Mass transfer in atmospheric water harvesting systems

Abstract

In this work, the rate-limiting diffusion mechanisms of MOF-303, MOF-333, and a multivariate (MTV) version of these metal–organic frameworks (MOFs), where the organic linkers are present in a 50/50 ratio, are identified and quantified using concentration swing frequency response (CSFR). The data show that the single-atom precision of MOFs allows for precise tuning of the diffusion rate that is not easily achieved in traditional adsorbent materials. The Maxwell-Stefan diffusivity as a function of loading was calculated to decouple the influence of molecular mobility and equilibrium effects. To further understand the diffusion process in these MOFs, samples with different crystal sizes were synthesized and diffusion rates were measured. The results show that the controlling diffusion length scale is similar between the small and large crystal samples, as evidenced by similar diffusion rate constants. The MOFs were then incorporated into a film using a binder system and the mass transfer mechanisms were identified using CSFR. When placed in this particular binder system, the macropore diffusion behavior dominates over the MOF micropore diffusion. To illustrate how these diffusion parameters govern the adsorption rates and dynamics of water-harvesting systems, a model of the MOF-coated tube was developed. The results show that, with only milligrams of adsorbent, CSFR can quantify the diffusion rate needed to predict the adsorption times in a water-harvesting system. More broadly, the results illustrate the connectivity between atomically-precise reticular chemistry and water-harvesting system performance.