Synthesis of lignin-based carbon aerogels for gas filter application

Kurzfassung

In this study lignin-based aerogels were synthesized and characterized, focusing on the optimization of key parameters such as lignin type, lignin to formaldehyde ratio, aging temperature and solvent exchange conditions. The aim was to develop porous materials with high surface area and structural stability for future use in gas adsorption applications. Lignin-based aerogels were synthesized using the procedure described in the literature by G. Aufischer et al.[8] The first step was to adapt the procedure to the requirements and conditions of our lab. The updated synthesis route was then used to further investigate the different parameters of the precursors. Two types of lignin, Indulin AT and organosolv lignin, were investigated as precursors. Indulin AT proved to be more reactive under base-catalyzed conditions, forming stable gels at elevated temperatures (70 C and 90 C), while the organosolv lignin did not gel, but only showed a small amount of precipitated particles, likely due to different structural features. With the Indulin T a systematic study was conducted using a constant lignin concentration of 10 wt.% and varying the formaldehyde content in weight ratios LN/FA ranging from 1:0.1 to 1:3. The samples with weight ratio 1:0.3 and above were successfully gelled. The samples in this series showed different behavior regarding appearance, mechanical stability and physica characteristics. The porosity of the samples was derived from helium pycnometry and envelope density measurements, showing a trend of increasing porosity with increasing formaldehyde content. A decreasing of the porosity between the examined batches was observed, which leads to the conclusion that the complexity of the lignin macromolecule could results in varying final materials. Surface area and pore characteristics were analyzed via N2-gassorption measurements. The aerogels exhibited Type IV isotherms, indicating mesoporosity . The BET surface area reached up to 370 m2g-1 for samples aged at 90 °C and LN/FA ratio of 1:0.6 (Fig. 18). The pore size distribution ranges form approximately 10 to more than 120 nm. A separate study on varying lignin concentrations (4 – 16 wt.%) showed that samples with lignin concentration below 6 wt.% could not be fully gelled and they remained viscous liquids. The macroscopic appearance of the aerogel samples in this series shows no significant variation in color. The BET surface area of the samples with 6 – 12% lignin were between 300 and 340 m2·g-1 followed by slight decrease for samples with higher lignin concentrations. For the 16% lignin sample it was observed that the material was not completely consumed. Thermogravimetric analysis guided the carbonization protocol, howing major decomposition occurring between 260 °C and 600 °C, with residual masses ranging from 40 - 48%. Carbonization was performed at 900 °C under inert atmosphere. Unexpected combustion was observed, likely due to trapped oxygen and/or insufficient purging. Only from the sample containing 16 wt.% lignin a sufficient amount remained after the procedure (Fig. 27). Remarkably, this sample showed a BET surface area of 2,065 m2·g-1 and a pore volume of 3.1 cm3 g-1, with micropore contribution confirmed by t-plot showing 1,249 m2·g-1.