JPCL jz9003087 Coudert Presentation

Hot Paper Stress-Based Model for the Breathingof Metal–Organic Frameworks TOC Graphics A. V. Neimark, F.-X. Coudert, A. Boutin, A. H. Fuchs Rutgers, the State University of New Jersey CNRS & Chimie ParisTech, France École normale supérieure de Paris, FranceJ. Phys. Chem. Lett. 2010, 1, 445-449 1

Soft Porous Crystals  SPC’s are Metal–Organic Frameworks with large structural flexibility  SPC’s display a large variety of framework dynamics  Structural transitions can be induced by external stimuli: guest adsorption, pressure, temperature, … intraframework dynamics negative thermal expansion ZIF-8 IRMOF-1 T V swelling Cu(4,4′ -bipy)(dhbc)2 MIL-88 solvent gate +guest: V opening MIL-53Review: Coudert et al,Chem. Phys. Chem. 2011 breathing Proprietary and Confidential 2 American Chemical Society

MIL-53 and the “breathing” phenomenon MIL-53 are M(OH)(1,4-benzenedicarboxylate), with M = Al3+, Cr3+, Ga3+, Fe3+, … (MIL = Matériaux Institut Lavoisier, discovered in Férey’s group) The framework of MIL-53(Al) has 2 structures, with large and narrow pores MIL-53 (Al) at 220 K + Xe + Xe lp np lp Guest adsorption allows a double structural transitions: lp → np → lp Can we propose a mechanism, from the point of view of the host framework?Proprietary and Confidential 3American Chemical Society

What is adsorption stress?   Adsorption of the guest gives rise to stress on the host framework  This stress deforms the structure according to Hooke’s law  This stress can tend towards contracting the cell or expanding it, depending on guest loading From a thermodynamic point of view, stress is a derivative of the thermodynamic potential The thermodynamic potential itself is linked to adsorption isotherms:Proprietary and Confidential 4American Chemical Society

Our stress-based model We build a model based on the following features: ✓ Adsorption in each structure follows a simple isotherm model (here, Langmuir-type) ✓ Isotherms are fitted on experimental data ✓ Model parameter derivatives are considered as adjustable parameters We obtain a profile of the adsorption-induced stress as a function of gas pressure, for each phase These nonmonotonic profile are characteristic of adsorption in nanopores Contraction of the material at low loading Swelling at higher pressureProprietary and Confidential 5American Chemical Society

Our stress-based model On top of that, we formulate the stress-threshold ansatz: For each phase, there is a given stress threshold that it can withstand. When adsorption stress reaches that threshold, the structure collapses. np critical stress Adsorption Desorption lp critical stressProprietary and Confidential 6American Chemical Society

Conclusions drawn from the model The model is consistent with experimental data ✓ Validation of the stress-threshold ansatz ✓ Accounts for the hystereses observed Experimental adsorption isotherms steps are not steep ✓ This can be explained by gradual transitions between phases ✓ Coexistence of lp/np phases in the region of the transition The experimental isotherms can be converted into “phase composition” profiles showing gradual lp→np and np→lp conversions.Proprietary and Confidential 7American Chemical Society

Conclusions  We propose that the driving force behind SPC’s breathing is adsorption-induced stress  The hysteretic behavior of the structural transition can be explained by a critical stress threshold that the structures can sustain. Our other work on this topic…  OFAST: a thermodynamic model to predict mixture adsorption in Soft Porous Crystals Coudert et al, J. Am. Chem. Soc. 2009  Constructing (T, P) phase diagrams for adsorption in Soft Porous Crystals from experimental data Boutin et al, Angewandte Chemie 2009  Physical mechanism and multiscale model for the breathing phenomenon Triguero et al, J. Phys. Chem. Lett. 2011Proprietary and Confidential 8American Chemical Society

JPCL jz9003087 Coudert Presentation

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