The methanol-to-olefins reaction is an important industrial process for the production of light olefins (C2-C4). Silicoaluminophosphates are the most common catalysts for this process with SAPO-34 (CHA), SAPO-18 (AEI) and their intergrowths being considered the most catalytically active and selective. Understanding the crystal growth of such materials is important for control of the structure and defect incorporation, which can have a large effect on the catalytic behaviour. In this thesis, the synthesis, characterisation, catalysis and crystal growth of such materials are investigated. A series of CHA/AEI intergrowth materials were synthesised by sequential increases in silicon content, where low silicon content led to formation of AEI and higher silicon content led to CHA and intergrowth formation. X-ray diffraction and MAS-NMR were used to quantify the amount of intergrowth and there was a strong correlation between both techniques. Atomic Force Microscopy (AFM) revealed the mechanism by which these intergrowth structures grow. There is competition at the surface between the spiral-growth and layer-growth mechanisms, which has a significant effect on the resulting intergrowth, as intergrowth formation is only permitted with a layer-growth mechanism. Intergrowth on screw dislocations is not allowed, and thus discrete blocks of pure-phase AEI or CHA form. These intergrowth materials were tested for their performance in the methanol-to-olefins reaction. With a higher level of silicon, the catalysts had a larger acid site density but equivalent acid strength. The conversion of methanol over the catalysts correlated with the acid site density, where a greater acid site density led to higher conversion and faster deactivation. The selectivity over time was similar for all catalysts, with a high selectivity to ethylene and propylene. However, at the same percentage conversion, the C2/C3 ratio showed a strong correlation to the cage shape. Catalysts with a higher ratio of AEI cages had a higher selectivity to C3 and C4 products than the other catalysts, owing to the larger size of the internal AEI cage compared to the CHA cage. The crystal growth mechanism on SAPO-18 was investigated in detail to interrogate the complex spiral pattern that forms on the surface. Spirals form in a triangular type pattern due to differences in growth rates in different crystallographic directions. Interlaced terraces were also present. The unit cell and the relative orientation of the AEI cages define the different growth rates. In-situ AFM was used to investigate the dissolution behaviour of SAPO-18 and SAPO-34. In both cases, dissolution occurred via classical step retreat. The similarity in the layer stacking in both materials led to equivalent structure dissolution in both cases. The 0.9 nm layers dissolved first to 0.7 nm (closed cages) then to 0.4 nm (unstable intermediates). Dissolution of SAPO-18 revealed unusual spiral dissolution pits near the core of the dislocations. CHA/AEI intergrowth materials were also prepared using a dual-template method, where two templates, morpholine for CHA and N,N-diisopropylethylamine for AEI, were combined during synthesis. The phase transition from CHA to AEI occurred at different molar ratios with different synthesis procedures. XRD modelling confirmed the synthesis of an intergrowth phase at a molar ratio of 70% morpholine and 30% DPEA. Changes in chemical shift in the 13C MAS-NMR were used to observe the different template interactions with the framework as the ratio of CHA and AEI cages changed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:679995 |
| Date | January 2016 |
| Creators | Smith, Rachel |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/probing-crystal-growth-in-methanoltoolefins-catalysts(83b999a1-775e-4e55-8e94-47403d2a86c6).html |
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