SORPTION OF WATER, METHANOL, AND AMMONIA ON ALPO4-5 AS STUDIED BY MULTINUCLEAR NMR-SPECTROSCOPY

Multinuclear NMR measurements were performed on several samples of AlPO4-5: samples that are as synthesized, calcined and rehydrated, dehydrated and those obtained after adsorption of methanol and ammonia. The dynamics of the adsorbates was followed by H-2 NMR, which was correlated with the changes the framework undergoes upon adsorption, as manifested by the P-31 and Al-27 MAS NMR spectrum. Water adsorption up to 20% of the total AlPO4-5 capacity did not affect the Al-27 and P-31 spectra, and the H-2 NMR spectrum indicates the presence of rather mobile water molecules. When the samples are completely hydrated, the Al spectrum shows the presence of octahedral Al, the P-31 line is significantly broadened, and the H-2 NMR shows the appearance of "bound" water molecules undergoing a local motion, which could be a pi-flip about the DOD bisector. These molecules are assigned to water coordinated to the octahedral Al and the flip is about the Al-O axis. The H-2 NMR spectrum of adsorbed ND3 indicates the presence of two types of molecules, just as in the case of water, with the bound molecules undergoing a rotation about the C3 axis, along the Al-N axis. In this case three types of Al were detected, tetrahedral, octahedral, and pentacoordinated. While water and ammonia behaved similarly, methanol adsorption did not produce octahedral Al and the methanol molecules were found to freely reorient within the AlPO4-5 channels. P-31 T2 measurements attributed the changes in the P-31 line width to inhomogeneous broadening due to dispersion of chemical shifts. This dispersion was assigned to the changes in the Al neighbors.