Microwave-Assisted Conversion of Low-Rank Lignite into Hierarchical Activated Carbon: Molecular Insights into Efficient Post-Combustion CO2 Capture

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 (Iᴰ/Iᴳ = 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.