Wolfgang Meiler

Carbon-13 nmr investigations of structure and mobility of adsorbed hydrocarbons

Abstract Carbon-13 NMR investigations of olefins adsorbed on synthetic faujasites (NaX, NaY, TIX) and on commercial alumina catalysts impregnated by Na + ions suggest the existence of a specific interaction between the olefins and Na + . The dominant effect on carbon-13 line shifts in the highly resolved spectra is attributed to olefin—sodium bonds. ln Na + -containing solutions no influence on carbon-13 lines due to olefin complexing has been found. Measurements on silver—olefin complexes in solution and in the adsorbed state yield comparable chemical shifts indicating a similar structure for the complexes in both cases. In addition, the first reported studies of the mechanism of the isomerization of n-butene by carbon-13 NMR methods are described.

13C NMR investigations of carbon monoxide adsorbed in zeolites

13C NMR spectra of CO and CO2 molecules adsorbed in zeolites of A, X, Y type were measured as a function of temperature and pore filling. In contrast to other systems, strong resonance shifts to lower fields appear when CO is adsorbed in decationated zeolites. These shifts can be interpteted by an interaction with adsorption sites of Lewis type.

NMR of Organic Compounds Adsorbed on Porous Solids

Publisher Summary This chapter focuses on nuclear magnetic resonance (NMR) of organic compounds adsorbed on porous solids. Porous solids, such as zeolites (molecular sieves), silica, and alumina, play an important role in many industrial processes, especially in the chemical industry, where they are used mainly as catalysts or as adsorbents. Most of the molecules make the use of classical thermodynamic methods, such as the measurement of adsorption isotherms and adsorption heats, or of well-established spectroscopic techniques, mainly with infrared spectroscopy. Moreover, from the 13 C relaxation times and the nuclear overhauser enhancement (NOE) factors, additional information about the geometry and the microdynamic behaviour of the adsorbed molecule can be derived. The chapter mainly emphasizes those reviews that are connected with a study of highly resolved NMR spectra of organic compounds adsorbed in porous solids and their theoretical interpretation in terms of a quantum mechanical treatment of the surface complexes formed.

Ab initio calculation of magnetic properties of CO, the equilibrium geometry and the 13C, 17O and 27Al NMR shielding tensors of the system

Abstract The susceptibility and chemical shielding of CO were determined with regard to the experimental and optimized bond length using coupled Hartree—Fock

NMR of Coals and Coal Products

Publisher Summary The chapter discusses the nuclear magnetic resonance (NMR) of coals and coal products. Of the various analytical methods available, NMR spectroscopy has proved to be of special importance in coal research. Different NMR techniques allow the structure of coal and coal products to be investigated in a direct and nondestructive way. Moreover, besides the investigation of pit coal extracts and a variety of coal-derived materials, the measurements made on the samples in connection with the technological coal conversion and refinement processes are also possible. The application of the NMR techniques to the study of fossil carbonaceous materials is of interest from the standpoint of both basic theoretical and applied research. The chapter demonstrates the application of the NMR techniques to brown coals from westelbian coalfields near Leipzig. A short survey is given of the origin, the composition of coals, and the processing technologies of coal conversion. A few general remarks are discussed in the chapter on the application of the NMR techniques to coal research together with some experimental details. The chapter elaborates on the NMR experiments carried out on coals, with regard to studying the carbon network, the proton system, including the water content, and the heteroatoms. The applicability of the NMR techniques in the field of physical and chemical treatment of coals and of the investigation of coal-derived products is also described in this chapter.

13 C nuclear magnetic resonance investigations of carbon monoxide in decationated zeolites of type Y

The 13C n.m.r. of carbon monoxide adsorbed on decationated zeolites of type Y is characterized by extremely large resonance shifts to lower magnetic field. The strong dependence of this resonance shift on temperature and on coverage can be explained by means of an exchange model described in the literature. This treatment yields the resonance shift of carbon monoxide molecules in the adsorption complex (350 ppm relative to gaseous CO) and the number of sites in zeolites effective for the complex formation (0.07 active sites per cavity). Investigations of the 13C nuclear magnetic relaxation of CO and CO2 adsorbed on zeolite NaY and its decationated forms suggest that the large resonance shifts are due to an interaction with Lewis-type centres in the form of extra-lattice aluminium ions. Thus, investigations of the CO adsorption by means of the 13C n.m.r. method are suitable for a characterization of Lewis-acid sites in zeolites.

Study of n-butene isomerization and polymerization on CaNaY-type zeolites by means of 13C-N.M.R. spectroscopy

Abstract With highly resolved carbon-13 n.m.r. spectroscopy, the isomerization and polymerization of but-1-ene sorbed in CaNaY-type zeolites have been investigated. The application of molecules partially enriched with carbon-13 nuclei allowed a detailed study of the transformtion of labelled groups. At lower temperatures (290 – 350 K) only the double bond isomerization of butenes proceeds in CaNaY-type zeolites with 65 – 70% of their sodium ions replaced by calcium. Within the limits of experimental error an enrichment of 13C nuclei was found in the CH3 and CH groups of but-2-ene molecules if the but-1-enes were enriched in the positions CH2 and CH, respectively. Relative equilibrium concentrations as determined from n.m.r. line intensities were in fair agreement with known data for the thermal equilibrium of the n-butene isomers in the gas phase. Thus, possible differences between Overhauser factors of the different carbon-13 resonance lines were so small that they did not give rise to significant errors in determining concentrations via the enchanced resonance line intensities. Polymerization reactions occurred at higher temperatures. They gave rise to mutually overlapping lines of saturated carbons which were difficult to detect unless carbon-13 enriched butene molecules were applied. Using pore filling factors of ≈ ≈ 0.6, partly branched octanes were formed. This is at variance with investigations at low pore filling factors of ≈ ≈ 0.1 where predominantly octenes were found. These experimental results are explained by a different availability of hydrogen as an important controlling factor in these reactions.

Ab initio calculation of the equilibrium geometry and the 13C, 17O, 23Na and 7Li NMR shielding tensors of the systems Na+/CO and Li+/CO

Abstract Using ab initio SCF calculations two minima have been found in Na + /CO and Li + /CO. The minimum corresponding to CO…Na + is slightly deeper than in Na + …CO. The minimum corresponding to CO…Li + is deeper than in Li + …CO. The 13 C, 17 O, 23 Na and 7 Li NMR shielding tensors were computed with the coupled Hartree—Fock method with large gaussian basis sets.

Molecular mobility in coal-solvent multicomponent systems

Abstract The n.m.r. pulsed field gradient technique is applied to study molecular mobility in mixtures of coal with organic solvents. In this way it is possible to measure the self-diffusion coefficient of dissolved coal constituents, of residual water and of the solvent itself. It has been found that for sufficiently large concentrations of the solvent and/or residual water (≈20 mass%) the mobility of these three components increases with both increasing humidity and solvent concentration, while for smaller concentrations in most cases a distinct minimum in molecular mobility is observed.

Zur Adsorption von Olefinmolekülen in Zeolithen

The interaction between 1-olefine molecules and Na+ ions in zeolites of type NaY were studied by13C NMR spectroscopy and quantum chemical calculations. Characteristic chemical shifts of olefinic carbon atoms indicate a terminal arrangement of the cation near the olefinic part of the molecule.