Translational and Rotational Motion of C8 Aromatics Adsorbed in Isotropic Porous Media (MOF-5): NMR Studies and MD Simulations

Abstract

We combined nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation to study xylene behavior in MOF-5, probing the effects of adsorbate geometry in a weakly interacting model isotropic metalorganic framework (MOF) system. We employed NMR diffusometry and relaxometry techniques at low field (13 MHz) to quantify the self-diffusion coefficients (D s ) and the longitudinal relaxation times (T 1 ) of xylenes in MOF-5 as a function of temperature at saturated loading for each xylene. These experiments reveal the translational motion activation energies to be 15.3, 19.7, and 21.2 kJ mol −1 and the rotational activation energies to be 47.26, 12.88, and 11.55 for the (p-,m-,o-) xylene isomers respectively. Paraxylene exhibits faster translational motion, yet shows four times the activation energy barrier for rotational motion vis-à-vis the other isomers. MD simulations performed on these model systems corroborate the findings for paraxylene and suggest that paraxylene has the a lower free energy barrier for hopping away from its binding sites. These simulations show that paraxylene has the slowest rotational motion in the plane of the xylene molecule while it actually has the fastest out-of-plane rotational motion.

Low-Field Functional Group Resolved Nuclear Spin Relaxation in Mesoporous Silica

“…9 Further well-established applications include the observation of cement and plaster hydration kinetics, 10,11 while emergent fields within the materials chemistry research space now include characterisation of bespoke porous architectures including heterogeneous catalysts, 12,13 zeolites 14,15 and metal-organic frameworks. 16,17 The most data-rich methods for elucidating spin relaxation processes in porous media are multidimensional relaxation correlation measurements. [18][19][20] Indeed, two-dimensional (2D) relaxation-relaxation (𝑇 1 − 𝑇 2 [21][22][23] and 𝑇 2 − 𝑇 2 24,25 ) and diffusion-relaxation (𝐷 − 𝑇 2 [26][27][28][29] ) measurements are widely applied to characterise the dynamics of confined fluids, while higher-dimensional experiments, [30][31][32][33] including spatially-resolved variants, 34,35 have also been reported.…”

Low‐Field NMR Relaxation Analysis of High‐Pressure Ethane Adsorption in Mesoporous Silicas

“…These time constants exhibit well‐defined relationships with molecular rotational and translational dynamics within the unrestricted bulk liquid phase [18] . For fluids confined to porous solids, however, the correspondence between time constants and molecular dynamics is perturbed by the attendant pore structure, [19,20] facilitating characterisation of a range of material and interfacial properties including pore sizes [21,22] and connectivity, [23–25] surface areas, [26,27] confinement effects [28–30] and adsorption interactions [31–37] . Papaioannou et al [38] .…”

Screening Metal–Organic Frameworks for Separation of Binary Solvent Mixtures by Compact NMR Relaxometry

“…Among existing analytical technics, nuclear magnetic resonance (NMR) has strongly advanced our understanding about the MOF materials in terms of structure, dynamics of the linker, and motion and diffusion of adsorbed molecules by taking advantage of dedicated high-field NMR devices and sophisticated experimental techniques [36][37][38][39][40][41][42][43]. Yet, useful microscopic information can also be gained with the help of less expensive compact low-field NMR sensors with open and closed magnet geometries.…”

“…Such sensors are well suited to interrogate the behavior of protonated materials and have been already successfully applied for a detailed characterization of various materials [36][37][38][39][40]. However, their application in the context of MOFs is still relatively scarce with existing studies focusing on the relaxation and diffusion behavior of gases and solvents in MOFs [41][42][43].…”