Hydrophobic Aerogels and Xerogels based on Trimethoxybenzene-Formaldehyde.

Abstract

Phenolic aerogels based on resorcinol-formaldehyde (RF) are among the best thermally insulating materials. However, the hydrophilicity inherent to the free phenolic moiety of RF gels generally limits their actual range of applications. Prior efforts to render phenolic gels hydrophobic are restricted to post-synthetic functionalizations of hydrophilic gels, processes that are often limited in efficiency, scope, and/or longevity. Here, an acid-mediated conversion of 1,3,5-trimethoxybenzene with formaldehyde is reported, yielding monolithic trimethoxybenzene-formaldehyde (TMBF) aerogels and xerogels with low density (0.11-0.30 g cm^-3), high porosity (74-92 %), inner surface areas (S_BET) of up to 284 m² g^-1, and thermal conductivity of 34.5-43.9 mW m^-1 K^-1. For a monolithic xerogel based on TMBF xerogels an unprecedently low thermal conductivity of 34.5 mW m^-1 K^-1 could be achieved. In addition, all TMBF gels are thermally stable (degradation >280-310 °C) and highly hydrophobic (water contact angles 130°-156°). As such, TMBF serves as a new class of inherently hydrophobic aerogels and xerogels and useful complement to RF materials.