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

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.