No abstract

The substitution of the divalent cations Mg 2+ and Zn 2+ into the aluminophosphate (AlPO) framework of STA-2 has been studied using an "NMR crystallographic" approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and firstprinciples calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH  or F  , and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined.   For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg 2+ for Al 3+ and, where required, by hydroxide anions coordinated to the framework. 27 Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations.The calculations also show that 31 P chemical shifts depend not only on the topologicallydistinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)2OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the 31 P isotropic chemical shift and the average bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured.

Cu2+ cations complexed by linear polyamines have been studied as structure-directing

The use of copper polyamine complexes as structure directing agents for microporous solids offers a direct route to the inclusion of Cu 2+ complex cations in their pores: upon calcination, this gives active catalysts for the selective catalytic reduction of NO with NH 3. In situ synchrotron IR absorption spectroscopy on crystals of dimensions 25-35 μm has been used to monitor the dehydration of the Cu 2+-cyclam complex that acts as a co-template for the silicoaluminophosphate SAPO STA-7 and, at higher temperatures (400 °C), the calcination that gives the active catalyst Cu,H-SAPO STA-7. Polarised synchrotron IR microspectroscopy reveals strong alignment of N-H bonds of the Cu 2+ cyclam in the larger cages of as-prepared STA-7, and complementary X-ray diffraction, ESR and UV-visible spectroscopy and computer simulation indicates that hydrated complexes act as co-templates during crystallisation: dehydration leads to removal of their coordinated water by 200 °C.