|Ph.D Student||Pri-Gal Efrat|
|Subject||A study of the Surface Properties of Mesoporous Carbon|
Based Materials by Solid State NMR
|Department||Department of Chemistry||Supervisor||Professor Asher Schmidt|
|Full Thesis text|
The phenomenon of surface binding and reactivity raises a wide range of fundamental questions: the types of interactions involved, the system components that imply selectivity, and structural and chemical modifications to tailor surface properties. Applications are within diverse fields such as separation processes, pollutants removal, electrode materials and catalysis.
As model systems I have synthesized variants of the high surface area (400-1000 m2/gr) mesoporous carbon-based FDU-15. Adsorbing small organic molecules (benzene, p-xylene, iso-propanol; specifically deuterated) as surface probes and employing primarily solid state 2H MAS NMR as molecular eyes I have studied the surface properties as function of calcination/carbonization temperatures (TC = 400, 600 / 800°C) and chemical activation.
The NMR confirms the increasing carbon character - larger conjugated aromatic domains - with increased calcination temperature. Atop the intrinsically disordered surfaces we identify the occurrence of two classes of sites: one that provides strong binding with high specificity, and a second class of weaker and no detectable specificity. This distinction is facilitated by 2H MAS NMR showing uniform surface-induced immobilization of the adsorbate molecules at the strong-binding sites, yet none for the weak. At the low calcination temperature surface densities of the strong binding sites are high (10-50%) and are of structural irregularities of molecular dimensions. Their density, abundance, substantially decreases with calcination temperature, with the weak-binding sites becoming dominant. Ring current effects sensed by the probe adsorbate molecules (upfield chemical shifts vs. bulk) report that the chemical character of the strong binding sites is of reduced aromaticity, while that of the weaker binding sites is typified by larger conjugation domains. The NMR clearly identifies for the benzene adsorbate molecules p-p interactions at the strong binding sites, however atop the second class of sites all adsorbates undergo rapid isotropic reorientation and exchange. As the investigated FDU-15 materials proved potent adsorbents, the NMR shows that the adsorption mechanisms are determined by calcination temperature: for low TC strong hydrophobic interactions at specific sites, yet weaker interactions plus confinement at the high TC. These distinctions are possible by accounting also for desorption kinetic effects as examined by employing mild overnight evacuation. Noteworthy is the fact that earlier adsorption studies attributed potency of these materials to the micropore content, whereas our study highlights the critical role of the surface chemical-structural properties irrespective of pore type.
In spite of the high potency, differences in binding strengths were found insufficient to provide selectivity. Inspecting competitive binding of benzene vs. p-xylene (on 400FDU-15) the NMR shows no thermodynamic preference. When combined with kinetic desorption effects, retention selectivity with 10-fold preference for p-xylene was obtained. Combined with surface activation, pronounced selectivity towards the less hydrophobic iso-propanol adsorbate was demonstrated.
2H MAS NMR, as tailored in this study, provides a sensitive high-resolution means to probe the functional properties of the surfaces at the molecular level. This study emphasizes the importance of molecular level characterization of adsorbent-adsorbate (surface-molecule) interactions for both - the understanding of the complex phenomena of adsorption-desorption and their practical utilization as in rationally designed functional materials.