PNNL

Abstract

In this article we report a detailed study of resorcinol-formaldehyde (RF) aerogels prepared under different processing conditions, [resorcinol]/[catalyst] (R/C) ratios in the starting sol-gel solutions, using continuous flow hyperpolarized 129Xe NMR in combination with solid-state 13C and two-dimensional wide-line separation (2D-WISE) NMR techniques. The degree of polymerization and the mobility of the cross-linking functional groups in RF aerogels are examined and correlated with the R/C ratios. The origin of different adsorption regions is evaluated using both co-adsorption of chloroform and 2D EXSY 129Xe NMR. A hierarchical set of Xe exchange processes in RF aerogels is found using 2D EXSY 129Xe NMR. The exchange of Xe gas follows the sequence (from fastest to slowest): mesopores with free gas, gas in meso- and micro-pores, free gas with micropores, and, finally, among micropore sites. The volume-to-surface-area (Vg/S) ratios for aerogels are measured for the first time without the use of geometric models. The Vg/S parameter, which is related both to the geometry and the interconnectivity of the pore space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about a 5-fold rise in Vg/S.