Investigation on pore size regulation of carbon molecular sieves at sub-nanometer scale and their separation performance for C8 cycloalkane isomers

The precise regulation of sub-nanometer pore structures in carbon molecular sieves (CMS) is pivotal for the efficient separation of C8 cycloalkane isomers. This study addresses the separation challenge between ethylcyclohexane (ECH) and its cis-1,2-dimethylcyclohexane(cis-1,2-DMCH) and trans-1,3-dimethylcyclohexane(trans-1,3-DMCH) counterparts. We systematically investigate the structural properties of CMS with tunable pore sizes in the sub-nanometer range (5.4–29 Å) and their impact on the adsorptive separation performance for C8 cycloalkanes. A material system comprising six CMS samples with distinct pore dimensions was constructed. By integrating static adsorption, kinetic analysis, dynamic breakthrough experiments, and molecular dynamics simulations, we elucidate the structure–performance relationships. The results demonstrate that CMS-4, featuring a narrowly distributed ultramicropore size of 6.4 Å and a high micropore volume (0.42 cm 3  g −1 ), exhibits optimal adsorption capacity for ECH (19.4 mg g −1 ) and superior selectivity (with selectivity coefficients of 1.98 and 2.02 over cis-1,2-DMCH and trans-1,3-DMCH, respectively). Adsorption kinetic studies reveal a diffusion rate order of ECH > trans-1,3-DMCH > cis-1,2-DMCH, which aligns well with the hierarchy of effective molecular widths derived from their minimum cross-sectional areas, confirming the regulatory role of molecular geometry in diffusion behavior. Dynamic breakthrough experiments further validate the selective separation capability of CMS-4 for ECH under continuous flow conditions. Molecular simulations clarify that the differences in diffusion behavior originate primarily from steric hindrance effects related to molecular cross-sectional size, while the stronger interaction energy between ECH and the carbon framework provides a thermodynamic basis for its preferential adsorption. The three-dimensional "pore-size matching–molecular size–diffusion kinetics" correlation model established in this work provides both a theoretical foundation and experimental support for the rational design and performance optimization of high-performance carbon molecular sieves aimed at efficient liquid-phase isomer separation.