Fine-Tuning Window Apertures in ZIF-8/67 Frameworks by Metal Ions and Temperature for High-Efficiency Molecular Sieving of Xylenes

Abstract

Separation of structurally similar components from their mixtures is one of the most promising applications of metal−organic frameworks (MOFs). A high efficiency of such molecular sieving requires fine tuning of the MOF structure. In this work, we investigate subtle metal-and temperature-induced changes in window dimensions of zeolitic imidazolate frameworks (ZIF-8(Zn) and ZIF-67(Co)) and apply such structural tuning for efficient separation of xylene isomers (p-, m-, and o-xylenes). The use of Co instead of Zn favorably modifies window geometry: it accelerates the diffusion of all components by a factor of 2−3 while maintaining closely the same separation efficiency as that of ZIF-8(Zn). Outstanding selectivity above 18:1 and faster isolation of demanded p-xylene from the ternary mixture using ZIF-67(Co) have been demonstrated at room temperature, opening new horizons for its energy-efficient xylene separation. More generally, our findings suggest the prospective ways to tune various MOFs for target liquidstate separations.

A gating ultramicroporous metal-organic framework showing high adsorption selectivity, capacity and rate for xylene separation

“…The very different pX selectivity between MAF-88 and MAF-89 highlights the critical role of pore size especially at the apertures. High pX selectivity of 81.2/98.9 and promissing pX capacitiy of 2.2/2.7 mmol g −1 were recently reported for SOD-[Zn- pores, but they need more than 8 h to reach equilibrium [38].…”

“…While many thermodynamic, kinetic, and even molecularsieving separation mechanisms have been reported [23][24][25][26], only ultramicropores with sizes similar to that of pX achieved high pX selectivity over 10 [2,3,21,22,27-34]. The size/shape-matched pores can achieve better guest recognition, while the ultra-small pore apertures enlarge the difference of guest adsorption/diffusion rates through the gating mechanism (transisent opening/closing of relatively small pore apertures), although these structural responses are often ignored due to the difficulty of characterization [22,23,[35][36][37][38][39]. However, such adsorbents generally possess discrete (0D) or one-dimensional (1D) pores with low porosities (void ratio < 20%) and high gating barriers (relatively rigid), giving low adsorption capacities (< 2.0 mmol g −1…”

Discrimination of xylene isomers in a stacked coordination polymer

“…High diffusion rates drastically decrease the adsorption-desorption time, which is crucial for industrial applications ( 32 ). In this regard, operation at elevated temperature is necessary to achieve fast kinetics and sufficient diffusion rate for practical separations of xylene mixtures.…”

Shaping 90 wt% NanoMOFs into Robust Multifunctional Aerogels Using Tailored Bio‐Based Nanofibrils

“…[ 27,28 ] The effective aperture of the cavity windows of ZIF‐8 has been calculated to be ≈0.7 nm. [ 29 ] However, there are several studies where ZIF‐8 has been observed to adsorb larger molecules than its pores. [ 30 ] This suggests that the adsorption occurs both on exterior surfaces of nZIF‐8 particles and on CNFs, owing to charge exchange, and at least partially inside the pores (Table S7, Supporting Information).…”