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Open Access Publications from the University of California

Molecular Simulations of Microporous Materials for Energy and Environmental Applications

  • Author(s): Liu, Yifei
  • Advisor(s): Smit, Berend
  • et al.

Microporous materials are a diverse class of materials that includes porous carbons, zeolites, metal-organic frameworks (MOFs), and zeolite-templated carbons (ZTCs). These materials are characterized by a high surface area and diversity of pore geometry, as well as chemical tunability in MOFs, which provides a large design space for applications in gas storage and separations, catalysis, energy storage, sensing, and medicine.

The first chapter of this dissertation introduces microporous materials, their applications, and methods for studying them. In Chapter 2, I discuss a study that investigated the potential to tune the product distribution of the propene dimerization reaction via shape selectivity in MOFs and zeolites. We showed that the shape of the pore indeed exerts a large influence on the distribution of branched and linear products from propene dimerization. We demonstrated, moreover, that it is possible to predict the experimental linear conversion through the use of Monte Carlo simulations to compute the contribution of the heat of adsorption of the different products to their free energies of formation. Using this approach, we screened a library of MOFs and zeolites and identified materials which could further increase the linear conversion of propene dimerization beyond the highest-performing materials identified in the literature.

In the next chapter, Chapter 3, I detail our investigation into the adsorption of Δ9-tetrahydrocannabinol (THC) in MOFs, in order to find a material that can be used in a THC breathalyzer. We tested two models of THC, one with only the bulky, rigid head group, and another describing the full molecule, including the flexible alkyl tail; we found that the simplified THC-head model is sufficient in most cases to rank the adsorption energy and Henry coefficient of materials. We also compare NVT and Widom insertion methods for computing the heat of adsorption of the materials, finding that in general the NVT method provides more statistically reliable values from shorter simulation times than Widom insertions. We introduced a novel method to evaluate the pore accessibility to nonspherical adsorbates, based on calculating the overlap area between the pore and an ellipsoidal probe molecule, and used this criterion to further narrow the pool of promising candidates. From our screening, we identified three motifs promoting high THC adsorption, and highlight the highest-performing MOFs exhibiting these motifs. Finally, we computed the Henry coefficients of water in our materials and determined a selectivity threshold that would enable the detection of THC from humid breath.

In Chapter 4, I present results from reactive force field simulations of a set of amine-appended Mg2(dobpdc) MOFs, which have shown promising performance for carbon capture. We studied the mobility of the the amine functional groups in the MOFs, and found that they undergo a hopping motion in which the ends of the amine swap their coordination bond with the metal center of the MOF. We computed the rates of amine hopping and the activation enthalpies and entropies of the transition, and found that it is possible to tune them by using functional groups with more or less steric hindrance at the amine ends, giving us insight into how different functional groups affect the step temperature and pressure of CO2 adsorption.

In Chapter 5, I discuss the performance of ZTCs as the electrode material in an electrical-double layer capacitor (EDLC). We built EDLC simulation cells from a library of ZTC materials and investigated multiple equilibration protocols, determining that equilibrating with a constant-potential simulation is the most suitable choice. We then showed that the charging rate is strongly correlated with the pore size of the materials, which suggests that ion diffusion within the pores is the limiting process for electrode charging. Unlike the charging rate, the equilibrium capacitance of the materials was not clearly correlated to common geometric descriptors, but we found that it was strongly correlated to the charge compensation per carbon (CCpC), which indicates the efficiency of charge storage in the materials. In order to better understand the charge storage mechanisms in ZTCs, we examined the local charges and ion adsorption sites of individual pores in the materials, and identified characteristics of pores with higher and lower than average CCpC. We found that the CCpC is enhanced for adsorption sites near pockets with a small radius of curvature, and suggest this as a guideline for the design of improved EDLC electrode materials.

The final chapter concludes the dissertation and presents some perspectives on the outlook of these results.

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