Selective Aerobic Oxidation of a Metal–Organic Framework Boosts Thermodynamic and Kinetic Propylene/Propane Selectivity

Abstract

Efficient adsorptive separation of propylene/propane (C 3 H 6 /C 3 H 8 )i sh ighly desired and challenging.K nown strategies focus on either the thermodynamic or the kinetic mechanism. Here,w er eport an interesting reactivity of am etal-organic framework that improves thermodynamic and kinetic adsorption selectivity simultaneously.W hen the metal-organic framework is heated under oxygen flow, half of the soft methylene bridges of the organic ligands are selectively oxidized to form the more polar and rigid carbonyl bridges. Mixture breakthrough experiments showed drastic increase of C 3 H 6 /C 3 H 8 selectivity from 1.5 to 15. Forc omparison, the C 3 H 6 /C 3 H 8 selectivities of the best-performing metal-organic frameworks Co-MOF-74 and KAUST-7 were experimentally determined to be 6.5 and 12, respectively.G as adsorption isotherms/kinetics,s ingle-crystal X-ray diffraction, and computational simulations revealed that the oxidation gives additional guest recognition sites,w hich improve thermodynamic selectivity,a nd reduces the framework flexibility,w hich generate kinetic selectivity.

A Hydrogen‐Bonded yet Hydrophobic Porous Molecular Crystal for Molecular‐Sieving‐like Separation of Butane and Isobutane

“…To study the potential of 1′ for separation applications, adsorption kinetics of typical C4 hydrocarbons were measured at 298 K and ambient pressure (Figure S9a). The obtained diffusional time constant ( D′ = D / r 2 ) increased as the molecular size decreased (Figure S9b), being consistent with the normal trend used for ordinary kinetic separation [20a] . Interestingly, the largest molecule i ‐C 4 H 10 was completely excluded, indicating potential molecular sieving, the ideal case of kinetic separation with infinite selectivity.…”

Separation of propylene and propane with a microporous metal–organic framework via equilibrium‐kinetic synergetic effect

“…After fitting the dynamic adsorption profiles with micropore diffusion model (Figure S5‐S6), it can be obtained the diffusion time constants (D') of C 3 H 6 are 1.26 × 10 −6 s −1 at 298 K and 9.81 × 10 −6 s −1 at 323 K. Meanwhile, the corresponding values for C 3 H 8 are 1.17 × 10 −7 s −1 and 3.15 × 10 −7 s −1 , apparently lower than C 3 H 6 . Consequently, the kinetic selectivity of ZnAtzPO 4 can be determined as 11 under ambient temperature and up to 31 at 323 K. These results prove ZnAtzPO 4 offers a synergetic effect of equilibrium and kinetics to maximize its performance for C 3 H 6 /C 3 H 8 separation, which has rarely been observed in MOF materials 32,33,38 . Different from traditional kinetically selective adsorbents, which typically adsorb comparable amounts of C 3 H 6 and C 3 H 8 under equilibrium state, setting a limit for their utmost separation capability, ZnAtzPO 4 displays an excellent gas uptake ratio (C 3 H 6 /C 3 H 8 ) up to 1.79 at 298 K and 1.0 bar, superior to other reported kinetically driven materials such as ZIF‐8 (1.05), 31 ELM‐12 (1.08) 3 and DBTO‐MOF (1.30) 30 .…”

“…To assess the interaction energies between gas molecules and the pore of ZnAtzPO 4 , Clausius‐Clapeyron equation was employed to calculate the isosteric heat of adsorption ( Q st , Figure S7‐S9). Interestingly, the material manifests very modest Q st for C 3 H 6 , only 27.5 kJ mol −1 at zero loading (Figure 2c), prominently lower than many well‐known equilibrium‐based materials such as Fe‐MOF‐74 (44 kJ mol −1 ), Cu 3 (BTC) 2 (49 kJ mol −1 ), Mg‐MOF‐74 (60.5 kJ mol −1 ) and MIL‐100(Fe) (70 kJ mol −1 ) 1‐3,21‐23,28,32,40 . The modest Q st illustrates that ZnAtzPO 4 selectively adsorbs C 3 H 6 through mild physisorption interactions, which may greatly facilitate the desorption of gas molecules under mild conditions and reduce the energy input for the regeneration process.…”

“…Besides, most of the kinetically selective adsorbents are reported to have comparable C 3 H 6 /C 3 H 8 equilibrium uptake capacities 29‐31 . Such disadvantageous thermodynamic effect may limit the utmost separation performance of porous materials, and implies there is still broad space to further prompt the separation capability by fine‐tuning the pore chemistry 32,33 . Materials qualified with molecular sieving effect are ideal for gas separation and can be reckoned as an extreme case of kinetically driven media 1,2,34,35 .…”

Differential guest location by host dynamics enhances propylene/propane separation in a metal-organic framework

“…This is currently accomplished by energy-intensive cryogenic distillation 1 , based on the small boiling point difference between the two compounds. Separation methods based on porous materials that utilise the difference between molecular sizes and diffusivities [2][3][4][5] or attractive host-guest interactions 6,7 are of great interest as energy-efficient alternatives.…”