Structural and Dynamical Properties of Guests in Metal Organic Frameworks

Mukherjee, Piyali (2023) Structural and Dynamical Properties of Guests in Metal Organic Frameworks. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Piyali Mukherjee (Roll No. 15RS005)) 15RS005.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

Diffusion plays an important role in processes like molecular sieving, adsorption, separation, ion exchange, shape-selective catalysis, etc. These processes generally occur within the microporous solids such as aluminosilicates (zeolites), aluminophosphates, metal-organic frameworks, etc. Though many factors control the diffusion process inside these porous materials, the size and shape of the diffusing particles and the nature of the host structures have attracted considerable interest in the past. It is reported that self-diffusivity in microporous materials does not vary monotonically with the size of the diffusing particles. This is due to the cancellation of the force on the diffusing particle at the window of the microporous materials. This explains the existence of an anomalous diffusion regime. We examine whether this type of anomalous diffusion exists in the metal organic frameworks (MOFs) or not. It is known that the window of a microporous material is the bottleneck for diffusion process. The window of a MOF has a significant effect in the diffusion process. We show that the interaction between the host and the guests depends on the particle size as well as concentration and this interaction is different for different host-guest combinations. We have performed all-atom molecular dynamic (MD) simulations to understand the diffusion process of different molecules through different MOFs. Chapter 1 presents literature survey on different MOFs and their sub-groups, such as zeolite imidazolate frameworks (ZIFs). At first we describe the key features, classification, and applications of the porous materials. Then we explain the different structural aspects, topologies, and building units of MOFs and ZIFs, along with necessary examples of each type. Both MOFs and ZIFs have a large variety of applications, such as gas separation, catalysis, sensing, drug delivery, proton conductivity, etc. We have described how different MOFs and ZIFs have been selectively used for different applications. In brief, chapter 1 offers a revisit to some of the most notable contributions in the applications of MOFs and ZIFs. Chapter 2 presents a brief review of Molecular Dynamics (MD) simulations, different numerical techniques associated with the MD Simulations and their numerical forms. This chapter also describes the methods and models, as well as the characterization of the microscopic properties that have been employed in this thesis. The computed radial distribution functions (RDFs) indicate the locations of the guests at the windows of the host during the diffusion process. The different regions in the Mean Square Displacement (MSD) plots explain the different forms of diffusion of particles inside the host. The calculated self-diffusivity (D) values show the effect of temperature and concentration of guests on the diffusion of molecules in different conditions. Chapter 3 shows molecular dynamics simulation study of size-dependent diffusion of model guest particles in a metal-organic framework. For this study, we have taken zeolite imidazolate framework-8 (ZIF-8) as the host structure and some model guests with different sizes as our guest particles. Our target was to study exclusively the effect of guest size on the diffusion process, so we have varied the guest size up to a smaller value than the window diameter of the ZIF-8. We have performed an extensive MD simulation at different temperatures and concentrations of the guest particles to explore the effect of size of the guests as well the structural properties of the window of the host on the diffusion process. It is also observed that the guest-host interaction significantly depends on the concentrations of the guests at all temperatures. Chapter 4 describes a computational investigation of the separation of trans-1, 3-butadiene (t-C₄H₆) from the C₄ hydrocarbons mixture using a ZIF-71. Separation of C₄ hydrocarbons, particularly trans-1, 3-butadiene (t-C₄H₆) from other C4 hydrocarbons mixtures such as 1-butene (1-C₄H₈), iso-butene (i-C₄H₈) and n-butane (n-C₄H₁₀) is extremely important in the oil and gas processing industry. We have performed extensive molecular dynamics simulation to explore the possibility of using the metal–organic framework (ZIF-71) as a host for the separation of C₄ hydrocarbons from their mixtures. From the self-diffusion coefficient value, we show that even at room temperature ZIF-71 is an excellent host to separate trans-1, 3-butadiene (t-C₄H₆) from the other C₄ hydrocarbons. It is also observed that at the appropriate thermodynamic conditions all the C₄ molecules can be individually separated from their mixtures by using only ZIF-71 metal organics framework. Chapter 5 explains the separation of CO₂/CH₄ by the same ZIF (that is, ZIF-71) as that used in the previous study in chapter IV. As ZIF-71 has been proven to be a convenient option for the separation of C₄ hydrocarbon mixtures, we have used this to see whether it can also be used for other separation processes also. A molecular dynamics simulation study shows that the separation of CO₂/CH₄ by ZIF-71 is possible at lower temperature. The self-diffusivity values of the two components show that CO₂ moves much faster than CH₄ through the host structure of ZIF-71. It is also observed that the effect of temperature on the diffusion is negligible for CO₂, but it has a significant effect on the diffusion of CH₄ at a concentration under study. Chapter 6 describes some of the future work which can be done from the understanding we obtain from this thesis.

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