Chitrakshi Goel

Synthesis of nitrogen doped mesoporous carbons for carbon dioxide capture

Nitrogen doped mesoporous carbons were prepared by nanocasting method at varying carbonization temperatures followed by characterization in terms of their structural, textural and chemical properties. Melamine-formaldehyde resin and mesoporous silica were used as the polymeric precursor and hard template respectively. Meso-structural ordering of the template was retained by the prepared carbons as suggested by the structural analysis. Evolution of nitrogen and oxygen functionalities along with textural properties of nitrogen doped carbons were regulated by the carbonization temperature. The prepared carbons obtained by carbonization at 700 °C exhibited a maximum surface area of 266 m2 g−1 along with a nitrogen content up to 21 weight%. CO2 adsorption was studied in a fixed-bed column at several temperatures (30 to 100 °C) and CO2 concentrations (5 to 12.5%). Adsorbent reusability was examined by carrying out multiple adsorption–desorption cycles. MF-700 showed the highest CO2 adsorption capacity of 0.83 mmol g−1 at 30 °C. CO2 adsorption kinetics were investigated by fitting experimental CO2 uptake data to different adsorption kinetic models, out of which the fractional order model was found to fit over the complete adsorption range with the error% between experimental and model predicted data within the range of 5%. In addition, the isosteric heat of adsorption was estimated to be around 17 kJ mol−1, confirming the occurrence of the physiosorption process.

Mesoporous carbon adsorbents from melamine-formaldehyde resin using nanocasting technique for CO2 adsorption

Mesoporous carbon adsorbents, having high nitrogen content, were synthesized via nanocasting technique with melamine-formaldehyde resin as precursor and mesoporous silica as template. A series of adsorbents were prepared by varying the carbonization temperature from 400 to 700°C. Adsorbents were characterized thoroughly by nitrogen sorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), elemental (CHN) analysis, Fourier transform infrared (FTIR) spectroscopy and Boehm titration. Carbonization temperature controlled the properties of the synthesized adsorbents ranging from surface area to their nitrogen content, which play major role in their application as adsorbents for CO2 capture. The nanostructure of these materials was confirmed by XRD and TEM. Their nitrogen content decreased with an increase in carbonization temperature while other properties like surface area, pore volume, thermal stability and surface basicity increased with the carbonization temperature. These materials were evaluated for CO2 adsorption by fixed-bed column adsorption experiments. Adsorbent synthesized at 700°C was found to have the highest surface area and surface basicity along with maximum CO2 adsorption capacity among the synthesized adsorbents. Breakthrough time and CO2 equilibrium adsorption capacity were investigated from the breakthrough curves and were found to decrease with increase in adsorption temperature. Adsorption process for carbon adsorbent-CO2 system was found to be reversible with stable adsorption capacity over four consecutive adsorption-desorption cycles. From three isotherm models used to analyze the equilibrium data, Temkin isotherm model presented a nearly perfect fit implying the heterogeneous adsorbent surface.

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

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.

Novel nanostructured carbons derived from epoxy resin and their adsorption characteristics for CO2 capture

In this work, a nanocasting technique has been used to synthesize oxygen enriched carbon adsorbents with epoxy resin as the precursor and mesoporous zeolite as a template. Carbonization and physical activation with CO2 was carried out to prepare different carbon adsorbents. Characterization of the synthesized adsorbents was done using N2 sorption, XRD, SEM, TEM, TGA, FTIR spectroscopy, CHN analysis, and XPS. The surface area and pore volume of the synthesized adsorbent prepared at 600 °C were found to be a maximum of 686.37 m2 g−1 and 0.60 cm3 g−1, respectively, but showed a lower adsorption capacity due to lesser oxygen content as compared to the sample prepared at 700 °C. The sample prepared at 700 °C exhibited the highest CO2 uptake, approximately 0.65 mmol g−1, at 30 °C due to the high oxygen content, which was estimated to be about 53.98% determined using CHN analysis and also due to high surface basicity confirmed by XPS. The sample prepared by direct carbonization of the polymeric precursor shows a completely non-porous and highly acidic material having the least adsorption capacity. It was found that an increase in concentration of CO2 increases adsorption capacity and an increase in adsorption temperature decreases adsorption capacity. CO2 adsorption kinetics were performed by using three kinetic models and from the correlation coefficient, adsorption kinetics were found to obey fractional order with error% within the range of 4.24%. For checking the regenerability, four adsorption–desorption cycles were examined. It was found that the adsorbents exhibit easy regenerability, stable adsorption capacity and good selectivity for CO2–N2 separation. The experimental data are well fitted with the Freundlich isotherm, showing a heterogeneous adsorbent surface. The isosteric heat Qst of CO2 is 9.09 kJ mol−1, which indicates the presence of the physisorption process. The negative value of Gibbs free energy suggests the spontaneous nature of the process. The values of ΔH° and ΔS° were found to be −2.562 kJ mol−1 and 0.033 kJ mol−1 K−1, respectively. The negative value of ΔH° suggests the exothermic nature of the adsorption process.