Activation of Prussian blue analogues:

Abstract

Prussian blue analogues (PBAs) are low-cost porous materials with promising potential in gas separation, owing to their abundance of open metal sites. To access these open metal sites PBAs need to be activated through removal of water from the porous structure. However, the conditions required for effective activation and their structural consequences remain poorly understood, with existing effective methods being costly. Therefore, to gain insights in the activation process and its effects on structure, we systematically investigated the effect of activation temperature on three different ferricyanide-based PBAs (FePBA, CoPBA, and CuPBA) using a simple nitrogen flow activation procedure. We successfully synthesised these structures and optimised the activation procedure for micropore capacity and crystallinity. Infrared spectroscopy as well as CO2 and CO adsorption measurements revealed that, even under optimised conditions, water remained within the PBAs, with only a limited number of open metal sites accessible. At higher activation temperatures, micropore capacity and crystallinity decrease. This hydrophilicity, however, also showed positives, for example in the application of PBAs as desiccants. CuPBA, specifically, boasts a water adsorption of 355 mg g􀀀 1 at a relative humidity of ≤10%, competitive with zeolites and silica per weight unit, but higher per volume unit. Overall, however, while PBAs offer promise for gas adsorption owing to their high surface area and low cost, practical utilisation of their open metal sites remains challenging due to their strong affinity for water.