Time-efficient Acid-catalyzed Synthesis of Resorcinol-formaldehyde Aerogels

Resorcinol (R)−formaldehyde (F) aerogels are pursued as precursors of carbon aerogels, which are electrically conducting. They are usually prepared via a week-long base-catalyzed gelation process from an aqueous sol. For this work, we reasoned that because both the reaction of R with F and the condensation of the resulting hydroxymethyl resorcinol with R are electrophilic aromatic substitutions, they should proceed easily by acid catalysis in one pot. Thereby, we have developed and reported an HCl-catalyzed gelation process in CH3CN, which is completed in about 2 h at room temperature or in 10 min at 80 °C. The final aerogels are chemically indistinguishable (by IR and 13C CPMAS NMR) from typical base-catalyzed samples. In analogy to phenol−formaldehyde resin formation, the mechanism may involve o-quinone methide intermediates (hence the red color prevailing throughout the process). The effect of aging is discussed in terms of shrinkage and is attributed to further reaction and incorporation of more formaldehyde into wet gels, followed by syneresis (reaction with one another of dangling oligomeric appendices on the skeletal framework).