in the effluent stream. The separation selectivity at 298 K and 1 bar, 78, is the third best value yet reported for C2H2 selective physisorbents whereas the mid‐loading performance sets a new benchmark. The performance of sql‐16‐Cu‐NO3‐α′ is attributed to a new type of C2H2 binding site in which CH···ONO2 interactions enable moderately strong sorbent‐sorbate binding (Qst (C2H2) = 38.6 kJ/mol) at low loading. Conversely, weak CO2 binding (Qst (CO2) = 25.6 kJ/mol) at low loading means that (ΔQst)AC [Qst (C2H2)–Qst (CO2)] is 13 kJ/mol at low coverage and 11.4 kJ/mol at mid‐loading. Analysis of in situ powder X‐ray diffraction and modelling experiments provide insight into the sorption properties and high C2H2/CO2 separation performance of sql‐16‐Cu‐NO3‐α′