(102f) Metal-Organic Frameworks As a Platform to Study Impact of Surface Heterogeneity on Adsorption of Ideal Mixtures

Adsorption-based separation processes are one of the most promising approaches to reduce global energy consumption. Industrial processes involving the separation of multi-component gas streams are particularly energy-intensive. To replace these with large-scale adsorption separation systems, more reliable multi-component adsorption data are needed, yet are challenging to obtain experimentally. Thus, predictive methods using single-component isotherms are the most common approach to study mixture adsorption, with the Ideal Adsorbed Solution Theory (IAST) as the most prominent prediction method. However, previous studies have revealed deviations where IAST predictions exceed experimentally measured multi-component adsorption loadings by more than 100%. Surface heterogeneity, which describes the distribution of different adsorption sites with different binding energies in adsorbents, has been identified as a contributor to these deviations. Metal-organic frameworks (MOFs) are a class of adsorbents that exhibit tunable modularity. In particular, varying the ratios of linkers present in the material to control their surface heterogeneity makes MOFs a powerful platform to systematically investigate this contribution to the non-ideal adsorption behavior. Understanding surface heterogeneity in adsorption is crucial to leveraging the outstanding properties of MOFs in industrial processes under real conditions. This study narrows the gap between the development of high-performance adsorbents and application of those adsorbents in separations of complex mixtures as it aids in evaluating the accuracy and limitations of mixture theories like IAST. Here we focus on contributions from the adsorbent to non-ideal behavior by comparing binary adsorption isotherms from breakthrough experiments with single-component based IAST adsorption predictions. Varied compositions of an ideal, binary gas mixture are studied in mixed-linker derivatives of the UiO-66 series. Deviations between experimentally-determined and IAST-predicted loadings are used to correlate systematically induced heterogeneity to the non-ideal behavior of the adsorption system and efficacy of IAST.