Property Comparison of Base- Versus Acid-Catalyzed RF Aerogels

Property Comparison of Base- Versus Acid-Catalyzed RF Aerogels 

Hydroxymethylated resorcinol in either base- or acid-catalyzed sol does not accumulate because of its instability, p-Hydroxymethyl groups undergo rapid conversion to carbocations or o-quinone methides. This explains how identical condensation products are obtained under acid or alkaline reaction conditions [19]. This fact is evident in the NMR spectra of RF aerogels prepared under acid- or base-catalyzed conditions [33-38]. 

On comparing the solid NMR spectra of base-catalyzed RF aerogels with those of the HCl-catalyzed process, it is evident that the latter, being completed in minutes rather than in days, yields materials that are chemically indistinguishable from those obtained by the usual base-catalyzed process in a week. 

Berthon-Fabry et al. have studied multiscale structure formation as a function of the concentration of monomers and of acid. The % mass ratio (percent of monomers to the total mass of the sol) was used as a controlling parameter for the effect of monomer concentration. Formulations with % mass ratio 35 and relatively low acid catalyst concentration (R/C 50) gave homogenous monoliths. With a higher monomer % mass ratio ( 55) and acid catalyst concentrations RjC 1), a very viscous resin-like material was obtained. Varying the concentration of monomers and acid is an effective method to produce carbon aerogels with different morphologies [11, 37]. 

Incoiporation of metal precursors into carbon aerogel is traced back to incorporation of metal salts into RF solution before the sol-gel process. Simple physical adsorption of metal salts on wet gel matrices caused leaching of the metal ions during washings required by the sol-gel process. This resorted researchers to use derivatives of resorcinol (e.g., 

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