Advancing the characterization of open framework compounds by Positron Annihilation Spectroscopy (PAS)

• Applicants: Dr. Ahmed Gamal Attallah Elsherif, Ph.D.; Professor Dr. Stefan Kaskel
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Synthesis and Properties of Functional Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464857745

Positron annihilation lifetime spectroscopy (PALS) is an effective porosimetry technique complementing standard methods when they lose their applicability. PALS is able to detect open and closed pores, resolve depth-dependent porosity, and identify defects and grain boundaries. Furthermore, it can operate under variable pressures, temperatures, and atmospheres for monitoring structural changes during in situ conditions. These features render PALS as a valuable porosimetry tool particularly in the case of closed pores or open to close pore transitions where only PALS can be used. In this project we aim to apply PALS to foster the understanding of pore and defect contents and dynamics in Metal-Organic Frameworks (MOFs). MOFs are superior to many porous materials because of their extremely high porosity and structure tunability and functionality. We focus on two MOF categories: Switchable MOFs and core-shell MOFs. Switchable MOFs are responsive materials that undergo distinct, repeatable, structural transformations under external stimuli, yielding interesting applications for gas separation, purification, and storage. Core-shell MOFs benefit from the dual-property of the shell and the core elements. However, little research has been devoted to these structures. Reversible switchable MOFs undergo repeated closed-open pore transformation cycles and it is essential to quantify the residual porosity not measurable by other techniques. Additionally, the pore transformation process requires in situ evaluation to provide a mechanism of pore transformation. Moreover, depth-resolving analysis of core-sell MOFs will explain why some members are inaccessible to gas molecules while X-ray shows a perfectly intact structure. This behavior is attributed to humidity-sensitive surfaces that degrade upon exposure. Controlled depth-resolved porosimetry of open and closed pores will advance the understanding of the core-shell behavior. PALS is a promising tool to address these open questions about switchable and core-shell MOFs. However, PALS porosimetry has some challenges including the effect of the chemical quenching of the metal nodes on o-Ps lifetime and the role of particle and pore size on o-Ps migration into interparticle spaces. Both factors underestimate MOFs pore sizes determined from PALS and have never been quantitatively determined. We provide an intensive plan to deal with these challenges. Sharing our outputs will help the community to apply PALS in a trustable way. After PALS standardization, our plan includes depth-profiling porosimetry of the humidity-degradable surface layer and SiO2 and polymer surface-coated core-shell for gas encapsulation technology. In situ adsorption of N2 (77 K), CH4 (111 K), CO2 (195 K), n-heptane (298 K), butane (298 K) will be studied by PALS in switchable MOFs. These studies will provide mechanistic insights for dynamic transformations in MOFs.

• DFG Programme: Research Grants