Fast microwave-assisted synthesis of tailored mesoporous carbon xerogels

Visual illustration of the different stages involved in the organic xerogels synthesis with the determination of the gelation point and the observed mesoporosity of carbon xerogels in a wider pH for the microwave than for conventional synthesis. [Display omitted] ► Mesoporosity can be tailored using MW synthesis of carbon xerogels. ► A wider range of pH than conventional methods can be used with MW synthesis. ► MW synthesis allows straightforward determination of the gelation point. ► Faster and cheaper production of carbon xerogels can be achieved using MW synthesis. Resorcinol–formaldehyde carbon xerogels with several initial pH were synthesized using two different heating methods (conventional and microwave heating). The effect of the pH of the precursor solution and the method of synthesis employed on the textural and chemical properties of the final materials was evaluated. It was found that both methods produce tailored carbon xerogels depending on the initial pH and that the pores of the carbon xerogels become larger as the initial pH decreases. High pHs result in exclusively microporous carbon xerogels, while a decrease in the amount of NaOH added, i.e. lower pH, causes the materials to evolve firstly into micro–mesoporous samples and then into micro–macroporous carbon xerogels. The main difference between the two heating methods studied, apart from the duration of the synthesis (i.e. approximately 5h for the microwave-assisted synthesis as opposed to several days by conventional methods) lies in the meso–macroporosity of the resulting materials, since microwave radiation produces mainly mesoporous carbon xerogels with a specific mesopore size over a wider range of pH than conventional synthesis. For example, the pH range for mesoporous MW samples is 4.5–6.5 while equivalent samples that are conventionally synthesized require an initial pH of between 5.8 and 6.5. This work also illustrates a simple and precise method for determining the gelation point (tg) of different pH resorcinol–formaldehyde mixtures, based on varying the energy consumed by the microwave device during the synthesis of organic gels, without the need for other more complicated techniques.