New possibilities of NMR spectroscopy in studies of adsorption and catalysis

Abstract

By using modern Fourier spectrometers operating at high (about 7 T) magnetic field and having special devices for magic angle spinning (MAS), one can obtain within reasonable periods of time, the NMR spectra for nuclei of various elements present in heterogeneous catalysts, including nuclei with small magnetic moments and low natural abundance, as well as the high resolution spectra of these catalysts and of molecules adsorbed on their surfaces. The following examples of such studies are presented: A comparison of 51V-, 17O- and 23NA-NMR spectra of vanadium catalysts, for SO 2 oxidation to SO 3 with those of various reference compounds made it possible to identify some chemical compounds present in the active component of these catalysts, as well as to elucidate the interaction between the active component and the support. Coordination of vanadium by pyrosulphate has been studied in the melt of the active component under typical conditions (500°C) at which the catalysts usually operate. By using 27Al-MAS NMR, it is possible to resolve the lines from aluminium ions in tetrahedral and octahedral positions for various crystalline modifications of Al 2O 3. For zeolites using 27Al NMR, some of the aluminium was found on certain treatments to escape from tetrahedral positions in the lattice and to acquire octahedral coordination in the presence of water. The 29Si-MAS spectra support this conclusion, indicating a simultaneous decrease in the fraction of silicon atoms of the lattice that have four and three aluminium atoms in the second coordination sphere, and than increase in the fraction of silicon atoms that have two and one aluminium atoms in the second coordination sphere. In 1H-MAS spectra of zeolites, lines from various hydroxyl groups can be resolved. The higher sensitivity of modern spectrometers, together with the use of MAS techniques, open novel possibilities for the study of adsorbed species. As an example, results obtained for the adsorption of hydrogen, methane, carbon monoxide and propylene on heterogenous catalysts are presented.