Sorption of water, methanol, and ammonia on AlPO4-5 as studied by multinuclear NMR spectroscopy

Multinuclear NMR measurements were performed on several samples of AlPO{sub 4}-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 {sup 2}H NMR, which was correlated with the changes the framework undergoes upon adsorption, as manifested by the {sup 31}P and {sup 27}Al MAS NMR spectrum. Water adsorption up to 20% of the total AlPO{sub 4}-5 capacity did not affect the {sup 27}Al and {sup 31}P spectra, and the {sup 2}H 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 {sup 31}P line is significantly broadened, and the {sup 2}H 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 {sup 2}H NMR spectrum of adsorbed ND{sub 3} indicates the presence of two types of molecules, just as in the case of water, with the bound molecules undergoing a rotation about the C{sub 3}more » 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 AlPO{sub 4}-5 channels. {sup 31}P T{sub 2} measurements attributed the changes in the {sup 31}P line width to inhomogeneous broadening due to dispersion of chemical shifts. This dispersion was assigned to the changes in the Al neighbors.« less