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Spatiotemporal and In-situ Spectroscopic Crystallization Studies of Microporous Materials

Periodic Reporting for period 1 - ZeoSynMech (Spatiotemporal and In-situ Spectroscopic Crystallization Studies of Microporous Materials)

Reporting period: 2016-09-01 to 2018-08-31

Zeolite based catalysts are used globally in large quantities across a wide range of applications including chemical production from both renewable and more traditional fossil feedstocks, automotive emissions reduction, gas separation and water treatment. They are able to serve such diverse applications as they are crystalline, microporous materials (pores less than 2 nm) that exhibit robust hydrothermal stability, allowing them to be used under demanding process conditions. Zeolites are naturally occurring minerals, but most that are industrially used are synthetically manufactured. New materials and compositions are demanded to optimize processes and serve new applications, and are often developed by trial and error methods guided by researcher experience since the crystallization process is at best partially understood. The inefficiency of this method makes the development of new materials a time-consuming, costly process. Therefore, the primary scientific objectives of this project were to investigate the synthesis of zeolites using advanced characterization techniques, and especially monitor heteroatom incorporation (the catalytic active site), with the ultimate goal to bring greater insight to the process so that more advanced materials can be engineered.

During this project we have investigated the full lifetime of zeolite catalysts from crystallization to deactivation using advanced characterization techniques including atom probe tomography (APT) and scanning transmission X-ray microscopy (STXM). APT is a type of 3-D microscopy that can produce atom-by-atom material reconstructions with sub-nanometer resolution, and STXM is able to produce spatially resolved (50 nm spot size) XANES (X-ray absorption near edge structure) maps of materials to learn more about the local environment of specific elements. Using these advanced characterization techniques, we have added insight to the underlying mechanisms of material crystallization, the distribution of active sites in materials and the deactivation of catalysts. Some of the notable catalysts that were investigated include SAPO-34, which is industrially used to convert methanol (sourced from both renewable and non-renewable feedstocks) into desirable commodity chemicals such as propylene and ethylene, as well as copper-exchanged zeolite SSZ-13 that is used to reduce NOX emissions in diesel vehicles. These applications are top priorities for the European Union within Horizon 2020 as they allow for increasing use of renewables as well as reducing air pollution.
The research work began with synthesizing several different zeolites in order to monitor the crystallization process. This was primarily done by analyzing the conformation of the template molecules inside of the materials using Raman spectroscopy. The influence of heteroatoms such as cobalt and silicon was also investigated. Then we investigated the distribution of silicon (the active site) in SAPO-34 using atom probe tomography (APT). By doing this we could gain greater insight into the distribution and size of the active sites in the material. We also used APT to study how the automotive emissions catalyst, copper-exchanged zeolite SSZ-13 is able to resist deactivation under demanding tailpipe conditions while similar materials are not as resilient. Then, scanning transmission X-ray microscopy (STXM) was also used to study the same catalyst to understand nanoscale changes that occur to the material during a simulated aging procedure. The insights gained during these studies contribute to our overall understanding of zeolites, and this fundamental knowledge is important to engineer superior functional materials.

The acquired results were communicated to the scientific community through publishing the work in the peer-reviewed highly prestigious scientific journals, as well as presenting in multiple international conferences, via oral presentations as well as poster presentations. The outcomes of the project were also communicated to the general public through press releases and social media platforms.
The work performed in this project has led to greater understanding of solid catalysts, which are relevant for using sustainable chemical feedstocks as well as clean air. Specifically, we have investigated: 1) The importance of the conformation, that is spatial arrangement, of template molecules in zeolite synthesis. 2) The distribution of catalytically active heteroatoms in catalysts using atom probe tomography (APT), which is the highest resolution, element specific 3-D microscopy available. We have applied this to isotopically labelled materials to gain the most accurate information possible and complement the APT results with solid-state nuclear magnetic resonance (solid-state NMR) spectroscopy investigations. 3) Copper-exchanged zeolites, that are known to be used as diesel vehicle emissions reduction catalysts, were investigated in a first of its kind study that used scanning transmission X-ray microscopy (STXM) and APT to study the same zeolite crystal and correlate the findings. Therefore, this project has led to increased understanding of relevant catalysts as well as important method development. We believe these findings will assist researchers from both academia and industry in designing better catalysts, and have also shown new ways to correlate the results of advanced characterization techniques that may be applicable to a wide variety of functional materials, including batteries, fuels cells and membranes.