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

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• 作者：Daniel M. Dawson ; John M. Griffin ; Valerie R. Seymour ; Paul S. Wheatley ; Mahrez Amri ; Teresa Kurkiewicz ; Nathalie Guillou ; Stephen WimperisRichard I. Walton ; Sharon E. Ashbrook
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：January 26, 2017
• 年：2017
• 卷：121
• 期：3
• 页码：1781-1793
• 全文大小：685K
• ISSN：1932-7455

文摘

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 structural H₂O is present. H₂O is detected in AlPO-34 prepared with mor as the SDA, although this material was previously reported as anhydrous, suggesting that this form of AlPO-34 may be hygroscopic despite the presence of the SDAs within the pores.