Correlating MOF-808 parameters with mixed-matrix membrane (MMM) CO2 permeation for a more rational MMM development

Abstract

Consistent structure–performance relationships for the design of MOF (metal–organic framework)-based mixed-matrix membranes (MMMs) for gas separation are currently scarce in MMM literature. An important step in establishing such relationships could be to correlate intrinsic MOF parameters, such as CO₂ uptake and the CO₂ adsorption enthalpy (Q_st), with the separation performance indicators of the MMM (i.e. separation factor and permeability). Such a study presumes the availability of a platform MOF, which allows systematic comparison of the relevant MOF parameters. MOF-808 can take up the role of such a platform MOF, owing to its unique cluster coordination and subsequent ease of introducing additional functional molecules. For this purpose, formic acid (FA) modulated MOF-808 (MOF–FA) was post-synthetically functionalized with five different ligands (histidine (His), benzoic acid (BA), glycolic acid (GA), lithium sulfate (Li₂SO₄) and trifluoroacetic acid (TFA)) to create a series of isostructural MOFs with varying affinity/diffusivity properties but as constant as possible remaining properties (e.g. particles size distribution). CO₂ uptake and CO₂ adsorption enthalpy of the MOFs were determined with CO₂ sorption experiments and Clausius–Clapeyron analysis. These MOF properties were subsequently linked to the CO₂/N₂ separation factor and CO₂ permeability of the corresponding MMM. Unlike what is often assumed in literature, MOF-808 CO₂ uptake proved to be a poor indicator for MMM performance. In contrast, a strong correlation was observed between Q_st at high CO₂ loadings on one hand and CO₂ permeability under varying feed conditions on the other hand. Furthermore, correlation coefficients of Q_st,15 and Q_st,30 (Q_st at 15 and 30 cm³ (STP) g⁻¹) with the separation factor were significantly better than those calculated for CO₂ uptake. The surprising lack of correlation between membrane performance and CO₂ uptake and the strong correlation with Q_st opens possibilities to rationally design MMMs and stresses the need for more fundamental research focused on finding consistent relationships between filler properties and the final membrane performance.