Precise control of pore hydrophilicity enabled by post-synthetic cation exchange in metal-organic frameworks.

Ashley M Wright,Sungwoo Yang,Mircea Dincă,Adam J Rieth,Evelyn N Wang

doi:10.1039/c8sc00112j

Ashley M Wright, Sungwoo Yang + Show 3 more

Open Access

https://doi.org/10.1039/c8sc00112j

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Abstract

The ability to control the relative humidity at which water uptake occurs in a given adsorbent is advantageous, making that material applicable to a variety of different applications. Here, we show that cation exchange in a metal-organic framework allows precise control over the humidity onset of the water uptake step. Controlled incorporation of cobalt in place of zinc produces open metal sites into the cubic triazolate framework MFU-4l, and thereby provides access to materials with uptake steps over a 30% relative humidity range. Notably, the MFU-4l framework has an extremely high water adsorption capacity of 1.05 g g-1, amongst the highest known for porous materials. The total water capacity is independent of the cobalt loading, showing that cation exchange is a viable route to increase the hydrophilicity of metal-organic frameworks without sacrificing capacity.

Highlights

Mainstream deployment of devices such as adsorption heat pumps (AHPs) and atmospheric water generators (AWGs) require materials that adsorb water vapor below 100% relative humidity (RH).[1,2,3] The salient properties of the adsorbent depend on the speci c application but generally require water stability, water sorption with a steep uptake step at a desirable relative humidity, and a high total uptake capacity
Very little water is adsorbed between 0–65% RH, but once pore lling is initiated at 65% RH, the uptake step is large and steep with 1.01 gH2O gMOFÀ1, adsorbed by 70% RH (Fig. 1)
The saturation uptake capacity at 95% RH is 1.05 gH2O gMOFÀ1 indicating most of the water is adsorbed during the sharp uptake step

Summary

Introduction

Mainstream deployment of devices such as adsorption heat pumps (AHPs) and atmospheric water generators (AWGs) require materials that adsorb water vapor below 100% relative humidity (RH).[1,2,3] The salient properties of the adsorbent depend on the speci c application but generally require water stability, water sorption with a steep uptake step at a desirable relative humidity, and a high total uptake capacity. Sorbents that are very hydrophilic may adsorb water at low RH, which is advantageous for applications such as water vapor capture in desert areas, or AHP systems requiring large temperature gradients. These advantageous properties come with the caveat that high adsorbent regeneration

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