Temperature dependence of adsorption hysteresis in flexible metal organic frameworks

Shamsur Rahman,Arash Arami-Niya,Gongkui Xiao,Xiaoxian Yang,Eric F May,Gang Li

doi:10.1038/s42004-020-00429-3

Abstract

“Breathing” and “gating” are striking phenomena exhibited by flexible metal-organic frameworks (MOFs) in which their pore structures transform upon external stimuli. These effects are often associated with eminent steps and hysteresis in sorption isotherms. Despite significant mechanistic studies, the accurate description of stepped isotherms and hysteresis remains a barrier to the promised applications of flexible MOFs in molecular sieving, storage and sensing. Here, we investigate the temperature dependence of structural transformations in three flexible MOFs and present a new isotherm model to consistently analyse the transition pressures and step widths. The transition pressure reduces exponentially with decreasing temperature as does the degree of hysteresis (c.f. capillary condensation). The MOF structural transition enthalpies range from +6 to +31 kJ·mol−1 revealing that the adsorption-triggered transition is entropically driven. Pressure swing adsorption process simulations based on flexible MOFs that utilise the model reveal how isotherm hysteresis can affect separation performance.

Highlights

“Breathing” and “gating” are striking phenomena exhibited by flexible metal-organic frameworks (MOFs) in which their pore structures transform upon external stimuli
We show the new model more accurately represents the separation performance achievable in pressure swing adsorption processes based on flexible MOFs
The number of data acquired along each branch of each hysteretic sorption isotherm is sufficient to clearly resolve the characteristic parameters ptr, δptr, and σ, as well as their respective temperature dependences

Summary

Introduction

“Breathing” and “gating” are striking phenomena exhibited by flexible metal-organic frameworks (MOFs) in which their pore structures transform upon external stimuli These effects are often associated with eminent steps and hysteresis in sorption isotherms. Pressure swing adsorption process simulations based on flexible MOFs that utilise the model reveal how isotherm hysteresis can affect separation performance. The most obvious difference is the drastic increase in uptake that occurs when the number of accessible adsorption sites in the framework suddenly changes in response to external stimuli, such as the adsorption or desorption of guest molecules[13,14,15] This feature of flexible MOFs allows the adsorption of guest molecules seemingly larger than the nominal crystallographic pore diameter to suddenly increase.

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Conclusion