H NMR, computational modeling, and long-term cycling experiments, we disentangled how substituting Zr with Hf and changing topology affect the sorption behavior. Single-crystal measurements reveal substantial crystal-to-crystal variations in residual DMF, which directly reduce the accessible pore volume and alter adsorption isotherms-effects hidden in ensemble-averaged data. In both MOFs, Zr-based materials exhibit higher uptake due to defect-induced porosity. Hf substitution lowers the intrinsic defect density and improves cycling stability, but also leads to stronger solvent coordination and reduced pore accessibility. This trade-off is most pronounced in MOF-808(Zr), which shows high uptake but collapses during cycling, whereas the Hf analogue remains structurally stable at reduced capacity. By resolving how defects, guest molecules, and metal identity interplay at the single-crystal level, this work provides molecular design rules for balancing stability and performance in next-generation AWH sorbents.
Tittel et al. (Wed,) studied this question.