Search

The preparation of stable large pore aluminophosphate (AlPO) zeotypes offers materials for applications in adsorption and catalysis. Here we report the synthesis of the pure AlPO with the SAO topology type (AlPO STA-1) using N,N′-diethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidine (DEBOP) as the organic structure directing agent in the presence of fluoride. The AlPO STA-1 can be rendered microporous (pore volume 0.36 cm3 g−1) via calcination and the calcined form remains stable in the presence of moisture. The structure of the dehydrated form has been established by Rietveld refinement (tetragonal P4¯n2, a = 13.74317(10) Å, c = 21.8131(5) Å, V = 4119.94(16) Å3). Multinuclear 27Al and 31P MAS NMR, together with 2D COSY and CASTEP NMR calculations, enables resolution and assignment of the signals from all crystallographically distinct Al and P framework sites. Structural elucidation of the as-prepared aluminophosphate-fluoride is more challenging, because of the presence of partially protonated OSDA molecules in the 3D-connected channel system and in particular because the fluoride ions coordinate with positional disorder to some of the Al atoms to give 5-fold as well as tetrahedrally-coordinated framework Al species. These are postulated to occupy Al–F–Al bridging sites, where they are responsible for distortion of the framework [P4¯n2, a = 13.3148(9) Å, c = 22.0655(20) Å, V = 3911.9(7) Å3]. Calcination and removal of fluoride ions and OSDAs allows the framework to expand to its relaxed configuration. The SAO topology type aluminophosphate can also be synthesised with small amounts of Si and Ge in the framework, and these SAPO and GeAPO STA-1 materials are also stable to template removal. IR spectroscopy with CO as a probe at 123 K indicates all have weak-to-mild acidity, increasing in the order AlPO < GeAPO < SAPO. These STA-1 materials have been investigated for their activity in the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam at 598 K: while all are active, the AlPO form is favoured due to its high selectivity and slow deactivation, both of which are a consequence of its very weak acid strength, which is nevertheless sufficient to catalyse the reaction. [ABSTRACT FROM AUTHOR]

The fully La-exchanged form of the large pore zeolite L, La3.0Al9.0Si27.0O72, has been prepared by repeated aqueous ion exchange and internal cation exchange resulting from calcination to give a low silica zeolite (Si/Al = 3) with all cations hidden from available porosity. This results from the migration of La3+ from sites in the large 12-membered ring (12R) channel (exchangeable via aqueous solution) to 'closed' sites in the structure. The mechanism for this internal cation exchange, which has been elucidated by Rietveld analysis of powder X-ray diffraction data, comprises sequential La3+ migration, first to ste cages between cancrinite (can) cages in the framework and then into the can cages. In each case K+ cations must first leave the closed cages, in the first case through 8Rs, in the second via puckered 6Rs. The first migration step is achieved by 573 K, the second by 1073 K. Decrease of the K+ occupation in accessible main channel sites of zeolites K9.0–, K5.7La1.1–, K2.7La2.1– and La3.0–L strongly changes the shape of the CO2 adsorption isotherms, as the strength of interaction decreases. The Henry law constant for K9.0–L zeolite is 36.60(2) mol (kg bar)−1, whereas for La3.0–L it is 1.80(3) mol (kg bar)−1, if an initial uptake of 0.1 mmol g−1 is neglected. The isosteric heat of adsorption is correspondingly strongly reduced. A CO2 isotherm shape similar to that observed for La–L is also shown by a fully-exchanged La–Y, La18.7Al56.0Si136.0O384, prepared from Na–Y by repeated aqueous and internal ion exchange. In this case the internal exchange requires the egress of Na+ cations from sodalite cages via planar 6R windows and so is possible at lower temperatures (<673 K). The ability to modify CO2 isotherm shape in low silica zeolites by hiding cations in inaccessible cages can be of interest to tailor pressure swing adsorbents for a range of CO2-containing gas streams. [ABSTRACT FROM AUTHOR]