Towards general network architecture design criteria for negative gas adsorption transitions in ultraporous frameworks

Simon Krause,Paul Iacomi,Stefan Kaskel,Jürgen Getzschmann,Sebastian Ehrling,Philip L Llewellyn,Alexandra Franz,Guillaume Maurin,Volodymyr Bon,Irena Senkovska,Felicitas Kolbe,Dirk Wallacher,François-Xavier Coudert,Pascal G Yot,Eike Brunner,Erik Troschke,Jack D Evans,Daniel M Többens

doi:10.1038/s41467-019-11565-3

Abstract

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.

Highlights

Switchable metal-organic frameworks (MOFs) have been proposed for various energyrelated storage and separation applications, but the mechanistic understanding of adsorptioninduced switching transitions is still at an early stage
Rationalisation of adsorption-induced structural transformations has been achieved by analysing their thermodynamics via density functional theory (DFT)[20] and molecular dynamic (MD) simulations[22], demonstrating the importance of micromechanics in responsive frameworks
Reducing the pore size and ligand length in an isoreticular network (DUT-48) increases the framework rigidity and decreases the energetic driving force for the contraction, suppressing the adsorption-induced structural transition[32], while the external pressure, inducing the compaction is increased from 35 MPa (DUT-49) to 65 MPa (DUT-48) using mercury as the pressure transducing medium

Summary

Introduction

Switchable metal-organic frameworks (MOFs) have been proposed for various energyrelated storage and separation applications, but the mechanistic understanding of adsorptioninduced switching transitions is still at an early stage. 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 Xray and neutron diffraction, NMR spectroscopy, and calorimetric techniques parallelised to adsorption for a series of six isoreticular networks. DUT-49 expels previously adsorbed gas from the framework leading to a stepwise desorption in the adsorption branch of the isotherm corresponding to a pressure amplification in the closed measuring cell[3] Such NGA transitions require the system to traverse through metastable states. This finding indicates NGA to be a general phenomenon observable for a wider class of highly porous materials satisfying specific structural design rules

Methods

Results

Conclusion