Metal organic frameworks for greenhouse gas capture: from atomic-level understanding to device design using computational methods

Abstract

The significant challenges facing our society include global warming and climate change, both stemming from the increasing worldwide emission of greenhouse gases and demand for energy worldwide. Since global warming results from the greenhouse effect, a natural process that warms Earth's surface by trapping heat in the atmosphere through greenhouse gases. Human activities, particularly the burning of fossil fuels driven by energy demand, significantly contribute to the increase in greenhouse gases, intensifying this effect and causing the planet to heat up. To address these challenges, our research have two main focuses:

i. Our first area of investigation involves using molecular simulation methods to understand how greenhouse gases can be adsorbed in adsorbents. Metal-organic frameworks (MOFs) have emerged as promising candidates for selective gas separations due to their tuneable pore size and chemistry. MIL-101(Cr), a Cr-based MOF composed of chromium ions and terephthalic acid ligands, possesses an exceptionally high specific surface area, large pore size, significant pore volume, superior separation selectivity, ease of regeneration, and excellent thermal, chemical, and water stability. Numerous studies have not considered the influence of the size and configuration of gas molecules and the topology structure of MIL-101(Cr) on the adsorption process. Moreover, there is a lack of quantitative analysis of the effect of different atoms of MIL-101(Cr) on gas adsorption, and these atoms are regarded as different adsorption sites. Additionally, understanding how gas mixtures (such as binary and ternary mixtures) are adsorbed remains a challenge in research. To address these issues, we employed Monte Carlo and molecular dynamics simulations to investigate how greenhouse gases (e.g. CO₂, CH₄) are adsorbed in MIL-101(Cr), including adsorption isotherms, density distribution profiles, self-diffusion coefficients, radial distribution functions (RDF), and gas selectivity.

ii. The second area of investigation mainly studies the feasibility of using MIL-101(Cr) as a solid adsorbent for Direct Air Capture (DAC) process. For effective process design and optimization, it is crucial to have a thorough understanding of the dynamic behavior of these adsorption systems. So, it's crucial to determine the effluent concentration profile, commonly known as the breakthrough curve. Through analyzing the breakthrough curve predictions of MIL-101(Cr) for two-component and three-component mixed gases, can a new DAC process be proposed to separate CH₄ and CO₂ from gas mixture? This will lay a theoretical foundation for practical industrial applications (e.g. biogas and coal mine gas upgrading). Additionally, another investigation explores the feasibility of a low-energy consumption device as a novel air intake unit to process large-scale airflow by utilizing several methods, including Computational Fluid Dynamics (CFD), mathematical models and experiments. Various solar updraft designs have been explored in recent years, and the Trombe wall is considered one of the most versatile formats. It offers advantages such as low building material costs, a small footprint, simple structure, and high thermal efficiency. The research question is whether this technology can be deployed globally, as in regions with moderately low solar radiation (typical of many European countries), can it generate sufficient updraft? Our investigation aims to answer this crucial question and provide insights into the potential applicability of this technology worldwide.

This extensive investigation started with the first focus, we systematically explored how various gas mixtures, including methane (CH₄), nitrogen (N₂), and carbon dioxide (CO₂), interact with MIL-101(Cr), a type of Metal Organic Framework. The study revealed that the primary adsorption sites for CH₄ and N₂ are associated with the C=C double bond in the benzene ring of MIL-101(Cr), where Cr and O atoms play a secondary role. Conversely, for CO₂ adsorption, the primary adsorption site is the open metal center (Cr), while the C=C double bond serves a secondary role. CH₄ demonstrated a preference for occupying large and medium cages, while N₂ and CO₂ exhibited more uniform distribution patterns.

Expanding on this research, a subsequent study focused on CO2 and CH4 mixtures as well as CH₄ and N₂ gas mixtures in MIL-101(Cr) for highly selective separation, particularly in biogas and coal mine methane applications. In binary gas mixtures, MIL-101 demonstrated higher selectivity for CO₂ over CH₄ and for CH₄ over N₂. Interestingly, the presence of CH₄ in both cases further reduced the diffusion coefficient of the respective gases.

To address the gap in understanding ternary gas mixtures, a multi-scale simulation (e.g. molecular, device and process scale) approach was employed. The study demonstrated MIL- 101's higher affinity for adsorbing CO₂ in ternary mixtures. N2 demonstrated higher mobility within MIL-101 compared to CH₄ and CO₂. The adsorbent's topological structure primarily determines the upper limit of gas adsorption capacity. Density distribution diagrams illustrated the spatial occupation of gas molecules changed with the quantity of other gas molecules within MIL-101. An increase in CH₄ constrained CO₂ and N₂ diffusion space, reducing their self- diffusion coefficients. This microscopic understanding contributes to a comprehensive view of MIL-101's potential for selective gas capture.

Back to the second focus, Chapter 6 shows that a mathematical model was developed in RUPTURA code to describe the flow and mass transfer of CO₂-CH₄-N₂ ternary gas mixtures, incorporating an adsorption isotherm model to study the performance of DAC processes. A pioneering approach involves the integration of molecular-scale and macro-scale simulations to predict breakthrough curves of gas mixtures. Additionally, a two-adsorption-column DAC system with Temperature Swing Adsorption cycle is proposed and the results demonstrate that the predicted breakthrough curve validates the significant potential of MIL-101(Cr) solid adsorbent in efficiently purifying CH₄ and CO₂ from coal mine methane and biogas at ambient conditions with a simple step process. Chapter 7 shows that a simplified solar updraft device, inspired by the Trombe wall concept, was built as an air intake unit to replace the function of the fan from DAC system, investigating its performance under low solar intensities. Experimental results demonstrated significant airflow generation even with low solar radiation, and numerical models provided insights for designing future devices. The study explored the effects of solar radiation intensities, air channel height, and thickness on updraft performances, offering valuable guidance for practical applications.

In summary, the important findings of this work are as follows: (1) Gas diffusion is influenced by pore size, molecular size, and the co-existence of another gas. (2) Gas distribution in MOF depends on gas molecular configurations (e.g. size and shape). (3) CH4 and N2 adsorption depends on C=C bond (major) and Cr, O atoms (secondary) of MIL-101(Cr). (4) CO₂ adsorption depends on C = C bond (secondary) and Cr, O atoms (major) of MIL-101(Cr). (5) The adsorbent's topological structure primarily determines the upper limit of gas adsorption capacity. (6) A two-stage DAC system is proposed and the results confirm MIL-101's potential in purifying CH₄ efficiently from coal bed methane and biogas under ambient condition. (7) The new design of air intake unit, a simplified Trombe wall, can generate enough airflow under low solar conditions. These findings provide valuable insights into the complex interactions within gas mixtures and the selective capabilities of MIL-101(Cr), offering implications for various applications in gas separation processes.