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首页> 外文学位 >Physicochemical, Spectroscopic Properties, and Diffusion Mechanisms of Small Hydrocarbon Molecules in MOF-74-Mg/Zn: A Quantum Chemical Investigation
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Physicochemical, Spectroscopic Properties, and Diffusion Mechanisms of Small Hydrocarbon Molecules in MOF-74-Mg/Zn: A Quantum Chemical Investigation

机译：MOF-74-Mg / Zn中小烃分子的理化性质，光谱性质和扩散机理：量子化学研究

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摘要

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.
机译：在石油精炼工业中，压裂过程允许将长链烃裂化为小烯烃和石蜡分子的混合物，然后通过需要能量和金钱的低温蒸馏过程将其分离。为了减少能源消耗和资本消耗，可调谐吸附剂（例如金属有机骨架（MOF））对轻烃的选择性吸附似乎是更有效的气体分离工艺的最有希望的替代方法。 MOF是由有机连接剂连接的无机砖组装而成的新型多孔材料。从晶体工程的角度来看，MOF在创建微孔（0.2--2.0 nm）到中孔（> 50 nm）空隙腔的范围内具有优势，为有机连接基和空隙的功能化提供了独特的机会。 MOF-74-M系列（M =金属）具有非常重要的意义，其特征是开放金属位点的密度很高，而不是完全协调的金属中心。 MOF的这一家族也称为CPO-27-M。与其他已知的MOF和沸石相比，MOF-74具有更大的分离潜力。在原子轨道（LCAO）线性组合方法中实施的密度泛函理论（DFT）已用于研究MOF-74-Mg / Zn中C1-C4烃的选择性吸附。该研究首先通过采用分子簇方法进行，然后通过应用周期性边界条件（PBC）进行。尽管两种模型方法均一致认为显示出MOF的开放金属位点处吸附的烯烃与石蜡之间的结合能存在显着差异，但与在PBC水平上获得的结果相比，分子簇水平上报道的结果却被低估了。 PBC模型的使用可校正基集叠加误差（BSSE），分子横向相互作用（LI），零点能量（ZPE）和热能（TE）贡献的结合能。这样，在PBC水平上获得的结果与实验量热值（即，吸附热）直接可比。这项工作首次讨论了通过分子簇获得的结合能与实验吸附热之间的虚构协议的起源，确定了其起源是由于误差补偿。提出了基于相对于气相分子的强度和频移的光谱学研究，作为对小烃（C1-C 2）与MOF-74-Mg中开放金属位点相互作用的进一步研究。为了提供对烃分子行为的更全面描述，还提出了扩散机理研究的结果。扩散机制的研究基于攀爬图像微动弹性带（CI-NEB）模拟，并结合了在平面波（PW）中实现的范德华函数（vdW-DF）和超软伪势）DFT方法。 CI-NEB研究表明，相对于其烯烃对应物，石蜡分子更倾向于在MOF-74-Mg的腔内和沿腔扩散。

著录项

  • 作者

    Degaga, Gemechis D.;

  • 作者单位

    Michigan Technological University.;

  • 授予单位 Michigan Technological University.;
  • 学科 Physical chemistry.
  • 学位 Ph.D.
  • 年度 2018
  • 页码 145 p.
  • 总页数 145
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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