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Investigation of porous solids applying 129Xe NMR
Molecular sieves are a class of microporous (pore diameter < 20 Å) open-framework solids, which include aluminosilicates (zeolites), aluminophosphates, and related materials. Zeolites are built from corner-sharing SiO4 and AlO4 tetrahedra linked by the apical oxygen atoms to form a framework of high internal surface area with channels, cages, and cavities of molecular dimensions. Aluminophosphates (AlPO) consist of linked TO4 tetrahedra, where aluminium and phosphorus alternate as T atoms. Molecular sieves belong to the most important inorganic compounds, and possess a wide range of practical applications, for example as catalysts, adsorbents, ion exchangers and detergent ingredients.
These microporous materials have huge internal surface areas capable of adsorbing guest atoms (or molecules) with proper size and shape. These "spies", such as noble gases, can indirectly acquire valuable information about the host structure by detecting the changes in the NMR spectrum compared to free bulk gas state. It has turned out that 129Xe NMR of adsorbed xenon gas is the most applicable and convenient method. The nuclear shielding - and therefore also NMR chemical shift - of adsorbed 129Xe is known to depend on the size and shape of the void space, the chemical composition of the adsorbent, and the nature and concentration of coadsorbed molecules. The 129Xe NMR signals are averaged by fast exchange between adsorption sites and even with the gas phase, and thus cannot usually be assigned to a particular site or to structural features. Another problem is to account contributions arising from Xe-Xe interactions to the observed chemical shift.
The main interest of our studies is in various aluminophosphate molecular sieves, including AlPO4/SAPO-5, -11, -31, and -41 materials. For the silicoaluminophosphates (SAPO), negative lattice charges due to silicon is compensated by framework cations, which affect significantly the catalytic activity, hydrothermal stability and adsorptive properties of the material. The properties of these cations can be studied with different probe atoms and NMR spectroscopy. We have also done experimental work with other microporous materials, such as H-ZSM-5, NaA, 13X, and Abscents. The main part of the experimental techniques include temperature and loading dependent measurements of chemical shifts and anisotropic powder patterns (with lineshape fittings), 1D/2D exchange spectroscopy and magic angle spinning (MAS).
Static 129Xe NMR spectra of adsorbed xenon in most of the materials usually reveal a temperature and loading dependent anisotropic and asymmetric nuclear shielding tensor. The deformation of the shielding tensor is often related to the interactions between the interiors of the sieve channels (and for example with the charge-compensating cations) and the mutual probe atom interactions. Besides Xe-129, other noble gases can be used as well; 21Ne, 83Kr and 131Xe NMR experiments have been carried out for gases adsorbed in several zeolites and molecular sieves.
Computational studies of zeolites have been done and so far include molecular dynamics simulations of 129Xe adsorbed in zeolite NaA and AlPO4-11 as well as ab initio calculation of 129Xe shielding.
Investigations of mesoporous materials by xenon porometry are descriped here.