Conducting macroporous carbon foams derived from microwave-generated caramel/silica gel intermediates

Abstract

The current work reports the preparation of macroporous carbon/silica foams using sucrose as precursor in the presence of silica gel and water. After a preliminary fast transformation of sucrose into caramel by MW irradiation, the carbon/silica materials are obtained by thermal treatment at 800 °C under nitrogen atmosphere. In contrast to carbon/clay materials prepared by the same synthetic route, the use of microporous silica gel as inorganic support for the chemical transformation of sucrose gives rise to foam-like materials without requiring the addition of blowing agents. The resulting materials are low-density monoliths (0.17 g cm−3) that show several populations of macropores centered at 150 and in the range of 30–5 µm, as well as mesopores around 5–6 nm. The porosity is close to 97%, as determined from helium pycnometry, which gave a skeletal density value of 6.08 g cm−3. The resulting foams show electrical conductivity values around 3 mS cm−1 due to the generated graphene-like materials, as corroborated by Raman spectroscopy. A preliminary evaluation of these macroporous materials as adsorbents of aromatic pollutants was carried out using methylene blue (MB) as a model dye. The regeneration of the MB-loaded foams to make possible their application in successive adsorption cycles was assessed by solid–liquid extraction and electrochemical regeneration. In both cases, the treatment produced an increase in the uptake capacity, which could be due to an increment in porosity and specific surface area, from 360 to 440 m2 g−1, as determined from mercury porosimetry and N2 adsorption measurements. In addition, the electrochemical method allowed a partial degradation of the pollutant, as determined by UV–Vis and HPLC–MS. Thus, the carbon/silica materials here developed are promising adsorbents for treatment of polluted effluents.