Microwave-Assisted Conversion of Low-Rank Lignite into Hierarchical Activated Carbon: Molecular Insights into...

Lignite-derived activated carbon (L-AC) was fabricated via a microwave-assisted KOH activation process using a low-rank Mae Moh lignite and explored its potential as an adsorbent solid for post-combustion CO₂ capture. Optimization of the KOH ratio, microwave irradiation power, and activation time gave rise to a product with a BET surface area of 1349 m² g^-1 and total pore volume of 0.78 cm³ g^-1, which represented 165 times and 78 times enhancement compared with that of the initial lignite, respectively. Scanning electron microscope (SEM) images proved the formation of a hierarchical macropore-mesopore-micropore structure, whereas Raman (I_D/I_G = 1.83) and Fourier-transform infrared spectroscopy analyses revealed a graphitic-like structure rich in defects with the existence of C=O and C-O-C functional groups involved in the Lewis acid-base interaction between L-AC and CO₂ molecules. Dynamic fixed-bed breakthrough tests performed at temperatures of 298, 328, and 353 K under post-combustion relevant conditions (CO₂ concentration: 15%, pressure: 1 atm) yielded CO₂ equilibrium uptake capacities of 47.34, 34.37, and 21.34 mg g^-1, respectively, with outstanding cyclic stability achieved after six consecutive adsorption-desorption cycles of temperature swing adsorption-desorption at 393 K. Among the seven nonlinear kinetic models, the Avrami, FL-PFO, and general-order models exhibited the highest fitting accuracy (R² = 0.9994-0.9998), suggesting that CO₂ adsorption onto L-AC proceeds through heterogeneous, multi-stage adsorption kinetics. A Weber-Morris intra-particle diffusion analysis identified a three-stage sequential transport mechanism in which mesopore diffusion constitutes the primary rate-limiting step. Thermodynamic parameters confirmed spontaneous (ΔG° = -24.20 to -26.87 kJ mol^-1), exothermic (ΔH° = -9.42 kJ mol^-1), and entropy-assisted adsorption (ΔS° = +49.93 J mol^-1 K^-1) consistent with a physisorption mechanism, corroborated by a low activation energy of 9.11 kJ mol^-1. These findings demonstrate the viability of low-rank lignite as a low-cost precursor for the scalable synthesis of high-performance carbonaceous CO₂ adsorbents for post-combustion capture applications.