Templating approaches to the synthesis of new microporous materials for gas adsorption and separation

Molecular sieves--Synthesis;Chemical templates;Gases--Absorption and adsorption

Castro, María ; Wright, Paul Anthony ; Wright, Paul Anthony

University:University of St Andrews

Department:Chemistry (School of)

关键词: Molecular sieves--Synthesis;    Chemical templates;    Gases--Absorption and adsorption;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/851/MariaCastroPhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

Structure direction in the synthesis of phosphate-based materials(aluminophosphates, AlPOs; magnesiumaluminophosphates, MgAPOs;silicoaluminophosphates, SAPOs; magnesiumsilicoaluminophosphates, MgAPSOs), hasbeen investigated through co-templating synthesis studies supported by molecularmodelling. These solids have been characterised by diffraction and solid-state NMR, andtheir properties in gas adsorption and catalysis have been measured.The parameters in the hydrothermal synthesis of SAPO STA-7, St Andrewsporous solid number 7, (SAV), in which the macrocycle 1,4,7,11-tetraazacyclotetradecane (cyclam) and tetraethylammonium (TEA) cations are used asco-templates, were investigated in detail. A new route involving a reversal of the mixingorder of reagents leads to the formation of single crystals up to 50 μm with perfecttetragonal prismatic morphology that was not achieved via previous synthetic routes. Forthe first time in SAPO STA-7, X-ray diffraction locates the tetraethylammonium cation(TEA) in tg.tg. conformation. The synthesis and full characterisation of a novelaluminophosphate structure designated STA-14 (KFI) represents the first example of adesigned synthesis of a zeotype. The synthesis route is based on a co-templating approachsupported by molecular modelling to design the specific template for one of the two typesof cages within the structure. The first, a larger type of cage, also present in AlPO-42(LTA), is templated by the azaoxacryptand 4,7,13,16,21,41-diaza-1,10-bicyclo[8,8,8]-hexocosane (‘Kryptofix 222’, hereafter K222). The modelled co-template configuration,in this case TEA in the tt.tt configuration, was experimentally observed by X-raydiffraction. Modifying the gel chemistry leads to SAPO and MgAPSO STA-14, whichdisplay high pore volumes for N₂ adsorption, similar to those of STA-7 and SAPO-34(CHA). Furthermore, during these synthetic studies, a novel fully tetrahedrally-coordinated magnesiumaluminophosphate layer phase has been prepared, with a structureof relevance to hypothetical VPI-5 (VFI) type extended structures.Molecular modelling was also applied in another aluminophosphate-basedmaterial, that of STA-2 (SAT), to predict a template that could be prepared frominexpensive reagents. Existing routes required the use of expensive quinuclidine as a precursor to the template 1,4-bisquinuclidinium butane. The template suggested bymodelling, 1,4-diazabicyclo (2,2,2)octane butane (NC₆H₁₂N⁺-C₄H₈-⁺NC₆H₁₂N), labelledDABCO_C4, templated AlPO STA-2 successfully. Structure characterisation of the as-prepared form of AlPO STA-2 using X-ray synchrotron data suggest the formation of Al-OH-Al units to accommodate the positively-charged template within the neutralframework and a combination of ¹³C, ¹⁴N and ¹⁵N NMR studies have been used to givefurther details of the template environment in the cages.The gas adsorption behaviour of the stable materials STA-7, STA-14 and STA-2was evaluated for CO₂. High pressure adsorption (0 to 40 bars) on STA-7 and STA-14shows similar behaviour due to their structural and chemical similarities. The total uptakeof CO₂ for SAPO STA-7 is less than for the zeolite NaX (FAU) (3.4 and 5.2 mmolg-1respectively at 373 K and 12 bars) but the usable capacity for pressure swing adsorptiontechnology (PSA) between 1 to 20 bar for STA-7 is twice the value for NaX. The affinityof adsorption towards CO₂ and its low uptake at 1 bar made SAPO STA-7 a desirablesorbent for PSA. The zeotype affinity of adsorption for different probe gases is different,CO₂ >> CH₄ > CO, to that for zeolite NaX CO₂ >> CO > CH₄. Low pressure CO₂adsorption (0 to 1 bar) in STA-7, STA-14 and STA-2 at temperatures between 273 and303 K demonstrates that the topology and therefore the total free pore volume accessibleto the gas molecules is the most important factor in determining the uptake in these solids,but that the composition and distribution of the silicon cations within the framework alsohas an important effect. For example at 273 K and 1 bar, the uptake of the STA-2framework in the SAPO form is ca. 2wt% higher than in the AlPO form, but comparedwith SAPO STA-7, the uptake due to pore volume limitations is 10wt% lower undersame conditions. In addition, the high quality of the SAPO STA-7 crystals obtained bythe new route made them suitable in collaborations for the direct observation of diffusionof methanol by interference microscopy (IFM) and the study of their crystal growth bycombined atomic force microscopy (AFM) and high resolution scanning electronmicroscopy (HRSEM). The catalytic applications of the STA-7 and STA-14 for themethanol-to-olefins reaction (MTO) and the argon adsorption at 87 K were alsoperformed collaboratively. The results are reported and discussed here in the light of theirstructure and composition.