Structure and Reactivity of Zeolites using Solid-State NMR Spectroscopy

纳米多孔材料是现代科学中最令人兴奋的一类固体。特定的孔径（与小分子大致相同）和高内表面积使它们特别适用于纳米级反应容器和催化剂、分子筛以及储存和运输工具。将所有类型的多孔材料联系在一起的一个特点是，它们的结构与任何应用都密切相关，因此详细的、原子水平的知识对于开发这些迷人的固体的新用途至关重要。核磁共振波谱能够提供局部原子尺度环境的详细信息，而不需要任何远程顺序或周期性，使其成为多孔材料结构，无序性和反应性的理想探针。在某些情况下，感兴趣的核磁共振活性物质的天然丰度很低，需要同位素富集（起始物质或反应中使用的试剂）才能在合理的时间尺度上获得具有良好灵敏度的光谱。在这个项目中，我们将利用固体核磁共振波谱法探索沸石的结构和反应性。硅基沸石在广泛的工业过程中起着至关重要的作用。然而，尽管有数十万种假设的沸石结构是可能的，但其中许多不能用传统的水热合成方法制备。最近开发的ADOR（组装，拆卸，组织和重组）工艺克服了这一限制，通过将已知的硅酸盐基母沸石分解成其组成部分，然后以新的方式组织这些部分，然后重新组装形成新材料。一个重要的目标是准确地了解这个过程是如何在分子水平上发生的。通过富集初始母固体的不同部分，例如29Si，然后通过使用17o富集的反应物开始分解过程，我们可以显著提高核磁共振实验的灵敏度，并使新的实验成为可能，否则不可能。这项工作将集中在两个关键领域：(1)调查ADOR进程的早期阶段。使用Ge-UTL沸石作为模型系统，将进行不同持续时间的反应（重点是反应的早期阶段），并研究固体材料和反应溶液（使用NMR和XRD），以深入了解ADOR过程的机理和形成的中间物质。通过改变反应发生的条件（如温度和酸浓度），就有可能了解如何控制反应和最终形成的产物。(2)利用原位核磁共振波谱技术了解ADOR过程。虽然可以通过研究水解不同时间的样品来获得信息，但在停止反应和准备样品进行核磁共振分析的时间内，信息可能会丢失，如果需要在反应和随后的分析之间存储样品，则可能会看到变化。了解反应机理的一个更直观的方法是在原地跟踪它，即在核磁共振转子内。这带来了许多挑战，包括需要在较小的体积下研究反应，需要快速获得具有良好灵敏度的光谱，以及需要快速旋转固体和溶液的非均质混合物。这项工作将为沸石水解的原位核磁共振研究制定一个方案，最初使用29si富集的Ge-UTL作为模型系统。然后将应用所开发的方法在不同条件下（例如，不同酸浓度或温度）和使用同位素富集试剂（例如，D2O和H217O）研究ADOR过程。

Nanoporous materials are one of the most exciting classes of solids in modern science. The specific pore sizes (which are about the same as small molecules) and high internal surface areas make them particular useful as nanosize reaction vessels and catalysts, molecular sieves, and as storage and delivery vehicles. One feature that connects all the types of porous materials is that their structure is intimately connected with any application, and so detailed, atomic-level knowledge of this is vitally important in developing new uses of these fascinating solids. The ability of NMR spectroscopy to provide detailed information on the local, atomic-scale environment, without the need for any long-range order or periodicity, makes it an ideal probe of the structure, disorder and reactivity of porous materials. In some cases the NMR-active species of interest have low natural abundance and isotopic enrichment (either of the starting material or of the reagents used in a reaction) is required to acquire spectra with good sensitivity on a reasonable timescale. In this project we will explore the structure and reactivity of zeolites using solid-state NMR spectroscopy. Silica-based zeolites play a vital role in a wide range of industrial processes. However, although many hundreds of thousands of hypothetical zeolite structures are possible, many of these cannot be prepared using traditional hydrothermal synthesis. The recently developed ADOR (Assembly, Disassembly, Organisation and Reassembly) process overcomes this limitation by disassembling a known silicate-based parent zeolite into its constituent parts, then organising these parts in a new way before reassembling them to form a new material. An important goal is to understand exactly how the process occurs at the molecular level. By enriching different parts of the initial parent solid with, e.g., 29Si and then by using an 17O-enriched reactant to start the disassembly process we can increase the sensitivity of the NMR experiments significantly and enable new experiments that would not otherwise be possible. The work will focus on two key areas:(1) Investigating the early stages of the ADOR process. Using the Ge-UTL zeolite as a model system, reactions will be carried out for varying durations (focussing on the early stages of the reaction) and both the solid material and the reaction solution studied (using NMR and XRD) to gain insight into the mechanism of the ADOR process and the intermediate species that form. By varying the conditions (e.g., temperature and acid concentration) under which the reaction takes place it will be possible to understand how to control the reaction and ultimately the products formed. (2) Understanding the ADOR process using in situ NMR spectroscopy.While information can be obtained by studying samples that have been hydrolysed for different times, information can be lost in the time taken to stop the reaction and prepare samples for NMR analysis, and changes may be seen if samples are required to be stored between the reaction and subsequent analysis. A more intuitive approach for understanding the mechanism of reaction would be to follow it in situ, i.e., within the NMR rotor. This poses a number of challenges, including the need to study reactions at lower volume, the need to acquire spectra with good sensitivity rapidly, and the need for rapid spinning of a heterogeneous mixture of solid and solution. This work will develop a protocol for in situ NMR studies of zeolite hydrolysis, initially using 29Si-enriched Ge-UTL as a model system. The methods developed will then be applied to study the ADOR process under different conditions (e.g., varying acid concentration or temperature) and using isotopically-enriched reagents (e.g., D2O and H217O).