Aromatic compounds dramatically suppress diffusion by forming strong π–π interactions with carbon nanotube walls and with each other, while higher temperatures can partially reverse this effect. The findings provide molecular-level guidance for optimizing supercritical water gasification and related energy technologies.
Supercritical water—formed when water exceeds its critical point (647.1 K, 22.1 MPa)—has become a powerful green medium for converting biomass, plastics, and fossil resources into fuels and chemicals. Under these conditions, water’s density, viscosity, dielectric constant, and hydrogen-bond network change dramatically, making it less polar, highly diffusive, and capable of dissolving a wide range of organic compounds and gases. These unique properties underpin technologies such as waste plastic recycling, in situ resource extraction, and supercritical water gasification (SCWG). In practical applications, however, SCW and organic intermediates coexist within nanoporous structures, where confinement significantly alters diffusion and reaction pathways. How molecular structure—particularly differences between alkanes and aromatics—regulates transport under such confinement remains a critical question for optimizing SCWG performance.
A study (DOI: 10.48130/scm-0025-0015) published in Sustainable Carbon Materials on 15 January 2026 by Hui Jin’s team, Xi'an Jiaotong University, reveals how molecular structure controls mass transport in nanoconfined supercritical water systems, providing molecular-level insights to optimize supercritical water gasification and related energy conversion processes.
Molecular dynamics simulations were first validated by calculating the self-diffusion coefficients of bulk water, benzene, and methane and comparing them with available experimental data, using diffusion as the primary metric to assess force field reliability. The simulated values deviated by less than 7% from experiments, confirming that the model accurately reproduces the transport behavior of both water and organic molecules and providing a solid basis for subsequent confined-system analysis. Using this validated framework, the study systematically examined the effects of confinement size, solute concentration, and temperature on SCW–organic mixtures inside carbon nanotubes under typical SCWG conditions. Increasing nanotube diameter significantly enhanced diffusion for both solutes and SCW, reflecting weakened wall restrictions; however, aromatics consistently showed much lower diffusivities than alkanes, and the suppression intensified with increasing ring number. For example, compared with methane systems, anthracene reduced solute diffusion by over 80% and SCW diffusion by up to 50%. Energy analysis revealed that CNT–solute interactions dominate (60–80% of total energy) and are an order of magnitude stronger in aromatic systems, driven by π–π interactions. Radial density profiles confirmed dense near-wall adsorption layers for aromatics at ~3.3–3.5 Å from the surface, consistent with π–π stacking distances, explaining their restricted mobility. Increasing solute concentration (1–30%) further reduced diffusion by disrupting the SCW hydrogen-bond network and shifting the system’s energy contribution toward solute-dominated interactions, particularly for multi-ring aromatics. Finally, raising temperature from 673 to 973 K significantly increased diffusivity by weakening wall adsorption and promoting desorption, reducing near-wall aromatic fractions by 16–22%. Together, these results demonstrate that molecular structure, confinement scale, concentration, and temperature collectively regulate mass transport in nanoconfined SCW–organic systems.
In conclusion, this study demonstrates that molecular structure critically regulates mass transport in nanoconfined supercritical water systems. Aromatic hydrocarbons strongly adsorb onto pore walls and form π–π stacked clusters, significantly suppressing diffusion and potentially reducing gasification efficiency or promoting coking. By elucidating the underlying adsorption and clustering mechanisms, the findings provide theoretical guidance for optimizing SCWG conditions through temperature control and pore design to enhance mass transfer and reaction performance.
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References
DOI
10.48130/scm-0025-0015
Original Souce URL
https://doi.org/10.48130/scm-0025-0015
Funding information
This work was financially supported by the National Key R&D Program of China (Grant No. 2020YFA0714400), and Shaanxi Science & Technology Co-ordination & Innovation Project (Grant No. 2024GX-YBXM-518).
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