An ²⁷Al MQMAS NMR Study of the Thermal Transformation between the Microporous Aluminium Methylphosphonates AlMePO-β and AlMePO-α

Abstract The 27Al MQMAS NMR technique is used to investigate the thermal transformation between AlMePO-β and AlMePO-α. The required spectral resolution is achieved using a novel split-t1 version of the basic five-quantum MAS experiment. The five-quantum split-t1 27Al MAS NMR spectrum of a sample where the thermal transformation has been interrupted before complete conversion is compared with that of a simple physical mixture of the α and β polymorphs. The two spectra are shown to be different, a result which does not appear to support the proposition by Carter et al. (J. Mater. Chem. 1997, 7, 2287) of a topotactic reconstructive transformation. Based purely on our 27Al MQMAS results, we outline a possible alternative (and rather more commonplace) mechanism for the thermal transformation.

In (a), (b), (c) and (d), projections onto the F1 axis of the two-dimensional 27Al five-quantum MAS NMR spectra of AlMePO-α, AlMePO-β, a physical mixture of AlMePO-α and AlMePO-β in a mass ratio of 1:2 and intermediate α/β form of AlMePO, respectively. The projections shown in (c) and (d) are overlaid in (e) with the former being shown as a dashed line.