Optimized synthesis of highly microporous activated carbon from residual biomass lignin for efficient CO2 capture

A versatile innovative biorefinery model for the complete valorization of lignocellulosic giant reed (Arundo donax L.) to give valuable bio-based products was implemented and optimized. The hemicellulose fraction was selectively fractionated and depolymerized by microwave-assisted FeCl3-catalyzed hydrolysis to give xylose-rich hydrolysates, which were then converted to new generation bio-oil (triglycerides) and biodiesel (fatty acids methyl esters) by fermentation with the oleaginous yeast Lipomyces starkeyi.1 The cellulose fraction of the pretreated solid was then exploited by MW-assisted FeCl3-catalyzed hydrolysis to give levulinic and formic acid.2 Finally, the lignin-rich solid residue obtained at the end of the proposed cascade process was chemically activated to produce activated carbon with a microporous structure suitable for CO2 adsorption. The activation protocol was optimized by a chemometric approach based on the Response Surface Methodology (RSM). Activation temperature and KOH/lignin weight ratio were selected as independent variables. The reaction time was fixed at 60 min. The selected responses were: i) activated carbon yield (wt%); ii) carbon yield (wt%); iii) CO2 uptake (mg/g). Under the optimized process conditions (633 °C, KOH/lignin 3.0 wt/wt, 60 min) the AC yield was 34.4 wt% and the CO2 uptake was 72.3 mg/g confirming the promising application of the residual lignin. Moreover, the obtained material (Fig. 1) showed similar CO2 uptake values over 10 cycles of adsorption and desorption tests, demonstrating the regeneration of the bio-based material obtained without losing its gas uptake capacity. The complete conversion of each main fraction of the starting raw material represents a crucial strategy for favoring the economic and environmental sustainability of integrated biorefinery processes in the perspective of the Green Chemistry principles. Figure 1. SEM images of the activated carbon under the optimised process conditions. References 1. Di Fidio, N., Ragaglini, G., Dragoni, F., Antonetti, C., Raspolli Galletti, A. M. Bioresour. Technol. 2021, 325, 124635-124644. 2. Di Fidio, N., Antonetti, C., Raspolli Galletti, A. M. Bioresour. Technol. 2019, 293, 122050-122058. Acknowledgements: This work was funded by PRA 2020/2021 project “New horizons in CO2 chemistry: from capture to fine chemicals and metal based drugs” (code PRA_2020_39) of the University of Pisa.