Formation of Porous Aluminophosphate Frameworks Monitored by Hyperpolarized 129Xe NMR Spectroscopy

Abstract

The formation of two aluminophosphate frameworks, AlPO4-5 and AlPO4-18, has been monitored using hyperpolarized 129Xe NMR as a probe. The reaction progress from an amorphous phase to AlPO4-5 and its subsequent conversion to AlPO4-18 is observed for xenon adsorbed in samples that have been reacted for varying lengths of time. The extreme sensitivity of 129Xe chemical shifts to local environments of the xenon leads to separate NMR peaks for xenon adsorbed in the two different aluminophosphate frameworks present in a single reaction mixture. For xenon adsorbed in AlPO4-5, an anisotropic 129Xe NMR line shape at δiso ≈ 63 ppm is observed, whereas for xenon in AlPO4-18, a peak with a small anisotropy at δiso ≈ 74 ppm is observed. Grand canonical Monte Carlo simulations provide a theoretical description of xenon adsorbed in the two aluminophosphate frameworks, and the resulting simulated 129Xe NMR line shapes are in good agreement with experimental data. Two-dimensional exchange NMR spectroscopy was used to examine the exchange of xenon between AlPO4-5, AlPO4-18, and bulk xenon gas. The results indicate that the domains of AlPO4-5 are in intimate contact with those of AlPO4-18, facilitating a higher intercrystallite exchange than with the bulk xenon gas.