From layer sliding to near-zero compressibility: novel high-pressure flexibility and CO₂ site evolution in pre-ELM-11 and ELM-11

Abstract

Understanding how mechanical stress reshapes flexible metal–organic frameworks (MOFs) is important for designing robust, stimuli‑responsive CO₂ sorbents. Here we compare the high‑pressure flexibility of the elastic layered MOF ELM‑11 and its hydrated precursor pre‑ELM‑11 using in situ synchrotron powder X‑ray diffraction (PXRD) and Fourier‑transform infrared (FTIR) spectroscopy. Pre‑ELM‑11 shows a pronounced change in anisotropic response at 2.15 GPa: compression along the c axis switches from positive linear compressibility (PLC) to negative linear compressibility (NLC), consistent with a transition from interlayer compression to pressure‑driven layer sliding. In contrast, activated ELM‑11 undergoes continuous anisotropic compression from 0.12 to 4.28 GPa dominated by interlayer contraction, and principal‑axis analysis reveals near‑zero linear compressibility (ZLC) along X3. To probe pressure‑tuned host–guest interactions, we further monitor CO₂‑loaded ELM‑11 by in situ FTIR. Deconvolution of the ν₃ band of natural‑abundance ¹³CO₂ resolves an increase in distinct adsorption environments from two to three at 3.44 GPa, with additional sites appearing above 9.59 GPa. Together, these results map distinct pressure‑activated deformation pathways in closely related layered frameworks and demonstrate that mechanical pressure can reveal and create new CO₂ binding environments, informing the design of flexible 2D MOFs for pressure‑responsive gas adsorption. More broadly, the comparative results identify interlayer hydrogen‑bond pinning and initial interlayer spacing as key molecular levers that select pressure‑activated deformation pathways and thereby tune pressure‑dependent CO₂ site heterogeneity in layered flexible MOFs.