Transition metal inclusion in RhoZMOF materials

Abstract

Porous RhoZMOF is a negatively charged zeolite-like MOF material susceptible to post-synthesis modification by an ion-exchange process replacing as-synthesized organic cations with transition metals. This material exhibiting open transition metal sites is a very attractive candidate for industrial applications related to clean energy technologies, like electrochemistry and gas storage. In the present work, partial or total Co²⁺, Ni²⁺ and Cu²⁺ inclusions have been successfully carried out in the RhoZMOF structure by ion-exchange treatment. Particularly, complete exchange of HDMA⁺ with Co²⁺ could be reached at room temperature after 12 h of contact time. For Cu²⁺ exchange, a rapid transformation of the RhoZMOF structure into a new Cu-rich, crystalline and practically non-porous phase was observed at room temperature; its appearance can be avoided and the desired structure can be preserved when ion-exchange treatment is carried out at −20 °C for 6 h, reaching a partial ion-exchange degree of 30%. Moreover, for Ni²⁺ ion-exchange treatment at 25 and 0 °C, a covering of an amorphous Ni-rich organometallic phase over the crystals was detected by SEM analysis; its appearance can be prevented by reducing the amount of HDMA⁺ in the media by developing the ion-exchange process in two steps: exchanging HDMA⁺ with Na⁺ and Na⁺ with Ni²⁺. The results provide a post-synthesis modification route for obtaining porous MOF materials containing open transition metal sites accessible for redox catalysis and selective gas adsorption.