A joint experimental–modelling study has demonstrated a large flexibility of the MIL-47(V^IV) upon mechanical pressure which strongly deviates from its rigid behaviour in presence of guest molecules. A structural transition suspected by mercury intrusion and further confirmed by X-ray powder diffraction and molecular dynamics simulations, leads to a closed MIL-47(V^IV) form never observed so far corresponding to a cell contraction of up to 43%. The microscopic key features that govern this transition are then elucidated from complementary Raman experiments.

Investigating the irreversible pressure induced structural transformation of MIL-53(Al).

Switchable metal-organic frameworks (MOFs) have been proposed for various energy-related storage and separation applications, but the mechanistic understanding of adsorption-induced switching transitions is still at an early stage. Here we report critical design criteria for negative gas adsorption (NGA), a counterintuitive feature of pressure amplifying materials, hitherto uniquely observed in a highly porous framework compound (DUT-49). These criteria are derived by analysing the physical effects of micromechanics, pore size, interpenetration, adsorption enthalpies, and the pore filling mechanism using advanced in situ X-ray and neutron diffraction, NMR spectroscopy, and calorimetric techniques parallelised to adsorption for a series of six isoreticular networks. Aided by computational modelling, we identify DUT-50 as a new pressure amplifying material featuring distinct NGA transitions upon methane and argon adsorption. In situ neutron diffraction analysis of the methane (CD₄) adsorption sites at 111 K supported by grand canonical Monte Carlo simulations reveals a sudden population of the largest mesopore to be the critical filling step initiating structural contraction and NGA. In contrast, interpenetration leads to framework stiffening and specific pore volume reduction, both factors effectively suppressing NGA transitions.

Determination of the mechanical energy storage performance of the aluminum fumarate metal–organic framework A520.