Nanotechnology and Nanoelectronics - Materials, Devices, Measurement Techniques Part 8

Tham khảo tài liệu 'nanotechnology and nanoelectronics - materials, devices, measurement techniques part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 132 6 Selected Solid States with Nanocrystalline Structures All relevant atomic nuclei which are built into the frame structure of the zeolites the so-called framework can be detected with the help of the NMR . 29Si 27Al 17O 31P . The natural abundance of 27Al and 31P lies within 100 therefore the appropriate NMR spectra can be measured with good . short measurement times. However 27Al manifests a quadrupole moment which can lead to a widening of the NMR line by interacting with the electrical field gradients. NMR analysis requires an enrichment of oxygen with the 17O isotope since it naturally occurs only in very small quantities 158 . The NMR lines of 29Si and 31P are normally narrow. These two elements beside 27Al play an important role as framework atoms in the zeolite structures. The 29Si and 31P NMR lines are very often used for the analysis of zeolites. The importance of the 29Si NMR is based on the fact that the sensitivity of the chemical shift of 29Si correlates with the degree of condensation of the Si-O tetrahedrons . the number and the type of tetrahedrally coordinated atoms which are bonded with a given SiO4 complex. The signal of the chemical shift of 29Si in 29Si n Al with n 0 1 2 3 4 number of aluminum atoms which share any oxygen atoms with the concerned Si-O tetrahedron covers a range from -80 to -115 ppm. The highest signal occurs for n 0 . if no aluminum atom shares oxygen atoms with the Si-O tetrahedrons. An important measure that can be obtained in the long run in this way is the Si Al ratio of the zeolite frame. The existence of the so-called extra framework aluminum atoms can be proven by the 27Al NMR . Al atoms which exist in the investigated structure in addition to those tetrahedrally built into the zeolite frame. Regarding catalytic applications in particular it is of great significance that the important dealumination process be pursued with the 29Si and the 27Al NMR 158 . In this connection it can be mentioned .

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