Dissertation/Thesis Abstract

Physicochemical, Spectroscopic Properties, and Diffusion Mechanisms of Small Hydrocarbon Molecules in MOF-74-Mg/Zn: A Quantum Chemical Investigation
by Degaga, Gemechis D., Ph.D., Michigan Technological University, 2018, 145; 10791501
Abstract (Summary)

In petroleum refining industries, the fracturing process allows for the cracking of long-chain hydrocarbons into a mixture of small olefin and paraffin molecules that are then separated via the energetically and monetarily demanding cryogenic distillation process. In an attempt to mitigate both energetic and capital consumptions, selective sorption of light hydrocarbons by tunable sorbents, such as metal-organic frameworks (MOFs), appears to be the most promising alternative for a more efficient gas separation process. MOFs are novel porous materials assembled from inorganic bricks connected by organic linkers. From a crystal engineering stand point, MOFs are advantageous in creating a range of microporous (0.2–2.0 nm) to mesoporous (>50 nm) void cavities, presenting unique opportunities for the functionalization of both the organic linkers and the void. Of significant importance is the MOF-74-M family (M = metal), characterized by a high density of open metal sites, that is not fully coordinated metal centers. This family of MOF is also known as CPO-27-M. MOF-74 have demonstrated more separation potential than other known MOFs and zeolites. Density functional theory (DFT), as implemented within a linear combination of atomic orbital (LCAO) approach, has been used to investigate the selective sorption of C1-C4 hydrocarbons in MOF-74-Mg/Zn. The study was first implemented by adopting a molecular cluster approach, and later by applying periodic boundary conditions (PBC). While both modellistic approaches agree in showing significant differences in binding energies between olefins and paraffins adsorbed at the MOFs’ open metal sites, results reported at the molecular cluster level show underestimation when compared to those obtained at the PBC level. The use of PBC models allow for the correcting of binding energies for basis set superposition error (BSSE), molecular lateral interaction (LI), zero-point energy (ZPE), and thermal energy (TE) contributions. As such, results obtained at the PBC level are directly comparable to experimental calorimetric values (i.e., heat of adsorptions). This work discusses, for the first time, the origin of the fictitious agreement between binding energies obtained with molecular clusters and experimental heats of adsorption, identifying its origin as due to compensation of errors. Spectroscopy studies based on the intensities and frequency shifts with respect to the molecules in the gas phase are presented as a further investigation of the interaction of the small hydrocarbons (C1-C 2) with the open metal sites in MOF-74-Mg. In an attempt to provide a more comprehensive description of the behavior of the hydrocarbon molecules, results from diffusion mechanism studies are also presented. The investigations of the diffusion mechanisms are based on the use of climbing-image nudge elastic band (CI-NEB) simulations, coupled with van der Waals functional (vdW-DF) and ultra-soft pseudopotentials as implemented within the plane-wave (PW) DFT approach. The CI-NEB studies showed that paraffin molecules are more energetically favored to diffuse within and along the cavity of MOF-74-Mg with respect to their olefin counterparts.

Indexing (document details)
Advisor: Valenzano, Loredana
Commitee: Luck, Rudy L., Pandey, Ravindra, Perrine, Kathryn A.
School: Michigan Technological University
Department: Chemistry
School Location: United States -- Michigan
Source: DAI-B 79/10(E), Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Physical chemistry
Keywords: CI-NEB, DFT, Diffusion, Hydrocarbons, MOFs, Separation
Publication Number: 10791501
ISBN: 9780355979534
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