Ultraslow Dynamics of a Framework Linker in MIL-53 (Al) as a Sensor for Different Isomers of Xylene

Abstract

is an important example of metal−organic frameworks (MOFs) with a flexible framework capable to efficiently separate ortho and para isomers of xylene at moderate temperatures. The MIL-53 MOF contains mobile terephthalate phenylene fragments that can be used as a dynamic probe to investigate the guest−host interactions and the origin of the separation selectivity. Here 2 H NMR spin alignment echo technique for the first time was applied to probe ultraslow structural mobility (0.1−1 kHz) in MOFs materials, with particular application to MIL-53(Al) saturated with ortho or para isomers of xylene. A specific influence of different isomers of xylene adsorbed in the MOF pores on the rotation of the phenylenes in MIL-53 for the temperature range with proved separation selectivity (T < 393 K) is shown. It has been established that the rotation of phenylene fragments is sensitive to the type of xylene isomer. The phenylenes' rotation performs 1 order of magnitude slower in the presence of o-xylene (k ortho = 70 s −1 ) compared to the same rotation in the presence of the other isomer, p-xylene (k para = 800 s −1 ) at T ∼ 373 K. This is rationalized by a stronger interaction of the ortho isomer with the linker than the para isomer. This finding offers an understanding of the molecular mechanism of p-and o-xylene separation by MIL-53: stronger interaction of o-xylene with organic linker compared to p-xylene provides separation of these isomers on MIL-53.

Rotational Dynamics of Linkers in Metal–Organic Frameworks

“…The same mechanism of complete rotation has been observed in several other terephthalate-based MOFs, including the well-known MIL-47(V), MIL-53 [18,19,20], and UiO-66 [8,21,22] families. The rotational barriers for all these systems are within 30–50 kJ mol −1 , regardless of the metal centers, which suggests the barrier is in fact intrinsically determined by the linker, with some variations due to metal electronegativity and possible steric effects from the crystal lattice.…”

Emergence of Cooperative Rotor Dynamics in Metal–organic Frameworks via Tuned Steric Interactions

“…20) and NH 2 -MIL-53(Al)-d 3 ( Supplementary Fig. 21) are in line with literature 42,54,55 as well as our expectations based on the DFT and BDS analyses. Supplementary Fig.…”

“…Experimental work on MIL-53 [31][32][33] has established that its p-phenylene groups undergo rotation in the form of π-flips, exhibiting similar behavior to several other terephthalate-based MOFs. 29,[34][35][36] The largepore conformation of MIL-53 suggests that the rotating rings should not be subject to significant steric effects.…”

Efficient adsorption separation of xylene isomers using a facilely fabricated cyclodextrin‐based metal–organic framework

“…Terephthalic acid is primarily used in the plastics industry for the production of polyester fibers and polyethylene terephthalate (PET) resins. [20][21][22][23][24][25][26][27][28][29] Some MOFs show the separation mechanisms of commensurate stacking separation and molecular sieving. 8,9 Current industrial practice for separating xylenes involves slow and energy-consuming processes such as fractional crystallization, or adsorption separation on zeolites.…”

“…[10][11][12] As an emerging family of porous materials with zeolite topologies, metal-organic frameworks (MOFs) possess well-defined channels or cavities that are constructed by metal cations/clusters and functional organic linkers, making them highly suitable for diverse adsorption applications, [13][14][15][16][17][18][19] such as xylenes separation. [20][21][22][23][24][25][26][27][28][29] Some MOFs show the separation mechanisms of commensurate stacking separation and molecular sieving. 12,30 The adsorption of oX and pX over Cu 3 (BTC) 2 (HKUST-1), Ni 2 (dobdc) (CPO-27-Ni), ZIF-76, or RHO-ZMOF (In 3+ ) favor oX due to the commensurate stack effect following the difference in the isomers' overall dipole moments or boiling points.…”