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.