Structure and Reactivity of Zeolites using Solid-State NMR Spectroscopy

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).