NMR studies of benzene mobility in metal-organic framework MOF-5

Abstract

Abstract. The concentration and temperature dependence of the self-diffusion of benzene adsorbed in the metal-organic framework MOF-5 (IRMOF-1) is studied by pulsed field gradient (PFG) NMR spectroscopy.When increasing the loading from 10 to 20 molecules per unit cell of MOF-5, the experimental diffusion data drop by a factor of about three while current molecular dynamic (MD) simulations predict slightly increasing diffusion coefficients for this range of loadings. The observation is rationalized using the recently predicted clustering of adsorbate molecules in microporous systems for temperatures well below the adsorbate critical temperature. Necessary improvements of molecular simulation models for predicting diffusivities under such conditions are discussed.

Assessing Guest‐Molecule Diffusion in Heterogeneous Powder Samples of Metal–Organic Frameworks through Pulsed‐Field‐Gradient (PFG) NMR Spectroscopy

“…The investigation of guest diffusion through PFG NMR is notably facilitated when the host material has a known and well‐ordered structure, such as narrow particle size distribution, homogenous and large host particle sizes or an easy‐to‐infer diffusivity inside particles . Thus, much effort has been made to study such idealized systems . It is true that recent progress in PFG NMR methodology has facilitated diffusion studies in also more complex pore architectures just as the investigation of highly heterogeneous systems being characterized by a distribution of diffusivities .…”

“…[1,2] Thus, much effort has been made to study such idealizedsystems. [8][9][10][11] It is true that recent progress in PFG NMR methodology [12] has facilitated diffusion studies in also more complex pore architectures [13][14][15][16] just as the investigationof highly heterogeneouss ystemsb eing characterized by ad istribution of diffusivities. [16,17] However,a pplication of such advanced techniques requires high accuracyi nt he spinecho attenuations, which cannotb ee xpectedt ob ea ttainable as ar ule.…”

Nanoporous Materials Can Tune the Critical Point of a Pure Substance

“…Exchange peaks are prominent in Figure 4 B, confirming that the molecules are exchanging between the liquid and vapor phases within a timescale of t exch =100–500 ms (SI Discussion). This time scale, along with a representative diffusion coefficient6a,b of benzene in IRMOF‐1 (≈10 −9 m 2 s −1 ) gives us ca. 10 −5 m as the characteristic length scale over which a molecule diffuses between both phases.…”

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

“…Hence, different techniques, a summary of which can be found in a review by Ka ¨rger et al, 179,180 have been used to study guest diffusivities as well as possible molecular transport resistances in porous materials. Among them, in this review we highlight quasi-elastic neutron scattering (QENS), [181][182][183][184][185][186][187][188][189][190][191][192][193][194][195][196][197][198] pulsed field gradient (PFG)-NMR 179,[199][200][201][202][203][204][205][206][207] as well as interference 208 and IR microscopy. 179,[208][209][210][211][212][213][214][215][216][217][218][219][220][221] QENS allows to determine the diffusion of guest molecules under equilibrium conditions on a...…”

“…For PFG NMR, however, the measurement time scale allows for different diffusion path scenarios (within and outside the crystals, see review by Chmelik et al), 179 and only for sufficient large crystals, together with short observation times and low diffusivities, intracrystalline self-diffusivities can be obtained. PFG NMR has been used to determine the intracrystalline self-diffusivities of different guest molecules in several well-known MOFs, 199,201,202,205,229 and in the particular case of ZIF-8, self-diffusivities of different hydrocarbons, 204 small alcohols, 200 CO 2 207 and CH 4 203,206 have been reported.…”