Tuning the switching pressure in square lattice coordination networks by metal cation substitution

Abstract

Coordination networks that undergo guest-induced switching between “closed” nonporous and “open” porous phases are of increasing interest as the resulting stepped sorption isotherms can offer exceptional working capacities for gas storage applications. For practical utility, the gate ad/desorption pressures (P_ga/P_gd) must lie between the storage (P_st) and delivery (P_de) pressures and there must be fast switching kinetics. Herein we study the effect of metal cation substitution on the switching pressure of a family of square lattice coordination networks [M(4,4′-bipyridine)₂(NCS)]_n (sql-1-M-NCS, M = Fe, Co and Ni) with respect to CO₂ sorption. The Clausius–Clapeyron equation was used to correlate P_ga/P_gd and temperature. At 298 K, P_ga/P_gd values were found to vary from 31.6/26.7 bar (M = Fe) to 26.7/20.9 bar (M = Co) and 18.5/14.6 bar (M = Ni). The switching event occurs within 10 minutes as verified by dynamic CO₂ sorption tests. In addition, in situ synchrotron PXRD and molecular simulations provided structural insight into the observed switching event, which we attribute to layer expansion of sql-1-M-NCSvia intercalation and inclusion of CO₂ molecules. This study could pave the way for rational control over P_ga/P_gd in switching adsorbent layered materials and enhance their potential utility in gas storage applications.