Water and small organic molecules as probes for geometric confinement in well-ordered mesoporous carbon materials

Abstract

Mesoporous carbon materials were synthesized employing polymers and silica gels as structure directing templates. The basic physico-chemical properties of the synthetic mesoporous materials were characterized by (1)H and (13)C MAS solid-state NMR, X-ray diffraction, transmission electron microscopy (TEM) and nitrogen adsorption measurements. The confinement effects on small guest molecules such as water, benzene and pyridine and their interactions with the pore surface were probed by a combination of variable temperature (1)H-MAS NMR and quantum chemical calculations of the magnetic shielding effect of the surface on the solvent molecules. The interactions of the guest molecules depend strongly on the carbonization temperature and the pathway of the synthesis. All the guest-molecules, water, benzene and pyridine, exhibited high-field shifts by the interaction with the surface of carbon materials. The geometric confinement imposed by the surface causes a strong depression of the melting point of the surface phase of water and benzene. The theoretical calculation of (1)H NICS maps shows that the observed proton chemical shifts towards high-field values can be explained as the result of electronic ring currents localized in aromatic groups on the surface. The dependence on the distance between the proton and the aromatic surface can be exploited to estimate the average diameter of the confinement structures.

NMR Study of Ion Dynamics and Charge Storage in Ionic Liquid Supercapacitors

“…Two spectral features are clearly visible for the TFSI anions, with the broad feature at −84.9 ppm assigned to adsorbed anions nearby carbon surfaces within the carbon micropores (referred to as in-pore) and the narrow feature at −78.3 ppm assigned to nonadsorbed anions occupying large reservoirs of ionic liquid between primary carbon particles (referred to as ex-pore). 38 − 41 As discussed in the Introduction , the in-pore feature is shifted relative to the ex-pore due to the local magnetic field arising from the circulation of delocalized π electrons in the carbon. The ex-pore anions on the other hand are remote from carbon surfaces and therefore have a chemical shift that is essentially identical to the neat ionic liquid (−78.3 ppm).…”

“… 38 − 40 Density functional theory (DFT) calculations of nucleus-independent chemical shifts (NICSs) nearby various sp 2 -hybridized carbon structures have offered insights into the experimentally observed spectra and suggest that the shifts observed for adsorbed species depend primarily on the structure of the carbon adsorbent. 38 , 41 − 44 NMR experiments on typical activated carbons soaked with organic electrolytes have shown that the carbon micropores are wetted in the absence of an applied potential, i.e., they may be considered as ionophilic. 38 , 39 , 45 , 46 Both in situ 47 , 48 and ex situ 49 NMR studies of supercapacitors with organic electrolytes have shown that the populations of anions and cations inside the carbon micropores change when a voltage is applied and that the exact charge storage mechanism is sensitive to the electrolytes used and their concentration.…”

Lattice simulation method to model diffusion and NMR spectra in porous materials

“…28 Moreover, nanoporous carbons are known to be highly disordered: they present a range of pore sizes and geometries, as well as features such as five-membered rings, sheet edges, and curved sheets. 32,33 DFT calculations have shown that curvature 8 and the presence of sheet edges 34 have a large effect on the local magnetic field and thus on the chemical shifts. The variety of local structures will thus lead to a large range of chemical shifts for adsorbates and may affect the observed lineshapes.…”

1H NMR Investigations of Activated Carbon Loaded with Volatile Organic Compounds: Quantification, Mechanisms, and Diffusivity Determination

“…For the three VOCs, higher amounts were associated with an enlargement towards high ppm values, which induced a shift of approximately 1 ppm of the MAS averaged signal. This phenomenon was already obtained on mesoporous and microporous carbons 32,24 . Thirdly, for increasing loadings of VOCs, the initial adsorbed water signal vanished while a new peak located a few ppm away appeared.…”

“…Such a signal can be generated by a part of the adsorbed molecules exhibiting large dipolar coupling 31,32 . For this specific purpose, high field 1 H NMR recordings correlated with very high MAS frequencies were carried out with the aim of detecting such interaction effects.…”