The molecule 4-piperidinopiperidine was predicted by the computer methodology ZEBEDDE^ to be a suitable template for the synthesis o f an aluminophosphate with a chabazite-type structure, the template should completely fill the CHA cage, thus forming the product efficiently and preventing the competitive formation o f other phases. A cobalt-substituted aluminophosphate was synthesised using this template; the product was a blue crystalline phase with an X-ray pattern similar to that o f the
43
mineral chabazite . The material formed much more quickly than other chabazite- type materials, in a few hours compared to several days. There was no evidence of competitive formation o f any other phases. This material was designated DAF-5. From a Rietveld refinement o f the high-resolution powder diffraction data collected for this material, it was shown that this material did indeed have a framework
structure similar to chabazite. Micro-single crystal diffraction (on a crystal of size 30
X 30 X 30|im) located only one template molecule per cage, and, as predicted this fully occupied the cage.
Combined XAS/XRD was used to study the stability o f the material during the calcination process; the framework started collapsing at ca. 400°C, which is the temperature we would expect the template molecule to be lost from the framework. Following the example o f Barrett et al^, we attempted to stabilise the structure by incorporating silicon into the structure so the cobalt concentration could be lowered, whilst keeping the overall heteroatom concentration the same.
Two CoAPSO’s were prepared, with the cobalt concentrations reduced from 20% to 4% and 10%. Combined XAS/XRD showed that both o f these materials were stable and retained their structure throughout the calcination process; the instability of DAF-5 CoAPO was almost certainly caused by having too much cobalt in the aluminophosphate framework. Analysis o f the EXAFS data showed that there is a slight decrease in the Co-0 bond length during the calcination for both materials; therefore only a small percentage o f the Co(II) is being oxidised to Co(III). Also, there was no shift in the edge positions after the calcination, which is what we would expect if a change in oxidation state is occurring. Elemental analysis showed a significant amount (20 to 35%) of silicon replaces aluminium as well as phosphorus in the aluminophosphate framework with the formation o f silicon islands. This is confirmed from the infra-red spectra: the bands one would expect if Bronsted acidity
56 59-64
is present ’ are very low in intensity compared to other CHA-type
aluminophosphates. If silicon clusters are present, acidity will only occur at the edges o f the Si-rich regions, thus greatly reducing the overall acidity.
In summary, from the results obtained we can conclude that on comparison of the DAF-5 materials with other chabazite-type structures, DAF-5 is not likely to show catalytic activity in either acid-catalysed reactions e.g. conversion of methanol to light alkenes, or in oxidation reactions where good redox properties are required. However, our work has demonstrated the power of the de-novo design technique in assisting the synthesis o f microporous materials and has once more emphasised the important role of silicon in stabilising aluminophosphate materials.
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