Physical and Analytical Chemistry Seminar
Lecturer: Efrat Pri-Gal
Location: Faculty Seminar Room
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, pollutant 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-151,2. 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 (400, 600, 800C) and chemical activation. Atop the intrinsically disordered surfaces we identify the occurrence of two classes of sites: one that provides strong binding with high binding specificity, and a second class of weaker and no detectable specificity. The NMR exposes increasing carbon character (large conjugated aromatic domains) with increased calcination temperature, relatively high surface density of the strong binding sites (10-50%) and structural irregularities of molecular dimensions.
Surprisingly, we further find that surface structures with “defected” carbon character are the strong binding strength sites (- interactions) while the large conjugation domains (as in high carbon character) provide the weaker binding sites.
The investigated FDU-15 materials proved as potent binders, however just based on the differences in binding strengths they do not provide high selectivity. However, when combined also with kinetic effects, retention selectivity with 10-fold preference for p-xylene over benzene was obtained. Combined with surface activation3, pronounced selectivity towards the less hydrophobic iso-propanol adsorbate was demonstrated.
This work 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.
- Yan Meng, † et al. A Family of Highly Ordered Mesoporous Polymer Resin and Carbon Structures from Organic−Organic Self-Assembly. chem. Mater 18, 4447–4464 (2006).
- Xue, C., Tu, B. & Zhao, D. Facile fabrication of hierarchically porous carbonaceous monoliths with ordered mesostructure via an organic organic self-assembly. Nano Res. 2, 242–253 (2009).
- Wu, Z., Webley, P. A. & Zhao, D. Comprehensive Study of Pore Evolution, Mesostructural Stability, and Simultaneous Surface Functionalization of Ordered Mesoporous Carbon (FDU-15) by Wet Oxidation as a Promising Adsorbent. Langmuir 26, 10277–10286 (2010).