Fast synthesis of micro/mesoporous xerogels: Textural and energetic assessment

Abstract

The sol-gel polymerization of resorcinol/formaldehyde mixtures to obtain porous gels is typically a long process performed throughout several days. In this work, we have explored an experimental approach to reduce the time necessary to obtain porous gels based on mild polymerization conditions and direct drying. We have analyzed the effects of the temperature and time of the gelation/aging step on the porosity of the gels, as well as the impact on the overall energetic cost of the process. Data have shown that well-developed micro-mesoporous architectures can be obtained within less than a day. The temperature of the gelation/aging step mainly affects the mesopore network, whereas the microporosity is determined by the composition of the precursor's mixture. The exclusion of the solvent exchange step yields soft mechanically fragile porous gels with structural limitations upon carbonization at high temperature in inert atmosphere, due to the surface tensions applied to the backbone during the evolution of volatiles. The mesopore structure lost during carbonization is not recovered upon activation in CO 2 atmosphere, but it is preserved upon chemical activation in K 2 CO 3 and the resulting gel exhibits a bimodal micro-mesoporous distribution. Furthermore, the energy savings of this route are similar to those obtained using microwave-heating in terms of grams of xerogel per kilowatt hour of energy consumed for similar textural properties. The correlation between the energy power consumed and the textural parameters is a useful tool to optimize the synthesis.

Carbon Black as Conductive Additive and Structural Director of Porous Carbon Gels

“…An important feature of the gas adsorption isotherms is the evolution of the shape and the position of the hysteresis loop (Figure 3) with the amount of conductive additive. The pristine gel (sample PG) exhibited a narrow loop (type H2) between 0.4 and 0.7 of relative pressures, in agreement with previous studies using a similar reactants molar ratio [31]. For the samples prepared in the presence of CB, the hysteresis loop broadened significantly, spanning from 0.4 up to 0.9 of relative pressures.…”

Nanoconfinement of glucose oxidase on mesoporous carbon electrodes with tunable pore sizes

“…All the carbon gels displayed similar values of BET surface area and micropores volumes, whereas the total pore volumes increased with the R/C molar ratio. This behavior is a common characteristic of this type of carbon materials obtained from sol-gel polycondensation of RF mixtures [18][19][20][21].…”

“…Nanoporous carbon gels were synthesized by the sol-gel polymerization of resorcinol (R) and formaldehyde (F) in water (W), using sodium carbonate (C) as catalyst, as reported elsewhere [20,21]. Briefly, the precursors were mixed at fixed molar ratios (R/W of 0.06, R/F of 0.5 and R/C of 50, 75, 100 and 200) under magnetic stirring and immediately heated in airtight sealed glass vessels for gelation/aging at 95°C for 4 h in an oven.…”

“…The effect of the various synthesis parameters -such as the molar ratio of reactants, pH, surfactant concentration, aging temperature-on the textural and morphological features of carbon gels has been widely discussed in the literature [18][19][20][21], and remains out of the scope of this study. We herein introduce the main physicochemical parameters of the prepared NPC used as electrodes for the immobilization of GOX for data interpretation and aiming at elucidating the role of nanopore confinement on the activity of the immobilized enzyme.…”

“…To attain this goal, nanoporous carbon gels prepared by solgel polycondensation of resorcinol-formaldehyde mixtures were used as electrode materials for the immobilization of the enzyme. These materials can be easily synthesized with tunable pore architectures by adjusting the synthesis conditions [18][19][20][21], thus allowing to obtain carbons combining large specific surface areas with a proper network of transport pores to favor the diffusion and adsorption of bulky molecules [22,23]. The novelty of the work resides on providing a deep insight on the role of the tight nanoconfinement of an immobilized enzyme inside the porosity of carbon electrodes, aiming at correlating the electrochemical and enzymatic response with the pore dimensions of the carbon-based electrode materials.…”