27Al multiple-quantum magic angle spinning NMR study of the thermal transformation between the microporous aluminum methylphosphonates AlMePO-β and AlMePO-α

The Al-27 multiple-quantum ma,alc angle spinning (MQMAS) NMR technique is used to investigate the thermal transformation between AlMePO-beta and AlMePO-alpha. The required spectral resolution is achieved using a novel split-t(1) version of the basic five-quantum MAS experiment. The five-quantum split-t(1) Al-27 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 alpha- and beta-polymorphs. The two spectra are shown to be different, a result that does not appear to support the proposition by Carter et al. (J. Mater. Chem. 1997, 7, 2287) of a topotactic reconstructive transformation. On the basis of our Al-27 MQMAS results, we outline a possible alternative (and rather more commonplace) mechanism for the thermal transformation.