Resorcinol–formaldehyde based nanostructured carbons for CO2 adsorption: kinetics, isotherm and thermodynamic studies

Abstract

Silica templated nanostructured carbons were developed from a resorcinol–formaldehyde polymeric precursor by varying the carbonization temperature from 400 °C to 800 °C. The prepared carbons were characterized thoroughly for their textural, surface and chemical properties followed by dynamic CO₂ capture performance at various adsorption temperatures from 30 °C to 100 °C under simulated flue gas conditions. Among the prepared carbons, carbonization at 700 °C resulted in the nanostructured carbon material, as indicated by XRD and TEM results, having the best textural properties i.e. specific surface area and total pore volume around 435 m² g⁻¹ and 0.22 cm³ g⁻¹, respectively. The sample obtained by carbonization at the most severe conditions (≥800 °C) exhibited textural properties comparable to that of RF-700 but showed lower CO₂ adsorption capacity on account of reduction in surface basicity at higher temperatures. On the other hand, preparation of the carbon material by direct carbonization of the polymeric precursor, i.e. without using a template, resulted in a completely non-porous material with very low CO₂ adsorption capacity. Moreover, both the textural properties and the surface chemistry had an effect on the CO₂ adsorption performance of the prepared carbons. RF-700 exhibited the highest dynamic CO₂ adsorption capacity of 0.761 mmol g⁻¹ at 30 °C in a binary mixture of 12.5% CO₂ in N₂ attributed to a well-developed porous structure and high surface basicity of 1.93 meq g⁻¹. It also demonstrated high selectivity towards CO₂ over N₂ and stable adsorption capacity over multiple adsorption–desorption cycles. CO₂ adsorption on the prepared carbons was well described by a fractional order kinetic model. Fitting of equilibrium data of CO₂ adsorption by Temkin isotherm model and variation in isosteric heat of adsorption with surface coverage indicated an energetically heterogeneous adsorbent surface. Thermodynamics of CO₂ adsorption on the carbon material suggested an exothermic, random and spontaneous nature of the process. The thermal energy required for desorption of CO₂ was also estimated to be around 1.9 MJ per kg CO₂.