Synthetic Control and Characterization of NU-1000

Abstract

The production and release of greenhouse gasses has become a major issue in today’s society. Methane is a powerful greenhouse gas and is the main component of natural gas. Natural gas is often collected and transported to be used as a fuel, but leaks result in release of some of that methane into the atmosphere. Work is underway to develop an efficient catalyst capable of selective oxidation of methane to methanol. Metal-organic frameworks have become popular candidates for catalysts and catalyst scaffolds. The Zr-based metal-organic framework NU-1000 is a robust, mesoporous material that can be used in a variety of applications, including catalysis, sensing, gas separation and storage, and scaffolds. It can be synthesized by combining a solution of hexazirconium nodes ([Zr6O16H16]8+) and organic acid modulator in N,N-dimethylformamide with a solution of linker (1,3,6,8-tetrakis(p-benzoic acid)pyrene) and aging at elevated temperature. The typical product is 1-3 μm crystals that are primarily composed of NU-1000 but that contain domains of an impurity phase called NU-901 that is a polymorph of NU-1000. The ideal NU-1000 synthesis will yield phase-pure particles and enable control over crystal size. The structural differences between NU-1000 and NU-901 lead to a hypothesis that changing the organic acid modulator from benzoic acid to a larger and more rigid carboxylic acid might lead to steric interactions between the modulator coordinating on the node and linkers bound to nodes, inhibiting growth of the more dense NU-901-like material and resulting in phase-pure NU-1000. Side-by-side reactions comparing the products of synthesis using benzoic acid or biphenyl-4-carboxylic acid as organic acid modulator produce structurally heterogeneous crystals and phase-pure NU-1000 crystals, respectively. NU-1000 particles synthesized in the range of 1-3 μm, while useful for many applications, are not large enough for single-crystal X-ray diffraction and are not small enough for nanomaterial applications like drug delivery. The synthesis of NU-1000 provides a variety of experimental handles that can be tuned to produce a wide range of particle sizes. For example, the rate of nucleation and growth is closely tied to the concentration of modulator. This is because NU-1000 is formed via a competitive reaction between modulator and linker molecules for the binding sites on the hexa-Zr nodes. By changing the concentration of the linker, modulator, and any additives, the nucleation and growth rates can be altered to produce the desired particle size. The choice of Zr precursor between ZrOCl2 • 8 H2O and ZrCl4 also plays a significant role in determining the resulting particle size. People acquire a wide range of data like crystal size and morphology, crystallographic information, and elemental quantification and distribution using techniques like transmission electron microscopy and energy-dispersive X-ray spectroscopy. The characterization of size, size distribution, crystallinity, and chemical composition are critical to studying the catalytic properties of product materials. However, due to the delicate nature of MOFs, gathering this data can be very challenging. MOFs commonly undergo radiation damage under a focused electron beam causing a loss of crystallinity. While various techniques can circumvent this damage like cryogenic transmission electron microscopy and low-dose electron microscopy, this dissertation focuses on analyzing the damage and ensuring the data collected remains reliable.