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

Abstract

Adsorptive separation of p-xylene (pX) from xylene isomers is a key process in chemical industry, but known adsorbents cannot simultaneously achieve high adsorption selectivity, capacity, and rate. Here, we demonstrate gating ultramicropore as a solution for this challenge. Slight modification of the synthetic condition gives rise to isomeric metal-organic frameworks α-[Zn(pba)] (MAF-88, H 2 pba = 4-(1H-pyrazol-4-yl)benzoic acid) and β-[Zn(pba)] (MAF-89) possessing similar pillared-column structures, porosities, and high pX capacities of 2.0 mmol g −1 , but very different framework/pore topologies, pore sizes, and pX selectivities. For binary and ternary mixtures of liquid xylene isomers, MAF-88 with narrow one-dimensional (1D) channels shows pX selectivity of 11 and 1.6, while MAF-89 with 3D-connected quasi-discrete pores shows pX selectivity up to 221 and 46, respectively. Thermogravimetry, differential scanning calorimetry, and time-dependent separation experiments reveal that the kinetic effects of the gating pores play more important roles than the thermodynamic effects, which is further confirmed by single-crystal X-ray diffraction and computational simulations.

Highly Selective p-Xylene Separation from Mixtures of C8 Aromatics by a Nonporous Molecular Apohost

“…PXRD data revealed that TPBD-αI had converted to TPBD-PX upon exposure to an equimolar quaternary mixture of C8 aromatic isomers within 35 min (Figure S58), indicating relatively fast kinetics under slurry-assisted immersion at room temperature. This sorption time is comparable to MOFs, PCPs, and PMCs with high selectivities. ,,, In the liquid phase, MAF-36 - 3C ( S PX/OX/MX = 51.3) and 3B ( S PX/OX/MX = 16) exhibited varying adsorption rates, taking 24 and 3 h, respectively, whereas form 1B ( S PX/OX/MX = 15) exhibited fast uptake in 2 min attributed to smaller crystal size .…”

“…These values surpass the selectivity of current benchmark sorbents MAF-36−3C (51.3 at 25 °C), 19 Mn-dhbq (48.3 at 120 °C), 31 and MAF-89 (46.4 at 35 °C). 20 Furthermore, while TPBD-αII (S PX/OX/MX = 43.6) displayed a slightly lower selectivity than previous benchmarks, its performance remains high (Figures 4a and S55). Concerning purification of PX from EB, literature reports are limited, likely reflecting that sorbents tend to exhibit relatively low PX/EB selectivity, making them unsuitable for purifying PX when EB is present.…”

“…In previous work, sorbents that exhibit discrimination for PX, typically adapt to the size and shape of PX molecules, controlling their pore structure via a gating mechanism, leading to a guest-loaded complex with the lowest binding energy and preference for PX over other C8 aromatic isomers. ,, PX clathrate TPBD-PX offers precise size/shape matching to accommodate PX molecules, establishing favorable positions for C–H···π interactions along the a -axis. CH···O and CH···N interactions formed between the host and guest serve to anchor the PX molecules.…”

“…Physisorbents such as metal–organic materials (MOMs), − especially flexible MOMs (FMOMs), − have exhibited promising characteristics for C8 separation. ,− Notably, two-dimensional (2D) layered coordination networks, , one-dimensional (1D) chain coordination polymers, , and zero-dimensional (0D) coordination complexes − can offer selective binding of C8 guests through C8-induced structural phase transformations from guest-free apohosts (α-phases) to inclusion compounds (β-phases) . Organic molecular solids can also undergo phase transformations induced by guest molecules − and offer selective binding of C8 isomers. ,,− In terms of selectivity, a cage host with intrinsic porosity was found to offer across-the-board PX selectivity ranging between 6.60 and 12.10 .…”

Scalable and Depurative Zirconium Metal–Organic Framework for Deep Flue-Gas Desulfurization and SO2 Recovery

“…Over the past decades, the great potential of energy-saving physical adsorption and separation technology has encouraged fast development of task-specific adsorbents, ,,,,− and many porous solids, such as zeolites, , mesoporous silica, metal oxides, ionic microgels, activated carbons, and polymers, , have been tested for SO 2 adsorption. However, these traditional materials tend to show low SO 2 uptake capacities due to limited surface areas and porosities, high regeneration energy, and, even worse, irreversible structural degradation under harsh conditions required to remove adsorbed SO 2 .…”