Templating approaches to the synthesis of new microporous materials for gas adsorption and separation
Abstract
Structure direction in the synthesis of phosphate-based materials
(aluminophosphates, AlPOs; magnesiumaluminophosphates, MgAPOs;
silicoaluminophosphates, SAPOs; magnesiumsilicoaluminophosphates, MgAPSOs), has
been investigated through co-templating synthesis studies supported by molecular
modelling. These solids have been characterised by diffraction and solid-state NMR, and
their properties in gas adsorption and catalysis have been measured.
The parameters in the hydrothermal synthesis of SAPO STA-7, St Andrews
porous solid number 7, (SAV), in which the macrocycle 1,4,7,11-
tetraazacyclotetradecane (cyclam) and tetraethylammonium (TEA) cations are used as
co-templates, were investigated in detail. A new route involving a reversal of the mixing
order of reagents leads to the formation of single crystals up to 50 μm with perfect
tetragonal prismatic morphology that was not achieved via previous synthetic routes. For
the 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 novel
aluminophosphate structure designated STA-14 (KFI) represents the first example of a
designed synthesis of a zeotype. The synthesis route is based on a co-templating approach
supported by molecular modelling to design the specific template for one of the two types
of 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-ray
diffraction. Modifying the gel chemistry leads to SAPO and MgAPSO STA-14, which
display 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 structure
of relevance to hypothetical VPI-5 (VFI) type extended structures.
Molecular modelling was also applied in another aluminophosphate-based
material, that of STA-2 (SAT), to predict a template that could be prepared from
inexpensive reagents. Existing routes required the use of expensive quinuclidine as a precursor to the template 1,4-bisquinuclidinium butane. The template suggested by
modelling, 1,4-diazabicyclo (2,2,2)octane butane (NC₆H₁₂N⁺-C₄H₈-⁺NC₆H₁₂N), labelled
DABCO_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 neutral
framework and a combination of ¹³C, ¹⁴N and ¹⁵N NMR studies have been used to give
further details of the template environment in the cages.
The gas adsorption behaviour of the stable materials STA-7, STA-14 and STA-2
was evaluated for CO₂. High pressure adsorption (0 to 40 bars) on STA-7 and STA-14
shows similar behaviour due to their structural and chemical similarities. The total uptake
of CO₂ for SAPO STA-7 is less than for the zeolite NaX (FAU) (3.4 and 5.2 mmolg
-1
respectively at 373 K and 12 bars) but the usable capacity for pressure swing adsorption
technology (PSA) between 1 to 20 bar for STA-7 is twice the value for NaX. The affinity
of adsorption towards CO₂ and its low uptake at 1 bar made SAPO STA-7 a desirable
sorbent 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 CO₂
adsorption (0 to 1 bar) in STA-7, STA-14 and STA-2 at temperatures between 273 and
303 K demonstrates that the topology and therefore the total free pore volume accessible
to 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 also
has an important effect. For example at 273 K and 1 bar, the uptake of the STA-2
framework in the SAPO form is ca. 2wt% higher than in the AlPO form, but compared
with SAPO STA-7, the uptake due to pore volume limitations is 10wt% lower under
same conditions. In addition, the high quality of the SAPO STA-7 crystals obtained by
the new route made them suitable in collaborations for the direct observation of diffusion
of methanol by interference microscopy (IFM) and the study of their crystal growth by
combined atomic force microscopy (AFM) and high resolution scanning electron
microscopy (HRSEM). The catalytic applications of the STA-7 and STA-14 for the
methanol-to-olefins reaction (MTO) and the argon adsorption at 87 K were also
performed collaboratively. The results are reported and discussed here in the light of their
structure and composition.
Type
Thesis, PhD Doctor of Philosophy
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