Dynamic mixed gas breakthrough studies using monolithic MOFs

https://doi.org/10.1039/D1FD00017A

“At this point, we examined the gas separation performances of the _monoMOFs using experimental breakthrough studies on the pristine samples of powdered and monolithic variants of _monoHKUST-1, _monoUiO-66 and _monoUiO-66-NH₂. We examined gas mixtures associated with post-combustion carbon capture (15/85 v/v CO₂/N₂, dry and 74% relative humidity) and natural gas purification (50/50 v/v CO₂/CH₄) at room temperature (Fig. 4, 5 & S33–S41†). All of the materials examined under both dry and moist conditions achieved efficient CO₂/N₂ and CO₂/CH₄ separation. N₂ and CH₄ gases eluted through the bed immediately, whereas CO₂ was retained in the adsorbent bed. For the 15/85 v/v CO₂/N₂ gas mixture, _monoHKUST-1 and the powdered variant achieved CO₂ uptake capacities of 21.3 and 24.7 cm³ g⁻¹, respectively, under dry conditions (Table 1). The CO₂ uptake value for _monoHKUST-1 was in agreement with the IAST predicted value (Fig. S27†). Importantly, when the bulk densities of the monolithic and powdered variants of HKUST-1 were taken into account, the _monoHKUST-1 material displayed a volumetric CO₂ uptake of 22.6 cm³ cm⁻³, which is nearly double that of the HKUST-1 packed powder at 12.4 cm³ cm⁻³. Similar trends were observed for the volumetric CO₂ uptake performance of the UiO-66 (monolith = 16.0 cm³ cm⁻³; powder = 10.0 cm³ cm⁻³) and UiO-66-NH₂ (monolith = 20.0 cm³ cm⁻³; powder = 11.6 cm³ cm⁻³) materials for the 15/85 v/v CO₂/N₂ gas mixture (Table 1 & Fig. S34, S35†). When we exposed the materials to a humid (ca. 74% relative humidity) 15/85 v/v CO₂/N₂ gas stream, both the packed powder and monolithic MOF materials displayed a ca. 40% drop in performance compared to the dry gas mixture (Fig. S36–S38†). The drop in performance is attributed to the competitive adsorption between CO₂ and H₂O molecules.^9,32,33 Interestingly, despite this reduction in performance, the _monoMOFs exhibited nearly double the volumetric CO₂ uptake performance under humid conditions compared to the powders for each of the MOF variants studied.”

“Fig. 4 Gravimetric (A–C) and volumetric (D–F) CO₂ breakthrough curves for a 15/85 v/v CO₂/N₂ gas stream at 298 K for monolithic and powdered HKUST-1, UiO-66 and UiO-66-NH₂ materials.”

“Fig. 5 Gravimetric (A–C) and volumetric (D–F) CO₂ breakthrough curves for a 50/50 v/v CO₂/CH₄ gas stream at 298 K for monolithic and powdered HKUST-1, UiO-66 and UiO-66-NH₂ materials.”