Chapter 12: Gated and Stepwise Sorption Processes in Functional Metal-organic Frameworks

Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) are a fascinating class of materials whose constituent motifs, metal–metal clusters and organic pillars, arrange themselves in a periodic fashion in space to generate a wide range of crystalline architectures. An intriguing subclass of these materials is flexible or soft MOFs, also popularly known as soft porous crystals that can change or adapt crystal structure in response to various external stimuli such as guests, pressure, temperature, light etc. This softness enables MOFs to switch over between different crystalline phases resulting in many interesting properties which are rarely observed in rigid MOFs. Among the various studies devoted to the soft MOFs, gas and vapour sorption are of special interest because of their importance in the separation and purification processes. The sorption processes displayed by the soft MOFs are unusual compared to the rigid MOFs and exhibit many interesting features such as steps and hysteresis. This book chapter is focussed on two such sorption processes known as stepwise and gated sorption, unusual and unique sorption phenomena that deviate from general IUPAC classification.