A Multinuclear NMR Study of Six Forms of AlPO-34: Structure and Motional Broadening

Abstract

We report a study of the CHA-type aluminophosphate AlPO-34, prepared with six different structure-directing agents (SDAs): piperidine (pip), morpholine (mor), pyridine (pyr), 1,4,8,11-tetraazacyclotetradecane (cyclam), 1,3-dimethylimidazolium (dmim) chloride, and 1-ethyl-3-methylimidazolium (emim) bromide. Using a combination of solid-state NMR spectroscopy, periodic density functional theory (DFT) calculations, and synchrotron X-ray diffraction, we show that, even in crystallographically well-ordered materials such as AlPO-34 with dmim as the SDA, local disorder may be present. For such disordered structures, where it is challenging to use DFT to assign NMR spectra, we show that the 31P isotropic chemical shift can be predicted accurately using the mean P–O bond length and P–O–Al bond angle, in an extension of previous work. Variable-temperature 27Al NMR reveals the presence of microsecond-time scale dynamics in all forms of AlPO-34, with two different motional regimes observed, depending on whether stru...