Using Site Heterogeneity in Metal–Organic Frameworks with Bimetallic Open Metal Sites for Olefin/Paraffin Separations

Abstract

It is generally thought that homogeneous adsorbents allow higher performance than heterogeneous materials for gas storage, but it is less clear whether this concept extends to adsorption-based separations. We chose a specific application, adsorptive separation of ethylene and ethane by metal–organic frameworks (MOFs) with open metal sites (OMS), to study in silico whether using a heterogeneous blend of OMS provides advantages. We studied 12 metal-substituted M-BTCs as homogeneous adsorbents and evaluated 11 mixed-metal pairs as energetically heterogeneous materials. We aimed to understand whether mixed-metal materials (MM′-BTC) could ever outperform pure-metal M-BTC MOFs for ethylene/ethane separations. We used both a simplified model of a two-step saturation–desaturation cycle and detailed process model of a four-step vacuum-pressure swing adsorption process to examine the process performance of M-BTC and MM′-BTC MOFs for this separation. The adsorption isotherms are the primary inputs for process models of equilibrium adsorption-based separations. We developed density functional theory (DFT) derived adsorption models because FF-based GCMC simulations cannot accurately describe OMS and are not useful for our materials. We observed that the simplified process model can be used to prescreen mixed-metal MOF candidates. One of our main results is that Cr–Mg–BTC gave significantly higher purities than both Cr-BTC and Mg-BTC as described by both the simplified and full process models. This provides a specific example for which in some circumstances energetically homogeneous adsorbents do not necessarily outperform energetically heterogeneous adsorbents for adsorptive separations.