Controlling Crystallization and Transport in Metal–Organic Frameworks
This dissertation documents efforts to control and take advantage of crystallization handles in metal–organic framework synthesis. Understanding fundamental processes during synthesis allows for directed, predictable control over crystallite size and shape, which is demonstrated to play a role in intracrystalline diffusion.
Chapter 1 first introduces a summary of crystallization and transport in porous materials, including classical and non-classical models of crystallization, homogeneous and heterogeneous nucleation, and mass transfer resistances. The application of these concepts to metal–organic frameworks is presented alongside common synthetic strategies, coordination modulation and other strategies to control crystallite size and shape, and the potential for non-coordinating bases and buffers to help alleviate some complicating factors common to framework synthesis. Finally, because crystallite size and shape can control intracrystalline path lengths, an introduction to mass transfer resistances is given, including intracrystalline diffusion. Different methods of measuring diffusion in porous materials are presented, including the technique applied in this work, zero-length column chromatography (ZLC).
Chapter 2 details success in deconvoluting solution equilibria during hydrothermal metal–organic framework synthesis. The use of non-coordinating bases and anions allows for generalizable increase in crystallite size. Further, non-coordinating buffers may be used during synthesis to add or subtract individual coordinating anions at a given pH. This strategy allows for tunable and predictable control over aspect ratio in a one-dimensional metal–organic framework.
Chapter 3 describes the discovery and utilization of interfacial effects during hydrothermal synthesis to control crystalline phase and size. Controlling the interface between reaction vessel and solution via silanization is found to decrease morphological distribution, change the phase produced for stock solutions, and in some cases, increase crystallite volume by several orders of magnitude.
Chapter 4 details the assembly and usage of a ZLC instrument capable of differentiating between different mass transfer resistances. Design considerations and instrument improvements are described. The instrument is used to probe CO2 diffusion within very large crystallites (up to 700 microns in length) of Zn2(dobdc), where surface resistances and defects are proposed to account for a lower diffusivity measured via ZLC versus pulsed-field gradient NMR. Synthetic control over Co2(dobdc) path length is demonstrated to bring about improved mass transfer resistances via path length control for the industrially important molecule m-xylene.