N-doped sponge-like biochar: A promising CO2 sorbent for CO2/CH4 and CO2/N2 gas separation

Sponge-like biochar sorbents were prepared from the dissolution of chitosan followed by freeze-drying methodology and pyrolysis at three different temperatures (400, 600, and 800 degrees C) to produce sustainable N-enriched carbon materials with enhanced CO2 uptake from CO2/CH4 and CO2/N-2 gas mixtures. The pyrolysis process was reproduced by operando TGA-IR to study the gas evolved from the pyrolysis process. It was found that the pyrolysis temperature highly influences the textural properties of the chitosan sponge-like biochar materials, impacting mainly the amount and type of the N-species on the sample but also at the microporosity. XPS revealed the transformation of the amino groups from chitosan into pyridinic-N, pyrrolic-N, graphitic center-N, and graphitic valley-N or pyridine-N oxide species during the pyrolysis process. Increasing the pyrolysis temperature enhanced the quantity of the latter two N-type species. All sponge-like biochars adsorbed higher amounts of CO2 compared with CH4 and N-2 gases, with maximum CO2 uptake (similar to 1.6 mmol.g(-1)) at 100 kPa and 25 degrees C for the sample pyrolyzed at 600 degrees C (named CTO_P600). Biochar produced at 800 degrees C showed no longer adsorption capacity for CH4 and N-2, having the highest selectivity value for CO2/N-2 separation under continuous flux conditions among all prepared biochar sorbents. Isobaric CO2 adsorption measurements on the CTO_P600 sorbent revealed that physisorption phenomena predominantly governed the CO2 adsorption process, which was confirmed by its consistent adsorption capacity after 10 consecutive adsorption-desorption cycles. Moreover, the biochar exhibited tolerance to water vapor adsorption, indicating its suitability to work under moisture-rich conditions.

Automated summary

Sponge-like biochar sorbents with enhanced CO2 uptake were prepared from chitosan through dissolution, freeze-drying, and pyrolysis at different temperatures. The pyrolysis temperature influenced the textural properties of the biochar, as well as the type and amount of N-species present. XPS analysis revealed the transformation of amino groups into different N-species during pyrolysis. The biochar adsorbed higher amounts of CO2 compared to CH4 and N-2 gases, with the highest selectivity for CO2/N-2 separation observed for the sample pyrolyzed at 800 degrees C. The adsorption process was predominantly governed by physisorption, and the biochar showed tolerance to water vapor adsorption, indicating its suitability for use in moisture-rich environments.