M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

7 0 H I G H L I G H T S 2 0 2 3 I

Retrieving thermodynamic information of adsorption processes from high-resolution powder X-ray diffraction adsorption isotherms

Gas adsorption isotherms are the standard technique to characterise porous solids. However, no atomic-level adsorbent-adsorbate interactions can be extracted therefrom. High-resolution powder X-ray diffraction, applied on variable gas pressure datasets, was exploited not only to localise and quantify the guest molecules in the host framework, but also to shed light on the thermodynamics of the process.

Understanding adsorption processes at the molecular level is at the frontier of porous materials research [1]. For this reason, much research is focused on finding appropriate techniques to properly characterise pore surface [2]. In recent years, in-situ synchrotron radiation high-resolution powder X-ray diffraction (HR-PXRD) has emerged as a powerful tool to unveil the main host- guest interactions and primary adsorption sites in a wide range of inorganic, organic and metal-organic porous materials [3]. The information typically retrieved was limited to the localisation of the guest molecules and the modification of the host framework upon adsorption. Recently, efforts were made to model and understand the whole adsorption process, including the construction of PXRD adsorption isotherms [4]. However, this approach had not yet been stretched to the limit, going beyond crystal structure determination, host-guest interactions and guest quantification to study the thermodynamics of the adsorption process.

In this work, the iron-based metal-organic framework Fe2(BDP)3 [H2BDP = 1,4-bis(1H-pyrazol-4-yl)benzene] [5] (Figure 51a) was chosen as a case study due to its high crystallinity and extremely high stability. Nevertheless, the proposed method could potentially be applied to any porous microcrystalline material and with probe molecules different from the one adopted here. In-situ HR-PXRD measurements were performed at beamline ID22 to collect data at two different temperatures, T = 273 and 298 K, while varying the CO2 loading in the

Fig. 51: a) Representation of the crystal structure of Fe2(BDP)3 viewed along the [100] crystallographic direction. b) HR-PXRD patterns acquired at 273 K and variable CO2 pressure. c) Position of the three independent CO2 molecules, together with the main host-guest (green dashed lines) and guest-guest (light blue dashed lines)

interactions, obtained from the Rietveld refinement at 273 K and pCO2 = 1 bar. d) HR-PXRD adsorption isotherms in the range 0-8 bar at 273 and 298 K. The dashed lines depict the fit obtained using the Freundlich-Langmuir model. e) Comparison between the isosteric heat of adsorption as a function of the CO2 uptake for Fe2(BDP)3

calculated from the conventional adsorption isotherms (blue circles) and retrieved from the HR-PXRD experiment (orange diamonds). Colour codes: Fe, orange; N, blue; C, grey.