The location of active sites in microporous solids
Abstract
The location of heteroatoms within porous materials was examined in chosen zeolite
and zeotype materials. For zeolites, the substitution of aluminium (mainly, but also
gallium and boron) for silicon was examined in materials with the AFX and EUO
topology types. For the zeotypes, magnesium and zinc substitution for aluminium was
studied in MAPO STA-2 (SAT framework) as well attempts to include silicon into the
material. In addition to the investigation of heteroatoms within the framework, the role
of templating has been explored in (Cu-)SAPO STA-7 and phosphonium cation
templated materials, where calcination leaves extra-framework Cu²⁺ cations and
phosphorus species, respectively. Solid-state NMR spectroscopy, X-ray diffraction
(single-crystal and powder) and IR spectroscopy were used for the characterisation of
these structures.
For MAPO and ZnAPO STA-2, the location of the heteroatoms in the framework was
confirmed and their role in charge balancing established. Additionally, for MgAPO STA-2, the NMR indicates preferential substitution of Mg in one of the two
crystallographically distinct Al sites. The zeolite SSZ-16 (AFX structure type, which like
SAT has two topologically-distinct Al sites) was synthesised with two closely related
diquaternary templates The synthesis was optimised with respect to purity,
crystallisation conditions, and time, giving a range of materials with different Si/Al
contents. Based on ²⁷Al and ¹¹B MAS NMR, it was not possible to resolve between the
two crystallographic sites. Full details are given of the crystallisation using these
organic structure directing agents. Other templated materials crystallised included
mordenite, EU-1, L and offretite. For the more complex and higher Si/Al EUO materials
(10 crystallographically-distinct tetrahedral sites), it was attempted to locate heteroatom
positions in the isostructural EU-1 and ZSM-50, which are prepared with
hexamethonium and dibenzyldimethyl ammonium cations, respectively. These zeolites
show considerable charge-balancing of the charged template by framework defects
over the Si/Al range. These defects can be healed by post-synthetic treatment with
aqueous NH₄F solutions. In all samples examined, the ²⁷Al MAS NMR shows
significant differences between the EU-1 and ZSM-50 materials, which is attributed to
the role of the different charge distribution of the templates.
With focus on extra-framework species, the calcination and subsequent dispersion of
Cu²⁺ cations throughout the pore space of Cu-SAPO STA-7 was examined by
Synchrotron IR spectroscopy of microcrystals at the Diamond Light Source. Applying
polarised IR radiation, direction-dependent information was extracted. Combining IR
spectroscopic data with complementary techniques such as EPR spectroscopy, X-ray
diffraction and computational modelling, the dehydration state of the templating Cu-cyclam complex and the change in orientation of the complex was determined based
on the characteristic N-H stretching vibration.
Additionally, a range of commercially-available and custom-made phenylphosphonium-
based organics have been examined as templates for MAPO based materials. A
layered phase based on tetraphenylphosphonium and methyltriphenylphosphonium
cations was synthesised and the structure was confirmed by Rietveld refinement on
powder X-ray diffraction data. For diphenylphosphonium templates, framework MAPOs
have successfully been prepared, including those with the ATS topology type (MAPO-36). The phosphonium-based templates were removed by calcination, leaving
phosphate species in the pores behind according to solid-state NMR.
Type
Thesis, PhD Doctor of Philosophy
Rights
Embargo Date: 2019-01-11
Embargo Reason: Thesis restricted in accordance with University regulations. Print and electronic copy restricted until 11th January 2019
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