Impact of Carbon Dioxide Loading on the Thermal Conductivity of Metal Organic Frameworks

Organic Polymeric Materials Physical Chemistry Chemistry Materials Science Materials Chemistry
Metal Organic Frameworks Thermal Conductivity Carbon Dioxide Molecular Dynamics Simulations Gas Adsorption

Academic Background

The issue of global warming is becoming increasingly severe, and the research on carbon dioxide (CO₂) capture and storage technologies has become a hotspot in the scientific community, as CO₂ is one of the most significant greenhouse gases. Metal-Organic Frameworks (MOFs), with their exceptionally high porosity and surface area, are considered ideal materials for capturing and storing CO₂. However, the adsorption process of CO₂ is exothermic, which may lead to an increase in material temperature, thereby affecting its adsorption efficiency. Therefore, understanding the impact of CO₂ loading on the thermal conductivity of MOFs is crucial for optimizing their performance in practical applications. Previous studies have mainly focused on the thermal conductivity of MOFs without gas loading, while systematic research on the thermal conduction mechanisms of gas-loaded MOFs is lacking. This paper employs molecular dynamics simulations and lattice dynamics calculations to deeply explore the influence of CO₂ loading on the thermal conductivity of MOF-5, revealing the key role of temperature and gas diffusivity in thermal conduction.

Source of the Paper

This paper is co-authored by Sandip Thakur and Ashutosh Giri, both affiliated with the Department of Mechanical, Industrial, and Systems Engineering at the University of Rhode Island, USA. The paper was published on April 15, 2025, in the Journal of Chemical Physics and is part of the 2024 JCP Emerging Investigators Special Collection.

Research Process

1. Molecular Dynamics Simulations and Thermal Conductivity Calculations

The study first utilizes the Reactive Force Field (ReaxFF) to conduct molecular dynamics (MD) simulations, examining the thermal conduction behavior of MOF-5 under different CO₂ loadings. The specific process is as follows: - System Equilibration: The system is equilibrated using the NPT (constant temperature and pressure) and NVT (constant temperature and volume) ensembles to ensure stability. - Thermal Conductivity Calculation: The Green-Kubo (GK) method is employed to calculate thermal conductivity, deriving the values through the heat current autocorrelation function (HCACF). - Vibrational Mode Analysis: The longitudinal and transverse current correlation functions are calculated using the Dynasor software to analyze the phonon dynamics of MOF-5.

2. Gas Diffusivity Calculations

The study derives the diffusion coefficient of CO₂ molecules by calculating their mean square displacement (MSD), analyzing the diffusion behavior of CO₂ under different temperatures and gas densities.

3. Minimum Thermal Conductivity Model

The study also compares the experimental results with the classical minimum thermal conductivity model, validating the changes in thermal conductivity of MOF-5 at low and high temperatures.

4. Spectral Heat Flux Calculations

Spectral heat flux calculations quantify the contribution of different vibrational frequencies to the total heat flux, revealing the role of CO₂ molecules in thermal conduction.

Key Findings

1. Influence of Temperature and Gas Density on Thermal Conductivity

The study finds that at low temperatures (<200 K), CO₂ molecules adsorb onto the pore walls of MOF-5, leading to enhanced solid-gas interactions and increased phonon scattering, significantly reducing thermal conductivity. At high temperatures (>200 K), the diffusivity of CO₂ molecules increases, allowing them to move freely within the pores, providing additional channels for heat conduction, thereby increasing thermal conductivity with higher gas densities.

2. Vibrational Mode Analysis

Vibrational mode analysis shows that at low temperatures, gas loading significantly affects the phonon dynamics of MOF-5, leading to shortened phonon lifetimes and reduced thermal conductivity. At high temperatures, gas loading has a minimal impact on phonon modes, and thermal conductivity is primarily contributed by the heat conduction of gas molecules.

3. Relationship Between Gas Diffusivity and Thermal Conductivity

By calculating the diffusion coefficient of CO₂, the study finds that gas diffusivity is extremely low at low temperatures but significantly increases at high temperatures. This result explains the reduction in thermal conductivity at low temperatures and the increase at high temperatures.

Conclusion

This study, through systematic molecular dynamics simulations and lattice dynamics calculations, reveals the mechanisms by which CO₂ loading affects the thermal conductivity of MOF-5. The research finds that temperature and gas diffusivity are key factors determining changes in thermal conductivity. At low temperatures, gas adsorption leads to increased phonon scattering and significantly reduced thermal conductivity, while at high temperatures, the free movement of gas molecules provides additional channels for heat conduction, increasing thermal conductivity with higher gas densities. These findings provide important theoretical insights for optimizing the application of MOFs in gas storage, separation, catalysis, and thermoelectrics.

Research Highlights

  1. Innovative Methodology: This paper is the first to combine reactive molecular dynamics simulations and lattice dynamics calculations to systematically study the impact of gas loading on the thermal conductivity of MOFs.
  2. Significant Findings: It reveals the critical role of temperature and gas diffusivity in thermal conduction, offering new perspectives for the thermal management of MOFs.
  3. Application Value: The results contribute to optimizing the application of MOFs in CO₂ capture and storage, while also providing a reference for thermal conduction research in other gas-loaded materials.

Additional Valuable Information

The paper also provides detailed supplementary materials, including vibrational density of states, bulk modulus, Green-Kubo method, mean square displacement, and gas diffusivity calculations, offering rich data support for researchers in related fields.

Through this study, we not only gain a deeper understanding of the mechanisms by which CO₂ loading affects the thermal conductivity of MOFs but also provide important theoretical guidance for the future design and application of MOF materials. This research holds significant scientific and practical value in addressing global warming and developing novel gas storage technologies.