Oral Presentation The 35th Biennial Conference of the Society of Crystallographers in Australia and New Zealand 2024 (Crystal 35)

The role of hydrophobic channel moieties on the adsorption and structural transitions in STAM-1 and STAM-17 frameworks during compression in N2 (108956)

Isabelle Jones 1 2 , Gemma Turner 2 , James Brookes 2 , Aston Summers 2 , Russell Morris 3 , Lauren McHugh 4 , Stephanie Bird 5 , Rosemary Young 5 , Rachel Williamson 5 , Alan Riboldi-Tunnicliffe 5 , Stephen Moggach 2
  1. School of Earth and Planetary Sciences, Curtin University, Bentley, WA, Australia
  2. School of Molecular Sciences, University of Western Australia, Crawley, WA, Australia
  3. School of Chemistry, University of St Andrews, St Andrews, United Kingdom
  4. Department of Chemistry, University of Liverpool, Liverpool, United Kingdom
  5. Australian Synchrotron, Clayton, Victoria, Australia

Metal-organic frameworks (MOFs) with customizable pore sizes offer versatile solutions for selective adsorption, separation processes, and energy storage applications. One such example is the Cu(II) and monomethyl ester benzene tricarboxylate based MOF STAM-1,1 and its isostructural equivalent STAM-17, which utilises a 5-substituted alkoxy isophthalate ligand.2 Both crystalise in P-3m1 and form a Kagomi-type layered structure with alternating hydrophobic and hydrophilic channels. These structures provide remarkable air- and hydrostability that are otherwise absent in Cu-based MOFs with similar motifs, such as HKUST-12–4. The change of ligand between STAM-17 and STAM-1, alters the overall size and accessibility of the hydrophobic pores2,3. The compressibility, phase behaviour, and gas uptake at pressure have, however, not been explored for MOFs isostructural to STAM-1.

Here we present the first study to examine how variations in the moiety of the hydrophobic channel functional group affects structural behaviours in STAM-1 and its isostructural equivalents STAM-17-OEt and STAM-17-OnPr, using a small guest probe (N2) to 3.5 GPa. This work demonstrates that addition of larger functional moieties negatively impacts guest intrusion into the MOF structure, while increasing the compressibility of the material, with the bulk moduli decreasing from 25(3) GPa in STAM-1 to 11(4) GPa in STAM-17-OEt. In this study, all frameworks exhibited the highest compressibility along their channels. Notably, STAM-17-OEt showed a 1.3-fold volume reduction compared to STAM-1 under similar pressure. Increasing the size of the hydrophobic pore functional group (STAM-1 ≤ STAM-17-OEt < STAM-17-OnPr) correlated with decreased N2 uptake and greater compression. The size of the ligand also altered adsorption sites within the framework. STAM-1 and STAM-17-OEt both readily filled two adsorption sites at the centre of the hydrophobic pore. STAM-17-OnPr blocked N2 adsorption initially, only permitting adsorption into interstitial sites after 0.6 GPa. Through this study, we showed that the ability guest uptake and compressibility could be tuned through ligand substitution, which may be advantageous for developing frameworks for bespoke application.

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